JP5937995B2 - Manufacturing method of internal electrode of pump cell - Google Patents

Description

  The present invention relates to a method for producing an internal electrode of a pump cell and the pump cell.

  2. Description of the Related Art Conventionally, NOx sensors that detect NOx concentration in a gas to be measured such as automobile exhaust gas are known. Such NOx sensors generally include a main pump cell and a measurement pump cell. In the main pump cell, oxygen is pumped out of the measurement gas so that the oxygen concentration of the measurement gas introduced into the sensor becomes zero. In the measurement pump cell, NOx contained in the gas to be measured after being processed in the main pump cell is decomposed into nitrogen and oxygen, and the gas to be measured is based on the current flowing through the measurement pump cell according to the oxygen concentration after decomposition. The NOx concentration is detected. Here, the main pump cell has a structure in which the solid electrolyte layer is sandwiched between the outer pump electrode exposed to the outer space and the inner pump electrode exposed to the inner space into which the gas to be measured including NOx is introduced. The inner pump electrode of the main pump cell is required not to decompose NOx into nitrogen and oxygen. An inner pump electrode that satisfies these requirements is manufactured, for example, by printing and baking an electrode paste in which a trace amount of Au powder is mixed with platinum powder (see Patent Document 1).

Japanese Patent Laid-Open No. 9-288086

  However, since Au powder tends to aggregate during storage, the dispersion form of the Au powder in the electrode paste varies depending on the Au powder production lot and storage period. If the dispersion state of the Au powder in the electrode paste is changed, the property required for the inner pump electrode of the main pump cell, that is, the property of suppressing the decomposition of NOx may be impaired, which is not preferable.

  The present invention has been made to solve such a problem, and is an internal electrode of a pump cell capable of pumping out oxygen in the internal space to the external space while keeping decomposition of NOx in the internal space of the NOx sensor low. The purpose is to provide a stable supply.

The method for producing the internal electrode of the pump cell of the present invention is as follows.
It has a structure in which a solid electrolyte layer is sandwiched between an external electrode exposed to the external space and an internal electrode exposed to the internal space into which the measurement gas containing NOx is introduced, and pumps oxygen from the internal space to the external space A method of manufacturing the internal electrode of a pump cell capable of:
A heat treatment step of heating the Au powder at 150 to 300 ° C. so that the particle size d50 of the Au powder by laser diffraction / scattering method is 2 to 7 μm;
Forming an electrode paste having a platinum group element as a main component and Au powder after the heat treatment step as a subcomponent, printing the electrode paste, and firing to form the internal electrode; and
Is included.

  The internal electrode obtained by this manufacturing method can pump out oxygen in the internal space to the external space while keeping the decomposition of NOx in the internal space low regardless of the production lot and storage period of the Au powder to be used. . That is, an internal electrode with stable electrode performance can be provided.

  The “platinum group element” is ruthenium, rhodium, palladium, osmium, iridium, or platinum. Of these, platinum is preferable.

  In the method for producing an internal electrode of the pump cell of the present invention, in the heat treatment step, the Au powder is preferably heat-treated at 150 to 300 ° C. under atmospheric pressure in the atmosphere. By doing so, since the heat treatment is performed in the atmosphere under normal pressure, a special atmosphere is unnecessary, and the cost of the heat treatment process is not increased.

  The pump cell of this invention is a pump cell provided with the internal electrode manufactured by the manufacturing method mentioned above. Since the performance of the internal electrode of this pump cell is stabilized, the detected NOx concentration is highly reliable.

2 is a schematic cross-sectional view of the gas sensor 100. FIG. 4 is a particle size distribution diagram of Examples 8 and 9 and Comparative Example 3. FIG. SEM photographs of Examples 8 and 9 and Comparative Example 6.

  Next, a schematic configuration of the gas sensor 100 as an example of the embodiment of the present invention will be described. FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100. The gas sensor 100 includes a sensor element 101 that detects the NOx concentration in the gas to be measured.

The sensor element 101 includes a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, and a first solid electrolyte layer 4 each made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO ₂ ). The spacer layer 5 and the second solid electrolyte layer 6 are elements having a structure in which the layers are laminated in this order from the bottom in the drawing. The solid electrolyte forming these six layers is dense and airtight. The sensor element 101 is manufactured, for example, by performing predetermined processing and circuit pattern printing on a ceramic green sheet corresponding to each layer, stacking them, and firing and integrating them.

