Kinetics and mechanism of laser-driven powder synthesis from organosilane precursors
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Laser-driven synthesis of refractory powders from the organosilicon compounds ((CH3)3Si)2NH and ((CH3)3Si)2O has been studied as prototypical of powder syntheses from large molecules. A cw CO2 laser is used for powder syntheses and pulsed TEA CO2 laser-driven homogeneous pyrolysis is used to study the initiation chemistry. Initial decomposition of the organosilanes by Si-C bond cleavage initiates free radical chain reactions which produce organosilicon polymers and subsequent refractory phases. A qualitative model is proposed which provides a framework for understanding the parameters influencing powder synthesis from large molecules.
I. INTRODUCTION The application of lasers to chemical processing is currently an area of active research.1 Preparation of solid powders from gaseous reactants using laser-driven thermal reactions is a laser-driven chemical process which shows particular promise.2"4 Powders made by the method apparently meet most of the criteria of "ideal" powders, for the preparation of superior quality technical ceramics, and for the reduction of sintering temperature and/or times.5'6 An ideal powder 7 is defined as fine, pure, equiaxed, and nonagglomerated with a narrow size distribution. Such a powder packs well with a large number of particle-particle contacts, and should sinter to near theoretical density. Lasers, and particularly CO2 lasers, offer several advantages in driving thermal processes. High temperatures are obtained readily, heating and quenching are fast, and there is control over the time scale of the reactions. Additionally, the hot zone is confined to the interaction region of gaseous reactants and the laser beam, and does not contact any surfaces. This eliminates reactor corrosion, contamination of products through such corrosion, and the need for a high temperature furnace. The best-developed powder synthesis chemistry with lasers employs silane as the principal reactant, alone to yield silicon, or with ethylene or ammonia to yield silicon carbide or nitride, respectively.2 While use of binary hydrides in laser synthesis could be extended to other maingroup elements, transition metals (for which volatile hydrides are known) offer intriguing possibilities for the synthesis of novel refractory compounds. Organometallic molecules, in which organic ligands confer volatility, afford the opportunity to pursue laser-driven synthesis of metal-containing materials. The pyrolysis chemistry of such species is necessarily more complex than that of sima)Current
address: Research Department, Engelhard Corporation, Edison, New Jersey 08818.
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J. Mater. Res., Vol. 4, No. 6, Nov/Dec 1989
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pie hydrides due to myriad available reaction pathways. Our initial studies8 have employed the organosilicon compounds (Me3Si)2NH (HMDS) and (Me3Si)2O (HMDSO) as prototypical species for laser pyrolysis of large molecules. The early reaction chemistry of these molecules has been studied in a static reactor using a pulsed TEA CO2 laser for infrared mu
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