Aerogel-based thermal superinsulation: an overview
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- Koebel, M., Rigacci, A. & Achard, P. J Sol-Gel Sci Technol (2012) 63: 315. doi:10.1007/s10971-012-2792-9
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This review is focused on describing the intimate link which exists between aerogels and thermal superinsulation. For long, this applied field has been considered as the most promising potential market for these nanomaterials. Today, there are several indicators suggesting that this old vision is likely to become reality in the near future. Based on recent developments in the field, we are confident that aerogels still offer the greatest potential for non-evacuated superinsulation systems and consequently must be considered as an amazing opportunity for sustainable development. The practical realization of such products however is time-consuming and a significant amount of R&D activities are still necessary to yield improved aerogel-based insulation products for mass markets.
KeywordsAerogelComposite materialsSuperinsulationThermal insulationInsulation marketEnergy efficient buildingsCommercializationSol–gelThermal conductivityStructure dependenceAmbient pressure dryingSupercritical CO2Hydrophobization
1 Global need for superinsulation solutions
1.1 Why superinsulation?
Ever since the first global oil crisis in the seventies, the scarcity of fossil fuels, which is the number one resource for our chemical industry and energy carrier, has underlined the dependence of modern society on cheap energy and resources . Over short or long term, that very fact is forcing humanity to rethink global energy strategies and consequently take appropriate measures. In addition to a limited supply of carbon based fuels worldwide, the effect of the carbon footprint i.e. the influence of a rising carbon dioxide (CO2) concentration in the earth’s atmosphere  and its effect on the global climate [3, 4], has become indisputably clear: wide media coverage has inseminated public awareness at the turn of the millennium. Pictures displaying the melting polar cap went around the world leaving its spectator in a state of awe: the effects of the humans triumph through stellar advancements in the technological age can no longer be denied. Suddenly it is becoming clear that our precious technology and free market economy is threatening the continuity of our very existence.
Having come to this realization, international political efforts are asking for immediate solutions to the global warming and climate change problems. A goal often cited in this context is the stabilization of the atmospheric CO2 concentration below 500 parts per million . The necessity for action and a rapidly increasing demand for renewable energy sources (sources with no net emission of CO2) led to an overestimation of the potential and speed of implementation of such technologies, a statement which describes the current situation quite adequately. Even though absolutely necessary for our advancement and an essential investment in our future, the development of alternative energy schemes is an arduous task and will take decades if not centuries to completely replace current technology. Our dependence on oil, gas and coal (>85 % of the world energy demand in 2003) is much deeper than most people feel comfortable to admit. It is therefore completely unrealistic to assume that renewable energies will be able, like certain groups claim, to replace a significant fraction of carbon based energy carriers in the next 10–20 years. The main reasons for an immediate implementation and delayed action are technological difficulties and economic barriers . This realization automatically leads to a limited number of reasonable strategies for a global reduction of CO2 and greenhouse gas emissions. An ideal course of action requires short, medium and long-term strategies to bridge gaps while developing long-term renewable energy supply systems for the blue planet.
1.2 Buildings as a tremendous market opportunity
The use of superinsulation is restricted to areas where it can offer a cost advantage due to a space-saving effect, improved service-life (lower servicing/support cost) or advanced product properties (resistance against chemicals, high or low temperature, etc.). In the building sector, space saving is the number one reason for the use of superinsulation. Typical examples include side-on balcony and accessible roof balcony constructions, interior insulation solutions for building retrofit as well as slim façade insulation for the renovation of historical buildings. A number of other niche insulation markets outside of the building sector such as thermal insulation for apparel, aerospace, petrochemical pipelines and pumping fluid media in industry applications as well as low-temperature processes are worth mentioning and will be discussed in more detail later on.
2 High-performance insulation or superinsulation: concept and examples
2.1 Definition and materials classification
Overview of insulation materials according to their ambient thermal conductivities: conventional (Top five) and superinsulation materials and components (bottom three ) can be classified by their respective thermal conductivity values
λamb (Wm−1 K−1)
0.034 … 0.045
0.031 … 0.043
0.038 … 0.050
Expanded polystyrene (EPS)
0.029 … 0.055
Extruded polystyrene (XPS)
0.029 … 0.048
Phenolic resin foam
0.021 … 0.025
0.020 … 0.029
SiO2 based aerogel
0.012 … 0.020
Aerogels derived from organic compounds
0.013 … 0.020
Vacuum insulation panels (VIP)
Silica core sealed and evacuated in laminate foil
0.003 … 0.011(*)
Vacuum glazing (VG)
Double glazing unit with evacuated space and support pillars
0.0001 … 0.0005(#)
0.003 … 0.008(##)
conduction through the solid material,
conduction through the pore medium (interstitial fluid e.g. air, water)
convective transport by the pore medium,
radiative transport from solid surfaces through the pore fluid,
radiative transport from the solid through the solid network or bulk.
Superinsulating SiO2 aerogels are low-density (typically in the 0.08–0.2 gcm−3 range) nanostructured solids with a porosity >90 % and typical mesopore diameters between 4 and 20 nm. Those mesopores can add up to more than 90 % of the total pore volume. Aerogels owe their extremely low thermal conductivity to the combination of low density and small pores. In other words, the small pores effectively limit conductive and convective gas transport, the low density on the other hand implies that the solid network is delicate providing only limited pathways for conduction though it. Several research groups have studied thermal insulation aspects of aerogel materials in great detail, the leading contributions in the field coming from the group of Fricke, Reichenauer, Weinläder at the Bavarian Center for Applied Energy Research ZAE (see , chapter 21). In the following, let us briefly recapitulate the heat transfer pathways in aerogels.
