JPS6326767Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to an oxygen concentration sensor element (hereinafter referred to as an oxygen sensor element or simply a sensor element) using a solid electrolyte, and in particular an oxygen sensor that can continuously and highly accurately detect the oxygen concentration in automobile exhaust gas. Regarding elements. For measuring oxygen concentration in gas, an oxygen sensor using an oxygen concentration battery using a zirconia electrolyte is known. This device forms a cylindrical container with an electrolyte closed at one end, and platinum electrodes are formed on both the inside and outside of this container.A standard gas with a known oxygen concentration is brought into contact with the electrode inside the container, and a gas to be measured is brought into contact with the other electrode. The oxygen concentration in the gas to be measured is measured from the difference in electromotive force generated between the two electrodes when they are brought into contact. Conventionally, automobile exhaust gas treatment has been performed according to the oxygen concentration signal detected by the concentration cell type oxygen sensor mentioned above, but from the standpoint of energy conservation, oxygen partial pressure has been changed from extremely low oxygen concentration ranges to high concentrations of several 10%. There has been a strong desire for an oxygen sensor that can detect oxygen continuously and with high precision. Contrary to the oxygen concentration battery mentioned above, it is known that when voltage is applied between both electrodes of a solid electrolyte with electrodes formed on both sides, oxygen permeates from one electrode (cathode) to the other electrode (anode). It is being Using this principle, by limiting the amount of oxygen flowing into the cathode, a limiting current is generated in the sensor and a constant voltage is applied.
Since the amount of current between the electrodes varies depending on the O ₂ concentration, a method for measuring oxygen concentration based on changes in this current has been developed and is called a limiting current type oxygen sensor. As a result of various research on this limiting current type oxygen sensor, the present inventors formed electrode layers on both sides of a solid electrolyte in the form of a plate, for example, a disk, and formed a porous ceramic layer on the cathode electrode layer. We have discovered that by restricting the inflow of oxygen, an oxygen sensor that can accurately measure various oxygen concentrations can be obtained. This limiting current type oxygen sensor element does not require a reference gas and can continuously and accurately measure oxygen concentration in a wide range of gases, from very low oxygen concentrations to high concentrations of several tens of percent. Therefore, when measuring the oxygen concentration in the exhaust gas of an internal combustion engine, the air-fuel ratio of the mixed gas is lean.
This has the advantage that it can also be operated on the side. However, when the oxygen sensor element is used for a long period of time, carbon, etc. in the exhaust gas adheres to the oxygen sensor element, causing clogging, making it impossible to obtain reliable values. The present invention aims to prevent the adhesion of carbon and the like by coating the element surface with a rougher porous inorganic coating layer in the limiting current type oxygen sensor element. That is, the oxygen sensor element of the present invention has a solid electrolyte formed in a plate shape that allows oxygen ions to pass therethrough, and on both sides of the solid electrolyte,
Provide electrodes and lead wires for applying voltage,
The surface of the cathode of the electrode is coated with a porous inorganic coating layer for restricting the amount of oxygen inflow, and the entire surface of the device, including the coating layer on the cathode surface and the anode surface, is coated with a more porous inorganic coating. It is characterized by having layers. The oxygen sensor element of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing an example of the sensor element of the present invention. The sensor element A of the present invention includes metal electrodes 2 and 3 provided on both sides of a disk-shaped oxygen ion permeable body 1 made of a solid electrolyte by, for example, a sputtering method.
