Materials with viscoelastic stiffness greater than diamond
Answers to some questions
Rod Lakes, University of Wisconsin
Why use tin?
Tin as a matrix material is convenient because it is relatively stiff (one quarter as stiff as steel; about 70% as stiff as aluminum) and it can be easily melted to make a casting. More recently other materials have been used as a matrix.
Why use barium titanate?
Barium titanate changes from one solid crystal form to another as it is heated or cooled: a phase transformation. The freezing of water to form ice is a phase transformation of a different kind. Barium titanate is a rather well known material which has been studied by others. It shows phase transformations near 110 degrees C and near 5 degrees C. In composites, however, the transformation is restrained and the temperatures do shift. Moreover, barium titanate exhibits a volume change during transformations. The volume change occurs even in pieces large enough to have many domains.
How are the materials combined?
The tin is melted and the grains of barium titanate are kept dispersed in the tin by ultrasound. The material is then cast into an ingot.
Can the temperature of operation be varied?
Based on the underlying concept, this should certainly be possible. Indeed, more recent experiments show that the temperature can be shifted over a considerable range for which high stiffness and high damping are maintained.
Will these materials stop bullets?
We don't know; we have not shot anything into them.
Will these materials scratch glass?
The materials are stiffer than diamond, not harder. We have not measured properties such as hardness, strength, or toughness. We do not have a compelling reason to expect extreme hardness or strength. High toughness is a distinct possibility.
What is the advantage of high stiffness?
Stiffness is not the same as strength. 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. High stiffness is a major advantage in computer disk drives, in micro-manipulator devices, in engine parts, and in golf clubs since it is desirable that these items not deform excessively in response to force.
How is the extreme behavior achieved?
Negative stiffness of inclusions is combined with positive stiffness of the matrix to obtain a small compliance hence a large stiffness. This was shown to be possible in a theoretical article by Lakes in 2001. The present Science article represents the first experimental demonstration of extreme stiffness in such a composite. Publications by Lakes and co-workers on this idea are here .
If one were to add a positive and a negative stiffness, the stiffness would be small or zero. It is also possible to add compliances in a composite. Compliance is the inverse of stiffness. In our composites, positive and negative compliances are combined to obtain a small compliance corresponding to a large stiffness.
Are these materials ready for commercial use?
Not yet. The extreme stiffness reported in the Science article occurs in a narrow temperature range of about 3 degrees F to exceed the stiffness of diamond, and 12 degrees F to exceed the stiffness of steel. There is potential, however, to substantially increase the range. Further experiments show a wider range of 10 degrees F to exceed the stiffness of diamond. Recent theoretical study by W. Drugan in Engineering Physics shows it is possible to stabilize such composites without the need for a narrow temperature range.
What do the materials look like?
They do not look like diamond. The present materials have a silvery metal appearance with white grains embedded. Other materials based on this concept may have a different appearance.
Have other researchers studied related materials and structures?
Yes. By now there is a considerable follow on literature on negative stiffness, snap through, metamaterials, and related concepts.
Will these materials make my teeth whiter?
I do not think so.
More extremal materials