Effect of micro/nano La ₂ O ₃ addition on the sintering and properties of mullite synthesized

Abstract The aim of this work is to prepare interlocking prismatic mullite from low‐cost associated rare‐earth kaolin. High‐performance mullite was synthesized by reaction sintering using calcined associated rare‐earth kaolin and α‐Al ₂ O ₃ as raw materials and micro‐ and nano‐La ₂ O ₃ as additives. The mineralogical, morphological, and chemical characteristics of calcined associated rare earth kaolin were analyzed, and the effects of the addition of micro‐ and nano‐La ₂ O ₃ and sintering temperature on the phase transformation, sintering process, microstructure, and mechanical properties of mullite were studied. The results show that the associated rare earth elements Ce, Th, and Y in calcined kaolin are mainly light rare earth elements, which are mainly endowed in the interlayer structure of K‐bearing clay minerals. The addition of La ₂ O ₃ can promote the growth of secondary mullite grains and accelerate the secondary mullitization process. The relative densities of mullite samples adding 4 wt.% micro/nano La ₂ O ₃ and sintered at 1500°C for 4 h were 92.72% and 93.4%, and the micro Vickers hardnesses were 11.32 and 12.35 GPa, respectively. The mullite sample with 2 wt.% nano‐La ₂ O ₃ had the most interlaced prismatic grains, resulting in the highest flexural strength of 157.8 MPa.