The deposition mechanisms of boron from boron trichloride and hydrogen vapors are investigated by a stagnation flow technique. The characteristics of gas flow are first deduced from experimental deposition rates. Then, using the Stefan-Maxwell equations for diffusion, the interfacial conditions of deposition, equilibrium, or supersaturation are inferred from experimental rates of deposition in the stagnation zone. Three mechanisms of deposition are deduced, mass transfer-interracial equilibrium, mass-transfer-surface kinetics, and surface kinetics. They occur as a function of three experimental parameters; temperature, mass flow rate, and inlet composition BC13/H2. A correlation for mass transfer in impinging jets is obtained.

Compact coatings Of amorphous boron were deposited on the smooth surface of substrates such as graphite, refractory metals, iron, and stainless steel using the reduction of boron trichloride by hydrogen. With a constant surface temperature and a low deposition efficiency, a constant thickness is obtained if transport limitations are avoided at all points of the surface. Extension of the results is proposed for a deposition on larger surfaces and substrates of complex shape. The rate of deposition was studied at temperatures ranging from 950 ~ to 1200~ The apparent activation energy was found equal to 31.4 kcal/mole. Adherence and absence of cracks are a function of the specific nature of the substrate. The most satisfactory coatings were obtained on graphite, refractory metals of Group Vb, iron, and stainless steel. On iron and stainless steel, a diffusion barrier was first deposited by pack cementation, which avoids the transport of the metals and slows down the boron diffusion. Boron morphology regularity is shown to be a function of nucleation.The rate at which a substance is deposited from chemically reactive vapors can be governed by either chemical kinetics or mass transport of reactants and products. The relative rate of transport to and from the reaction surface and the rate of chemical conversion in turn determine local concentrations and supersaturation which may influence deposit morphology. The relative importance of the diffusional and kinetic factors during chemical vapor deposition of boron by hydrogen reduction of boron trichloride on a hot filament was studied in detail by P. E. Gruber (1) and H. E. Carlton et al. (2). Generally a horizontal resis~t -ance heated wire is used. In this case, mass transport in the vapor phase is important whatever the geometry of the deposition cell. It is always eclual to or greater than the mass transport by natural convection, and up to 1050~176 the deposition rate is governed by chemical kinetics (1-3).On a massive substrate, the rate of deposition is generally limited by mass transport at these temperatures and crystals of boron nucleate very rapidly (4). To avoid this crystallization, a deposition cell was built to produce a stream of reactants flowing at high velocity normal to the center of the substrate, then flowing at high velocity in a parallel direction. In this case, it is very difficult to specify mass transport properties, but it was possible to increase experimentally the mass transfer by increasing the mass flow rate. The rate of mass flow was determined by the rate of reactants metered with flowmeters and by the shape and size of the deposition cell. In all cases where the deposition rate is governed by chemical kinetics at all points of the substrate, uniform coatings of "amorphous" boron are produced. Experimental TechniquesThe experimental set up is shown in Fig. 1. The details of the deposition cell are shown in Fig. 2. The apparatus is made of Pyrex glass except for the reactor which is made of silica. The desired flow rates of B C13 and H2 a...

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