Lead wires 6, 6 for voltage application and output detection are connected to the electrodes 2, 3, a porous inorganic coating layer 4 is provided on the surface of the electrode (cathode) 2 on the oxygen inflow side, and this coating layer 4 A second coating layer 5 made of a coarser porous inorganic material is provided so as to cover the entire outer surface of the element including the top. Electrolyte 1 is an oxygen ion permeator
Oxides such as ZrO ₂ , HfO ₂ , ThO ₂ , Bi ₂ O ₃ and other
A dense sintered body containing MgO, Y ₂ O ₃ , Yb ₂ O ₃ , etc. as a solid solution is used. As the electrodes 2 and 3 formed on both sides of the disc-shaped electrolyte 1, it is appropriate to use Pt, Pd, and Ag singly or in a mixture of two or more. Coating layers 4 and 5 provided on the outer surface of the electrode
may be any porous heat-resistant inorganic substance, such as α-Al ₂ O ₃ , MgO-Al ₂ O ₃ ,
SiO ₂ , ZrCaO ₃ and the like are preferred. Among these, the first coating layer 4 provided on the surface of the cathode 2 is for regulating the amount of oxygen reaching the electrode 2, and the second coating layer 4 is formed of a fine porous inorganic material and covers the entire element. layer 5
is made of coarsely porous inorganic material. The particle size of the porous inorganic material forming the coating layer 4 on the cathode 2 side is preferably about 10 to 100 microns, preferably about 20 to 70 microns. Also, the thickness of coating layer 4 is 200~
Set to 2000μ. The porous inorganic material forming the second coating layer 5 provided on the entire surface of the element has an average particle size of 50 to 200 microns, and is provided so as to have a thickness of approximately 20 to 100 microns. To measure the oxygen concentration in exhaust gas using sensor element A of the present invention, as shown in FIG. The voltage applied between the anode 3 and the anode 3 is measured with a voltmeter 9, and the current that changes depending on the oxygen concentration in the exhaust gas is measured with an ammeter 8. When actually measuring the oxygen concentration, in the sensor having the structure shown in FIG. 3, the heating element 10 generates heat to heat the element A to a predetermined temperature of approximately 750°C. FIG. 3 shows the state in which the sensor element A is attached to the sensor body, and FIG. 4 is a partially enlarged exploded perspective view of FIG. 3. As shown in the figure, there are heat-resistant metal lead wires 11, 1 made of stainless steel inside.
A disk-shaped sensor element A is arranged at the tip of a substantially cylindrical alumina insulator tube 12 in which wires 1, 11, 11 are wired in the longitudinal axis direction, and the lead wires 6, 6 are parallel to the longitudinal axis direction. A heating element 10 consisting of a coiled sheathed metal heater or ceramic heater made of Nichrome, Kanthal, etc. is arranged so as to surround the outer periphery of the sensor element A, and these are connected to the lead wires 6, 6 of the sensor element A and both ends 10a, 10b of the heating element. A lead wire 11 wired to the alumina insulator tube 12,
11, 11, 11 is attached to the tip of the alumina insulator tube 12 by welding (welding parts 13, 1
3, 13, 13). Lead wire 11, 11, 11,
One end of 11 is connected to conductive wires 16, 16, 16, 16 located above the alumina insulator tube 12, and a connector 17,
These conductive wires 16, 16, 16, 16 are used to extract sensor signals to the outside and input electric power necessary for heating the heating element 10. The outer periphery of the alumina insulator tube 12 is covered with a protective cover 15 made of a metal that does not easily undergo oxidation deformation even at high temperatures, such as stainless steel, and a plurality of ventilation holes 14 are opened in the vicinity of the sensor element A and the heating element 10 at the tip. are doing. In addition, in the figure, 18 is a flange, 19 is a mounting hole, and 2
0 is a waterproof tube, 21 and 22 are Teflon bushes, and 23 is an insulating rubber tube. Next, an example of a method for manufacturing the sensor element A of the present invention will be described below. The solid electrolyte of sensor element A has a purity of 99.9.
% zirconium oxide powder and the same purity 99.9%
Yttrium oxide powder was collected at a molar ratio of 9:1, and ground and mixed in a wet ball mill for 5 hours.
Dry at 150°C for 6 hours. This powder was heated to 1200℃ for 4 hours.
Time: Calcinate, then grind for 5 hours in a wet ball mill, make the particle size relatively fine, and heat again at 150℃ for 6 hours.
Dry for an hour. The obtained powder was molded under a pressure of 1200Kg/cm ²
Pressure mold it into a disk shape with a thickness of 1.5 mm and a diameter of 4.0 mm. This molded body is fired in air at 1800°C for 3 hours to form a sintered body. A circular platinum electrode 2 with a diameter of 3 mm and a thickness of 1 μ is placed on both the upper and lower surfaces of the disk-shaped solid electrolyte 1 thus obtained.
