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 sinter- able composite tungsten-copper 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 values for electrical and thermal conductivity.
• Tungsten-copper based composite materials are used for the production of heat exchangers suitable for electrical devices and for the production of electrodes and power electrical contacts. Because tungsten and copper do not alloy together, various methods have been developed to bond them 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 most widely used method for obtaining this bonding comprises: i) sintering of a tungsten metal powder at a such temperature to obtain a porous tungsten structure; ii) infiltrating said structure by molten copper, the pores of the structure being filled by means of 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.
• Another set of methods for obtaining tungsten-copper composite powders include the steps of mixing/grinding and 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 resulted tungsten-copper powder can be directly pressed and sintered to density values exceeding 95 % with respect to theoretical ones.
• Compounds as copper tungstate (CuWO ), wherein copper and tungsten are mixed in the atomic range, can also 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 using balls made of hard metal or other ceramic material, resulting in a potentially contaminated mixture. Furthermore 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 the use as precursors, instead of conventional oxides, of hydrate 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).
• hydrate 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 treating of such a mixture of said hydrate oxides results in water development with formation of CuO and WO 3 oxides having 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 which includes the use of ammonium tungstate (both meta- tungstate, AMT, and para-tungstate, APT) as tungsten precursors, meanwhile both CuO and Cu 2 O can be used as copper precursors.
• Tungsten oxide (WO3) obtained by 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.
• Cu ⁇ /V ratio in CuWO is fixed (25,7 % by weight in the W-Cu final 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. 543938 suggests a proc- ess for the production of composite tungsten-copper 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 tungsten metal, cuprous oxide and, if so desired, 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.
• 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.
• a technique partially similar to the above mentioned various methods, which includes a step for a dry or aqueous phase powder mixing, followed by high temperature reduction, is described in European Patent Publication EP- A-0806489. According to it W/Cu products, having density values above 97 % with respect to theoretical ones, are directly obtained by using starting mix- tures containing copper and a transition metal (as W or Mo), provided that the mixture contains also chemically bonded oxygen, for example in the form of copper oxide, in such amounts to improve the sinterability thereof.
• the de- scribed procedure preferably includes mechanical mixing of tungsten metal 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.
• mono-metal powders from gold, palladium, platinum, iridium, osmium, copper, silver, nickel, cobalt, lead or cadmium
• mono-metal powders can be produced by precursor reduction by using an organic liquid phase made up of one or a mixture of polyols. More particularly a compound of desired metal selected from oxides, hydroxides and metal salts is reduced in organic liquid phase by heating to a temperature of at least 85°C. Owing to the reduction, the metal is separated in the form of high purity powder.
• the method is suggested for the production of films and powders in nanometric range 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 con- sisting of refractory metals (W, Ti, Mo, Re, Ta) or their alloys produced from salts or acids which oxy-anions contain said metals.
• tungsten-copper composite powders suitable to be used directly for the production of sintered products by using a reduction process in an organic liquid phase consisting of one or a mixture of polyols wherein copper is added as precursor compound whereas tungsten is added as metal.
• an organic liquid phase consisting of one or a mixture of polyols wherein copper is added as precursor compound whereas tungsten is added as metal.
• tungsten metal is necessary in order to carry out the reduction of copper precursor at reasonably low temperature and times, as tungsten itself takes part in the copper compound reduction, so allowing the reduction reaction be carried out at lower temperatures.
• the organic phase reaction can be carried out below the lowest temperature values known in the art (85°C).
• a specific object of the present invention 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: a) suspend a tungsten metal powder in one or a mixture of liquid polyols.; b) add to the thus obtained suspension a copper precursor and possi- bly minor amounts of other metal precursors; c) heat the resulting suspension to a temperature of at least 60°C and keep it under stirring at a such temperature for a sufficient time to allow the reduction of said copper and other possible metal precursors be carried out; d) separate the solid phase resulting from the suspension of preceding step and wash it by using organic solvent, thus obtaining a powder consisting of a tungsten-copper mixture possibly containing other metal elements, wherein all the metals are highly interspersed.
