Fabrication of diamond/aluminum composites by vacuum hot pressing: Process optimization and thermal properties
Introduction
Efficient heat removal is becoming increasingly urgent for the reliability and life spans of electronic components, owing to the continuously increased power density. It is essential to develop thermal management materials with high thermal conductivity (TC) and tailored coefficient of thermal expansion (CTE), which can not only dissipate the generated heat of electronic components, but also minimize thermal stress between the electronic components and the thermal management materials, to ensure their reliability and lifetime. With rather high TC (1300–2200 W/mK) and low CTE (2 × 10−6 K−1), diamond particles are ideal reinforcements to prepare metal matrix composites (MMCs) for applications of heat sink and electronic packaging [1], [2], [3]. Compared with other metallic matrices such as Cu and Ag, diamond/Al composites have drawn much more attentions than ever before due to their low density, low cost and easy fabrication [4], [5], [6]. Meanwhile, powder metallurgy (PM) methods are becoming prevalent as means of fabrication of diamond/Al composites to avoid the formation of Al4C3 as a detrimental interface reaction product in the liquid infiltration process. At present, spark plasma sintering (SPS) is the mostly used PM method to sinter diamond/Al composites [7], [8], [9], [10]. However, due to the very short sintering time, it always results in insufficient interface diffusion and poor interfacial bonding, and thus much lower TC than expected [8], [9]. Although the TC of the SPS composites could be enhanced to some extent, either by alloying of the Al matrix [3], [10] or by surface metallization of diamond [9], negative effects are brought on the TC as well. Alloying will cause the TC degradation of Al matrix, while the metallic coating, such as Ti, may behave as an additional thermal barrier. Therefore, it is essential to develop an alternative PM method that enable controllable diamond/Al interface to obtain high TC.
Recently, a process of vacuum hot pressing (VHP) was proved effective in achieving much more enhanced TC than that of SPS [7]. In the VHP process, diamond/Al composites were sintered for rather a longer time than SPS, which enable larger processing windows to control the interface diffusion and interfacial bonding. To take full potential of VHP process, the effect of the processing parameters such as sintering temperature, pressure and time on the microstructure and thermal property of the VHP diamond/Al composites were studied in this paper, and the optimized processing parameters were determined. As results, TC over 90% of the theoretical prediction is achieved in diamond/Al composites when sintered at 650 °C/67 MPa for 90 min. Al4C3 is detected in the composites sintered at 650 °C for a prolonged time up to 180 min, and it turns out to be negative to obtain high thermal properties.
Section snippets
Materials
The raw materials used are atomized pure Al powders and synthetic diamond particles, and their morphologies were shown in Fig. 1a. The atomized pure Al powders, whose chemical composition was tested by inductively coupled plasma mass spectrometer (ICP-MS), as tabulated in Table 1, were used as matrix. With Fe, Si and V elements as the main impurities, the Al powders were about 99.84% in purity. Synthetic diamond particles (Type HWD40, Henan Huanghe Whirlwind International Co. Ltd., China) with
Effect of sintering temperature on thermal properties and microstructures
Sintering temperature is the key parameter to consolidate the powder billets of diamond and pure Al by VHP. Fig. 2 shows the relative density and TC of diamond/Al composites with 40 vol% diamond sintered at different temperatures for 30 min under 67 MPa. With the sintering temperature increasing from 550 °C to 655 °C, both the relative density and TC of the composites increase. The composite sintered at 550 °C has a TC of 79 W/mK, even lower than that of pure Al (218 W/mK by the same VHP method). With
Conclusion
Diamond/aluminum composites were fabricated by vacuum hot pressing the powder billets of diamond and pure Al. The sintering temperature, pressure and time in the process were optimized to achieve high thermal conductivity. As results, sintered at 650 °C/67 MPa for 90 min, the composites with 20–55 vol% diamond possess thermal conductivities over 90% of the theoretical predictions, and the composite with 55 vol% diamond exhibits a thermal conductivity of 567 W/mK. The high thermal conductivity
Acknowledgments
The authors would like to acknowledge the financial support of the National Basic Research Program (973 Program) (No. 2012CB619600), the National Natural Science Foundation (Nos. 51071100, 51131004, 50890174), the National High-Tech R&D Program (863 Program) (No. 2012AA030311), the International S&T Cooperation Program (Nos. 2010DFA52550, 2009DFA52410) of China, and Shanghai Science & Technology Committee (No. 11JC1405500).
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