Scientists have for the first time developed a transparent sample of a popular industrial ceramic that is the second hardest material after diamond and can withstand substantially higher temperatures. The super-hard window made of cubic silicon nitride can potentially be used under extreme conditions like in engines, researchers said. "Silicon nitride is a very popular ceramic in industry," said Norimasa Nishiyama from German Research institute DESY. "It is mainly used for ball bearings, cutting tools and engine parts in automotive and aircraft industry," said Nishiyama, who now is an associate professor at Tokyo Institute of Technology in Japan. The ceramic is extremely stable, because the silicon nitrogen bond is very strong. At ambient pressures, silicon nitride has a hexagonal crystal structure and sintered ceramic of this phase is opaque. Sintering is the process of forming macroscopic structures from grain material using heat and pressure.
The technique is widely used in industry for a broad range of products from ceramic bearings to artificial teeth. At pressures above 130 thousand times the atmospheric pressure, silicon nitride transforms into a crystal structure with cubic symmetry that experts call spinel-type in reference to the structure of a popular gemstone. Artificial spinel (MgAl2O4) is widely used as transparent ceramic in industry. "The cubic phase of silicon nitride was first synthesised by a research group at Technical University of Darmstadt in 1999, but knowledge of this material is very limited," said Nishiyama. The team used a large volume press (LVP) to expose hexagonal silicon nitride to high pressures and temperatures. At about 156 thousand times the atmospheric pressure and a temperature of 1,800 degrees Celsius a transparent piece of cubic silicon nitride formed with a diameter of two millimetres. Analysis of the crystal structure showed that the silicon nitride had completely transformed into the cubic phase. "The transformation is similar to carbon that also has a hexagonal crystal structure at ambient conditions and transforms into a transparent cubic phase called diamond at high pressures," said Nishiyama. "Cubic silicon nitride is the hardest and toughest transparent spinel ceramic ever made," he said. Cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride," Nishiyama said. "But boron compounds are not transparent, and diamond is only stable up to approximately 750 degrees Celsius in air. Cubic silicon nitride is transparent and stable up to 1,400 degrees Celsius," he added. The study was published in the journal Scientific Reports.
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