Studies of the nanoscale behaviour of silver led to the creation of a type of the metal 42 per cent stronger than any previously recorded, but which retained all of the electrical conductivity of softer types of silver which, according to project coleader Frederic Sansoz, a materials scientist and mechanical engineer from the University of Vermont, could lead to advances in applications from aerospace to energy.
The key to the discovery, a collaboration between Vermont and the Lawrence Livermore National Laboratory, was the defects which naturally occur in every metal, caused by imperfect formation of the geometrical lattice of atoms that form the material. Sometimes these defects lead to unwanted properties, such as brittleness or softening. Sansoz and Lawrence Livermore lead scientist Morris Wang were attempting to solve a problem which has dogged material scientists: whenever an alloy is made to overcome one of these unwanted properties, the electrical conductivity drops.
In a paper in Nature Materials, Sansoz and Wang describe how they doped silver with a small percentage of copper to control the behaviour of defects in the silver lattice. Only a trace amount of copper was required – less than one per cent by weight – but this produced a marked improvement in conductivity. The copper impurity turned two types of inherent nanoscale defect in the silver into a much stronger internal structure, the team explained.
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The research exploited a property known as the Hall-Petch relation, where the smaller the micro crystals – regions of defect-free order – that compose a metal become, the stronger it gets. The Hall-Petch relation breaks down when the micro crystals are smaller than tens of nanometres across, as the boundaries between micro-crystals become unstable. When copper is introduced into the silver, it is attracted to the defects in the lattice, Sansoz said, and stops the boundaries between micro-crystals and another form of defect, known as coherent twin boundaries, from moving.
Coherent twin boundaries are structures of paired atoms that form a symmetrical mirror-like crystalline interface that under normal circumstances strengthens the metal by stabilising crystalline interfaces, but which also weaken as the space between the paired atoms falls below a few nanometres. Copper atoms, being slightly smaller than silver atoms, can stabilise the lattice but are at such low concentration that they do not interrupt the conductivity.
"The copper atom impurities go along each interface and not in between," Sansoz explained. "So they don't disrupt the electrons that are propagating through."
The team called the new type of alloy a "nanocrystalline-nanotwinned metal" and in their paper they claim that these have "unprecedented mechanical and physical properties". Not only does it overcome the softening previously seen as micro crystals and twin boundaries get too small, it overcomes the theoretical Hall-Petch limit at which crystal size reduction ceases to strengthen the material.
The team reports that an "ideal maximum strength" can be reached in metals with twin boundaries only 7nm apart. Moreover, a heat-treated version of the silver-copper alloy had a hardness above what had previously been thought to be the theoretical maximum.
"We've broken the world record, and the Hall-Petch limit too, not just once but several times in the course of this study, with very controlled experiments," said Sansoz.
Sansoz said that this new class of metal could have many applications, and that the principle could be applied to other metals.
“This is a new class of materials and we're just beginning to understand how they work," he said. "When you can make material stronger, you can use less of it, and it lasts longer, and being electrically conductive is crucial to many applications."
Some possibilities include lighter aircraft and more efficient solar cells, and there may also be applications in nuclear reactors and the other systems in nuclear power stations.
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