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Dajie Zhang and Dennis C. Nagle Advanced Technology Laboratory, Johns Hopkins University, Baltimore, Maryland 21211

Caitlin E. Feeser Department of Chemical Engineering, Widener University, Chester, Pennsylvania 19013 (Received 2 July 2007; accepted 19 October 2007)

Fully dense and net-shaped silicon carbide monoliths were produced by liquid silicon infiltration of carbon preforms with engineered bulk density, median pore diameter, and chemical reactivity derived from carbonization of crystalline cellulose and phenolic resin blends. The ideal carbon bulk density and minimum median pore diameter for successful formation of fully dense silicon carbide by liquid silicon infiltration are 0.964 g cm−3 and approximately 1 ␮m. By blending crystalline cellulose and phenolic resin in various mass ratios as carbon precursors, we were able to adjust the bulk density, median pore diameter, and overall chemical reactivity of the carbon preforms produced. The liquid silicon infiltration reactions were performed in a graphite element furnace at temperatures between 1414 and 1900 °C and under argon pressures of 1550, 760, and 0.5 Torr for periods of 10, 15, 30, 60, 120, and 300 min. Examination of the results indicated that the ideal carbon preform was produced from the crystalline cellulose and phenolic resin blend of 6:4 mass ratio. This carbon preform has a bulk density of 0.7910 g cm−3, an actual density of 2.1911 g cm−3, median pore diameter of 1.45 ␮m, and specific surface area of 644.75 m2 g−1. The ideal liquid silicon infiltration reaction conditions were identified as 1800 °C, 0.5 Torr, and 120 min. The optimum reaction product has a bulk density of 2.9566 g cm−3, greater than 91% of that of pure ␤–SiC, with a ␤–SiC volume fraction of approximately 82.5%.

I. INTRODUCTION

The conversion of porous carbon preforms to dense SiC by liquid Si infiltration (LSI) can become a low-cost and reliable method to form SiC components of complex shape and high density. This is achieved by reacting micron-porous carbon preforms with desirable density and geometry [Fig. 1(a)] with liquid Si [Fig. 1(b)]. The reduction in surface energies of both the liquid Si and solid carbon leads to reactive wetting and infiltration of porous carbon by liquid Si. The free energy reduction associated with the highly exothermic reaction, T艌TM = 1414 °C

Si共l兲 + C共s兲 → SiC共s兲 , ⌬G°f = 共−27100 − 2.73T log T + 18.1T 兲 cal mol−1

leads to the formation of SiC. It is of special significance that with well-controlled reaction parameters such as carbon microstructure and Si infiltration temperature, the macrostructure and geometry of the carbon preforms can be retained during the SiC formation process [Fig. 1(c)]. In this article, we describe a new method of creating porous carbon preforms in an effort to better engineer the carbon microstructure to minimize the amount of residual carbon and Si phases in the LSI reaction product. We also identify carbon atomic ordering as a key factor determining the carbon chemical reactivity with liquid Si, and prelimin