Continuous alumina fibers show a great potential for high-temperature insulation and debris shielding in spacecraft. Their advantages include high strength, high modulus, and excellent oxidation resistance at high temperatures. The sol-gel method is the main process for producing these fibers. This method involves several steps, i.e., thermal decomposition, phase transformation, densification, and grain growth. These steps make it difficult to precisely control the fiber microstructure and optimize the mechanical properties.

This review summarizes recent development on controlling the microstructures of alumina fibers. These include selecting appropriate precursors, optimizing processing parameters, and using sintering aids. Sol-gel precursors are usually formed via hydrolyzing and condensing inorganic aluminum salts, carboxylates, or alkoxides. Gelation proceeds by natural aging or concentration, resulting in three-dimensional networks. After dry spinning or electrospinning, pre-sintering removes most organics. Residual hydroxyl groups are eliminated at high temperatures, affecting fiber densification. Metastable alumina phases convert to a stable α-Al₂O₃ at approximately 1200 ℃ because of a high energy barrier. Additives such as α-Fe₂O₃, α-Al₂O₃, or α-Cr₂O₃ aid nucleation and increase nucleation density. The phase transformation process produces a stable α-Al₂O₃ phase, but at this stage the material still contains a significant amount of porosity, resulting in a poor mechanical strength. Producing dense, high-performance alumina fibers requires high-temperature treatment for mass transport and pore removal. However, densification and grain growth occur together. A rapid grain growth at high temperatures can trap pores, making their removal difficult. Understanding and balancing the relationship between the two is thus central to the sintering process. The strategies to restrain grain growth and control microstructure include two-step sintering and the addition of SiO₂, MgO, or ZrO₂.

This review also briefly addresses the mechanisms underlying the performance degradation of alumina fibers under high-temperature service conditions. The microstructural evolution of alumina fibres under high-temperature conditions exerts a dominant influence on their mechanical properties. Regarding failure induced by abnormal grain growth in single-phase alumina fibers at high temperatures, the incorporation of a second phase can regulate fiber microstructure and mechanical properties. For instance, within the Al₂O₃-SiO₂ system, Nextel 720 fibers demonstrate markedly a superior high-temperature creep resistance, compared to Nextel 610 fibers. Nevertheless, studies on the creep failure behavior of such fibers remain limited. Based on the systematic microstructural characterization of high-performance alumina fibers, the elucidation of microstructural evolution mechanisms during sol-gel synthesis, and the in-depth analysis of the intrinsic relationships between microstructural evolution and mechanical property degradation under high-temperature service conditions, this review establishes a theoretical foundation for the fabrication of high-performance continuous alumina fibers and promotes their engineering application in extreme environments.