US3414704A - Self-regulating heating device - Google Patents

Description

Dec. 3, 1968 c. D. FLANAGAN SELF-REGULATING HEATING DEVICE 3 Sheets-Sheet 1 Filed Feb. 25, 1965.

Inventor, Charles D. Flanagan, -'///"'IJZ/// y Ayeni'.

Dec. 3, 1968 c. D. FLANAGAN 7 3,414,704

SELF-REGULATING HEATING DEVICE Filed Feb. 25, 1965 3 Sheets-Sheet 2 Inventor, Charl D. Flanagan,

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1968 c. D. FLANAGAN SELF-REGULATING HEATING DEVICE 3 Sheets-Sheet Filed Feb. 25, 1965 55 Stu \CS Rm w$ I50 200 TEMPERA TURE 7 Inventor, Flanagan, 2/

Charles D Q7 ,4 K M A3672 t United States Patent 3,414,704 SELF-REGULATING HEATING DEVICE Charles D. Flanagan, Attleboro, Mass., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Feb. 25, 1965, Ser. No. 435,165 18 Claims. (Cl. 219210) ABSTRACT OF THE DISCLOSURE Method and means for providing a relatively constant ambient temperature for temperature sensitive devices is disclosed. The method includes the steps of forming an enclosure, in which the controlled ambient will be effected, at least partially, of a positive temperature coefficient of resistance (hereinafter referred to as PTC) material and applying a voltage to the PTC material. When used as taught by the invention, the material acts as a heater and also as its own temperature regulator. The enclosure is formed, at least partially, by various structures or means including a single hollow cylindrical element and a disc-shaped PTC block sandwiching a thermal conductor in which is located a cavity. In these embodiments, the direction of current flow through the PTC elements is approximately parallel to the direction of heat fiow into the cavity.

This invention relates to heating elements, and particularly to ovens used to provide a constant temperature for temperature sensitive devices located therein. There are many electronic components which must be contained in a constant ambient for effective operation in certain applications. Examples of such components are crystals, diodes, transistors and so on.

It is known to provide ovens as described above which utilize a heater and a thermostat. The thermostat keeps the inside oven temperature within a certain range by turning on and off the heater current by use of movable contacts. This type of oven has certain inherent disadvantages, viz., the temperature varies as a result of the characteristics of the thermostat-from a maximum to a minimum back to a maximum and so on. Also, since there is mechanical movement, the longevity of the device is limited.

Another approach has been what is commonly known as proportional control whereby relatively complex electrical circuits serve to limit the power input to the heater to equal the heat loss from the oven. This is done, for example, by providing a bridge containing a temperature-sensing device which is used to balance a circuit containing the heater. This type of control eliminates the on/ofi moving contacts and therefore provides more precise temperature control with no overshoot or thermal cycling, more constant power requirement and no noise due to mechanical operation although many devices of this description do emit electrical noise. Also, the device is relatively complex and expensive.

It is an object of the invention to provide an oven which is simple, highly reliable, long lasting, mechanically and electrically silent operating and displays a closely controlled, relatively constant oven temperature.

It is an object of the invention to provide a heating element which has a self-regulated temperature.

It is a further object to provide an oven which is characterized by being a self-regulating temperature nature.

It is another object of the invention to provide an oven which will maintain a relatively constant inside Cir 3,414,704 Patented Dec. 3, 1968 ice temperature for temperature sensitive devices contained therein regardless of change in heat demand which is simple, reliable, long lasting, silent operating and has no moving parts.

The invention accordingly comprises the elements and combinations of elements, steps and sequence of steps, features of construction and manipulation, and arrange ments of parts, all of which will be exemplified in the structures and methods hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings:

FIG. 1 is a vertical cross section through one embodiment of the invention;

FIG. 2 is a cross section through 2-2 of FIG. 1;

FIG. 3 is a top plan view of a second embodiment of the invention;

FIG. 4 is a cross section through 4-4 of FIG. 3; and

FIG. 5 is a chart plotting logarithms of resistivity against temperature of a steep sloped PTC material usable in accordance with the invention.

