The Effects of Composition and Microstructure on the Thermal Conductivity of Liquid-Phase-Sintered W-Cu
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COMPOSITES of W-Cu and Mo-Cu are commonly used for thermal management applications due to their high thermal conductivities and low thermal expansion coefficients, which can be tailored by varying the Cu content. Copper contents for heat sinks generally range from 10 to 20 wt pct. These composite heat sinks can be processed by infiltrating a W or Mo skeleton with Cu,[1–5] liquid-phase sintering of milled,[6–8] co-reduced,[9–16] or mechanically alloyed powders,[17–22] and powder rolling.[23–25] Liquid-phase sintering is desirable for producing net-shape heat sinks,[8,26–29] but it can result in residual porosity since the low solubility of W or Mo in Cu limits densification via solution reprecipitation. Submicron W or Mo powders are required to achieve near full density without sintering aids.[30,31] Wide ranges of thermal conductivities have been reported for specific Cu contents depending on the processing conditions.[2,7,8,11,13,16,19,25,32,33] Transition metal impurities, either added intentionally as a sintering aid or from contamination during processing, are especially detrimental to thermal conductivity,[6–8,13,22,33] JOHN L. JOHNSON, R&D Director, is with ATI Engineered Products, Huntsville, AL 35806. Contact e-mail: John.Johnson@ ATImetals.com SEONG JIN PARK, Assistant Professor, is with the Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, 790-784, S. Korea. YOUNG-SAM KWON, President, is with CetaTech, Inc., Sacheon, Gyungnam, 664-953, S. Korea. RANDALL M. GERMAN, Associate Dean, is with the College of Engineering, San Diego State University, San Diego, CA 92182-1326. Manuscript submitted September 23, 2009. Article published online April 7, 2010 1564—VOLUME 41A, JUNE 2010
with as little as 100 ppm Fe causing a 16 pct decrease. Oxygen is another common impurity, but it has much less effect on the thermal conductivity.[32] Microstructural features, such as grain size, connectivity, and contiguity, can also affect thermal conductivity, but they are often masked by other factors, including the precision of measurement techniques. Many models[34–40] have been developed for the thermal conductivity of composites depending on the geometry of the different phases. Modification of these models has led to the analysis of the effect of specific microstructural features, such as grain size, on the thermal conductivity.[41,42] However, liquid-phasesintered composites such as W-Cu and Mo-Cu have unique microstructures due to the interaction of the reinforcement (W or Mo) during processing. Contacts between the W and Mo particles grow via solid-state diffusion resulting in a highly contiguous microstructure.[30,31] A more continuous matrix phase is expected to provide improved conductivity, but the effect of contiguity on thermal conductivity is not adequately predicted by previous models. Increasing contiguity has been shown to decrease the electrical conductivity of infiltrated W-Cu.[5] In this work, a model is presented that enables evaluation of the role of cont
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