JP5965564B1 - Ammonia gas sensor and ammonia gas concentration measurement method - Google Patents

Abstract

A gas sensor capable of suitably detecting ammonia gas and having excellent durability is provided. A mixed potential gas sensor includes a sensor element including an oxygen ion conductive solid electrolyte as a constituent material and a heater provided in the element, and the sensor element includes Pt, Au, and a solid electrolyte having oxygen ion conductivity. And a reference electrode comprising a cermet of Pt and an oxygen ion conductive solid electrolyte, and a porous electrode protective layer covering at least the detection electrode, and constituting a detection electrode The Au abundance ratio, which is the area ratio of the Au coating portion to the Pt exposed portion on the surface of the precious metal particles to be performed, is 0.4 or more, the porosity of the electrode protective layer is 5 to 40%, and the sensor element is in the gas to be measured And a potential difference generated between the detection electrode and the reference electrode when heated to an element control temperature of 400 ° C. to 800 ° C. by a heater. There are, determining the ammonia gas concentration and manner. [Selection] Figure 4

Description

  The present invention relates to a gas sensor that detects ammonia gas.

  There are various types of gas sensors, such as a semiconductor type, a catalytic combustion type, an oxygen concentration difference detection type, a limiting current type, and a mixed potential type, for detecting the concentration of a predetermined gas component in the gas to be measured and obtaining its concentration. Among them, there is one that uses a sensor element whose main constituent material is ceramic that is a solid electrolyte such as zirconia.

  Among these, the mixed potential type gas sensor using ammonia gas as a detection target component uses a metal oxide as a material of the detection electrode (see, for example, Patent Document 1) and a composite oxide of gold and metal oxide. There are broadly divided into those using noble metals (see, for example, Patent Document 3 and Patent Document 4).

  Further, as an ammonia gas sensor that does not use a mixed potential, one that detects ammonia gas by measuring the impedance of a sensitive part is already known (for example, see Patent Document 5).

Special table 2009-511859 gazette JP 2011-47756 A Japanese Patent No. 4914447 JP 2012-211928 A Japanese Patent Laid-Open No. 2005-83817

  For example, there is a need to reliably detect a small amount (for example, 100 ppm or less) of ammonia gas contained in exhaust gas from an internal combustion engine such as an automobile engine. However, it is desirable that the sensor element of the gas sensor to meet such needs is durable (long-term reliability) because it is constantly exposed to a high-temperature exhaust gas atmosphere and is not necessarily easily replaced. .

  Many ammonia gas sensors in which the detection electrode is made of a metal oxide, such as the ammonia gas sensor disclosed in Patent Document 1, have been studied. All of them are from the viewpoint of electrode durability (adhesion and stability). Therefore, there is a problem that it cannot endure the automobile exhaust environment.

The ammonia gas sensor disclosed in Patent Document 2 is said to have little sensitivity fluctuation due to H ₂ O in the gas to be detected, but has a problem in durability because a complex oxide is used for the detection electrode. . In addition, since Au (gold) and BiVO ₄ , which are listed as the main constituent materials of such complex oxides, have melting points of 1064 ° C. and 947 ° C., respectively, they are not suitable for automobile exhaust gas applications where high-temperature gas is measured. There is also the problem of being.

  The gas sensor disclosed in Patent Document 3 is a mixed potential type gas sensor in which a detection electrode is made of a noble metal and ammonia gas can be measured with good sensitivity. The gas component concentration is obtained on the assumption that the two electrodes have catalytic activity to some extent.

  Specifically, the first electrode is formed by integral firing (co-firing) with a solid electrolyte constituting the sensor element after application of the Pt—Au paste to the sensor element. The second electrode has a two-layer structure in which Au plating is performed after the Pt paste is applied to the sensor element and co-fired with the solid electrolyte. And has a “slight catalytic activity”. In the second electrode, it is thought that it is the Au plating part that actually contributes to the electrochemical reaction. Therefore, the second electrode is not an alloy electrode, but at least the part that contributes to the electrochemical reaction is substantially In particular, it can be said that the electrode has an Au composition ratio of 100%, that is, an Au electrode.

  The gas sensor disclosed in Patent Document 4 employs Au as a noble metal component of the detection electrode. The detection electrode is formed by applying a paste to the solid electrolyte body after firing and then firing it. By so-called secondary firing. In order to ensure the selectivity of ammonia gas, a selective reaction layer made of a metal oxide is provided on the detection electrode.

  Since Au has a low melting point, it has a high vapor pressure, and single substance cannot be co-fired with a solid electrolyte body. Therefore, in the techniques disclosed in Patent Document 3 and Patent Document 4, the Au electrode is formed by plating or secondary firing. In any of these aspects, the gas sensor is in a high temperature state when used. There is a problem in long-term reliability because Au may be detached from the sensor element.

  In order to use the Au electrode stably, it is effective to raise the melting point by alloying with Pt and co-fire with the solid electrolyte body. However, if the amount of Pt in the electrode is too large, There is also a problem that ammonia burns due to catalytic activity.

  As a result of intensive studies, the inventor of the present invention can co-fire the detection electrode with the solid electrolyte body by using a metal component of the detection electrode of the gas sensor as a Pt—Au alloy with an increased Au abundance ratio on the surface. In addition, the present inventors have found that it is possible to disable the catalytic activity for ammonia gas and develop a hybrid potential correlated with the ammonia gas concentration. Based on such knowledge, it has been thought that a mixed potential gas sensor capable of detecting ammonia gas with high sensitivity and ensuring durability can be realized.

On the other hand, in order to ensure the detection sensitivity of ammonia gas, it is important to reduce the interference of other gases, particularly the influence of hydrocarbons and H ₂ O, which is also a problem in Patent Document 2.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a gas sensor that can suitably detect ammonia gas and has excellent durability.

  In order to solve the above problems, a first aspect of the present invention is a mixed potential type gas sensor for detecting ammonia gas in a gas to be measured, the sensor comprising an oxygen ion conductive solid electrolyte as a constituent material. And a heater for heating the sensor element, the sensor element having a detection electrode made of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity on the surface. A reference electrode composed of a cermet of Pt and a solid electrolyte having oxygen ion conductivity, and an electrode protective layer that is a porous layer covering at least the detection electrode, wherein the noble metal is Pt and Au, The presence of Au, which is the area ratio of the portion covered with Au to the portion where the Pt is exposed in the surface of the noble metal particles constituting the detection electrode 0.4 or more, the porosity of the electrode protective layer is 5% or more and 40% or less, the sensor element is disposed in the gas to be measured, and the element having a temperature of 400 ° C. or more and 800 ° C. or less by the heater The concentration of the ammonia gas is obtained based on a potential difference generated between the detection electrode and the reference electrode when heated to a control temperature.

  A second aspect of the present invention is an ammonia gas sensor according to the first aspect, wherein the element control temperature is 450 ° C. or higher and lower than 700 ° C.

  A third aspect of the present invention is the ammonia gas sensor according to the second aspect, wherein the element control temperature is 450 ° C. or higher and 650 ° C. or lower.

  According to a fourth aspect of the present invention, there is provided a mixed potential gas sensor for detecting ammonia gas in a gas to be measured, the sensor element comprising an oxygen ion conductive solid electrolyte, and the sensor element A heater for heating the sensor element, the sensor element having a detection electrode made of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity on the surface, and Pt and oxygen ion conductivity A reference electrode composed of a cermet with a solid electrolyte having a noble metal, wherein the noble metal is Pt and Au, and the Au is coated on a portion of the surface of the noble metal particle constituting the detection electrode where the Pt is exposed. The abundance ratio of Au, which is the area ratio of the portion that is present, is 0.4 or more, the sensor element is disposed in the gas to be measured, and is heated by the heater. When heated to element control temperature of less than 450 ° C. or higher 700 ° C., based on the potential difference between the reference electrode and the sensing electrode to determine the concentration of the ammonia gas, characterized in that.

  A fifth aspect of the present invention is the ammonia gas sensor according to the fourth aspect, wherein the element control temperature is 450 ° C. or higher and 650 ° C. or lower.

