JPH09278559A - Porous substrate for forming thin film, method for producing the same and oxygen pump apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a porous substrate, having small degrees of surface roughness and suitable for an oxygen pump apparatus by forming a layer of fine particles on a surface of a porous board, bonding them with a bonding agent and being impregnated with a solution of a metal having oxidation-reduction activity on oxygen. SOLUTION: An alumina paste prepared by dispersing alumina fine particles 7 having particle diameters of about 1μm is applied on a porous aluminum board 6 of about 0.5mm in thickness formed by sintering, the obtained alumina- coated board is dried by air and subsequently the resultant board is sufficiently dried at about 300-1000 deg.C by using an electric furnace. The process is repeated 2-5 times so that the thickness of the layers of alumina fine particles 7 is made larger than the unevenness of the surface of the porous alumina board 6. Chloroplatinic acid solution in ethylene glycol is dropped on the obtained board to impregnate the porous aluminum board 6 and the alumina fine particles 7 and the resultant board is heated at about 300-1000 deg.C. Thus, platinum deposits on the inner surfaces of pour holes formed by alumina particles 9 and on the surfaces of the alumina fine particles 7.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a porous substrate used for forming a thin film and an oxygen pump using the porous substrate.

[0002]

2. Description of the Related Art As a technique for forming a thin film on a porous substrate, an oxygen sensor using a zirconia thin film which is a solid electrolyte having oxygen ion conductivity or a CaZrO3 thin film (for example, Technical Digest of th.
e 9th Sensor Symposium, 19
90. pp213 and Proceeding of
he 21th Chemical Sensor S
ymposium, 1995. pp145) etc. exist conventionally. In the oxygen sensor, as shown in FIG. 5, a porous first platinum electrode thin film 52, a solid electrolyte thin film 53, and a porous second platinum electrode thin film 54 are sequentially laminated on a porous substrate 51. Oxygen gas in the atmosphere that is configured and is in contact with one surface of the solid electrolyte thin film 53 is solid electrolyte thin film 53.
This utilizes the oxygen pumping phenomenon in which ions move inside and are released as oxygen gas from the other surface. As described above, in the solid electrolyte thin film 53,
Oxygen moves as ions, but since oxygen does not exist in the ionic state in the first place,
There is a need to pre-ionize the oxygen before it passes through the solid electrolyte. The platinum electrodes 52 and 5 play this role.
4, and the platinum electrodes 52 and 54 are used as electrodes for performing a redox reaction for converting oxygen gas into oxygen ions and vice versa.

Further, in the oxygen sensor shown in FIG. 5 described above, the substrate forming the solid electrolyte thin film 53 must be made of a porous material permeable to gases such as oxygen, and particles such as alumina are generally used. Is formed by sintering
A porous material whose surface is polished is used. As described above, the reason why alumina is generally used as a substrate is that the material called alumina is a material that is resistant to oxidation and has heat resistance to about 300 ° C. Because there is.

[0004]

On the other hand, the thin film material of solid electrolyte such as zirconia used for the above oxygen sensor, oxygen pump, etc. has a film thickness as thin as possible from the viewpoint of performance such as response speed and oxygen transfer speed. It is desirable to do. For example, since the oxygen transfer rate is inversely proportional to the film thickness, the oxygen transfer rate is 2 if the film thickness of the solid electrolyte can be halved.
Can be improved by a factor of two. Therefore, oxygen pump performance,
That is, further thinning is indispensable for improving the oxygen transfer amount.

However, in order to form a thin film on a porous substrate, the unevenness present on the surface of the porous substrate is made sufficiently smaller than the thickness of the thin film formed on it in terms of the coverage of the thin film. I need to put it. For example, thickness 1
In order to form a thin film of μm, the roughness of the porous substrate needs to be suppressed to about 0.1 μm or less. However, even if the surface of the porous material is polished, the degree of unevenness cannot be made smaller than the size of the particles constituting the porous material. Therefore, the film thickness of the solid electrolyte must be unavoidably formed, and it must be formed sufficiently thicker than the roughness of the substrate, and the effect of thinning cannot be exhibited.

