JP7070517B2 - Gas sensor element and gas sensor - Google Patents

Description

The present invention relates to a gas sensor element and a gas sensor.

Conventionally, as a gas sensor arranged in the exhaust pipe of an internal combustion engine, for example, in order to detect a solid electrolyte composed of a zirconia solid electrolyte having oxygen ion conductivity and a concentration of a specific gas in the gas to be measured. A gas sensor element provided with the detection electrode of the above and a reference electrode provided on the other surface of the solid electrolyte body and exposed to the reference gas, and a gas sensor using the gas sensor element are known.

For example, in Patent Document 1, a gas sensor element having a detection electrode having a noble metal region composed of a noble metal, a solid electrolyte region composed of a solid electrolyte, and a mixed region in which a noble metal and a solid electrolyte are mixed, the gas sensor. A gas sensor using an element is disclosed. In the same document, the gas sensor is configured to measure the NOx concentration of the gas to be measured by a gas sensor element built in the gas sensor. As the noble metal, a noble metal containing at least platinum (Pt) and rhodium (Rh) having high NOx decomposability is used.

Japanese Unexamined Patent Publication No. 2018-100879

In the gas sensor element provided with the detection electrode having the mixed region described above, when rhodium is unevenly distributed in the mixed region, the unevenly distributed rhodium affects the movement of oxygen ions (O2 ^- ions), and charges tend to be accumulated. That is, the electrode capacitance of the detection electrode becomes large. As the electrode capacitance of the detection electrode increases, the noise of the current increases and the static detection accuracy decreases. In addition, the hysteresis of the current also increases, and the dynamic detection accuracy during transient behavior such as a sudden change in NOx concentration decreases.

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 element capable of reducing the electrode capacitance of a detection electrode, and a gas sensor using the gas sensor element.

One aspect of the present invention is a solid electrolyte body (26) composed of a zirconia solid electrolyte having oxygen ion conductivity.
A detection electrode (220) provided on one surface of the solid electrolyte body for detecting the concentration of the specific gas in the gas to be measured (G), and
A gas sensor element (2) provided on the other surface of the solid electrolyte body and provided with a reference electrode (27) exposed to the reference gas (A).
The detection electrode has a noble metal region (221) composed of a noble metal containing at least platinum and rhodium, a solid electrolyte region (222) composed of the solid electrolyte, and a mixed region in which the noble metal and the solid electrolyte coexist. Has (223) and
The electrode capacitance of the detection electrode is predetermined in the dL Cartesian coordinate system represented by the width d of the mixed region and the ratio L of the rhodium unevenly distributed portion in the mixed region in which the rhodium is unevenly distributed to the entire mixed region. When the curve representing the correlation between the width d of the mixed region and the ratio L of the rhodium uneven distribution portion when it becomes a value is defined as the correlation curve (R),
The detection electrode is located in a region below the correlation curve when the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are 300 μF.
It is in the gas sensor element (2).

Another aspect of the present invention is the gas sensor (1) having the gas sensor element.

The gas sensor element has the above configuration. Therefore, the gas sensor element can reduce the electrode capacitance of the detection electrode.

The gas sensor has the gas sensor element capable of reducing the electrode capacitance of the detection electrode. Therefore, the gas sensor suppresses current noise and hysteresis by reducing the electrode capacitance of the detection electrode, and can detect the concentration of the specific gas in the gas to be measured with high accuracy.

The reference numerals in parentheses described in the scope of claims and the means for solving the problem indicate the correspondence with the specific means described in the embodiments described later, and limit the technical scope of the present invention. It's not a thing.

FIG. 1 is a cross-sectional view of a gas sensor of the embodiment having the gas sensor element according to the first embodiment. 2A and 2B are cross-sectional views showing a detailed configuration of the gas sensor element according to the first embodiment, FIG. 2A is a cross-sectional view taken along the line aa along the longitudinal direction of the gas sensor element, and FIG. The cross-sectional view taken along the line is a cross-sectional view taken along the line cc perpendicular to the longitudinal direction of the gas sensor element. FIG. 3 is an explanatory diagram schematically showing the fine structure of the detection electrode in the gas sensor element according to the first embodiment. FIG. 4 is an explanatory diagram schematically showing an enlarged portion F surrounded by a square in FIG. FIG. 5 is an explanatory diagram for explaining a method of measuring the width d of the mixed region. 6A and 6B are explanatory views for explaining a method for measuring the ratio L of the rhodium uneven distribution portion, in which FIG. 6A is a STEM image of a mixed region, and FIG. 6B is an EDS mapping image of Zr element in the mixed region. c) is an example of an EDS mapping image of Pt element in the mixed region, and (d) is an example of an EDS mapping image of Rh element in the mixed region. FIG. 7 is an explanatory diagram for explaining a correlation curve showing the correlation between the width d of the mixed region and the ratio L of the rhodium uneven distribution portion when the electrode capacitance of the detection electrode becomes a predetermined value in the dL Cartesian coordinate system. Is. FIG. 8 is an explanatory diagram showing an equivalent circuit model of the detection electrode when measuring the electrode capacitance of the detection electrode. FIG. 9 is an explanatory diagram schematically showing a Core-Cole plot diagram when measuring the electrode capacitance of the detection electrode. FIG. 10 is an explanatory diagram schematically showing a Bode diagram when measuring the electrode capacitance of the detection electrode. FIG. 11 is a diagram showing the relationship between the width d (nm) of the mixed region and the ratio L (%) of the rhodium uneven distribution portion obtained in the experimental example. FIG. 12 is a diagram showing the relationship between the electrode capacitance (μF) of the detection electrode and the NOx output noise (ppm) obtained in the experimental example. FIG. 13 is a diagram showing the relationship between the width d (nm) of the mixed region and the electrode reaction resistance (Ω) of the detection electrode obtained in the experimental example. FIG. 14 is a diagram showing a line of the width d of the mixed region = 30 nm in the relationship between the width d (nm) of the mixed region and the ratio L (%) of the rhodium uneven distribution portion obtained in the experimental example. ..

