WO2017034037A1

WO2017034037A1 - A/f sensor and method of manufacturing same

Info

Publication number: WO2017034037A1
Application number: PCT/JP2016/075136
Authority: WO
Inventors: 竜己宇治山; 鈴木　康文
Original assignee: 株式会社デンソー
Priority date: 2015-08-27
Filing date: 2016-08-29
Publication date: 2017-03-02

Abstract

An A/F sensor 1 is provided with a solid electrolyte body 2 and a heater 6. The A/F sensor 1 is provided downstream, in exhaust gas g, of a purifying device 10 which purifies the exhaust gas g. The solid electrolyte body 2 is formed in the shape of a cup. A measuring electrode 4 which comes into contact with the exhaust gas g is formed on the outer surface 21 of the solid electrolyte body 2, and a reference electrode which comes into contact with a reference gas is formed on the inner surface 22 of the solid electrolyte body 2. The solid electrolyte body 2 is formed from zirconia. The solid electrolyte body 2 is provided with a detecting portion 20 which is sandwiched between the measuring electrode 4 and the reference electrode 5, and through which oxygen ions are conducted. The detecting portion 20 has a cubic phase proportion of at least 88 mol%.

Description

A / F sensor and manufacturing method thereof

The present disclosure relates to an A / F sensor for measuring an air-fuel ratio of exhaust gas and a manufacturing method thereof.

As an A / F sensor for measuring the air-fuel ratio of exhaust gas, a configuration including a solid electrolyte body having oxygen ion conductivity is known (see Patent Document 1). The solid electrolyte body is made of, for example, zirconia and is formed in a plate shape or a cup shape. A measurement electrode that contacts the exhaust gas is provided on one surface of the solid electrolyte body. On the other surface, a reference electrode that contacts a reference gas such as the atmosphere is provided.

Part of the solid electrolyte body is a detection part sandwiched between the measurement electrode and the reference electrode. In the A / F sensor, when the detection unit is heated to the activation temperature, oxygen ions move in the detection unit from the reference electrode to the measurement electrode or from the measurement electrode to the reference electrode. By measuring the current generated at this time, the oxygen concentration in the exhaust gas is measured, and the air-fuel ratio is calculated.

The A / F sensor is used in an engine control system such as a vehicle. In the engine control system, feedback control of the engine is performed using the air-fuel ratio value measured by the A / F sensor. Thus, the air-fuel ratio of the exhaust gas is controlled to reduce harmful substances in the exhaust gas.

Also, the A / F sensor is attached to the exhaust pipe of the engine. The exhaust pipe is provided with a purification device for purifying the exhaust gas. The A / F sensor is often provided on the upstream side of the exhaust gas from the purification device.

In recent years, developments have been made to further reduce harmful substances in exhaust gas. For this purpose, it is considered effective to control the air-fuel ratio of exhaust gas with higher accuracy. For example, an A / F sensor is provided not only on the upstream side of the purification device but also on the downstream side, and the A / F sensor on the downstream side is used to control the air-fuel ratio of the exhaust gas that has passed through the purification device with higher accuracy. To do. Thereby, it has been studied to further reduce harmful substances contained in the exhaust gas that has passed through the purification device.

JP 2014-122878 A

However, with the conventional A / F sensor, it is difficult to obtain a high air-fuel ratio measurement accuracy required for the A / F sensor provided on the downstream side. That is, the conventional A / F sensor has a relatively high electrical resistance of the detection unit. When the electric resistance of the detection unit is high, the value of the electric resistance tends to vary from one A / F sensor to another. For this reason, the value of the current flowing through the detection unit is likely to vary for each A / F sensor, and the air-fuel ratio measurement accuracy is likely to be lowered.

The present disclosure aims to provide an A / F sensor provided on the downstream side of the exhaust gas purifying device and capable of more accurately measuring the air-fuel ratio of the exhaust gas, and a manufacturing method thereof.

A first aspect of the technology of the present disclosure is an A / F sensor (1) that is provided on the downstream side of the exhaust gas with respect to the purification device (10) that purifies the exhaust gas and measures the air-fuel ratio of the exhaust gas, A cup-shaped solid electrolyte body (2) whose front end is closed and whose base end is open, a reference gas chamber (3) formed inside the solid electrolyte body and into which a reference gas is introduced, and an outer surface of the solid electrolyte body The measurement electrode (4) formed in (21) and in contact with the exhaust gas, the reference electrode (5) formed in the inner surface (22) of the solid electrolyte body and in contact with the reference gas, and the reference gas chamber And a heater (6) for heating the solid electrolyte body, wherein the solid electrolyte body is composed of zirconia, and the solid electrolyte body is interposed between the measurement electrode and the reference electrode, Detection unit (20) that conducts oxygen ions Includes, 該検 knowledge unit, the proportion of the cubic phase is not less than 88 mol%.

A second aspect of the technology of the present disclosure is a method for manufacturing the A / F sensor, in which a fired body (29) is manufactured by firing the unfired body (28) of the solid electrolyte body. Performing a firing step, and an energization step of causing a ratio of the cubic phase in the detection unit to be 88 mol% or more by flowing a current between the measurement electrode and the reference electrode formed on the fired body. .