To solve the above problems, the particles of alumina or the like which form the porous substrate are made finer (for example, 0.1 μm).
The surface roughness can be reduced to reduce the thickness of the thin film, but in that case, the alumina particles are formed too densely and it is difficult for gases such as oxygen to permeate through the porous substrate. Then, this becomes the rate-determining factor, and the amount of oxygen supplied to the solid electrolyte decreases, which causes another new problem that the oxygen pump performance deteriorates.

Also, it is relatively coarse (for example, about 10 μm)
A fine (eg, about 0.1 μm) alumina particle layer is formed by sintering on the surface of a porous substrate formed of alumina particles, and the surface is polished to produce a porous substrate having a small surface roughness. It can also be used. However, in this case, 1500 ° C for sintering alumina.
Since such a high temperature is required, it is necessary to sinter the alumina particles having different particle diameters a plurality of times, and there is a problem that the substrate tends to warp due to the sintering. Further, since alumina is a hard material, it takes a long time for the polishing process.

On the other hand, as described above, in order to construct the oxygen pump, electrodes on both sides of the solid electrolyte thin film that convert oxygen gas into oxygen ions and electrodes that have an oxidation-reduction effect that converts oxygen ions into oxygen gas are used. Need to be sandwiched between. Platinum is generally used for these electrode materials, but it is necessary to bring the oxygen gas or oxygen ions into sufficient contact with the platinum electrode in order to carry out the reaction sufficiently. That is, it is necessary to increase the contact area between the oxygen gas or oxygen ions and the platinum electrode, and if this is small, the conversion reaction becomes rate-determining and the oxygen pump performance deteriorates.

In order to solve the above problems, in order to increase the contact area between the oxygen ions and the platinum electrode, it is necessary to make the platinum electrode thick. However, when a platinum electrode is formed on the surface of a porous substrate by a sputtering method or the like, the surface roughness of the formed platinum electrode becomes rougher than the unevenness of the underlying porous substrate, so this electrode cannot be formed thick. .

Similarly, the thickness of the other platinum electrode formed on the surface of the solid electrolyte thin film must be increased to some extent. Although the thickness of this electrode is not limited as long as it is porous, it is necessary to minimize the amount of platinum used since it is expensive. However, in order to obtain a sufficient effect, the amount of platinum that forms a certain thickness is necessary, and there is a problem that the cost becomes high.

[0011]

In order to solve the above-mentioned problems, in the present invention, as a first means for obtaining a porous substrate suitable for an oxygen pump, the surface of a porous plate is covered with fine particles to form oxygen. Against the inner surface of the pores in the porous plate and the surface of each of the fine particles with a continuous film of a metal having a redox action against the porous plate and the fine particles and the fine particles to each other. A porous substrate having a small surface roughness and suitable for an oxygen pumping device is used by bonding.

This porous substrate comprises a step of covering the surface of the porous plate with fine particles, and a step of impregnating the porous plate and the fine particles with a solution of a metal having an oxidation-reduction effect on oxygen. , A step of depositing a continuous film of the metal by heating the solution.

As a second means for obtaining a porous substrate suitable for an oxygen pump, the surface of the porous plate is covered by binding fine particles with a binder to have a redox effect on oxygen. A porous substrate having a small surface roughness and suitable for an oxygen pump device is used by covering the inner surfaces of the pores of the porous plate and the surfaces of the fine particles and the binder with a continuous metal film.

This porous substrate has a step of covering the surface of the porous plate with fine particles, a step of binding the porous plate and the fine particles with a binder, and an oxidation-reduction effect for oxygen. It is manufactured by a step of impregnating the porous plate and the fine particles with a solution of a metal, and a step of depositing a continuous film of the metal by heating the metal.

As a concrete constitution of the oxygen pump using the above-mentioned porous substrate, a continuous film of a metal having a redox action on oxygen is formed on the porous plate and the surface of the porous plate. A solid electrolyte thin film having oxygen ion conductivity and a metal electrode having a redox action on oxygen are laminated on a porous substrate formed by combining fine particle layers.

