JPH0717780A - Method for producing porous ceramic film - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a method for easily and easily producing a porous ceramic membrane having a small pressure loss and an excellent gas separation efficiency as a gas separation membrane in a short time. [Structure] 400-1200 on a substrate made of a porous ceramics sintered body by a CVD method using alkoxides such as Si, alkoxides such as B, and oxygen gas as a source gas.
By forming a composite oxide film such as SiO ₂ —B ₂ O ₃ at a temperature of ℃, and etching the composite oxide film with vapor of HCl or HF by utilizing the difference in acid resistance,
Only specific oxides such as ₂ O ₃ are removed. By this method,
The pores formed in the trace after the removal of the specific oxide such as B ₂ O ₃ have a uniform pore size of nanometer size, which communicates in a substantially linear manner from the bonding surface with the base material to the surface of the other film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a porous ceramic membrane having nanometer-sized pores useful as a gas separation membrane.

[0002]

2. Description of the Related Art In recent years, as the global warming phenomenon has become a problem, the emission control of CO ₂ gas, which is the main cause of the global warming phenomenon, has become stricter, and a technology for separating CO ₂ from exhaust gas after combustion is required in a thermal power plant or the like. Is becoming. For such gas separation, a porous membrane having pores of nanometer size is required to allow only specific gas molecules to pass through.

Porous glass is well known as a porous material having pores of nanometer size. Porous glass is synthesized by a phase separation method or a sol-gel method. That is, in the phase separation method, the sodium borosilicate glass is heat treated to form a SiO ₂ -rich phase and Na ₂ O.B ₂ O ₃
After separating into a phase rich in Na ₂ O and treating with an acid or alkali aqueous solution to elute the phase rich in Na ₂ O.B ₂ O ₃ , porous quartz glass is obtained. Also by the sol-gel method, a porous glass can be synthesized by hydrolyzing a solution of an alkoxide of various metals such as Si into a sol state, coating the solution on a substrate or the like, and drying and gelling the solution.

However, although the pores of the porous glass synthesized in this manner can be made to have a size of nanometer, the pores 1 existing in the glass skeleton 2 are directional as shown in FIG. When used as a gas separation membrane, the pressure loss when passing through the gas increases,
There is a problem that the amount of separation processing is small. Further, since the pores 1 have a poor directionality, if a small amount of suspended fine particles are present in the gas to be separated, clogging occurs and the filter function cannot be sufficiently exhibited.

Further, in order to use the porous glass for gas separation, it is necessary to make it into a thin film, but it is difficult to make it into a thin film by the phase separation method. From this point as well, the porous glass by the phase separation method can pass through the gas. There was a drawback that the pressure loss at that time became larger. On the other hand, since it is difficult to make the film thickness uniform in the sol-gel method,
When the porous glass obtained by the sol-gel method was used as the gas separation membrane, it was extremely difficult to control the pressure loss value.

Further, the porous glass has a drawback that it takes a very long time to manufacture. That is, in the phase separation method, heating for a long time is required to separate the glass, and a long time is required for the subsequent etching treatment with an acid or alkali aqueous solution. For example, etching is often performed using an aqueous HCl solution, but since it cannot be heated to 100 ° C. or higher, a processing time of 24 hours or more is generally required. On the other hand, in the sol-gel method, the process of immersing the substrate in the sol after adjustment and then pulling it up is repeated, and it takes a long time to dry the sol applied to the substrate. in this way,
Porous glass is not easy to manufacture and has a drawback that the cost of the gas separation membrane is high.

In addition to these problems, in order to achieve high separation efficiency as a gas separation membrane, it is preferable that the pore size of nanometer-sized pores be uniform, but porous glass has a uniform pore size. It was difficult to form pores. That is, an aqueous solution of acid or alkali is generally used for the etching treatment for forming pores, but since it is difficult for water molecules to penetrate into narrow gaps in etching of such an aqueous solution, the average pore diameter is particularly 2 nm. If it was less than (nm), it was not possible to obtain a porous glass having a uniform pore size.

[0008]

In view of such conventional circumstances, the present invention has a uniform film thickness, has pores of uniform nanometer size, has a small pressure loss, and is free from clogging. An object of the present invention is to provide a method for easily and easily producing a porous ceramic membrane useful as a small gas separation membrane in a short time.

