2-D SAXS INVESTIGATION OF POLARIZING GLASSES. S. Polizzi, P. Riello, G. Fagherazzi, Universita di Venezia (Italy), M.Bark, Universitat Ulm & Hasylab, Hamburg (Germany), N. F. Borrelli, Corning Glass R. & D., Corning, (N.Y.-USA)
The investigated materials (Polarcor(TM)) are alkali aluminoborosilicate glassy platelets where an Ag(Cl,Br) microcrystalline phase (~ 1 wt.%) was thermally developed at 725deg.C. Redrawing these samples, at temperature above their softening point, causes the microcrystalline particles to assume very elongated ellipsoidal shape with the major axis parallel to the stretching direction. A chemical reduction of this phase with H2 at 430deg.C, gives rise, inside the superficial layers of the platelet, to Ag oriented particles which are responsible for the observed polarizing properties. Because of the higher density of Ag in comparison with Ag(Cl,Br), particles are expected to shrink or broken up during reduction. The present structural investigation has characterised this process by means of a bidimensional SAXS study accompanied by WAXS analysis. The elliptic bidimensional section of the ellipsoidal object obtained in the the reciprocal space of measure using the X-ray Hasylab synchrotron source (point-like beam), was scanned with a 2D-detector. These experimental intensities, properly corrected, were fitted with the following equation:
I(h) = ## D(a, b)Io (h, a,b)dadb
where Io(h,a,b) is the scattering intensity of an ellipsoidal particle with semi-axes a and b, D(a,b) is a Weibull distribution function (|h|=4[[pi]]sin([[theta]])/[[lambda]] with 2[[theta]] the scattering angle). Since upon strecthing the volume of the original droplets must keep constant, we introduced a constraint between a and b based on fluidodynamic arguments (T. P. Seward, J. Non-Cryst. Sol. 15, 1974, 487).
The so-obtained particle distribution functions before chemical reduction have volumetric average dimensions of 25 x 370 nm, with the distribution of the major axis skewed up to length values of 1200 nm. After reduction, a second distribution of aligned short particles (average dimensions of 15 x 30nm) grows up to the detriment of the main distribution, whose shape however remains practically constant in the un-reduced core of the glassy platelet.