Synthesis of low dimensional boron nitride nanomaterials for thermal management applications

This thesis investigates the synthesis of boron nitride (BN) nanomaterials using the chemical vapour deposition (CVD) technique, in order to propose an efficient means of generating high-quality BN nanomaterials of different morphologies whose properties can be controlled, so that they can be optimised for real-world applications, particularly for thermal management. It is found out that the growth of BN nanotubes (BNNTs) followed a tip-growth mechanism and this was confirmed by varying the duration of reaction studied through scanning electron microscopy (SEM). This study shows that the powder precursor of boron and magnesium oxide generated gaseous boron oxide and magnesium that was found to be deposited on the surface of the SiO2 target substrate in the form of magnesium borate. This observation was investigated by thermal gravimetric analysis (TGA), energy-dispersive x-ray spectroscopy (EDX), and X-ray photoelectron (XPS) depth profiling. The lower melting point of magnesium borate compared to the synthesis temperature enabled the formation of catalyst droplets that was found to be crucial to generating the nanotubular structure. Hindering the formation of catalyst droplets resulted in the synthesis of BN nanosheets. Hybrid one-dimensional nanostructures such as BNNTs coated with CNT were synthesised by adding methane as carbon source in the synthesis. The properties of BN nanomaterials can be tailored by controlling their morphology that has been observed by the increase of thermal conductivity of BN nanotubes compared to BN nanosheets at the same weight percent in isopropanol that was measured using the transient plane source (TPS) method. These controllable properties make BN nanomaterials a promising material that can be used as fillers in composites and liquid solutions for thermal management and other applications. Performing Density Functional Theory (DFT) calculations showed insights regarding BN bond formation and it was found out that a small difference of energy barrier was present between the synthesis with and without the catalyst droplet, from which it can be inferred that BN bond formation can still occur in the absence or presence of the catalyst droplet. BN bond formation occurring on the MgO (100)-(111) step-edge surface was shown to have more stable energy than those on the MgO (111) facet surface, which led to the development of zigzag chirality. These theoretical findings are found to be in good agreement with the empirical results. Lastly, it was found out that the activation energy for BNNT growth is 58.88 kcal/mol by correlation of the growth rate and the Arrhenius equation.