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention can greatly reduce the electrical resistance of the detection unit when the proportion of the cubic phase in the detection unit is 88 mol% or more. We found that it can be reduced. That is, zirconia crystals include a cubic phase (hereinafter also referred to as “C phase”), a monoclinic phase (hereinafter also referred to as “M phase”), and a tetragonal phase (hereinafter also referred to as “T phase”). The M phase and the T phase have high electric resistance, but the C phase has low electric resistance. Therefore, in the A / F sensor of the present disclosure, the electrical resistance of the detection unit can be lowered by sufficiently increasing the amount of the C phase in the detection unit. Therefore, variation in electrical resistance in the detection unit can be reduced, and variation in current flowing in the detection unit can be reduced. Thereby, the air-fuel ratio of exhaust gas can be measured with high accuracy. Therefore, in the A / F sensor of the present disclosure, the air-fuel ratio of the exhaust gas can be controlled with higher accuracy, and harmful substances in the exhaust gas can be further reduced.

Moreover, in the said 2nd aspect in the technique of this indication, the said baking process and the said electricity supply process are performed.
It is difficult to make the proportion of the C phase of the detection part 88 mol% or more only by performing the firing step. However, if the energization process in which a current is passed between the measurement electrode and the reference electrode is performed, the C phase of the detection unit can be easily increased to 88 mol% or more. Therefore, in the sensor manufacturing method of the present disclosure, the A / F sensor can be easily manufactured.

As described above, according to the technology of the present disclosure, it is possible to provide an A / F sensor that is provided on the downstream side of the exhaust gas purification device and that can more accurately measure the air-fuel ratio of the exhaust gas, and a method for manufacturing the A / F sensor.
In addition, the code | symbol in the parenthesis described in the means to solve a claim and a subject shows the correspondence with the specific means as described in embodiment mentioned later. Therefore, these codes do not limit the technical scope of the present disclosure.

FIG. 1 is an enlarged cross-sectional view of a main part of a solid electrolyte body in the first embodiment. FIG. 2 is a side view of the solid electrolyte body in the first embodiment. FIG. 3 is a cross-sectional view of the A / F sensor according to the first embodiment. FIG. 4 is a view for explaining the mounting position of the A / F sensor in the first embodiment. FIG. 5 is a diagram for explaining a manufacturing process of each sample in the first embodiment. FIG. 6 is a graph showing the relationship between the C phase ratio of the detection unit and the electrical resistance of the detection unit in Experimental Example 1. FIG. 7 shows the XRD analysis results of the detection unit in the range of 20 to 90 ° when 2θ is in the range of 20 to 90 °, where θ is the incident angle of X-rays (CuKα rays) in Experimental Example 1. FIG. 8 is a graph showing the relationship between the electrical resistance of the detection unit, the ratio of the C phase, and the temperature of the detection unit in Experimental Example 2. FIG. 9 is a graph showing the relationship between the electrical resistance of the detection unit, the ratio of the C phase, and the area of the detection unit in Experimental Example 3. FIG. 10 is a graph showing the relationship between the electrical resistance of the detection unit, the ratio of the C phase, and the thickness of the detection unit in Experimental Example 4. FIG. 11 is a graph showing the relationship between the bending strength of the solid electrolyte body and the temperature for each fracture probability in Experimental Example 5.

The A / F sensor of the present disclosure can be applied to an in-vehicle sensor for measuring an air-fuel ratio of exhaust gas discharged from a vehicle engine.

(Embodiment 1)
An embodiment according to the A / F sensor of the present disclosure will be described with reference to FIGS. In the following description, the “front end side” is one side of the A / F sensor in the axial direction (Z1 side in the Z direction) and refers to the side where the sensor is exposed to the exhaust gas that is the gas to be measured. The “base end side” refers to the opposite side (Z2 side in the Z direction). As shown in FIG. 4, the A / F sensor 1 of the present embodiment is provided on the downstream side of the exhaust gas g with respect to the purification device 10 that purifies the exhaust gas g in the flow path of the exhaust gas g. The A / F sensor 1 is provided for measuring the air-fuel ratio of the exhaust gas g.

As shown in FIG. 3, the A / F sensor 1 includes a solid electrolyte body 2 and a heater 6. The solid electrolyte body 2 is formed in a cup shape in which the distal end is closed and the proximal end is opened. A reference gas chamber 3 into which a reference gas such as the atmosphere is introduced is formed inside the solid electrolyte body 2. A heater 6 is disposed in the reference gas chamber 3.

As shown in FIG. 1, a measurement electrode 4 that contacts the exhaust gas g is formed on the outer surface 21 of the solid electrolyte body 2. A reference electrode 5 in contact with the reference gas is formed on the inner surface 22 of the solid electrolyte body 2.

The solid electrolyte body 2 is composed of zirconia (ZrO ₂ ). The solid electrolyte body 2 includes a detection unit 20 that is interposed between the measurement electrode 4 and the reference electrode 5 and that conducts oxygen ions. The detector 20 has a cubic phase ratio of 88 mol% or more.

The A / F sensor 1 of the present embodiment is an in-vehicle sensor for measuring the air-fuel ratio of exhaust gas generated from a vehicle engine.

