Samuel KISTLER : Aerogels ~ Articles, videos, 2 patents & 6 books.

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Samuel KISTLER
Aerogel

https://www.nature.com/articles/127741a0

Kistler, S. S. (1932). "Coherent Expanded-Aerogels". Journal of Physical Chemistry. 36 (1): 52–64.
doi:10.1021/j150331a003.

Abstract -- THE continuity of the liquid permeating jellies is demonstrated by diffusion, syneresis, and ultrafiltration, and the fact that the liquid may be replaced by other liquids of very diverse character indicates clearly that the gel structure may be independent of the liquid in which it is bathed. Hitherto the attempt to remove the liquid by evaporation has resulted in shrinkage so great that the effect upon the structure may be profound...

US2093454 -- METHOD OF PRODUCING AEROGELS

Samuel KISTLER

This Invention relates to improvements in the art and process of producing dry gels from colloidal solutions, and the present specification is Particularly directed to the production of a gel, one continuous phase of which is a gas, and which I therefore define as an aerogel.

Whenever a colloidal solution is precipitated, the product formed is usually defined as a gel.

It is distinct from the precipitates from crystallo loidal solutions by containing large quantities of the solvent in a soft "gelatinous" mass, usually microscopically heterogeneous and presenting some rigidity. Gels may be divided into two classes, the gelatinous precipitates and the jellies according to whether the product formed settles out in a flocculent mass or occupies the entire volume that the original solution occupied and appears microscopically homogeneous.

Gels may be again divided into elastic and non-elastic groups according to whether the gel will re-swell or not, after being dried, when placed in the original solvent. Examples of the elastic. gel ate gelatine and egg albumin, while examples of the non-elastic gel are silica and alumina gels.

The present invention applies to a new mode of removing the solvent from a gel, either elastic or non-elastic, in preparation for its use in the solid dried state, and the provision of a new class of products obtained thereby.

The most widely used of the inorganic gels in the dried condition for the purposes for which my product is most suited is that of. silica, so that I shall describe something of its properties and mode of production, with explanation of the properties as they are now understood. I shall also list some of its commercial uses in order to indicate what may be the uses of my product.

Most of the gel now commonly used commercially is made by mixing rapidly. a solution of water glass with a solution of some suitable salt or acid, usually the latter, and allowing the mixture to stand until it becomes a stiff jelly. The jelly is then broken up, washed free of salts and allowed to dry siowly. It shrinks greatly in drying and when completely dry it is a hard glassy mass. A great number of variations have been tried in the conditions of precipitation and-drying but the properties of the final product can be affected only to a very limited extent. These materials are designated in the art as xerogels and are characterized and distinguishable from my products in that they can be wetted with the same liquid menstrutun as employed in their preparation, and subsequently dried by simple 5,) volatilization of the liquid without changing the (Cl. 252--6) physical characteristics of the product materially.

Thus ordinary silica gel (xerogel) can be immersed in water and subsequently dried at an elevated temperature and atmospheric pressure without changing materially its apparent density and other physical properties. On the other hand, an aerogel cannot be wetted and dried without loss of its characteristic properties. On the contrary when an aerogel is wetted and dried -the drying operation results in shrinkage and other changes incident to its conversion to an xerogel.

The best theoretical treatment sets forth that the jelly is a mass of interlacing fibres. Upon removal of the liquid by drying, the surface of the liquid in trying to withdraw within the mass of submicroscopic fibres forms concave menisci, just as it forms in a small tube, and tends to draw the fibrils with it so that there is not only a shrinkage of the water, or solvent volume, but shrinkage of the entire mass. This pull of the liquid surface on the f1brils can easily be understood when it is remembered that the smaller a tube is the higher will water rise in it by capillarity. The spaces between the fibrils are the counterparts of exceedingly minute tubules. It is therefore clear that the gel will shrink, until the f1brils are sufficiently compacted to withstand the compressive force due to the liquid surface. This force Is very large, being of the order of thousands of pounds per square inch in easily imagined circumstances. With a substance as strong as silica, a point is reached where the shrinkage ceases and the water dries out, leaving a porous mass. Usually this mass contains from 30-50% voids by volume. In the extreme case that of a gel in which the drying was carried out with ferric oxide in the meshes and later the ferric oxide was dissolved out, a very fragile product was obtained having approximately 75% of its volume void.

The uses for silica gel in the dried condition depend upon the tendency of all molecules, either gaseous or in solution, to attach themselves to a surface. For a given volume, of dried silica gel the internal surface is enormous so that easily conden,sed vapors show a very decided tendency to collect in the gel, and substances in solution can often be almost compl6tely removed by the gel. In addition to the surface effect, there is the tendency for vapors to condense in fine capillaries much more readily than they condense in ordinary vessels.

These properties of the gel make it valuable for removing certain undesirable compounds from lubricating olls; for decolorizing sugar solutions during refining; for removal of vapors valuable or otherwise from gases such, for instance, as the removal of benzene from coke oven gas; and for drying of air, since the gel will remove water almost completely from air and can then be reactivated by simply heating and driving off the water. Its power to absorb vapors has made It serviceable in automatic refrigerator cars where it Is used Instead -of water to absorb ammonia, and In gas masks it could easily prove valuable for removal of toxic gases.

Another Important use of silica gel Is as a carrier of contact catalysts. A contact catalyst is a substance that influences a chemical reaction at its surface without itself being consumed by the reactioft. Since large surface is. the principal requirement and since many contact catalysts are expensive methods have been devised for coating, the immehse inner surface of silica. gel with such catalysts, thus effecting' considerable economies.

The primary object of the present invention therefore is to produce the gel by a method which will enable one at all times to have complete control thereof, whereby gels may be obtained with void,space ranging all the way from the now usual 30-50% found'in all commercial gels, up to 99% or higher. Another object of my invention is to provide a new blass of gels--which are referred to herein as aerogels-,and a method for their preparation, characterized in that they have inordinate adsorptive characteristics even as compared to the most porous active present day gels.

An additional object of my inventibn resides in,producing a new class of colloidal products of an amorphous or essentially amorphous character, the degree of subdivision and spacial relationship of whose individual particles is the same as that In an undried gel or jelly or substantially of that order.

A further object is to produce the gels in an effective and economical manner.

The following examples illustrate embodiments of my invention:

Example 1-1 first form a hydrogel, such as silica hydrogel or jelly, In a suitable liquid medium, for example: water. For this purpose I may simply acidify water glass with sufuric acid In the well known manner employed in manufacturing silica gel of commerce. The gel is then washed free of soluble substances. In certain cases the gel may be, If desh!, partially dried before washing in order to strengthen ffie mass to stand the washing action or to permit advantageous handling of the same. After being washed the gel Is placed In a strong. autoclave. The autoclave is nearly filled with a liquid and then closed. The liquid used may be either the water usually used in the precipitating operation or a substituted 'liquid more suited from a mechanical standpoint to carrying qut my method as will be pointed out later. The whole mass In the, autoclave Is then slowly heated. Due to the expansion of the liquid with temperature, the gas space in the autoclave may be completely filled by the liquid considerably before the critical temperature is reached.

In such a case continued heating would tend to cause further expansion of the liquid and thereby subject the vessel to excessive pressures. It therefore becomes necessary to release some of the liquid through a suitable valve. In practice the pressure is preferably maintained at or slighoy above the critical pressure of theL liquid.

The minimum quantity of liquid that is advisable to use in the pressure vessel Is that quantity 2,098,454, which, when expanded to completely fill the vessel, will have the critical density. If the gel present contains this limiting quantity of liquid it is not necessary to add more.

