Synthesis of Metal Carbides Using Biological Templates
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Synthesis of metal carbides using biological templates Yongsoon Shin,* Xiaohong S. Li, William D. Samuels, Yong Wang, Larry R. Pederson, Greg J. Exarhos Pacific Northwest National Laboratory, 902 Battelle Blvd, MS K2-44, Richland, WA 99354 ABSTRACT Nanocrystalline metal carbides (MC: M=Si, Ti) have been prepared using cellulose network mineralized with silica and titania by carbothermal reduction at high temperature in Ar. Hierarchical biological structures indigenous to the cellulose precursor were completely replicated after the reaction. Cubic phase MC composites show relative low oxygen content 0.24 wt% and a high lattice parameter of 4.327Å in TiC. The particle sizes of the MC composites are 200-700nm for SiC and 10-50nm for TiC, and high BET surface area, up to 150m2/g. INTRODUCTION The use of natural biological materials as templates to construct novel hierarchical inorganic materials is an emerging area because of their unique and complex microstructures [1]. Compare to artificial templates, biological materials are hierarchical, abundant, complex, renewable, and environmentally benign. So far, several types of biological materials such as diatoms [2], bacteria [3,4], pollen [5], cornstarch [6], chitin [7], and wood [8,9] have been utilized to prepare hierarchical inorganic materials. Highly anisotropic wood cellular structures are good hierarchical templates to generate novel ceramics with a micro-, meso-, and macro-structures. Much attention has been paid on the conversion of biological wood tissue to ceramic materials. For example, SiO2 [8,9], TiO2 [10,11], ZrO2 [12], Al2O3 [13], zeolite [14] were produced through solgel mineralization or infiltration into pyrolyzed wood. Mineralization of such intricate structures with metal oxides at low pH was found to replicate exactly the wood architecture in the ceramic following burn-out in air at high temperature [8]. The stable semi-crystalline cellulose networks persisted following this treatment. However, under chemically-induced mineralization at high pH, the negative replica of the structure could be formed [9]. Solution pH was found to be critical in controlling leaching of lignin from the wood and also for controlling the kinetics of the hydrolysis-condensation interfacial reactions that drive formation of the ceramic phase. In both cases, however, hierarchical structures were produced with high surface area and interconnected porosity. It is required to have carbon/metal oxide composites with homogeneously distributed metal oxides through cellulose network to produce high quality metal oxides and MC. Here we describe the synthesis of MC by the carbothermal reduction of mineralized cellulose with silica or titania. The biomorphic cellular MC ceramics were prepared by controlling the amount of silica and titania, and the size of MC nanoparticles. The biomorphic structures such as cells, lumen, and pits were mainly retained through agglomeration of nanoparticles. Nanosized MC powders (SiC: 200-700nm; TiC: 1050nm) were synthesized and TiC showed mini
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