Solid-phase synthesis of cupric tungstate
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The high degree of mixing of W and Cu phases in copper tungstates makes them an attractive source for manufacturing W–Cu composite powders. Hydrogen reduction of copper tungstates provides composite W–Cu powder products with a uniform, homogeneous dispersion of the metal phases. This paper presents test results for a variety of solid-phase reactions to synthesize cupric tungstate (CuWO4). Hydrated, dehydrated, and complex oxides of tungsten and copper have been used as solid reactants. With stoichiometric ratios of reactants, synthesis in air at 800 °C produced 96% to 100% conversion to CuWO4. Heterogeneous synthesis of CuWO4 with the participation of three solid phases (S1 + S2 → S3) required the simplest, most inexpensive equipment. The end product properties of synthesized CuWO4 could be controlled by the proper choice of reactants.
I. BACKGROUND A. Information survey
For many years W–Cu composites have been used as electrical contact materials and electrodes.1,2 The basic methods for fabrication of W–Cu composites are infiltration of a porous tungsten skeleton with liquid copper, hot pressing, and various techniques incorporating liquidphase sintering, re-pressing, explosive pressing, etc.2 It has been established that, to improve the thermophysical properties of W–Cu composites, it is necessary to increase their density. In addition to improved mechanical properties, higher density leads to improved thermal conductivity. This is critical for expanding the application of W– Cu composites as heat sink materials for electronics. A very promising method of producing high-density W–Cu composites consists of liquid-phase sintering of ultrafine composite powders.2 A number of studies have been aimed at comparing the pressibility and sinterability of W–Cu composite powders made by hydrogen coreduction of tungsten and copper oxides versus powders prepared by mechanical alloying.3,4,5 An appreciable increase in the homogeneity of W–Cu co-reduced powders has been demonstrated. At sintering, the densification in W–Cu systems prepared by mechanical alloying was found to be very slow compared to the densification in composites produced from co-reduced powders. To achieve optimal homogeneity of W–Cu composites, studies have been conducted on direct reduction of metallic constituents forming the composite powders from the respective copper tungstates. Copper tungstates 1096
http://journals.cambridge.org
J. Mater. Res., Vol. 16, No. 4, Apr 2001 Downloaded: 02 Apr 2015
provide a metallurgical environment where copper and tungsten are mixed together at an atomic level. It has been demonstrated that, upon hydrogen reduction of metals from the tungstates, the finest possible distribution of product phases may be obtained. This imparts a high degree of phase dispersion and homogeneity to the W–Cu composites.2,6 Studies on making W–Cu composite powders by hydrogen reduction of copper tungstates have been conducted at Manchester University, U.K.,1 Institute for Problems in Materials Science, Kiev, Ukraine,2 and, most recently, a
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