INFLUENCE OF TEMPERATURE AND TIME ON THE STABILITY OF SILVER IN SILICA SOL-GEL GLASSES

The darkening of silica sol-gel glasses doped with 0.05 mol% silver wa s studied. Six sols were prepared from TEOS and silver nitrate. Differ ent additives were used to influence the chemical and physical states of silver: oxidizing or reducing agents (H2O2, As2O5), colloid stabili zer (sodium citrate) and network modifiers (Li2O, CaO). Sols were geli fied at 60 degrees C and densified at 600 degrees C. The samples witho ut additives and those prepared with H2O2 or As2O5 darkened their yell owish colour. The intensity of their absorption band at 410 nm increas ed with time at room temperature even if they were protected from ligh t. With increased temperature, the darkening became faster. In contras t, samples kept at temperatures below 10 degrees C did not show any ch ange. When the darkened samples were heated above 400 degrees C, rever sible bleaching took place. This darkening-bleaching is of thermal nat ure (''thermochromic effect'') and seems to be determined by a reversi ble aggregation-disaggregation of tiny silver particles. The presence of sodium citrate as an additive delayed the darkening effect and the presence of CaO delayed it even further. Lithium oxide inhibited it to tally.