Process for producing porous sintered aluminum, and porous sintered aluminum

Abstract

This method for producing porous sintered aluminum includes: mixing aluminum powder with a sintering aid powder containing titanium to obtain a raw aluminum mixed powder; mixing the raw aluminum mixed powder with a water-soluble resin binder, water, and a plasticizer containing at least one selected from polyhydric alcohols, ethers, and esters to obtain a viscous composition; drying the viscous composition in a state where air bubbles are mixed therein to obtain a formed object prior to sintering; and heating the formed object prior to sintering in a non-oxidizing atmosphere, wherein when a temperature at which the raw aluminum mixed powder starts to melt is expressed as Tm (° C.), a temperature T (° C.) of the heating fulfills Tm−10 (° C.)≤T≤685 (° C.).

Claims

1. A method for producing porous sintered aluminum comprising: mixing aluminum powder with a sintering aid powder containing titanium to obtain a raw aluminum mixed powder; mixing the raw aluminum mixed powder with a water-soluble resin binder, water, and a plasticizer containing at least one selected from polyhydric alcohols, ethers, and esters to obtain a viscous composition; drying the viscous composition in a state where air bubbles are mixed therein to obtain a formed object prior to sintering; and heating the formed object prior to sintering in a non-oxidizing atmosphere, wherein when a temperature at which the raw aluminum mixed powder starts to melt is expressed as Tm (° C.), a temperature T (° C.) of the heating fulfills Tm−10 (° C.)≤T≤685 (° C.).

2. The method for producing porous sintered aluminum according to claim 1, wherein an average particle diameter of the aluminum powder is in a range of 2 to 200 μm.

3. The method for producing porous sintered aluminum according to claim 1, wherein when an average particle diameter of the sintering aid powder is expressed as r (μm), and a mixing ratio of the sintering aid powder is expressed as W (% by mass), r and W fulfill 1 (μm)≤r≤30 (μm), 1 (% by mass)≤W≤20 (% by mass), and 0.1≤W/r≤2.

4. The method for producing porous sintered aluminum according to claim 1, wherein the sintering aid powder is either one or both of titanium and titanium hydride.

5. The method for producing porous sintered aluminum according to claim 1, wherein non-water-soluble hydrocarbon system organic solvent containing 5 to 8 carbons is added to the viscous composition.

6. The method for producing porous sintered aluminum according to claim 1, wherein the water-soluble resin binder is contained at a content in a range of 0.5% to 7% of a quantity (mass) of the raw aluminum mixed powder.

7. The method for producing porous sintered aluminum according to claim 1, wherein surfactant is added to the raw aluminum mixed powder at a content in a range of 0.02 to 3% of a quantity (mass) of the raw aluminum mixed powder.

8. The method for producing porous sintered aluminum according to claim 1, wherein the viscous composition is extended to have a thickness of 0.05 mm to 5 mm, and then the viscous composition is dried to produce the formed object prior to sintering as a plate shape formed object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an SEM photograph of foamed aluminum in Example 1.

(2) FIG. 2 is a partially enlarged SEM photograph of FIG. 1.

(3) FIG. 3 is an SEM photograph of foamed aluminum in Comparative Example 1.

(4) FIG. 4 is a photograph of foamed aluminum obtained by a combined method which includes a fifth method in the conventional art as a method of performing free sintering of aluminum powder and a slurry foaming method.

BEST MODE FOR CARRYING OUT THE INVENTION

(5) Hereinafter, one embodiment of the method for producing porous sintered aluminum according to the present invention will be described.

(6) The method for producing aluminum of the present embodiment includes the following steps:

(7) a process of providing raw aluminum mixed powder in which aluminum power is mixed with titanium and/or titanium hydride to obtain raw aluminum mixed powder;

(8) a process of providing a viscous composition in which the raw aluminum mixed powder is mixed with water-soluble resin coupler, water, and plasticizer to provide a slurry viscous composition;

(9) a process prior to sintering in which the viscous composition is dried in a state where air bubbles are mixed therein to obtain a formed object prior to sintering; and

(10) a sintering process in which the formed object prior to sintering is heated at a temperature T that fulfills Tm−10 (° C.)≤heating temperature T≤685 (° C.) in a non-oxidizing atmosphere.

(11) Here, Tm (° C.) represents the temperature at which the raw aluminum mixed powder starts to melt.

(12) In the process of providing the raw aluminum mixed powder, an aluminum powder having an average particle diameter of 2 to 200 μm is used. The reason is as follows. In the case where the average particle diameter is small, it is necessary to add a large amount of water-soluble resin coupler to the aluminum powder in order that the viscous composition has a viscosity at which the viscous composition can be formed in to a desired shape and the formed object prior to sintering has a handling strength. However, in the case where a large amount of water-soluble resin coupler is added, an amount of carbon remaining in the aluminum is increased when the formed object prior to sintering is heated, and the remained carbon inhibits the sintering reaction. On the other hand, in the case where the particle diameter of the aluminum powder is excessively large, the strength of the foamed aluminum is lowered. Accordingly, as described above, the aluminum powder having the average particle diameter in a range of 2 to 200 μm is used, and the average particle diameter is more preferably in a range of 7 to 40 μm.

