Alumina ceramic continuous fiber is one of the most important materials because of its high strength and modulus, high temperature resistance, chemical inertness in both oxidizing and reducing atmospheres even at high temperature, resistance to attack from non-oxide materials, good electrical insulation and low thermal conductivity. The high strength and high-temperature resistance have allowed the development of metal and ceramic matrix composites with high tensile strength or high-temperature load-bearing capability. Thus, alumina ceramic continuous fiber is of great importance in aerospace, advanced manufacturing, and other fields.
The Al2O3 content in alumina ceramic fiber is usually more than 70% (mass fraction), and generally contains a small amount of other oxides, such as SiO2, B2O3, Fe2O3, ZrO2, and etc. Amorphous SiO2, transition phase alumina, α-Al2O3 and mullite are the common phases in alumina ceramic fibers, resulting in various phase compositions and microstructures. Chemical composition and the microstructure have great effects on the fibers’ mechanical performance and temperature resistance. Depending on the chemical composition and the microstructure, the alumina ceramic fibers show the tensile strength from 1.5 GPa to 3.1 GPa, and a long-term using temperature varied from 1 000 ℃ to 1 300 ℃. The careful selection is necessary according to the unique circumstances and conditions involved in the use and processing of the alumina fibers. Furthermore, the preparation of alumina ceramic fibers with controlled composition and microstructure is critical.
Main methods for manufacturing alumina ceramic fibers can be classified as slurry process, melt-spinning process, prepolymerization process and sol-gel spinning process. The melt spinning method was limited in the length of the fibers, thus, it was not suitable for the preparation of the alumina continuous fibers. While it is difficult to prepare alumina fibers with high strength from the slurry method. Excellent alumina continuous fibers can be obtained through the pre-polymerization process and sol-gel spinning process, in which the sol-gel method shows the advantages of adjustable composition, uniform composition at the atomic level, relative moderate preparation conditions and low cost, becoming the most important preparation method for alumina ceramic continuous fibers.
In the past decades, a family of alumina fibers have been developed, typically as the commercial 3M NextelTM alumina ceramic fibers, which have been used in a variety of areas. However, its export to China is restricted by developed countries because of the application in military field, such as aerospace. To develop the local production, many researchers of China have devoted to the preparation of alumina continuous ceramic fibers, mainly by new sol, that is, how to transform from the gel fiber to ceramic fiber. Fortunately, significant progress has been made recently, so that commercialization of the ceramic fiber comes to be true. This further promotes the applications of alumina ceramic fibers. However, there is still a certain gap in performance between the local produced fibers and the fibers abroad. Continued technological innovation is still an important task in the future.
Summary and prospects At present, the development of alumina based ceramic continuous fiber in China is mainly carried out under the national academic financial support. The main work at current stage mainly focuses on how to produce the fibers which could meet the basic properties. Most research and development usually rely on model products. Few attention is paid to the research on relevant scientific mechanism and theory, resulting in lack of theoretical support for new applications and in lack of independent innovation ability. Since most of the applications of alumina continuous fiber are based on its performance of high temperature resistance and high strength, it can be predicted that improving the tensile strength and thermal resistance of alumina continuous fiber is still the main goal in the future. Therefore, it is believed as important work to carry out key basic theoretical research, to clarify the intrinsic relationship between the composition-structure-processing-performance of alumina ceramic fibers, and to establish a theoretical model of high-performance alumina ceramic continuous fiber. These investigations can direct the design and preparation of the fibers, to achieve breakthroughs hopefully in the strength and temperature resistance of alumina ceramic continuous fiber, and to furtherly promote the application of fiber and the composite in aerospace and the related fields.