The gas sensor element of the present embodiment is provided on one surface of a solid electrolyte composed of a zirconia solid electrolyte having oxygen ion conductivity and a solid electrolyte, and detects the concentration of a specific gas in the gas to be measured. It is provided with a detection electrode for the purpose and a reference electrode provided on the other surface of the solid electrolyte body and exposed to the reference gas. The detection electrode has a noble metal region composed of a noble metal containing at least platinum (Pt) and rhodium (Rh), a solid electrolyte region composed of a solid electrolyte, and a mixed region in which the noble metal and the solid electrolyte are mixed. ing.

Here, in the dL Cartesian coordinate system defined by the width d of the mixed region and the ratio L of the rhodium unevenly distributed portion where rhodium is unevenly distributed in the mixed region to the entire mixed region, the electrode capacitance of the detection electrode is a predetermined value. A curve showing the correlation between the width d of the mixed region and the ratio L of the rhodium uneven distribution portion is defined as a correlation curve. In the detection electrode, the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are in the region below the correlation curve .

In the gas sensor element of the present embodiment, since the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are in the region below the correlation curve in which the electrode capacitance of the detection electrode is a predetermined value, the electrode capacitance of the detection electrode is present. Can be reduced. This can be considered to be due to the following reasons.

A detection electrode using a noble metal containing at least Pt and Rh contains two or more kinds of metal elements. In such a detection electrode, separation of each metal element may occur due to the difference in diffusion rate of each metal element at the time of forming a mixed region at the interface between the noble metal region and the solid electrolyte region. In particular, the rhodium uneven distribution portion in which Rh is unevenly distributed among the separated metal elements inhibits the reoxidation of Zr generated by the reduction of ZrO ₂ in the process of forming the mixed region. That is, when the rhodium uneven distribution portion increases, Rh is preferentially oxidized at the time of reoxidation, and the reoxidation of Zr is inhibited. As a result, Zr remains in the mixed region and oxygen ions do not move, and the electrode capacity of the detection electrode increases. However, when the relationship between the width d of the mixed region and the ratio L of the rhodium uneven distribution portion in the detection electrode is set as described above, the ratio of the rhodium uneven distribution portion is reduced, so that in the process of forming the mixed region. Zr produced by the reduction of ZrO ₂ is easily reoxidized. Therefore, according to the gas sensor element of the present embodiment, it is possible to reduce the electrode capacitance of the detection electrode.

The gas sensor of the present embodiment has the gas sensor element of the present embodiment capable of reducing the electrode capacitance of the detection electrode. Therefore, in the gas sensor of the present embodiment, the noise and hysteresis of the current are suppressed by reducing the electrode capacitance of the detection electrode, and the concentration of the specific gas in the gas to be measured can be detected with high accuracy.

Hereinafter, specific examples of the gas sensor element and the gas sensor of the present embodiment will be described in detail with reference to the drawings. The gas sensor element and the gas sensor of the present embodiment are not limited by the following examples.

The gas sensor element and the gas sensor of the first embodiment will be described with reference to FIGS. 1 to 10. As illustrated in FIG. 1, the gas sensor 1 has a gas sensor element 2. The gas sensor 1 of the first embodiment can be, for example, a NOx sensor that detects the amount of nitrogen oxides (NOx). Such a gas sensor 1 is arranged and used in an exhaust pipe through which exhaust gas flows, for example, in an internal combustion engine. The gas to be measured G is an exhaust gas, and the specific gas whose concentration is to be detected is NOx. Hereinafter, the description will be made on the premise that the specific gas in the measured gas G is NOx. However, the specific gas to be detected is not limited to NOx, and can be implemented as, for example, a gas sensor for detecting ammonia or other gas by selecting an appropriate element for the constituent material of the solid electrolyte or the electrode. ..

As illustrated in FIG. 1, the gas sensor 1 is installed in the exhaust passage of an internal combustion engine in a vehicle. Exhaust gas flowing through the exhaust passage is introduced into the gas sensor 1 as the measured gas G. As a result, the gas sensor 1 measures the NOx concentration of the gas G to be measured by the gas sensor element 2 built therein. Specifically, the gas sensor 1 includes a sensor housing 101, an insulating insulator 102, element covers 103A, 103B, 103C, and a sensor harness 104 in addition to the gas sensor element 2. FIG. 1 shows an example in which the sensor control circuit 105 is connected to the sensor harness 104. Note that Y in FIG. 1 indicates the gas flow direction in which the gas to be measured G flows inside the gas sensor element 2.

The sensor housing 101 holds the gas sensor element 2 inside via the insulating insulator 102. The element covers 103A, 103B, and 103C are fixed to the sensor housing 101. The element covers 103A and 103B cover the outer peripheral side of the upstream element end portion 10a of the gas sensor element 2 in the gas flow direction Y. The element covers 103A and 103B have gas introduction holes 103a and 103b for introducing the exhaust gas from the exhaust pipe as the measured gas G into the upstream element end portion 10a housed therein. The element cover 103C covers the outer peripheral side of the downstream element end portion 10b of the gas sensor element 2 in the gas flow direction Y. The element cover 103C has an atmosphere introduction hole 103c for introducing the atmosphere as the reference gas A into the downstream element end portion 10b housed therein. A plurality of sensor harnesses 104 are provided across the inside and outside of the element cover 103C. The sensor control circuit 105 is connected to the gas sensor element 2 via a plurality of sensor harnesses 104 outside the sensor housing 101 and the element cover 103C. The sensor control circuit 105 in the first embodiment controls the supply of voltage to the detection cell 22, the monitor cell 23, and the pump cell 24, which will be described later.

