Thomas F. Soules

A molecular dynamic calculation of the structure of sodium silicate glasses

Molecular dynamic calculations of the structure of a series of sodium silicate glasses, a common soda–lime glass, a sodium borosilicate glass and a sodium–potassium silicate glass were made. Calculated radial distribution functions are in good agreement with existing x‐ray diffraction data and a previous MD calculation on silica glass. During the dynamical runs alkali and alkaline earth cations cluster around nonbridging oxygen atoms. In the borosilicate glass studied, the boron atoms enter the silica network and are tetrahedrally coordinated by oxygen. Other features of the simulated glasses include the coordination number of cations, the degree of inhomogeneity, the distribution of bridging and nonbridging oxygen atoms, the Si–O–Si bond angles, the ring structure, etc. Some preliminary diffusion studies will be presented.

Molecular dynamic calculations of glass structure and diffusion in glass

Abstract During the past few years several studies have been made of the structure of network forming glasses using the method of molecular dynamics (MD). In general, the MD structures closely resemble the random network structures first proposed by Zachariasen. For example, silica glass has a cage-like structure resembling a randomly distorted high-cristobalite. In simulated sodium silicate glasses, Na ions create non-bridging oxygen atoms and occupy interstitial sites with no well-defined coordination but a most probable coordination number of five. Vitreous B 2 O 3 contains planar regular triangles linked at apices to form a random network of ribbons and sheets. The MD structure of vitreous B 2 O 3 does not contain boroxyl rings. In sodium borosilicate glasses the trigonal to tetrahedral conversion of boron with the addition of sodium was observed to agree with the NMR results. In sodium alumino-silicate glass simulations, aluminum prefers four coordination, regardless of the concentration of Na 2 O. The mechanism is discussed. The MD structure can be used to interpret many physical properties. All of the simulated glasses undergo a kinetic glass transition as they are cooled. Glass formation occurs when the system reaches a temperature at which the network forming cations and anions cease to diffuse on the time scale of the MD runs. This occurs at temperatures corresponding to viscosities of 1 P or less diffusion constants of around 10 −5 cm 2 /s. Hence, the MD glasses have very high fictive temperatures. Sodium and other alkali ions, however, can be observed to diffuse at much lower temperatures. “Experiments” can be performed on the computer simulated network glasses and liquids.

A molecular dynamic calculation of the structure of B2O3 glass

A molecular dynamic calculation on vitreous B2O3 reveals a structure in which boron is trigonally coordinated to oxygen. Boron atoms are at the center of approximately regular triangles. The triangles are connected at apices to form a three‐dimensional network of interconnected ribbons and sheets. No tendency to form boroxol groups is found and a reinterpretation of the x‐ray data of Mozzi and Warren is suggested. A comparison is made with the molecular dynamic structure of silica.

The luminescent properties of antimony in calcium halophosphates

Abstract Antimony may be present at calcium sites in the halophosphate lattice with anion (oxygen) compensation or cation (sodium) compensation, or as a SbCl 3 vapor-doped activator. Oxygen compensated antimony is present at calcium (II) sites with the oxygen substituting for the adjacent halide ion. The antimony-oxygen system thus formed has its own unique emission, excitation, and decay properties. Sodium compensated and vapor-doped antimony in halophosphate have similar luminescent properties which are, however, distinctly different from those of oxygen compensated (normal) phosphors. Antimony which is not oxygen compensated emits at ∾400 nm and probably substitutes at the calcium (II) site also.

The rheological properties and fracture of a molecular dynamic simulation of sodium silicate glass

We extend earlier molecular dynamic calculations to a study of the rheological properties and failure of sodium silicate glass. Under biaxial compressions glass flow and stress relaxation are observed. The simulated glass ‘‘breaks’’ by forming cavities when subjected to a sudden large biaxial expansion. A microscopic model of a fiber behaves elastically up to stresses of near one million psi and then breaks with significant plastic deformation. The strength of the fiber is related to the ultimate strength of the glass. Under severe uniaxial compression the microscopic fiber undergoes a change in its atomic structure to a layered structure.

Sodium diffusion in alkali silicate glass by molecular dynamics

Sodium ion diffusion in a sodium silicate glass (22Na2O ⋅ 53SiO2) was studied by molecular dynamics. Sodium diffuses by hopping from one interstitial site to another. At high temperatures reliable sodium diffusion coefficients and activation energies are determined. These are in good agreement with those extrapolated from experimental data. It is suggest that simple Born–Mayer potentials to describe ionic bonding can be used for estimating transport properties. Glass forming is followed as the temperature is lowered in simulated cooling schedules.

Computer simulation of glass structures

There are several computer simulation methods which can be used to generate glass structures. Among these are the methods which are used together with interatomic or intermolecular potential functions and which starting with an arbitrary initial configuration find structures corresponding to representative local minima in the potential energy surface. These include relaxation techniques, Monte-Carlo (MC) methods and molecular dynamics (MD). These are discussed. MC and MD simulations have the advantage that they sample configurations with thermodynamic probabilities and can be cooled through the glass transition. If the data are obtained carefully, MD simulations are found to display the time dependent character of the experimental glass transition. Glass structures obtained by cooling MD simulations of the fluid are discussed and compared with diffraction results.

An approximation of the Coulomb force for molecular dynamic calculations

An approximate expression is given for the long range attractive Coulomb froce. This expression can conveniently be used in molecular dynamics calculations and avoids ambiguities of other approximations. (AIP)

Models of glass strength and relaxation phenomena suggested by molecular dynamic simulations

A model is proposed to explain several properties of MD ensembles representing silica and silicate glasses. The model involves the breaking of bridging SiO bonds. The breaking of a bond is here defined as stretching the bond to an inflection point in the interatomic potential. This model is proposed to explain: (1) the theoretical or intrinsic strength of the glasses, (2) flow and stress and structural relaxation, (3) melting and devitrification, (4) why silica based compounds form thetrahedral network glasses easily whereas other analogous compounds, AlPO4 for example, does not and (5) the thermal expansion of silica. The model is not new and only suggested by the molecular dynamic calculations, that is, we have not been able to observe the mechanism of glass failure and flow in the simulations. More extensive analysis and simulations are needed in the future.

Molecular Orbital Model for Antimony Luminescent Centers in Fluorophosphate

A detailed semiempirical molecular orbital description of the low‐lying excited states of the antimony luminescent center in fluorophosphate is presented. The MO model is used to interpret both the excitation and emission spectra. Good agreement with experiment is found. The model also explains differences in the characteristics of the excitation and emission spectra which are observed when the phosphor is prepared in the presence or absence of charge‐compensating oxygen.