Scientists have developed the first transparent industrial ceramic material. Made of cubic silicon nitride, the material is second only to diamonds in hardness, but can withstand substantially higher pressures.
The super-hard window offers potential use in extreme conditions like engines. Silicon nitride, a popular ceramic, is mainly used for ball bearings, cutting tools and engine parts in automotive and aircraft industry.
The material, developed by a team of Japanese and German scientists, is extremely stable because the silicon nitrogen bond is exceptionally strong. At ambient pressures, silicon nitride has a hexagonal crystal structure and is opaque. However, at pressures above 130 thousand times the atmospheric pressure, silicon nitride transforms into a spinel-type crystal structure with cubic symmetry called spinel-type. The sintered ceramic of this phase is widely used in industry for a broad range of products from ceramic bearings to artificial teeth.
Norimasa Nishiyama, the study’s lead author and an associate professor at Tokyo Institute of Technology, says, “The cubic phase of silicon nitride was first synthesized by a research group at Technical University of Darmstadt in 1999, but knowledge of this material is very limited.”
Nishiyama’s team used a large volume press to expose hexagonal silicon nitride to high pressures and temperatures. At about 156 thousand times the atmospheric pressure and a temperature of 1800 degrees Celsius, a transparent piece of cubic silicon nitride formed with a diameter of about two millimeters.
A scanning transmission electron microscope showed that the high-pressure sample has thin grain boundaries— the average grain size was about 150 nanometres (millionths of a millimeter). The silicon nitride had completely transformed into the cubic phase.
According to Nishiyama, the high-pressure phase oxygen impurities are distributed throughout the material and don’t accumulate at the grain boundaries like in the low-pressure phase—a crucial requirement for the transparency.
“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,” explains Nishiyama. “However, the transparency of silicon nitride strongly depends on the grain boundaries. The opaqueness arises from gaps and pores between the grains.”
Cubic silicon nitride is the hardest and toughest transparent spinel ceramic ever made. The scientists foresee diverse industrial applications for their super-hard windows.
“Cubic silicon nitride is the third hardest ceramic known, after diamond and cubic boron nitride,” Nishiyama explains. “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 1400 degrees Celsius.”
However, the possible window size is limited due to the large pressure needed to synthesize transparent cubic silicon nitride.
“The raw material is cheap, but to produce macroscopic transparent samples we need approximately twice the pressure as for artificial diamonds,” Nishiyama says. “It is relatively easy to make windows with diameters of one to five millimeters. But it will be hard to reach anything over one centimeter.”
The Tokyo Institute of Technology, Ehime University, University of Bayreuth, Japanese National Institute for Materials Science, and Hirosaki University developed the material.
BCC Research forecasts a booming
global market for transparent ceramics. The market, which reached $194.1 million in 2016, is expected to reach $227.3 million and $548.7 million in 2017 and 2022, respectively, corresponding with a compound annual growth rate (CAGR) of 19.3%.