Similar references characters indicate corresponding parts throughout the several views of the drawings.

Dimensions of certain of the parts as shown in the accompanying drawings may have been modified and/ or exaggerated for the purposes of clarity of illustration.

Referring to FIGS. 1 and 2, the first embodiment is shown. The crystal oven or chamber 10 consists of heating element 12 supported on a base 14. Heating element 12 is composed of a cylinder 16 and discs 18 and 20 all composed of positive temperature resistivity coefficient material, hereinafter referred to as PTC material. Cylinder 16 is coated by any known method, such as firing or ultrasonic soldering with an outer electrically conductive metal layer 22 and an inner electrically conductive metal layer 24 providing a good electrical connection with the PTC cylinder 16. The ends of cylinder 16 are coated in any known manner with electrically insulating layers 26 and 28 of resin, ceramic or other suitable material. Discs 18 and 20' are circular in shape and are coated on their faces with electrically conductive metal layers 30, 32 and 34, 36 respectively. The discs close the ends of cylinder 16 and form a cavity 11 therein. Disc 20 mounts bracket 38 whch is formed of an electrically conductive and spring material such as beryllium copper. The bracket 38 is telescopically received in the cylinder 16 and fingers 40 of the bracket electrically connect layer 34 of disc 20 and inner layer 24 of cylinder 16. Lead 42 electrically connects inner layer 24 of cylinder 16 and layer 32 of disc 18 so that the entire inner surface of the heating element forming the cavity is electrically connected. The outer surface of the heating element is electrically connected in like manner. Lead 44 electrically connects layer 30 of disc 18 and outer layer 22 of cylinder 16. Outer layer 22 of cylinder 16 is electrically connected to layer 36 of disc 20 by clips 46 which are composed of electrically conductive spring material. Clips 46 are suitably attached to layer 36 as at 48 and releasably grip cylinder 16 at 50. Cylinder 16 and disc 18 are encased in a conventional potting compound 52 of suitable electrical and heat insulating characteristics, leaving spaces 54 for reception of clips 46. Metal cap 56 covers the encased heating members 16 and 18. Heating members 16 and 18 along with the potting material 52 and cap 56 are releasably supported on base 14 which is composed of a conventional molded phenolic resin or similar material. Terminals 60, 62, 64 and 66 of a good electrically conductive material are inserted in base 14 to provide electrical connections with outside power sources. L1 is connected to terminal 60 and L2 is connected to terminal 62 by screws 58. Disc 20 is provided with bore 76 through which conductor 68 passes and electrically connects terminal 60 and bracket 38. Conductor 70 electrically connects terminal 62 and layer 36 of disc 20. A complete circuit is thereby produced between Ll, terminal 60, conductor 68, bracket 38, inner layers 24, 32 and 34, PTC elements 16, 18 and 20, outer layers 22, 30 and 36, conductor 70, terminal 62 and L2. It may be seen therefore, that current passes through the heating element 12 thereby generating heat which causes the oven to heat up as explained infra. Bracket 38 also serves to mount support or socket 80 which receives the electronic or other component 90 that is desired to be kept at a constant ambient. Conductors 72 and 74 pass through bore 78 in disc 20 and electrically connect terminals 64 and 66 which provide the electrical connections for the electronic component.

