US20050281699A1

US20050281699A1 - Method for producing porous metal with micro-holes

Info

Publication number: US20050281699A1
Application number: US10/872,318
Authority: US
Inventors: Shang Kang; Shao Tseng; Hsin Tseng; Te Yuan; Ching Lee
Original assignee: CHIN CHU HUNG
Current assignee: CHIN CHU HUNG
Priority date: 2004-06-18
Filing date: 2004-06-18
Publication date: 2005-12-22

Abstract

A method for producing porous metal with micro-holes comprises the steps of selecting a base material, sintering or smelting, removing the filling material, washing, drying and reduction. The base material is selected from pure metal and alloy powder for sintering or casting. The filling material is fine powder solvable in water or in alkaline solvents. The sintering step further includes steps of mixing and metal powder metallurgy. The casting utilizes high or low waves to fuse the base material particles. A selected solvent removes the filling material in the raw metal bulk produced in the sintering or casting step, and a product of porous metal with micro-holes is formed.

Description

FIELD OF THE INVENTION

The present invention relates to methods for producing porous metal with micro-holes, and more particularly to a method for producing porous metal with micro-holes of micron order (250-0.1 μm). The method is easy to apply and can control the distribution density of mutually connected micro-holes, and the materials thereby produced are of low cost and can be used in catalysts, filters, and electrodes for batteries. They can be also used as anti-shock, anti-electromagnetic wave, heat-diffusing and sound-damping materials.

BACKGROUND OF THE INVENTION

The so-called porous metal or foam metal in the market refers to sintered metals of which the ratio of occupancy of the remnant holes therein exceeds 10%. The holes consist of randomly distributed inter-connected holes and isolated holes. The method cannot control the shape, the diameter and the distribution ratio of the holes it produces.
The methods for producing porous metal of the prior art includes:
(1) powder metallurgy that sinters a plurality of metal or alloy powder;
(2) sintering using metallic fibers as a replacement for the aforesaid metal powder;
(3) porous casting method that heats and cools a metal around the phase transition point thereof so as to cause the metal to expand and contract;
(4) foam or porous aluminum added into a molted metal;
(5) reducing the metal compounds in foam iron or foam titanium.
The current porous metallic materials are mainly produced by means of powder metallurgy sintering. The steps of powder metallurgy is mixing metal power with a binding material, sufficiently stirring the mixture and filling the mixture in to a mold, punching the mold as that the mixture takes form, and then sintering the mixture. In the sintering process, metal powder 9 integrated by a binder 22 will transit from an initial loose state (FIG. 1(a)), to a state in which metal powder starts to merge (FIG. 1(b)), and then to a state in which merged metal powder increases gradually (FIG. 1(c)), and finally to a state in which the merged metal powder forms a solid bulk (FIG. 1(b)).
In the aforesaid sintering process of powder metallurgy, the metal powder particles 9 a firstly contact with each other at contact points 91 a; between the contact points there form holes 92 a, as shown in FIG. 2(a). As the temperature increases, the contact points 91 a extend gradually to form neck portions 91 b, as shown in FIG. 2(b). As the neck portions 91 b, 91 c, 91 d continue to extend, the bulk of metal powder becomes more and more solid and the holes 92 b, 92 c, 92 d become smaller and smaller, as shown in FIG. 2(b), (c) and (d). Thereby, the closeness of metal powder particles is enhanced, but the hole distribution is poor.
Therefore, the conventional method of powder metallurgy enhances the closeness of metal particles but do little about upgrading the hole distribution.
Further, the uniformity of the mixture of metal powder and binding medium is susceptible to the mobility, the size and the shape of the metal powder particles, as well as the friction between the particles and the surfaces of the molding machine, resulting in uneven distribution of stress and strain and low material density. Take iron powder particles as an example, FIG. 3(a)-(b) shows the density distribution (in units of g/cm³) with respect to cylindrical materials made of iron powder under a variety of pressures, which is rather non-uniform. It can be further shown in FIG. 4 that the density of iron powder in a pressurized chamber is rather non-uniform. As shown in FIG. 5(a)-(c) are the pressure distributions in iron powder contained in a pressurized chamber at a pressure of 7 ton/cm². The height to diameter ration of the pressurized chamber H/D is 0.42 in FIG. 5(a), 0.79 in FIG. 5(b) and 1.66 in FIG. 5(c). Therefore, the pressure distributions as shown in FIG. 5(a)-(c) are rather non-uniform.
Further, depending on manufacturing methods, the shape of metal powder particles can be spherical, polygonal, slab-shaped, liquid-drop-shaped. For achieving better shaping, ventilating property and hole distribution for ordinary metal powder, gas-cooling spray is utilized to create spherical metal particles. It is known that there are several ways to fill with spherical particles, such as cubic, rectangular, square and hexagonal arrangements. According to experiments, the hole diameter is 41.4% of the particle diameter when the cubic piling is used. The formation and scale of hole diameter are shown in FIGS. 6(a) and (b), wherein
FIG. 6(a) discloses that D₁=D₀(2/√{square root over ( )}3−1)=0.156D₀;
FIG. 6(a) discloses that D₂=D₀(√{square root over ( )}2−1)=0.414D₀;
where D₁and D₂are the hole diameters between metal particles;
where D₀is the particle diameter.
Therefore, it is known theoretically that the hole diameters between metal particles D₁and D₂are 15.6-41.4% of the particle diameter D₀. However, because of non-uniform particle diameters, the actual percentage lies between 16-20%. It is a further effect that, as the particles are oxidized and become wet, the stress and strain between the particles become uneven, resulting in deformation and breakdown. Therefore, the hole diameter deduced by maximum particle diameter or estimated by probability is 18% of the spherical metal particle diameter. Accordingly, the hole occupancy ratio of the porous metal sintered from metal powder using common metal metallurgy methods is less than 50%. However, it can attain more than 90% using metal fiber sintering. Therefore, it is a common practice that metal fibers, bridging materials and foam materials are added to powder-binder mixture for enhancing hole distribution; the added materials are then burn away by heating. However, it is still difficult to control the hole diameter and the hole distribution. At the same time, burning the bridging materials may produce poisonous gases and oxidization, accumulating choral substance in the holes, which is difficult to clean away. This effect causes troubles in utilizing porous metal materials. For example, the function and quality of catalyses or battery electrodes made of porous metal materials thereby produced may be seriously affected.
Since the holes in conventional porous metals are spacing between powder particles, fibers and other constitutes, they are irregular in shape and distribution. Therefore, the hole distribution is of poor uniformity and therefore low quality.
Further, there exists a manufacturing method for spongy metal in which uniform hole shape and diameter and high density can be attained, which is disclosed by R.O.C. patent numbers 434323 and 491903. Those two methods utilize an organic material of desired size, shape and particle diameter as the filling material. The material is then soaked with fire-resistant mud and then goes through a piling process to attain desired shapes and diameters for isolated holes and interconnecting holes. However, as the diameter of the organic particles becomes too small (say, approaching millimeter scale), the material density is difficult to control. This is disadvantageous in mass production of materials of fine isolated holes and interconnecting holes.
Therefore, if the hole diameter and distribution can be effectively and stably controlled, the porous metals thereby produced can be used in catalyst filters, and electrodes for batteries, anti-shock, anti-electromagnetic wave, heat-diffusing and sound-damping materials, of which the functions are significantly enhanced. But a method for producing porous metal with micro-holes is still absent, therefore motivating the present invention.