  A sixth aspect of the present invention is an ammonia gas sensor according to any of the first to fifth aspects, wherein the sensor element is isolated from a space in which the gas to be measured exists, and a reference gas is introduced. A reference gas introduction space, wherein the reference electrode is arranged in an atmosphere of the reference gas.

  A seventh aspect of the present invention is the ammonia gas sensor according to the sixth aspect, wherein the sensor element further comprises a reference gas introduction layer that is a porous layer communicating with the reference gas introduction space, An electrode is covered with the reference gas introduction layer.

  An eighth aspect of the present invention is the ammonia gas sensor according to the sixth aspect, characterized in that the reference electrode is disposed so as to be exposed to the reference gas introduction space.

  A ninth aspect of the present invention is the ammonia gas sensor according to any one of the first to fifth aspects, wherein the detection electrode and the reference electrode are disposed on a surface of the sensor element. And

  A tenth aspect of the present invention is the ammonia gas sensor according to the ninth aspect, characterized in that the detection electrode and the reference electrode are covered with an electrode protective layer.

  An eleventh aspect of the present invention is a method for measuring the concentration of ammonia gas in a gas to be measured using a mixed potential type gas sensor, wherein the gas sensor comprises a solid electrolyte having oxygen ion conductivity. A sensor element; and a heater provided inside the sensor element for heating the sensor element, the sensor element having a detection electrode made of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity on the surface. And a reference electrode composed of a cermet of Pt and a solid electrolyte having oxygen ion conductivity, and an electrode protective layer that is a porous layer covering at least the detection electrode, wherein the noble metal is Pt and Au. The surface of the surface covered with the Au with respect to the portion where the Pt is exposed in the surface of the noble metal particle constituting the detection electrode The Au abundance ratio as a ratio is 0.4 or more, the porosity of the electrode protective layer is 5% or more and 40% or less, the sensor element is disposed in the gas to be measured, and the sensor is provided by the heater. The ammonia gas concentration is obtained based on a potential difference generated between the detection electrode and the reference electrode while the element is heated to an element control temperature of 400 ° C. or higher and 800 ° C. or lower.

  A twelfth aspect of the present invention is the ammonia gas concentration measurement method according to the eleventh aspect, wherein the element control temperature is set to 450 ° C. or higher and lower than 700 ° C.

  A thirteenth aspect of the present invention is an ammonia gas concentration measurement method according to the twelfth aspect, wherein the element control temperature is 450 ° C. or higher and 650 ° C. or lower.

  A fourteenth aspect of the present invention is a method for measuring the concentration of ammonia gas in a gas to be measured using a mixed potential type gas sensor, wherein the gas sensor comprises an oxygen ion conductive solid electrolyte as a constituent material. A sensor element; and a heater provided inside the sensor element for heating the sensor element, the sensor element having a detection electrode made of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity on the surface. And a reference electrode composed of a cermet of Pt and a solid electrolyte having oxygen ion conductivity, wherein the noble metal is Pt and Au, and the Pt of the surfaces of the noble metal particles constituting the detection electrode is The Au abundance ratio, which is the area ratio of the portion covered with the Au to the exposed portion, is 0.4 or more, and the sensor element is Based on a potential difference generated between the detection electrode and the reference electrode in a state where the sensor element is placed in a measurement gas and the sensor element is heated to an element control temperature of 450 ° C. or more and less than 700 ° C. by the heater, the ammonia The gas concentration is obtained.

  A fifteenth aspect of the present invention is the ammonia gas concentration measurement method according to the fourteenth aspect, wherein the element control temperature is 450 ° C. or higher and 650 ° C. or lower.

  According to the first to fifteenth aspects of the present invention, the influence of interference by other gas components is reduced, and ammonia gas can be suitably detected, and an ammonia gas sensor with excellent durability is realized.

  In particular, according to the first to third aspects of the present invention, an ammonia gas sensor is realized in which at least the influence of interference by hydrocarbon gas is reduced.

  In particular, according to the second and third aspects of the present invention, an ammonia gas sensor is realized in which the influence of interference due to water vapor is reduced in addition to the hydrocarbon gas.

  In particular, according to the fourth and fifth aspects of the present invention, an ammonia gas sensor is realized in which the influence of water vapor interference is reduced.

  Moreover, according to the 11th thru | or 15th aspect of this invention, the density | concentration of ammonia gas can be calculated | required in the state which suppressed the influence of the interference by another gas component.

  In particular, according to the eleventh to thirteenth aspects of the present invention, the concentration of ammonia gas can be determined in a state where at least the influence of interference by hydrocarbon gas is suppressed.

  In particular, according to the twelfth and thirteenth aspects of the present invention, it is possible to determine the concentration of ammonia gas in a state where the influence of water vapor interference is suppressed in addition to the hydrocarbon gas.

  In particular, according to the fourteenth and fifteenth aspects of the present invention, the concentration of ammonia gas can be determined in a state where the influence of interference due to water vapor is suppressed.

It is a cross-sectional schematic diagram which shows roughly an example of a structure of 100 A of gas sensors which concern on a 1st structure. It is a cross-sectional schematic diagram which shows roughly an example of a structure of gas sensor 100B which is a modification of gas sensor 100A. It is a cross-sectional schematic diagram which shows roughly an example of a structure of gas sensor 100C which concerns on a 2nd structure. It is a figure which illustrates the sensitivity characteristic in five types of sensor elements 101A which each made Au different in the detection electrode 10. It is a figure which shows the sensitivity characteristic with respect to ammonia gas, and the sensitivity characteristic with respect to ethylene gas about three gas sensors 100A from which the porosity of the surface protective layer 50 differs. FIG. 5 is a graph plotting changes in sensor output with respect to water vapor concentration at each element control temperature when the gas sensor 100A is driven at different element control temperatures in a plurality of gas atmospheres having a constant ammonia gas concentration and different water vapor concentrations. is there. It is the figure which plotted the change of the sensor output with respect to the ammonia gas density | concentration for every gas atmosphere with the same water vapor | steam density | concentration when driving the gas sensor 100A by setting element control temperature to 380 degreeC. It is the figure which plotted the change of the sensor output with respect to ammonia gas density | concentration for every gas atmosphere with the same water vapor | steam density | concentration when driving the gas sensor 100A by making element control temperature 420 degreeC. It is the figure which plotted the change of the sensor output with respect to ammonia gas density | concentration for every gas atmosphere with the same water vapor | steam density | concentration when driving the gas sensor 100A by making element control temperature 450 degreeC. It is the figure which plotted the change of the sensor output with respect to ammonia gas density | concentration for every gas atmosphere with the same water vapor | steam density | concentration when element control temperature is 5000 degreeC and the gas sensor 100A is driven. It is the figure which plotted the change of the sensor output with respect to ammonia gas density | concentration for every gas atmosphere with the same water vapor | steam density | concentration when driving the gas sensor 100A with element control temperature being 650 degreeC. It is the figure which plotted the change of the sensor output with respect to ammonia gas density | concentration for every gas atmosphere with the same water vapor | steam density | concentration when driving the gas sensor 100A with element control temperature being 750 degreeC. It is a figure which shows the flow of a process at the time of producing sensor element 101A thru | or 101C. It is the figure which plotted Au abundance ratio in the detection electrode 10 formed using the said conductive paste with respect to Au addition rate in the case of producing the conductive paste for detection electrode formation by Au liquid mixing.

<First configuration>
FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100A according to the first configuration of the present invention. FIG. 1A is a vertical cross-sectional view along the longitudinal direction of a sensor element 101A that is a main component of the gas sensor 100A. FIG. 1B is a diagram including a cross section perpendicular to the longitudinal direction of the sensor element 101A at the position AA ′ in FIG.

The gas sensor 100A according to the first configuration of the present invention is a so-called mixed potential type gas sensor. Generally speaking, the gas sensor 100A includes a detection electrode 10 provided on the surface of a sensor element 101A mainly composed of a ceramic that is an oxygen ion conductive solid electrolyte such as zirconia (ZrO ₂ ), and the inside of the sensor element 101A. Between the reference electrode 20 and the reference electrode 20 provided in the above, based on the principle of the mixed potential, a potential difference is generated due to a difference in concentration of the gas component to be measured in the vicinity of both electrodes. The concentration of the gas component is obtained.

