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DEVELOPMENT OF THE GAS-FILM LEVITATION (GFL) TECHNIQUE TO PRODUCE HIGH QUALITY STRUCTURAL AND OPTICAL GLASSES

Objective


The GFL technique has been successfully applied to the preparation of reference samples of model and industrial product SLS (oxide) glasses from precursor disks and many SLS discs have been prepared by this process at high temperatures (up to 1450°C). Up-grading of the levitation facility has allowed contactless processing of large SLS glass samples which corresponds to an improvement of an order of magnitude in weight for the levitation technique. The development of a prototype to levitate large float glass plates was not found to be industrially relevant but a flotation technique derived from the GFL technique for the shaping of large SLS float glass was studied.

The reproducible preparation by levitation of ZBLAN (fluoride) discs of high optical quality was demonstrated in a one step procedure using NF3 as reactive atmosphere on selected raw materials. Bulk scattering losses of 0.016 dB/km (extrapolated at the 2.55 um wavelength) have been achieved along with concentrations of scattering defects as low as 25 per cm{3}. These results means that a factor fifty improvement has been achieved compared to conventional routes and that the theoretical intrinsic limit is quite close.

Following those results with ZBLAN, the technological development of vertical levitators was engaged either for the molding of glass rods or for the continuous synthesis of glass rods starting from powders. Difficulties were encountered with ZBLAN glasses and only small cylinders could be synthesised, 10 to 20 mm in height or pre-cast rods molded over 10 mm in height. A major breakthrough was however demonstrated : liquid columns up to 15 cm in height have been successfully molded during orientations tests with water at room temperature. This last result confirms the real capacity of the technique to confine long liquid columns.

Levitated SLS and ZBLAN glass discs have shown improved optical homogeneity properties compared to precursor glass and the levitation technique was found to be the only reproducible preparation route at this time to obtain ultra-homogeneous ZBLAN glasses.

New luminescence and refractive index data have been recorded for ZBLAN and float glass as a means of making quality control for the levitation and float processes. Tin profiles at the upper and lower surfaces of SIV glass products have been particularly characterised. For ZBLAN glasses, the lack of reference data has not allowed to obtain directly usable information.

Besides these achievements, a better and more realistic view of the industrial interest of the GFL technique has emerged and through the perfecting of the workprogramme, cross - fertilization between the SLS and ZBLAN activities has increasingly grown through characterisation activities and Gas Film Levitation of glass discs. New ideas have emerged as the low cost planar waveguide and the dissemination of results through publications is in rapid progress. Finally, contacts established with university groups turned out to be positive. To day, several European groups are developing GFL activities directed towards material solidification and basic studies in hydrodynamics.
The present proposal is articulated about the development of the Gas-Film-Levitation (GFL) technique applied to the preparation of two glasses of industrial relevance. One glass family is a sodalime float glass (flat glass used in the car and building industries); the other glass family is a halide glas, known as ZBLAN, which has considerable potential for the preparation of ultra low loss optical fibres at 2.55um wavelength.

The interest of the GFL technique in the processing of glasses from the melt stems from the fact that being a containerless technique it virtually suppresses wall (such as tin in float glass) or crucible induced flaws. These flaws often dramatically limit the glass quality, such as its mechanical strength (reduction of over two orders of magnitude for the present flat glass) and its surface properties or its optical scattering loss factor (increase of typically two orders of magnitude in conventional ZBLAN glass fibres). Recently, the lowest value of scattering loss ever reported for ZBLAN glass was measured on a ZBLAN glass made by the GFL technique. The scattering loss had a pure Rayleigh behaviour and its extrapolated value at 2.55um, i.e. 0,015dB/km, wase close to the theoretical predictions.

As opposed to other levitation techniques, the GFL technique is the only one which has the potential of processing large samples. Therefore, it appears to be a master technique for R&TD activities with a possible extension to industrial applications.

The development of the GFL technology and the demonstration of clear improvements of the bulk and surface properties of the two selected glasses are two major objectives of this proposal. The third important objective is to evaluate the possible applications of this technique in areas such as flat glass processing, fibre drawing, integrated optics, optical components and microgravity experiments.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Alcatel Alsthom Recherche
Address
Route De Nozay
91460 Marcoussis
France

Participants (4)

COMMISSARIAT A L'ENERGIE ATOMIQUE
France
Address
Rue Des Martyrs 17
38054 Grenoble
Società Italiana Vetro SpA
Italy
Address
Zona Industriale
66050 San Salvo (Chieti)
Technische Universität Clausthal
Germany
Address
Zehntnerstraße 2A
38678 Clausthal-zellerfeld
University of Sussex
United Kingdom
Address

BN1 9RH Brighton