This is an image showing the reaction between a grain of sodium carbonate (red) and two grains of silica (blue and yellow). These reactions produced sodium silicates, the precursors of glass. The grain of sand measures about 100 ?m across. Credit: The American Ceramic Society
have 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.
The results are published in the Journal of the American Ceramic Society.
team of scientists was led by Emmanuelle Gouillart from the joint
research unit between CNRS and Saint Gobain, a global glass
manufacturer, and included scientists from the Universities of Toulouse
and Grenoble, INRIA Saclay and the European Synchrotron Radiation
Facility (ESRF) in Grenoble.
is one of the oldest man-made materials, use of which spread during
ancient Egyptian and Roman cultures. A non-crystalline amorphous
material, it is produced by the fusion of crystalline powder mixtures
heated to high temperatures. These ingredients are quartz sand (silica,
SiO2), sodium and calcium carbonates (Na2CO3, CaCO3), and minor more
industrial foundries, the powder mixture is heated to about 1500 C and
kept at this elevated temperature for many days to eliminate bubbles and
unmolten grains. This consumes a lot of energy and one of the current
industrial challenges is obtaining glass of good quality at lower
temperatures. For example, the global glass industry’s energy
consumption (86.5 TWh in 2005) compares with the entire electricity
production of the Netherlands (108 TWh in 2008).
individual grain of silica normally melts at very high temperatures
(1700 C). Adding carbonates triggers chemical reactions that lower this
temperature. However, the interplay between the geometry of the grains
and the rate of chemical reactions during the early stages of the
melting which starts already well below 1000 C, have remained a mystery
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.
make visible chemical reactions between individual grains, the
scientists used X-ray microtomography, a technique allowing visualizing
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 3D”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.
This is a photo of the beamline ID15A at the ESRF where the experiment on the glass formation were performed. Credit: A. Molyneux/ESRF
the ESRF, we can take a microtomography image with a spatial resolution
of 1.6 ?m 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.
sequences of microtomography images confirmed the importance of good
contact between grains of different substances, as it is these contacts
which 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.
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 to 930 C.
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.
scientists now wish to vary the sizes of the grains and the way in
which they ramp up the temperature. In the long term, these fundamental
studies will tell us how to reduce the number of defects produced at the
start of the glass formation process, and help to find faster and less
energy consuming manufacturing processes.
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