More specifically, the gas sensor 100A preferably uses an exhaust gas existing in an exhaust pipe of an internal combustion engine such as a diesel engine or a gasoline engine as a measurement gas, and preferably adjusts the concentration of ammonia (NH ₃ ) gas in the measurement gas. It is for seeking.

  In addition to the detection electrode 10 and the reference electrode 20 described above, the sensor element 101A is mainly provided with a reference gas introduction layer 30, a reference gas introduction space 40, and a surface protective layer 50.

  In the first configuration of the present invention, the sensor element 101A includes a first solid electrolyte layer 1, a second solid electrolyte layer 2, and a third solid electrolyte layer 3 each made of an oxygen ion conductive solid electrolyte. The fourth solid electrolyte layer 4, the fifth solid electrolyte layer 5, and the sixth solid electrolyte layer 6 are laminated in this order from the lower side in the drawing, and mainly include those layers. It is assumed that other components are provided on the interlayer or on the outer peripheral surface of the element. Note that the solid electrolyte forming these six layers is dense and airtight. The sensor element 101A 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.

  However, it is not an essential aspect that the gas sensor 100 </ b> A includes the sensor element 101 </ b> A as a laminate of such six layers. The sensor element 101A may be configured as a stacked body of a larger or smaller number of layers, or may not have a stacked structure.

  In the following description, for convenience, a surface located above the sixth solid electrolyte layer 6 in the drawing is referred to as a surface Sa of the sensor element 101A, and a surface located below the first solid electrolyte layer 1 is referred to as the sensor element 101A. This is referred to as back surface Sb. Further, when the concentration of ammonia gas in the gas to be measured is obtained using the gas sensor 100A, a predetermined range including at least the detection electrode 10 from the tip portion E1 which is one end portion of the sensor element 101A is within the gas to be measured. The other part including the base end part E2 which is the other end part is arranged so as not to come into contact with the gas atmosphere to be measured.

  The detection electrode 10 is an electrode for detecting the gas to be measured. The detection electrode 10 is formed as a porous cermet electrode of Pt containing Au at a predetermined ratio, that is, a Pt—Au alloy and zirconia. The detection electrode 10 is provided on the surface Sa of the sensor element 101 </ b> A in a substantially rectangular shape in plan view at a position near the distal end E <b> 1 that is one end in the longitudinal direction. When the gas sensor 100A is used, the sensor element 101A is arranged in such a manner that at least the portion where the detection electrode 10 is provided is exposed in the gas to be measured.

  In addition, the detection electrode 10 is made incapable of catalytic activity against ammonia gas in a predetermined concentration range by suitably determining the composition of the Pt—Au alloy as the constituent material. That is, the decomposition reaction of ammonia gas at the detection electrode 10 is suppressed. Thereby, in the gas sensor 100A, the potential of the detection electrode 10 is selectively changed with respect to the ammonia gas in the concentration range in accordance with the concentration (has a correlation). In other words, the detection electrode 10 has a characteristic that the potential dependence of the concentration of the ammonia gas in the concentration range is high while the dependence of the potential of the other measurement gas components is small. It is provided so that it may have.

  More specifically, in the sensor element 101A of the gas sensor 100A according to the first configuration of the present invention, by appropriately determining the Au abundance ratio on the surface of the Pt—Au alloy particles constituting the detection electrode 10, at least 0 ppm to The detection electrode 10 is provided so that the dependence of the potential on the ammonia gas concentration is significant in the concentration range of 100 ppm. Although details will be described later, this means that the detection electrode 10 is provided so that ammonia gas can be suitably detected in a concentration range of 0 ppm to 100 ppm.

In the present specification, the Au abundance ratio means an area ratio of a portion covered with Au to a portion where Pt is exposed in the surface of the noble metal particle constituting the detection electrode 10. . In the present specification, detection values for Au and Pt in an Auger spectrum obtained by performing AES (Auger electron spectroscopy) analysis on the surface of the noble metal particle are used.
Au abundance ratio = Au detection value / Pt detection value (1)
The Au abundance ratio is calculated by the following formula. When the area of the part where Pt is exposed is equal to the area of the part covered with Au, the Au abundance ratio is 1.

  The Au abundance ratio is calculated using the relative sensitivity coefficient method from the peak intensities of detection peaks for Au and Pt obtained by performing XPS (X-ray photoelectron spectroscopy) analysis on the surface of the noble metal particles. It is also possible. The value of the Au abundance ratio obtained by such a method can be regarded as substantially the same as the value of the Au abundance ratio calculated based on the result of the AES analysis.

  Details of the detection electrode 10 will be described later.

  The reference electrode 20 is an electrode having a substantially rectangular shape in plan view, which is provided inside the sensor element 101A and serves as a reference when determining the concentration of the gas to be measured. The reference electrode 20 is formed as a porous cermet electrode of Pt and zirconia.

  The reference electrode 20 may be formed so as to have a porosity of 10% to 30% and a thickness of 5 μm to 15 μm. Further, the plane size of the reference electrode 20 may be smaller than that of the detection electrode 10 as illustrated in FIG. 1, or may be approximately the same as that of the detection electrode 10 as in a second configuration described later (FIG. 3). reference).

  The reference gas introduction layer 30 is a layer made of porous alumina provided so as to cover the reference electrode 20 inside the sensor element 101A, and the reference gas introduction space 40 is on the base end E2 side of the sensor element 101A. It is an internal space provided in. In the reference gas introduction space 40, the atmosphere (oxygen) as a reference gas for obtaining the ammonia gas concentration is introduced from the outside.

  Since the reference gas introduction space 40 and the reference gas introduction layer 30 are in communication with each other, when the gas sensor 100A is used, the periphery of the reference electrode 20 is constantly atmospheric (through the reference gas introduction space 40 and the reference gas introduction layer 30). Oxygen). Therefore, when the gas sensor 100A is used, the reference electrode 20 always has a constant potential.

  Since the reference gas introduction space 40 and the reference gas introduction layer 30 are not in contact with the gas to be measured by the surrounding solid electrolyte, even when the detection electrode 10 is exposed to the gas to be measured, The reference electrode 20 does not come into contact with the gas to be measured.

  In the case illustrated in FIG. 1, the reference gas introduction space 40 is provided in such a manner that a part of the fifth solid electrolyte layer 5 is a space communicating with the outside on the base end E2 side of the sensor element 101A. It becomes. The reference gas introduction layer 30 is provided between the fifth solid electrolyte layer 5 and the sixth solid electrolyte layer 6 so as to extend in the longitudinal direction of the sensor element 101A. The reference electrode 20 is provided at a position below the center of gravity of the sensor element 101A as viewed in the drawing.

  The surface protective layer 50 is a porous layer made of alumina, provided in a manner covering at least the detection electrode 10 on the surface Sa of the sensor element 101A. The surface protective layer 50 is provided as an electrode protective layer that suppresses deterioration of the detection electrode 10 due to continuous exposure to the measurement gas when the gas sensor 100A is used. In the case illustrated in FIG. 1, the surface protective layer 50 is provided in a manner to cover not only the detection electrode 10 but also almost all of the surface Sa of the sensor element 101 </ b> A except for a predetermined range from the tip end portion E <b> 1. It becomes.

  The surface protective layer 50 should just be provided in the thickness of 10 micrometers-50 micrometers, the pore diameter should just be 1 micrometer or less, and it is suitable that a porosity is 5% or more and 40% or less. If the porosity is less than 5%, the gas to be measured does not suitably reach the detection electrode 10 and the responsiveness of the gas sensor 100A deteriorates, which is not preferable. When the porosity exceeds 40%, it is not preferable because poisonous substances are likely to adhere to the detection electrode 10 and the function of protecting the detection electrode 10 cannot be performed sufficiently.

  In addition, when the porosity of the surface protective layer 50 is 40% or less, as described later, there is also an effect that the influence of interference of other gas components can be suppressed.