Most commercially relevant superinsulating SiO2 aerogels have densities between 80 and 200 kg m−3. Their thermal conductivity values are dominated by conduction through the solid silica particle network at high densities and a combination of radiation and gaseous conduction through the air inside the pores at low densities. To produce the lowest conductivity materials it is necessary to find an optimum between those two types of contributions. It seems apparent that the areogel density has established itself as a central parameter in the discussion of thermal transport properties of these materials. Note that for reasons of clarity and simplicity we are basing our discussion solely on room temperature thermal conductivity values. They are relevant for ambient applications, which, led by building insulation, are representative of the main share of the world’s insulation markets. Because of a growing interest in superinsulation products, we shall also provide a brief description of the other two evacuated superinsulation systems which are complementary to areogels namely vacuum insulation panels and vacuum glazing to complete the picture.
2.2 Pressure dependence of the thermal conductivity λ
The idea of achieving superinsulating properties by evacuating a vessel and thus limiting gaseous heat transport is not exactly a novel concept: the first Dewar flask, an evacuated glass bottle, was invented by Sir James Dewar more than 100 years ago. The space inside the glass receptacle is evacuated to about 10−4 Pa (10−6 Torr) in order to eliminate gaseous convection and conduction. The inner glass surfaces are coated with an IR reflecting thin metal film which reduces radiative heat transport. One realizes quickly that in the case of an evacuated vessel, a high quality vacuum is necessary to curb gaseous heat transport. However, if a porous material was to be placed inside and evacuated, heat transfer by gas molecules is already effectively suppressed at much higher pressures (lower quality vacuum levels). In porous solids, the gaseous heat transfer is determined by the number density (pressure) of gas molecules as a transfer medium as well as by the number of “walls” or solid skeleton connection pathways between hot and cold sides. At a high pressure, the mean free path of the gas molecules is much smaller than the size of the pores. This means that the collision (momentum transfer) between the gas particles is limiting the total heat transfer. Under atmospheric conditions, this is true for most conventional porous insulation materials. If one were now to reduce the gas pressure by evacuation, the gaseous conductivity remains more or less constant until the mean free path of the gas molecules attains values which are on the order of the size of the pores of the solid. At this point, communication or heat exchange through gas molecules drops significantly because of collisions with the pore walls. This means that an evacuated solid with small enough pores can become a superinsulating material.
For standard applications, i.e. at near ambient temperatures, the gaseous conduction is typically the largest contributor to the overall heat transport. Nevertheless, when creating the perfect superinsulating material or component, the conduction through the bulk material needs to be minimized as well, which is synonymous with the requirement of a high porosity (and ideally an intrinsically low bulk conductivity) of the evacuated solid. In a similar manner, radiative transport needs to be taken into account. The latter is particularly relevant at elevated temperatures. Having covered the basics of evacuated superinsulation, let us briefly zoom in on the two most typical systems and their applications: Vacuum Insulation Panels (VIPs) and Vacuum Glazing (VG).
2.3 Evacuated superinsulation systems
Vacuum Insulation Panels (VIPs) and Vacuum Glazing (VG) are the incarnations of the concepts shown in Fig. 3. For a VIP, a core of a pressed mesoporous powder, typically fumed silica, is wrapped in a multilayer laminate barrier foil and evacuated to submillibar (<1 hPa) pressures and sealed . The barrier foil consists of a multilayer polymer foil sandwich construction with Al diffusion barrier layers in between them. A vacuum glazing unit consists of two glass panes separated by a very thin (0.2–0.8 mm) gap and held together by an absolutely hermetic edge seal. A cavity pressure on the order of ~10−1–10−2 Pa (10−3–10−4 Torr) is required for optimal VG insulation performance; at higher pressures the thermal performance rapidly deteriorates.
The main advantage of both types of evacuated superinsulation systems is their extreme performance. Substitution of conventional double glazing units with vacuum glazing (VG), can bring about a reduction of a factor of 2–5 of heating and cooling energy per area of glazed building surface . Vacuum insulation panels offer a 4–10 times higher simulation performance per cm of material thickness used when compared to standard insulation materials such as polymer foams or mineral/glass wool.
Both VIP and VG are nevertheless quite sensitive to damage and subject to aging effects . This can lead to a partial or complete loss of the thermal insulation performance: VIP barrier foils are extremely sensitive to mechanical piercing or other forms of damage (aging, loss of integrity, delamination). The great sensitivity and aging effects which are due to gas permeability (as the pressure inside the panel rises, so does the thermal conductivity) are among the main disadvantages of vacuum insulation. Where suitable, VIP are extremely powerful insulation systems if properly implemented and mounted. Current VIP-technology uses mostly pyrogenic silica as a core material. It is the only evacuated system available today which can fulfill the requirements for long-term applications in building insulation with a service life period of 30 years or more . Other mineral–based materials such as glass fibers, porous minerals or expanded clays are used as alternative inexpensive core materials and/or fillers. For vacuum glazing, the situation is quite similar. The high vacuum environment must be maintained inside the glazing cavity over a prospected service life period of 30 years without the need for additional pumping or servicing. Those stringent requirements make a practical realization of vacuum glazing on a commercial scale a Gordian task. From a practical point of view, the lack of suitable (vacuum compatible, hermetic, glass-to glass, full perimeter) edge sealing technologies have inhibited the large scale commercial realization of superinsulating vacuum glazing systems with the desired performance. Currently the only commercial VG products which are being manufactured in Asia (Japan, China and Korea) have an insulation performance which is significantly inferior to that of Ar or Kr filled state-of-the-art commercial double glazings. They are being sold in the Asian markets as a replacement for single pane glass. Nevertheless, VG still offers great potential to revolutionize the glazing and building insulation markets, however new, advanced edge-sealing and integrated manufacture concepts are necessary. At best, the realization and implementation of distinct technological advances  could reach the public markets within the next 10 years. Alternative highly insulating translucent glazing systems are aerogel-filled windows which are already available on the market.
