JP2005336539A - Porous sintered compact and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous sintered compact in which the gradient distribution of pore sizes is improved, thus sintered compact strength and compact strength are high and sinterability is high, and to provide its production method. <P>SOLUTION: The porous sintered compact has a mean grain size of ≤1 μm, a residual carbon content of ≤100 ppm, and a porosity of 10 to 40%. The sintered compact preferably has electrical conductivity, comprises iron-group metal as the main component, and also has a specific surface area of ≥0.5 m<SP>2</SP>/g. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a porous sintered body and a method for producing the same, and more particularly to a porous sintered body that can be suitably used as an electrode for a fuel cell and a method for producing the same.

  Conventionally, various methods have been considered as a method for forming a porous sintered body. For example, a method of pressing a ceramic powder and making it porous by controlling the sintering temperature, or reducing the pressure when pressing the ceramic powder and then firing it to obtain a porous sintered body There are known a method, a method of blending a binder when forming ceramic powder, and gasifying and removing in sintering.

  When such a binder is used for molding, the pores are not uniformly distributed, and large pores are locally formed. Therefore, there is a problem that the strength is low even when firing.

  Therefore, a molded product is proposed in which the pore distribution form can be controlled by using an aqueous solution of a natural polysaccharide such as agar having good thermal decomposability as the binder (see, for example, Patent Document 1).

  In addition, a method has been proposed in which a foamed product and an aqueous solution of an induced protein are added and gelled using a stirred slurry (see, for example, Patent Document 2).

Furthermore, Ni fine powder is mixed into a thermoplastic resin such as polyethylene and then extruded to form a fiber, and the short fiber formed by irradiating with ultraviolet rays is mixed with water, a packing agent, a binder, and a dispersant. There has been proposed a method for producing a metal porous plate by forming into a green tape, degreasing and sintering in a reducing atmosphere (for example, see Patent Document 3).
JP-A-2001-261463 JP-A-6-293572 JP 2000-54005 A

  However, although the porous sintered body described in Patent Document 1 has an advantage that it is easy to obtain a high porosity, since there is a gradient in the pore diameter distribution, the outermost surface is easily densified, so the gas permeability is low. There was a problem of becoming.

  Although the porous sintered body described in Patent Document 2 can obtain a light-weight fired ceramic molded body having good thermal conductivity and thermal shock, the pore size is as large as 10 μm or more, so that the strength is lowered. there were.

  In addition, since the porous sintered body described in Patent Document 3 is sintered in a vacuum or in an inert atmosphere, residual organic components or residual carbon, which is a decomposition product of the resin component, remains, resulting in a decrease in sinterability. In addition, there is a problem that the strength decreases.

  Furthermore, in order to reduce the residual carbon, it is necessary to reduce the content of the resin component in the slurry. However, if the resin component is reduced to several percent or less, the strength of the green body before sintering formed from the slurry decreases. Therefore, there is a problem that the molded body is easily destroyed by external force applied to the molded body, such as vibration and contact, before the molded body is fired.

  Accordingly, an object of the present invention is to provide a porous sintered body having improved pore diameter gradient distribution, high sintered body strength and high molded body strength and high sintered body, and a method for producing the same.

  The present invention is based on the novel finding that by controlling the amount of residual carbon using the atomization of the crystals and the gelling agent, it is possible to suppress the strength reduction and to uniformly generate fine pores. In addition, the pore diameter gradient distribution can be improved, and the sintered compact strength and the molded article strength are high, and the porous sintered compact having a high sinterability and the manufacturing method thereof can be realized.

  That is, the porous sintered body of the present invention is characterized by comprising a porous sintered body having an average particle diameter of 1 μm or less, a residual carbon amount of 100 ppm or less, and a porosity of 10 to 40%.

  In particular, the porous sintered body is preferably composed mainly of an iron group metal. Thereby, it can be used for a fuel cell, an oxygen separator, a humidity sensor, a gas sensor, and the like.

  The iron group metal is preferably Ni. Ni has high electrical conductivity and is stable in a reducing atmosphere, so that it can be suitably used as an electrode material, particularly an electrode material for a fuel cell.

