New 'extralibral' composite 'stiffer than diamond'

Materials World magazine
1 Apr 2007

Diamond's title as the world's stiffest material may have been usurped by a new composite made up of molten tin and the mineral barium titanate.

A team of researchers from Washington State University and the University of Wisconsin-Madison, both in the USA, and Ruhr-University Bochum, Germany, created the material, which they called ‘extralibral' as it is ‘on the boundary of balance,' says Dr Roderic Lakes, a Professor of Engineering Physics at the University of Wisconsin-Madison.

To create the super-stiff material, particles of barium titanate, which is often used as an insulator in electronic components such as microphones or mobile phone speakers, were evenly dispersed through tin and heated to about 300ºC. Once ingots of the new composite had cooled, three centimetre by two millimetre samples were tested for stiffness at various temperatures - between 58 and 59ºC, they were shown to be stiffer than diamond. Some were nearly 10 times more resistant to bending.

The secret to these results is the way that tin and barium titanium are bonded. The team chose the materials based on their unique characteristics.

‘I contemplated the composite equations and thought, "what will happen if I reverse the stiffness of one constituent?"' explains Lakes.

Tin is a relatively stiff material that can easily be melted, while barium titanate changes its form as it is warmed or cooled. When the heated ceramic material is embedded in tin, however, its phase transformation is held back as it cools, creating stored energy.

According to researchers, the negative stiffness, or instability, of the barium titanate, matched with the stiffness, or stability, of the tin, creates a level of stiffness that goes far beyond that achieved by any other material.

‘This was not an accident,' says Lakes. ‘The system was chosen because barium titanate undergoes a volume change during phase transformation. The stiffness of each constituent was balanced with the other. We did expect to see extreme stiffness and made sure the apparatus was capable of measuring it.'

High stiffness does not, however, equate to increased strength, and it is unlikely that extralibral will match diamond in terms of hardness. Stiffness is nevertheless a desired trait for computer disk drives, micro-manipulator devices, engine parts and golf clubs, as these items need to withstand force without deforming. It is possible that the new material could provide a cheaper alternative to diamond for such applications.

The composite is not yet ready for commercial use, as its extreme stiffness is only achieved within a narrow temperature range of just over 1ºC. Further experimentation (which has yet to be published) has shown the material to exceed the stiffness of diamond within a wider range of 10ºF (around 5.5ºC). The researchers plan to seek federal support to continue their work.