EP1140397B1

EP1140397B1 - Process for the production of tungsten-copper composite sinterable powders

Info

Publication number: EP1140397B1
Application number: EP99954333A
Authority: EP
Inventors: Gualtiero Unv. di Roma "Tor Vergata" GUSMANO; Alessandra Unv. di Roma "Tor Vergata" BIANCO; Riccardo Unv. di Roma "Tor Vergata" POLINI
Original assignee: Celsia SpA
Current assignee: Celsia SpA
Priority date: 1998-12-16
Filing date: 1999-10-12
Publication date: 2003-01-02
Anticipated expiration: 2019-10-12
Also published as: AU1073800A; EP1140397A1; IT1302926B1; ATE230321T1; CZ20012180A3; DE69904757D1; WO2000035616A1; ITRM980776A1

Abstract

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 exceeding 96 % with respect to the theoretical one and high values for electrical and thermal conductivity, the method essentially comprising the reduction of a copper precursor in the presence of tungsten metal suspended in an organic liquid phase produced by one or a mixture of polyols. The reaction is carried out by heating the suspension to a temperature of at least 60 DEG C and keeping it at a such temperature for a sufficient time to bring about the reduction of the copper precursor by means of the organic phase and tungsten therein.

Description

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 (known as infiltration technique) 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. In addition, although tungsten porous structures having shapes similar to those of end products can be produced, following the infiltration process further machining of products is necessary in order to remove the excess of copper, flowed out of the pores of the tungsten structure, and in order to obtain again the desired shapes. Also in view of this aspect, the use of the infiltration process is convenient from an economic standpoint, only for producing products having quite simple shapes.

The latter limitation, together with the need to carry out two high temperature treatments (the tungsten sintering and the following infiltration of the resulting porous structure by molten copper), as well as the machining needed for re-obtaining the product with desired size, reduce the commercial applicability of the infiltration process and result in a need for a direct sintering of mixtures of tungsten and copper powders.

However, the conventional powder metallurgy methods for the production of components by mixing, pressing and sintering mixtures of tungsten and copper as elemental powders did not prove commercially convenient, particularly where the copper content is low (from 5 to 20% by weight). In fact the high residual porosity in the products obtained by using the pressing and sintering process, which impairs not only mechanical strength properties but also and particularly the electrical and thermal conductivity, results in the need for further re-pressing and re-sintering processes. (D. L. Houck, L.P. Dortiman, M. Paliwal, "Tungsten/Copper Composites for Heat Sinks and Electrical Contacts", in Proceedings of 7^th International Tungsten Symposium, pages 390-409, 24-27 September 1996, Goslar, Germany).

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.

Also compounds as copper tungstate (CuWO₄), wherein copper and tungsten are mixed at the atomic level, can be reduced to obtain tungsten-copper composite powders having good sintering properties. According to conventional methods, copper tungstate is produced by reacting in the solid phase CuO with WO₃; in order to obtain an intimate contact between the two oxide phases, however, it is necessary to grind for a long time the CuO-WO₃ mixture by means of balls made of hard metal or ceramic material, thus resuiting 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.

In order to reduce temperatures and calcining times, 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.H₂O) and tungstic acid, H₂WO₄ (i.e. WO₃.H₂O). The heat treatment of such a mixture of hydrated oxides results in water development with formation of CuO and WO₃ with high surface area, which assures the advantage of higher reactivity in the following step at higher temperatures (600-800°C).

Furthermore, with reference to the production of the CuWO₄ mixed oxide, 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 Cu₂O can be used as copper precursors. Tungsten oxide (WO₃), 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 CuWO₄ 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 WO₃ to the tungstate.

In all the above described processes the production of composite tungsten-copper powders having suitable interspersion of the two metallic phases requires carrying out of at least two of the following preparative steps:

i) mixing and possible dry milling of copper oxides or hydroxides with tungsten oxides, hydrated oxides or other tungsten compounds, such as AMT or APT;

ii) solid state reacting at temperatures between 600 and 800°C;

iii) treating in hydrogen atmosphere for a sufficient time to achieve a complete reduction of oxide type compounds.