As shown in FIG. 2, the solid electrolyte body 2 includes a diameter-enlarged portion 25 that is partially enlarged. A portion 23 located on the distal end side (Z1 side in the Z direction) with respect to the enlarged diameter portion 25 is exposed to the exhaust gas g. On the other hand, an output extraction portion 24 is formed at a position closer to the base end side (Z2 side in the Z direction) than the enlarged diameter portion 25. The output extraction part 24 is not exposed to the exhaust gas g. A terminal portion 42 is formed on the outer surface of the output extraction portion 24. The measurement electrode 4 and the terminal part 42 are connected by a lead part 41.

The measurement electrode 4 is formed in an annular shape so as to surround the solid electrolyte body 2. The length L (see FIG. 1) of the measurement electrode 4 in the axial direction (Z direction) of the solid electrolyte body 2 is set to 3 mm or less. In the present embodiment, the reference electrode 5 is formed on the entire inner surface 22 of the solid electrolyte body 2. The measurement electrode 4 and the reference electrode 5 are each made of platinum (Pt).

As shown in FIG. 1, a diffusion layer 211 and a trap layer 212 are formed on the outer surface 21 of the solid electrolyte body 2. The diffusion layer 211 and the trap layer 212 cover the measurement electrode 4. The diffusion layer 211 is made of aluminum oxide, magnesium oxide, and spinel (alumina-magnesia-spinel), and the trap layer 212 is made of porous alumina. The exhaust gas g contacts the measurement electrode 4 through the diffusion layer 211 and the trap layer 212. The diffusion layer 211 is provided to control the diffusion rate of the exhaust gas g. The trap layer 212 is provided to collect poisonous substances in the exhaust gas g.

In the A / F sensor 1, when the temperature of the detection unit 20 is raised to the activation temperature using the heater 6 (see FIG. 3), oxygen ions are converted into the interior of the solid electrolyte body 2 when the exhaust gas g is in a rich atmosphere. Is moved from the reference electrode 5 to the measurement electrode 4. On the other hand, when the exhaust gas g is in a lean atmosphere, oxygen ions move from the measurement electrode 4 to the reference electrode 5. At this time, the A / F sensor 1 measures the oxygen concentration in the exhaust gas g by measuring the value of the current flowing between the measurement electrode 4 and the reference electrode 5, and calculates the air-fuel ratio of the exhaust gas g. It is configured.

As described above, in the detection unit 20 of the present embodiment, the C phase is 88 mol% or more. In the solid electrolyte body 2 other than the detection unit 20, the C phase is less than 88 mol%. More specifically, it is 87 mol% or less. Further, the solid electrolyte body 2 of the present embodiment contains yttrium oxide (Y ₂ O ₃ ) in the range of 4.5 to 6 mol%.

In this embodiment, the area of the detection unit 20 is in the range of 20 to 40 mm ² , and the thickness Th of the detection unit 20 is in the range of 0.5 to 2 mm.

Next, a method for manufacturing the solid electrolyte body 2 will be described. In order to manufacture the solid electrolyte body 2, a process as shown in FIG. 5 is performed. In this manufacturing method, first, ZrO ₂ powder and Y ₂ O ₃ powder are mixed and formed into a cup shape, and the green body 28 of the solid electrolyte body 2 is produced. Thereafter, the green body 28 is fired (firing step). Then, the measurement electrode 4 and the reference electrode 5 are formed on the fired body 29 by plating. Thereafter, the diffusion layer 211 is plasma sprayed on the surface of the fired body 29, and further a slurry that becomes the trap layer 212 is applied, followed by drying and firing.

In the fired body 29, the proportion of the C phase is usually 87 mol% or less in all the parts. Such a fired body 29 can be formed by the following method, for example. Specifically, when the composition of the unfired body 28 is 94% ZrO ₂ and 6% Y ₂ O ₃ and fired at 1100 ° C. for about 24 hours, the proportion of the C phase is 87 mol% in all parts. A fired body 29 is formed.

Next, in this manufacturing method, the heater 6 is disposed inside the fired body 29, and a current is passed between the measurement electrode 4 and the reference electrode 5 while the fired body 29 is heated by the heater 6 (energization process). In the energization process, for example, the temperature of the detection unit 20 is set to 850 ° C., and a current of 260 mA is passed between the measurement electrode 4 and the reference electrode 5. If it does in this way, the crystal structure of the detection part 20 will change and the ratio of C phase will increase. In this manufacturing method, the ratio of the C phase in the detection unit 20 can be 88 mol% or more by performing the energization process for a predetermined time.

When the air / fuel ratio of the exhaust gas g is measured using the A / F sensor 1, the heater 20 is used to heat the detection unit 20 within the range of 600 to 1000 ° C. When measuring the air-fuel ratio, the current flowing through the detection unit 20 is very small (the flowing current is small). Therefore, the ratio of the C phase does not change while measuring the air-fuel ratio. In contrast, in the energization step, the temperature of the detection unit 20 is set to 850 ° C., and a larger current (a current of about several hundred mA) is passed through the detection unit 20 than when the air-fuel ratio is measured. As described above, when a large current is passed, the crystal structure of the detection unit 20 changes, and the proportion of the C phase increases.

Next, the overall structure of the A / F sensor 1 will be described. As shown in FIG. 3, the A / F sensor 1 includes the solid electrolyte body 2, the housing 14, the wiring 15 (15a, 15b), the heater wiring 16, the cover 17 (17a, 17b), and the atmosphere. A side cover 18 (18a to 18c) and a seal portion 19 are provided. The solid electrolyte body 2 is fixed in the housing 14.