[0009]

In order to achieve the above object, the method for producing a porous ceramic film provided by the present invention comprises at least one selected from the group consisting of Si, Zr, Ti, Hf, Pb and Al. Alkoxides of seed elements,
B, a base made of a porous ceramics sintered body having continuous ventilation holes by a CVD method using an alkoxide of at least one element selected from the group consisting of alkali metals and alkaline earth metals and oxygen gas as raw material gases. By forming a composite oxide film of the elements of both groups on the material at a temperature of 400 to 1200 ° C. and subjecting the composite oxide film to an etching treatment using vapor of hydrogen chloride or hydrogen fluoride, the B, alkali It is characterized in that only oxides of metals or alkaline earth metals are removed.

A porous ceramics sintered body as a base material is already known and is used as a filter material, a carrier, a sensor and the like. Originally, since the ceramics sintered body is obtained by sintering powder, there are a large number of pores having a diameter of about several μm. However, the pore diameter is controlled by the filling rate of the green compact, the sintering conditions, etc. The porous ceramics sintered body is formed as a result. However, since the purpose of the present invention is gas separation, a porous ceramic sintered body having a continuous ventilation hole is used.

[0011]

In order to obtain a gas separation membrane with a small pressure loss, it is necessary to make the pores present in the membrane directional. As a result of researching a method for forming pores having such a directionality, the present inventors have found that a CVD method (chemical vapor deposition method) using two or more kinds of metal alkoxide as a source gas is used to form a substrate. A film formed of a remaining oxide by forming a complex oxide film and etching away only a specific oxide with vapor of HCl or HF by utilizing the difference in acid resistance of each oxide in the complex oxide film. The inventors have found that directional pores can be formed therein, and have reached the present invention.

The CVD method itself is known, and the formation of an oxide film by the CVD method using a metal alkoxide is also used for the formation of an insulating film such as SiO _{2 in the} process of manufacturing a semiconductor device. However, oxide films in the field of semiconductors are often single oxides such as SiO ₂ and may be doped with a small amount of elements such as B and P in order to improve the smoothness of the film surface and the film strength. Not too much. On the other hand, the method of the present invention forms a macroscopic composite structure in which a large amount of two kinds of oxides are combined with each other by the CVD method, and only the specific oxide of the composite oxides is removed by etching.

That is, according to the CVD method using two or more kinds of metal alkoxides of the present invention as the source gas, each oxide phase in the complex oxide formed on the substrate is finely divided into the order of nanometers. It becomes a mixed pseudo-phase separation state. Moreover,
By setting the temperature of the base material for forming the composite oxide film to 400 ° C. or higher, each oxide phase can have a columnar structure elongated in the film thickness direction. Pores having a directivity extending linearly in the film thickness direction are obtained after the etching removal. However, when the temperature of the base material exceeds 1200 ° C., a powdery substance is generated without being formed into a film, so it is preferable to set the temperature to 1200 ° C. or lower.

Vapor of HCl or HF is used for the etching treatment for removing only the specific oxide having poor acid resistance in the complex oxide. In particular, since HF is highly corrosive, the etching time can be shortened. The etching temperature, that is, the temperature of the vapor of HCl or HF varies depending on the composition of the composite oxide, the pore size of the pores, etc., but is preferably 100 ° C. or higher in order to shorten the etching time. By increasing the temperature of the vapor, the etching rate is increased, and the etching can be completed in a much shorter time compared with the etching using the acid aqueous solution, but a temperature higher than 400 ° C. should be avoided.

Further, since the etching is performed by vapor, it is easy to penetrate into a narrow gap, so that extremely fine pores can be formed, and even if the average pore diameter is 2 nm or less, the pores having a uniform pore diameter are formed. Can be obtained. Moreover, by using HCl or HF vapor for etching, C
Since the complex oxide forming step by the VD method and the etching treatment step can be carried out in the same reaction vessel,
The manufacturing process is simplified and efficient processing is possible in a short time.

