阅读说明：本技术 固体电解质、气体传感器 (Solid electrolyte and gas sensor ) 是由 吉田充宏 铃木聪司 于 2020-02-18 设计创作，主要内容包括：一种由稳定化氧化锆或部分稳定化氧化锆形成的固体电解质(1)以及具备固体电解质(1)的气体传感器。将固体电解质(1)从室温加热至1200℃前后的构成固体电解质(1)的晶粒(2)中的立方晶相(21)及正方晶相(23)中的至少一者的微晶粒径的变化率为10％以下。固体电解质优选由部分稳定化氧化锆形成。(A solid electrolyte (1) formed of stabilized zirconia or partially stabilized zirconia and a gas sensor provided with the solid electrolyte (1). The rate of change in crystallite particle size of at least one of a cubic phase (21) and a tetragonal phase (23) of crystal grains (2) that constitute the solid electrolyte (1) is 10% or less when the solid electrolyte (1) is heated from room temperature to about 1200 ℃. The solid electrolyte is preferably formed from partially stabilized zirconia.)

1. A solid electrolyte which is a solid electrolyte (1) formed of stabilized zirconia or partially stabilized zirconia,

wherein the rate of change in crystallite particle size of at least one of a cubic crystal phase (21) and a tetragonal crystal phase (23) in the crystal grains (2) that constitute the solid electrolyte before and after heating from room temperature to 1200 ℃ is 10% or less.

2. The solid electrolyte according to claim 1, wherein the solid electrolyte is formed of partially stabilized zirconia having an yttria content of 4.5 to 8 mol%.

3. A gas sensor (5) provided with the solid electrolyte of claim 1 or 2.

Technical Field

The present invention relates to a solid electrolyte formed of stabilized zirconia or partially stabilized zirconia, and a gas sensor provided with the solid electrolyte.

Background

In an exhaust system of an internal combustion engine or the like, a gas sensor is used for the purpose of detecting an oxygen concentration, an air-fuel ratio, and the like in exhaust gas. Such a gas sensor uses an oxide ion-conductive solid electrolyte such as zirconia. Solid electrolytes are often used in environments accompanied by rapid temperature changes. However, since zirconia is likely to crack due to phase change caused by temperature change, improvement of thermal shock resistance has been sought.

For example, patent document 1 discloses a technique of improving thermal shock resistance by forming a monoclinic zirconia layer on the surface of a zirconia oxygen sensor element in which a stabilizer is solid-dissolved by an aging treatment.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 63-210063

Disclosure of Invention

In recent years, solid electrolytes are required to be used under higher loads, and further improvement in thermal shock resistance has been desired. Specifically, the following tendency is exhibited: the solid electrolyte is exposed to a higher temperature environment due to, for example, a change in the mounting position of the in-vehicle gas sensor. For example, in a hybrid vehicle and an idle reduction vehicle, the following tendency is exhibited: the solid electrolyte is frequently exposed to a cold-hot cycle with frequent start and stop of the engine and the heater.

The purpose of the present application is to provide a solid electrolyte and a gas sensor having excellent thermal shock resistance.

One embodiment of the present invention relates to a solid electrolyte comprising stabilized zirconia or partially stabilized zirconia, wherein a rate of change in crystallite particle size of at least one of a cubic phase and a tetragonal phase in crystal grains constituting the solid electrolyte before and after heating from room temperature to 1200 ℃ is 10% or less.

Another aspect of the present application relates to a gas sensor including the solid electrolyte.

As described above, the solid electrolyte is prepared by adjusting the rate of change in crystallite size to a predetermined value or less. Therefore, the strength reduction against the cooling-heating cycle can be suppressed, and the occurrence of cracks can be suppressed. As a result, the solid electrolyte has excellent thermal shock resistance, and can maintain high strength even when exposed to a high-temperature environment or an environment having a high cooling-heating cycle frequency, for example. The reason for this is considered to be that relaxation of internal energy associated with phase transition of the crystal phase reduces strain, and the mechanism thereof will be described later.

The gas sensor includes the solid electrolyte having excellent thermal shock resistance. Therefore, the gas sensor has high reliability against pressure increase in the cooling-heating cycle, and can accurately measure the gas concentration over a long period of time.

As described above, according to the above configuration, a solid electrolyte and a gas sensor having excellent thermal shock resistance can be provided.

In the claims, the parenthesized symbols indicate correspondence with specific means described in the embodiments described below, and do not limit the technical scope of the present application.

Drawings

The above objects, and other objects, features and advantages in the present application will become more apparent from the following detailed description with reference to the accompanying drawings. The attached drawings are as follows:

fig. 1 is a schematic diagram showing the microstructure of a strain-reduced solid electrolyte in embodiment 1.

Fig. 2 is an explanatory diagram showing the volume change and the thermal expansion coefficient of the crystal phase in embodiment 1.

FIG. 3 shows ZrO in embodiment 1₂-Y₂O₃Is a phase diagram.

FIG. 4 shows ZrO in embodiment 1₂Is an explanatory diagram of the thermodynamic explanation of the phase transition of (1).

