Continuum model for nanocolumn growth during low-pressure vapour deposition.
| dc.contributor.author | Huang, Yicun | |
| dc.date.accessioned | 2020-02-27T22:42:06Z | |
| dc.date.available | 2020-02-27T22:42:06Z | |
| dc.date.issued | 2019 | en |
| dc.description.abstract | Controlling the interfaces, including the free surfaces, of nanostructured thin films is of crucial importance for applications such as photocatalysis, single-crystalline substrates and energy materials. Some polycrystalline TiO₂ thin films grown by chemical vapour deposition at low pressures possess unusual anatase nanocolumns with well-aligned, faceted, secondary features. One variant grown by pulsed pressure metalorganic vapour deposition has excellent photocatalytic and antimicrobial activity. The striking anatase morphology has a 110 growth direction, and the activity is thought to be related to the crystallography of the exposed surfaces and nanostructure. The unusual microstructure is hypothesised to result from the interplay between the kinetics of preferential deposition caused by the shadowing effect at low pressures and finite surface diffusion driven by highly anisotropic surface energies. There is no existing modelling work that explores these detailed effects and the resulting exposed surfaces for realistic, material-specific deposition systems. In this thesis, a continuum model that incorporates both process-specific and materials-specific effects is developed. A spectral method is used to solve coupled partial differential equations for the 3D evolution of structure. The model was validated for a well-known MgO microstructure produced by oblique angle deposition. A long-standing discrepancy between the existing theories and experimental observations regarding the column tilt angle and in-plane texturing in this cubic 100 -habit material is resolved with the new model. A finite surface diffusion, together with shadowing due to ballistic transport, is shown to give rise to the biaxial texture and the roof-tile surface morphology experimentally observed for MgO. The evolution of anatase-TiO₂ nanocolumns during low pressure chemical vapour deposition was studied. The anisotropic interfacial energies for important surfaces in this tetragonal material were calculated using density functional theory. A numerical perturbation study showed the influence of the initial condition and growth regime on stabilising experimentally consistent morphologies. Plate-like branched structures were stabilised with large surface areas of high energy free surfaces such as 116 . Plate growth directions consistent with available scanning electron microscopy were predicted. Many nanostructured materials with interesting morphologies and surface-dominated properties are made using low pressure chemical or physical vapour deposition. The model developed and validated here can be applied to understand how to select processing parameters such as temperature and pressure in order to control the resultant morphologies and engineering properties. | en |
| dc.identifier.uri | http://hdl.handle.net/10092/18618 | |
| dc.identifier.uri | http://dx.doi.org/10.26021/1616 | |
| dc.language | English | |
| dc.language.iso | en | |
| dc.publisher | University of Canterbury | en |
| dc.rights | All Right Reserved | en |
| dc.rights.uri | https://canterbury.libguides.com/rights/theses | en |
| dc.title | Continuum model for nanocolumn growth during low-pressure vapour deposition. | en |
| dc.type | Theses / Dissertations | en |
| thesis.degree.discipline | Mechanical Engineering | en |
| thesis.degree.grantor | University of Canterbury | en |
| thesis.degree.level | Doctoral | en |
| thesis.degree.name | Doctor of Philosophy | en |
| uc.bibnumber | 2829882 | |
| uc.college | Faculty of Engineering | en |