Allan J. Bruce

The influence of glass composition on the crystal growth kinetics of heavy metal fluoride glasses

Abstract The kinetics of crystallization of two heavy metal fluoride glasses, ZrF 4 BaF 2 LaF 3 AlF 3 LiF and ZrF 4 BaF 2 LaF 3 AlF 3 LiFPbF 2 , which are of potential use as core and clad materials for fiber optic waveguides, have been studied by differential scanning calorimetry. The devitrification processes follow the Johnson-Mehl-Avrami relation, and the values of the kinetic parameters for isothermal and non-isothermal techniques are in excellent agreement. The crystallization activation energies for these glasses are about two-thirds those previously reported for a ternary ZrF 4 BaF 2 LaF 3 glass. The implications of the experimental results for glass stability against devitrifications are discussed.

Crystallization of fluorozirconate glasses

The crystallization of a number of glasses of the fluorozirconate family has been studied using powder X-ray diffraction and differential scanning calorimetry, as a function of time and temperature of heating. The main crystalline phases were ..beta..-BaZrF/sub 6/ and ..beta..-BaZr/sub 2/F/sub 10/. Stable and metastable transformations to the low-temperature ..cap alpha..phases were also investigated. The size of crystallites in fully devitrified glasses was calculated to be about 600 A from line broadening of the X-ray diffraction peaks.

Determination of the mass fraction of crystals in partly frozen hydrocarbon fuels

Abstract The per cent by mass of crystalline solid in a partly frozen hydrocarbon fuel cannot be determined by a filtration experiment alone because the precipitate retained by the filter entraps large amounts of the liquid phase. A technique has been developed which augments the filtration experiment with an independent determination of the amount of liquid in the precipitate and hence allows calculation of the true value of the mass percent of crystalline solid, % S . This method involves the addition to the initial fuel of a small amount of a non-crystallizing dye as a tracer and the determination of the amount of entrapped liquid in the precipitate fraction by a spectrophotometric comparison of the dye concentrations in the precipitate and filtrate. The technique has been tested at several temperatures on both a diesel and a jet fuel. % S increased with decreasing temperature below the melting point. The % S values for the jet fuel were in good agreement with results obtained in another laboratory using a filtration/g.c. analysis method. Precipitates obtained by filtration of the partly frozen fuels were found to contain 50–80% entrapped liquid.

Comparative study of BaF2/ThF4 glasses containing YF3, YbF3 and LuF3

Abstract A series of glasses of composition (mol %) (x)BaF 2 − (33.3−x/3)ZnF 2 − (33.3−x/3) − (33.3−x/3)ThF 4 , where M = Y or Yb and x = 10–25 were synthesized. Optical, thermal and microhardness measurements were made and the data compared with that for fluorozirconate, fluorohafnate, and glasses where M = Lu. The subject materials have higher glass transition and crystallization temperatures, higher hardness and somewhat lower UV absorption coefficients than typical ZrF 4 /HfF 4 -based glasses.

Crystallization of heavy metal fluoride glasses

The kinetics of crystallization of a number of fluorozirconate glasses were studied using isothermal and dynamic differential scanning calorimetry, and X-ray diffraction. The addition of the fluorides LiF, NaF, AiF3, LaF3 to a base glass composition of ZrF4-BaF2 reduced the tendency to crystallize, probably by modifying the viscosity-temperature relation. ZrF4-BaF2-LaF3-A2,F3-NaF glass was the most stable against devitrification and perhaps is the best composition for optical fibers with low scattering loss. Some glasses first crystallize out into metastable --BaZr2F10 and ,.---BaZrF6 phases, which transform into the most stable a-phases when heated to higher temperatures. The size of the crystallites was estimated to be (≈ 600 Å from X-ray diffraction.

Sub-Tg relaxations in heavy metal fluoride glasses

Structural relaxation studies during annealing of a series of ZrF4-based glasses below the glass transition temperature have been carried out. Indications are that no property changes due to structural relaxation are likely to occur at ambient temperature over periods of tens of years. Some of the lower Tg glasses, however, did exhibit detectable structural relaxation on annealing at temperatures as low as 100°C over roughly a one year time period.

Intrinsic Instabilities Of Heavy Metal Fluoride Glasses

Heavy metal fluoride glasses (HMFG) are potentially useful as optical components in a wide range of devices. Their utilization has so far been delayed mainly because of insufficient material purity and inadequate processing conditions. However, as the result of numerous research efforts, these problems are gradually diminishing, and it now seems likely that the ultimate limitations for use of HMFG components, at least in those applications in which high optical transparency is not a prerequisite, will be imposed by more intrinsic instabilities of the glasses themselves. These include their strong tendency to crystallize on quenching and subsequent reheating, low mechanical and chemical durability, and the possibility that they will undergo significant physical aging in situ. Experimental data relating to these problems have now been obtained, and their relative importance is assessed in this paper.

Structural Relaxation in Fluoride Glasses

Structural relaxation studies during annealing of a series of ZrF4-based glasses below the glass transition temperature have been carried out. Indications are that no property changes due to structural relaxation are likely to occur at ambient temperature over periods of tens of years. Some of the lower Tg glasses, however, did exhibit detectable structural relaxation on annealing at temperatures as low as 100°C over roughly a one year time period.

Paramagnetism in heavy metal fluoride glasses

Abstract The preparation and characterization of heavy metal fluoride glasses in the system BaF2MnF2ThF4 (BMT) are discussed. Rare earth ions, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb are incorporated in BMT composition to study the effect of these ions on the magnetic and magneto-optic properties of the base glass. Glass transition and crystallization temperatures (Tg and Tx) are determined. The measured glass samples have low Verdet constant, exhibit strong paramagnetic behavior and are transparent in the infrared spectral region (2–9 μm).

Intrinsic Instabilities Of Heavy Metal Fluoride Glasses (HMFG)

Heavy metal fluoride glasses (HMFG) are potentially useful as optical components in a wide range of devices. Their utilization has so far been delayed mainly because of insufficient material purity and inadequate processing conditions. However, as the result of numerous research efforts these problems are gradually diminishing and it now seems likely that the ultimate limitations for use of HMFG components, at least in those applications where high optical transparency is not a prerequisite, will be imposed by more intrinsic instabilities of the glasses themselves. These include their strong tendency to crystallize on quenching and subsequent reheating, low mechanical and chemical durability and the possibility that they will undergo significant physical aging in situ. Experimental data relating to these problems have now been obtained and in the light of these we wish to present an assessment of their relative importance.