• the organic phase wherein the oxidation-reduction reaction and concurrent interspersion of produced copper and starting tungsten occur is constituted of ethylene glycol, neat or in admixture with other polyols, as for example diethylene glycol.
• the starting tungsten metal 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, for copper (II) acetate monohydrate (Cu(CH 3 COO) 2 .H 2 ⁇ ) or insoluble in the polyol, as is the case for cupric and cuprous oxide (CuO).
• the method suggested in accordance with the present invention allows the preparation of tungsten-copper composite powders having a broad composition range because, 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 organic phase suspen- sion/solution.
• starting tungsten metal being active in the copper reduction, undergoes to a partial solution as tungstate and therefore is diminished in the final metallic product.
• Suitable starting amounts of tungsten metal 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 which is the object of the present invention can be stored for a long time if wet with same organic solvent, avoiding thus the hazard for the spontaneous ignition of dry powders. Possible residues of organic phases which can be present after composite tungsten-copper powder washings are removed during the sintering cycle.
• microstructure of the final pow- der by modifying: i) the grain size of tungsten starting powder, //) the composition of organic phase employed, Hi) the copper precursor and concentration thereof, iv) the temperature and reaction time.
• composite tungsten-copper powders containing suitable additives for reducing temperatures or sintering times and/or improving tech- nological and utilization properties of the product obtained therefrom can be produced.
• a suitable amount of a cobalt (II) compound as, for example, tetrahydrate cobalt (II) acetate
• cobalt metal is formed, which, in small amounts, allows to lower the W-Cu compound sintering temperature and/or times (S. K. Joo, S. W. Lee, T. H. Ihn, "Effect of Cobalt Addition on the Liquid Phase Sintering of W-Cu Prepared by the Fluid- ized Bed Reduction Method"; Met. Mater. Trans. Vol. 25A, pages 1575-1578 (1994).
• EXAMPLE 1 670 g of tungsten powder having a grain size of 1 ⁇ m (H. C. Stark, grain size distribution 0,1-2,5 ⁇ m) are charged in a Pyrex glass reactor with 3.3 I of ethylene glycol (Carlo Erba). Thereafter to the suspension under stir- ring are added 315 g of monohydrate copper acetate (corresponding to 100 g of copper metal). The reaction is carried out at 70°C over 6 hours.
• the reaction provides 670 g of product, weighted after the resulting composite powder has been separated, washed with acetone and air dried.
• the atomic absorption analysis shows a copper content of 15 % by weight.
• EXAMPLE 2 The same procedure as in the example 1 has been applied except that using 397 g of tungsten, whereas the copper amount, in the form of monohydrate copper acetate, has been kept constant (315 g). The reaction provides 400 g of product, weighted after the resulting composite powder has been separated, washed with acetone and air dried. The atomic absorption analysis showed a copper content of 25 % by weight and the reaction yield was 81 %.
• 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 % with respect to theoretical one.
• the electrical conductivity of the sintered product was 46 % IACS.
• COMPARATIVE EXAMPLE 1 The same procedure as in the example 1 is except that in the absence of tungsten. After six hours at 70°C no formation of copper metal is observed. This result proves that at the employed temperature the presence of tungsten is required in order to perform the reduction of cupric precursor to copper metal. COMPARATIVE EXAMPLE 2
• COMPARATIVE EXAMPLE 4 4.00 g of tungstic acid (H 2 WO4) are charged in a Pyrex glass flask with 60 ml of ethylene glycol. After two hours at the glycol boiling temperature (198°C) no chemical reaction is observed. This result proves that under the indicated conditions neither tungstic acid, another potential tungsten precursor, can be reduced to tungsten metal by ethylene glycol.
