C. Austen Angell

Generation of fractal structures by negative pressure rupturing of SiO2 glass

Abstract Recently there have been several studies of the dynamics of fractal structures utilizing SiO2 in low-density gel and ‘smoke’ forms (1,2). It is found that over a range of densities, down to a twentieth of the density of silica glass, mechanically stable structures can be formed whose characteristics satisfy the criteria of fractal structures and whose range of self-similarity increases with decreasing density. In this work we test the possibility of producing the fractal low-density silica aggregates by catastrophically rupturing a condensed amorphous silica phase utilizing the methods of molecular dynamics computer simulation. The isotropic expansion causes bond angle opening without rupture up to a tensile limit of −70 kbar. Beyond this point (instability or spinodal) the structure ruptures in a very specific way, viz., by developing a self similar void structure which can be well described by a fractal dimension changing linearly with density. The structure maintains the preferred bond distances and coordination numbers, but permits the density to change by formation of internal surface at a minimum cost in energy. Down to a threshold density, the resulting structures span the simulation box and form a continuous network by sharing oxygens between the basic SiO4 tetrahedra but gradually lose connectivity in higher dimensions. An analysis of the density correlation functions shows the structures to have fractal dimensions. The bond stretching vibrations and the density of states are monitored over a range of densities. The simulated density of states is interpreted by reference to theoretical studies of fracton dynamics (1,2) in aggregates near the percolation threshold and compared with data obtained from neutron and light scattering on the experimental low-density amorphous silicas (1) of the aerogel type.

Structural incompatibilities and liquid–liquid phase separation in molten binary silicates: A computer simulation

We have carried out a series of ion dynamics simulations on binary silicate systems ranging from MnO–SiO2, which shows a large liquid–liquid miscibility gap, to K2O–SiO2, which does not unmix even below the stable liquidus. We have used Coulomb attractions plus Born–Mayer–Huggins repulsive pairwise‐additive potentials, evaluating the energies with a full Ewald summation. Energies of the systems were determined at a constant pressure using the Andersen algorithm. In the simulated systems (which are too small to permit phase separation) results show that in the MnO–SiO2 system the heats of mixing are indeed positive, and greater than any conceivable T⋅ΔS value, in the composition range 0.5<XSiO2 <1.0 in which phase separation occurs in practice. In K2O–SiO2, by contrast, ΔHm is always negative. Analysis of angular correlations as a function of temperature using new ‘‘circular distribution functions,’’ allows the origin of the unfavorable mixing energy to be determined in terms of structural/geometrical inco...

A structural model for prototypical fluorozirconate glass

Existing interpretations of structural data and computer simulation results for binary BaF2-ZrF4 glasses are reviewed. Detailed comparisons of X-ray scattering results and new simulation results are made to determine the important structural features that a suitable model should be able to reproduce. A bipolyhedral module-based model for the glass which satisfies these constraints is then described and proposed as an idealization of the BaF2-2ZrF4 glass structure. Generalizations to other compositions and densities are considered. Key relationships to, and differences from, the bipolyhedral Zr2F124− module-based layer lattice recently determined for β-BaZr2 F10 are examined. The model is used as a basis for discussion of optical properties and crystallization kinetics.

Fluoride bridging modes in fluorozirconate glasses by x-ray and computer simulation studies

A second characteristic Zr-Zr distance due to edge-sharing (ZrF /SUB n/ ) polyhedra, which is observed in crystalline ..cap alpha..-ZrF/sub 4/ (high-temperature form) and is also suggested by recent X-ray studies of BaF/sub 2/-ZrF/sub 4/ glasses, has been observed in ion dynamics computer simulation studies. Experimental X-ray and computer simulation results both imply that about one out of each four bridges between Zr pairs involves two fluoride ions, and that this proportion is almost independent of composition in the binary system.

Generation of Fractal Silicas by Negative Pressure Rupturing of SiO2 Glass

Abstract Stimulated by the finding that SiO2 aggregates (e.g. SiO2 “smokes” or “snow”) assembled under highly non-equilibrium conditions have fractal geometries and dynamics—and retain them under annealing densification almost until the density of silica glass is reached –we have examined the consequences of an inverse procedure. Using the power of molecular dynamics to perform otherwise difficult experiments we have subjected normal density vitreous SiO2 at 300 K to isotropic expansions (which generate negative pressures) to and well beyond the tensile limit at –70 kbar. As the tensile limit is exceeded, void-containing structures are produced which indeed prove to be fractal in character, and the pressure trends towards zero. The fractal dimension is determined concordantly from two different density correlation relationships and is found to change smoothly with density from 3.0 at the tensile limit down to 1.7 at the limiting density of 0.1 g/cm3 set by our system size. Our study suggests that the path...

Nonvibrational Nondiffusional Modes of Motion in Hydrated Calcium Nitrate Melts

Interest in highly concentrated aqueous solutions, in which all or most of the water is directly bound to ions, is relatively recent and has focused to a large extent on hydration coordination numbers and the extent to which cation-bound hydration water molecules are replaced by anions. For example, infrared and Raman spectroscopy of nitrate vibrations in nitrate melts1,2 have provided evidence that the nitrate ion competes effectively with water for the first coordination shell of Ca2+ in Ca(NO3)2 3 solutions and their mixtures with KNO3.4 From the relative intensities of vibrational absorptions due to cation-perturbed and -unperturbed anion normal modes, equilibrium constants can be estimated for water-anion exchange.