Homogeneity of a ZrF4-based glass at the nano-scale

A glass of composition 53ZrF(4)-20BaF(2)-4LaF(3)-3AlF(3)-20NaF (T-g=260 degrees C) was prepared by careful crucible melting. High-resolution atomic force microscopy of fracture surfaces displayed the presence of nano-pores with diameters of 20-50 nm, being 4-10 nm deep, in all glasses. It was further found that only glasses without annealing and glasses with an annealing step considerably below T-g showed a distinct pattern, i.e. ripples of approximate to 20 nm in diameter and an rns roughness of approximate to 0.6 nm. Glasses annealed either near T-g or at the temperatures of maximum nucleation or maximum crystal growth rates showed both regions with the ripple pattern and regions with nano-hillocks, growing in size with increasing annealing temperature and time. Thus these hillocks nearly reach micro-dimensions of approximate to 270 nm in diameter and approximate to 65 nm in height following a 90 min annealing step at 343 degrees C, the temperature of maximum crystal growth. These findings give evidence that the glass system, which is thought to be one of the most suitable for fiber drawing, is much less stable against nucleation and crystallization than anticipated. (C) 1999 Elsevier Science B.V. All rights reserved.