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NASA NTRS2001

High Temperature Elastic Properties of Single Crystal Mullite (Approximately 2.5Al2O3.SiO2) by Brillouin Spectroscopy

The complete elastic tensor of mullite has been determined by brillouin spectroscopy at room temperature and elevated temperatures up to 1200C. Equivalent, isotropic moduli (bulk, shear, and Young's) have been calculated. The room temperature values obtained using Voigt-Reuss-Hill averaging are: K(sub VRH) = 173.5 + 6.9 GPa, G(sub VRH) = 88.0 + 3.5 GPa, E(sub VRH) = 225.9 + 9.0 GPa. All moduli show relatively gradual decreases with temperature. The temperature derivatives obtained for the equivalent, isotropic moduli are: dK(sub VRH)/dT = - 17.5 + 2.5 MPa/deg. C, dG(sub VRH)/dT = -8.8 + 1.4 MPa/deg. C, dE(sub VRH)/dT = -22.6 + 2.8 MPa/deg C. Substantial differences between bulk properties calculated from the single crystal measurements in this study and the properties reported in the literature for polycrystalline sintered mullite are identified, indicating the importance of factors such as microstructure, intergranular phases, and composition to the elasticity of mullite ceramics.

Palko, James W.↗

NASA NTRS1988

Orbital magnetoconductance in the variable-range-hopping regime

The orbital magnetoconductance (MC) in the variable-range-hopping (VRH) regime is evaluated by use of a model which approximately takes into account the interference among random paths in the hopping process. Instead of logarithmic averaging the MC is obtained by the critical percolating resistor method. The small-field MC is quadratic in H; it is positive deep in the VRH regime and changes sign when the zero-field conductivity is high enough. This behavior (except for the sign change) and the relevant magnetic field scale are in agreement with recent experiments. The calculated MC is always positive for strong fields and is predicted to saturate at sufficiently large fields.

Sivan, U.↗

NASA NTRS2000

The Single-Crystal Elasticity of Yttria (Y2O3) to High Temperature

The single-crystal elastic moduli of yttria have been measured by Brillouin spectroscopy up to 1200 C. The room temperature values obtained are C11 = 223.6 +/- 0.6 GPa, C44 = 74.6 +/- 0.5 GPa, and C12 = 112.4 +/- 1.0 GPa. The resulting bulk and (Voigt-Reuss-Hill) shear moduli are K = 149.5 +/- 1.0 GPa and G(sub VRH) = 66.3 +/- 0.8 GPa, respectively. These agree much more closely with experimental values reported for polycrystalline samples than do previous single-crystal measurements. Linear least squares regressions to the variation of bulk and shear moduli with temperature result in derivatives of dK/dT = -17 +/- 2 MPa/degC and dG(sub VRH)/dT = -8 +/- 2 MPa/degC. Elastic anisotropy was found to remain essentially constant over the temperature range studied.

Kriven, Waltraud M.↗

NASA NTRS2001

Single-Crystal Elastic Constants of Yttria (Y2O3) Measured to High Temperatures

Yttria, or yttrium sesquioxide (Y2O3), has been considered for use in nuclear applications and has gained interest relatively recently for use in infrared optics. Single crystals of yttria have been grown successfully at the NASA Glenn Research Center using a laser-heated float zone technique in a fiber and rod. Such samples allow measurement of the single-crystal elastic properties, and these measurements provide useful property data for the design of components using single crystals. They also yield information as to what degree the elastic properties of yttria ceramics are a result of the intrinsic properties of the yttria crystal in comparison to characteristics that may depend on processing, such as microstructure and intergranular phases, which are common in sintered yttria. The single-crystal elastic moduli are valuable for designing such optical components. In particular, the temperature derivatives of elastic moduli allow the dimensional changes due to heating under physical constraints, as well as acoustic excitation, to be determined. The single-crystal elastic moduli of yttria were measured by Brillouin spectroscopy up to 1200 C. The room-temperature values obtained were C(sub 11) = 223.6 + 0.6 GPa, C(sub 44) = 74.6 + 0.5 GPa, and C(sub 12) = 112.4 + 1.0 GPa. The resulting bulk and (Voigt-Reuss-Hill) shear moduli were K = 149.5 + 1.0 GPa and G(sub VRH) = 66.3 + 0.8 GPa, respectively. Linear least-squares regressions to the variation of bulk and shear moduli with temperature resulted in derivatives of dK/dT = -17 + 2 MPa/C and dG(sub VRH)/dT = -8 + 2 MPa/ C. Elastic anisotropy was found to remain essentially constant over the temperature range studied.

Sayir, Ali↗