Aerosol Synthesis of Aluminum Nitride Powders
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AEROSOL SYNTHESIS OF ALUMINUM NITRIDE POWDERS
ALBERT A. ADJAOTTOR AND GREGORY L. GRIFFIN Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803
ABSTRACT We describe a new laboratory-scale aerosol process for producing AIN powder. A two-stage reactor design is used. In the first stage, triethyl aluminum (TEA = AI(CCH ).) 2 5 and NH 3 react to form an aerosol adduct in a laminar flow diffusive mixing zone. The aerosol then enters the furnace stage, where it is converted to AIN. We have examined the influence of the major operating variables (e.g., inlet TEA concentration, reactor residence time, and furnace temperature) on the particle size and distribution, yield, and efficiency. For example, increasing the TEA concentration from 0.12 to 1.30 pImol/cm' causes an increase in the mean particle diameter (from 0.07 to 0.13 Pim), a slight increase in polydispersity (from 0.31 to 0.43), and a decrease in yield efficiency (from 90% to 73%). In contrast, decreasing the reactor residence time (by increasing the flow rate) has little effect on mean particle diameter, but causes a significant increase in yield efficiency (approaching 100%). The overall behavior of the reactor suggests a model in which the particle size distribution of the final product is determined primarily by the aerosol formation steps in the mixing stage (i.e., nucleation, growth, and coalescence), while the composition and crystallinity of the product are determined by furnace conditions.
INTRODUCTION Aluminum nitride is a non-oxide high-tech ceramic material that has recently generated considerable interest within the ceramic community 11]. Because of its high thermal conductivity, AIN is currently the subject of intense international research as a replacement for A12 0 3 in heat-dissipating substrates for microelectronic device packaging. If sintered to high density in transparent form, the material has potential applications in the area of electrooptics. Because of its excellent corrosion resistance properties, AIN is also a potential structural material, in either monolithic or composite form, for applications such as heat engine parts, ballistic armor, and refractory crucibles. Aluminum nitride powder is currently manufactured commercially using either the direct nitridation of aluminum in the presence of N2 or NH 3, or by the carbothermal reduction of A12 0, in the presence of N 2 [1]. The former procedure involves reacting aluminum powder with N 2 gas at temperatures above 1473K. Carbothermal reduction involves reacting finely mixed A12 0 3 and carbon powder in a N2 -containing gas at temperatures between 1373-2023K; an intermediate oxidation step at 873-973K may be used to remove unreacted carbon. Both of these processes are usually performed as batch operations, which are generally viewed as being less efficient and subject to greater quality control problems than continuous operations. Both processes are also limited by the impurity level and morphology (i.e., particle size, shape, and uniformity) of the starti
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