Tungsten-copper based composite materials are used for the produc- tion 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 ad- vantageous mechanical properties of tungsten are coupled to the high electri- cal and thermal conductivity of copper.

The most widely used method for obtaining this bonding (known as infiltration technique) 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. Fur- thermore, in order that pores can be filled by molten copper, these must be opened or it is necessary to minimize the fraction of closed pores in starting sintered tungsten. Where there are closed pores, through which copper can not flow and therefore not fill them, it results in fragile products. Thus the need to minimize the presence of closed pores results in a critical first step during

the production process 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. By considering also 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 of carrying out two high temperature treatments (the tungsten sintering and the following infiltration of the resulting porous structure by molten copper) and with machining needed for re-obtaining the product having desired sizes, reduce the commercial ap- plicability of the infiltration process and result in a need for a direct sintering of mixtures of tungsten and copper powders.

However, the conventional methods in the powder metallurgy for the production of components by mixing, pressing and sintering mixtures of tung- sten and copper metal powders did not prove commercially convenient, par- ticularly 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 sin- tering 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. Dorfman, M.

Paliwal,"Tungsten/Copper Composites for Heat Sinks and Electrical Con- tacts", in Proceedings of 7th International Tungsten Symposium, pages 390- 409, on 24-27 September, Goslar, Germany.

Another set of methods for obtaining tungsten-copper composite pow- ders include the steps of mixing/grinding and following co-reduction in hydro- gen atmosphere of copper oxide and tungsten oxide powders. The thus ob- tained metal particles are in more intimate contact than that obtainable by using only mechanical grinding of copper and tungsten metals and the re- sulted tungsten-copper powder can be directly pressed and sintered to density values exceeding 95 % with respect to theoretical ones.

Compounds as copper tungstate (CuW04), wherein copper and tung- sten are mixed in the atomic range, can also 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 WOs; 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 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.

In order to reduce temperatures and calcining times, 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. H20) and tungstic acid, H2W04 (i. e. WOs. H20). The heat treating of such a mixture of said hy- drate oxides results in water development with formation of CuO and WOs oxides having high surface area, which assures the advantage of higher reac- tivity in the following step at higher temperatures (600-800°C).

Furthermore with reference to the production of CuW04 mixed oxide, U. S. patent No. 54670549 discloses an alternative route with respect to the above mentioned which inclues the use of ammonium tungstate (both meta- tungstate, AMT, and para-tungstate, APT) as tungsten precursors, meanwhile both CuO and Cu20 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. As Cu/W ratio in CuW04 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 WOs to the tungstate.

In all the above described processes the production of composite tungsten-copper powders having suitable interspersion of two metallic phases

requires carrying out of at least two of two following preparative steps: i) mixing and possible dry grinding of copper oxides or hydroxides with tungsten oxides, hydrate oxides or compounds, as AMT od APT; ii) solid phase reacting at temperatures between 600 and 800°C; iii) treating in hydrogen atmosphere for a suffiicent time to achieve a complete reduction of oxide type compounds.

An usual drawback for such methods (in addition, of course, to the need for high temperatures) is because they require a careful control of oper- ating conditions in order to obtain a composite metallic powder having a suit- able grain size. Furthermore, the thus obtained tungsten-copper powders have tendency to form aggregates, which reduces their usefulness for pro- ducing small components as required in electronic industry.

As an alternative to methods according to which tungsten and copper precursors are dry mixed/grounded, 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 pow- ders comprising tungsten metal, cuprous oxide and, if so desired, cobalt pow- der 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 cu- prous oxide (Cu20) is reduced in hydrogen atmosphere at 700-730°C to pro- duce 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 tem- perature treating in hydrogen atmosphere, results in the formation of a sin- tered 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 mono-metal powders (from gold, palladium, platinum, iridium, osmium, copper, silver, nickel, cobalt, lead or cadmium) 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.

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 tempera- tures which are always higher than 85°C (preferably in the range 100-350°C).

A further development of the above described"polyol method"is re- ported in U. S. Patent No. 5759230, which is relevant to the production of me- tallic films and powders with sizes in 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 polyol, wherein one or more precursors are suspended (typically in the form of metal salt, hydrate salt or oxy-anion). 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. Ac- cording to another specific embodiment 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.

Although the general method is described with reference to various

single metals and alloys or metal composites, its application does not allow the co-reduction of W and Cu precursors in order to produce tungsten and copper sinterable 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 proce- dure for the production of composite directly sinterable tungsten-copper metal powders based on a liquid phase reduction of precursors thereof, which pro- cedure takes advantages of polyol method for non requiring high temperature treatments and of carrying out in a single step both the mixing and reduction steps of two powders and is suitable specifically for the mentioned metals.

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

Therefore it is 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 possibie metal precursors be carried out; d) separate the solid phase resulting from the suspension of preced- ing step and wash it by using organic solvent, thus obtaining a powder con- sisting of a tungsten-copper mixture possibly containing other metal elements, wherein all the metals are highly interspersed.

Preferably 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 sium. The copper compound can be either soluble in the polyol, as is the case, for example, for copper (II) acetate monohydrate (Cu (CH3CO0) 2. H20) or insoluble in the polyol, as is the case for cupric and cuprous oxide (CuO).