(13) The aluminum powder is mixed with titanium and/or titanium hydride. The reason is as follows. In the case where the aluminum powder is mixed with titanium and the formed object prior to sintering is heated at the heating temperature T which fulfills Tm−10 (° C.)≤heating temperature T≤685 (° C.), it is possible to perform free sintering of aluminum without generating lumps of liquid droplets. In addition, titanium hydride (TiH.sub.2) contains titanium at a content of 47.88 (molecular weight of titanium)/(47.88+1 (molecular weight of hydrogen)×2), which is 95% by mass or greater, and dehydrogenation of the titanium hydride occurs at a temperature of 470 to 530° C. to convert into titanium. Therefore, the titanium hydride is thermally decomposed into titanium by the aforementioned heating. Accordingly, it is possible to perform free sintering of aluminum without generating lumps of liquid droplets even in the case where the titanium hydride is mixed thereinto.

(14) Here, when the average particle diameter of titanium or titanium hydride is expressed as r (μm), and the mixing ratio of titanium or titanium hydride is expressed as W (% by mass), the following equations are fulfilled: 1 (μm)≤r≤30 (μm), 0.1 (% by mass)≤W20 (% by mass), and 0.1≤W/r≤2. For example, in the case where the titanium hydride powder has an average particle diameter of 4 μm, 0.1≤W/4≤2 is to be fulfilled; and therefore, the mixing ratio W becomes in a range of 0.4 to 8% by mass. In the case where the titanium powder has an average particle diameter of 20 μm, 0.1≤W/20≤2 is to be fulfilled; and therefore, the mixing ratio W becomes in a range of 2 to 40% by mass. However, since 0.1 (% by mass)≤W≤20 (% by mass) is to be fulfilled, the mixing ratio becomes in a range of 2 to 20% by mass.

(15) The average diameter of titanium hydride is set to fulfill 0.1 (μm)≤r≤30 (μm), the average diameter is preferably set to fulfill 1 (μm)≤r≤30 (μm) and is more preferably set to fulfill 4 (μm)≤r≤20 (μm). The reason is as follows. In the case where the average diameter is 1 μm or smaller, there is a concern of spontaneous combustion. After sintering, the titanium hydride becomes titanium grains covered with a compound of aluminum and titanium. In the case where the average particle diameter exceeds 30 μm, the compound phase of aluminum and titanium is easily peeled off from the titanium grains; and thereby, a desired strength of the sintered body cannot be obtained.

(16) The reason that 0.1 (% by mass)≤W≤20 (% by mass) is set is as follows. In the case where the mixing ratio W of the sintering aid powder exceeds 20% by mass, the sintering aid particles contact with each other in the raw aluminum mixed powder; and thereby, the reaction heat between aluminum and titanium cannot be controlled, and a desired porous sintered body cannot be obtained.

(17) Even in the case where the mixing ratio fulfills 0.1 (% by mass)≤W≤20 (% by mass), the reaction heat between aluminum and titanium became excessively high in some cases depending on the particle diameter of the sintering aid powder. In these cases, the temperature of melted aluminum due to the reaction heat further rose; and thereby, the viscosity thereof was lowered. As a result, liquid droplets were generated in some cases.

(18) In view of these, test pieces were manufactured under various conditions, and the test pieces were observed by an electron microscope. As a result of the observation, it was found that only a surface layer portion having a substantially constant thickness from the exposed surface side of the titanium particle reacted with aluminum in the case where the amount of heat generation was controlled to be in a range controllable by the mixing ratio of titanium and the particle diameter of titanium. From the experimental results, it was found that the conditions of 1 (μm)≤r≤30 (μm) and 0.1 (% by mass)≤W/r≤2 (% by mass) are preferable in order to prevent the occurrence of liquid droplets.

(19) Hereinafter, the meaning of 0.1≤W/r≤2 in the case of using titanium as the sintering aid powder will be described. When the average particle diameter of titanium is expressed as r, the number of titanium particles is expressed as N, the additive quantity (mass) of titanium is expressed as w, the specific weight of titanium is expressed as D, and the reduction amount in the titanium particle diameter due to the reaction with aluminum is expressed as d, the reaction heat amount Q fulfills Q∝4πr.sup.2dN since the reaction heat amount Q is proportional to the volume of reacted titanium. Moreover, since the additive amount of the titanium particles is calculated as a product of the average volume of one titanium particle and the number of titanium particles, w=4/3πr.sup.3DN is obtained. Accordingly, if the latter equation is substituted into the former equation, Q∝3wd/rD is obtained. Here, Q∝w/r is further obtained based on the fact that 3/D is a constant and the observation result that d is substantially constant regardless of the sintering conditions. Therefore, the range of W/r in which the liquid droplets are not generated is experimentally determined and the range is limited as described above. Thereby, the generation of liquid droplets due to the excessively high reaction heat between aluminum and titanium is prevented.