Further, the technique of forming the porous substrate of the present invention is applied to suppress the amount of platinum used in the platinum electrode formed on the surface of the solid electrolyte thin film. That is, the electrode formed on the solid electrolyte thin film has an oxygen pump structure using a layer of particles on the surface of which a continuous metal film having a redox effect on oxygen is formed.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION A case in which a porous substrate and an oxygen pump device using the same in an embodiment of the present invention are applied to a deoxygenated atmosphere storage will be described below with reference to FIG. Note that the deoxygenated storage is to prevent the oxidation of foods, etc., and the storage for such a purpose is conventionally prepared by replacing the air in the storage with an inert gas such as nitrogen gas or carbon dioxide gas. Although a deoxidized atmosphere was realized, the technique of the present invention made it possible to obtain the same effect without using an inert gas.

In FIG. 1, reference numeral 1 is a wall surface of the deoxidized atmosphere storage cabinet, the lower side in the figure is the internal atmosphere side, and the upper side is the atmospheric atmosphere side. Reference numeral 2 denotes an oxygen pump device using the porous substrate 3 of the present embodiment. The oxygen pump device 2 has an oxygen pump unit 4 and a heating unit 5, and stores oxygen gas in the deoxygenated storage chamber. It moves in the direction of arrow A and discharges to the outside of the store indicated by arrow B.

The porous substrate 3 is obtained by polishing the surface of a porous alumina plate 6 obtained by sintering relatively large alumina particles (for example, 10 μm) with alumina fine particles 7 having a particle diameter of 0.1 μm. In addition, the structure is such that the inner surfaces of the pores of the porous alumina plate 6 and the surfaces of the individual particles of the alumina fine particles 7 are connected by a continuous platinum film (see FIG. 3). The porous substrate 3 has a function of converting the oxygen gas in the deoxygenated storage into negative oxygen ions by the function of the platinum film.

Next, a method of manufacturing the above-mentioned porous substrate 3 will be described below with reference to FIG.

First, as shown in FIG. 2A, the thickness is 0.5.
On the surface of the porous alumina plate 6 (the particle diameter of alumina is, for example, about 10 μm) formed by sintering of 0.1 mm, the particle diameter of 0.
Alumina paste in which 1 μm alumina fine particles 7 are dispersed in a solvent is applied and naturally dried, and then 300 using an electric furnace.
Sufficiently dry at a temperature of about 1000 ° C. This step is repeated about 2 to 5 times to make the layer of the alumina fine particles 7 thicker than the size of the irregularities on the surface of the porous alumina plate 6 to form the alumina fine particle layer. In this state, the alumina fine particles 7 adhere to the porous alumina plate 6 relatively hard, but are so brittle that they are easily scratched. Further, since the dried alumina fine particles 7 adhere to the surface of the underlying porous alumina plate 6, the irregularities on the surface of the alumina fine particle 7 layer are large.

Next, as shown in FIG. 2 (b), chloroplatinic acid (hexachloroplatinic (IV) acid hexahydrate) 8 dissolved in ethylene glycol is dropped onto the porous alumina plate 6 and the alumina fine particles 7. Impregnate and 300 to 10 in an electric furnace
Heat at a temperature of 00 ° C. This process is repeated about 2 to 5 times. As a result, platinum is deposited in a film form on the surfaces of the alumina particles 9 constituting the porous alumina plate 6, that is, the inner surfaces of the pores formed by the alumina particles 9 and the surfaces of the alumina fine particles 7 (see FIG. 3). As shown in FIG. 3, the deposited platinum forms a continuous platinum film 10 and firmly bonds the alumina fine particles 7 to each other and the alumina fine particles 7 and the porous alumina plate 6 firmly. As a result, the porous alumina plate 6 and the alumina fine particles 7 become an integrated conductor. Since the thickness of the platinum film 10 can be formed uniformly with respect to the alumina particles, the thickness of the platinum film 10 can be controlled by the concentration of chloroplatinic acid and the number of repetitions of this step.

In the method of the present embodiment described above, the alumina particles 7 are not joined by sintering, but
Since they are bonded by platinum, they can be solidified at a lower temperature than in the case of sintering alumina, and as a result, the disadvantage that the alumina plate is warped does not occur.
The platinum film 10 is composed of alumina fine particles 7 and alumina particles 9
Since it covers the surface of, it has a vast area.

