Interaction of NF 3 with melt confinement materials in fluoride glass processing
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I. INTRODUCTION Heavy metal fluoride glasses are currently under intense investigation because of their theoretical capability of offering lower optical attenuation than silica, as well as an extended IR transparency compared to the oxide glasses. Particularly, fluorozirconate glasses (based on ZrF 4 as the main glass former) are of interest for both fiber optic and bulk transmission applications.1'2 These fluoride glasses are marginal glass formers and very hygroscopic compared to silicate glasses. Both the tendency to devitrify as well as the IR transparency are strongly affected by the presence of oxide and hydroxide impurities. The former lead to enhanced light scattering, whereas the latter cause the multiphonon absorption edge to shift towards shorter wavelengths compared to the intrinsic edge of the fluorides. One approach to minimize these deleterious effects is the use of a reactive atmosphere during the melting of these glasses (reactive atmosphere processing, or RAP). 3 In this technique a halogenated gas (e.g., CC14, CF 4 , SF6, F 2 ) is used to scavenge hydroxide groups and (preferably) fluorinate oxide impurities. An additional benefit of this method is the control of oxidation states of both glass former and (optically active) impurity cations. Particularly, Zr 4 + has a tendency to be reduced, resulting in black-speckled glasses.3 These reactive (i.e., oxidizing) gases prevent this and simultaneously oxidize, for instance, Fe 2 + (a strongly absorbing impurity between 2 and 3 pvci, the region of minimum attenuation ) to Fe 3 + . The latter has no significant absorption in this spectral region, and hence higher levels of iron contamination in the fluoride precursors can now be tolerated.4 Recently, nitrogen trifluoride (NF 3 ) has been used to provide a reactive atmosphere during fluoride meltJ. Mater. Res. 3 (2), Mar/Apr 1988
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ing.5 In this paper we study the dissociation and reaction of NF 3 with several processing confinement and crucible materials typically used in the melting of these glasses. II. NF3 REACTIVE ATMOSPHERE PROCESSING A disadvantage of all RAP species, with the exception of fluorine itself, is the possible incorporation of dehydration and oxide removal reaction products in the melt. For instance, the use of SF6, CS2, or CC14 may lead to extrinsic absorption from their corresponding oxides.3'6 Similarly, inclusion of carbon, sulfur, or chlorine increases the optical attenuation.7'8 In addition, chlorinated species (e.g., ZrF 4 _ x Cl^.) may not be desirable, since chlorine in the glass appears to enhance ionizing radiation damage.9'10 This is particularly of great importance to ultra-low-loss fiber applications of these glasses, where radiation sensitivity is a determining factor in long-term reliability.'' Although, in principle, fluorine itself, would of course, be the RAP agent of choice, its handling requirements detract from its practical attractiveness. For this and the above reasons, a more suitable fluorine source appears to be5 NF 3 , which at e
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