• COMPARATIVE EXAMPLE 5 5.30 g of dihydrate sodium tungstate and 1.60 g of copper acetate monohydrate are charged in a Pyrex glass flask with 60 ml of ethylene glycol. After two hours at the glycol boiling temperature (198°C) no chemical reaction is observed. The experiment proves that it is not possible to obtain W-Cu composite powders by co-reduction of copper and tungsten precursors using ethylene glycol.
• EXAMPLE 3 670 g of tungsten powder, having an average grain size of 4 ⁇ m, are charged in a Pyrex glass reactor with 12 I of ethylene glycol (Carlo Erba). To the suspension under stirring then are added 315 g of monohydrate copper acetate (corresponding to 100 g of copper metal). The reaction is carried out at the boiling temperature of ethylene glycol over 15 minutes. The reaction provides 670 g of product, the yield being therefore 87
• EDS Energy dispersion microanalysis
• 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 also sites suitable for the hetero- geneous nucleation of the copper metal formed by reduction of copper compound.
• Example 4 The procedure as in example 3 is applied with exception that the reaction is carried out at 110°C over two hours. After separation and washing with acetone of the obtained powder, microscopic analysis (SEM, EDS) shows that its microstructure and the interspersion of the two metals are the same as in example 3.
• EXAMPLE 6 The same procedure as in example 3 is applied with exception that 125 g of CuO (Aldrich) as copper precursor and tungsten having an average grain size of 0,5 ⁇ m (Good Fellow) are used. The reaction yield is 87 % and tungstate ions in the organic phase are detected.
• CuO Aldrich
• tungsten having an average grain size of 0,5 ⁇ m (Good Fellow) are used.
• the reaction yield is 87 % and tungstate ions in the organic phase are detected.
• the thus obtained 85W-15Cu composite powder is showed as being completely similar to that obtained in example 5, indicating that also copper precursors insoluble in reaction medium can be used for producing composite powders having highly interspersed metal phases.
• EXAMPLE 7 670 g of tungsten powder having an average grain size of 1 ⁇ m (H. C. Stark) are been charged in a Pyrex glass reactor with 3.3 I of ethylene glycol (Carlo Erba). Then to the suspension under stirring 315 g of monohydrate copper acetate are added (Carla Erba). The reaction is carried out at the boiling temperature of glycol (198°C) over two hours.
• tungsten-copper composite powder (670 g) are separated and washed with acetone. Pressing and sintering tests in hydrogen atmosphere as well as conductivity measurements are carried out and results thereof are reported in the following table. Pressing Load Sintering Relative Electrical conduc- (ton/cm 2 ) temperature density tivity (% IACS)
• EXAMPLE 8 The procedure of example 7 is repeated with the only difference that tetrahydrate cobalt (II) acetate has been added also in amount corresponding to a cobalt content in the final composite of 0.5 % by weight.
• the powder is pressed at 2.39 ton/cm 2 and sintered in hydrogen atmosphere at 1300°C, the density being 97 % with respect to the theoretical one.
• suitable sintering additives as cobalt metal, herein formed by co-reduction of the corresponding salt in the organic phase.
• EXAMPLE 9 670 g of tungsten powder, having an average grain size of 1 ⁇ m (H. C. Stark) are charged in a Pyrex glass reactor with 3.3 I of ethylene glycol (Carlo Erba). Then to the suspension under stirring 125 g of CuO (Aldrich) and 14 g of tetrahydrate cobalt (II) acetate (Carlo Erba) are added, the amounts being such to have in the final product a copper content of 15 % by weight and a cobalt content of 0.5 % by weight. The reaction is carried out at the boiling temperature of ethylene glycol (198°C) over two hours. The thus obtained tungsten-copper composite powder is separated and washed with acetone.
• ethylene glycol ethylene glycol
• the method of the invention allows the production of W-Cu composite powder 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 in object 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, avoiding thus any preliminary process for the powder mixing and/or grinding.

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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
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