JPH01176937A - Solid electrolyte enzyme sensor - Google Patents

Abstract

PURPOSE:To enhance the accuracy of measurement and to increase response speed by providing porous electrodes and fine particles of a solid electrolyte, etc., and operating the title sensor at a relatively low temp. CONSTITUTION:The electromotive force based on a difference between the oxygen partial pressures of a reference gas of a known oxygen partial pressure and a gas to be measured is generated between the porous electrodes 2 and 3 when the above-mentioned reference gas is introduced into the sensor from a reference gas introducing pipe 9 and the gas to be measured is introduced into an outside pipe 8. This electromotive force is, therefore, measured by an electromotive force meter 10 to determine the oxygen partial pressure of the gas to be measured. The gaseous oxygen in closed cells 7 is discharged by the oxygen pump effect of the fine particles 5 of the solid electrolyte or the gaseous oxygen is introduced therein from the contact gas even if the closed cells 7 exist in the electrodes 2, 3 and, therefore, the oxygen partial pressure of the gas in the cells 7 equilibrates rapidly with the oxygen partial pressure of the contact gas. The sensor is operated at the relatively low temp. in such a manner. The accuracy of the measurement is thus enhanced and the response speed is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an oxygen sensor using a solid electrolyte having oxygen ion conductivity.

(Prior art) As is well known, a solid electrolyte oxygen sensor is composed of a partition wall made of a solid electrolyte that has oxygen ion conductivity at high temperatures, and porous electrodes provided on both sides of the partition wall. When one electrode is brought into contact with a reference gas with a known oxygen partial pressure and the other electrode is brought into contact with a gas to be measured whose oxygen partial pressure is unknown, the difference in oxygen partial pressure between the two gases will be This takes advantage of the fact that a corresponding electromotive force appears. Therefore, the magnitude of the electromotive force is expressed by the well-known Nernst equation, and if the temperature of the partition wall and the oxygen partial pressure of the reference gas are known, then from the magnitude of the electromotive force, It is possible to determine the oxygen partial pressure in the gas to be measured, that is, the oxygen concentration.

Incidentally, in such solid electrolyte oxygen sensors, the porous electrode is generally made of Pt from the viewpoint of catalytic activity and chemical stability, but oxygen sensors generally operate at high temperatures of 800°C or higher. Therefore, if the gas to be measured contains CH4, Co, H2, etc., which have low ignition points, these components will burn during the measurement, and the combustion reaction will consume oxygen, resulting in poor accuracy. There is a problem that high oxygen concentration cannot be measured.

In order to solve these problems and enable operation at lower temperatures that do not involve combustion reactions, Japanese Patent Laid-Open No. 50-91.
In the No. 389 invention, the porous electrode is made of solid electrolyte powder having oxygen ion conductivity and Pt, Pd, Pt-
It is proposed that the material be formed by baking a mixture with powder or fine particles of a platinum group metal such as a Pd alloy. However, platinum group metals are not only extremely expensive but also have a problem of high baking temperatures. Also, because oxygen diffusion is relatively slow, even though it can be operated at low temperatures, the 90% response time (the time it takes for the electromotive force to reach 90% of the theoretical value) at 400°C is, for example, 10 minutes. The problem is that it is extremely long.

(Problems to be Solved by the Invention) The purpose of the present invention is to solve the above-mentioned problems of conventional oxygen sensors, and to provide a solid electrolyte that not only operates at a relatively low temperature and has high measurement accuracy but also has a very fast response speed. To provide oxygen sensors.

(Means for Solving the Problems) This invention for achieving the above-mentioned object comprises a partition made of a solid electrolyte having oxygen ion conductivity, a reference gas side porous electrode provided on one wall of the partition, and and a porous electrode on the gas-to-be-measured side provided on the other wall, and at least the porous electrode on the gas-to-be-measured side is made of a mixed material of solid electrolyte fine particles having oxygen ion conductivity and silver. The solid electrolyte oxygen sensor is characterized in that the amount of the fine particles is 5 to 15% by weight.

To roughly explain the invention in detail, the oxygen sensor of the invention includes a partition wall made of a solid electrolyte having oxygen ion conductivity, a reference gas side porous electrode provided on one wall surface of the partition wall, and a reference gas side porous electrode provided on one wall surface of the partition wall. and a porous electrode on the gas-to-be-measured side.