3 was formed, and a platinum wire with a diameter of 0.3 mm was crimped onto this to form lead wires 6, 6. The porous inorganic coating layers 4 and 5 are formed by spraying MgO-Al ₂ O ₃ to a predetermined thickness using a plasma spraying method. The spraying conditions and spray layer specifications are as follows: Γ Plasma arc current 500A Γ Plasma arc voltage 65V Γ Usage gas N ₂ 100SCFH H ₂ 15SCFH (SCFH…Standard Cubic Feet/
Hour) Γ Distance from plasma gun to object to be sprayed: approx. 80 mm Γ Spraying agent average particle size Cathode surface 40μ Entire element surface 100μ Γ Spray thickness Cathode surface 900μ Entire element surface 70μ Next, in the same process as above, the fifth A sensor element B having a structure as shown in the figure is manufactured (comparative example sensor). The cathode 2 and anode 3 are made of MgO with an average particle size of 40μ.
Coating layers 4 and 5' are provided by coating Al ₂ O ₃ to a thickness of 900 μm and 70 μm, respectively, and no coating layer is provided on the other surface portions of the device. Sensor element A of the present invention obtained as above
and Comparative Example Sensor Element B are used to measure the oxygen concentration in automobile exhaust gas and evaluate their respective performances. Figure 6 shows the current value of the sensor at each voltage when the engine speed is kept constant and the voltage applied to the sensor is changed in a sensor having the sensor element A of the present invention (having the structure shown in Figure 3). measure,
The results are shown below. This measurement was performed at various engine loads to vary the oxygen concentration in the exhaust gas. In the figure, a, b, c, d, e, f
are curves showing the relationship between voltage and current measured under the oxygen concentration and engine load shown in Table 1 below, respectively.

[Table] In Figure 6, the horizontal axis is voltage (Volt) and the vertical axis is current (mA). In the figure, the values showing almost parallel measurement lines are the limiting current values at each oxygen concentration (each engine load). Oxygen concentration (%) obtained by the above measurement method
The relationship between the limit current value (mA) and the sensor A of the present invention
For each of Comparative Example Sensor B, changes between the initial period and after durability (actual vehicle durability 50,000 km) are shown in FIGS. 7 and 8. As is clear from these figures, in Comparative Example Sensor B, there is a decrease in sensor output due to clogging due to adhesion of carbon, etc., but in Sensor A of the present invention, there is almost no change in output even after durability, and it can be used for a long period of time. It is clear that it can withstand the use of As is clear from the above description, since the sensor element of the present invention covers the entire outer surface of the element with the rough second porous inorganic coating layer, clogging due to adhesion of carbon, etc. does not occur, and therefore reliability is improved. In addition to being able to obtain a certain value, since the layer also covers the anode surface, it is sufficient in terms of protection of the anode, so it has various advantages.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of the sensor element A of the present invention, and FIG.
The figure is a circuit diagram for detecting the output of the sensor element, FIG. 3 is a side view showing the structure of a sensor having the sensor element, and FIG. 4 is an enlarged exploded perspective view showing the portion shown in FIG. 3. , FIG. 5 is a cross-sectional view of a comparative example oxygen sensor element, FIG. 6 is a graph showing the relationship between current and voltage at each oxygen concentration of the sensor of the present invention having sensor element A, and FIG. 7 is a graph of the sensor element of the present invention. FIG. 8 is a graph showing the relationship between the oxygen concentration and the limiting current at the initial stage and after the durability test, and FIG. In the figure, 1... solid electrolyte, 2... cathode, 3...
Anode, 4...Porous coating layer (first), 5
...Porous coating layer (second), 6,6...
Lead wire, 12...Alumina insulator tube.

Claims (1)

[Scope of utility model registration request] Electrodes and lead wires for applying voltage are provided on both sides of a solid electrolyte sintered body formed into a plate shape and permeable to oxygen ions, and the cathode surface of the electrode is porous to limit the amount of oxygen inflow. An oxygen sensor element coated with an inorganic coating layer, further comprising a more porous inorganic coating layer provided over the entire element surface including the coating layer on the cathode surface and the anode surface.