  In the present embodiment, the porosity is evaluated by image analysis of an enlarged image of a cross-sectional SEM image (secondary electron image) (“Ceramic Processing” by Kaoru Mizutani et al. (Gihodo Publishing) The description is used as a reference).

  Further, as shown in FIG. 1B, the gas sensor 100A includes a potentiometer 60 that can measure a potential difference between the detection electrode 10 and the reference electrode 20. In FIG. 1B, the wiring between the detection electrode 10 and the reference electrode 20 and the potentiometer 60 is shown in a simplified manner. However, in the actual sensor element 101A, the surface Sa on the base end E2 side is shown. Alternatively, a connection terminal (not shown) is provided on the back surface Sb so as to correspond to each electrode, and a wiring pattern (not shown) that connects each electrode and the corresponding connection terminal is formed on the surface Sa and inside the element. The detection electrode 10, the reference electrode 20, and the potentiometer 60 are electrically connected through a wiring pattern and a connection terminal. Hereinafter, the potential difference between the detection electrode 10 and the reference electrode 20 measured by the potentiometer 60 is also referred to as a sensor output.

  Further, the sensor element 101A includes a heater unit 70 that plays a role of temperature adjustment for heating and maintaining the sensor element 101A 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 back surface Sb (the lower surface of the first solid electrolyte layer 1 in FIG. 1) of the sensor element 101A. The heater electrode 71 is electrically connected to an external power source 80 so that power can be supplied from the external power source 80 to the heater unit 70 through the heater electrode 71.

  The heater 72 is an electrical resistor provided inside the sensor element 101A. The heater 72 is connected to the heater electrode 71 through the through-hole 73, and generates heat by being supplied with power from the external power source 80 through the heater electrode 71, thereby heating and keeping the solid electrolyte forming the sensor element 101A. .

  In the case illustrated in FIG. 1, the heater 72 is sandwiched between the second solid electrolyte layer 2 and the third solid electrolyte layer 3 from above and below, and is detected from the proximal end portion E2 to the vicinity of the distal end portion E1. It is buried over a position below the electrode 10. By appropriately controlling the voltage value applied to the heater 72 by the external power source 80 by a control means (not shown) and flowing a heater current according to a desired temperature, the entire sensor element 101A is brought to a temperature at which the solid electrolyte is activated. It is possible to adjust.

  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 solid electrolyte layer 2 and the heater 72 and electrical insulation between the third solid electrolyte layer 3 and the heater 72. Yes.

  The pressure diffusion hole 75 is a portion that is provided so as to penetrate the third solid electrolyte layer 3 and communicate with the reference gas introduction space 40, and has the purpose of alleviating the increase in internal pressure accompanying the temperature increase in the heater insulating layer 74. It is formed by.

  When obtaining the ammonia gas concentration in the gas to be measured using the gas sensor 100A having the above-described configuration, as described above, only a predetermined range including at least the detection electrode 10 from the tip E1 of the sensor element 101A. While the gas to be measured is disposed in a space where the gas to be measured exists, the base end E2 side is disposed so as to be isolated from the space, and the atmosphere (oxygen) is supplied to the reference gas introduction space 40. Further, the heater element 72 heats the sensor element 101A to a predetermined temperature of 400 ° C. or higher and 800 ° C. or lower, preferably 450 ° C. or higher and lower than 750 ° C., more preferably 450 ° C. or higher and 650 ° C. or lower. Note that the temperature of the sensor element 101A when heated by the heater 72 is also referred to as an element control temperature. In the present embodiment, the element control temperature is evaluated based on the surface temperature of the detection electrode 10. The surface temperature of the detection electrode 10 can be evaluated by infrared thermography.

  In the state described above, a potential difference is generated between the detection electrode 10 exposed to the gas to be measured and the reference electrode 20 disposed in the atmosphere. However, as described above, the potential of the reference electrode 20 arranged in the atmosphere (constant oxygen concentration) is kept constant, while the potential of the detection electrode 10 is changed to the ammonia gas in the gas to be measured. On the other hand, since it has concentration dependency selectively, the potential difference (sensor output) is substantially a value corresponding to the composition of the gas to be measured existing around the detection electrode 10. Therefore, a certain functional relationship (this is called sensitivity characteristic) is established between the ammonia gas concentration and the sensor output. In the following description, the sensitivity characteristic may be referred to as a sensitivity characteristic for the detection electrode 10 or the like.

  When actually obtaining the ammonia gas concentration, the sensitivity characteristics are experimentally specified in advance by measuring the sensor output using a plurality of different mixed gases with known ammonia gas concentrations as the gas under measurement. . Thus, when the gas sensor 100A is actually used, the sensor output that changes momentarily according to the concentration of the ammonia gas in the gas to be measured is converted into the ammonia gas concentration based on the sensitivity characteristics in the arithmetic processing unit (not shown). Thus, the ammonia gas concentration in the gas to be measured can be obtained almost in real time.

<Modification of the first configuration>
FIG. 2 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor 100B that is a modification of the gas sensor 100A. FIG. 2A is a vertical cross-sectional view along the longitudinal direction of the sensor element 101B, which is a main component of the gas sensor 100B. FIG. 2B includes a cross section perpendicular to the longitudinal direction of the sensor element 101B at the position BB ′ in FIG.

  The gas sensor 100B extends the reference gas introduction space 40 of the sensor element 101A of the gas sensor 100A to the lower side of the detection electrode 10, while omitting the reference gas introduction layer 30 and making the reference electrode 20 the reference gas introduction space 40. It is provided in such a manner that it is exposed. Other configurations are the same as those of the gas sensor 100A. Therefore, how the sensor output is generated is the same as that of the gas sensor 100A. That is, the gas sensor 100B is also a so-called mixed potential type gas sensor, like the gas sensor 100A.

  Therefore, also in the gas sensor 100B having the above-described configuration, the ammonia gas concentration in the gas to be measured can be obtained by specifying the sensitivity characteristics in advance as in the gas sensor 100A.

<Second configuration>
FIG. 3 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100C according to the second configuration of the present invention. FIG. 3A is a vertical cross-sectional view along the longitudinal direction of the sensor element 101C, which is the main component of the gas sensor 100C. FIG. 3B includes a cross section perpendicular to the longitudinal direction of the sensor element 101C at the CC ′ position in FIG.

  Similarly to the gas sensors 100A and 100B, the gas sensor 100C is a so-called mixed potential type gas sensor. However, unlike the sensor element 101A and the sensor element 101B described above, the sensor element 101C of the gas sensor 100C includes not only the detection electrode 10 but also the reference electrode 20 disposed on the surface Sa of the sensor element 101C and the surface protective layer 50. It is coated with. The constituent material of each electrode is the same as that of the gas sensors 100A and 100B.

  On the other hand, the reference gas introduction space 40 (and the reference gas introduction layer 30) and the pressure diffusion hole 75 are not provided inside. Other components are the same as those of the gas sensors 100A and 100B. In the case shown in FIG. 3, the detection electrode 10 and the reference electrode 20 are provided at the same position in the longitudinal direction (see FIG. 3B), but this is not an essential aspect, and the longitudinal direction It may be arranged along.

  When obtaining the ammonia gas concentration in the measurement gas using the gas sensor 100C having the above-described configuration, the reference electrode 20 is exposed to the measurement gas in addition to the detection electrode 10 unlike the gas sensors 100A and 100B. In this manner, the sensor element 101C is arranged. Therefore, the detection electrode 10 and the reference electrode 20 are exposed to the same atmosphere. However, since the constituent materials of both electrodes are the same as those of the gas sensors 100A and 100B, the potential of the detection electrode 10 is relative to the ammonia gas concentration. This also applies to the point of selective variation. On the other hand, in the reference electrode 20 formed as a porous cermet electrode of Pt and zirconia, unlike the detection electrode 10, the catalytic activity is not suppressed for a specific gas component. The behavior with respect to gas components other than ammonia gas is the same between the detection electrode 10 and the reference electrode 20. Therefore, also in the gas sensor 100C, the sensor output substantially varies depending on the ammonia gas present in the gas to be measured.

