Browsing by Author "Nel, J.T."

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    Influence of the substrate gas-inlet gap on the growth rate, morphology and microstructure of zirconium carbide films grown by chemical vapour deposition
    (Ceramics International, 2017) Biira, S.; Alawad, B.A.B.; Bissett, H.; Nel, J.T.; Ntsoane, T.P.; Hlatshwayo, T.T.; Crouse, P.L.; Malherbe, J.B.
    The influence of the gap between the gas inlet and the substrate in an in-house developed thermal chemical vapour deposition (CVD) reactor, on the growth rate, surface morphology, phase composition and microstructure of deposited ZrC films was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The ZrC films were grown on high density graphite substrates at different substrate-inlet gaps, viz. 70 mm, 90 mm, 120 mm, 145 mm and 170 mm, at substrate temperatures of 1200 °C and 1400 °C. The growth rate of ZrC films prepared at 1400 °C was observed to be higher than at 1200 °C., and was found to decrease with increase in substrate-inlet gap at both temperatures. The boundary layer thickness increased with an increase in substrate-inlet gap. The diffusion coefficients of the reactants were found to be 0.176 cm2/s and 0.200 cm2/s for the ZrC films deposited at 1200 °C and 1400 °C respectively. A model illustrating the diffusion of source materials through the boundary layer to the reacting surface was also given. The XRD results of ZrC films showed that at both 1200 °C and 1400 °C the (111) plane was the less preferred orientation, while (200) and (220) were the preferred planes. The degree of preferred orientation of ZrC films was found to decrease with increasing substrate-inlet gap. SEM results indicated that as the substrate-inlet gap increased from 70 mm to 170 mm for 1400 °C, the films became more uniform with increased particle agglomeration. The cauliflower like clusters of particles grew larger in size and covered the whole surface. By contrast, at 1200 °C the surface crystallites had complex facets that decreased in size as the substrate-inlet gap increased from 70 mm to 170 mm.
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    The role of ZrCl4 partial pressure on the growth characteristics of chemical vapour deposited ZrC layers
    (Ceramics International, 2017) Biira, S.; Crouse, P.L.; Bissett, H.; Hlatshwayo, T.T.; Njoroge, E.G.; Nel, J.T.; Ntsoane, T.P.; Malherbe, J.B.
    ZrC layers were deposited in a chemical vapour deposition (CVD) reactor on graphite substrates using a ZrCl4- Ar-CH4-H2 precursor mixture. The deposition was conducted at different ZrCl4 partial pressures at a constant substrate temperature of 1400 °C for 2 h at atmospheric pressure. The deposited ZrC layers were characterised using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The effect of ZrCl4 partial pressure on the growth rate, microstructure and surface morphology of the deposited layers was studied. The ZrCl4 partial pressure was manipulated by changing the flow rate of the argon carrier gas through the sublimation chamber. The boundary layer thickness decreased as ZrCl4 partial pressures increased due increased argon flows. The increased ZrCl4 partial pressure increased the growth rate of ZrC layers linearly. It was found that the transport process of the source materials was laminar and forced convection flow. The flow process of source materials through the boundary layer to the reacting surface was also illustrated using a model. The average crystallite size increased with ZrCl4 partial pressures, whereas the lattice parameter, lattice strain and dislocation density decreased as ZrCl4 partial pressure increased. The surface morphology of the asdeposited ZrC layers varied with the ZrCl4 partial pressure. The size of crystals grew larger and the cavities surrounding them decreased in number and size as the ZrCl4 partial pressure increased.