Experimental study of the role of hydroxyl radical in silicon dioxide particle nucleation in silane combustion using UV absorption spectroscopy.

Abstract

Gas-phase combustion synthesis (GPCS) of nanostructured materials is a powerful synthesis method, capable of generating high purity materials with controlled particle size, size distribution, morphology and composition. However, the fundamental mechanisms governing the formation of nanosized materials are not well understood, particularly the process of particle nucleation. Experimental methods and diagnostic tools, which can be used to improve our knowledge of the physical and chemical processes important in GPCS, are vital to developing combustion synthesis technologies and realizing the potential of nanostructured materials. The research presented in this work demonstrates experimental methods and diagnostic tools that are used to improve our knowledge of the physical and chemical processes that occur in GPCS. In particular, an <italic>uv</italic> absorption diagnostic is developed for use in measuring hydroxyl radical mole fractions and temperature in the harsh conditions found in GPCS systems. This diagnostic offers a non-intrusive method of making <italic> in situ</italic> measurements of key radical species in flames with high particle loadings. Additionally, a free-piston rapid-compression facility (RCF) has been modified for use in generating conditions representative of the high temperatures found in GPCS systems. The RCF is used to generate a sustained (&sim;50 ms) high temperature/pressure environment in which the early steps of SiO₂ particle nucleation and growth phenomena are explored. The test times achievable with the RCF are more than an order of magnitude greater than achievable via shock tube experiments and allow the study of particle nucleation and growth over a much longer period. The aforementioned <italic> uv</italic> absorption diagnostic is used in conjunction with RCF experiments to obtain time dependant concentrations of OH, a radical thought to play a key role in of the kinetics of SiO₂ particle formation and growth. The experimental data, combined with thermo-fluid modeling of the RCF, and chemical kinetic modeling of SiH₄ combustion, are used to further develop our understanding of particle nucleation and growth in silicon based GPCS systems.