2.4 Aerogel based superinsulation
Monolithic aerogels: large (>1 cm), homogeneous blocks of a particular aerogel material.
Divided materials: finely divided aerogels are random “monolithic” pieces or crumbs of aerogel with typical diameters below 1 cm for granules and 1 mm for powders.
Composite materials: homogeneous or heterogeneous aerogel phases with at least one additive incorporated either into the gel matrix (e.g. during synthesis) or added to the gel as a second distinct phase such as fibers, blankets, a fleece or also by a subsequent modification by compounding.
Monoliths are primarily obtained via supercritical drying. The synthesis of large monolithic pieces of aerogels is both time and cost intensive. For this reason, there are virtually no commercial monolithic aerogel products available at the moment. Today’s top-selling commercial products are either granular or blanket-type materials. The three classes of aerogel morphologies will be discussed in more detail in the following chapter.
2.5 Market considerations
Going back to the global insulation data, niche products, which are denoted “other insulation” in the graph, account for less than 4 % of the global market. Still, this sector has an annual turnover in excess of 1.1 billion US$ (2008). In this sector, aerogels account currently for approximately 5 %.
In 2008, aerogel materials and products sales were in excess of 80 million US$ , with thermal insulation products making up for approximately 60 % of this number or 50 million US$. The aerogels sector being an emerging market, tremendous annual growth rates from 50 to 75 % have been seen in recent years and similar developments are expected for the near future. In 2004 for example, global aerogel sales accounted for only 25 million US$. Recent market studies project worldwide sales of aerogel products in excess of 500 million US$ for the year 2013. This development is consistent with a number of new production facilities which have recently been coming online. These developments shall be taken as a sign that global markets are ready for high-priced, aerogel based superinsulation products. It is expected that, starting from today, the aerogel markets will continue to grow much more rapidly than the “conventional” insulation business for at least a decade up to the point when markets will begin to saturate. Saturation will certainly depend on a continuous reduction of product cost which goes hand in hand with increasing production capacities. A current benchmark value for a cubic meter of silica aerogel is on the order of 4,000 US$. With increasing commercialization, this value could drop below the 1,500 US$ mark by the year 2020. To gain more insight into the materials side, let us discuss superinsulating aerogels, their chemical composition and synthesis as well as application-geared improvement strategies.
3 Superinsulating aerogel materials
3.1 Overview of common chemical synthesis methods
Silica aerogels are the most widely studied aerogel materials and the only ones “really” available commercially today. For this reason, we are limiting the discussion of chemical synthesis methods to this class of materials. Generally, the most widely used sources of silicon, the so-called precursors are alkoxysilanes such as tetraethoxysilane or tetraethylsilicate TEOS and tetramethoxysilane TMOS [24, 25] or the inorganic sodium silicate (waterglass) . More recently, waste products such as oil shale ash , rice husk ash [28, 29] or fly ash  have been investigated as inexpensive alternatives. From a more general point of view, such exotic precursors make part of an eco-design strategy for silica aerogels elaboration . For reasons of simplicity we focus here on the description of the two more common waterglass and alkoxide precursors only:
Both precursors allow either single or two-step preparation. In a single step reaction, the formation of colloidal particles or larger silica building blocks (generally called clusters ) and their random aggregation to form a three-dimensional network structure, the gel, are occurring simultaneously. In a two-step process, a colloidal particle solution, the sol, is obtained through hydrolysis or acidification/ion exchange respectively. Such a sol state can be stable over weeks or months, because the aggregation of colloids is inhibited through repulsive interparticle potentials (surface charges). Partial neutralization of the colloid surface charges induced by base addition typically induces gelation in the second step. This happens when the pH of the solution is near the isoelectric point (pI) of the metal oxide sol. Because sol formation and gelation are well separated, two step processes are in general more versatile and allow better control over the resulting gel network morphology. The main chemical difference between waterglass and alkoxide based methods is the nature of the solvent medium as described here-after.
Sodium silicate is an inorganic, water soluble compound and hence waterglass based gels are typically synthesized in aqueous solution. It is prepared industrially by reacting quartz sand with soda lime at high temperatures (~1,100 °C) and dissolving the resulting solid “cullet” and finally adjusting the pH-value/sodium content by addition of sodium hydroxide . One important fact to keep in mind is the chemical complexity and diversity of the silicate system: silicic acid (Si(OH)4) and silicate as an acid base pair do not behave like typical inorganic mineral acids due to its complex isomerism and structure.
Both, TEOS and TMOS find wide use in the synthesis of silica-based materials such as mesoporous materials , aerogels , films  and more recently organically modified silica (Ormosil) [41, 42] and composite materials [43, 44].
However, in reality silicic acid monomer forms are known to condensate rapidly (formation of dimers, trimers etc.) via siloxane bonding through water (≡Si–OH/≡Si–OH reaction) and/or alcohol (≡Si–OH/≡Si–OR reaction) condensation. Hence a more or less complete hydrolysis of silicon alkoxides leads to the formation of silica oligomers and eventually a colloidal silica solution or sol. The chemical nature, diversity and composition of such a sol and the resulting aerogels compare quite well to materials obtained by acidificication (or ion exchange) of aqueous sodium silicate  with two main differences—the solvent medium and the presence of residual unhydrolyzed alkoxy groups which are isotropically distributed within the silica phase. Note that the use of the term “hydrolysis” is also sometimes used incorrectly in conjunction with sodium silicate systems where no actual chemical hydrolysis is involved in the activation/sol formation.
