seminar phd (report)

7/29/2019 Seminar PHD (report)

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Silica Aerogel Synthesis and Applications

A. IntroductionAn aerogel is an open-celled, mesoporous, solid foam that is composed of a network of interconnecte

nanostructures and that exhibits a porosity (non-solid volume) of no less than 50% (aerogel.org). The discret

liquid in a hydrogel is replaced by air in aerogel.

Silica aerogel comprises of rigid three dimensional network of contiguous particles of colloidal silica (1

100 nm in diameter) (Iler 1978). The colloidal particles are connected via siloxane (Si-O-Si) bridge whic

constitutes the framework and decides the properties of silica aerogel. Silicon (Si) atom being tetravalent, i

capable of forming four covalent bonds, and this leads to variation in arrangement of silica particle with

general formula of (SiO2). In general, silica can be broadly characterized as crystalline and amorphous; densit

of crystalline quartz, which is hardest form of silica, is 2.66 g/cm3

whereas density of amorphous clay is 2.

g/cm3. Silica aerogel is categorized under amorphous silica due of absence of ordered arrangement. Silic

aerogel on an average posses high surface area (500-1200m2/g), high porosity (80-99.8%), low densit

(~0.003g/cm3), low thermal conductivity (0.005W/mK), low dielectric constant (k=1.0-2.0) and low refractiv

index (~1.5) (Dorcheh and Abbasi, 2008).

Silica aerogel in recent has received lot of attention due to its unusual properties. The properties of silic

aerogel are largely determined by the protocol of its synthesis. Evaluating its chemical and structural propertie

is crucial where newer technologies are pouring in to accurately estimate its parameters and make measuremen

easier and quicker. Another area which interests the scientific community is application of aerogel. Newer nich

are emerging in field of silica aerogel research. This seminar report presents a brief summary of importan

aspects of aerogel, viz. synthesis, properties and applications.

B. SynthesisSilica aerogel was first prepared by Samuel Stephens Kistler in year 1931 as a result of obtaining liquid fre

gel. He started with silica hydrogel and used ethyl alcohol at critical point to replace water, without shrinkage i

pore structure (Kistler, 1932). Silica aerogel is now largely produced using sol-gel process. Sol-gel process is

technique to get integrated network of particle or polymer (gel) from a finely dispersed colloidal solution (sol

Three reactions used to describe the sol-gel process are, (Brinker and Scherer, 1990)

a. Hydrolysis of silica source to form silica monomer (silicic acid).b. Polymerization of silica monomer by water condensation reaction.c. Polymerization of silica source by alcohol condensation reaction (Figure 1)
http://www.aerogel.org/http://www.aerogel.org/


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Figure 1: Sol gel reactions

Synthesis of silica aerogel can be divided into three main regimes: (gel formation, aging and drying)

1. Gel formationAlkoxysilane (Si-(OR)4), watergalss (Na2SiO3) and sand (SiO2) are the prominent precursors fo

silica gel formation and alkoxysilane are widely used because of their high purity and ease of operation

Among the alkoxysilane group, low molecular weight members such as tetramethoxysilane (TMOS

tetraethoxysilane (TEOS), methyltrimethoxysilane (MTMS) are preferred choices (Figure 2).

Figure 2: Silica precursor (alkoxysilane)

Preparation of sol involves forming homogeneous mixture of silica source in suitable medium

Alkoxysilane are immiscible in water and hence organic solvents such as alcohols, acetone, dioxane

tetrahydrofurane are used to solubilize water and alkoxysilane both, so that hydrolysis (reaction 1 i