The method suggested in accordance with the present invention al- lows 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. With reference to this object it is necessary to have in mind that, besides the fact that the copper is added to the reaction mixture in the form of a precursor compound, starting tungsten metal, being active in the copper reduction, undergoes to a partial solution as tungstate and therefore is dimin- ished 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.

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 occurs depends on the selected temperature; for exam- ple, at 70°C the reduction is completed over 4-6 hours, whereas at the boiling

temperature of the used polyol (preferably ethylene glycol, Teb = 198°C) the reaction is completed over 5-15 minutes.

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 sinter- ing cycle.

Furthermore it possible to control the microstructure of the final pow- der by modifying: i) the grain size of tungsten starting powder, ii) the composi- tion of organic phase employed, iii) the copper precursor and concentration thereof, iv) the temperature and reaction time.

Furthermore composite tungsten-copper powders containing suitable additives for reducing temperatures or sintering times and/or improving tech- nological and utiiization properties of the product obtained therefrom can be produced. 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, tetrahydrate cobalt (II) acetate, at the reaction temperature 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).

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 for obtained products are re- ported.

EXAMPLE 1 670 g of tungsten powder having a grain size of 1 m (H. C. Stark, grain size distribution 0,1-2,5 j. m) are charged in a Pyrex glass reactor with 3.3 1 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.

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 cuprous compound.

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

Pressing Load Sintering Relative Electrical conduc- (ton/cm2) temperature density tivity (% IACS) 2.39 1350°C 97 % 38 The above data point out that the obtained composite powder is suit- able both for pressing and sintering applications, relative density values above 96 % being obtained. Correspondingly high values for electrical conductivity, required for applications to which these materials are designed, are obtained.

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/cm2 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 5.30 g of dihydrate sodium tungstate (Na2W04.2H20) are charged in a Pyrex glass flask with 60 mi of ethylene glycol. After two hours at the glycol boiling temperature (198°C) no chemical reaction is observed. This result proves that glycol is not suitable to carry out the reduction of sodium tungstate to tungsten metal during the experienced reaction time neither at its boiling temperature.

COMPARATIVE EXAMPLE 3 5.30 g of dihydrate sodium tungstate (Na2W04.2H20) are 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 is observed. This result proves that neither by substituting the polyol it is possible to carry out the reduction of sodium tungstate to tungsten metal powder.

COMPARATIVE EXAMPLE 4 4.00 g of tungstic acid (H2W04) 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 precur- sor, 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 mi 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 pm, are

charged in a Pyrex glass reactor with 12 1 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 %. Chemical analysis of organic phase shows 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 analysis have been carried out. X-ray diffractome- try (XRPD) showed that only W and Cu phases are present. Micrographs obtained by scanning electron microscopy (SEM) show that, owing to the redox reaction to which they underwent, the tungsten grains have modified their morphology showing corrugated surfaces. Furthermore the obtained copper metal nucleates on the tungsten products in the form of small crystal- line aggregates.

Energy dispersion microanalysis (EDS) shows a ratio between the CuKal, 2 and W Lai peaks higher than that corresponding to powder mixtures containing 15 % by weight of Cu and 85 % by weight of W (85W-15Cu), con- firming that the copper has nucleated on W pproducts 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 tung- state. Such a surface corrugation provides also sites suitable for the hetero- geneous nucleation of the copper metal formed by reduction of copper com- pound. Thus it is possible to achieve such a interspersion between two metal- lic phases that results in the sinterability of the resulting W-Cu composite powder.

Example 4 The procedure as in example 3 is applied with exception that the re- action 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.

The organic solution contains tungstate ions and the reaction yield is 87 %.

EXAMPLE 5 The procedure as in example 3 is applied, using tungsten having an average grain size of 0,5 pm (Good Fellow). The reaction yield is 87 % and again tungstate ions are present in the organic phase.

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

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 lim (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 1 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 boil- ing temperature of glycol (198°C) over two hours.

The thus obtained 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/cm2) temperature density tivity (% 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 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.

Following the reaction, the powder is pressed at 2.39 ton/cm2 and sintered in hydrogen atmosphere at 1300°C, the density being 97 % with respect to the theoretical one. This result proves that the process of the inven- tion allows concurrent adding of suitable sintering additives as cobalt metal, 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 Ex- ample 7, it clearly appears that by adding cobalt in small amounts it is possible to achieve the same relative densities as obtained at lower sintering tem- perature.

EXAMPLE 9 670 g of tungsten powder, having an average grain size of 1, um (H. C.

Stark) are charged in a Pyrex glass reactor with 3.3 1 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.

Thereafter the powder is pressed at 2.39 ton/cm2 and sintered in hy- drogen atmosphere at 1350°C obtaining a density value of 98 % with respect to theoretical one.

The obtained result proves that 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.

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 compos- ite powders, according to which the mixing and reduction by hydrogen gas of oxide precursors are carried out at high temperature, 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 preiiminary process for the powder mixing and/or grinding.

Although the present invention has been described with reference to its specific embodiments it should be understood that changes or modifica- tions may be made by those skilled in the art without departing from the scope of the invention.