A reconsideration on the definition of the term aerogel based on current drying trends

Kistler, in his original definition, stated that aerogels are obtained from gels in which the pore liquid is replaced by air with moderate shrinkage of the matrix. However, aerogels have been commonly seen as materials that are obtained at supercritical drying (SCD) condition, as this is typically the only one generating minimal impact on the porous structure. Other approaches to dry the gels have been introduced long ago, though their employment meant that the gel would suffer substantial changes during drying. Current developments in drying procedures have allowed to dry monolithic gels with a much more moderate impact on the porous matrix without SCD. In this article, several works where monolithic materials dried with non-supercritical conditions are presented and some of their properties are compared against typical properties of aerogels. It is found that many of these feature aerogel-like properties. Based on these findings, a critical reflection on the definitions of aerogels is presented and the suitability of the terms aerogel, xerogel and cryogel is discussed. Aerogels should designate materials derived from gels that are dried with a confirmed moderate impact on the solid network, regardless of the drying approach used. As such, Kistler's definition is the most suited even today. Instead of using a qualitative view of the impact that the solid network suffers during drying, the relevant structural and performance properties of the material must be determined, as well as those of the SCD-dried counterpart, for comparative validation regarding the type of material. (C) 2017 Elsevier Inc. All rights reserved.