(20) The following components are added to the raw aluminum mixed powder in the process of providing the viscous composition: at least one kind selected from polyvinyl alcohol, methylcellulose, and ethylcellulose as a water-soluble resin coupler; at least one kind selected from polyethyleneglycol, glycerin, and di-N-buthyl phthalate as a plasticizer; distillated water; and alkylbetaine as a surfactant.

(21) In the case where at least one kind selected from polyvinyl alcohol, methylcellulose, and ethylcellulose is used as the water-soluble resin coupler, a relatively small additive amount is sufficient. Therefore, the additive amount (ratio) thereof is set to be in a range of 0.5% to 7% of the quantity (mass) of the raw aluminum mixed powder. In the case where the additive amount of the water-soluble resin coupler exceeds 7% of the quantity (mass) of the raw aluminum mixed powder, the amount of carbon remaining in the formed object prior to sintering is increased during heating, and the remained carbon inhibits the sintering reaction. In the case where the additive amount of the water-soluble resin coupler is less than 0.5%, the handling strength of the formed object prior to sintering cannot be secured.

(22) Alkylbetain is added at an amount (ratio) of 0.02% to 3% of the quantity (mass) of the raw aluminum mixed powder. In the case where the amount (containing ratio) is set to be 0.02% or higher of the quantity (mass) of the raw aluminum mixed powder, air bubbles are effectively generated during mixing a non-water-soluble hydrocarbon system organic solvent which will be described later. By setting the amount (containing ratio) to be 3% or lower, the inhibition of the sintering reaction due to the increased amount of carbon remaining in the formed object prior to sintering can be prevented.

(23) After kneading the mixture, foaming is performed by further mixing the non-water-soluble hydrocarbon system organic solvent containing 5 to 8 carbons; and thereby, a viscous composition including air bubbles mixed thereinto is prepared. As the non-water-soluble hydrocarbon system organic solvent containing 5 to 8 carbons, at least one kind selected from pentane, hexane, heptane, and octane can be used.

(24) Next, in the step prior to sintering, a strip-shaped polyethylene sheet is prepared of which the surface is coated with separating compound, and the viscous composition is extended to have a thickness of 0.05 mm to 5 mm by coating the viscous composition on the surface of the strip-shaped polyethylene sheet. Then, the temperature and moisture of the surrounding (circumferential atmosphere) are controlled for a specific time period so as to make the dimensions of air bubbles uniform. Thereafter, the resulting object is dried at a temperature of 70° C. by an air dryer. Here, the viscous composition is coated by a doctor blade method, a slurry extrusion method, or a screen printing method.

(25) The dried viscous composition is peeled off from the polyethylene sheet, and then, if necessary, the dried viscous composition is cut out into a predetermined shape such as a circle having a diameter of 100 mm. Thereby, the formed object prior to sintering is obtained.

(26) Next, in the sintering process, zirconia spinkle powder is spread on an alumina setter, and the formed object prior to sintering is placed on the alumina setter. Then, pre-sintering is performed by holding the formed object prior to sintering at 520° C. for one hour in an argon atmosphere whose dew point is −20° C. or lower. Thereby, the water-soluble resin coupler, a binder solution of the plasticizer component, the distillated water, and alkylbetaine are evaporated (removal of binder). In addition, dehydrogenation proceeds in the case where titanium hydride is used as the sintering aid powder.

(27) Thereafter, the formed object prior to sintering which is pre-sintered is heated at a heating temperature T which fulfills Tm−10 (° C.)≤heating temperature T≤685 (° C.) to obtain foamed aluminum.

(28) This is based on the following reason. It is considered that the reaction between aluminum and titanium starts by heating the formed object prior to sintering up to the melting temperature Tm (° C.). However, aluminum contains a very small amount of eutectic alloy elements such as Fe, Si, and the like as impurities in practice; and thereby, the melting point thereof is lowered, Therefore, it is considered that the reaction between aluminum and titanium starts by heating up to Tm−10 (° C.) and foamed aluminum is formed. In practice, the melting point of aluminum is 660° C.; however, the melting start temperature of an atomized powder having a purity of about 98% to 99.7%, which is marketed as a pure aluminum powder, is about 650° C.