Fig. 5 is an explanatory diagram showing a temperature profile in the case where a cooling-heating cycle is performed after the firing step without performing the annealing treatment in embodiment 1.

Fig. 6 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in VI of fig. 5.

Fig. 7 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in VII in fig. 5.

Fig. 8 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in VIII of fig. 5.

Fig. 9 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in IX of fig. 5.

Fig. 10 is a schematic diagram showing the microstructure of a solid electrolyte having strain in embodiment 1.

Fig. 11 is an explanatory diagram showing a temperature profile in the case where the annealing treatment is performed after the firing step in embodiment 1, and then the cooling-heating cycle is performed.

Fig. 12 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in XII of fig. 11.

Fig. 13 is a schematic diagram showing a state of a crystal phase constituting the solid electrolyte in XIII in fig. 11.

Fig. 14 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in XIV of fig. 11.

Fig. 15 is a schematic diagram showing the state of the crystal phase constituting the solid electrolyte in XV of fig. 11.

Fig. 16 is a schematic diagram showing a state of a crystal phase constituting the solid electrolyte in XVI in fig. 11.

Fig. 17 is a schematic diagram showing a state of a crystal phase constituting the solid electrolyte in XVII of fig. 11.

Fig. 18 is an explanatory diagram showing changes in crystallite size caused by annealing treatment in embodiment 1.

Fig. 19 is a sectional view of the gas sensor in embodiment 2.

Fig. 20 is a sectional view of the laminated gas sensor element according to embodiment 2.

Fig. 21 is a sectional view of a cup-shaped gas sensor element in embodiment 2.

Fig. 22 is an explanatory view showing a method for producing a solid electrolyte in experimental example 1.

Fig. 23 is a diagram showing an example of a peak derived from {111} of the C phase in the XRD pattern of the solid electrolyte in experimental example 1.

Fig. 24 is a graph showing the relationship between the rate of change in crystallite particle size in the C phase and the rate of decrease in strength in experimental example 1.

Fig. 25 is a graph showing the relationship between the annealing temperature and the change rate of the crystallite grain size of the C phase in experimental example 1.

Fig. 26 is a graph showing the relationship between the annealing temperature and the strength reduction rate in experimental example 1.

Fig. 27 is a graph showing the relationship between the holding time of the annealing temperature and the change rate of the crystallite grain size of the C phase in experimental example 1.

Detailed Description

< embodiment 1>

Embodiments of the solid electrolyte will be described with reference to fig. 1 to 18. In the following description, the cubic phase is suitably referred to as "C phase", the monoclinic phase is suitably referred to as "M phase", and the tetragonal phase is suitably referred to as "T phase". As illustrated in fig. 1, the solid electrolyte 1 is composed of a plurality of crystal grains 2. The crystal phase of the crystal grains 2 has a form of a C phase 21, an M phase 22, and a T phase 23.

The solid electrolyte 1 is formed of stabilized zirconia or partially stabilized zirconia. Stabilized zirconia and partially stabilized zirconia are so-called sintered bodies, and a stabilizer is solid-dissolved in zirconia.

Examples of the stabilizer include yttrium oxide, calcium oxide, magnesium oxide, scandium oxide, and ytterbium oxide. The stabilized zirconia, partially stabilized zirconia may contain at least 1 of the above as a stabilizer. The stabilizer is preferably yttria from the viewpoint of chemical stability. When the stabilizing agent is yttria, partially stabilized zirconia is formed when the content of yttria is 8 mol% or less, and stabilized zirconia is formed when the content of yttria exceeds 8 mol%.

Generally, the component is zirconium oxide (ZrO)₂) The crystal grains 2 of (2) include 3 kinds of M-phase 22, T-phase 23 and C-phase 21. The phase changes in the order of the M phase 22, T phase 23, and C phase 21 with the temperature increase, and the stabilization is performed. The C phase 21 and the T phase 23 are stabilized and metastable even at room temperature by the stabilizing agent such as yttria being dissolved in zirconia as a solid solution. The room temperature is, for example, 25 ℃ and the same applies to the following description.

Among the crystal phases, the C phase 21 has the highest ion conductivity but has low strength and a high thermal expansion coefficient as shown in fig. 2. From the viewpoint of reducing the thermal expansion coefficient, the solid electrolyte 1 is preferably formed of a mixed phase of the C phase 21 and the M phase 22. In this case, alumina, spinel (MgAl) can be reduced ₂O₄) The difference in thermal expansion between the member of different material and the solid electrolyte 1. As a result, it is possible to prevent cracks and separation from occurring at the contact portion between the solid electrolyte 1 and the dissimilar material member due to the difference in thermal expansion.

