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Modern Aerogels

By Ramakrishnan K, CS04B021 Anush Krishnan, AE04B002 Vijay Shankar V, EP04B011 Ishan Srivastava, EP04B007 Aakashdeep Singh, MM04B001 Radha R, CH04B034

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1. Introduction Aerogels are low-density solid-state materials derived from gel in which the liquid component of the gel has been replaced with gas. Aerogel materials possess a wide variety of exceptional properties, hence a striking number of applications have developed for them. Many of the commercial applications of aerogels such as catalysts, thermal insulation, windows, and particle detectors are under development and new applications have been publicized since the ISA4 Conference in 1994: e.g., supercapacitors, insulation for heat storage in automobiles, electrodes for capacitive deionization, etc. More applications are evolving as the scientific and engineering community becomes familiar with the unusual and exceptional physical properties of aerogels. In addition to growing commercial applications of aerogels, there are also scientific and technical applications, as well. Aerogels were first created by Steven Kistler in 1931, as a result of a bet with Charles Learned over who could replace the liquid inside a jam (jelly) jar with gas without causing shrinkage. The first results were silica gels. Aerogel can be made of many different materials; Kistler's work involved aerogels based on silica, alumina, chromia, and tin oxide. Carbon aerogels were first developed in the early 1990s. Aerogels exhibit some very exotic properties, much of which is attributed to their structure. They consist mainly of chains of constituent material which are joined together to form a large structure with comparatively large-sized pores interspersed filled with air. This gives rise to a low density, very low thermal conductivity, low refractive index and dielectric constant and large surface area per unit weight. Aerogels can also be modified by different means of synthesis to obtain more varied and hybrid properties, thus giving rise to more kinds of aerogels which find use in various fields.

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2. Structure Aerogels consist of a complicated cross-linked internal structure of chains of the aerogel constituent molecules with a large number of air filled pores that take up most of the volume. Observations by scanning electron microscopy and transmission electron microscopy lead to the following conclusions about the structure of an aerogel (of density 0.23 gm cm-3) studied by J L Rousset et al. Primary chains, 30-50 A in diameter, form the substructures of grains whose size varies from 100 to 500 A, depending on density. The grains form large-scale aggregates, leading to pores on scales exceeding 1000 A. Most aerogels have a structure that is similar to that described, with changes only in scale.

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Information on pore volume and pore size distribution is determined by thermoporometry. From the results of J L Rousset et al, to be consistent with the measured overall aerogel density of 0.23 g cm-3 for the given sample, 27% of the total pore volume must correspond to pores of radius larger than 350 A. This amount of macroporous volume is close to that estimated from SEM pictures.

Fig Cumulated porous volume against the pore radius obtained from thermometry

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3. Synthesis of Aerogels 3.1 Sol-Gel Process Aerogels are commonly synthesized by the Sol-gel process. The Sol-gel process could be described as formation of an oxide network through polycondensation reactions of a molecular precursor in a liquid. The formation of aerogels, in general, involves two major steps, the formation of a wet gel, and the drying of the wet gel to form an aerogel. Silica aerogels are generally prepared from silicon alkoxide precursors. The most common of these are tetramethyl orthosilicate (TMOS, Si (OCH3)4), and tetraethyl orthosilicate (TEOS, Si (OCH2CH3)4). However, many other alkoxides, containing various organic functional groups, can be used to impart different properties to the gel. The initial step in the formation of aerogels is hydrolysis and condensation of alkoxide. As condensation reactions progress the sol will set into a rigid gel.

The kinetics of the above reaction is impracticably slow at room temperature, often requiring several days to reach completion. For this reason, acid or base catalysts are added to the formulation. These catalysts speed up the hydrolysis of silicon alkoxide. In acidic environments the oxygen atom in Si-OH or Si-OR is protonated and H-OH or H-OR are good leaving groups. The electron density is shifted from the Si atom, making it more accessible for reaction with water. In basic environments nucleophilic attack by OH- occurs on the central Si atom. The amount and type of catalyst used play key roles in the microstructural, physical and optical properties of the final aerogel product. For example aerogels prepared with acid catalysts often show more shrinkage during supercritical drying and are less transparent than base catalyzed aerogels. As reaction progresses, the sol reaches the gel point, that is, the point in time at which the network of linked oxide particles spans the container holding the Sol. At the gel point the Sol becomes an Alcogel.

