On controlling aerogel microstructure by freeze casting

Aerogels, the lightest solids with practicable mechanical properties and functionality, are rapidly expanding the family members and applications these years. The reason can be attributed to the simple and green preparation by freeze casting (FC) method and the excellent properties of the resultant materials. Although inorganic aerogels, metal aerogels, polymer aerogels and composite aerogels towards different goals, such as thermal isolation, sensors, electromagnetic interference shielding, absorbent, separation, life science, supercapacitor, catalysis, etc., have been fabricated by this methodology, the definition and influence factors during the FC process are not clarified in the literature and become a remarkable bottleneck for further development. This review differentiates three different types of freeze. And for the FC type, it collects the scattered factors in literature that influence the microstructure of aerogels and makes combinations and comments between the microstructure and the resulting performances. Three kinds of FC methods, i.e. traditional unidirectional freezing casting (UFC), recent bidirectional freeze casting (BFC) and orthogonal freeze casting (OFC), are clarified. We hope this review contribute the development of FC method and design of novel aerogels.