Patent Publication Number: US-2016221078-A1

Title: Aluminum-based porous body and method for manufacturing same

Description:
TECHNICAL FIELD 
     The present invention relates to a porous body having a strut, which is connected three-dimensionally, and having a three-dimensional network structure, which includes communicating holes that are formed by the strut so as to three-dimensionally communicate with each other. In particular, the present invention relates to an aluminum-based porous body, which has a strut made of aluminum or aluminum alloy, and relates to a production method therefor. 
     BACKGROUND ART 
     Porous bodies having a strut, which is connected three-dimensionally, and having a three-dimensional network structure, which includes communicating holes that are three-dimensionally formed by the strut, may be used as a filter (Japanese Unexamined Patent Applications Laid-Open Nos. 05-339605 (Patent document 1) and 08-020831 (Patent document 2)), a catalyst carrier (Patent document 2), etc. The filter allows a fluid such as a gas or a liquid to pass through the communicating holes and to filter the fluid. The catalyst carrier changes the fluid using a catalyst carried on the surface of the strut. 
     The porous bodies having such a three-dimensional network structure may be produced by the following method. In one method (Japanese Unexamined Patent Application Laid-Open No. 57-174484 (Patent document 3)), after the surface of a foamed resin strut having communicating holes is made conductible and is electroplated, the resin is decomposed by heating so as to be removed. In another method (Patent documents 1 and 2, and Japanese Examined Patent Application Publication No. 61-053417 (Patent document 4)), after a mixture of an organic polymer binder and micro metal bodies is coated on a foamed resin having communicating holes by immersing, spraying, or the like, the resin is decomposed by heating so as to be removed, while the micro metal bodies are sintered. In another method (Japanese Unexamined Patent Application Laid-Open No. 06-235033 (Patent document 5)), after the surface of a foamed resin strut having communicating holes is provided with adhesiveness and is adhered with powder, the resin is decomposed by heating so as to be removed, while the powder is sintered. 
     The porous bodies having such a three-dimensional network structure have a large contact area with a fluid, and therefore, use of the porous bodies as a heat exchanger component of a heat exchanger has been investigated (Japanese Examined Patent Application Publication No. 06-089376 (Patent document 6)). A heat exchanger is a device that is used for heating or cooling by efficiently transferring heat from a higher temperature object to a lower temperature object. In general, a fluid such as a gas or a liquid is used as a medium for heat exchange, and the heat exchanger heats or cools by providing heat to (heat) or removing heat from (cool) the fluid. In such a heat exchanger, the contact area with the fluid is increased by providing fins or the like, which are made of a metal material having a high thermal conductivity, whereby the heat exchange efficiency is increased. Nevertheless, alternatively, porous bodies having a three-dimensional network structure which is made of a metal material with a high thermal conductivity, may be used instead of the fins or the like, so that a fluid passes through communicating holes thereof. In this case, the contact area between the metal material with a high thermal conductivity and the fluid may be further increased, whereby the heat exchange efficiency may be greatly improved. 
     In view of this, since aluminum is light in weight and has a high thermal conductivity, use of aluminum for a porous body having a three-dimensional network structure has been proposed. However, since aluminum is difficult to electroplate, production using the electroplating as in Patent document 3 is difficult to perform. 
     On the other hand, in Patent document 4, a mixture of an organic polymer binder and an aluminum powder is coated on a foamed resin, which has communicating holes, by immersing, spraying, or the like. Then, the resin is decomposed by heating at 520° C. for 2 hours in a hydrogen flow so as to be removed, while the micro metal bodies are sintered. In this method, since the aluminum powder particles have a strong oxide film (alumina: Al 2 O 3 ) on their surfaces, only a small portion of the aluminum powder particles are bonded to each other even by sintering, and only porous bodies, which are brittle and have very low strength, may be produced. 
     DISCLOSURE OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an aluminum-based porous body having a three-dimensional network structure with sufficient strength and to provide a production method therefor. In this three-dimensional network structure, aluminum powder particles or aluminum alloy powder particles, which initially have a strong oxide film on their surfaces, are used, and they are strongly bonded to each other. 
     The present invention provides an aluminum-based porous body having a strut, which is connected three-dimensionally, and having a three-dimensional network structure, which includes communicating holes that are formed by the strut so as to three-dimensionally communicate with each other. The strut is made of aluminum or aluminum alloy having a density ratio of not less than 90% and contains an aluminum oxide that is dispersed in an inner part thereof. It should be noted that the “aluminum” of the present invention is defined as aluminum, which consists of not less than 95 mass % of Al and the balance of impurities such as C and N and does not contain other metal elements. 
     In the aluminum-based porous body of the present invention, since the density ratio of the strut is not less than 90%, the strut has high strength. Moreover, since the aluminum oxide (alumina: Al 2 O 3 ) is dispersed in the strut, the aluminum matrix is strengthened, whereby the strut has further high strength. 