Heating is continued (only enough liquid being released to prevent excessive pressures but not enough to produce substantial drying of the gel) -until the temperature exceeds the critical temperature of the liquid in the pressure vessel. The gas Is then released at a rate insufficlent, to damage. the gel. The gel is left behind In a dried condition but having suffered little, If any, shrinkage.

If the gel Is put Into the vessel without added liquid, the liquid in the gel will expand with rising temperature more rapidly than evaporation into the closed space will occur, provided the total quantity of liquid present is equal to the minimum described above, and the gel will thereby. not be subjected to' compressive forces due to CaPillarity.

The principle upon which my method works Is as follows:

Above the critical temperature no gas can be liquefied, regardless of how great the pressure.

If the gel is covered with liquid in the autoclave, there will be no liquid-gas surface coinciding with the gel surface, and therefore there can be no compression of the gel structure due to capillarity.

As the temperature rises the liquid Is never allowed to evaporate down to where the gel Is exposed. Now asthe critical temperature is passed, the liquid is transformed Into a gas imperceptibly so that whereas only a fraction of a degree below that temperature one could say that the gel 9 Z Is filled with liquid, when that temperature Is exceeded by only the slightest amount one must say that the gel is now filled with gas. If the pressure Is maintained above the critical point, no surface can form and the gel has no way of "knowing" when the liquid that It held in its meshes has been converted to a gas. Slow removal of the gas, slow only to prevent the disruption of the gel, leaves the gel in the expanded condition but dry, having at no time in th6 proc- 4r) ess experienced forces that would tend to compress it.

Since the surface tension of a liquid decreases as the temperature rises, becoming zero at the critical temperature, it would be possible to ob- r)O tain gels In a partially shrunken condition by allowing the liquid to slowly evaporate at some temperature below the critical temperature. The nearer to that t. emperature that evaporation was allowed to occur, the smaller force would the gel have to withstand and consmuently the less compression would it experience. The most practical method of controlling the density, however, seems to be to allow evaporation to proceed at ordinary temperatures under controlled conditions until (10 the gel has shrunk as much as Is desired, and then to place it in the autoclave and remove the remainder of the liquid as described above.

I As previously stated, most gels are primarily formed in water, but water has an Inconveniently (;5 high critical temperature and the critical pressure -is very high, which would require very strong apparatus, If the water was continued as the liquid in carrying out the method here disclosed. Furthermore, water exerts a very power- 7( ful solvent action as the temperature rises so that some gels, for example that of silica, would dissolve before the-critical temperature Is reached.

Silica is then precipitated as -a very voluminous powder when the water Is released above the 2,093,454 critical temperature. This powder Is of extremely fIn6 texture and is valuable as a catalyst carrier in vanadium sulfuric acid catalysts. So far as I am aware this material has never been manur) factured heretofore.. However, In order to obtain the aerogels It is desirable and preferable, in my method, to substitute another liquid for the water.

This is easily done by replacing the water In a gel with some liquid, such as an alcohol, that Is very soluble or completely soluble in water. This liquid may then be replaced by other liquids miscible with it but insoluble in water.

In addition to the fact that my method enables one to obtain aerogels that could not have been prepared by known methods, It may beapplied to partially shrunken gels. It therefore enables one to control completely the gel and obtain a product of just the desired density or porosity desired for a given purpose. The silica gel previously produced and which showed the maximum porosity so far obtained, possessed very little strength and has therefore not proved commercially practical. On the other hand, silica gel produced by my method and having the same apparent density-, will have considerable strength and can be obtained in relatively large lumps or pieces which is not possible when made by present day methods..

As stated above, the only methods In existence prior to my invention for the control of the character of the final dried gel are crude In nature and Ineffectual except over a limited range of properties. My invention enables complete control of the gel. The drying process may be stopped at any predetermined point and the liquid removed without further shrinkage of the gel skeleton. I have obtained silica gels with void space ranging all the way from the usual 30-50%, found in the present commercial gels, up to 99%, and I have certainly not reached the limit. If there were any reason for doing so, It Is certain that I could obtain dry gels whose volume would be only 0.5% silica. My improved method is applicable to all gels, and numerous gels that have never been produced in the dried condition with appreciable free space within them can now be obtained In as voluminous condition as desired. For example, aerogels, of cellulose, collodion, gelatine, albumin, alumina, nickel hydroxide, thoria, titania, stannic oxide, magnesium hydroxide, chromic oxide are some of the other aerogels that I have prepared, and there is no reason to believe that the list cannot be extended almost indefinitely. The production -55 of such gels as pyroxylin and cellulose in the distended condition offers a large Reld for investigation, and the - probabilities are that such aerogels will prove valuable.

The aerogel seems to be particularly suited to the support of contact catalysts, especially for gaseous reactions, since gases can pass through the gels of low density with great facility and yet each molecule must cbm6-very close to a surface many, many times during the passege.

Many of. the catalysts can themselves be produced in the form of gels and thus do away with the necessity of supporting on silica to give them large surfaces.

In gas masks the voluminous gel may be used as an ultra-fIlter for the removal of submicroscopic particles from the air. Toward the end, of the last war the masks were useless in combatipg certain extremely fine powders and fogs that were used to produce snee7Ang.

When a non-elastic aerogel of low density, such as that of silica, is powdered, the product is of extreme fineness and is useful as a polishing abrasive. The voluminous gel is a very excellent heat insulator.

Example 2Dlssolve 44 grams of magnesium nitrate Mg(N03)2.6%0 in a small amount of glycerin and make up to a volume of 50 cc.

Thereafter add 35 cc. of diethyl &mine diluted with glycerin to 100 cc. in a manner whereby the diethylamine solution floats on top of the magnesium nitrate solution. Now shake the two layers vigorously for a few seconds and allow the resulting mixture to set to a firm jelly. To remove the glycerin the jelly is extracted with alcohol until the glycerin is completely removed.

The alcohol is then extracted by means of ethyl ether until most of the alcohol is removed. The resulting ether gel is then placed in a pressure vessel with excess ether, if,necessary, and the temperature raised to 195 C. while maintaining the pressure sufficiently high to prevent evapo ra,tion of the ether. After the temperature has reached the critical point the vapor Is permitted to escape. TWe resulting gel is very light, coherent, though fragile, elastic and transparent or translucent.

Example 3Dissolve 83 grams of chromic nitrate Cr(N03)3.9H20 in 100 cc. of water to which there is then added, while stirring vigorously, 35 grams of ammonium acetate dissolved in 50 cc. of water. After the mixture Is uniform It Is permitted to set. A jelly will form In about 5 minutes. To remove the soluble salts the gel is broken up and washed with water, after which the water is replaced by alcohol and the alcohol Is replaced, in part at. least, by diethyl ether as described in Example 2. The etheralcohol gel Is then converteC to the aerogel as described heretofore. The resulting product, when broken to pass a 10 mesh screen, hat an apparent density of.25 gram per cc.

Example 4.--82 grams of stannic chloride SnC14.5H20 are dissolved in 200 grams of water and placed in a dialyzer, such as a sack made of regenerated cellulose as, for example, "Cellophane", and is then suspended in 2500 cc. of distilled water. After about 24 hours the contents of the sack will have set to a fim Jelly.

This jelly is then washed with methyl alcohol until most of the water has been, removed, after which it is converted to an aerogel by heating to 260 C. before the vapors are released. The aerogel is very light, colorless and transparent.