Just like in the waterglass case, a two-step alkoxyde process permits to separate hydrolysis/condensation reactions from network formation/gelation  and thus, permit to “control” in a better way the texture of the dried material. The classical two-step process is often referred to as acid/base catalyzed synthesis. Hydrolysis and condensation of so-formed silicic acid oligomers is initiated by catalytic amounts of acid and is believed to follow an SN1 limited mechanism with a trialkoxysiliconium cation as a rate determining transition step . The sol formed in this way carries a certain amount of positive surface charge (e.g. in the form of protonated silanol groups) which leads to repulsion of individual colloidal particles causing their kinetic stabilization. Base addition in catalytic amounts neutralizes these surface charges and initiates gelation in the second step. The kinetics and reaction mechanisms of the hydrolysis and condensation reactions has been widely described in the literature [48, 49]. Nuclear magnetic resonance is a great tool for this type of investigations as it allows a quantitative in situ observation  and modelling  of the various bonding states (alkyoxyde, silanol, siloxane) of the silicon centers. Small angle X-Ray scattering allows to obtain information about the particle sizes and morphology of sol and gel states during the synthesis reaction . The choice and concentration of base used in both single and two-step alkoxide processes is a central parameter in the synthesis of alkoxide-based gels.
In a single step process, the alkoxide is hydrolysed by a base catalyzed reaction. Typically Lewis bases such as fluorides  or mixed ammonium fluoride and hydroxide [53, 54] but also alkylamines  are used. In the former case, the hydrolysis goes through an SN2 type pathway, that is through nucleophilic attack of the silicon atom. Because hydrolysis/condensation and gelation processes are not separated, the gel morphology of materials produced by single step methods is generally less regular and the pore size distribution is wider and contains a larger fraction of (macro)pores with a diameter >100 nm. For this reason, aerogels coming out of a two step process are known to exhibit better optical transparency than their single-step analogues . In the aerogel domain, some pre-polymerized TEOS-based precursors can be commercially found. They have been developed from the early nineties on by the French fine chemicals manufacturer PCAS (Longjumeau, France). The corresponding commercial solutions are composed of silica-based oligomers obtained by sub-stoichiometric reaction of TEOS with water in ethanol under mineral acid catalysis . These precursors are named PEDS (for polyethoxydisiloxanes). They are roughly composed of 5–10 monomer units and some 29Si NMR studies have shown that these oligomers appear weakly ramified (with Si-atoms exhibiting 2–4 siloxane bonds) . So-to-say, such precursors are ready-made sols for performing the second base-catalysed step at the laboratory scale. Amongst others, they have been widely studied for elaboration of transparent super-insulating silica aerogel panes for highly-efficient double-glazings .
3.2 Super-insulating silica-based aerogels
3.2.1 General overview
All these properties have a strong influence on the thermal characteristics. The effective thermal conductivity is strongly dependent on the aerogel apparent density . Indeed superinsulating silica aerogels with λ <0.020 Wm−1 K−1 can only be found within a rather narrow range of densities (Fig. 6), usually located between 0.10 and 0.20 g cm−3.
Finally, it must be added that the flexibility of the sol–gel process also permits to disperse particulate matter into the sol to improve both thermal and mechanical characteristics of the resulting insulators : for example, powders and nanoparticules (e.g. C, TiO2, ZrO2) for infrared opacification or fibers (e.g. ceramics, organics, glass) or inorganic clays such as kaolin or attapulgite for mechanical strengthening. More recent activities are focusing on the dispersion of nanofibers to reinforce significantly mechanics without inducing damaging increase of the thermal conductivity .
3.2.3 Granular materials
Thanks to the induced spring-back effect , thermal conductivities equivalent to these of large silica aerogel plates can be achieved with ambient dried analogues today. Furthermore, as a consequence of these direct/indirect sylilation processes, the dried materials exhibit hydrophobic properties. Indeed, because of replacements of hydrophilic silanols by nonpolar ≡Si–R groups (with –R being an alkyl group) the hydrophilicity decreases drastically. This has significant consequences on thermal insulation applications under real-life conditions. The main difference between supercritical superinsulating aerogels lies in the fact that so far the subcritical route only permits to obtain granular pieces of material (Fig. 13) as well as, in some rare cases, some small monoliths .
As previously described in many papers , packed beds of granular beads present effective thermal conductivities slightly larger than their monolithic parents at atmospheric pressure and within the low-vacuum range because of the trapped air in the inter-granular macropores, even if the granules themselves offer the same thermal conductivity values as the monoliths. Of course, the overall packed bed conductivity in this range can be significantly decreased by simple compression and/or packing which reduces the volume fraction of air . It can also be underlined that, still because of the inter-granular macroporosity, without significant external load, under vacuum, effective thermal conductivities of packed beds of granules are generally lower than the ones of their monolithic counterparts .
3.2.4 Aerogel infused blankets
3.3 Hybrid and organic aerogels
For various reasons, it seems at present that superinsulating solutions at atmospheric pressure could merge from composites and hybrids and, more particularly, from coupling inorganic and organic chemistry. Among others, pure silica systems are mechanically too weak, blankets are dusty, purely organic derived materials are flammable. Combining the advantages from both ends, inorganic–organic composites and/or hybrids could permit to obtain superior superinsulators, particularly from a mechanical and thermo-mechanical compromise point of view.
As an illustration of studies involving organo-mineral composites, the following approaches must be mentioned: (i) preparation of foam-based composites through elaboration of superinsulating silica aerogels directly within the pore network of open organic foams (polyolefin, polyurethane, etc.) , (ii) dispersion of finely divided silica aerogels in organic sols (which have previously proven to lead to intrinsically low conductivities ) and (iii) polymeric binding of superinsulating silica granules (for example, with poly-tretrafluoroethylene (PTFE)  or epoxides ). All these types of composites can have extremely low effective thermal conductivities under ambient conditions.