  One end of the sensor element 101, and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, is a gas inlet 10, a first diffusion rate limiting unit 11, and a buffer space. 12, the second diffusion rate limiting part 13, the first internal space 20, the third diffusion rate limiting part 30, and the second internal space 40 are adjacently formed in such a manner that they communicate in this order.

  The gas introduction port 10, the buffer space 12, the first internal space 20, and the second internal space 40 are provided on the lower surface of the second solid electrolyte layer 6 with the upper portion provided in a state in which the spacer layer 5 is cut out. The space inside the sensor element 101 is defined by the lower part being the upper surface of the first solid electrolyte layer 4 and the side parts being the side surfaces of the spacer layer 5.

  Each of the first diffusion rate controlling unit 11, the second diffusion rate controlling unit 13, and the third diffusion rate controlling unit 30 is provided as two horizontally long slits (the opening has a longitudinal direction in a direction perpendicular to the drawing). . In addition, the site | part from the gas inlet 10 to the 2nd internal space 40 is also called a gas distribution part.

  Further, at a position farther from the front end side than the gas circulation part, the side part is partitioned by the side surface of the first solid electrolyte layer 4 between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5. The reference gas introduction space 43 is provided at the position. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

  The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is formed so as to cover the reference electrode 42.

  The reference electrode 42 is an electrode formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, the reference electrode 42 leads to the reference gas introduction space 43. An air introduction layer 48 is provided. Further, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 using the reference electrode 42.

  In the gas circulation part, the gas inlet 10 is a part opened to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas inlet 10.

  The first diffusion control unit 11 is a part that provides a predetermined diffusion resistance to the gas to be measured taken from the gas inlet 10.

  The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate controlling unit 11 to the second diffusion rate controlling unit 13.

  The second diffusion rate limiting unit 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20.

  When the gas to be measured is introduced from the outside of the sensor element 101 to the inside of the first internal space 20, the pressure fluctuation of the gas to be measured in the external space (exhaust pressure pulsation if the gas to be measured is an automobile exhaust gas) ), The gas to be measured that is suddenly taken into the sensor element 101 from the gas inlet 10 is not directly introduced into the first internal space 20, but the first diffusion control unit 11, the buffer space 12, the second After the concentration variation of the gas to be measured is canceled through the diffusion control unit 13, the gas is introduced into the first internal space 20. As a result, the concentration fluctuation of the gas to be measured introduced into the first internal space is almost negligible.

  The first internal space 20 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured introduced through the second diffusion rate limiting unit 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

  The main pump cell 21 includes an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, and an upper surface of the second solid electrolyte layer 6. An electrochemical pump cell comprising an outer pump electrode 23 provided in a manner exposed to the external space in a region corresponding to the ceiling electrode portion 22a, and a second solid electrolyte layer 6 sandwiched between these electrodes. is there.

  The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first inner space 20, and the spacer layer 5 that provides side walls. Yes. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom portion is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Spacer layers in which the electrode portions 22b are formed and the side electrode portions (not shown) constitute both side walls of the first internal space 20 so as to connect the ceiling electrode portions 22a and the bottom electrode portions 22b. 5 is formed on the side wall surface (inner surface), and is disposed in a tunnel-shaped structure at the portion where the side electrode portion is disposed.

The inner pump electrode 22 and the outer pump electrode 23 are formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO ₂ containing 1% of Au). The inner pump electrode 22 in contact with the gas to be measured is formed using a material that has a reduced reduction ability for the NOx component in the gas to be measured.

  In the main pump cell 21, a desired pump voltage Vp 0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and the pump current is positive or negative between the inner pump electrode 22 and the outer pump electrode 23. By flowing Ip0, oxygen in the first internal space 20 can be pumped into the external space, or oxygen in the external space can be pumped into the first internal space 20.

  In order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte 6, the spacer layer 5, the first solid electrolyte 4, The third substrate layer 3 and the reference electrode 42 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for main pump control.

  By measuring the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known. Further, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 of the variable power source 24 so that the electromotive force V0 is constant. Thereby, the oxygen concentration in the first internal space 20 can be kept at a predetermined constant value.

  The third diffusion control unit 30 provides a predetermined diffusion resistance to the gas under measurement whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20, and the gas under measurement is supplied to the gas under measurement. This is the part that leads to the second internal space 40.