The PTC elements 16, 18 and 20 act as a heater and also as their own temperature regulator. The PTC material which is a semi-conductive, steep sloped material such as, for example, Ba La TiO the resistivitytemperature curve for which is shown in FIG. 5, has'a low resistance in the cold state and when power is applied through the heating circuit relatively large currents are drawn and consequently relatively high power and heat are dissipated. As cavity 11 heats up due to the thermal dissipation (electrical losses), the portions or layers nearest the cavity of the PTC elements will also increase in temperature and will increase abruptly in resistance when the anomaly temperature is reached. An equilibrium condition is soon reached with the inner portions or layers of the PTC elements in a high temperature, high resistance state as at d on curve C and the portions or layer furthest from the cavity (outside portions) in a low temperature, low resistance state as at a on curve C, where the thickness of the portion in the PTC element in the high resistance state adjusts itself so that the total PTC element resistance presented to the essentially constant voltage external circuit is such that thermal dissipation just offsets the thermal losses to the ambient thereby maintaining the inside surface of the PTC elements at the desired controlled temperature. The anomaly point b on curve C, or the temperature at which the resistance starts to increase abruptly with increasing temperature, will be located intermediate the inner and outer layers of the PTC elements. Should conditions change, the PTC elements will react to compensate for these changes so as to maintain the temperature of the internal surface, the surface defining the cavity, essentially constant. If the thermal losses increase due to a lower ambient temperature, increased circulation of air around the device or some other cause the temperature of the outer portions is, of course, lowered. This in turn lowers the temperature in the region where the anomaly point was located, moving the anomaly point further inwardly toward the cavity thus reducing the total resistance of the PTC elements which permits an increased current flow with concomitant increased thermal dissipation of a magnitude which will just maintain the inside portions of the PTC elements at essentially the same temperature, that is, the temperature associated with that portion of the PTC resistance-temperature curve in the high resistance state. In other words, the anomaly point (temperature b) moves inwardly providing lower total resistance and increased current with concomitant increased thermal dissipation to compensate for the increased thermal losses.

If the thermal losses decrease due to higher ambient temperature, decreased air circulation around the device or some other cause, the anomaly point moves outwardly raising the total resistance and decreasing the thermal dissipation with a balance again attained between the thermal losses and the thermal dissipation with the inner portions of the PTC elements still at the essentially constant temperature associated with the high resistance portion of the PTC resistance temperature curve, thereby maintaining cavity 11 at essentially the same temperature.

Another embodiment of the invention is indicated by numeral in FIGS. 3 and 4. The heating element consists of disc shaped blocks 101 and 102 of PTC material of the same type used in the first embodiment which is provided, as seen in FIG. 4, on the top surface of block 101 and the bottom surface of block 102 with a layer of an electrically conductive material 104 and 106 respectively, such as silver, to which are joined respectively power lines L3 and L4. A disc shaped block 108 of material of good electrical and thermal conductivity such as copper is sandwiched between blocks 101 and 102 and held tightly together by encapsulating material 110. The contiguous surfaces 103 and of blocks 101, 108 and 108, 102 respectively, are smooth and flat insuring maximum physical contact to provide a good electrical and thermal connection. Alternatively, if a better electrical and thermal connection is desired the bottom surface of block 101 and the top surface of block 102 may be provided with a layer of an electrically conductive material (not shown). Block 108 serves to even out any fluctuations in temperature which might exist on the inner surfaces of blocks 101, 102 due to imperfections in the material of blocks 101, 102 such as inclusions or other non-uniform density areas and the like. Block 108 is provided with a centrally located bore passing through the center of the disc shaped block 108 defining a cavity 112 which is maintained at a relatively constant temperature by the self-regulating heating elements 101, 102. Device 100 is encapsulated by a suitable electrical and heat insulation material keeping bore 114 in encapsulation 110 and 112 in block 108 free of material to provide access to the cavity 112 formed within block 108. A junction of a thermocouple is shown in broken lines located within the cavity 112 and is kept at a relatively constant predetermined temperature by the heating action of PTC element 102. Of course, devices other than thermocouple junctions may be inserted in the cavity 112 to be maintained at a constant temperature.

For the successful operation of the oven within the purview of the invention, the self-regulating heating element 12 in the first embodiment and 101, 102 in the second, must be constructed of material having as a characteristic a large positive temperature coefficient of resistance (PTC); that is, material in which the percent change in resistance per degree change in temperature in the so-called break-point range is very large, as an example per degree centigrade. This break-point range occurs near the Curie point of the material and is sometimes referred to as the PTC anomaly. Reference may be had to a copending case Ser. No. 508,643, filed Oct. 24, 1965 which is a continuation-in-part of abandoned case Ser. No. 435,166, filed Feb. 25, 1965, assigned to the assignee of this case for a resistivity-temperature curve of such a material, viz. lanthanum doped barium titanate. This material has been successfully used as a self-regulating heating element.

