Cuprous Halide Microcrystals Containing Photochromic Sol-Gel Glass

CuBr containing photochromic sol-gel glasses were synthesized by the introduction of CuBr dissolved in CH₃CN into a sol-gel of Al₂O₃-SiO₂ type. Microcrystals of CuBr were developed by a two step heat treatment, the first step in air initially at 280°C/2 h and then at 410°C/2 h for the removal of the organics and the second step at 700°C/2 h in nitrogen atmosphere for the reduction of the copper compounds to Cu₊. Photometric measurements of the small gel pieces show a distinct darkening of 24% and a remarkable fading at room temperature of 13% within 10 minutes.