| Powder metallurgical W(Mo)-Cu alloys exhibit excellent physical property, high thermal and electrical conductivity, low and alterable thermal expansion coefficient. They are widely used for heavy-duty electronic contacts, electronic packing devices, heat sink materials and are of great interest in aeronautics and some other advanced fields. In most of these applications, high-dense W(Mo)-Cu materials with homogeneous microstructure are required for high performance. However, because of the mutual insolubility and big difference of melting points between W(Mo) and Cu, it is difficult to sinter W(Mo)-Cu materials to full density by the traditional liquid phase sintering or copper infiltration sintering. Therefore, much work has been do to obtain nearly full density W(Mo)-Cu composite with homogenous microstructure.It has been shown that using of homogeneous and ultra-fine composite powder can effectively improve sinterability of a powder compact, especially in a liquid phase sintering system such as W(Mo)-Cu system, in which the dominant sintering mechanism is known to be particle rearrangement. Therefore, attempts have been made to prepare ultra-fine and well-dispersed W(Mo)–Cu powder to facilitate the rearrangement of W(Mo) particles during liquid phase sintering to increase sintering densification. Recently, some wet-chemical methods have been tried to synthesize superfine W(Mo)-Cu powders with high purity and homogeneous distribution of W(Mo) and Cu. In this dissertation, precipitation-reduction and sol-gel methods were designed to prepare W-Cu and Mo-Cu powders, respectively.First, a precipitation-reduction process was employed to prepare W-Cu nanopowders. Precipitates was synthesized by adding aqueous ammonia into a complex solution containing ammonium metatungstate and copper nitrate, then heating the complex solution. The precipitates powders were then calcined and hydrogen-reduced to convert into W-Cu powders. Phase constitution and morphology of the precipitates, the calcined powder, as well as the reduced powder were characterized. Influence of ratio of W to Cu on phase conversion of the precursor during calcination was investigated. Effects of reduction temperature and H2 flow rate on the hydrogen reduction kinetic were also studied. It is shown that the precipitation-reduction process produces nanosized W-Cu powders with particle size of about 100nm. The composition of the precipitates changes with the ratio of W to Cu in the complex solution, and only CuWO4 was found in the calcined powder when weight percentage of Cu ion content is 26wt.%. The reduction temperature and H2 flow rate have a great influence on the kinetics of the hydrogen-reduction process. The obtained W-Cu powder shows good sinterability. A relative density of 98.15% of the theoretical was achieved for W-Cu powder compacts sintered at 1200℃in H2 atmosphere for 90min. Furthermore, the sintered W-Cu parts exhibit excellent physical and mechanical properties. Electrical conductivity and thermal conductivity of the W-Cu parts sintered at 1200℃is 36.88%IACS and 167W/ (m·K), respectively, and bending strength and Vickers hardness for the W-Cu compacts sintered at 1200℃were 1012.2MPa and 264HV, respectively. Thermal expansion coefficient is between 7.0×10-6 K-1 and 7.8×10-6K-1 from room temperature to 450℃.Mo-Cu superfine powder was then synthesized by a sol-gel method, in which sols were formed and transformed to gels by heating a complex solution containing ammonium heptamolybdate, copper nitrate, citric acid and aqueous ammonia. The gels were then calcined and hydrogen-reduced to convert into Mo-Cu powder. Phase constitution and morphology of the gels, the calcined powder, as well as the resulting Mo-Cu powder were characterized by x-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). Microstructure of sintered Mo-Cu parts was observed by scanning electron microscopy (SEM). It is shown that the sol-gel process produces superfine Mo-Cu powder with a mean particle size of about 100nm. The Mo-Cu powder shows good sinterability. Relative density of 99.78% of the theoretical was achieved for Mo-Cu powder compacts sintered at 1200℃in H2 atmosphere for 90min. Furthermore, the sintered Mo-Cu parts exhibit excellent physical and mechanical properties. Electrical conductivity and thermal conductivity of the Mo-Cu parts sintered at 1200℃is 42.56%IACS and 157W/ (m·K), respectively, and bending strength and Vickers hardness for the Mo-Cu compacts sintered at 1200℃were 988MPa and 227HV, respectively. Thermal expansion coefficient is between 6.7×10-6 K-1 and 7.6×10-6K-1 from room temperature to 450℃.
|
PDF Full Text Request