Next, the detailed configuration of the gas sensor element 2 will be described. As illustrated in FIG. 2, the gas sensor element 2 includes a measurement gas chamber 20, a reference gas chamber 21, a detection cell 22, a monitor cell 23, a pump cell 24, and a heater 25. The gas sensor element 2 is configured by laminating a heater 25, a solid electrolyte body 26 described later, and an insulating layer 28. The measurement gas chamber 20 is formed as a space surrounded by the solid electrolyte body 26 and the insulating layer 28 , and the reference gas chamber 21 is formed as a space surrounded by the heater 25 and the solid electrolyte body 26. Hereinafter, each element of the gas sensor element 2 will be described in detail.

The measurement gas chamber 20 is a space into which the exhaust gas as the gas to be measured G is introduced. The measurement gas chamber 20 is formed as a space sandwiched between the solid electrolyte body 26 and the insulating layer 28 . The insulating layer 28 is plate-shaped and is laminated on the plate-shaped solid electrolyte body 26 via the first spacer 201. When the solid electrolyte body 26 is viewed from the front, the first spacer 201 has a C shape with one side open, whereby the measurement gas chamber 20 has a box shape with a part open. The opened portion is referred to as an exhaust gas introduction port 202. A diffusion resistor 203 is arranged at the introduction port 202 in the present embodiment, and the exhaust gas passes through the diffusion resistor 203 and is introduced into the measurement gas chamber 20. Therefore, the exhaust gas is introduced into the measurement gas chamber 20 by the diffusion resistor 203 under a predetermined diffusion resistance.

The solid electrolyte body 26 is composed of a zirconia solid electrolyte having oxygen ion conductivity. Specifically, for example, yttria-stabilized zirconia (YSZ) or the like can be adopted as the solid electrolyte. As the solid electrolyte, for example, calcium oxide-stabilized zirconia, alumina-stabilized zirconia, or the like can also be adopted. Alumina and other generally known insulators can be used for the above-mentioned insulating layer 28 and the first spacer 201.

The reference gas chamber 21 is a space into which the reference gas A for generating a reference potential for calculating the concentration of NOx, which is a specific gas, is introduced. For example, the atmosphere is introduced into the reference gas chamber 21 as the reference gas A. The reference gas chamber 21 is formed as a space sandwiched between the heater 25 and the solid electrolyte body 26. The solid electrolyte body 26 is laminated on the heater 25 formed in a plate shape via the second spacer 211. When the solid electrolyte body 26 is viewed from the front, the second spacer 211 is formed on the side where the diffusion resistor 203 is formed, and the introduction port (not shown) of the reference gas A is the introduction port 202 of the measurement gas chamber 20. Is designed to open on the opposite side.

The solid electrolyte body 26 is formed so as to separate the measurement gas chamber 20 and the reference gas chamber 21, and is exposed to both the measurement gas chamber 20 and the reference gas chamber 21. As a result, in the gas sensor 1, oxygen ions move in the solid electrolyte body 26 according to the difference between the NOx concentration in the exhaust gas and the NOx concentration in the atmosphere, and a sensor current is generated.

The detection cell 22 has a detection electrode 220, a solid electrolyte body 26, and a reference electrode 27. The detection electrode 220 is formed on one side exposed to the measurement gas chamber 20 in the solid electrolyte body 26. On the other hand, the reference electrode 27 is formed on one side exposed to the reference gas chamber 21 in the solid electrolyte body 26. That is, in the detection cell 22, the solid electrolyte body 26 is sandwiched between the detection electrode 220 and the reference electrode 27. The monitor cell 23 and the pump cell 24, which will be described later, also include the solid electrolyte body 26 and the reference electrode 27 as components, but in the first embodiment, the detection cell 22, the monitor cell 23, and the pump cell 24 are the solid electrolyte body 26 and the pump cell 24. The reference electrode 27 is shared.

The detection electrode 220 contains a noble metal containing platinum (Pt) and rhodium (Rh), and a solid electrolyte having the same composition as the solid electrolyte constituting the solid electrolyte body 26. That is, the detection electrode 220 is an electrode composed of platinum and rhodium acting as catalysts and a zirconia solid electrolyte having oxygen ion conductivity. The solid electrolyte contained in the detection electrode 220 is integrally coupled with the solid electrolyte 26 constituting the detection cell 22, and is in a state where ion conduction is possible between the solid electrolytes. NOx contained in the exhaust gas introduced into the measurement gas chamber 20 is adsorbed on the surface of the exposed noble metal and is ionized into nitrogen ions and oxygen ions by catalytic action. Of these, oxygen ions conduct in the solid electrolyte contained in the detection electrode 220. This charge is further conducted to the solid electrolyte body 26, which is detected as a sensor current. Then, the concentration of NOx is detected based on the magnitude of the sensor current. The detailed fine structure of the detection electrode 220 will be described later.

The monitor cell 23 has a monitor electrode 230, a solid electrolyte body 26, and a reference electrode 27. As described above, the monitor cell 23 shares the solid electrolyte body 26 and the reference electrode 27 with the detection cell 22. The monitor electrode 230 is formed on one side exposed to the measurement gas chamber 20 in the solid electrolyte body 26. The monitor electrode 230 is, for example, an electrode containing platinum (Pt) and gold (Au), and although it does not have the ability to decompose NOx, it can decompose oxygen molecules so that a current caused by oxygen ions flows. It has become.

