Microwave sintering and properties of AlN/TiB 2 composites
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Yuval Carmel and Tayo Olorunyolemi Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742
Isabel K. Lloyd and Otto C. Wilson, Jr. Department of Materials and Nuclear Engineering and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 (Received 13 November 2001; accepted 10 October 2002)
The effect of TiB2 on the densification behavior and properties of microwave-sintered AlN/TiB2 ceramic was investigated. The densification of the composite was significantly retarded in nitrogen atmosphere due to strong nitridation of TiB2 compared to sintering in argon atmosphere. The densities of the AlN/TiB2 composites containing different amounts of TiB2 all reached 99% of the theoretical density during 2 h of sintering at 1850 and 1900 °C. Microstructure analysis revealed that the TiB2 particles were dispersed in the AlN matrix while AlN grains retained its contiguity. This microstructure led to a composite with superior properties; thermal conductivity as high as 149 W/(m K) was achieved. The microwave sintered composites are harder and tougher than pure AlN. Microwave-sintered AlN/TiB2 composite is a promising material for structural applications in which high thermal conductivity and controlled dielectric loss are important.
I. INTRODUCTION
It is increasingly recognized that properties such as thermal and electrical conductivity can be of considerable significance for advanced ceramic composites.1,2 For example, the vacuum electronics tube industry requires vacuum-compatible, electrically insulating materials that provide superior thermal management and controlled microwave dielectric losses for component performance and reliability.3 Traditionally, the BeO/SiC composite was the material of choice for power applications in this industry. Unfortunately, BeO is toxic and long-term carcinogenic. Of all electrically insulating ceramic materials, only AlN has shown high thermal conductivity [approximately 150 W/(m K)] comparable to that of BeO and, thus, has potential to replace BeO in these applications. AlN is potentially useful due to its combination of high thermal conductivity, good mechanical properties, high electrical resistivity, and chemical and thermal stability.4 Like BeO in its homogeneous form, AlN has a low dielectric loss. Generally, to
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J. Mater. Res., Vol. 18, No. 1, Jan 2003 Downloaded: 15 Mar 2015
increase and control the dielectric loss it is necessary to add high-loss materials like SiC and TiB2 into the AlN matrix. However, including secondary phase particles into AlN matrix is likely to decrease its thermal conductivity. Thus, the microstructure must be designed to maintain high thermal conductivity along with desired dielectric loss. Thermal properties of AlN reinforced with hard particles such as SiC and TiB2 are of interest, from both practical and theoretical view
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