  The porous sintered body preferably contains at least one of zirconia, alumina, and titania as a main component.

It is preferable that the specific surface area of the porous sintered body is 0.5 m ² / g or more. Thereby, when it uses as a porous electrode, electrical conductivity can be made high.

  The porous sintered body preferably has electrical conductivity. Thereby, a conductive porous sintered body that is cheaper than plating or sputtering can be formed and used as a moisture adjusting sheet.

  It is preferably used as an electrode. Thereby, since the size of the pores can be reduced, the thickness can be reduced, and the electrical resistance can be reduced.

  The method for producing a porous sintered body according to the present invention comprises mixing a powder having an average particle size of 0.1 μm or less, a gelling agent and a solvent at a temperature not higher than the melting temperature of the gelling agent to produce a mixture. A molding step in which the mixture is put in a specific mold and molded to produce a molded body, a calcining step of firing the molded body to obtain a sintered body, and the gelation from the sintered body And a gelling agent removing step for removing the agent. Thereby, it is possible to improve the gradient distribution of the pore diameter, and to produce a porous sintered body having high sintered body strength and molded body strength and high sinterability.

  In the molding step, it is preferable that the gelling agent is cooled to a temperature equal to or lower than the gel point of the gelling agent and solidified to be molded. Thereby, powder scattering of 0.1 μm or less can be prevented, and the gelling agent can be uniformly dispersed.

  After the gelling agent removing step, it is preferable to include a heat treatment step for performing a heat treatment at a temperature higher than that of the calcining step. Thereby, the amount of residual carbon contained in a porous sintered compact can be made low.

  It is preferable that the calcination temperature of the calcination step is 600 ° C. or less. Thereby, uniform pores can be obtained.

  In the gelling agent removing step, the sintered body is preferably immersed in an acidic solution. Thereby, since the gelling agent melts at a temperature below the gel point, carbon remaining in the molded body can be reduced.

  The raw material powder is preferably an iron group metal. Thereby, since it is stable and has high conductivity even in a reducing atmosphere, it can be suitably used as an electrode of a fuel cell.

  The present invention improves the sinterability using fine powder with high activity, suppresses the grain growth of fine powder, and uniformly generates pores that form a fine network, and as a binder a gelling agent such as agar And the residual carbon can be reduced by removing the gelling agent from the sintered body.

  The present invention relates to a porous sintered body, and it is important that the average particle diameter of crystal grains constituting the porous sintered body is 1 μm or less. When the average particle diameter of the crystal exceeds 1 μm, the contact area between the particles becomes small, and when used for a porous body, there is a disadvantage that the pores become large.

  It is important that the amount of carbon remaining is 100 ppm or less. If the amount of residual carbon exceeds 100 ppm, it may be disadvantageous for use as an electrode because coarse particles may be generated or patterns may be broken.

  It is important that the porosity of the porous sintered body is 10 to 40%. If the porosity is less than 10%, there is a drawback that it is difficult to form continuous pores in the porous sintered body, and if it exceeds 40%, it is difficult to keep the molding strength of the porous sintered body high.

  It is preferable that the porous sintered body has conductivity. By providing conductivity, it can be used as a protective film, a sensor, and an electrode, and can be applied to, for example, a protective film of a gas turbine used at a high temperature, a spark plug, and the like.

  In particular, when a porous sintered body is used as an electrode, it becomes possible to extend the life of the element because of the porous sintered body having a small size and high molding strength, and as a fuel cell, humidity sensor, gas sensor, etc. Available.

It is preferable that the specific surface area of the porous sintered body is 0.5 m ² / g or more, particularly 0.6 m ² / g or more, more preferably 0.8 m ² / g or more. When the specific surface area is 0.5 m ² / g or more, when used as a porous electrode, a high current can flow because of high conductivity.

  According to the present invention, the crystal particles constituting the porous sintered body are made of metal or ceramics.

Examples of the metal include gold, silver, copper, platinum, Fe, Ni, Co, Mn, and Al, but an iron group metal is preferably used as a main component from the viewpoint of obtaining high conductivity.

