DISCOLORATION OF ALUMINA CERAMICS WITH ULTRAVIOLET-RADIATION AND ELUCIDATION OF ITS MECHANISM

Discoloration in sintered alumina ceramics from colorless to brownish yellow by ultraviolet radiation was found and its mechanism was examin ed. UV-induced optical absorption bands were observed around 330 and 4 80 nm. An intense red luminescence peak appeared around 650-690 nm upo n exciting UV region, and intensity of the luminescence band decreased with duration of UV-illumination time. ESR hyperfine structures of a Mn-55(2+) appearing in the spectrum of the specimen before illuminatio n disappeared after UV-illumination. It is concluded that the discolor ation is attributed to photochemical conversion of Mn2+ to Mn3+ by the following reaction: Mn2+ + Fe3+ = Mn3+ + Fe2+. Here, Mn and Fe are co ntained in the specimen as trace impurities (in Referceram concentrati ons of Mn and Fe are 3 ppm and 80 ppm, respectively). Extinction coeff icients of visible bands in Mn3+ are larger by 2 to 3 orders of magnit ude than those of Mn2+. Therefore, the key species of the solarization of alumina is considered to be trace impurity of Mn2+.