Studies on the Reproducible Production of Grin (Gradient Index) Glass Rods By A Sol-Gel Process
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STUDIES ON THE REPRODUCIBLE PRODUCTION OF GRIN (GRADIENT INDEX) GLASS RODS BY A SOL-GEL PROCESS
J. Brian Caldwell, Tessie M. Che, Richard W. Cruse, Robert M. Mininni, Randall E. Nikles, Victor N. Warden and Mark A. Banash, Enimont America Inc., 2000 Cornwall Road, Monmouth Junction, NJ 08852 ABSTRACT Optical quality radial gradient-index (GRIN) glass can be made by first leaching a TiO 2-AI2 0 3-SiO 2 gel in aqueous sulfuric acid solution, then drying and firing the gel. In this paper, we present details of a study on the reproducible production of GRIN glasses by this sol-gel process. Physical and optical data on the intermediate gels and resulting gradient-index glass will be discussed. INTRODUCTION A sol-gel technique has been used recently to make radial gradient index glass rods of good quality with the TiO 2-A1 2 0 3 -SiO 2 system [1].
The technique involves
acid-leaching a multicomponent silicate gel to create a radial composition gradient of TiO 2 within the gel. This gel is then fixed, dried and sintered to make gradient-index glass. The product is highly refractory, has a very low coefficient of thermal expansion and possesses optical properties suitable for use in imaging arrays and fiber optics. In this paper, we address the problem of refractive index profile reproducibility. EXPERIMENTAL PROCEDURE A TiO 2-A12 0 3-SiO 2 gel was synthesized using an established procedure [1] for preparing multicomponent silicate gels from metal alkoxides. Tetramethoxysilane (Huls Petrarch Systems), methanol, N,N-dimethylformamide (Fisher), titanium isopropoxide and aluminum di(sec-butoxide)acetoacetic ester chelate (Alfa Products) were used without further purification. With vigorous stirring, 118.5 ml (0.796 moles) of tetramethoxysilane (TMOS) were added to a solution containing 191.3 ml (4.723 moles) of methanol and 191.3 ml (2.47 moles) of N,N-dimethylformamide in a 1000 ml round-bottom flask. After 20 minutes, 14.4 ml of 0.1M HCI were added to the reaction mixture. After 30 minutes, a mixture consisting of 13.6 ml (0.046 moles) titanium isopropoxide (Ti(OC3H7 )3 ) and 28.52 g (0.094 moles) of aluminum di(secbutoxide) acetoacetic ester chelate was added, and the hydrolysis and polycondensation reactions were allowed to proceed for one hour. Finally 53.7 ml (2.98 moles) of water Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
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were added to complete the reaction. Gelation occurred within several hours at room temperature. REPRODUCIBILITY STUDY Two separate batches of solution were prepared, each having a volume of 611.4 ml. Twenty (20) cylindrical gel rods, each 15 x 96 mm long, were made from each batch by casting the material in polypropylene centrifuge tubes. All gel rods were treated during the leaching and fixing steps in the following manner. After a gelation period of approximately 2 to 3 hours, the samples were allowed to age for 24 hours at 25 0 C and then at 600 C for 24 hours. The gels were allowed to cool to 250 C over a 24 hour period and then were transferred to 25 x 120 mm
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