Theory for incongruent crystallization: application to a ZBLAN glass

Equations which describe incongruent nucleation and subsequent crystal growth are derived. A ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass was used to test the validity of these equations. Nucleation rate measurements were fitted to theory and some growth rate measurements were in reasonable agreement with theor. predictions. Both nucleation theory and crystal growth theory were used for computer simulations of the crystn. behavior during heat treatments. Some heat treatments were performed in a DSC app. to verify the theories. The exptl. results were in good agreement with the numerical data. Using these theor. results it is possible to est. fiber scattering losses due to crystn. Depending on drawing temp., estd. losses can vary from 0.014 (310 Deg) to >=25 decibel/km (320 Deg). [on SciFinder (R)

Nieuw criterium voor glasvorming

In this article it is shown that the electronegativity values of the constituent atoms of a binary compound can be used to predict the formation of fluoride and other halide glasses. For a more accurate and fundamental prediction of glass formation, a new criterion is proposed then. This new criterion, defined as the ratio of covalent bond strength to liquidus temperature, is meant to replace the electronegativity criterion. In combination with the total bond strength criterion, accurate predictions of glass formation can be made, as is shown by several examples

Glass formation in halide systems

A review and discussion, with 2 refs., of the application of accepted criteria used to predict glass formation in oxide systems to that of halide systems including the electronegativity and single-bond energies-to-melting temp. ratios. A new criterion is proposed defined as the ratio of the covalent part of the single-bond energy to the melting temp. [on SciFinder (R)

Yield improvement for an industrial-oxy fuel fired glass furnace

A glass prodn. plant had about 10.5% prodn. loss due to glass defects. The furnace was about 7() years old and produced soda-lime glass. TNO has improved the line yield by redn. of these glass defects. TNO investigated the prodn. facility and found pull variations which were not compensated well enough by energy corrections. By measuring the response of the melting tank at disturbances originating from pull variations and energy input, a black-box-model of the furnace was made. This model was used for an improved control system for the melting tank. Furthermore, the glass defects were studied in more detail and it was found that they originated from above glass level in the melting tank. With the use of the TNO-Glass Tank Model, the behavior of the temp. and the flow of the melt in the melting tank were studied and predicted the effect of a change in the process settings on the melting process. The main result was a change in current flow of the glass melt which resulted in a better soly. of the glass defects. Also a thorough anal. of selected historical data of the furnace was made, which showed a correlation between process settings and glass defects. This correlation was in agreement with lab. experience available at TNO on refractory corrosion by sodium vapors under oxy-fuel firing conditions. The empirical model developed by TNO to predict sodium evapn. and refractory corrosion were in good agreement with data obtained from prodn. By changing the process settings, the fast increasing corrosion was stopped and the amt. of the prodn. loss decreased from 10.5% to 5%. This successful cooperation between a glass prodn. plant and TNO showed that theor. knowledge of the glass prodn. and the refractory corrosion combined with measurements, historical data anal. and modeling can lead to improvement in case more complex mechanisms are responsible for prodn. losses. [on SciFinder (R)

Nucleation in ZBLAN glasses

Nucleation rates were detd. in a ZrF4-BaF2-NaF-LaF3-AlF3 glass (ZBLAN) using an optical method. The results were compared with a similar glass having a slightly different compn. The difference in the nucleation rate is explained by classical nucleation theory using calcd. free-energy differences between the ZBLAN liq. and the BaZrF6 crystal pptg. phase. [on SciFinder (R)

Evaluation of glass formation criteria

In this paper it is shown that the electronegativity values of the constituent atoms of a binary compound can be used to predict the formation of fluoride and other halide glasses. It is also shown that bond strength can be used as a secondary criterion, to explain most discrepancies. In trying to obtain more fundamental insight towards glass formation, special empahsis has been placed on the covalent part of the bond energies. It is shown to what extent knowledge of these energies provides a useful basis for prediction of glass formation