     In the aluminum-based porous body, pores in the inner part of the strut preferably have a size of not greater than 10 μm from the viewpoint of the strength of the strut. Also, since the strength of the strut is undesirably decreased if the aluminum oxide (alumina: Al 2 O 3 ) is coarse in size although being used in expectation of strengthening the aluminum matrix, the aluminum oxide preferably has a size (outermost diameter) of not greater than 10 μm. In addition, the aluminum oxide is desirably contained in the strut at an area ratio of 5 to 20% in cross section. If the area ratio of the aluminum oxide is less than 5%, the effect for strengthening the matrix is not sufficiently obtained. On the other hand, if the area ratio of the aluminum oxide is greater than 20%, the strut is difficult to produce. The area ratio of the oxide in the cross section of the strut can be measured by using image analyzing software (for example, “WinROOF” produced by Mitani Corporation) such that a cross sectional image of the strut is automatically binarized or the image is converted into a gray scale image, and an appropriate threshold value is set. The aluminum-based porous body of the present invention may have a strut that is hollow, in one embodiment. 
     The present invention also provides an aluminum-based porous body having a strut, which is connected three-dimensionally, and having communicating holes, which are made to three-dimensionally communicate with each other by the strut. This strut has a three-dimensional network structure that is made of aluminum or aluminum alloy. This aluminum-based porous body shows a stress-strain diagram, in which a stress amount is increased with the increase in a strain amount when a load is applied, and the stress becomes approximately constant due to crushing of the communicating holes and is then increased. 
     For instance, in the aluminum porous body as disclosed in Patent document 4, since a small portion of the aluminum powder particles are bonded to each other, the bonding between the aluminum powder particles breaks when a stress is applied and breaks continuously in accordance with the increase in the stress. In this case, numerous crushed pieces are generated from the aluminum porous body due to the breakage. 
     In order to avoid this situation, the strut should be made so that the bonding between the aluminum powder particles does not break and the stress is increased in accordance with the increase in a strain amount, when a load is applied. By forming a structure, in which such a strut is three-dimensionally connected, breakage of the bonding between the aluminum powder particles is unlikely to occur, and generation of numerous crushed pieces is avoided. 
     In the aluminum-based porous body having such a strut, the strut is elastically deformed until a strain that is applied to the strut reaches a specific amount, and the strut is plastically deformed and the communicating holes, which three-dimensionally communicate with each other, start to be crushed, when the strain amount exceeds the specific amount. The deformation of the aluminum-based porous body progresses in a condition in which the stress is not greatly increased (approximately constant) even when the strain amount is further increased. Then, the communicating holes, which three-dimensionally communicate with each other, are crushed completely when the strain amount applied to the aluminum-based porous body is increased. Thereafter, when a strain is further applied to the aluminum-based porous body, strain occurs in the aluminum-based porous body including the communicating holes, which initially three-dimensionally communicate with each other but are closed, and stress is increased with the increase in the strain amount. Here, the elastic limit of the deformation of the strut is desirably not less than 0.5 MPa. 
     In the aluminum-based porous body showing such a stress-strain diagram, the strut is difficult to rupture and plastically deforms when a strain is applied at a specific amount or greater, because the aluminum powder particles are strongly bonded to each other. 
     The aluminum-based porous body preferably has a density ratio of the strut at not less than 90% from the viewpoint of the strength of the strut. The pores in the inner part of the strut preferably have a size of not greater than 10 μm. 
     The amount of the crushed pieces, which are generated when a load is applied until the stress is increased after the stress is approximately constant in the stress-strain diagram, is preferably not more than 5 mass % of the aluminum-based porous body. 
     The aluminum-based porous body according to the present invention can be obtained by a production method for an aluminum-based porous body of the present invention. This production method includes: using a resin three-dimensional network structure body, which has a resin strut that is connected three-dimensionally and which includes communicating holes that are formed by the resin strut so as to three-dimensionally communicate with each other, as a substrate, adhering at least one of aluminum powder and aluminum alloy powder on a surface of the resin strut of the substrate, and heating the adhered powder to not less than the melting point thereof in a nonoxidizing atmosphere so as to eliminate and remove the substrate and to melt the adhered powder. 
     In the production method for the aluminum-based porous body of the present invention, the substrate having a resin strut, which is connected three-dimensionally, and having a resin three-dimensional network structure, which includes communicating holes that is formed by the resin strut so as to three-dimensionally communicate with each other, is used. Then, at least one of the aluminum powder and the aluminum alloy powder is adhered on the surface of the resin strut of the substrate, and the resin substrate is then eliminated and removed by heating in the nonoxidizing atmosphere. These steps are the same as in the method disclosed in Patent document 4. However, in the present invention, the adhered powder is heated to not less than the melting point thereof so as to be melted. In the present invention, the aluminum powder and the aluminum alloy powder preferably have an average particle diameter of 1 to 50 μm. The heating temperature is preferably set in a range of from the melting point to not higher than the melting point+100° C. 
