One important characteristic of all aerogels is their large specific surface area. Almost in every paper on aerogels, not only the envelope density is reported but also the specific surface area in terms of inner surface per unit mass. We first present some fundamental relations for the specific surface area of particulate aerogels, such as silica or RF aerogels, and fibrillar aerogels such as cellulose. We then define terms and present selected experimental results to compare the models with reality.Techniques to measure the surface area by nitrogen adsorption and the fundamental equations behind them such as the famous BET theory or the t-plot method are derived and compared with experimental results.

Numerous daily-life materials exhibit a porous structure, e.g., foams made from different polymers (polystyrene, polyurethane), clays, tiles, bricks, oxide ceramics, bones, sponges, wood or diatoms. In many cases, the mechanical properties can be described by simple scaling laws with the relative or envelope density being the decisive factor. It is generally agreed that similar scaling laws apply to aerogels and xerogels, but the special nanostructured nature of aerogels and the mode used to form them out of a solution of monomers or polymers make an essential difference. A brief discussion of the conventional approach for closed and open cell foams or honeycombs based on the famous book written by Lorna J.Gibson and Michael F. Ashby on porous materials is given and extended to aerogels. Beforewe discuss aerogels, we briefly give for newcomers in the field of mechanical properties some textbook knowledge about mechanical testing. The chapter deals then with elastic and plastic properties of aerogels, compares modelling with experimental results and discusses deviations from classical porous media theory observed in aerogels.

The general thermodynamic concept of solutions is applied to polymer solutions. The famous Flory–Huggins theory is explained and binodal and spinodal lines are determined as they depend on the degree of polymerisation. Polymer solutions can exhibit not only an upper critical point but also a lower critical point. Some aerogels seem to exhibit such a phase behaviour.

Aerogels are famous for theirlow thermal conductivity making them the super-isolation materials of the future. The extreme small conductivity, which can be even close to the conductivity of vacuum isolation panels, poses, however, problems to many conventional measurement techniques, since even smallest heat leaks might give erroneous results by easily 20–30%. This chapter therefore is divided into several sections: first, general aspects of heat conduction are treated; and second, models are discussed explaining the thermal conductivity, specific heat and thermal diffusivity of porous materials, especially aerogels. The experimental techniques to measure thermal conductivity of isolating materials are discussed in detail and the theoretical background explained.

A wide class of aerogels starts from solution of monomers in which the monomers react, forming oligomers, polymers, particles and eventually a spanning cluster or a solid network embedded in a solution: a wet gel. Meanwhile,the two classical aerogels prepared in this way are the silica and resorcinol-formaldehyde ones. In the first section, silica aerogels, silica being the most often used precursor, are treated: the reaction between them in a solution, hydrolysis and polycondensation, the growth of fractal and compact structures, their gelation and ageing after the gel point has passed. Finally, the chemistry of silica aerogels with lower functional silanes is briefly discussed. In the second section, the chemistry of resorcinol (R) and formaldehyde (F) is presented, as well as the reaction between both molecules under basic and acidic conditions and how polymers develop from monomers. The effect of various process parameters, the ratio of R to F or the concentration of a catalyst, the dilution ratio with water and the influence of temperature on gelation are treated in detail. Finally, some thoughts on the thermodynamics of RF gels are presented.

Aerogel technology provides lightweight materials with an outstanding combination of properties. One major problem for the preparation of aerogels is how to eliminate the liquid solvent from the wet gel while avoiding shrinkage, cracking and collapse of the gel structure. Several techniques have been used and are still under development. The chapter discusses three techniques to dry a wet gel: ambient or evaporative drying, freeze drying and supercritical drying. All aspects of each drying technique are explained in detail, andvarious effects of drying routines on the final aerogel structure and thus properties are discussed.

Pressure-driven flow through porous media is a well-investigated subject of fluid and gas dynamics. Since aerogels possess a nanostructure and porosities above 90%, the flow through the pores needs special consideration. We only discussgas flow through aerogels. First, there is of course the conventional viscous flow determined mainly by the pressure gradient and the viscosity, as in Hagen–Poisseuille flow. In such a flow situation, the molecules interact with each other more frequently than with pore walls. Knudsen flow is determined by the interaction of molecules with pore walls, meaning collision events between themselves are negligible. The third possibility is a sliding of molecules along the surface of the pore walls determined by the friction coefficient between molecules and the pore surface. The essential characteristic property determining the flow through a porous body is the so-called permeability. The chapter derives not only the basic flow equations for porous mediabut also discusses experimental approaches to determine gas phase permeability and compare experimental results with theoretical models.

