George F. Neilson

A test of classical nucleation theory: crystal nucleation of lithium disilicate glass

For several inorganic glasses predictions have been made of the homogeneous crystal nucleation rate using classical nucleation theory. However, in none of these cases were comparisons made with experiment, due primarily to the inability of being able to guarantee homogeneous conditions. Evidence has been provided that crystalline formation in Li2O · 2 SiO2 glass may initiate by a homogeneous mechanism. Thus, we have computed the nucleation rate curve of lithium disilicate crystals in this glass. It is found that not only do all forms of the classical theory predict nucleation rates many orders of magnitude smaller than those observed, but also that the temperature dependence of the theoretical rate is quite different from that observed.

Crystallization of Na2OSiO2 gel and glass

Abstract The crystallization behavior of a 19 wt% soda silica gel and gel-derived glass was compared to that of the ordinary glass of the same composition. Both bulk and ground glass samples were utilized. X-ray diffraction measurements were made to identify the crystalline phases and gauge the extent of crystallization. It was found that the gel crystallized in a distinctive manner, while the gel glass behavior was not qualitatively different from that of the ordinary glass.

The preparation and characterization of a lithium borate glass prepared by the gel technique

The preparation of an amorphous lithium borate gel by the metal organic procedure is described. In addition, a preliminary evaluation of the behaviour of the gel upon heating is given. In particular the crystallization tendency of the gel is studied with the aid of DTA and X-ray diffraction, and the structural changes in the gel are monitored with the aid of IR spectroscopy. The glass produced from the lithium borate gel is compared to both the gel precursor material and a glass of similar composition prepared by conventional techniques. Specifically, the relevant water contents, crystallization behaviour, and structural features are contrasted.

The crystallization behavior and kinetics of a barium fluorozirconate type glass

Abstract An investigation of the crystallization behavior of a Zr-Ba-La-Al-F glass which is subjected to isothermal heat treatments is presented. The number and nature of the crystalline phases which form and their nucleation sites are determined at three temperatures. Also, the growth rate and volume fraction of crystals which nucleate internally in the glass, when heated at 320°C, are determined as a function of time.

Crystallization of barium fluorozirconate based glasses

Abstract The crystallization behavior of glass in the barium fluorozirconate family is examined. In particular, the number and type of crystalline phases which form upon isothermal heat treatment is studied. In addition, the sequence of crystallization events, and the nature of the potential crystallization sites are discussed. Finally, the random (non-reproducible) nature of the crystallization behavior is described and analyzed.

Homogeneous versus heterogeneous crystal nucleation in Li2O-2SiO2 glass

Abstract When crystal nucleation occurs in glasses, it is difficult to ascertain whether the crystallization has initiated homogeneously or by a heterogeneous mechanism. Here we suggest a method which might enable one to make this choice, and it is applied to the bulk crystal nucleation observed in Li2O · 2SiO2. It is concluded that crystal nucleation in the latter system most probably occurs homogeneously.

Effect of OH content on phase separation behavior of soda-silica glasses

The kinetics of phase separation in a gel and ordinary soda-silica glass are followed by use of small angle X-ray scattering. Also, the influence of OH on the phase separation behavior is studied. It is found that OH accelerates the growth of the secondary phase, and that gel and ordinary glasses of similar composition and OH concentration exhibit similar phase separation characteristics.

On the possibility of critical radius measurements in the homogeneous crystal nucleation of glass

The potential of determining the liquid-crystal surface tension during crystal nucleation in inorganic glasses without measuring the nucleation rate is examined analytically. Attention is focused on the feasibility of measuring the critical radius of the crystal nuclei as a function of temperature in order to obtain the surface tension values. Two approximate expressions are defined for the upper and lower bounds of the bulk free energy of crystallization per unit volume. The expressions, when used for a validated approximation for the critical radius, show that the critical radius is a function of the undercooling and the molar volume. The minimum undercooling necessary for detecting homogeneous crystal nucleation is then calculated, which yields an upper limit to the critical radius in the range 10-15 A. Since the size range is too small for detection, the critical radius is not a useful measure for crystal nucleation studies.

Fluoride glass starting materials: Characterization and effects of thermal treatment

The production of heavy metal fluoride (HMF) glasses, and the effects of thermal treatments on the HMF glasses are investigated. ZrF4, BaF2, AlF3, LaF3, and NaF were utilized in the synthesis of zirconium-barium-lanthanum-aluminum-sodium fluoride glass. The purity of these starting materials, in particular ZrF4, is evaluated using XRD analysis. The data reveal that low temperature heating of ZrF4-H2O is effective in removing the water of hydration, but causes the production of ZrF4 and oxyfluorides; however, dehydration followed by sublimation results in the production of monoclinic ZrFe without water or oxyfluoride contaminants.

The effects of hydrated raw materials upon heavy metal fluoride glass quality

Abstract The consequences of using hydrated raw materials for the preparation of ZBLAN glass were qualitatively assessed in terms of effect upon 1) general appearance, 2) hydroxyl content, 3) oxide content and 4) presence of inclusions. Glasses were prepared using ZrF 4 ·H 2 O and sublimed ZrF 4 and dry N 2 and dry N 2 + CCl 4 atmospheres. The resulting glasses were examined with the aid of IR spectroscopy and optical microscopy. It was found that the use of hydrated raw materials did not significantly enhance the OH- or O--concentration of the glass, but did produce a degradation of glass quality.