  Among iron group metals, Ni is stable in a reducing atmosphere, and therefore can be suitably used as an electrode material, and is particularly preferable in that it can be suitably used as an electrode of a fuel cell.

  The ceramics may be oxide ceramics or non-oxide ceramics such as nitride ceramics, carbide ceramics, and carbon ceramics. For example, oxide ceramics include alumina, mullite, zirconia, titania, silica, magnesia, ferrite, cordierite, yttria and other rare earth oxides, and nitride ceramics include aluminum nitride, silicon nitride, Examples of the sialon include silicon carbide, boron carbide, and tungsten carbide as the carbide ceramics, and examples of the carbon ceramic include carbon nanotubes.

  Alumina is excellent in high hardness, high strength, high wear resistance, insulation, and the like, and is low in cost. Therefore, it can be widely applied as a structural material or a substrate material. Moreover, since zirconia has high toughness and high strength and high ion conductivity, it can be suitably used as an electrode material or a solid electrolyte material. Furthermore, since titania is accompanied by a change in valence, it can be used suitably as a conductor material, a catalyst, or the like.

  FIG. 1 is a schematic cross-sectional view showing an example of a porous sintered body, which has metal particles 1 and ceramic particles 2 and includes a plurality of pores 3 in a state where only the neck of each particle is sintered. Shows the case. As shown in FIG. 1, the porous sintered body is not limited to a composite material of metal and ceramics, and may be a single metal or a single ceramic.

  As the metal particles 1 used in FIG. 1, for example, at least one kind of metal such as nickel, iron, platinum or the like and the ceramic particles 2 can be constituted by combining at least one kind from the above-mentioned ceramics, for example.

  In particular, by using Ni as the metal particles 1 and zirconia as the ceramic particles 2 of the porous sintered body in which pores are three-dimensionally formed, Ni having conductivity by electron conduction and conductivity by ion conduction are provided. And can be suitably used as an electrode material such as a fuel electrode of a fuel cell.

  Next, a method for producing the above porous sintered body will be described. First, raw material powder is prepared. As the raw material powder, a metal powder or a ceramic powder as described above is prepared. The average particle size of the raw material powder is preferably 100 nm or less, particularly 80 nm or less, and more preferably 50 nm or less. When mixing a powder of 100 nm or less with a solvent and a gel-forming substance, the smaller the particle size of the molding powder, that is, the finer the particles, the more the solvent is taken away from the periphery of the gel-forming substance. Since the powder easily aggregates strongly around the gel-forming substance particles, the advantage of suppressing the strength reduction when producing the molded body is obtained.

  Next, it is important to prepare a mixture (mixing step) by mixing the raw material powder with a gelling agent and a solvent at a temperature not higher than the melting temperature of the gelling agent. By using the gelling agent in this way, a porous body free from particle aggregation can be obtained.

  By heating and kneading the mixture of powder, gelling agent and solvent at a temperature above the melting temperature of the gelling agent to form a sol, and then cooling the sol to a temperature below the gel point of the gel forming material A molded body is obtained.

  A natural or synthetic hydrocolloid is preferably used as the gelling agent. Hydrocolloids include polysaccharides such as agar, carrageenan, farseleran, alginic acid and its derivatives, azodobacter vinelandie gum, pectin, curdlan, starch and its derivatives, proteins such as gelatin, chitin, chitosan and modified whey protein Etc. In particular, it is preferable to use agar as a binder. For example, it is preferable to use an agar having an average molecular weight of 30,000 to 150,000, a gel strength of 200 to 1000 g / cm 2 at an agar concentration of 5%.

  The solvent is a gel generating solvent that dissolves the gelling agent, generates a sol when heated, and generates a gel when the temperature drops. The solvent is particularly preferably water, but may be a hydrophilic organic compound solvent such as alcohols, or a mixture of water and a hydrophilic organic compound solvent. As the hydrophilic organic compound, alcohols such as methanol, ethanol and propanol are preferable. The reason for this is that alcohol enters the agar molecule and further substitutes for water and swells, so that it does not absorb water rapidly and it is difficult to produce aggregates.