  Therefore, in the gas sensor 100C, similarly to the gas sensors 100A and 100B, the ammonia gas concentration in the gas to be measured can be obtained by specifying the sensitivity characteristics in advance.

<Details of detection electrode>
As described above, in the gas sensors 100A to 100C, the detection electrode 10 is formed so that the catalytic activity for ammonia gas is disabled for a predetermined concentration range. This is realized by containing gold (Au) as a conductive component (noble metal component) of the detection electrode 10 in addition to platinum (Pt) as a main component.

  In addition, there exists a tendency for Au to concentrate on the surface of the noble metal particle which comprises the detection electrode 10, so that Au abundance ratio is large. More specifically, there is a tendency that an Au-rich Pt—Au alloy is formed in the vicinity of the surface of the Pt-rich Pt—Au alloy particles. And the tendency which the catalyst activity in the detection electrode 10 becomes impossible becomes large, so that the tendency which concerns is large.

  FIG. 4 is a diagram illustrating sensitivity characteristics (changes in sensor output with respect to ammonia gas concentration) in five types of sensor elements 101A with different Au abundance ratios in the detection electrode 10. The measurement conditions of the sensor output when obtaining such sensitivity characteristics and the analysis conditions of the Au abundance ratio are as follows. Note that the Au abundance ratio shown in FIG. 4 is an average value of values calculated by the equation (1) from the respective AES analysis results at three different precious metal portions existing on the fracture surface of the detection electrode 10.

(Measurement conditions for sensor output)
Element control temperature: 500 ° C .;
Gas atmosphere: O ₂ = 10%, H ₂ O = 5%, NH ₃ = 0-100 ppm (25 ppm step), the balance is N ₂ ;
Gas flow rate: 0.5 L / min;
Pressure: 1 atm;
Surface protective layer: Porosity 12%, thickness 10 μm.

(Au abundance analysis conditions)
Apparatus: Field emission Auger electron spectrometer (SAM680, manufactured by Physical Electronics, USA);
Measurement conditions: acceleration voltage 20 keV;
Analysis region: about 50 nmφ (spot analysis of noble metal particles exposed on the surface of the detection electrode 10).

  According to FIG. 4, when the Au abundance ratio in the detection electrode 10 is 0 (that is, when the metal component in the detection electrode is only Pt) and 0.20, the graph is flat, that is, the ammonia gas concentration is Even if it is high, no sensor output can be obtained.

  However, when the Au abundance ratio is 0.28, the graph is inclined on the high concentration side, and when the Au abundance ratio is 0.36 and 1.09, a remarkable output change is seen on the low concentration side, and the high concentration side However, the sensor output increases as the ammonia gas concentration increases.

  Based on the relationship between the Au abundance ratio of the detection electrode 10 and the sensitivity characteristics of the gas sensor, ammonia gas in a low concentration range of 0 ppm to 100 ppm can be obtained by setting the Au abundance ratio in the detection electrode 10 to 0.4 or more. The above-described gas sensors 100A to 100C can measure the concentration.

  If the Au abundance ratio is too large, it becomes difficult to form the detection electrode 10 itself, and the melting point of the detection electrode 10 as a whole is lowered, so that the detection electrode 10 can function properly. Therefore, the Au abundance ratio should be 2.4 or less at most.

  Further, as shown in FIG. 4, when the Au abundance ratio is small, the concentration dependence of the sensor output becomes remarkable on the high concentration side, and when the Au abundance ratio is large, the concentration dependence of the sensor output is noticeable on the low concentration side. The reason for this is that in the former case, since Pt is large in the detection electrode 10, the ammonia in the exhaust gas burns by the catalytic activity of Pt until it reaches the three-phase interface and causes an electrochemical reaction. On the other hand, in the latter case, a part of ammonia in the exhaust gas does not burn and reaches the three-phase interface without being burned, and therefore it is considered that an electrochemical reaction occurs and a potential is developed.

  The volume ratio between the noble metal component and zirconia in the detection electrode 10 may be about 4: 6 to 8: 2.

  Further, in order for the gas sensors 100A to 100C to exhibit their functions appropriately, the porosity of the detection electrode 10 is preferably 10% or more and 30% or less, and the thickness of the detection electrode 10 is preferably 5 μm or more.

  The planar size of the detection electrode 10 may be determined as appropriate. For example, the length in the longitudinal direction of the sensor element is about 2 mm to 10 mm, and the length in the direction perpendicular thereto is about 1 mm to 5 mm. .

<Suppression of interference of other gas components>
Since the gas sensors 100A to 100C are intended to detect ammonia gas, ideally, the sensor output desirably reflects only the concentration of ammonia gas. In other words, the detection electrode 10 is highly potential dependent on the concentration of ammonia gas, while not having potential dependency on the concentration of other measured gas components. Means that it is desirable to be provided.

However, because the principle of the mixed potential is adopted, in the gas sensors 100A to 100C, in addition to the ammonia gas, other gas components expressing a mixed potential similar to that of the ammonia gas are mixed in the gas to be measured. In this case, in principle, an output value (potential difference between the detection electrode 10 and the reference electrode 20) derived from the other gas component can be superimposed on the obtained sensor output. That is, interference of other gas components can occur in the sensor output. When such interference occurs, the concentration of ammonia gas is excessively calculated from the actual value. Therefore, from the viewpoint of ensuring measurement accuracy, it is desirable to suppress the influence as much as possible even if the concentrations of other gas components are reflected in the sensor output. In practice, the interference of hydrocarbon gas and water vapor (H ₂ O gas) tends to be a problem.

(Interference of hydrocarbon gas)
FIG. 5 is a diagram showing sensitivity characteristics with respect to ammonia gas and sensitivity characteristics with respect to ethylene (C ₂ H ₄ ) gas, which is a kind of hydrocarbon gas, for three gas sensors 100A having different porosity of the surface protective layer 50. is there. 5 (a), 5 (b), and 5 (c), respectively, a gas sensor 100A in which the surface protective layer 50 is not provided (it can be assumed that the porosity is 100%), a gas sensor 100A in which the porosity of the surface protective layer 50 is 40%, and This is the result for the gas sensor 100A in which the porosity of the surface protective layer 50 is 12%.

  The measurement conditions of sensor output when each sensitivity characteristic is obtained are as follows. The Au abundance ratio was about 0.5 in any gas sensor 100A.

Element control temperature: 500 ° C .;
Gas atmosphere: O ₂ = 10%, H ₂ O = 5%, NH ₃ or C ₂ H ₄ = 0-100 ppm (NH ₃ is set to 25 ppm step, C ₂ H ₄ is set to the same concentration as NH ₃ except 25 ppm), The balance is N ₂ ;
Gas flow rate: 0.5 L / min;
Pressure: 1 atm;
Surface protective layer: thickness 10 μm.

  The evaluation of the porosity was performed by taking a cross-sectional SEM image of the surface protective layer 50 under the condition of an acceleration voltage of 5 kV and analyzing the 7500-fold magnified image.

  5A to 5C, both ammonia gas and ethylene gas have a higher sensor output as the concentration increases, and ammonia gas has a larger output value when compared at the same concentration. Yes. In the case of ethylene gas, the rate of change (slope) of the sensor output with respect to the concentration value is almost constant, whereas in the case of ammonia gas, the change is greatest from 0 ppm to 25 ppm, and the concentration is further increased. In a large range, the rate of change is small.

  From the viewpoint of avoiding the interference of ethylene gas with the detection of ammonia gas, the output value in the sensitivity characteristic of ammonia gas should be as large as possible, and the difference between the output value of ammonia gas and the output value of ethylene gas should be as large as possible. desirable.

  From this point, when comparing FIGS. 5A to 5C, in the case of FIG. 5A showing the sensitivity characteristic in the gas sensor 100A not provided with the surface protective layer 50, the ammonia gas and the ethylene gas are generally up to 100 ppm. The difference in sensor output value is smaller than in other cases, and the sensor output value when the concentration of ethylene gas is 100 ppm is almost the same as the sensor output value of ammonia gas when the concentration is 25 ppm. I'm stuck.

