Surface Brillouin scattering studies of elastic properties of materials at high temperature
Date
2009-02-04T10:17:39Z
Authors
Mathe, Bhekumusa Abraham
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Abstract
Brillouin scattering from surfaces, more often called surface Brillouin scattering (SBS)
has been widely used to investigate elastic properties of thin films, interfaces, layered
systems. In this thesis, the focus is on bulk specimens, ranging from large oriented
crystals to polycrystalline layers. Well-characterised titanium alloys (TiC and TiCN) are
shown to have elastic moduli that obey the quasi-harmonic approximation while retaining
their refractory properties. Iron (IV) sulphide, is investigated at ambient and high
temperatures where it is demonstrated that the elastic constant C12 has a positive value
even at high temperatures, and comparison of the high temperature elastic constants data
to theoretical results obtained from models developed by Sithole et. al. shows reasonably
good agreement. An amorphous boron carbide film deposited on silicon is shown to
undergo a phase change at about 350 - 400oC to a possibly nanocrystalline state, a
process that is accompanied by an increase in Young’s modulus of about 14 GPa from
ambient value. The combination of SBS dispersion results and Green’s function analysis
of organ-pipe modes observed for the boron carbide film, has the potential of being used
to extract elastic constants of thin layers, where appropriate conditions for the observation
of 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-pipe
modes demonstrates the applicability of this approach to the analysis of some layered
systems. Boron-doped polycrystalline silicon (polysilicon in short) is a popular MEMS
structural material, and high temperature measurements of this material are reported for
the first time for the temperature range beyond 100oC where the Young’s moduli is
observed to remain consistent at about 163 GPa. The increase in bulk modulus observed
at about 300oC is associated with growth of crystallites as a result of the very long
thermal-soaking periods. High temperature Raman measurements conducted parallel to
the SBS measurements show an unusual recovery of the asymmetry of the Fano-line
shape on cooling down to room temperature. Grain boundary diffusion of the smaller
boron atoms and lattice relaxation effects are considered to explain the phenomenon, and
the results are related to the corresponding SBS elasticity measurements.