Mechanism And Characterization Studies on Boron Carbides Deposited by Chemical Vapor Deposition Technique
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Mechanism And Characterization Studies on Boron Carbides Deposited by Chemical Vapor Deposition Technique Mustafa Karaman, H. Önder Özbelge, N. Aslı Sezgi, Timur Doğu Department of Chemical Enginering, Middle East Technical University 06531, Ankara, Turkey ABSTRACT Boron carbide was produced in an impinging jet CVD reactor from a gas mixture of BCl3, CH4 and H2. The mass transfer limitations on the reaction kinetics were minimised by the jet impingement on the substrate surface. XPS characterization of the produced deposits revealed a nearly pure boron carbide phase containing small amounts of oxy-boron and oxy-carbon species. After a detailed kinetic study, a reaction model was proposed to predict the rates of boron carbide and dichloroborane formation reactions. In this model, boron trichloride is adsorbed on the surface non-dissociatively, whereas hydrogen and methane are adsorbed dissociatively. BC is formed on the solid surface through the reaction of adsorbed boron trichloride with adsorbed methane in the form of CH3(s) (adsorbed CH3 on the surface). Produced BC is reacted in successive series reactions including adsorbed boron trichloride and adsorbed hydrogen. In the proposed mechanism, dichloroborane is produced only through the gas phase reaction between boron trichloride and hydrogen. The simultaneous fit of the experimental data to the model expression gave good fits for the boron carbide and dichloroborane formation reactions with the high correlation coefficient values. INTRODUCTION Boron carbide is a special material with outstanding mechanical, physical and chemical properties. It has high elastic modulus, low density, and high resistance to both alkali and acids. It has high neutron capture section owing to high boron content in its structure. A major part of nuclear power is produced in reactors controlled by boron carbide based absorbing materials [1]. It is one of the hardest material in nature, surpassed by diamond and cubic boron nitride. However, the hardness of diamond and cubic boron nitride gradually decreases as temperature rises. Boron carbide keeps its hardness at elevated temperatures and in fact it is the hardest material above 1100oC [2]. Considering its high-temperature stability, large Seebeck coefficient and low thermal and high electrical conductivity, boron carbide could find potential use as hightemperature thermoelectric material for energy converters [3]. EXPERIMENTAL DETAILS The experimental work was carried out to produce boron carbide on tungsten substrates from a gas mixture of BCl3, CH4 and H2. Experimental set-up is shown in Figure 1. The most important part of the set-up is dual impinging-jet reactor, which was utilised to minimise the effect of mass transport limitations on the reaction kinetics. Reactant gas mixture is fed to the reactor through the two orifices having 1 mm diameter each. Substrate was held hanging in the middle of the reactor between the two electrodes, upper one was fixed and lower one was dipped into mercury pool. Weight of the lower
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