Chapter 48. Optimization and Scale-Up of Si-N-O Fiber Synthesis
- Ersan Ustundag,
- Gary Fischman
Published Online: 26 MAR 2008
Copyright © 1999 The American Ceramic Society
23rd Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: B: Ceramic Engineering and Science Proceedings, Volume 20 Issue 4
How to Cite
Vital, A., Ewing, H. C., Vogt, U. and Reh, L. (2008) Optimization and Scale-Up of Si-N-O Fiber Synthesis, in 23rd Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: B: Ceramic Engineering and Science Proceedings, Volume 20 Issue 4 (eds E. Ustundag and G. Fischman), John Wiley & Sons, Inc., Hoboken, NJ, USA. doi: 10.1002/9780470294574.ch48
- Published Online: 26 MAR 2008
- Published Print: 1 JAN 1999
Print ISBN: 9780470375648
Online ISBN: 9780470294574
- silicon oxynitride;
Silicon oxynitride (Si-N-O) fibers exhibit excellent high-performance features and thus are a promising candidate for high temperature application in CMC's. In the original one-stage process developed by EMPA, amorphous Si-N-O fibers grow at 1723 K under flowing ammonia on a precursor powder mixture which consists of silica, silicon carbide and titanium. It is suggested that the fiber-forming steps of the reaction sequence involve ammonolysis of SiO(g) by NHx-radicals. SiO(g) is produced in the precursor bed and the NHx-radicals result from the dissociation of the ammonia. While the resulting fibers show promising high-temperature properties, the process has significant drawbacks including insufficient fiber yield, inhomogeneous fiber growth morphology and especially lack of process control and possibilities for analysis. Attempts to optimize the process conditions did not result in enhanced fiber growth.
Consequently a two-stage fiber growth process was developed which allows the SiO(g) generating stage and the fiber growth stage to be separated. Fiber nucleation in the new set-up results on a porous ceramic filter substrate placed at the confluence of the SiO(g) gas flow from the first stage and a separate stream of NHx-radicals. A Si-N-O layer condenses on the filter surface which functions as a fiber nucleation area from which long Si-N-O single fibers can grow. Analysis of the sub-stages and modeling of the two-stage process with Chemsage™ allowed the correlation of the fiber composition with the fugacity of the SiO(g) produced in the first stage, thereby facilitating process control.