Recent research at the National Institute of Materials Science in Japan on negative thermal expansion (NTE) materials has led to the discovery of alloys exhibiting unexpectedly large thermal contraction. |
One
common reason that people with fillings experience toothache is that
their fillings expand at a different rate to the original tooth when,
for example, drinking a hot drink. Contrary to intuition, however, not
all materials expand when heated—some actually contract. Recent research
on these so-called negative thermal expansion (NTE) materials has led
to the discovery of alloys exhibiting unexpectedly large thermal
contraction.
Controlling
the thermal expansion of composites is important for producing
nanometer-scale electronic circuits, as well as the next-generation fuel
cells and thermoelectric devices. An ability to combine NTE materials
with ‘normal’ materials which expand upon heating ensures a reduction in
thermal expansion in a composite material—something that people with
tooth fillings would appreciate. An example of such a composite is
Invar, an iron-nickel alloy with a uniquely low coefficient of thermal
expansion. As a result it is used where high dimensional stability is
required, such as precision instruments, clocks or seismic creep gauges.
Koshi
Takenaka at the Department of Crystalline Materials Science, Nagoya
University in Japan works on NTE materials for practical applications.
In the latest issue of Science and Technology of Advanced Materials he
summarizes the physical mechanisms governing NTE with emphasis on recent
developments.
Takenaka
notes that, “NTE materials will expand our capability of
thermal-expansion control, opening a new paradigm of materials science
and technology thermal-expansion-adjustable composites”. One challenge
facing the scientist is that the addition of NTE materials to composites
leads to undesirable instabilities at interfaces. New methods for
producing stable interfaces between the host composite and NTE
compensators are of critical importance. Nevertheless, the so-called
‘one-component’ materials—such as manganese antiperovskites, zirconium
vanadates, and hafnium tungstates—exhibiting negligible thermal
expansion offer a promising route towards achieving this goal.
Negative thermal expansion materials: Technological key for control of thermal expansion