Example 5.-53 grams of a commercial grade of dry thorium nitrate are dissolved in 35 cc. of methyl alcohol. The solution Is allowed to stand a few hours before further use. 10 parts by volume of the aforement4oned, solution is then thoroughly mixed with 10 parts by volume of redistilled aniline. Subsequently 1.8-2.0 parts by volume of water are added whereby the water floats on top of the mixture. The resulting mixture is shaken vigorously for 1 to 2 seconds, and finally permitted to stand. After a very short period of time it will set into a jelly which is broken up. and covered with a mixture of 9 volumes of methyl alcohol and one volume of concentrated aqueous ammonia. This mixture is changed from time to time over a period of several days until practically all of -the aniline has been leached out. Finally, it is washed with pure methyl alcohol or acetone or a mixture of these two, and autoclaved to form thorium. oxide aerogel.

Example 6.-380 grams of aluminum n1trate ARNOD 3.9H20 are first dissolved In 1200, cc. of methyl alcohol, cooled to about -10 C. and stirred vigorously while adding 280. grams of redistilled aniline. The resulting mixture Is allowed to attain room temperature at which time a firm Jelly will have formed that can be extracted with metbyl alcohol until free of aniline and salts. The alcohol-aluminum hydroxide Jelly, or alcogel as It Is referred to in the art, Is then autoclaved to form an aerogel whereby an elastic, transparent product, having an apparent specifle gravity of less than.1, which is relatively strong considering Its low density, is obtained. Example 7.--Sufflcient gelatin is dissolved in 1,5 -hot ethyl alcohol'containing sufficient acetic acid to form a firm Jelly upon cooling. This Jelly is then hardened by covering It with one of the higher alcohols, such as propyl or butyl, and allowing it to stand for several days. It is then extracted with ether to displace the alcohols.

Alternatively, one may employ liquid propane or liquid dimethyl ether to displace, at least In part, the alcohol or alcohol-ether mixture. The resulting mixture Is autoclaved at about 110 C.

in the case of propane, or 130 C. In the case of dimethyl ether. Some shrinkage will be observed, but an aerogel having about 70% void space, which is strong, tough and transparent and through which igases diffuse readily, Is obtained.

Example 8.-5 to 10 grams of agar-agar are dissolved in 100 cc. of water at its normal boiling temperature, and the solution Is then allowed to cool to form a stiff Jelly. The Jelly is extracted with alcohol -and then with ethyl ether, and finally with propane or dimethyl ether before autoclaving the same substantially as described in Example 7. The aerogel is a light, pithlike, opaque product.

4o Example 9.-A beaker Is filled with a viscous collodion solution to a depth of 2 cn-L Several centimeters of benzene are then carefully floated on top. After several days' standing the collodion has set to a clear, stiff Jelly. The benzene containing gel Is treated with liquid propane to displace the benzene, at least in part, and subsequently is converted to an aerogel which is light, strong and translucent.

In the foregoing description and examples,, a number of solvents have been disclosed and a number of media having more or less favorable critical temperatures have been enumerated. It will be understood that these are exemplary only and that in addition to alcohol (ethyl alcohol), methyl alcohol, acetone, dimethyl ketone, diethyl ethbr, dimethyl ether, benzene and propane, many other liquids may be employed. Similarly, many other combinations may be employed with the view of obtaining the aerogel forming phenomenon ujader the most favorable temperature and pressure conditions.

Reference Is herein made to the Miller and Connolly,Patent #1,77.2,055, wherein a silica hydrogel is heated In the presence of liquid water for the purpose of effecting a hardening of the gel. However, in this process the final removal of water Is effected by the more or less conventional means and the shrinking phenomenon oc- casioned by the high surface tension of liquid water manifests Itself. Accordingly, one obtains by this procedure a product which has an apparent specific gravity of as low as.5 but does not have the properties of aerogels. The Miller and Connolly product may be treated with water and dried, repeatedly, to obtain a somewhat sim2,098,454 ilar product each time whereas my aerogel, when treated with water and subsequently dried, Is converted to the xerogel having a substantially higher apparent specific gravity, otherwise lacking the characteristics of the aerogel and rather simulating the Miller and Connolly product so far as specific gravity Is concerned. Por example, if a silica aerogel with a specific gravity of 1 is wetted with water and subsequently dried, the resulting dried product resembles Miller's product Land has a specific gravity of approximately.6. 1 have not been able to produce an aerogel from a gel not previously dried that will not shrink in this manner when wetted and subsequently dried in the conventional manner.

PInally, it is to be noted that the AMer and Connolly process is-not capable of producing a product having the low apparent specific gravities which I am able to obtain since the limiting factor of the surface tension of the water is not overcome by Miller and Connolly. Consequently, shrinking, which attends the removal of water by'conventional means, manifests Itself. In general, and in the absence of purposeful preliminary shrJn age, my aerogel products are characterized In part by the fact that their apparent specific gravity is not substantially greater than 15% of the actual specific gravity of thesubstance.

The aerogels are distinguishable from ordinary xerogels by their inordinate Catalytie activity.

For example, in the vapor phase oxidation of acetaldehyde to acetic acid, a silica aerogel performs more satisfactorily than the ordinary silica gel due in part tothe fact that the aerogel does not become fouled as rapidly. In comparative tests the aerogel did not show any signs of fouling during the course of the runs while the ordinary gel turned yellow and showed resinous materials deposited thereon. Thoria aerogels convert carboxylic, acids, as for example acetic acid, to ketones with practically quantitative yields at 300 C., while the best form of thoria, that was obtained by heating the oxalate required a reaction temperature Of L at least 50 higher, the conversion was incomplete andside reactions resulted in the formation of by-products. It is to t6 be understood that for certain catalytic purposes it is desirable to have a mixed aerogel, that is, an aerogel containing more than one metallic component. These may be produced, preferably, by mixing the alcogels or ether gels before the autoclaving operation. The following example illustrates an alternative procedure for preparing one such mixed gel:

Example 10.-A silica hydrogel was soaked In a strong solution of aluminum sulphate for 24 hours, after which it was dropped into a concentrated ammonia solution. The resulting gel was washed with water to remove soluble salts, extracted with alcohol and autoclaved in the usual manner. This gel was found to be more active in the oxidation of acetaldehyde to acetic acid than either a silica aerogel or an alumina aerogel.

In addition to the foregoing distinguishing characteristics of the aerogels, their thermal conductivity further emphasizes the fundamental difference between aerogels as a class and xerogels or products such as Miller and Connolly describe. To illustrate: silica aerogel, having an apparent specific gravity of.18, shows a heat conductivity lower than any solid which has ever been measured so far as I have been able to ascertain. Its heat conductivity at 34 C. is4.85XIO-5 cal./see./'C./cm. This constant compares very favorably withsoine of the best known heat insulators of which the following are representative:

Cork ---------------- ------ -------- lox Asbt;slos ------------ ---------------- 22 X 10-5 Best grade of --r--.;.neral wool_____ ------ 7 ' 6 x 10--5 Loose cellulose fibres ----------------- 8Axl0-5 One of the advantagqs of the silica aerogel re-, sides in its ability to withstand elevated temperatures of at least 800' c. indefinitely while other aerogels, such as' alun-Ana aerogels, will. stand even higher temperatures. Ordinary silica gels (xerogels) 'are not much better than ordinary sand and possess no usefulness in the field of heat insulation.

Another feature of the aerogel products of the. inorganic type resides in the fact that they do not pack or settle. It is well known that many 20' fmely divided materials tend to pack, leaving void spaces, and for this reason are not entirely satisfactory heat insulating,media. Aerogels, on the other hand, show little or no such tendency.

Moreover, while the materials are fragile, never2 theless they will support considerable weight, particularly when supported and in this respect are distinctly superior to materials such as mineral wool and asbestos which lose their valuable heat insulating properties as they are compressed or as they may settle. In other words, it is absolutely necessary, in oider to preserve the heat insulating value of these materials, to preserve their fluffy nature.

