Crystallization in fluoride glasses

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