MOST materials expand when they are heated, as their constituent atoms
vibrate more and move farther apart. But scientists in the US have discovered a
material that does exactly the opposite. When the temperature rises, it
shrinks.
鈥淭his compound is unique,鈥� says Arthur Sleight of Oregon State University in
Corvallis, who heads the team. The new ceramic material, called zirconium
tungstate (ZrW2O8), is more than just a curiosity. By mixing
it with other materials, researchers should be able to form composites that will
neither expand nor contract when the temperature changes. This could be valuable
in a number of temperature-sensitive products, from circuit boards to
telescopes.
For years, materials scientists have known of a few substances that expand in
one direction but contract in another when heated. Indeed, one company, Corning
Glass, manufactures a range of cooking ware made from tiny particles of one such
ceramic oriented in random directions so the dishes expand or contract very
little with changes in temperature.
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Several years ago, Sleight began trying to understand these unusual ceramics
and while developing computer models to simulate their behaviour, stumbled
across a substance that contracted on heating in all three dimensions. The
鈥渘egative thermal expansion鈥� occurred only well above room temperature, and
Sleight鈥檚 team has since been looking for a material showing thermal contraction
in a more useful range.
They have now succeeded. In last week鈥檚 issue of Science (vol 272, p
90), the researchers describe zirconium tungstate as contracting by some 0.75
per cent when it is heated from less than half a degree above absolute zero to
777 掳C, at which point it decomposes into the oxides ZrO2 and
WO3. 鈥淲e never expected to find materials with this quality over such a
broad temperature range,鈥� says Sleight.
Sleight believes the explanation behind the ceramic鈥檚 strange qualities lies
with the behaviour of zirconium-oxygen-tungsten 鈥渓inkages鈥� in the
material鈥攑oints at which a zirconium atom and a tungsten atom are coupled
via an atom of oxygen. Each can be pictured as a zirconium atom and tungsten
atom pulling on opposite ends of a rope, with an oxygen atom at a point on the
rope between them.
At low temperatures, the oxygen atom is still and the rope鈥攚hich
corresponds to the chemical bonds between the atoms鈥攊s straight. But as
the temperature increases, the oxygen atom begins to move and push on the rope,
causing it to bend. This brings the zirconium and tungsten atoms closer
together. Because the other chemical bonds in the material are not strongly
affected by increasing temperature, the overall effect is a contraction.