Chemical Synthesis of Aluminum Nitride Powders
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CHEMICAL SYNTHESIS OF ALUMINUM NITRIDE POWDERS PHILLIP R. COFFMAN%, WILLIAM T. PETUSKEY*, AND SANDWIP K.DEY** *DEPARTMENT OF CHEMISTRY "DEPARTMENT OF CHEMICAL, BIO, AND MATERIALS ENGINEERING ARIZONA STATE UNIVERSITY, TEMPE, AZ 85287 ABSTRACT An organoaluminum precursor was synthesized by a low temperature reaction and characterized by NMR, FTIR and mass spectrometry. This compound was pyrolyzed under several experimental conditions to obtain aluminum nitride powders. The pyrolyzed products were subsequently characterized by XRD and SEM. The mechanisms of precursor development and subsequent pyrolysis are discussed. INTRODUCTION Aluminum nitride, AIN, is a ceramic being developed for use inthe electronics industry as a substrate material for VLSI packaging (1]. The current method of substrate production involves dispersing AIN powders ina slurry, tape casting into green sheets, electroding, stacking and co-firing. The powders used inthis process are usually synthesized by the direct nitridation of aluminum or the carbothermal reduction of alumina. There are several detriments to these powders; they contain residual reactants (alumina, carbon, or aluminum) and they have large and irregular particle sizes, making them difficult to densify. The synthesis of AIN powders by chemical methods offers the distinct advantage of being able to control the morphology and size distribution of the powder. Several groups have made aluminum nitride powders by the pyrolysis of organometallic precursors [2-4]. The precursor chosen for pyrolysis inthis study was dimethylaminoalane, AIH 2 N(CH 3)2. Itwas first synthesized by Wiberg and May [5] in1955 and then by Ruff and Hawthorne in 1960 [6]. Itis formed inhigh yields (63-89%) from the reaction of lithium aluminum hydride, LiAIH 4, and dimethylamine hydrochloride, [NH 2 (CH 3 )2 ]CI. LiAIH 4 + [NH 2 (CH3 )2 ]Cl
---- >
AIH 2 N(CH 3 )2 + 2H2 + LiCI
Dimethylaminoalane was reported by Wiberg to have a melting point of 89 -901C, to decompose at around 1300C,and to sublime under vacuum at 40- 600C. There is a discrepancy between Wiberg and Ruff as to what is the degree of association insolution. Indiethylether, Wiberg stated that it was 2.16, and inbenzene 2.36. Ruff reported that the degree of association is 2.99 inbenzene. Amass spectral study was done of this compound and it was found to be trimeric inthe gas phase [7]. There are a number of reasons why dimethylaminoalane was selected as the precursor to aluminum nitride. First, aluminum-nitrogen bonds are formed quite readily at a low temperature. The ease of formation of amine adducts to aluminum is AI-H > Al-C >AI-X where Xis a halogen. Dimethylaminoalane is quite volatile and thus can be easily purified by sublimation. The theoretical ceramic yield of aluminum nitride from dimethylamionoalane is 56.1%. Insolution, the trimeric form already has formed the six-membered ring that is found inthe wurtzite structure of aluminum nitride (Fig. 1). Consequently, polymerization at low temperatures followed by pyrolysis could be a facile, alt
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