     In the condition in which at least one of the aluminum powder and the aluminum alloy powder is adhered on the surface of the resin strut of the substrate before heating, the surfaces of the powder particles are covered by an oxide film, and each of the powder particles contacts the other powder particle via the oxide film. Then, by heating the substrate, in which at least one of the aluminum powder and the aluminum alloy powder is adhered on the surface of the strut, to the melting point of the powder, the resin substrate is decomposed and eliminated in the heating step, and the melted powder particles break the oxide film, which is formed on their surfaces, and wet and cover their surfaces of powder particles. At this time, the oxide film that is formed on the surfaces of the powder particles becomes a strut of the aluminum-based porous body in place of the resin strut, and the melted powder particles wet the outside of this strut, whereby adjacent powder particles are bonded with the melted powder particles. Therefore, the aluminum-based porous body that is obtained after the heating has strong metallurgical bonding, and a sufficient bonding strength is obtained. On the other hand, when the inventors of the present invention performed an experiment by using a copper powder in the same manner as in the production method of the present invention, the copper powder particles fell off when melted, and a porous body could not be formed. Accordingly, the capability of maintaining the shape even when the powder is melted is a specific effect of aluminum and aluminum alloy having the oxide film. 
     The strut of the aluminum-based porous body thus obtained has a density ratio of, for example, 90% or more, and is made of aluminum or aluminum alloy, which contains the oxide film, that is, alumina (Al 2 O 3 ), that is formed on the surfaces of the original powder particles, in the inner part thereof. The alumina is hard, and it is dispersed in the matrix of the aluminum or the aluminum alloy and strengthens the matrix, whereby the aluminum or the aluminum alloy has high strength. It should be noted that the density ratio of the strut cannot be measured by the Archimedes method, and therefore, the density ratio is calculated as a difference between an area (matrix portion except for hollow portions) in the cross section of the strut and an area ratio of pores that are dispersed in the matrix portion in the cross section of the strut by observing the cross section of the strut. The area in the cross section of the strut and the area ratio of the pores of the strut may be measured by using image analyzing software (for example, “WinROOF” produced by Mitani Corporation) such that a cross sectional image of the strut is automatically binarized or the image is converted into a gray scale image, and an appropriate threshold value is set. 
     The final three-dimensional network structure of the aluminum-based porous body is obtained by melting the powder, which is at least one of the aluminum powder and the aluminum alloy powder and is adhered on, and is supported by, the surface of the strut of the substrate. Therefore, the three-dimensional network structure of the substrate affects the final three-dimensional network structure of the aluminum-based porous body. Accordingly, by changing the three-dimensional network structure of the substrate, an aluminum-based porous body having a desired three-dimensional network structure can be obtained. 
     If the strut of the aluminum-based porous body having the three-dimensional network structure is excessively thin, the strength of the aluminum-based porous body is decreased. On the other hand, if the strut of the aluminum-based porous body is excessively thick, the fluid is prevented from passing through the communicating holes, and the pressure loss is increased. Considering the use of the aluminum-based porous body in a heat exchanger, the thickness of the strut is preferably set at 50 to 500 μm. 
     The strut of the aluminum-based porous body is formed by adhering at least one of the aluminum powder and the aluminum alloy powder on the surface of the resin strut of the substrate and by melting the powder. In this case, if the amount of the powder that is adhered on the surface of the resin strut of the substrate is great, an excessive amount of the melted adhered powder is generated, whereby the shape of the strut that is formed by the melted adhered powder is difficult to be maintained by the surface tension and tends to deteriorate. In view of this, it is preferable to adhere the powder of at least one of the aluminum powder and the aluminum alloy powder on the surface of the resin strut so that the thickness will be 100 to 1000 μm from the surface of the resin strut. In this case, the strut is made of aluminum or aluminum alloy with a preferable thickness of 50 to 500 μm after the melting. 
     The method for adhering at least one of the aluminum powder and the aluminum alloy powder on the surface of the resin strut of the substrate is exemplified below. That is, at least one of the aluminum powder and the aluminum alloy powder is dispersed in a dispersion medium and is then adjusted so that the viscosity will be 50 to 1000 Pa·s under a temperature condition of 25° C., whereby a dispersion liquid is obtained. Then, after a substrate is immersed in the dispersion liquid, the substrate is dried, whereby at least one of the aluminum powder and the aluminum alloy powder is adhered on the surface of the resin strut of the substrate. 
     Effects of the Invention 
     According to the present invention, the aluminum-based porous body of the present invention has high strength, and the aluminum-based porous body having such high strength is produced in a simple manner and at a low cost and is excellent in mass productivity, by the production method for the aluminum-based porous body of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a bonding condition between powder particles according to the production method for the aluminum-based porous body of the present invention. 
         FIG. 2  is a schematic view showing a bonding condition between powder particles according to a conventional production method for an aluminum-based porous body. 
