Sinterable powders from laser driven reactions : annual report

Prepared for U.S. Department of Defense under Contract no. N00014-77-C-0581.Novel methods for producing ideal powders for fabricating Si3N ceramic parts have been investigated. The characteristics sought were principally uniformly small diameter, equiaxed, high purity particles which are free of agglomerates. Two laser processes were studied. In the first, a CO2 laser source was used to heat the: reactant gases by coupling directly to them. Silicon (Si) and Si3N4 powders have been synthesi- zed directly from SiH4 and NH 3/SiH4 mixtures respectively. The spatially well- defined reaction zone and the combination of the rapid heating rates with the short times at reaction temperatures yielded very uniform and small diameter particles. Si N powders were produced with particle diameters entirely with- in the range o 00-200 A. The particles are equiaxed and nearly spherical. The Si powders had the same general characteristics, but exhibited a slightly larger range of diameters. An analytical model gives an approximate description of the process. The reaction proceeds as a thermal reaction at thresholds which agree with measured optical absorptivities. The absorptivities are high, which will permit good efficiencies to be achieved. We were not successful in inducing a multiphoton, unimolecular reaction. The second process uses selective heating to reduce the size of over- sized particles or agglomerates as well as modifying the shape of high aspect ratio particles. This process is based on the decreasing absorption efficiency of a particle to light when its diameter is less than the wavelength cf the incident light. Light induced comminution was demonstrated. The power required for comminution is higher than was originally anticipated, but it agrees with present analytical models. While the process was demonstrated with a 150 watt CO2 laser, this power limitation precluded an orderly investigation of process variables. It appears that 750-1500 watts are required to operate continuously. These and higher power C02 lasers are commercially available. We have successfully demonstrated both processes for producing superior Si ane Si3N4 powders. The results of the direct synthesis approach are viewed as extremely important. Their uniform, small particle sizes make them unique with respect to all other powders. While demonstrated with Si and Si3N4 powders, the process appears applicable to other materials. More work is required to determine the ultimate significance of the laser commi- ntion process. It appears that the process will work as anticipated and will induce the desired size and shape changes. It is apparent that it will be an energy intensive process