SUMMARY OF THE INVENTION

Accordingly, the primary objective of the present invention is to provide an innovative method for producing porous metal with micro-holes, in contrast to the conventional metal powder metallurgy.
The secondary objective of the present invention is to provide a sintering method for producing porous metal with micro-holes.
The method for producing porous metal with micro-holes according to the present invention has the following advantages.

- 1. It is easy and cheap to acquire the pure metal or alloy powder used as the base material for the sintering process or the casting process. Including metal powder, alloy powder and solvent fine powder, the suitable material is selected to produce porous metals of desired distribution of the fine holes.
- 2. Metal powder metallurgy utilizes cold or hot press forming in the sintering process, thereby the base material and filling material attain perfect mixing. It is equivalent that high or low frequency sound waves are used in the sintering process in which the filling material is mixed uniformly with the molten base material according to a selected proportion. Therefore, the method according to the present invention can effectively control the distribution of the interconnecting holes in the porous metals produced, further enhancing the density and stability of the fine holes.
- 3. Ultrasonic waves are used to remove the filling material distributed uniformly in the metal raw bulk, which is a stable technology for lowing production cost and fault rate.
- 4. The oxidation layers (films) on the surface of the porous metals produced is removed by injecting reduction gas in a vacuum chamber or being dissolved in a solvent. The product therefore has nice looking out look and is suitable for making catalyst filters, and electrodes for batteries, anti-shock, anti-electromagnetic wave, heat-diffusing and sound-damping materials.

To achieve the object, the present invention provides a powder metallurgy method for producing porous metal with micro-holes, comprising the steps of: (a) selecting materials, a base material being selected from pure metal powder and alloy powder for sintering; a filling material being fine powder solvable in water or in alkaline solvents, said filling material having a melting point higher than that of said base material; (b) sintering, said sintering further including a mixing step for mixing said base material and said filling material and in which a binding material for making the piling of said base and said filling materials uniform and dense, said mixture being dried and preheated to remove the water vapor attached to said filling material, said mixture being heated to sintering temperature to become a dense metal bulk having micro-hole texture; (c) removing said filling material by immerging said metal bulk in a washing tank of a selected solvent and then applying ultrasonic waves for shaking away said filling material, whereby a porous metal half-product with micro-holes configured by said sintered base material is formed; (d) washing away remained solvent between micro-holes of said porous metal half-product; and (e) drying away remained water drops between micro-holes of said porous metal half-product from said washing step. The product of porous metals with micro-holes can be formed.
Moreover, the present invention provide a smelting method for producing porous metal with micro-holes. The method comprises the steps of: (a) selecting materials, a base material being selected from pure metal and alloy of a predetermined melting point for smelting; a filling material being fine powder solvable in water or in alkaline solvents, said filling material having a melting point higher than that of said base material; (b) smelting, said smelting further including a mixing step for mixing said base material and said filling material in an oven equipped with high or low frequency waves for inducing a desired magnetic stirring effect, said smelted mixture being disposed in a thermo chamber equipped with ultrasonic waves for uniformly stirring, said mixture undergoing a fast casting process whereby solid particles of said filling material can be uniformly distributed between molten smelting base material, said mixture undergoing being cooled to form a dense metal bulk having micro-hole texture; (c) removing said filling material by immerging said metal bulk in a washing tank of a selected solvent and then applying ultrasonic waves for shaking away said filling material, whereby a porous metal half-product with micro-holes configured by said sintered base material is formed; (d) washing away remained solvent between micro-holes of said porous metal half-product; and (e) drying away remained water drops between micro-holes of said porous metal half-product from said washing step. The product of porous metals with micro-holes can be formed.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) shows the loosely distributed metal powder particles, connected by a binding material, before the conventional process of metal powder metallurgy.
FIG. 1(b) shows that the metal powder particles in the raw material start to fuse with each other in an initial phase of the conventional process of metal powder metallurgy.
FIG. 1(c) shows that the metal powder particles in the raw material become denser in a medium phase of the conventional process of metal powder metallurgy.
FIG. 1(d) shows that the metal powder particles in the raw material become solidly sintered after the conventional process of metal powder metallurgy.
FIG. 2(a) shows the contact points of and the holes between metal powder particles in an initial phase of metal powder metallurgy.
FIG. 2(b) shows the contact points in FIG. 2(a) extend to form contact faces and neck-like portions.
FIG. 2(c) shows that the neck-like portions in FIG. 2(b) continue to enlarge and the volume of holes shrinks gradually.
FIG. 2(d) shows that the neck-like portions in FIG. 2(c) maximize and the volume of holes shrinks to minimum.
FIG. 3(a) shows the unevenness in density of a cylindrical object formed by iron powder under a pressure of 10 ton per centimeter squared.
FIG. 3(b) shows the unevenness in density of a cylindrical object formed by iron powder under a pressure of 15 ton per centimeter squared.
FIG. 3(c) shows the unevenness in density of a cylindrical object formed by iron powder under a pressure of 20 ton per centimeter squared.
FIG. 3(d) shows the unevenness in density of a cylindrical object formed by iron powder under a pressure of 25 ton per centimeter squared.
FIG. 4 shows the unevenness in density of iron powder in a pressurized chamber from measurement.
FIG. 5(a) shows the unevenness in pressure distribution of iron powder disposed in an H/D=0.42 pressurized chamber having a pressure of 7 tons per centimeter squared.
FIG. 5(b) shows the unevenness in pressure distribution of iron powder disposed in an H/D=0.79 pressurized chamber having a pressure of 7 tons per centimeter squared.
FIG. 5(c) shows the unevenness in pressure distribution of iron powder disposed in an H/D=1.66 pressurized chamber having a pressure of 7 tons per centimeter squared.
FIG. 6(a) shows spherical particles expressing metal powder and filling material in rhomb piling.
FIG. 6(b) shows spherical particles expressing metal powder and filling material in cubic piling.
FIG. 7 is the flow chart illustrating the method of metal powder metallurgy of the present invention.
FIG. 8 is the flow chart illustrating the method of sintering of the present invention.
FIG. 9 is a local enlarged view of the base material for sintering stuffed uniformly within filling material.
FIG. 10 is a local enlarged view of the texture of the metal raw bulk.
FIG. 11 is a local perspective enlarged view of the texture of the metal raw bulk.
FIG. 12 shows the relation between filling material particle diameter D and the neck portion diameter X.
FIG. 13 is a local perspective enlarged view of the texture of a porous metal thereby produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for producing porous metal with micro-holes may have at least two preferred embodiments; one is a powder metallurgy method, and the other is a smelting method.
The powder metallurgy method comprises the steps of material mixing 1, sintering fusing process 2, filling material removing 3, washing 4, drying 5 and reduction 6. The sintering step 2 further comprises steps of mixing and stirring 21, form fixing 25, preheating 26 and sintering 27, as shown in FIG. 7.
The smelting method comprises the steps of material mixing 10, smelting fusing process 7, filling material removing 30, washing 40, drying 50 and reduction 60. The smelting fusing process 7 is bifurcated into two paths, one using high frequency oven and the other using low frequency oven. There is then a process of casting for form fixing, as shown in FIG. 8.
The first step of the above two methods are all material mixing 1, 10.
For the material mixing 1 of the powder metallurgy method, a pure metal or alloy powder must be selected as a sintering base material 11. And a filling material 12 that is solvable in water or alkaline solvents and having a melting point higher than that of the sintering base material 11 must be provided, so that the filling material 12 will not be sintered during the sintering process.
For the material mixing 10 of the smelting method, a pure metal or alloy powder must be selected as a smelting base material 13. And a filling material 12 in fine powder form that is solvable in water or alkaline solvents and having a melting point higher than that of the smelting base material 13 must be provided, so that the filling material 12 will not be sintered when the mixture is disposed in a high frequency or low frequency oven for smelting.

