WO2005090953A1

WO2005090953A1 - Method of stabilizing output of resistance type oxygen sensor utilizing cerium oxide

Info

Publication number: WO2005090953A1
Application number: PCT/JP2005/005034
Authority: WO
Inventors: Noriya Izu; Norimitsu Murayama; Woosuck Shin; Ichiro Matsubara
Original assignee: National Institute Of Advanced Industrial Science And Technology
Priority date: 2004-03-22
Filing date: 2005-03-18
Publication date: 2005-09-29
Also published as: JP4431680B2; TW200537092A; TWI295371B; JP2005308711A

Abstract

With respect to a resistance type oxygen sensor utilizing an oxide semiconductor composed mainly of cerium oxide, there is provided a method of stabilizing the output of resistance type oxygen sensor, comprising adding an oxide containing tetravelent metal ion for resistance lowering to the oxide semiconductor so as to inhibit any variation of sensor output to sulfur dioxide, thereby attaining stabilization of sensor output. This method can appropriately be used as an output stabilization technique for resistance type oxygen sensor for use in an air/fuel ratio feedback control system for automobiles and boilers, or an automobile exhaust gas catalyst deterioration system.

Description

Specification

Output stabilization method of resistance type oxygen sensor using cerium oxide

Technical field

The present invention relates to a method for stabilizing the output of a resistance-type oxygen sensor having a gas detection portion made of an oxide semiconductor whose resistance value changes according to the oxygen partial pressure of an atmospheric gas. Specifically, the output stabilization of a resistance-type oxygen sensor that measures oxygen partial pressure, used in an air-fuel ratio feedback control system that controls the air-fuel ratio of automobile exhaust gas to improve exhaust gas purification efficiency and fuel efficiency. The present invention relates to a dani method.

The present invention relates to a technical field of a resistance-type oxygen sensor based on measuring a partial pressure of oxygen in an atmosphere using a change in resistivity or electric conductivity of an oxide semiconductor. Considering the problem that the output of the sensor fluctuates with respect to sulfur dioxide (SO) gas having a concentration of, for example, 500 ppm or lppm,

2

It is intended to provide a new sensor output stabilization technique capable of drastically solving various problems.

[0003] The present invention is applied to, for example, a vehicle exhaust gas catalyst deterioration detection system for detecting deterioration of a vehicle exhaust gas purification catalyst, an air-fuel ratio feedback control system for optimizing combustion efficiency of a boiler, and the like. It is useful as one suitably applied as an output stabilization technique for a resistance type oxygen sensor or the like to be used.

Background art

[0004] Conventionally, as an oxygen gas sensor for automobiles, for example, a solid electrolyte sensor has been mainly used as described in a prior art document (see Patent Document 1). This type of sensor measures the difference in oxygen partial pressure between the reference electrode and the measurement electrode as an electromotive force, and therefore requires a reference electrode. Therefore, this type of sensor has a problem that the structure is complicated and miniaturization is difficult! /

[0005] In order to overcome this problem, for example, a resistance-type oxygen gas sensor that does not require a reference electrode, as described in a prior art document, has been developed (see Patent Document 2). . To briefly explain the measurement principle of this resistance type oxygen gas sensor, first, the oxygen partial pressure of the atmosphere Changes, the oxygen vacancy concentration of the oxide semiconductor changes. The resistivity or electrical conductivity of an oxide semiconductor has a one-to-one correspondence with the oxygen vacancy concentration, and the resistivity of the oxide semiconductor changes with a change in the oxygen vacancy concentration. By measuring the resistivity, the oxygen partial pressure of the atmosphere can be known.

The present inventors have been conducting research and development on a resistance-type oxygen sensor using cerium oxide as an oxide semiconductor, but cerium oxide has a diffusion coefficient of oxygen. This is because it is expected that the response speed is higher than that of titanium oxide. In a resistance-type oxygen sensor using cerium oxide, reducing the particle size of cerium oxide to 200 nm has been a component of improving response speed (see Patent Document 3).

