Aerogel Seminar Report PDF

AEROGEL

2015-2016

CHAPTER 1

INTRODUCTION Aerogel is defined as a group of extremely light and porous solid materials. Silica-based aerogels are among the lightest ones, can be less than four times as dense as dry air, and some are nearly transparent, its nickname is “solid smoke” or “frozen smoke”. By Technical Definition, An aerogel is an open-celled, mesoporous, solid foam that is composed of a network of interconnected nanostructures and that exhibits a porosity (non-solid volume) of no less than 50%. The term “mesoporous” refers to a material that contains pores ranging from 2 to 50 nm in diameter.Since this definition is good for most porous materials, the term aerogels became reserved for the porous gels obtained by removing solvent from highly swollen gels at the conditions that no or minimal collapse occurs, which causes the liquid in the gel to become supercritical (in a state between a liquid and a gas) and lose its surface tension. The result is an open porous material with a backbone morphology that can be modeled in terms of three dimensionally interconnected strings of nanoscopic pearls. The length scale of both the “pearls” as well as the interconnected voids can be independently tailored over a wide range, i.e. from a few nanometers to several microns.One of the striking advantages of aerogels compared to other porous materials is that both porosity and inner surface area can be tuned independently. Porosities of up to 99.9 % are achievable; when microporosity is present, the specific surface area can exceed 1500 m2/g. Because of their unique properties, i.e., large surface area, very small pores and very low bulk density, aerogels are potentially important candidates for a wide range of applications.

DEPARTMENT OF MECHANICAL, MVJCE

Page 1

AEROGEL

2015-2016

CHAPTER 2 HISTORY

Steven. S. Kistler Steven. S. Kistler of the College of the Pacific in Stockton, California set out to prove that a "gel" contained a continuous solid network of the same size and shape as the wet gel. It is believed that Kistler's interest was stimulated by a friendly wager with fellow worker Charles Learned. They competed to see if one of them could replace the liquid inside a jelly jar with gas without causing any shrinkage. Kistler won the bet, and published his findings in a 1931 edition of the journal Nature. As is often the case, the obvious route included many obstacles. If a wet gel were simply allowed to dry on its own, the gel would shrink, often to a fraction of its original size. This shrinkage was frequently accompanied by severe cracking of the gel. Kistler surmised, correctly, that the solid component of the gel was microporous, and that the liquid-vapor interface of the evaporating liquid exerted strong surface tension forces that collapsed the pore structure. Kistler then discovered the key aspect of aerogel production:

DEPARTMENT OF MECHANICAL, MVJCE

Page 2

AEROGEL

2015-2016

CHAPTER 3 What Does an Aerogel Feel Like? How Strong Are They? To the touch, an inorganic aerogel (such as a silica or metal oxide aerogel) feels something like a cross between a Styrofoam® peanut, that green floral potting foam used for potting fake flowers, and a Rice Krispie®. Unlike wet gels such as Jell-O®, inorganic aerogels are dry, rigid materials and are very lightweight. In general aerogels are pretty fragile. Inorganic aerogels are friable and and will snap when bent or, in the case of very low density aerogels, when poked, cleaving with an irregular fracture. This said, depending on their density, aerogels can usually hold a gently applied load of up to 2,000 times their weight and sometimes more. But since aerogels are so low in density, it doesn’t take much force to achieve a pressure concentration equivalent to 2,000 times the material’s weight at a given point. The amount of pressure required to crush most aerogels with your fingers is about what it would take to crush a piece of Cap’n Crunch® cereal. Organic polymer aerogels are less fragile than inorganic aerogels and are more like green potting foam in consistency in that they are squish irreversibly. Carbon aerogels, which are derived from organic aerogels, have the consistency of activated charcoal and are very much not squishy. There are several examples, however, of remarkably strong aerogels that can withstand tens of thousands of times their weight in applied force. A class of polymer-crosslinked inorganic aerogels called x-aerogels are such materials and can even be made flexible like rubber in addition to being mechanically robust. One type of x-aerogel made from vanadia (vanadium oxide) is extraordinarily strong in compression with the highest compressive strength to weight ratio of any known type of aerogel and rivals that of materials such as aerospace-grade carbon fiber composites! Regardless of composition, most types of aerogel can be made stronger simply by making them denser (between 0.1 and 0.5 g cm-3), however only at the expense of their light weight and ultralow thermal conductivity.

DEPARTMENT OF MECHANICAL, MVJCE

Page 3

AEROGEL

2015-2016

CHAPTER 4 Aerogel 4.1 What Are Aerogels Made Of? The term aerogel does not refer to a particular substance, but rather to a geometry which a substance can take on–the same way a sculpture can be made out of clay, plastic, papier-mâché, etc., aerogels can be made of a wide variety of substances, including:         

Silica Most of the transition metal oxides (for example, iron oxide) Most of the lanthanide and actinide metal oxides (for example, praseodymium oxide) Several main group metal oxides (for example, tin oxide) Organic polymers (such as resorcinol-formaldehyde, phenol-formaldehyde, polyacrylates, polystyrenes, polyurethanes, and epoxies) Biological polymers (such as gelatin, pectin, and agar agar) Semiconductor nanostructures (such as cadmium selenide quantum dots) Carbon Carbon nanotubes

4.2 Types of Aerogel 1) 2) 3) 4) 5) 6) 7)

Silica Aerogel Carbon Aerogel Metal oxide Aerogel Organic Aerogel Alumina Aerogel Semiconducting Metal Chalcogenide Aerogels Metal Aerogels

DEPARTMENT OF MECHANICAL, MVJCE

Page 4

AEROGEL

2015-2016

4.2.1 Silica Aerogel Silica aerogel is the most common type of aerogel and the most extensively studied and used. It is a silica-based substance, derived from silica gel. The lowest-density silica nanofoam weighs 1,000 g/m3, which is the evacuated version of the record-aerogel of 1,900 g/m3. The density of air is 1,200 g/m3 (at 20 °C and 1 atm). As of 2013, aero graphene had a lower density at 160 g/m3, or 13% the density of air at room temperature. The silica solidifies into three-dimensional, intertwined clusters that comprise 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. It has remarkable thermal insulative properties, having an extremely low thermal conductivity: from 0.03 W/mK in atmospheric pressure down to 0.004 W/m·K in modest vacuum, which correspond to R-values of 14 to 105 (US customary) or 3.0 to 22.2 (metric) for 3.5 in (89 mm) thickness. For comparison, typical wall insulation is 13 (US customary) or 2.7 (metric) for the same thickness. 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.