  The second internal space 40 is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured introduced through the third diffusion control unit 30. The NOx concentration is measured mainly in the second internal space 40 in which the oxygen concentration is adjusted by the auxiliary pump cell 50, and further by measuring the pump cell 41 for measurement.

  In the second internal space 40, after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal space 20 in advance, the auxiliary pump cell 50 further supplies the gas to be measured introduced through the third diffusion control unit. The oxygen partial pressure is adjusted. Thereby, since the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, the gas sensor 100 can measure the NOx concentration with high accuracy.

  The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, and an outer pump electrode 23 (outer pump electrode 23). The auxiliary electrochemical pump cell is configured by the second solid electrolyte layer 6 and the sensor element 101 and an appropriate electrode on the outside.

  The auxiliary pump electrode 51 is disposed in the second internal space 40 in the same tunnel configuration as the inner pump electrode 22 provided in the first internal space 20. That is, the ceiling electrode portion 51 a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40 is formed on the first solid electrolyte layer 4. The bottom electrode part 51b is formed, and the side electrode part (not shown) connecting the ceiling electrode part 51a and the bottom electrode part 51b is provided on the spacer layer 5 that provides the side wall of the second internal space 40. It has a tunnel-type structure formed on both wall surfaces.

  Note that the auxiliary pump electrode 51 is also formed using a material having a reduced reducing ability with respect to the NOx component in the gas to be measured, like the inner pump electrode 22.

  In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal space 40 is pumped to the external space, or It is possible to pump into the second internal space 40 from the space.

  Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte. The layer 4 and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an auxiliary pump control oxygen partial pressure detection sensor cell 81.

  The auxiliary pump cell 50 performs pumping by the variable power source 52 that is voltage-controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. Thereby, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

  At the same time, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and the electromotive force V0 is controlled, so that the third diffusion rate limiting unit 30 controls the second internal space. The gradient of the oxygen partial pressure in the gas to be measured introduced into the gas 40 is controlled so as to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

  The measurement pump cell 41 measures the NOx concentration in the gas to be measured in the second internal space 40. The measurement pump cell 41 includes a measurement electrode 44 provided on a top surface of the first solid electrolyte layer 4 facing the second internal space 40 and spaced from the third diffusion rate-determining portion 30, an outer pump electrode 23, The electrochemical pump cell is constituted by the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

  The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the second internal space 40. Further, the measurement electrode 44 is covered with a fourth diffusion rate controlling part 45.

The fourth diffusion rate-determining part 45 is a film composed of a porous body mainly composed of alumina (Al ₂ O ₃ ). The fourth diffusion control unit 45 plays a role of limiting the amount of NOx flowing into the measurement electrode 44 and also functions as a protective film for the measurement electrode 44. In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the generated amount can be detected as the pump current Ip2.

  In order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, that is, a first solid electrolyte layer 4, a third substrate layer 3, a measurement electrode 44, and a reference electrode 42, that is, A measurement pump control oxygen partial pressure detection sensor cell 82 is configured. The variable power supply 46 is controlled on the basis of the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82.

The gas to be measured introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion rate-determining unit 45 under the condition where the oxygen partial pressure is controlled. Nitrogen oxide in the gas to be measured around the measurement electrode 44 is reduced (2NO → N ₂ + O ₂ ) to generate oxygen. The generated oxygen is pumped by the measurement pump cell 41. At this time, the variable power source is set so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes constant. 46 voltage Vp2 is controlled. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxide in the gas to be measured, the nitrogen oxide in the gas to be measured using the pump current Ip2 in the measurement pump cell 41. The concentration will be calculated.

  In addition, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detecting means as an electrochemical sensor cell, The electromotive force according to the difference between the amount of oxygen generated by the reduction of the NOx component in the surrounding atmosphere and the amount of oxygen contained in the reference atmosphere can be detected, thereby the concentration of the NOx component in the gas to be measured Is also possible.

  The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83. The oxygen partial pressure in the gas to be measured outside the sensor can be detected by the electromotive force Vref obtained by the sensor cell 83.

  In the gas sensor 100 having such a configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx). A gas to be measured is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas is determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41 in proportion to the NOx concentration in the measurement gas. You can know.

  Furthermore, the sensor element 101 includes a heater unit 70 that plays a role of temperature adjustment for heating and maintaining the sensor element 101 in order to increase the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

  The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to an external power source, power can be supplied to the heater unit 70 from the outside.

