View source for Aerogel

{{short description|Synthetic ultralight solid material}}
{{Use dmy dates|date=July 2016}}
[[File:Aerogel hand.jpg|right|thumb|A block of silica aerogel in a hand.]]
{{Quote box|width = 35%
|title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
|quote = '''aerogel''': [https://doi.org/10.1351/goldbook.G02600 gel] {{sic|comprised |hide=n|of}} a microporous solid in which the dispersed phase is a gas. (See Gold Book entry for note.)
<ref name='Gold Book "aerogel"'>{{cite book |chapter=aerogel |chapter-url=https://goldbook.iupac.org/terms/view/A00173 |title-link=IUPAC Compendium of Chemical Terminology| title =Gold Book | edition=3rd |date=2019|orig-date=2014 |publisher=IUPAC |access-date=1 April 2024 |ref=Gold Book A00173|isbn=978-0-9678550-9-7 |url-status=live |archive-url=https://web.archive.org/web/20121130224939/http://goldbook.iupac.org/A00173.html |archive-date=30 November 2012 |doi=10.1351/goldbook.A00173}}</ref>
}}
'''Aerogels''' are a class of [[manufacturing|synthetic]] porous [[ultralight material]] derived from a [[gel]], in which the [[liquid]] component for the gel has been replaced with a [[gas]], without significant collapse of the gel structure.<ref name="goldbook007">{{cite journal |title=Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC Recommendations 2007) |journal=[[Pure and Applied Chemistry]] |date=2007 |volume=79 |issue=10 |pages=1801–1829 |doi=10.1351/pac200779101801| doi-access =free |df=dmy-all <!-- sigh -->  |first1=J. V.| last1= Alemán |first2= A. V.|last2= Chadwick |first3= J. |last3=He |first4= M. |last4=Hess |first5= K. |last5=Horie |first6= R. G. |last6=Jones |first7= P. |last7=Kratochvíl |first8= I. |last8=Meisel |first9= I.|last9= Mita |first10= G.|last10= Moad |first11= S. |last11=Penczek |first12= R. F. T. |last12= Stepto}}</ref> The result is a solid with extremely low [[density]]<ref name="GuinnessRecord">{{cite web |url=http://stardust.jpl.nasa.gov/news/news93.html |title=Guinness Records Names JPL's Aerogel World's Lightest Solid |date=7 May 2002 |publisher=Jet Propulsion Laboratory |work=NASA |access-date=25 May 2009 |archive-url=https://web.archive.org/web/20090525181226/http://stardust.jpl.nasa.gov/news/news93.html |archive-date=25 May 2009 |url-status=live}}</ref> and extremely low [[thermal conductivity]]. Aerogels can be made from a variety of [[Chemical compound|chemical compounds]].<ref name="Aerogels Handbook">{{cite book |title=Aerogels Handbook |editor1-last=Aegerter |editor1-first=M.A. |date=2011 |publisher=Springer publishing |isbn=978-1-4419-7477-8 |editor2=Leventis, N. |editor3=Koebel, M. M.|doi=10.1007/978-1-4419-7589-8 }}</ref> [[Silica]] aerogels feel like fragile [[expanded polystyrene|styrofoam]] to the touch, while some [[polymer]]-based aerogels feel like rigid foams.
Aerogels are produced by extracting the liquid component of a gel through [[supercritical drying]] or [[freeze-drying]]. This allows the liquid to be slowly dried off without causing the solid matrix in the gel to collapse from [[capillary action]], as would happen with conventional [[evaporation]]. The first aerogels were produced from [[silica gel]]s. Kistler's later work involved aerogels based on [[alumina]], [[Chromium(III) oxide|chromia]], and [[tin dioxide]]. [[Carbon]] aerogels were first developed in the late 1980s.<ref>{{cite journal |last=Pekala |first=R. W. |title=Organic aerogels from the polycondensation of resorcinol with formaldehyde |journal=Journal of Materials Science |language=en |volume=24 |issue=9 |pages=3221–3227 |doi=10.1007/BF01139044 |issn=0022-2461 |bibcode=1989JMatS..24.3221P |year=1989 |s2cid=91183262}}</ref>
== History ==
The first documented example of an aerogel was created by [[Samuel Stephens Kistler]] in 1931,<ref name=":1">{{cite journal |last=Pajonk |first=G. M. |date=1991-05-16 |title=Aerogel catalysts |url=https://dx.doi.org/10.1016/0166-9834%2891%2985054-Y |journal=Applied Catalysis |language=en |volume=72 |issue=2 |pages=217–266 |doi=10.1016/0166-9834(91)85054-Y |issn=0166-9834}}</ref> as a result of a bet<ref>{{cite book |last1=Barron |first1=Randall F. |url=https://books.google.com/books?id=exRjDAAAQBAJ&q=kistler+charles+learned&pg=PA41 |title=Cryogenic Heat Transfer |last2=Nellis |first2=Gregory F. |publisher=[[CRC Press]] |year=2016 |isbn=9781482227451 |edition=2nd |page=41 |archive-url=https://web.archive.org/web/20171122171437/https://books.google.com/books?id=exRjDAAAQBAJ&pg=PA41&dq=kistler+charles+learned&hl=en&sa=X&redir_esc=y |archive-date=22 November 2017 |url-status=live |df=dmy-all}}</ref> with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.<ref>{{cite journal |author=Kistler, S. S. |date=1931 |title=Coherent expanded aerogels and jellies |journal=[[Nature (journal)|Nature]] |volume=127 |issue=3211 |page=741 |bibcode=1931Natur.127..741K |doi=10.1038/127741a0 |s2cid=4077344 |doi-access=free}}</ref><ref>{{cite journal |author=Kistler, S. S. |date=1932 |title=Coherent Expanded-Aerogels |journal=[[Journal of Physical Chemistry]] |volume=36 |issue=1 |pages=52–64 |doi=10.1021/j150331a003}}</ref>
==Properties==
[[File:Aerogelflower_filtered.jpg|right|thumb|A flower resting on a piece of silica aerogel, which is suspended over a flame from a [[Bunsen burner]]. Aerogels have excellent insulating properties, and the flower is protected from the heat of the flame.]] Despite the name, aerogels are solid, rigid, and dry materials that do not resemble a gel in their physical properties: the name comes from the fact that they are made ''from'' gels. Pressing softly on an aerogel typically does not leave even a minor mark; pressing more firmly will leave a permanent depression. Pressing extremely firmly will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass (a property known as ''[[friability]]''), although more modern variations do not suffer from this. Despite the fact that it is prone to shattering, it is very strong structurally. Its impressive load-bearing abilities are due to the [[dendrite (metal)|dendritic]] microstructure, in which [[spherical]] particles of average size 2–5&nbsp;[[nanometre|nm]] are fused together into clusters. These clusters form a three-dimensional highly [[porosity|porous]] structure of almost [[fractal]] chains, with pores just under 100&nbsp;nm. The average size and density of the pores can be controlled during the manufacturing process.
An aerogel material can range from 50% to 99.98% air by volume, but in practice most aerogels exhibit somewhere between 90 and 99.8% porosity.<ref>{{cite web |website=Aerogel.org |title=What is Aerogel? |url=http://www.aerogel.org/?p=3 |access-date=2023-01-22 |language=en-US}}</ref> Aerogels have a porous solid network that contains air pockets, with the air pockets taking up the majority of space within the material.<ref>{{cite web |url=http://www.azom.com/article.aspx?ArticleID=6499 |title=What is Aerogel? Theory, Properties and Applications |date=12 December 2013 |publisher=azom.com |access-date=5 December 2014 |url-status=live |archive-url=https://web.archive.org/web/20141209123257/http://www.azom.com/article.aspx?ArticleID=6499 |archive-date=9 December 2014 |df=dmy-all}}</ref>
Aerogels are good [[thermal insulation|thermal insulators]] because they almost nullify two of the three methods of [[heat transfer]] – conduction (they are mostly composed of insulating gas) and convection (the microstructure prevents net gas movement). They are good [[heat conduction|conductive]] insulators because they are composed almost entirely of gases, which are very poor heat conductors. (Silica aerogel is an especially good insulator because silica is also a poor conductor of heat; a metallic or carbon aerogel, on the other hand, would be less effective.) They are good [[convection (heat transfer)|convective]] inhibitors because air cannot circulate through the lattice. Aerogels are poor [[thermal radiation|radiative]] insulators because [[Infrared|infrared radiation]] (which transfers heat) passes through them.
Owing to its [[hygroscopic]] nature, aerogel feels dry and acts as a strong [[desiccant]]. People handling aerogel for extended periods should wear gloves to prevent the appearance of dry brittle spots on their skin.
The slight colour it does have is due to [[Rayleigh scattering]] of the shorter [[wavelength]]s of [[visible light]] by the nano-sized dendritic structure. This causes it to appear smoky blue against dark backgrounds and yellowish against bright backgrounds.
Aerogels by themselves are [[hydrophilic]], and if they absorb moisture they usually suffer a structural change, such as contraction, and deteriorate, but degradation can be prevented by making them [[hydrophobic]], via a chemical treatment. Aerogels with hydrophobic interiors are less susceptible to degradation than aerogels with only an outer hydrophobic layer, especially if a crack penetrates the surface.
