Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F9/16—Making metallic powder or suspensions thereof using chemical processes
  • B22F9/18—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds
  • B22F9/24—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds starting from liquid metal compounds, e.g. solutions
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/17—Metallic particles coated with metal
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/045—Alloys based on refractory metals
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
  • H01H1/00—Contacts
  • H01H1/02—Contacts characterised by the material thereof
  • H01H1/021—Composite material

Definitions

• the present invention relates to a process for the production of sinterable tungsten-copper composite powders. More particularly the invention relates to a method for the production of a composite powder consisting of finely interspersed tungsten and copper, which powder can be directly pressed and sintered to provide products having density values near to theoretical ones and showing high electrical and thermal conductivity.
• Tungsten-copper composite materials are used for the production of heat exchangers for electrical devices and for the production of electrodes and power electrical contacts. Since alloying does not occur between tungsten and copper, various methods have been developed to combine these metals in order to obtain products wherein the low coefficient of thermal expansion and the advantageous mechanical properties of tungsten are coupled to the high electrical and thermal conductivity of copper.
• the method most widely used to this aim comprises: i) sintering a tungsten metal powder at such a temperature to obtain a porous tungsten structure; ii) infiltrating said structure with molten copper, the pores of the structure being filled by the liquid metal (see, for example, Randall M. German, "Sintering Theory and Practice", pages 385-389, John Wiley & Sons, Inc., New York (1996).
• the amount of copper which can be incorporated in sintered tungsten depends, however, on the porosity of the latter, which in turn depends on the starting grain size of tungsten powder and on the sintering conditions. Furthermore, in order to be filled by molten copper, the pores must be open or it is necessary that the fraction of closed pores in the starting sintered tungsten be minimal. Where there are dosed pores, through which copper cannot flow thus filing them, fragile products are obtained. Thus the need to minimize the presence of closed pores makes the first step during the production process a critical one, and limits the range of the obtainable tungsten-copper compositions.
• Another set of methods for obtaining tungsten-copper composite powders include the steps of mixing/grinding and the following co-reduction in hydrogen atmosphere of copper oxide and tungsten oxide powders.
• the thus obtained metal particles are in more intimate contact than that obtainable by using only mechanical grinding of copper and tungsten metals and the resulting tungsten-copper powder can be directly pressed and sintered to density values exceeding 95 % of the theoretical ones.
• copper tungstate (CuWO 4 ), wherein copper and tungsten are mixed at the atomic level, can be reduced to obtain tungsten-copper composite powders having good sintering properties.
• copper tungstate is produced by reacting in the solid phase CuO with WO 3 ; in order to obtain an intimate contact between the two oxide phases, however, it is necessary to grind for a long time the CuO-WO 3 mixture by means of balls made of hard metal or ceramic material, thus resuiting in a potentially contaminated mixture.
• high temperatures and long calcining times impair the process for producing W-Cu powders from an economic standpoint, although metallic powders obtained from tungstate have good interspersion and sintering properties.
• U.S. patent No. 5468457 suggests to use as precursors, instead of conventional oxides, hydrated oxides, i.e. copper hydroxide, Cu(OH) 2 (i.e. CuO.H 2 O) and tungstic acid, H 2 WO 4 (i.e. WO 3 .H 2 O).
• hydrated oxides i.e. copper hydroxide, Cu(OH) 2 (i.e. CuO.H 2 O) and tungstic acid, H 2 WO 4 (i.e. WO 3 .H 2 O).
• the heat treatment of such a mixture of hydrated oxides results in water development with formation of CuO and WO 3 with high surface area, which assures the advantage of higher reactivity in the following step at higher temperatures (600-800°C).
• U.S. patent No. 54670549 discloses an alternative route with respect to the above mentioned one, which includes the use of ammonium tungstate (both meta-tungstate, AMT, and para-tungstate, APT) as tungsten precursors, while both CuO and Cu 2 O can be used as copper precursors.
• Tungsten oxide (WO 3 ) obtained from the ammonium tungstate decomposition at temperatures higher than 250°C, shows a high reactivity and therefore there is no more the need for the starting grinding step to promote the contacting and the following reaction between the oxide precursors.
• As the Cu/W ratio in CuWO 4 is fixed (25.7% by weight in the final W-Cu powder), in order to obtain metal powders with different copper content it is necessary to modify the amount of copper oxides or to add WO 3 to the tungstate.