As shown in fig. 4, the transformation is performed to form a more stable state with a lower internal energy in the crystal grains 2. At ZrO₂And Y₂O₃In the solid solution firing process of (2), as shown in FIG. 3, the crystal grains 2 of the M-phase 22 in a stable state in the room temperature region exist as the T-phase 23 in the firing temperature region (specifically, 1400 ℃ or higher). Then, at the time of temperature decrease after completion of firing, the phase changes from the T-phase 23 to the M-phase 22 with expansion. Since expansion is accumulated as strain in the crystal grains 2 near the grain boundaries without being released, the internal energy in the crystal grains 2 is high. Therefore, the crystal grains 2 of the M phase 22 are easily phase-changed into the T phase 23. That is, the phase transition temperature from the M-phase 22 to the T-phase 23 is higher than the temperature specified in the phase diagram of FIG. 3, as shown in FIG. 4, from T₀To T₀' reduction of only Δ T₀。

For such a transformation peculiar to zirconia, it is effective to reduce the strain to reduce the internal energy of the crystal grains 2, and the transformation can be suppressed. For reducing the internal energy, for example, annealing treatment is effective. The reason is that: the thermal energy in the annealing process rearranges the crystal grains 2 in the thermal vibration, and the strain is reduced. That is, the internal energy is reduced by reducing the strain, and as a result, the phase transition temperature is increased and the phase transition is suppressed. The following description will be made in detail.

As shown in fig. 5, in the case where, for example, annealing treatment is not performed after firing, the solid electrolyte 9 has strain S in, for example, the grain boundary of the crystal grains 2 due to transition from the T-phase 23 to the M-phase 22 accompanied by expansion at the time of temperature reduction in the firing step, as illustrated in fig. 6. Then, the strain energy becomes internal energy. As shown in fig. 5, if a cooling-heating cycle from room temperature to 1200 ℃ is applied to the solid electrolyte 9, the M phase 22 of the crystal grains 2 is transformed into the T phase 23 as illustrated in fig. 6 and 7, and, for example, a volume shrinkage of about 4% occurs at the temperature rise of the cooling-heating cycle. As shown in fig. 5, the temperature is further raised after the phase transition occurs, and when the temperature is 1200 ℃, for example, the C phase 21 thermally expands as shown in fig. 8. At this time, the void generated by volume contraction in the phase transition from the M-phase 22 to the T-phase 23 is filled by the thermal expansion of the C-phase 21. As shown in fig. 5, after cooling in the cooling-heating cycle, for example, in a state at room temperature, phase transformation accompanied by expansion from the T-phase 23 to the M-phase 22 accompanying expansion occurs as illustrated in fig. 9, but as described above, the C-phase 21 also expands, and therefore the strain S of the crystal grains 2 becomes larger than that after firing. That is, as illustrated in fig. 10, the solid electrolyte 9 has strain at, for example, grain boundaries of the crystal grains 2, and the internal energy of the crystal grains 2 becomes larger than that after firing.

On the other hand, as shown in fig. 11, for example, when annealing treatment is performed, the internal energy can be reduced by the following mechanism. As illustrated in fig. 12, when the temperature is decreased in the firing step, the crystal grains 2 of the solid electrolyte 1 have strain S due to the phase transition from the T phase 23 to the M phase 22 accompanied by expansion. In the solid electrolyte 1, as shown in fig. 11, for example, by annealing, the crystal grains 2 are rearranged by thermal vibration as shown in fig. 13, and the strain S is reduced. As a result, after the annealing treatment, the solid electrolyte 1 has a state in which the internal energy of the crystal grains 2 is low, as illustrated in fig. 14. In such a solid electrolyte 1, as shown in fig. 11 and 15, since the phase transition temperature from the M-phase 22 to the T-phase 23 is high, if a cooling-heating cycle from room temperature to 1200 ℃ is applied to the solid electrolyte 1, the phase transition from the M-phase 22 to the T-phase 23 becomes difficult during temperature rise. This makes it difficult to generate voids by volume shrinkage at the time of phase transition. When the temperature is further increased, for example, in a state of 1200 ℃, the C phase 21 thermally expands as illustrated in fig. 16, but since voids are generated by volume contraction, the volume change of the C phase 21 filling the voids is less likely to occur. That is, by performing the annealing treatment, the influence of the thermal expansion of the C phase 21 is reduced to such an extent that the transformation temperature from the M phase 22 to the T phase 23 is increased. As shown in fig. 11, after cooling in the cooling-heating cycle, for example, in a state at room temperature, the transition from the T-phase 23 to the M-phase 22 occurs with expansion, but the influence of thermal expansion of the C-phase 21 is reduced (see fig. 16), and therefore the strain S of the crystal grains 2 is reduced. That is, as illustrated in fig. 1 and 17, the internal energy of the crystal grains 2 of the solid electrolyte 1 is small.