=== Structure ===
Aerogel structure results from a [[sol-gel]] [[polymerization]], which is when [[monomer]]s (simple molecules) react with other monomers to form a sol or a substance that consists of bonded, cross-linked [[macromolecule]]s with deposits of liquid solution among them. When the material is critically heated, the liquid [[evaporates]] and the bonded, [[cross-linked]] macromolecule frame is left behind. The result of the polymerization and critical heating is the creation of a material that has a porous strong structure classified as aerogel.<ref>[https://str.llnl.gov/str/Foxhighlight.html Aerogel Structure] {{webarchive|url=https://web.archive.org/web/20141225170824/https://str.llnl.gov/str/Foxhighlight.html |date=25 December 2014}}. Str.llnl.gov. Retrieved on 31 July 2016.</ref> Variations in synthesis can alter the surface area and pore size of the aerogel. The smaller the pore size the more susceptible the aerogel is to fracture.<ref>{{cite web |url=http://www.aerogel.org/?p=16 |title=Silica Aerogel |website=Aerogel.org |url-status=live |archive-url=https://web.archive.org/web/20160404111603/http://www.aerogel.org/?p=16 |archive-date=4 April 2016 |df=dmy-all}}</ref>
=== Porosity of aerogel ===
There are several ways to determine the porosity of aerogel: the three main methods are gas [[adsorption]], mercury porosimetry, and scattering method. In gas adsorption, nitrogen at its boiling point is adsorbed into the aerogel sample. The gas being adsorbed is dependent on the size of the pores within the sample and on the partial pressure of the gas relative to its [[saturation pressure]]. The volume of the gas adsorbed is measured by using the Brunauer, Emmit and Teller formula ([[BET theory|BET]]), which gives the specific [[surface area]] of the sample. At high partial pressure in the adsorption/desorption the Kelvin equation gives the pore size distribution of the sample. In mercury porosimetry, the [[mercury (element)|mercury]] is forced into the aerogel porous system to determine the pores' size, but this method is highly inefficient since the solid frame of aerogel will collapse from the high compressive force. The scattering method involves the angle-dependent deflection of radiation within the aerogel sample. The sample can be solid particles or pores. The radiation goes into the material and determines the fractal geometry of the aerogel pore network. The best radiation wavelengths to use are X-rays and neutrons. Aerogel is also an open porous network: the difference between an open porous network and a closed porous network is that in the open network, gases can enter and leave the substance without any limitation, while a closed porous network traps the gases within the material forcing them to stay within the pores.<ref>[https://pamelanorris.wordpress.com/resources/pore-structure-of-silica-aerogels/ Pore Structure of Silica Aerogels] {{webarchive|url=https://web.archive.org/web/20141201064113/http://energy.lbl.gov/ECS/aerogels/sa-pore.html |date=1 December 2014}}. Energy.lbl.gov. Retrieved on 31 July 2016.</ref> The high porosity and surface area of silica aerogels allow them to be used in a variety of environmental filtration applications.
=== Knudsen effect ===
Aerogels may have a [[thermal conductivity]] smaller than that of the gas they contain.<ref>{{cite journal |last1=Zhang |first1=Hu |last2=Zhang |first2=Chao |last3=Ji |first3=Wentao |last4=Wang |first4=Xian |last5=Li |first5=Yueming |last6=Tao |first6=Wenquan |date=2018-08-30 |title=Experimental Characterization of the Thermal Conductivity and Microstructure of Opacifier-Fiber-Aerogel Composite |journal=Molecules |volume=23 |issue=9 |page=2198 |doi=10.3390/molecules23092198 |issn=1420-3049 |pmc=6225116 |pmid=30200271 |doi-access=free}}</ref><ref>{{Citation|last1=Caps|first1=R.|title=Aerogels for Thermal Insulation|date=2004|url=https://doi.org/10.1007/978-0-387-88953-5_46|work=Sol-Gel Technologies for Glass Producers and Users|pages=349–353|editor-last=Aegerter|editor-first=Michel A.|place=Boston, MA|publisher=Springer US|language=en|doi=10.1007/978-0-387-88953-5_46|isbn=978-0-387-88953-5|access-date=2021-03-29|last2=Fricke|first2=J.|editor2-last=Mennig|editor2-first=Martin}}</ref> This is caused by the [[Knudsen number|Knudsen effect]], a reduction of thermal conductivity in gases when the size of the cavity encompassing the gas becomes comparable to the [[mean free path]]. Effectively, the cavity restricts the movement of the gas particles, decreasing the thermal conductivity in addition to eliminating convection. For example, thermal conductivity of air is about 25&nbsp;mW·m<sup>−1</sup>·K<sup>−1</sup> at standard temperature and pressure (STP) and in a large container, but decreases to about 5&nbsp;mW·m<sup>−1</sup>·K<sup>−1</sup> in a pore 30 nanometers in diameter.<ref>Berge, Axel and Johansson, Pär (2012) [http://publications.lib.chalmers.se/records/fulltext/local_159807.pdf Literature Review of High Performance Thermal Insulation] {{webarchive|url=https://web.archive.org/web/20141121114200/http://publications.lib.chalmers.se/records/fulltext/local_159807.pdf |date=21 November 2014}}. Department of Civil and Environmental Engineering, Chalmers University of Technology, Sweden</ref>
=== Waterproofing ===
Aerogel contains particles that are 2–5&nbsp;nm in diameter. After the process of creating aerogel, it will contain a large amount of [[hydroxyl groups]] on the surface. The hydroxyl groups can cause a strong reaction when the aerogel is placed in water, causing it to catastrophically dissolve in the water. One way to waterproof the [[hydrophilic]] aerogel is by soaking the aerogel with some chemical base that will replace the surface hydroxyl groups (–OH) with non-polar groups (–O''R''), a process which is most effective when ''R'' is an [[aliphatic]] group.<ref>[http://www.vsl.cua.edu/cua_phy/images/c/cf/Aerogel_Aerlon_SilicaAerogels.pdf The Surface Chemistry of Silica Aerogels] {{webarchive|url=https://web.archive.org/web/20141201035010/http://energy.lbl.gov/ECS/aerogels/sa-chemistry.html |date=1 December 2014}}. Energy.lbl.gov. Retrieved on 31 July 2016.</ref>
==Production==
[[File:Aerogel fabrication strategies Polymers 2019.png|thumb|Comparison of aerogel fabrication strategies showing typical transitions into an aerogel: (a) the supercritical drying process where precursor materials undergo gelation prior to supercritical drying. (b) A standard freeze-drying technique where an aqueous solution is frozen.]]
[[File:Phase diagram gel to aerogel transition Polymers 2019.png|thumb|A typical phase diagram for pure compounds. Two methods are shown for the gel to aerogel transition: The solid-gas transition (during freeze-drying) and the transition from a liquid to gas during supercritical drying.]]
=== Overview ===
The preparation of silica aerogels typically involves three distinct steps:<ref>Araby, S.; Qiu, A.; Wang, R.; Zhao, Z.; Wang, C.H.; Ma, J. Aerogels based on carbon nanomaterials. J. Mater. Sci. 2016, 51, 9157–9189.</ref> the sol-gel transition (gelation),<ref>Pierre, A.C. History of Aerogels. In Aerogels Handbook. Advances in Sol-Gel Derived Materials and Technologies; Aegerter, M., Leventis, N., Koebel, M., Eds.; Springer: New York, NY, USA, 2011; pp. 3–18.</ref> the network perfection (aging), and<ref>Zhang, M.; Fang, S.; Zakhidov, A.A.; Lee, S.B.; Alieve, A.E.; Williams, C.D.; Atkinson, K.R.; Baughman, R.H. Strong, transparent, multifunctinoal, carbon nanotube sheets. Science 2005, 209, 1215–1220.</ref> the gel-aerogel transition (drying).
=== Gelation ===
Silica aerogels are typically synthesized by using a sol-gel process. The first step of the sol-gel process is the creation of a [[colloidal]] [[suspension (chemistry)|suspension]] of solid particles known as a "sol". The precursors are a liquid [[alcohol (chemistry)|alcohol]] such as ethanol which is mixed with a [[silicon alkoxide]], such as [[tetramethoxysilane]] (TMOS), [[tetraethoxysilane]] (TEOS), and polyethoxydisiloxane (PEDS) (earlier work used sodium silicates).<ref>{{cite journal |last1=Dorcheh |first1=Soleimani |last2=Abbasi |first2=M. |date=2008 |title=Silica Aerogel; Synthesis, Properties, and Characterization |journal=Journal of Materials Processing Technology |volume=199 |issue=1–3 |pages=10–26 |doi=10.1016/j.jmatprotec.2007.10.060}}</ref> The solution of silica is mixed with a catalyst and allowed to gel during a [[hydrolysis]] reaction which forms particles of silicon dioxide.<ref name="Making silica aerogels">{{cite web |url=http://eetd.lbl.gov/ECS/Aerogels/sa-making.html |title=Making silica aerogels |publisher=Lawrence Berkeley National Laboratory |archive-url=https://web.archive.org/web/20090514144121/http://eetd.lbl.gov/ecs/aerogels/sa-making.html |archive-date=14 May 2009 |df=dmy-all |access-date=28 May 2009}}</ref> The oxide suspension begins to undergo [[condensation reaction]]s which result in the creation of metal oxide bridges (either [[Oxo ligand|M–O–M, "oxo" bridges]], or M–OH–M, "[[-ol|ol]]" bridges) linking the dispersed colloidal particles.<ref>{{cite journal |date=2002 |title=Chemistry of Aerogels and their Applications |journal=[[Chemical Reviews]] |volume=102 |issue=11 |pages=4243–4265 |doi=10.1021/cr0101306 |pmid=12428989 |last1=Pierre |first1=A. C. |last2=Pajonk |first2=G. M.}}</ref> These reactions generally have moderately slow reaction rates, and as a result either acidic or basic [[catalyst]]s are used to improve the processing speed. Basic catalysts tend to produce more transparent aerogels and minimize the shrinkage during the drying process and also strengthen it to prevent pore collapse during drying.<ref name="Making silica aerogels" />
For some materials, the transition from a colloidal dispersion into a gel happens without the addition of crosslinking materials.<ref>Hüsing, N.; Schubert, U. Aerogels—Airy Materials: Chemistry, Structure, and Properties. Angew. Chem. Int. Ed. 1998, 37, 22–45.</ref> For others, crosslinking materials are added to the dispersion to promote the strong interaction of the solid particles in order to form the gel.<ref name="auto1">Capadona, L.A.; Meador, M.A.B.; Alunni, A.; Fabrizio, E.F.; Vassilaras, P.; Leventis, N. Flexible, low-density polymer crosslinked silica aerogels. Polymer 2006, 47, 5754–5761.</ref><ref>Leventis, N.; Lu, H. Polymer-Crosslinked Aerogels. In Aerogels Handbook. Advances in Sol-Gel Derived Materials and Technologies; Aegerter, M., Leventis, N., Koebel, M., Eds.; Springer: New York, NY, USA, 2011; pp. 251–285.</ref> The gelation time depends heavily on a variety of factors such as the chemical composition of the precursor solution, the concentration of the precursor materials and additives, the processing temperature, and the pH.<ref name="auto1"/><ref>Hench, L.L.; West, J.K. The sol-gel process. Chem. Rev. 1990, 90, 33–72.</ref><ref>Mulik, S.; Sotiriou-leventis, C.; Leventis, N. Time-Efficient Acid-Catalyzed Synthesis of Resorcinol—Formaldehyde Aerogels. Chem. Mater. 2007, 19, 6138–6144.</ref><ref>Zhang, J.; Cao, Y.; Feng, J.; Wu, P. Graphene-oxide-sheet-induced gelation of cellulose and promoted mechanical properties of composite aerogels. J. Phys. Chem. C 2012, 116, 8063–8068.</ref><ref>Hdach, H.; Woignier, T.; Phalippou, J.; Scherer, G.W. Effect of aging and pH on the modulus of aerogels. J. Non-Cryst. Solids 1990, 121, 202–205.</ref> Many materials may require additional curing after gelation (i.e., network perfection) in order to strengthen the aerogel network.<ref name="auto1"/><ref>Einarsrud, M.; Nilsen, E.; Rigacci, A.; Pajonk, G.M.; Buathier, S. Strengthening of silica gels and aerogels by washing and aging processes. J. Non-Cryst. Solids 2001, 285, 1–7.</ref><ref>Soleimani Dorcheh, A.; Abbasi, M.H. Silica aerogel; synthesis, properties and characterization. J. Mater. Process. Technol. 2008, 199, 10–26.</ref><ref>Hæreid, S.; Anderson, J.; Einarsrud, M.A.; Hua, D.W.; Smith, D.M. Thermal and temporal aging of TMOS-based aerogel precursors in water. J. Non-Cryst. Solids 1995, 185, 221–226.</ref><ref>Omranpour, H.; Motahari, S. Effects of processing conditions on silica aerogel during aging: Role of solvent, time and temperature. J. Non-Cryst. Solids 2013, 379, 7–11.</ref><ref>Cheng, C.-P.; Iacobucci, P.A. Inorganic Oxide Aerogels and Their Preparation. U.S. Patent 4,717,708, 5 January 1988.</ref>
=== Drying ===
Once the gelation is completed, the liquid surrounding the silica network is carefully removed and replaced with air, while keeping the aerogel intact. It is crucial that the gel is dried in such a way as to minimize the surface tension within the pores of the solid network. This is typically accomplished through supercritical fluid extraction using [[supercritical carbon dioxide]] (scCO<sub>2</sub>) or freeze-drying.This section briefly describes and compares the processing strategies of supercritical drying and freeze-drying.
