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 deg rees C) was prepared by careful crucible melting. High-resolution atomic fo rce microscopy of fracture surfaces displayed the presence of nano-pores wi th diameters of 20-50 nm, being 4-10 nm deep, in all glasses. It was furthe r found that only glasses without annealing and glasses with an annealing s tep considerably below T-g showed a distinct pattern, i.e. ripples of appro ximate 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 nucleati on or maximum crystal growth rates showed both regions with the ripple patt ern and regions with nano-hillocks, growing in size with increasing anneali ng temperature and time. Thus these hillocks nearly reach micro-dimensions of approximate to 270 nm in diameter and approximate to 65 nm in height fol lowing 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 s table against nucleation and crystallization than anticipated. (C) 1999 Els evier Science B.V. All rights reserved.