The monitor electrode 230 is formed adjacent to the detection electrode 220 in a direction orthogonal to the flux direction of the exhaust gas flowing from the introduction port 202 to the detection cell 22. That is, the detection electrode 220 and the monitor electrode 230 are under equivalent exposure conditions with respect to the flux of the exhaust gas uniformly introduced into the measurement gas chamber 20. The monitor cell 23 detects the concentration of residual oxygen contained in the exhaust gas after the oxygen concentration is adjusted by the pump cell 24. Specifically, the monitor cell 23 detects the current flowing through the solid electrolyte 26 caused by the residual oxygen. In this gas sensor element 2, by subtracting the output of the monitor cell 23 from the output of the detection cell 22, the offset of the output of the detection cell 22 due to the residual oxygen can be canceled and the concentration of NOx can be detected. It has become.

The pump cell 24 is located upstream of the detection cell 22 and the monitor cell 23 in introducing the exhaust gas. The pump cell 24 has a pump electrode 240, a solid electrolyte body 26, and a reference electrode 27. The pump electrode 240 is formed on one side exposed to the measurement gas chamber 20 in the solid electrolyte body 26. Like the monitor electrode 230, the pump electrode 240 is an electrode containing platinum (Pt) and gold (Au), and reduces oxygen to generate oxygen ions. Oxygen ions are conducted through the solid electrolyte body 26, move to the reference electrode 27 side, and are discharged to the reference gas chamber 21. As described above, the pump cell 24 is a cell that adjusts the oxygen concentration in the measurement gas chamber 20 by its pumping action. That is, the pump cell 24 adjusts the oxygen concentration in the exhaust gas on the upstream side with respect to the flow of the exhaust gas, and the detection cell 22 and the monitor cell 23 adjust the current caused by NOx with respect to the exhaust gas after the oxygen concentration is adjusted. And the current caused by residual oxygen is output respectively.

The pump cell 24 in the first embodiment has a function of decomposing a substance existing in the exhaust gas to generate a reducing gas. Specifically, the pump cell 24 decomposes water molecules contained in the exhaust gas to generate hydrogen gas. Hydrogen gas has a reducing property, and by using this function at the time of starting the gas sensor element 2, oxygen stored in the detection electrode 220 is reduced and removed.

The heater 25 maintains the temperature of the solid electrolyte body 26 at a temperature of 600 ° C. or higher to function as a solid electrolyte. The heater 25 is formed by providing a conductor layer 252 that generates heat by energization between the ceramic substrates 251. The conductor layer 252 is formed so as to overlap the solid electrolyte body 26 when the surface on which the various electrodes 220, 230, 240, 27 are formed is viewed from the front, and at least the various electrodes 220, 230, 240, 27 are formed. It is possible to maintain the temperature of the portion where the is formed and its vicinity at the active temperature. The temperature distribution of the solid electrolyte body 26 realized by the heater 25 can be appropriately set according to the required performance, and the routing of the conductor layer 252 is set according to the required temperature distribution. Can be done.

Next, the detailed fine structure of the detection electrode 220 will be described with reference to FIGS. 3 to 10.

As illustrated in FIGS. 3 to 6, the detection electrode 220 includes a noble metal region 221 composed of a noble metal containing at least platinum and rhodium, a solid electrolyte region 222 composed of a solid electrolyte, and a noble metal and a solid electrolyte. Has a mixed region 223 in which the above is mixed. Further, as illustrated in FIG. 3, the detection electrode 220 has pores 224. In the first embodiment, the solid electrolyte constituting the solid electrolyte region 222 is specifically composed of a solid electrolyte having the same composition as the solid electrolyte constituting the solid electrolyte body 26. Specifically, the mixed region 223 is formed along the interface I between the noble metal region 221 and the solid electrolyte region 222. Further, the mixed region 223 can be specifically composed of a Pt—Rh alloy and ZrO ₂ .

Here, the width of the mixed region 223 is d. The width d of the mixed region 223 is measured as follows. As illustrated in FIG. 5, five visual fields of reflected electron images of a cross section along the thickness direction of the detection electrode 220 are acquired by observation with a scanning electron microscope (SEM). At this time, the observation magnification is 100,000 times. Next, for each acquired backscattered electron image, the shortest distance between the interface I1 between the mixed region 223 and the noble metal region 221 and the interface I2 between the mixed region 223 and the solid electrolyte region 222 is set at 10 points at equal intervals. taking measurement. The average value of the obtained measured values for the five visual fields (that is, the measured values at 50 points in total) is defined as the width d (nm) of the mixed region 223.

Further, let L be the ratio of the rhodium uneven distribution portion in the mixed region 223 to which the rhodium is unevenly distributed with respect to the entire mixed region 223. The ratio L of the rhodium uneven distribution portion is measured as follows. Using a scanning transmission electron microscope (STEM) and energy dispersive X-ray analysis (EDS) device, one field of EDS point analysis (measurement range: diameter 10 nm) is performed on the mixed region 223 in the cross section along the thickness direction of the detection electrode 220. 20 points or more per (magnification: for example, 200,000 times). FIG. 6 (a) shows an example of an STEM image of a mixed region, FIG. 6 (b) shows an example of an EDS mapping image of Zr element in a mixed region, and FIG. 6 (c) shows an EDS mapping image of Pt element in a mixed region. As an example, FIG. 6D shows an example of an EDS mapping image of the Rh element in the mixed region. As the points to be analyzed, for example, eight lines can be drawn at equal intervals in each of the vertical and horizontal directions of the image, and the points to be analyzed can be determined from the line intersections in the mixed region 223. At this time, the number of lines can be increased or decreased as necessary so that the number of points to be analyzed is 20 or more. Next, the Rh ratio at each measurement point is calculated from the following formula 1.
Rh ratio (%) = 100 × (Rh [wt%]) / (Rh [wt%] + Pt [wt%]) ... Equation 1
Next, the measurement points where the Rh ratio exceeds 80% are defined as the rhodium uneven distribution portion, and the ratio of the rhodium uneven distribution portion to the total number of measurement points is calculated from the following formula 2, and the calculated value is the ratio L (%) of the rhodium uneven distribution portion. ).
Rhodium uneven distribution ratio L (%) = 100 × (points of measurement points where Rh ratio exceeds 80%) / (total number of measurement points) ... Equation 2

