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CA1225434A - Method and apparatus for integrally heated electrochemical sensors - Google Patents

Method and apparatus for integrally heated electrochemical sensors

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Publication number
CA1225434A
CA1225434A CA000447373A CA447373A CA1225434A CA 1225434 A CA1225434 A CA 1225434A CA 000447373 A CA000447373 A CA 000447373A CA 447373 A CA447373 A CA 447373A CA 1225434 A CA1225434 A CA 1225434A
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Canada
Prior art keywords
cell
electrolyte
electrode
heating
layer
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Expired
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CA000447373A
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French (fr)
Inventor
Harold A. Brouneus
William T. Kane
Margaret M. Layton
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Corning Glass Works
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Corning Glass Works
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  • Measuring Oxygen Concentration In Cells (AREA)

Abstract

ABSTRACT

The invention is, in part, a unique, solid electro-lyte-integral cell electrode/heater configuration which pro-vides a zone of uniform maximum heating at a predetermined location within the electrolyte and which, when used with a suitable resistive heating electric current, confines the current to the integral electrode/heater prolonging cell life. Another aspect of the invention is the use of a radio frequency alternating electric current for resistively heating an electrochemical cell. Preferably, the radio frequency selected is sufficiently high so as to eliminate any offsets in the emf developed by the cell which are caused by the heater current. The invention also includes an apparatus for measuring the concentration of particular gases, such as oxygen, incorporating either and, preferably, both other inventive aspects of the invention. The preferred cell configuration reduces the complexity of such an apparatus by eliminating the auxiliary heat source and provides a more accurate and reliable electrochemical sensing cell. Radio frequency heating allows the operations of cell heating and emf measurement to proceed independently and concurrently and provides a continuously responding, self-heating detection apparatus.

Description

e - Y l~S43~

IMPROVED METHOD AND APPA~ATUS FOR INTEGRALLY
HEATED ELECTROCHEMICAL SENSORS

FIELD OF THE I~VENTION

The invention -elates to electrochemical cells oper-ated at elevated temperatures and, in particular, to method and apparatus for heating electrochemical cells such as those used for detecting and/or mea~uring the concentration of oxy-gen or other gaseous compounds to an elevated temperature for proper operation.

B~CKGROU~D OF THE INVENTION

The use of electrochemical cells with solid electro-lytic elements and gas porous platinum electrodes for detect-ing or measuring the content of oxygen or certain other gase ous compounds in a sample gas is well known. See, for example, U.S. Patent No. 3,928,161 to McIntyre et al. and ~o. 4,282,078 to Chamberland et al. ~aterials such as zirconia and yttria-thoria are good oxygen ion conductors at elevated temperaturesbut are less conductive or essentially non-electronically con-ductive at temperatures between the elevated temperatures and room temperature. Where the electrodes of such a cell are sub-jected simultaneously to differing oxygen concentrations, an emf is developed across the cell between the electrodes the value of which can be determined by the ~ernst equation as follows:
RT Pl (O~) ?
emf = 4F ln P2 (O2) +

where emf = sensor output in volts T = absolute temperature R = gas constant F = Farraday constant Pl (2) = reference gas (oxygen) partial pressure P2 (2) = sample gas (oxygen) partial pressure r = cell constant ~ .

.

r ~ S 4 3 ~L

Similar equations can be developed for other electrochemical systems. The cell constant C is a correction factor which reflects the inability of a particular cell to perform to theoretical limits represented by the preceding term of the equation. Cells used for oxygen concentration determination should be designed so as to consistently exhibit a known, predetermined cell constant value under all operating condi-tions in order that the bias in emf might ~e corrected or, preferably, eliminate such bias entirely (and thus the con--stant C).
A particularly useful application of oxygen concen-tration measuring apparatus is for the measurement of the oxygen content of exhaust gases from boilers, furnaces, glass ~urnaces, etc. in order that the combustion or smelting pro-c~ss may ~e optimally controlled. Cl~sed loop oxygen sensing systems for combustion control in large steam ~lants used for power generation or industrial heating have been commercially available for at least ten years. Many of the systems avail-able use a zirconia (ZrO2) based solid electrolyt~ sensing cell installed in either an in si~u or extractive mode. An in situ sensor is physically located within the boiler exhaust gas stream. Extractive systems are located out~ide the stack and require "plumbing" to carry a representatiYe sample of the flue gas from the stack to the sensor. Currently avail-able commercial units of bo,h designs typically are provided 21 with an auxiliary electric furnace to heat the sensor cell to a satisfactory operating temperature. In in situ sènsors, the outer surfac~ of the furnace also typically acts to pro-tect the sensor probe from damage due to the high velocity particles in the flue gas and from dust caking which would ~5 effect the accuracy of the sensorO
The auxiliary electric furnaces used by currently available oxygen measuring systems have certain undesirable characteristics. They are an added component to the system and represent a significant fraction of its total cost. They are cum~ersome to mount around the electrochemical sensor cell anà require considerable design work to assure providing pro~er .. .. .. . .. .. . . . .. . . . . .. .

:~22543~

heating with simultaneous sample gas exchange. They tend to be bulky and heavy, particularly when used with an in situ sensor which typically extends a meter or more into a flue or stack gas stream. Moreover, such furnaces consume substantial amounts of electrical power, typically between 200 and 400 watts, to maintain a sensor cell at a conventional 800C op-eratins temperature. Lastly, they are the least reliable com-ponent of the system and are a significant source of repair problems and cost.
Various attempts have been made to eliminate or at least reduce the size and co~plexity of the auxiliary furnace.
For example, U.S. Patent 4,098,650 to Sayles depicts an in situ oxygen measuring device in which the sensing cell is formed with a hollow interior into which is inserted a coil resistance wire heater. The aoresaid U.S. Patent 3,g28,161 to McIntyre et al. shows a different configuration for an in situ oxygen detector apparatus incorporating a resistance wire heater located wi~hin a tubular member supporting a disk shaped solid electrolyte sensor cell. U.S. Patent 4,334,940
2~ .to Habdas et al. depicts yet another solid electrolyte oxygen sensor in which a resistance wirs hea~er is incorporated in-ternally into a tubular support member containing the ~olid electrolyte sensor cell. While these heater configurations would appear to use less power than auxiliary furnaces, the series resistance windings employed by each would continue to consume significant amounts of electrical power. Moreover, incorporating the heater windings as indicated requires addi-tional manufacturing steps and increasing manufacturing com-plexity, particularly where t~e wires are ~o be threaded through fine holes. Lastly, heatin~ of the solid electrolyte sensor cell would appear to be uneven and unpredictable. The sensor output is dependent upon temperature (see the Nernst equation, above) and failure to maintain the sensor at a spe-cific temperature or to accurately measure the temperature of the sensor will lead to errors in the detector output.

.... . . . . . . .. .. . . . . . . . .. ... ..

4 ~2S~34 OBJECTS OF THE INVENTION
-It i5 an object of an aspect of the invention to provide a highly reliable and accurate oxygen concentration measuring apparatus with a self-heating electrochemical cell sensor.
It is an object of an aspect of the invention to provide a novel integral heater/cell electrode configuration for a solid electrochemical cell providing more uniform and controllable heating of the cell.
It is an object of an aspect of the invention to provide a novel integral cell electrode/heater configuration for a solid electrochemical cell providing an enlarged area or zone of uniform heating of the electrolyte of the cell.
It is an object of an aspect of the invention to provide an integral cell electrode/heater configuration for a solid electrochemical cell which diminishes the likelihood of a resistive heating current flowing through the solid electrolyte of the cell.
It is an object of an aspect of the invention to provide an integral cell electrode/heater configuration for a solid electrochemical cell wherein adequate heating of the cell is provided by resistive heating of only the integral cell electrode/heater.
It is an object of an aspect of the invention to provide a novel method of and apparatus for resistively heating an electrochemical sensor of a solid state gas concentration sensing apparatus with an alternating electric current which minimizes or eliminates the effect of the alternating current on the magnitude of the emf developed by the sensor.
It is an object of an aspect of the invention to provide a novel method of and apparatus for measuring the concentration of particular gaseous substances using a solid electrochemical cell which is simultaneously, in a nonparallel fashion, resistively heated.

~L2Z5 ~39L
SUMMARY OF THE INVENTION
The aforesaid disadvantages of the prior art are overcome and the above-indicated and other objects are provided by the subject invention which is hereinafter S described with .~Z2S43~L

respect to a preferred low temperature oxygen concentration measuring apparatus.
One important aspect of the invention is a novel solid electrochemical cell with integral cell electrode/heater which allows the cell to be heated by resistively heating only the integral electrode. The preferred electrochemical cell is formed by a solid electrolyte having a hollow tubular portion with an outer tubular surface and opposing inner tubular surface. The cell further includes an integral cell elec-trode/heater layer which covers substantially all but a pair of substantially parallel, opposing, longitudinally extending strips of the outer tube surface of the electrolyte. Pre-ferably, the tubular portion of the solid electrolyte has a su~stantially circular cross-section with a uniform cross-sectional thickness. This allows substantially uniform hea~ing of the electrolyte. Preferably too, the integral cell electrode/heater layer is also of a substantially unifonm compositi~n and thickness along at least a major portion of the outer tube surface so as to provide a uniform power density and uniform heating of the electrolyte~
The pre~erred cell is closed at one end by a hollow, substantially hemispherical portion of electrolyte integrally formed with the tubular portion at the one endO The hemi$pher-ical portion of the electrolyte has a convex outer surface and an opposing concave inner surface and the integral electrode/heater layer covers at least a major portion of the convex outer surface so as to minimize the amount of heat lost through the hemispherical tip of the cell. In the preferred cell configuration, the opposing strips of exposed electrolyte outer tube ~urface extend from the remaining open end of the tubular portion of the electrolyte along the tube portion of the electrolyte to the hemispherical portion.
In the preferred cell, the integral layer covers the entire convex outer surface of the hemispherical portion of the electrolyte tube again to reduce heat loss through the tip and there~y minimize the generation of thermal gradients within the electrolyte.

... .. . . . .