[0007] However, the resistance type oxygen sensor using cerium oxide has a problem that the output of the sensor fluctuates with respect to sulfur dioxide (SO 2) gas having a concentration of 500 ppm or lppm. There is

2

(See FIG. 1), a solution in the art was sought.

[0008] Patent Document 1: JP-A-55-137334

Patent Document 2: JP-A-62-174644

Patent Document 3: JP 2003-149189 A

Disclosure of the invention

Problems to be solved by the invention

[0009] Under such circumstances, the present inventors have made it possible to stabilize the output of a resistance-type oxygen sensor using cerium oxide in view of the above conventional technology. In the process of conducting intensive research with the goal of developing new technologies, the addition of oxidized oxides containing tetravalent metal ions to reduce the resistance to cerium oxide, an oxide semiconductor, We found that the intended purpose could be achieved, and conducted further research to complete the present invention o

[0010] It is an object of the present invention to provide a method for suppressing a change in output of a resistance type oxygen sensor using cerium oxide on sulfur dioxide gas.

Means for solving the problem

[0011] The present invention for solving the above-mentioned problems is directed to a resistance-type oxygen sensor using cerium oxide as a main component as an oxide semiconductor, for reducing the resistance of the oxide semiconductor. A method for stabilizing the output of a resistance-type oxygen sensor, characterized in that by adding an oxide having a tetravalent metal ion, the fluctuation of the sensor output with respect to sulfur dioxide is suppressed and the sensor output is stabilized. is there.

[0012] The present method includes: (1) an oxide having tetravalent metal ions, an oxide having zirconium ions or hafnium ions, and (2) an air-fuel ratio of an automobile or a boiler. In a preferred embodiment, the resistance type oxygen sensor is used in a feedback control system, and (3) the resistance type oxygen sensor is a resistance type oxygen sensor used in an automobile exhaust gas catalyst deterioration system.

Next, the present invention will be described in more detail.

The present invention provides a resistance-type oxygen sensor using cerium oxide as a main component as an oxide semiconductor, by adding an oxide containing a tetravalent metal ion for reducing resistance to the oxide semiconductor. The present invention is characterized in that the fluctuation of the sensor output with respect to sulfur dioxide is suppressed and the sensor output is stabilized.

[0014] In the present invention, a method of adding an acid compound having a tetravalent metal ion for lowering resistance to cerium oxide, which is an oxide semiconductor, is optional. Forces such as knotting and sedimentation methods are not limited to these. However, the oxide containing the tetravalent metal ion and the cerium oxide need to be dissolved in a solid solution instead of merely mechanically mixing the oxide containing the tetravalent metal ion with the cerium oxide to reduce the resistance. There is. The structure of the oxide semiconductor is also arbitrary, and examples thereof include a thin film, a thick film, a barta, a dense body, and a porous body.

[0015] However, the surface area per unit weight is as small as possible! In addition, since the effect of sulfur diacid is considered to be the effect on the outermost surface, the effect of the surface is reduced if the surface area per unit weight is small. Preferable examples of the oxidizing compound having a tetravalent metal ion for reducing the resistance to cerium oxynitride added to the oxide semiconductor include, for example, zirconium oxidization and hafnium oxidization. It should be noted that the present invention is not limited to these, and any equivalent or similar one having the same function can be used.

[0016] A tetravalent metal ion for reducing the resistance to cerium oxide is added to the oxide semiconductor. The following shows the concentration of the soybean noodles. An oxide having a tetravalent metal ion generally has a chemical composition of MO 2. Here, M is a tetravalent metal element. X concentration of MO

twenty two

%, The upper limit of X is 50 mol%. If it exceeds 50 mol%, the characteristics of MO are considered to be more remarkable than cerium oxide, and the characteristics of cerium oxide are lost.

2

That's why.