(29) On the other hand, in the case where the temperature reaches 665° C. which is the peritectic temperature of aluminum and titanium, and the melting latent heat is further input, the sintered aluminum (aluminum sintered body) is melted. Therefore, it is necessary to keep the temperature of a furnace atmosphere in a range of 685° C. or lower.

(30) In addition, it is necessary to perform the heating of the sintering process in a non-oxidizing atmosphere in order to suppress the growth of oxide layers on the aluminum particle surface and the titanium particle surface. However, the oxide layers on the aluminum particle surface and the titanium particle surface do not remarkably grow even in the case of heating in the air under the conditions where the heating temperature is 400° C. or lower and the holding time is about 30 minutes. Therefore, the formed object prior to sintering may be heated and held at a temperature in a range of 300° C. to 400° C. for about 10 minutes in the air (removal of binder), and then the formed object prior to sintering may be heated at a predetermined temperature in an argon atmosphere.

(31) The thus obtained foamed aluminum includes metal skeletons having a three-dimensional network structure of perforated sintered metal (perforated metal sintered body), and pores are included between the metal skeletons. In addition, Al—Ti compound is dispersed in the perforated sintered metal, 20 or more pores are formed per linear length of 1 cm, and the foamed aluminum has an overall porosity of 70 to 90% and is suitably used as a current collector of a lithium-ion secondary battery or an electrical double layer capacitor.

(32) The present invention is not limited to the aforementioned embodiments, and a sintering aid powder other than titanium and titanium hydride may be used as long as a sintering aid powder which contains titanium as a sintering aid element is used.

(33) In the method for producing foamed aluminum of the present embodiment, the aluminum powder is mixed with the titanium and/or titanium hydride as the sintering aid powder to prepare the raw aluminum mixed powder in the process of providing the raw aluminum mixed powder. Then, the viscous composition produced in the process of providing the viscous composition is foamed, and the viscous composition is heated at a temperature T which fulfills Tm−10 (° C.)≤T≤685 (° C.) in the sintering process. Thereby, it is possible to obtain uniform foamed aluminum which has a high porosity and includes two different kinds of pores. The two different kinds of pores includes pores which are surrounded by the sponge skeletons and have pore diameters of less than 600 μm and uniform dimensions, and fine pores which are formed in the sponge skeleton itself at an amount of two or more pores per linear length of 100 μm. The titanium hydride contains titanium at a content of 95% by mass or greater and dehydrogenation of the titanium hydride occurs at a temperature of 470 to 530° C. to convert into titanium. Therefore, the titanium hydride is thermally decomposed into titanium by the aforementioned heating. Accordingly, it is considered that the titanium hydride contributes to producing foamed aluminum by the aforementioned heating in the same manner as titanium.

(34) Moreover, the pre-sintering process is provided between the process of providing the viscous composition and the sintering process, and in the pre-sintering process, the strip-shaped polyethylene sheet is prepared of which the surface is coated with separating compound, and the viscous composition is extended to have a thickness of 0.05 mm to 5 mm by coating the viscous composition on the strip-shaped polyethylene sheet to obtain the formed object prior to sintering having a predetermined shape. In this case, by performing the sintering process as the post processing, it is possible to obtain foamed aluminum which is suitably used as a current collector of a lithium-ion secondary battery or an electrical, double layer capacitor.

EXAMPLES

Examples 1 to 16

(35) Al powders having average particle diameters of 2.1 μm, 9.4 μm, 24 μm, 87 μm, and 175 μm, Ti powders having average particle diameters of 9.8 μm, 24 μm, and 42 μm, and TiH.sub.2 powders having average particle diameters of 4.2 μm, 9.1 μm, and 21 μm were prepared. Then, in accordance with the aforementioned embodiment, the Al powder was mixed with the Ti powder and/or the TiH.sub.2 powder at the ratios shown in Table 1 to prepare raw aluminum mixed powders 1 to 10, and binder solutions 1 to 5 having the compounding compositions shown in Table 2 were prepared. They were kneaded with a non-water-soluble hydrocarbon system organic solvent at the ratios shown in Table 3 to manufacture viscous compositions of Examples 1 to 16.