The solid electrolyte that makes up the partition walls is made of so-called base materials such as ZrO2, HfO2, B i 203, CeO, etc., in order to stabilize the oxygen ion conductive crystal structure that is stable at high temperatures even at room temperature. Oxides having +2 or +3 valence cations as agents, such as Y2O3, cao, Mo
o, Yb2O3, SrO, 5C203, Sm2O3, M
It is made by dissolving oo3 and WO3 in solid solution. When the so-called base material is Zro2 or HfO2, Y2O3,
GapSMQOSYb203 is suitable. Also, Bi
2O:+ is Y2O3, Moo3, WO3, CeO
Y2O3 is suitable for each. The solid solution amount of these stabilizers is about 5 to 45 mol%. Such a solid electrolyte can be obtained by firing a powder prepared by a well-known method such as an oxide mixing method or a wet synthesis method such as a neutralization coprecipitation method.

The partition wall made of the solid electrolyte mentioned above can be plate-shaped, cylindrical, or
It can take a well-known shape, such as a tubular shape with one end closed and a U-shaped longitudinal section. The above-mentioned molding into such a shape using the so-called raw material powder can be performed by a well-known method such as a hot press method or a hot isostatic pressing method (HIP method).

The porous electrode provided on the wall of the partition is made of a mixed material of solid electrolyte fine particles that have oxygen ion conductivity at high temperatures and silver (Act>.Such an electrode is made of a mixture of solid electrolyte fine particles and silver The mixture with the paste can be formed on the wall surface, for example, by applying, spraying, printing, and then baking. In this case, the fine particles of the solid electrolyte constitute the partition wall. The electrode may be made of the same solid electrolyte as the solid electrolyte or a different solid electrolyte.The thickness of the electrode is about 5 to 10 μm.

The size of the fine particles of the solid electrolyte is suitably about 0.1 to 2 μm. Fine particles of less than 0.1 μm tend to aggregate and are therefore difficult to disperse in the silver paste. In addition, it is likely to be buried in the eye, making it difficult to exert the oxygen pumping effect described below. On the other hand, if the diameter exceeds 2 μm, the silver is likely to be fragmented and the current collecting function will deteriorate.

The amount of solid electrolyte fine particles in the mixed material forming the porous electrode should be 5 to 15% by weight. That is, if it is less than 5% by weight, the electrode will not easily become porous, and if it exceeds 15% by weight, the current collection function will be greatly reduced. Furthermore, as shown in the Examples described later, if the amount is less than 5% by weight or more than 15% by weight, the response speed will decrease significantly in either case, making it impossible to achieve the object of the present invention. .

In the porous electrode, of the reference gas side and the measurement gas side N electrodes, only the electrode on the measurement gas side can be made of the above-mentioned mixed material. This is because the reference gas is generally air or a mixture of oxygen gas and an inert gas such as nitrogen gas, so on the reference gas side, the consumption of oxygen gas due to the combustion reaction mentioned above is This is because there is no need to take this into consideration, and since the oxygen partial pressure of the reference gas is always kept constant, there is no need to take into account the delay in response speed due to open pores present in the electrode, which will be described later.

Note that when only the electrode on the gas to be measured side is formed of the above-mentioned mixed material, the electrode on the reference gas side is formed of the conventional material such as Pt mentioned above.

An oxygen concentration measuring device can be constructed using the oxygen sensor of the present invention. That is, the oxygen sensor of the present invention is constructed by placing it in, for example, an electric furnace to maintain the partition wall at a constant temperature, introducing a reference gas to one side of the partition wall, and
In addition, by introducing the gas to be measured on the other side and measuring the electromotive force generated between the porous electrodes on the reference gas side and the gas to be measured side, it is possible to measure the oxygen partial pressure of the reference gas, that is, the oxygen concentration. can be measured.