Of the two wires 15 (15a, 15b), one wire 15a is electrically connected to the terminal portion 42 (see FIG. 2). The other wiring 15 b is electrically connected to the reference electrode 5 formed on the inner surface of the output extraction portion 24. The heater wiring 16 is electrically connected to the heater 6.

The tip of the solid electrolyte body 2 is protected by two covers 17 (17a, 17b). Openings 170 are respectively formed in the

covers

17a and 17b. The exhaust gas g enters the inside of the cover 17 through the opening 170.

Further, a shoulder 140 is formed at a portion located on the base end side (Z2 side in the Z direction) of the housing 14 in the axial direction (Z direction). A spring member 141 is arranged at a position on the tip side of the shoulder 140 (Z1 side in the Z direction). In the present embodiment, the shoulder portion 140 is caulked to pressurize the solid electrolyte body 2 toward the distal end side in the axial direction (Z direction) and press the expanded diameter portion 25 against the housing 14. Thereby, the exhaust gas g is prevented from leaking from between the enlarged diameter portion 25 and the housing 14.

Three atmosphere side covers 18 (18a, 18b, 18c) are provided at a position on the base end side of the housing 14 in the axial direction (Z direction). A seal portion 19 is disposed at a base end side portion of the atmosphere side covers 18b and 18c. The wiring 15 and the heater wiring 16 pass through the inside of the seal portion 19. In the present embodiment, the seal portion 19 is fixed by caulking the atmosphere side covers 18b and 18c. In addition, a through portion 180 is formed in the atmosphere side covers 18b and 18c. In the present embodiment, the atmospheric air, which is a reference gas, is introduced into the reference gas chamber 3 inside from the outside of the A / F sensor 1 through the penetration part 180.

Next, the mounting position of the A / F sensor 1 will be described. As shown in FIG. 4, the A / F sensor 1 of this embodiment is attached to the exhaust pipe 12. The exhaust pipe 12 is connected to the engine 11 (internal combustion engine). The exhaust pipe 12 is provided with a purification device 10 that purifies the exhaust gas g.

The exhaust pipe 12 includes an upstream portion 12a that connects between the purification device 10 and the engine 11 and a downstream portion 12b that is provided on the downstream side of the purification device 10 in the flow path of the exhaust gas g. The A / F sensor 1 is attached to the downstream portion 12 b of the exhaust pipe 12. An upstream air-fuel ratio sensor 8 is attached to the upstream portion 12a.

The A / F sensor 1 and the upstream air-fuel ratio sensor 8 are connected to the control circuit unit 13. Thus, in the present embodiment, the A / F sensor 1, the upstream air-fuel ratio sensor 8, and the control circuit unit 13 constitute an engine control system 100 that controls the engine 11.

The control circuit unit 13 calculates the air-fuel ratio of the exhaust gas g upstream of the purifier 10 based on the output signal of the upstream air-fuel ratio sensor 8. The control circuit unit 13 calculates the air-fuel ratio of the exhaust gas g on the downstream side of the purification device 10 based on the output signal of the A / F sensor 1. The control circuit unit 13 performs feedback control of the engine 11 using these measured values of the air-fuel ratio. In the engine control system 100 of the present embodiment, the engine 11 is roughly controlled using the value of the air-fuel ratio measured by the upstream air-fuel ratio sensor 8. Further, the engine 11 is precisely controlled using the air-fuel ratio value measured by the A / F sensor 1. As a result, the engine control system 100 accurately controls the air-fuel ratio of the exhaust gas g. For this reason, the A / F sensor 1 of the present embodiment is required to have high air / fuel ratio measurement accuracy.

The purification device 10 includes a honeycomb structure 101 and a catalyst layer formed on the surface of the honeycomb structure 101. The honeycomb structure 101 is made of cordierite or the like, and has a plurality of cells through which the exhaust gas g passes. The catalyst layer contains a noble metal catalyst such as Pt or palladium (Pd). The exhaust gas g comes into contact with the noble metal catalyst when passing through the cell. Thereby, the engine control system 100 of the present embodiment is configured to purify harmful substances such as NOx and CO contained in the exhaust gas g.

The effect of the A / F sensor 1 of the present embodiment will be described. As described above, the detection unit 20 in the A / F sensor 1 of the present embodiment has a C phase ratio of 88 mol% or more. If it does in this way, in A / F sensor 1, the electrical resistance of detection part 20 in solid electrolyte object 2 can be reduced significantly, and the variation in this electrical resistance can be reduced. That is, zirconia crystals include a C phase, an M phase, and a T phase. The M phase and the T phase have high electric resistance, but the C phase has low electric resistance. Therefore, in the A / F sensor 1, the electrical resistance of the detection unit 20 can be lowered by sufficiently increasing the amount of the C phase in the detection unit 20. Therefore, variation in electrical resistance in the detection unit 20 can be reduced, and variation in current flowing in the detection unit 20 can be reduced. Thereby, the air-fuel ratio of the exhaust gas g can be measured with high accuracy. Therefore, in the A / F sensor 1 of the present embodiment, the air-fuel ratio of the exhaust gas g can be controlled with higher accuracy, and harmful substances in the exhaust gas g can be further reduced.