(36) TABLE-US-00001 TABLE 1 Raw aluminum mixed powder Sintering aid powder Aluminum powder Average Composition (% by weight) Average particle Sintering Al and particle Mixing ratio diameter aid inevitable diameter (% by weight) (μm) Aluminum powder Fe Si Ni Mg Cu impurities (μm) Ti TiH.sub.2 r powder W W/r Raw aluminum 0.15 0.05 0.01 — — remainder  24  0 100  9.1 remainder  1  0.11 mixed powder 1 of the present invention Raw aluminum 0.15 0.05 0.01 — — remainder  24  0 100 21  remainder  5  0.24 mixed powder 2 of the present invention Raw aluminum 0.15 0.05 0.01 — — remainder  24  0 100 21  remainder 15  0.71 mixed powder 3 of the present invention Raw aluminum 0.15 0.05 0.01 — — remainder  24  0 100  9.1 remainder 10  1.1 mixed powder 4 of the present invention Raw aluminum 0.15 0.05 0.01 — — remainder  24  0 100  4.2 remainder  5  1.2 mixed powder 5 of the present invention Raw aluminum 0.15 0.05 0.01 — — remainder  24  0 100  2.8 remainder  5  1.8 mixed powder 6 of the present invention Raw aluminum 0.16 0.08 — — — remainder   9.4 50 50 23  remainder  0.5 0.022 mixed powder 7 of the present invention Raw aluminum 0.18 0.06 0.01 0.4 1.6 remainder  87  100 0 24  remainder  1  0.042 mixed powder 8 of the present invention Raw aluminum 0.2 0.3 1.6 0.4 0.1 remainder 175  100 0 23  remainder  5  0.22 mixed powder 9 of the present invention Raw aluminum 0.2 0.05 — — — remainder   2.1 0 100  4.2 remainder  1  0.24 mixed powder 10 of the present invention Comparative raw 0.11 0.05 — — — remainder .sup.  220*.sup.1  100 0 24  remainder  5  0.21 aluminum mixed powder 31 Comparative raw 0.15 0.05 0.01 — — remainder  24  0 100 21  remainder  0.1 0.005*.sup.2 aluminum mixed powder 32 Comparative raw 0.15 0.05 0.01 — — remainder  24  0 100  4.2 remainder 15  3.6*.sup.2 aluminum mixed powder 33 Comparative raw 0.15 0.05 0.01 — — remainder  24  0 100 21  remainder .sup.  25*.sup.2  1.2 aluminum mixed powder 34 Comparative raw 0.15 0.05 0.01 — — remainder  24  100 0 .sup.  42*.sup.2  remainder 15  0.36 aluminum mixed powder 35 *.sup.1out of the scope of Claim 3; average particle diameter of aluminum powder: 2 μm to 200 μm *.sup.2out of the scope of Claim 4; average particle diameter and mixing ratio of sintering aid powder: 1 ≤ r ≤ 30 and 0.01 ≤ W/r ≤ 2

(37) TABLE-US-00002 TABLE 2 Compounding composition of binder solution (% by weight) Water-soluble resin coupler Plasticizer Surfactant MC EC PVA Gr PEG AB Water Binder 5 — — 3 3 0.1 remainder solution 1 Binder 0.1 2.9 — 3 3 0.5 remainder solution 2 Binder 0.2 — 4.8 1 5 2 remainder solution 3 Binder 9 — — 7 5 0.5 remainder solution 4 Binder 5 — — 3 3 5 remainder solution 5 MC: methylcellulose EC: ethylcellulose PVA: polyvinyl alcohol Gr: glycerin PEG: polyethyleneglycol AB: alkylbetaine