Among the various hybridization approaches performed with the primary goal to reinforce the tenuous solid skeleton of silica aerogels by an organic chemical modification without degrading the superinsulating property, we shall begin with the Ormosil (short for organically modified silica) methodology. The primary works of Mackenzie  have been significantly improved towards superinsulation applications, for example via the use of specific trialkoxysilyl  or polymethacrylate  derivatization agents. Conductivities as low as 0.015 Wm−1 K−1 have been obtained after supercritical CO2 drying. Besides, some recent sol–gel studies performed with methylsilsesquioxane, assisted with surfactants (CTAB and poly-ethylene glycol) and urea, have permitted to obtain large monoliths with a typical bulk density of 0.10 g cm−3 through ambient pressure drying . This could also represent a promising way to elaborate large superinsulating plates via a subcritical Ormosil process. Parallel to these promising studies, some cross-linking methods have recently been adapted for use in the aerospace/aeronautic thermal insulation field. They consist in reacting the remaining silanols present at the surface of the pores of silica aerogels with reactive molecules such as isocyanate and/or amines so that the fragile skeleton’s inner surfaces end up coated with an organic layer [94, 95]. For the moment, such a technique has demonstrated its efficiency through drastically superior mechanical properties such as stress at failure and flexural modulus, however generally the density of this type of material also increases significantly. Some recent works have demonstrated it is clearly possible to decrease this dramatic densification. For example, using several bis-alkyl silanes together with polymer reinforcement by epoxy has permitted to reach minimum density of 0.2 gcm−3 . Some hybrid cogelation works have also been performed with modified alkoxysilanes comprising either at least one isocyanate group or, in an alternative approach, at least one amine group . The so-promoted urea cross-linking permits to improve significantly the initial mechanics of the silica aerogel while maintaining thermal conductivity as low as 0.013 Wm−1 K−1 at room temperature. Anyway, it must be underlined here that the huge majority of these brand new promising materials are processed through supercritical drying paths. Rare are the studies published on elaboration of such hybrid silica-based materials through ambient drying . Recent attempt in this direction has shown that a density lower than 0.4 g cm−3 can be obtained through evaporative drying of polymer-crosslinked amine-modified wet gels . This type of work lets one think that superinsulating hybrid or composite silica-based ambigels are not too distant from being accessible at the laboratory scale.
Since then, this system has been intensively studied. By introducing for example polyamines in the sol together with polyol hardeners, some polyurea aerogels offering conductivities as low as 0.013 Wm−1 K−1 at ambient conditions have recently been obtained . Among the few other works initiated during this period on organic aerogel-like materials for thermal insulation, a number of studies performed on the melamine–formaldehyde system are worth mentioning. Through a microemulsion templating sol–gel route , some thermal conductivities slightly below 0.025 Wm−1 K−1 have been obtained .
Because all these synthetic pathways use quite costly and also harmful reagents (resorcinol, formaldehyde, isocyanate, etc.) more recent endeavors in this field have been concentrating on the utilization of inexpensive and/or non- or less toxic precursors. Among the most recent studies, the work by Lee and Gould on polycyclopentadiene-based aerogels should be pointed out . As a byproduct of the petrochemical industry, the monomer (dicyclopentadiene) is readily available and thus rather inexpensive. A sol–gel synthesis with this precursor can be conducted in alcohols according to a quite simple ring opening metathesis polymerization process which is catalysed by transition metal complexes of Ruthenium. The corresponding aerogels can present effective thermal conductivities lower than 0.015 Wm−1 K−1 at room temperature. They consequently appear extremely promising for thermal insulation applications but one must note here that, whatever the organic sol–gel synthetic method, no subcritically dried superinsulating organic aerogels are currently available even at the laboratory scale.
In parallel, recent progress in nanostructured sol–gel cellulosic materials reveals a significant potential of this type of brand new green aerogels for thermal insulation product applications.
3.4 Commercial products
With a large number of synthetic methods available to fabricate superinsulating aerogels let us now focus on products which are available commercially today. Those are almost exclusively based on silica. The first commercial aerogels were based on SiO2, following a recipe similar to Kistler’s original procedure and were produced after 1940 by the Monsanto Chemical Corp., Everett, Massachussetts, USA. Production stopped in the sixties for more or less two decades but started again in the early eighties (Thermalux L.P., Airglass A.B. ) and during the nineties (BASF , Hoechst). Different types of aerogels were then produced for various applications (silica aerogels for the agricultural sector  and paints , carbon aerogels for energy storage and conversion devices, etc.) but a large-scale commercial adaptation by the thermal insulation trade took place only recently. At present, the two main players involved in this field are the North American industrials Cabot Corporation  and Aspen Aerogels . They both sell silica-based aerogel products which are available in the form of a granular material or flexible blankets. Cabot Corporation has developed a family of silica aerogels under the trade name of Nanogel™ which is now being sold under the trade name Lumira™, while Aspen Aerogels is focussing on flexible aerogel blankets. Lumira™ materials are obtained after subcritical or supercritical drying of sylilated gels synthesized from silicic acid (waterglass based). Aspen Aerogels blanket products are obtained by supercritical CO2 drying of gels generally made from alkoxysilanes. More recent competitors such as Nano Hi-Tech in China  and EM-Power  in Korea are also supplying sol–gel based superinsulating materials, however their production volume at present does not compare with the ones of the main players Cabot and Aspen. In addition there are a number of specialty aerogels companies with extremely small sale volumes. One example is Airglass AB (Sweden) which can supply large monolithic flat plates of silica aerogel. These monoliths have been designed and developed initially for very specific scientific applications such as elementary particle physics counters at CERN, Geneva, Switzerland, however in a later stage their properties were optimized for applications in supersinsulating glazings, so-called aerogel glazings. Due to the tremendous production effort required to manufacture such monoliths defect free, this particular application will most likely not become industrially competitive in the near future.