The sintering base material 11 and the smelting base material 13 can be pure Al metal or alloys of Al and another metal selected from Si, Mg, Cu and Zn. The sintering base material 11 and the smelting base material 13 can also be pure Cu metal or alloys of Cu and another metal selected from Zn, Sn, Al and P. They can also be pure Fe metal powder or pure Pb metal or alloys of Pb and another metal selected from Sn and Zn. They can be pure Mg metal or alloys of Mg and another metal selected from Li, Al and Cu. They can be pure Mn metal or alloys of Mn and another metal selected from Fe and Ni. They can be pure Mo metal or alloys of Mo and another metal selected from Fe, Ni, Co and Cr. They can be pure Ni metal or alloys of Ni and another metal selected from Fe, Cr, Mo and Co. They can be pure Ag metal or alloys of Ag and Cu. They can be pure Sn metal or alloys of Sn and another metal selected from Pb, Cu and Bi. They can be pure W metal and alloys of W and another metal selected from Ni, Cu and Ag. They can be pure V metal or alloys of V and another metal selected from Mo, Ni, Co and Ti. They can be pure Ti metal or alloys of Ti and another metal selected from Al, Ni and Co. These materials are listed in Table 1.

TABLE 1


The sintering and smelting base materials used in the present invention and their properties.

Sintering and

Melting

Boiling

Solvable solvents

Insolvable

	smelting	Specific	point	point	Cold	Hot	Other	solvents
Names	materials	weight	° C.	° C.	water	water	solvents	Solvents