- ~Z~5~3 The described apparatus is heated by passing an electric (i.e., electronic) current thorugh at least a portion of the cell. In the preferred embodiment apparatus, an electric current is passed through a pair of leads in electrical contact with the electrode layer, each lead being electrically connected at the open end of the electrolyte with an end of one of the two halves of the layer formed by the division of the layer at that open end by opposing strips of exposed electrolyte tube surface. The integral layer preferably has a total maximum resistance at room temperature of about one-half ohm or less and thus a resistance less than or equal to that amount between the lead contact points.
When used as a gas concentration difference cell, the preferred embodiment further includes a second electrode layer contacting the inner tube surface of the electrolyte opposite the outer layer along and extending along at least a portion of the length of the tube exposed by the two opposing strips.
In the preferred embodiment cell, the electrode layers are gas porous and are formed from a material whch is more than one-half by weight of platinum and the solid electrolyte is of a material which is more than one-half by weight zirconia.
The preferred cell is unique in that it is the first configuration known in which an yttria stabilized zirconia solid electrolyte can be heated to an operating temperature of about 800 C or more by resistively heating only one of the two cell electrodes and no more that negligibly resistively heating the electrolyte. In other cell designs using an yttriazirconia electrolyte, partial short circuiting of the cell heater current through the electrolyte and, on occasions, through the electrolyte and second cell '`~' ,, 5~3~

electrode has typically if not invariably occurred.
Inthe preferred embodiment, no more than a negligible por-tion of the heater current passes through the electrolyte. The preferred embodiment allows heating of the electrolyte to a maximum temperature in a zone extending through the electrolyte between the heater/electrode and a second opposing electrode and covering an area of at least several square centimeters, the temperature of the electrolyte in the zone varying about 10 C or less from the maximum temperature. Important to achieving the creation of such a zone is the supplying of a substantially uniform electric power density over a major proportion of the integral electrode/heater layer contacting the solid electrolyte surface. This is achievable, in part, by the design of the preferred embodiment which minimizes the likelihood of short circuiting of the heater current through the electrolyte.
Other important aspects of the invention are a method of and apparatus for resistively heating an electrochemical cell in a manner so as to minimize or totally eliminate any effect of the heater current on the value of the emf developed by the cell. According to the invention, a radio frequency alternating current is generated by a suitable radio frequency alternating electric current source and is passed through at least a portion of the cell by suitable circuitry connecting the radio frequency alternating electric current source in a circuit through at least a portion of the cell for resistively heating at least a portion of a cell by the current. Preferably, the radio frequency alternating electric current source generates an alternating electric current with a frequency sufficiently high as to eliminate any offset in the magnitude of the emf generated by the electrochemical cell which is caused -S43~

by the frequency of the alternating current. It appears that direct current and alternatiny currents having a low frequency of al-ternation disrupt the ionic equilibrium at the electrolyte/electrode interface of the cell. It has been discovered that by raising the frequency of the alternating current, this disruption of equilibrium can be reduced if not effectively eliminated. It further appears for the cells studied thus far, that low radio frequency range alternating currents (3,000 to 300,000 hertz), specifically in range between about 3000 and 200,000 hertz, have proved to have a sufficiently high frequency as to totally eliminate any apparent effect of the alternating heater current on the emf developed by the cell. The particular minimum frequency required varies at least with electrode composition. A current having a frequency between about 30,000 and 100,000 hertz is preferred for ease of interfacing the current source with the cell.
The electrochemical cell is preferably operated at a predetermined temperature. To accomplish this, temperature sensing means such as a thermocouple is provided for generating a signal related to the temperature of the cell and a control circuit responsive to the temperature signal is provided for controlling the power level of the alternating current supplied through at least a portion of the cell. In the described preferred embodiment, cyclical on/off switching of the alternating current is used to modulate the power supplied to the cell.
Preferably too, the electrochemical cell includes an integral cell electrode/heater through which the radio frequency alternating current is circuited. Again, the preferred embodiment cell allows the circuiting of all or all but an negligible portion ~Z~5~3~
-8a-of -the alternating current through the integral electrode/heater and avoids the circuiting of anything more than a negligible portion of the current through the electrolyte.
A significant aspect of the invention is the cell/electrode configuration and the manner in which the alternating current source is supplied to the cell so as -to minimize the generation of excessive heat at the contact points where the current enters and exits the cell. Preferably, the cell includes a solid electrolyte material in a hollow tubular form having opposing inner and outer tube surfaces and a cell electrode (integral electrode/heater) is formed, at least in part, by an electrically conductive layer lS covering a major proportion of the outer tubular surface of the electrolyte. A pair of strips of exposed electrolyte outer tube surface ~L2;~5.~3~
g extending along at least a portion of the tubular electrolyte, divide the electrode layer into two end portions beginning near one open end of the tubular portion of the electrolyte.
The electrode layer is continuous where the two portions meet at the remaining end of the electrolyte. A pair of leads extend from the current source to each of the two end portions of the electrode layer near the one open end, This con$iguration connects substantially all of the el~c-trode layer in a circuit with the current source. Prefer-ably again, the total resistance of the cell electrode between the two lead contact points is about one-half ohm or less at room temperature.
Yet another important aspect of the invention is an oxygen concentration measurement apparatus incorporating either, and preferably, both the radio frequency heating feature and novel cell-electrode configuration of the inven-tion. Radio frequency heating and/or the preferred cell configuration can be adapted to similar appara~us known and used for the detection and/or measurement of the concentration of other types of gaseous chemical substances. The basic ~0 apparatus comprises an electrochemical cell having a solid electrolyte exhibiting an increased conductivity to ions of the predetermined gaseous substance when heated. A first gas electrode at a first location on the s~rface of the solid electrolyte provides an interface between the sample gas and the cell. A second reference electrode contacting the solid ele~trolyte at a second location is provided for generating a known, predetermined potential wit~ the electro-lyte. The cell develops an emf between the two electrodes having a magnitude related to the concentration of the prede-termined gaseous substance in the sample gas. The apparatusfurther comprises, in one basic form, an electric current source connected in a circuit across the cell for passing a radio frequency alternating electric current through a portion of the cell, resistively heating that portion of the cell.
3; In tne 2referred apparatus, a pair of electrically conductive ~'~Z5434 lea~s extend from different portions of one of the two electrodes to the electric current source so that the radio frequency alternating current may be passed through at least a portion of the one eIectrode. Preferably too, the apparatus is designed so that the radio frequency alternating current S passes only through the one electrode of ~he cell. Again, the preferred sensor apparatus includes an electrochemical cell having a solid electrolyte ~ormed, at least in part, by a ~ollow tube shape having an outer tubular surface and an opposing inner tubular surface. The one resistively heated electrode is prefe~ably formed by an electrically conductive layer which is porous to the gaseous substance and covers substantially all but a pair of narrow longitudinally extend-ing strips of the outer tube surface dividing the layer into two sections extending along the tube portion of the electro-15 lyte. A~ain, the tubular portion of the elec~rolyte ispreferably substantially circular in cross-section with a sub-stantially uniform cross-sectional thickness at each cross--section ahd the integral electrode/heater layer is of a substantially uniform composition and thickness along a major portion of the outer tube surface for uniform heating of the electrolyte. Preferably too, the electrolyte is formed with an integral, hcllow and substantially hemispherically por-tion closing one open end o~ the tubular portion of the electrolyte~ the hemispherical portion having opposing outer convex and inner concave surfaces. The layer covers at least a major portion of the outer convex surface so as to minimize the generation of temperature gradients within the electrolyte. In the preferred embodiment, the longitudi-nally extending strips of exposed outer tube surface of the electrolyte are parallel dividing the layer into equal end sections and extend from the remaining open end of the tube portion of the electrolyte to the hemispherical portion of the electrolyte. Moreover, the layer covers the entire outer convex surface of the hemispherical portion of the ... . . . .. .. . . . .

electrolyte further minimizing the generation of temperature differentials in the electrolyte.
In the preferred apparatus, a pair of electrically conductive leads extend from the current source to the one electrode layer and the one electrode further includes a pair of metal foil pads, each pad joining an end of one lead to an end of the electrode layer. This reduces the effective surface resistivity of the layer in the vicinity of the lead contact points reducing the generation of heat in that area and thereby minimizing the likelihood of shorting of the alternating current through the electrolyte in the vicinity of the lead contact points.
The preferred apparatus is operated as a concentration differencing cell and is provided with a second, gas porous, electrically conductive layer circumferentially covering a portion of the inner tube surface over an area extending opposite at least a portion of the tube surface exposed by the two opposing strips. The opposing strips define, to some extent, the active zone of the cell. To prevent emf offset, the second electrode layer must be positioned opposite the point of maximum temperature of the outer electrode. Again, the preferred oxygen sensing apparatus has an electrolyte formed from a material comprising a major portion by weight of zirconia and electrode layers formed from a material comprising a major proportion by weight platinum.
Other important aspects of the invention are the method of heating an electrochemical cell and the specific related method of operating a gas concentration detecting apparatus which generates a signal related to the concentration of a particular gas and which includes the steps of generating an alternating current having a radio frequency level of lZZ5~3~
-lla-alternation and passing the current through at least a portion o-f the cell to resistively heat the cell.
Preferably, the frequency is between about 30,000 to 100,000 hertz for simplifyin~ the problem of connecting the cell to the current source. Preferably too, the frequency of the current is sufficiently high as to eliminate any offset in that the magnitude of the emf developed by the 12 ~Z~5~3~

electrochemical cell which is caused by the frequency of the current. Viewed in another way, the alternating current frequency is sufficiently high that the magnitude of the cel] emf is unaffected hy the further frequency increase. It is further preferred b~at not required that the alternating current be passed through one of the two cell electrodes of the cell while passing no more than a negligible portion of the current through the electrolyte. Control of the temperature of the electrochemical cell is accomplished during the generating and passing steps by the contemporaneous steps of sensing the temperature of the electrochemical cell and varying, during the generating step, the power of the alternating electric current in response to the temperature sensing step.
According to another important aspect of the invention, the generating of an alternating current having a frequency sufficiently high such that the magnitude of the emf developed by a gas sensor cell is substantially unaffected by the current frequency allows the output of the cell to be measured during the step of passing the alternating current through at least a portion of the cell. Thus, the invention further includes during the heater current passing step the steps of sensing the emf developed by the electrochemical cell and generating a signal related to the magnitude of the emf for indicating the concentration of the particular gas in the sample gas.
Other aspects of this invention are as follows:
An electrochemical apparatus comprising:
an electrochemical cell;
generating means for supplying a radio frequency alternating electric current; and circuit means connecting said generation means in a circuit through said cell for resistively heating at least a portion of the cell with said radio frequency alternating electric current.

12a 1~5~3'~

An electrochemical apparatus responding to a predetermined gaseous substance in a sample gas comprising:
an electrochemical cell, the cell comprising:
5a solid electrolyte exhibiting an increased conductivity to ions of the predetermined gaseous substance when heated;
first electrode means contacting a surface of the solid electrolyte at a first location for forming a 0 gas electrode with the sample gas and the electrolyte;
second electrode means contacting a surface of the solid electrolyte at a second location for generating a known predetermined potential with said solid electrolyte;
15the electrochemical cell developing an emf between the two electrode means related to the concentration of the predetermined gaseous substance in the sample gas at the first electrode means; and electric current source means connected in an electrical circuit across at least a portion of the cell for passing a radio frequency alternating electric current through at least a portion of the cell.

12Z5~3'~

-12b-An improved method of heating an electro-chemical cell, said cell comprising an electrolyte, a first cell electrode and a second cell electrode, said electrodes contacting said electrolyte, characterized in that at least a portion of one of said electrodes contacting said electrolyte is heat-ed, the heating being carried out by generating an alterna-ting electric current having a radio frequency level of alternation and passing said alternating electric current through at least a portion of one o~ said electrodes.
An improved method of heating an electro-chemical cell, said cell comprising an electrolyte, a first cell electrode and a second cell electrode, said electrodes contacting said electrolyte~ characterized in that at least a portion of one of said electrodes contacting said electrolyte is heated, the heating being carried out by resistively heating at least one of said electrodes ~hile no more than negligibly resistively heating said electrolyte.
An improved method of heating an electro-chemical cell, said cell comprising an electrolyte, a first cell electrode and a second cell electrode, said electrodes contacting said electrolyte, characterized in that at least a portion of one of said electrodes contacting said electrolyte is heate~. by heating means constitutiny an integral part o~ said one electrode.

., BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view of the preferred apparatus, ln situ oxygen sensor incorporating a solid state electrochemical sensing cell with integral heater/electrode;
Fig. 2 is a sectioned view of the apparatus depicting the in situ sensor probe assembly;
Fig. 2a is an expanded view of the probe assembly of Fig. 2 in the area 2a;
Fig. 3 is a view of the preferred embodiment solid electrochemical cell;

l~ZS4~34 Fig. 4 is a lateral sectioned view of the cell in Fig, 3;
Fig. 5 lS a cross-sectional view of the cell of Fig.
3 and Fig. 4 along ~he lines 5-5;
Fig. 6 depicts graphically, the temperature within tne cell of Figs. 3-5 as a function of position from the clos-ed end of the cell;
Fig. 7 depicts schematically the electrochemical cell temperature control circuitry of Fig. l;
Fig. 8 depicts schematically a second gas detector embodiment incorporating a solid state electrochemical sensing cell with in~egral heater/electrode and disk shaped solld elec-trolyte;
Fig. 9 ~epicts schematically yet another gas detec-l; tor apparatus incorporating a solid s~ate electrochemical sensor cell with integral heater/electrode and solid electro lyte with an open tube configuration;
Fig. 10 .is a schematic view of a prototype oxygen sensor apparatus;
Fig. 11 is a view of a prototype electrochemical cell with inteyral heater/ele~trode used in the pro~otype apparatus or Fi~. ln; and Fig. lla is an expanded view o~ a portion of one of two helically wound electrode layer strips forming a part of the ~ntegral cell ~lectrode/heater of the prototype cell of Fig. 11.

D~TAILED DESCRIPTION OF THE INVE~TION

PREFERRED OXYGEN DETECTOR
Figs. 1 through 5 and 7 depict the components of the pre-ferred embodiment of ths present invention, a solid sta~e, in situ, low temperature oxygen sensor for monitoring oxygen con-tent in boiler stacks and the like. The apparatus is depicted schema'ically in Fig. 1 and includes a probe assembly 20, which is depicted in greater detail in Fig. 2 and is inserted through he wall 23 of b~iler ~lue or chimney 24 into the com-5~4 bustion gas stream, and related electronics 22 depicted in greater detail in Fig. 7. The probe assembly 20 includes a tubular electrochemical sensor cell 30 together with protec-tive and supporting elements further described in Fi3. 2 to support and protect the electrochemical cell 30 in the flue 24. As is depicted in greater detail in Figs. 3 through 5, the electrochemical cell 30 includes a solid electrolyte ele-ment 31 having a hollow tubular form with an'open end 32 and an integral hemispherically formed closed end 34. The elec-trolyte tube 31 is an yttria (8% by weight) stabilized zirco~nia substrate fabricated by conventional ceramic forming tech-niques. As is better seen in Figs. 4 and 5, an electrically conductive outer electrode layer 37 covers substantially all of the outer.surface of ~he electrolyte element 31. A similar ~ut separate layer 36 is provided on the inner surface of the electrolyte substrate 31 within the hemispherically closed half of the cell.
Returning to Fig. i, a thermocouple 39 or other suitable temperature sensing device is provided within the electrochemical cell 30 for sensing the electrolyte tempera-ture. Leads 40 and 41 carry an electrical signal generated by the thermocouple 39 to a temperature compensator 43. The compensator 43 develops a voltage level signal passed on line 43a to a thermal controller 42 which controls the heating of the call 30 by controlling the amount of electrical current circuit~d through the outer electrode layer 37. The outer electrode layer 37 is used as a sa~ple gas electrode and is exposed to the gases within the flue 24. The inner electrode layer 36 is used as a reference gas electrode and is supplied with ambient (outside) air, having a known concentration (20.95%) o oxygen, by a supply tube 44 or other appropriate means. A pair of pure nickel bus leads 48 and 50 are attached by means of pure platinum foil pads 52 and 54, respectively, to the outer electrode layer 37 and connect the outer elec-trode layer 37 across a power interface circuit 56. The powerinterface 56 passes or circuits a radio frequency alternating 12~S434 electric current through the leads 48 and 50, pads 52 and 54, and outer electrode layer 37, thereby resistively heating the outer electrode layer 37 and raising the temperature of the covered solid electrolyte element 31 to an elevated level at which it becomes suitably conductive to oxygen ions. The layer 37, pads 52 and ;4, and the portions of the leads 48 an~
;0 contacting the pads 52 and ;4, respectively, together form the integral cell electrode/heater 35 of the subject inven-tion. The inner electrode layer 36 is connected by mean~ of yet another electrical lead 58 in a circuit with the outer electrode layer 37, foil pad 52 and lead 48 across a suitable electrical or electromechanical device for measuring or re-sponding to the emf of the cell 30 developed between the inner and outer electrode layers 36 and 37. For combustion control operations, the emf output of the cell 30 is preferably first amplified by a linear amplifier 59 and then converted to a signal linearly related to oxygen content of the gas in the flue 24 by a logarithmic amplifier 60. The output of the logarithmic amplifier 60 is passed via a line 60a to a conven-tional closed loop combustion process controller (not depict-ed), which forms no part of the present invention. Alterna-tively or in addition, the inner and outer electrode layers 36 and 37 may be circuited across a suitable measuring device such as a high impedance voltmeter 61 for a visual indication of oxygen concentration in the gases within the flue 24. The output of the cell 30, measured by the emf developed between the outer electrode layer 37 and inner electrode layer 36, is representative of the difference in oxygen concentration be-tween ambient air and the gases within the flue ~4. The apparatus may be configured for operation at a fixed cell temperature or, if desired, the thermal controller 42 may be provided with a set point input 67 to con~rol the operating temperature of the electrochemlcal cell, Completins the appa-ratus in Fig. 1 are a timer circuit 62, a modulator circuit 64 and a radio frequency signal generator 66 which together contro lably supply a radio frequency digital sisnal to the ...... . . . . .. ..

~2ZS4134 power interface 56 which convsrts the digital signal to a similar frequency~ low-voltage alternating current.
Operation of the Fig. 1 apparatus is as follows.
The yttria stabilized zirconia electrolyte 31 of the sensor cell 30 exhibits an extremely low oxygen ion conductivity until it is heated above a temperature of about 600C. A
voltage dif~erence signal from the thermocouple 39, indicating the maximum temperature of the electrolyte, is passed via leads 40 a~d 41 to the temperature compensator circuit 43 which compensates the thermocouple signal for the effect o~
connecting the leads 40 and 41 to the circuit elements 2~.
The compensated signal is amplified to a magnitude such that it equals the voltage level signal outpu~ted by the set power circùit 46 along line 46a for an equal temperature. The am-piified compensated thermocouple voltage level signal is out-putted along line 43a to the thermal controller. The thermal controller 42, timer circuit 62 and modulator circuit 64 con-trol the on-off cycling of the radio frequency signal gener~-tor 66. As presently embodied, the timer circuit 62 generates a digital timing pulse every 7 milliseconds on line 62a. The modulator circuit 64 responds to the pulse and switches the high frequency signal source 66 on for a fraction of the 7 millisecond period. The radio frequency signal generator 66 can be activated between about 12 1/2% and 97% of the 7 milli-Z5 second timing cycle. The duration of a high level signal out-puttsd on the line 64a for activating generator 66 is control-led by the magnitude of the voltage level signal outputted by the thermal controller on line 42a. The radio frequency sig-nal generator 66 outputs on line 66a, in the described embodi-ment, a 50 kilohertz square wave signal fluctuating between 0and 15 volts The square wave signal is converted by suitable circuitry in the power interface circuit 56 to a low voltage level (nominal +6 volts to -6 vol~s), alternating square wave current of the same frequency, Leads 48 and 50 circuit the high frequency alternating current generated by the power interface circuit 56 through the outer electrode layer 37.
It will be appreciated that as the electrolyte 31 exhibits 5~3~

a relatively low electrical- conductivity at or below the operating temperature of the sensor 30, which is about 800C, that there is no flow or at most a negligible flow of alter-nating current through the electrolyte 31 or inner electrode layer 36 in the preferred embodiment at the indicated voltage levels.
Referring now to Fig. 2, the probe assembly 20 of Fig. 1 is depicted in a side-sectioned view. The major pur-pose of the probe assembly 20 is to support and extend the electrochemical sensor cell 30 sufficiently far into the stack 24 to obtain a good reading while protecting the cell 30 from abrasive particles in the stack gases and cooling from the stack sas flow. The electrochemical sensor cell 30 is affix-ed by suitable means, such as by a layer 68 of an electrically insulative ceramic cement, within the bore 69 of an extension tube 70. The tube 70 consists of an electrically insulative alumina tube section 72 of a convenient length so as to allow disassembly of the probe assembly 20 and the removal and/or replacement of the cell 30. The alumina tube section 72 is connected to a second extension tube section 74 of stainless steel or other suitable material by a Parker coup~
ling 76 or other suitable means. The cell 30 a~d extension tube sections 72 and 74 are encased in a protective outer tube 80, formed in the preferred embodiment by lengths of stainless steel tubing 82 and 84 joined by a threaded coupling 83 welded to tube section 84. A wire mesh screen 86 is provided at ~he open end of the tube 80 to admit stack gases and to retain an insulative material 96, only a portion of which is depicted. The screen 86 is held in position by appropriate means such as pins 88, one of which is depicted in Fig. 2. The present embodiment of the preferred oxygen sensor also presently includes a gas porous ceramic cup 90 surrounding the cell 30 and proximal tip of the alumina extension tube section 72. A hollow tube 92, typically of 3s stainless steel or other suitably heat and corrosion resistant material, is also currently proYided with the cup 90 for sen-sor calibration purposes. The tube 92 carries gas mixtures ........ .

