Abstract
Aerogels, the three-dimensional materials, have been considered to be revolutionary solid-state materials due to their highly porous structure, low density, large surface area, and low thermal conductivity, which can be applied in the many modern fields of industry. Since its discovery in 1931, research related to aerogels has expanded from the exploration of starting materials to synthesis routes and applications (e.g., thermal, sound insulation materials, and wastewater treatment). The limitations of conventional aerogels (silica, resorcinol/formaldehyde), which are unsustainable, non-biodegradable, and expensive materials has motivated researchers to seek alternative materials. In this regard, lignocellulose is a better candidate and thus has attracted the attention of the community.
INTRODUCTION
High-porosity materials are very common in nature (from plants to animals) and have been used by humans for thousands of years. Aerogel, an artificial solid material with surprisingly high porosity (>95%), was created by Kistler in the 1930s. Typically, aerogel is derived from a gel in which the liquid components in the gel are ultimately replaced by a gas. The result is a solid with super-light density, ultrahigh porosity, ultralow thermal conductivity, and highly three-dimensional (3D) nanoporous network. Nowadays, a variety of aerogels with adjustable chemical composition, tailorable properties, and various complex shapes have been manufactured. Moreover, these new aerogels show strong application prospects in the fields of thermal insulation, sound absorption, purification and separation, energy conversion, and biomedicine.
Aerogels are nanostructured, open porous solids formed by slow replacement of liquid phase in a gel with gas through CO2 supercritical drying, freeze-drying, or ambient drying. Typically, most of the aerogels are prepared based on sol–gel process, including dissolution of precursors, sol–gel formation, and the subsequent gel drying as schemed in Figure . In the sol–gel process, nanosized sol particles (colloidal particles) are formed spontaneously in the precursor solution or through hydrolysis and polycondensation reactions initiated by a certain catalyst. The sol particles gradually gather and grow into small particle clusters, and the small particle clusters collide with each other to form a larger particle cluster, and finally form a continuous network structure. Most inorganic aerogels are obtained by the sol–gel method. As we all know, silica aerogel is a highly porous and open-pored material made of amorphous silica nanoparticles by the sol–gel method, which are connected to each other in a 3D random network. However, it exhibits inherent structural brittleness, making their processing and handling difficult, and their manufacturing costs are relatively high, which limits their large-scale practical use.Recent studies have shown that adding flexible organic polymers to a silica gel solution will be an effective mechanical reinforcement method for silica aerogels.