2,093,454 i)ue to the porous nature of the aerogel much of the heat that is transferred passes through the gas phase. Accordingly, by substituting a' gas of low thermal conductivity for air it is possible to improve the thermal resistance of the aerogels appreciably. For example, a. gel that has a conductivity of 4.85 with air,' has a conductivity of 4.33 when saiurated with carbon dioxide and 3.71 when filled with dieblorodifluoromethane. Where the material is to serve as an insulator for refrigerator units and the like it is possible to lodge the insulator within a metal case which can be evacuated, the residual gas present being preferably one of low heat conductivity. PTom the foregoing description it will, be eviw dent that I have provided a new class of products, aerogels, and various methods for their preparation. it will likewise be apparent that the aerogels are characterized by the. fact that. they are formed from colloidal gels In which the liquid menstruum Is removed, at least in part, by heating the liquid under pressure beyond its critical temperature, and subsequently releasing the liquid thus heated. In this manner it is possible to produce 4 skeleton of the solid component of the gel as- it actually exists before treatment and differing only In that the liquid medium is displaced by a vapor or gas. I have also shown that it is possible to produce materials in voluminous precipitated form by heating a solution thereof to or beyond the critical temperature, and releasing the sol in its gaseous state.

The invention affords a novel class of materials. having unique properties which render.them of special importance in the field of catalysis, thermal insulation, or a. variety of other uses where a high degree of. porosity or fineness of subdivision is of importance.

US2188007 -- INORGANIC AEROGEL COMPOSITIONS

This invention relates to improvements in the art and process of producing dry gels from colloidal solutions, and the present specification is particularly directed to the production of a gel, one continuous phase of which is a gas, and which I therefore define as an aerogel.

This application is a continuation-in-part of my copending application, Serial Number 746,412, filed October 1, 1934, which in turn is a continuation of my application Serial Number 508,811, filed January 14, 1931.

Whenever a colloidal solution is precipitated, the product formed is usually defined as a gel.

It is distinct from the precipitates from crystalloidal solutions by containing large quantities of the solvent in a soft "gelatinous" mass, usually microscopically heterogeneous and presenting some rigidity. Gels may be divided into two classes, the gelatinous precipitates and the jellies according to whether the product formed settles out in a flocculent mass or occupies the entire volume that the original solution occupied, and appears microscopically homogeneous.

Gels may be again divided into elastic and nonelastic groups according to whether the gel will re-swell or not, after being dried, when placed in the original solvent. Examples of the elastic gel are gelatine and egg albumin, while examples of the nonelastic gel are silica and alumina gels.

The present invention applies to a new mode of removing the solvent from a gel, either elastic or nonelastic, in preparation for its use in the solid dried state, and the provision of a new class of products obtained thereby.

The most widely used of the inorganic aerogels is that of silica, so that I shall describe something of its properties and mode of production, with explanation of the properties as they are now understood. I shall also list some of its commercial uses in order to indicate what may be the uses of my product.

Most of the gel now commonly used commercially is made by mixing rapidly a solution of water glass with a solution of some suitable salt or acid, usually the latter, and allowing the mixtur6 tostand until it becomes a stiff Jelly. The jelly is then broken up, -washed free of salts and allowed to dry slowly. It shrinks greatly in drying and when completely dry it is a hard glassy mass. A great number of variations have been tried in the conditions of precipitation and drying but the properties of the final product can be affected only to a very limited extent. These U materials are designated in the art as xerosels (CL 252--6) and are characterized and distinguishable from my products in that they can be wetted with the same liquid menstruum. as employed in their preparation, and subsequently dried by simple volatilization of the liquid without changing the 5. physical characteristics of the product materially. Thus, ordinary silica gel (xerogel) can be immersed in water and subsequently dried at an elevated temperature and atmospheric pressure without changing materially its apparent density and other physical properties. On the other hand, an aerogel cannot be wetted and dried without loss ' of its characteristic properties. On the contrary, when an aerogel is wetted and dried the drying operation results in shrinkage and other changes incident to its conversion to an xerogel.

The best theoretical treatment sets forth that the jelly is a mass of Interlacing fibres. Upon removal of the liquid by drying, the surface of the liquid in trying to withdraw within the mass of submicroscopic fibres forms concave menisci, just as it forms in a small tube, and tends to draw the fibrils with it so that there is not only a shrinkage of the water, or solvent volume, but shrinkage of the entire mass. This pull of the liquid surface on the fibrils can easily be understood when it is remembered that the smaller the tube the higher the water will rise in it by capillarity. The spaces between the fibrils are the counterparts of exceedingly minute tubules.

It is, therefore, clear that the gel will shrink until. the fibrils are sufficiently compacted to withstand the compressive force due to the liquid surface. This force is very large, being of 36 the order of thousands of pounds per square inch in easily imagined circumstances. With a substance as strong as silica, a point is reached where the shrinkage ceases and the water dries out, leaving a porous mass. Usually this mass contains from 30-50 per cent voids by volume.

In the extreme case, that of a gel in which the drying was carried out with ferric oxide in the meshes, and later the ferric oxide was dissolved out, a very fragile product was obtained having approximately 75 per cent of its volume void.

The uses for silica gel in the dried condition depend upon the tendency of all molecules, either gaseous or in solution, to attach themselves to a surface. For a given volume of &led silica 410 gel the internal surface is enormous so that easily condensed vapors show a very decided tendency to collect in the gel, and substances In solution can often be almost completely removed by the gel. In addition to the surface eff ect, there is the tendency for vapors to condense in fine capillaries much more readily than they condense in ordinary vessels.

These properties of the gel make it valuable for removing certain undesirable compounds from lubricating oils; for decolorizing sugar solutions during refining; for removal of vapors valuable or otherwise from gases such, for instance as the removal of benzene from coke oven gas; and for drying of air, since the gel will remove water almost completely from air and can then be reactivated by simply-heating and driving off the water. Its power to absorb vapors has made it serviceable in automatic refrigerator cars where it is used instead of water to absorb ammonia, and in gas masks it could easily prove valuable for removal of toxis gases.

Another important use of silica gel is as carrier of contact catalysts. A contact catalyst is a substance that influences a chemical reaction at its surface without itself being consumed by the reaction. Since large surface is the principal requirement and since many contact catalysts axe expensive, methods have been devised for coating the immense inner surface of silica gel with such catalysts, thus effecting considerable economies.

The primary object of the present invention therefore is to produce the gel by a method which will enable me at all times to have complete control thereof, whereby gels may be obtained with void space ranging all the way from the now usual 30-50 per cent found in all commercial gels, up to 99 per cent or higher. Another object of my invention is to provide a new class of gelswhich are referred to herein as aerogels-and a method for their preparation, characterized in that they have inordinate adsorptive characterIstics even as compared to the most porous ac40' tive present day gels.

An additional object of my invention iesides in producing a new class of colloidal products of an amorphous or essentially amorphous character, the degree of subdivision and special relationship of whose individual particles is the same as that in an undried gel or jelly or substantially of that order.

A further object is to produce the gels in an effective and economical manner.

The following examples illustrate embodiments of my Invention:

Example 1.-I first form a hydrogel, such as silica hydrogel or jelly, in a suitable liquid medium, for example: water. For this purpose I may wsimply acidify water glass with sulfuric acid in the well known manner employed in manufacturing silica gel of commerce. The gel is then washed free of soluble substances. In certain cases the gel may be, if desired, partially dried before washing in order to strengthen the mass to stand the washing action or to permit advantageous handling of the same. After being washed the gel is placed in a strong autoclave. The autoclave is nearly filled with a liquid and then closed. The liquid used may be either the water usually used in the precipitating operation or a substituted liquid more suited from a mechanical standpoint to carrying out my method as will be pointed out later. The whole mass in the autoclave is then Toslowly heated. Due to the expansion of the liquid with temperature, the gas space in the autoclave may be completely filled by the liquid considerably before the critical temperature is reached. In such a case continued heating would T6 tend to cause further expansion of the liquid and 2,188,007 thereby subject the vessel to excessive pressures.