[0017] The lower limit is not particularly limited numerically. Compared to cerium with no MO

2

Any concentration may be used as long as the resistivity can be reduced to half or less. MO with added acid

2

Cerium can be in either a single phase or two phases. The structure and form of the resistance type oxygen sensor are arbitrary, and the present invention is also applicable to a resistance type oxygen sensor having a temperature compensating material.

Next, the operation of the present invention will be described.

2

SO is adsorbed, and at the same time, H 2 O in the atmosphere is also adsorbed to generate H +, which is

twenty two

It is thought to be a body. This conductivity is considered to be higher than the conductivity of cerium oxide without any addition, and since the current flows through the surface, the overall resistance is considered to decrease. However, by adding an oxide containing a tetravalent metal ion for reducing resistance, the conductivity of the oxide semiconductor becomes

(2) The total resistance does not fluctuate because it is larger than that of the product generated on the surface by introduction, and it is thought that output fluctuation due to SO can be suppressed

2

According to the method of the present invention, output fluctuations with respect to sulfur dioxide of 500 ppm or less, more effectively 1 ppm or less, can be suppressed. According to the method of the present invention, in a resistance-type oxygen sensor using cerium oxide as an oxide semiconductor as a main component, it is possible to stabilize the sensor output with respect to sulfur dioxide. The method of the present invention is suitably used, for example, as a resistance-type oxygen sensor used in an air-fuel ratio feedback control system of an automobile or a boiler, and an output stabilization technique of a resistance-type oxygen sensor used in an automobile exhaust gas catalyst deterioration system. be able to.

The invention's effect

According to the present invention, 1) an oxide having a tetravalent metal ion for lowering resistance is added to cerium oxide cerium, which is an oxide semiconductor, so that cerium oxide with respect to sulfur dioxide gas is added. It is possible to suppress the fluctuation of the output of the resistance type oxygen sensor using lithium.2) The fluctuation of the output for sulfur dioxide below several hundred ppm is eliminated.3) The sulfur dioxide adheres to the surface of the oxide semiconductor. 4) A small and simple oxygen sensor can be provided.5) The method of the present invention is a vehicle or boiler air-fuel ratio feedback control system or a vehicle exhaust gas catalyst deterioration system. It has a special effect that it can be applied to a resistance type oxygen sensor used for

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention will be specifically described based on examples, but the present invention is not limited by the following examples.

Hereinafter, first, the background to the present invention will be described based on Reference Examples 1 to 4. Reference Example 1

A cerium nitrate aqueous solution having a concentration of 0.1 OlOmolZdm ³ was sprayed, introduced into an electric furnace heated to 973 K, thermally decomposed, and recovered to obtain a powder of cerium oxide. The particle size of the obtained powder was about 200 nm to 300 nm. A paste obtained by mixing the obtained fine particle powder and a vehicle of an organic solvent (a mixture of ethyl cellulose and terbineol) was printed by screen printing on an aluminum oxide substrate.

Next, the printed matter was heated at 773 K in the air, and subsequently heated at 1473 K in the air to obtain a thick film. The obtained thick film was a porous body and had a particle size of 200 nm to 300 nm. An electrode was needed to measure the resistivity of the oxygen gas detection part, and a comb-shaped platinum electrode was provided by screen printing. By the above method, a resistance type oxygen sensor (CelOO) using cerium oxide was manufactured.

[0024] A measuring chamber capable of changing the oxygen partial pressure P (0), the SO partial pressure P (SO), and the temperature T is provided with a cell.

2 2 2

The electric resistance between the platinum electrodes was measured as a sensor output by a DC two-terminal method. Figure 1 shows the output change (resistance) of the CelOO oxygen sensor. At 873 or 1073 K, the gas was switched to A, B, C or C, B, A.

[0025] Here, A is 100% O (P (O) = 10 ⁵ Pa), B is 10% O + N (P (O) = 10 ⁴ Pa), C

2 2 2 2 2

Is lppmSO + 10% O + N (P (0) = 10 ⁴ Pa ゝ P (SO) = 0. LPa) ゝ C is 500 ppm SO + 10% O + N (P (0) = 10 Pa, P (SO) = 50 Pa).