Additional developments are going on worldwide to develop improved insulating components and systems based on current silica aerogel technology, both at the materials and component levels. As far as application is concerned, Nanogel™ granules can be directly used as superinsulating filling materials (such as for example, in daylighting panels) but CABOT is also marketing Nanogel™ -based components (such as Thermal Wrap™ and Compression Pack™) for specific applications like pipe-in pipe and cryogenic insulation systems. For its part, Aspen is commercializing blanket-based components for building insulation (so-called Spaceloft) as well as materials optimized for use in extremely hot and cold environments (Pyrogel and Cryogel products).
The role of superinsulation as niche products in the insulation business and their particular importance for space saving applications has been presented previously. A lower thermal conductivity means that the same insulation performance is achieved with a thinner insulation layer, or, that the same thickness of superinsulation is used to ensure far better insulation performance. This advantage is contrasted by a significantly higher cost per installed performance as shown below. This assessment is based on data from 2009.
Typical aerogel insulation product branches, their space savings benefit and economic relevance
Installation and assembly cost
Off-shore oil and gas
Smaller pipe diameter, lower weight, more pipes per installation round trip, fewer trips
Superior lifetime, improved degradation resistance
Simplification of overall design, light construction, size reduction lowers materials/assembly cost
Smaller gross weight results in results in fuel savings or additional capacity
Comparable to conventional insulation, currently more elaborate due to lack of experience
Reduction of heating/cooling energy and/or larger useable building/exterior volume
High temperature insulation
Smaller overall pipe diameter or exterior dimensions, easier installation
Reduced surface area per unit length, lower radiative losses, improved resistance and lifetime
Smaller overall pipe diameter or exterior dimensions, easier installation
Reduced sensitivity to cryo-embrittlement, increased lifetime, energy and/or space savings
Appliances and apparel
Significantly more complex than standard technology
Energy savings/increased thermal comfort for lightweight extreme performance personal wear/gear
For most aerogel insulation products, the reduction of the operating cost is higher than the savings accrued during installation. However in the case of off-shore oil and gas pipeline insulation, the installation part is the key factor in favor of aerogel-based products: With significantly smaller pipe diameters and reduced weight, assembly ships can carry significantly more pipeline with each trip, thus significantly cutting down on the number of trips and the overall installation cost. This is a model example of how an extremely specialized application can open up opportunities for high-performance high-cost products. In the following we shall briefly discuss the various types of aerogel insulation products which are available commercially today. Of course, it must be preliminary said that all current commercial products are silica-based.
4.1.1 Off-shore oil and gas
This is one of the oldest fields of activity of the Aspen Aerogels Spaceloft thermal insulation blanket. Cabot Corporation is offering their Nanogel-filled Compression Pack as a promising alternative solution to the blanket product. The superior thermal performance and improved chemical/pressure resistance of aerogels combined with assembly cost savings make them ideal candidates for this type of application. To this day, this is the undisputed model example for finding a niche market for aerogel insulation products and exploring it successfully.
4.1.2 Aeronautics and aerospace applications
One of the traditional fields of aerogels during their revival in the 1960s following Kistler’s discovery in 1925, the materials was extensively studied by the US space administration NASA. Large monolithic aerogel blocks are being used to collect stellar dust and particles in outer space (, on the Mars Rover roving vehicle silica aerogels were used for thermal insulation. Given their space savings potential and relatively low weight, aerogel insulation systems could also find applications in civil and military aviation, however little activity is known in this field so far, most likely for cost reasons.
4.1.3 High temperature
There are a number of industrial processes which require pumping hot fluids from one place to another. Many older plants were designed and built with little or no insulation, especially at times where energy was all abundant and cheap. These days, significant savings can be generated by insulating the piping systems of steam cycles, chemical processes or oil and gas processing refineries. This opens up a large potential for high performance insulation systems, particularly for older or already existing plants with restricted access and tightly spaced arrangements of individual pipes. Aspen’s Pyrogel is a product which was developed especially for high-temperature applications.
4.1.4 Cryogenic applications
In the field of cryotechnology, which includes for example transport and storage of liquefied gases or frozen biomedical specimens, aerogel insulation offers numerous advantages. Besides its superinsulation properties, aerogels tend to embrittle less with decreasing temperature than for example polymer foam insulation. Aspen Aerogels have developed a special low-temperature aerogel blanket insulation product, the Cryogel Z. Granular products such as Cabot’s Nanogel offer great versatility for lining nontrivial vessel and piping geometries which are part of more complex cryogenic systems. Cryogenic aerogel insulation is also investigated by the NASA’s rocket engine research teams for liquid hydrogen and liquid oxygen propelled drive systems .
4.1.5 Apparel and appliances (refrigeration systems, outdoor clothing & shoes)
Superinsulation properties can find use in a number of everyday products. Particularly personal apparel such as performance outdoors equipment for example clothing, shoes, gloves, foot warmers, tents, sleeping bags etc. could be outfitted with aerogel thermal insulating layers. The same holds for household appliances such as refrigerators or outdoors cooling boxes. A number of such products have recently reached consumer markets. With increasing market volume and increasing customer awareness, such products have a significant potential to compete in the high-priced products sector. However it is unlikely that aerogel based consumer products will conquer the middle and low-cost sectors.
