Surface Brillouin scattering studies of elastic properties of materials at high temperature

Mathe, Bhekumusa Abraham

University of the Witwatersrand

Others  :  http://wiredspace.wits.ac.za/bitstream/10539/6002/1/BAMAtheThesisPhD2008.pdf

瑞士|英语

来源: University of the Witwatersrand

【 摘 要 】

Brillouin scattering from surfaces, more often called surface Brillouin scattering (SBS)has been widely used to investigate elastic properties of thin films, interfaces, layeredsystems. In this thesis, the focus is on bulk specimens, ranging from large orientedcrystals to polycrystalline layers. Well-characterised titanium alloys (TiC and TiCN) areshown to have elastic moduli that obey the quasi-harmonic approximation while retainingtheir refractory properties. Iron (IV) sulphide, is investigated at ambient and hightemperatures where it is demonstrated that the elastic constant C12 has a positive valueeven at high temperatures, and comparison of the high temperature elastic constants datato theoretical results obtained from models developed by Sithole et. al. shows reasonablygood agreement. An amorphous boron carbide film deposited on silicon is shown toundergo a phase change at about 350 - 400oC to a possibly nanocrystalline state, aprocess that is accompanied by an increase in Young’s modulus of about 14 GPa fromambient value. The combination of SBS dispersion results and Green’s function analysisof organ-pipe modes observed for the boron carbide film, has the potential of being usedto extract elastic constants of thin layers, where appropriate conditions for the observationof these modes are met by the layer/substrate system. An agreement to within 6%between elastic moduli determined using SAWs and with those extracted from organ-pipemodes demonstrates the applicability of this approach to the analysis of some layeredsystems. Boron-doped polycrystalline silicon (polysilicon in short) is a popular MEMSstructural material, and high temperature measurements of this material are reported forthe first time for the temperature range beyond 100oC where the Young’s moduli isobserved to remain consistent at about 163 GPa. The increase in bulk modulus observedat about 300oC is associated with growth of crystallites as a result of the very longthermal-soaking periods. High temperature Raman measurements conducted parallel tothe SBS measurements show an unusual recovery of the asymmetry of the Fano-lineshape on cooling down to room temperature. Grain boundary diffusion of the smallerboron atoms and lattice relaxation effects are considered to explain the phenomenon, andthe results are related to the corresponding SBS elasticity measurements.