(38) TABLE-US-00003 TABLE 3 Components of viscous composition Non-water-soluble hydrocarbon system Ratio Raw aluminum mixed powder A Binder solution organic solvent waret-soluble Ratio of Mixing ratio Mixing ratio Mixing ratio resin coupler surfactant Type (% by weight) Type (% by weight) Type (% by weight) to A (%) to A (%) Example 1 Raw aluminum 50 Binder 49 hexane 1 2.8 0.49 mixed powder 1 of solution 2 the present invention Example 2 Same as above 50 Binder 49 heptane 1 2.8 0.49 solution 2 Example 3 Same as above 50 Binder 49 heptane 1 2.8 0.49 solution 2 Example 4 Same as above 49 Binder 49 octane 2 2.8 0.49 solution 2 Example 5 Same as above 50 Binder 49 octane 1 4.9 0.098 solution 1 Example 6 Same as above 50 Binder 49 hexane 1 4.9 0.098 solution 1 Example 7 Same as above 50 Binder 49 pentane 1 4.7 1.96 solution 3 Example 8 Raw aluminum 50 Binder 49 hexane 1 4.9 0.098 mixed powder 2 of solution 1 the present invention Example 9 Raw aluminum 50 Binder 49 hexane 1 4.9 0.098 mixed powder 3 of solution 1 the present invention Example 10 Raw aluminum 50 Binder 49 pentane 1 4.9 0.098 mixed powder 4 of solution 1 the present invention Example 11 Raw aluminum 50 Binder 49 heptane 1 4.9 0.098 mixed powder 5 of solution 1 the present invention Example 12 Raw aluminum 50 Binder 49 heptane 1 4.9 0.098 mixed powder 6 of solution 1 the present invention Example 13 Raw aluminum 50 Binder 49 octane 1 4.9 0.098 mixed powder 7 of solution 1 the present invention Example 14 Raw aluminum 50 Binder 49 octane 1 4.9 0.098 mixed powder 8 of solution 1 the present invention Example 15 Raw aluminum 50 Binder 49 pentane 1 4.9 0.098 mixed powder 9 of solution 1 the present invention Example 16 Raw aluminum 50 Binder 49 octane 1 4.9 0.098 mixed powder 10 of solution 1 the present invention Conditions of producing formed object prior to sintering Process of adjusting dimensions of air bubbles uniformly Drying process Thickness of formed Temperature Moisture Holding time Temperature Holding time coating (mm) (° C.) (%) (minute) (° C.) (minute) Example 1 0.35 35 90 20 70 50 Example 2 0.35 35 90 20 70 50 Example 3 0.35 35 90 20 70 50 Example 4 0.35 35 90 40 70 50 Example 5 0.2 35 90 20 70 50 Example 6 0.2 35 90 20 70 50 Example 7 0.2 35 90 20 70 50 Example 8 0.2 35 90 20 70 50 Example 9 0.2 35 90 20 70 50 Example 10 0.2 35 90 20 70 50 Example 11 0.2 35 90 20 70 50 Example 12 0.2 35 90 20 70 50 Example 13 0.2 35 90 20 70 50 Example 14 0.2 35 90 20 70 50 Example 15 0.2 35 90 20 70 50 Example 16 0.2 35 90 20 70 50 Heating conditions Degreasing process Sintering process Temperature Holding time Temperature Holding time Atmosphere (° C.) (minute) Atmosphere (° C.) (minute) Example 1 Ar 520 30 Ar 683 30 Example 2 Ar 520 30 Ar 650 30 Example 3 Ar 520 30 Ar 683 30 Example 4 Ar 520 30 Ar 675 30 Example 5 Ar 520 30 Ar 670 30 Example 6 Air 350 30 Ar 670 30 Example 7 Air 350 30 Ar 670 30 Example 8 Air 350 30 Ar 670 30 Example 9 Air 350 30 Ar 670 30 Example 10 Air 350 30 Ar 670 30 Example 11 Air 350 30 Ar 670 30 Example 12 Air 350 30 Ar 670 30 Example 13 Air 350 30 Ar 670 30 Example 14 Ar 520 30 Ar 655 30 Example 15 Ar 520 30 Ar 651 30 Example 16 Air 350 30 Ar 670 30

(39) Next, polyethylene sheets were prepared of which the surfaces were coated with separating compound, and the viscous compositions of Examples 1 to 16 were coated and extended on the surface of the polyethylene sheet by the doctor blade method, and the temperature and the moisture were controlled to be predetermined values for a specific time period so as to adjust the dimensions of air bubbles uniformly. Then, the viscous compositions were dried at 70° C. in an air dryer. The coating thicknesses of the viscous compositions, temperatures, moistures, and holding times at that time are shown in Table 3. Thereafter, the dried viscous compositions were peeled off from the polyethylene sheets and cut out into circular shapes having diameters of 100 mm to obtain formed objects prior to sintering in Examples 1 to 16.

(40) Then, zirconia spinkle powder was spread on an alumina setter, and the formed objects prior to sintering in Examples 1 to 16 were placed on the alumina setter. The formed objects prior to sintering in Examples 1 to 16 were subjected to debinding in an atmosphere where argon flowed or in air. Thereafter, the formed objects prior to sintering in Examples 1 to 16 were heated to obtain foamed aluminums. The heating temperatures and heating holding times are also shown in Table 3.

(41) Next, the contraction percentages and porosities of the obtained foamed aluminums in Example 1 to 16 were calculated. In addition, the number of three-dimensional pores was measured in a stereoscopic microscope photograph, and the number of pores in the skeletons was measured in a scanning electron microscope (SEM) photograph. The obtained SEM photograph was observed to confirm whether solidification of liquid droplets occurred. Moreover, surface analyses were conducted by an electron probe microanalyzer (EPMA) to confirm whether Al—Ti compound existed on the surface of the skeletons of the foamed aluminums. The results are shown in Table 5, the SEM photograph of the foamed aluminums in Example 1 is shown in FIG. 1, and a partially enlarged photograph thereof is shown in FIG. 2.

(42) Next, rolling extension tests were performed on the foamed aluminums in Examples 1 to 16 at a rolling reduction rate of 20%, and whether cracking occurred was visually confirmed. Thereafter, rectangular samples having dimensions of 20 mm×50 mm were cut out from the foamed aluminums, and the electrical resistances between opposed corners were measured. Then, the rectangular samples of the foamed aluminums were wound around an outer circumference of a cylindrical object having a diameter of 5 mm, and whether cracking occurred was visually confirmed. The results are shown in Table 5.