It, therefore, becomes necessary to release some of the liquid through a suitable valve. In practice the pressure is preferably maintained at or slightly above the critical pressure of the liquid. A The minimum quantity of liquid that is advisable to use in the pressure vessel Is that quantity which, when expanded to completely fill the vessel, will have the critical density. If the gel present contains this limiting quantity of liquid lo it is notnecessary to add more.

Heating is continued (only enough liquid being released to prevent excessive pressures, but not enough to produce substantial drying of the gel) until the temperature exceeds the critical 13 temperature of the liquid in the pressure vessel.

The gas is then released at a rate insufficient to damage the gel. The gel is left behind in a dried condition but having suffered little, if any, shrinkage.

If the gel is put into the vessel without added liquid, the liquid in the gel will expand with r.,SIng temperature more rapidly than evaporation into the closed space will occur, provided the total quantity of liquid present is equal to the minimum described above, and the gel will thereby not be subjected to compressive forces due to the capillarity.

The principle upon which'my method works is as follows:

Above the critical temperature no gas can be liquefied, regardless of how great the pressure.

If the gel is covered with liquid in the autoclave, there will be no liquid-gas surface coinciding with the gel surface, and therefore there can be no compression of the gel structure due to capillarity. As the temperature rises the liquid is never allowed to evaporate down to where the gel is exposed. Now as the critical temperature is passed, the liquid is transformed into a gas imperceptibly so that whereas only a fraction of a degree below that temperature one could say that the gel is filled with liquid, when that temperature is exceeded by only the slightest amount one must say that the gel is now filled 43 with gas. If the pressure is maintained above the critical point, no surface can form and the gel has no way of "knowing" when the liquid that it held in its meshes has been converted to a gas. Slow removal of the gas, slow only to prevent the disruption of the gel, leaves the gel in the expanded condition but dry, having at no time in the process experienced forces that would tend to compress it.

Since the surface tension of a liquid decreases as the temperature rises, becoming zero at the critical temperature, it would be possible to obtain gels in a partially shrunken condition by allowing the liquid to slowly evaporate at some temperature below the critical temperature. The Go nearer to that temperature that evaporation was allowed to occur, the smaller force would the gel have to withstand and consequently the less compression would it experience. The most practical method of controlling the density, howev. er, or, seems to be to allow evaporation to proceed at ordinary temperatures under controlled conditions until the gel has shrunk as much as is desired, and then to place it in the autoclave and remove the remainder of the liquid as described above.

As previously stated, most gels are primarily formed in water, but water has an inconveniently high critical temperature and the critical pressure is very high, which would require very strong apparatus, if the water was continued as Ig 2,188,007 the liquid in carrying out the method here disclosed. Furthermore, water exerts a very powerful solvent action as the temperature rises so that some gels, for example, that of silica, would dissolve before the critical temperature is reached.

Silica is then precipitated as a very voluminous powder when the water is released above the critical temperature. This powder is of extremely fine texture and is valuable as a catalyst carrier in vanadium sulfuric acid catalysts. So far as I am aware, this material has never been manufactured heretofore. However, in order to obtain. the aerogels, it is desirable and preferable, in my method, to substitute another liquid for the water.

This is easily done by replacing the water in a gel with some liquid, such as an alcohol, that is very soluble or completely soluble in water. This liquid may then be replaced by other liquids miscible with it but insoluble in water.

In addition to the fact that my method enables one to obtain aerogels that could not have been prepared by known methods, it may be applied to partially shrunken gels. It therefore enables one to control completely the gel and obtain a product of just the desired density or porosity desired for a given purpose. The silica gel previously produced and which showed the maximum porosity so far obtained, possessed very little strength and has, therefore, not proved comSomercially practical. On the other hand, silica gel produced by my method and having the same apparent density, will have considerable strength and can be obtained in relatively large lumps or pieces which is not possible when made by present day methods.

As stated above, the only methods in existence prior to my invention for the control of the character of the final dried gel are crude in nature and ineffectual except over a limited range of properties. My invention enables complete control of the gel. The drying process may be stopped at any predetermined point and the liquid removed without further shrinkage of the gel skeleton. I have obtained silica gels with void space ranging all the way from the usual 30-50 per cent, found in the present commercial gels, up to 99 per cent, and I have certainly not reached the limit. If there were any reason for doing so, it is certain that I could obtain dry gels whose volume would be only 0.5 per cent silica, My improved method is applicable to all gels, and numerous gels that have never been produced in the dried condition with appreciable free space within them can now be obtained in as voluminous condition as desired. For example, I have produced aerogels of cellulose, collodion, gelatine, albumin, alumina, nickel bydroxide, thoria, titania, stannic oxide, magnesium ydroxide, chromic oxide and others, and there is no reason to believe that the list cannot be extended almost indefinitely. The production of such gels as pyroxylin And cellulose in the distended condition offers a large field for investigation, and the probabilities are that such aerogels will prove valuable. Similarly, aerogels of compounds of iron, cobalt, zinc, cadmium, barium, manganese, vanadium, and copper may be prepared.

The aerogel seems to be particularly suited to the support of contact catalysts, especially for gaseous reactions, since gases can pass through the gels of low density with great facility and yet each molecule must come very close to a surface many, many times during the passage., Many of the catalysts can themselves be produced in the form of gels and thus do away with the necessity of supporting on silica to give them large surfaces.

In gas masks the voluminous gel may be used as an ultm-filter for the removal of submicroseopic particles from the air. Toward the end of the last war the masks were useless in combating certain extremely fine powders and fogs that were used to produce sneezing.

When a nonelastic aerogel of low density, such as that of silica, is powdered, the product is of extreme fineness and is useful as a polishing abrasive. The voluminous gel is a very excellent heat insulator. I Example 2.-Dissolve 44 grams of magnesium nitrate. Mg(NO3)2.6H20 in a small amount of glycerin and make up to a volume of 50 cc.

Thereafter add 35 cc. of diethyl amine diluted with glycerin to 100 cc. in a manner whereby the diethylamine solution floats on top of the magnesium nitrate solution. Now shake the two layers vigorously for a few seconds and allow the resulting mixture to set to a firm jelly. To remove the glycerin the jelly is extracted with alcohol until the glycerin is completely removed.

The alcohol is then extracted by means of ethyl ether until most of the alcohol is removed. The resulting ether gel is then placed in a pressure vessel with excess ether, if necessary, and the temperature raised to 195 C. while maintaining th6 pressure sufficiently high to prevent evaporation of the ether. After the temperature has reached the critical point the vapor is permitted to escape. The resulting gel is very light, coherent, though fragile, elastic and transparent or translucent.

Example 3.-Dissolve 83 grams of chromic nitrate Cr (NOD 3.9H20 in 100 cc. of water to which there is then added, while stirring vigorously, 35 grams of ammonium acetate dissolved in 50 cc. of water. After the mixture is uniform it is permitted to set. A jelly will form in about minutes. To remove the soluble salts the gel is broken up and washed with water, after which the water is replaced by alcohol and the alcohol is replaced in part at least, by diethyl ether as described in Example 2. The ether-alcohol gel is then converted to the aerogel as described heretofore. The resulting product, when broken to pass a 10 mesh screen, has an apparent density of.25 gram per cc.