  The heater 72 is an electric resistor formed in a form sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater electrode 71 through the through-hole 73, generates heat when power is supplied from outside through the heater electrode 71, and heats and keeps the solid electrolyte forming the sensor element 101.

  The heater 72 is embedded over the entire area from the first internal space 20 to the second internal space 40, and the entire sensor element 101 can be adjusted to a temperature at which the solid electrolyte is activated. ing.

  The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 by an insulator such as alumina. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72.

  The pressure dissipating hole 75 is a portion that is provided so as to penetrate the third substrate layer 3 and communicate with the reference gas introduction space 43, and is for the purpose of alleviating the increase in internal pressure accompanying the temperature increase in the heater insulating layer 74. Formed.

  Next, a method for manufacturing the inner pump electrode 22 of the main pump cell 21 of the sensor element 101 and the auxiliary pump electrode 51 of the auxiliary pump cell 50 will be described. As described above, the inner pump electrode 22 and the auxiliary pump electrode 51 are formed using a material having a reduced reducing ability with respect to the NOx component in the gas to be measured. Below, although the manufacturing method of the inner side pump electrode 22 is demonstrated, the auxiliary pump electrode 51 is manufactured similarly.

  First, the Au powder is preliminarily heat treated. A particle size d50 (by laser diffraction / scattering method, the same applies hereinafter) of the Au powder before the heat treatment is less than 2 μm. The heat treatment is performed at 150 to 300 ° C. so that the particle size d50 of the Au powder becomes 2 to 7 μm. Specifically, heating is performed at 150 to 300 ° C. under normal pressure in the atmosphere. When the particle size d50 of the Au powder after the heat treatment is less than 2 μm, the ability to decompose NOx into nitrogen gas and oxygen gas cannot be sufficiently suppressed, which is not preferable, and when it exceeds 7 μm, it becomes difficult to control the film thickness thinly. It is not preferable because it is difficult and economically disadvantageous. If the temperature during the heat treatment is less than 150 ° C., the particle size d50 of the Au powder after the heat treatment is less than 2 μm, which is not preferable. If the temperature exceeds 300 ° C., the Au powder after the heat treatment is sintered and the particle size d50 exceeds 7 μm. It is not preferable. What is necessary is just to set the heat processing time suitably in the range of 1 to 10 hours, for example so that the particle size d50 may be set to 2-7 micrometers. The heat treatment of the Au powder may be performed in an inert atmosphere such as a nitrogen atmosphere, but since an air atmosphere is sufficient, it is preferably performed in an air atmosphere in consideration of cost. This heat treatment may be performed in a pressurized atmosphere, but since normal pressure is sufficient, it is preferably performed at normal pressure in consideration of cost.

  Next, an electrode paste having Pt powder as a main component and heat-treated Au powder as a subcomponent is prepared, and the electrode paste is screen-printed on a ceramic green sheet so as to have a predetermined pattern, followed by firing. The electrode paste is prepared by putting Pt powder, Au powder after the heat treatment step and solid electrolyte powder in a binder solution obtained by dissolving a binder in a solvent, and mixing the resulting mixture. The Au powder is preferably used in a mass ratio of 0.01% or more and less than 2% with respect to the Pt powder. As the solid electrolyte powder, the same component as that of the second solid electrolyte layer 6 is used. As the ceramic green sheet, one that becomes the second solid electrolyte layer 6 when fired is used. The electrode paste is printed so that the film thickness after printing and drying is 5 to 20 μm.

  Thereafter, the ceramic green sheet on which the electrode paste is printed is fired at 1300 to 1400 ° C. under normal pressure in an air atmosphere. Thereby, the ceramic green sheet becomes the second solid electrolyte layer 6, and the printed electrode paste becomes the inner pump electrode 22. Actually, the sensor element 101 is laminated after ceramics green sheets corresponding to each layer are subjected to predetermined processing and circuit pattern printing, and the laminated body is fired and integrated under the above-described conditions. To make it. For this reason, the firing of the electrode paste is performed simultaneously with the firing of such a laminate.

  Here, the correspondence between the components of the present embodiment and the components of the present invention will be clarified. Both the main pump cell 21 and the auxiliary pump cell 50 of the present embodiment correspond to the pump cell of the present invention, the outer pump electrode 23 corresponds to the outer electrode, and both the inner pump electrode 22 and the auxiliary pump electrode 51 serve as the inner electrode. Both the first internal space 20 and the second internal space 40 correspond to the internal space.

