A salt-stratified tank for measuring the dynamic response of conductivity probes

A 2 1/2-m-high salt-stratified tank has been developed to measure the dynamic response functions of conductivity probes. By using stirring grids to sharpen a two-layer interface, it has been possible to obtain the response functions over the wavenumber range of 0.5 to 300 cycles/m. Test probes, which are mounted on the end of a vertically oriented ram, have been fired through the interface at speeds from 5 mm/s to 3.1 m/s.     

A CNDO/2 molecular orbital study of the silica polymorphs quartz,cristobalite, and coesite

CNDO/2 MO calculations on H₁₂Si₅O₁₆ clusters modeling silicate tetrahedral linkage in the silica polymorphs show total energy minima at bent SiOSi angles and a correlation between the Si-O bond lengths, d(Si-O), used in the calculation and the minimum energy value of the SiOSi angle. Calculations on hydrogen saturated Si₅O₁₆ clusters isolated from the structures of low quartz, low cristobalite and coesite which were adjusted by DLS methods so that all d(Si-O) equal 1.61 Å and all _L OSiO equal 109.47° yield Mulliken bond overlap populations, n(Si-O), and Si-O two-center energies, E(Si-O), which correlate with observed bond lengths; shorter bonds involve larger n(Si-O) values and more negative E(Si-O) values.

     

Calculation of the equation of state and elastic moduli of MgO using molecular orbital theory

The success of molecular orbital theory in calculating crystal properties such as bond lengths and atomic force constants has been well documented in the literature. Calculations can be extended to crystals under simulated compression and strain to determine elastic moduli and their pressure derivatives. Comparison of the molecular orbital results with both experimental values and results obtained by calculations such as the potential induced breathing model provides insight into the nature of chemical bonding in MgO. In this study, several molecular clusters were investigated as possible models for MgO; the cluster Mg₄O₄H₂₄ was chosen as the best model. Molecular energies were calculated with respect to bond length for both compression and expansion based on clusters that had been optimized for minimum energy. The resulting energy-volume curve was fitted to a recently derived equation of state (Brown, in preparation) to derive the values of K ₀ and dK₀/dP and the individual elastic moduli and their pressure derivatives were calculated by applying strain to the molecular cluster at both zero and elevated pressures. Agreement between theory and experiment varies between parameters, but the overall trend is encouraging. Since the molecular orbital model includes only short range interactions, its ability to approximating model the elastic moduli of MgO suggests a strong contribution to the elastic energy from short range interactions.