An usual drawback of such methods (in addition, of course, to the need for high temperatures) is due to the fact that they require a careful control of operating conditions in order to obtain a composite metallic powder having a suitable grain size. Furthermore, the thus obtained tungsten-copper powders tend to form aggregates, which reduces their usefulness for producing small components as required in electronic industry.

As an alternative to methods wherein tungsten and copper precursors are dry mixed/grounded, 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 (Cu₂O) 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 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

Besides by reduction in hydrogen atmosphere at high temperatures, metallic powders can be produced by liquid phase reduction using an alcohol solvent as reducing agent.

For example, according to the method suggested in European Patent EP-B-0113281 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.

Such a patent, however, does not disclose nor suggest the production of mixed metallic powders, for example by co-reducing two different metal precursors. Furthermore the process suggested requires the use of temperatures which are always higher than 85°C (preferably in the range 100-350°C).

A further development of the above described "polyol method" is reported in U.S. Patent No. 5759230, which is relevant to the production of metallic films and powders with sizes in the nanometric range comprising one or more metal elements, sometimes alloyed. In this case, as above, 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. According to a specific embodiment 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.

Although the general method is described with reference to several single metals and alloys or metal composites, its application does not allow the co-reduction of W and Cu precursors in order to produce sinterable tungsten-copper composite powders and the specific examples in fact do not disclose such a combination.

Accordingly it is an object of the present invention to provide a procedure for the production of directly sinterable tungsten-copper composite metal powders based on a liquid phase reduction of precursors thereof, which procedure takes advantage from the polyol method, in that it does no require high temperature treatments and affords to obtain in a single step both the mixing and the reduction of the two powders and, at the same time, is suitable specifically for the two mentioned metals.

According to the invention it has been found that it is possible to produce 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. In fact it has been found that the presence of 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. In fact it has been found that in the presence of elemental tungsten the organic phase reaction can be carried out below the lowest temperature values known in the art (85°C).

Therefore 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:

a) suspending an elemental tungsten powder in one or a mixture of liquid polyols;

b) adding to the thus obtained suspension a copper precursor and, optionally, minor amounts of other metal precursors;

c) heating the resulting suspension to a temperature of at least 60°C and keeping it under stirring at such a temperature for a sufficient time to allow the reduction of said copper precursor and of said other optional metal precursors;

d) separating the solid phase from the suspension obtained from the preceding step and washing it with an organic solvent, thus obtaining a powder consisting of a tungsten-copper mixture optionally containing other metal elements, wherein all the metals are highly interspersed.

Preferably 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 µm. The copper compound can be either soluble in the polyol, as is the case, for example, of copper (II) acetate monohydrate (Cu(CH₃COO)₂.H₂O) or insoluble in the polyol, as is the case of cupric and cuprous oxides (CuO and Cu₂O 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. In this connection it is to be noted, besides the fact that copper is added to the reaction mixture in the form of a precursor compound, that 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.

As above specified, the temperature of the organic phase, wherein the copper compound reduction occurs, is at least 60°C. The time during which the reduction process is achieved depends on the selected temperature; for example, at 70°C the reduction is completed over 4-6 hours, whereas at the boiling temperature of the polyol employed (preferably ethylene glycol, T_eb = 198°C) the reaction is completed over 5-15 minutes.

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.

Furthermore it is possible to control the 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.

It is also possible to obtain composite tungsten-copper powders containing suitable additives for reducing sintering temperatures or times and/or improving technological and utilization properties of the product obtained. For example by adding to the tungsten suspension (in addition to the copper precursor) a suitable amount of a cobalt (II) compound, as, for example, cobalt (II) acetate tetrahydrate, at the reaction temperature 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)).

Specific embodiments of the method in accordance with the invention are described in the following by way of example only, and analytical data as well as results of experimental tests or the products obtained are reported.

EXAMPLE 1

670 g of tungsten powder having a grain size of 1 µm (H. C. Stark, grain size distribution 0.1-2.5 µm) was charged in a Pyrex glass reactor with 3.3 l of ethylene glycol (Carlo Erba). Thereafter 315 g of copper acetate monohydrate (corresponding to 100 g of copper metal) was added to the suspension under stirring. The reaction was carried out at 70°C over 6 hours.

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 reaction yield (87%) and the presence of tungsten ions in the organic phase suggest that tungsten takes an active part in the reduction reaction of the cuprous compound.

