High pressure combustion synthesis of aluminum nitride
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We report initial results on the synthesis of monolithic aluminum nitride by burning A1-A1N mixtures in high pressure nitrogen. The objective is to synthesize economically large, near-theoretical density AIN parts. In this work, we begin with compacted mixtures of 10 ^m Al and 3 nm AIN powder formed into 7.62 cm diameter by 3.81 cm thick disks having densities up to 60% of theoretical. Then, at N 2 pressures up to 180 MPa (26 000 psi), we ignite the disk on one face. The fraction of Al converted to AIN, density, and severity of macroscopic cracking vary with N 2 pressure and heat transfer from the sample. Presently, products are inhomogeneous, showing regions of relatively high porosity, regions with no porosity but with AIN in a matrix of Al, and regions of nearly theoretical density AIN.
I. INTRODUCTION
II. EXPERIMENTAL
The thermomechanical properties of aluminum nitride make it attractive for a variety of uses, ranging from microcircuit substrates to battlefield armor.1"3 Since the bonding in AIN is predominately covalent, it is difficult to sinter without sintering aids, which affect its thermomechanical properties. Large, fully-dense AIN parts can be fabricated by hot pressing AIN powders, a relatively expensive process. Combustion synthesis (also known as gasless combustion, self-propagating high temperature synthesis, or SHS) is a rapidly developing technology to produce oxides, carbides, silicides, borides, and nitrides in systems that have a highly exothermic reaction. The reactants (normally powders) are mixed intimately and ignited. The heat of reaction is adequate to ignite nearby, unreacted regions while sustaining heat losses from the system. The important advantages of the process are high purity products and low energy consumption. Some problems encountered in forming monolithic parts (as opposed to powders) using combustion synthesis are porosity, less than full reaction, and cracking owing to thermal shock. This research is a continuation of the work of Dunmead et al.,4 in which the final part density and fraction reacted were studied as a function of N 2 pressure, green density, and dilution by AIN. Their experiments were made on right cylinders 1.9 cm diameter and about 2 cm long and demonstrated that it is possible to obtain regions of fully reacted AIN, regions having greater than 90% theoretical density, and regions of A1N-A1 cermet. The objective of this work is to fabricate large parts with densities greater than 90% of theoretical to be used in ballistic tests. We report the results of experiments to make 7.62 cm diameter, 3.81 cm thick disks using combustion synthesis under high N 2 pressure.
A. Materials We used the same Al and AIN as Dunmead et al.4 The Al powder is from Alcoa Co., with an average particle size of 9.9 ^m, a BET surface area of 1.10 m2 g"1, and major impurities of Fe (100 ppm) and Si (30 ppm). The AIN is from Advanced Refractories Technology, with an average particle size of 2.9 ^m and a BET of 2.53 m2 g - 1 . Major impurities were Fe (600 ppm) and Si (200 ppm). A ne
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