  Further, the amount of the gelling agent is preferably 12 parts by weight or less, and more preferably 8 parts by weight or less with respect to 100 parts by weight of the powder. Thus, when mixing the gelling agent powder, the amount of water lost from the slurry is reduced, so that microcracks in the sintered body are suppressed. In the mixture of the present invention, the amount of the solvent is preferably 40 parts by weight or less, and more preferably 30 parts by weight or less with respect to 100 parts by weight of the powder. By reducing the amount of solvent, the pore size can be kept small.

  The temperature at which the molding powder, the gelling agent and the solvent are mixed is not higher than the melting temperature of the gelling agent. Even with the same gel-forming substance, the dissolution temperature may vary if the solvent is different. In the present invention, it is particularly preferable to mix at a temperature slightly lower than the dissolution temperature. For example, in the case of agar, the dissolution temperature is preferably 50 ° C. or lower, more preferably 40 ° C. or lower. .

  The dissolution temperature of the gelling agent is a temperature at which the gelling agent dissolves in the solvent and becomes a sol when the temperature is raised after mixing the gelling agent and the solvent.

  It is also important that the mixture produced in the mixing step is put into a specific mold and molded to produce a molded body (molding step).

  In the molding step, it is preferable to cool the gelling agent to a temperature equal to or lower than the gelling point of the gelling agent and to solidify and mold. A gelling agent is a substance that generates a fluid sol by heating above the melting temperature and then forms a gel by cooling the sol to a temperature below the gel point. It is preferable to use a substance that generates a sol at a melting temperature or higher and generates a gel at a temperature lower than the gel point. When such a reversible gelling agent is used, the gelling agent can be easily removed.

  It is important to sinter the molded body obtained in the molding process to obtain a sintered body (calcination process). The firing temperature in this calcining step is a temperature at which particles adhere to each other.

  The calcining temperature in the calcination step is preferably 600 ° C. or lower, particularly 500 ° C. or lower, and more preferably 300 ° C. or lower. By setting the temperature to 600 ° C. or lower, a module using an organic material can be used, and carbon nanotubes (CNT) can be added without being decomposed.

  Next, it is also important to remove the gelling agent from the sintered body obtained in the calcination step (gelling agent removal step). Thereby, a gelatinizer can be reused.

  In the gelling agent removing step, the sintered body is preferably immersed in an acidic solution. In acidic solutions, gels can be easily unnetworked. Therefore, the gelling agent can be efficiently removed. For example, when removing the gelling agent, it is preferable to immerse the sintered body in an acidic solution. Thereafter, the solvent may be dried through a water washing step.

  After the gelling agent removing step, firing at a temperature higher than the calcining step (heat treatment step) is preferable. This makes it possible to completely remove the water and solvent used when removing the gelling agent.

  The firing temperature in the post-baking step is preferably not less than the firing temperature in the calcining step and not more than 1000 ° C. By setting the temperature to 1000 ° C. or less, it becomes easy to keep the crystal grain size of the porous crystal body to 1 μm or less. Furthermore, a uniform porous body can be obtained.

(Raw material powder)
TiO ₂ powder having a purity of 99.9% and an average particle diameter of 3 μm, 100 nm, and 50 nm, and a purity of 99.99% and an average particle diameter of 30 nm and 100 nm, produced by a gas phase method (flame method) as a molding powder Prepared.

Further, NiO powder having a purity of 99.9% and an average particle diameter of 100 nm, YSZ (yttrium-stabilized zirconia: Y2O3: 8%) powder having an average particle diameter of 30 nm, and alumina having a purity of 98% and an average particle diameter of 50 nm A mixed powder prepared by mechanical bonding was prepared. Incidentally, YSZ powder and _Al 2 _{O 3} were respectively added 10 wt% with respect to the Ni powder.

  Agar, Yamato (manufactured by Ina Food Industry Co., Ltd., dissolution temperature 100 ° C.), and UM-11 (immediately soluble agar: manufactured by Ina Food Industry Co., Ltd., dissolution temperature 100 ° C.) were prepared as gelling agents.