• U.S. patent No. 5439638 suggests a process for the production of tungsten-copper composite powders having copper contents in the range between 5 and 60% by weight, wherein the starting ingredients are wet mixed. More particularly the process uses starting powders comprising elemental tungsten, cuprous oxide and, optionally, cobalt powder at level less than 0.5% by weight. The powders are first interspersed in an aqueous medium, then the liquid is removed by spray-driyng; in a such way a flowable powder comprising spherical aggregates is obtained. Ultimately cuprous oxide (Cu 2 O) is reduced in hydrogen atmosphere at 700-730°C to produce a tungsten-copper sinterable powder, in the form of spherical aggregates too.
• starting powders comprising elemental tungsten, cuprous oxide and, optionally, cobalt powder at level less than 0.5% by weight.
• the powders are first interspersed in an aqueous medium, then the liquid is removed by spray-driy
• EP-A-080648 A technique, partially similar to the various methods mentioned above, including a step for a dry or aqueous phase powder mixing, followed by high temperature reduction, is described in European Patent Publication EP-A-0806489.
• the latter teaches that W/Cu products, having density values above 97% of theoretical, are directly obtained by using starting mixtures containing copper and a transition metal (as W or Mo), provided that the mixture also contains chemically bonded oxygen, for example in the form of copper oxide, in such amounts to improve the sinterability thereof.
• the described procedure preferably includes mechanical mixing of elemental tungsten and cuprous oxide powders, which, following their pressing and high temperature treating in hydrogen atmosphere, results in the formation of a sintered product
• metallic powders can be produced by liquid phase reduction using an alcohol solvent as reducing agent.
• monometallic powders (of gold, palladium, platinum, iridium, osmium, copper, silver, nickel, cobalt, lead or cadmium) can be produced by reduction from a precursor by using an organic liquid phase made up of one or a mixture of polyols. More particularly a compound of the desired metal selected from oxides, hydroxides and metal salts is reduced by the organic liquid phase by heating the mixture to a temperature of at least 85°C. Owing to the reduction, the metal is separated in the form of high purity powder.
• the reducing agent is formed by an alcohol phase, typically a polyol, wherein one or more precursors are suspended (typically in the form of metal salt, hydrated salt or oxy-anion).
• the method is suggested for the production of nanostructured films and powders of one or more metals selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Ta, W, Re, Os, Ir, Pt and Au.
• the method provides powders consisting of refractory metals (W, Ti, Mo, Re, Ta) or their alloys produced from salts or adds which contain said metals in the corresponding oxy-anions.
• tungsten-copper composite powders suitable to be used directly for the production of sintered products, by using a reduction process in a liquid organic phase consisting of one or a mixture of polyols wherein copper is added as precursor compound whereas tungsten is added as metal.
• a reduction process in a liquid organic phase consisting of one or a mixture of polyols wherein copper is added as precursor compound whereas tungsten is added as metal.
• elemental tungsten is necessary in order to achieve the reduction of the copper precursor at reasonably low temperature and short time, as tungsten itself takes part in the copper compound reduction, thus allowing the reduction reaction to occur at lower temperatures.
• the organic phase reaction can be carried out below the lowest temperature values known in the art (85°C).
• the present invention specifically provides a method for the production of tungsten-copper composite powders suitable to be pressed and sintered and having a copper content from 5 to 35% by weight, the method comprising the following steps:
• the organic phase wherein the oxidation-reduction reaction and concurrent interspersion of the produced copper and the starting tungsten occur consists of ethylene glycol, pure or in admixture with other polyols, as for example diethylene glycol.
• the starting elemental tungsten powder can be any commercially available powder having an average grain size preferably in the range from 0.5 to 6 ⁇ m.
• the copper compound can be either soluble in the polyol, as is the case, for example, of copper (II) acetate monohydrate (Cu(CH 3 COO) 2 .H 2 O) or insoluble in the polyol, as is the case of cupric and cuprous oxides (CuO and Cu 2 O respectively).
• the method suggested in accordance with the present invention allows the preparation of tungsten-copper composite powders having a broad composition range since, in order to obtain the desired proportions in the final composite powder, it is only required to modify the starting relative amounts of tungsten and copper compound present in the organic phase suspension/solution.
• the starting elemental tungsten being active in the copper-reduction, undergoes a partial solubilization as tungstate and therefore its concentration in the final metallic product is reduced.
