| Molecular dynamics (MD) simulations have been used to study the structure and dynamics of sodium tetrasilicate (Na2Si4O9 ) liquid as a function of pressure, ranging from I atmosphere to 100 GPa, at a temperature of 6000 K. The calculated self-diffusivity of the ions increases with increasing pressure, up to a maximum of approximately 10--15 GPa. Above this pressure, the O2- diffusivity decreases slightly with increasing pressure. The results of the simulations allow the distinction of two different mechanisms for the pressure-induced coordination change of silicon. The first, occurring at lower pressures, involves the formation of V-coordinated silicon, via reaction with non-bridging oxygens. The high pressure mechanism involves a reaction of bridging oxygens, which results in the formation of III-coordinated oxygen.;MD simulations were carried out in order to investigate the structure and transport properties of boron oxide melt, as a function of pressure. The simulations show a rapid initial increase in the diffusion coefficients of boron and oxygen ions to ∼5--7 GPa, followed by a slower increase from 7--14 GPa. The increase in ion diffusivities is correlated with an increase in the proportion of BO4 to BO3 units. These results can be used to help rationalize an increase in growth rate of boron suboxide (B6O) crystals, observed from B2O3-B 6O melts in the 0--4 GPa pressure range.;Structural characterization has also been carried out on a decomposed alumina ceramic precursor material, which is synthesized via thermal decomposition of an aluminum nitrate, nanohydrate [Al(NO3)3·9H 2O] salt to yield an x-ray amorphous, water-soluble precursor. Characterization of the solid precursor is presented, along with an in-depth study on the aluminum speciation in solution. Although the solid precursor contains entirely VI-coordinated aluminum, the solution phase contains IV-, V-, and VI-coordinated aluminum, whose relative abundance does not change with increasing thermal decomposition of the starting material. Furthermore, it is the change in solution concentration which alters the relative ratios of the structural units present in solution. |