According to the method of the present invention, since the specific oxide to be removed has a directionality and exists in a finely divided pseudo-phase state in the composite oxide on the order of nanometers, it is present. It is possible to control the pore size of the pores formed by removal to the order of nanometer (nm = 10 ⁻⁹ m) and to give directionality to almost all the pores. Therefore, the porous ceramic membrane obtained by the method of the present invention has pores of uniform nanometer-sized pores which are communicated in a substantially linear manner from the joint surface with the base material to the surface of the other membrane. When used as a separation membrane, pressure loss is small and highly efficient gas separation is possible.

Since the pores of the known porous ceramics sintered body as the base material are much larger than those of the porous ceramics film of the present invention, the pressure loss due to the base material can be almost ignored. Moreover, since the porous ceramic film formed by the CVD method is bonded to the base material, it has excellent mechanical strength. Further, since the composite oxide film having a uniform film thickness can be formed along the shape of the substrate by the CVD method, the obtained porous ceramic film has a uniform film thickness and a flat plate shape. However, it is possible to obtain a porous ceramics film having various shapes such as a curved surface and a cylindrical shape by simply selecting the shape of the base material.

Only specific oxides from the composite oxide film are HC
In order to remove by etching with 1 or HF vapor,
It is preferable that the difference in acid resistance between two or more kinds of oxides constituting the composite oxide is large. As such a combination, SiO ₂ , ZrO ₂ , TiO ₂ , which are oxides excellent in acid resistance,
HfO ₂ , PbO ₂ or Al ₂ O ₃ and B ₂ O ₃ which is an oxide having poor oxide resistance, alkali metals such as Li, Na and K and alkaline earth metals such as Mg, Ca, Sr and Ba. There is an oxide. In particular, SiO ₂ having excellent acid resistance is preferable as a component that remains after the etching treatment to form the porous ceramic film.

Therefore, as the raw material gas for the CVD method in the present invention, Si which is a raw material of an oxide having excellent acid resistance,
An alkoxide of at least one element selected from the group consisting of Zr, Ti, Hf, Pb, and Al, and B selected as a raw material of an oxide having poor acid resistance, an alkali metal, and an alkaline earth metal. Also, two kinds of metal alkoxides are necessary, together with at least one kind of elemental alkoxide. In addition, it is necessary to use oxygen gas together with two kinds of metal alkoxides in order to obtain a porous ceramic film that is dense and has excellent adhesion to a substrate.

These raw material gases are carried by a carrier gas of an inert gas such as Ar, He, N _2, etc., but the flow rate ratios of the two kinds of metal alkoxides among the raw material gases are in the pseudo-phase-separated state. It is important in determining the fineness of the columnar structure of the oxide and thus the pore size of the finally obtained pores. That is, the smaller the proportion of B, the alkali metal or alkaline earth metal alkoxide, the finer the columnar structure of the oxide having poor acid resistance and the smaller the pore size of the obtained pores, but the proportion of the alkoxide is If the amount is too small, it will be impossible to obtain pores that are continuous in the film thickness direction from the bonding surface with the base material to the surface of the other film.

If the ratios of the two kinds of metal alkoxides are kept constant, the compositions of the two kinds of oxides, which are excellent in acid resistance and inferior in acid resistance, become almost constant. Therefore, the pore size and the porosity of the obtained pores. Becomes almost constant. On the other hand, if this ratio is changed on the way, that is, the gas flow rate of at least one metal alkoxide is increased or decreased stepwise or continuously, or the flow rate of one of both alkoxides is increased stepwise or continuously. And the other flow rate is decreased stepwise or continuously, the composition of the two kinds of oxides changes stepwise or continuously, so that the porosity and pore size of the obtained film are different from those of the base material. It is possible to change stepwise or continuously along the film thickness direction from the bonding surface of No. 1 to the other film surface.

For example, every time a predetermined film forming time elapses,
When the flow rate of the Si alkoxide is increased stepwise and the flow rate of the B alkoxide is decreased stepwise, B ₂ O ₃ becomes smaller toward the film surface side (upper side of the drawing), as shown in FIG. A SiO ₂ —B ₂ O ₃ composite oxide film having a columnar structure in which the composition of SiO ₂ and B ₂ O ₃ is changed stepwise is formed. Therefore, when this composite oxide film is etched with, for example, HCl vapor, B ₂ O _{3 which} is weak against acid is dissolved and removed, and pores having a stepwise change in pore diameter are formed in that portion, and thus the remaining SiO ₂ is formed. The porosity of the porous ceramic film naturally changes in a stepwise manner.

