Novel Molten Salt Synthesis of ZrB2 and ZrC powders and Molten Salt Synthesis of Novel TiC

Abstract

Pure submicron-sized zirconium diboride (ZrB2) powder was synthesised via a molten salt mediated reduction route using ZrO2, Na2B4O7, and Mg powders as the starting raw materials and MgCl2 as the reaction medium. By using appropriately excessive amounts of Mg and Na2B4O7 to compensate for their evaporation losses, ZrO2 can be completely converted into ZrB2 after 3 h of firing at 1200 C. This synthesis temperature is 100-500 C lower than that of other conventional synthesis techniques. In addition, the formation of undesirable Mg3B2O6 can be effectively suppressed. To a large extent, the prepared ZrB2 particles preserved the shapes and sizes of the original ZrO2 particles, indicating a template growth mechanism for their formation in which ZrO2 functions as the reaction template. Using this developed synthesis method, submicron-sized and nanosized zirconium carbide (ZrC) powders were synthesised in the reaction system of ZrO2-Mg-carbon black using NaCl-KCl as the reaction medium. The synthesis temperatures were 950 and 850 C for the former and the latter powder size, respectively, which are much lower temperatures than those used in most of the reported methods. Compared with the submicron-sized ZrO2 powders, the finer ZrO2 particles considerably enhanced the reaction rate and thus the completion of the reaction at a lower temperature. The resulting ZrC particles exhibited two different morphologies: one retained the shapes and sizes of the original ZrO2, and the other retained those of the starting carbon black, suggesting that both ZrO2 and carbon black had acted as reaction templates. In addition, the 2D-nanostructure of a non-layered structure material, titanium carbide (TiC), was fabricated. The novel TiC nanosheets (TNS) and TiC-coated graphite nanosheets (TCNS) were produced at 950 C for 8 h and 900 C for 5 h, respectively, in KCl molten salt using graphite nanosheets (GNS) as both a carbon source and reaction templates. The produced TNS and TCNS retained the shapes, sizes and thickness of the original GNS to a high degree, indicating that the GNS had acted as the reaction template. For TCNS, a lower molar ratio of Ti/C required a lower synthesis temperature and/or a shorter holding time. This effective processing technique was also employed to produce TiC foams at 1050 C for 4 h using carbon foam as the reaction template. This synthesis temperature is significantly lower than that (>1450 °C) used in most of the other techniques. The resultant pores were clear of any undesired phases such as impurities and/or membranes, and the cell-networks were free of surface cracks and holes. The cell-networks, pore sizes, and cell sizes of the synthesised foams were well defined by those of the original carbon foam, suggesting a template growth mechanism of the formation of the TiC foam.