4.2 Zoom on building insulation
At the beginning of this chapter the need for superinsulation was founded on the imminent necessity for a global CO2 reduction. It was shown that insulating the existing building stock can bring about substantial energy savings and that it is amongst the most sensible short-term measures to curb CO2 emissions quickly. Given the tremendous volume of insulation materials needed for building applications, this is clearly one of the fields with a huge potential for the worlds growing areogel insulation markets.
renovation of historical buildings (exterior insulation)
interior insulation and transparent/translucent daylighting and windows
flat-roof balcony constructions (aerogels as a less damage sensitive alternative to VIP)
Aestethic architecture with slim exterior insulation, light weight elements
The bulk of newly constructed buildings and most renovations will still go on using conventional insulation products, because the extra space gained by a slimmer insulation only compensates the added cost in high-prized locations (inner city). This is the main reason why aerogels have only very hesitantly found their way into the building insulation sector, nevertheless, this trend is currently changing with more and more new aerogel based insulation products and demonstration projects appearing these days.
4.2.1 Monolithic aerogel windows
More recently, Aspen Aerogels has engaged in the development of reinforced but highly translucent, monolithic aerogel panels for “aerogel window” applications. The technology is currently still under development, however the fact that one of the major players in the aerogel business is involved in this type of activity raises hope that a commercial solution may be within reach. But what about the cost? In Europe, the going rate for a standard triple glazed window is around 800 $ per square meter. A 2.5 cm thick monolithic aerogel plate sandwiched in between two glass panes could reach a U-value below 0.5 Wm−2 K−1. The difficulty is to produce monoliths of high optical quality (no defects, cracks, inhomogeneities) in the form of perfect plates of at least 0.5 m by 0.5 m dimension. If we use a bulk volume price of silica aerogel of 4,000 $ per m3, the additional cost originating from the aerogel itself of around 100 $ per m2 could be attractive. However, high optical quality, large area monolith are most likely going to come with a significantly higher price tag due to the more delicate handling and processing steps and perhaps also a non-negligible reject rate. From a market perspective, a lighter double paned aerogel monolith glazing offers great promise. So far, the slow processing due to the extremely delicate nature of the gels as well as their supercritical drying process have rendered any attempts for commercialization unsuccessful.
4.2.2 Transparent daylighting applications
Granular translucent aerogel materials are much easier to handle, as they can be poured like a powder. The first and largest industrial supplier of high optical quality granular silica aerogels is Cabot. Their Nanogel® (now Lumira) products have been at the forefront of transparent daylighting applications. By filling a transparent cavity or profile, highly insulating modules or windows can be produced which still allows for a high transmission of diffusely scattered light. The main difference to a monolithic aerogel plate is that the granular material no longer remains optically transparent, and thus no longer allows to recreate an optical image, as if viewed through a milk glass. Due to the numerous air/aerogel interfaces, a packed bed of aerogel granules scatters light much more which results in exactly this type of translucent properties. Diffuse lighting offers the advantage of a softer light and reduced shadowing and glare when compared to a standard window.
Another type of transparent daylighting solution for roof constructions are the use of textile roof elements. In North America, Birdair  is a pioneer in the construction of suspended, tent-like aerogel roofing solutions. To date a number of different applications for these lightweight textile roofing systems with superior insulation performance have been realized covering industrial buildings, event halls, convention centers, hotels and sports complexes. One of the better known examples today is the retrofit of the roughly 5,000 m2 large Dedmon Athletic Center of Radford University in Radford, VA, USA. The use of Birdair’s Tensotherm system resulted in a reduction of the total heating and cooling energy demand by roughly three quarters. At the moment, a much larger sports facility, the Talisman Centre in Calgary, Canada, is being retrofitted which spans an area more than three times that of the Dedmon Athletic Center. Transparent textile roofing is in fact an excellent example for the overall economic efficiency of aerogel solutions, which despite the high materials costs are offset by their low weight, simple installation, longevity, optical transmission and water vapor permeability. All these factors combined make Tensotherm a highly competitive product for large and heated/cooled spaces.
Last but not least, granular silica aerogel can also be used to fill the cavity in a double glazing, resulting in an opaque but highly insulating window element. The German company Okalux  has developed an aerogel filled window termed Okagel which offers thermal transmittance values down to U = 0.3 Wm−2 K−1. Their most famous demonstrator unit to date is the window elements used in the British Haley VI Research Station in the Antarctic. In less adverse climatic conditions, the Tengelmann Climatic Market in Mühlheim, Germany, was outfitted with Okagel roof window elements. Aerogel filled windows offer an additional quite substantial thermal performance advantage over standard window elements when installed in a non-vertical fashion. For tilted and flat-roof standard windows, the certified U-values can no longer be upheld, because of a substantial increase in the convective heat transfer of the filler gas which is suppressed under normal vertical installation conditions. In an aerogel filled window, convective heat transfer does not notably contribute to the overall thermal performance.
4.2.3 Other aerogel granulate based insulation
4.2.4 Composite building materials and products based on aerogel granulate and powders
More recently, a number of building material manufacturers have realized the importance of aerogel insulation due to their excellent overall properties and thus a number of silica aerogel granulates based composites have been developed. In 2010 the mineral wool fiber producer Rockwool  has announced their Aerowool product , board/like product made by compounding aerogel granulates with mineral wool and resin based binders. In this way, a sturdy, lightweight insulation board with a thermal conductivity around 0.020 Wm−1 K−1 is produced.
The compounding of granular aerogels today has penetrated virtually all types of construction materials. Much in the same way as the render and mortar products, Ratke  demonstrated proof of concept of ultralightweight concrete by mixing sand, cement and up to 70 % of silica aerogel granulate by volume. This astonishing material offers still excellent mechanical strength and thermal conductivity values as low as 0.1 Wm−1 K−1, making it a perfect candidate for extreme fire resistant concrete construction.