Al and	Al,	2.7	660(low)	2056	No	No	In HCl,	CH₃CH₂OH
Al alloys	Al-Si, Mg,Cu,Zn	≅2.7	<660(low)	<2056			NaOH, KOH	(alcohol),
								glycerine
Cu and	Cu,	8.92	1083(high)	2300	No	No	In HNO₃	NH₄Cl, NaOH,
Cu alloys	Cu-Zn, Sn, Al, P	≅8.92	<1083(high)	<2300			hot H₂SO₄	KOH, NH₄OH
Fe and	Fe	7.86	1535(high)	3000	No	No	In H₂SO_4,	NH₄Cl, NaOH,
Fe alloys		7.6 ˜ 7.8	1575(high)	<3000			HNO₃, HCl	KOH, NH₄OH
Pb and	Pb	11.34	327.5(low)	1620	No	No	In HNO₃	CH₃CH₂OH
Pb alloys	Pb-Sn, Zn	<11.34	<327(low)	<1620				(alcohol),
								glycerine
Mg and	Mg	1.74	651(low)	1110	No	Yes,	In H₂SO₄,	CH₃CH₂OH(al-
Mg alloys	Mg-Li, Al, Cu	≅1.74	<651(low )	<1110		mildly	HNO₃,dilute	cohol), H₄Cl,
							HCl	glycerine,
								NaOH, KOH
Mn and	Mn	7.2	1260(high)	1900	Yes	No	In	NH₄Cl, NaOH,
Mn alloys	Mn-Fe, Ni	≅7.2	<1260(high)	<1900			dilute H₂SO₄,	KOH, NH_4OH
							Dilute HNO₃,
							dilute HCl
Mo and	Mo	10.2	2620(high)	3700	No	No	In hot and	NH₄OH,
Mo alloys	Mo-Fe,Ni, Co,Cr	≧10.2	<2620(high)	<3700			thick H₂SO₄	NH₄Cl
Ni and	Ni	8.90	1452(high)	2900	No	No	In dilute	NH₄Cl, NaOH,
Ni alloys	Ni-Fe,Cr,Mo,Co	≧8.90	<1452(high)	<2900			HNO₃	KOH, NH₄OH
Ag and	Ag	10.5	960.5(low)	1950	No	No	In HNO₃,	NaOH, KOH,
Ag alloys	Ag-Cu	≧10.5	<960.5(low)	<1950			hot H₂SO₄	NH₄OH,
								NH₄Cl
Sn and	Sn	7.31	232(low)	2260	No	No	In HCl,	CH₃CH₂OH
Sn alloys	Sn-Pb, Cu, Bi	≅7.31	<232(low)	<2260			H₂SO₄,	(alcohol),
							dilute HNO₃	glycerine
W and	W	19.3	3370(high)	5900	No	No	In hot and	NH₄Cl, NaOH,
W alloys	W-Ni, Cu, Ag	<19.3	<3370(high)	<5900			thick KOH	KOH,
								NH₄OH,
								NH₄OH

	PS: To lower the sintering temperature of W and W alloys to below 1500° C., Ni powder is added. The
	filling material is selected to be solvable in alkaline solvents. The filling material should have a
	melting point higher than 1600° C., such as MnO, TiO₂, V₂O₃, ZnO, MgO, and ZrO₂

V and	V	5.96	1710(high)	3000	No	No	In HNO₃,	NaOH, KOH,
V alloys	V-Mo, Ni, Co,Ti	≧5.96	≧1710(high)	<3000			H₂SO₄	NH₄OH,
								NH₄Cl
Ti and	Ti	4.5	1800(high)	>3000	No	Yes	In HCl, HNO₃,	NaOH, KOH,
Ti alloys	Ti-Al, Ni, Co	≧4.5	≧1800(high)	≧3000			H₂SO₄	NH₄OH, NH₄Cl

The filling material 12 can be selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO, which are fine powder solvable in water or alkaline solvents, as listed in Table 2.

TABLE 2


The filling materials of the present invention.

Melting

Boiling

Solvable solvents

Corresponding

	Chemical	Specific	point	point	Cold	Hot		sintering of smelting
Names	formula	weight	° C.	° C.	water	water	Other solvents	base materials

Calcium	CaCl₂	2.15	772	>1600	Partially	Partially	In CH₃CH₂OH	For Al, Pb, Mg,
chloride							(alcohol)	Sn and their alloys
Manganese	MnO	5.18	1650	>1600	No	No	in NH₄Cl	For Cu, Fe, Mg,
monoxide								Mn, Ni and their
								alloys
Salt	NaCl	2.163	801	1413	Yes	Yes	In glycerine	For Al, Pb, Mg,
								Sn and their alloys
Titanium	TiO₂	4.26	1640	<3000	No	No	in NaOH, KOH	For Cu, Fe, Mn,
dioxide								Ni, Mg and their
								alloys
Tungsten	WO₃	7.16	2130	<3000	No	No	in NaOH, KOH	For Cu, Fe, Mn,
trioxide								Ni, Ag, V, Ti and
								their alloys
Vanadium	V₂O₃	4.87	1970	<3000	Yes,	Yes	in NaOH, KOH	For Cu, Fe, Mn,
trioxide					mildly			Ni, Ag, V, Ti and
								their alloys
Zinc	ZnO	5.61	1800	<3000	Lightly	No	in Na0OH, KOH	For Cu, Fe, Mn,
monoxide								Ni, Ag, V, Ti and
								their alloys
Magnesium	MgO	3.65	2800	3600	Lightly	No	in NH₄OH,	For Cu, Fe, Mn,
monoxide							NH₄Cl	Ni, Ag, V, Ti, Mo and
								their alloys

