Fluoride Glasses: Synthesis and Properties
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Brazilian Jmrnal of Physics, vol. 22, no. 3, Sep tember, 1992 Fluoride Glasses: Synthesis and Properties M. Poulain and A. Soufiane Centre d'Etude des Matériaux Auancés, Rennes, France Y. Messaddeq and M. A. Aegerter Instituto de Física e Química de São Carlos, Universidade de São Paulo Caixa Postal 369, São Carlos, 13560-970, SP, Brasil Received July 16 , 1992 The discovery of heavy metal fluoride glasses has opened new prospects for fiber optics operating beyond 2 pm with expected losses less than 1od2 dB/Km. The main interest of fluoride glasses lies in their infrared transmission up to 8 pm in the bulk form and 4.5 pm for optical fibers. We have reported here the preparation, the glass forming systems and properties of heavy metal fluoride glasses. with respect to divitrification. Consequently, process- ing may be critica1 for the quality of the final material. The development of optoelectronic devices and sys- It starts with batch preparation and ends with sample tems for tel~communications,sensing and miscellaneous polishing. application~has stimulated intense research on vitre- ous materials. Insofar as it becomes possible to reach the theoretit:al limits of silica-based glasses, further im- 11.1.1. Starting materials provements in performances depend only on the dis- The manufacturing of high quality optical compo- covery of new glasses and their optimization. The dis- nents such as optical fibers implies that starting mate- covery of uiiexpected glases at Rennes University in rials meet severe purity requiremerits. However, they 1974l was tlie beginning of numerous researches on flu- are different from current chemical specifications be- oride and, more generally, halide glasses, resulting in cause one may tolerate significant amounts of diamag- the description of hundreds of new glass forming sys- netic cations, while trace levels of optically abscrbent ten~.Fluor de glass fiber technology was investigated, impurities must be reduced drasticdlly. As an exam- especially in its specific aspects. Glass formation, which ple, there is no need to remove alkali or alkaline earth was considered as an exceptional event, appears now as elements from starting rnaterials. a common f\:ature in many fluoride systems. This de- Fluoride glasses may contain a small amount of an- velopment has expanded the horizons of the traditional ionic oxygen which reduce slightly glass forming ability field of glass science. Also, infrared fibers are now avail- in most cases, but is believed to generate wavelength able for vari~stechnical purposes. Prospects for long independent scattering. Anionic impurities, such as ni- haul repeaterless telecommunications are promising, in trates, carbonates, and sulfates may be chemically ac- spite of extrinsic scattering losses which keep the ac- tive and should be avoided since they are a source of tua1 losses al~ovethe theoretical limits. The closest ap- anionic oxygen in the final glass even when a fluori- plications of fluoride glasses in this field of telecommu- dating step is carried out. Chlorine anions have less nications relate to optical amplification, as they offer influence upon optical transmission, except in the UV possibilities st the 1.31 micrometer wavelength2. spectrum. Another group of insidious impurities consists in 11. Glass synthesis and ~rocessin~ gaseous species because they are usually neglected. Carbon dioxide may be present in fairly large amounts 11.1. Genera aspects of fluoride glass preparation in ianthanum oxide. The most drastic impurity is wa- The classical way to prepare a glass sample consists ter which is common both in many so-called anhydrous in rnixing tk e basic glass components, heating, cool- fiuorides and also in reportedly non-hydroscopic com- ing, casting ztnd annealing. Although the synthesis of a pounds. Depending on the process, wat,er contamina- fluoride glasr encompasses this sequence of operations, tion may largely determine the final leve1 of hydroxyl it requires scme special processing in relation with the and anionic oxygen. chernical reactivity of fluoride powders and melts, and Processing atmosphere is critica1 for achieving an also with thc relatively low stability of fluoride glasses optimum glass quality. For example, hydrogen fluoride 206 M. Poulain et al. HF is commonly contaminated by H2 and H20, and However, there are some limitation and problerns most pressured gases contain traces of water. associated with the casting method. First, atmospheric In summary, the selection and the handling of start- contamination is enhanced as the melt surface is in- ing materials require much attention. Oxides should creased during casting. Hydrolysis can occur and more preferentially be reheated at 1000°C before use. Alkali volatile fluorides such as ZrF4 go into the vapor phase and alkaline earth fluorides rnay also be dehydrated by which locally modifies the chemical composition. More- heating them in an oven. Other fluorides, e.g. those