543~
.

having a known oxygen content, such as air, to the interior of the thimble 90 an~ outer electrode layer 37 o~ the cell 30.
In this way, ln situ calibration o the apparatus can be ac-complished. The space between the thimble 90, sensor 30 and extension tube 72 is filled with an appropriate electrically and thermally insulative material 94 such as alumina silicate only a portion of which is illustrated in Fig. 2 for purposes of clari~y. Similarly, the space between the porous ceramic cup 90 and extension tu~e 70, section tube 72, coupling 76, and metal tube section 74, and the outer stainless steel pro-tective tube 82 ic also filled with a gas porous, thermally and electrically insulative material 96, again such as alum-ina silicate and again only a portion of which is illustrated in Fig 2 for clarity. The screen 86, outer insulation 96, porous ceramic cup 90 and inner insulation 94 are all gas porous allowing stack gases to reach the outer electrode .layer 37 (see Figs. 3-5) of the sensor cell 3U~ These ele-ments also filter solid particulates from the stack gases entering the probe 20.
2~ Inserted into the hollow interior of the cell 30 is a four ~ore alumina tube 100. Two bores of the tube 100 con-tain leads 40 and 41 forming the thermocouple 39 which is exposed, as indicated in Fig. 2, at the so-called "active zonel' of the sensor 31. That is the location where the solid electrolyte element 31 is heated to its maximum temper-ature by the outer electrode layer 37 acting as a resistive heater. A third bore of the alumina tube 100 acts as the air supply tube 44 carrying ambient air to the inner electrode layer 36 of the sensor cell 30. The fourth bore of the alumina tube 100 carries the lead 58 contacting the inner electrode layer 36 of the cell 30. Fig. 2a, an enlaryed.
view of the area 2a of Fig. 2, shows one-half of the thermo-couple 39, formed by the lead 40. Chromel-P~ and Alumel'~
leads are used for a k-type thermocouple 39 and are joined ~5 where exposed from the four bore alumina tube 100. The inner electrode lead 58 has a pure platinum wire tip ex-tendiny from the tube 100 which is joined within the tube r~ S~39C

100 to a pure nickel wire at a point 58a i~ the vicinity o~
the junctions between the pure nickel bus leads 48 and 50 and the pure platinum pads 52 and 54, respectively, The purpose is to provi~e an inner electrode nickel~platinum junction which produces, at the sensor's operating temperature, a compensating emf that is essentially of equal magnitude and opposite polarity to that generated by the junction o~
the platinum foil pad 52 and nickel lead 48. The purpose is to minimize the magnitu~e of the cell constant, C, of the apparatus as well as to reduce construction costs of the apparatus. A fine platinum wire mesh 102 is positioned about the tip of the tu~e 100 so as to be compressed against the inner electrode layer 36 and lead 58 when the tube 100 is inserted into the cell 30. ~n electrical junction is thus formed between th~ inner electrode layer 36, platinum mesh 102 and inner electrode lead 58. Leads 48 and 50 ~rom the outer electrode layer are not depicted in Fig. 2, but are covered/ at reast in part, by the c~ment layer 68 and extend down the bore 69 of the tube 70. An electrically insulative covering is provided as the leads 48 and 50 exten~ from the sensor 30 to prevent shorting or grounding.
While the preferred embodiment is an 1n situ oxygen sensor, the invention is not limited to in situ sensors or to in situ sensors having the indicated configuration. For ex-ample, the sensor 30 is easily adapted to an extractive modeO
A sample gas may be supplied to the interior electrode layer 36 or exterior electrode layer 37, with air heing supplied to the remaining electrode layer. Alternatively, the config-uration of the in situ oxygen sensor 30 may be varied by in-verting the orientation of the sensor cell 30 so that the cell 30 extends into and is protected by the alumina tube section 72. This would require modification to the pQsitiOnin3 and configuration of the various electrode leads 48, 50, and 58 as well as to the thermocouple 39. In thls o~ientatlon, the inner electrode layer 36 would be used as the sample gas elec-trode while the outer electrode layer 37 would be exposed to ambient air and used as the reference gas electrode of the ~LZ~S~3~

cell 30. Moreover, while an ambient air reference gas electrode is formed in the interior of the preferred embodiment cell 30, it is conceivable that a solid state reference electrode can be formed by using methods and materials known to the art eliminating the need to supply air to the cell 30 interior. See, for example, the solid, oxygen reference electrode revealed in U.S. Patent No. 3,883,408 to Kim et al. It is further envisioned that the present invention may be usefully employed in other areas, particularly in the detection and measurement of other gaseous compounds, apparatus which typically employ similar solid state electrochemical cells 30 utilizing electrolyte and electrode materials selected for the gaseous compound to be detected. See, for example, U.S. Patent No.
4,282,078 to Chamberland et al. It is further believed - that at least some inventive aspects of the preferred embodiment oxygen detector apparatus may be usefully employed in the area of oxygen and oxide concentration detection and measurement in fluids, such as molten metals.
PREFERRED ELECTROCEEMICAL CELL A~lD
INTEGRAL ELECTRODE/HEATER
Figs. 3-5 depict in greater detail the configuration and construction of the preferred embodiment electrochemical cell 30. The configuration of the cell 30 including the integral cell electrode/
heater 35 comprising the outer electrically conductive, gas porous layer 37, pure platinum lead attachment pads 52 and 54 and the connected portions of pure nickel leads 48 and 50, are significant inventive aspects of the preferred embodiment. The emf developed by the cell 30 is controlled, in part, by the maximum temperature of the electrolyte 31. This is the temperature T of the Nernst equation previously referred to. It has been observed that thermal gradients in the solid electrolyte 31 leading to differing maximum temperatures for the inner and outer electrode layers 36 and 37 cause an offset in the ~ magnitude of the emf actually developed by the cell 30
5 ''' from that predicted by the Nernst ~ZS434 equation. The Nernst equation i5 only valid where the maxi-mum temperature of the two electrodes are approximately equal.
Furthermore, failure to locate and measure maximum temperature generated by the integral electrode/heater can lead to a run-away neating situation. In order to provide a reliable andaccura~e oxygen measuring apparatus, it is necessary to create an isothermal zone or enlarged area of maximum temperature ex~ending su~stantially uniformly through and across the elec-trolyte (i.e., an "active region") between the electrode lay-ers 36 and 37 at a known location. The creation of such azone allows the temperature to be accurately determined an~
regulated and thenmal gradients between the inner and outer electrodes to be minimized so as to reduce or eliminate a changing and unpredictable cell emf offset, It has been discovered that the best manner to pro-vi~e such an isothermal region in the electrolyte is to pro-~ide an integral heater/electrode covering a large surface area of the electrolyte 31 with a uniform power density. The preferred design minimizes the length to width ratio of the effective heater/electrode thereby minimizing total electrode resistance whilè still providing a substantially uniform power ~ensity over a large area of the electrolyteO A solid stabil-ized zirconia electrolyte 31 is provided in the shape of a hollow tubular section 33 and integral, hollow hemispherical section 34. The preferred embodiment sensor uses a partially yttria stabilized zirconia isopressed tubular substrate havin~
a 38 mm overall length, 6 mm outer diameter and maxim~
inner diameter of 4 mm. The tubular portion 33 of the electro-lyte 31 is preferably of a uni~orm circular cross-section, as is best seen in Fig. 5, and has a substantially uniform cross-sectional thickness, t, at each cross-section. The solid electrolyte tube 31 is formed by eonven-tional ceramic techniques, preferably isopressing for greater dimensiona' accuracy and more consistent quality. The cross-3~ sectional thickness, t, is substantially the same along mostof the length of the tubular portion 33 of the electrolyte 31 but lncreases near the hemispherical end 34 as a result of the .. .. . . .. ... . . . . . ..

~ZZ~434 .

isopressing proce~s. It is desirable to form the electolyte 31 with as substantially a uniform thickness, t, throughout as possible for uniform heating. The preferred integral heat~
. er/electrode 35 is ormed in part by the layer 37 of an oxygen porous, electrically conductive material applied to all but a pair of narrow diametrically opposing strips 110 of the outer surface o~ the electrolyte 31. A me~allic (preferably plati-num) based paste or ink composition is coated as uniformly as possi~le tb the outer surface of the electrolyte 31 so ~s to 0 form an electrode layer 37 of substantially uniform thickness and uniform electrical resistance. The coating is dried and, if ap~ropriate, heated to sinter the electrode layer material to the electrolyte 31. An ink of the ~ame material having a greater liquid content is used to "cement" the foil pads 52 15 and 54 to the electrode layer 37. Suitable means such as spot welds 112 are used to affix the lea~s 48 and 50 to the foil pads 52 and 54, respectively, before attachment of ~he pa~s to the electrode layer 37. The inner electrode layer 36 ls formed by coating the inner surface of the electolyte 31 with tha same paste or ink.
~ he outer electrode layer 37, being primarily metal.-lic ~n all the embo~iments being discussed, has a positive temperature coefficient of resi~tance, as do the metallic-foil pads 52 and 54 and leads 48 and 50. Thus, the resistance of the electrode 35 increases with temperature. By employing a s~bstantially uniformly thick outer electrode layer 37, a resistive heater having a substantially uniform power density is formed over the tubular portion 33 of the electrolyte 31.
The metallic foil pads 52 and 54 effectively reduce the sur-face resistivity of the integral heater/elec.~rode 35 near theopen end 32 of the electrolyte tube 31. This reduces the electric power density and the resulting generation of heat in the vicinity of the pads 52 and 54. The strips 106 cause the alternating current supplied by the power interface 56 to travel along the length of the layer 37 and over the hemispher-ical end 34 of the electrolyte 31. At ~he operating tempera-ture of the sensor 30 (about 800C), the outer electrode layer ;~2~543~

, .
37 acts as a black body, radiator. The rate of heat loss through radiation from the outer slectrode layer 37 is greatest at the hemispherical end 34 than along the length of the tubu-lar portion 33 of the cell 30. Thus, the resistivity and power density of the electrode layer 37 is effectively reduced at that end 34, Maximum heating therefore occurs in the region between the hemispherical en~ 34 and the foil pads 52 and 54.
Fig. 6 depicts diagrammatically the form of the sensor 30 and the resulting nominal temperature distribution through the electrolyte 31 as a function of position along the electrolyte length. In embodiments of the sensor 30 formed from zirconia tubes haviny an outer diameter of 6 millimeters, an inner ~i-ameter of 4 millimeters and a length of approximately 40 mil-limeters, an "active zone" 114 of maximum temperature having no more than about a 10C variation over about a 6 millimeter length of the electrolyte 31 can be generated beginning approx-imately 9 to 12 millimeters from the hemispherical tip 34.
In the depicted preferred embodiment of the cell 30, - the strips 106 extend along the length of the outer surface of the electrolyte 31 from the open end 32 to the hemispheri-cal portion 34~ While it is not believed necessary to cover the entire hemispherical portion 34 with the electrode layer 37, as depicted, it is presently su~ested to do so as it is - believed that covering the entire surfaca of the hemispheri~
cal section 34 with the outer electrode layer 37 reduces heat conduction from the "active zone" to the tip 34 thus minimiz-ing the thermal gradient in the "active zone". It also has the effect of distributing the current ~ensity over a broader area preventing the possible generation of hot spots at the ends of the strips 106 where the hemispherical por~ion 34 meets the tubular portion 33 of the electrolyte body 31~
The strips 106 are no wider than is necessary to prevent short-ing of the electrode layer 37 at the operating temperature o the sensor 30 and at voltage of the alternating current used to heat the outer electrode layer 37. For the preferred electrode layer material and an yttria stabilized zirconia electrolyte substrate 3; o~erated at a temperature of about 5439t 800C with an alternating current voltage fluctuating between about ~ 6V, gaps approximately .76 millimeters wide in thé
circum~erential direction of the electrolyte 31 were found to be suitable. Gaps 106 should be kept as narrow as Dossible in the circumferential direction to reduce thermal ~radients developing due to electrolyte cooling in the unheated gap5 106.
The area of attachment of the bus strips 48 and 50 to the electrodq layer 37 is a potential source of high elec-trical resistance and therefore of excessive and undesirableheat yeneration. For this reason and to reduce the overall power requirements of the cell 30, it is suggested that the resistance per unit length of the outer electrode 35 in the region of the intermediate foil pads 52 and 54 and bus strips 48 and 50 be kept to a value less than about l/lOth that in the "active zone~O This is substantially achieved by the foil pads 52 and 54 bonded to the outer layer 37. If neces-sary, it is believed that further reduction in the resistance of the integral heater/electrgde 35 can be achieved by over-laying the bus leads 48 and 50 with another layer of similarmetal foil. Surface resistivity of the integral heater/elec-trode 35 and of the electrode layer 37 can be measured by the conventional four probe techni~ue. The surface resistivity measurements reflect a changing resistance in the heater/elec-trode 35 and layer 37, The surface resistivity of the pre-ferred embodiment platinum-bismuth trioxide electrode material is about 0.05 ohm~square ~ 50%) or less for a typical .001 inch ~ilm at room temperature and provides an electrode layer in the preferrPd apparatus having a total resistance of less than about one-half ohm at room temperature between the first and second nickel lead/ platinum foil junctions (48/50 and 49/51). The test may also be used to determine the uniformity of the layer 37 as surface resistivity of the layer 37 is directly proportional to its thickness.

