Composite Si/C/N powder production by laser induced gas phase reactions
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Composite Si/C/N powder production by laser induced gas phase reactions E. Borsella, S. Botti, R. Fantoni, R. Alexandrescu,a) and I. Morjana) ENEA, Area INN, Dip. Sviluppo Tecnologie di Punta, C. R. E. Frascati, P. O. Box 65-00044 Frascati, Rome, Italy
C. Popescu Research Centre for Quality and Environmental Protection Siret Str. 95, Bucharest, Romania
T. Dikonimos-Makris and R. Giorgi ENEA, Area INN, Dip. Proc. Chim., Mat. non Met. e Relat. Trat., C. R. E. Casaccia, Rome, Italy
S. Enzo Dip. Chimica Fisica, University of Venice, Italy (Received 1 August 1991; accepted 20 April 1992)
Ultrafine amorphous Si/C/N ternary powders have been prepared in a CO 2 laser assisted process. We demonstrate the possibility to drive the C/N ratio in the powder by properly choosing the experimental conditions and the gaseous reactant ratio in the initial mixture containing silane, dimethylamine, and ammonia. A kinetic model that accounts for reaching the equilibrium between the gaseous reaction intermediates and the solid products is proposed.
I. INTRODUCTION CO2 laser driven synthesis of ceramic powders from gaseous reactants is currently an area of active research.1"6 In the last years extensive experimental work has been carried out with SiH4/hydrocarbons and SiH4/ammonia as principal reactants to yield silicon carbide or nitride, respectively.1"3 The replacement of SiH4 by organosilanes or halogen derivatives of silane is now under investigation4 aiming at increasing safety, lowering production costs, and improving the properties of the final powders (i.e., carbon addition to silicon nitride improves the sinterization properties). In recent experimental studies,5'6 simple hydrides have been replaced by amines as reaction partners of SiH4 and the possibility to synthesize ternary Si/C/N powders has been demonstrated. It has been shown that the chemical composition can to some extent be controlled as a function of reaction parameters (laser power, gas flow rates, reactor pressure). In previous papers,5 we have reported a detailed study of the effect of the reaction parameters on ultrafine, ternary Si/C/N powder production after CO 2 laser induced reactions between silane and dimethylamine (CH3)2NH. It has been observed that decreasing the SiH 4 /DMA (dimethylamine) ratio has the same effect on the powder final composition as increasing the total gas pressure, i.e.: (i) decrease of the O relative content,
^Permanent address: Institute of Atomic Physics, Laser Department, Bucharest, Romania. J. Mater. Res., Vol. 7, No. 8, Aug 1992
(ii) increase of the total SiN + SiC content with respect to SiO, (iii) decrease of the SiN content with respect to the total SiN + SiC content, and (iv) decrease of the free Si content. A higher productivity, decreased free Si content, and O contamination are typical features of reactions occurring at higher temperatures. Reaction temperature was increased either by lowering the total gas flow or by increasing the CO 2 laser power in the reaction volume. XRD (X-Ray Diffraction) analysis indicated t
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