Gels where the liquid is allowed to evaporate at a natural rate are known as [[xerogel]]s (i. e. are not aerogels). As the liquid evaporates in such manner, forces caused by [[surface tension]]s of the liquid-solid [[interface (chemistry)|interfaces]] are enough to destroy the fragile gel network. As a result, xerogels cannot achieve the high porosities and instead peak at lower porosities and exhibit large amounts of shrinkage after drying.<ref>{{cite journal |date=1992 |title=Aerogels |journal=[[Journal of the American Ceramic Society]] |volume=75 |issue=8 |pages=2027–2036 |doi=10.1111/j.1151-2916.1992.tb04461.x |last1=Fricke |first1=Jochen |last2=Emmerling |first2=Andreas}}</ref> To avoid the collapse of fibers during slow solvent evaporation and reduce surface tensions of the liquid-solid interfaces, aerogels can be formed by [[freeze-drying|lyophilization]] (freeze-drying). Depending on the concentration of the fibers and the temperature to freeze the material, the properties such as porosity of the final aerogel will be affected.<ref>{{cite journal |last1=Zhang |first1=Xuexia |last2=Yu |first2=Yan |last3=Jiang |first3=Zehui |last4=Wang |first4=Hankun |date=2015-12-01 |title=The effect of freezing speed and hydrogel concentration on the microstructure and compressive performance of bamboo-based cellulose aerogel |journal=Journal of Wood Science |language=en |volume=61 |issue=6 |pages=595–601 |doi=10.1007/s10086-015-1514-7 |s2cid=18169604 |issn=1611-4663|doi-access=free |bibcode=2015JWSci..61..595Z }}</ref>
In 1931, to develop the first aerogels, Kistler used a process known as [[supercritical drying]] which avoids a direct phase change.<ref name=":4"/> By increasing the temperature and pressure he forced the liquid into a [[supercritical fluid]] state where by dropping the pressure he could instantly gasify and remove the liquid inside the aerogel, avoiding damage to the delicate three-dimensional network. While this can be done with [[ethanol]], the high temperatures and pressures lead to dangerous processing conditions. A safer, lower temperature and pressure method involves a solvent exchange. This is typically done by exchanging the initial aqueous pore liquid for a [[carbon dioxide|CO<sub>2</sub>]]-miscible liquid such as ethanol or [[acetone]], then onto [[liquid carbon dioxide]], and then bringing the carbon dioxide above its [[critical point (thermodynamics)|critical point]].<ref>{{cite journal |last1=Tewari |first1=Param H. |last2=Hunt |first2=Arlon J. |last3=Lofftus |first3=Kevin D. |date=1985-07-01 |title=Ambient-temperature supercritical drying of transparent silica aerogels |url=https://dx.doi.org/10.1016/0167-577X%2885%2990077-1 |journal=Materials Letters |language=en |volume=3 |issue=9 |pages=363–367 |doi=10.1016/0167-577X(85)90077-1 |bibcode=1985MatL....3..363T |issn=0167-577X}}</ref> A variant on this process involves the direct injection of supercritical carbon dioxide into the pressure vessel containing the aerogel. The result of either process exchanges the initial liquid from the gel with carbon dioxide, without allowing the gel structure to collapse or lose volume.<ref name="Making silica aerogels"/>
==== Supercritical drying ====
To dry the gel, while preserving the highly porous network of an aerogel, supercritical drying employs the use of the liquid-gas transition that occurs beyond the critical point of a substance. By using this liquid-gas transition that avoids crossing the liquid-gas phase boundary, the surface tension that would arise within the pores due to the evaporation of a liquid is eliminated, thereby preventing the collapse of the pores.<ref name="auto3">Gurav, J.L.; Jung, I.K.; Park, H.H.; Kang, E.S.; Nadargi, D.Y. Silica aerogel: Synthesis and applications. J. Nanomater. 2010, 2010, 23.</ref> Through heating and pressurization, the liquid solvent reaches its critical point, at which point the liquid and gas phases become indistinguishable. Past this point, the supercritical fluid is converted into the gaseous phase upon an isothermal de-pressurization. This process results in a phase change without crossing the liquid-gas phase boundary. This method is proven to be excellent at preserving the highly porous nature of the solid network without significant shrinkage or cracking. While other fluids have been reported for the creation of supercritically dried aerogels, scCO<sub>2</sub> is the most common substance with a relatively mild supercritical point at 31&nbsp;°C and 7.4 MPa. CO<sub>2</sub> is also relatively non-toxic, non-flammable, inert, and cost-effective when compared to other fluids, such as methanol or ethanol.<ref>Beckman, E.J. Supercritical or near-critical CO2 in green chemical synthesis and processing. J. Supercrit. Fluids 2004, 28, 121–191.</ref> While being a highly effective method for producing aerogels, supercritical drying takes several days, requires specialized equipment, and presents significant safety hazards due to its high-pressure operation.
