Diamond Loses Its Crown
Diamond loses its stiffness crown to new material
16:22 02 February 2007
A material that is stiffer than diamond has been created by mixing particles of the mineral barium titanate and molten tin. Diamond was previously the stiffest material known.
The new material was made by a team from Washington State University and Wisconsin-Madison University, both in the US, and from Ruhr-University Bochum in Germany.
They mixed molten tin, heated to about 300ºC, with pieces of a ceramic material called barium titanium - often used as an insulator in electronic components. The particles were each about one-tenth of a millimetre in diameter and were dispersed evenly through the tin using an ultrasonic probe.
Once ingots of the new composite had cooled, rectangular or cylindrical samples 3 centimetres long and 2 millimetres across were tested for stiffness. The response of the samples to bending was tested by gluing one end to a strong support rod and the other to a magnet with a small mirror attached.
An electromagnet was used to exert a rhythmic force on the material one hundred times per second. The resistance of the composite to the bending force - called the Young's modulus - was recorded by a light sensor monitoring laser light bouncing off the mirror.
The tests were carried out at a variety of temperatures. Between 58ºC and 59ºC the samples became stiffer than diamond. Some were nearly 10 times as resistant to bending.
"This is very clever," says composite materials researcher Mark Spearing of Southampton University, UK. "They've come up with an interesting material."
The material's stiffness results from the properties of the barium titanate pieces, Spearing says. As the material cools, its crystal structure changes, causing its volume to expand.
"Because they are held inside the tin matrix, strain builds up inside the barium titanate," Spearing explains, "at a particular temperature that energy is released to oppose a bending force."
Since energy has to be stored in the material to make it super-stiff, the creators have only really measured an "apparent Young's modulus", says Spearing. A true Young's modulus is an inherent property of a material, and would also be more constant across a greater range of temperatures, he notes.
Nevertheless, the new material could still have useful applications, says Spearing, perhaps for making shock-protective casings. "You might be able to make a tune-able damper that transmits force very well under certain conditions but behaves differently and is softer the rest of the time," he says.
Journal reference: Science (DOI: 10.1126/science.1135837)