Brillouin spectroscopic studies of the elastic properties of artificial and natural ice samples

Gammon, Peter Henry (1981) Brillouin spectroscopic studies of the elastic properties of artificial and natural ice samples. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The method of Brillouin spectroscopy has been used to measure the dynamic elastic moduli of local homogeneous regions in ice samples representing four different environments of formation. These included artificial ice frozen from distilled water, clear monocrystalline glacial ice, bubbly lake ice and sea ice. Results show Brillouin spectroscopy to be an effective technique for accurately measuring the small scale elastic properties (between inclusions) of natural ice samples. – The four samples studied were found to have identical microscopic elastic properties, that is, the respective measured sets of elastic moduli showed agreement within experimental uncertainty. Accordingly, the elastic properties of homogeneous monocrystalline ice have been found not to vary with sample age, with impurities present at the time of freezing or with crystal quality. The bulk elastic properties of ice remain of course, subject to modification by differing crystal grain textures and by the presence of inclusions of various sorts. – Weighted mean adiabatic elastic moduli for homogeneous mono-crystalline ice (equivalently, local regions in polycrystalline or heterogeneous ice) have been calculated from the combined Brillouin spectroscopic data for the four types of samples. The values at -16°C were determined to be c₁₁ = 139.25 ± .41, c₁₂ = 70.82 ± .39, c₁₃ = 57.65 ± .23, c₃₃ = 150.10 ± .46, c₄₄ = 30.14 ± .11 (units of 10⁸ N/m² or kbar). A simplified linear temperature correction relation for the elastic moduli has been determined from Brillouin data obtained at -3°C and at -16°C. A full range of derived elastic parameters for monocrystalline ice and for homogeneous isotropic polycrystalline ice has been calculated. – The values for elastic moduli obtained in the present work are subject to smaller overall uncertainty than are values obtained by previous authors. Agreement of the present results with previous measurements is generally good but not in all cases within experimental uncertainty. Unnoticed systematic error in previous measurements provides the most probable explanation for the discrepancies. However, variation of elastic moduli induced by unusual sample preparation techniques cannot be entirely ruled out. Prior to the present application of Brillouin spectroscopy no single technique for measuring the elastic moduli of ice has been applied in a uniform manner to ice samples from several sources. Hence, the present results are unique in that their applicability to ice from a wide range of sources has been experimentally verified. – In a departure from the primary objective of the Brillouin spectroscopic studies (measurement of elastic moduli) the method has also been used to investigate the local acoustic properties of the crystal grain boundary region in a bicrystalline sample of river ice. Spectra from the boundary region showed a distinct and unexpected asymmetry in the Brillouin frequency shifts of the upshifted versus the downshifted longitudinal components. The precise physical processes underlying the observation have not been determined. However, it has been verified that the effect was not instrumental. Interaction between bulk and surface acoustic modes propagating in the grain boundary region appears to provide a plausible line of explanation. Additional theory and experimental work, continuing from the present work can be expected to yield valuable direct information on the structure and physical properties of the crystal grain boundary region in polycrystalline ice.

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