.. . . ., , ~ . .. . ... . .. . .. . .. ...... .. . . . . . . . . . ... . . . . . . . .

i;~ZS434 The depicted embodiment of the cell 30 with integral heater/cell elec~rode 35 is presently preferred for its supe-rior performance given the present fabricating constraints.
Other electrode layer application techniques are being studied which may provide a means for controllably varying the thickness of the electrode layer 37. When accomplished, this may allow variation in the construction of the integral heater electrode 35 such as the elimination o~ the foil pads 50 and 52, the e~pansion of the active region or the use of even smaller electrolyte shapes. .
The proper functioning of the preferred embodiment cell results from the proper balancing of a number of inter-.
related factors including tube and heater layer size, shape, composition and electrical resistance/conductivity character-istics, sensor operating.temperature and heating current vol-tage~ Variation of any parameter may result in some signifi-cant portion of the heating current shorting through the elec-trolyte or the electrolyte and remaining electrode. This will necessarily effect the accuracy of the cell and will prob-ably reduce its operating lifeO
Models of the preferred embodiment apparatus arepresently being fabricated from yttria stabilized (8% by weight) zirconia substrate tubes which are readily available in the si2e and shape of the solid electrolyte element 31 being employed. However, other suitable oxygen ion conducting solid electrolyte materials such as ~irconia stabilized with other compounds and other oxides known in the art may ~e em-ployed. Indeed, it has been observed that at the operatingtemperature of the cell 30 of ap~roximately 800C, that calcia stabilized ~irconia is les~ electronically conductive than the yttria stabilized zirconia, a factor not.deemed significant for the preferred em.bodiment of the cell 30 and integral electrode/
heater layer 37, but which may be important in other configur-ations to prevent a short circuiting of the alternating cur-rent through the solid electrolyte 31 during heating~ Forexample, successful operation of a calcia stabilized zirconia electrolyte cell 30 has been demonstrated. At an operating lZf~5'134 - 26 ~

temperature of approximately 800C, the calcia stabilized zirconia is an order of magnitude less electrically conduc~
tive than the yttria stabilized zirconia. However, the calcia stabilized zirconia appears to be more susceptible to electrolytic decomposition from an alternating electric vol-tage than does the yttria material. The calcia stabilized electrolyte may ke useful in a short opexating life applica-tion and/or in a different electrode/electrolyte configuration where the reduced electrical conductivity of the calcia .
stabilized zirconia might prevent undesired shorting of the heater alternating electric current.

.
ELECTRODE MATERIAL
Another inventive aspect of the preferred apparatus is the material employed for the electrode layers 36 and 37.
The material has been found to be one of the key components effecting the accuracy, sensitivity and longevity of the sen-sor. The ideal material used has to be applicable in control-lable thicknesses on th~ electrolyte 31, form a strong bond (preferably chemical) with the electrolyte to resist spalling 20- or other forms of physical deterioration, have the same ther-mal coefficient of expansion or otherwise be sufficiently ductile in the thicknesses used to prevent spalling or crack-ing and have the appropriate el-ec~rical resistivity, preferably about .05 ohms/square or less again for a typical .001 inch thick film. For oxygen concentration sensing with the inte-gral heater/electrode 35 the layer 37 is desirably permeable to gaseous oxygen. Because of its high catalytic activity for the oxygen equilibration reaction, platinum is a preferred oxygen detector electrode material. Nevertheless, platinum cannot be used alone as a heater material because it does not form a chemical bond to the zirconia substrate electrolyte 31 and, if resistively heated, will spall.
Initial experiments were performed using a previous-ly known resistive heater material demonstrating a long-term physical stability with zirconia when heated. When dried, the ~Z5~34 material essentially comprised two parts by weight powdered platinum, one part by weight powderea gold an~ a small portion by weight of a glass ceramic frit ~inder. The ma~erial was applied as an ink by brushing or dipping to form a coating approximately 25 microns thick which consolidated to a film approximately 10 microns thick when dried and fired. However, to obtain a one-half ohm resistance desired for the integral heater/ electrode 35, thicker films were required. These were formed by coating and then firing successive layers o~to the slectrolyte 31. The most uniform coating results were obtained by dipping the electrolyte tube into the ink, slowly withdrawing it at a uniform rate and then rotating the tube in a furnace while dryingO The particular composi-tion used was a number 2408 Hanovia pas~e consisting of ~wo 15parts platinum, one part gold with 307% by weight Corning Glass Works #186 BED glass frit to which was added approxi-mately 45% of a proprietary Hanovia liquid medium comprising essentially Texanol. The frit is a lead oxide, titania, silica glass which crystalizes to form PbTiO3 at about 20700DC and which melts at about 1109C. Three such coatings were applied to the outer and inner surfaces of a zirconia substrate tube. The substrate was dried and fired at approxi-mately 900C for one hour after each coating. The electrode - layer material was extremely adherent and could not be scraped ~5off with a knife. The lead in the slass frit appeared to react with the gold and platinum to form a low melting temperature alloy chemically bonding the layer to the sub~trate but also acting as a gaseous oxygen block. The prototype sensor was slow to reach equilibrium and, accordingly, slow 30to respond to oxygen concentration changes. As other, more promising electrode materials were discovered, further experi-ment~tion with the glass frit based material was suspended.
However, it is believed that some improvement in the oxygen permeability of the glass frit material might be achieved 35by a variation of the CQmpOnents, In particular, a reduc-tion in the amount of glass ~ri~ used might be made to balance impro~ed ntesral cell eiectrode/heater layer oxygen porosity :~ZZ5~3~
,, .

with reduced adherability of the layer to the zirconia sub-strate. Scoring of the layer also improved oxygen permeabil-ity. The glass frit material perfor~ed adeguately when used as an lntegral heater/reference electrode with a conventional platinum layer sensor electrode.
Initial experiments were directed in part to demon-strate that oxygen concentration could be measured using an electrode formed of this material. Although this proved to be true, the search continued for materials providing greater adhesion to the zirconia electrolyte substrate r lower resisti-vity and, in particular, greater gaseous oxygen permeability for quicker sensor response~
A preferred integral electrode/heater layer material has been discovered possessing these qualities. In addition, the new material is used to provide equal resistivity in thin ner layers than the glass frit material thereby reducing costs.
The preferred material paste is supplied by Engelhard Indus-tries, Inc. and prepared by adding to their standard platinum sensor electrode paste, a small mount of bismuth trioxide~
The paste is formed from a mixture of dry components compris-in~ a major proportion by weight powdered platinum and a minor proportion by weight bismuth trioxide (Bi2o3). Trace amounts of gold (0.1-0.5% by weight) and other materials may be pre~ent. The dry components are mixed with a suitable liquid vehicle, again such as Texanol, to-form a fluid mixture which can be applied to the zirconia substrate electrolyte 31 by a variety of methods. It was found that a coating apDroximately .001 inch thick (about .025 mm) provides an electrode layer 37 with an approximately one-half ohm or less total resistance at room temperature which could be heated to an operating temperature of about 800C with as little as 15 watts of ener3y. The paste appears ~o form a dark residual layer, which may be an alloy, on the substrate after firing. Electron microprobe analysis has shown that the layer forms a chemical bond at the ~irconia interface and that the bismuth is concentrated at the interface.

.. , . . . . . , . .. . , . .. . .. . . ., ~ . . ..

~Z25~3~
- 2~ -Moreover, the material exhibited extremely high gaseous oxy~
gen permeability as indicated by a virtually instantaneous response of test sensors to variations in oxygen concentra-tions in sample gases employed.
Bismuth trioxide is added amounts sufficient to improve the adherence or bonding of the resulting electrode layer to the soli~ electrolyte. It is believed that bismuth trioxide in amounts of up to 10% by weight and preferably - between about 1% and 5~ by weight may be beneficially employed to form an oxygen porous integral heater/cell electrode layer. The bismuth trioxide appears to act as a flux, ~s does the gold. Thus, it is belisved that the amounts of bismuth trioxide and gold may be varied from the indicated amounts, at least to some extent, without ~eleterious effect on the characteristics of the resulting layer. It is also suspected, but has not been verified to date, that the bismuth trioxide may be useful in forming electrode la~ers wi~h base as well as noble metals, where base metals have heretoforP been employed in the past. Moreover, the dis-covered composition may be advantageously employed as a gas electrode material where an integral heater/electrode appli-cation is not needed by virtue o~ its ability to chemically bond the preferred platinum electrode material to a zirconia substrate in an oxygen porous layera The preferred paste electrode material is presently being hand applied. A single application provides a layer approximately 0.001 inches (25 microns) thick after firing.
Variations in the thickness of the outer electrode layer 37 of tne preferred embodimant res~lting from these application methods have been observed to produce a variation in surface resistivity of within about +50% from an average value of about 0.05 ohms~square/mil or less at room temperature. This has proved to provide adequately uniform heatins. In this regard, it is believed that the zirconia electrolyte substrate 31 acts as a heat sink diffusing heat rapidly away from hot Z5~

spots in the electrode layer 37 resulting from uneven layer thickness, Before application o~ the electrode paste to the outer surface of the electrolyte tube 31, the surface was masked for the slots 106. After each application of the elec-trode paste, the coated electrolyte substrate was heat-treated at about 980C +15C for about 15 minutes to sinter the pasteO
Overfiring appear to reduce the oxygen permeability of the resulting layer. One coat formed the outer`layer 37; three coats o~ the paste were applied to the inner surface of the electrolyte substrate 31 to form the inner electrode la~er 36.
The coated substrate was heat-treated as previously indicated between each application. Three coats were used to form the inner electrode layer 36 because of the difficulty of applying the paste to the tube interior and the desire to ensure conti-nuity o~ the inner electrode layer over the inner surface ofthe electrolyte substrate 31.