The annealing temperature is, for example, the melting point of stabilized zirconia or partially stabilized zirconia or less, and the annealing treatment is a concept different from the concept of removing the strain S by heating the ceramic to the vicinity of the melting point to soften the ceramic. The higher the annealing temperature, the better, but the higher the annealing temperature, and the higher the annealing temperature, and the lower the annealing temperature, the higher the annealing temperature, and the higher the annealing temperature, and the lower the annealing temperature, the higher the annealing temperature, and the lower the annealing temperature, and the lower the annealing temperature, and the lower the temperature. Further, it can be seen that: in the case where the yttrium oxide concentration is 2.5 to 7 mol% as shown in fig. 3, the mixed phase of the M phase 22 and the C phase 21 is stable in the room temperature region, and the upper limit of the annealing temperature in the mixed phase is lowered to 600 ℃. However, the phase diagram of fig. 3 is a theoretical value in an equilibrium state, and in actual stabilized zirconia or partially stabilized zirconia, there is a tendency that: the statistical concentration distribution of yttrium oxide is caused by the non-uniformity of the solid solution reaction due to the non-uniform mixing of the raw material powders. That is, in practice, it is difficult to achieve an equilibrium state as in the phase diagram shown in fig. 3, and the temperature at which the phase transition from the M-phase 22 to the T-phase 23 tends to be higher than the theoretical value temperature in fig. 3. Therefore, the actual phase transition temperature becomes higher than 600 ℃, and the annealing temperature can be set to a temperature exceeding 600 ℃. In practice, the annealing temperature is preferably 1150 ℃ or less, more preferably 1100 ℃ or less, and still more preferably 1000 ℃ or less.

As described above, the phase transition temperature decreases as the internal energy increases, and the strain S generated in the cooling-heating cycle increases. Therefore, the strain S in the cold-hot cycle becomes an alternative index of the internal energy. Specifically, the strain S may be evaluated before and after heating of the solid electrolyte 1 from room temperature to 1200 ℃, more specifically, by applying a cold-heat cycle of room temperature → 1200 → room temperature to the solid electrolyte 1. Strain S can be trapped as interatomic distances in the crystal. Therefore, the strain S can be evaluated by measuring and calculating the crystallite diameter by X-ray diffraction (XRD). The larger the change rate of the crystallite particle diameter before and after the cooling-heating cycle, the larger the internal energy. The method for measuring and calculating the crystallite diameter will be described with reference to experimental examples, and it can be calculated from the scherrer equation based on the half width and diffraction angle in the XRD pattern.

Although the M phase 22 expands due to the phase transformation, the crystallite size of any of the C phase 21, the M phase 22, and the T phase 23 may be evaluated from the viewpoint of the balance of the forces due to the pressing of the surrounding crystal grains 2 accompanying the expansion. On the other hand, the generation of the microcracks becomes a factor of the decrease in the strength of the solid electrolyte 1, and the microcracks due to the strain S are generated inside the crystal grains 2 of the C phase 21 having a smaller strength than the M phase 22 and the T phase 23 and in the grain boundaries in the vicinity of the C phase 21. Therefore, the crystallite size of the C phase 21 is preferably evaluated. The C phase 21 has a large symmetry of crystal lattice, so that the crystal is easily cracked, and further, Y passes through ₂O₃The solid solution of (2) improves the reactivity, and the strength is reduced due to an increase in particle size during sintering.

The C-phase 21 and the T-phase 23 have similar crystal structures, and can be distinguished from each other by the miller index on the high-angle side in the evaluation of XRD, but the peak intensity is weak, and the measurement accuracy is lowered. Therefore, from the viewpoint of facilitating measurement and calculation of the crystallite diameter by XRD, it is more preferable to evaluate the rate of change in the crystallite diameter of at least one of the C-phase 21 and the T-phase 23. In XRD, the crystallite diameter can be measured and calculated based on peaks derived from the {111} plane of C phase 21 and the {101} plane of T phase 23, which have the highest peak intensity.

In the solid electrolyte 1 of the present embodiment, the rate of change in crystallite size of at least one of the C phase and the T phase in crystal grains constituting the solid electrolyte 1 when the solid electrolyte 1 is heated from room temperature to 1200 ℃ is 10% or less. If the rate of change in crystallite particle size exceeds 10%, the strain energy is large, and the thermal shock resistance is insufficient. From the viewpoint of further improving the thermal shock resistance, the rate of change in crystallite particle size is preferably 6% or less, more preferably 2% or less.

As shown in fig. 17, for example, the crystal of the solid electrolyte 1 is stabilized by annealing treatment, and the crystallite diameter is increased. As a result, the rate of change in crystallite particle size after the thermal history of the solid electrolyte 1 due to the cooling-heating cycle is smaller than in the case where the annealing treatment is not performed. In the present embodiment, the solid electrolyte 1 in which the internal energy is reduced by the annealing treatment and the change rate of the crystallite particle size is reduced has been described, but the solid electrolyte 1 exhibits excellent thermal shock strength even in a method other than the annealing treatment as long as the change rate of the crystallite particle size is 10% or less.

The solid electrolyte 1 is suitable for a gas sensor that detects vehicle exhaust gas, for example. The reason for this is as follows. In the use environment of the gas sensor, it is expected that: in the future, the cycle of starting and stopping the engine further increases, and the thermal efficiency of the engine increases, resulting in an increase in the exhaust gas temperature. That is, the engine is at high load and the exhaust temperature rises. As a result, the solid electrolyte of the gas sensor reaches the phase transition temperature of the T-phase 23 from the M-phase 22 as described above, and the T-phase 23 is again changed to the M-phase 22 upon cooling. There is a possibility that the strength may be reduced by the propagation of the micro cracks due to the strain S generated by the contraction and expansion at this time, but since the solid electrolyte 1 having the rate of change in crystallite size of 10% or less under the above-mentioned predetermined conditions can increase the phase transition temperature of the M phase 22 by, for example, 60 ℃ at the maximum, the strength is less likely to be reduced even when the exhaust temperature increases in the future.