The present invention utilizes the above-mentioned sintering base material 11 and filling material 12, and the process of the powder metallurgy method is described as follows and shown in FIG. 7.
In the material mixing step 1, the sintering base material 11 is selected to be Al metal or alloys of Al and another element selected from Al, Si, Mg, Cu and Zn, in powder form. Since the melting point of the base material is less than 660° C., the filling material 12 is chosen to be of higher melting point. According to Table, available filling materials 12 are fine powder of CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.
In another case, when Cu metal or alloys of Cu with another element selected from Zn, Sn, Al and P in powder form is selected a sintering base material 11, the preferred filling materials 12 listed in Table 2 are MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO, since the melting point of the base material is less than 1083° C.
According to the principle just described, other sintering base materials 11 listed in Table 1 can form the following combinations with the filling materials listed in Table 2.
As the sintering base material is pure Fe metal powder, the filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure Pb metal or an alloy of Pb and another metal selected from Sn and Zn, the filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure Mg metal or an alloy of Mg and another metal selected from Li, Al and Cu, the filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure Mn metal or an alloy of Mn and another metal selected from Fe and Ni, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure Mo metal or an alloy of Mo and another metal selected from Fe, Ni, Co and Cr, the filling material is fine powder of MgO. As the sintering base material is pure Ni metal or an alloy of Ni and another metal selected from Fe, Cr, Mo and Co, the filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the. sintering base material is pure Ag metal or an alloy of Ag and Cu, the filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure Sn metal or an alloy of Sn and another metal selected from Pb, Cu and Bi, the filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure W metal or an alloy of W and another metal selected from Ni, Cu and Ag, the filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure V metal or an alloy of V and another metal selected from Mo, Ni, Co and Ti, the filling material is fine powder selected from WO₃, V₂O₃, ZnO and MgO. As the sintering base material is pure Ti metal or an alloy of Ti and another metal selected from Al, Ni and Co, the filling material is fine powder selected from WO₃, V₂O₃, ZnO and MgO.
When choosing the above materials, the sintering base material 11 of pure metal or alloy in powder form and the filling materials 12 in powder form can be acquired at low prices in the market, the diameter of which ranging from 250 to 0.1 micron. And the shape of the particles has already attained spherical, suitable for dense piling up.
Further, in addition to the diameter of the particles, when choosing the above mixing materials, the smaller the angle of repose of the particles, the better the mobility of the particles during stirring is. And the friction between particles is smaller.
As an even further consideration, the occupied volume of the filling material 12 should exceed 50%, namely the volume ratio of the filling material to the sintering base material must be greater than 1. The mixing and stirring 21 is then performed, producing a porous metal with hole ratio greater than 50%.
In the step of mixing and stirring 21, as shown in FIG. 7, the selected sintering base material 11 and filling material 12 are disposed in a stirring container and added with binder 22 and plasticizer 23, so that the base material 11 and filling material 12 can be uniformly mixed, as shown in FIG. 9.
Binder 22 provides a sintering base material 11, in form of pure metal or alloy powder, and a filling material 12 with binding force and flexibility in the step of dry molding 25. In the later step of sintering 27, the binder 22 will be removed from the mixture through high temperature oxidation. The binder 22 can be selected form polyvinyl alcohol, wax, cellulose, dextrin, thermosetting plastic, calcium chloride compounds, algae gel, lignin, rubber, starch, flour, gelatin, protein, asphalt, acrylic, polystyrene, paraffin wax and beeswax.
If polyvinyl alcohol is chosen to be the binder 22, the polymerization number as defined in the molecular formula [—CH₂—CH—]_nis preferably, around 1500-1700. However, polyvinyl alcohol is not suitable to be used with alkaline oxides, such as MgO, CaO, BaO, ZnO, borate and phosphate.
On the other hand, if polyvinyl acetate is chosen to be the binder 22, the polymerization number as defined in the molecular formula:

