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Title: Low temperature synthesis and characterisation of novel complex carbide- and boride-based materials
Author: Liu, C.
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2019
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This thesis work dealt with four novel types of carbide- and boride-based materials which are considered to be candidate materials for a variety of important applications. They were synthesized by using a low temperature "universally" applicable molten salt synthesis technique, and characterised systematically. In the first part of this thesis work, phase pure well-dispersed Al8B4C7 particles with the average size of about 200 nm were synthesized from Al, B4C and C after firing in NaCl-NaF at 1250 °C for 6 h. Under the optimal condition, Al initially diffused rapidly through the molten salt onto the surface of C to form Al4C3, and also diffused through the salt rapidly onto the surface of B4C and reacted with it to form Al3BC and AlB2. B from the decomposition of AlB2 and Al3BC also slightly dissolved in the salt, diffused onto the surface of Al4C3 formed earlier (from the Al-C reaction and decomposition of Al3BC), and reacted with it to form Al8B4C7. Compared to the conventional synthesis techniques, the synthesis temperature in the present case was about 500 oC lower, which was attributable to the great accelerating-effect of the molten salt containing NaF. In the second part of this thesis work, molybdenum aluminum boride (MoAlB) fine powders were synthesized from Al, B and Mo in molten NaCl. The effects of key processing parameters on the phase evolution and morphology of product powder were investigated and the relevant reaction mechanisms discussed. As-prepared MoAlB 2 particles exhibited three different morphologies: rounded particles (1~3 μm), plate-like particles (<5 μm in diameter) and columnar crystals with various lengths (up to 20 μm) and diameters (up to 5 μm), resultant from different reaction routes. The optimal synthesis condition for synthesis of phase pure MoAlB was: using 1.4 times excessive Al and firing at 1000 oC for 6 h. This synthesis temperature was much lower than required by other synthesis techniques. Under the optimal condition, Mo initially reacted with Al and B, forming respectively Al8Mo3 and MoB which further reacted with excessive Al to form MoAlB and Al-rich Al-Mo phases (such as Al4Mo). The latter further reacted with the residual B and form secondary MoAlB. The molten salt played an important role in the whole synthesis process by improving the mixing between the reactant species and facilitating their diffusion processes. The third part of this thesis work deals with nanocarbon supported tungsten carbide nanocatalysts for hydrogen generation. By using WO3, Mg and C as starting materials and KCl as reaction media, tungsten carbides (WC with W2C) nanoparticles (< 5 nm) were in-situ formed/anchored on nanosized carbon black (CB) and carbon nanotube (CNT). Owing to this special hybrid structure, both the exposed surface area of active species and the electrical conductivity of the catalysts were increased effectively, making the catalysts perform considerably better in HER than pure WC and WC based catalysts prepared via other conventional routes. WC nanocrystals in-situ formed/anchored on CNTs showed small overpotential (90 mV), low Tafel slope (69 mV dec-1), high current density (93.4 and 28 mA cm-2 at 200 and 300 mV, respectively) and excellent stability 3 (remaining stable even after 3000 cycles). Such a performance is one of the best among those of WC based electrocatalysts developed to date. We demonstrate here significantly improved HER performances of inexpensive tailored WC materials, along with a facile synthesis strategy which could be also readily extended to prepare a range of other types of mono-dispersed nanocatalysts for more potential applications. In the final part of this thesis work, novel 2D SiC nanosheet (SNS), ZrC nanosheet (ZNS), and SiC- and ZrC-coated graphite nanoplatelets were successfully prepared at relatively low temperatures. The effects of processing parameters such as firing temperature, time, and salt on the reaction/synthesis processes were examined, and the relevant mechanisms proposed. In all the cases, Si or Zr slightly dissolved in the molten salt and diffused rapidly through it onto the surface of graphite nanoplatelet, and then reacted in-situ to form SiC or ZrC which retained the morphology and size of the original graphite nanoplatelet, i.e., a template growth mechanism had functioned in all the cases and the original graphite nanoplatelet acted as the template. In the initial stage, as no barrier layer was built up on the surface of the graphite nanoplatelet, so the reaction was rapid. However, with the enhancement in the reaction extent, more and more carbide product would be formed, leading to the formation of a carbide barrier layer the remaining graphite nanoplatelet. Then, the reaction process would be controlled by the diffusion process of the metal and/or carbon. Nevertheless, as the molten salt medium could improve the mixing between the metal and the graphite nanoplatelet, and accelerate the diffusion process of the metal dissolved in it, the overall reaction still remained very 4 rapid, as verified by the much lowered synthesis temperature, especially in the case of ZrC formation where Zr has a sufficiently "high" solubility in the molten salt (1250 and < 850 oC respectively in the cases of SiC and ZrC formation). By controlling the ratio between the metal and graphite nanoplatelet, both carbide nanosheets and carbide-coated graphite nanoplatelet/graphene nanosheets could be readily synthesized.
Supervisor: Zhang, S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available