Moreover, in this embodiment, the length L (refer FIG. 1) of the measurement electrode 4 in the axial direction (Z direction) of the solid electrolyte body 2 is 3 mm or less.
Therefore, in this embodiment, the usage-amount of the noble metal which comprises the measurement electrode 4 can be reduced, and the manufacturing cost of the A / F sensor 1 can be reduced. If the length L is shortened, the measurement accuracy of the air-fuel ratio tends to be lowered, but in this embodiment, the amount of phase C is set to 88 mol% or more, so that the measurement accuracy of the air-fuel ratio can be improved. Therefore, in the present embodiment, it is possible to achieve both a reduction in manufacturing cost of the A / F sensor 1 and an improvement in air-fuel ratio measurement accuracy.

Moreover, the detection part 20 of this embodiment has 1 mol% or more C phases more than parts other than the detection part 20 among the solid electrolyte bodies 2. FIG.
Therefore, in this embodiment, the solid electrolyte body 2 can be manufactured easily. That is, when the solid electrolyte body 2 is manufactured, as described above, the unfired body 28 (see FIG. 5) is fired to create the fired body 29. However, it is difficult to obtain a fired body 29 having a C phase ratio of 88 mol% or more. On the other hand, it is relatively easy to obtain a fired body 29 having a C phase ratio of 87 mol% or less. Therefore, in the present embodiment, for example, a fired body 29 having a C phase ratio of 87 mol% is created, and then the above energization process is performed to increase the C phase in the detection unit 20 by 1 mol% or more. Thereby, in this embodiment, the solid electrolyte body 2 whose C phase of the detection part 20 is 88 mol% or more can be manufactured easily.
As will be described later, when the C phase is increased from 87 mol% to 88 mol% or more, the electrical resistance of the detection unit 20 is greatly reduced (see FIG. 6). Therefore, if the proportion of the C phase in the detection unit 20 is increased by 1 mol% or more from 87 mol%, which is the proportion of other parts, the electrical resistance of the detection unit 20 can be greatly reduced.

Further, the solid electrolyte body 2 of the present embodiment contains Y ₂ O ₃ in the range of 4.5 to 6 mol%. In this way, in the A / F sensor 1, the thermal expansion coefficient of the solid electrolyte body 2 can be made substantially equal to the thermal expansion coefficient of the porous alumina constituting the diffusion layer 211 (see FIG. 1) and the trap layer 212. Therefore, in the present embodiment, when the solid electrolyte body 2 is heated by the heater 6, it is difficult for thermal stress to be applied to the solid electrolyte body 2.

Moreover, in the detection part 20 of this embodiment, it is preferable that the ratio of C phase shall be 95 mol% or less.
When the solid electrolyte body 2 is manufactured, if the amount of current flowing through the detection unit 20 is increased, the proportion of the C phase increases. However, if the C phase is increased to 95 mol% or more, problems such as reduction of ZrO ₂ to zirconium (Zr) occur. Therefore, the ratio of the C phase is desirably 95 mol% or less.

Further, the A / F sensor 1 of the present embodiment is configured to heat the detection unit 20 within the range of 600 to 1000 ° C. by the heater 6 when measuring the air-fuel ratio of the exhaust gas g.
When the temperature of the detection unit 20 is less than 600 ° C., the electrical resistance of the detection unit 20 cannot be sufficiently reduced as will be described later. Moreover, when exceeding 1000 degreeC, temperature is too high and the intensity | strength of the solid electrolyte body 2 will fall easily. Furthermore, when the temperature of the detection part 20 exceeds 1000 degreeC, the problem that the power consumption of the heater 6 becomes high also arises. Therefore, when measuring the air-fuel ratio, it is preferable to set the temperature of the detection unit 20 within the range of 600 to 1000 ° C. It is more preferable that the temperature of the detection unit 20 when measuring the air-fuel ratio is in the range of 650 to 800 ° C.

In the present embodiment, the area of the detection unit 20 (the area of the measurement electrode 4) is set to 40 mm ² or less. When the area of the detection unit 20 exceeds 40 mm ² , the area is too large, so that the amount of noble metal used for the measurement electrode 4 is likely to increase. Therefore, the manufacturing cost of the A / F sensor 1 is likely to increase. Therefore, the area of the detection unit 20 is preferably 40 mm ² or less.

In the present embodiment, the area of the detection unit 20 (the area of the measurement electrode 4) is set to 20 mm ² or more. Since the area of the measurement electrode 4 varies in size (manufacturing variation), if the area is reduced to less than 20 mm ² , the influence of this variation increases. For this reason, the variation in electric resistance in the detection unit 20 becomes large, and the measurement accuracy of the air-fuel ratio tends to be lowered. Therefore, the area of the detection unit 20 is preferably 20 mm ² or more.

In the present embodiment, the thickness Th of the detection unit 20 is 2 mm or less. When the thickness Th of the detection unit 20 exceeds 2 mm, the electrical resistance of the detection unit 20 becomes too high, and the measurement accuracy of the air-fuel ratio tends to decrease. Therefore, the thickness Th of the detection unit 20 is preferably 2 mm or less.

In this embodiment, the thickness Th of the detection unit 20 is set to 0.5 mm or more. When the thickness Th of the detection unit 20 is less than 0.5 mm, the strength of the detection unit 20 tends to decrease as will be described later. Therefore, the thickness Th of the detection unit 20 is preferably 0.5 mm or more.