Comparative Examples 1 to 9

(43) Comparative raw aluminum mixed powders 31 to 35 were prepared by using the same Al powder, Ti powder, and TiH.sub.2 powder as those in Examples. Either one of the comparative raw aluminum mixed powders 31 to 35 and the raw aluminum mixed powder 1 of the present invention was mixed and kneaded with either one of the binder solutions 1 to 5 shown in Table 2 and the non-water-soluble hydrocarbon system organic solvent at the mixing ratios shown in Table 4. Other conditions were same as those in Examples. Thereby, foamed aluminums in Comparative Examples 1 to 9 were produced. The foamed aluminums in Comparative Examples 1 to 9 were evaluated by the same methods as those for Examples. The evaluation results are shown in Table 5, and an SEM photograph of the foamed aluminum in Comparative Example 1 is shown in FIG. 3.

(44) TABLE-US-00004 TABLE 4 Components of viscous composition Non-water-soluble hydrocarbon system Ratio Raw aluminum mixed powder A Binder solution organic solvent waret-soluble Ratio of Mixing ratio Mixing ratio Mixing ratio resin coupler surfactant Type (% by weight) Type (% by weight) Type (% by weight) to A (%) to A (%) Comparative Raw aluminum 50 Binder 49 hexane 1 2.8 0.49 Example 1 mixed powder 1 of solution 2 the present invention Comparative Same as above 50 Binder 49 heptane 1 2.8 0.49 Example 2 solution 2 Comparative Same as above 50 Binder 49 octane 1 8.82*.sup.4 0.49 Example 3 solution 4 Comparative Same as above 49 Binder 49 pentane 1 4.9 4.9*.sup.5 Example 4 solution 5 Comparative Comparative raw 50 Binder 49 pentane 1 4.9 0.098 Example 5 aluminum mixed solution 1 powder 31 Comparative Comparative raw 50 Binder 49 hexane 1 4.9 0.098 Example 6 aluminum mixed solution 1 powder 32 Comparative Comparative raw 50 Binder 49 heptane 1 4.7 0.098 Example 7 aluminum mixed solution 1 powder 33 Comparative Comparative raw 50 Binder 49 octane 1 4.9 0.098 Example 8 aluminum mixed solution 1 powder 34 Comparative Comparative raw 50 Binder 49 pentane 1 4.9 0.098 Example 9 aluminum mixed solution 1 powder 35 Conditions of producing formed object prior to sintering Process of adjusting dimensions of air bubbles uniformly Drying process Thickness of formed Temperature Moisture Holding time Temperature Holding time coating (mm) (° C.) (%) (minute) (° C.) (minute) Comparative 0.35 35 90 20  70 50 Example 1 Comparative 0.35 35 90 20  70 50 Example 2 Comparative 0.35 35 90 20  70 50 Example 3 Comparative 0.2 35 90 20  70 50 Example 4 Comparative 0.2 35 90 20  70 50 Example 5 Comparative 0.2 35 90 20  70 50 Example 6 Comparative 0.2 35 90 20  70 50 Example 7 Comparative 0.2 35 90 20  70 50 Example 8 Comparative 0.2 35 90 20  70 50 Example 9 Heatiing conditions Degreasing process Sintering process Temperature Holding time Temperature Holding time Atmosphere (° C.) (minute) Atmosphere (° C.) (minute) Comparative Ar 520 30 Ar .sup.  690*.sup.3 30 Example 1 Comparative Ar 520 30 Ar .sup.  620*.sup.3 30 Example 2 Comparative Ar 520 30 Ar 683 30 Example 3 Comparative Ar 520 30 Ar 670 30 Example 4 Comparative Air 350 30 Ar 670 30 Example 5 Comparative Air 350 30 Ar 670 30 Example 6 Comparative Air 350 30 Ar 670 30 Example 7 Comparative Air 350 30 Ar 670 30 Example 8 Comparative Air 350 30 Ar 670 30 Example 9 *.sup.4out of the scope of Claim 7 *.sup.5out of the scope of Claim 8 *.sup.3out of the scope of Claim 1