• Suitable starting amounts of elemental tungsten and copper compound to be used for producing a composite powder having desired W/Cu ratios can be easily established by those skilled in the art on the basis of reaction yields for a few exemplary experiments, as illustrated in the following examples.
• the temperature of the organic phase, wherein the copper compound reduction occurs is at least 60°C.
• the composite powders obtained by using the method of the present invention can be stored for a long time wet with same organic solvent, thus avoiding any risk of spontaneous ignition of the dry powders. Possible residues of organic phases which can be present after the composite tungsten-copper powder washings are removed during the sintering cycle.
• microstructure of the final powder by modifying: i) the grain size of the tungsten starting powder, ii) the composition of the organic phase employed, iii) the copper precursor and concentration thereof, iv) the reaction temperature and time.
• tungsten-copper powders containing suitable additives for reducing sintering temperatures or times and/or improving technological and utilization properties of the product obtained.
• a suitable amount of a cobalt (II) compound as, for example, cobalt (II) acetate tetrahydrate
• cobalt metal is formed, which, in small amounts, allows to lower the W-Cu composite sintering temperature and/or time (S. K. Joo, S. W. Lee, T. H. Ihn, "Effect of Cobalt Addition on the Liquid Phase Sintering or W-Cu Prepared by the Fluidized Bed Reduction Method"; Met Mater. Trans. Vol. 25A, pages 1575-1578 (1994)).
• the reaction provided 670 g of product, weighed after the resulting composite powder had been separated, washed with acetone and air dried.
• the atomic absorption analysis showed a copper content of 15% by weight
• the 75W-25Cu powder thus obtained has been pressed at 2.39 ton/cm 2 and sintered in hydrogen atmosphere at 1300°C, obtaining a density value of 98% of the theoretical one.
• the electrical conductivity of the sintered product was 46% IACS.
• the tungsten particles take an active part in the copper reduction, modifying their morphology with surface corrugations because of the oxidation to tungstate.
• Such a surface corrugation provides sites suitable for the heterogeneous nucleation of the elemental copper formed by reduction of the copper compound.
• example 3 The procedure of example 3 was repeated except that the reaction was carried out at 110°C over two hours. After separation and washing with acetone of the obtained powder, microscopic analysis (SEM, EDS) showed that its microstructure and the interspersion of the two metals were the same as in example 3.
• the organic solution contained tungstate ions and the reaction yield was 87%.
• the thus obtained 85W-15Cu composite powder was showed to be completely similar to that obtained in example 5, indicating that also copper precursors insoluble in the reaction medium can be used for producing composite powders having highly interspersed metal phases.
• tungsten-copper composite powder (670 g) was separated and washed with acetone. Pressing and sintering tests in hydrogen atmosphere as well as conductivity measurements were carried out and the results thereof are reported in the following table. Pressing Load (ton/cm 2 ) Sintering temperature Relative density Electrical conductivity (% IACS) 0.95 1300°C 96% 40 1350°C 97% 41 2.39 1300°C 96% 39 1350°C 97% 41
• the obtained result proves that the method of the invention allows the production of W-Cu composite powders having high sinterability also by using a copper precursor which is insoluble in the organic phase wherein the reaction occurs.
• the method according to the invention allows the production of tungsten-copper composite powders suitable for the production of sintered products, having also complex shapes, without the need of using the conventional and more expensive infiltration method.
• the method of the invention has furthermore the advantage of carrying out both the copper reduction and the W and Cu interspersion in an organic liquid phase wherein tungsten powder is present, thus avoiding any preliminary process for the powder mixing and/or grinding.

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• Chemical & Material Sciences (AREA)
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• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
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• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)

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• 1998
  • 1998-12-16 IT IT1998RM000776A patent/IT1302926B1/it active IP Right Grant
• 1999
  • 1999-10-12 WO PCT/IT1999/000321 patent/WO2000035616A1/en not_active Application Discontinuation
  • 1999-10-12 EP EP99954333A patent/EP1140397B1/en not_active Expired - Lifetime
  • 1999-10-12 DE DE69904757T patent/DE69904757D1/de not_active Expired - Lifetime
  • 1999-10-12 CZ CZ20012180A patent/CZ20012180A3/cs unknown
  • 1999-10-12 AT AT99954333T patent/ATE230321T1/de not_active IP Right Cessation
  • 1999-10-12 AU AU10738/00A patent/AU1073800A/en not_active Abandoned

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