2 2 2 2 4 2

When the gas was switched from A to B, the output of the oxygen sensor changed because the oxygen partial pressure changed by one digit. This is a normal response. Next, the B force also switched the gas to C or C '. At this time, if there is no influence of SO gas, the resistance is not changed because the oxygen partial pressure has not changed.

2

Force that should not change The result is different, the resistance changes after switching to C or C '. [0027] That is, CelOOi, 873 or 1073K

2 2 There is an effect, especially under conditions other than 1073K and lppmSO,

twenty two

. For this reason, if the exhaust gas contains SO at lppm or more, the output may fluctuate.

2

I found out.

Reference Example 2

To investigate the reaction product of SO and CeO, fine powder of CeO was added to 500ppmSO + 10% O

2 2 2 2 2

Annealed in + N and the reaction products were evaluated by XRD. CeO used in this test

2

The powder was prepared by a precipitation method and had a primary particle size of 20 nm or less. Also,

2

For comparison, a commercial reagent, Ce (SO) · 8Η, was annealed in 773Κ air.

2 4 3 2

[0029] In Fig. 2 (a) and (b), the CeO fine powder was added to 500 ppm SO at 1073 and 873K, respectively.

twenty two

9 shows the XRD pattern of a sample obtained by annealing in + 10% O + N for 24 hours. At 1073K

twenty two

In the sample after annealing, no surface discoloration and no reaction phase was observed.

[0030] The CeO powder is a pale yellow color.

2

Only a very weak part of the exposed surface turned white. When a portion containing a large amount of it was taken out and examined by XRD, there was a reaction phase indicated by ☆. In the sample containing no white discoloration, the reaction phase marked with a star was not observed.

When annealing at a lower temperature of 823K, the reaction phase increased a little more. The reaction phase is only a small part of the surface, and the powder is taken out so as to contain a large amount of it, and XRD analysis shows that the reaction phase is a mixture of Ce (SO) 4Η, CeOSO, Ce (SO) 4ΗO, etc. Do you know

2 4 3 2 4 4 2

I got it. This is the same as Ce (SO) · 8Η ァ which is annealed at 773Κ in air. [0032] In lppmSO + 10% O + N, the reaction temperature was

2 2 2

The phase response was unobservable. That is, in exhaust gas containing less than lppm SO,

2

It was confirmed that no phase occurred. In lppmSO, a reaction phase is formed on cerium oxide.

2

Despite this, the production of cerium (CelOO) by SO gas shown in Reference Example 1

2

The occurrence of force fluctuations indicated that the power fluctuations and the reaction phase formation were independent.

[0033] Reference Example 3

Ce (SO) · 8Η Η molded body (9.9mm, 11.66mmL) is annealed at 873Κ, lOh

2 4 3 2

, And measuring the resistance at 873 K, was determined conductivity, 1. 4x10- ⁶ SZm der ivy. The conductivity of CeO at this temperature 1x10- ⁴ - since it is 5x10- ⁴ SZm, Ce (S

twenty two

The resistivity of reactants such as O 2) is very high. This indicates that the resistance at 873K is low.

4 3

It was confirmed that the bottom was unrelated to the formation of reactants such as Ce (SO).

2 4 3

[0034] From the above Reference Examples 2 and 3, the decrease in the resistance of cerium oxide by the SO gas was caused by the reaction phase.

2

It was confirmed that it was not related to generation.

Reference Example 4

When an electrode was attached to the aluminum substrate and the resistance was measured at room temperature, the resistance was 500 ppm SO + 10% O + N gas in 873K, which was over the detection limit of the voltmeter (120ΜΩ).

2 2 2

And the resistance between the electrodes was measured at room temperature to show 70ΜΩ. When the surface of the substrate was wiped with ethanol, the detection limit was again exceeded (120 Ω). From this, it is necessary to anneal in 500 ppm SO + 10% O + N gas, so that a conductive substance adheres.