Figure 1) reaction can be initiated. Water to alkoxysilane (H2O/Si(OR)4) ratio should be at least 4:1 focomplete hydrolysis. Hydrolysis of alkoxysilane is a facilitated by acid and base catalyst. Hydrochlori

acid (HCl), sulphuric acid (H2SO4), formic acid (CH2O2), acetic acid (C2H4O2), oxalic acid (H2C2O4) etc

are widely used as acid catalysts. For hydrolysis under basic condition, ammonia (NH3) in dilut

concentration is preferred over sodium hydroxide (NaOH) and ammonium fluoride (NH4F). Both

hydrolysis and condensation reactions (reaction 2 in Figure 1) occur simultaneously. Rate of hydrolysi

and condensation varies non-monotonously with pH (as shown in Figure 3). Hydrolysis is favored i

acidic pH where as condensation is favored in basic pH, hence, mixed (combination of acid and base


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to a shrinkage of framework, which could be avoided by using low vapor pressure solvents. Usage o

ionic solvents, that have low vapor pressure, has also been reported in literature for silica aeroge

synthesis (Dorcheh and Abbasi, 2008).

3. DryingThe major challenge in drying is the capillary pressure exerted on the silica structure due t

presence of liquid-vapour interface. Capillary pressure depends on, liquid-vapour interfacial tension (lv

contact angle () and size of pore (rp), which is illustrated by Young-Laplace equation (Brinker an

Scherer, 1990),

() , (2

where, Pc is differential capillary pressure. For pores ranging from 2-50 nm, the differential pressur

ranges from 100 to 200 MPa. This large capillary force is sufficient to destroy nano structure of silic

framework, leading to a dense solid, known as xerogel. Such destructive forces need to be avoided t

preserve the porous structure of wet gel.

Since the major parameter responsible for capillary forces are contact angle and interfacial tension, on

can mitigate the capillary forces by changing these two parameters, and device appropriate dryin

protocol based on them.

a. By changing the surface characteristic of the silica aerogel, it can be changed from hydrophilic thydrophobic, which will either reduce or reverse the direction of capillary forces (Figure 4).

Figure 4: Capillary forces in hydrophilic and hydrophobic silica aerogel

In this method surface of silica network is modified using suitable silylating agents which replaces pola

hydroxyl group (-OH) with non polar group as shown in Figure 5. Due to inverted liquid-vapour interfac


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the capillary pressure helps in expanding the pore, rather than collapsing and interns strengthen the silic

aerogel.

Figure 5: Silylating of silica gel

Figure 6: Silylation agents

Silylating agents such as trimethylchlorosiliane (TMCS), hexymethyldisilazane (HMDS) etc whos

chemical structures are shown in Figure 6 are often used. Silylating agent should have at least one fre

site to react with terminal (-OH) in silica gel replace it with its non polar group. This modified silica ge

can be dried at ambient pressure at 50 -100 C (Mahadik et al., 2011). Silylation of silica aerogel could b

carried out either by adding surface modifying agents at the gel fromation step or by keeping wet gel i

bath of silylating agents after it has been obtained by convential method. Gels obtained by this method i

relatively less denser than conventional method and has larger pore volume.

b. The other parameter to control capillary pressure is liquid vapour interfacial tension (lv), whicvanishesh when interfacial boundary is dissolved. By operating at supercritical conditions of solvent, th

liquid-vapor interfacial boundary can be dissolved, and hence avoid damages the due to capillary force

Such successful attempts was first demonstrated by Kistler (Kistler, 1932), where he used low boilin

solvents (ethanol) to prepare wet gel, and subsequently brought to critical conditions by slowl

increasing the temperature and pressure. Such modes of drying (where organic solvents are frequentl

used) is known as high temperature supercritical drying (HTSCD), since it has to be operate at very hig

temperature (>500 K). Since organic solvents are flammable, they are prone to accidents, and fire ris

increases exponentially with increase in temperature. Low temperature supercritical drying (LTSCD

uses carbon dioxide (CO2) gas, whose critical point is close to ambient condition, hence can be operate

at low temperature and pressure (around 310 K and 80 bar). It requires an additional step of replacin

pore solvent by carbon dioxide (CO2) by flushing it at high pressure of 100 bar.