Moreover, in this embodiment, when manufacturing the A / F sensor 1, the said baking process and the said electricity supply process are performed. In the firing step, the unfired body 28 (see FIG. 5) of the solid electrolyte body 2 is fired to produce a fired body 29. In the energization process, a current is passed between the measurement electrode 4 and the reference electrode 5 formed on the fired body 29 so that the ratio of the C phase in the detection unit 20 is 88 mol% or more. Thereby, the solid electrolyte body 2 is formed.
Thus, in this embodiment, the ratio of the C phase in the detection part 20 can be increased easily by performing the said electricity supply process. Therefore, it is easy to manufacture the solid electrolyte body 2 in the manufacturing method of the present embodiment.

In addition, the ratio of the C phase in the detection unit 20 is more preferably 88.5 mol% or more. In the case of 88.5 mol% or more, the variation in the electrical resistance of the detection unit 20 can be further suppressed (see FIG. 6).

As described above, according to the present embodiment, it is possible to provide an A / F sensor 1 that is provided on the downstream side of the exhaust gas purification device 10 and that can more accurately measure the air-fuel ratio of the exhaust gas, and a manufacturing method thereof.

(Experimental example 1)
An experiment was conducted to confirm the operational effects of the A / F sensor 1 of the present embodiment. First, in this experiment, as shown in Table 1 below, five types of A / F sensor 1 samples (samples 1 to 5) having different C phase ratios in the detection unit 20 were manufactured. And the electrical resistance of each detection part 20 was measured for these samples. Thereby, in this experiment, the relationship between the ratio of C phase and the variation in electrical resistance was investigated.

First, a method for manufacturing the sample will be described. In producing Samples 1 to 5, first, ZrO ₂ powder and Y ₂ O ₃ powder were mixed and formed into a cup shape. Thereby, the unsintered body 28 (refer FIG. 5) of the solid electrolyte body 2 was manufactured. The composition of the green body 28 was such that ZrO ₂ was 94 mol% and Y ₂ O ₃ was 6 mol%.

Thereafter, a firing step was performed in the above-described sample manufacturing method. For Samples 2 to 5, the green body 28 was fired within the range of 1100 to 1185 ° C. for 24 hours. On the other hand, Sample 1 was fired at the same temperature for 6 hours. Thereby, the fired body 29 was created. When the firing step is performed under the above conditions, the sample 1 has a C phase ratio of 86 mol% in the fired body 29. On the other hand, in the samples 2 to 5, the ratio of the C phase of the fired body 29 is 87 mol%.

In the above sample manufacturing method, after firing the unfired body 28, the measurement electrode 4 and the reference electrode 5 were formed by plating. Thereafter, the diffusion layer 211 was plasma sprayed on the surface of the fired body 29, and a slurry to be the trap layer 212 was applied, followed by drying and firing.

For

Samples

1 and 2, the fired body 29 was not subjected to an energization step, and the fired body 29 was used as the solid electrolyte body 2 as it was. On the other hand, for Samples 3 to 5, the fired body 29 was subjected to an energization process. Thereby, in the sample manufacturing method, the ratio of the C phase in the detection unit 20 was adjusted. For example, for sample 5, a current of 260 mA was passed between measurement electrode 4 and reference electrode 5 for 25 seconds while heating fired body 29 to 850 ° C. using heater 6 (first energization). Thereafter, at the same temperature, the direction of current was reversed, and a current of 260 mA was applied for 25 seconds (second energization). Then, it heated at 850 degreeC for 5 minutes using the heater 6 in the state which does not flow an electric current. When the energization process is performed under these conditions, the ratio of the C phase in the detection unit 20 is increased by the current and becomes 88.5%. In this way, Sample 5 was prepared. For

samples

3 and 4, the temperature and current values were the same as in sample 5, and the current flow time was set to the values shown in Table 1. Thereby,

samples

3 and 4 were prepared.

In this experiment, after preparing Samples 1 to 5, the ratio of the C phase in the detection unit 20 of each sample was measured by X-ray diffraction (XRD). For the measurement, X-rays (CuKα rays) having a wavelength of 0.15418 nm were used. Then, the X-rays were applied to the detectors 20 of the samples 1 to 5, and the diffraction angle was measured by changing the incident angle θ within the range of 2θ = 20 to 90 °. As a result, as shown in FIG. 7, peaks corresponding to the crystal planes of the C phase, the M phase, and the T phase appear at a predetermined angle 2θ.

In this experiment, the intensity of the peak corresponding to each crystal plane of the M phase, the T phase, and the C phase is measured, and the following equations [1] to [3] are used to determine the phase of each phase in the detection unit 20. The percentage was calculated. In the following formula, m represents the peak intensity of the M phase, t represents the peak intensity of the T phase, c represents the peak intensity of the C phase, and the numerical value in () represents the Miller index of the crystal plane. Moreover, M means the proportion of the M phase, T means the proportion of the T phase, and C means the proportion of the C phase.
M = {m (111) + m (−111)} / {m (111) + m (−111) + t (111) + c (111)} × 100 (1)
T = (100−M) × {t (400) + t (004)} / {t (400) + t (004) + c (400)} [2]
C = (100−M) × c (400) / {t (400) + t (004) + c (400)} [3]

In this experiment, after measuring the ratio of the C phase in the detection unit 20, the electrical resistance of the detection unit 20 was measured. At this time, the temperature of the detection unit 20 was set to 700 ° C. using the heater 6, and the electrical resistance of the detection unit 20 was measured in this state. Table 1 shows the average value of electrical resistance and 3σ, which are the measurement results of this experiment. Moreover, the relationship between the ratio of C phase and electrical resistance is shown in FIG. In this experiment, the area of the detection unit 20 was 28.26 mm ² and the thickness of the detection unit 20 was 0.5 mm. The thickness of the measurement electrode 4 was 1.6 μm.