         FIG. 3  is a view showing an aluminum-based porous body of an example of the present invention. 
         FIG. 4  is a view of a SEM image and a distribution of each element of a strut of an aluminum-based porous body of an example of the present invention, which was observed by an EPMA. 
         FIG. 5  is a view showing an aluminum-based porous body of a comparative example. 
         FIG. 6  is a stress-strain diagram of an aluminum-based porous body of each of an example of the present invention and a comparative example. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be described hereinafter. 
     (Substrate) 
     As a substrate, a three-dimensional network structure body, which has a strut that is connected three-dimensionally and which includes holes formed by the strut so as to three-dimensionally communicate with each other, is used. Since the substrate is used for supporting at least one of the aluminum powder and the aluminum alloy powder that is adhered on the surface of the strut, and the substrate should be decomposed and eliminated by heating, the substrate is made of resin. Specifically, polyurethane foam may be generally used as the substrate, but a foam of silicone resin or polyester resin may also be used. 
     (Aluminum Powder or Aluminum Alloy Powder) 
     As described above, an aluminum powder is used as the powder to be adhered on the resin strut of the substrate in view of the balance of the thermal conductivity and the specific gravity. However, an aluminum alloy powder, in which aluminum is preliminarily alloyed with a component for strengthening aluminum, may be used instead of the aluminum powder. For example, when an aluminum alloy powder, in which Al is preliminarily alloyed with an alloying element such as Cu, Mn, Mg, Si, or the like, is used, the strut of the aluminum-based porous body is made of aluminum alloy, and the strength of the aluminum-based porous body is improved. By adding the alloying element such as Cu, Mn, Mg, Si, or the like, to Al, although the thermal conductivity is decreased compared to the case of using only Al, the thermal conductivity is still sufficiently high because the base metal is Al. As the aluminum powder and the aluminum alloy powder, a commonly used powder, that is, a powder having an oxide film (alumina: Al 2 O 3 ) of approximately 10 Å on the surfaces of the powder particles, is used. 
     The powder of at least one of the aluminum powder and the aluminum alloy powder to be adhered on the resin strut of the substrate is preferably a fine powder because the fine powder can be adhered densely on the surface of the resin thin strut of the substrate. If the powder particles are large in size, the powder is difficult to adhere densely on the surface of the resin strut of the substrate. Moreover, since the mass of the powder is increased, the powder is difficult to adhere on the surface of the resin strut of the substrate and tends to peel off easily. In view of this, a powder having an average particle diameter of not greater than 50 μm is preferably used as the aluminum powder and the aluminum alloy powder. Moreover, it is more preferable to use a powder which has an average particle diameter of not greater than 50 μm and which does not include powder particles that have a particle diameter of greater than 100 μm. It should be noted that excessively fine powder is difficult to handle because Al is an active metal. In view of this, a powder having an average particle diameter of not less than 1 μm is preferably used as the aluminum powder and the aluminum alloy powder. 
     (Adhering Step) 
     In order to adhere at least one of the aluminum powder and the aluminum alloy powder on the resin strut of the substrate, each kind of the methods that are conventionally used may be applied. Typical methods are described below. 
     (1) Wet Method 
     The wet method may be found in Patent documents 1, 2, 4, and the like, and is performed such that a dispersion liquid is prepared by dispersing at least one of an aluminum powder and an aluminum alloy powder in a dispersion medium, and a substrate is immersed in the dispersion liquid and is then dried. As the dispersion medium, a liquid, which includes a volatile liquid such as alcohol, or water, as a solvent and includes a binder that is dissolved in the solvent, may be used. In this case, a dispersion agent may be added in the dispersion medium so as to prevent the powder particles from precipitating. In addition, a solution of an organic polymer such as phenol resin may be used as the dispersion medium. 
     At this time, the amount of the powder of at least one of the aluminum powder and the aluminum alloy powder to be adhered on the surface of the resin strut of the substrate can be controlled by the viscosity of the dispersion liquid. That is, if the viscosity of the dispersion liquid is high, the amount of the powder that is adhered on the surface of the resin strut of the substrate is great, and in contrast, if the viscosity of the dispersion liquid is low, the amount of the powder that is adhered on the surface of the resin strut of the substrate is small. It should be noted that if the viscosity of the dispersion liquid is excessively high, the amount of the powder that is adhered on the surface of the resin strut of the substrate is excessively increased, and the thickness from the surface of the resin strut exceeds 1000 urn, whereby the shape of the strut tends to be deteriorated in the heating step, which is described later. In view of this, the viscosity of the dispersion liquid is preferably set at not greater than 1000 Pa·s under a temperature condition of 25° C. On the other hand, if the viscosity of the dispersion liquid is excessively low, the amount of the powder that is adhered on the surface of the resin strut of the substrate is insufficient, whereby an aluminum-based porous body having a three-dimensional network structure with a thin strut is obtained after the heating step, and the strength of the aluminum-based porous body is decreased. In view of this, the viscosity of the dispersion liquid is preferably set at not lower than 50 Pa·s under a temperature condition of 25° C. The viscosity can be measured by using a viscometer of TVB10 model produced by TOKI SANGYO CO., LTD., or the like, such that a torsion angle of two slit disks due to viscous torque is measured and is converted into the viscosity. 