Fig 4.2.1: Silica Aerogel

DEPARTMENT OF MECHANICAL, MVJCE

Page 5

AEROGEL

2015-2016

4.2.2 Carbon Aerogel 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 400–1,000 m2/g. They are often manufactured as composite paper, non-woven made of carbon fiber, impregnated withresorcinol 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.

Fig 4.2.2: Carbon Aerogel

DEPARTMENT OF MECHANICAL, MVJCE

Page 6

AEROGEL

2015-2016

4.2.3 Metal Oxide Aerogel 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 difference 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

Samarium Oxide

Yellow, transparent

Holmium Oxide, Erbium Oxide

Pink, transparent

Table 4.2.3: Different color of different metal oxide aerogels.

DEPARTMENT OF MECHANICAL, MVJCE

Page 7

AEROGEL

2015-2016

4.2.4 Organic Aerogel Organic aerogels have been around as long as any aerogel-in fact, the first aerogel Samuel Kistler is believed to have prepared was aerogel made from jelly. Kistler also prepared aerogels of gelatin and rubbers, both of which are composed of organic polymers. Basically, an organic aerogel is any aerogel with a framework primarily comprised of organic polymers. Generally, organic aerogels have very different properties from inorganic aerogels such as silica aerogel and metal oxide aerogels. They are generally less friable and less fragile than inorganic aerogels, instead squishing when compressed. The term “organic aerogel” can refer to one of many different kinds of aerogels, each with properties arising from the polymer. Organic aerogels can be made from resorcinol formaldehyde, phenol formaldehyde, melamine formaldehyde, cresol formaldehyde, phenol furfuryl alcohol, polyacrylamides, polyacrylonitriles, polyacrylates, polycyanurates, polyfurfural alcohol, polyimides, polystyrenes, polyurethanes, polyvinyl alcohol dialdehyde, epoxies, agar agar, agarose, and many others. Although organic aerogels have been around since the first aerogels were prepared, they were, for the most part, overlooked until the 1980’s when Lawrence Livermore National Laboratory scientists began producing organic aerogels made of phenolic resins. The bulk of this work was done by scientists Dr. Rick Pekala and Dr. Joe Satcher, who synthesized the first resorcinolformaldehyde polymer aerogels (or RF aerogels for short).-essentially, aerogels composed of the same material as the plastic “Bakelite”. Depending on their density, RF aerogels range from light orange to deep red to black in color and range from translucent to opaque. Low density organic aerogels (0.5 g cm-3) can be extremely robust and very hard to squeeze, almost like a car seat cushion

4.2.5 Alumina Aerogel These aerogels are made with aluminium oxide and are used as catalysts, especially when "metal-doped" with another metal. Alumina aerogels have many fascinating properties, such as high temperature stability and high surface area, which result in them having great potential applications. However, their mechanical properties are very poor, which greatly limits their practical application and commercialization. In this communication we successfully synthesized monolithic alumina aerogels via ambient pressure drying by using attapulgite (ATP) as a reinforcing agent. The resulting attapulgite/alumina composite aerogels exhibit strong mechanical properties. TEM/SEM analysis showed that alumina particles firmly adhered to the surface of the rod shaped crystal of attapulgite and attapulgite played a supporting role as the skeleton in the structure of the composite aerogels.

4.2.6 Semiconducting metal chalcogenide Aerogel Semiconducting metal chalcogenide aerogels possess a unique combination of porosity, optical translucency, and photoluminescence, and show great promise for use as chemical sensors and energy, applications such as photovoltaics and extraction of hydrogen from water using DEPARTMENT OF MECHANICAL, MVJCE

Page 8

AEROGEL

2015-2016

sunlight as the energy source. They consist of a network of interconnected nanoparticles which form a sponge-like, open-celled, nonporous framework. Semiconducting metal chalcogenide (say “kal-kah-jih-nide”) aerogels are a new, exciting class of aerogels materials. First prepared in 2002, these aerogels possess a unique combination of porosity, optical translucency, and photoluminescence, and show great promise for use as chemical sensors (artificial nose?) and energy applications such as photovoltaics (solar cells) and extraction of hydrogen from water using sunlight as the energy source. Like other aerogels, metal chalcogenide aerogels consist of a network of interconnected nanoparticles which form a sponge-like, open-celled, nonporous framework. But it is the carefully-engineered nanoparticles used to make these aerogels that makes them so special

4.2.7 Metal Aerogel Metal aerogels combine the unique properties of metals with the unique properties of aerogels. They exhibit high specific surface areas and electrically conductive. Until very recently, metal aerogels did not exist. This is partly because there were (and still are) no known synthetic routes for producing wet gels of metals, although numerous synthetic routes for preparing solutions of metal nanoparticles (sols) do exist. In principle, if a gel composed of a network of metal nanoparticles could be prepared, it could also be supercritical dried to produce a metal aerogel. This said, like enlightenment and entropy, aerogel is a state function-it doesn’t matter how you get there but just that you do. The most consistent definition of aerogel implies that for a material to qualify as an aerogel, it must possess no less than 50% liquid-free porosity by volume and must be primarily mesoporous. There are now a number of ways to make nonporous metal foams that almost fit both criteria, however, and one way that unambiguously fits both. Here we will discuss the three major approaches:   

