Rutgers University have produced fine particles of SiC and Si3N4 by reacting SiH4, NH3, and C2H4 in a CO2 laser beam. The powders produced are typically less than 0.1 µm in diameter.26 In other work, water vapor has been reacted with aerosol droplets of alkoxides to produce either pure or mixed oxide powders of the order of 1 µm in diameter.27

Although the approaches just described are technically promising, their cost-effectiveness remains a problem. Thus it is sometimes worthwhile for the industrial scientist to revisit a long-established approach and see if improved mechanistic understanding can reveal some new opportunity for improving its efficiency. For example, it is known that silica and carbon can interact through the gaseous phase SiO, which adsorbs on the carbon and reacts to produce SiC and CO. If the carbon particles are small, this reaction can be controlled so that the size and shape of the product SiC particle is strongly determined by that of its carbon precursor. In this way, SiC particles 1 µm in diameter have been produced at 1,600°–500°C below traditional SiC production temperatures.28 Estimates indicate that this approach, scaled up, could produce submicron-sized SiC for sale at $5 to $7 per pound.

CERAMICS AND METALLURGY: METAL-MATRIX COMPOSITES

The classic challenge to the metallurgist has been to increase the strength and stiffness of metallic alloys without significantly reducing their ductility. Considerable success has been achieved with respect to strength, and ferrous alloys exhibiting yield strengths greater than 1,800 MPa and fracture toughness of 80 MPa m1/2 are now available. But economical ways of increasing alloy stiffness have been more difficult to come by. Over the past 20 years, the development of metals reinforced with graphite or ceramic fibers has been vigorously pursued, yet the resulting materials remain expensive, and the production of complex shapes from them is difficult. Joining fiber-reinforced metal components also presents problems.

However, new approaches to improving specific stiffness are emerging, driven by the need for lightweight space structures. One such approach has resulted in the new aluminum-lithium alloys now coming to market. These alloys exhibit a specific modulus more than 20 percent greater than that of aluminum. Another approach, using rapid solidification technology, has produced aluminum-base materials of complex nonequilibrium chemistry with strengths greater than 600 MPa, ductilities greater than 9 percent at 20°C, and substantial strength retention to about 350°C. One might conjecture, however, that the intrinsic chemical instability of such materials could lead to unpredictable behavior during complex operating conditions involving elevated temperatures, cyclic stressing, and active environments.

A third approach, now under development at Martin Marietta Laboratories, is based on the intrinsic stability of ceramic particles and their known ability



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