The solid electrolyte 1 is preferably formed of partially stabilized zirconia, and the content of yttria in the partially stabilized zirconia is preferably 2 to 8 mol%. In this case, alumina, spinel (MgAl) can be reduced₂O₄) The difference in thermal expansion coefficient between the solid electrolyte 1 and the member of different material. As a result, the occurrence of cracks due to the difference in thermal expansion coefficient can be prevented. Therefore, the solid electrolyte 1 is suitable for use in joining members of dissimilar materials. As such an application, for example, a sensor element of a gas sensor can be exemplified. The content of yttria in the partially stabilized zirconia is more preferably 4.5 to 8 mol%. In this case, the thermal expansion coefficients of the dissimilar material member and the solid electrolyte 1 are more matched, and the occurrence of cracks can be further prevented.

The solid electrolyte 1 can be produced by performing a mixing step, a firing step, and an annealing step. In the mixing step, zirconia and a stabilizer are mixed. Thus, a mixture was obtained. The mixture can be formed into a desired shape to make a shaped body.

In the firing step, the mixture or the molded body thereof is fired to obtain a fired body. The fired body is formed of stabilized zirconia or partially stabilized zirconia.

In the annealing step, the fired body is heated. The annealing temperature is, for example, 800 to 1150 ℃. In the case where the annealing temperature is lower than 800 ℃, the internal energy of the crystal grains 2 becomes insufficient. On the other hand, when the annealing temperature exceeds 1150 ℃, a phase transition from the M-phase 22 to the T-phase 23 occurs in the annealing treatment, and the effect of reducing the internal energy cannot be obtained. From the viewpoint of sufficiently obtaining the effect of reducing the internal energy by the annealing treatment, the annealing temperature is preferably 900 to 1100 ℃, and more preferably 950 to 1000 ℃. As for a specific example of the method for producing the solid electrolyte 1, description will be made by way of an experimental example.

< embodiment 2>

Embodiments of the gas sensor will be described. Note that, among the symbols used in embodiment 2 and thereafter, the same symbols as those used in the present embodiment represent the same constituent elements and the like as those in the present embodiment unless otherwise specified.

The gas sensor 5 of the present embodiment includes the sensor element 6 as shown in fig. 19 and 20. The sensor element 6 of the present embodiment is a gas sensor element that detects gas. The sensor element 6 has a solid electrolyte 1, a detection electrode 62, a reference electrode 63, and a diffusion barrier layer 66. That is, the gas sensor 5 has the solid electrolyte 1 in the sensor element 6. The detection electrode 62 and the reference electrode 63 are formed on both surfaces 601A, 602A of the solid electrolyte 1, respectively. The detection electrode 62 and the reference electrode 63 form a pair of electrodes formed at positions facing each other. The diffusion barrier layer 66 restricts the flow rate of the measurement gas such as the exhaust gas G reaching the detection electrode 62. The gas sensor 5 is a limiting current type gas sensor that detects the oxygen concentration (i.e., the air-fuel ratio) of the exhaust gas G based on the magnitude of a limiting current generated between a pair of electrodes 62, 63 in a state where a voltage is applied between the electrodes 62, 63.

The gas sensor 5 of the present embodiment will be described in detail below. In the following description, the side of the gas sensor 5 in the axial direction X exposed to the measurement gas such as the exhaust gas G is referred to as a tip side X1, and the opposite side is referred to as a base side X2.

(gas sensor)

The gas sensor 5 is used in an exhaust pipe of an internal combustion engine such as a vehicle. The limiting current type gas sensor 5 is used as an air-fuel ratio sensor for quantitatively detecting the air-fuel ratio of the exhaust gas G flowing through the exhaust pipe as in the present embodiment. The gas sensor 5 can quantitatively determine the air-fuel ratio of the exhaust gas G in both the case where the air-fuel ratio is on the rich side and the case where the air-fuel ratio is on the lean side.

Here, the air-fuel ratio of the exhaust gas G refers to a mixture ratio of fuel and air at the time of combustion in the internal combustion engine. The rich side is a side where the air-fuel ratio of the exhaust gas G is higher than the theoretical air-fuel ratio when the fuel and air are completely combusted. The lean side refers to a side where the air-fuel ratio of the exhaust gas G is less than the stoichiometric air-fuel ratio.

In the gas sensor 5 of the present embodiment, the air-fuel ratio of the exhaust gas can be detected by detecting the oxygen concentration of the exhaust gas. The gas sensor 5 as an air-fuel ratio sensor substantially detects the oxygen concentration of the exhaust gas G on the lean side, while detecting the unburned gas concentration of the exhaust gas G on the rich side.

As shown in fig. 19, the gas sensor 5 includes a housing 71, a distal end side cover 72, a proximal end side cover 73, and the like, in addition to the sensor element 6. The housing 71 is attached to the exhaust pipe and holds the sensor element 6 via an insulator 74. The front end cover 72 is attached to the front end side X1 of the housing 71 and covers the sensor element 6. The front end cover 72 has a double structure and is composed of an inner cover 721 and an outer cover 722. The base end cover 73 is attached to the base end side X2 of the housing 71 and covers the terminals 75 for electric wiring of the sensor element 6 and the like.