     (2) Dry Method 
     The dry method may be found in Patent document 5 and is performed as follows. That is, an adhesive solution such as of acrylic type or rubber type, or an adhesive resin solution such as of phenol resin, epoxy resin, or furan resin, is coated on the surface of the substrate so as to provide adhesiveness. Then, the substrate is vibrated in powder or is sprayed with the powder so that the powder adheres the surface of the strut. 
     (Heating Step) 
     After at least one of the aluminum powder and the aluminum alloy powder is adhered on the surface of the strut, the substrate is heated to not less than the melting point of the adhered powder in a nonoxidizing atmosphere. The resin substrate is decomposed, and it is eliminated and removed, while the temperature is increased to the melting point. 
     When the heating temperature exceeds the melting point of aluminum (melting point: 660.4° C.) or the aluminum alloy, the inner parts of the aluminum powder particles or the aluminum alloy powder particles are melted. That is, since the surfaces of each of the aluminum powder particles and the aluminum alloy powder particles are covered with an oxide film (alumina: Al 2 O 3 ), and the melting point of alumina is such a high temperature as 2072° C., the oxide film on the surfaces of each of the aluminum powder particles and the aluminum alloy powder particles is not melted, but the inner part of these powder particles is melted. The aluminum or the aluminum alloy that is thus melted in the inner part of the powder particles breaks the oxide film of the surfaces of the powder particles and wets and covers the surfaces of the powder particles, and the melted aluminum or the melted aluminum alloy, which is generated from respective powder particles, is mixed and is bonded together, as shown in  FIG. 1 , At this time, the oxide film that is formed on the surfaces of the powder particles becomes a strut of the aluminum-based porous body in place of the resin strut and maintains the shape of the resin strut. The surface of this strut is relatively smooth due to the surface tension of the melted aluminum or the melted aluminum alloy, which is bonded to each other, and is also a continuous metal surface because neck portions are eliminated. 
     The strut of the aluminum-based porous body that is thus obtained is made of aluminum or the aluminum alloy, which contains the oxide film, that is, alumina (Al 2 O 3 ), that is formed on the surfaces of the original powder particles, in the inner part thereof. The alumina is hard and is dispersed in the matrix of the aluminum or the aluminum alloy, thereby strengthening the matrix. Moreover, this strut includes cavities at the portions where the resin strut existed, and is hollow, and therefore, this strut may be effectively used in cases requiring reduction in weight. 
     In contrast, if the heating temperature is less than the melting point of aluminum or the aluminum alloy, as shown in  FIG. 2 , the strong oxide film that is formed on the surfaces of the aluminum powder particles or the aluminum alloy powder particles act as a barrier. Therefore, the aluminum powder particles or the aluminum alloy powder particles are prevented from being diffusion-bonded to each other, whereby the sintering is difficult to proceed. 
     If the heating step is performed in an oxidizing atmosphere such as air, the melted aluminum or the melted aluminum alloy, which breaks the oxide film on the surfaces of the powder particles and is thereby exposed, is immediately oxidized. Therefore, the melted aluminum or the melted aluminum alloy is prevented from wetting and covering the surfaces of the powder particles and is also prevented from being mixed together, whereby the bonding between the powder particles is prevented. Accordingly, the heating step is desirably performed in a nonoxidizing atmosphere such as of nitrogen gas, inert gas, or the like. It is not necessary to use a reducing atmosphere such as of hydrogen gas, hydrogen mixed gas, or the like, in the heating step because the oxide film on the surfaces of the aluminum powder particles or the aluminum alloy powder particles should not be removed. Nevertheless, since the reducing atmosphere is a nonoxidizing atmosphere, the reducing atmosphere may be used. Alternatively, a reduced-pressure atmosphere (vacuum atmosphere), in which the pressure is not greater than 10 −3  Pa, may be used. 
     The aluminum powder or the aluminum alloy powder, which is adhered on the substrate, can be melted at a heating temperature that is higher than the melting point thereof. However, if the heating is performed at a temperature that is much higher than the melting point, extra energy is correspondingly required, and the shape of the strut tends to be deteriorated due to decrease in the viscosity of the melted aluminum or the melted aluminum alloy. Therefore, the heating temperature is preferably set in a range of from the melting point to the melting point+100° C. 
     If the strut of the aluminum-based porous body having the three-dimensional network structure is excessively thin, the strength of the aluminum-based porous body is decreased. On the other hand, if the strut of the aluminum-based porous body is excessively thick, the fluid is prevented from passing through the communicating holes, and the pressure loss is increased. The strut of the aluminum-based porous body is formed by adhering at least one of the aluminum powder and the aluminum alloy powder on the surface of the resin strut of the substrate and by melting the powder. In this case, when the amount of the powder that is adhered on the surface of the resin strut of the substrate is increased, the amount of the melted adhered powder is increased. If the melted adhered powder is excessively generated, the shape of the strut that is formed by the melted adhered powder is difficult to maintain by the surface tension and tends to be deteriorated. Therefore, the thickness of the strut of the aluminum-based porous body is preferably set at 50 to 500 μm. Moreover, it is preferable to adhere at least one of the aluminum powder and the aluminum alloy powder on the surface of the resin strut so that the thickness will be 100 to 1000 μm from the surface of the resin strut. In this case, the strut is made of aluminum or the aluminum alloy with a preferable thickness of 50 to 500 μm after the melting. 
