RESUMO
We have used fluctuation electron microscopy (FEM) to investigate the nucleation stage of TiO2 crystal formation in binary TiO2-SiO2 glasses with heat treatment. It was found that spatial fluctuations of electron scattering in the glass with 13 wt% TiO2 increases with heat treatment above 800 °C but before any crystals precipitate, i.e. before crystals are detectable by electron diffraction. We have attributed this to TiO2 clustering and increasing medium-range order due to gradual ordering of TiO2 phase. Moreover, we have found that FEM is sensitive to structural changes at temperatures as low as 400 °C but the nature of the changes yet to be determined. This demonstrates that FEM can be sensitive to structural changes in oxide glasses occurring during thermal treatment but preceding detection of the first crystals.
RESUMO
Structural anisotropy induced by steady shear and its mechanistic relation with shear thinning are investigated in AsxSe100-x glasses (5 ≤ x ≤ 30) quenched from parent liquids subjected to shear rates ranging between 0 and 104 s-1 using polarized Raman spectroscopy and two-dimensional X-ray diffraction. When taken together, the results demonstrate significant shear-induced partial alignment of -Se-Se-Se- chain moieties in the flow direction of the extruded fibers. This alignment is reminiscent of nematic liquid crystals where orientational order exists without positional order. The degree of this structural alignment in quenched glasses appears to be practically independent of the shear rate, although the parent liquids undergo shear thinning at the highest shear rates. It is conjectured that any causal relationship between structural alignment and shear thinning in the liquid may be masked in the glassy state by the postextrusion structural relaxation of the parent liquid.