Pressing and sintering tests in hydrogen atmosphere as well as conductivity measurements have been carried out on the 85W-15Cu powder obtained and the results thereof are reported in the following table.

Pressing Load (ton/cm²)	Sintering Temperature	Relative density	Electrical conductivity (% IACS)
2.39	1350°C	97%	38

The above data point out that the obtained composite powder is suitable for pressing and sintering applications, relative density values above 96% being obtained. Correspondingly high values of electrical conductivity, required for the intended applications of these materials are obtained.

EXAMPLE 2

The same procedure of example 1 was repeated except that 397 g of tungsten was used, whereas the copper amount, in the form of copper acetate monohydrate, was kept constant (315 g). The reaction provided 400 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 25% by weight and the reaction yield was 81%.

The 75W-25Cu powder thus obtained has been pressed at 2.39 ton/cm² 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.

COMPARATIVE EXAMPLE 1

The same procedure of example 1 was repeated except that tungsten was absent After six hours at 70°C no formation of elemental copper was observed. This result proves that at the employed temperature the presence of tungsten is required in order to perform the reduction of the copper precursor to metallic copper.

COMPARATIVE EXAMPLE 2

5.30 g of sodium tungstate dihydrate (Na₂WO₄.2H₂O) were 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 was observed. This result proves that glycol is not suitable to carry out the reduction of sodium tungstate to elemental tungsten in the experienced reaction time, not even at its boiling temperature.

COMPARATIVE EXAMPLE 3

5.30 g of sodium tungstate dihydrate (Na₂WO₄.2H₂O) were charged in a Pyrex glass flask with 60 ml of diethylene glycol. After two hours at the glycol boiling temperature (245°C) no chemical reaction was observed. This result proves that not even by substituting the polyol it is possible to carry out the reduction of sodium tungstate to elemental tungsten powder.

COMPARATIVE EXAMPLE 4

4.00 g of tungstic acid (H₂WO₄) were 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 was observed. This result proves that under the indicated conditions neither tungstic add, another potential tungsten precursor, can be reduced to tungsten metal by ethylene glycol.

COMPARATIVE EXAMPLE 5

5.30 g of sodium tungstate dihydrate and 1.60 g of copper acetate monohydrate were 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 was observed. The experiment proves that it is not possible to obtain W-Cu composite powders by co-reduction of copper and tungsten precursors by using ethylene glycol.

EXAMPLE 3

670 g of tungsten powder, having an average grain size of 4 µm, was charged in a Pyrex glass reactor with 12 I of ethylene glycol (Carlo Erba). To the suspension under stirring 315 g of copper acetate monohydrate (corresponding to 100 g of copper metal) were then added. The reaction was carried out at the boiling temperature of ethylene glycol over 15 minutes.

The reaction provided 670 g of product, the yield being therefore 87%. Chemical analysis of the organic phase showed the presence of tungstate ions and the absence of cupric ions.

In order to establish the microstructure of the thus obtained W-Cu composite powder different analyses have been carried out. X-ray diffractometry (XRPD) showed that only W and Cu phases were present Micrographs obtained by scanning electron microscopy (SEM) showed that, owing to the redox reaction they underwent, the tungsten grains had a modified morphology showing corrugated surfaces. Furthermore the obtained metallic copper nucleated on the tungsten particles in the form of small crystalline aggregates.

Energy dispersion microanalysis (EDS) showed a ratio of the Cu K_α1,2 to the W L_α1 peaks higher than that corresponding to powder mixtures containing 15% by weight of Cu and 85% by weight of W (85W-15Cu), confirming that the copper had nucleated on W particles with formation of a coating.

It is apparent, therefore, that in the process according to the invention 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. Thus it is possible to achieve such an interspersion between the two metallic phases that results in the sinterability of the resulting W-Cu composite powder.

EXAMPLE 4

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%.

EXAMPLE 5

The procedure of example 3 was repeated, using tungsten having an average grain size of 0.5 µm (Good Fellow). The reaction yield was 87% and again tungstate ions were present in the organic phase.

The structural characterization of the thus obtained 85W-15Cu composite powder was carried out by electron scanning microscopy and energy dispersion microanalysis and both these techniques showed excellent interspersion of the two metal phases.