Ordinarily, the barium titanate family of ceramics has an electrical resistivity of a magnitude greater than 10' ohm cm., however, proper doping can reduce the resistivity to less than 10 ohm cm. Lanthanum doping of barium titanate produces an exceptionally steep slope of the resistivity-temperature curve at temperatures above the anomaly point combined with a relatively low base resistivity at temperatures below the anomaly point. Other doping elements may be used to modify the anomaly temperature, the base resistivity and the rate of change of resistivity with temperature above the anomaly temperature. I

As an example, one self-regulating heater element 18 was made using Ba La TiO as follows.

The raw materials used were reagent grades of barium carbonate (BaCO lanthanum carbonate and titanium dioxide (TiO These were weighed out to an accuracy of about .25% to form stoichiometric mixtures, plus 0.1 mole percent excess TiO in order to assure the formation of a liquid phase during final firing. These materials are mixed and a sutficient amount of distilled water was added to form a 20% solid mixture by weight. This mixture was ball milled and dried. The dried product was powdered and calcined in order to convert the material into the desired doped compound (Ba La TiO by firing at approximately 1100 C. in air and cooled. The material, in the form of a porous cake, similar in texture and appearance to soft blackboard chalk, was broken up and wet milled as above, dried, comminutedand sieved from :40 to -270 (U.S. standard sieves). The resulting powder was again immediately dried to, drive off any moisture which might have been absorbed during comminution and sieving and finally pressed into the desired cylindrical shape using conventional closed die ceramicpressing techniques on a hydraulic press. The resulting compacted powderv body was fired to the ceramic state at about 1500 C. Further details regarding the preparation of similar PTC material may be found in copending application, filed Apr. 13, 1964, Ser. No. 359,370, assigned to the assignee of the instant invention.

Any material which displays a relatively steep positive sloped resistivity-temperature curve can be used as the heat generating, self-regulating element in accordance with the present invention.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

I claim:

1. The method of providing a relatively constant temperature in a cavity comprising the steps of defining the cavity at least partially with a semi-conductive, steep sloped PTC material having a positive temperature coefficient greater than 30% per degree centigrade above an anomaly temperature, applying suflicientvoltage to the PTC material whereby current will flow through the material heating the material up due to heat generated in the PTC material, reaching an equilibrium condition where heat loss from the material equals the heat generated and thereby maintaining the cavity at approximately a constant temperature regardless of changes in heat demand.

2. The method of providing a relatively constant temperature in a cavity comprising the steps of defining the cavity at least partially with a semi-conductive, steep sloped PTC material having a positive temperature coefiicient greater than 30% per degree centigrade above an anomaly temperature, applying sufficient voltage to the material in such a manner that current will flow through the material in a direction parallel to heat flow in the material causing the material to heat up due to heat generated in the material by the current flow, reaching an equilibrium condition where heat loss from the material equals the heat generated and a boundary forms between material of high and low resistance state thereby maintaining' the cavity at approximately a constant temperature in spite of changes in heat demand.

3. A self-regulating heating oven comprising:

(a) a steep-sloped PTC heating element composed of material having a positive temperature coefficient greater than 30% per degree centigrade above an anomaly temperature and formed with a cavity defined therein; and

(b) means to apply voltage across the element whereby the resulting current will cause the element to heat up and an electrical, thermal equilibrium will be reached so that the temperature in the oven will be approximately constant regardless of changes in heat demand.

4. A self-regulating heating device comprising:

(a) a semi-conductive steep sloped PTC element composed of material having a positive temperature coefficient greater than 30% per degree centigrade above an anomaly temperature and formed with a cavity defined therein;

(b) means to apply voltage across the element from inside the cavity to the outside of the element whereby the resulting current will flow through the element in a direction parallel to heat flow in the element and will cause the element to heat up and an equilibrium with heat losses will be reached so that the temperature in the cavity will be approximately constant regardless of changes in heat demand.