(Function) With the partition wall heated to a certain temperature, for example, 300 to 850°C, a reference gas with a known oxygen partial pressure is brought into contact with the porous electrode on the reference gas side, while a porous electrode on the gas to be measured side is brought into contact with the reference gas. A gas to be measured with an unknown oxygen partial pressure is brought into contact. Then, an electromotive force corresponding to the difference in oxygen partial pressure between the two gases appears between the two electrodes. The magnitude of this electromotive force E is determined by the well-known Nernst equation, that is, E = (RT / 4 F > I n (P m /
Ps) However, R: Gas constant d: Absolute temperature of the partition wall F: Faraday constant Pm: Oxygen partial pressure of the measured gas PS: Expressed as the oxygen partial pressure of the reference gas. Here, since the temperature of the partition wall and the oxygen partial pressure of the reference gas are kept constant during the measurement, the electromotive force E7'J1 and the like can be used to determine the oxygen partial pressure of the gas to be measured, that is, the oxygen concentration.

At this time, in the porous electrode, the fine particles of the solid electrolyte act in the opposite manner to that of an oxygen sensor, that is, act as an oxygen pump, and the gas at the interface between the partition wall and the porous electrode is quickly replaced. The oxygen pumping action is an action that occurs because the above-mentioned Nernst equation is reversible as is well known. Therefore, even if closed pores exist in the porous electrode, the oxygen partial pressure of the gas in the closed pores quickly equilibrates with that of the contact gas due to the oxygen pumping action.

(Embodiment) In FIG. 1, a reference gas side porous electrode 2 is formed on one wall surface on the closed end side of a partition wall 1 made of a solid electrolyte having a U-shaped cross section and having one end closed. . Similarly,
A reference gas side porous electrode 3 is formed on the other wall surface of the partition wall 1, and an oxygen sensor 4 is configured.

Referring to FIG. 2, the porous electrode 2 (3) is made of a mixed material of solid electrolyte fine particles 5 and silver 6. As shown in FIG.

Thus, although the electrode 2 (3) is porous when viewed as a whole, there are portions where closed pores 7 are present.

The oxygen sensor described above constitutes an oxygen concentration measuring device, for example, as follows. That is, as shown in FIG.
Insert the reference gas introduction pipe 9 into the inside of the cell so that its tip is near the closed end of the partition wall 1, and further connect the electromotive force meter 10 to the porous electrode 2.3 via a lead wire. do. On the other hand, the partition wall 11 can be heated to a constant temperature of 300 to 850°C using the furnace 11.

In the state shown in FIG. 1, a reference gas with a known oxygen partial pressure is introduced from the reference gas introduction tube 9 as shown by the arrow, and a gas to be measured is introduced into the outer tube 8 as shown by the arrow. An electromotive force is generated between the electrodes 2 and 3 based on the difference in the oxygen partial pressures of both gases, so this is measured with an electromotive force meter 10, and from the Nernst equation (above), the oxygen partial pressure of the gas to be measured ( Oxygen concentration). At this time, even if the porous electrode 2 (3) has closed pores 7 as shown in FIG. 2, the oxygen gas in the closed pores 7 is exhausted by the oxygen pumping action of the solid electrolyte 5.
Alternatively, since oxygen gas is introduced therein from the contact gas, the oxygen partial pressure of the gas in the closed pores 7 quickly equilibrates with the oxygen partial pressure of the contact gas.

(Example 1) Powder with a solid solution amount of Y2O3 of 8 mol% was prepared by a well-known neutralization coprecipitation method from a ZrOCl2 aqueous solution with a purity of 99.9% and a YCl3 aqueous solution with a purity of 99.9%. The powder was fired at 1000°C for 2 hours to obtain a raw material powder.

Next, the raw material powder and ethyl alcohol were mixed in a weight ratio of 2.
= 1, crushed in a ball mill for 24 hours to give an average particle size of about 0.3 μm, dried, added polyvinyl alcohol as a binder, and molded using a rubber press to obtain a cross-sectional shape. A molded article having a U-shape and one end closed was obtained. At this time, the molding pressure is 100
0K (1/Cm2).

Next, the molded body was placed in an electric furnace and sintered at 1700'C for 2 hours to obtain solid electrolyte partition walls as shown in FIG. This partition wall has an outer diameter of approximately Bmm.

The inner diameter was about 5mm and the length was about 110mm.