Typical acid or base catalyzed TEOS gels are often classified as "single-step" gels, referring to the "one-pot" nature of this reaction. A more recently developed approach uses pre-polymerized TEOS as the silica source. Pre-polymerized TEOS is prepared by heating an ethanol solution of TEOS with a sub-stoichiometric amount of water and an acid catalyst. This material is redissolved in ethanol and reacted with additional water under basic conditions until gelation occurs. Gels prepared in this way are known as "two-step" acid-base catalyzed gels. These slightly different processing conditions impart subtle, but important changes to the final aerogel product. Single-step base catalyzed aerogels are typically mechanically stronger, but more brittle, than two-step aerogels.

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While two-step aerogels have a smaller and narrower pore size distribution and are often optically clearer than single-step aerogels.

The final, and most important, process in making silica aerogels is supercritical drying. This is where the liquid within the gel is removed, leaving only the linked silica network. The process can be performed by venting the ethanol above its critical point (high temperature-very dangerous) or by prior solvent exchange with CO2 followed by supercritical venting (lower temperatures-less dangerous). The alcogels are placed in the autoclave (which has been filled with ethanol). The system is pressurized to at least 750-850 psi with CO2 and cooled to 5-10 degrees C. Liquid CO2 is then flushed through the vessel until all the ethanol has been removed from the vessel and from within the gels. When the gels are ethanol-free the vessel is heated to a temperature above the critical temperature of CO2 (31 degrees C). As the vessel is heated the pressure of the system rises. CO2 is carefully released to maintain a pressure slightly above the critical pressure of CO2 (1050 psi). The system is held at these conditions for a short time, followed by the slow, controlled release of CO2 to ambient pressure. Under these conditions, the network structure is retained and a gel with large pores is formed. The density of the resulting aerogel will be very low generally somewhere around 0.1 g/cm3. If the gel is dried by evaporation, then the capillary forces will result in shrinkage, the gel network will collapse, and a xerogel is formed. This supercritical besides being a critical step in the production of aerogels is also one of the major obstacles in the mass production of aerogels. This process is both time consuming (several days) and costly. 3.2 Preparation of Carbon Aerogels The above mentioned method of preparing silica aerogels is not very successful in the case of carbon aerogels mainly due to the effects of steric hindrance in tetra alkyl ethers. Instead a variant of the Sol-gel process is used. The precursor that is generally used in the synthesis of carbon aerogels is resorcinol-formaldehyde solution. Polycondensation of resorcinol with formaldehyde in aqueous solutions leads to gels that can be super critically dried with CO2 to form organic aerogels which are called resorcinol-formaldehyde (RF) aerogels. Carbon aerogels can be obtained by pyrolysis of resorcinol formaldehyde aerogels in an inert atmosphere.

Carbon aerogels are considered to be ideal electrode materials for super capacitors and rechargeable batteries. However they also have the same disadvantage as that of silica aerogels as their preparation also involves supercritical drying. Another disadvantage is the high cost of resorcinol. 3.3 Aerogel Composites Incorporation of additives into aerogels, whether it is during their synthesis or after, results in a final product which is actually a composite of the substrate (the aerogel) and one or more additional phases. As all these materials have an aerogel substrate, there is always at least one phase with physical structures with dimensions on the order of nanometers. Hence these composites are often referred to as nanocomposites.

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The role of the additives is to enhance the properties of pure aerogels or to impart additional desirable properties depending on the application. One of the first and also one of the most interesting aerogel composites to be prepared is platinum loaded carbon aerogel nanocomposite which is used as an electro catalyst in polymer electrolyte membrane fuel cells. 3.3.1 Aerogel nanocomposites through Sol-Gel processing In this method soluble inorganic or organic compounds are added to the sol before gelation. There are two criteria that must be met to prepare a composite by this route. First, the added component must not interfere with the gelation chemistry of the silica precursor. The second is that the added phases must not be leached out during the alcohol soak and supercritical drying steps. When the added component is a metal complex, the second problem could be easily tackled by using a binding agent, such as (CH3O)3SiCH2CH2NHCH2CH2NH2. This can bind with the silica backbone through the hydrolysis of its methoxysilane groups and chelate the metal complex with its dangling diamine. After the gel has been dried, the resulting composite consists of a silica aerogel with metal ions atomically dispersed throughout the material. Thermal post-processing induces thermal diffusion and reduction of the metal ions, forming nanometer-scale metal particles within the aerogel matrix. These composites are being extensively used as catalysts for gas-phase reactions.