RADIO FRI~QUENCY HEATING
Other important inventive aspects of the preferred embodiment oxygen sensor and integral heater/electrode cell are the method and apparatus used to resistively heat the cell. It has been observed, in the course of the development of the preferred embodiment apparatusr that the magnitude of the emf developed ~y the electrochemical cell was effected by the relatively low frequency alternating current (convention-al 60 Hz line current) initially used for resistively heating prototype cells. When not influenced by a strong electric or magnetic field, it is bslieved the -oxygen ions in the solid zirconia electrolyte immediately adjacent each electrode lay-er 36 an~ 37 will be in equilibrium with the oxygen partialpressure in the gas adjacent the electrode layer 36 and 37 and tnat the existence of a strong electric or magnetic field will disturb that equilibrium. It was observed that when a steady, low frequency, sinusoidal electric current (i.e., conventional 60 hertz line current) was applied to the heater electrode layer, it was found impossible to simultaneously obtain a meaningful measure of ~he potential developed between .. . . . . ..... . .... . . .. . ... . . . .. . . .. . . .. ... ... . .. . ..

lZ25~3'~

the inner and outer electrodes. A digital typ~ voltmeter, which is in essence a periodic sampling instrument, will read out a wide range of potential levels. An analog type voltme-ter will read out a steady average potential which, however, is far different from the potential which would be produced by that sensor when heated lsothermally by a separate heat source such as a conventional auxiliary electric furnace. The shift in the average sensor emf developed due to the low fre-quency electromagnetic field surrounding the integral heater/
electrode is in that direction (i.e., more negative) which is indicative of a lowering of the partial pressure of oxygen adja-cent that heater/electrode. On some of the earliest embodi-ment sensors, the 60 cycle current shifted the average emf by as much as about 225 millivolts. This necessitated following a repetitive heat first then measure cycle in using the early embodiments of the apparatus, It was discovered that as the frequency of the sinusoidal heating current is increased, the average value of the sensor potential shifts in the direction which is indicative of a more correct reading o~ the oxygen partial pressure at the heater/electrode~ Above some suffi-ciently high frequency, the emf developed by th~ cell becomes stable and uneffected by the frequency of the heater curren~.
It llas been observed in various embodiments of the subject oxygen sensing apparatus that the permeability of the elect~ode/heater layer to oxygen h~s a bearin~ on the minimum frequency of the heater alternating current that can be em~
ployed. The platinum-gold-glass frit material initially employed exhibited a rather poor oxygen permeability. The minimum usable alternating frequency current was in the Yicin-ity of 200 KHz. This is in what is conventionally understood to be the low frequency portion ~30-300 KHz) portion of the radio frequency spectrum. It was found that with respect to the preferred platinum-bismuth trioxide electrode/heater layer material, the minimum usable frequency was below l0 KHz which lies within what is generally understood to be the very low ~requency portion (3-3~ KHz) of the radio frequency spectrum.

... .. . .. . . . . . . .. .

~ZZ5~34 For ease of coupling between the power source and electrode/
heater layer, it has been found preferable to employ a square wave alternating curren~ in a range of about 30 KHz to 100 RHz. As previously indicated, the power in~erface circuit S6 of the preferred embodiment apparatus generates a S0 KHz square wave alternating current~ Higher frequency alternating currents can be employed if power source switching losses are not excessive or interference with radio communication servic--es does not pose a problem. Moreover, a sinusoidal alternat-ing current may be employed in place of a square wave currentO
There is a significant amount of odd harmonic energy in square wave currents typically generated by existing equipmentr There-fore, the fundamental frequency for a square wave current may be somewhat lower than that of a sinusoidal current to have an equivalent ~nd effect on the observed electrochemical cell potential~ As the cell developed emf can be made insensitive to the heater current by employing a sufficiently high fre-quency current, a-variety of oxygen concentration apparatus operating methods can be employed. For example, the alternat-ing heater current can be .continuously circuited through the integral heater/electrode layer and its amplitude varied in order to provide the required heat input to maintain a stable electrolyte temperature. Alternatively, a constant amplitude alternating current may be switched on and off for variable lengths of time in order to supply the required heat input.
In that scheme, which is incorporated into the preferred em-bodiment apparatus disclosed, the off-time ~nterval is held short enough, for example about 50 milliseconds or less, that the transient temperature excursions in the active part of .30 the sensor ceIl are below the levels which would produce unacceptable variations in the emf developed by the cell.
As an example of the performance capability of the preferred ambodiment apparatus, the preferred embodiment elec-trochemical cell with platinum-bismuth trioxide electrode lavers was configured in a heat and measure mode of operation where the instantaneous cell emf was interrogated by a sample and hold read out system immediately prior to the reapplica-~5~34 tion of each burst of radio frequency heater current. For achange in the off-interval period length from approximately 0.03 to 0.27 seconds, the change in the measured instantane-ous potential for one typical experimental cell was in the worst instance 0.7 millivolts. Changing the partial pressure of the oxygen to the inner electrode from a level of 20.95%
to 2.79% for cell operating nominal temperature 1073K, the actual change in instantaneous potential developed by the experimental cell varied from 46.1 to 46.3 millivolts over the above indicated range of off-time interval leng~hs. According to the Nernst equation, such a conc~ntration change a~ the indicated temperature should produce a theoretical change in the sensor potential of 46.5 millivolts. Over the same range of of-time interval lengths, the value of the potential ob-served with an average reading me~er was essentially constantat 46.2 to 46~3 millivolts. Without correction, these numbers represent, in t~le worst case, an error in the determination of partial pressure of oxygen of less than about 2~. ~
Referring now to Fig. 7, there is depicted in great-er detail the components of the thermal controller 42, temper-ature compensa~or circuit 43, timer circuit 62, modulator circuit 64, high frequency signal source 66 and power inter-face 56 which together act as a closed loop temperature con-trol circuit for the cell 30. ~he set point input circuit 46 is optionally provided in order to adjust the operating temperature of the cell 30. A suitable voltage level signal might otherwise be generated by another circuit .element to represent a fixed operating temperature for the cell 30. The overall function of the circuit elements 42, 43, 46, 56 and 62 through 66 have been previously described.
~ s is indicated in Fig. 7, the set point circuit 46 includes a set point calibration circuit formed by a variable resistance 131 and fixed resistance 132. Also included in the circuit 46 are an operator controlled potentiometer 133 with temperature indicating control dial (not depicted) for opera-tor selection of the cell operating temperature, automated set point input junctions 134 and 134a where an externally ..... ... . . .. . .. ..... .. . .... . . .. . .. . . ..

4;3~

generated set point voltage signal may be inputted into the set point circuit 46 for automatic temperature control and a manually ope.rated switch 13i for selecting operator or auto-matic set point control. The circuit 46 generates a voltage 5 level signal passed on line 46a to the thermal controller circuit 42.
Also providing an input to that circuit 42 is the thermocouple temperature compensation circuit 43 which includ-es a cold junction compensation diode (type IN916) 136, opera-tional amplifier (type OP07) 137, capacitor 138, variable resistor 138a and fixed resistors 139 through 143. Screwheads 116 and 118 represent a circuit junction block to which the thermocouple leads 40 and 41 are connected. The diode 136 is, in reality, located at the junction block so as to be at an identical temperature to that of the negative (Alumel~)the~ocouple lead 41. The operational amplifier 137 accepts at its negative input, the voltage level signal on the posi-tive (Chromel~) lead 40 of the thermocouple 39 and a tempera-ture compensated voltage level derived from the cold compensa-tion diode 136. These voltage level signals are summed togeth-er and amplified so that a compensating voltage due to change in room temperature emf at the termination of the input of the thermocouple 39 is added to the thermocouple signal thus restoring the reverse emf effect caused by connecting ths thermocouple leads 40 and 41 to the room temperature junction block. Capacitor 138 is provided to decouple the thermocouple emf from any fluctuating emf component which might be induced in the thermocouple 39 by the alternating heater current.
Variable resistor 138a provides means for adjusting the temperature compensation. The remaining resistors provide, with the other elements, an effective gain of 122. The amplified and compensated voltage of the thermocouple 39 is passed on line 43a to the thermal controller 42.
The thermal controller circuit 42 is formed by first and second operational amplifiers (type 347) 146 and 147 with related feedback circuits and resistors 144, 145 and 151 ad-.. . ..

~Z~25434 justing the gain of signals on lines 43a, 46a and 146a, re-spectively. The first operational amplifier 146 together with diode 148, capacitor 149 and resistances 15Q and 151 form a novel combination deviation amplifier and two mode (percent proportional band and integral functions~ controller. The room temperature compensated thermocouple voltage outputted on line 43a is combined with the set point voltage level sig-nal outputted on lines 46a at junction 152 and the combined voltages, together with feedback voltage are fed into the negative input of the first operational amplifier 146 The set point circuit and temperature compensation circuit voltage level signals are scaled for 5 millivolts/C. The se~ point voltase level signal is the opposite polarity of that output-ted by the compensation circuit amplifier 137 so that the algebraic sum of the output of the compensator circuit 43 and set point circuit 46, as computed by the amplifier 146, is - equal to the proportional deviation error selected for optimal temperature control loop stability and desired whole tempera-ture accuracy. The resistors 151 and 144 provide a fixed proportional band error (1%) which represents the allowable percentage difference between the voltage levels on lines 46a and 43a. Ra~et action occurs as a function of error integra-tion across the capacitor 149. The combination of proportion-alizing and reset action is selected to provide, in the pre-ferred embodiment being described, optimal loop stability and whole temperature accuracy within +1C with expected oxygen sensor upsets. Diode 148 is provided to improve the response time o~ the two mode controller. The diode prevents the out-put of the amplifier 146 from rising much above zero, as would occur if there were a sudden call for a lower control tempera-ture. The diode 148 reduces the integration requirement im-posed on the capacitor 149 under such conditions allowing a faster return to control. The second operational amplifier 147 together with resistors 152 and 153 form a unity gain inverter which reverses the polarity c,f the output of the first amplifier 146 for driving the modulator circuit 64.
The timer circuit 62 and modulator circuit 64 are lZ'~543~L

formed from a single integrated circuit timer element (type 556) together with related capacitors an~ resistances. The single integrated timer circuit is represented functionally ~y two separate timer elements 155 and 162 in Fig. 7, The ; timer circuit 62 is formed by the timer element 15; together with capacitors 156, 157, 158 and resistors 159, 160 and 161~
Resistor 160 is connecte~ between the first reset and dis-charge function of the integrated circuit timer while resistor 161 is connected between the first discharge and first thres-hold functions of the integrated circuit timer. Resistors 160 an~ 161 define the width (which is arbitrary) of the tim-ing pulse outputted ~y the timer circuit 62. Capacitor 158 is ~rovided between an input voltage source and the first trigger function of the integrated circuit timer and defines the time period (7 milliseconds in the described embodiment) between consecutive pulses outputted by the timer circuit 62.
Capacitor 157 is provided to by-pass the first control voltage function. Resistor 159 and capacitor 156 smooth and shape the signal outputted ~y the timer circuit 62 on the line 62a, The output of the timer 62 is passed via the line 62a ~o the modulator circuit 64 which includes the second timer element 162, capacitor 163 and variable resistor 164.
The timer element 162 is formed by sui.tably wiring ~starting from upper left side of 162) the second reset, discharge~
threshold, control voltage and output functions of the 556 integrated circuit timer. The capacitor 163 and variable resistor 164 determine the proportion of the 7 millisecond period for the output of the timer element 162 is high on line 64a, The resistor 164 is adjusted to provide for a maximum high output duration of 98% of the 7 millisecond pulse.
The timer element 155 is connected as a free running oscillator. The timer element 162 is connected in a monosta-ble mode so as to be triggered by the negative going edge of each of the consecutive pulses from the timer element 155.
The threshold triggering voltage of the timer element 162 is further modulated by the voltage level signal passed from the thsrmâl controller circuit 42 along the line 42a. The timer ... .. . .. .. . ... . . . ~ . . . . ... . . . . . . . ..