As illustrated in FIG. 7, in the dL Cartesian coordinate system represented by the width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion, the mixed region when the electrode capacitance of the detection electrode 220 becomes a predetermined value. A curve representing the correlation between the width d of 223 and the ratio L of the rhodium uneven distribution portion is defined as the correlation curve R. In the dL Cartesian coordinate system, the horizontal axis is the width d of the mixed region 223, and the vertical axis is the ratio L of the rhodium uneven distribution portion. Specifically, the correlation curve R has the width d of the mixed region 223 as the horizontal axis, plots the ratio L of the rhodium uneven distribution portion at least two points or more, and performs multiple regression analysis from the electrode capacitance of the detection electrode 220 at each plot point. It can be obtained by finding a correlation line having the same electrode capacitance at. That is, the correlation curve R can be obtained by obtaining the correlation line having the same electrode capacitance by multiple regression analysis using the coordinates of at least two points consisting of the combination of the width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion. Can be done. More specifically, in FIG. 7, for example, the coordinates (d ₁ , L ₁ ) that are the electrode capacitance X ₁ (μF) in the dL Cartesian coordinate system and the coordinates (d) that are the electrode capacitance X ₃ (μF). An example is shown in which a correlation curve R having the same electrode capacitance X ₂ (μF) is obtained by multiple regression analysis using two plot points of ₃ , L ₃ ). The number of coordinates of the plot points in the multiple regression analysis can be preferably 3 or more from the viewpoint of obtaining a higher correlation.

The electrode capacitance of the detection electrode 220 can be obtained by performing equivalent circuit fitting on the Core-Cole plot using an impedance analyzer. Specifically, the measurement of the electrode capacitance of the detection electrode 220 is carried out at the sensor operating temperature of 700 to 900 ° C. and in a NO gas atmosphere (for example, 2000 ppm). FIG. 8 shows an equivalent circuit model of the detection electrode 220. Further, FIG. 9 shows a schematic diagram of a Core-Cole plot diagram. Further, FIG. 10 shows a schematic diagram of a Bode diagram. In the equivalent circuit model, R1 is the intragranular resistance of the solid electrolyte. R2 is the grain boundary resistance between the noble metal particles of the electrode and the solid electrolyte particles. R3 is the total of the interfacial resistance between the noble metal particles of the electrode and the gas, the resistance due to the adsorption of NO gas molecules, and the resistance due to surface diffusion, that is, the electrode reaction resistance. C2 is the capacitance of the grain boundary between the noble metal particles of the electrode and the solid electrolyte particles. C3 is the capacitance at the interface between the noble metal particles of the electrode and the gas. The electrode capacitance Ce of the detection electrode 220 is the sum of _C2 and C3 ( _Ce = C2 + C3). The resistance (R1, R2, R3) can be obtained from the chord of each semicircle in the Core-Cole plot diagram. If the real component of impedance Z in the Core-Cole plot diagram is Z _re and the imaginary component of impedance is Z _im , the relationship of | Z | = √ (Z _re ² + Z _im ² ) is satisfied. For the capacitance (C2, C3), find the frequency (f2, f3) at which the curve bends in the Bode diagram, and substitute the corresponding resistance and frequency values into the formula of C = 1 / (2πf · R). Can be obtained by. That is, the electrode capacitance _Ce of the detection electrode 220 can be calculated from the following equation.
C _e = C2 + C3 = 1 / (2πf2 ・ R2) + 1 / (2πf3 ・ R3)

In the gas sensor element 2, as illustrated in FIG. 7, the detection electrode 220 has the width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion in the region below the correlation curve R described above . That is, in the dL Cartesian coordinate system, the gas sensor element 2 has a mixed region 223 from each coordinate (d, L) located below the correlation curve R (inside the shaded area in FIG. 7). The width d and the ratio L of the rhodium uneven distribution portion are selected. The region below the correlation curve R is L <f when the correlation curve R is represented by L = f (d) (where f (d) is a function of the width d of the mixed region 223). This is the area (d). As will be described later in an experimental example, the correlation curve R is a curve in which the proportion L of the rhodium uneven distribution portion decreases as the width d of the mixed region 223 increases. Further, the correlation curve R is located upward as the electrode capacitance of the detection electrode 220 increases, and is located downward as the electrode capacitance of the detection electrode 220 decreases. Therefore, for example, when there is a request to set the electrode capacitance of the detection electrode 220 to a predetermined value or less, the electrode capacitance of the detection electrode 220 is set to be within the region below the correlation curve R when the predetermined value is reached. The width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion may be set. More specifically, as shown in FIG. 7, when the width of the mixed region 223 is d'when the electrode capacitance of the detection electrode 220 is requested to be a predetermined value (for example, X2) or less. Rhodium so as to be smaller than the ratio L'of the rhodium uneven distribution portion when the width of the mixed region 223 is d', which is obtained from the correlation curve R when the electrode capacitance of the detection electrode 220 becomes a predetermined value (X2). The ratio of the uneven distribution portion may be set. Similarly, when there is a request for the electrode capacitance of the detection electrode 220 to be a predetermined value (for example, X2) or less and the ratio of the rhodium uneven distribution portion is L', the electrode capacitance of the detection electrode 220 is a predetermined value (X2). ), The width of the mixed region 223 may be set so as to be smaller than the width d'of the mixed region 223 when the ratio of the rhodium uneven distribution portion is L'.