3.3.2 Aerogel nanocomposites through Chemical Vapor Infiltration The open pore network of aerogels allows for easy transport of vapors throughout the entire volume of the material. This provides another route to an aerogel nanocomposite. Virtually any compound with at least a slight vapor pressure can be deposited throughout a silica aerogel. To prevent subsequent desorption of the added phase, it is useful to convert the adsorbed material into a non-volatile phase by thermal or chemical decomposition. The figure shows the B-H curve of silica aerogel to which magnetite has been added by CVI to introduce magnetic properties in the aerogel.

3.3.3 Aerogel nanocomposites through Energized Gas Treatment. This process utilizes an energized reducing gas to form thin films of new material on the interior surface of the aerogel. In the simplest case, silica aerogel monoliths are partially reduced by energized hydrogen. The resulting composite consists of a silica aerogel with a thin layer of oxygen-deficient silica (SiOx) on the interior surface. As with other reduced silica materials, this material exhibits strong visible photoluminescence at 490-500 nm when excited by ultraviolet (330 nm) light.

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4. Aerogels as Catalysts Aerogels aid in heterogeneous catalysis, when the reactants are either in gas or liquid phase. They are characterized by very high surface area per unit mass, high porosity which makes them a very attractive option for catalysis. Some of the reactions catalyzed by aerogels are listed below and their activities/selectivities are compared with conventional catalysts. 4.1 Some Examples of aerogels in catalysis 4.1.1 Synthesis of nitrile from hydrocarbons using Nitric Oxide (NO) [1!] It is seen that Nitriles can be produces from hydrocarbons (aliphatic olefins paraffins substituted benzene) by reaction with nitric oxide instead of conventional ammonia and oxygen/air mixture using PbO2-ZrO2 Aerogel. The advantages are this reaction is less exothermic, there is no free oxygen involved in the reaction and it does not produce hydrogen cyanide. A comparison of porosity selectivity and conversion of PbO2 – ZrO2 aerogel to corresponding xerogel (obtained by drying the gel in normal conditions) is as follows

The reaction considered here is the production of acrylonitrile from propylene. Surface area, selectivity and conversion are enhanced by the use of aerogel. When only NO is sent through the aerogel it forms N2O and O2 through disproportionation. It is believed that this formation is responsible for CO2 production in the previous reaction thereby reducing selectivity. Isobutene can be converted into methacrylonitrile by reacting it with NO on Zinc Oxide aerogel [2!]. 4.1.2 Synthesis of methanol from CO using Copper Zirconia aerogel [3!] It is known that copper catalysts aid hydrogenation of CO to produce methane and methanol. When Cu is doped in Zirconia, the activity of the catalyst is seen to increase. This is attributed to both increase in surface area and metal – support interaction which somehow affects the synthesis of methanol. The catalyst shows high total area even after exposed to CO for a long time (It is resistant to sintering). When the catalyst is used for the first time it goes through a maximum in activity and then stabilizes to a smaller value. On post treatment, it retains 75% of its initial activity. But on exposure to air the activity

PbO2-ZrO2 Specific Surface Area

Selectivity Conversion

Aerogel 168 86 2.3 Xerogel 43 79 0.9

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reduces to a quarter of the original value. The air exposed catalyst however produces significantly less methane. A detailed investigation of this process is still in progress.

CZ 300 is the catalyst used just after production. It shows high selectivity and conversion CZ 301 is the catalyst after post treatment (evacuated at 230º C). It shows close to three-fourths the conversion but poor selectivity. CZ 302 is that catalyst that is exposed to air after post treatment. It shows very low conversion but higher selectivity 4.1.3 Isomerisation of Butene using Zirconia Aerogel Zirconia in the form of aerogel can be used as a catalyst in isomerisation of butene in He environment. A study of the catalytic activity of Zirconia aerogel was done by G.M. Pajonk and A. E1 Tanany [5!] and was later compared to that of Zirconia Xerogel aerogel Xerogel Minimum Activation Temperature