Further, as shown in FIG. 2 (c), the surface of the alumina fine particles 7 is polished by using a polishing machine. Since the alumina fine particles 7 are bonded to each other by the platinum film 10, they can be easily polished. In this embodiment, the alumina particles 7 are bonded with platinum as described above, and then the surface is polished. However, after the alumina paste layer before bonding with platinum is used, the alumina particle layer is formed and then polished. May be performed.

The particle size of the alumina fine particles 7 is 0.
Since it is 1 μm, the roughness of the polished surface can be suppressed to about 0.2 μm or less, and a very smooth surface can be obtained. The layer of the alumina fine particles 7 has a thickness of the porous alumina plate 6
The thickness should be so thin that it is not exposed. If the alumina fine particles 7 are sintered at a high temperature without using the technique of the present invention, this polishing becomes difficult and it takes a long time.

The porous substrate 3 produced by the above method itself serves as a platinum electrode. As described above, the porous substrate 3 needs to have an action of converting the oxygen gas in the deoxygenated storage into negative oxygen ions. Since the platinum electrode formed in the present embodiment has a large surface area, it can provide a sufficient contact area for the oxygen gas and the platinum film 10 to come into contact with each other, and the conversion reaction can be carried out quickly and completely. It has the performance that can be done. Moreover, since the layer of the alumina fine particles 7 is thin, there is a feature that the resistance against movement of oxygen gas is small.

A zirconia thin film 11, which is a solid electrolyte having oxygen ion conductivity, is formed on the surface of the porous substrate 3 by a sputtering method, and a second platinum electrode 12 is formed on the surface (FIG. 1). Since the surface of the porous substrate 3 is very smooth, the thickness of the zirconia thin film 11 can be reduced to about 1 μm.

The second platinum electrode 12 operates to convert negative oxygen ions into oxygen gas. In order to perform this conversion reaction quickly and completely, the second platinum electrode 12 also needs a large contact area. Therefore, the second platinum electrode 12
Is used by sintering a platinum paste applied relatively thickly. In this embodiment, in order to reduce the amount of expensive platinum used, the second platinum electrode can also be manufactured by the same method as the method of forming the platinum film on the porous substrate 3. A method of manufacturing the second platinum electrode 12 will be described below with reference to FIG.

First, as shown in FIG. 4 (a), the surface of the zirconia thin film 11 is coated with an alumina paste in which fine alumina particles 13 having a particle size of 0.1 μm are dispersed in a liquid and naturally dried, and then an electric furnace is used. And fully dry at a temperature of about 300 to 1000 ° C. This process is repeated 2 to 5 times to obtain a layer of alumina fine particles 13 (for example, 0.1 m thick).
m) is formed to a predetermined thickness.

Next, as shown in FIG. 4B, chloroplatinic acid 8 dissolved in ethylene glycol is added to the alumina fine particles 13.
Is dripped to impregnate and heated at a temperature of 300 to 1000 ° C. in an electric furnace. This step is repeated about 2 to 5 times, whereby platinum is deposited in a film form on the surface of the alumina fine particles 13. The deposited platinum film 10 firmly bonds the alumina fine particles 13 to each other and the zirconia thin film 11 and the alumina fine particles 13 firmly. Moreover, platinum becomes a continuous film and forms the second platinum electrode 12. The thickness of the platinum film can be controlled by the concentration of chloroplatinic acid and the number of repetitions of this step, as in the case of forming the porous substrate 3 described above.

The second platinum electrode 12 formed in this embodiment has a large surface area because the surface of the alumina fine particles 13 is covered with a platinum film, and the oxygen gas and the platinum film come into contact with each other. Sufficient contact area can be provided. Therefore, it has the ability to convert oxygen ions to oxygen gas quickly and completely.
Moreover, since platinum is only formed on the surface of the alumina fine particles 13, the amount of platinum used can be extremely small.