In the gas sensor element 2, specifically, the detection electrode 220 is a region below the correlation curve R when the width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion L are the electrode capacitance of the detection electrode 220 of 300 μF. It is said to be the configuration in.

According to this configuration, it is possible to ensure the improvement of the detection accuracy of the NOx gas concentration by reducing the noise of the current and the hysteresis. Therefore, according to this configuration, it becomes easy to obtain the gas sensor element 2 and the gas sensor 1 which are advantageous for detecting the NOx gas concentration.

This is due to the following reasons. The low frequency noise from the outside is amplified by the electrode capacitance of the detection electrode 220. Therefore, by reducing the electrode capacitance of the detection electrode 220, NOx output noise is reduced and the static accuracy of NOx output is improved. Further, when an electric charge is accumulated in the electrode capacitance of the detection electrode 220, the current value is delayed with respect to the voltage change when there is a transient voltage change, and hysteresis occurs. Therefore, the dynamic accuracy of NOx output is improved by reducing the electrode capacitance of the detection electrode 220.

The detection electrode 220 is preferably configured such that the width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion are in the region below the correlation curve R when the electrode capacitance of the detection electrode 220 is 250 μF. It is preferable that the width d of the mixed region 223 and the ratio L of the rhodium uneven distribution portion are in the region below the correlation curve R when the electrode capacitance of the detection electrode 220 is 200 μF , more preferably the mixture. The width d of the region 223 and the ratio L of the rhodium uneven distribution portion can be configured to be in a region below the correlation curve R when the electrode capacitance of the detection electrode 220 is 150 μF .

In the gas sensor element 2, the width d of the mixed region 223 can be specifically 30 nm or more. According to this configuration, it becomes easy to make a sufficient amount of the three-phase interface between the gas phase containing the specific gas in the measured gas G in the mixed region 223, the noble metal region 221 and the solid electrolyte region 222. Therefore, it becomes easy to reduce the electrode reaction resistance of the detection electrode 220, and the activity of the detection electrode 220 can be improved. The width d of the mixed region 223 can be preferably 40 nm or more, more preferably 50 nm or more, still more preferably 100 nm or more, from the viewpoint of improving the activity of the detection electrode 220. Further, since Rh has a reducing property, it has a property of aggregating and stabilizing, and when a heat load is applied by maintaining a high temperature, Rh particles aggregate with each other and the reaction resistance increases. If the width d of the mixed region 223 becomes too large, condensation becomes more likely to proceed. From the viewpoint of durability against heat load due to high temperature maintenance, the width d of the mixed region 223 is preferably 3000 nm or less, more preferably 2000 nm or less, still more preferably 1500 nm or less, and even more preferably 1000 nm or less. be able to.

In the gas sensor element 2, the electrode reaction resistance of the detection electrode 220 can be specifically 15000Ω or less. According to this configuration, it becomes easy to obtain a highly active and highly accurate NOx output .

The electrode reaction resistance of the detection electrode 220 is R3 described in the above-mentioned measurement of the electrode capacitance of the detection electrode 220, and this R3 can be calculated together with the measurement of the electrode capacitance of the detection electrode 220. Further, as shown in the experimental example described later, the electrode reaction resistance of the detection electrode 220 can be set to 15000Ω or less by setting the width d of the mixed region 223 to 30 nm or more.

The electrode reaction resistance of the detection electrode 220 can be preferably 14000 Ω or less, more preferably 13000 Ω or less, still more preferably 12000 Ω or less, from the viewpoint of improving the accuracy of NOx output. Since the smaller the electrode reaction resistance of the detection electrode 220 is, the better, the lower limit is not particularly limited.

In the gas sensor element 2, the noble metal contained in the detection electrode 220 can be configured to contain 30 to 70% by mass of rhodium and 70 to 30% by mass of platinum. Specifically, the noble metal contained in the detection electrode 220 can be made of a Pr—Rh alloy containing 30 to 70% by mass of rhodium and 70 to 30% by mass of platinum. According to these configurations, the balance between the NOx decomposability by Rh and the securing of the peel strength of the detection electrode 220 due to the oxidative expansion of Rh is excellent.

In the above-described embodiment, the configuration in which the gas sensor element 2 has the monitor cell 23 has been described, but the monitor cell 23 is not an essential configuration from the viewpoint of detecting the concentration of the specific gas in the gas to be measured G. However, it is preferable to have the monitor cell 23 in that the oxygen concentration in the gas after the oxygen concentration is adjusted by the pump cell 24 is correctly detected and the background is corrected for the output of the detection cell 22. ..

Further, in the above-described embodiment, the precious metals constituting the detection electrode 220 containing Pt and Rh have been described, but even if the detection electrode 220 is configured to contain palladium (Pd), ruthenium (Ru), or the like. good.

Further, in the measurement gas chamber 20, the space in which the detection cell 22 is formed and the space in which the pump cell 24 is formed may be configured to be separated so that the gas to be measured G can come and go. Specifically, for example, the gas sensor element 2 may have a configuration in which a diffusion rate-determining body (not shown) is formed between the space in which the detection cell 22 is formed and the space in which the pump cell 24 is formed. The diffusion rate-determining body can separate the measurement gas chamber 20 into two spaces by separating the detection cell 22 and the pump cell 24, and allow the measured gas G to permeate while adjusting the diffusion resistance.