150º C 400º C

Activation temp corresponding to maximum conversion

430º C ~900º C

The maximum conversion attained is 88%. The cis-trans selectivity is 0.61. 4.1.4 Hydrogenation of Butene using Zirconia aerogel [5!]: When the previous reaction is carried out in H2 environment instead we see that hydrogenation of butene is more prominent (55.5% selectivity). The ratio of cis-trans but-2-ene remains the same showing that there is no change is pore structure, except now the same adsorption sites also catalyze hydrogenation reaction. 4.1.5 Hydrogenation of cyclohexene using nickel-alumina aerogel [6!]: On carrying out hydrogenation of cyclohexene on aerogel and xerogel of the same volume (aerogel wt 0.05g xerogel wt 0.2), xerogel gives more conversion (37.1%-50%) But conversion per unit mass of aerogel is much higher (about four times higher). The

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method of preparation also affects the catalyst activity. Aerogels produced by impregnation give more conversion than those produced by coagulation. 4.1.6 CH4-CO2 reforming using ultra-fine NiO-La2O3-Al2O3 [7!] Methane, when reformed using water, produces syngas with high H2/CO2 ratio. It is desired to produce syngas with high CO as it can be used to produce long chain hydrocarbons. This can be achieved by reacting methane with CO with CO2. This reaction however is prone to coking. A comparison of the conventional catalyst and the aerogel is done in this paper

The discrepancy in the aerogel-1 is attributed to defects in preparation process. In general it is seen that aerogels have very high surface area, high porosity and leads to very less coking as compared to non aerogel catalyst. (Note La2 O3 leads t better CO2 adsorption) 4.2 Absorption Characteristics of Aerogels: 4.2.1 Super hydrophobic aerogels-Use in absorption of organic solvents and oils A. Venkateshwara Rao et al [8!] studied the absorption and desorption capacity of super hydrophobic silica aerogel using eleven solvents and here oils.

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The aerogel sample is kept in a given liquid till it is fully wetted. It is then kept at different temperatures, in filter paper and butter paper and desorption properties are monitored. After complete desorption infra red spectrum is analyzed to see if additional bonds are formed and TEM is used to see if the microstructure has changed. Absorption characteristics: The amount of liquid absorbed by aerogel depends on the surface tension on the liquid. Liquids with low surface tension wet the aerogel surface (water does not). Once the aerogel surface is wetted, the amount of absorption varies linearly with surface tension of liquid.

Desorption Characteristics: Rate of evaporation is determined by both surface tension and vapor pressure. The lesser the surface tension, the molecules can reach the surface faster and get desorbed better. More the vapor pressure better is the evaporation. A typical desorption curve looks like a decay curve. Picture showing various stages of desorption

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Important inferences drawn were: 1. Aerogels can absorb fifteen times their own mass of the liquid. 2. They retain their original structure after the adsorbate is fully desorbed. 3. They tend to shrink when used with oils. 4. They can be reused at least three times without causing much damage to the

structure. 5. Rate of desorption is enhanced by increasing temperature or by using ordinary

filter paper. 4.2.2Ag doped carbon aerogel: Use in water treatment [9!]. The presence of Bromide and Iodide ions in water may lead to formation of undesired products which maybe carcinogenic. Therefore it is necessary to remove the halide ions from water. For this purpose, a study of absorption characteristics of Ag doped carbon aerogel is done and compared with the commercially used sorber. Aerogel shows much higher absorption even though the porosity and surface area of the two are comparable. This is attributed to the slight acidic nature of the surface of aerogel, due to the presence of Ag in I oxidation state. This means, the adsorbate is chemically bonded to aerogel rather than by electrostatic interaction like in the case of “sorbo-activated Carbon”. Even though they are promising materials for halide removal, the possibility of elucidation of organic molecules and reusability are yet to be evaluated. 4.2.3 Aerogels as humidity sensors Aerogels have high overall porosity, good pore accessibility, and high surface active sites. They are therefore potential candidates for use as sensors. A study by Chein – Tsung Wang et al [10!] on silica nanoparticle aerogel thin films shows that their electrical resistance markedly decreases with increasing humidity. They are highly sensitive to 40% RH and greater and operate with a 3.3% hysterisis which is attributed to their pore structure. Xerogel of the same material on the other hand shows very low sensitivity.