of over, there rnay be some condenses around the upper zirconium, aluminum and rare-earth, cannot be easily part of the crucible, leading to oxides or oxyfluoride dehydrated. For this reason, the ammonium bichloride phases which can be incorporated into the melt flow process which works with oxides or hydrated rnaterials, and resulting in microcrystalline phases. Finally, has been widely used for current preparations. the liquid motion rnay generate small bubbles which do not always reach the sample surface before the glass 11.1.2. Melting and fining is frozen. - Melting is usually implemented in platinum, gold or It is also possible to manufacture homogeneous vitreous carbon crucibles. When there is no fluorination and defect-free glass samples using the "mold-crucible" step, the heating rate rnay be fast. The critica1 point method: in this method, the molten glass is simply of this step is the dryness of the working atmosphere. cooled inside the crucible in which it was melted. Thus, Flowing a dry gas into the melting enclosure rnay not the sample replicates the crucible shape and has much be always sufficient because water can remain adsorbed more limited exchange with the atmosphere because on the walls. the melt remains static. Indeed homogeneous, cord A raw glass is obtained at the end of the initial and defect-free samples have been obtained in this way. melting process. It appears often grey or black and ex- By comparison with the casting method, lower cooling hibits a rather high divitrification rate. Consequently, rates must be used and it is difficult to avoid bubbles on batches cooled within the crucible rnay be crystalline. the walls. Therefore, the outer part of the glass sample Also, optical scattering and hydroxyl absorption at 2.9 has to be removed by polishing. micrometers rnay be significant. When glasses exhibit high divitrification rate, rapid Most of these defects are removed during the fin- quenching is needed. This rnay be achieved simply by ingstage. It consists in heating the melt above squeezing the melt between two metal plates or by us- the liquids temperature in an oxidizing atmosphere. ing classical quenching devices such as cooled rollers or The viscosity decreases and the melt is homogenized splat quenching. without stirring. Volatile species are eliminated, and In order to get samples free of interna1 stresses, an reduced phases which give rise to scattering are oxi- annealing stage is usually needed, especially before cut- dized and dissolved. Most of hydroxyl decompose into ting and polishing. Temperature is adjusted empiri- gaseous HF and anionic oxygen. Time and temperature cally, commonly around the glass transition tempera- are adjusted according to glass composition, batch size ture, Tg. For current (i.e. small) samples, annealing and crucible geometry. With fluorozirconate glasses, time appears to be less important than slow cooling to care must be taken regarding ZrF4 volatilization which room temperature in order to avoid the formation of occurs at high temperature. new thermal stresses. 11.1.3. Casting, cooling and annealing 11.2. Chemical reactivity After fining, glass melt is cooled at a temperature In laboratory conditions, oxide materials appears for which nucleation rate is still low. It must not be kept much more inert than fluorides. Chemical reactions in this situation for a too long time. It is poured onto between fluorides and water are the most important. a metallic mold - usually brass - which has been pre- The reaction rates are critically dependent on temper- heated around glass transition temperature. Graphite ature and also on cation nature. For example, alkali rnay also be used as a mold material although its sur- and alkaline earth fluorides which rnay adsorb water at face is easily contained. room temperature are easily dehydrated. On the con- Most fluoride glass samples are prepared in this way. trary, niobium pentafluoride is hygroscopic even at low By comparison with classical glasses, the low melt vis- temperature giving rise to oxyfluoride and gaseous HF. cosity makes it possible to fill molds of small size or In standard fluoride glasses, there are two possible re- complex shape. The large difference between the solid actions between molten glass and water3 and liquid volumes implies that the volume of the melt poured is larger than that of the final sample. There rnay be some problems at the end of the solidification process when the entire outer surface has just solidified but the inside has not. For example, "vacuum" bubbles Reaction 1 occurs at lower temperatures, for exam- may be formed along the axis in cylindrical samples. ple around Tg,while reaction 2 becomes predominant Brazilian Jmrnal of Physics, vol. 22, no. 3, September, 1992 207 at higher tcmperatures. Then, in the molten state, hy- The chemical reaction between oxides and ammo- droxyl groups become reactive with fluorine anions, ac- nium bifluoride follows the general scheme: cording to i,he relation MO, + (n + x/2)NH4HF2 -+ MF2,xNH4F + (2n + x/4)H20 + (n - x/2)NH3.