Microwave sintering of high-density, high thermal conductivity AlN
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Tayo Olorunyolemi Institute of Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742
Otto C. Wilson and Isabel K. Lloyd Department of Materials and Nuclear Engineering, and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742
Yuval Carmel Institute of Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 (Received 17 January 2002; accepted 6 August 2002)
Microwave energy was used to sinter high thermal conductivity AlN ceramics (160–225 W/mK). The effects of sintering time, temperature, and amount of additive on phase composition, phase distribution, densification behavior, grain growth, and thermal conductivity were studied. The thermal conductivity of AlN was greatly improved by the addition of Y2O3, extended sintering time, and higher sintering temperatures. Thermal conductivity development in Y2O3-doped AlN showed two distinctive time regimes: (i) densification, where full densification, secondary phase formation, concentration and segregation, and rapid purification of AlN grains occur, accompanied by a large increase in thermal conductivity; (ii) postdensification, where grain growth and secondary phase sublimation/evaporation occur, yielding a further increase in thermal conductivity. Our results indicate that microwave sintering is a promising approach for synthesis of high thermal conductivity AlN ceramics.
I. INTRODUCTION
Aluminum nitride is an excellent candidate for substrates in many electronic packaging applications due to its high thermal conductivity (theoretically, 319 W/MK), a thermal expansion coefficient close to that of silicon, high electrical resistivity, and a low dielectric permittivity.1–6 The development of AlN sintering technology and the understanding of critical factors controlling the thermal conductivity of AlN ceramics have led to significant opportunities for AlN products in other thermal management applications. These include windows for microwave sources where high thermal conductivity, high electrical resistivity, vacuum compatibility, and structural integrity are required. In addition, AlN can be mixed with dielectrically lossy components to make high thermal conductivity, controlled electrical resistivity composites for use in power microwave tubes.7 Roomtemperature thermal conductivities ranging from 70 to a)
Present address: Fuel Cell Energy, Inc., Danbury, CT 06813. J. Mater. Res., Vol. 17, No. 11, Nov 2002
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180 W/MK have been routinely reported for AlN fabricated by pressureless sintering with post heat-treatment, hot pressing, and hot isostatic pressing.2,5,6,8–11 Lower than theoretical thermal conductivity in polycrystalline AlN is usually attributed to microstructure factors,5,6,8 including lattice impurities or defects, second phases, grain boundaries, and porosity. Sintering conditions, the quality of the starting powder, the type of sintering aids, forming techn
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