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5. Thermal properties of Aerogels One of the extraordinary properties that Aerogels have is their very low thermal conductivity. Also their thermal conductivity decreases even further under vacuum. The passage of thermal energy through an insulating material occurs through three mechanisms; solid conductivity, gaseous conductivity, and radiative (infrared) transmission. The sum of these three components gives the total thermal conductivity of the material. Aerogels are remarkable thermal insulators because they almost nullify all three methods of heat transfer (convection, conduction, and radiation). Solid conductivity is an intrinsic property of a specific material. For dense silica, solid conductivity is relatively high (a single-pane window transmits a large amount of thermal energy). However, silica aerogels possess a very small (~1-10%) fraction of solid silica. Additionally, the solids that are present consist of very small particles linked in a three-dimensional network with many "dead-ends". Therefore, thermal transport through the solid portion of silica aerogel occurs through a very tortuous path and is not particularly effective. The space not occupied by solids in an aerogel is normally filled with air (or another gas) unless the material is sealed under vacuum. These gases can also transport thermal energy through the aerogel. The pores of silica aerogel are open and allow the passage of gas (albeit with difficulty) through the material. The final mode of thermal transport through silica aerogels involves infrared radiation. They are reasonably transparent in the infrared (especially between 3-5 microns). At low temperatures, the radiative component of thermal transport is low, and not a significant problem. At higher temperatures, radiative transport becomes the dominant mode of thermal conduction, and must be dealt with.

5.1 Minimizing the solid component of thermal conductivity

There is little that can be done to reduce thermal transport through the solid structure of silica aerogels. Lower density aerogels can be prepared (as low as 0.003 g/cm3), which reduces the amount of solid present, but this leads to aerogels that are mechanically weaker. Additionally, as the amount of solids decreases the mean pore diameter increases (with an increase in the gaseous component of the conductivity). These are, therefore, generally not suitable for insulation applications. However, as noted above, the tortuous solid structure of silica aerogels leads to an intrinsically low thermal transport. Granular aerogels have an extremely low solid conductivity component. This is due to the small point of contact between granules in an aerogel bed. However, in granular aerogel, the inter-granule voids increase the overall porosity of the material thereby requiring a higher vacuum to achieve the maximum performance.

5.2 Minimizing the gaseous component of thermal conductivity

A typical silica aerogel has a total thermal conductivity of ~0.017 W/mK (~R10/inch). A major portion of this energy transport results from the gases contained within the aerogel. This is the transport mode that is most easily controllable.

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As a consequence of their fine pore structure, the mean pore diameter of an aerogel is similar in magnitude to the mean free path of nitrogen (and oxygen) molecules at standard temperatures and pressures. If the mean free path of a particular gas were longer than the pore diameter of an aerogel, the gas molecules would collide more frequently with the pore walls than with each other. If this were the case, the thermal energy of the gas would be transferred to the solid portion of the aerogel (with its low intrinsic conductivity). Lengthening the mean free path relative to the mean pore diameter can be accomplished in three ways: by filling the aerogel with a gas with a lower molecular mass (and a longer mean free path) than air, by reducing the pore diameter of the aerogel, and by lowering the gas pressure within the aerogel.

The first of these methods is generally not practical, as light gases are relatively expensive and would eventually escape the system. The mean pore diameter can be reduced by increasing the density of the aerogel. However, any benefit from a lower gaseous conductivity component is counteracted by an increase in the solid conductivity component. The pore diameter can be reduced somewhat (while keeping the aerogel's density constant) by using the two-step process to prepare the aerogel. The greatest improvement is found by reducing the gas pressure. Vacuum insulations are commonplace in various products (such as Thermos bottles). These systems generally require a high vacuum to be maintained indefinitely to achieve the desired performance. In the case of aerogels, however, it is only necessary to reduce the pressure enough to lengthen the mean free path of the gas relative to the mean pore diameter. This occurs for most aerogels at a pressure of about 50 Torr. This is a very modest vacuum that can be easily obtained and maintained (by sealing the aerogel in a light plastic bag).

The graphic below shows Thermal Conductivity vs. Pressure for single-step and two-step silica aerogels. The minimum value of ~0.008 W/mK corresponds to ~R20/inch.

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5.3 Minimizing the radiative component of thermal conductivity

As noted above, the radiative component of thermal conductivity becomes more important as temperatures increase. If silica aerogels are to be used at temperatures above 200 degree C, this mode of energy transport must be suppressed. This can be accomplished by adding an additional component to the aerogel, either before or after supercritical drying. The second component must either absorb or scatter infrared radiation. A major challenge for this process is to add a component that does not interfere with the mechanical integrity of the aerogel or increase its solid conductivity. One of the most promising additives is elemental carbon. Carbon is an effective absorber of infrared radiation and, in some cases, actually increases the mechanical strength of the aerogel.