Combustion Synthesis of Metal Nanofoams Dealloying of Templated Au Alloys Nanosmelting of Iron Aerogel

DEPARTMENT OF MECHANICAL, MVJCE

Page 9

AEROGEL

2015-2016

4.3 PHYSICAL PROPERTIES OF AEROGEL

1.1.1 Values

1.1.2 Unit s

1.1.3 Silica Aerogel

Class

Silica

Composition

SiO2

Density Range

g cm-3

C 0.02-0.5 600-800

500-950

Pore Volume

cm g-1

Primary Particle Size

nm

2.0-3.0

Average Pore Size

nm

20

Transparency

Clear to foggy

Appearance

Transparent or white with blue cast from Rayleigh scattering

Monolithicity

Monolithic

Flexibility

Rigid, friable Hydrolysis of silicon alkoxide or acid-driven condensation of waterglass Supercritical CO2 or hightemperature drying from organic solvent

Gel Synthesis

Drying Method

Index of Refraction

Carbon

0.0011-0.650

Surface Area

Thermal Conductivity at Room Temperature Electrical Conductivity

1.1.4 Carbon Aerogel

W m-1 K-1

0.016-0.03

S cm-1

1×10-18

Dimensionl ess

1.002-1.046

3.0-20 7.0-20.0 Opaque Opaque black, shiny or matte Monolithic, charcoal like Rigid, breaks like charcoal Polymerization of 1,3dihydroxybenzene with methanal Supercritical CO2 followed by pyrolysis at 400°C1050°C under inert gas 120-320

1-14.7

Table 4.3: Physical property of Carbon and Slilica Aerogels

DEPARTMENT OF MECHANICAL, MVJCE

Page 10

AEROGEL

2015-2016

4.4 World records hold by Aerogels. Many aerogels boast a combination of impressive materials properties that no other materials possess simultaneously. Specific formulations of aerogels hold records for the lowest bulk density of any known material (as low as 0.0011 g cm-3), the lowest mean free path of diffusion of any solid material, the highest specific surface area of any monolithic (non-powder) material (up to 3200 m2 g-1), the lowest dielectric constant of any solid material, and the slowest speed of sound through any solid material. It is important to note that not all aerogels have record properties. By tailoring the production process, many of the properties of an aerogel can be adjusted. Bulk density is a good example of this, adjusted simply by making a more or less concentrated precursor gel. The thermal conductivity of an aerogel can be also be adjusted this way, since thermal conductivity is related to density. Typically, aerogels exhibit bulk densities ranging from 0.5 to 0.01 g cm-3 and surface areas ranging from 100 to 1000 m2 g-1, depending of course on the composition of the aerogel and the density of the precursor gel used to make the aerogel. Other properties such as transparency, color, mechanical strength, and susceptibility to water depend primarily on the composition of the aerogel. Records held by some specially-formulated silica aerogels: 

Lowest density solid (0.0011 g cm-3)



Lowest optical index of refraction (1.002)



Lowest thermal conductivity (0.016 W m-1 K-1)



Lowest speed of sound through a material (70 m s-1)



Lowest dielectric constant from 3-40 GHz (1.008)



Highest specific surface area for a monolithic material (3200 m g-1)

DEPARTMENT OF MECHANICAL, MVJCE

Page 11

AEROGEL

2015-2016

CHAPTER 5 HOW IS AEROGEL MADE As described in the What is Aerogel? , an aerogel is the intact, dry, ultralow density, porous solid framework of a gel (that is, the part that gives a gel its solid-like cohesiveness) isolated from the gel’s liquid component (which takes up most of the volume in the gel). But how do you isolate such a material from a gel?

5.1 The Start of an Aerogel: A Gel Aerogels start their life out as a gel, physically similar to Jell-O®. A gel is a colloidal system in which a nanostructured network of interconnected particles spans the volume of a liquid medium. Gels have some properties like liquids, such as density, and some properties like solids, such as a fixed shape. In the case of Jell-O, this network of particles is composed of proteins and spans the volume of some sort of fruit juice. A gel is structurally similar to a wet kitchen sponge, only with pores a thousand to a million times smaller. Because a gel’s pores are so small, the capillary forces exerted by the liquid are strong enough to hold it inside the gel and prevent the liquid from simply flowing out. It’s important to remember that gelatin isn’t the only type of gel–in fact, chemists can prepare gels with backbones composed of many organic and inorganic substances and many liquid interiors. Once a gel is prepared, it must be purified prior to further processing. This is because the chemical reactions that result in the formation of a gel leave behind impurities throughout the gel’s liquid interior that interfere with the drying processes used to prepare aerogel (as described below). Purification is done by simply soaking the gel under a pure solvent (depending on the gel this could be acetone, ethanol, acetonitrile, etc.), allowing impurities to diffuse out and pure solvent to diffuse in. The solvent in which the gel is soaked is typically exchanged with fresh solvent multiple times over the course several days. Depending on the volume and geometry of the gel, diffusive processes can take any where from hours to weeks. A ice-cube size sample can usually be purified in 1 or 2 days.