The detection electrode 220 can be formed, for example, as follows. A mixture containing noble metal particles containing at least Pt and Rh such as Pt—Rh alloy particles, zirconia-like solid electrolyte particles such as yttria-stabilized zirconia particles, and, if necessary, a pore-forming agent, is formed in the solid electrolyte body 26. Apply to the surface and bake in a nitrogen atmosphere. Next, a voltage is applied between the baked detection electrode 220 (strictly speaking, the electrode before the detection electrode 220) and the reference electrode 27 in a nitrogen gas atmosphere to perform an energization process. As a result, a part of ZrO ₂ is reduced to Zr at the interface between the noble metal particles and the zirconia solid electrolyte particles, and Pt and Rh are solidly dissolved in Zr. Then, oxygen gas is introduced into the atmosphere, and the reduced Zr is reoxidized to obtain ZrO ₂ . As a result, a mixed region 223 of Zr-Pt-Rh is formed at the interface between the noble metal region 221 and the solid electrolyte region 222. Here, in the noble metal particles, Pt and Rh are completely solid-solved, but since the solid-solution rate of each element is different, the difference in the solid-solution rate increases as the temperature during the energization treatment increases. Therefore, when Pt and Rh are solid-solved in Zr, if the electrode temperature is too high, the Pt-Rh alloy that has been completely solid-solved due to the difference in solid-solution rate may be separated, and Rh tends to be unevenly distributed. Become. Further, if the electrode temperature is too low, solid solution does not proceed. In order to suppress uneven distribution of Rh and obtain a sufficient width of the mixed region 223, the electrode temperature is set low and the energization time is relatively long. When it is desired to set the electrode temperature higher, the uneven distribution of Rh can be minimized by making the energization time relatively short. The electrode temperature, energization time, and applied voltage can be appropriately set in consideration of these phenomena. A known technique can be applied to the method of forming each configuration other than the detection electrode 220.

(Experimental example)
According to the manufacturing method described above, four gas sensor elements having different electrode capacities of detection electrodes were manufactured. In this example, a paste-like mixture containing Pt—Rh alloy particles, yttria-stabilized zirconia, and a pore-forming agent is used, and by changing the energization treatment conditions, the surface of the solid electrolyte composed of yttria-stabilized zirconia is formed. Each detection electrode was formed. For each gas sensor element, the electrode capacitance of the detection electrode was measured by the above-mentioned measurement method, and the gas sensor element having the measured electrode capacitance of 144 μF was sample 1, and the gas sensor element having the detected electrode capacitance of 155 μF. The gas sensor element having an electrode capacitance of 410 μF was designated as sample 1C, and the gas sensor element having an electrode capacitance of a detection electrode of 390 μF was designated as sample 2C.

From the results of STEM observations and EDS analysis, the gas sensor elements of each sample all contained a Pt—Rh alloy, yttria-stabilized zirconia and pores, similar to that shown in FIG. 6 above. It is composed of a noble metal region composed of a Pt—Rh alloy, a solid electrolyte region composed of yttria-stabilized zirconia, and a mixed region in which a Pt—Rh alloy and yttria-stabilized zirconia are mixed. It was confirmed that there was.

For the detection electrode in the gas sensor element of each sample, the width d of the mixed region and the ratio L of the rhodium uneven distribution portion were measured according to the above-mentioned measurement method. Next, the value of the ratio L of the rhodium uneven distribution portion was plotted with the width d of the mixed region as the horizontal axis. Next, for the four levels of sample 1, sample 2, sample 1C, and sample 2C, the electrode capacitance of the detection electrode is set as the objective variable y, the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are set as explanatory variables (x ₁ , x). By performing multiple regression analysis as ₂ ), the following multiple regression equation was obtained.
y = α ₀ + α ₁ x ₁ + α ₂ x ₂ (However, α ₀ , α ₁ , and α ₂ are partial regression coefficients)
Furthermore, since the electrode capacitance of the detection electrode is affected by the total amount of the rhodium uneven distribution portion, that is, it changes depending on the product of the width d of the mixed region and the ratio L of the rhodium uneven distribution portion, the interaction of the two explanatory variables is also considered. By performing multiple regression analysis, the following multiple regression equation was obtained.
y = α ₀ + α ₁ x ₁ + α ₂ x ₂ + α ₃ x ₁ x ₂ (However, α ₀ , α ₁ , α ₂ , and α ₃ are partial regression coefficients)
Each correlation curve R was obtained by transforming this multiple regression equation as follows and substituting an arbitrary electrode capacitance into y. The results are shown in FIG.
Correlation curve R: x ₂ = (y-α ₀ -α ₁ x ₁ ) / (α ₂ + α ₃ x ₁ )
(However, α ₀ , α ₁ , α ₂ , and α ₃ are partial regression coefficients)

As shown in FIG. 11, it can be seen that the proportion L of the rhodium uneven distribution portion decreases as the width d of the mixed region increases. Further, it can be seen that the correlation curve is located upward as the electrode capacitance of the detection electrode increases, and is located downward as the electrode capacitance of the detection electrode decreases. From this result, when there is a request to set the electrode capacitance of the detection electrode to a predetermined value or less, the region from the region below the correlation curve (negative side of the correlation curve) where the electrode capacitance of the detection electrode becomes the predetermined value to the mixed region. It can be seen that by setting the width d and the ratio L of the rhodium uneven distribution portion, the electrode capacity can be reduced more than the electrode capacity of the correlation curve. That is, the combined coordinates (c, d) of the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are on the negative side with respect to the width d of the mixed region and the negative side with respect to the ratio L of the rhodium uneven distribution portion. By setting the width d of the mixed region and the ratio L of the rhodium uneven distribution portion, it can be said that the electrode capacity can be reduced more than the electrode capacity of the correlation curve.