The graphic below shows Thermal Conductivity vs. Pressure for pure single-step silica aerogel and single-step silica aerogel with 9% (wt/wt) carbon black. At ambient pressure the addition of carbon lowers the thermal conductivity from 0.017 to 0.0135 W/mK. The minimum value for the carbon composite of ~0.0042 W/mK corresponds to ~R30/inch.

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5.4 Application of thermal properties of aerogels

In the 1930's, when Kistler prepared and studied the first silica aerogels, thermal 4.4 insulation was a low priority and applications of aerogels in insulation systems was not pursued. The renaissance of aerogel technology around 1980 coincided with an increased concern for energy efficiency and the environmental effects of chlorofluorocarbons (CFC's).

It was then readily apparent that silica aerogels were an attractive alternative to traditional insulation due to their high insulating value and environment-friendly production methods. Unfortunately, the production costs of the material were prohibitive to cost-sensitive industries such as housing. Research is continuing to improve the insulative performance and lowering the production costs of aerogels.

5.5 Proposed and prospective uses of Aerogels

In space

In space, aerogels have already been used as thermal isolation material in the Mars Rover of the Pathfinder mission. NASA used aerogel to insulate the electronics on the intrepid Sojourner from the chisel cold of the Martian night. A disadvantage of conventional aerogels is their brittleness and small mechanical stability. Recent developments demonstrate, however, that the mechanical characteristics of aerogels can be improved significantly by using inorganic and organic material combinations (e.g. silicate/Polyurethane) substantially. Therefore, in the future, aerogels may find applications as high strength, ultra-light, thermally insulating structure material in space.

In Refrigerators

Aerogels are a more efficient, lighter-weight, and less bulky form of insulation than the polyurethane foam currently used to insulate refrigerators, refrigerated vehicles, and containers. And, they have another critical advantage over foam. Foams are blown into refrigerator walls by chlorofluorocarbon (CFC) propellants, the chemical that is the chief cause of the depletion of the earth's stratospheric ozone layer. Replacing chlorofluorocarbon-propelled refrigerant foams with aerogels could help reduce this toll.

In Housing, Skylights, Windows

Factors which make aerogels suitable for use in housing: (i) Made of inexpensive silica, aerogels can be fabricated in slabs, pellets, or most any shape desirable and have a range of potential uses (ii) By mass or by volume, silica aerogels are the best solid insulator ever discovered. Aerogels transmit heat only one hundredth as well as normal density glass.

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Associated problems:

(i) Aerogels are friable, i.e. they are prone to shattering like glass. (ii) Aerogels are hygroscopic in nature and on absorbing moisture; they will

usually cause a structural change of contraction etc. and deteriorate. After the first rain they would turn to sludge and ooze down the side of the house

(iii) Aerogels are transparent but they are not transparent enough to be used in double-paned windows.

(iv) Despite its superior efficiency, cost of production of Aerogel panes will be pretty high compared to other existing cheaper alternatives.

Proposed solution:

It has been shown that producing aerogel in a weightless environment can produce particles with a more uniform size and reduce the Rayleigh scattering effect in silica aerogel, thus making the aerogel less blue and more transparent. Sandwiched between two layers of glass, transparent compositions of aerogels make possible double-pane windows with high thermal resistance. Furthermore, it is also possible to make aerogels hydrophobic by chemical treatment.

In Clothing, Apparels, Blankets

Aspen Aerogels Inc. of Marlborough, Massachusetts has produced a Spaceloft product, an inexpensive, flexible blanket that incorporates a thin layer of aerogel embedded directly into the fabric. Spaceloft is relatively inexpensive, flexible, hydrophobic, and breathable. It is also three times more effective than the best commercially available clothing insulation. Jackets made out of this material are intended for wear in extremely harsh conditions and activities, such as Antarctic expeditions. As the price of Spaceloft comes down with mass production, it is expected to be more widely used in everyday winter clothing. Recently, NASA's Johnson Space Center used Spaceloft to construct mittens as a precursor to space gloves for Mars exploration. Another type of aerogel is organic, which are made of carbon and hydrogen atoms. Organic aerogels are stiffer and stronger than silica aerogels and are measurably better insulators. Organic aerogels have extremely high thermal resistance (six times higher than fiberglass) and can be converted to pure carbon aerogels while still retaining many properties of the original aerogel - in addition to becoming electrically conductive.

6. Aerogels: optical properties and applications Aerogels show very interesting optical properties. For e.g. we take silica aerogels. They are transparent since they are made of the same material as glass. It has a bluish appearance off reflected light and causes reddening of transmitted light. These effects can easily be explained using Rayleigh scattering effects.