~ZZ5~3'~

circuit 162 thus configured acts as a pulse width modulated oscillator. With consecutive pulses being outputted by the timer element 155 at 7 millisecond intevals, the maximum pulse width of the high output level signal outputted by the timer element 162 is adjusted to 6.8 milliseconds by the variable resistor 164 with a threshold voltage of 12.5 sup-plied from the thermal controller circuit 42. The minimum stable pulse width supplied by the timer element 162 in the 'depicted configuration is one millisecond with a threshol~
voltage of 2 from the thermal controller 42. Thus, the total range of effective thermal controller control of the modulator,circuit 64 is approximately between 12.5% and 97~
of the 7 millisecond duty cycle. The on/off (high/low) vol-tage level signal outputted by ,the timer element 162 is passed via line 64a to the radio frequency signal generator circuit 66.
The circuit 66 is formed by an inte~rated circuit timer 166 (type 5~5), variable resistor 167, diodes 168 a~d ,169, capacitors 170 and 171 and~fixed resistance 172. The timer 166 is also wired as a free running oscillator ~enerat-ing a square wave output signal fluctuatins between levels of, about 0 and +15 volts at a frequency of 50 Khz. The output of the oscillator 162 is switched on and of~ by applying ~he width modulated pulse~ outputted by the modulator cirsuit 64 on line 64a to the reset function of the type 555 timer ele~
ment 166r The frequency of the output of the timer circuit 166 is determined by the combination of capacitor 170 and resistors 167 and 172. The variable resistor 167 is connected between the power input (Vcc) and discharge functions of the 555 timer. The resistor 172 and diodes 168 and 169 are con-nected between the discharge and threshold functions of the timer while the line 64a feeds into the reset functions. Capa-citor 171 by-passes the voltage control function. The variable resistor 167, fixed resistor 172 and diodes 168 and 169 are provided to cause the timer element 166 to generate a symmet-rical square wave with equal on and off time periods. Actual adjustment is provided by variable resistor 167. Diodes 168 ... .. . . .. ~ .. . .. . . . . . . .. ..

~ 5~3~ .

and 169 provide an alternative discharge path for the vol-- tage charge on capacitor 170 which itself is selected to determine the frequency of the signal (50 Kh7) outputted on the line 66a. This assures that the signal generated by the signal generator 66 has an on time of less than or equal to 50% of the duty cycle. This is necessary in order to control symmetric AC heating current through a power interface circuit drive coupling capacitor 1790 The power interface circuit 56 is formed in the pre-ferred embodiment by a novel charge-discharge square wave al-ternating current signal generator formed by a first transis-tor (type 2N2218) 175, second and third transistors (type 2N6284~ 176 and 177, AC non-polarized capacitors (polyfilm) 178 and 179, electrolytic capacitor 180, and fixed resistor elements 181 and 182. The transistors 175, 176 and 177 form a drive circuit and act as switches which control the alter-nate char~ing and discharging of capacitance 179. The tran-sistor 175 is an inverter the purpose of which is to provide a 180 phase inversion of the 50 Khz oscillator signal out-putted by the radio frequency signal source 66, along with necessary current amplification to drive the transistor 176.
The oscillator 166 drives the transistor 177 through the coupling capacitor 178r The action of driving transistors 176 and 177 alternately cause a direct current to flow through the transistor 176 into capacitor 179 through ~he oxygen sensor integral cell electrode/heater 35 to a common return and then to cause another direct current to flow in the opposite direction back through the element 35, capacitor 179 ana transistor 177 to a common circuit return. Thus, the capacitor 179 is alternately charged by the transistor 176 and discharged through the transistor 177 at a frequency equal to that of the driving frequency (50 Khz) provided by the modulator 66. Capacitor 179 acts like an alternating current generator causing an alternating electric current to flow through the load comprising the transformers 183 and 184.
Resistors 181 and 182 adjust the voltages passed to the base and collec~or, respectively, of transistor 175. Capaci-tor 180 provides a by-pass for a +45 volt power source fed 1~5~3~

into the power inter~ace 56 at location 126. The drive coupling capacitor 179 decouples the DC component from the AC current generated by the transistors 176 and 177 causing an alternating sguare wave current fluctuating between about .+20 and -20 volts to flow through the toroid auto trans-former 182. The charge-discharge action of the capacitor 179, unlike the conventional push-pull configuration, requires only one power source supplied at the location 126. The cir-cuit as configured outputs a maximum power of about 30 to 40 watts. The present drive circuit has the advantages of reduc-ed cost, size and improved reliability over more complicated push-pull power circuits and is an improved device where a simple on/off square wave alternating current is needed~ The toroid auto transformer 182 steps up the voltage passed to the 15conventional step-down (isolating toroidal) transformer 183 which, in turn, increases the current passed through the heat-er circuit 35 of the cell 30 from a level of about 1 amp to about 6 to 7 amps and reduces the voltage accordingly.
The depicted circuit utilizes three power sources:
20+15 volts denoted by junctions 122, -15 volts denoted by junc-tion 124 and +45 volts denoted by junction 126. The following other circuit elements were used.
Resistance Element Resistors 1 ohms 139 25200 ohms 138a ~aximum) 330 ohms 182 lk ohms 150 & 181 5k ohms 131 (maximum) 8.2k ohms 141, 142 30lOk ohms 133 (potentiometer), 140, 152, & 153 15k ohms 159, 172 18k ohms 132 20k ohms 160 3525k ohms 167 (maximum) lOOk ohms 144, 145, 161, &
164 (maximum) 1 mega ohm 143 .

.. .. ~ , ~ . . ., ... . . , . . , . . . . . . . .. . ~ .. . .

~5'~3~

CAPACITORS
Capacitance Element 1 microfarad 178 microfarad 138 & 179 5 100 microfarad 180 0.001 farads 149 & 170 0.0015 farads 156 0.1 farad 157, 158, 163 & 171 ALTERNATE EMBODIMENTS
10 While the preferred embodiment of the invention has been described in terms of a low temperature gaseous oxygen detector incorporating a close ended tubular electrochemical sensor cell 30, other sensor cell configurations are envisioned to employ the inventive aspects heretofore described. For example, one common configuration for an oxygen or gaseous oxide detector is depicted schematically in Fig. 8 and comprises a solid state electrochemical cell 230 having a disk shaped solid electrolyte 231, a first gas electrode layer 237 on one outer surface of the electrolyte disk 231 and a second gas electrode layer 236 formed on the opposing outer surface of the electrolyte disk 231. Commonly used in an extractive type detector, suitable means such as a non-conductive alumina tube 272 is used to fixably mount the cell 230 and to seal opposing chambers 250 and 252 in which a sample gas and a re~erence gas, respectively, are circulated. The gases are brought into the chambers 250 and 252 to the respective electrode layers 237 and 236 by suitable piping such as quartz or alumina tubes 254 and 256. For further information regarding the construction of this type of a sensor, see, for example, the U.S. Patent 4,282,078 to Chamberland et al. One electrode layer 237 is wired by leads 260 and 262 to an alternating electric current source 260 in order that the layer 237 may be used as an integral cell electrode/heater. In addition, the lead 162 branches into a potentiometer 166 or other suitable high impedance device for responding to emf developed 43~

between the two electrode layers 237 and 236. Lead 268 completes the circuit between the electrode layers 237 and 236 through the potentiometer 266. The solid electrolyte wafer 231 is envisioned to have circular outer surfaces upon which the electrode layers 237 and 236 are applied. Electrode patterns which have been devised previously for providing substantially large areas of uniform heating for use as stove elements and the like are described in U.S. Patent Nos. 3,813,520 and 3,848,111, both to Brouneus. It is envisioned that these patterns or simple modifications to them may be employed to provide sufficiently large, essentially isothermal active areas in the cell 230 to allow use of the apparatus as an accurate measuring device. Again, lS the technology has application both to the detection and measurement of oxygen and to the detection and measurement of other gaseous compounds including various gaseous oxides. Again, a solid reference electrode electrochemically suited for the chemical reaction driving the sensor 230 may be employed in place of the gaseous reference electrode layer 236.
Also, the two electrode layers 237 and 236 may be circuited through separate alternating current generators and used as combination heater/electrodes.
Yet another potential sensor configuration employing an integral heater/electrode which is envisioned is a flow through tube such as is depicted in Fig. 9. An electrochemical sensor cell 330 is provided with a tubular solid electrolyte substrate 331, again of zirconia or other solid electrolyte material suitable for the electrochemical application, having opposing open ends 332 and 333. An inner electrode layer 337 is provided along the inner tubular surface of the substrate 331 while a second electrode layer 336 is applied to the outer surface of the tubular substrate 331. The interior of the tubular substrate 331 would be sealed from its exterior by suitable means such as alumina tubes 338 and 340 circulating a reference gas or sample gas mixture ;,.

~22S43~
41a through the interior of the cell 330. The cell 330 i5 again heated by resistively heatlng one or both of the electrode layers 334 or 336. In the depicted embodiment, a high Erequency current - ~225439L
.

source 342 is connected by means of bus leads 344 and 346 to circular terminals 348 and 350 at opposing ends 332 and 333 of the inner electrode layer 337. Again, the inner electrode 337 is electrically connected with the outer electrode 336 across a potentiometer 352 or other high impedance circuit for responding to the emf developed by cell 330.

PROTOTYPE APPARATUS
Depicted diagrammatically in Fig. 10 is an appara-tus conceptualizin~ the manner of operation of the first suc-cessful prototype oxygen detection apparatus with integral -heater/electrode cell 530. Figs. ll and lla depict the con-figuration of that prototype cell 530O The apparatus depicted in Fig. ll includes the sensor cell 530, a thermocouple 539 positioned within the sensor at about the location of the junctions between the leads 548 and 550 and an outer double spiral electrode layer 537 for detecting the maximum tempera-ture of the cell 530. Leads 540 and 541 of the thermocouple 539 are carried to a standard furnace controller 542. A tube 544 carries a sample gas having a known oxygen content to the interior of the cell 530 while the exterior of the cell 530 is exposed ambient air. The atmospheres inside and outside the cell 530 were isolated from one another by suitable means not shown. Leads 548 and 550 connected a step-down transform-er 556 across and the integral cell electrode~heater layer 537 on the outer surface of the cell 530. Yet another lead 558 connected an inner electrode layer 536 on the inner surface of the cell 530 and the integral Plectrode/heater layer 537 on the exterior of the cell 530 across a potentiometer 561.
Conventional 60 Hz alternating line current was supplied via plug 566 to the step-down transformer 556. A conceptual switch assembly 600' between the plug 566 and transformer 556 controls the flow of current to the transformer 556. The first switch 601' is operated by the action of the temperature controller 542 while a second switch 602' is manually operated.
The operation of switch 602' is linked with that of a switch 603' so that when the alternating current is switched off by - iL2Z5~3~
- ~3 -switch 602' the potentiometer 561 is activated by switch 603'.
F'ig. 11 is a plan View of the prototype electrochem-ical cell 530. The cell 530 includes a solid yttria stabiliz-ed (8~ by weight) zirconia substrate solid electrolyte 531 in a tubular configuration similar to that of the solid electro-lyte 31 in the cell 30 in Figs. 1 through 5 having a 6 inch length and 3/8 inch outer diameter. An outer integral cell electrodc/heater layer 537 is formed on the exterior surface of the electrolyte 531 and includes a portion 600 covering the hemispherical tip of the electrolyte 531, a pair of opposing, helically extending electrode spirals 601 and 602 about 1/8 inch wide and extend'ing approximately half the length (3 inch-es) of the tube and ending in opposing parallel strips 603 and 604, respectively, extending longitudinally for the remainder of the tube length (about 3 inches)~ The leads 548 and 550 are connected to the longitudinally extending parallel strip porti~ns 603 and 604. The helical nature of the strip 601 is depicted in Fig~ lla. The hidden portion of the strip 6~1 in, Fig. 11 being indicated by the phantom element 601a in Fig.
2~ lla. Narrow spirals 531a of exposed electrolyte separa~ed adjoining spirals 601 and 602 of the electrode layer 537. A
solid electrode layer 536 is provided entirely across the' interior surface of the electrolyte 531 extending from the hemispherically closed end ~34 o~ the electrolyte to a point 2~ beyond where the longitudinally extending parallel strip por-tion 603 and 604 begin. The extent of the inner electrode layer 536 is indicated representatively by the dotted line 605 in Fig. lla.
The operation of the apparatus in Fig. 10 was as follows. Switches 6011 and 602' were closed allowing the step-down transformer 556 to supply an alternating voltage with a 60 Hz frequency and amplitude of about 5 to 20 volts. This current was passed via the leads 548 and 550 through the out~
er electrode layer 537. The outer and inner layers 536 and 537 were each formed by three applications of the a~oresaid platinum-gold-glass frit material. The sensor 530 was slowly heat~d (about 300-400,C per hour). The controller 542 moni-.. .. .. . , ... . .. .. . ~, .... . . . . . ...... .. . . . . . . . . . . .

