Grin Rod of Large Diameter and Large Delta-N
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GRIN ROD OF LARGE DIAMETER AND LARGE DELTA-N MASAYUKI YAMANE, ATSUO YASUMORI, MITSUNOBU IWASAKI AND KAZUTAKA HAYASHI Tokyo Institute of Technology, Dept. of Inorganic Materials, 2-12-1, Ookayama, Meguro-ku, Tokyo 152, JAPAN ABSTRACT A gradient-index glass rod of about 13 mm in diameter and 20 mm in length with the refractive index difference of about 0.05 between center and perimeter has been prepared by the solgel process from a precursor solution consisted of tetramethoxysilane, tetraethoxysilane, boron ethoxide and aqueous solution of lead acetate. A bubble free wet gel of about 35 mm in diameter and 50 mm in length was obtained by adding acetic acid to the precursor solution as a buffering agent. The liquid in the micropores of the wet gel was totally replaced with acetone. Then the compositional gradient of lead was formed in radial direction of the gel by soaking in an ethanolic solution of potassium acetate. A transparent r-GRIN rod of about 10 mm in diameter was obtained by the sintering of the gel at 580'C. INTRODUCTION A gradient index (GRIN) glass rod having a continuous variation in refractive index, particularly those having a large diameter with large delta-n , is one of the materials which are advantageously produced by the sol-gel process over conventional methods [1]. The preparation of r-GRIN rods of about 7 mm in diameter with delta-n of about 0.04 was possible by the sol-gel process using an aqueous metal salt solution as a source for index-modifying cations [2]. The method which was developed by the authors is potentially applicable to the production of a material having a diameter larger than 10 mm with delta-n of 0.05 or more, although the technique has not yet been established. The problems to be solved in this method in order to produce a GRIN material of this size are inclusion of bubbles during casting of a sol, migration of index-modifying cations during drying, deformation of a gel in the initial stage of drying due to its own weight, and the onset of fracture during drying. This paper reports the study on the elimination of these problems in order to establish the procedure for the production of r-GRIN rod having a diameter larger than 10 mm with delta-n of 0.05, which is hardly attainable by the conventional process of GRIN preparation such as ion-exchange, molecular stuffing, and so on [3]. The main techniques introduced in the modification were; (1) the introduction of acetic acid in the precursor sol as a buffer to control gelling time for the elimination of bubble inclusion, (2) total replacement of water in a gel with acetone prior to the formation of concentrational gradient of lead, in order to minimize the distortion of the formed concentrational profile due to the migration of lead ions and to eliminate the deformation of the gel upon drying, (3) the employment of ethanolic solution of potassium acetate as a source for Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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for of acetone replacement and (4) the total ion, potassium ethanol after the form
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