==== Freeze-drying ====
Freeze-drying, also known as freeze-casting or ice-templating, offers an alternative to the high temperature and high-pressure requirements of supercritical drying. Additionally, freeze-drying offers more control of the solid structure development by controlling the ice crystal growth during freezing.<ref>Jin, H.; Nishiyama, Y.; Wada, M.; Kuga, S. Nanofibrillar cellulose aerogels. Colloids Surfaces A Physicochem. Eng. Asp. 2004, 240, 63–67.</ref><ref name="auto">Jiménez-Saelices, C.; Seantier, B.; Cathala, B.; Grohens, Y. Effect of freeze-drying parameters on the microstructure and thermal insulating properties of nanofibrillated cellulose aerogels. J. Sol-Gel Sci. Technol. 2017, 84, 475–485.</ref><ref name="auto4">Wang, C.; Chen, X.; Wang, B.; Huang, M.; Wang, B.; Jiang, Y.; Ruoff, R.S. Freeze-Casting Produces a Graphene Oxide Aerogel with a Radial and Centrosymmetric Structure. ACS Nano 2018, 12, 5816–5825.</ref><ref name="auto2">Simon-Herrero, C.; Caminero-Huertas, S.; Romero, A.; Valverde, J.L.; Sanchez-Silva, L. Effects of freeze-drying conditions on aerogel properties. J. Mater. Sci. 2016, 51, 8977–8985.</ref> In this method, a colloidal dispersion of the aerogel precursors is frozen, with the liquid component freezing into different morphologies depending on a variety of factors such as the precursor concentration, type of liquid, temperature of freezing, and freezing container.<ref name="auto"/><ref name="auto4"/><ref name="auto2"/> As this liquid freezes, the solid precursor molecules are forced into the spaces between the growing crystals. Once completely frozen, the frozen liquid is sublimed into a gas through lyophilization, which removes much of the capillary forces, as was observed in supercritical drying.<ref>Deville, S. Ice-templating, freeze casting: Beyond materials processing. J. Mater. Res. 2013, 28, 2202–2219.</ref><ref>Deville, S. The lure of ice-templating: Recent trends and opportunities for porous materials. Scr. Mater. 2018, 147, 119–124.</ref> Though typically classified as a “cryogel”, aerogels produced through freeze-drying often experience some shrinkage and cracking while also producing a non-homogenous aerogel framework.<ref name="auto3"/> This often leads to freeze-drying being used for the creation of aerogel powders or as a framework for composite aerogels.<ref>Shen, C.; Calderon, J.E.; Barrios, E.; Soliman, M.; Khater, A.; Jeyaranjan, A.; Tetard, L.; Gordon, A.; Seal, S.; Zhai, L. Anisotropic electrical conductivity in polymer derived ceramics induced by graphene aerogels. J. Mater. Chem. C 2017, 5, 11708–11716.</ref><ref>Ali, I.; Chen, L.; Huang, Y.; Song, L.; Lu, X.; Liu, B.; Zhang, L.; Zhang, J.; Hou, L.; Chen, T. Humidity-Responsive Gold Aerogel for Real-Time Monitoring of Human Breath. Langmuir 2018, 34, 4908–4913.</ref><ref>Cong, L.; Li, X.; Ma, L.; Peng, Z.; Yang, C.; Han, P.; Wang, G.; Li, H.; Song, W.; Song, G. High-performance graphene oxide/carbon nanotubes aerogel-polystyrene composites: Preparation and mechanical properties. Mater. Lett. 2018, 214, 190–193.</ref><ref>Cao, N.; Lyu, Q.; Li, J.; Wang, Y.; Yang, B.; Szunerits, S.; Boukherroub, R. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem. Eng. J. 2017, 326, 17–28.</ref><ref>Jia, J.; Wang, C. A facile restructuring of 3D high water absorption aerogels from methoxy polyethylene glycol-polycaprolactone (mPEG-PCL) nanofibers. Mater. Sci. Eng. C 2019, 94, 965–975.</ref>
=== Preparation of non-silica aerogels ===
[[Resorcinol]]–[[formaldehyde]] aerogel (RF aerogel) is made in a way similar to production of silica aerogel. A carbon aerogel can then be made from this resorcinol–formaldehyde aerogel by [[pyrolysis]] in an [[inert gas]] atmosphere, leaving a matrix of [[carbon]].<ref>{{cite journal |last1=Gan |first1=Yong X. |last2=Gan |first2=Jeremy B. |date=June 2020 |title=Advances in Manufacturing Composite Carbon Nanofiber-Based Aerogels |journal=[[Journal of Composites Science]] |language=en |volume=4 |issue=2 |pages=73 |doi=10.3390/jcs4020073 |doi-access=free}}</ref> The resulting carbon aerogel may be used to produce solid shapes, powders, or composite paper.<ref>{{cite web |title=Carbon Aerogel - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/engineering/carbon-aerogel |access-date=2021-03-29 |website=[[ScienceDirect]]}}</ref> Additives have been successful in enhancing certain properties of the aerogel for the use of specific applications. Aerogel [[composite material|composites]] have been made using a variety of continuous and discontinuous [[reinforcement]]s. The high aspect ratio of fibers such as [[fiberglass]] have been used to reinforce aerogel composites with significantly improved mechanical properties.
==Materials==
[[File:Aerogelbrick.jpg|thumb|A 2.5 kg [[brick]] is supported by a piece of aerogel with a mass of 2&nbsp;g.]]
===Silica aerogel===
Silica aerogels are the most common type of aerogel, and the primary type in use or study.<ref name=":4">{{cite journal |last1=Nguyen |first1=Hong K. D. |last2=Hoang |first2=Phuong T. |last3=Dinh |first3=Ngo T. |last4=Nguyen |first4=Hong K. D. |last5=Hoang |first5=Phuong T. |last6=Dinh |first6=Ngo T. |date=August 2018 |title=Synthesis of Modified Silica Aerogel Nanoparticles for Remediation of Vietnamese Crude Oil Spilled on Water |journal=[[Journal of the Brazilian Chemical Society]] |language=en |volume=29 |issue=8 |pages=1714–1720 |doi=10.21577/0103-5053.20180046 |issn=0103-5053 |doi-access=free}}</ref><ref>{{cite web |date=2015-04-15 |title=Aerogels: Thinner, Lighter, Stronger |url=http://www.nasa.gov/topics/technology/features/aerogels.html |access-date=2021-03-29 |website=[[NASA]] |language=en}}</ref> It is [[silica]]-based and can be derived from [[silica gel]] or by a modified [[Stober process]]. Nicknames include ''frozen smoke'',<ref name="Times081907">{{cite news |last=Taher |first=Abul |date=19 August 2007 |title=Scientists hail 'frozen smoke' as material that will change world |work=Times Online |location=London |url=http://www.timesonline.co.uk/tol/news/uk/science/article2284349.ece |url-status=dead |access-date=22 August 2007 |archive-url=https://web.archive.org/web/20070912234840/http://www.timesonline.co.uk/tol/news/uk/science/article2284349.ece |archive-date=12 September 2007 |df=dmy-all}}</ref> ''solid smoke'', ''solid air'', ''solid cloud'', and ''blue smoke'', owing to its [[transparency (optics)|translucent]] nature and the way [[light]] [[scattering|scatters]] in the material. The lowest-density silica nanofoam weighs 1,000&nbsp;g/m<sup>3</sup>,<ref name="terms">[https://web.archive.org/web/20050718075757/http://www.llnl.gov/IPandC/technology/profile/aerogel/Terms/index.php Aerogels Terms]. LLNL.gov</ref> which is the evacuated version of the record-aerogel of 1,900&nbsp;g/m<sup>3</sup>.<ref name="llnl03">{{cite web |url=http://www.llnl.gov/str/October03/NewsOctober03.html |title=Lab's aerogel sets world record |date=October 2003 |publisher=LLNL Science & Technology Review |url-status=live |archive-url=https://web.archive.org/web/20061009154049/http://www.llnl.gov/str/October03/NewsOctober03.html |archive-date=9 October 2006 |df=dmy-all}}</ref> The density of [[air]] is 1,200&nbsp;g/m<sup>3</sup> (at 20&nbsp;°C and 1&nbsp;atm).<ref>Groom, D.E. [http://pdg.lbl.gov/2007/reviews/atomicrpp.pdf Abridged from Atomic Nuclear Properties] {{webarchive|url=https://web.archive.org/web/20080227212418/http://pdg.lbl.gov/2007/reviews/atomicrpp.pdf |date=27 February 2008}}. Particle Data Group: 2007.</ref>
The silica solidifies into three-dimensional, intertwined clusters that make up only 3% of the volume. Conduction through the solid is therefore very low. The remaining 97% of the volume is composed of air in extremely small nanopores. The air has little room to move, inhibiting both convection and gas-phase conduction.<ref>{{cite news |url=http://www.aerogel.com/resources/about-aerogel/ |title=About Aerogel |newspaper=Aspen Aerogels |publisher=ASPEN AEROGELS, INC. |access-date=12 March 2014 |url-status=live |archive-url=https://web.archive.org/web/20140526131958/http://www.aerogel.com/resources/about-aerogel/ |archive-date=26 May 2014 |df=dmy-all}}</ref>
Silica aerogel also has a high optical transmission of ~99% and a low refractive index of ~1.05.<ref name=":3" /> It is very robust with respect to high power input beam in continuous wave regime and does not show any boiling or melting phenomena.<ref>{{cite journal |last1=Gentilini |first1=S. |last2=Ghajeri |first2=F. |last3= Ghofraniha |first3=N. |last4=Falco |first4=A. Di |last5=Conti |first5=C. |date=2014-01-27 |title=Optical shock waves in silica aerogel |url= https://www.osapublishing.org/oe/abstract.cfm?uri=oe-22-2-1667 |journal=Optics Express |language=EN |volume=22 |issue=2 |pages=1667–1672 |doi=10.1364/OE.22.001667 |pmid=24515173 |bibcode=2014OExpr..22.1667G |issn=1094-4087 |hdl=10023/4490 |hdl-access=free}}</ref> This property permits to study high intensity nonlinear waves in the presence of disorder in regimes typically unaccessible by liquid materials, making it promising material for nonlinear optics.
This aerogel has remarkable thermal insulative properties, having an extremely low [[thermal conductivity]]: from 0.003&nbsp;[[watt|W]]·m<sup>−1</sup>·[[kelvin|K]]<sup>−1</sup><ref>"Thermal conductivity" in {{RubberBible86th}} Section 12, p. 227</ref> in atmospheric pressure down to 0.004&nbsp;W·m<sup>−1</sup>·K<sup>−1</sup><ref name="terms"/> in modest vacuum, which correspond to [[R-value (insulation)|R-values]] of 14 to 105 (US customary) or 3.0 to 22.2 (metric) for {{convert|3.5|in|mm|0|abbr=on}} thickness. For comparison, typical wall insulation is 13 (US customary) or 2.7 (metric) for the same thickness. Its [[melting point]] is {{convert|1473|K|C F|0|abbr=on}}. It is also worth noting that even lower conductivities have been reported for experimentally produced monolithic samples in the literature, reaching 0.009&nbsp;W·m<sup>−1</sup>·K<sup>−1</sup> at 1atm.<ref>{{cite journal |author1=Cohen, E. |author2=Glicksman, L. |date=August 1, 2015 |title=Thermal Properties of Silica Aerogel Formula |journal=Journal of Heat Transfer |publisher=ASME International |volume=137 |issue=8 |page=081601 |doi=10.1115/1.4028901 |hdl=1721.1/106629 |s2cid=55430528 |hdl-access=free}}</ref>
Until 2011, silica aerogel held 15&nbsp;entries in ''[[Guinness World Records]]'' for material properties, including best insulator and lowest-density solid, though it was ousted from the latter title by the even lighter materials [[aerographite]] in 2012<ref>{{cite journal |last=Mecklenburg |first=Matthias |date=July 2012 |title=Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance |journal=Advanced Materials |volume=24 |issue=26 |pages=3486–90 |doi=10.1002/adma.201200491 |pmid=22688858 |bibcode=2012AdM....24.3486M |s2cid=2787227}}</ref> and then [[aerographene]] in 2013.<ref>Whitwam, Ryan (26 March 2013). [http://www.geek.com/articles/chips/graphene-aerogel-is-worlds-lightest-material-20130326/ Graphene aerogel is world's lightest material] {{webarchive|url=https://web.archive.org/web/20130327134015/http://www.geek.com/articles/chips/graphene-aerogel-is-worlds-lightest-material-20130326/ |date=27 March 2013}}. gizmag.com</ref><ref>Quick, Darren (24 March 2013). [http://www.gizmag.com/graphene-aerogel-worlds-lightest/26784/ Graphene aerogel takes world's lightest material crown] {{webarchive|url=https://web.archive.org/web/20130325182654/http://www.gizmag.com/graphene-aerogel-worlds-lightest/26784/ |date=25 March 2013}}. gizmag.com</ref>
===Carbon===
[[Carbon]] aerogels are composed of particles with sizes in the [[nanometer]] range, [[covalent bond|covalently bonded]] together. They have very high [[porosity]] (over 50%, with pore diameter under 100&nbsp;nm) and surface areas ranging between 400 and 1,000&nbsp;m<sup>2</sup>/g. They are often manufactured as composite paper: non-woven paper made of [[carbon fibers]], impregnated with [[resorcinol]]–[[formaldehyde]] aerogel, and [[pyrolisis|pyrolyzed]]. Depending on the density, carbon aerogels may be electrically conductive, making composite aerogel paper useful for electrodes in [[capacitor]]s or deionization electrodes. Due to their extremely high surface area, carbon aerogels are used to create [[supercapacitor]]s, with values ranging up to thousands of [[farad]]s based on a capacitance density of 104&nbsp;F/g and 77&nbsp;F/cm<sup>3</sup>. Carbon aerogels are also extremely "black" in the infrared spectrum, reflecting only 0.3% of radiation between 250&nbsp;nm and 14.3&nbsp;μm, making them efficient for [[Solar thermal energy|solar energy]] collectors.