Classical technique to describe time-dependent changes in solution, the formation of particles, clusters and networks are scattering techniques. Most often used is small angle X-ray scattering (SAXS). In this appendix, the basics of SAXS are reviewed and the essential equations, used quite often in the aerogel literature, are derived. The dynamic light scattering used to study gel forming solutions is reviewed at the end.

The nanostructure of aerogels is most impressive. We present some microscopic pictures to illustrate the variety of structures observed in inorganic and organic aerogels. The pictures are accompanied with a brief discussion of the techniques used for imaging and gives several practical hints to achieve excellent pictures in transmission or scanning electron microscopy.

Aerogels are fascinating materials. Give a piece a silica aerogel into someone's hands, and that person is immediately fascinated and curious about how such a solid, stiff material can be that light and transparent able to withstand a burning flame of a welding torch, and that they can still hold it in their hands without any feeling of it becoming hot. They feel the same experience if they hold, for instance, a cellulose aerogel in their hands: it is equally stiff, light, white and feels like a marshmallow without having a glueing touch to the fingers. The introduction discusses the understanding and various definitions of aerogels and classifies the types of aerogels developed so far. Finally, the chapter gives a brief overview of the history of aerogels.

The isolation of a hot tube is a standard task in industry. Here we derive the fully time dependent solution of a tube with an aerogel cover whcih shows the importance of thermal diffusivity in instationary isolation tasks.

Gelation describes the transformation from a liquid or fluid state to a somewhat solid state, well known from daily experience, making, for instance, a jelly pudding. Gels and gelation are studied quite extensively in chemistry and physics, and especially over the past three decades theoreticians have discussed the formation of gels and the relation between structure and properties quite extensively (percolation theory, fractals). In this chapter, we will first discuss the viscosity of solutions and how it changes during gelation. For an understanding of modern equipment, to analyse gelation, it is important to briefly discuss viscoelasticity andsimple models for a viscous fluids. We thendescribe how gelation is measured, from very simple methods to more elaborate ones. The chapter closes with a survey of theoretical models for gelation such as percolation, diffusion-limited cluster aggregation, mean field theory using the Smoluchowski equation, scaling analysis and polymerisation-induced phase transformation (PIPS). For all these models, predictions of gel time can be made, showing how the composition of the solution, the viscosity and the temperature affect it.

The seemingly simplest property of any material and aerogels especially is the density, defined as the ratio of mass and volume. For any regularly shaped body such as a cube, sphere or cylinder, the volume is readily determined and the mass obtained by simply weighing the body. For a porous material, especially if the shape is not regular, the density is not that easy to determine. For aerogels, two different values are usually determined: the so-called envelope densityandthe skeletal density. The envelope density is defined as the massdivided by the total volume enclosing the porous structure. The skeletal density instead is the density of the solid backbone of the aerogel, i.e., the sum of the volume of all nanoparticles making up the aerogel. The chapter discusses techniques to measure both densities and all aspects of these techniques and closes with a section discussing how to estimate the final aerogel density from the known composition of the monomeric precursor solution or in the case of biopolymers that of a polymeric solution.

In synthesis, processing and applications of aerogels transport of liquids and gases in and through a wet gel or aerogel quite often determine the time scale and the properties. The transport of molecules is a diffusive process, meaning that the molecules move randomly in the solvent biased by concentration or chemical potential gradients. They typically diffuse from points of higher potential (mostly also higher concentration) to regions of lower potential or concentration. Similarly, a diffusion process occurs when a wet gel is washed, for instance, in an ethanol bath to exchange the pore fluid after gelling and ageing to one which is miscible with, for instance, carbon dioxide. In such a situation, the wet gel is overlaid by ethanol, which then diffuses into the pore space. Inasmuch as it diffuses inwards, the gel fluid moves outwards into the ethanol layer. During adsorption and desorption studies on aerogels, nitrogen diffuses into the pore system and will then be adsorbed or desorbed there. This transport takes time, and wediscuss characteristic times for such a process. This chapter discusses concepts of diffusion of species in general and in aerogels especially.