Thermal conductivity of AlN ceramic with a very low amount of grain boundary phase at 4 to 1000 K
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Hiromi Nakano Electron Microscope Laboratory, Ryukoku University, Seta Otsu, 520-2194, Japan
Kazuyori Urabe Department of Materials Chemistry, Ryukoku University, Seta Otsu, 520-2194, Japan
Kozo Ishizaki School of Mechanical Engineering, Nagaoka University of Technology, Nagaoka, 940-2188, Japan
Shixun Cao and Katsunori Mori Faculty of Engineering, Toyama University, 930-8555, Japan (Received 16 May 2002; accepted 26 August 2002)
An AlN ceramic fired at 2173 K for 100 h under a reduced N2 atmosphere with carbon possessed a room-temperature conductivity of 272 Wm−1K−1, slightly lower than the value for high-purity, single-crystal AlN. However, the thermal conductivity of the ceramic at temperatures below 100 K was much lower than that of single crystal. This is mainly due to phonon scattering by grain junctions that possess an amorphous film with a thickness of under 1 nm. At 500 to 1000 K, no significant difference in the conductivity was observed between the ceramic and the single crystal.
I. INTRODUCTION
Thermal conductivity measurements of high-purity single crystals and theoretical calculations have revealed the intrinsic thermal conductivity of AlN at room temperature to be 320 Wm−1K−1. Oxygen is the most detrimental impurity with respect to thermal conductivity. Oxygen atoms are incorporated into the AlN lattice by substitutional solution at nitrogen sites, creating aluminum vacancies. The presence of these defects results in decreased thermal conductivity of AlN single crystals.1 Research into the processing and thermal conductivity of AlN ceramics was actively carried out during the last two decades of the 20th century. In 1981, the thermal conductivity achieved for AlN ceramics was about 40 Wm−1K−1 at room temperature.2 This figure was then enhanced via three development processes to promote the densification and decrease the oxygen impurity content of AlN ceramics 3 and has at present reached 272 Wm−1K−1,4 81% of the intrinsic conductivity of AlN, and slightly lower than the conductivity of highpurity single crystal.1 The processes include (i) addition of effective sintering aids such as Y2O3 and CaO,5–8 (ii) use of high-purity fine powders,9 and (iii) firing in a reduced N2 atmosphere with carbon.10,11 Process (iii) contributes most strongly to the enhancement of conductivity, since significant reduction occurs in the oxygen 2940
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J. Mater. Res., Vol. 17, No. 11, Nov 2002 Downloaded: 14 Mar 2015
content of ceramics. The migration of the grain boundary phase to the surface of the sample is also found during this process.12 In this case, an AlN ceramic with an extremely low volume of grain boundary phase was obtained.4,10–12 In the present study, using the above processes, we fabricated an AlN ceramic with high thermal conductivity and an extremely low-volume grain boundary phase. The thermal conductivity of the ceramic at temperatures between 4 and 1000 K was measured, and its temperature dependence is discussed. II. EXPERIMENTAL
The fabrication procedure and charact
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