The term "aerogel" to describe airy masses of [[carbon nanotube]]s produced through certain [[chemical vapor deposition]] techniques is incorrect. Such materials can be spun into fibers with strength greater than [[Kevlar]], and unique electrical properties. These materials are not aerogels, however, since they do not have a monolithic internal structure and do not have the regular pore structure characteristic of aerogels.
===Metal oxide===
[[Metal oxide]] aerogels are used as catalysts in various chemical reactions/transformations or as precursors for other materials.
Aerogels made with [[aluminium oxide]] are known as alumina aerogels. These aerogels are used as catalysts, especially when "doped" with a metal other than aluminium. [[Nickel]]–alumina aerogel is the most common combination. Alumina aerogels are also being considered by [[NASA]] for [[Stardust (spacecraft)#Sample collection|capturing]] hypervelocity particles; a formulation doped with [[gadolinium]] and [[terbium]] could [[fluoresce]] at the particle impact site, with the amount of fluorescence dependent on impact energy.
One of the most notable differences between silica aerogels and metal oxide aerogel is that metal oxide aerogels are often variedly colored.<ref>{{cite web |url=http://www.aerogel.org/?p=44 |title=Metal Oxide Aerogels |publisher=Aerogel.org |access-date=12 June 2013 |url-status=live |archive-url=https://web.archive.org/web/20130812045126/http://www.aerogel.org/?p=44 |archive-date=12 August 2013 |df=dmy-all}}</ref>
{| class="wikitable"
|-
! Aerogel !! Color
|-
| [[Silica]], [[alumina]], [[titanium dioxide|titania]], [[zirconia]] || Clear with Rayleigh scattering blue or white
|-
| [[Iron oxide]] || Rust red or yellow, opaque
|-
| [[Chromium(III) oxide|Chromia]] || Deep green or deep blue, opaque
|-
| [[Vanadia]] || Olive green, opaque
|-
| [[Neodymium oxide]] || Purple, transparent
|-
| [[Samarium(III) oxide|Samaria]] || Yellow, transparent
|-
| [[Holmium(III) oxide|Holmia]], [[erbium(III) oxide|erbia]] || Pink, transparent
|}
===Other===
Organic polymers can be used to create aerogels. [[SEAgel]] is made of [[agar]]. AeroZero film is made of [[polyimide]]. Cellulose from plants can be used to create a flexible aerogel.<ref>{{cite journal |last2=Saito |first2=Tsuguyuki |last3=Isogai |first3=Akira |date=2014 |title=Aerogels with 3D Ordered Nanofiber Skeletons of Liquid-Crystalline Nanocellulose Derivatives as Tough and Transparent Insulators |journal=Angewandte Chemie International Edition |volume=53 |issue=39 |pages=10394–7 |doi=10.1002/anie.201405123 |last1=Kobayashi |first1=Yuri |pmid=24985785}}
*{{lay source |template=cite news |author=Manisha Lalloo |title=Plant material aligns to make tough aerogels |url=http://www.rsc.org/chemistryworld/2014/07/plant-material-aligns-make-tough-aerogels-nanocellulose |url-access=registration |work=ChemistryWorld |publisher=Royal Society of Chemistry |date=10 July 2014}}</ref>
GraPhage13 is the first graphene-based aerogel assembled using [[graphene oxide]] and the [[M13 bacteriophage]].<ref>Passaretti, P., et al. (2019). "Multifunctional graphene oxide-bacteriophage based porous three-dimensional micro-nanocomposites." Nanoscale 11(28): 13318-13329. https://doi.org/10.1039/C9NR03670A</ref>
[[Chalcogel]] is an aerogel made of [[chalcogen]]s (the column of elements on the periodic table beginning with oxygen) such as sulfur, selenium, and other elements.<ref>Biello, David [http://www.scientificamerican.com/article/heavy-metal-filter-made-largely-from-air/ Heavy Metal Filter Made Largely from Air.] {{webarchive|url=https://web.archive.org/web/20150226102809/http://www.scientificamerican.com/article/heavy-metal-filter-made-largely-from-air/ |date=26 February 2015}} ''Scientific American'', 26 July 2007. Retrieved on 2007-08-05.</ref> Metals less expensive than platinum have been used in its creation.
Aerogels made of [[cadmium selenide]] [[quantum dots]] in a porous 3-D network have been developed for use in the semiconductor industry.<ref>{{cite journal |last1=Yu |first1=H |last2=Bellair |first2=R |last3=Kannan |first3=R. M. |last4=Brock |first4=S. L. |author-link4=Stephanie Brock |date=2008 |title=Engineering Strength, Porosity, and Emission Intensity of Nanostructured CdSe Networks By Altering The Building Block Shape |journal=[[Journal of the American Chemical Society]] |volume=130 |issue=15 |pages=5054–5055 |doi=10.1021/ja801212e |pmid=18335987|bibcode=2008JAChS.130.5054Y }}</ref>
Aerogel performance may be augmented for a specific application by the addition of [[dopant]]s, reinforcing structures, and hybridizing compounds. For example, Spaceloft is a composite of aerogel with some kind of fibrous batting.<ref>{{cite web |url=http://www.aerogel.org/?p=1058 |title=Strong and Flexible Aerogels |website=Aerogel.org |access-date=17 July 2014 |url-status=live |archive-url=https://web.archive.org/web/20141011004216/http://www.aerogel.org/?p=1058 |archive-date=11 October 2014 |df=dmy-all}}</ref>
[[Amyloid fibril]]s from food waste ([[whey]]) have the potential for use in [[AF aerogel]]s for gold extraction from [[e-waste]].<ref name="Peydayesh2024">{{cite journal |last1=Peydayesh |first1=Mohammad |last2=Boschi |first2=Enrico |last3=Donat |first3=Felix |last4=Mezzenga |first4=Raffaele |title=Gold Recovery from E‐Waste by Food‐Waste Amyloid Aerogels |journal=Advanced Materials |date=May 2024 |volume=36 |issue=19 |doi=10.1002/adma.202310642|hdl=20.500.11850/658267 |hdl-access=free }}</ref>
==Applications==
<gallery>
File:Stardust Dust Collector with aerogel.jpg|The "[[Stardust (spacecraft)|Stardust]]" dust collector with aerogel blocks. (NASA)
File:Stardust-particle-Tsou060207b.jpg|Cosmic dust caught in aerogel blocks from "Stardust". (NASA)
File:Oil absorption by BN aerogel.jpg|Oil absorption by an aerogel.<ref name="Song 10337">{{cite journal |last1=Song |first1=Yangxi |last2=Li |first2=Bin |last3=Yang |first3=Siwei |last4=Ding |first4=Guqiao |last5=Zhang |first5=Changrui |last6=Xie |first6=Xiaoming |date=2015-05-15 |title=Ultralight boron nitride aerogels via template-assisted chemical vapor deposition |journal=Scientific Reports |language=en |volume=5 |issue=1 |pages=10337 |doi=10.1038/srep10337 |pmid=25976019 |pmc=4432566 |bibcode=2015NatSR...510337S |issn=2045-2322}}</ref> (''Scientific Reports'')
File:BN aerogel on hair.jpg|An aerogel held up by hair.<ref name="Song 10337"/> (''Scientific Reports'')
File:Aerogel crayons.jpg|An aerogel holding crayons, with a flame lit underneath, demonstrating its excellent insulation from heat. (NASA)
</gallery>Aerogels are used for a variety of applications:
* [[Thermal insulation]]; with fibre reinforced silica aerogel insulation boards insulation thickness can be reduced by about 50% compared to conventional materials. This makes silica aerogel boards well suited for the retrofit of historic buildings<ref name="Ganobjak et al 2020">{{cite journal |last1=Ganobjak |first1=Michal |last2=Brunner |first2=Samuel |last3=Wernery |first3=Jannis |title=Aerogel materials for heritage buildings: Materials, properties and case studies |journal=Journal of Cultural Heritage |date=2020 |volume=42 |issue=March–April |pages=81–98 |doi=10.1016/j.culher.2019.09.007 |s2cid=209375441 |doi-access=free}}</ref> or for the application in dense city areas.<ref name="Wernery et al 2021">{{cite journal |last1=Wernery |first1=Jannis |last2=Mancebo |first2=Francisco |last3=Malfait |first3=Wim |last4=O'Connor |first4=Michael |last5=Jelle |first5=Bjørn Petter |title=The economics of thermal superinsulation in buildings |journal=Energy & Buildings |date=2021 |volume=253 |issue=December 2021 |page=111506 |doi=10.1016/j.enbuild.2021.111506 |s2cid=239117650 |doi-access=free|bibcode=2021EneBu.25311506W |hdl=11250/2789460 |hdl-access=free }}</ref> As other examples, aerogel has been added in granular form to [[window#Skylight|skylights]] for this purpose. [[Georgia Institute of Technology]]'s 2007 [[Solar Decathlon]] House project used an aerogel as an insulator in the semi-transparent roof.<ref>[https://web.archive.org/web/20080216122656/http://solar.gatech.edu/light_roof.php Solar Decathon 2007]. GATech.edu</ref>
