Scientists synthesise first sample of transparent silicon nitride
Scientists have synthesised the first transparent sample of a popular industrial ceramic at DESY. The result is a super-hard window made of cubic silicon nitride that can potentially be used under extreme conditions like in engines, as the Japanese-German team writes in the journal Scientific Reports.
Cubic silicon nitride (c-Si3N4) forms under high pressure and is the second hardest transparent nanoceramic after diamond but can withstand substantially higher temperatures.
"Silicon nitride is a very popular ceramic in industry," explains lead author Dr. Norimasa Nishiyama from DESY who now is an associate professor at Tokyo Institute of Technology. "It is mainly used for ball bearings, cutting tools and engine parts in automotive and aircraft industry."
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," says Nishiyama.
His team used a large volume press (LVP) at DESY to expose hexagonal silicon nitride to high pressures and temperatures. At approximately 156 thousand times the atmospheric pressure (15.6 gigapascals) and a temperature of 1800 degrees Celsius a transparent piece of cubic silicon nitride formed with a diameter of about two millimetres. "It is the first transparent sample of this material," emphasises Nishiyama.
Analysis of the crystal structure at DESY's X-ray light source PETRA III 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," explains Nishiyama. "However, the transparency of silicon nitride strongly depends on the grain boundaries. The opaqueness arises from gaps and pores between the grains."
Investigations with a scanning transmission electron microscope at the University of Tokyo showed that the high-pressure sample has only very thin grain boundaries. "Also, in the high-pressure phase oxygen impurities are distributed throughout the material and do not accumulate at the grain boundaries like in the low-pressure phase. That's crucial for the transparency," says Nishiyama.
"Cubic silicon nitride is the hardest and toughest transparent spinel ceramic ever made," summarises Nishiyama. 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," explains Nishiyama. "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, because of the large pressure needed to synthesise transparent cubic silicon nitride, the possible window size is limited for practical reasons. "The raw material is cheap, but to produce macroscopic transparent samples we need approximately twice the pressure as for artificial diamonds," says Nishiyama. "It is relatively easy to make windows with diameters of one to five millimetres. But it will be hard to reach anything over one centimetre."
Tokyo Institute of Technology, Ehime University, the University of Bayreuth, Japanese National Institute for Materials Science, and Hirosaki University were also involved in this research.
Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 per cent) and the German federal states of Hamburg and Brandenburg (10 per cent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY's combination of photon science and particle physics is unique in Europe.
"Transparent polycrystalline cubic silicon nitride"; Norimasa Nishiyama et al.; Scientific Reports, 2017; DOI: 10.1038/srep44755
Thomas Zoufal | EurekAlert!
Hidden talents: Converting heat into electricity with pencil and paper
20.02.2018 | Helmholtz-Zentrum Berlin für Materialien und Energie
Contacting the molecular world through graphene nanoribbons
19.02.2018 | Elhuyar Fundazioa
For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.
In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...
Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale
Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...
For photographers and scientists, lenses are lifesavers. They reflect and refract light, making possible the imaging systems that drive discovery through the microscope and preserve history through cameras.
But today's glass-based lenses are bulky and resist miniaturization. Next-generation technologies, such as ultrathin cameras or tiny microscopes, require...
Scientists from the University of Zurich have succeeded for the first time in tracking individual stem cells and their neuronal progeny over months within the intact adult brain. This study sheds light on how new neurons are produced throughout life.
The generation of new nerve cells was once thought to taper off at the end of embryonic development. However, recent research has shown that the adult brain...
Theoretical physicists propose to use negative interference to control heat flow in quantum devices. Study published in Physical Review Letters
Quantum computer parts are sensitive and need to be cooled to very low temperatures. Their tiny size makes them particularly susceptible to a temperature...
15.02.2018 | Event News
13.02.2018 | Event News
12.02.2018 | Event News
20.02.2018 | Life Sciences
20.02.2018 | Medical Engineering
20.02.2018 | Physics and Astronomy