  According to this embodiment described in detail above, the inner pump electrode 22 and the auxiliary pump electrode 51 are provided in the first internal space 20 and the second internal space 40 regardless of the production lot and storage period of the Au powder to be used. Oxygen in each of the internal cavities 20 and 40 can be pumped out to the external space while keeping NOx decomposition low. That is, the inner pump electrode 22 and the auxiliary pump electrode 51 with stable electrode performance can be obtained.

  In addition, since the main pump cell 21 and the auxiliary pump cell 50 are pump cells including the inner pump electrode 22 and the auxiliary pump electrode 51 manufactured by the above-described manufacturing method, the detected NOx concentration is highly reliable.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the above-described embodiment, the auxiliary pump cell 50 is provided. However, when the oxygen concentration in the second internal space 40 can be sufficiently reduced only by the main pump cell 21, or when the NOx concentration measurement with high accuracy is required. If not, the auxiliary pump cell 50 may be omitted.

  In the embodiment described above, the inner pump electrode 22 and the auxiliary pump electrode 51 are made of a material in which a small amount of Au powder is mixed with Pt powder, but may contain other platinum group elements (for example, Pd). .

[General manufacturing procedure]
The sensor element 101 of each example and each comparative example is laminated on a ceramic green sheet corresponding to each layer after performing predetermined processing and printing of a circuit pattern, and the laminated body is subjected to atmospheric pressure and normal pressure. It was manufactured by firing at 1350 ° C. and integrating. In particular, the inner pump electrode 22 was prepared by preparing an electrode paste, screen-printing the electrode paste on a ceramic green sheet so as to have a predetermined pattern, and then firing it. The electrode paste was prepared by adding 60 g of Pt powder, 0.6 g of Au powder and 15 g of zirconia powder to a binder solution prepared by dissolving 3 g of butyral resin as a binder in 18 g of butyl carbitol as a solvent, and crushing the resulting mixture ( The mixture was prepared by dispersing with a tri-roll mill after mixing with Raikaiki. The electrode paste was printed so that the film thickness after printing and drying was 9 to 16 μm. The electrode paste was fired at the same time as the laminate in which the ceramic green sheets were laminated.

  As Au powder before the heat treatment, five types of lots A to E were used. Table 1 shows the value of the particle size d50 before heat treatment of each lot.

[Comparative Examples 1-5, Examples 1-9]
In Comparative Examples 1 to 5, the sensor element 101 was produced using the Au powders of lots A to E as they were as the components of the electrode paste without being heat-treated. A gas containing N ₂ gas as a base and containing O ₂ : 18%, NO: 500 ppm, H ₂ O: 3% (both volume ratios) is used as the measurement gas, and the pump current Ip0 of the main pump cell 21 is used. The pump voltage Vp0 was measured at a current of 3.0 mA. The results are shown in Table 2. The particle size distribution of Comparative Example 3 is shown in FIG.

  In Examples 1 to 5, as shown in Table 2, by treating the Au powders of lots A to E in the atmosphere at 150 ° C. for 2 to 4 hours, the particle size d50 after the heat treatment is 2.6 to 3 .. 3 μm. The sensor element 101 was manufactured using the Au powder thus heat-treated as a component of the electrode paste. And the pump voltage Vp0 when the pump current Ip0 of the main pump cell 21 was 3.0 mA was measured using the gas to be measured. The results are shown in Table 2.

  In Examples 6 to 9, as shown in Table 2, by treating the Au powder of lot C at 150 ° C. under atmospheric pressure for 3 to 10 hours, the particle size d50 after the heat treatment is 3.0 to 7.0 μm. It was made to become. The sensor element 101 was manufactured using the Au powder thus heat-treated as a component of the electrode paste. And the pump voltage Vp0 when the pump current Ip0 of the main pump cell 21 was 3.0 mA was measured using the gas to be measured. The results are shown in Table 2. Moreover, the particle size distribution and SEM photograph of Examples 8 and 9 are shown in FIGS. 2 and 3, respectively.

