A rapid, direct nitridation process for the manufacture of sinterable aluminum nitride (AIN) powder was developed at the pilot scale. Atomized aluminum metal and nitrogen gas were heated and reacted rapidly to synthesize AIN while they passed through the reaction zone of a transport flow reactor. The heated walls of the reactor simultaneously initiated the reaction and removed the generated heat to control the exotherm. Several variations of the process were required to achieve high conversion and reduce wall deposition of the product. The fine AIN powder produced did not require a postreaction grinding step to reduce particle size. However, a secondary heat treatment, following a mild milling step to expose fresh surface, was necessary to ensure complete conversion of the aluminum. In some instances, a final air classification step to remove large particles was necessary to promote densification by pressureless sintering. The A1N powder produced was pressureless sintered with 3 wt% yttria to fabricate fully dense parts which exhibited high thermal conductivity. The powder was shown to be less sinterable than commercially available carbothermally produced powders.
The carbothermal nitridation synthesis of ␣-Si 3 N 4 is studied using electron microscopy techniques (FEG/SEM and TEM) and chemical composition analysis to characterize the reaction at various degrees of conversion. The reaction follows a nucleation-growth mechanism. Without ''seed'' ␣-Si 3 N 4 in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermediate phase. In the presence of ''seed,'' the limiting step is growth of ␣-Si 3 N 4 onto the ''seed'' nuclei. Growth appears to follow a gas-phase route and is characterized by an irregular porous layer which grows onto the ''seed.'' The porous structure is the result of reaction around carbon particles which are consumed during the process. The presence of admixed ''seed'' Si 3 N 4 in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activation energy of E = 457 ± 55 kJ/mol for the overall reaction closely approximates that previously reported for the formation of SiO. This result, along with the finding that residual crystalline SiO 2 is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO 2 . Since reaction at the carbon and SiO 2 contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO 2 with CO.
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