Hierarchically structured aluminum borate whisker aerogel via sol–gel method and in-situ reaction

Abstract Ultralight, thermally insulating, and temperature-resistant inorganic aerogels, including particulate and fibrous aerogels, serve as excellent thermal protection materials and hold significant applications in aerospace, energy, and environmental engineering. In this work, hierarchical aluminum borate whisker (ABOw) aerogels were prepared via a sol–gel process, followed by freeze-drying and high-temperature in-situ reaction, in which ABOw uniformly grows within the matrix, interspersed with either mullite fibers covered by ABOw or rod-like ABOw assemblies. The size of ABOw within the matrix and the phase transformation temperature (Al4B2O9 → Al18B4O33) decrease with increasing Al/B ratio. ABOw growth follows the V–S–L mechanism: Within the matrix, whiskers nucleate uniformly around alumina particles and then grow; in contrast, whiskers on the fiber surface nucleate heterogeneously on preferentially oriented mullite grains, grow in clusters, and gradually merge into a single whisker of increasing length. The density of the ABOw aerogel ranges from 0.173 to 0.243 g/cm3, which decreases with increasing calcination temperature and decreasing Al/B ratio. The thermal conductivity lies between 0.0560 and 0.0873 W/(m·K) and increases with increasing calcination temperature. The ABOw aerogels calcined at 1200 °C have a compressive strength of 97.8–214.3 kPa, and the main failure modes are matrix fracture and interfacial cracking. The aerogel with an Al/B ratio of 2 : 1 exhibits a linear shrinkage of less than 5% without obvious welding phenomena after high-temperature heat treatment (1400 °C/20 h, 1500 °C/2 h). This high-temperature resistant, lightweight, and thermally insulating ABOw aerogel is an excellent thermal protection material and is also suitable for high-temperature filtration, adsorption, and catalysis.