CRYSTALLIZATION IN FLUORIDE GLASSES

Fluoride glasses which are used for passive and active optical fibers are subject to devitrification, especially during fiber-drawing. Kinet ic parameters may be obtained through isothermal and nonisothermal met hods. Basic relations deriving from the Mehl-Avrami-Johnson approach a llow the determination of activation energy E(a) and Avrami exponent n . The paradox of activation energy is expressed in terms of correlatio n between E(a) value and binding energy, which raises the question of the physical meaning of activation energy. Various stability factors a re listed and their validity discussed. The important problem of nucle ation in fluoride glasses emphasizes the role of anionic oxygen in the nucleation mechanism. The crystallization behavior of the main fluori de glasses, including cadmium fluorochloride glasses, is reported in r elation to fiber-drawing ability.