5.2 The Dire Consequences of Evaporativelly Drying a Gel Now, if you’ve ever left Jell-O uneaten and uncovered in the refrigerator for a long while (on the order of a week or so), you may have observed the gel shrinks gradually. This occurs when the liquid trapped in the gel evaporates from the gel’s surface. As molecules of liquid escape into the air, the surrounding liquid molecules are pulled together by capillary action and tug on the framework of the gel. Continued evaporation results in collapse of the framework of the gel, forming a dense, hard substance with less than 10% of the volume of the original gel. This is called xerogel (pronounced zeroGEL). In fact, 1980’s-style hard contact lenses used to be manufactured by drying silica gels into lens-shaped silica xerogels. Aerogel is the solid framework of a gel isolated from its liquid component, prepared in such a way as to preserve the framework’s pore structure (or at least most of it). In other words,

DEPARTMENT OF MECHANICAL, MVJCE

Page 12

AEROGEL

2015-2016

Fig 5.1: Formation of Xeroxel Aerogel is what would be left over if you could remove the liquid from a gel without causing it to shrink. This is most effectively done through a special technique called supercritical drying (although as you will see below, there are other ways to make aerogel as well).

5.3 Supercritical Drying In general, supercritical drying is used when liquid needs to be removed from a sample that would be damaged by evaporative or other drying techniques. Biological specimens, for example, are often preserved through supercritical drying. Supercritical drying is a clever technique by which we can pull the rug out from under capillary action (so to speak). As mentioned earlier, capillary action induced by liquid evaporating from a gel’s pores causes the gel to shrink. So what if there were some way to avoid capillary forces to begin with? This is where supercritical drying comes in. All pure substances (that won’t decompose) have what’s called as acritical point–a specific and characteristic pressure and temperature at which the distinction between liquid and gas disappears. For most substances, the critical point lies at a fairly high pressure (>70 atmospheres) and temperature (>400°F). At the critical point, the liquid and vapor phases of a substance merge into a single phase that exhibits the behavior of a gas (in that it expands to fill the volume of its container and can be compressed) but simultaneously possesses the density and thermal conductivity of a liquid. This phase is called supercritical fluid. Say we have a sealed container containing a liquid below its critical point inside and equipped with a pressure gauge on top. In fact, a certain amount of liquid will evaporate in the container until the vapor pressure of the liquid is reached in the container, after which no more liquid will evaporate and the gauge will read a corresponding stable pressure. Now if we heat this container, we will notice the pressure in the container increases, since the vapor pressure of a liquid increases with increasing temperature. As the critical point draws near, the pressure in the container squeezes molecules in the vapor close enough together that the vapor becomes almost as dense as a liquid. At the same time, the temperature in the container gets high enough DEPARTMENT OF MECHANICAL, MVJCE

Page 13

AEROGEL

2015-2016

that the kinetic energy of the molecules in the liquid overwhelms the attractive forces that hold them together as a liquid. In short, as the pressure and temperature in the container get closer to the critical point, the liquid phase becomes more gas-like and the vapor phase more liquidlike. Finally, the critical point is reached and the meniscus dividing the two phases blurs away, resulting in a single supercritical phase. As this occurs, the surface tension in the fluid gradually drops to zero, and thus the ability of the fluid to exert capillary stress does too.

Fig 5.2: Supercritical Drying

5.4 Aerogelification In the case of making aerogels, a gel is placed in a pressure vessel under a volume of the same liquid held within its pores (let’s say ethanol for example). The pressure vessel is then slowly heated to the liquid’s critical temperature. As this happens, the vapor pressure of the liquid

DEPARTMENT OF MECHANICAL, MVJCE

Page 14

AEROGEL

2015-2016

increases, causing the pressure in the vessel to increase and approach the critical pressure of the liquid. The critical point is then surpassed, gently transforming the liquid in the gel (as well as the liquid and vapor surrounding the gel) into a supercritical fluid. Once this happens, the ability of the fluid in the gel to exert capillary stress on the gel’s solid framework structure of the gel has decreased to zero. With supercritical fluid now present throughout the entire vessel and permeating the pores of the gel, the fluid in the gel can be removed. This is done by partially depressurizing the vessel, but not so much as to cause the pressure in the vessel to drop below the critical pressure. The temperature of the vessel must also remain above the critical temperature during this step. The goal is to remove enough fluid from the vessel while the fluid is still supercritical so that when the vessel is fully depressurized/cooled down and drops below the fluid’s critical point, there will simply not be enough substance left in the vessel left for liquid to recondense. This might require several cycles of heating (and thus pressurizing) followed by depressurization (again all done above the critical point). Once enough fluid has been removed from the vessel, the vessel is slowly depressurized and cooled back to ambient conditions. As this happens, the fluid in the vessel passes back through the critical point, but since much of the fluid has been removed and the temperature is still elevated as the vessel depressurizes, the fluid reverts to a gas phase instead of a liquid phase. What was liquid in the gel has been converted into a gas without capillary stress every arising, and an aerogel is left behind. It is important to note, however, that most of the liquids used in the preparation of gels are organic solvents such as methanol, ethanol, acetone, and acetonitrile, and such liquids are potentially dangerous at the temperatures and pressures required to make them supercritical. To make the aerogelification process less dangerous, the liquid component of a gel can be exchanged with a non-flammable solvent that mixes well with organic solvents–liquid carbon dioxide

