Grain boundary segregation for enhancing the thermal stability of alumina-mullite diphasic fibers by La2O3 addition

Improved thermal stability of fibers is increasingly demanded for high-temperature applications. In this study, alumina-mullite diphasic fibers with 0-5 wt % La2O3 addition were synthesized via the sol-gel method. The precipitation of LaAl11O18 occurred when the content of added La2O3 exceeded a critical range. Subsequently, the effects of 1 wt % La2O3 on alumina-mullite diphasic fibers were systematically discussed in terms of pyrolysis removal, phase transition pathways, microstructure and thermal stability. The results indicated that alumina-mullite diphasic fibers consisted of & gamma;, & theta;-Al2O3 and mullite phases. The spherical & gamma;, & theta;-Al2O3 grains, ranging in size from 20 to 100 nm, were dispersed within the mullite matrix with an irregular shape. Additionally, the La element was segregated into the & gamma;, & theta;-Al2O3 grain boundaries during the crystallization process. This resulted in stabilized & gamma;, & theta;-Al2O3 and mullite grains and an improved strength retention rate of 84 % after thermal exposure at 1200 & DEG;C for 5 h.