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Production of a novel X-Aerogel for Ballistics

Shahnawaz Vahora 0508464

B.Eng Mechanical Engineering with Aeronautics

Supervisors: Dr.Phillip Harrison,

Prof. Duncan Gregory Dr. Robert Hughes

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Content List

Abstract Page 3

Introduction Page 4

1. Silica Aerogels Page 5

2. X-Aerogels Page 5

3. Supercritical and Ambient drying methods Page 8

4. Fibre Reinforced Aerogels Page 10

5. Mechanical Testing Page 12

6. Applications Page 15

Aims Page 18

Experimental Page 19

Mechanical Testing Methodology Page 30

Results and Discussion Page 32

Costs Page 41

Conclusions Page 42

Glossary of Chemistry terms Page 43

References Page 45

Appendix Page 48

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Abstract

The aim of this project is to produce and mechanically characterize X-Aerogels

and fibre reinforced aerogels to use them in energy absorption applications and in

ballistic armour. Problems encountered were that crosslinking and substitute process

for supercritical drying, ambient drying were described ambiguously in literature [6]. A

lot of experiments were needed to identify the variables to change and set a standard

procedure. Other problems found were that when procedure did work, the density of

the aerogel was found to be more than double compared to literature value [6] while

Fibre reinforced samples were hard to mould with homogenous fibre matrix

distribution. The final samples produced were characterized through SEM,

compression tests, Vickers hardness test and 3 point flexural test.

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Introduction Aerogels were first made in the 1930s by Samuel S. Kistler. [1] Aerogel is a low-

density solid-state material derived from gel in which the liquid component of the gel

has been replaced with gas. We can essentially summarize them as dried gels. They

are chemically inert and with a low dielectrical constant, with high pore volumes and

are hence extremely lightweight ceramic materials. So far one important application of

aerogels has been as an insulator, this use has varied from basic skylights to NASA’s

pathfinder mission’s sojourner Mars Rover [2]. Other applications vary from

construction of supercapacitors [3] to radiators in Cherenkov Effect [4] to space dust

particle trappers in NASA’s stardust spacecraft. Despite aerogels possessing such

remarkable properties their application has been severely limited due to their weak and

fragile mechanical strength. This has been mainly due to the aerogels brittle and

hygroscopic nature, this leads them to absorb moisture from the environment and

leads to structural collapse due to capillary forces developing in the pores. There have

been many attempts to correct this and utilise their aforesaid advantages, such as:

• Crosslinking Aerogels with Diisocyanate

• Crosslinking Aerogels with Epoxides

• Fibre reinforcing Aerogels with glass wool

• Fibre reinforcing Aerogels with Carbon Fibre Belts.

• Fibre reinforcing Aerogels with Carbon nanotubes

One of the main methods to be looked at to strengthen the Aerogels is cross-linking

the nanoparticle building blocks of silica hydrogels with a polymer (hexamethylene

diisocyanate) [5] and epoxy materials [6]. Another method is fibre reinforcement for

aerogels using glass wool fibres [7]. Recent research done using these crosslinking

techniques show that the aerogels can be made 300x stronger while only being 3x

denser with added properties of being less hydrophilic and flexibility. These properties,

high energy absorption, low density, high insulation, flexibility and stability in standard

atmospheric conditions, combine to put Aerogels in a new light.

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1. Silica Aerogels

Aerogels are the product of a SOL-GEL process, whose final stage involves replacing

the pore-filling solvent with liquid CO2, which is later gasified supercritically leaving

behind a very low density solid. The Sol-Gel process for creating silica Aerogels starts

with mixing a Silica precursor like silica alkoxide Tetramethylorthosilicate TMOS (see

Fig 1) with an Alcohol (methanol), deionised water and a catalyst (Ammonium

Hydroxide). The SOL is created by the hydrolysis [A] and polycondensation [B]

reactions forming silica particles in the solution. While the GEL is created as the

condensation reactions continue and more and more

deposits of silica are formed and the particles can bond

together to form a three dimensional structure. The chemical

bonding between the silica particles is known as covalent

bonding [C] while the physical bonding is due to Van der

Waals forces [D]. The SOL-GEL phase essentially produces a Hydrogel [E]. They only

become Aerogel once they go through the supercritical drying phase. If the Hydrogels

at that stage are left to dry with unhindered shrinkage through evaporation, the

resulting material is called Xerogel. Xerogels may retain their original shape, but often

crack. The shrinkage during drying is often extreme (~90%). [8]

Aerogels can be produced by either one step base catalyzed process [F] or two

step acid-base catalyzed process [G] [1]. Single-step base catalyzed aerogels are

typically mechanically stronger, but more brittle, than two-step aerogels. [1] While two-

step aerogels have a smaller and narrower pore size distribution and are often optically

clearer than single-step aerogels.Once the silica nanostructure is formed all that is left

to do is to dry the structure and obtain an Aerogel. Traditionally that is done by

supercritical drying (see section 3)

2. X-Aerogels

Silica Aerogels made by supercritical drying however are fragile and environmentally

unstable materials (e.g., hygroscopic), thus their practical use is limited to specialized

environments. “The desirable properties of silica Aerogels (low density, low thermal

conductivity, and low dielectric constant) stem from the significant amount of

mesoporosity [H] (see dark areas in Fig. 2a) rendering the bulk mostly empty space

filled with air. For example, it can be calculated based on the skeletal density of