Fig 5.3 : State of Aerogel during processs

DEPARTMENT OF MECHANICAL, MVJCE

Page 15

AEROGEL

2015-2016

5.5 The Sol-Gel Process The term sol-gel (say “sahl-jell”) refers to a process in which solid nanoparticles dispersed in a liquid (a sol) agglomerate together to form a continuous three-dimensional network extending throughout the liquid (a gel). The term sol-gel is sometimes used as a noun to refer to gels made through the sol-gel process, but this is somewhat of an abuse of the term, since pretty much all gels are made through the sol-gel process. 5.5.1 Sols, Gels, and Aerogels are Colloids: A colloid is a mixture in which at least two different phases are intimately mixed at the nanolevel. The term “phase” generally refers to a solid, liquid, or gas form of some substance. A colloid typically has a continuous phase in which something else with a different phase is dispersed (the “dispersed phase”). Different phases can still be the same phase of matter, for example, two different phases could both be liquids, just not miscible liquids. Colloids are different from homogeneous solutions, in which a substance is dissolved or mixed with another substance and does not separate out, in that the components of colloids are nanoparticles or macromolecules (giant molecules), typically with a length or diameter ranging from a few nm to several hundred nm in diameter. Sols: A sol is a liquid. The continuous phase in a sol is a liquid and the dispersed phase is a solid. The difference between a sol and a non-colloidal liquid is that solid nanoparticles are dispersed throughout the liquid in a sol. If you put a sol in a centrifuge, you can force the nanoparticles dispersed in the liquid to precipitate out. This will not happen for a non-colloidal liquid solution, for example, salt dissolved in water. An example of a sol is black inkjet ink (carbon black dispersed in water). Gels: A gel is a wet solid-like material in which a solid network of interconnected nanostructures spans the volume of a liquid medium. The continuous phase is a solid network and the dispersed phase is a liquid. Gels tend to be mostly liquid in composition and typically exhibit the density of a liquid as result but have cohesiveness like a solid. An example of a gel is Jell-O gelatin. Aerogels: An aerogel is solid with air pockets dispersed throughout. Aerogels are essentially the solid framework of a gel isolated from the gel’s liquid medium. Some aerogels, such as carbon aerogels and iron aerogels, are derived from other types of aerogels, but the aerogels they’re derived from came from a gel directly. Production of Sols Sols of all sorts of compositions can be made several different ways. Nanoparticles of any solid dispersed in any liquid in such a way that the solid phase does not spontaneously precipitate or settle out is considered a sol.

DEPARTMENT OF MECHANICAL, MVJCE

Page 16

AEROGEL

2015-2016

5.5.2 Generally there are two ways sols are made: (a)Nanoparticles are grown directly in a liquid. This happens when you make Jell-O, or a silica gel. Basically you mix ingredients that contain molecules that can interconnect together to form bigger molecules and eventually nanoparticles. These nanoparticles then hook up together to form a gel network. See the Silica Aerogel article under Flavors of Aerogel for a detailed example. (b)Nanoparticles are synthesized and then dispersed in a liquid. This is how more advanced gels are made. Nanoparticles such as quantum dots or carbon nanotubes are made through some process and then dissolved in a liquid directly or dispersed using the help of a surfactant (a detergent, like shampoo or dish soap). This is how metal chalcogenide aerogels and carbon nanotube aerogels start out. In the case of quantum dots, these nanoparticles are usually synthesized in a liquid, centrifuged out, and then redissolved in the desired liquid medium for whatever you’re using them for. Carbon nanotubes, on the other hand, are grown at high temperatures outside of a liquid medium. 5.5.3 The Sol-Gel Transition A sol can become a gel when the solid nanoparticles dispersed in it can join together to form a network of particles that spans the liquid. This requires that the solid nanoparticles in the liquid, which are constantly bouncing around in random directions because of temperature (that is, they are undergoing Brownian motion), bump into each other and stick together when they do. For some nanoparticles this is easy, almost automatic, since they contain reactive surface groups that condense together to form bonds. For other nanoparticles, however, this can be tricky and requires the addition of an additive to “glue” the particles together or removal something from the particle surfaces so that they stick together when collide, either by bonding together or by electrostatic forces (static electricity). As a sol becomes a gel, its viscosity approaches infinity and finally becomes immobile (that it is, it stops being able to flow and fill its container, although it might still wobble back and forth). This transition from sol to gel is called gelation. The point in time when the particle network extends across the entire volume of the liquid causing it to immobilize is called the gel point. The time required for a gel to form after mixing stuff together to make the gel is called the gel time. 5.5.4 Factors that Affect Sol-Gel Chemistry Sol-gel chemistry tends to be particularly sensitive to the following parameters: pH. Any colloidal chemistry that involves water is sensitive to pH. In the case of silica gel formation, this has to do with the hydrolysis step of the silica precursor that results in silanol groups, which are what connect together to produce silica nanoparticles and eventually the gel network. See the Silica Aerogel article under Flavors of Aerogel for more details.

DEPARTMENT OF MECHANICAL, MVJCE

Page 17

AEROGEL

2015-2016

Solvent. As molecules assemble together (polymerize) into nanoparticles, the solvent needs to be able to keep the nanoparticles dissolved so that they don’t precipitate out of the liquid. Also, the solvent can play a role in helping nanoparticles connect together. As a result, the solvent makes a big difference in ensuring a gel network can form. Temperature. The chemical kinetics of the different reactions involved in the formation of nanoparticles and the assembly of nanoparticles into a gel network are accelerated by temperature, meaning the gel time is affected by temperature. If the temperature is too low, gelation may take weeks or months. If the temperature is too high, the reactions that join nanoparticles together into the gel network occurs so quickly that clumps form instead and solid precipitates out of the liquid. Reaction-generated heat Heat released from chemical reactions involved in the formation of nanoparticles and gel networks can feed back into the solution and cause things to react faster, releasing even more heat, causing things to react even faster, etc. Time. Depending on the type of gel being made, different steps in the gel formation process work differently over different time scales. In general, slower is better for sol-gel. If a gel is allowed to form slowly, it usually has a much more uniform structure. This often means a stronger gel and, in the case of potentially transparent gels like silica, results in a clearer gel that Rayleigh scatters less (appears less blue). Speeding reactions up too much causes precipitates to form instead of gel network, and can make a gel cloudy and weak or simply not form. Catalysts. A catalyst is a chemical that accelerates a chemical reaction but does not get used up in doing so. In a lot of sol-gel chemistry, both acids (H+) and bases (OH–) are catalysts, but accelerate chemical reactions by different mechanisms. This is another reason why sol-gel chemistry is usually pH sensitive. In silica gel chemistry, fluoride ion (F–) can catalyze gel formation, as it exploits a special ability of silicon to temporarily form five bonds. Small amounts of catalysts (“catalytic amounts”), on the order of milligrams per tens of milliliters of solution can cause drastic changes in gel time–in many cases reducing gel time from hours, days, or weeks to minutes. Agitation. Mixing a sol as it gels is important to ensure that the chemical reactions in the solution occur uniformly and that all molecules receive an adequate supply of chemicals they need for the reactions to proceed properly. However, as a sol gels, there are microscopic and macroscopic domains of partially-formed gel network throughout the liquid, and agitation can sometimes disrupt the formation of these domains, meaning they get broken up, however usually the broken network fragments regrow and a solution-wide network results.

