Synthesis Of Carbon, Silicon Carbide And Transition Metal Carbides

Carbonaceous materials, such as carbon, silicon carbide and transitional metal carbides, have been the research hot in the field of materials science owing to their extensive applications in absorption, electrode materials, semiconductors, superconductors and ultra-hard wear-resistant materials. Based on the full investigation on the current state of the above-mentioned carbonaceous materials, we select the carbonaceous materials as the research object in this thesis. Adopting the bottom-up synthetic methodology of nanomaterials, we explore easily accessible, environment-friendly and cost-effective synthetic routes and have successfully prepared carbonaceous materials including silicon carbide and transitional metal carbide nanomaterials, honeycomb-like graphitic macroporous carbon materials and graphene-like carbon films. Relatively big breakthroughs have been made in the aspects of low-temperature synthesis of carbonaceous materials. The main work contents are summarized as follows:1. Utilizing autoclave technology,3C-SiC nanostructure has been prepared at120℃in the existence of iodine using silicon powders as silicon sources, glucose or maltose as carbon sources and magnesium powders as reducing agent. The as-obtained3C-SiC sample is the mixtures of wire-like (over70%) and tower-shaped nanostructure. TEM analysis shows that the wire-like nanostructures which are about25-100nm in diameter and several tens of micrometers in length with a zigzag structure are formed by the leaning-type packing of triangular nanosheets with the average diameter of60nm. And the tower-shaped nanostructures are formed by the level-type packing of nanosheets with the average diameter of750nm and thickness of about40nm.The terrace-like side profiles of the tower-shaped nanostructures with a decreasing size from the bottom to the tip just reflect their layered structures and thickness of about40nm for single layer.The packing directions of both wire-like and tower-shaped nanostructures are along [111].The addition of iodine in this route decreased markedly the synthetic system temperature. The role of iodine was similar to transport agent of chemical vapor transport reaction.This synthetic route provides a cost-effective and relatively mild method for preparing3C-SiC nanostructures and exhibits obvious advantage over traditional high-temperature synthetic methods. Inspired by this methodology, we attempted to develop this synthetic route into the synthesis of transitional metal carbides. Via selecting corresponding metal oxides as metal sources, we have successfully synthesized a series of carbide materials (VC, WC, NbC, TiC) with high purity. Overall, this methodology provides new routes for synthesizing carbide materials.2. Via simple solvothermal method, honeycomb-like graphitic macroporous carbon materials was prepared at600℃by one-step reaction of magnesium powders, magnesium ethoxide and ethylene glycol. The diameter of pores is about500-800nm and the average thickness of pore walls is about5nm. SEM photos display interconnected cavity channels and a part of cavities collapsed. The as-prepared macroporous carbon materials exhibit high specific surface area of54.3m2/g. Through detailed characterization of untreated product we deduced that the in situ generated MgO or MgCO3particles played as template in the formation of macroporous carbon materials. This synthetic method is easily accessible to large-scale preparation and provides new routes for industrial fabrication of porous carbon materials. Moreover, this route achieves graphitization of macroporous carbon materials at low temperature, which provides certain lessons for studies on the graphitization mechanism of carbon materials. Moreover, the as-prepared macroporous carbon materials present superior adsorption capacity for Congo red dye, indicating potential applications in water treatment.3. Graphene-like carbon films with average thickness of about4nm were prepared at500℃via hydrothermal pyrolysis of hexamethylenetetramine using magnesium as reducing agent. The experiment results showed that the generated MgO acted as in situ template. Meanwhile, we measured the electrochemical properties of graphene-like carbon films as anode materials for Li-ion batteries. The measurement result showed that the first discharge and charge specific capacity was1417mAh/g and966mAh/g, selectively. The first charge-discharge efficiency was merely68%. However, with gradual formation and stabilization of SEI films, the charge-discharge curve was almost uniform since the second cycle and the charge-discharge efficiency increased markedly. After4charge-discharge cycles, the charge-discharge efficiency was stabilized at above95%. The specific capacity was550mAh/g after60charge-discharge cycles, which was higher than the theoretical value of commercial anode graphite materials. Moreover, the as-prepared carbon films exhibited excellent rate properties by measuring the specific capacity and cycle life of this graphene-like carbon films at different current density from100mA/g to5000mA/g. Overall, high specific capacity and excellent rate performance make the graphene-like carbon films applicable in Li-ion batteries.