FIG. 6 is a graph in which the horizontal axis represents the C phase ratio and the vertical axis represents the electrical resistance. In FIG. 6, the relationship between the ratio of the C phase and the electric resistance is indicated by plot points, and the maximum value and the minimum value of the electric resistance are indicated by error bars. The measurement results of this experiment are shown in FIG. From FIG. 6, it can be seen that if the proportion of the C phase in the detection unit 20 is 88 mol% or more, the electrical resistance is sufficiently low and the variation in the electrical resistance is also small. As described above, from the measurement result of this experiment, when the A / F sensor 1 having a C phase ratio of 88 mol% or more in the detection unit 20 is used, the variation in the current flowing through the detection unit 20 is measured when the air-fuel ratio is measured. It can be seen that the air-fuel ratio can be accurately measured. Therefore, it can be seen that the A / F sensor 1 of the present embodiment can accurately control the air-fuel ratio of the exhaust gas g and can reduce harmful substances in the exhaust gas g.

(Experimental example 2)
Next, an experiment for confirming the relationship between the temperature of the detection unit 20 and the electrical resistance in the A / F sensor 1 was performed. First, in this experiment, a plurality of samples of the A / F sensor 1 were prepared by performing the same process as in Experimental Example 1. The ratio of the C phase in the detection part 20 of each sample was 85, 85.5, 86, 86.5, 87, 87.5, 88, 88.5, 89 mol%. And these samples were heated to 500 degreeC, 600 degreeC, 700 degreeC, 800 degreeC, 900 degreeC, 1000 degreeC using the heater 6, and the electrical resistance of the detection part 20 in each temperature was measured. Incidentally, each sample, the area of the detection unit 20 is set to 28.26 mm ^2, the thickness of the detecting portion 20 was set to 0.5 mm.

The measurement results of this experiment are shown in FIG. As shown in FIG. 8, when the temperature of the detection unit 20 is 500 ° C., the electric resistance of the detection unit 20 is high regardless of the proportion of the C phase. On the other hand, when the temperature of the detection unit 20 is 600 ° C. or higher, the electrical resistance of the detection unit 20 can be greatly reduced by setting the ratio of the C phase to 88 mol% or higher.

Note that FIG. 8 shows that the electrical resistance of the detection unit 20 is almost the same at 900 ° C. and 1000 ° C. Thus, from the measurement results of this experiment, it can be seen that even if the detection unit 20 is heated to 1000 ° C. or higher, the electrical resistance cannot be greatly reduced. Further, as described above, when the temperature of the detection unit 20 is higher than 1000 ° C., problems such as a decrease in strength of the detection unit 20 and an increase in power consumption of the heater 6 are likely to occur. Therefore, when measuring the air-fuel ratio, it is preferable to set the temperature of the detection unit 20 to 1000 ° C. or less.

(Experimental example 3)
Next, an experiment for confirming the relationship between the area of the detection unit 20 and the electrical resistance in the A / F sensor 1 was performed. First, in this experiment, a plurality of samples of the A / F sensor 1 were created by performing the same process as in Experimental Example 1. For each sample, the ratio of the C phase in the detection unit 20 and the area of the detection unit 20 were as follows. The ratio of the C phase was set to 85, 85.5, 86, 86.5, 87, 87.5, 88, 88.5, and 89 mol%, as in Experimental Example 2. The area of the detection unit 20 was set to 20, 25, 28, 30, 40 mm ² . And the heater 6 of each sample was heated and the temperature of the detection part 20 was 700 degreeC. In this state, the electrical resistance of the detection unit 20 was measured. In addition, the thickness of the detection part 20 of each sample was 0.5 mm.

The measurement results of this experiment are shown in FIG. FIG. 9 shows that the electrical resistance of the detection unit 20 can be reduced as the area of the detection unit 20 is increased. However, when the area of the detection unit 20 is larger than 40 mm ² , the area of the measurement electrode 4 is increased as described above, and the amount of noble metal used to configure the measurement electrode 4 is increased. Therefore, the manufacturing cost of the A / F sensor 1 increases. Further, as described above, the area of the measurement electrode 4 has variations (manufacturing variations). Therefore, when the area of the detection unit 20 (the area of the measurement electrode 4) is smaller than 20 mm ² , the influence of this variation becomes large. As a result, the variation in the electrical resistance of the detection unit 20 becomes large, and the measurement accuracy of the air-fuel ratio tends to decrease. Therefore, the area of the detection unit 20 is preferably in the range of 20 to 40 mm ² .

