Reaction of NF 3 and SF 6 with some oxides connected with heavy-metal fluoride glass processing
- PDF / 583,794 Bytes
- 6 Pages / 593.28 x 841.68 pts Page_size
- 24 Downloads / 155 Views
NTRODUCTION The preparation of heavy-metal fluoride glass fibers and bulk optics requires a strict atmosphere control both during the melting of the glasses and the drawing of the preform in the case of fiber applications.' Typically a reactive atmosphere is used to getter both oxide and hydroxide impurities which increase the extrinsic light scattering as well as influence the infrared (IR) absorption properties. An additional benefit of these reactive species is control of the cationic valencies, both of glassforming constituents and impurity ions. '~* For instance, in the case of the former, Zr 4 + (in ZrF4-based fluoride glasses) has a tendency to be reduced resulting in black particles in the glass. The Fe 2 + impurities absorb strongly in the region of minimum optical attenuation, whereas F e 3 + does not. Although pure F 2 is the most suitable fluorination agent, SF6, NF 3 , CF 4 have been used more frequently.1'25"7 Additionally, CS2 and Cl2 have also been used as oxidizing and OH-removing agents. 68 Of these compounds NF 3 was used only recently,7 and its particular advantage is that inclusion of carbon, sulfur, and chlorine (or related complexes) in the melt is avoided. In a previous study we reported9 on the interaction of NF 3 with several crucible and reaction vessel materials, which are typically used in fluoride glass melting. This information is important for the possible incorporation of impurities resulting from these reactions, which ultimately determines the minimum optical attenuation. In this paper we report on two other important aspects of the use of NF 3 . The first is the reaction of several glass-forming cation oxides with NF 3 as a measure to remove oxide impurities during melting. In addition the effectiveness of sulfur hexafluoride (SF 6 ) for this purpose is also investigated. Second, the J. Mater. Res. 3 (4), Jul/Aug 1988
http://journals.cambridge.org
reactive behavior of these gases during the possible use as fluorination agents of the individual fluoride precursors (e.g., oxides or hydrated and/or oxide containing fluorides) is investigated. II. MELTING AND PRECURSOR REACTIVE ATMOSPHERE The previous study on the interaction of NF 3 with Ni, Pt, SiO2, and vitreous carbon showed9 that NF 3 does not thermally decompose until approximately 850 °C, in contrast to previous claims,8 which stated that NF 3 provides F 2 above 500 °C. This implies that the observed effectiveness8 in reducing oxide impurities in fluoride melts is the result of direct reaction between NF 3 and the particular oxide. It is the purpose of this paper to quantitatively determine the temperature range over which NF 3 converts ZrO 2 (zirconia) and A12O3 (alumina) to their respective fluorides and additional reaction products. The latter again is important for possible melt contamination. Another fluorination step that can be performed with NF 3 occurs during the drying of fluoride precursors. Several fluoride glass precursors (such as ZrF 4 ) are made through a wet process, which results in hydrated fluorides.6'10 Up
Data Loading...
