Advances in nuclear reactor technology and the use of gas-cooled fast reactors require the development of new materials that can operate at the higher temperatures expected in these systems. These include refractory alloys based on Nb, Zr, Ta, Mo, W, and Re; ceramics and composites such as those based on silicon carbide (SiC f -SiC); carbon-carbon composites; and advanced coatings. Besides the ability to handle higher expected temperatures, effective heat transfer between reactor components is necessary for improved efficiency. Improving thermal conductivity of the materials used in nuclear fuels and other temperature critical components can lower the center-line fuel temperature and thereby enhance durability and reduce the risk of premature failure.Silicon carbide (SiC) is an important ceramic, most commonly used because of its excellent properties such as high strength, high modulus, excellent creep resistance and its high temperature stability. Moreover, crystalline silicon carbide is attracting wide attention as a promising candidate for several applications in nuclear reactors owing to its high thermal conductivity, high melting temperature, good chemical stability, and resistance to swelling under heavy ion bombardment. There have been many efforts to develop SiC based composites in various forms for use in advanced energy systems. However, fabricating SiC based composites by traditional powder processing route generally requires very high temperatures for pressureless sintering. In recent years, with the development of high yield preceramic precursors, the polymer infiltration and pyrolysis (PIP) method has aroused interest for the fabrication of ceramic based materials. Polymer derived ceramic materials offer unique advantages such as ability to fabricate net shaped components, incorporate reinforcements, along with lower processing temperatures, and perhaps most importantly relatively easy control over the microstructure. The raw materials are element-organic polymers whose composition and architecture can be tailored and varied.The primary focus of this work is to use a pyrolysis-based process to fabricate a host of novel silicon carbide-metal carbide/oxide composites, and to synthesize new materials based on mixed-metal (metal/silicon) carbides that cannot be processed using conventional techniques. These mixedcarbide material systems are expected to offer improved material properties for higher-temperature applications. Our fabrication technique resulted in both amorphous and nano-grained SiC matrix composites by controlled pyrolysis of allylhydridopolycarbosilane (AHPCS), the precursor for SiC for our study, under inert argon atmosphere.Cylindrical pellets, φ25 × 25 mm, were fabricated for bulk scale characterization. These samples were analyzed in terms of density and porosity, thermal conductivity, and flexural strength under multiaxial loading conditions. The final composition in the fabricated composite was studied use Xray diffraction (XRD). This was useful in understanding the relative stabil...