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6.1 Rayleigh scattering The vast majority of light that we see everyday around us is scattered light. As we already know, scattering leads to various phenomena like blue skies, red sunsets, white or grey color of clouds, poor visibility on foggy days etc. Scattering of light takes place due to its interaction with inhomogeneities in solids, liquids or gases. The entity that causes scattering, the scattering center, can be a single large molecule or a group of many molecules which are arranged in a non-uniform way. Scattering is more effective when the size of the scattering center and the wavelength of the incident light are comparable. In silica aerogels, the primary particles have a diameter of 2-5 nm, so they don’t contribute much to scattering. However, scattering need not arise only due to solid structures. As it is an aerogel, there is a network of pores in it, which themselves can act as scattering centers. Majority of them are smaller (~20 nm) than the wavelength of visible light. There are, however, invariably some number of pores that are larger than the wavelength of visible light. The number and size of the pores in an aerogel can be controlled to some extent by modifying the process through which they are made, i.e. by modifying the sol-gel chemistry used to prepare aerogels. As scattering efficiency is dependent on the size of the scattering center, different wavelengths will scatter with different magnitudes. This causes reddening of transmitted light (since with a long wavelength, it is scattered less by fine structure of aerogels) and bluish appearance of reflected light.

6.2 Parameters To quantitatively measure Rayleigh scattering we need to take into account the two components that contribute to it viz. Rayleigh scattering and the wavelength independent transmission factor (due to surface damage and imperfections). The transmission spectrum of the slab (to be analyzed) is taken and plotted against inverse fourth power of the wavelength.

where T = transmission A = wavelength independent transmission factor C = intensity of Rayleigh scattering t = thickness of the slab λ = wavelength of light

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C and A can be determined from the plot. So we can say that aerogels with a high value of A and a low value of C will be the most transparent. Now we will look into the behavior of aerogels in two regions of electromagnetic spectrum which are of interest i.e. visible and infrared.

6.3 Visible transmission spectrum The intrinsic absorbance of silica is low in visible region. So the transmission effects are mainly attenuated by scattering effects. Transmittance is cut off near 300nm where scattering dominates and is again cutoff at around 2700-3200nm where absorbance dominates. There is then a “visible window” of transmission that is an interesting feature of this material.

6.4 Infrared spectrum In the infrared part of the spectrum, scattering becomes less significant and standard molecular vibrations become more important. Significant features in the fig. are at 3500 cm-1 strong absorption band due to O-H stretching vibrations,1600 cm-1 weak absorption band due to adsorbed water and surface O-H groups,~1100 cm-1 strong absorption band due to Si-O-Si fundamental vibration. So we get a region between 3300 cm-1 and 2000 cm-1 with high infrared transparency which lowers its insulative performance. This can be rectified by adding additives which absorb in this range. 6.5 Optical oxygen sensors Photoluminescence occurs when a material absorbs a photon of sufficient energy. It goes into an excited state. Now it relaxes back after sometime (generally microseconds or nanoseconds) by radiating this excess energy. But if some oxygen molecule collides with it while it is in excited state, this excess energy is passed onto oxygen molecule and so the material relaxes in a non-radiative manner. So the efficiency of photoluminescence quenching is directly dependent on the number of collisions between material molecules and oxygen molecules and consequently, the photoluminescence intensity is determined by the number of oxygen molecules in the surrounding.

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7. Carbon Aerogel Electrochemical Double Layer Capacitors

Electrochemical double layer capacitors (Supercapacitors) are capacitors with large capacitances of the order of thousands of Farads. They are very promising materials as a replacement for electric batteries. Their potential use in electric vehicles has caused a lot of excitement as they are very efficient and are also able to regenerate energy lost in breaking. They have the ability to store greater amounts of energy than conventional capacitors and are also able to deliver more power.

Fig. Ragone plot for different energy storage devices. Fig. Conceptual fig of an EDLC construction

Supercapacitors store charge in a way that is analogous to the electrostatic capacitor but instead of charge accumulating on the two conductors it accumulates at the interface between the surface of the conductor and the electrolytic solution. The accumulated charge hence forms and electric double layer, the separation between the layers being of the order of a few angstroms. The double layer model proposed by Helmholtz in 1853 estimates the specific capacitance (for high concentrations) to be

4CA

επδ

=

Where C is the capacitance. A is the surface area. ε is the relative dielectric constant of the medium between two layers (electrolyte). δ is the distance between the two layers.

