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Transparent ceramics make super-hard windows

March 21, 2017

Transparent ceramics make super-hard windows:
Scientists synthesise first sample of transparent silicon nitride at DESY

Transparent polycrystalline cubic silicon nitride.

Figure 1. Transparent polycrystalline cubic 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 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.

A bright-filed TEM image of polycrystalline cubic silicon nitride.

Figure 2. A bright-filed TEM image of polycrystalline cubic silicon nitride.
The average particle diameter is about 150 nanometers.

"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 spinel-type crystal structure with cubic symmetry. Spinel (MgAl2O4) is a popular gemstone and the most popular transparent ceramic in industry. It is the namesake for a whole class of materials with similar crystal structure, where the oxygen (O) atoms form a cubic close-packed lattice and metal atoms — like manganese (Mg) and aluminium (Al) — occupy certain positions within the lattice. In the case of c-Si3N4, the nitrogen (N) atoms form a cubic lattice and the silicon (Si) atoms are sitting within the lattice. Cubic silicon nitride has the same inner structure as spinel.

"The cubic phase 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 submit hexagonal silicon nitride to high pressures and temperatures. At approximately 156 thousand times the atmospheric pressure (15.6 gigapascals) and a temperature of 1,800 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 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 forsee diverse industrial applications for their super-hard windows. "Cubic silicon nitride is the third hardest ceramic known, after diamond an 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 1,400 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.

Mechanical properties of cubic silicon nitride.

Figure 3. Mechanical properties of cubic silicon nitride.

Reference

Authors:
Norimasa Nishiyama1, 7, *, Ryo Ishikawa2, Hiroaki Ohfuji3, Hauke Marquardt4, Alexander Kurnosov4, Takashi Taniguchi5, Byung-Nam Kim5, Hidehiro Yoshida5, Atsunobu Masuno6, Jozef Bednarcik1, Eleonora Kulik1, 4, Yuichi Ikuhara2, Fumihiro Wakai7, Tetsuo Irifune3, 8
Title of original paper:
Transparent polycrystalline cubic silicon nitride
Journal:
Scientific Reports, 2017
DOI :
Affiliations :
1 Deutsches Elektronen-Synchrotron DESY,Germany
2 Institute of Engineering Innovation, The University of Tokyo,Japan
3 Geodynamics Research Center, Ehime University, Japan
4 Bayerisches Geoinstitut, Universität Bayreuth, Germany
5 National Institute for Materials Sciences, Japan
6 Graduate School of Science and Technology, Hirosaki University, Japan
7 Laboratory for Materials and Structures, Tokyo Institute of Technology, Japan
8 Earth-Life Science Institute, Tokyo Institute of Technology, Japan

Further information

Specially Appointed Associate Professor Norimasa Nishiyama
Institute of Innovative Research

Email nishiyama.n.ae@m.titech.ac.jp
Tel +81-45-924-5337

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