* A chemical [[adsorption|adsorber]] for cleaning up spills.<ref>{{cite journal |last1=Gan |first1=Guoqiang |last2=Li |first2=Xinyong |last3=Fan |first3=Shiying |last4=Wang |first4=Liang |last5=Qin |first5=Meichun |last6=Yin |first6=Zhifan |last7=Chen |first7=Guohua |date=2019 |title=Carbon Aerogels for Environmental Clean-Up |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/ejic.201801512 |journal=[[European Journal of Inorganic Chemistry]] |language=en |volume=2019 |issue=27 |pages=3126–3141 |doi=10.1002/ejic.201801512 |s2cid=191132567 |issn=1099-0682}}</ref> Silica aerogels may be used for filtration; They have a high surface area, porosity, and are [[ultrahydrophobicity|ultrahydrophobic]]. They may be used for the removal of heavy metals. This could be applied to [[wastewater treatment]].<ref name=":2">{{cite journal |last1=Shi |first1=Mingjia |last2=Tang |first2=Cunguo |last3=Yang |first3=Xudong |last4=Zhou |first4=Junling |last5=Jia |first5=Fei |last6=Han |first6=Yuxiang |last7=Li |first7=Zhenyu |date=2017 |title=Superhydrophobic silica aerogels reinforced with polyacrylonitrile fibers for adsorbing oil from water and oil mixtures |journal=[[RSC Advances]] |language=en |volume=7 |issue=7 |pages=4039–4045 |doi=10.1039/C6RA26831E |bibcode=2017RSCAd...7.4039S |doi-access=free}}</ref>
* As a [[passive daytime radiative cooling|daytime radiative cooling]] surface that is designed to be efficient in [[solar radiation]] and [[thermal emittance]]. Aerogels may be lower in cost and negative environmental impacts than other materials.<ref name=":28">{{cite journal |last1=Liu |first1=Xianhu |last2=Zhang |first2=Mingtao |last3=Hou |first3=Yangzhe |last4=Pan |first4=Yamin |last5=Liu |first5=Chuntai |last6=Shen |first6=Changyu |date=September 2022 |title=Hierarchically Superhydrophobic Stereo-Complex Poly (Lactic Acid) Aerogel for Daytime Radiative Cooling |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202207414 |journal=Advanced Functional Materials |volume=32 |issue=46 |doi=10.1002/adfm.202207414 |s2cid=252076428 |via=Wiley}}</ref><ref name=":27">{{cite journal |last1=Li |first1=Tao |last2=Sun |first2=Haoyang |last3=Yang |first3=Meng |last4=Zhang |first4=Chentao |last5=Lv |first5=Sha |last6=Li |first6=Bin |last7=Chen |first7=Longhao |last8=Sun |first8=Dazhi |title=All-Ceramic, Compressible and Scalable Nanofibrous Aerogels for Subambient Daytime Radiative Cooling |url=https://www.sciencedirect.com/science/article/abs/pii/S138589472204997X |journal=Chemical Engineering Journal |year=2023 |volume=452 |page=139518 |doi=10.1016/j.cej.2022.139518 |bibcode=2023ChEnJ.45239518L |s2cid=252678873 |via=Elsevier Science Direct}}</ref>
* A [[catalyst]] or a catalyst carrier.<ref name=":3">{{cite journal |last1=Gurav |first1=Jyoti L. |last2=Jung |first2=In-Keun |last3=Park |first3=Hyung-Ho |last4=Kang |first4=Eul Son |last5=Nadargi |first5=Digambar Y. |date=2010-08-11 |title=Silica Aerogel: Synthesis and Applications |journal=[[Journal of Nanomaterials]] |volume=2010 |pages=1–11 |language=en |doi=10.1155/2010/409310 |doi-access=free}}</ref><ref>{{cite journal |last1=Choi |first1=Jinsoon |last2=Suh |first2=Dong Jin |date=2007-09-01 |title=Catalytic Applications of Aerogels |url=https://doi.org/10.1007/s10563-007-9024-2 |journal=Catalysis Surveys from Asia |language=en |volume=11 |issue=3 |pages=123–133 |doi=10.1007/s10563-007-9024-2 |s2cid=97092432 |issn=1574-9266}}</ref><ref name=":1"/>
* Silica aerogels can be used in imaging devices, optics, and light guides.<ref name=":3" />
* [[Thickening agent]]s in some [[paint]]s and [[cosmetics]].<ref>{{cite web |last=Spoon |first=Marianne English |date=25 February 2014 |title='Greener' aerogel technology holds potential for oil and chemical clean-up |url=http://www.news.wisc.edu/22566 |url-status=live |archive-url=https://web.archive.org/web/20150428193731/http://www.news.wisc.edu/22566 |archive-date=28 April 2015 |access-date=29 April 2015 |website=University of Wisconsin Madison News |df=dmy-all}}</ref><ref name=":3"/><ref>{{cite web |date=2006-04-01 |title=Taking control |url=https://www.cosmeticsbusiness.com/technical/article_page/Taking_control/47075 |access-date=2021-03-29 |website=Cosmetics Business |archive-date=6 November 2020 |archive-url=https://web.archive.org/web/20201106232500/https://www.cosmeticsbusiness.com/technical/article_page/Taking_control/47075 |url-status=dead }}</ref>
* As components in energy absorbers.<ref>{{cite journal |last1=Chen |first1=Hao |last2=Xu |first2=Yuanming |last3=Tong |first3=Yan |last4=Hu |first4=Junhao |date=2019-03-15 |title=The investigation of nanofluidic energy absorption system based on high porosity aerogel nano-materials |url=https://www.sciencedirect.com/science/article/pii/S1387181118305183 |journal=Microporous and Mesoporous Materials |language=en |volume=277 |pages=217–228 |doi=10.1016/j.micromeso.2018.09.032 |bibcode=2019MicMM.277..217C |s2cid=105477931 |issn=1387-1811}}</ref>
* Laser targets for the United States [[National Ignition Facility]] (NIF).<ref>{{cite journal |last1=Remington |first1=Bruce A. |last2=Park |first2=Hye-Sook |last3=Casey |first3=Daniel T. |last4=Cavallo |first4=Robert M. |last5=Clark |first5=Daniel S. |last6=Huntington |first6=Channing M. |last7=Kuranz |first7=Carolyn C. |author7-link=Carolyn Kuranz |last8=Miles |first8=Aaron R. |last9=Nagel |first9=Sabrina R. |last10=Raman |first10=Kumar S. |last11=Smalyuk |first11=Vladimir A. |date=2019-09-10 |title=Rayleigh–Taylor instabilities in high-energy density settings on the National Ignition Facility |journal=[[Proceedings of the National Academy of Sciences]] |language=en |volume=116 |issue=37 |pages=18233–18238 |doi=10.1073/pnas.1717236115 |issn=0027-8424 |pmid=29946021 |pmc=6744876 |bibcode=2019PNAS..11618233R |doi-access=free}}</ref>
* A material used in impedance matchers for transducers, speakers and range finders.<ref>{{cite journal |last=Hrubesh |first=Lawrence W. |date=1 April 1998 |title=Aerogel applications |url=https://zenodo.org/record/1259629 |journal=Journal of Non-Crystalline Solids |volume=225 |issue=1 |pages=335–342 |doi=10.1016/S0022-3093(98)00135-5 |bibcode=1998JNCS..225..335H}}</ref>
* According to [[Hindawi (publisher)|Hindawi]]'s ''[[Journal of Nanomaterials]]'', aerogels are used for more flexible materials such as clothing and blankets: "Commercial manufacture of aerogel 'blankets' began around the year 2000, combining silica aerogel and fibrous reinforcement that turns the brittle aerogel into a durable, flexible material. The mechanical and thermal properties of the product may be varied based upon the choice of reinforcing fibers, the aerogel matrix and [[opacifier|opacification additives]] included in the composite."<ref name=":3"/>
* Silica aerogel has been used to capture [[cosmic dust]], also known as space dust.<ref>{{cite journal |last1=Hüsing |first1=Nicola |last2=Schubert |first2=Ulrich |date=1998 |title=Aerogels—Airy Materials: Chemistry, Structure, and Properties |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291521-3773%2819980202%2937%3A1/2%3C22%3A%3AAID-ANIE22%3E3.0.CO%3B2-I |journal=Angewandte Chemie International Edition |language=en |volume=37 |issue=1–2 |pages=22–45 |doi=10.1002/(SICI)1521-3773(19980202)37:1/2<22::AID-ANIE22>3.0.CO;2-I |pmid=29710971 |issn=1521-3773}}</ref><ref>{{cite journal |last=Tsou |first=Peter |date=1995-06-02 |title=Silica aerogel captures cosmic dust intact |url=https://dx.doi.org/10.1016/0022-3093%2895%2900065-8 |journal=Journal of Non-Crystalline Solids |series=Proceedings of the Fourth International Symposium on AEROGELS |language=en |volume=186 |pages=415–427 |doi=10.1016/0022-3093(95)00065-8 |bibcode=1995JNCS..186..415T |issn=0022-3093}}</ref> [[NASA]] used an aerogel to trap space dust particles aboard the [[Stardust (spacecraft)|Stardust]] spacecraft.<ref>{{cite web |title=NASA - Catching Comet Dust With Aerogel |url=https://www.nasa.gov/mission_pages/stardust/mission/index-aerogel-rd.html |access-date=2021-03-29 |website=[[NASA]] |language=en}}</ref> These aerogel dust collectors have very low mass.<ref>{{cite web |last=Tsou |first=Peter |title=Silica Aerogel Captures Cosmic Dust Intact |url=https://trs.jpl.nasa.gov/bitstream/handle/2014/33446/94-1484.pdf?sequence=1&isAllowed=y |access-date=2021-03-29 |website=[[NASA]]}}</ref> The particles vaporize on impact with solids and pass through gases, but can be trapped in aerogels. NASA also used aerogel for thermal insulation for the [[Mars rover]]s.<ref>[http://marsrovers.jpl.nasa.gov/mission/sc_rover_temp_aerogel.html Preventing heat escape through insulation called "aerogel"] {{webarchive|url=https://web.archive.org/web/20071013103911/http://marsrovers.jpl.nasa.gov/mission/sc_rover_temp_aerogel.html |date=13 October 2007}}, ''NASA CPL''</ref><ref>[http://www.aero.org/publications/crosslink/fall2006/backpage.html Down-to-Earth Uses for Space Materials] {{webarchive|url=https://web.archive.org/web/20070930011123/http://www.aero.org/publications/crosslink/fall2006/backpage.html |date=30 September 2007}}, ''The Aerospace Corporation''</ref><ref name=":3"/>