5. A- self-regulating heating device according to claim 4 in which the element is a hollow cylinder and the means to apply voltage includes electrically conductive layers electrically connected to the inner and the outer surfaces of the cylinder.

6. A self-regulating heating device comprising:

(a) a semi-conductive steep-sloped PTC element in the form of a hollow cylinder defining a cavity therein;

(b) means to apply voltage across the element from inside the cavity to the outside of the element including electrically conductive layers electrically connected to the inner and outer surfaces of the cylinder and each of the two ends of the cylinder contacts and is closed by a disc composed of a semi-conductive steep-sloped PTC material, the faces of the discs are electrically connected to electrically conductive material forming inner and outer conductive layers whereby the inner conductive layers defining the cavity is at one electrical potential and the outer conductive layers are at another electrical potential whereby the resulting current will flow through the element in a direction parallel to heat flow in the element and will cause the element to heat up and an equilibrium with heat losses will be reached so that the temperature in the cavity will be approximately constant regardless of changes in heat demand.

7. A self-regulating heating device according to claim 6 in which the ends of the cylinder are faced with insulation and electrical leads join the inner and outer cylinder layers to the respective conductive layers on the discs.

'8. A self-regulating heating device according to claim 6 in which the PTC material is a doped barium titanate.

9. A self-regulating heating device according to claim 8 in which the PTC material is doped with lanthanum.

10. A self-regulating heating oven comprising:

(a) a steep-sloped PTC heating element with a cavity defined therein;

(-b) two electrically conductive layers electrically connected to opposite sides of the element;

(0) means including a lead connected to each of the conductive layers, to apply voltage across the element whereby the resulting current will cause the element to heat up and an electrical, thermal equilibrium will be reached so that the temperature in the oven will beapproximately constant regardless of changes in heat demand;

((1) a thermally conductive material lining the cavity;

and

(e) thermal and electrical insulation encapsulating the oven and provided with at least one opening in alignment with the cavity in the PTC element.

11. A self-regulating heating device according to claim defined therein consisting of a hollow cylinder of PTC material coated on the inner and outer surfaces with a conductive layer and the end faces with an insulating layer, and two discs of PTC material coated on both faces with a conductive layer, a disc juxtaposed to each end of the cylinder to form the cavity therein defined by inner and outer conductive layers with PTC material sandwiched therebetween;

(b) means to apply voltage across the element includ- (i) conductors connecting the inner conductive layers; (ii) conductors connecting the outer conductive layers; and (iii) a lead connected to the inner and the outer conductive layers; and

(c) a socket adapted to receive electronic components supported on one of the discs and telescopically inserted within the cylinder whereby the resulting current will cause the element to heat up and an electrical, thermal equilibrium will be reached so that the temperature in the oven will be approximately constant regardless of changes in heat demand. 15. A self-regulating heating device according to claim 14 further including:

(d) a base .of electrical insulation provided with a recess to receive socket supporting disc and a plurality of terminals; (e) the conductors connecting the conductive layers connected to two of the terminals; (f) conductors connecting the socket to two more terminals; (g) heat and electrical insulation means cupping the cylinder and other disc; and (h) means removably gripping the cupped cylinder and the disc.

16. A self-regulating heating device according to claim 15 in which the; PTC material is a doped barium titanate.

17. A self-regulating heating device according to claim 16 in which the PTC material is doped with lanthanum. 18. A self-regulating electrical heating oven according 20 to claim 3 in Which the PTC material is Ba La TiO References Cited UNITED STATES PATENTS Kohler 338-22 X Landis et a1 338---2-8 X Cox 1322 X Schusterius 219505 X Johnston 252-520 X RICHARD M. WOOD, Primary Examiner. C. L. ALBRITTON, Assistant Examiner.

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