Next, in the same manner as described above, a plurality of gas sensor elements having different electrode capacities of the detection electrodes were manufactured, and NOx output noise was measured. The measurement of NOx output noise was performed by driving the sensor in N ₂ atmosphere, O ₂ atmosphere, and NO atmosphere. The NOx output noise was calculated from the difference between the maximum peak and the minimum peak in the waveform of the obtained NOx output waveform for 20 sec. The results are shown in FIG. As shown in FIG. 12, according to the relationship line between the electrode capacitance of the detection electrode and the NOx output, it can be seen that the NOx output noise increases almost proportionally as the electrode capacitance of the detection electrode increases. Therefore, according to FIGS. 11 and 12, the noise and hysteresis of the current can be suppressed by reducing the electrode capacitance of the detection electrode, and the concentration of the specific gas in the gas to be measured can be detected with high accuracy. Understand. Further, according to FIG. 12, it can be seen that it is desirable to set the electrode capacitance of the detection electrode to 300 μF or less in order to reduce the NOx noise to 10 ppm or less in order to improve the NOx detection accuracy.

Next, in the same manner as described above, a plurality of gas sensor elements having different electrode capacities of the detection electrodes were produced, and the width d of the mixed region and the electrode reaction resistance of the detection electrodes were measured according to the measurement method described above. The result is shown in FIG. As shown in FIG. 13, according to the relationship between the width d of the mixed region and the electrode reaction resistance of the detection electrode, it can be seen that the electrode reaction resistance of the detection electrode decreases as the width d of the mixed region increases. .. Further, according to FIG. 13, it can be seen that it is desirable that the width d of the mixed region is 30 nm or more in order to make the electrode reaction resistance of the detection electrode 15,000 Ω or less in order to improve the NOx detection accuracy. Note that FIG. 14 shows a line having a width d of the mixed region = 30 nm in relation to the width d (nm) of the mixed region and the ratio L (%) of the rhodium uneven distribution portion. As illustrated in FIG. 14, the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are lower than the correlation curve when the electrode capacitance of the detection electrode is 300 μF, and the width d of the mixed region is By being in the region of 30 nm or more, it becomes easy to obtain a gas sensor element which is advantageous for improving NOx detection accuracy by reducing the electrode capacitance of the detection electrode.

The present invention is not limited to the above embodiments and experimental examples, and various modifications can be made without departing from the gist thereof. In addition, the configurations shown in the embodiments and experimental examples can be arbitrarily combined.
Hereinafter, an example of the reference form will be added.
Item 1.
A solid electrolyte body (26) composed of a zirconia solid electrolyte having oxygen ion conductivity, and
A detection electrode (220) provided on one surface of the solid electrolyte body for detecting the concentration of the specific gas in the gas to be measured (G), and
A gas sensor element (2) provided on the other surface of the solid electrolyte body and provided with a reference electrode (27) exposed to the reference gas (A).
The detection electrode has a noble metal region (221) composed of a noble metal containing at least platinum and rhodium, a solid electrolyte region (222) composed of the solid electrolyte, and a mixed region in which the noble metal and the solid electrolyte coexist. Has (223) and
The electrode capacitance of the detection electrode is predetermined in the dL Cartesian coordinate system represented by the width d of the mixed region and the ratio L of the rhodium unevenly distributed portion in the mixed region in which the rhodium is unevenly distributed to the entire mixed region. When the curve representing the correlation between the width d of the mixed region and the ratio L of the rhodium uneven distribution portion when it becomes a value is defined as the correlation curve (R),
In the detection electrode, the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are set from the region below the correlation curve.
Gas sensor element (2).
Item 2.
Item 2. The gas sensor element according to Item 1, wherein the detection electrode has a width d of the mixed region and a ratio L of the rhodium uneven distribution portion set from a region below the correlation curve when the electrode capacitance is 300 μF. ..
Item 3.
Item 2. The gas sensor element according to Item 1 or Item 2, wherein the width d of the mixed region is 30 nm or more.
Item 4.
Item 2. The gas sensor element according to any one of Items 1 to 3, wherein the electrode reaction resistance of the detection electrode is 15,000 Ω or less.
Item 5.
Item 6. The gas sensor element according to any one of Items 1 to 4, wherein the precious metal contains 30 to 70% by mass of rhodium and 70 to 30% by mass of platinum.
Item 6.
The gas sensor (1) having the gas sensor element according to any one of Items 1 to 5.

1 Gas sensor 2 Gas sensor element 26 Solid electrolyte 220 Detection electrode 221 Noble metal region 222 Solid electrolyte region 223 Mixed region R Correlation curve 27 Reference electrode G Measured gas A Reference gas

Claims (5)

A solid electrolyte body (26) composed of a zirconia solid electrolyte having oxygen ion conductivity, and
A detection electrode (220) provided on one surface of the solid electrolyte body for detecting the concentration of the specific gas in the gas to be measured (G), and
A gas sensor element (2) provided on the other surface of the solid electrolyte body and provided with a reference electrode (27) exposed to the reference gas (A).
The detection electrode has a noble metal region (221) composed of a noble metal containing at least platinum and rhodium, a solid electrolyte region (222) composed of the solid electrolyte, and a mixed region in which the noble metal and the solid electrolyte coexist. Has (223) and
The electrode capacitance of the detection electrode is predetermined in the dL Cartesian coordinate system represented by the width d of the mixed region and the ratio L of the rhodium unevenly distributed portion in the mixed region in which the rhodium is unevenly distributed to the entire mixed region. When the curve representing the correlation between the width d of the mixed region and the ratio L of the rhodium uneven distribution portion when it becomes a value is defined as the correlation curve (R),
The detection electrode is located in a region below the correlation curve when the width d of the mixed region and the ratio L of the rhodium uneven distribution portion are 300 μF.
Gas sensor element (2). The gas sensor element according to claim 1 , wherein the width d of the mixed region is 30 nm or more. The gas sensor element according to claim 1 or 2 , wherein the electrode reaction resistance of the detection electrode is 15,000 Ω or less. The gas sensor element according to any one of claims 1 to 3 , wherein the precious metal contains 30 to 70% by mass of rhodium and 70 to 30% by mass of platinum. A gas sensor (1) having the gas sensor element according to any one of claims 1 to 4 .

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