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Mechanical Properties of Hot-Pressed B-BF Materials1 49 4 4% IRON 0 NO IRON AH* = 86 kcollmolr , 4 , 56 57 58 59 I/,T I 105 (OK-') Fig. 4. Arrhenius plot for SiO formation with and without catalyst for SiC-SiOl mixtures; k represents percent completiodmin from Fig. 3.Silica, and carbon. This surface-area dependence and the fact that reactions between carbon and silica or silicon carbide and silica proceed to complete vaporization of all constituents indicate gaseous intermediates in the reduction mechanism. IV. ConclusionSilica and carbon act similarly to silica and silicon carbide in producing silicon monoxide. When an excess of silica is present, i.e. the molar ratio of silica to carbon is > 1 :3, the reaction proceeds in two steps: the formation of silicon carbide by reaction with all of the available carbon and the reduction of the remaining silica by the silicon carbide formed.The following reactions occur:Initial SiOz+3C+SiC+2C0(4)Final 2SiOz+SiC-+3Si0+C0(3)Overall SiO,+C+SiO+CO (5) These reactions may result in failure of silicate refractories by volatilization at > 1 W C . Iron acts as a catalyst for the reduction reaction of silica by silicon carbide. References ' J. G. Lee and 1. B. Cutler. "Formation of Silicon Carbide from Rice Hulls." A m Ccmin. Soc Bid/.. 54 [Z] 195-98 (1975).Elastic modulus and flexural strength were measured at room temperature for solid pieces hot-pressed at 1900 to 2600 K from mixtures of B,C powder and 515 wt% B powder. Regression analysis showed that elastic modulus and flexural strength are not significantly affected by these boron additions. Elastic modulus is related to porosity and flexural strength to porosity and grain size. The fracture surface energy of boron carbide was evaluated. X-ray diffraction and chemical analyses showed that the specimens were composed entirely of boron carbide after hot-pressing. Lattice constants increased with the initial boron content.

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