SA scientists lead breakthrough in understanding of expansion/contraction of materials

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SA scientists lead breakthrough in understanding of expansion/contraction of materials

Engela Duvenhage

Photomicrographs of the same crystal showing the decrease in size as the temperature is lowered from 50°C (323 K) to -28°C (245 K).

Chemists know that most materials expand when warmed, while only a handful of materials, do the reverse and shrink when the heat is on. No definitive answers have emerged regarding the extent to which some solids will expand or shrink when heated, but a group of Stellenbosch scientists have made significant inroads into this fundamental question.

Under leadership of Prof Len Barbour, Stellenbosch chemists have recorded the largest positive (PTE) and negative (NTE) thermal expansion in any known pure organic material.

When warmed, the majority of materials expand, while they shrink when cooled. This is because the amplitude of atomic or molecular vibrations is increased by heat energy in a process called positive thermal expansion.
In negative thermal expansion, the reverse happens, with the molecule contracting as it is heated. This phenomenon, which is very rare, is generally limited to inorganic framework materials and a few polymers.

Prof Barbour, holder of the South African Research Chair (SARChI) research chair in Nanostructured Functional Materials in the SU Department of Chemistry and Polymer Science, carried out the research jointly with Drs Dinabandhu Das and Tia Jacobs.

The results of their fundamental research appear in the January 1 edition of the highly respected international journal Nature Materials.

Anomalously large thermal expansion (NTE and PTE) has been reported almost exclusively for polymers and inorganic materials, but such properties are virtually unknown and unexplored for organic systems.

The study was carried out on diyn diol, which was synthesised in Barbour’s Stellenbosch laboratory.

“What is particularly unusual about this material is the magnitude of the expansion and contraction,” he says. “Also, we have been able to explain the cause of the large NTE and PTE by tracking the behaviour of individual molecules with changing temperature.”

The expansion coefficient is between eight and twenty times as large as that for a typical material, whereas the contraction coefficients are between twice and ten times as large.

The dumb-bell-shaped diyn diol forms crystals in which the molecules are stacked at an angle. At 225 K, this angle is 54.2°, but as the crystal is warmed the angle decreases, reaching 51° at 330 K. This means that, like a rubber band, the crystal stretches in the direction of the axis along which the molecules are stacked, and contracts in the directions of the other two axes.

“There might be many more materials that have even larger thermal expansions,” Prof Barbour speculates. He believes that these research results might one day feed into the development of heat-activated molecular switches and other thermomechanical devices.

More information:

The paper is available online at

http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat2583.html.

Stellenbosch University: www.sun.ac.za

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February 2010	Article

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