Classification of Aerogels

There are various methods for classifying aerogels. According to the appearance of aerogels, they can be divided into monoliths, powders, and membranes. According to different microstructures, they can be classified into microporous (<2 nm) aerogels, mesoporous (2-50 nm) aerogels, and mixed porous aerogels. The most commonly used method for classifying aerogels is to distinguish them by their components, as shown in the following figure: Aerogels can be divided into two categories: single-component aerogels and aerogel composites. Single-component aerogels are divided into inorganic aerogels (silica-based and non-silica) and organic aerogels (resin-based and cellulose-based). Aerogel composites include multi-component aerogels, gradient aerogels, and micro/nano aerogel composites.

Classification 1 Inorganic aerogel

Inorganic aerogels are a class of porous materials with unique physical and chemical properties that have attracted much attention due to their extremely low density, high specific surface area and excellent thermal insulation properties. Since their first preparation by Samuel Kistler in 1931, aerogels have demonstrated a wide range of potential applications in the fields of materials science, energy, environmental engineering and biomedicine. The uniqueness of inorganic aerogels lies in their nanoscale porous structure, which not only gives the materials extremely low density and high specific surface area, but also enables them to exhibit excellent thermal, acoustic, optical and mechanical properties.

With the progress of science and technology, the preparation methods of inorganic aerogels have been optimized, their performance has been improved, and their application fields have been expanding. From the initial silicon-based aerogel to today's metal-oxide and carbon-based aerogels, researchers have endowed aerogels with more versatility and application possibilities by regulating the material composition and structure. For example, in the field of energy, aerogels are used as electrode materials for supercapacitors and lithium-ion batteries; in environmental engineering, the high adsorption properties of aerogels make them ideal for treating water and air pollution; in the field of biomedicine, the porous structure of aerogels provides new solutions for drug slow release and tissue engineering.

In the future, through the development of low-cost preparation processes, the design of multifunctional composites, and in-depth research on the environmental and biological safety of aerogels, inorganic aerogels are expected to realize breakthrough applications in more fields.

Classification 2 Organic Aerogel

Organic aerogel was firstly obtained by Lawrence Livermore National Laboratory in the United States with resorcinol and formaldehyde as raw materials, and resorcinol-formaldehyde ( RF) aerogel was obtained, which marked the introduction of organic aerogel. With the emergence of RF aerogels, organic aerogels such as phenol-furan-formaldehyde and cresol-formaldehyde were gradually prepared. In recent years, with the extensive use of polymer materials, organic polymer aerogels have been studied. Organic aerogel is different from inorganic aerogel's biggest feature is that organic polymers have flexible molecular design, which makes the performance of organic aerogel become easier for people to control, can be diversified through the molecular design to get more diversified performance of the product. There are many kinds of organic polymers, and the types, properties and applications of several common organic synthetic polymer aerogels are shown in the table below.

Organic Aerogel Types

Organic Aerogel Types and Applications

Classification 3 Composite Aerogel

Composite aerogel is a class of materials that combines aerogel with other materials to enhance its performance or give it new functions. Through compounding, the mechanical strength, thermal conductivity, electrical conductivity, adsorption properties of aerogel can be significantly improved, thus expanding its scope of application. Mainly divided into the following categories: organic-inorganic composite aerogel, carbon-based composite aerogel, polymer composite aerogel, metal oxide composite aerogel, bio-based composite aerogel, etc.

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