DEPARTMENT OF MECHANICAL, MVJCE

Page 18

AEROGEL

2015-2016

5.6 How Big Can an Aerogel Be Made? Just as you can only bake a pie as big as your oven, you can only supercritically dry an aerogel as large as your pressure vessel. This means one of three things–either you need a big supercritical dryer, you limit yourself to making small aerogels, or you use a non-supercritical drying technique (see below). Additionally, large continuous volumes (such as cubes or spheres) are generally difficult to make since it takes exponentially longer for solvent from the interior of the gel to diffuse out of the gel as the gel thickness is increased. However, hollow cubes and spheres, flat plates and discs, and rods with thicknesses less than two inches (5 cm), regardless of how big the gel’s other dimensions are, can be easily made. This said, there are many techniques for preparing aerogel materials called ambigels (often just referred to as aerogels) with subcritical drying techniques. These materials typically have porosities of 50-95% and so they are usually (but not always) a little less dense than supercritically-dried aerogels. Subcritical drying techniques typically require speciallymodified gels, in which the solid framework of the gel is chemically changed so that liquid is less able to stick to it and thus exerts only minimal stress on the gel upon evaporation. Additionally, the liquid in the pores of the gel is frequently replaced with a liquid that has a low surface tension, such as pentane or hexane, so that when the liquid is evaporated little capillary stress can result. Cabot Corp.’s Nano gel aerogel granules are made through a subcritical drying technique.

5.7 STRUCTURE OF AEROGEL

Fig 5.4 High-porosity aerogel

3D Computer simulation of high Porosity aerogel showing fractal Structure of the SiO2 clusters

DEPARTMENT OF MECHANICAL, MVJCE

2D slice with strand like structure

Page 19

AEROGEL

2015-2016

CHAPTER 6 AEROGEL APPLICATIONS A nearly transparent, very lightweight material that is a dry gel principally made from silica (silicon dioxide) and 96% air. Dubbed a "Super Material", aerogel is the world's lightest solid, weighing as little as three times that of air and exhibiting superb insulating properties. Although aerogel looks like it could float away, it has very high compression strength. Theoretically, a block weighing less than a pound could support a weight of half a ton. Aerogels real strength is its incredible insulating effects on any kind of energy transfer; thermal, electrical or acoustic. Aerogel can damp out almost any kind of energy. A one-inch thick Aerogel window has the same insulation value as 15 panes of glass and trapped air - which means a conventional window would have to be ten-inches thick to equal a one-inch thick aerogel window.The following applications for aerogels are associated with certain properties of aerogel materials. In many cases, the application is associated with a single property even if the aerogels have a combination of properties appropriate to the given application.

6.1 THERMAL APPLICATIONS Aerogels are good thermal insulators because they almost nullify the three methods of heat transfer (convection, conduction, and radiation). They are good conductive insulators because they are composed almost entirely from a gas, and gases are very poor heat conductors. Silica aerogel is especially good because silica is also a poor conductor of heat (a metallic aerogel, on the other hand, would be less effective). They are good convective inhibitors because air cannot circulate through the lattice. Carbon aerogel is a good radiative insulator because carbon absorbs the infrared radiation that transfers heat at standard temperatures. The most insulative aerogel is silica aerogel with carbon added to it.

Fig 6.1: The crayons on top of the aerogel are protected from the flame underneath. Similar silica aerogels were used to insulate the Mars rover

DEPARTMENT OF MECHANICAL, MVJCE

Page 20

AEROGEL

2015-2016

6.2 POROSITY AND SURFACE AREA APPLICATIONS Due to their high porosity, their very large inner surface area (easily accessible because of the open porosity), and the controllable dispersion of the active component, they are especially active catalysts or catalytic substrates. There are numerous references of this application for various aerogels and doped aerogels. Moreover, the high porosity and large surface areas lead to applications as filters, absorbing media for desiccation, filters, reinforcement agents, pigments, jellifying agents, waste containment, encapsulation media, and pesticides. The carbon aerogels have been used as electrodes capacitors in energy storage devices known as double layer capacitor because they are electrically conductive with a very large surface area. The stored energy in these devices can be released faster than conventional batteries with high power densities. Thus, have potential application in electric vehicles, microelectronics, and hydrogen fuel storage. One of the promising new applications for aerogels is in a cost-effective purification process. The carbon aerogel capacitive deionization process works by sending solutions with various positively and negatively charged ions through an electrochemical cell consisting of numerous electrodes containing carbon aerogels in the form of sheets. The aerogel process can have a variety of uses ranging from extracting harmful contaminants from industrial waste water to desalinizing seawater.

