Ceramic fibers stability

Glass Ceramic fibers and particles Mechanical strength Temperature resistance Chemical resistance Thermal stability... 

Ceramic composites, which use ceramic fiber or whisker reinforcement in a ceramic matrix, are less susceptible to brittle failure since the reinforcement intercepts, deflects and slows crack propagation. At the same time, the load is transferred from the matrix to the fibers to be distributed more uniformly. These ceramic composites are characterized by low density, generally good thermal stability, and corrosion resistance. 

Yajima [6-8] reported further that heteroatoms such as titanium, zirconium or vanadium could be incorporated into the polymers and ceramic fibers to enhance their thermal stability. 

High temperature stability of these nonoxide fibers in air is another critical problem. Thermal stability of ceramic fibers derived from polymeric precursors is of special concern mainly because, as mentioned above, they frequently have undesirable phases present in them. Polycarbosilane-derived SiC fibers, such as Nicalon or Tyranno, involve a thermal oxidation curing process as described above and can contain as much as 10 wt % oxygen (Okamura and Seguchi, 1992). Such fibers decompose at temperatures above 1200 C in a nitrogen or argon atmosphere with SiO and CO gas evolution ... 

When the mesogen moiety is included into the main chain of the polymer, the obtained macromolecule contains inherently rigid units, which usually result in remarkable mechanical properties and thermal stability. Fibers made by these polymers compete with the best ceramic fibers and are far superior to metal fibers [83]. They therefore are ideal candidates as reinforcements for polymer-based composites. However, these materials often have a poor miscibility and adhesion to other polymeric substrates, limiting the range of their applications. This problem basically arises from weak intermolecular interactions either within the liquid-crystalline polymer itself or with the matrix of the composite. Strong ionic... 

The potential for further extensive development in ceramic-fiber composites is thus dependent upon the production of a wide array of fiber compositions as a significant existing problem is of property mismatch between the fiber and matrix. It has, therefore, been emphasized that the challenge is to either obtain new fibers with improved thermoxidative stability in comparison to the limited number currently available, or to suitably modify existing fiber materials. 

Assessing the state of the art in ceramic fibers requires a thoughtful definition of performance requirements and comparisons of these requirements to current capabilities. Defining the state of the art also requires the rigorous characterization of available materials, particularly the properties that are relevant to CMC applications. Microstructural stability, stiffness (elastie modulus), and creep behavior, as a function of temperature and time at temperature in actual service environments, are the most eritieal fiber properties. Therefore, research should focus on determining these properties, including understanding mierostruetural and compositional relationships to these properties. 

Over the past 30 years, however, the performance of ceramic fibers has continued to improve. Today s low modulus carbon fiber has a modulus that was attainable only by the highest grades of early carbon fiber. The latest version of 3M Nextel oxide ceramic fibers show thermal stability far above the Nextel grades that were first commercialized. The Hi-Nicalon SiC fibers manufactured by Nippon Carbon Company are superior to the original Nicalon fibers. It is possible, therefore, that ceramic fibers will eventually reach a level of performance that will justify their price to design engineers of thermostructural components. 

Everitt, G.F. 1988. Stabilized aluminum acetate used for an alumina source in ceramic fibers. Pp. 463-476 in Ultrastructure Processing of Advanced Ceramics, J.D. Mackenzie and D.R. Ulrich (eds). New York John Wiley and Sons. 

Freeman, H.A., N.R. Langley, C.T. Li, J. Lipowitz, and J.A. Rabe. 1993. Polymer derived ceramic fibers having improved thermal stability. U.S. PATENT 5,238,742, 1993 August 24. 

The dimensional stability of plastic structural components - especially when integrated with other parts to make up components or a system, and in particular in combination with different materials (steel, aluminum, magnesium, ceramic, fiber composites) - is often a factor of central importance in technical applications (Fig. 75) [16]. 

Uses Lubricant for mbber molds, textile fibers, metalworking binder, solvent in cosmetics hair preps. pharmaceutic aid in gas chromatography in paints paper coatings polishes ceramics foam stabilizer in fermented malt beverages in food-pkg. adhesives... 

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