 
New High Temperature Materials Pave Way for Next Generation Energy and Environment Systems
Recent work by CU-Boulder mechanical engineering Professor Rishi Raj, in collaboration with R. Riedel at the Technical University of Darmstadt and H.J. Kleebe at the University of Bayreuth in Germany, has focused on silicon carbonitrides (SiCN), a new class of materials with highly unusual properties at high temperatures. SiCN remains thermally stable to 1600 degrees C and higher, and has an oxidation rate nearly 10 times slower than that of polycrystalline silicon carbide and silicon nitride, according to Raj, an expert in high temperature research in the field of metals and ceramics.
"Carbon is an outstanding material for building high temperature systems, except that it burns in air above about 800 degrees C," Raj says. "The SiCN materials have all the good attributes of carbon except that they do not burn at temperatures approaching 2000 degrees C." The SiCN materials are also unconventional in the way they are fabricated. Instead of using the traditional powder route, SiCNs are made from polymer precursors that can be cast directly into net shape, applied as coatings, or drawn into fibers like polymers.
The SiCN materials also are emerging as one of the most chemically inert materials. This property, along with low cost processing, is leading Raj and his students to consider biomedical applications such as minimally invasive technologies and implants.  Engineering Home |
Critical
advances in energy, environment, and space technologies often emerge
from new developments in high temperature materials. The efficiency
of gas turbines, for example, and the extent of pollution they produce,
is directly related to the combustion temperature. In situ combustion
of carbon in a high temperature filter can eliminate particulate emissions
from diesel engines. Space propulsion also is critically dependent on
lightweight, ultrahigh temperature materials.
The
remarkable properties of SiCN derive from their amorphous structure
in contrast to the crystalline structure of silicon nitride and silicon
carbide, which represent state-of-the-art high temperature materials.
As shown in the accompanying high resolution transmission electron micrographs,
the crystalline materials contain grain boundaries which weaken their
high temperature mechanical properties. The SiCN materials remain predominantly
amorphous, allowing them to retain their properties to very high temperatures.
SiCN also has an ultra low coefficient of thermal expansion, which would
impart outstanding thermal shock resistance. Lastly, SiCN exhibits almost
zero deformation at very high temperature.
Early
applications are likely to emerge in coatings, fibers, and MicroElectroMechanical
Systems (MEMS) technologies requiring materials that can perform at
ultrahigh temperatures, strong temperature gradients and extreme environments.
Raj is leading a proposal to create a new laboratory at CU-Boulder for
scientific and application research in carbon based materials, such
as silicon carbonitrides, boron carbonitrides, and transition metal
carbides, for application at temperatures up to 4000 degrees C. This
lab will provide the tools for fundamental research and the development
of specific materials with unique properties, he says.