     The three-dimensional network structure of the aluminum-based porous body that is produced by the above production method maintains the three-dimensional network structure of the resin substrate as it is. Therefore, by changing the three-dimensional network structure of the resin substrate, the three-dimensional network structure of the aluminum-based porous body can be changed, and the hole ratio of the entirety of the aluminum-based porous body and the size of the holes can be adjusted as desired. Specifically, the aluminum-based porous body can be made so as to have a hole ratio of 80 to 95%, preferably 85 to 95%, and so as to include the holes having sizes of 30 to 4000 μm, and a porous body of 6 to 80 ppi (cells/25.4 mm) is easily produced. 
     In the case of forming the aluminum-based porous body by using the aluminum alloy, the following idea may be conceived. That is, a component such as Cu, Mg, or the like, which generates a liquid phase of a eutectic composition in combination with Al, may be used as a raw powder in the form of a single powder or an aluminum alloy powder. This raw powder may be added in the aluminum powder, whereby an aluminum based mixed powder may be prepared. Then, the aluminum based mixed powder may be adhered on the surface of the resin substrate having the three-dimensional network structure and be sintered at a temperature at which the liquid phase of the eutectic composition is generated. In this idea, the distributions of the component elements in the aluminum-based porous body tend to not be uniform, and the aluminum oxide is not dispersed in the inner part of the strut, whereby a desired strength is difficult to obtain. 
     In contrast, by using the aluminum pre-alloyed powder, in which Al is preliminarily alloyed with the component element, as described above, the distributions of the component elements in the aluminum-based porous body is made uniform. In addition, the aluminum oxide that is generated due to the production method is dispersed in the inner part of the strut. Therefore, high strength can be obtained compared to the case of performing the idea of using the aluminum based mixed powder and sintering by generating a liquid phase of the eutectic composition. 
     EXAMPLES 
     As a resin substrate having a three-dimensional network structure, polyurethane foam (Product name: Everlight SF, produced by Bridgestone Corporation) having a length of 10 mm, a width of 20 mm, and a thickness of 10 mm, was prepared. The polyurethane foam had a hole ratio (ratio of the volume of communicating holes to the volume of the entirety) of 95% and had communicating holes with a circle equivalent diameter of 3000 Then, polyvinyl alcohol (Product name: GOHSENOL GH-23, produced by The Nippon Synthetic Chemical Industry Co., Ltd.) containing resin at 1 mass was prepared as a dispersion medium. The dispersion medium and an aluminum powder having an average particle diameter of 6 μm were mixed in a mass ratio of 1:1, whereby an aluminum powder dispersed liquid (viscosity at 25° C.: 50 to 75 Pa·s, measured by a viscometer of TVB10 model produced by TOKI SANGYO CO., LTD.) was obtained. After the substrate was immersed in the aluminum powder dispersed liquid, extra slurry was removed by a roll, and the substrate was dried at 100° C. for 120 minutes, whereby the substrate on which the aluminum powder was adhered, was prepared. The substrate, on which the aluminum powder was adhered, obtained thus, was heated at a heating temperature shown in Table 1 for 210 minutes in a reduced-pressure atmosphere (vacuum atmosphere) at a pressure of 10 −3  Pa, whereby porous samples Nos. 01 to 07 were formed. 
     The hole ratio of each of these porous samples was measured by the Archimedes method. In addition, the size of pores and the circle equivalent diameter of the holes (communicating holes) of the three-dimensional network structure were measured by observation using an optical microscope and by using image analyzing software (“WinROOF” produced by Mitani Corporation), and an average value of each of the size of the pores and the circle equivalent diameter of the holes was calculated. 
     Moreover, the porous samples were embedded in resin, and the porous samples were mirror polished and were etched with Keller&#39;s reagent (hydrochloric acid: 0.5 ml, nitric acid: 2.5 ml, hydrofluoric acid: 1.5 ml, distilled water: 95 ml). Then, the metallic structure of the strut portion was observed. In this case, by using the image analyzing software (“WinROOF” produced by Mitani Corporation), the image was binarized, and an area ratio of the strut portion (matrix portion except for hollow portions) and an area ratio in the cross section of the pores, which were dispersed in the strut portion (matrix portion except for hollow portions) were measured, whereby a density ratio of the strut portion was calculated. Meanwhile, the metallic structure of the matrix portion except for the hollow portions in the cross section of the strut portion was observed by an EPMA at a magnification of 5000 times. Then, also, by using the image analyzing software (“WinROOF” produced by Mitani Corporation), the areas of regions of 180 or larger (180 to 255) were measured by setting a threshold value at 180 by a valley method, whereby the size and the area ratio in the cross section of the oxides that were dispersed in the cross section of the strut portion were measured. These results are shown in Table 1. 
     Furthermore, each of the aluminum-based porous body samples of samples Nos. 01 to 07 was subjected to a compressive yield test, and a strain amount and a stress were measured while a compressive load was increased, whereby a stress-strain diagram was formed. Then, the stress when entering a plateau region, in which the stress was approximately constant, was calculated from the stress-strain diagram, and the result is also shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                   
                   