Example 4.--82 grams of stannic chloride SnCI4.5H20 are dissolved in 200 grams of water and placed in a dialyzer, such as a sack made of regenerated cellulose as, for example, Cellophane, and is then suspended in 2500 cc. of distilled water. After about 24 hours the contents of the sack will have set to a firm jelly.

This jelly is then washed with methyl alcohol (10 until most of the water has been removed, after which it is converted to an aerogel by heating to 2601 C. before the vapors are released. The aerogel is very light, colorless and transparent.

Example 5-53 grams of a commercial grade of dry thorium nitrate is dissolved in 35 cc. of methyl alcohol. The solution is allowed to stand a few hours before further use. 10 parts by volume of the aforementioned solution is then thoroughly mixed with 10 parts by volume of redistilled aniline. Subsequently 1.8-2.0 parts by volume of water are added whereby the water floats on top of the mixture. The resulting mixture is shaken vigorously for I to 2 seconds, and finally permitted to stand. After a very short 4, period of time It will set Into a jelly which is broken up and covered with a mixture of 9 volumes of methyl alcohol and one volume of concentrated aqueous ammonia. This mixture is changed from time to time over a period of several days until practically all of the aniline has been leached out. Finally, it Is washed with pure methyl alcohol or acetone or a mixture of these two, and autoclaved to form thorium. oxide aerogel.

Example 6380 grams of aluminum nitrate Al(N03)3.9H20 are first dissolved in 1200 cc. of methyl alcohol, cooled to about -10 C. and stirred vigorously while adding 280 grams Igof redistilled aniline. The resulting mixture is allowed to attain room temperature at which time a firm jelly will have formed that can be extracted with methyl alcohol until free of aniline and salts. The alcoholaluminum hydroxide jelly,-or alcogel as it is referred to in the art, is then autoclaved to form an aerogel whereby an elastic, transparent product, having an apparent specific gravity of less than.1, which Is relatively strong considering its low density, is obtained.

In the foregoing description and examples a number of solvents have been disclosed and a number of media having more or less favorable critical temperatures have been enumerated. It Sowill be understood that these are exemplary only and that in addition to alcohol (ethyl alcohol), methyl alcohol, acetone, dimethyl ketone, diethyl ether, dimethyl ether, benzene and propane, many other. liquids may be employed. Similarly, many other combinations may be employed with the view of obtaining the aerogel forming phenomenon under the most favorable temperature and pressure conditions.

It is to be understood that the particular 48method of preparing the hydrogel, alcogel, ethergel or other,gels, in addition to its conversion to the aerogel, forms no part of and does not limit the present Invention.

Reference is herein made to the Miller ard Connolly Patent No. 1,772;055, wherein a silica hydrogel is heated in the presence of liquid water for the purpose of effecting a hardening of the gel. However, in this process the final removal of water is. effected by the more or less conventional means and the shrinking phenomena occasioned by the high surface tension of liquid water manifests itself. Accordingly, one obtains by this precedure a product which has an apparent specific- gravity of as low as.5 but does ggnot have the properties of aerogels. The Miller and Connolly product may be treated with water and dried, repeatedly, to obtain a somewhat similar product each time whereas my aerogel, when treated with water and subsequently dried, is con_ eoverted to the xerogel having a substantially higher apparent specifIc gravity, otherwise lacking the characteristics of the aerogel and rather simulating the Miller and Connolly product so far as specific gravity is concerned. For example, if a silica aerogel with a specific gravity of.1 is wetted with water and subsequently dried, and the resulting dried product resembles Miller's product and has a specific gravity of approximately.6. 1 have not been able to produce an aerogel from a gel not previously dried that will not shrink in this manner when wetted and sub_ sequently dried in the conventional manner.

Finally,,it is to be noted that the Miller and Connolly process is not capable of producing a product having the low apparent specific gravi2,188,007 ties which I am able to obtain since the limiting factor of the surface tension of the water is not overcome by Miller and Connolly. Consequently, shrinking, which attends the removal of water by conventional means, manifests itself. In general, and in the absence of purposeful preliminary shrinkage, my aerogel products are characterized In part by the fact that their apparent specific gravity is not substantially greater than 15% of the actual specific gravity of the substance.

The aerogels are distinguishable from ordinary xerogels by their inordinate catalytic activity.

For example, In the vapor phase oxidation of acetaldehyde to acetic acid, a silica aerogel performs more satisfactorily than the ordinary silica gel due in part to the fact that the aerogel does not become fouled as rapidly. In comparative tests the aerogel did not show any signs of fouling during the course of the runs while the ordinary gel turned yellow and showed resinous materials deposited thereon. Thoria aerogels convert carboxylic acids, as, for example, acetic acid, to ketones with practically quantitative yields at 300 C., while the best form of thoria that was obtained by heating the oxalate required a reaction temperature of at least 500 higher, the conversion was incomplete and side reactions resulted in the formation of byproducts.

It is to be understood that for certain catalytic go purposes it is desirable to have a mixed aerogel, that is, an aerogel containing more than one metallic component. These may be produced, preferably, by mixing the alcogels or ether gels before the autoclaving operation., The following example Illustrates an alternative procedure for preparing one such mixed gel: Example 7.-A silica hydrogel was soaked in a strong solution of aluminum sulphate for 24 hours, after which it was dropped into a concentrated ammonia solution. The resulting gel was washed with water to remove soluble salts, extracted with alcohol and autoclaved in the usual manner. This gel was found to be more active in the oxidation of acetaldehyde to acetic acid than either a silica aerogel or an alumina aerogel.

In addition to all of the foregoing characterIstics which distinguish my aerogels from xerogels, the difference in thermal conductivity further emphasizes the fundamental difference between aerogels as a class and xerogels, or products such as Miller and Connolly describe.

To illustrate: silica aerogel, having an apparent specific gravity of.18, shows a heat conductivity lower than any solid which has ever been measured so far as I have been able to ascertain. Its heat conductivity at 34 C. is 4.85x10-5 cal./sec./C./cm. 00 This constant compares very favorably with some of the best known heat insulators of which the following are representative:

Cork................10 X 10Asbestos --------------------------- 22 X 10--5 Best grade of mineral wool ----------- 7.6 X 10-5 Loose cellulose f1bres --------------- 8.4 X 10-5 One of the advantages of the silica aerogel resides in its ability to withstand elevated temperatures of at least 800 C. indefinitely while other aerogels, such as alumina aerogels, will stand even higher temperatures. Ordinary silica gels (xerogels) are not much better than ordinary V# 2188,007 sand and possess: no usefulness in the field of heat Insulation.

Another feature of the aerogel products of the inorganic type resides in the fact that they do not pack or settle. It is well known that many finely divided materials tend to pack, lea;ving void spaces, and for this reason are not entirely satisfactory heat insulatinOmedia. Aerogels, on the. other hand, show little or no such tendency.

Moreover, while the materials are fragile, nevertheless they will support considerable weight, particularly when supported and in this respect are distinctly superior to materials such as mineral wool and asbestos which lose their 16 valuable heat insulating properties as they are compressed or as they may settle. In other words, it is absolutely necessary, in order to preserve the heat insulating value of these materials, to preserve their fluffy nature.

Due to the porous nature of the aerogel much of the heat that is transferred passes through the gas phase. Accordingly, by substituting a gas of low thermal conductivity for air it is possible to improve the thermal resistance of the aerogels appreciably. For example, a gel that has a conductivity of 4.85 with air, has a c6nductivity of 4.33 when saturated with carbon dioxide and 3.71 when filled with dichlorodifluoromethane. Where the material is to serve as an insulator for refrigerator units and the like it is possible to lodge the insulator within a metal case which can be evacuated, the residual gas present being preferably one of low heat conductivity.

