Rapid microwave-assisted synthesis and characterization of transition metal carbides and nitrides

摘要

The aim of this thesis is to describe the rapid microwave synthesis of a number of transition metal carbides and nitrides as well as their structural characterization and develop reproducible procedures that can cut processing times and, hence, reduce the energy consumption. Specifically, 4 binary systems are investigated: V–C, Zr–C, Hf–C and Zr–N. Carbide syntheses were conducted using either elemental or oxide precursors under argon, whereas the nitride system was investigated from zirconium powder under either nitrogen or ammonia gas.ududMicrowave syntheses were conducted using both multi-mode cavity (MMC) and single- mode cavity (SMC) microwave reactors at a power of 800 W and 1 kW, respectively, with an operating microwave frequency of 2.45 GHz. Vanadium carbide production from both oxide and elemental precursors was achieved in 6 minutes for MMC experiments and 2 minutes for SMC experiments. Zirconium carbide was obtained from zirconium powder and graphite in 20 minutes in a MMC reactor and 6 minutes in a SMC reactor. Unfortunately, the carbothermal reduction of ZrO2 to ZrC was not successful as the starting materials did not react with each other and no product formation was observed. Similar results were obtained for the carburization of HfO2. However, hafnium carbide was synthesized combining graphite with hafnium metal in 20 minutes in a MMC reactor and 6 minutes in a SMC reactor but the formation of additional oxide phases (i.e. HfO2) was also observed. Finally, zirconium nitride production was investigated in a MMC reactor and prepared in 20 minutes from zirconium metal under either N2 or NH3 gas. Generally, oxygen inclusion was observed in all experiments either in the form of oxycarbide or additional oxide phase(s).ududOnce a reproducible experimental technique was established, products were characterized by several analytical techniques. Powder X-ray diffraction (PXRD) was used to identify product phases, study the phase evolution of the microwave processes and refine the MW-synthesized structures by Rietveld method. Powder neutron diffraction (PND) was used on the V-C and Zr-C samples to evaluate product purity and the C and O occupancies of the final products. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) provided information about product morphology, particle size and purity. EDX supported the evidence for oxygen inclusion across samples. Supporting information in favour of this was additionally offered by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface of products together with the chemical state of the elements present in it.