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The History, Physical Properties, and Energy-Related Applications of Aerogels

Ai Du1,2 and Chengbin Wu1,2

1 Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai, 200092, China

2 School of Physics Science and Engineering, Tongji University, Shanghai, 200092, China

Chinese physicist Prof. Kun Huang mentioned in his course "Solid State Physics" before 1964 that "here we only discuss the crystal, not because non-crystalline solids are unimportant but because they are over-complicated" (translated) [1]. Aerogels, normally nanoporous noncrystalline solids, exhibit numerous unique properties but are not fully understood. Among them, their physical properties are fundamental to aerogel science, which strongly affects the noncrystalline theory and related applications. The aerogel science has been booming recently. Plenty of aerogels with novel compositions, structures, properties, and applications have joined the community. Research papers and patents related to aerogels have increased sharply. A symposium mainly referring to aerogel was added to the 2017 MRS Spring Meeting & Exhibit, and two series of international conferences (International Seminar on Aerogels, and International Conference on Aerogel-Inspired Materials) and one series of regional conferences (Sino-International Symposium on Aerogels) has been held biennially. Many new findings and concepts have come to the fore but lack timely updates about the definition and theory.

Thus, in this chapter, we will briefly introduce the history, physical properties, and applications, especially for the energy-related applications. The definition of aerogels, mechanisms, and prospects will also be discussed. We hope the audience can learn something and perhaps come up with novel ideas based on the historical progress, developing theory, smart design for specific applications, and selected works in this chapter. We hope more researchers join us and paint a bright future for the aerogel science.

1.1 Definition and History of the Aerogels

1.1.1 Basic Characteristics and Definition of Aerogels

The aerogel is a very special solid whose physics properties could be much different from its solid and gas components. One of the most notable samples is the sonic velocity in aerogels. As we know, the sound velocity through silica aerogels could be as low as 100 m s-1, which is much lower than that in the dense silica (>5000 m s-1) and the air (~340 m s-1) included. Therefore, in our previous review, we suggested that aerogel is not only a novel material but also a new state of matter due to its unique position in the phase diagram and the diverse compositions [2].

There is no uniform definition of the term aerogel since the concept is still developing. Traditional academicians think that aerogel is a supercritical fluid-dried gel, while the gels with freeze drying, air drying, and ambient drying without large shrinkage are regarded as cryogel, xerogel and ambi-gel, respectively. The public may think the classifications are complex and prefer simple and identifiable definitions. Thus, the aerogel is defined as "a light, highly porous solid formed by replacement of liquid in a gel with a gas so that the resulting solid is the same size as the original" and "a solid material of extremely low density, produced by removing the liquid component from a conventional gel" by Merriam-Webster Dictionaries and Oxford Dictionaries, respectively. These definitions indicate a wet process the aerogel has undergone and a distinctive feature of ultralight. Similarly in the Aerogel Handbook, Pierre applied the initial idea of Kistler to define it as the "gels in which the liquid has been replaced by air, with very moderate shrinkage of the solid network" [3]. A longer definition in Hüsing's review (also in Ullmann's Encyclopedia of Industrial Chemistry) designates the aerogel as the "materials in which the typical structure of the pores and the network is largely maintained . while the pore liquid of a gel is replaced by air."

Recently, several studies have used the term aerogel to refer to the solid formed from a gel by nonsupercritical drying [4]. Thus, the academic community of aerogel science tends to approve the definition of the aerogels identified by the specific structure but not the preparation or drying method. IUPAC (international union of pure and applied chemistry) gave aerogel a definition of "gel comprised of a microporous solid in which the dispersed phase is a gas," seeming not to mention the forming or drying method [5]. However, the word gel refers to a wet sol-gel process. Indeed, most aerogels reported are derived from the wet gel via a sol-gel process. Some are not, however, For example, Gao's group developed a "sol-cryo" method to construct ultra-flyweight carbon aerogels by direct cryodesiccation of the aqueous, fluid solutions of carbon nanotubes (CNTs) and graphene oxide(GO) without undergoing the gelation process. That means the aerogel is not necessarily derived from a gel [6]. The other representative sample is that Aliev et al. developed a dry method (catalytic chemical vapor deposition) to prepare straight sidewalls of multi-walled nanotube forests and corresponding transparent carbon nanotube aerogel. The wet sol-gel process is not necessary to form an aerogel as well. Thus, in a broad sense, aerogel-related porous materials classified originally as xerogel or cryogel are gradually accepted as aerogels. Nowadays, aerogel is increasingly recognized as a matter with gel-like structure and unique characteristics, without considering the preparation or drying method.

Here, the definition of an aerogel in a broad sense should be regarded as a state of matter whose structure is similar to the solid networks of a gel with gas or vacuum in-between [2]. This definition ensures the aerogel in a high-vacuum environment could be still called "aerogel." Moreover, this definition does not emphasize the wet sol-gel process but focuses on the gel-like structure. According to IUPAC, gel means a "non-fluid colloidal network or polymer network that is expanded throughout its whole volume by a fluid." To induce the concept of gel-like structure could further avoid discussing the preparation process.

But it is not easy to describe the gel structure due to its complexity. In our opinion, as shown in Figure 1.1, a typical gel-like structure should have the following characteristics: (i) highly dispersed, coherent, and randomly distributed networks and pores that are expanded throughout its whole volume; (ii) hierarchical structure ranging from nanoscale primary structure (building blocks and pores) to its monolithic appearance; (iii) fractals in-between different hierarchies; (iv) normally composed of noncrystalline or nanocrystalline matter. Normal nanoporous powders with porosity could not be identified as the aerogel since they cannot be monolithic.

Figure 1.1 Typical gel-like structure of aerogels.

Traditional foams or cellular solids, even though their density is ultralow, cannot be recognized as aerogels probably due to the closed pores or large primary structure. Biology-derived porous materials with fine and hierarchical structure (like woods) could not be regarded as aerogels because of their relatively ordered structure and lack of microscopic fractal features. It is worth noting that fractals are emphasized because they are usually derived from multi-body random movements of the building blocks normally limited by diffusion or reaction. The forming process, named self-organized criticality by Bak, leads to a significantly complex structure, which may be extremely important for the unique properties or special behavior of the aerogel [7].

The gel-like structure leads to some property characteristics of aerogels, such as ultralow density, ultralow thermal conductivity, ultralow modulus, ultralow refractive index, ultralow dielectric constant, ultralow sound speed, high specific surface area and ultrawide adjustable ranges of physical properties. As one frequently mentioned characteristic, apparent density of the aerogel could be lower than air density. However, ultralow density is not a necessary feature, since many kinds of aerogels show relatively high density. Also, the aerogel could be, but not necessarily, formed as a monolith. The common forms of the aerogel include thin film, granule, powder, and sheet, for example.

The gel-like structure could be characterized by using different characterizations, among which nitrogen adsorption/desorption and small-angle X-ray scattering (SAXS) analysis are the most powerful tools in our opinion. By using BET (Brunner-Emmet-Teller), BJH (Barrett-Joyner-Halenda), DFT (Density Functional Theory), or FHH (Frenkel-Halsey-Hill) method to treat nitrogen adsorption/desorption results, we could statistically analyze the pore structure, getting abundant information including specific surface area, pore volume, average pore size, surface interaction, pore size distribution, micropore size distribution, surface fractal dimension, and...