33 From the foregoing description it will be evident that I have provided a new class of products, aerogels, and various methods for their preparation. It will likewise be apparent that the gels are characterized by the fact that they are formed from colloidal solutions or gels In which the liquid menstruum. is replaced by a gas such as air. I In my copending application referred to herein I have claimed aerogels generically and the tetra valent oxide forming elements of the fourth group of the periodic system, specifically. In this ap

https://www.youtube.com/watch?v=xSaPJuRM8no

Turn Beach Sand Into Aerogel Insulation With Vinegar. 10x Better Than Fiberglass. Hidden Since 1931   // Uncover Vault and StoneShift Science

...Aerogel outperforms fiberglass insulation by a factor of 2.5 to 3 per inch of thickness. NASA put it on the Mars rovers. The military used it in World War Two. And then the construction industry — worth over $230 billion a year — made sure you never heard about it again. Not because it failed. Because it works, and you can make it from materials that cost under $8 total...

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Making aerogel

NileRed explores the complex chemistry behind creating high-quality silica aerogel. The process involves crafting an alcohol-based gel and utilizing custom-built high-pressure equipment for supercritical drying, a specialized method required to prevent the delicate material from collapsing.

https://www.youtube.com/watch?v=J87Qyxzm_fQ

DIY Potato Aerogel: Free Cooling from Your Kitchen? // NightHawkInLight

https://pubs.rsc.org/en/content/articlelanding/2025/ta/d4ta08793c

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https://doi.org/10.1039/d4ta08793c

Ultrawhite structural starch film for sustainable cooling
Yang Liu, et al.

Reducing human reliance on high-electricity-consuming cooling technologies like air conditioning is crucial for reshaping the global energy paradigm. Through utilizing natural starch gelatinization, freeze-drying and densification processes, we fabricated an ultrawhite cooling starch film with an ultrahigh solar reflectance of 0.96 and strong infrared emittance of 0.94. The porous structure of the cooling starch film, systematically controlled by the mechanical pressing processing, allows for effective scattering of solar radiation while emitting strongly during the atmospheric transparency window, thereby contributing to high-efficiency daytime radiative cooling capacity. Furthermore, the cooling starch film exhibits excellent mechanical tensile strength, measuring at up to 38.5 megapascals, which is more than twice the strength of natural wood. The ultrawhite radiative cooling starch film holds significant promise for optimizing cooling energy usage, especially in hot and arid climates.

https://en.wikipedia.org/wiki/Aerogel

Aerogel

Aerogels are a class of 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.The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile 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 gels. Kistler's later work involved aerogels based on alumina, chromia, and tin dioxide. Carbon aerogels were first developed in the late 1980s.

History

The first documented example of an aerogel was created by Samuel Stephens Kistler in 1931, as a result of a bet with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.

Properties

Despite the name, aerogels are solid, rigid, and dry materials that do not resemble a gel in their physical properties: the name is because 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 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. Even though it is prone to shattering, it is very strong structurally. Its impressive load-bearing abilities are due to the dendritic microstructure in which spherical particles of average size 2–5 nm are fused together into clusters. These clusters form a three-dimensional highly porous structure of almost fractal chains, with pores just under 100 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. Aerogels have a porous solid network that contains air pockets, with the air pockets taking up the majority of space within the material.

Aerogels are good 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 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 convective inhibitors because air cannot circulate through the lattice. Aerogels are poor radiative insulators because 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 color it does have is due to Rayleigh scattering of the shorter wavelengths 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 monomers (simple molecules) react with other monomers to form a sol or a substance that consists of bonded, cross-linked macromolecules 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.[14] 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.

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), which gives the specific surface area of the sample.[16] At high partial pressure in the adsorption/desorption the Kelvin equation gives the pore size distribution of the sample. In mercury porosimetry, the 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. 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. This is caused by the 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 mW·m−1·K−1 at standard temperature and pressure (STP) and in a large container, but decreases to about 5 mW·m−1·K−1 in a pore 30 nanometers in diameter.

Waterproofing

Aerogel contains particles that are 2–5 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 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 (–OR), a process which is most effective when R is an aliphatic group.

Production

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.

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.

The preparation of silica aerogels typically involves three distinct steps: the sol-gel transition (gelation),the network perfection (aging), and 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 of solid particles known as a "sol". The precursors are a liquid 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). The solution of silica is mixed with a catalyst and allowed to gel during a hydrolysis reaction which forms particles of silicon dioxide. The oxide suspension begins to undergo condensation reactions which result in the creation of metal oxide bridges (either M–O–M, "oxo" bridges, or M–OH–M, "ol" bridges) linking the dispersed colloidal particles. These reactions generally have moderately slow reaction rates, and as a result either acidic or basic catalysts 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.

For some materials, the transition from a colloidal dispersion into a gel happens without the addition of crosslinking materials. For others, crosslinking materials are added to the dispersion to promote the strong interaction of the solid particles in order to form the gel. 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. Many materials may require additional curing after gelation (i.e., network perfection) in order to strengthen the aerogel network.

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 (scCO2) 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 xerogels (i. e. are not aerogels). As the liquid evaporates in such manner, forces caused by surface tensions of the liquid-solid 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. To avoid the collapse of fibers during slow solvent evaporation and reduce surface tensions of the liquid-solid interfaces, aerogels can be formed by 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.

In 1931, to develop the first aerogels, Kistler used a process known as supercritical drying which avoids a direct phase change. 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 CO2-miscible liquid such as ethanol or acetone, then onto liquid carbon dioxide, and then bringing the carbon dioxide above its critical point. 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.

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. 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, scCO2 is the most common substance with a relatively mild supercritical point at 31 °C and 7.4 MPa. CO2 is also relatively non-toxic, non-flammable, inert, and cost-effective when compared to other fluids, such as methanol or ethanol.[44] 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. 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.[46][47][48] 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. 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.This often leads to freeze-drying being used for the creation of aerogel powders or as a framework for composite aerogels.

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.[56] The resulting carbon aerogel may be used to produce solid shapes, powders, or composite paper.[citation needed] Additives have been successful in enhancing certain properties of the aerogel for the use of specific applications. Aerogel composites have been made using a variety of continuous and discontinuous reinforcements. The high aspect ratio of fibers such as fiberglass have been used to reinforce aerogel composites with significantly improved mechanical properties.
Materials

Silica aerogel

Silica aerogels are the most common type of aerogel, and the primary type in use or study. It is silica-based and can be derived from silica gel or by a modified Stober process. Nicknames include frozen smoke, solid smoke, solid air, solid cloud, and blue smoke, owing to its translucent nature and the way light scatters in the material. The lowest-density silica nanofoam weighs 1,000 g/m3, which is the evacuated version of the record-aerogel of 1,900 g/m3.[60] The density of air is 1,200 g/m3 (at 20 °C and 1 atm).

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.

Silica aerogel also has a high optical transmission of ~99% and a low refractive index of ~1.05. It is very robust with respect to high power input beam in continuous wave regime and does not show any boiling or melting phenomena.[63] 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, although its thermal conductivity depends on formulation, density, pressure and measurement method. In one transient hot-wire study of monolithic silica aerogel, Cohen and Glicksman reported that the thermal conductivity decreased from 9.3 mW·m−1·K−1 (0.0093 W·m−1·K−1) at 1 atm to 3.2 mW·m−1·K−1 (0.0032 W·m−1·K−1) at 0.1 atm; the same aerogel in granular form had a thermal conductivity of 15.0 mW·m−1·K−1 at ambient gas pressure under modest compression. Its melting point is 1,473 K (1,200 °C; 2,192 °F).

Until 2011, silica aerogel held 15 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[ and then aerographene in 2013.