                 Strut portion 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Overall 
                   
                   
                   
                 Area ratio 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Size of 
                   
                   
                   
                 of oxides  
                   
                   
               
               
                   
                 Heating 
                   
                 commun- 
                   
                   
                   
                 in 
                   
                   
               
               
                   
                 temper- 
                 Hole 
                 icating 
                 Density 
                 Size of  
                 Size of  
                 cross 
                 Plateau 
                   
               
               
                 Sample 
                 ature 
                 ratio 
                 holes  
                 ratio 
                 pores 
                 oxides 
                 section 
                 region 
                   
               
               
                 No. 
                 ° C. 
                 % 
                 μm 
                 % 
                 μm 
                 μm 
                 % 
                 MPa 
                 Notes 
               
               
                   
               
               
                 01 
                 520 
                 95 
                 3000 
                 50 
                 30 
                 — 
                 — 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 02 
                 600 
                 95 
                 2800 
                 60 
                 25 
                 — 
                 — 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 03 
                 640 
                 95 
                 2600 
                 70 
                 20 
                 — 
                 — 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 04 
                 661 
                 92 
                 2500 
                 90 
                 10 
                 10 
                 8 
                 0.5 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 present invention 
               
               
                 05 
                 680 
                 90 
                 2500 
                 90 
                  8 
                 10 
                 8 
                 0.8 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 present invention 
               