Next, on each wall surface of the partition wall, mix the solid electrolyte fine particles that have been ground and dried in a ball mill and silver paste so that the solid electrolyte fine particles are 2.5% by weight, and apply the mixture with a brush. was baked at 700'C for 1 hour to form a porous electrode, and an oxygen sensor as shown in FIG. 1 was obtained. In exactly the same way, the amount of solid electrolyte fine particles was 5.0% by weight, 7°5% by weight, 10.0% by weight, and 12.5% by weight.
, 15.0% by weight, 17.5% by weight, and 20.0% by weight, and a total of 8 types of oxygen sensors are used for 1 q. For comparison, an oxygen sensor was also obtained in which a porous electrode was formed by baking only silver paste, which did not contain fine particles of solid electrolyte.

Next, the apparatus shown in Fig. 1 was constructed using these nine types of oxygen sensors in total, and air was used as the reference gas, and nitrogen gas containing 1% by volume of oxygen gas was used as the gas to be measured.
The response speed, that is, the time required for the electromotive force to reach 90% and 95% of the theoretical value, was measured from the electromotive Kerr time curve at 'C.

The measurement results are shown in Figure 3. In FIG. 3, C on the horizontal axis is the amount of solid electrolyte fine particles (% by weight), and ↑ on the vertical axis is time (seconds). Also, the solid line indicates that the electromotive force is 90% of the theoretical value.
The dotted line also shows the time required to reach 95%. Note that the flow rates of both the reference gas and the gas to be measured were 200 m1/min. Furthermore, a silver wire was used for the lead wire.

As is clear from Figure 3, there are 5 rivers of solid electrolyte particles.
The response speed is extremely fast in the range of ~15% by weight.

(Example 2) In Example 1, the gas to be measured was replaced with nitrogen gas containing 10% by volume of oxygen gas. The measurement results are shown in Figure 4.

(Example 3) From a ZrOCl2 aqueous solution with a purity of 99.9% and a CaCl2 aqueous solution with a purity of 99.9%, a solid solution amount of CaO of 12 mol% was obtained by a well-known neutralization coprecipitation method. After forming a certain powder, the powder was fired at 1000°C for 2 hours to obtain a raw material powder.

Next, the raw material powder and ethyl alcohol were mixed in a weight ratio of 2.
: 1, milled in a ball mill for 24 hours to give an average particle diameter of about 0.3 μm, dried, added polyvinyl alcohol as a binder, and then molded the cross section using a rubber press method. A molded article having a U-shape and one end closed was obtained. At this time, the molding pressure is 100
It was set to 0KMCm2.

Next, the above molded body was placed in an electric furnace and sintered at 1600° C. for 2 hours to form solid electrolyte partition walls as shown in FIG. This partition had an outer diameter of about 8III1, an inner diameter of about 6 mm, and a length of about 110 mm.

Hereinafter, a total of nine types of oxygen sensors were made in the same manner as in Example 1, and the same measurements as in Example 1 were carried out for each oxygen sensor.

The measurement results are shown in FIG.

(Example 4) In Example 3, the gas to be measured was replaced with nitrogen gas containing 10% by volume of oxygen gas. The measurement results are shown in Figure 6.

(Example 5) BizO3 powder with a purity of 99.9% and a powder with a purity of 99.9%
．． 9% Y2O3 powder was weighed so that the solid solution amount of Y2O3 powder was 25 mol%, ethyl alcohol was added thereto, and mixed for 30 minutes in an alumina mortar.
Ethyl alcohol was volatilized in an oil bath at 0'C, and the mixture was further calcined at 700'C for 3 hours to obtain a raw material powder.

Next, the above raw material powder was pre-pulverized in an alumina mortar for 30 minutes, crushed in a ball mill and molded by a rubber press method in the same manner as in Example 1, and the molded body was sintered at 900°C for 20 hours. A partition wall as shown in the figure was obtained. The particle size after pulverization using a ball mill was 0.5 μm.

Hereinafter, a total of nine types of oxygen sensors were made in the same manner as in Example 1 using the above-mentioned partition wall, solid electrolyte fine particles after being ground by a ball mill, and silver paste, and the same measurements as in Example 1 were carried out for each oxygen sensor. The measurement results are shown in Figure 7.

(Example 6) In Example 5, the gas to be measured was replaced with nitrogen gas containing 10% by volume of oxygen gas. The measurement results are shown in FIG.

