Rock-solid option

A unique barium titanate and tin composite developed by US researchers could provide building materials with diamond-style toughness. Siobhan Wagner reports

In an ideal world, where money was no object and engineering techniques had advanced, diamond would be used to build bridges, buildings and even aircraft. It is not only the stiffest and hardest material around but it also has the highest thermal conductivity.

Of course, any notion of using diamond for such large applications does not make sense in the real world. Still, there does appear to be some glimmer of hope for a commercial alternative to the valuable gemstone that combines some of its greater benefits with cost efficiency.

Engineers at the University of Wisconsin-Madison in the US have recently developed a unique barium titanate and tin composite that is the first material known to be stiffer than diamond.

While diamond achieves its rock-solid stability by directional and strong bonds, tying every carbon atom to four others, the Wisconsin engineers created their composite from ordinary materials held together in an extraordinary way.

Barium titanate is a well-researched crystalline material previously used in applications such as microphones or mobile phone speakers.

Lead researcher Prof Rod Lakes decided to use it for his composite because it can change volume during phase transformation. It changes from one solid crystal form to another as it is heated to 110°C or cooled to near 5°C. The researchers used tin as their matrix material because it is relatively stiff (about 25 per cent as stiff as steel and about 70 per cent as stiff as aluminium) and can be easily melted to make a casting.

When pieces of barium titanate are embedded in a tin matrix, the phase transformation, or shift in the arrangement of atoms, is held back, creating stored energy. Lakes compares the process to water that seeps its way into the cracks of a road and freezes. It is unable to expand because it is held in place.

The blocked phase transformation creates negative stiffness, or instability, within the barium titanate, while the tin has positive stiffness, or stability. In the university lab tests, the researchers negated previous scientific findings when they demonstrated a composite that is stiffer than either of its components. This was because in all previous composites, the components were in a minimum energy state. There was no stored energy and both stiffness values were positive.

‘What’s novel here is that we stabilised the negative behaviour by enclosing it into material that has positive behaviour,’ said Lakes. His team demonstrated that if it embedded barium titanate within the tin, the resulting composite material achieves stiffness approaching 10 times that of diamond.

Lakes explained that compliance, both positive and negative, is the inverse of stiffness. When positive compliances and negative compliances are added together, the result is a sum that is close to zero — which corresponds to extreme stiffness.

While the advantage of these materials over diamonds is stiffness, Lakes is keen to point out that this is not the same as strength.

On his university website, Lakes explains it thus: ‘A porcelain coffee cup is stiff: it does not deform easily. But it is not strong: it can break easily. Mountain climbing rope made of nylon is strong: it supports considerable weight. The rope is not very stiff, so it stretches and protects a falling climber from injury.’

These super-stiff materials will be useful applications in products such as computer disk drives, micro-manipulator devices, engine parts and golf clubs, where it is desirable that the items do not deform excessively in response to force.

Lakes said he is not comfortable speculating about future applications for these materials at such an early stage but did give one example that would probably excite civil engineers and architects.

‘One possibility [for these materials] is extremely tall buildings that don’t flex because now, they tend to flap in the wind,’ he said. ‘The problem is now ameliorated by using a damper so it doesn’t vibrate as much. But you could just use a stiff element that could perhaps prevent the occurrence of flapping.’

With the way the material is now configured, however, there are some limitations. Like the phase transformation of water to ice at 0°C, the barium titanate phase transformation is also governed by temperature. The current composite exhibits extreme stiffness only within a narrow temperature range of less than 10ºC.

Lakes said the temperature at which this material works is like a hot day in the desert, about 65ºC. But theoretically, he said, the material can be made stable in different ways, so the researchers may not have those temperature limitations with some future tune-ups they may make to the material in the future.

While the material has proved to be stiffer than diamond, the composite does not have all the attributes of the gemstone. It is doubtful the material — with its silvery metal appearance and embedded white grains — will ever be used to make engagement rings. Also unlike diamond, the material is not expected to be able to scratch glass.

The material is stiffer than diamond, not harder, explained Lakes, adding that the researchers have not measured properties such as hardness, strength or toughness but they do not have a compelling reason to expect extreme hardness or strength.

They plan to test these properties in the future, but it is unlikely they will put their material under any sort of bullet-proof test. There is no need to shoot the composite with a pistol, said Lakes, if mathematics and theoretical models prove their assertions.

If any continued work with the material is to go on, however, the researchers will need additional funding, so they are now writing to US agencies for support.