FEATURE
BY CHRIS RUTKOWSKI
GLASS
ACTION:
Understanding the properties of glasses
Most people associate magnets and their properties
with physics, but on the fourth floor of the Parker
Building in the Faculty of Science, it’s all about how
magnets can be used in chemical research.
Chemistry professor Scott Kroeker’s research on the
structure of glasses uses huge superconducting magnets
to understand these unique materials. “Nuclear magnetic
resonance is at the heart of all my research,” he says.
Kroeker explains that most solids are conceptualized
as crystalline. A simple example is sodium chloride—
ordinary table salt—where it’s easy to see that it’s made
of tiny crystals.
Glass is formed when certain materials are melted at
a high temperature then rapidly cooled, preventing
crystallization. Instead, the material “freezes” into a
disordered solid state with very different properties.
23 Summer 2014
The most common glass in use today is soda-lime glass,
made of mostly silicon dioxide (SiO2), sodium oxide
(Na2O) and lime (CaO). When cooled from the melt
phase, it produces the window glass that we have in all
our offices and homes.
Materials considered glasses do not have crystalline
order and are difficult to study on a molecular scale
because their local structure varies subtly in different
parts of the glass.
In fact, X-ray diffraction can’t resolve the structure
of glasses, but a completely different process known as
nuclear magnetic resonance spectroscopy (NMR) can be
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Mike Latschislaw
FEATURE
used to probe their molecular structure
by looking at the various frequencies
within the material.
Kroeker’s lab contains three NMR
spectrometers. Unlike the magnetic
resonance imaging (MRI) systems
found in many hospitals, the magnets
in Kroeker’s lab stand vertically.
In MRI systems, the magnets are
oriented on their sides with a large hole
into which patients pass for internal
analysis. MRI uses radio frequency
fields to map the local environments
of the hydrogen nuclei of water in
25 Summer 2014
different parts of a body, providing
images representing the internal
structure of the tissue.
The NMR instruments used to
study the structure of glasses don’t use
magnetic field gradients to produce an
image, but instead use homogeneous
fields to measure absorbed electromagnetic radiation at frequencies that
depend on the isotopes under observation. The structural information comes
from the chemical shift of a nucleus, the
characteristic frequency with which the
atom resonates and which can be mea-
sured with great accuracy. The NMR
spectrometer detects such frequencies
and can determine the local molecular
structure of materials by comparing
frequencies of known elements.
Although much of his research
involves glassy materials, Kroeker
considers himself principally an
NMR spectroscopist.
“What I do is very specialized.
Without an effective and efficient NMR
spectrometer, much of this research
would be impossible, and I’d have
nothing to do!” he laughs.
Facing page: Light through an optical fibre.
Part of Kroeker’s research on
glasses is concentrated on fibre
optics, the thin transparent wiry
material that is widely used in information technology. Optical fibres
are much more efficient than copper
wire for transmitting information,
but even they have their limits.
Whereas copper wire needs periodic
repeaters to boost electrical signals
in order to transmit long distances,
light travelling through optical fibres
doesn’t need as many repeaters, but
it does need some for long-distance
transmission. The distance the
light can travel before regeneration
depends on the purity of the
glass fibres.
Fibre optic repeaters are usually
inline lasers in which photons are
reflected back and forth until they
burst out of the cavity as a new
laser pulse. These lasers are typically
spaced about 100 kilometres apart,
so that very long distances can be
covered at the speed of light.
Kroeker uses NMR spectroscopy
to study how the optical properties
of new laser glasses are influenced by
the composition and structure of the
glass. More effective laser materials will improve long-distance data
transmission.
A second aspect of Kroeker’s
research looks at exactly the reverse:
studying how to add impurities into
melted materials as they cool so that
they don’t crystallize but instead
form glass. This is important in a
number of processes, but none so
critical as in the long-term storage
of nuclear waste.
Kroeker explains that most
countries reprocess radioactive
waste from the spent-fuel rods used
in nuclear power. Canada stores its
nuclear waste underground rather
than reprocessing.
In countries where nuclear waste
is reprocessed, such as France, the
procedure generates waste products
in liquid form.
“It’s necessary to deal with this
nuclear waste properly,” says Kroeker,
“because it can remain dangerous to
humans for up to a million years.”
The best way to contain such liquid nuclear waste is to immobilize it
in glass. Typically, the waste is mixed
in with liquid borosilicate glass at a
temperature of about 1100°C. This
temperature is hot enough that the
nuclear waste products dissolve
within the liquid glass and then
harden into a more compact and
durable material.
However, there are some radioactive elements that don’t dissolve
well in the liquid glass. Instead, they
create crystalline regions within the
glass that can, unfortunately, dissolve
in water. Obviously, this would be
a problem if the hardened glass
came in contact with water while in
storage. Kroeker’s research involving nuclear waste is determining
the nature of the crystalline regions
within glass.
“X-ray diffraction can’t do it,”
he says. “The only way to probe the
resulting complex glass-crystalline
composites is NMR spectroscopy.”
To study how the crystalline deposits form inside the glass, Kroeker
uses the cesium-133 isotope, which is
not radioactive but reacts chemically
exactly like its radioactive cousins,
cesium-137 and cesium-135, found
in nuclear waste. He does NMR of
the solid glasses at room temperature
and of the melted glass using laserheated equipment, so he can mimic
“We are trying to understand
the chemistry of partly
crystallized glass to improve
its durability. My research
focus is to determine the
chemical composition of
the crystalline phases and
ultimately, to eliminate
crystallization so that all
the dangerous constituents
are securely locked up in
the glass.”
the industrial formation of these
materials.
Kroeker notes: “We are trying
to understand the chemistry of
partly crystallized glass to improve its
durability. My research focus is to determine the chemical composition of
the crystalline phases and ultimately,
to eliminate crystallization so that
all the dangerous constituents are
securely locked up in the glass.”
He adds: “While it seems complicated, our hope is that the work
we are doing here in Manitoba will
create a safer world for generations
to come.”
The main funding for Kroeker’s
work is through grants from the
Natural Sciences and Engineering Research Council of Canada
(NSERC) and Canada Foundation
of Innovation (CFI). Additional
funding has facilitated his travel and
research visits to the UK and France
to work with their nuclear waste
organizations. n
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