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TMOS hydrolysis rapidly and gives narrow and uniform pores with high surface area than TEOS

MTMS gives high surface area with more flexible network than TEOS. Addition of MTMS to TMOS

yields more hydrophobic aerogel, compare to TMOS alone. Addition of dimethyldiethoxysilane t

TEOS shifts pore size to larger pore radii. Alkoxides are highly hazardous and causes blindness. It i

also expensive and hence not commercialized, therefore now trend is to search for cleaner and les

expensive silica source. Waterglass is cheaper and gives monolithic and large pore size aerogeImportant parameter while using waterglass is Na2O/SiO2 molar ratio which gives best results at 1:3.

and Na2SiO3/H2O > 8:1000 (Dorcheh and Abbasi, 2008).

4. AdditivesAdditive such as polyethylen glycol(PEG), polyvinyl alcohol (PVA), methyltriethoxysilan

(MTEOS) have been used during synthesis to imporve properties of resulting aerogel. PEG reacts wit

silica structures to forms a spacing between them, thus, acting as a template for pore. In smal

concentration it strenthen the solid matrix however, in large concentration it weakens the solid matrix

Water soluble polymer such as PVA can be used for controlleing pore size of aerogel. PEG and PVA

promotes lipase activity and therfore can be used as biocatalyst support. Surfactant in water- ethano

solution gives surfactant templated aerogel.

C. CharacterizationWith advent of sophisticated instruments it is now possible to accurately measure different properties o

silica aerogel. This section explains important properties of silica aerogel and their characterization techniqu

used.

1. Pore structureThe porous silica aerogel is termed by IUPAC as mesoporous materials as its majority of pore

lies in the range of 2 to 50 nm. Silica aerogel is also termed as open pore of interconnected network

because fluid can flow from one pore to another. The structural properties of interest relates pore size

crystallinity and porosity.

BET characterization based on nitrogen adsorption/desorption technique is used to estimate por

size, size distribution and surface area of aerogel. This method is suitable for silica aerogel because of it

smaller pore size where conventional technique such as Mercury intrusion technique is not suitable

Typically silica aerogel have average pore diameter in range of 20-40 nm and BET surface area of 600

1000 m2/g and porosity of 99%. Scanning electron microscopy (SEM) and transmission electro

microscopy (TEM) is used for studying micro structure of silica aerogel. Although it gives direc

examination of aerogel there may be electrostatic loading of fine aerogel powder and may alter th


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observation. In addition, predicting 3D structure from 2D images is difficult, however primary structure

of (2-3 nm) which have micro pores and secondary structure are obtained as a network of primar

structures thus giving low density can be observed from the images. Atomic force microscopy (AFM

can be used to study densification of aerogel. Nuclear magnetic resonance (NMR) is used to study stage

of condensation reactions.

2. Thermal conductivitySilica aerogel has very low thermal conductivity, even lesser than air (Dorcheh and Abbas

2008). Its thermal conductivity is in order of 0.02 W/mK at ambient condition and 0.01 W/mK when ai

from the pores is evacuated. Transmission of energy in form of radiation is also less, because aerogel

significantly opaque in infrared (IR) region of spectrum. Being open pore structure, hot gases can pas

through and still conduct energy. To calculate total thermal conductivity all the three modes are to b

coupled which is carried out in vacuum insulation conductivity tester (VICTOR).

3. Optical PropertiesSilica microstructures are smaller than wave length of visible light, therefore it appears a

transparent porous material. It has small amount of light scattering in blue region thus giving it

characteristic blue tint. Scattering characteristic is measured using Rayleigh scattering apparatu

Refractive index of aerogel is calculated using Clausis-Mosotti equation (Poelz and Riethmuller, 1982);

, (3

where, n is refractive index, is density of aerogel, ns = 1.46 is the refractive index of crystalline silica o

density, s = 2.2 g/cm3, which depends only on density of aerogel.