  In Table 2, when comparing Examples 1 to 5 and Comparative Examples 1 to 5 using Au powder of different lots, the pump voltage Vp0 was 530 to 580 mV in Examples 1 to 5, whereas In Examples 1 to 5, it was 660 to 760 mV. The high pump voltage Vp0 means that the NOx component in the gas to be measured is easily decomposed into nitrogen gas and oxygen gas at the inner pump electrode 22. When the Au powder is heat-treated in advance as in Examples 1 to 5 so that the particle size d50 falls within the range of 2.6 to 3.3 μm, the first internal space 20 regardless of the lot of Au powder. It was found that oxygen in the first internal space 20 can be pumped out to the external space while suppressing decomposition of NOx. In Examples 5 to 9, the Au powder of the same lot C was used and the holding time at 150 ° C. was changed so that the particle size d50 of the Au powder after heat treatment was in the range of 2.6 to 7.0 μm. I made it. As a result, it was found that as the holding time at 150 ° C. becomes longer, the value of the particle size d50 increases and the pump voltage Vp0 decreases. From the above Examples 1 to 9, even if Au powders of different lots are used, if the heat treatment conditions are set appropriately, the pump voltage Vp0 is set to substantially the same value, that is, the decomposition of the NOx component in the gas to be measured is reduced. It turned out that the ability to suppress can be made comparable.

[Comparative Example 6]
In Comparative Example 6, when the Au powder of lot C was treated at 150 ° C. under atmospheric pressure for 10 hours, the particle size d50 became unmeasurable. This is because the Au powder was sintered due to excessive heating and the particles became coarse. The SEM photograph of this comparative example 6 is shown in FIG.

[Examples 10 and 11 and Comparative Examples 7 and 8]
In Example 10, the Au powder of lot D was treated at 250 ° C. under atmospheric pressure for 2 hours in the atmosphere, and the particle size d50 of the Au powder after the heat treatment was set to 2.7 μm. In Example 11, the temperature was changed to 300 ° C., and the particle size d50 of the Au powder after the heat treatment was set to 4.3 μm. Using these Au powders as a component of the electrode paste, the sensor element 101 was fabricated and the pump voltage Vp0 was measured under the above-described conditions. The pump voltage Vp0 was 550 mV and 420 mV, respectively (see Table 2). On the other hand, in Comparative Example 7, the temperature was changed to 350 ° C., and in Comparative Example 8, the temperature was changed to 400 ° C. As a result, the sintering of Au powder proceeded, and the particle size d50 could not be measured. From these results, it was found that the upper limit of the heat treatment of the Au powder was 300 ° C.

  The present invention is applicable to NOx sensors.

DESCRIPTION OF SYMBOLS 1 1st board | substrate layer, 2nd 2nd board | substrate layer, 3rd board | substrate layer, 4th 1st solid electrolyte layer, 5 spacer layer, 6 2nd solid electrolyte layer, 10 gas inlet, 11 diffusion control part, 12 buffer space, 13 diffusion limiting part, 20 first internal space, 21 main pump cell, 22 inner pump electrode, 22a ceiling electrode part, 22b bottom electrode part, 23 outer pump electrode, 24 variable power supply, 30 diffusion limiting part, 40 second internal space , 41 Measurement pump cell, 42 Reference electrode, 43 Reference gas introduction space, 44 Measurement electrode, 45 Diffusion limiting part, 46 Variable power supply, 48 Air introduction layer, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 51a Ceiling electrode part, 51b Bottom electrode part, 52 Variable power supply, 70 Heater part, 71 Heater electrode, 72 Heater, 73 Through hole, 74 Heater insulation layer, 75 Pressure dissipation , 80 main pump-controlling oxygen-partial, 81 auxiliary-pump-controlling oxygen-partial-pressure detection sensor cell 82 measuring-pump-controlling oxygen-partial-pressure detection sensor cell 83 sensor cell 100 gas sensor 101 sensor elements.

Claims (2)

It has a structure in which a solid electrolyte layer is sandwiched between an external electrode exposed to the external space and an internal electrode exposed to the internal space into which the measurement gas containing NOx is introduced, and pumps oxygen from the internal space to the external space A method of manufacturing the internal electrode of a pump cell capable of:
A heat treatment step of heating the Au powder at 150 to 300 ° C. so that the particle size d50 of the Au powder by laser diffraction / scattering method is 2 to 7 μm;
Forming an electrode paste having a platinum group element as a main component and Au powder after the heat treatment step as a subcomponent, printing the electrode paste, and firing to form the internal electrode; and
Of the internal electrode of the pump cell containing In the heat treatment step, the Au powder is heat treated at 150 to 300 ° C. under atmospheric pressure in the atmosphere.
The manufacturing method of the internal electrode of the pump cell of Claim 1.