Structural Stability of Tellurium Halide Glasses
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STRUCTURAL STABILITY OF TELLURIUM HALIDE GLASSES JAY J.L. YI* and PETER R. STRUTT The University of Connecticut, Institute of Materials Science, Storrs, CT 06269, U.S.A. *now with Circon ACMI Corp., Stamford, CT 06904. ABSTRACT In quantitative studies of the structural stability of Te-Cl and Te-Br glasses, specific attention has been devoted to determining the kinetics of their overall crystallization process and the glass composition range in which chemical decomposition occurs at elevated temperatures. The calorimetric studies reveal that the tendency of crystallization or thermal decomposition of these glasses is related to phase-diagram characteristics. It seems that the structure of these glasses is not an idealized random arrangement as described by the classic model. Underlaying causes for this deviation from a perfect random arrangement to one involving a degree of ordering ( structural group ) is discussed. INTRODUCTION Glasses based on tellurium halide are characterized by interesting infrared transmission properties, since their multiphonon absorption edge extends to longer wavelength ( 15-20mg ) [I]. It is worthwhile to have a better understanding of the fundamental aspects of these new glasses. In the present work, a sensitive calorimetric method is used for determining the crystallization tendency or structural stability of these glasses; the composition range at which the glasses undergo a chemical decomposition at elevated temperatures is also determined by this method. An attempt is made to give some structural interpretation on the basis of these experiment results, which cannot be explained by the classic structural model. EXPERIMENT RESULTS Glasses of the Te-Cl and Te-Br systems were prepared for these experiments. Te of 99.999% purity and Br2 of 99.99% purity were used as starting materials. TeCI4 was used for introducing Cl, it was carefully dryed and handled in a glove box filled with dry argon. The materials were loaded into Pyrex ampules which were pre-heated in vacuum for several hours. The ampules were then sealed under a vacuum of 10-5 Torr, they were heated up to 350-390 C, kept rotating for several hours, and then quenched in air. Te-Cl Glass Fig.l shows the differential scanning calorimetry (DSC) curves of Te60C140 and Te 5 0C150 glasses. Besides the endothermic peak at Tg ( about 80 C ), the Te 6 0C140 curve has an exothermic peak of crystallization and another endothermic peak at the Mat. Res. Soc. Symp. Proc. Vol. 172. 01990 Materials Research Society
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T03C0
I
T.UCI
II 100.0 180.0 200.0 250.0
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Tomparturi
300.0
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(b) Fig.l
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liquidus temperature; on the other hand, the Te 5 0C150 curve has only a very large peak of chemical decomposition. The entirely different shape of the two curves indicates probably the difference in structural features between the two glasses. A structural-sensitive parameter Eopt ( optical band-gap energy ) was estimated by measuring the absorption coefficient a of gl
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