* The [[US Navy]] evaluated use of aerogels in undergarments as passive thermal protection for divers.<ref>{{cite journal |last=Nuckols |first=M. L. |date=2005 |title=Manned Evaluation of a Prototype Composite Cold Water Diving Garment Using Liquids and Superinsulation Aerogel Materials |url=http://archive.rubicon-foundation.org/3487 |journal=United States Navy Experimental Diving Unit Technical Report |volume=NEDU-05-02 |author2=Chao J. C. |author3=Swiergosz M. J. |access-date=21 April 2008 |archive-url=https://web.archive.org/web/20080820004306/http://archive.rubicon-foundation.org/3487 |archive-date=20 August 2008 |url-status=usurped |df=dmy-all}}</ref><ref name=":3"/> Similarly, aerogels have been used by NASA for insulating [[space suit]]s.<ref>{{cite journal |last1=Trevino |first1=Luis A. |last2=Orndoff |first2=Evelyne S. |last3=Tang |first3=Henry H. |last4=Gould |first4=George L. |last5=Trifu |first5=Roxana |date=2002-07-15 |title=Aerogel-Based Insulation for Advanced Space Suit |url=https://doi.org/10.4271/2002-01-2316 |journal=SAE Technical Paper Series |volume=1 |language=en |location=Warrendale, PA |publisher=SAE International |doi=10.4271/2002-01-2316}}</ref><ref name=":3"/>
* In [[particle physics]] as radiators in [[Cherenkov effect]] detectors, such as the ACC system of the Belle detector, used in the [[Belle experiment]] at [[KEKB (accelerator)|KEKB]].<ref>{{cite journal |last1=Iwata |first1=S. |last2=Adachi |first2=I. |last3=Hara |first3=K. |last4=Iijima |first4=T. |last5=Ikeda |first5=H. |last6=Kakuno |first6=H. |last7=Kawai |first7=H. |last8=Kawasaki |first8=T. |last9=Korpar |first9=S. |last10=Križan |first10=P. |last11=Kumita |first11=T. |date=2016-03-01 |title=Particle identification performance of the prototype aerogel RICH counter for the Belle II experiment |journal=[[Progress of Theoretical and Experimental Physics]] |volume=2016 |issue=33H01 |pages=033H01 |doi=10.1093/ptep/ptw005 |issn=2050-3911 |doi-access=free|arxiv=1603.02503 }}</ref> The suitability of aerogels is determined by their low [[index of refraction]], filling the gap between gases and liquids, and their transparency and solid state, making them easier to use than [[cryogenic]] liquids or compressed gases.<ref>{{cite journal |last1=Wang |first1=Jieyu |last2=Petit |first2=Donald |last3=Ren |first3=Shenqiang |date=2020 |title=Transparent thermal insulation silica aerogels |journal=Nanoscale Advances |language=en |volume=2 |issue=12 |pages=5504–5515 |doi=10.1039/D0NA00655F |pmid=36133881 |pmc=9417477 |bibcode=2020NanoA...2.5504W |doi-access=free}}</ref>
* [[Resorcinol]]–[[formaldehyde]] aerogels (polymers chemically similar to [[phenol formaldehyde resin]]s) are used as precursors for manufacture of carbon aerogels, or when an organic insulator with large surface is desired.<ref>{{Citation|last1=Mulik|first1=Sudhir|title=Resorcinol–Formaldehyde Aerogels|date=2011|url=https://doi.org/10.1007/978-1-4419-7589-8_11|work=Aerogels Handbook|pages=215–234|editor-last=Aegerter|editor-first=Michel A.|series=Advances in Sol-Gel Derived Materials and Technologies|place=New York, NY|publisher=Springer|language=en|doi=10.1007/978-1-4419-7589-8_11|isbn=978-1-4419-7589-8|access-date=2021-03-29|last2=Sotiriou-Leventis|first2=Chariklia|editor2-last=Leventis|editor2-first=Nicholas|editor3-last=Koebel|editor3-first=Matthias M.}}</ref>
* Metal–aerogel [[nanocomposite]]s prepared by impregnating the hydrogel with solution containing ions of a [[transition metal]] and irradiating the result with [[gamma ray]]s, precipitates [[nanoparticle]]s of the metal. Such composites can be used as catalysts, sensors, and [[electromagnetic shielding]], and in waste disposal. A prospective use of platinum-on-carbon catalysts is in [[fuel cell]]s.<ref>{{cite journal|journal=Nat Commun|volume=13|doi=10.1038/s41467-022-34444-w|title=An integrated platinum-nanocarbon electrocatalyst for efficient oxygen reduction|publisher=[[Nature (journal)|Nature]]|doi-access=free|first1=Let|last1=Huang|first2=Min |last2=Wei |first3=Ruijuan |last3=Qi |first4=Chung-Li |last4=Dong |first5=Dai |last5=Dang |first6=Cheng-Chieh |last6=Yang |first7=Chenfeng |last7=Xia |first8=Chao |last8=Chen |first9=Shahid |last9=Zaman |first10=Fu-Min |last10=Li |first11=Bo |last11=You |first12=Bao Yu |last12=Xia|date=2022 |issue=1 |page=6703 |pmid=36344552 |pmc=9640595 |bibcode=2022NatCo..13.6703H }}</ref>
* As a drug delivery system owing to its [[biocompatibility]]. Due to its high surface area and porous structure, drugs can be adsorbed from supercritical {{chem|CO|2}}. The release rate of the drugs can be tailored by varying the properties of the aerogel.<ref>{{cite journal |date=2004 |title=Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems |journal=Journal of Non-Crystalline Solids |volume=350 |pages=54–60 |bibcode=2004JNCS..350...54S |doi=10.1016/j.jnoncrysol.2004.06.031 |author=Smirnova I. |author2=Suttiruengwong S. |author3=Arlt W.}}</ref>
* Carbon aerogels are used in the construction of small electrochemical double layer [[supercapacitor]]s. Due to the high surface area of the aerogel, these capacitors can be 1/2000th to 1/5000th the size of similarly rated electrolytic capacitors.<ref>{{cite web |url=http://powerelectronics.com/portable_power_management/batteries/power_aerogel_capacitors_support/ |title=Aerogel Capacitors Support Pulse, Hold-Up, and Main Power Applications |date=1 February 2002 |author=Juzkow, Marc |work=Power Electronic Technology |url-status=live |archive-url=https://web.archive.org/web/20070515141549/http://powerelectronics.com/portable_power_management/batteries/power_aerogel_capacitors_support/ |archive-date=15 May 2007 |df=dmy-all}}</ref> According to Hindawi's ''Journal of Nanomaterials,'' "Aerogel supercapacitors can have a very low [[electrical impedance|impedance]] compared to normal supercapacitors and can absorb or produce very high peak currents. At present, such capacitors are [[electrical polarity|polarity-sensitive]] and need to be wired in series if a working voltage of greater than about 2.75&nbsp;[[Volt|V]] is needed."<ref name=":3"/>
* [[Dunlop Sport]] uses aerogel in some of its [[rackets (sport)|racquets]] for sports such as tennis.<ref>{{cite web |date=July 2007 |title=Dunlop Expands Aerogel Line - Tennis Industry |url=http://www.tennisindustrymag.com/articles/2007/07/dunlop_expands_aerogel_line.html |access-date=2021-03-29 |website=Tennis Industry Magazine}}</ref>
* In water purification, [[chalcogel]]s have shown promise in absorbing the heavy metal pollutants mercury, lead, and cadmium from water.<ref>Carmichael, Mary. [http://www.msnbc.msn.com/id/20123389/site/newsweek/ First Prize for Weird: A bizarre substance, like 'frozen smoke,' may clean up rivers, run cell phones and power spaceships.] {{webarchive |url=https://web.archive.org/web/20070817195536/http://www.msnbc.msn.com/id/20123389/site/newsweek/ |date=17 August 2007}} Newsweek International, 13 August 2007. Retrieved on 2007-08-05.</ref> Aerogels may be used to separate oil from water, which could for example be used to respond to [[oil spill]]s.<ref>{{cite journal |last1=Mazrouei-Sebdani |first1=Z. |last2=Salimian |first2=S. |last3=Khoddami |first3=A. |last4=Shams-Ghahfarokhi |first4=F. |date=2019-08-01 |title=Sodium silicate based aerogel for absorbing oil from water: the impact of surface energy on the oil/water separation |url=http://adsabs.harvard.edu/abs/2019MRE.....6h5059M |journal=Materials Research Express |volume=6 |issue=8 |pages=085059 |doi=10.1088/2053-1591/ab1eed |bibcode=2019MRE.....6h5059M |s2cid=155307402 |issn=2053-1591}}</ref><ref name=":2"/><ref name="Song 10337"/> Aerogels may be used to disinfect water, killing bacteria.<ref>{{cite journal |last1=Wang |first1=Fei |last2=Dai |first2=Jianwu |last3=Huang |first3=Liqian |last4=Si |first4=Yang |last5=Yu |first5=Jianyong |last6=Ding |first6=Bin |date=2020-07-28 |title=Biomimetic and Superelastic Silica Nanofibrous Aerogels with Rechargeable Bactericidal Function for Antifouling Water Disinfection |url=https://doi.org/10.1021/acsnano.0c03793 |journal=[[ACS Nano]] |volume=14 |issue=7 |pages=8975–8984 |doi=10.1021/acsnano.0c03793 |pmid=32644778 |s2cid=220474580 |issn=1936-0851}}</ref><ref>{{cite web |last=Patel |first=Prachi |date=2020-08-21 |title=Loofah-inspired aerogel efficiently filters microbes from water |url=https://cen.acs.org/materials/nanomaterials/Loofah-inspired-aerogel-efficiently-filters/98/web/2020/08 |access-date=2021-03-29 |website=[[Chemical & Engineering News]]}}</ref>