lf~'~543~1 .

tored the temperature of the cell 530 by means of the thermo-couple 539 and controlled the on/off operation of the step-down transforme.r 556 by the switch 601'. When it was desired to measure the emf ~eveloped by the cell 530 across the two 5electrode layers 536 and 537, the alternatiny current was switched off manually, such as by the switch 602', and the emf developed between layers 536 and 537 measured (in the depic~ed apparatus by closing the swi~h 603'). The apparatus generated a two millivolt emf (cell constant C of the Nernst equation) 10when ambient air was supplied to both electrodes 536 and 537.
Heating of the cell 530 by the depicted outer elec-trode layer 537 configuration was less optimal than that of the preferred embodiment outer eIectrode 37 :configuration depicted in Figs. 2 through 4. It was found that when a vol-tage of sufficient magnitude to heat the cell 530 was circuit-ed through the platinum-gold-glass frit outer electrode layer 537 at the. opera~ing temperature of the sensor (nominally 760C) shorting of the heater across the electrolyte surface between the helical spiral strip 601 and 602 occurred. Short~
ing across the electrolyte might occur anywhere along the spiral ~ut once initiated would inevitably travel back toward the junction between the leads 548 and 550 and longitudinally extending parallel strips 603 and 604 of the outer electrode 537 where the voltage difference was greatest. Measurement-c indicated that when a desired temperature of about 760C was eventually produced at the center of the helical windings 601 and 602, tha temperature of the electrode at the hemispherical tip 534 was approximately 500C and in the vicinity of the junction between the leads 548 and 550 and parallel extènding 30strips 603 and 604 was about 1000C. Heating of the cell 530 . caused the resistivity of the outer electrode layer 537 to rise and the electronic resistivity of the substrate 531 to fall thus forminy, in effect, a pair of variable resistors in parallel. The apparatus was made to operate by locating the sensing portion of the thermocouple 539 in the vicinity of the junction between the leads 548 and 550 and the outer elec-.. . ........ .. . ... .. . . .. .. .. . . ... .. . .. . . .

~LZ25434 - 45 ~

trode layer 537. Once the thermocouple 539 was moved into the vicinity of the junction ~etween the electrode layer 537 and leads 548 and S50 and the voltage adjusted to hold the sensor at about 800C, tha ~ensor 530 was successfully operat~
ed continually for one week without failure. A 60 Hz alternat-ing current of between a~out 1.5 and ~.0 amps at about 14 volts was used to heat the appara~us. In another experiment, a cal-cia stabilized zirconia substrate solid electrolyte was sub-stituted for the yttria stabilized elec~rolyte 531. in the sensor 530. The calcia stabilized zirconia has an electric conductivity an order of magnitude less than that of the yt-tria stabilized zirconia at the operating temperatures of the cell 530. The calcia stabilized electrolyte cell provided an active zone about one half inch long located about one inch back from the tube tip in the spiral resion. However, the calcia stabilized zirconia exhibited susceptibility to elec-trolys s from the alternating (60 H~) heater current.
While various embodiments of the subject invention have been described and modifications thereto suggested, it will be appreciated that other changes and improvements will be apparent to one skilled in the art. The invention is therefore not limited to the disclosure but is set forth in the appended claims.

, . . . . . ~ . . . . . .....

Claims (26)

WHAT IS CLAIMED IS:
1. An electrochemical cell with integral heater comprising:

a solid electrolyte having at least a hollow tubular portion with an outer tubular surface and opposing inner tubular surface; and an integral cell electrode/heater layer cover-ing substantially all but a pair of opposing, sub-stantially parallel longitudinally extending strips of the outer tubular surface.
2. The electrochemical cell of claim 1 wherein said electrolyte tubular portion has a substantially circular cross-sectioned with a substantially uniform cross-sectional thick-ness and said layer has a substantially uniform composition and thickness along at least a major proportion of the outer tubular surface.
3. The electrochemical cell of claim 1 wherein said solid electrolyte further comprises a hollow, substantially hemispherical portion integrally formed with the tubular portion closing one open end of the tubular portion, the hemispherical portion having a convex outer surface and an opposing concave inner surface and wherein the integral layer covers at least a major proportion of the convex outer surface.
4. The electrochemical cell of claim 3 wherein said strips extend from a remaining open end of the tubular portion of the electrolyte to the hemispherical portion of the electro-lyte and said integral layer covers the entire convex outer surface.
5. The electrochemical cell of claim 3 wherein the opposing parallel strips of exposed electrolyte extend from near the remaining open end of the tube portion of the electrolyte and divide said layer into two portions, the two layer portions beginning near the remaining open end of the tubular electrolyte and joining near the hemisphere portion of the electrolyte to form a single continuous layer, and said integral layer has a maximum total resistance of about one-half ohm or less at room temperature.
6. The electrochemical cell of claim 1 further comprising a second electrode layer contacting said inner tube surface of the electrolyte opposite said first electrode layer and extending along at least a portion of the length of the tubular portion exposed by the two opposing strips.
7. The electrochemical cell of claim 6 wherein the two electrode layers are each formed from a material comprising a major proportion by weight of platinum and are porous to gaseous oxygen, and said solid electrolyte is formed from a material comprising a major proportion by weight of zirconia.
8. An electrochemical apparatus comprising:

an electrochemical cell according to claim 1;

generation means for supplying a radio frequency alternating electric current; and circuit means connecting said generation means in a circuit through said cell for -47a-resistively heating at least a portion of the cell with said radio frequency alternating electric current.
9. The electrochemical apparatus of claim 8 where-in said cell further comprises a second electrode and said cell develops an emf between the two electrodes; and wherein said radio frequency alternating electric current has a fre-quency of alternation sufficiently high that the emf developed by the electrochemical cell is substantially uneffected by any further increase of the frequency of alternation of said alternating current.
10. The electrochemical apparatus of claim 8 wherein the frequency of the alternating current is between about 3000 and 200,000 hertz.
11. The electrochemical apparatus of claim 10 wherein the frequency of the alternating current is between about 30,000 and 100,000 hertz.
12. The electrochemical apparatus of claim 8 fur-ther comprising:

temperature sensing means for generating a sig-nal related to the temperature of the electrochemic-al cell; and control means responsive to the temperature signal for controlling the power level of the alter-nating current supplied by said generation means through said cell.
13. The electrochemical apparatus of claim 12 wherein said control means further comprises means for auto-matically switching the alternating current on and off.
14. The electrochemical apparatus of claim 8 wherein said electrochemical cell further comprises a cell electrode and said circuit means connects said generation means across the cell electrode for resistively heating at least a portion of the cell electrode.
15. The apparatus of claim 14 wherein said electro-chemical ceil further comprises:

a solid electrolyte having a hollow, tubular portion with an outer tubular surface and opposing inner tubular surface;
said cell electrode comprises:
an electrically conductive layer covering a major proportion of the outer tubular surface of the electrolyte;
the cell further comprises:

a pair of strips of exposed electrolyte outer tubular surface extending along at least a portion of the tubular portion of the electrolyte, the strips dividing the electrode layer into two end portions beginning at an open end of the tube, the electrode layer being circumferentially continuous about the electrolyte outer surface at the remain-ing end of the electrolyte; and said circuit means comprises:

a pair of leads, each lead extending from the generation means to one of the two portions of the electrode layer near the said open end of the tubular electrolyte whereby substantially all of the electrode layer is connected in a circuit with the generation means.
16. The electrochemical apparatus of claim 15 where-in the resistance of the cell electrode between the two lead contact points is about one-half ohm or less at room tempera-ture.
17. An improved method of heating an electro-chemical cell, said cell comprising an electrolyte, a first cell electrode and a second cell electrode, said electrodes contacting said electrolyte, characterized in that at least a portion of one of said electrodes contacting said electrolyte is heat-ed, the heating being carried out by generating an alternating electric current having a radio frequency level of alternation and passing said alternating electric current through at least a portion of one of said electrodes.
18. The improved method of claim 17 wherein the generating step further comprises the step of:
generating an alternating current with a frequency of alternation sufficiently high as to eliminate any offset in the magnitude of the emf developed by the electrochemical cell caused by the frequency of the alternating current.
19. The improved method of claim 17 wherein during passing of said alternating electric current through at least a portion of one of said electrodes no more than a negligible portion of said alternating electric current is passed through said electrolyte.
20. The improved method of claim 17 wherein said generating step further comprises generating an alter-nating electric current with a frequency of between about 30,000 and 100,000 hertz.
21. The improved method of claim 17 further com-prising the step of sensing the temperature of the cell and wherein said step of generating further comprises varying the power of the alternating electric current passed through the cell in response to said sensing step.
22. An improved method of heating an electro-chemical cell, said cell comprising an electrolyte, a first cell electrode and a second cell electrode, said electrodes contacting said electrolyte, characterized in that at least a portion of one of said electrodes contacting said electrolyte is heated, the heating being carried out by resistively heating at least one of said electrodes while no more than negligibly resistively heating said electrolyte.
23. The improved method of claim 22 wherein said resistive heating step further comprises the steps of:
generating an electric current; and passing said current through one cell electrode while passing no more than a negligible portion of said current through the electrolyte.
24. The improved method of claim 23 wherein said generating step further comprises the step of gene-rating an alternating electric current having a frequency of between about 3,000 and 300,000 hertz.
25. The improved method of claim 22 wherein the electrolyte is a solid and wherein the resistive heat-ing step further comprises the step of: heating the electrolyte to a maximum temperature in a zone, the zone extending through the electrolyte between the two electrodes and covering an area of at least several square centimeters and the temperature of the electro-lyte in the zone varying about 10°C or less from the maximum temperature.
26. The improved method of claim 22 wherein the electrolyte is a solid and at least one of the elec-trodes further comprises an electrically conductive layer covering at least a portion of an outer surface of the solid electrolyte and said step of resistively heating further comprises the steps of: passing a substantially uniform electric power density through a major proportion of the electrically conductive layer.
CA000447373A 1983-04-25 1984-02-14 Method and apparatus for integrally heated electrochemical sensors Expired CA1225434A (en)

Applications Claiming Priority (2)

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US488,094 1983-04-25

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