(sensor element)

As illustrated in fig. 20, for example, a stacked sensor element can be used as the sensor element 6. That is, the sensor element 6 may be formed of a laminate in which the reference electrode 63, the plate-shaped solid electrolyte 1, and the detection electrode 62 are laminated in this order.

As shown in fig. 20, the sensor element 6 has, for example, a plate-shaped solid electrolyte 1. The solid electrolyte 1 has a measurement gas surface 601A and a reference gas surface 602A. The measurement gas surface 601A is a surface exposed to a measurement gas such as exhaust gas G, and serves as a gas contact portion that contacts the measurement gas. On the other hand, the reference gas surface 602A is a surface exposed to a reference gas such as the atmosphere a. The measurement gas surface 601A and the reference gas surface 602A are surfaces of the solid electrolyte 1 that face each other.

The detection electrode 62 is provided on the measurement gas surface 601A of the solid electrolyte 1. On the other hand, the reference electrode 63 is provided on the reference gas surface 602A. When the sensor element 6 is formed of such a laminated sensor element, the heating element 641 constituting the heater 64 is laminated with the solid electrolyte 1 through the insulator 642. The insulator 642 is made of, for example, alumina.

The detection electrode 62 faces the measurement gas chamber 68. The measurement gas passing through the porous diffusion barrier 66 is introduced into the measurement gas chamber 68. The measurement gas chamber 68 is a space surrounded by the solid electrolyte 1, the measurement gas chamber formation layer 681, and the diffusion barrier layer 66. The detection electrode 62 is formed in contact with the solid electrolyte 1, and the measurement gas chamber formation layer 681, which is a structural member of the measurement gas chamber 68, is formed in contact with the solid electrolyte 1. The detection electrode 62 is exposed to a measurement gas such as exhaust gas G, and detects the gas together with the reference electrode 63. The detection electrode 62 is electrically connected to a terminal 75 to which a lead wire 76 is connected.

The reference electrode 63 faces the reference gas chamber 69. The reference gas such as the atmosphere a is introduced into the reference gas chamber 69 from the base end side X2 through the passage hole 731 of the base end side cover 73. In addition, as the sensor element 6, a cup-shaped sensor element described later may be used instead of the laminated sensor element.

The detection electrode 62 is exposed to the measurement gas such as the exhaust gas G flowing into the front end side cover 42 through the passage holes 723, 724, 725 provided in the front end side cover 72. The reference electrode 63 is exposed to a reference gas such as the atmosphere a flowing from the inside of the base end side cover 73 into the reference gas chamber 69 of the solid electrolyte 1 through a passage hole 731 provided in the base end side cover 73.

The heater 64 generates heat by energization, and heats the solid electrolyte 1 and the electrodes 62 and 63 to an active temperature at the time of starting the internal combustion engine and the gas sensor 5. The heater 64 includes an insulator 642 made of an alumina sintered body and a heating element 641 formed therein. The alumina sintered body constituting the insulator 642 is in contact with the solid electrolyte. The insulator 642 constituting the heater 64 is also a structural member forming the reference gas chamber 69, and also functions as a reference gas chamber forming layer.

Further, in the solid electrolyte 1, a measurement gas chamber formation layer 681 constituting the measurement gas chamber 68 is formed in a layered manner on the measurement gas surface 601A side. The measurement gas chamber formation layer 681 is formed of alumina. That is, the solid electrolyte 1 is in contact with the insulator 642 constituting the heater 64 on the reference gas surface 602A side, and is in contact with the measurement gas chamber formation layer 681 on the measurement gas surface 601A side. That is, the solid electrolyte 1 has a contact portion 1A that is in contact with the measurement gas chamber formation layer 681 and the insulator 642, which are different material members.

The diffusion barrier layer 66 is formed of, for example, a porous body of spinel. Further, a shield layer 60 made of alumina is provided on the surface of the diffusion barrier layer 66. The shielding layer 60 is made of a dense body that is impermeable to gas. The exhaust gas G flowing into the distal-side cover 72 reaches the measurement section 50 of the detection electrode 62 through the diffusion barrier 66. In the structure of the sensor element 6 illustrated in fig. 20, the diffusion barrier layer 66 is not in contact with the solid electrolyte 1, but a structure in which the diffusion barrier layer 66 is in contact with the solid electrolyte 1 may be employed.

(solid electrolyte)

The solid electrolyte 1 is formed of stabilized zirconia or partially stabilized zirconia. Specifically, the solid electrolyte 1 described in embodiment 1 is used. The solid electrolyte 1 is excellent in thermal shock resistance, and can maintain high strength even when exposed to a cold-heat cycle in a high temperature region exceeding 1050 ℃. Therefore, for example, even when the gas sensor 5 is used in an environment of over 1050 ℃, the gas sensor 5 can detect the measurement gas while maintaining high reliability. Further, the solid electrolyte can suppress a decrease in strength even in an environment where a cooling-heating cycle is repeated many times. Therefore, even when the gas sensor is used in an environment in which the heating and cooling frequencies are high, the gas sensor can detect the measurement gas while maintaining high reliability.

