Characterization and dehydration of zirconium tetrafluoride for fluoride glass preparation
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I. INTRODUCTION There is currently an intense effort to find the optimal heavy metal fluoride (HMF) glass compositions for the fabrication of mid-infrared (IR), ultra low-loss ( < 0.1 dB/km) optical fibers.1 Much of this attention is focused on multicomponent glasses in which zirconium tetrafluoride (ZrF 4 ) is the principal glass component. Such glasses are usually prepared by bulk melting procedures in which the individual batch components are first weighed and then carefully mixed in the appropriate stoichiometric amounts. It is common practice to use these components in the form "as received" from chemical suppliers, sometimes after long storage periods, without further characterization or purification. This practice should be avoided since it can lead to significant compositional uncertainty and serious contamination problems in the final glass. This is of particular concern when optimum stability and precisely tailored physical properties are required, as in the fabrication of optical fibers. Zirconium tetrafluoride is reported to exist not only as an anhydrous salt but also in several hydrated forms and HF adducts, e.g., ZrF 4 • H 2 O, ZrF 4 • 3H2O, and ZrF 4 • HF • 3H 2 O (Ref. 2). These compounds differ in molecular weight by 11 %, 32%, and 44%, respectively, from ZrF 4 , and their fraction present depends both on the synthetic route employed and the subsequent sample history. It is also reported that the anhydrous material cannot be obtained by heating the hydrated salts in air. Instead they convert first to ZrOF 2 at 300 °C and then to ZrO 2 at 350°C (Ref. 2). This is particularly serious for HMF glass optical fiber fabrication since oxidic impurities can lead to unacceptably high optical absorption and/or light scattering. In order to reduce the presence of these species, large amounts of ammonium bifluoride are often added to fluoride glass batches prior to melting with the intention of removing any oxygenJ. Mater. Res. 3 (4), Jul/Aug 1988
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containing species by their conversion to fluorides. Since the addition of any extra components to the glass melt can only serve to increase the chance of contamination by trace elements, and it is known that ammonium bifluoride addition can result in the presence of NH 4+ in the final glass,3 it would be desirable to minimize the use of this procedure. Similarly, reactive atmosphere processing (RAP) techniques4 are commonly used to alleviate the oxide impurity problems in HMF glasses; however, these techniques can also lead to further contamination of the glasses by absorbing and/or scattering impurities. The use of RAP should therefore be minimized and more attention paid to the purity and integrity of the individual starting materials for the glass synthesis. In this work we have studied samples from each of two batches of high-purity zirconium tetrafluoride obtained from Sassoon Metals & Chemicals, Inc., two batches of 99.5% purity from Cerac, Inc., and a sample of sublimed, special optical grade, material from EM Chemicals. These ma
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