The present invention relates generally to combustion synthesis and its use in preparing aluminum nitride (AlN) as powder, platelets or porous bodies, solid solution powders or porous bodies of AlN and another ceramic material such as silicon carbide (SiC) or powdered composites or porous bodies of AlN and transition metal borides or carbides. The present invention also relates generally to polymer-ceramic composites and metal-ceramic composites prepared by infiltration, or another conventional technique, of the porous bodies, either as synthesized or as coated with a silicate material. The present invention relates more particularly to combustion synthesis of said AlN powder, platelets, composites, porous bodies and solid solutions at gaseous nitrogen pressures of less than 30 atmospheres (3.0 Mpa).
A variety of refractory ceramic materials including nitrides, carbides, borides, nitride-oxide and carbide-oxide composites are known to be produced by combustion synthesis of powder compacts. The process uses heat evolved during spontaneous chemical reactions between mixtures of solids, solids and liquids or solids and gases. A combustion wave is initiated by an ignition source and rapidly propagates through the compact. The key to self-propagating high temperature synthesis (SHS) is that once initiated, highly exothermic reactions become self-sustaining and propagate through a reactant mixture in the form of a combustion wave. As the combustion wave or front advances, reactants are converted to products. A major advantage of SHS as a process for synthesizing materials stems from energy savings associated with a self-sustaining reaction.
The combustion reaction is initiated by one of two procedures. In either procedure, the reactant mixture is usually cold-pressed, or otherwise formed, into a powder compact, typically cylindrical in shape, prior to initiation. One procedure heats a small region, normally an upper surface portion, of the powder compact with a heated tungsten coil, or other ignition source, to an ignition temperature. Following ignition of that region or portion, the combustion wave advances throughout the compact and leaves behind a desired reaction product. The other procedure heats the entire compact up to an ignition temperature whereupon combustion occurs in a substantially simultaneous manner throughout the compact in a thermal explosion.
U.S. Pat. Ser. No. 4,988,645 teaches, at column 1, lines 42-51, that solid-gas combustion synthesis reactions must take place under a gas pressure that equals or exceeds a desired product's dissociation pressure at the adiabatic combustion temperature. At column 1, lines 50-51, it teaches that some materials require high pressures and notes that AlN is formed at 14 MPa and silicon nitride (Si.sub.3 N.sub.4) is formed at 50 MPa (500 atmospheres (atm)).
U.S. Pat. Ser. No. 4,877,759 teaches, at column 1, lines 30-48, the use of solid sources of nitrogen such as sodium azide (NaN.sub.3) in combustion synthesis. It also teaches, at column 1, lines 45-48, that neither silicon (Si) nor aluminum (Al) can be combusted at one atm of nitrogen even if a solid source of nitrogen is present.