MEASUREMENT OF AEROSOLS IN A SILICON-NITRIDE FLAME BY OPTICAL-FIBER PHOTON-CORRELATION SPECTROSCOPY

Concentrated aerosols in flames can be monitored by photon correlation spectroscopy (PCS), a dynamic laser light scattering technique. In th is study the growth of silicon nitride particles in an aerosol reactor is monitored using this method. The particles are formed by heating s ilane and ammonia in a strong infrared laser beam. A flame results in which the particles grow mainly by coagulation. Kruis et al. (1993, Ae rosol Sci. Technol. 19, 514) developed a coagulation model, which is u sed to predict the development of the particle size in the flame. A si ngle mode optical fiber in combination with a polarization filter and an interference filter significantly reduced the strong background fla me light. Hence, a good signal to noise ratio was obtained and optical alignment has become easy by using the fiber. Standard PCS software h as been used to obtain diffusion coefficients and velocities of the pa rticles from the light scattering data. Using this, the particle size has been calculated. The silicon nitride particles grow in less than 2 ms from 30 to 240 nm. The characteristic sintering time that has been found from matching the coagulation model from Kruis et al. to the ex perimental results is 2.5 x 10(-15) Td(p)3 exp(53.65/T) {+/-20%} [s], where T is the temperature [K] and d, the primary particle diameter [n m]. The high value of the pre-exponential factor compared to boron car bide, which sinters according to the same mechanism, is attributed to the different molecular properties of silicon nitride.