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Carbon aerogels because of their very high surface area per unit volume, low density and good electrical conductivity provide an interesting material for use in Supercapacitors. They also have a very efficient way of storing and releasing charge and can hence survive for a large number of charge/discharge cycles. One might think that the specific capacitance depends on the surface area of the electrode; this is not always the case. This happens because the actual double layer capacitance varies according to the process used to prepare carbon and it is the accessibility of the pores to the electrolyte that is important. The mobility of ions within the pores is different from the mobility in the electrolytic solution and is to Fig.Pore diameter and its effect on capacitance a large extent dependent on pore size. If the pores don’t allow easy access to the ions, then they won’t contribute to the double layer capacitance. Therefore the pore size must be chosen based on the electrolyte. Carbon aerogels are an exciting material to be considered for electrochemical capacitors because the synthesis method determines the microtexture of the aerogel; the pore size and the pore distribution. If the pore diameter is less than 2nm then it is called a micropore and those with diameter exceeding 50nm are called macropores and those in between are called mesopores. The porosity of carbon aerogels is based on the interconnection between nanoparticles of the same size that is at the origin of the uniform mesoporous microstructure with a specific surface area between 500 and 900 m2 per gram. The pore size in a carbon aerogel strongly depends on the preparation process i.e. the initial concentration of reactant and the Resorcinol to catalyst ratio (R/C). Smaller pore carbon aerogels are much better than larger pore aerogels because the pores are easily accessible for the electrolyte ions. The pore size in a carbon aerogel / xerogel or also depends on the elaboration method and the final temperature in the pyrolysis. A competing effect has been observed between particle size and bulk density on the specific capacitance of an aerogel. Capacitance increases linearly with surface area but after pore volume goes above 0.5cm3/g the capacitance largely remains constant. EDLC’s are expected to be used widely in a lot of applications like memory backup, electric vehicles, electromechanical actuators, portable power supplies etc.

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8. Use in RICH detectors Ring Imaging Cherenkov (RICH) detectors are detectors that can determine velocities of fundamental particles. This is done indirectly by measuring the Cherenkov angle of the moving particle – angle between its path and the Chrenkov radiation emitted. Cherenkov radiation is emitted when matter travels at a velocity greater than the speed of light in that medium. The Cherenkov angle is given by

cos ccnv

θ =

Where c is the speed of light in vacuum, n is the refractive index of the medium and v is the velocity of the particle in the medium. Aerogels have very low densities that range from as low as 0.003 gm/cm3 (about three times that of air) to 0.55 gm/cm3. The refractive index of an aerogel is related to its density: 1 0.21n ρ= +

The corresponding refractive indices lie in the range 1.006 – 1.11. Aerogels consist of grains of SiO2 in the size range 1 – 10 nm linked together in a three-dimensional structure filled by trapped air. The large amount of trapped air gives it very high porosity (upto 99.8% at times) and this allows high energy particles to travel through them for sufficient distance at a high velocity – higher than the speed of light in the aerogel. When the particle enters the aerogel, Cherenkov radiation gives rise to a cone of light which is the focus to obtain a ring image, from which the Cherenkov angle and thus, the velocity of the particle can be obtained. Aerogels are being developed with more desirable properties like high strength, high transparency and crack-free. They are far easier to handle than gases at high pressures and have lower refractive indices than liquids, and also allow particles to pass through more easily. They are seriously being considered for use as radiators in such detectors. Such detectors can also be used to identify and discriminate between atomic species, for e.g. to determine the ratio and activity of Sr-90 and its daughter nucleus Y-90 which are present in a system. Both are very good β-emitters but Y-90 produces beta radiaton of higher energy, that can be detected using a Cherenkov detector. This is very useful as Sr-90 is highly radiotoxic and harmful pollutant.

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The innovative set-up of the LHCb RICH with aerogel and gas radiators

Table: Expected performances of LHCb RICH detectors with n=1.03 aerogel radiator and CF4, C4F10 gas radiators. The following factors are listed: momentum thresholds for pions and kaons, maximum Cherenkov emission angle, contributions to the angle resolution from the uncertainty of the photon emission-point, from the radiator chromatic dispersion and from photon detector spatial resolution (assuming 2.5x2.5 mm2 pixel size), total angle resolution per photoelectron, and the momentum upper limit of 3 / Kπσ separation.

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