Such a porous ceramic film having a porosity and a pore diameter inclined in the film thickness direction can share the separating function according to the components of the gas to be separated. For example, a large number of suspended particulates are present in the exhaust gas of a thermal power plant, and this causes clogging of the filter. Therefore, conventionally, it was necessary to specially install a filter for separating particulates. However, if a porous ceramic film having a porosity and a pore diameter inclined in the film thickness direction is used, fine particles are first removed at a portion having a large pore diameter, and then CO ₂ is separated at a portion having a small pore diameter. it can.

[0024]

EXAMPLE 1 A bottomed cylindrical base material made of a porous alumina sintered body having a porosity of 40% and a pore diameter of about 1 μm and having an inner diameter of about 10 mm and a wall thickness of 1 mm closed at one end was used. , SiO ₂ —TiO ₂ —Na _{2 on} the outside of this substrate by the CVD method under the following conditions.
An O-based composite oxide film was formed to have a film thickness of about 100 μm.

Gas flow rate: Si (OC ₂ H ₅ ) ₄ 0.15 l / min Ti (OCH ₃ ) ₃ 0.12 l / min Na (OC ₂ H ₅ ) 0.07 l / min O ₂ 0.50 l / min Ar 1.20 l / min Temperature: 800 ° C Pressure: 100 Torr

The composition of the obtained SiO ₂ —TiO ₂ —Na ₂ O type composite oxide film was Si: Ti: Na = 36: in atomic ratio.
It was 22:42, and was almost constant throughout the film. This composite oxide film was made 1 by 100% hydrogen chloride (HCl) gas.
By performing an etching treatment at 50 ° C. for about 5 minutes to dissolve and remove the Na ₂ O phase, SiO ₂ − is substantially removed.
A porous ceramic film made of TiO ₂ having a porosity of about 46% and having pores with an average pore diameter of 1.5 nm and a thickness of about 100 μm was obtained.

The porous ceramic membrane formed on this substrate was used as a bottomed cylindrical gas separation filter, and the porous ceramic membrane 3 was incorporated into the apparatus shown in FIG. Then, the gas to be separated 5 in which H ₂ : N ₂ was mixed at a ratio of 1: 1 was introduced to the upstream side of the filter, and a vacuum pump 6 on the downstream side of the filter introduced a pressure of 3 kg / cm ² and 1.0 l /
While suctioning through the porous ceramics film 3 and the substrate 4 at a flow rate of min, the pressure on the upstream side of the filter was measured by the pressure gauge 7, and the gas analyzer 8 measured the composition of the gas passing through the filter.

For comparison, a bottomed cylindrical shape similar to that described above made of a porous glass (thickness: about 100 μm) having a porosity of about 45% and an average pore diameter of about 1.5 nm, which was manufactured by a normal phase separation method. A filter was prepared and incorporated into the apparatus of FIG. 3 in the same manner as above, and the pressure on the upstream side of the filter and the composition of the gas passing through the filter were measured under the same conditions as above. The results of these measurements are shown in Table 1.

[0029]

[Table 1]

From the results shown in Table 1, since the porous ceramic membrane of the present invention has a uniform pore size, it has a gas separation ability superior to that of the conventional porous glass membrane as a gas separation membrane, and at the same time, it is It can be seen that the pressure loss can be made smaller than that of the porous glass membrane.

Example 2 The same cylindrical bottomed cylindrical base material made of porous alumina as in Example 1 was used, and Z was formed on the outside of this base material by the CVD method under the following conditions.
The rO ₂ —TiO ₂ —CaO based composite oxide film was formed to a film thickness of about 100.
The film was formed to have a thickness of μm.

Gas flow rate: Zr (OC ₂ H ₅ ) ₄ 0.15 l / min Ti (OCH ₃ ) ₃ 0.15 l / min Ca (OC ₂ H ₅ ) 0.20 l / min O ₂ 1.20 l / min Ar 1.20 l / min Temperature: 700 ° C Pressure: 10 Torr

The obtained ZrO ₂ --TiO ₂ --CaO type composite oxide film was exposed to 100% HF vapor and subjected to etching treatment at 200 ° C. for about 3 minutes to dissolve and remove the CaO phase. The obtained porous ceramics film is substantially ZrO 2.
It was composed of _2- TiO ₂ , had a porosity of about 38%, and had an average pore diameter of 1.8 nm.