n is preferably around 400-600. It can also used with alkaline oxides. However, polyvinyl acetate is solvable only in organic solvents such as alcohol, benzene and methylbenzene.
Paraffin wax and beeswax can be also chosen as the binder 22, which have good fluidity when heated. They can damp a sintering base material 11 rather effectively, but the heating temperature should not exceed 130° C., or they will be oxidized quickly.
The plasticizer 23 enhances mobility between a sintering base material 11 and a filling material 12, lessening the problem of high viscosity induced by the binder 22. It can enhance the mixing uniformity in the step of mixing and stirring 21. The plasticizer 23 can be selected from glycol, glycerin, sulfuric acid, stearin acid, oleic acid, wood oil and other vegetable oils.
If glycerin is chosen to be the plasticizer 23, it can provide an additional effect of moisture absorption. If oleic acid is used as the plasticizer 23, it can provide an additional effect of lubricating and activating the surfaces of powder particles. If wood oil and other vegetable oils are chosen, they can lubricate, emulsify and bind powder particles.
After the step of mixing and stirring 21, the next step is dry molding 25, as shown in FIG. 7. In this step, a base material 11 and a filling material 12 are heated to 250° C. so that they become dry. Since they are bound by the binder 22, the uniform distribution and compact stacking are preserved. The filling material 12 acts as bridging material between particles of the base material 11.
After the step of dry molding 25, the molded filling material 12 is preheated at a temperature between 500 and 700° C. in a step of preheating 26, for removing water from the filling material 12 and therefore facilitating the subsequent step of sintering 27, as shown in FIG. 7.
In the step of sintering 27, the mixture of a base material 11 and a filling material 12 is cast into a dense raw metal bulk 28 by means of cold isostatic pressing or hot isostatic pressing. The sintering temperature is controlled above 50% of the melting point of the metal and below the melting point of the filling material 12. If the base material 11 is alloy powder, the sintering temperature is generally lower.
To prevent oxidation of the raw metal bulk 28 in the step of sintering 27, the process is performed in a reduction gas.
Under the above conditions for sintering, the filling material 12 and the base material 11 originally contacted at points (FIG. 9) become a structure in which the melted base material 11 fills the space between the particles of the filling material 12, as shown in FIG. 10. A multiple of neck portions 120 are formed between particles of the filling material 12 a and between the filling material 12 a and its surrounding base material 11 a. The raw metal bulk 28 after cooling possesses internal granular textures. This powder metallurgy method particularly suits for making high melting point metal or alloy parts with a complex shape under 500 grams.
The neck portions 120 become channels between the micro-holes after the filling material 12 a is removed. The distribution ratio of the holes can be obtained by X/D (FIG. 12), where D is the powder particle diameter of the filling material 12 and X is the diameter of the cross-section of the neck portions 120. The neck diameter is controlled by sintering temperature, time and mixing ratio of the base material 11 and the filling material 12. Therefore, the hole distribution ration can be determined before the step of sintering.
Further, the second embodiment of the present invention as a smelting method for producing porous metal is described below, as shown in FIG. 8.
The first thing to do before the step of mixing 10 is to know the melting point temperature of the smelting base material 13, a pure metal or an alloy. Then we can execute the step of smelting 7.
The smelting base materials 13, pure metals or their associated alloys, must be divided into two categories, those of low melting points and those of high melting points. The smelting base materials 13 listed in Table 1 having a melting point temperature of 1000° C. or above includes:
Cu (or alloys of Cu with Zn, Sn, Al or P);
Fe;
Mn (or alloys of Mn with Fe or Ni);
Mo (or alloys of Mo with Fe, Ni, Co or Cr);
Ni (or alloys of Ni with Fe, Cr, Mo or Co);
Ag (or alloys of Ag with Cu);
W (or alloys of W with Ni, Cu or Ag);
V (or alloys of V with Mo, Ni, Co or Ti);
Ti (or alloys of Ti with Al, Ni or Co)∘
The smelting base materials 13 listed in Table 1 having a melting point temperature below 1000° C. includes:
Al (or alloys of Al with Si, Mg, Cu or Zn);
Pb (or alloys of Pb with Sn or Zn);
Mg (or alloys of Mg with Li, Al or Cu);
Sn (or alloys of Sn with Pb, Cu or Bi )∘
The principle of choosing a filling material 12 for a particular smelting base materials 13 is the same as in the sintering method. That is, the melting point of the filling material 12 must be higher than that of the smelting base materials 13, so that the filling material 12 will not melt in the later smelting process.
To effectively execute the smelting process, the step of smelting 7 is divided into two independent paths, one using a high frequency oven 71 and the other using a low frequency oven 72, as shown in FIG. 8. The above smelting base materials 13 of high melting points are processed in the high frequency oven 71, and the above smelting base materials 13 of low melting points are processed in the low frequency oven 72.
When using the high frequency oven 71 or the low frequency oven 72 for smelting, the two ovens can respectively induce a periodic magnetic force to urge the desired stirring. Thereby, the smelting base materials 13 and its filling material 12 can mix uniformly.
To achieve an even better stirring effect, two thermos chambers 710 and 720 are respectively provided in front of the high frequency oven 71 and the low frequency oven 72. The filling material 12 is further exposed to ultrasonic waves so that it can even more uniformly distributed in the liquid of the smelting base material 13. In a later step of vacuum molding 74, the granular filling material 12 can quickly and uniformly distributed in the smelting base materials 13, forming a raw metal bulk 78 having an internal granular structure.
The raw metal bulk 78, of a low or high melting point, has the same structure of neck portions 120 as in the case of sintering, as shown in FIG. 10 and 11. Therefore, the hole distribution ratio can also be controlled. This method particularly suits making raw metal bulks 78 of high or low melting point and simple outlook, such as a slab or a plate.
The following steps are common to the sintering and the smelting methods. As shown in FIG. 7 and 8, the step of removing filling material 3 or 30 applies the property that the filling material 12 in a raw metal bulk 28 or 78 can dissolve in water or alkaline solvents. Therefore, we can choose a solvent 31 that is harmless to the smelting base materials 13 and then put the raw metal bulk 28 or 78 in a washing tank equipped with ultrasonic waves. The cavitation effect induced by the ultrasonic waves cleans the filling material 12 a in the raw metal bulk 28 or 78, as shown in FIG. 11. After the washing, we have a half-product of porous metal in which either a sintering base material 11 a or a smelting base material 13 a exhibit a micro-hole structure.
When the filling material 12 is CaCl₂, the corresponding solvent 31 is CH₃CH₂OH (alcohol), as shown in Table 2. The CH₃CH₂OH (alcohol) is suitable for using with smelting base materials 13 of Al, Pb, Mg and Sn metals and their associated alloys, as shown in Table 1.
When the filling material 12 is MnO, the corresponding solvent 31 is NH₄Cl, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Cu, Fe, Mg, Mn and Ni metals and their associated alloys, as shown in Table 1.
When the filling material 12 is NaCl, the corresponding solvent 31 is cold or hot water, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Al, Pb, Mg and Sn metals and their associated alloys, as shown in Table 1.
When the filling material 12 is TiO₂, the corresponding solvents 31 are NaOH and KOH, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Cu, Fe, Mg, Mn and Ni metals and their associated alloys, as shown in Table 1.
When the filling material 12 is WO₃, the corresponding solvents 31 are NaOH and KOH, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Cu, Fe, Mg, Mn, Ni, Ag, V and Ti metals and their associated alloys, as shown in Table 1.
When the filling material 12 is V₂O₃, the corresponding solvents 31 are NaOH and KOH, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Cu, Fe, Mg, Mn, Ni, Ag, V and Ti metals and their associated alloys, as shown in Table 1.
When the filling material 12 is ZnO, the corresponding solvents 31 are NaOH and KOH, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Cu, Fe, Mg, Mn, Ni, Ag, V and Ti metals and their associated alloys, as shown in Table 1.
When the filling material 12 is MnO, the corresponding solvents 31 are NH₄OH or NH₄Cl, as shown in Table 2. The solvent 31 is suitable for using with smelting base materials 13 of Cu, Fe, Mg, Mn, Ni, Ag, V, Ti and Mo metals and their associated alloys, as shown in Table 1.
The porous half-product 32 produced after the step of removing the filling material, 3 or 30, must undergo a step of water cleaning, 4 or 40, since the alkaline solvent 31 attached to the half-product 32 must be cleared. The step 4 or 40 is executed in a water tank equipped with ultrasonic waves.
The porous half-product 32 must go through a step of drying by heat, 5 or 50. This is done in an infrared or microwave ovens. The microwave frequency used in the step of microwave drying is between 100-140 GHz, and the wavelength is between 1 mm-1 m. After this step, a porous metal 8 having micro-holes 80 is formed.
Further, if the filling material 12 used is an oxide, a further step of reduction, 6 or 60, is necessary.
To execute the step of reduction, 6 or 60, the dried porous metal 8 is disposed in a vacuum chamber filled with a reduction gas (hydrogen gas, for example), whereby the oxidation layers or films between micro-holes 80 can be removed.
An alternative reduction method for the step 6 or 60 is using a solvent for dissolving the oxidation layers and films. The solvent can be NaOH, KOH, NH₄Cl and NH₄OH. This is done in a tank equipped with ultrasonic waves.
The present invention is thus described, and it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A powder metallurgy method for producing porous metal with micro-holes, comprising the steps of:

(a) selecting materials, a base material being selected from pure metal powder and alloy powder for sintering; a filling material being fine powder solvable in water or in alkaline solvents, said filling material having a melting point higher than that of said base material;

(b) sintering, said sintering further including a mixing step for mixing said base material and said filling material and in which a binding material for making the piling of said base and said filling materials uniform and dense, said mixture being dried and preheated to remove the water vapor attached to said filling material, said mixture being heated to sintering temperature to become a dense metal bulk having micro-hole texture;

(c) removing said filling material by immerging said metal bulk in a washing tank of a selected solvent and then applying ultrasonic waves for shaking away said filling material, whereby a porous metal half-product with micro-holes configured by said sintered base material is formed;

(d) washing away remained solvent between micro-holes of said porous metal half-product; and

(e) drying away remained water drops between micro-holes of said porous metal half-product from said washing step;

whereby product of porous metals with micro-holes can be formed.

2. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Al metal and alloys of Al and another metal selected from Si, Mg, Cu and Zn, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

3. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Cu metal and alloys of Cu and another metal selected from Zn, Sn, Al and P, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

4. The method for producing porous metal with micro-holes of claim 1 wherein said base material is pure Fe metal powder, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

5. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Pb metal and alloys of Pb and another metal selected from Sn and Zn, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

6. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Mg metal and alloys of Mg and another metal selected from Li, Al and Cu, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

7. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Mn metal and alloys of Mn and another metal selected from Fe and Ni, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

8. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Mo metal and alloys of Mo and another metal selected from Fe, Ni, Co and Cr, and wherein said filling material is fine powder of MgO.

9. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Ni metal and alloys of Ni and another metal selected from Fe, Cr, Mo and Co, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

10. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Ag metal and alloys of Ag and Cu, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

11. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Sn metal and alloys of Sn and another metal selected from Pb, Cu and Bi, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

12. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure W metal and alloys of W and another metal selected from Ni, Cu and Ag, wherein pure Ni metal powder is added, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

13. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure V metal and alloys of V and another metal selected from Mo, Ni, Co and Ti, and wherein said filling material is fine powder selected from WO₃, V₂O₃, ZnO and MgO.

14. The method for producing porous metal with micro-holes of claim 1 wherein said base material is in powder form and selected from pure Ti metal and alloys of Ti and another metal selected from Al, Ni and Co, and wherein said filling material is fine powder selected from WO₃, V₂O₃, ZnO and MgO.

15. The method for producing porous metal with micro-holes of claim 1 wherein volume ratio of said filling material to said base material is larger than 1.

16. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is CH₃CH₂OH when said filling material is CaCl₂.

17. The method for producing porous metal with micro-holes of claim 16 wherein said solvent is suitable for Al, Pb, Mg and Sn in pure and alloy forms as said sintering base material.

18. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is NH₄Cl when said filling material is MnO.

19. The method for producing porous metal with micro-holes of claim 18 wherein said solvent is suitable for Cu, Fe, Mg, Mn and Ni in pure and alloy forms as said sintering base material.

20. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is cold or hot water when said filling material is NaCl.

21. The method for producing porous metal with micro-holes of claim 20 wherein said-solvent is suitable for Al, Pb, Mg and Sn in pure and alloy forms as said sintering base material.

22. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is selected from NaOH and KOH when said filling material is TiO₂.

23. The method for producing porous metal with micro-holes of claim 22 wherein said solvent is suitable for Cu, Fe, Mn, Ni and Mg in pure and alloy forms as said sintering base material.

24. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is selected from NaOH and KOH when said filling material is WO₃.

25. The method for producing porous metal with micro-holes of claim 24 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V and Ti in pure and alloy forms as said sintering base material.

26. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is selected from NaOH and KOH when said filling material is V₂O₃.

27. The method for producing porous metal with micro-holes of claim 26 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V and Ti in pure and alloy forms as said sintering base material.

28. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is selected from NaOH and KOH when said filling material is ZnO.

29. The method for producing porous metal with micro-holes of claim 28 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V and Ti in pure and alloy forms as said sintering base material.

30. The method for producing porous metal with micro-holes of claim 1 wherein said solvent is selected from NH₄OH and NH₄Cl when said filling material is MgO.

31. The method for producing porous metal with micro-holes of claim 30 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V, Ti and Mo in pure and alloy forms as said sintering base material.

32. The method for producing porous metal with micro-holes of claim 1 wherein said preheat temperature for removing water attached to said filling material is from 500 to 700° C.

33. The method for producing porous metal with micro-holes of claim 1 wherein said sintering temperature is above half of the melting point of said base material and blow the melting point of said filling material.

34. The method for producing porous metal with micro-holes of claim 1 wherein said washing step is performed in a tank provided with ultrasonic waves.

35. The method for producing porous metal with micro-holes of claim 1 wherein said drying step is performed an infrared oven.

36. The method for producing porous metal with micro-holes of claim 1 wherein said drying step is performed in a microwave oven having a microwave frequency from 100 to 140 GHz and a wavelength from 1 mm to 1 m.

37. The method for producing porous metal with micro-holes of claim 1 further including a step of reduction for removing oxidation layers (films) when said filling material is an oxide.

38. The method for producing porous metal with micro-holes of claim 37 wherein said step of reduction is placing said porous metal after drying in a vacuum chamber, filling in hydrogen gas as a reduction gas for the reduction reaction by which said oxidation layers (films) between micro-holes of said porous metal are removed.

39. The method for producing porous metal with micro-holes of claim 37 wherein said step of reduction is performed using a solvent that dissolves said oxidation layers (films) between micro-holes of said porous metal.

40. The method for producing porous metal with micro-holes of claim 39 wherein said solvent is selected from NaOH, KOH, NH₄Cl and NH₄OH.

41. A smelting method for producing porous metal with micro-holes, comprising the steps of:

(a) selecting materials, a base material being selected from pure metal and alloy of a predetermined melting point for smelting; a filling material being fine powder solvable in water or in alkaline solvents, said filling material having a melting point higher than that of said base material;

(b) smelting, said smelting further including a mixing step for mixing said base material and said filling material in an oven equipped with high or low frequency waves for inducing a desired magnetic stirring effect, said smelted mixture being disposed in a thermo chamber equipped with ultrasonic waves for uniformly stirring, said mixture undergoing a fast casting process whereby solid particles of said filling material can be uniformly distributed between molten smelting base material, said mixture undergoing being cooled to form a dense metal bulk having micro-hole texture;

whereby product of porous metals with micro-holes can be formed.

42. The method for producing porous metal with micro-holes of claim 41 wherein a high frequency oven is used for said smelting step when said base material is of high melting point.

43. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure Cu metal and alloys of Cu and another metal selected from Zn, Sn, Al and P, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

44. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is pure Fe metal, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

45. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure Mn metal and alloys of Mn and another metal selected from Fe and Ni, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

46. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure Mo metal and alloys of Mo and another metal selected from Fe, Ni, Co and Cr, and wherein said filling material is fine powder of MgO.

47. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure Ni metal and alloys of Ni and another metal selected from Fe, Cr, Mo and Co, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

48. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure Ag metal and alloys of Ag and Cu, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

49. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure W metal and alloys of W and another metal selected from Ni, Cu and Ag, wherein pure Ni metal powder is added, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

50. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure V metal and alloys of V and another metal selected from Mo, Ni, Co and Ti, and wherein said filling material is fine powder selected from WO₃, V₂O₃, ZnO and MgO.

51. The method for producing porous metal with micro-holes of claim 41 wherein said base material for high melting-point smelting is selected from pure Ti metal and alloys of Ti and another metal selected from Al, Ni and Co, and wherein said filling material is fine powder selected from WO₃, V₂O₃, ZnO and MgO.

52. The method for producing porous metal with micro-holes of claim 41wherein said solvent is CH₃CH₂OH when said filling material is CaCl₂.

53. The method for producing porous metal with micro-holes of claim 52 wherein said solvent is suitable for Al, Pb, Mg and Sn in pure and alloy forms as said base material.

54. The method for producing porous metal with micro-holes of claim 41wherein said solvent is NH₄Cl when said filling material is MnO.

55. The method for producing porous metal with micro-holes of claim 54 wherein said solvent is suitable for Cu, Fe, Mg, Mn and Ni in pure and alloy forms as said base material.

56. The method for producing porous metal with micro-holes of claim 41 wherein said solvent is cold or hot water when said filling material is NaCl.

57. The method for producing porous metal with micro-holes of claim 56 wherein said solvent is suitable for Al, Pb, Mg and Sn in pure and alloy forms as said base material.

58. The method for producing porous metal with micro-holes of claim 41 wherein said solvent is selected from NaOH and KOH when said filling material is TiO₂.

59. The method for producing porous metal with micro-holes of claim 58 wherein said solvent is suitable for Cu, Fe, Mn, Ni and Mg in pure and alloy forms as said base material.

60. The method for producing porous metal with micro-holes of claim 41 wherein said solvent is selected from NaOH and KOH when said filling material is WO₃.

61. The method for producing porous metal with micro-holes of claim 60 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V and Ti in pure and alloy forms as said base material.

62. The method for producing porous metal with micro-holes of claim 41 wherein said solvent is selected from NaOH and KOH when said filling material is V₂O₃.

63. The method for producing porous metal with micro-holes of claim 62 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V and Ti in pure and alloy forms as said base material.

64. The method for producing porous metal with micro-holes of claim 41 wherein said solvent is selected from NaOH and KOH when said filling material is ZnO.

65. The method for producing porous metal with micro-holes of claim 64 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V and Ti in pure and alloy forms as said base material.

66. The method for producing porous metal with micro-holes of claim 41 wherein said solvent is selected from NH₄OH and NH₄Cl when said filling material is MgO.

67. The method for producing porous metal with micro-holes of claim 66 wherein said solvent is suitable for Cu, Fe, Mn, Ni, Ag, V, Ti and Mo in pure and alloy forms as said base material.

68. The method for producing porous metal with micro-holes of claim 41 wherein volume ratio of said filling material to said base material for high or low melting-point smelting is larger than 1.

69. The method for producing porous metal with micro-holes of claim 41 wherein said smelting temperature is above the melting point of said base material, and blow the melting point of said filling material.

70. The method for producing porous metal with micro-holes of claim 41 wherein said washing step is performed in a tank provided with ultrasonic waves.

71. The method for producing porous metal with micro-holes of claim 41 wherein said drying step is performed an infrared oven.

72. The method for producing porous metal with micro-holes of claim 41 wherein said drying step is performed in a microwave oven having a microwave frequency from 100 to 140 GHz and a wavelength from 1 mm to 1 m.

73. The method for producing porous metal with micro-holes of claim 41 further including a step of reduction for removing oxidation layers (films) when said filling material is an oxide.

74. The method for producing porous metal with micro-holes of claim 73 wherein said step of reduction is placing said porous metal after drying in a vacuum chamber, filling in hydrogen gas as a reduction gas for the reduction reaction by which said oxidation layers (films) between micro-holes of said porous metal are removed.

75. The method for producing porous metal with micro-holes of claim 73 wherein said step of reduction is performed using a solvent that dissolves said oxidation layers (films) between micro-holes of said porous metal.

76. The method for producing porous metal with micro-holes of claim 75 wherein said solvent is selected from NaOH, KOH, NH₄Cl and NH₄OH.

77. The method for producing porous metal with micro-holes of claim 41 wherein a low frequency oven is used for said smelting step when said base material is of low melting point.

78. The method for producing porous metal with micro-holes of claim 41 wherein said base material for low melting-point smelting is selected from pure Al metal and alloys of Al and another metal selected from Si, Mg, Cu and Zn, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

79. The method for producing porous metal with micro-holes of claim 41 wherein said base material for low melting-point smelting is selected from pure Pb metal and alloys of Pb and another metal selected from Sn and Zn, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

80. The method for producing porous metal with micro-holes of claim 41 wherein said base material for low melting-point smelting is selected from pure Mg metal and alloys of Mg and another metal selected from Li, Al and Cu, and wherein said filling material is fine powder selected from CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.

81. The method for producing porous metal with micro-holes of claim 41 wherein said base material for low melting-point smelting is selected from pure Ag metal and alloys of Ag and Cu, and wherein said filling material is fine powder selected from MnO, TiO₂, WO₃, V₂O₃, ZnO and MgO.

82. The method for producing porous metal with micro-holes of claim 41 wherein said base material for low melting-point smelting is selected from pure Sn metal and alloys of Sn and another metal selected from Pb, Ni, Cu and Bi, and wherein said filling material is selected from fine powder of CaCl₂, MnO, NaCl, TiO₂, WO₃, V₂O₃, ZnO and MgO.