The oxygen pump unit 4 is manufactured as described above, and the heating unit 5 having the heater 14 and the support substrate 15 (formed of alumina or the like) is arranged so as to face the oxygen pump unit 4, and the oxygen pump unit 2 is provided. (See FIG. 1). In addition, 16 is a power source for heating the heater 14,
Reference numeral 17 is an oxygen pump operating power source. The oxygen pump operating power source 17 is arranged so as to apply a positive voltage to the second electrode 12 and a negative voltage to the porous substrate 3.

Next, the operation of the oxygen pump device according to the present embodiment will be described below with reference to FIG.

First, the heater 14 and the heating power source 16 are used to heat the heating section 5, and the oxygen pump section 4 is heated to about 800 ° C. by its radiation and convection. Next, a positive voltage and a negative voltage are applied to the second platinum electrode 12 and the porous substrate 3, respectively, using the oxygen pump operating power supply 17. The oxygen gas in the deoxidized storage changes into negative oxygen ions in the porous substrate 3 and reaches the lower surface of the zirconia thin film 11. Oxygen ions pass through the oxygen ion defects in the zirconia thin film 11 and move to the second platinum electrode 12 to which a positive voltage is applied, and the oxygen ions change to oxygen gas on the surface of the second platinum electrode 12.
It is discharged to the outside.

In order for the oxygen pump device 2 to exhibit high performance (in other words, to take in oxygen at a high speed), it is necessary to reduce the thickness of the zirconia thin film 11 and to use oxygen gas on the surface of the platinum electrode. It is necessary to promptly carry out the oxygen ionization reaction and the opposite oxygen gasification reaction of oxygen ions. Since the surface roughness of the porous substrate 3 of the present embodiment can be made extremely small, the zirconia thin film 11 can be thinned to about 1 μm or less. Further, since the porous substrate 3 and the second platinum electrode 12 have a large area, the ionization reaction of oxygen gas and the opposite reaction can be performed quickly. Therefore, by using the technique of the present invention, a higher performance oxygen pump device can be obtained. Moreover, since the amount of platinum used for the second platinum electrode 12 is small, there is a feature that it can be manufactured at low cost.

Although the zirconia thin film 11 is formed directly on the porous substrate 3 in this embodiment, a platinum thin film is formed on the surface of the polished porous substrate 3 by a sputtering method or the like, if necessary. You may keep it. In this case, since the contact area between the zirconia thin film 11 and the electrode increases,
It is possible to reduce the resistance value when operating the oxygen pump.

In the present embodiment, platinum, which has a large action, is used as the electrode material having a redox action on oxygen, but it is easy to apply a material other than platinum to the technique of the present invention. Is. For example, other metal materials such as nickel, gold and titanium, oxide materials thereof, or platinum-
An alloy material such as palladium may be used.

Furthermore, in the present embodiment, the alumina fine particles 7 of the porous substrate 3 and the alumina fine particles 13 of the second electrode 12 and the like are bonded by a platinum film, but a bonding material other than platinum may be used. . For example, by a method similar to that using chloroplatinic acid, alumina particles may be impregnated with a molybdenum chloride solution and heated at a high temperature to form a molybdenum oxide film, and the particles may be bonded to each other. Of course, if the operating temperature of the oxygen pump device is low, particles may be bonded using a glass frit or the like. In this way, the alumina fine particles may be previously bound to each other with a binder to form a binder layer, and thereafter, chloroplatinic acid may be impregnated and heated to form a platinum film on the surface thereof.
By using this method, the amount of platinum used can be reduced significantly.

In the present embodiment, alumina is used for the porous plate 6, and alumina is used for both the fine particles covering the porous plate 6 and the fine particles covering the zirconia thin film 11.
Of course, other materials may be used. Further, in the present embodiment, the alumina fine particles 7 and 13 having a particle diameter of 0.1 μm are used, but this particle diameter may be selected as necessary, and naturally a plurality of particles having different particle diameters may be used as necessary. The fine particles may be layered.

Although zirconia was used as the solid electrolyte thin film, the present invention does not limit the material of the solid electrolyte at all.

[0041]

By using the present invention, a porous substrate having a small surface roughness can be obtained, and a thin solid electrolyte thin film can be produced. This allows
The oxygen pump performance can be greatly improved.