  In the case of such a gas sensor 100A, it means that the ammonia gas cannot be reliably detected unless it is guaranteed that the difference in concentration between the ammonia gas and the ethylene gas present in the gas to be measured is sufficiently large. .

  In contrast, in the case of FIG. 5B in which the surface protective layer 50 having a porosity of 40% is provided and in the case of FIG. 5C in which the surface protective layer 50 having a porosity of 12% is provided, ethylene gas is used. Even if the sensor output is a value when the concentration is 100 ppm, it is smaller than the sensor output value of ammonia gas when the concentration is 25 ppm.

  This means that when the gas sensor 100A includes the surface protective layer 50 and the porosity range of the surface protective layer 50 is at least 40% or less, even if a maximum of 100 ppm of hydrocarbon gas is present, If ammonia gas is present at a concentration of at least 25 ppm, this means that it can be reliably detected. In such a case, the calculated ammonia gas concentration may include an error. However, as compared with FIG. 5A, FIG. 5B, and FIG. 5C, the ammonia gas concentration is different from that of the gas sensor 100A that does not include the surface protective layer 50. Thus, it can be said that the maximum error is suppressed. That is, the gas sensor 100A including the surface protective layer 50 having a porosity of 40% or less from the viewpoint of protecting the detection electrode 10 for the above-described reason is used as a gas to be measured as compared with the gas sensor 100A not including the surface protective layer 50. It can be said that it is hard to receive the influence of the interference by the contained hydrocarbon gas.

  Of course, when the interference of hydrocarbon gas and other gases is not a problem, it is clear from the sensitivity characteristics shown in FIGS. 5B and 5C that the concentration of even a trace amount of ammonia gas can be measured. It is.

  FIG. 5 shows the result of the gas sensor 100A. However, since the arrangement relationship between the surface protective layer 50 and the detection electrode 10 is the same as that of the gas sensor 100A in the gas sensors 100B and 100C, also in the gas sensors 100B and 100C. As for the surface protective layer 50, the same effects as those of the gas sensor 100A can be obtained.

  However, in the actual usage phase of the gas sensors 100A to 100C, the interference between the ammonia gas and the hydrocarbon gas occurs when the ammonia gas and the hydrocarbon gas contained in the gas to be measured reach the detection electrode 10 at the same time. In the sensor elements 101A to 101C, the porosity of the surface protective layer 50 is set to 40% or less so that ammonia gas reaches the detection electrode 10 preferentially. . In the gas sensors 100A to 100C, the influence of interference by the hydrocarbon gas is also reduced from this viewpoint.

  Specifically, in the sensor elements 101A to 101C, since the porous surface protective layer 50 is provided so as to cover the detection electrode 10, the external measurement gas is formed in a mesh shape inside the surface protective layer 50. It passes through the formed pores and reaches the detection electrode 10. However, when the gas to be measured includes several gas components, the speed until each gas component passes through the surface protective layer 50 and reaches the detection electrode 10 is not the same. The arrival speed increases due to the small molecular size. For example, while the molecular weight of ammonia is 17, in the case of a hydrocarbon gas contained in the exhaust gas of an internal combustion engine, the molecular weight is 28 even for ethylene with a small molecular size, and a hydrocarbon gas with a larger molecular weight is also generated. Therefore, the ammonia gas reaches the detection electrode 10 with priority over the hydrocarbon gas.

  Therefore, if the concentration measurement is intermittently performed at a predetermined interval, the sensor output first generated at every measurement timing is derived from ammonia gas. If the concentration is calculated based on such sensor output, it is possible to calculate with high accuracy without the influence of hydrocarbon gas.

(Water vapor interference)
FIG. 6 shows the water vapor concentration at each element control temperature when the gas sensor 100A is driven at different element control temperatures in a plurality of gas atmospheres having a constant ammonia gas concentration and different water vapor concentrations (H ₂ O concentrations). It is the figure which plotted the change of the sensor output by making the horizontal axis into logarithmic scale.

  The measurement conditions when each sensor output is obtained are as follows. The Au abundance ratio in the detection electrode 10 of the gas sensor 101A used was 0.99, and the porosity of the surface protective layer 50 was 12%.

Element control temperature: 380 ° C., 420 ° C., 450 ° C. to 750 ° C. (50 ° C. step);
Gas atmosphere: O ₂ = 10%, H ₂ O = 1%, 5%, 10%, 15%, NH ₃ = 100 ppm, the balance is N ₂ ;
Gas flow rate: 0.5 L / min;
Pressure: 1 atm;
Surface protective layer: thickness 10 μm.

  From FIG. 6, the sensor output tends to be smaller as the element control temperature is higher, while the sensor output value tends to fluctuate particularly in a range where the water vapor concentration is small as the element control temperature is lower. It can be seen that the variation associated with the case is significant. Since the concentration of ammonia gas is constant at 100 ppm, the fluctuation is considered to be due to the difference in the concentration of water vapor. And since the water vapor concentration in the exhaust gas is usually about 5% to 15%, when considering the measurement of the ammonia gas concentration in the exhaust gas, the fluctuation affects the accuracy of the ammonia gas concentration. This indicates that there is a possibility that water vapor gas interference occurs when measuring ammonia gas.

  In view of the results shown in FIG. 6, the element control temperature is preferably 450 ° C. or higher and lower than 700 ° C., and 450 ° C. or higher and 650 ° C. or lower if the influence of water vapor interference is excluded when measuring the ammonia gas concentration. It can be said that it is more preferable. This is because when the element control temperature is determined within such a range, if the ammonia gas concentration is constant, the sensor output is substantially constant regardless of the water vapor concentration.

  This is shown in FIGS. 7 to 12 show the ammonia gas concentrations when the gas sensor 100A is driven at various element control temperatures in the range of 380 ° C. to 750 ° C. in a plurality of gas atmospheres having different ammonia gas concentrations and water vapor concentrations. It is the figure which plotted the change of the sensor output with respect to every gas atmosphere with the same water vapor | steam density | concentration. Specifically, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11 and FIG. 12 respectively set the device control temperatures to 380 ° C., 420 ° C., 450 ° C., 500 ° C., 650 ° C., and 750 ° C. Shows the results when. 7 (a), FIG. 8 (a), FIG. 9 (a), FIG. 10 (a), FIG. 11 (a), and FIG. (B), FIG. 8 (b), FIG. 9 (b), FIG. 10 (b), FIG. 11 (b), and FIG. 12 (b).

  The measurement conditions when each sensor output is obtained are as follows. The Au abundance ratio in the detection electrode 10 of the gas sensor 101A used was 0.99, and the porosity of the surface protective layer 50 was 12%.

Gas atmosphere: O ₂ = 10%, H ₂ O = 1%, 5%, 10%, 15%, NH ₃ = 0 ppm, 1 ppm, 3 ppm, 5 ppm, 10 ppm, 25 ppm, 50 ppm, 100 ppm, 500 ppm, the balance being N ₂ ;
Gas flow rate: 0.5 L / min;
Pressure: 1 atm;
Surface protective layer: thickness 10 μm.

  When comparing FIG. 7 to FIG. 12, first, when the element control temperatures shown in FIG. 7 and FIG. 8 are 380 ° C. and 420 ° C., the sensor output shows dependence on the ammonia gas concentration. It has also been affected by fluctuations in water vapor concentration.

  On the other hand, when the element control temperatures shown in FIGS. 9 to 11 are 450 ° C., 500 ° C., and 650 ° C., the sensor output depends on the ammonia gas concentration in all concentration ranges from 0 ppm to 500 ppm. In addition, almost the same sensor output is obtained regardless of the water vapor concentration. The error is about 10 mV at most, and such an error can be allowed in light of the measurement accuracy in the gas sensor 100A.

  On the other hand, when the element control temperature shown in FIG. 12 is 750 ° C., the sensor output does not depend on the ammonia gas concentration or the water vapor concentration, and is substantially constant at a small value of about 15 mV.