               
                 06 
                 700 
                 85 
                 2000 
                 92 
                  5 
                 10 
                 8 
                 1.4 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 present invention 
               
               
                 07 
                 760 
                 80 
                 1800 
                 95 
                  2 
                 10 
                 8 
                 1.7 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 present invention 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, in each of the porous samples of samples Nos. 01 to 07, the hole ratio was approximately the same as that of the urethane foam that was used as the substrate, and the size of the communicating holes was also approximately the same as that of the urethane foam of the substrate. According to this result, the hole ratio and the size of the communicating holes of the urethane foam of the substrate are maintained in the porous samples as they are. 
     On the other hand, in the samples Nos. 01 to 03, in which the heating temperature was less than the melting point (660° C.) of aluminum, the sintering did not sufficiently proceed, and the density ratio of the strut portion was low. 
     In contrast, in the samples Nos. 04 to 07, in which the heating temperature was higher than the melting point (660° C.) of aluminum, the density ratio of the strut portion was not less than 90% and was high. In the samples Nos. 04 to 07, the pores of the strut portion were as small as 2 to 10 μm. In the samples Nos. 04 to 07, oxides were dispersed in the inner part of the strut portion, and the oxide had a size of 10 μm and had an area ratio of 8% in cross section. On the other hand, in the samples Nos. 01 to 03, which are comparative examples, only a small portion of the aluminum powder particles were bonded, and the oxides existed only on the surfaces of the aluminum powder particles and were not dispersed in the inner part of the matrix. 
       FIG. 3  shows a view in which the condition of pores in the porous sample of sample No. 04 of the example of the present invention was observed. As shown in  FIG. 3 , in the porous sample of the example of the present invention, the melted aluminum bonded the adjacent powder particles, and the surface of the strut was relatively smooth due to the surface tension of the melted aluminum and had a metal continuous surface because neck portions were eliminated. 
       FIG. 4  shows a view of a SEM (Scanning Electron Microscope) photograph of a cross section of the strut portion of the aluminum-based porous body of the example of the present invention, which was observed by an EPMA (Electron Probe MicroAnalyser).  FIG. 4  also shows a mapped image showing distribution of each of the components of Al and O (Oxygen). As shown in  FIG. 4 , in the aluminum-based porous body of the example of the present invention, Al 2 O 3  (alumina) was dispersed in the matrix of aluminum. 
     On the other hand,  FIG. 5  shows a view in which the condition of pores in the porous sample of sample No. 03, which is a comparative example. As shown in  FIG. 5 , in the porous sample of the comparative example, only a part of the aluminum powder particles were bonded by solid phase diffusion, and neck portions (bonded portions of the powder particles) did not grow. Therefore, the shapes of the original powder particles were observed. 
     In the aluminum-based porous body samples of samples Nos. 04 to 07, the elastic limit was 0.5 MPa or higher until the stress reached the plateau region. The elastic limit was increased to 1.7 MPa in accordance with the increase in the density ratio. The result of the compressive yield test was described in detail with reference to  FIG. 6 . The aluminum-based porous body sample of sample No. 04, which is the example of the present invention, was elastically deformed at the initial stage of the deformation, and the stress was increased with the increase in the strain amount. Then, the stress becomes constant although the strain amount was increased. In this condition, the deformation proceeded while the communicating holes of the aluminum-based porous body sample were compressed and were closed. After the strain amount was increased and the aluminum-based porous body sample was densified by further increasing the load, the stress was increased in accordance with the increase in the strain amount while the load was increased, as in the case of an ordinary metallic sample. Such deformation behavior is typical for the aluminum-based porous body sample. When the aluminum-based porous body sample was observed after the test, the communicating holes were crushed, but the strut was not ruptured. 
     In contrast, in the aluminum-based porous body samples of samples Nos. 01 to 03, the plateau region was not observed. As shown in  FIG. 6 , in the aluminum-based porous body sample of the comparative example of sample No. 3, since the bonding strength between the powder particles was insufficient, the sample broke at the initial stage of the deformation when the compressive load was applied, and the sample was crushed into pieces and was in a powder condition after the test. 
     As described above, the aluminum-based porous body sample of the present invention showed a stress-strain diagram, in which the stress amount is increased with the increase in the strain amount when the load is applied, and the stress becomes approximately constant due to crushing of the communicating holes and is then increased. This aluminum-based porous body sample had higher strength than the conventional aluminum-based porous body sample. 
     INDUSTRIAL APPLICABILITY 
     The aluminum-based porous body of the present invention has high strength, and therefore, it is preferably used for various kinds of porous members.