23 - Continuous oxide fibers

Abstract

Continuous, polycrystalline ceramic oxide fibers are characterized by having a high alumina content, an ultra-fine or nanoscale microstructure, which provides good strength and flexibility up to 1200°C and above in oxidizing atmospheres. Continuous oxide fibers are produced by the sol/gel process. Fibers are spun into continuous rovings consisting hundreds of individual filaments, which are heat treated to convert them into ceramic. Fibers can be divided into two classes: alumina-silica fibers consisting of a mixture of transitional alumina and amorphous silica and crystalline α-Al2O3–based fibers. Alumina-silica fibers have their largest applications as high-temperature textiles for insulation, while crystalline α-Al2O3fibers have higher elastic modulus and strength that are more commonly used for composite reinforcement. The strength of oxide fibers is controlled by “weakest link” statistics, i.e., fracture occurs at the largest flaw. Minimizing flaw size during processing enables higher strengths. High-temperature fiber properties are controlled by the size and nature of the crystalline and amorphous phases present; fiber manufacturers are working to optimize microstructures to improve high-temperature performance.