6.3 OPTICAL PROPERTY APPLICATIONS Aerogel is transparent when its microstructural components are very small compared with the wavelength of light. Transparent aerogels, together with their exceptional thermal insulation ability, have been considered for use as super-insulating sheets in double walled window systems because help considerably to reduce thermal losses in windows. Translucent aerogels have been proposed to improve the efficiency of solar thermal energy storage devices. Moreover, the ultra-low density aerogels can be used as lightweight mirror backings. Aerogels have been used to prepare ultra-pure, full-density silica glass by sintering at temperatures below the melting temperature of silica. Silica aerogels with silicon exhibits strong photo-luminescence (luminescence stimulated by visible or ultraviolet radiation). Silica aerogel, doped with radioactive tritium and phosphor, makes an efficient radio-luminescent light source. There is also evidence for quantum confinement in nanoparticle-loaded silica aerogels for producing blue light emission

DEPARTMENT OF MECHANICAL, MVJCE

Page 21

AEROGEL

2015-2016

Fig 6.2: Rayleigh scattering of LASER beam by silica aerogel

6.4 ACOUSTICAL AND MECHANICAL APPLICATIONS Aerogels may also have acoustic and mechanical applications. Because of their unusual structure, aerogels have low sound velocities, as low as 30 meters per second. Another important acoustic property of aerogel is its mechanical impedance. The impedance is the product of density and the sound velocity of the material. Since both are low, silica aerogel has the lowest impedance of all solid material. This allows the aerogels to be used for coupling sound waves in air to a transducer (device that converts energy from one form to another), this may be useful either to generating or detecting sound. Therefore, they should be efficient ultrasonic devices as acoustic impedance matching, and sound absorption (anechoic chambers) Aerogels have also been proposed as a shock absorbing material. One of the earliest experiments was to measure shock compression in silica aerogels. The low density of the silica aerogel allowed more internal energy could be deposited in it. 6.5 ELECTRICAL AND ELECTRONIC APPLICATIONS Silica aerogel is an electrical insulator with a low dielectric constant, k (k is the measure of the ability of a material to store electrical potential energy under the influence of an electric field). The velocity of signal propagation in a chip is dependent on the dielectric constant of the surrounding electrical insulation. The lower the dielectric constant, the higher the velocity. Therefore, thin aerogel films are almost ideal dielectrics for ultra-fast integrated circuits. The bulk aerogels can be used for microwave electronics and high voltage insulators. The pure carbon aerogels are quite electrically conductive, so they have applications as electrodes for batteries, fuel cells, and capacitors. Other metal oxide aerogels have been made, which exhibit super-conducting behavior, thermoelectric behavior, and piezoelectric properties

6.6 SPACE APPLICATIONS Aerogels have already captured cosmic dust while on the European Retrieval Carrier (EURECA) satellite and in Space Shuttle experiments , and will capture cemetery’s dust in NASA's STARDUST project. Lightweight silica aerogels have also been proposed as a contaminant collector, to protect space mirrors from volatile organics.

DEPARTMENT OF MECHANICAL, MVJCE

Page 22

AEROGEL

2015-2016

MARS ROVER Aerogels were used to insulate the Mars Rover, where its lightness and strength were established as ideal. That's the primary reason aerogel was used as insulation on the Sojourner Mars rover in 1997. As night fell on Mars, the temperature dropped down to -67 C (-88 F). Although the temperature outside was colder than Antarctica in winter, it remained a balmy 21 C (70 F) inside the Rover, where sensitive electronics were protected from the hard freeze.

AEROGEL SAMPLE COLLECTOR Comet and interstellar particles are collected in ultra-low density aerogel. More than 1,000 square centimeters of collection area is provided for each type of particle (cometary and interstellar). The collector tray contains ninety blocks of aerogel in a metal grid. The appearance of the grid has been likened to an ice cube tray; the round collector is about the size of a tennis racket. When the spacecraft flew past the comet, the impact velocity of the particles in the coma as they were captured was 6100 meters per second, up to nine times the speed of a bullet fired from a rifle. Although the captured particles were each smaller than a grain of sand, high-speed capture could have altered their shape and chemical composition — or vaporized them entirely. Stardust capsule with aerogel collector deployed to collect the particles without damaging them, a silicon-based solid with a porous, sponge-like structure is used in which 99.8 percent of the volume is empty space. Aerogel is 1,000 times less dense than glass, another siliconbased solid. When a particle hits the aerogel, it buries itself in the material, creating a carrotshaped track up to 200 times its own length, as it slows down and comes to a stop — like an airplane setting down on a runway and braking to reduce its speed gradually. Since aerogel is mostly transparent — a property earning it the nickname "solid smoke" or "blue smoke" — scientists will use these tracks to find the tiny particles. The aerogel was packed in a Sample Return Capsule (SRC) which was released from the spacecraft just before reentry, for a separate landing on a parachute, while the rest of the spacecraft fired its engines, putting it into orbit around the sun.

Fig 6.3: Interstellar Particles trapped in the collector

DEPARTMENT OF MECHANICAL, MVJCE

Page 23

AEROGEL

2015-2016

CHAPTER 7 MARKET AND TREND OF AEROGEL 7.1 The Market for Aerogels With its unique properties of being extremely lightweight, a great thermal insulator and given the vast applications already discovered for this material type, the potential markets for aerogels look promising. According to Electronics.ca research network, the market for aerogels by 2013 is estimated to be $646.3 million with a compound annual growth rate of 50.8%. A number of industries have shown active interest in the material and applications have been extend to the automotive, oil and gas, constructions, cryogenics, shipping and other industries. With new applications being discovered and innovation activity around the development of aerogels still at a high, the production and demand for various types of aerogels will continue to be strong. The current high production costs associated with manufacturing aerogels makes it expensive often costing up to $1000 per ounce. However, this hasn’t appeared to have deterred demand and as with most new materials, we can perhaps expect costs to come down later in the life cycle.

DEPARTMENT OF MECHANICAL, MVJCE

Page 24

AEROGEL

2015-2016

7.2 Publication Trend Going by the patent publication trend, the first few decades from the early 1950’s right till the late 90’s saw consistent activity around the innovation of aerogels with an average of about 20- 30 patents published each year. From the year 2000 onwards the publications started to rise rapidly from about 80 published in 2000 to over 400 last year in 2009. The last 5 years have been the most remarkable clearly displaying just how significant aerogels can be for the future.