EXAMPLE 6

The same procedure of example 3 was repeated except that 125 g of CuO (Aldrich) as copper precursor and tungsten having an average grain size of 0,5 µm (Good Fellow) were used. The reaction yield was 87% and tungstate ions were detected in the organic phase.

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.

EXAMPLE 7

670 g of tungsten powder having an average grain size of 1 µm (H. C. Stark) were charged in a Pyrex glass reactor with 3.3 I of ethylene glycol (Carlo Erba). Then to the suspension under stirring 315 g of copper acetate monohydrate were added (Carlo Erba). The reaction was carried out at the boiling temperature of glycol (198°C) over two hours.

The thus obtained 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/cm²)	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

EXAMPLE 8

The procedure of example 7 was repeated with the only difference that cobalt (II) acetate tetrahydrate was also added, in amount corresponding to a cobalt content in the final composite of 0.5 % by weight.

Following the reaction, the powder was pressed at 2.39 ton/cm² and sintered in hydrogen atmosphere at 1300°C, the density being 97% of the theoretical one. This result proves that the process of the invention allows concurrent adding of suitable sintering additives as elemental cobalt, herein formed by co-reduction of the corresponding salt in the organic phase.

By comparing the above indicated density value, relevant to a test with sintering temperature of 1300°C, with those indicated in the table of Example 7, it clearly appears that by adding cobalt in small amounts it is possible to achieve the same relative densities at lower sintering temperature.

EXAMPLE 9

670 g of tungsten powder, having an average grain size of 1 µm (H. C. Stark) was charged in a Pyrex glass reactor with 3.3 l of ethylene glycol (Carlo Erba). Then to the suspension under stirring 125 g of CuO (Aldrich) and 14 g of cobalt (II) acetate tetrahydrate (Carlo Erba) were added, the amounts being such as to have in the final product a copper content of 15% by weight and a cobalt content of 0.5 % by weight. The reaction was carried out at the boiling temperature of ethylene glycol (198°C) over two hours. The thus obtained tungsten-copper composite powder was separated and washed with acetone.

Thereafter the powder was pressed at 2.39 ton/cm² and sintered in hydrogen atmosphere at 1350°C obtaining a density value of 98% of the theoretical one.

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.

As herein above pointed out, 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. With respect to the known methods for the production of tungsten-copper composite powders, according to which the mixing and reduction by hydrogen gas of oxide precursors are carried out at high temperature, 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.

Claims

A process 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, said process comprising the following steps:

a) suspending an elemental tungsten powder in a liquid polyol or in a mixture of liquid polyols;

b) adding to the thus obtained suspension a copper precursor and, optionally, minor amounts of other metal precursors;

c) heating the resulting suspension to a temperature of at least 60°C and keeping it under stirring at such temperature for a sufficient time to allow the reduction of said copper precursor of said and other optional metal precursors;

d) separating the solid phase obtained from the suspension obtained from the preceding step and washing the same with an organic solvent.
The process according to claim 1 wherein said polyol is ethylene glycol, pure or in admixture with other polyols.
The process according to claim 1 or claim 2, wherein said elemental tungsten powder has an average grain size in the range from 0.5 to 6 µm.
A process according to any one of claims 1-3, wherein said copper precursor is selected from the group consisting of cupric oxide (CuO), cuprous oxide (Cu₂O) and cuprous acetate monohydrate (Cu(CH₃COO)₂.H₂O).
A process according to any one of claims 2-4, wherein the heating of step c) is carried out at least at 70°C.
The process according to claim 5, wherein said step c) is carried out for a time between 4 and 6 hours.
A process according to any one of claims 2-4, wherein the heating of step c) is carried out at the boiling temperature of ethylene glycol (198°C) and said step is carried out for a time between 5 and 15 minutes.
A process according to any one of the preceding claims, wherein the tungsten-copper composite powder obtained from said step d) has a copper content of 15% by weight and a tungsten content of 85% by weight
A process according to any one of claims 1-7, wherein in said step b), besides said copper precursor, a minor amount of a cobalt (II) compound is also added.
The process according to claim 9, wherein the amount of said cobalt (II) compound is such that the resulting final composite powder has a content of cobalt metal of 0.5% by weight