2 2 2

I understood.

[0036] The details of the conductive substance are not known, but it is expected that the substance is a composite adsorbate of SO adsorbed on the oxide semiconductor surface and H 2 O adsorbed simultaneously from the atmosphere.

twenty two

. The charge carrier may be the force that is the H + of this adsorbate. The impact of SO on CelOO

The reason why 2 was given was considered to be that the resistivity of this conductive material was lower than that of CelOO. Therefore, if the resistivity of an oxide semiconductor can be reduced, the influence of SO can be suppressed.

2

And reached the present invention.

Example 1

[0037] Powder of an oxidized product containing cerium ions and zirconium ions was produced by the following procedure. Cerium nitrate aqueous solution and zirconium oxynitrate aqueous solution at the specified concentration (X)

Zr Z (x + X Zr Ce

: 20 mol%) to obtain a mixed aqueous solution in which the concentration of the sum of cerium ions and zirconium ions is 0.1 OlOmol Zdm ³ . Next, the mixed aqueous solution was sprayed, introduced into an electric furnace heated to 973K, thermally decomposed, and recovered to obtain a fine powder of an oxide containing cerium ions and zirconium ions. Here, the ZrO concentration of the oxide powder is the ZrO

2

It was the same as the ion concentration. The particle size of the obtained powder was about 200 nm and 300 nm.

[0038] A paste in which the obtained fine particle powder and a vehicle of an organic solvent (a mixture of ethyl cellulose and terbineol) were mixed was printed on an aluminum substrate by screen printing. Next, the printed matter was heated at 773K in the air and subsequently at 1473K in the air to obtain a thick film. The obtained thick film was a porous body and had a particle size of 200 nm to 300 nm. An electrode was required to measure the resistivity of the oxygen gas detection part, and a comb-shaped platinum electrode was provided by a screen printing method. By the above-mentioned method, a resistance type oxygen sensor (Z20) using cerium oxidized to which 20 mol% of oxidized zirconium was added was produced. Zirconium oxide and cerium oxide were completely solid-solved and were single-phase cubic crystals.

An experiment was performed using the same device as in Reference Example 1. The gas conditions are the same as in Reference Example 1.

Figure 3 shows the experimental results. Compared to the CelOO resistor at the same temperature, the resistance of the Z20 was more than an order of magnitude lower. Since the thick film shape of CelOO and Z20 are the same and the electrode shape is also the same, it can be concluded that the resistivity of Z20 is at least one order of magnitude lower than that of CelOO. With the Z20, 873K: almost no power except for 500ppmSO. 873K: Even at 500ppmSO, CelO

twenty two

0 of 873: Influence from 500ppmSO.

2

[0040] It is known that the SO concentration in exhaust gas is lppm or less [F. Hashimoto, T. T.

2

anaka, O. Asami, Development of continuous high-sensitivity exnaust S O analyzer, R & D Review of Toyota CRDL, 34 [3], (1999) 9-16]. Force S

2

With Z20, it was confirmed that there was no effect at any temperature of 873K and 1073K in the real environment, and the effect of the present invention was demonstrated.

Example 2

[0041] Cerium oxide powder and hafnium oxide powder were mixed with cerium ions and hafnium ions. The ratio was measured to 9: 1, and the mixture was wet-mixed using an agate mortar and ethanol as a dispersion medium. After mixing, the dried powder was press-molded to obtain a molded body. The molded body was fired in the air at 1400 ° C for 10 hours and solid-phase sintered. After cooling to room temperature, the sintered body was pulverized to obtain a powder. A paste in which the obtained powder and a vehicle of an organic solvent were mixed was previously printed by screen printing on an aluminum substrate on which a platinum comb-shaped electrode was formed. Next, the film was heated in air at 500 ° C and subsequently in air at 1300 ° C to obtain a thick film.