Carbon

Carbon aerogels are composed of particles with sizes in the nanometer range, covalently bonded together. They have very high porosity (over 50%, with pore diameter under 100 nm) and surface areas ranging between 400 and 1,000 m2/g. They are often manufactured as composite paper: non-woven paper made of carbon fibers, impregnated with resorcinol–formaldehyde aerogel, and pyrolyzed. Depending on the density, carbon aerogels may be electrically conductive, making composite aerogel paper useful for electrodes in capacitors or deionization electrodes. Due to their extremely high surface area, carbon aerogels are used to create supercapacitors, with values ranging up to thousands of farads based on a capacitance density of 104 F/g and 77 F/cm3. Carbon aerogels are also extremely "black" in the infrared spectrum, reflecting only 0.3% of radiation between 250 nm and 14.3 μm, making them efficient for solar energy collectors.

The term "aerogel" to describe airy masses of carbon nanotubes 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 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.

Aerogel    Color
Silica, alumina, titania, zirconia    Clear with Rayleigh scattering blue or white
Iron oxide    Rust red or yellow, opaque
Chromia    Deep green or deep blue, opaque
Vanadia    Olive green, opaque
Neodymium oxide    Purple, transparent
Samaria    Yellow, transparent
Holmia, erbia    Pink, transparent
Other

Organic polymers can be used to create aerogels. SEAgel is made of agar. Cellulose from plants can be used to create a flexible aerogel.

GraPhage13 is the first graphene-based aerogel assembled using graphene oxide and the M13 bacteriophage.

Chalcogel is an aerogel made of chalcogens (the column of elements on the periodic table beginning with oxygen) such as sulfur, selenium, and other elements. 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.

Aerogel performance may be augmented for a specific application by the addition of dopants, reinforcing structures, and hybridizing compounds. For example, Spaceloft is a composite of aerogel with some kind of fibrous batting.

Amyloid fibrils from food waste (whey) have the potential for use in AF aerogels for gold extraction from e-waste.Their use would have a less environmental impact than that of the conventional use of activated carbon as adsorbent

Applications

Aerogels are used in a wide range of applications because of their low density, low thermal conductivity, and high surface area. Silica aerogels were the first to be developed commercially and remain the most widely used type.

Flexible polymer-based aerogels have been developed to overcome the brittleness of traditional silica aerogels and enable thin, mechanically compliant insulating materials for aerospace and electronic systems. Polyimide aerogel films derived from NASA-developed aerogel technology have been commercialized for such applications; examples include the polyimide aerogel film AeroZero.

    Aircraft de-icing: An 80 grams (2.8 oz) carbon nanotube aerogel could cover the wings of a jumbo jet. Aerogel heaters could operate continuously at low power, preventing ice from forming.
    Catalyst or catalyst carrier.
    Chemical adsorber: Silica aerogels have high surface area, porosity, and are ultrahydrophobic. They may be used to remove heavy metals, for example in wastewater treatment.
    Cosmic dust capture: NASA used aerogel to trap space dust particles aboard the Stardust spacecraft.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 rovers.
    Drug delivery: Drugs can be adsorbed from supercritical CO
    2 with release rate controlled by varying the aerogel properties.
    Electrochemical double layer supercapacitors. Aerogels' high surface area allow capacitors that are .02-.05% the size of similarly rated electrolytic capacitors.
    Electromagnetic shielding
    Energy absorbers
    Fuel cells: platinum-on-carbon catalysts.
    Imaging devices, optics, and light guides.
    Impedance matchers for transducers, speakers and range finders
    Inertial Confinement Fusion (ICF) and X-ray laser targets: In ICF, it is used as low-density target materials to create foam targets that aid in simulating the conditions necessary for fusion. Their low-density structure allows for precise control over the fusion fuel, facilitating efficient compression and heating by the laser energy.
    Introducing disorder into superfluid helium-3.
    Organic insulator: valued for their large surface areas
    Radiative cooling: Aerogels can aid in thermal emittance at lower cost and environmental impact than other materials.
    Passive thermal protection: The US Navy evaluated aerogels for use in diver undergarments and by NASA for insulating space suits.
    Radiators in Cherenkov effect detectors: The ACC system of the Belle detector used aerogels in the Belle experiment at KEKB, because of 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.
    Sensors
    Sound insulation, on windows or during construction.
    Tennis rackets: Dunlop Sport uses aerogel in some racquets.
    Textiles and other flexible materials: "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 opacification additives included in the composite."
    Thermal insulation: fibre-reinforced silica aerogel insulation boards insulation can be about 50% as thick as conventional materials. They are -suited for historic building retrofit,. Aerogel has been added in granular form to skylights for this purpose. Georgia Institute of Technology's 2007 Solar Decathlon House project used an aerogel as an insulator in its translucent roof. Transmission tunnel of the Chevrolet Corvette (C7). CamelBak thermal sport bottle. 45 North uses aerogel to keep hands warm in its Sturmfist 5 cycling gloves.
    Thickening agents in paints and cosmetics.
    Warheads: Fogbank, a material of secret composition used in U.S. thermonuclear warheads, may be an aerogel.
    Waste disposal
    Water purification: Chalcogels can absorb metal pollutants such as mercury, lead, and cadmium. Aerogels can also absorb oil, for example to respond to oil spills. Aerogels can disinfect water.
Photothermal aerogels can purify water by accelerating evaporation.

Safety

Silica-based aerogels are not known to be carcinogenic or toxic. However, they are a mechanical irritant to the eyes, skin, respiratory tract, and digestive system. They can also induce dryness of the skin, eyes, and mucous membranes.[120] 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.

https://www.aerogel.org/
Open Source Aerogel

What, you may ask, is aerogel? Aerogels are the world's lightest solid materials, composed of up to 99.98% air by volume. Aerogels are a diverse class of amazing materials with properties unlike anything else. Transparent superinsulating silica aerogels exhibit the lowest thermal conductivity of any solid known. Ultrahigh surface area carbon aerogels power today's fast-charging supercapacitors. And ultrastrong, bendable x-aerogels are the lowest-density structural materials ever developed.

Welcome to Aerogel.org. Here you will find an encyclopedic reference about aerogels, how-to guides for making aerogels and building a do-it-yourself supercritical dryer, the world's most comprehensive aerogel image gallery, a podcast with the world's leading aerogel scientists, and more....

Silica Aerogel (TEOS, Base-Catalyzed)

Editor’s Note: This is an adaptation of the silica aerogel procedure from the Lawrence Berkeley National Laboratory site about aerogels, which for a long time was the only procedure for making aerogels publicly available. That procedure, we’re sorry to say, does not work. Maybe you’ve tried it. If you have, you’ll have noticed that the solution stays separated as two layers and a gel never forms. That’s because there’s not enough alcohol. Maybe it was a typo. So we modified that procedure and present the modified version that works for us below...

Books

Aerogel
Fouad Sabry

Aerogels for Energy Saving and Storage
Meldin Mathew, Hanna J. Maria, Ange Nzihou, Sabu Thomas

Biobased aerogels: polysaccharide and protein-based materials
Mavelil-Sam, Rubie, Pothan, Laly A., Thomas

The Chemistry and Physics of Aerogels: Synthesis, Processing, and Properties
Lorenz Ratke, Pavel Gurikov

Silica Aerogel Composites: Novel Fabrication Methods
Mahesh Sachithanadam, Sunil Chandrakant Joshi (auth.)

Aerogels I: Preparation, Properties and Applications
Inamuddin, Tauseef Ahmad Rangreez, Mohd Imran Ahamed

Aerogels II: Preparation, Properties and Applications
Inamuddin

More articles @: https://1lib.sk/s/aerogels?