Researchers at Germany’s University of Erlangen-Nuernberg have developed a sol-gel process for producing biomorphic (Si, Ti, Zr)-carbides. This process involves biotemplating, where biostructures are converted into ceramics by first pryolyzing a bioprecursor at 800°C, infiltrating this pyrolyzed product four times with a sol, followed by gelation and a final pyrolyzing step at 1600°C, which converts the material into a metal carbide with a microcellular structure. This process has the advantages of being inexpensive and being able to produce a large variety of microstructures since a large variety of biological templates are available.
Materials produced so far include ZrC-C (with a high volume of C), TiC-C, and SiC-C. The porosity and composition are a function of the porosity of the carbon precursor and the sol gel precursor. By using melt infiltration, composites can be made based on aluminum, magnesium, and silicon. Al2O3-SiC nanocomposites have also been produced at Germany's University of Darmstadt by infiltrating an alumina matrix by a liquid polycarbosilane.
Other research on nanocomposites has involved materials for photonic applications. For instance, Rutgers University is investigating incorporating passive nanoparticles in an active transparent matrix or active nanoparticles in a passive matrix. For the first approach, nanosized dopants are based on single wall carbon or VOx nanotubes. The second approach involves Eu3+, Nd3+, and similar rare earth dopants in a sol gel glass host.
The second approach has several major advantages. By mixing a variety of different types of particles, multiple emissions in a single host are possible. By controlling powder properties, the line width can be varied. The host matrix can also be optimized to improve other optical properties. Impurities are avoided and scattering is minimized by reducing the particle size of the dopants. Such nanocomposites have applications as ultrabroadband optical amplifiers.
Polymer matrices are also being considered, such as perfluorocyclobutane (PFCB) and toluene based polymers. These types of matrices are inexpensive to manufacture and are better index matched. Dopants include Ho-LaF3 and other rare earths.
The next meeting on composites sponsored by the American Ceramic Society is the 5th International Conference on High Temperature Ceramic Matrix Composites, Westcoast Grand Hotel, Seattle, Washington on September 12-15, 2004. For details see www.ceramics.org/htcmc5/.
About the Author
Laurel M. Sheppard is President of Lash Publications International (www.lashpublications.com) and Contributing Editor of Ceramic Industry (www.ceramicindustry.com). She has a B.S. in ceramic engineering and has written numerous articles on ceramic technology and manufacturing, as well as a market report on ceramic matrix composites for Business Communications Co. (www.bccresearch.com).