  That is, the results shown in FIGS. 7 to 12 show that the element control temperature is appropriately set in the range of 450 ° C. or higher and lower than 700 ° C., preferably in the range of 450 ° C. or higher and 650 ° C., so that the interference of water vapor is not considered. It shows that the concentration of ammonia gas can be measured.

  As described above, it is considered that the element control temperature is preferably 450 ° C. or higher and lower than 700 ° C. if the influence of water vapor interference is excluded when measuring the ammonia gas concentration. When the temperature is higher than or equal to ° C., ammonia gas cannot always be measured. That is, when the element control temperature is set to 700 ° C. or more, the influence of water vapor interference is reduced, but the sensor output itself is also reduced. However, if the ammonia gas concentration is in a range larger than 100 ppm, it is shown in FIG. This is because the sensor output becomes larger than the measured value, so that measurement in such a concentration range is possible in principle, and ammonia gas cannot always be measured.

  6 to 12 show the results of the gas sensor 100A. Since the element control temperature is set in the gas sensors 100B and 100C in the same manner as the gas sensor 100A, the element control temperature is also set in the gas sensors 100B and 100C. If the temperature is 450 ° C. or higher and lower than 700 ° C., preferably 450 ° C. or higher and 650 ° C. or lower, the same effect as the gas sensor 100A can be obtained.

  It should be noted that in the actual usage phase of the gas sensors 100A to 100C, the interference of the water vapor gas with the ammonia gas does not necessarily occur independently of the hydrocarbon gas interference described above. Arise. However, if the element control temperature is appropriately set as described above, the sensor output becomes independent of the concentration of the water vapor gas. Therefore, the porosity of the surface protective layer 50 is suitably determined in the range of 5% to 40%. If the gas to be measured contains both water vapor gas and hydrocarbon gas in addition to ammonia gas, the concentration of ammonia gas can be reduced while minimizing the interference of hydrocarbon gas. Can be measured.

<Manufacturing process of sensor element>
Next, a process for manufacturing the sensor elements 101A to 101C will be described by taking as an example the case of having a layer structure as illustrated in FIGS. Schematically speaking, the sensor elements 101A to 101C illustrated in FIGS. 1 to 3 form a laminate made of a green sheet containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component, and cut the laminate.・ Made by firing. Examples of the oxygen ion conductive solid electrolyte include yttrium partially stabilized zirconia (YSZ).

  FIG. 13 is a diagram showing a flow of processing when manufacturing the sensor elements 101A to 101C. When manufacturing the sensor elements 101A to 101C, first, a blank sheet (not shown), which is a green sheet on which no pattern is formed, is prepared (step S1). Specifically, six blank sheets corresponding to the first to sixth solid electrolyte layers 1 to 6 are prepared. In addition, a blank sheet for forming the surface protective layer 50 is also prepared. The blank sheet is provided with a plurality of sheet holes used for positioning during printing or lamination. The sheet hole is formed in advance by a punching process using a punching device. When the corresponding layer is a green sheet constituting an internal space, the penetrating portion corresponding to the internal space is also provided in advance by a similar punching process or the like. In addition, the thickness of each blank sheet corresponding to each layer of the sensor elements 101A to 101C does not have to be the same.

  When a blank sheet corresponding to each layer can be prepared, pattern printing / drying processing for forming various patterns is performed on each blank sheet (step S2). Specifically, electrode patterns such as the detection electrode 10 and the reference electrode 20, the reference gas introduction layer 30, internal wiring (not shown), and the like are formed. In addition, the cut mark used as the reference | standard of a cutting | disconnection position when cut | disconnecting a laminated body in a post process is also printed with respect to the 1st solid electrolyte layer 1. FIG.

  Each pattern is printed by applying a pattern forming paste prepared according to the characteristics required for each forming object to a blank sheet using a known screen printing technique. A known drying means can also be used for the drying process after printing.

  The sensor elements 101A to 101C are characterized by the preparation of the conductive paste used for forming the detection electrode 10. Details thereof will be described later.

  When the pattern printing is finished, printing / drying processing of an adhesive paste for laminating and bonding the green sheets corresponding to the respective layers is performed (step S3). A known screen printing technique can be used for printing the adhesive paste, and a known drying means can also be used for the drying process after printing.

  Subsequently, the green sheets to which the adhesive is applied are stacked in a predetermined order, and are subjected to pressure bonding by applying predetermined temperature and pressure conditions to perform a pressure bonding process to form a single laminate (step S4). Specifically, a green sheet to be laminated is stacked and held on a predetermined lamination jig (not shown) while being positioned by a sheet hole, and the lamination jig is heated and pressed together with a lamination machine such as a known hydraulic press machine. To do. The pressure, temperature, and time for performing heating and pressurization depend on the laminating machine to be used, but appropriate conditions may be determined so that good lamination can be realized.

  When a laminated body is obtained as described above, subsequently, a plurality of portions of the laminated body are cut and cut into individual units (referred to as element bodies) of the sensor elements 101A to 101C (step S5). The cut element body is baked under a predetermined condition to generate the sensor elements 101A to 101C as described above (step S6). That is, the sensor elements 101A to 101C are produced by integral firing (co-firing) of the solid electrolyte layer and the electrode. The firing temperature at that time is preferably 1200 ° C. or higher and 1500 ° C. or lower (for example, 1400 ° C.). In addition, by performing integral firing in such an aspect, in the sensor elements 101A to 101C, each electrode has sufficient adhesion strength. This contributes to improving the durability of the sensor elements 101A to 101C.

  The sensor elements 101A to 101C thus obtained are accommodated in a predetermined housing, and are incorporated in the main bodies (not shown) of the gas sensors 100A to 100C.

<Conductive paste for detection electrode formation>
Next, the conductive paste used for forming the detection electrode 10 will be described. The conductive paste for forming the detection electrode is prepared by using an Au ion-containing liquid as a starting material for Au, and mixing the Au ion-containing liquid with Pt powder, zirconia powder, and a binder. In addition, as a binder, what can disperse | distribute another raw material to such an extent that it can print, and should just be burned down by baking should just be selected suitably. The production of the conductive paste in such an embodiment will be referred to as Au liquid mixing.

Here, the Au ion-containing liquid is obtained by dissolving a salt or organometallic complex containing Au ions in a solvent. Examples of salts containing Au ions include tetrachloroauric (III) acid (HAuCl ₄ ), gold (III) chloride (NaAuCl ₄ ), potassium dicyanogold (I) (KAu (CN) ₂ ), and the like. Can do. Examples of organometallic complexes containing Au ions include diethylenediamine gold (III) chloride ([Au (en) ₂ ] Cl ₃ ), dichloro (1,10-phenanthroline) gold (III) chloride ([Au (phen) Cl ₂ ] Cl), dimethyl (trifluoroacetylacetonato) gold or dimethyl (hexafluoroacetylacetonato) gold can be used. From the standpoints that impurities such as Na and K do not remain in the electrode, are easy to handle, or are easily dissolved in a solvent, tetrachlorogold (III) acid and diethylenediamine gold (III) chloride. It is preferable to use ([Au (en) ₂ ] Cl ₃ ). In addition to alcohols such as methanol, ethanol, and propanol, acetone, acetonitrile, formamide, and the like can be used as the solvent.

  In addition, mixing can be performed using well-known means, such as dripping. Further, in the obtained conductive paste, Au exists in an ion (or complex ion) state, but the detection electrode formed on the sensor elements 101A to 101C obtained through the above-described manufacturing process. In No. 10, Au exists mainly in the form of a simple substance or an alloy with Pt.

  FIG. 14 shows the weight ratio of Au (hereinafter referred to as Au addition) with respect to the weight of all precious metal elements in the starting material (the sum of the weights of Pt and Au) in the case of producing a conductive paste for forming a detection electrode by mixing Au liquid. It is the figure which plotted Au abundance ratio in the detection electrode 10 formed using the said electrically conductive paste with respect to the ratio). Note that the Au abundance ratio shown in FIG. 14 is calculated based on the result of XPS analysis.