* Aerogel can introduce disorder into [[superfluid]] [[helium-3]].<ref>[[William Halperin|Halperin, W. P.]] and [[James Sauls|Sauls, J. A.]] [[arxiv:cond-mat/0408593v1|Helium-Three in Aerogel]]. Arxiv.org (26 August 2004). Retrieved on 7 November 2011.</ref>
* In aircraft de-icing, a new proposal uses a [[carbon nanotube]] aerogel. A thin filament is spun on a winder to create a 10&nbsp;micron-thick film. The amount of material needed to cover the wings of a jumbo jet weighs {{convert|80|g}}. Aerogel heaters could be left on continuously at low power, to prevent ice from forming.<ref>{{cite news |url=https://www.economist.com/blogs/babbage/2013/07/de-icing-aeroplanes |title=De-icing aeroplanes: Sooty skies |date=26 July 2013 |newspaper=The Economist |access-date=11 December 2013 |url-status=live |archive-url=https://web.archive.org/web/20131230212607/http://www.economist.com/blogs/babbage/2013/07/de-icing-aeroplanes |archive-date=30 December 2013 |df=dmy-all}}</ref>
* Thermal insulation transmission tunnel of the [[Chevrolet Corvette (C7)]].<ref>Katakis, Manoli. (11 July 2013) [http://gmauthority.com/blog/2013/07/what-does-nasa-have-to-do-with-the-2014-corvette-stingray/ NASA Aerogel Material Present In 2014 Corvette Stingray] {{webarchive|url=https://web.archive.org/web/20140222024500/http://gmauthority.com/blog/2013/07/what-does-nasa-have-to-do-with-the-2014-corvette-stingray/ |date=22 February 2014}}. GM Authority. Retrieved on 2016-07-31.</ref>
* [[CamelBak]] uses aerogel as insulation in a thermal sport bottle.<ref>[http://www.pinkbike.com/news/camelbak-podium-ice-insulated-bottle-review-2014.html Camelbak Podium Ice Insulated Bottle – Review] {{webarchive|url=https://web.archive.org/web/20141003153250/http://www.pinkbike.com/news/camelbak-podium-ice-insulated-bottle-review-2014.html |date=3 October 2014}}. Pinkbike. Retrieved on 31 July 2016.</ref>
* 45 North uses aerogel as palm insulation in its Sturmfist 5 cycling gloves.<ref>[http://45nrth.com/products/gloves/sturmfist-5 Unparalleled Cold Weather Performance] {{webarchive|url=https://web.archive.org/web/20160110032732/http://45nrth.com/products/gloves/sturmfist-5 |date=10 January 2016}}. 45NRTH. Retrieved on 31 July 2016.</ref>
* Silica aerogels may be used for [[soundproofing|sound insulation]], such as on windows or for construction purposes.<ref>{{cite web |title=Silica Aerogels - an overview |url=https://www.sciencedirect.com/topics/engineering/silica-aerogels |access-date=2021-03-29 |website=[[ScienceDirect]]}}</ref><ref>{{cite journal |date=2021-06-15 |title=A review on silica aerogel-based materials for acoustic applications |journal=[[Journal of Non-Crystalline Solids]] |language=en |volume=562 |pages=120770 |doi=10.1016/j.jnoncrysol.2021.120770 |issn=0022-3093 |last1=Mazrouei-Sebdani |first1=Zahra |last2=Begum |first2=Hasina |last3=Schoenwald |first3=Stefan |last4=Horoshenkov |first4=Kirill V. |last5=Malfait |first5=Wim J. |bibcode=2021JNCS..56220770M |s2cid=233562867 |doi-access=free}}</ref>
* It has been suggested that [[Fogbank]], a material of secret composition used in U.S. thermonuclear warheads, may be an aerogel.<ref>{{Cite magazine | last = Last | first = Jonathan V. | title = The Fog of War: Forgetting what we once knew | journal = [[The Weekly Standard]] | volume = 14 | issue = 33 | date = 18 May 2009 |url = https://www.weeklystandard.com/jonathan-v-last/the-fog-of-war | archive-url = https://web.archive.org/web/20181205161703/https://www.weeklystandard.com/jonathan-v-last/the-fog-of-war | archive-date = 2018-12-05}}</ref>
*Aerogels are used in [[Inertial confinement fusion|Inertial Confinement Fusion (ICF)]] and [[X-ray laser]] targets.<ref>{{cite web |url=https://www.samaterials.com/content/the-10-strongest-materials-known-to-man.html |title=The 10 Strongest Materials Known To Man |last=Trento |first=Chin |website=Stanford Advanced Materials |date=May 20, 2024 |access-date=June 22, 2024}}</ref> In ICF, it is used as low-density target materials to create foam targets that aid in simulating the conditions necessary for [[Nuclear fusion|fusion]]. Their low-density structure allows for precise control over the fusion fuel, facilitating efficient compression and heating by the laser energy.<ref>{{cite journal |last1=Hair |first1=L. M. |last2=Pekata |first2=R. W. |date=1988 |title=Low-density resorcinol–formaldehyde aerogels for direct-drive laser inertial confinement fusion targets |url=https://pubs.aip.org/avs/jva/article-abstract/6/4/2559/101149/Low-density-resorcinol-formaldehyde-aerogels-for?redirectedFrom=fulltext |journal=Journal of Vacuum Science & Technology |volume=6 |issue=4 |pages=2559–2563 |doi=10.1116/1.575547 |bibcode=1988JVSTA...6.2559H |access-date=June 22, 2024}}</ref><ref>{{cite thesis |last1=Braun |last2=Tom |date=2013 |title=Development of aerogel-lined targets for inertial confinement fusion experiments |url=https://www.osti.gov/biblio/1077169/ |publisher=U.S. Department of Energy |doi=10.2172/1077169 |osti=1077169 |access-date=June 22, 2024}}</ref>
==Safety==
Silica-based aerogels are not known to be [[carcinogenic]] or toxic. However, they are a mechanical [[irritation|irritant]] to the eyes, skin, respiratory tract, and digestive system. They can also induce dryness of the skin, eyes, and mucous membranes.<ref>{{cite journal |last1=Thapliyal |first1=Prakash C. |last2=Singh |first2=Kirti |date=2014-04-27 |title=Aerogels as Promising Thermal Insulating Materials: An Overview |journal=[[Journal of Materials]] |volume=2014 |pages=1–10 |doi=10.1155/2014/127049 |language=en |doi-access=free}}</ref> Therefore, it is recommended that protective gear including respiratory protection, gloves and eye goggles be worn whenever handling or processing bare aerogels, particularly when a dust or fine fragments may occur.<ref>[http://aerogel.com/products/pdf/Cryogel_5201_10201_MSDS.pdf Cryogel 5201, 10201 Safety Data Sheet] {{webarchive|url=https://web.archive.org/web/20101223111216/http://www.aerogel.com/products/pdf/Cryogel_5201_10201_MSDS.pdf |date=23 December 2010}}. Aspen Aerogels. 13 November 2007</ref>
==See also==
* [[Carbon nanofoam]]
* [[Fogbank]]
* [[Nanogel]]
==References==
{{CC-notice|cc=by4|url=https://www.mdpi.com/2073-4360/11/4/726|author=Elizabeth Barrios, David Fox, Yuen Yee Li Sip, Ruginn Catarata, Jean E. Calderon, Nilab Azim, Sajia Afrin, Zeyang Zhang and Lei Zhai}}
{{Reflist}}
; Further reading
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* {{cite journal |date=1998 |title=Aerogels – Airy Materials: Chemistry, Structure, and Properties |journal=[[Angewandte Chemie International Edition]] |volume=37 |issue=1/2 |pages=22–45 |doi=10.1002/(SICI)1521-3773(19980202)37:1/2<22::AID-ANIE22>3.0.CO;2-I |author=N. Hüsing |author2=U. Schubert |pmid=29710971}}
* {{cite journal |date=2002 |title=Chemistry of aerogels and their applications |journal=[[Chemical Reviews]] |volume=102 |issue=11 |pages=4243–4266 |doi=10.1021/cr0101306 |pmid=12428989 |author=Pierre A. C. |author2=Pajonk G. M.}}
{{refend}}
==External links==
{{Commons category|Aerogel}}
* [https://www.aerogel.org/ Open-source aerogel]
* [https://www2.lbl.gov/Science-Articles/Archive/aerogel-insulation.html Aerogel Research at LBL: From the Lab to the Marketplace], Jeffery Kahn, Summer 1991, Berkeley Lab (Lawrence Berkeley National Laboratory)
{{Emerging technologies|topics=yes|robotics=yes|manufacture=yes|materials=yes}}
[[Category:Aerogels]]
[[Category:Least dense things]]