4. Surface propertySilica aerogel are highly hydrophilic in nature due to presence of polar (-OH) group at terminal

of silica network. Surface (-OH) group of silica aerogel can be modified by a) adding silylating agen

during sol-gel synthesis process or b) passing methanol gas through the dried aerogel (Lee and Kim

1995). The silylated aerogel are hydrophobic because all (-OH) groups are replaced by non polar groups

Fourier transform infrared spectroscopy (FTIR) is used to confirm the presence/absence of non

polar/polar group in modified silica aerogel. The wetting behavior of aerogel is estimated by evaluatin

contact angles and surface energies (sv). Surface energy can be calculated by usingNeumanns equation

which incorporates general Youngs equation, (Mahadik, 2011)


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* + *

+

, (4

where, = 0.000125 (mJ/m2), is contact angle, sv and lv are surface energies of solid-vapour an

liquid-vapour respectively. Contact angle of hydrophobic silica aerogel is observed to be in range of 130

150 C. Hydrophobicity can be retained for higher temperature up to 673 K, after which alkyl group (

CH3) decomposes giving back hydrophilic aerogel.

Table 1: Properties of Silica aerogel

Property Values Instruments

Apparent density (g/cm ) 0.0011-0.65

Internal surface area(m /g) 500-1000 BET Surface area

Primary particle diameter(nm) 2-5 Electron microscopy

Refractive index 1.002-1.08

Coefficient of thermal expansion

(micro strain /C)

2-4 10-

Ultra sonic method

Thermal conductivity(W/mK) 0.016-0.03 VICTOR

Dielectric constant 1.008-2.27

Speed of sound(m/s) 70-130

D. ApplicationsDue to its versatile properties, silica aerogel have potential applications in various areas. However it no

commercial utilized because preparation of silica aerogel is expensive so as to use in day to day application. Thi

section will explore some of the applications of silica aerogel in representative areas.

1. Catalysis and reaction engineeringSilica aerogel poses high porosity (>85%) and very large surface area (1000m

2/g), which i

desirable property for catalytic application, and already being used for synthesis of nitrile from

hydrocarbon and nitric oxide (Rousset J. L., et al., 1990). Because of its three dimensioned network,

can be used as catalyst support as demonstrated in the study of aerogel supported copper zirconia catalys

for synthesis of methanol from carbon monoxide (Gurav, et al., 2010). Its porous nature can also be use

in adsorption based applications such as filters, adsorbing media and encapsulation medium etc.


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2. BiotechnologySilica aerogel being non toxic and chemically inert, can be used in biotechnology as a carrier fo

drug delivery. After coating its surface with Eudragit L, it can be used for targeted drug deliver

(Alnaief et al., 2012). Because of its large internal network it can be used as biosensor to recogniz

specific bioactive molecule, recently shown in case of recognizing human gene ATP50 by oligo

nucleotide immobilized in aerogel (Gurav, et al. 2010).

3. Architectural applicationBy incorporating hydrophobicity in silica aerogel, it has potential application as transparen

window insulating material, due to its low thermal conductivity (< 3W/m K). It is used as a template fo

dye sensitized solar cell (DSSC), when it is deposited with atomic layer of titanium dioxide (TiO 2

because of its large surface area to efficiently load TiO2. Brittleness of silica aerogel can be avoided b

obtaining composites with suitable polymer like polyester, leading to flexible insulating blankets. Suc

blankets have potentials to replace conventional insulators.

4. Physical scienceBecause of very low dielectric constant of silica aerogel, it is used in the form of thin film fo

inter layer dielectric spacer in integrated circuits. Being optically transparent, Cherenkov radiation

occurring due to particle passing through medium, could be detected. Aerogel doped with radioactiv

substance like tritium and phosphor can be used as efficient radio luminescent light source. Hydrophili

silica aerogel shows increase in electric conductivity with increase in relative humidity, and has potentia

to be used as humidity sensor (Gurav, et al. 2010).

