Monday, 17 October 2011

Secondary Science - The Nobel Prize for Chemistry

The “impossible” crystals -  The 2011 Nobel Prize for Chemistry

This year’s Nobel Prize for Chemistry has been awarded to Prof. Daniel Shechtman of Israel.

The Story

In 1982, forty-one year old Daniel Shechtman was spending  a couple of years in the USA, on sabbatical from his post in the Materials Science Department at Technion, the Israel Institute of Technology.  He was using electron diffraction to investigate the structure of crystals of some alloys of aluminium..  The results gave a surprising pattern.  Shechtman checked and re-checked his results but they still seemed to show that the arrangement of the atoms in the crystal was what had previously been thought to be impossible.  His American colleagues thought his results were ridiculous and a journal refused to publish them. 

Shechtman returned to Israel and with help from friends succeeded in getting his results published. Many scientists criticised his work but others found other examples of the impossible crystals.

At the same time as Shechtman’s discovery, scientist Alan Mackay was building models using unusual patterns devised by the mathematician Roger Penrose.  Mackay’s models matched Shechtman’s results – the crystals were not impossible after all. Now the study of quasicrystals is growing and uses are being found for the strange materials.  Daniel Shechtman is still Professor of Materials Science at Technion.

The Science
It had always been thought that the arrangement of particles in crystals must be orderly, repetitive and symmetrical.  This means that only a limited number of patterns were allowed.  If we use 2-D patterns as an example, triangles, squares or hexagons can fit together with no gaps and the pattern repeated time after time.  Pentagons will not fit.  However Roger Penrose showed that patterns made up of rhombuses or other repeated simple shapes could be built up into patterns containing pentagons, but each part of the pattern looked different.  Islamic artists had made mosaics of tiles in these patterns in medieval Iran.  Mackay showed that these same patterns could be built in 3D using balls to represent atoms.

Electron diffraction pictures are obtained when crystals scatter a beam of electrons.  The effect is similar to using X-rays.  The pictures appear as a pattern of bright spots.  Normal crystals produce patterns of spots with multiples of 4 of 6 spots in circles corresponding to the normal cubic or hexagonal arrangement of the particles.  Shechtman’s pictures had rings of ten spots showing that the atoms were arranged in the non-repeating patterns predicted by Mackay’s models.

The applications

Since Shechtman’s discovery many quasicrystalline materials have been found and are beginning to find uses.  Steel containing quasicrystals embedded in the normal crystalline structure is hard and durable.  It is used in razor blades and fine needles.

Other quasicrystalline materials are poor conductors of heat and electricity and have non-stick surfaces and high melting points. They are being used in non-stick cookware, in electronic devices such as LEDs and to insulate engines.

The lessons for science

We think of scientists as being methodical and sceptical.  Shechtman was a good scientist in the way that he checked his data but was intrigued by a surprising and perhaps anomalous result.  His colleagues do not come out of the episode as well.  Their immediate rejection of his conclusions because they disagreed with the textbook on crystal structures showed an unwillingness to consider new and different ideas.
The definition of crystals has had to be changed – it is now any structure that produces a distinctive diffraction pattern.

Further investigation

Go to the Nobel Prize website for more details about Shechtman’s prize winning work.
Search terms:  Professor Daniel Shechtman, quasicrystals,  Penrose tiling, electron diffraction,  Alan Mackay crystals.

Peter Ellis

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