The microstructure of borosilicate glasses containing elongated and oriented phase-separated crystalline particles

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Abstract

The microstructure of phase-separated and stretched silver halide particles, embedded in an aluminoborosilicate glass, is studied by small angle X-ray scattering. The data are quantitatively analysed, using a model of prolate ellipsoids of revolution generated by the elongation of a distribution of spheres. The average particle is 22 ± 3 × 160 ± 20 nm2, but the distribution is very much skewed so that a fraction of much longer particles is also present. Chemical reduction in hydrogen causes the formation of metal particles inside the rigid glass cavities formed during drawing. Two thirds of the volume of these cavities remains empty, due to the loss of the halide atoms and to the higher density of the silver crystals. The longer particles are lost, each one giving rise to two or three smaller ones. These data are compared with transmission electron microscopy (TEM) results.

Introduction

The precipitation of metal halide crystalline particles in alkali aluminoborosilicate glasses, containing small fractions (0.1–0.5 wt%) of a metal phase, allows the design of a variety of photosensitive materials with different interesting applications, depending on the particular preparation path [1]. If the glass is stretched and then chemically reduced in hydrogen, elongated metal particles, which are very well aligned in the stretching direction, are obtained [2]. These particles can induce polarising properties in different wavelength ranges, depending on the particular metal used [3]. The most classic examples are Ag-containing glasses, which are active in the infrared region [4].

In a recent paper [5], we investigated the latter material by two-dimensional small angle X-ray scattering (2D-SAXS). A model based on fluid-dynamical considerations was put forward and tested on as-stretched and reduced samples, giving a quantitative characterisation of the elongated particles. In a following paper [6], the same samples were further investigated by transmission electron microscopy (TEM), with microanalytical capabilities, confirming the results obtained by SAXS and adding information on the inner structure of the elongated particles.

However, a direct comparison of the as-stretched and the reduced material was still lacking. In the present work, a series of related samples (i.e. cut from the same piece of material) has been prepared to follow the morphological and microstructural changes along the preparation path. While, in the commercial material, the chemical reduction is limited to a thin surface layer of few microns, the reduced sample studied here was held in H2 for a long time, sufficient to assure a complete reduction throughout the material. This sample is compared with one prepared in air with the same thermal schedule, in order to separate the thermal effect from the chemical one. Furthermore, spheroidisation has been induced, by heating above the glass softening point, with the aim of reproducing the material before stretching (not available for technical reasons). The samples have been investigated by 2D-SAXS, using synchrotron radiation. Further developments in the method of data analysis are also discussed.

Section snippets

Sample preparation

The starting glass belongs to a class of well studied photochromic glasses [7]. Its nominal composition in wt% is: SiO2 (56), B2O3 (18), Al2O3 (6), K2O (6), ZrO2 (5), Na2O (4), Li2O (2), Ag (0.21), Cl (0.17), Br (0.14), CuO (0.006). The samples are obtained from glass slabs of high optical quality, which are cut, ground and polished.

All the samples have been preliminarily heated at 950 ± 5 K for 1 h and then drawn under an applied stress of 34.5 ± 0.5 MPa at 970 ± 5 K (the softening point is 920 K).

Results

A typical transmission electron micrograph (TEM) of an as-stretched sample is shown in Fig. 1. Fig. 2 shows, for sample I, contour plots of the SAXS intensity and of the fitted function, together with residuals relevant to the different contour levels. The calculated numerical

Discussion

TEM images of similar samples have been reported in Refs. 5, 6. The distributions of particle dimensions obtained by TEM were in very good agreement with those obtained by SAXS [5], in spite of the huge difference in particle counting statistics for the two techniques.

In the present paper, the validity of the model used to describe the SAXS data for the as-stretched material (a distribution of spheres drawn by a parametrised elongation law) is also demonstrated by the analysis of the scattering

Conclusions

The proposed model for analysing the SAXS data has been demonstrated to give an accurate quantitative description of the stretched material. The development of a more precise theoretical fluid-dynamics model is desirable to fully exploit the potentiality of this technique. The chemical reduction process causes a rearrangement of the metal atoms inside the rigid glass cavities formed during drawing, two third of whose volume remains empty, due to the loss of the halide atoms and to the higher

Acknowledgements

The assistance of Sabine Cunis and Dr. Ulrich Lode at HASYLAB is gratefully acknowledged. The TEM measurements have been carried out at CNR-LAMEL, Bologna (Italy) by Dr A. Armigliato on samples prepared by A. Garulli. The authors are thankful to them for their competence and kindness. The project was supported by the European Commission, under the framework of the Human Capital and Mobility Program (Access to Large-Scale Facilities), the Italian Research Council (CNR) and the Italian Ministry

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