When the structure of the thick film after firing at 1300 ° C. was observed with a scanning electron microscope, it was a porous body having a particle size of 1 to 2 m. The thickness was 20 m. An X-ray diffraction analysis of the fired thick film confirmed that hafnium ions were in solid solution! / ヽ. An experiment was performed using the same device as in Example 1. The gas conditions are the same as in Example 1. Figure 4 shows the experimental results at 1073K. As a comparative example, the change in resistance of cerium oxide without additive is also shown.

[0043] The resistance of the resistance-type oxygen sensor (Hf10) using hafnium-doped cerium oxynitride having a hafnium ion concentration of 10 mol% is determined by the resistance-type oxygen sensor (Ce100) using undoped oxidized cerium. The resistance was 30 times lower than that of the above, and it was confirmed that the resistance was reduced by the hafnium sardine-added kamuri.

[0044] The resistance of CelOO decreases as soon as 500 ppm SO gas is introduced,

2

As in Example 1, the effect of so was observed. On the other hand, the resistance of the hafnium-added oxygen sensor is

2

Even if 500 ppm SO is introduced, the resistance hardly changes, confirming that there is no influence of SO

twenty two

As a result, the effect of the present invention was demonstrated.

Industrial applicability

The present invention relates to a method for stabilizing the output of a resistance type oxygen sensor using cerium oxide, and according to the present invention, it is intended to reduce the resistance to cerium oxide, which is an oxide semiconductor. By adding an oxide having a tetravalent metal ion, it is possible to suppress fluctuations in the output of a resistance-type oxygen sensor using cerium oxide with respect to sulfur dioxide gas.

Further, in the present invention, it is necessary to provide a filter for preventing output fluctuation with respect to sulfur dioxide at a concentration of several hundred ppm or less and preventing the sulfur dioxide from adhering to the surface of the semiconductor. The advantages are that it is important and that a small and simple oxygen sensor can be provided. . The method of the present invention can be suitably applied to a resistance-type oxygen sensor used in an air-fuel ratio feedback control system of an automobile or a boiler or an automobile exhaust gas catalyst deterioration system. Brief Description of Drawings

FIG. 1 shows a change in output of an oxygen sensor according to the related art with respect to SO gas.

2

[Figure 2] Cerium oxide fine powder at 873 or 1073K, 500ppmSO + 10% O + N gas

The XRD pattern of the sample obtained by annealing at 22h in 222 is shown. (A) is the result at 873K, (b) is the result at 1073K.

FIG. 3 shows an output change with respect to SO gas of an oxygen sensor stabilized by the method of the present invention.

2

. In this case, zirconium oxide was added with 20 mol% to reduce the resistance. This was described as Z20.

FIG. 4 shows that the oxygen sensor stabilized by the method of the present invention with respect to SO gas at 1073 K

2

Indicates output change. In this case, 10 mol% of hafnium oxide was added to reduce the resistance. This was described as Hf10.

Claims

The scope of the claims

[1] In a resistance-type oxygen sensor using cerium oxide as an oxide semiconductor as a main component, an oxide semiconductor having a tetravalent metal ion for lowering resistance is added to the oxide semiconductor. A method for stabilizing the output of a resistance-type oxygen sensor, characterized by suppressing the fluctuation of the sensor output with respect to sulfur dioxide and stabilizing the sensor output.

2. The method for stabilizing the output of a resistance-type oxygen sensor according to claim 1, wherein the oxide having a tetravalent metal ion is an oxide having a zirconium ion or a hafnium ion.

3. The method for stabilizing the output of a resistance-type oxygen sensor according to claim 1, wherein the resistance-type oxygen sensor is a resistance-type oxygen sensor used in an air-fuel ratio feedback control system of an automobile or a boiler.

4. The method for stabilizing the output of a resistance-type oxygen sensor according to claim 1, wherein the resistance-type oxygen sensor is a resistance-type oxygen sensor used in an automobile exhaust gas catalyst deterioration system.