Homogeneity of a ZrF 4-based glass at the nano-scale

Abstract

A glass of composition 53ZrF 4–20BaF 2–4LaF 3–3AlF 3–20NaF ( T g=260°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 ≈20 nm in diameter and an rms roughness of ≈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 ≈270 nm in diameter and ≈65 nm in height following a 90 min annealing step at 343°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.