On the other hand, if the solid electrolyte is made thinner, the oxygen pumping performance is rate-limited by the conversion reaction of oxygen gas and oxygen ions carried out at the platinum electrode. However, by using the technique of the present invention, a small amount of platinum is used. Thus, a wide reaction area can be obtained, and the conversion reaction can be performed quickly. Therefore, a higher performance oxygen pump can be obtained in combination with a thinner solid electrolyte. Moreover, since the layer of alumina fine particles is thin, the particle layer does not become a resistance and do not hinder the permeation of oxygen gas.

Furthermore, since the temperature at which the alumina fine particles are bonded can be made much lower than the temperature at which the alumina is sintered, not only the power consumption is small, but also the inconvenience that the substrate warps due to heat does not occur.

Further, according to the technique of the present invention, since the alumina fine particles are bonded by using platinum or the like, the surface can be easily polished, and the productivity can be greatly improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing the configuration of an oxygen pump according to an embodiment of the present invention.

FIG. 2 is a sectional view of a manufacturing process of a porous substrate according to an embodiment of the present invention.

FIG. 3 is a structural cross-sectional view of a porous substrate according to an embodiment of the present invention.

FIG. 4 is a second oxygen pump according to the embodiment of the present invention.
Of platinum electrode manufacturing process

FIG. 5 is a sectional view showing the configuration of a conventional oxygen sensor.

[Explanation of symbols]

 2 Oxygen pump part 3 Porous substrate 5 Heating part 6 Porous alumina plate 7 Alumina fine particles 11 Zirconia thin film 12 Second platinum electrode

Claims (6)

[Claims] 1. A porous plate, a fine particle layer formed on the surface of the porous plate, an inner surface of pores in the porous plate and individual surfaces of fine particles in the fine particle layer. A thin film-forming porous substrate having a metal film having a redox effect on oxygen formed in, wherein the metal plate is used to bond the fine particles in the porous plate and the fine particle layer to each other. A porous substrate for forming a thin film, which is characterized in that 2. A step of forming a fine particle layer on the surface of a porous plate; a step of impregnating the porous plate and the fine particle layer with a solution containing a metal having a redox action on oxygen. A method for producing a thin film-forming porous substrate, comprising a step of depositing a continuous film of the metal by heating the solution. 3. A porous plate, a fine particle layer formed on the surface of the porous plate, an inner surface of pores in the porous plate and individual surfaces of fine particles in the fine particle layer. A thin film forming porous substrate having a binder layer formed on a substrate, and a metal film having a redox effect on oxygen formed so as to cover the binder layer, wherein the binder layer and the metal A porous substrate for forming a thin film, wherein the porous plate and the fine particles in the fine particle layer are bonded to each other by a film. 4. A step of forming a fine particle layer on the surface of a porous plate, a step of binding the porous plate and the fine particles with a binder, and containing a metal having a redox effect on oxygen. A method for producing a thin film-forming porous substrate, comprising: a step of impregnating a solution with the porous plate and the fine particle layer; and a step of heating the solution to deposit a continuous film of the metal. 5. A porous plate, a fine particle layer formed on the surface of the porous plate, an inner surface of pores in the porous plate and individual surfaces of fine particles in the fine particle layer. A thin film-forming porous substrate having a metal film having a redox effect on oxygen formed on the substrate, a solid electrolyte thin film having oxygen ion conductivity formed on the porous substrate, and the solid electrolyte thin film An oxygen pump device having a metal electrode having an oxidation-reduction effect on oxygen formed above, wherein the porous plate and the fine particles in the fine particle layer are bonded to each other by the metal film. An oxygen pump device characterized by the above. 6. A porous plate, a first electrode formed on the porous plate, a solid electrolyte thin film formed on the first electrode, and a fine electrode formed on the solid electrolyte thin film. An oxygen pump device having a particle layer and a thin film forming porous substrate having a metal film having a redox effect on oxygen formed so as to cover individual surfaces of the fine particles in the fine particle layer. An oxygen pump device characterized in that the fine particles in the fine particle layer are bonded to each other by the metal film.