(electrode)

The material of the detection electrode 62 of the present embodiment is not particularly limited as long as it is a material having catalytic activity for oxygen or the like. For example, the detection electrode 62 may contain any one of Pt (platinum), Au (gold), Ag (silver), a mixture or alloy of Pd (palladium) and Ag, and a mixture or alloy of Pt and Au as a noble metal component. The material of the reference electrode 63 is not particularly limited, and Pt, Au, Ag, Pd, or the like may be contained as the noble metal component.

As the sensor element 6, instead of the laminated sensor element, for example, a bottomed cylindrical (specifically, cup-shaped) sensor element may be used as illustrated in fig. 21. Such a cup-shaped sensor element has a solid electrolyte 1 having a bottomed cylindrical shape (specifically, a cup-shaped shape), a detection electrode 62, and a reference electrode 63. The detection electrode 62 is provided on the outer peripheral surface 601A of the solid electrolyte 1. The reference electrode 63 is provided on the inner peripheral surface 602A of the solid electrolyte 1. In such a cup-shaped sensor element, a rod-shaped heater, not shown, is inserted into the sensor element 6. The heater heats the sensor element 6 to a desired temperature.

The detection electrode 62 is provided on the outer peripheral surface 601A of the solid electrolyte 1. Further, a porous protection layer 625 is formed on the outer peripheral surface 601A of the solid electrolyte. In fig. 21, the protective layer 625 is a porous body, and is formed of, for example, spinel. In the example of fig. 21, the detection electrode 62 is present between the protective layer 625 and the solid electrolyte 1, but the detection electrode 62 is not necessarily formed on the entire outer circumferential surface 601A, and a non-formed portion is usually present. Therefore, although the configuration is not illustrated, there is a portion where the protective layer 625 is in contact with the solid electrolyte 1. That is, the solid electrolyte 1 has a contact portion 1A that is in contact with the protective layer 625 that is a member of a different material. The outer peripheral surface 601 of the tip side X1 of the solid electrolyte 1 serves as a contact portion with the measurement gas such as the exhaust gas G.

The reference electrode 63 is provided on the inner peripheral surface of the cup-shaped solid electrolyte 1, but the reference electrode 63 may be provided on the entire inner peripheral surface or a part of the inner peripheral surface. In the case of being provided on a part of the inner peripheral surface, the alumina constituting the heater and the solid electrolyte may come into contact with each other.

As in the case of the above-described laminated sensor element, the strength against the cooling-heating cycle is improved by using the solid electrolyte 1 of embodiment 1 also for the cup-shaped sensor element. Therefore, even in the gas sensor 5 including the cup-shaped sensor element, the gas sensor 5 can detect the measurement gas while maintaining high reliability.

< Experimental example 1>

A plurality of solid electrolytes were prepared, and their performance was compared and evaluated. The method for producing the solid electrolyte in this example will be described below. In the production of the solid electrolyte, as illustrated in fig. 22, at least the mixing step S1, the firing step S2, and the annealing step S3 are performed, but in the present example, the molding step is performed after the mixing step and before the firing step.

As illustrated in fig. 22, first, yttria powder is added to zirconia powder at a desired ratio, and the mixture is dry-mixed and pulverized. Thus, a mixed powder was obtained.

Subsequently, the mixed powder is mixed with water to obtain a mixed powder slurry. The mixed powder slurry is granulated to improve the fluidity of each raw material particle constituting the mixed powder and to facilitate molding into a desired shape. The granulation is carried out, for example, by spray granulation.

Next, the mixed powder was molded to obtain a molded body. The molding is performed by, for example, powder compaction. Further, grinding can be performed. In this example, the sample was formed into a shape used in the evaluation described later. In this molding step, a desired shape such as the element shape in embodiment 2 can be molded.

Next, the molded body was fired at a temperature of 1400 ℃. Thus, a fired body was obtained. Subsequently, annealing treatment of the fired body is performed. The annealing treatment is performed by heating the fired body at a predetermined annealing temperature. This operation gave a solid electrolyte. In this example, 13 kinds of solid electrolytes of samples 2 to 14 were prepared by changing the content of yttrium oxide, the annealing temperature, and the holding time at the annealing temperature. Further, sample 1 is a solid electrolyte that is not subjected to annealing treatment. Table 1 shows the production conditions of samples 1 to 14. The solid electrolytes of samples 1 to 14 were measured for the rate of change in crystallite size, initial strength, strength after endurance, and rate of decrease in strength in the following manner.

