Nucleation sites at cordierite glass surfaces

The nucleation density, N, of high-quartz solid solution crystals (''mu-cordierite'') at surfaces of cordierite glass (2MgO . 2Al(2)O(3) . 5SiO(2)) was studied by optical and electron microscopy for differently damaged surfaces prepared and thermally treated in controlled ambient atmosphere. (i) Sharp tips and edges are dominating nucleation sites at mechanical damaged cordierite glass surfaces. These sites are privileged due to a substantial reduce of the elastic energy that contributes to the work of forming a critical nucleus within the surrounding glass phase. (ii) Solid foreign particles are dominating nucleation sites at smooth surfaces. Oxide particles, dusted on to the glass surface prior to heat treatment, increase N in most cases. Their nucleating efficiency (ZrO2 (monoclinic)>(ZrO2 (tetragonal)>Al2O3 approximate to Fe2O3>CeO2>MgO) indicate their role as chemically stable nucleating substrates. Non-oxide particles have no influence (SiC, Si3N4) Or even suppress surface nucleation (WC, W) indicating chemical interaction with the glass. (iii) Both, no strong influence of ambient humidity on N and similar minimum values of N, N(min)approximate to 10(-8) mu m(-2), for free air and vacuum experiments indicate minor nucleation activity of flat pristine glass surfaces even in contact with air. On the other hand, N is influenced by the sample-crucible geometry.