(Mixing process)
5 times the weight of ethanol was added to the weight of the agar and mixed. (Agar A) Ion-exchanged water was added to the molded powder so that the water content was 30% by weight and mixed well. Then, pot mill mixing was performed for 12 hours using resin balls as media to obtain a slurry.

  The slurry was transferred to a mixing tank, and 5% by mass of Agar A in terms of dry powder was dropped and mixed while stirring. The temperature of the mixing tank to which agar was added was raised from room temperature to 95 ° C. over 30 minutes, and the slurry was kneaded while maintaining the temperature of the material at 95 ° C. Kneading was performed at 95 ° C. for 30 minutes to prepare a sol.

(Molding process)
The obtained sol was poured into a mold and cooled to room temperature to obtain a gel molded body.

(Calcination process)
Sintering was performed at each calcining temperature to obtain a porous sintered body.

(Gelling agent removal process)
Thereafter, this porous sintered body was immersed in an acidic solution (pH 3 acetic acid aqueous solution) for 30 minutes to remove the gelling agent. The acidic solution was washed with water.

(Heat treatment process)
After the gelling agent removal step, the porous sintered body was heat-treated as desired.

(Evaluation)
The amount of carbon remaining in the obtained sintered body was measured by gas analysis. The porosity was evaluated by the Archimedes method, the specific surface area was evaluated by the BET method, and the average particle size of the particles was also evaluated by the particle size measurement method. The results are shown in Table 1.

  Sample No. of the present invention. Samples Nos. 3 to 12 have a large average particle size. Compared to 2, the sinterability was enhanced, the pores were not inclined, and the strength of the sintered body was 4.8 MPa or more. It was within the scope of the present invention.

On the other hand, the sample No. 2 outside the scope of the present invention has a large average particle diameter of the raw material powder (Ni) of 3 μm. No. ² was densified and had a specific surface area as small as 0.2 m ² / g and could not be used as a porous material. In addition, since the amount of residual carbon was large, the sintered body strength was as small as 3 MPa.

  In addition, sample Nos. Out of the scope of the present invention using gelatin as a gelling agent. No. 1 could not obtain a sintered body because the strength of the compact was low.

It is a schematic sectional drawing which shows an example of the porous sintered compact of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Metal particle 2 ... Ceramic particle 3 ... Pore

Claims (13)

A porous sintered body comprising a porous sintered body having an average particle diameter of 1 μm or less, a residual carbon content of 100 ppm or less, and a porosity of 10 to 40%. The porous sintered body according to claim 1, wherein the porous sintered body contains an iron group metal as a main component. The porous sintered body according to claim 1, wherein the iron group metal is Ni. The porous sintered body according to any one of claims 1 to 3, wherein the porous sintered body contains at least one of zirconia, alumina, and titania as a main component. The porous sintered body according to claim 1, wherein a specific surface area of the porous sintered body is 0.5 m ² / g or more. The porous sintered body according to claim 1, wherein the porous sintered body has conductivity. The porous sintered body according to claim 6, which is used as an electrode. A mixing step in which a raw material powder having an average particle size of 100 nm or less, a gelling agent and a solvent are mixed at a temperature not higher than the dissolution temperature of the gelling agent to produce a mixture, and the mixture is put into a specific mold and molded. Forming a molded body, calcining the sintered body to obtain a sintered body, and a gelling agent removing step for removing the gelling agent from the sintered body. A method for producing a porous sintered body. The method for producing a porous sintered body according to claim 8, wherein, in the molding step, the gelling agent is cooled to a temperature equal to or lower than a gel point of the gelling agent and solidified to be molded. The method for producing a porous sintered body according to claim 8 or 9, further comprising a heat treatment step of performing a heat treatment at a temperature higher than that of the calcining step after the gelling agent removing step. The method for producing a porous sintered body according to any one of claims 8 to 10, wherein a firing temperature in the calcining step is 600 ° C or lower. The method for producing a porous sintered body according to any one of claims 8 to 11, wherein the gelling agent removing step immerses the sintered body in an acidic solution. The method for producing a porous sintered body according to any one of claims 8 to 12, wherein the raw material powder is an iron group metal.

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