A novel process for combustion synthesis of AlN powder
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A SHS process has been established for synthesis of AIN powder under low nitrogen pressures. Al and NaN3 powders were used as the metal and nitrogen sources, respectively. The compact of the mixture of the reactants was wrapped up with aluminum foil and then wrapped up with an igniting agent (i.e., Ti + C). The synthesis reaction was triggered by the combustion of the igniting agent. The wrappings were found necessary in achieving high product conversions under low nitrogen pressures ( < 1 MPa). The product conversion was also affected by the reactant composition and the nitrogen pressure. High conversions were obtained when the mass ratio of Al to NaN3 was 1/2 or lower and the nitrogen pressure was 0.5 MPa or higher. The AIN powders as synthesized were observed to have two major types of morphology, i.e., granular particles with 0.5-3 fim in diameter and fibers with aspect ratios of 10-800.
I. INTRODUCTION Recently, AIN has become a very important ceramic material in industrial applications due to its high thermal conductivity, electrical resistivity, excellent mechanical strength, and good oxidation and thermalshock resistance.1-2 It has been considered for many applications, such as electronic substrates, cutting tools, heat sinks, and many high-temperature structure components.3'4 Conventional methods5"7 for manufacturing AIN powder often require heating at elevated temperatures for long periods of time. In addition to high energy consumptions, the production rates are slow. In this work, we investigate the use of the combustion synthesis (SHS) method for the production of AIN powder. The SHS method, originally developed in the former USSR,8 has been applied to synthesis of various materials including ceramics, intermetallics, and composites. It has many potential advantages such as low processing cost, energy-efficiency, and high production rate.9 However, relatively little effort has been devoted to the study of the SHS method for the synthesis of AIN powder.10 A major problem encountered in the synthesis is that the metal (Al) particles melt and coalesce during reaction which inhibits diffusion of nitrogen from outside the metal compact due to collapse of the pore openings. A high pressure of N 2 gas is thus favorable to achieve high conversions when using N 2 gas as the nitrogen source.11"12 Recently, several other investigators13'14 demonstrated that use of solid-state nitrogen sources (e.g., metal azides) can significantly decrease the N 2 pressure a) Author
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http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 3, Mar 1995
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required for the synthesis of many nitride ceramic powders. The solid-state nitrogen source suitable for the synthesis must be an easily thermally decomposable compound in order that the synthesis reaction can proceed in a self-propagating combustion mode. A problem that must be considered in designing a SHS process is how to retain the nitrogen (produced by the decomposition) and how to make it reac
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