(45) TABLE-US-00005 TABLE 5 Evalutaion of current collector for positive electrode of lithium-ion Evaluation of foamed aluminum battery Presence or Minimum Presence or Presence absence of diameter at Number of pores absence of or absence cracking Filling which active Number of in skeleton per solidified of Al—Ti after 10% density material does three- skeleton length aluminum in compound Electric rolling and of active not fall in dimensional of 100 μm the form of on skeleton resistivity 5 mmφ material winding test pores (PPI.sup.*1) (pores/100 μm) liquid droplet surface (×10.sup.−6 Ωm) winding test (g/cm.sup.3) (mmφ) Example 1 52 2.9 Absent Present 3.1 Absent 4.8 2 Example 2 52 3.5 Absent Present 5.4 Absent 4.7 2 Example 3 52 2.2 Absent Present 2.2 Absent 4.6 1.5 Example 4 65 2.3 Absent Present 2.5 Absent 4.8 2 Example 5 56 2.5 Absent Present 2.6 Absent 4.2 2 Example 6 55 2.5 Absent Present 2.6 Absent 4.2 1.5 Example 7 77 2.7 Absent Present 2.7 Absent 4.2 2 Example 8 54 2.8 Absent Present 2.9 Absent 4.3 2 Example 9 55 2.3 Absent Present 2.3 Absent 4.3 2 Example 10 52 2.6 Absent Present 2.8 Absent 4.2 2 Example 11 53 2.2 Absent Present 3.2 Absent 4.2 2 Example 12 55 2.4 Absent Present 3.2 Absent 4.3 2 Example 13 53 2.8 Absent Present 3.4 Absent 4.1 2 Example 14 55 3.4 Absent Present 4.9 Absent 4.1 2.5 Example 15 55 3.2 Absent Present 4.3 Absent 4.2 2.5 Example 16 54 2.4 Absent Present 3.2 Absent 4.2 2 Comparative 70 2 Present* Present 2.9 Present* — — Example 1 Comparative 50 5.1 Absent Present 12.4* Present* — — Example 2 Comparative 51 4.6 Absent Present 11.9* Present* — — Example 3 Comparative 65 4.3 Absent Present 11.2* Present* — — Example 4 Comparative 52 1.8* Absent Present 8.9* Present* — — Example 5 Comparative 53 5.2 Absent Absent 12.2* Present* — — Example 6 Comparative 51 2.6 Present* Present 2.4 Absent — — Example 7 Comparative 51 2.2 Absent Present 2.8 Present* — — Example 8 Comparative 55 1.8* Absent Present 3.1 Present* — — Example 9 Comparative 30 0 Absent Absent 1.5 Absent 3.8 3.5 Example 1 .sup.*1PPI: number of pores per inch (25.4 mm)

(46) As can be understood from Table 5, with regard to the foamed aluminums in Examples 1 to 16, the numbers of pores per skeleton length of 100 μm of the perforated sintered metals were in a range of 2 to 4, and the numbers of three-dimensional pores per one inch were in a range of 52 or more, that is, the numbers of the three-dimensional pores per one centimeter in the metal skeletons were in a range of 20 or more. In addition, no lumps of liquid droplets were generated in the foamed aluminums, the electrical resistances were low, and no cracking due to the winding test were observed. Accordingly, the foamed aluminums in Examples 1 to 16 are suitable as a current collector for a positive electrode of a battery or a capacitor which requires high output and high energy density.

(47) Next, a lithium cobalt oxide (LiCoO.sub.2) powder as an active material, polyvinylidene fluoride (PVdE) as a coupler, artificial graphite powder as a conductive material were mixed at a ratio by weight of 86:6:8 to prepare a cathode material. N-methyl-2 pyrrolidone as a solvent was mixed with the cathode material to prepare a cathode active material slurry.

(48) Then, the foamed aluminums in Examples 1 to 16 and foamed aluminum in Conventional Example 1 were immersed into this cathode active material slurry for 10 minutes. The foamed aluminums were taken therefrom, and dried. Thereafter, the foamed aluminums were rolled to produce cathodes of lithium-ion batteries in Examples 1 to 16 having thicknesses of 0.5 mm.

(49) Here, as the foamed aluminum in Conventional Example 1, foamed aluminum of 30 PPI was used. The foamed aluminum was produced by a method of pressing aluminum into a casting mold having a core of sponge urethane which is mentioned as the second method in the related art. In addition, the filling densities of the cathode active materials of the foamed aluminum in Examples 1 to 16 and the foamed aluminum in Conventional Example 1 are shown in Table 5.

(50) Then, cylindrical objects having diameters of 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, and 5 mm were respectively prepared. The cathodes of lithium-ion batteries in Examples 1 to 16 and Conventional Example 1 were wound. Whether or not the active materials were peeled off was visually observed, and the minimum diameters with which peeling were not observed are shown in Table 5.

(51) As can be understood from the results in Table 5, with regard to the cathodes of the lithium-ion batteries in Examples 1 to 16, the active materials were not peeled off even in the case where the cathodes were wound around the cylindrical objects having diameters of 1.5 mm to 2.5 mm. On the other hand, with regard to the cathode in Conventional Example 1, the active material was peeled off when the cathode was wound around the cylindrical object having a diameter of 3 mm. In addition, the active material filling density of the cathode of the lithium-ion batteries in Examples 1 to 16 were in a range of 4.1 g/cm.sup.3 or greater. In contrast, the active material filling density of the cathode in Conventional Example 1 was 3.841 g/cm.sup.3, which was small.

INDUSTRIAL APPLICABILITY

(52) The present invention can be applied as a method for producing a current collector of a lithium-ion secondary battery or an electrical double layer capacitor as well as a method for producing foamed aluminum.