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Glass formation observed for the first time

A team of scientists headed by Emmanuelle Gouillart from the joint research unit between CNRS and Saint-Gobain has, for the first time, visualized the transformation of powder mixtures into molten glass. A better understanding of this process will make it possible to produce high quality glass at lower temperatures, leading to significant energy savings in industrial glass manufacturing.

A team of scientists has, for the first time, visualized the transformation of powder mixtures into molten glass. The team, led by Emmanuelle Gouillart from the joint research unit between CNRS and Saint-Gobain, a global glass manufacturer, included scientists from the Universities of Toulouse and Grenoble, INRIA Saclay and the European Synchrotron Radiation Facility (ESRF) in Grenoble. A better understanding of this process will make it possible to produce high quality glass at lower temperatures, leading to significant energy savings in industrial glass manufacturing.
Glass is a non-crystalline amorphous material, produced by the fusion of crystalline powder mixtures such as quartz sand (silica, SiO2), sodium and calcium carbonates (Na2CO3, CaCO3), and minor more specific additives, heated to high temperatures. Glass is also one of the oldest human-made materials, use of which spread during ancient Egyptian and Roman cultures.
The scientists set out to understand what exactly happens at the different stages of the transformation from powder to molten glass. For their experiment, they used mixtures of raw materials similar to that for making industrial window glass: two-thirds silica sand and one-third of sodium and calcium carbonates.
To make visible chemical reactions between individual grains, the scientists used X-ray microtomography, a technique enabling to visualize in real time changes in shape and positions of all grains in a given volume. These changes are probed by a fine, intense beam of X-rays sent through the sample. Like a three-dimensional ‘frame by frame’ sequence – tiny variations of the transmitted X-ray intensity are recorded when sand and carbonate grains start to react chemically, changing their shapes and transforming themselves into molten glass. “At the ESRF, we can take a microtomography image with a spatial resolution of 1.6 micrometres every few seconds. Observing fast changes with a high spatial resolution deep inside an oven held at close to 1000°C is impossible without X-rays,” says Marco Di Michiel from the ESRF.
The sequences of microtomography images confirmed the importance of good contact between grains of different substances, as these contacts determine whether or not the mixture turns into liquid glass. For example, a calcium carbonate grain can either incorporate itself into the highly reactive amorphous liquid or remain a crystalline defect, depending on the presence or absence of such contacts. The researchers were surprised by the high reactivity of sodium carbonate when still solid: these grains move just before the melting begins which increases the number of contacts with other grains and facilitates the reactions.
By merging hundreds of X-ray tomography images, the scientists produced a video sequence visualizing how different grains in the mixture move and fuse, one after the other, into molten glass as the temperature rose from 750°C to 930°C. “I have been working on these processes for many years, and it was absolutely fascinating to see like in a movie what happens at the onset of the powder/glass transition,” says Emmanuelle Gouillart.
The scientists now wish to vary the sizes of the grains and the way in which they ramp up the temperature, which, in the long term, will tell us how to reduce the number of defects produced at the start of the glass formation process, and help find faster and less energy consuming manufacturing processes. “We also wish to make X-ray imaging methods and data analyses a routine visualization tool for reactive granular mixtures. These are not only used in the manufacture of glass but also of other materials, and I see a huge industrial potential for optimizing these processes,” concludes Emmanuelle Gouillart.
The results are published in the Journal of the American Ceramic Society.

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