Combustion synthesis of AlN with melamine as an additive
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Melamine (C3H6N6) was used as an additive for synthesizing aluminum nitride by combustion synthesis. It was found that at least 0.1 M of melamine addition to 1 M of aluminum and 0.8 MPa of nitrogen pressure were necessary to initiate a stable reaction. In addition to melamine, however, use of aluminum nitride as a diluent and application of forced nitrogen flow yielded better results. Analyzing the reaction temperatures, reaction rates, nitrogen contents, x-ray diffraction patterns, and scanning electron microscopy photographs, the optimum conditions for synthesizing AlN were established: (i) reactants were 90 wt% (1 M Al + 0.1 M melamine) + 10 wt% AlN diluent, (ii) chamber pressure was 0.9 MPa, and (iii) nitrogen flow rate was 15 l/min. Under these conditions, aluminum nitride with the stoichiometric nitrogen content of 34 wt% could be obtained.
I. INTRODUCTION
Due to its high thermal conductivity, high electrical resistivity, and small dielectric constant, aluminum nitride (AlN) is considered a promising electronic packaging substrate.1–3 It is also attractive for other applications such as heat sinks, power transistor bases, and molten metal handling components. Despite its various advantages, however, AlN has not been widely used yet because of its high price and insufficient product reliability. To overcome some of the problems exposed in the conventional methods of producing AlN—the direct nitridation4,5 of aluminum and carbothermal reduction of alumina6,7—a variety of new production methods have been proposed. Some researchers have tried organochemical approaches that involve reactions between aluminum compounds and nitrogen-bearing organic materials.8,9 Others have proposed the combustion synthesis (SHS) method in which aluminum or its compounds are exothermically reacted with nitrogen.10–17 Combustion synthesis requires much less energy than other methods and demonstrates a possibility to produce purer AlN. However, both controlling the reaction process and maintaining sufficient contact between aluminum and nitrogen are very difficult. Due to the high reaction temperature, aluminum tends to melt and collapse during the earlier reaction period, effectively blocking the paths of nitrogen flow for further reactions. The pressure
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0093 J. Mater. Res., Vol. 21, No. 3, Mar 2006
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necessary for achieving a full contact between the reactants was calculated to be tens to several hundreds megaPascals by Dunmead et al.18 Since such high pressure is not so practical, many investigators have worked on the problem of reducing the necessary nitrogen pressure using additives such as carbon black, sodium nitride, ammonium chloride, urea, aluminum nitride, etc.15–17,20 However, these additives have some serious drawbacks. Carbon black is not very miscible with aluminum due to its lubricating effect and tends to fly away during the evacuation and ignition period. Sodium nitr
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