5. Space science researchThe insulating property of silica aerogel is manifested in providing thermal shield to space craft

space suits and Martian Rovers. Being highly porous with large network, high velocity cosmic particl

can be trapped inside silica aerogel matrix. Thus it is been used as hypervelocity particle collector in Spr

and Opportunity space programs. It was successfully employed in Stardust project to capture particle

from comet Wild2 (aerogel.org).


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1

E. Summary and ConclusionWe have discussed various aspects of silica aerogel synthesis such as raw material, different stages o

synthesis and effect of additive on properties of silica aerogel. Crucial properties of silica aerogel whic

gives detail insight of specific properties, its measurement and instrumentation required. We also highlighte

some of its potential and trapped applications of silica aerogel.

Silica aerogel is still in a developing stage. There are several issues pertaining during synthesis proces

and reproducibility, property optimization and overall cost of production. For commercializing the silic

aerogel such challenges are need to be addressed.

F.

Scope for future work

The challenge in the field of aerogel commercialization is its cost of production. This issue can b

address by exploring inexpensive raw materials and economical technology. Silica aerogel is also unsuitabl

in ambient condition because of its hydrophilic surface which adsorbs water onto it. Surface modification b

silylating agent is to used to overcome this problem. But in application of drug we need to alter its surfac

property as per local condition in this case we need to explore special coatings which alter its properties a

required.

There can be several modifications possible in conventional synthesis of aerogel to alter its properties a

recently demonstrated by addition of aluminum ions which alters the mean particle size distribution o

primary structure (Shyong and Chang, 2004). By incorporating various other parameters, tailor made silic

aerogel could be prepared as shown in recent study, where silica aerogel composites of metal organi

framework was prepared, (Ulker and Erucar, 2013).

Such tailor made aerogel can be utilized for trapping CO2 and catalytically converting in valuable fuel.


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Bibliography

Alnaief, M. and Antonyuk, S., (2012), A novel process fro coating of silica aerogel microspheres for controlle

drug release applications,Microporous and Mesoporous Materials, 160 , 167-173.

Brinker, C. J. and Sherer, G. W., (1990), The Physic and Chemistry of Sol-Gel Processing. Academic Pres

New York, USA.

Budunoglu, H., Yildirm, A., Guler, M. and Bayindir, M. (2011), Highly transparent, flexible, and thermall

stable superhydrophobic ORMOSIL Aerogel thin films,Applied materials and interfaces, 3, 539-545.

Dorcheh A.S. and Abbasi M. A., (2008), Silica aerogel: synthesis, properties and characterzation, Journal o

materials processing technology, 199 , 10-26.

Gurav J. L., Jung K., Park H., Kang E. S. and Nadargi D. Y., (2010), Silica aerogel:synthesis and applications

Journal of nanomaterials, 11,1-11.

Iler, R. K., (1978), The chemistry of silica, John Wiley and Sons, New York, USA.

Kistler, S. S., (1932), Coherent Expeanded Aerogels,Journal of Physical Chemistry, 36, 52-64.

Lee, K.H. and Kim, S.Y., (1995), Low density hydrophobic aerogels,Journal of Non crystalline solids, 186 , 18

22.

Mahadik, D. B., Rao V. A., Rao A. P. and Wagh, P. B., (2011), Effect of concentration of trimethylchlorosilan

(TMCS) and hexamethyldisilazane (HMDZ) silylating agents on surface free energy of silica aerogels, Journa

of Colloid and Interfacial Science, 356 , 289-302.

Ming-Shyong, T. and Wen-Chang, W., (2004), Aluminum nodified colloidal silica via sodium silicate, Materia

letters, 58 , 1881-1884.

Poelz, G. and Riethmuller, R., (1982), Preparation of silica aerogel for Cherenkov counter, Nuclea

Instrumentation Methods, 195, 491-503.

Ulker, Z. and Erucar, I., (2013), Novel nano sturctured composites of silca aerogels with a metal organi

framework,Microporous and Mesoporous Materials, 170 , 352-358.

www.aerogel.org (Retrieved on 16 April, 2013, from hhtp://www.aerogel.org).

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