  From FIG. 14, it can be seen that the Au abundance ratio tends to increase monotonously with respect to the Au addition ratio, and the sensing electrode 10 in which the Au abundance ratio is 0.4 or more when the Au addition ratio is 3 wt% or more. It can be seen that can be produced. Although illustration is omitted, an equivalent result is obtained when the Au abundance ratio is calculated based on the result of the AES analysis. That is, by using a conductive paste having an Au addition rate of 3 wt% or more, the detection electrode 10 having an Au abundance ratio of 0.4 or more can be suitably formed. For example, by using a conductive paste in which the Au addition rate is 20 wt%, the detection electrode 10 with an Au abundance ratio of 0.99 can be suitably formed.

  As described above, the value of the Au abundance ratio may be 2.4 or less at most, but such an upper limit value can be realized by setting the Au addition rate to 50 wt%.

<Another aspect of conductive paste production>
In producing the conductive paste for forming the detection electrode, instead of producing the conductive paste by Au liquid mixing as described above, a coating powder obtained by coating Pt powder with Au may be produced as a starting material. In such a case, a conductive paste for the pump electrode in the cavity is prepared by mixing the coating powder, zirconia powder, and a binder. Here, as the coating powder, an embodiment in which the surface of Pt powder particles is coated with an Au film may be used, or an embodiment in which Au particles are attached to Pt powder particles is used. You may do it.

  Also in this case, the detection electrode 10 having an Au abundance ratio of 0.4 or more and 2.4 or less can be suitably formed.

1-6 First to Sixth Solid Electrolyte Layers 10 Detecting Electrode 20 Reference Electrode 30 Reference Gas Introducing Layer 40 Reference Gas Introducing Space 50 Surface Protecting Layer 60 Potentiometer 70 Heater 71 Heater Electrode 72 Heater 73 Through Hole 74 Heater Insulating Layer 75 Pressure release hole 80 External power source 100A, 100B, 100C Gas sensor 101A, 101B, 101C Sensor element E1 (Sensor element) tip E2 (Sensor element) Base end Sa (Sensor element) Surface Sb (Sensor element) Back

Claims (15)

A mixed potential type gas sensor for detecting ammonia gas in a gas to be measured,
A sensor element comprising an oxygen ion conductive solid electrolyte as a constituent material;
A heater provided inside the sensor element for heating the sensor element;
With
The sensor element is
The surface is provided with a detection electrode composed of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity,
A reference electrode composed of a cermet of Pt and a solid electrolyte having oxygen ion conductivity;
An electrode protective layer that is a porous layer covering at least the detection electrode;
With
The noble metals are Pt and Au;
The Au abundance ratio, which is the area ratio of the portion covered with the Au to the portion where the Pt is exposed, of the surface of the noble metal particles constituting the detection electrode is 0.4 or more,
The electrode protective layer has a porosity of 5% to 40%,
Based on a potential difference generated between the detection electrode and the reference electrode when the sensor element is disposed in the gas to be measured and heated to an element control temperature of 400 ° C. or more and 800 ° C. or less by the heater. Determining the concentration of the ammonia gas;
An ammonia gas sensor characterized by that. The ammonia gas sensor according to claim 1,
The element control temperature is 450 ° C. or higher and lower than 700 ° C.,
An ammonia gas sensor characterized by that. The ammonia gas sensor according to claim 2,
The element control temperature is 450 ° C. or higher and 650 ° C. or lower.
An ammonia gas sensor characterized by that. A mixed potential type gas sensor for detecting ammonia gas in a gas to be measured,
A sensor element comprising an oxygen ion conductive solid electrolyte as a constituent material;
A heater provided inside the sensor element for heating the sensor element;
With
The sensor element is
The surface is provided with a detection electrode composed of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity,
A reference electrode comprising a cermet of Pt and a solid electrolyte having oxygen ion conductivity,
With
The noble metals are Pt and Au;
The Au abundance ratio, which is the area ratio of the portion covered with the Au to the portion where the Pt is exposed, of the surface of the noble metal particles constituting the detection electrode is 0.4 or more,
Based on a potential difference generated between the detection electrode and the reference electrode when the sensor element is disposed in the gas to be measured and heated to an element control temperature of 450 ° C. or higher and lower than 700 ° C. by the heater. Determining the concentration of the ammonia gas;
An ammonia gas sensor characterized by that. The ammonia gas sensor according to claim 4,
The element control temperature is 450 ° C. or higher and 650 ° C. or lower.
An ammonia gas sensor characterized by that. The ammonia gas sensor according to any one of claims 1 to 5,
The sensor element is
A reference gas introduction space that is isolated from the space in which the gas to be measured exists and into which the reference gas is introduced;
Further comprising
The reference electrode is disposed in an atmosphere of the reference gas;
An ammonia gas sensor characterized by that. The ammonia gas sensor according to claim 6,
The sensor element is
A reference gas introduction layer which is a porous layer communicating with the reference gas introduction space;
Further comprising
The reference electrode is coated on the reference gas introduction layer,
An ammonia gas sensor characterized by that. The ammonia gas sensor according to claim 6,
The reference electrode is disposed so as to be exposed to the reference gas introduction space.
An ammonia gas sensor characterized by that. The ammonia gas sensor according to any one of claims 1 to 5,
The sensing electrode and the reference electrode are disposed on a surface of the sensor element;
An ammonia gas sensor characterized by that. The ammonia gas sensor according to claim 9, wherein
The detection electrode and the reference electrode are covered with an electrode protective layer,
An ammonia gas sensor characterized by that. A method for measuring the concentration of ammonia gas in a gas to be measured using a mixed potential type gas sensor,
The gas sensor is
A sensor element comprising an oxygen ion conductive solid electrolyte as a constituent material;
A heater provided inside the sensor element for heating the sensor element;
With
The sensor element is
The surface is provided with a detection electrode composed of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity,
A reference electrode composed of a cermet of Pt and a solid electrolyte having oxygen ion conductivity;
An electrode protective layer that is a porous layer covering at least the detection electrode;
It is equipped with
The noble metals are Pt and Au;
The Au abundance ratio, which is the area ratio of the portion covered with the Au to the portion where the Pt is exposed, of the surface of the noble metal particles constituting the detection electrode is 0.4 or more,
The electrode protective layer has a porosity of 5% to 40%,
A potential difference generated between the detection electrode and the reference electrode in a state where the sensor element is disposed in the gas to be measured and the sensor element is heated to an element control temperature of 400 ° C. or more and 800 ° C. or less by the heater. Based on the above, obtain the concentration of the ammonia gas,
A method for measuring the concentration of ammonia gas. A method for measuring the concentration of ammonia gas according to claim 11,
The element control temperature is 450 ° C. or higher and lower than 700 ° C.
A method for measuring the concentration of ammonia gas. A method for measuring the concentration of ammonia gas according to claim 12,
The element control temperature is set to 450 ° C. or higher and 650 ° C. or lower.
A method for measuring the concentration of ammonia gas. A method for measuring the concentration of ammonia gas in a gas to be measured using a mixed potential type gas sensor,
The gas sensor is
A sensor element comprising an oxygen ion conductive solid electrolyte as a constituent material;
A heater provided inside the sensor element for heating the sensor element;
With
The sensor element is
The surface is provided with a detection electrode composed of a cermet of a noble metal and a solid electrolyte having oxygen ion conductivity,
A reference electrode comprising a cermet of Pt and a solid electrolyte having oxygen ion conductivity,
It is equipped with
The noble metals are Pt and Au;
The Au abundance ratio, which is the area ratio of the portion covered with the Au to the portion where the Pt is exposed, of the surface of the noble metal particles constituting the detection electrode is 0.4 or more,
A potential difference generated between the detection electrode and the reference electrode in a state where the sensor element is disposed in the gas to be measured and the sensor element is heated to an element control temperature of 450 ° C. or higher and lower than 700 ° C. by the heater. Based on the above, obtain the concentration of the ammonia gas,
A method for measuring the concentration of ammonia gas. A method for measuring the concentration of ammonia gas according to claim 14,
The element control temperature is set to 450 ° C. or higher and 650 ° C. or lower.
A method for measuring the concentration of ammonia gas.

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