(Experimental example 4)
Next, an experiment for confirming the relationship between the thickness of the detection unit 20 and the electrical resistance in the A / F sensor 1 was performed. First, in this experiment, a plurality of samples of the A / F sensor 1 were created by performing the same process as in Experimental Example 1. For each sample, the ratio of the C phase in the detection unit 20 and the thickness of the detection unit 20 were as follows. The ratio of the C phase was set to 85, 85.5, 86, 86.5, 87, 87.5, 88, 88.5, and 89 mol%, as in Experimental Example 2. Moreover, the thickness of the detection part 20 was 0.5, 0.8, 1.0, 1.5, 2.0 mm. And the heater 6 of each sample was heated and the temperature of the detection part 20 was 700 degreeC. In this state, the electrical resistance of the detection unit 20 was measured. Incidentally, each sample, the area of the detection portion 20 was set to 28.26 mm ^2.

The measurement results of this experiment are shown in FIG. From FIG. 10, it can be seen that the electrical resistance of the detection unit 20 increases when the detection unit 20 is thickened. When the thickness of the detection unit 20 exceeds 2 mm, the electrical resistance of the detection unit 20 and the variation thereof are too large. For this reason, it becomes difficult to accurately measure the air-fuel ratio of the exhaust gas g. Moreover, when the thickness of the detection unit 20 is less than 0.5 mm, the strength of the detection unit 20 tends to decrease. Therefore, the thickness of the detection unit 20 is preferably in the range of 0.5 to 2 mm.

(Experimental example 5)
Next, an experiment for confirming the relationship between the bending strength of the solid electrolyte body 2 and the temperature in the A / F sensor 1 was performed. First, in this experiment, a plurality of samples of the solid electrolyte body 2 were prepared by performing the same process as in Experimental Example 1. The thickness of the solid electrolyte body 2 was 0.5 mm. A four-point bending test of each solid electrolyte body 2 was performed on these samples. Thus, in this experiment, the maximum stress (bending strength) applied to the solid electrolyte body 2 before the solid electrolyte body 2 was broken was measured. In addition, this experiment was performed at room temperature (about 25 degreeC), 600 degreeC, 800 degreeC, and 1000 degreeC.

In this experiment, 8 samples were used at each temperature. Then, the bending strength at each temperature was evaluated, and the failure probability was 90%, 50%, 10%, 1%, 0.1%, 0.01%, 0.001%, 0.0001% from the Weibull plot. The value of bending strength was calculated. And this calculation result was made into the graph.

The calculation result of this experiment is shown in FIG. From FIG. 11, it can be seen that the bending strength decreases as the temperature of the solid electrolyte body 2 increases. In particular, it can be seen that when the temperature of the solid electrolyte body 2 is around 1000 ° C., it becomes half or less of room temperature. Therefore, when measuring the air-fuel ratio using the A / F sensor 1, the temperature of the solid electrolyte body 2 is preferably set to 1000 ° C. or less.

DESCRIPTION OF SYMBOLS 1 A / F sensor 10 Purification apparatus 2 Solid electrolyte body 20 Detection part 3 Reference gas chamber 4 Measurement electrode 5 Reference electrode 6 Heater

Claims

An A / F sensor (1) that is provided on the downstream side of the exhaust gas with respect to the purification device (10) that purifies the exhaust gas and measures the air-fuel ratio of the exhaust gas,
A cup-shaped solid electrolyte body (2) having a closed end and an open proximal end;
A reference gas chamber (3) formed inside the solid electrolyte body and into which a reference gas is introduced;
A measuring electrode (4) formed on the outer surface (21) of the solid electrolyte body and in contact with the exhaust gas;
A reference electrode (5) formed on the inner surface (22) of the solid electrolyte body and in contact with the reference gas;
A heater (6) disposed in the reference gas chamber and heating the solid electrolyte body,
The solid electrolyte body is composed of zirconia, and the solid electrolyte body includes a detection unit (20) that is interposed between the measurement electrode and the reference electrode and conducts oxygen ions, and the detection unit includes a cubic phase. A / F sensor whose ratio is 88 mol% or more.
The A / F sensor according to claim 1, wherein the detection unit includes the cubic phase in an amount of 1 mol% or more in the solid electrolyte body than the portion other than the detection unit.
The A / F sensor according to claim 1 or 2, wherein a length of the measurement electrode in the axial direction of the solid electrolyte body is 3 mm or less.
The A / F sensor according to any one of claims 1 to 3, wherein the solid electrolyte body contains Y 2 O 3 in a range of 4.5 to 6 mol%.
The A / F sensor according to any one of claims 1 to 4, wherein the detection unit has a cubic phase of 95 mol% or less.
6. The apparatus according to claim 1, wherein when the air-fuel ratio of the exhaust gas is measured, the detector is heated by the heater within a range of 600 to 1000 ° C. A / F sensor.
The area of the sensing portion is 40 mm 2 or less, A / F sensor according to any one of claims 1 to 6.
The A / F sensor according to any one of claims 1 to 7, wherein a thickness of the detection unit is 2 mm or less.
A method for manufacturing the A / F sensor according to any one of claims 1 to 8,
A firing step for producing a fired body (29) by firing the unsintered body (28) of the solid electrolyte body;
An energization step of bringing the ratio of the cubic phase in the detection unit to 88 mol% or more by flowing a current between the measurement electrode and the reference electrode formed on the fired body;
A method for manufacturing an A / F sensor.