Fig 7.2.1: Number of journals published on Aerogel as a main topic

Fig 7.2.2: Figure shows key players of Aerogel

DEPARTMENT OF MECHANICAL, MVJCE

Page 25

AEROGEL

2015-2016

CHAPTER 8 FUTURE Today significant efforts are underway to further mechanically strong aerogels, aerogels of new compositions for sensors and energy production, and to apply aerogels for use as hydrogen storage media. Metal aerogels are just around the corner. More advanced supercapacitors that rival today’s batteries are becoming likely. Hydrogen production using cleverly-engineered semiconductor aerogels will change the way we think about energy and fuel. Smart materials made possible by the unique combinations of materials properties exhibited by aerogels will enhance and impact our daily lives. Dr. Debra Rolison at the Naval Research Laboratory calls aerogel “the original nanotech”. But aerogels have come a long way since the days of Kistler, and there are endless possible applications of aerogel materials.

CONCLUSION Aerogel will probably be a common household name. Although it is not yet ready for commercial use, Americas Fortune magazine's "Technology to Watch" column mentioned 800 potential products that could be manufactured out of aerogel, citing everything from surfboards to satellites. With the use of aerogel in the Sojourner Mars rover, in the Star Dust spacecraft, and in the hundreds of other possible products and applications, the "unobtainium" once only dreamed about may finally be just around the corner.

DEPARTMENT OF MECHANICAL, MVJCE

Page 26

AEROGEL

2015-2016

GLOSSARY Hydrolysis: The reaction of a metal alkoxide (M-OR) with water, forming a metal hydroxide (M-OH). Condensation: A condensation reaction occurs when two metal hydroxides (M-OH + HO-M) combine to give a metal oxide species (M-O-M). The reaction forms one water molecule. Sol: A solution of various reactants that are undergoing hydrolysis and condensation reactions. The molecular weight of the oxide species produced continuously increases. As these species grow, they may begin to link together in a three-dimensional network. Gel Point: 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. Precursors: In the sol-gel process, the precursors (starting compounds) for preparation of a colloid consist of a metal or metalloid element surrounded by various ligands (appendages not including another metal or metalloid atom). The latter is an example of an alkoxide, the class of precursors most widely used in sol-gel research. Shrinkage: Shrinkage of a gel, either during syneresis or as liquid evaporates during drying, involves deformation of the network and transport of liquid through the pores. Aerosol: A colloidal suspension of particles in a gas (the suspension may be called a fog if the particles are liquid and a smoke if the are solid). Aging: The term aging is applied to the process of change in structure and properties after gelation. Bond formation does not stop at the gel point. In the first place, the network is initially compliant, so segments of the gel network can still move close enough together to allow further condensation (or other bond-forming processes). Moreover, there is still a sol within the gel network, and then those smaller polymers or particles continue to attach themselves to the network. Colloid: A mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. The dispersed phase is so small (1–1000 nm) that gravitational forces are negligible and interactions are dominated by short-range forces, such as van der Waals attraction and surface charges. The mixture is also called a colloidal

DEPARTMENT OF MECHANICAL, MVJCE

Page 27

AEROGEL

2015-2016

system, colloidal solution, or colloidal dispersion. Familiar colloids include fog, smoke, and homogenized milk.

Emulsion: Is a suspension of liquid droplets in another liquid, these types of colloids can be used to generate polymers or particles from which ceramic materials can be made Gel: A gel consists of two parts, a solid part and a liquid part. The solid part is formed by the threedimensional network of linked oxide particles. The liquid part (the original solvent of the sol) fills the free space surrounding the solid part. The liquid and solid parts of a gel occupy the same apparent volume. Thus, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. The gel can be removed from its original container and can stand on its own. The continuity solid structure gives elasticity to the gel (as in the familiar gelatin dessert). Alcogel (wet gel): At the gel point, the mixture forms a rigid substance called an alcogel. The alcogel can be removed from its original container and can stand on its own. An alcogel consists of two parts, a solid part and a liquid part. The solid part is formed by the three-dimensional network of linked oxide particles. The liquid part (the original solvent of the Sol) fills the free space surrounding the solid part. The liquid and solid parts of an alcogel occupy the same apparent volume. Supercritical fluid: A substance that is above its critical pressure and critical temperature. A supercritical fluid possesses some properties in common with liquids (density, thermal conductivity) and some in common with gases (fills its container, does not have surface tension). Aerogel: What remains when the liquid part of an alcogel is removed without damaging the solid part (most often achieved by supercritical extraction). If made correctly, the aerogel retains the original shape of the alcogel and at least 50% (typically >85%) of the alcogel's volume. Xerogel: What remains when the liquid part of an alcogel is removed by evaporation, or similar methods. Xerogels may retain their original shape, but often crack. The shrinkage during drying is often extreme (~90%) for xerogels.

DEPARTMENT OF MECHANICAL, MVJCE

Page 28

AEROGEL

2015-2016

REFERENCES Magda Moner i Gerona in: Phd thesis on SILICA AEROGELS: SYNTHESIS AND CHARACTERIZATION (university of Barcelona) C.J. Brinker, G.W. Sherer, Sol-Gel Science. Physics and Chemistry of Sol Gel Processing, Academic Press, New York, 1990 www.scribd.com www.wikipedia.org http://www.springerlink.com/content/0928-0707 http://www.aerogel.org http://www.tonyboon.co.uk/aerogel/aerogel.htm http://www.springerlink.com/content/0928-0707

DEPARTMENT OF MECHANICAL, MVJCE

Page 29