(Example 7) CeO powder with a purity of 99.9% and a purity of 99.9%
% Y2O3 powder so that the solid solution amount of Y2O3 powder is 5 mol%, add ethyl alcohol to this, mix in an alumina mortar for 30 minutes, and then volatilize the ethyl alcohol in an oil bath at 90 ° C. Let's make it 1000
The raw material powder was obtained by firing at ℃ for 2 hours.

Next, the above raw material powder was pre-pulverized in an alumina mortar for 15 minutes, crushed in a ball mill and molded by a rubber press method in the same manner as in Example 1, and the molded body was sintered at 1500°C for 2 hours. A partition wall as shown in the figure was obtained. The particle size after pulverization using a ball mill was 0.5 μm.

Hereinafter, a total of nine types of oxygen sensors were made in the same manner as in Example 1 using the above-mentioned partition wall, solid electrolyte fine particles after being ground by a ball mill, and silver paste, and the same measurements as in Example 1 were carried out for each oxygen sensor. The measurement results are shown in Figure 9. In addition, the solid electrolyte fine particles containing 20.0% by weight are as follows:
Even after 4 hours have passed since the start of measurement, the electromotive force remains at the theoretical value of 95.
% was not reached.

(Example 8) In Example 7, the gas to be measured was replaced with nitrogen gas containing 10% by volume of oxygen gas. The measurement results are shown in FIG.

In addition, in the case where the solid electrolyte fine particles were 20.0% by weight, the electromotive force did not reach 95% of the theoretical value even after 4 hours had passed from the start of the measurement.

(Example 9) Using the sintered body used in Example 5 and the mixed material obtained in Example 1, a total of nine types of oxygen sensors were made in the same manner as in Example 1, and an example was prepared for each oxygen sensor. The same measurements as in 1 were made. The measurement results are shown in FIG.

(Example 10) In Example 9, the gas to be measured was replaced with nitrogen gas containing 10% by volume of oxygen gas. The measurement results are shown in FIG.

(Effects of the Invention) In the oxygen sensor of the present invention, at least the porous electrode on the side of the gas to be measured is made of a mixed material of solid electrolyte fine particles having oxygen ion conductivity and silver, and the amount of the fine particles is 5.
~15% by weight, so as shown in the example, it operates satisfactorily even at relatively low temperatures, even when the gas to be measured contains CH4, Co, H2, etc., which have low ignition points. , it becomes possible to prevent the consumption of oxygen due to the combustion of these components during measurement, and the measurement accuracy is greatly improved. In addition, even if closed pores exist in the porous electrode, the oxygen partial pressure of the gas at the interface between the partition wall and the electrode increases due to the oxygen pumping action of the fine particles of the solid electrolyte contained in the solid electrolyte in an amount of 5 to 15% by weight. Since the response speed quickly comes to balance with that of , the response speed is greatly improved as shown in the embodiment.

[Brief explanation of the drawing]

FIG. 1 is a schematic vertical cross-sectional view showing an oxygen sensor according to an embodiment of the present invention and an oxygen concentration measuring device using the oxygen sensor, and FIG. 3 to 12, which are schematic vertical cross-sectional views showing model electrodes, are graphs showing response characteristics of an oxygen sensor according to an embodiment of the present invention and an oxygen sensor outside the scope of the present invention. 1: Solid electrolyte partition 2: Porous electrode 3: Porous electrode 4: Oxygen sensor 5: Solid electrolyte fine particles 6: Silver 7: Closed pores 8: Outer tube 9: Reference gas introduction tube 10: Electromotive force meter

Claims (2)

[Claims] (1) having a partition wall made of a solid electrolyte having oxygen ion conductivity, a reference gas side porous electrode provided on one wall surface of the partition wall, and a measured gas side porous electrode provided on the other wall surface of the partition wall; The porous electrode, at least the porous electrode on the gas-to-be-measured side, is composed of a mixed material of silver and solid electrolyte fine particles having oxygen ion conductivity, and the amount of the fine particles is 5 to 15% by weight. A solid electrolyte oxygen sensor featuring: (2) An oxygen concentration measuring device comprising the solid electrolyte oxygen sensor according to claim (1).