Ultrasonic-based Viscosity Measurements and the Devitirifcation of Heavy-Metal Fluoride Glasses
- Paper number
IAC-05-A2.3.06
- Author
Mr. Ian Dunkley, Queen's University, Canada
- Year
2005
- Abstract
Heavy-metal fluoride glasses have become very promising optical fibre materials due to their predicted ultra-low attenuation and near ultraviolet (UV) to mid-infrared (IR) optical transparency range. While conventional silica fibres have attained their theoretical minimum losses of 0.15 dB/km, fluoride glasses have the potential to yield losses of only 0.001 dB/km. Fluoride glasses also exhibit transparency into the mid-IR frequency, a region inaccessible to silica fibres. However, in this group of glasses, devitrification readily occurs during both bulk glass synthesis and fibre drawing, causing increased scattering and increased attenuation. Furthermore, when processed in the usual platinum crucible, fluoride glasses tend to absorb the crucible material, which severely limits their transparency range. Interestingly, it has recently been shown, and independently confirmed, that heating fluoride glass preforms into the crystallisation temperature range in a microgravity environment can completely suppress subsequent nucleation; in addition re-melting the glass in a containerless fashion can prevent crucible absorption. Commonly, in a condensed matter phase transformation, the appearance of an equilibrium phase in the transformation sequence is preceded by the formation of a metastable phase(s) with a smaller free energy barrier to mount prior to nucleation. However, whilst devitrification by the precipitation of various phases occurs readily when a ZBLAN glass is heated to temperatures in excess of 320°C in a terrestrial laboratory, the same operation may be performed in reduced gravity without the accompanying precipitation. It has been suggested that this arises because the buoyancy convection present in unit gravity is largely absent in reduced gravity, and so, the generation of super-critical size crystallisation embryos is much retarded. At unit-g, it is presumed that shear-thinning takes place leading to an increase in diffusion rates and a corresponding increase in both nucleation frequency and crystal growth rates. This paper examines the devitrification process(es) of ZBLAN glass by critically evaluating the current hypotheses of the role of gravity in the devitrification process and how the devitrification might be suppressed when processed terrestrially. Neither the shear thinning nor the density segregation explanations fully account for the role of gravity in devitrification and so an alternate explanation is proposed. A piezoelectric viscometer has been designed, and constructed to measure the low temperature viscosity of ZBLAN glass in preparation for reduced gravity experimentation. The viscosity estimates suggest that the optimum fibre drawing temperature region likely resides just inside the crystallisation temperature region, but below previously estimated values. However, the full viscosity-temperature relationship must be mapped in a microgravity environment to avoid unreliable interpolation techniques in estimating crtical viscosity values. A gas film levitator (GFL) was modified for use with fluoride glasses and evaluated with test stainless steel and flint glass preforms. The results of ground-based experimentation on fluoride glasses in preparation for microgravity testing will be presented along with the recent numerical modelling studies focused on the devitrification process in these glasses.
- Abstract document
- Manuscript document
IAC-05-A2.3.06.pdf (🔒 authorized access only).
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