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Manghnani, Murli H

Publications and source records attributed to Manghnani, Murli H.

3 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Hydrogen bond symmetrization and equation of state of phase D

We have synthesized phase D at 24 GPa and at temperatures of 1250-1100 C in a multianvil press under conditions of high silica activity. The compressibility of this high-silica-activity phase D (Mg{sub 1.0}Si{sub 1.7}H{sub 3.0}O{sub 6}) has been measured up to 55.8 GPa at ambient temperature by powder X-ray diffraction. The volume (V) decreases smoothly with increasing pressure up to 40 GPa, consistent with the results reported in earlier studies. However, a kink is observed in the trend of V versus pressure above {approx}40 GPa, reflecting a change in the compression behavior. The data to 30 GPa fit well to a third-order Birch-Murnaghan equation of state (EoS), yielding V{sub o} = 85.1 {+-} 0.2 {angstrom}{sup 3}; K{sub o} = 167.9 {+-} 8.6 GPa; and K{prime}{sub o} = 4.3 {+-} 0.5, similar to results for Fe-Al-free phase D reported by Frost and Fei (1999). However, these parameters are larger than those reported for Fe-Al-bearing phase D and for Fe-Al-free phase D. The abnormal volume change in this study may be attributed to the reported hydrogen bond symmetrization in phase D. Fitting a third-order Birch-Murnaghan EoS to the data below 30 GPa yields a bulk modulus K{sub o} = 173 (2) GPa for the hydrogen-off-centered (HOC) phase and K{sub o} = 212 (15) GPa for the data above 40 GPa for the hydrogen-centered (HC) phase, assuming K{prime}{sub o} is 4. The calculated bulk modulus K{sub o} of the HC phase is 18% larger than the bulk modulus K{sub o} of the HOC phase.

08 HYDROGEN↗
DOE OSTI2010

Compression studies of TiB[subscript 2] using synchrotron x-ray diffraction

The compressibility of TiB{sub 2} has been determined separately by synchrotron x-ray diffraction and ultrasonic measurements on samples loaded in diamond-anvil cells and in multianvil high-pressure apparatus up to pressures of 65.9 GPa and 13.9 GPa, respectively, at ambient temperature. The high-pressure x-ray diffraction measurements, carried out on a pure polycrystalline sample, show a monotonic decrease of the lattice parameters with pressure, with the c/a ratio variation suggesting an increase in the structural rigidity along the c direction. No structural transition is observed even after laser heating of the sample up to 2200 K at 40 GPa, indicating a phase stability of TiB{sub 2} in this pressure range. The ultrasonic measurements of compressional V{sub p} and shear V{sub s} velocities on a cylindrical specimen show linear pressure dependencies. Using the third-order Eulerian finite strain equation to fit the pressure-volume x-ray data, we obtain an isothermal bulk modulus of K{sub To} = 232.9 {+-} 4.2 GPa and its pressure derivative K{sub To} = 3.86 {+-} 0.23. Similarly, the parameters derived from fitting the finite strain equation to the ultrasonic data give an adiabatic bulk modulus of K{sub So} = 232.9 {+-} 1.6 GPa and K'{sub So} = 3.02 {+-} 0.24, in good agreement with the x-ray diffraction measurements. The shear modulus and its pressure derivative derived from the ultrasonic measurements are G{sub So} = 259.8 {+-} 0.4 and G{sub So} = 2.52 {+-} 0.01, respectively.

43 PARTICLE ACCELERATORS↗