Technion scientists resolve 133-year-old theory explaining the nature of layers found between different materials
Gold crystals equilibrated on a polycrystalline
In groundbreaking research reported in April 2011 by the top-tier journal Science, researchers at the Faculty of Materials Engineering describe the characteristics of nanometer-scale layers formed between different materials. They found these layers to have both solid and liquid properties, and to exist at equilibrium, thus supplementing Gibbs’ 1878 theory, which partially explained what happens when two materials come into contact.
sapphire (aluminum oxide) surface.
Using the Titan electron microscope,
a 1.2 nanometer layer was characterized
and its energy determined
“In the 1980s scientists showed that there is a very thin layer of matter between crystals, so thin that it could not be defined as liquid or solid,” explains Prof. Wayne D. Kaplan, faculty dean.
“Researchers around the world had been unable to understand why this layer exists and whether it is temporary or in a state of equilibrium, namely, permanent. Researchers knew that it exists at the interface between ceramic crystals, as well as on the surface of ice, but there was still much controversy over the cause of this phenomenon and its properties.”
Dr Mor Baram, in her doctoral work carried out under Kaplan’s supervision and in cooperation with Dr Dominique Chatain from the French research institute CNRS, proved through a long series of experiments, that such a layer of matter exists at the interface between metals and ceramic materials, and apparently also at the interface between metals and semiconductors.
“This phenomenon enables us to ice-skate, reduces the mechanical properties of ceramic materials at high temperatures, but seemingly contributes to the stability of innovative microelectronic devices,” says Kaplan.
At the Russell Berrie Nanotechnology Institute, the Technion team conducted novel experiments using the Titan electron microscope and FIB, the latter being kind of a workshop on the nanometer scale. They plated sapphire with a thin film of gold, 0.6 microns thick (for comparison, a single strand of hair is 80-100 microns thick). The researchers heated the samples until they reached equilibrium; that is, until the gold films broke up into billions of tiny gold crystals atop the sapphire. In parallel, there was a source of elements on top of the sapphire that the researchers already knew played a role in the layer between different materials (in their experiment: silicon and calcium). As the samples equilibrated, the calcium and silicon moved to the interface between the gold and sapphire and a thin layer, 0.0012 microns thick (1.2 nanometers), was created naturally, just four to five atoms wide.
“We successfully measured the energy stored between the gold and sapphire in the presence of the thin layer and thereby proved that its presence decreases the energy of the interface, and therefore improves its stability,” Kaplan explains. This also established that the layer of matter exists at equilibrium.
This scientific discovery has technological implications because it will enable scientists to improve the resilience of the bond between ceramic materials and metals, two types of materials that “do not like” to come into contact. Examples include the connection between metal conducting wires and chips in computers, and the protective ceramic coating on blades of jet engines.
This work was supported by the Russell Berrie Nanotechnology Institute at the Technion and the Israel Science Foundation, and the European Community’s Seventh Framework Programme. Dr Mor Baram acknowledges support from the Israeli Ministry of Science via an Eshkol Fellowship. She is currently conducting postdoctoral studies at Harvard University with Prof. David Clarke.