The porous ceramics membrane formed on this substrate was used as a bottomed cylindrical gas separation filter, and the porous ceramics membrane 3 was incorporated into the apparatus shown in FIG. It is. Next, the gas to be separated 5 in which H ₂ : N ₂ was mixed at 1: 1 was supplied to the upstream side of the filter, and the gas pressure on the upstream side was adjusted to 2.5 to 5.0 kg / cm by the vacuum pump 6 on the downstream side of the filter. 20 l / while changing in the range of ²
The composition of the gas passing through the filter was measured by the gas analyzer 8 for each gas pressure on the upstream side of the filter which was passed through the filter at a flow rate of min.

For comparison, a bottomed cylinder similar to the above, which is made of a normal porous glass (thickness: about 100 μm) having a porosity of about 39% and an average pore diameter of about 1.8 nm, manufactured by the phase separation method. A filter of the same shape was prepared and similarly incorporated in the apparatus of FIG. 3, and the composition of the gas passing through the filter was measured according to the gas pressure on the upstream side of the filter under the same conditions as above. The results of these measurements are shown in Table 2.

[0036]

[Table 2]

As can be seen from the results of Table 2, in the filter made of the porous ceramic membrane of the present invention, the composition of the gas passing through the filter hardly changes even if the gas pressure on the upstream side changes. In the filter made of the above porous glass, the H ₂ fraction of the permeated gas decreased sharply as the gas pressure on the upstream side increased. It is considered that this is because conventional porous glass has a wide pore size distribution of pores and includes a relatively large pore size, and thus N ₂ gas has passed through the large pores due to an increase in gas pressure on the upstream side.

[0038]

According to the present invention, the specific oxide in the complex oxide formed on the base material by the CVD method is converted into H by the difference in acid resistance.
By using the above method of etching removal of Cl or HF, a nanometer-sized pore diameter which is almost linearly communicated along the film thickness direction in a much shorter time and easily and easily as compared with the conventional production of porous glass. It is possible to manufacture the porous ceramics film having the fine pores in the form of being bonded onto the base material of the porous ceramics sintered body.

Since the porous ceramics membrane on the porous ceramics sintered body substrate has a uniform thickness and the nanometer-sized pores communicate with each other in a substantially linear manner along the thickness direction, the gas separation membrane is As a result, the pressure loss during gas permeation is extremely small, and since the pore diameters are uniform, high gas separation efficiency can be obtained. Further, the porous ceramic film having different pore diameters has an advantage that the pressure loss is small and the clogging due to the suspended fine particles is small.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view for explaining a complex oxide film formed by a CVD method according to the present invention.

FIG. 2 is a cross-sectional view of a porous glass manufactured by a conventional phase separation method.

FIG. 3 is a schematic cross-sectional view of a gas separation device used in an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pore 2 Glass skeleton 3 Porous ceramic film 4 Base material 5 Gas to be separated 6 Vacuum pump 7 Pressure gauge 8 Gas analyzer

Claims (3)

[Claims] 1. Si, Zr, Ti, Hf, Pb and Al
CVD using at least one alkoxide of an element selected from the group consisting of B, an alkoxide of at least one element selected from the group consisting of B, an alkali metal and an alkaline earth metal, and oxygen gas as source gases By the method, a composite oxide film of the elements of both groups is formed at a temperature of 400 to 1200 ° C. on a base material made of a porous ceramics sintered body having continuous pores, and the composite oxide film is subjected to hydrogen chloride or fluorination. A method for producing a porous ceramic film, characterized in that only the oxide of B, the alkali metal or the alkaline earth metal is removed by performing an etching treatment using hydrogen vapor. 2. The method for producing a porous ceramic film according to claim 1, wherein the temperature of the vapor of hydrogen chloride or hydrogen fluoride is 100 to 400 ° C. 3. The formation of the complex oxide and the subsequent etching treatment are performed in the same reaction vessel.
The method for producing the porous ceramic film according to claim 1.