(rate of change in crystallite diameter)

The following cooling-heating cycles were performed on the solid electrolytes of samples 1 to 14: heating from room temperature to 1200 ℃ at a heating rate of 300 ℃/h, and cooling to room temperature at a cooling rate of-300 ℃/h. The change rate of the crystallite particle size of the C phase 21 before and after the cooling-heating cycle was evaluated by X-ray diffraction (XRD). For the evaluation, an "Ultima III horizontal goniometer (D/teX)" manufactured by Rigaku corporation was used, and the measurement was performed by setting the sampling interval to 0.02 ° in the range of 29 to 31 ° in 2 θ/θ scanning continuous scanning. The radiation source uses CuK α radiation to accelerate the voltage: 40kV, current: the measurement was carried out at 30 mA. An example of an XRD pattern is shown in fig. 23. The crystallite diameter D is calculated by the scherrer equation of the following formula (I). In addition, as illustrated in fig. 23, the half width and diffraction angle in the XRD pattern subjected to the smoothing processing are used for calculation of the crystallite diameter. For the smoothing, a software "Peak search" manufactured by Rigaku corporation was used. The treatment conditions were: and (3) treatment: smoothing and weighted averaging: the number of smoothing points is 5 points, and the peak width threshold of BG is removed: 0.1, intensity threshold: 0.01. in this example, "the change rate of the crystallite particle size in the C phase" means "substantially the C phase and/or the T phase" from the viewpoint of the principle of measurement by XRD analysis.

D＝K×λ/(β×cosθ)(I)

D: crystallite particle size, K: constant (K ═ 0.9), λ: x-ray wavelength (λ ═ 1.5418nm), β: half width, θ: angle of diffraction

(initial Strength, Strength after durability)

The solid electrolytes of samples 1 to 14 were cut into pieces having a width of about 5mm, a length of 45mm and a thickness of 5mm to prepare measurement samples. The measurement sample was subjected to the following cooling-heating cycles 1000 times: heating from room temperature to 1100 deg.c at a heating rate of 300 deg.c/hr, and cooling to room temperature at a cooling rate of-300 deg.c/hr. Of the measurement samples not subjected to the cooling-heating cycle and the measurement samples subjected to the cooling-heating cycle 1000 times, the mass ratio of the sample to the sample was determined in accordance with JIS R1601: in the 4-point bending test of 2008, a strength evaluation sample was prepared, and the 4-point bending strength was measured. For the 4-point bending strength, 10 strength evaluation samples were prepared from each sample, and the arithmetic mean result thereof was used. The 4-point bending strength of the sample not subjected to the cooling-heating cycle was defined as the initial strength, and the 4-point bending strength of the sample subjected to the cooling-heating cycle 1000 times was defined as the post-endurance strength.

(intensity reduction ratio and judgment)

The reduction rate of the strength after endurance with respect to the initial strength was defined as the strength reduction rate. The case where the intensity reduction rate is 50% or more is determined as "C", the case where the intensity reduction rate is less than 50% and 40% or more is determined as "B", and the case where the intensity reduction rate is less than 40% is determined as "a".

TABLE 1

As is clear from table 1 and fig. 24, when the rate of change in crystallite particle size is 10% or less, the rate of decrease in strength is low. That is, the thermal shock resistance is excellent. From the viewpoint of further improving the thermal shock resistance, the rate of change in crystallite particle size is more preferably 6% or less, and still more preferably 2% or less.

As is clear from table 1, fig. 25, and fig. 26, the annealing temperature is preferably 800 to 1150 ℃, more preferably 900 to 1100 ℃, and still more preferably 950 to 1000 ℃ from the viewpoint of reducing the rate of change in crystallite particle size and sufficiently improving the thermal shock resistance. As is clear from table 1 and fig. 27, the holding time at the annealing temperature is preferably 0.75 hours or more, and more preferably 1 hour or more, from the viewpoint of sufficiently reducing the rate of change in the crystallite particle size. On the other hand, from the viewpoint of shortening the holding time to improve the productivity of the solid electrolyte, the holding time at the annealing temperature is preferably 3 hours or less, more preferably 2 hours or less.

From the viewpoint of becoming suitable for a solid electrolyte of a sensor element of a gas sensor, the initial strength is preferably 350MPa or more. The strength after aging is preferably 200MPa or more from the viewpoint of being suitable for a solid electrolyte of a laminated sensor element, and more preferably 250MPa or more from the viewpoint of being suitable for a laminated sensor element and a cup-shaped sensor element. The strength reduction rate is preferably less than 50%, more preferably less than 40%, and still more preferably 30% or less.

The present application is not limited to the above embodiments, and can be applied to various embodiments without departing from the scope of the present application. For example, the solid electrolyte in embodiment 1 can be used for a Solid Oxide Fuel Cell (SOFC). In this case, the solid electrolyte has, for example, contact surfaces with the anode layer and the cathode layer. Although the configuration is not shown, a solid electrolyte can be applied to a fuel cell in which an anode layer, an electrolyte layer made of a solid electrolyte, and a cathode layer are sequentially stacked. Further, a stack-type fuel cell can be constructed by stacking a plurality of fuel cells with separators interposed therebetween. Further, as the gas sensor, there are an oxygen sensor, an NOx sensor, and the like in addition to the air-fuel ratio sensor, and a solid electrolyte can be applied to these sensors.

The present application has been described with reference to the embodiments, but it should be understood that the present application is not limited to the embodiments and the structures. The present application also includes various modifications and variations within an equivalent range. In addition, various combinations and modes and other combinations and modes including only one element, more than one element or less than one element are also included in the scope and thought range of the present application.