4.2.5 Aerogel infused blanket products and derivatives
Together with a number of other central and northern European countries, Germany, Austria and Switzerland have taken a leading role in energy and environment related issues. Supported by their government’s trend-setting energy policy, innovation in the field of energy efficient building and construction have been subsidized and promoted in recent years. This has allowed for new, high-priced high-performance products and solutions to make their way into selected demonstration units which are often funded in part by the government. Even though a lot has happened within the past 5 years in the field of aerogel based insulation systems and components, aerogel superinsulation is still not ready for a broad application in the building market yet, mostly because of its price. Nevertheless, the speed and intensity of new developments precipitating onto the building market indicates that there is still plenty of room for innovation of aerogel superinsulation products in the not too distant future. With increasing market volume and falling aerogel prices, the saved space will more and more be able to compensate the extra cost. At this point, it may also be mentioned, that aerogel based insulation offers two inherent advantages over the most commonly used polymer foams, namely that it is both non-flammable and water vapor open. Even though almost forgotten over the past 20 years, one can observe a generally rising awareness on the side of the installers to the tight sealing of the gaps between insulation polymer foam plates (even though beneficial for optimized thermal insulation) as this situation inherently bears a substantial risk for condensation damage. Due to their open porous nature, this problem is eliminated when using aerogel based insulation. Today markets seem to indicate that aerogels are the insulation materials of the future—the question now remains to see which products are going to secure what section of the market and how quickly production volumes will rise and prices drop.
4.3 Safetly, health and toxicity considerations
Nanomaterials in general are currently being investigated worldwide for their potential health and environmental risks . As nanostructured materials, aerogels are nanomaterials and hence need close observation. Silica aerogels, the most prominent aerogel insulation materials, are made from colloidal silica nanoparticle building blocks. Now the question arises whether there is a significant health risk for end-users and the immediate environment where the product is applied. From a risk assessment point of view, the potential danger of a new nanomaterial is given by the product of intrinsic toxicity of the nanomaterial (nanotoxicity) and risk (probability) of exposure.
The toxicity of nanoparticulate silica has been widely studied in recent years . Generally amorphous silica nanoparticles are considered relatively safe, however the surface functionalization must be taken into account when assessing the specific (for example cell-) toxicity. Crystalline SiO2 nanoparticles are accompanied by a significantly higher risk for cell mutagenesis, however sol–gel processes produce amorphous materials unless they are sintered for longer periods of time at elevated temperature which can induce crystallization at the nanodomain level. From an exposure point of view, the risk of exposure to significant quantities of nanoparticulate silica when working with aerogels such as by inhalation is also quite low. Given the three dimensional network structure of aerogels, the resulting dust contains only very few individual particles in the nanometer size regime. By far more likely is the formation of chunks or fragments of the entire network in the micrometer size regime, consisting of many particles still covalently bound to each other. The dust produced in this way is in a way more similar to typical household dust.
Even though nanosafety does not seem an immediate concern for aerogel insulation, prototypes of each next generation product must be tested thoroughly. Manufacturers and businessmen alike must keep in mind the tremendous negative impact on the entire aerogels industry which can come from a single negative example. Because nanotoxicity is such a new field with a lot of unknowns, the power of bad news and their potential impact as a market inhibitor is tremendous. Therefore proper action must be taken so that the nanotoxicity problematic be addressed adequately in all instances. Of course, when more developed, organic or composite/hybrid aerogel insulators must be studied in the same way.
Aerogel based insulation products have established a place in various niche markets, particularly in the insulation products trade, over the past 10 years. Aerogel superinsulation affords superior insulation performance (lower thermal conductivity values) compared to conventional materials which translates into slimmer installed constructs and/or improved insulation performance. Motivated by global CO2 emission and space saving restrictions, aerogel superinsulation products will continue their advance into new market segments. The development of improved aerogel insulation systems and rapidly advancing process technology will allow manufacturers to promote next generation custom-tailored products. Inorganic (particularly SiO2 based) materials are likely to play a key role, also for years or decades to come. It is expected that in addition to the pure silica systems, organic aerogels and a variety of composites and/or hybrids (such as organic–inorganic or fiber-reinforced) materials and products will become available. Overall, the aerogels insulation products have a tremendous growth potential in excess of 50 % by volume per annum. For comparison conventional insulation products grow at an annual rate on the order of 5 %. In 2008 aerogel insulation products contributed less than 0.3 % to the total insulation products market volume.
Today, the materials cost for aerogel insulation is roughly twenty times higher (per installed performance) than that of standard insulation products. The main justification for the use of aerogel insulation systems therefore is space saving, reducing operating cost, longevity, and chemical resistance. Off-shore oil, high-temperature and building insulation as well as aeronautics/aerospace applications are the industry trades with the most significant growth potential for aerogel products in terms of estimated sales volume. Of all these, the building has certainly by far the largest growth potential but is also more sensitive to the materials cost than for example the aeronautics/aerospace field.
A current reference market price for a cubic meter of silica aerogel is on the order of 4,000 US$ (2008). With increasing commercialization, this value could drop below the 1,500 US$ mark by the year 2020. Lower prices will cause an equilibration or stabilization of the aerogels insulation market. The advance of organic and hybrid/composite systems could invigorate this development process additionally. In the meantime we shall look forward to seeing new aerogel insulation products and companies appear on the World markets.
The authors would like to acknowledge Springer publishing and the main editors Michel Aegerter and Nicholas Leventis for their involvement in the Aerogels Handbook by which this review article was inspired. Also, the European Commission, the French Agency for Environment and Energy Management (ADEME), the French National Research Agency (ANR), the French „Fonds Unique Interministériel “(FUI) fund and ARMINES (The Contract Research Association of MINES Schools) for their financial support since the early nineties through different projects (like HILIT, HILIT+, PACTE Aerogels, ISOCOMP and NANO-PU), MINES ParisTech/ARMINES/CEMEF for SEM/TEM characterization support and last but not least, the industrials PCAS FIXIT/HASIT and PAREXLANKO as well as the French Scientific and Technical Centre for Building (CSTB) are warmly acknowledged for fruitful collaborations.