Tungsten Micropowder/Copper Nanoparticle Core/Shell-Structured Composite Powder Synthesized by Inductively Coupled Therm
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INTRODUCTION
TUNGSTEN (W)-Copper (Cu) composite materials are widely used in electronic components for thermal/electrical management because W has a low coefficient of thermal expansion, while Cu has high thermal/electrical conductivity.[1] The physical properties of W-Cu composites can be varied by optimizing the phase composition and microstructure. The challenges in fabricating W-Cu composites, however, come from the mutual insolubility of W and Cu due to high positive heat of mixing. Also, the quite different thermo-physical properties such as density and melting temperature [W: 3683 K (3410 C), Cu: 1353 K (1080 C)] make it difficult to fabricate W-Cu composites. A large body of study has been carried out to develop processing methods to fabricate W-Cu composites. W-Cu composites can be fabricated by Cu infiltration sintering[2–4] or by liquid phase sintering of blended W and Cu powders.[5–7] The Cu infiltration method produces the W-Cu composite by a process in which molten Cu is poured into the sintered W porous structure. The infiltration process, however, requires high sintering temperature of 1473 K to 1673 K (1200 C to 1400 C) and even results in microstructural inhomogeneity. For another process, the liquid phase sintering with blends of W and Cu powder induces the selective melting of Cu KYOU-HYUN KIM and HANSHIN CHOI, Senior Researchers, and CHULWOONG HAN Researcher, are with the Advanced Process and Materials R&D Group, Incheon Regional Division, Korean Institute of Industrial technology, Incheon 406-840, South Korea. Contact e-mails: [email protected], [email protected] Kyou-Hyun Kim and Hanshin Choi contributed equally to this work. Manuscript submitted April 20, 2016. Article published online November 4, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
and the redistribution of molten Cu to fabricate W-Cu composite materials. During liquid state sintering, a few volume percent of the original solid is initially melted at the sintering temperature, while the solid state sintering is followed only by atomic diffusion. The volume of liquid is in general insufficient to reduce pore density so that additional processes are required to achieve full densification. The mutual insolubility between W and Cu, however, hinders densification during sintering process unlike other W-based binary system. For example, the W-Ni binary systems are further densified during sintering process as the small amount of W elements is dissolved into Ni.[8,9] On the other hand, Ryu and coworkers reported that the uniformly coated Cu nanoparticles on W can improve the densification by the capillary force between the W and the molten Cu.[10] The homogeneity of component phases in the W-Cu composite is therefore critical to determine the physical properties of W-Cu composites. In liquid phase sintering, the densification of W-Cu composites is achieved mainly by rearrangement of W particles due to the capillary force and surface tension of molten Cu particles.[11] The microstructure of the W-Cu composite is thus dependent largely o
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