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compact silica that the bulk of typical silica Aerogels with a

density of 0.1g/cm3 consists of 95% empty space”. [9] Its

important to take this important property of mesporosity which is

in a “pearl-necklace” network of microporous, secondary

particles further. During the aging process, the mesoporous

particles are connected by “necks” formed by dissolution and

reprecipitation. The obvious way to strengthen the chemical

structure of such monolithic Aerogels is by making those “necks”

wider. They can be made wider if the surface of silica is used as

the template for deposition and growth of the inter-particle cross

linker. This is done by surface terminating with silanols (SiOH)

and letting the secondary particles crosslink with Di-Isocynate [I]

(see Fig 3). [4]

The crosslinking process occurs by preparing Silica hydrogel monoliths from

the SOL-GEL process. Once they are aged for 48 hrs they are washed successively

with ethanol and then acetone at atmospheric pressure. A diisocyanate solution is then

prepared containing Desmodour N3200 in acetone 50% w/w [J] and the hydrogel is

placed in the solution oven cured at 53◦C for 48 hrs submerged in the isocyanate

solution. At the end of the period, gels are washed with acetone and are then dried

either with supercritical CO2 or by a hydrocarbon solvent such as pentane under

ambient conditions. [5]

The Di-Isocynate process (see Fig 4) then results in the production of tethers between

the aerogel nano-particles. It was shown that this resulting aerogels could be up to 3

times denser than aerogels based on the underlying silica framework, but up to 300

times stronger in terms of the force it takes to break them in a three-point bending test

configuration [10]. It was also discovered that the polymer accumulated on the surface

of the nanoparticles is not polyurethane throughout. It can be seen in fig 4 that the

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silica reacts with the cross linker

(isocyanate) to produce

carbamates. Specifically it is

made when the hydrogen atom

forms a bond with nitrogen atom

which breaks its double bond

with the carbon atom and the

silicon and oxygen atoms break

away from hydroxide group to

join with vacant bond left in the

carbon atom. Carbmate then

further reacts with water to

create NH2 an amino group and

carbon dioxide. This carbamate molecule then containing the amino group now further

reacts with the cross linker to create polyurea [K] chains which form the tethers. Since

cross-linking is mostly polyurea-based, more control of polyurea formation throughout

the crosslinking process may produce even stronger Aerogels. [10]

Another approach that has shown success and which builds on crosslinking with

diisocyanate is the production of robust, flexible aerogels by

surface modification of the skeletal silica nanoparticles with

amines [L] and then cross linking it with epoxides. This has been

done by co-gelation of tetramethoxysilane (TMOS) and 3-

aminopropyltriethoxylsilane (APTES), in which the resulting gels

have been cross-linked successfully with epoxides and

polystyrene. [6] It is important to note that tri-epoxy (see fig 5)

works the best because it has an increased reactivity with APTES

modified silica aerogels compared with Bi-Functional or Tetra-

Functional epoxides and produces similar pearl neckless structure

with large mesopores as the dissocynate crosslinker with added benefits of flexibility,

elasticity and retaining higher porosity than their isocyanate counterparts. [6]

The standard procedure (summarised in Fig 6) for creating epoxy cross linked aerogels

starts out just like diisocyanate cross linked aerogels by creating hydrogels through

combination of solution A and B which contain the same solvents (Methanol, TMOS,

Ammonium Hydroxide and deionised water) to undergo the Sol-Gel process. One

substitution that needs be done compared to the process described in the diisocyanate

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crosslinking, to prepare APTES modified

aerogels is the substitution of APTES 25-50%

of v/v [J] with TMOS in Solution A. This

eliminates the need for Ammonium hydroxide

as the catalyst, as now APTES acts like the

catalyst. APTES is important to incorporate

into the hydrogels as it prepares the surface

for epoxy functionality. Because APTES is

present in such high quantity the solutions

actually need to be cooled down so that the

gelation process is slowed down enough and

the solution can be placed in appropriate

moulds. The moulds are then covered and

aged for a 48 hour period after which they are

washed successively by Methanol (4X, 12

hours) and then THF (4X,12 hours) [M]

THF is used for its better mixing solubility with epoxides. It is found that

trifunctional epoxides (N,N¢-diglycidyl-4-glycidyloxyaniline) produce mechanically the

strongest samples. Therefore a solution is prepared which contains trifunctional

epoxides solution in THF and the hydrogel monoliths are submerged in them for 48

hours. Once the mixing is done, the solutions containing the hydrogels are then oven

cured for 44hours at 74C. Once they have been oven cured, they need to be washed

successively with THF (4X,12 hours) and then Acetone (4X,12 hours) and sent in an

autoclave for supercritical fluid extraction to finally produce epoxy cross linked

Aerogels [6].

3. Supercritical Drying and Ambient Drying

Supercritical drying is a process in which liquid can be removed from a gel

without causing the gel to collapse. Supercritical fluids behave like liquid and gas

simultaneously and usually occur at high pressures and high temperatures. [1]. As

can be seen in fig 7, they occur at the given critical point of a substance where high

pressure and temperature combine to make a supercritical fluid. Supercritical fluids

expand like gases, but have density and thermal conductivity closer to liquids.

Supercritical fluids also have lower surface tension than liquids which is what prevents

liquid-vapour meniscus [N] from occurring and hence preventing the capillary forces to

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collapse the pores. The Super critical process can be performed by prior solvent

exchange with CO2 followed by supercritical venting. The Hydrogels are placed in the

autoclave (which has been filled with alcohol (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 alcohol 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. Once this process is complete the Aerogels are made. [1]

With added robustness in crosslinked aerogels it is now also shown that we

could substitute the expensive Supercritical Fluid Extraction method with drying with

hydrocarbon solvents at ambient temperature. [5] It has been shown that for this

purpose hexane and pentane have had the best results. In particular drying it with

pentane has shown to result in aerogels that are both microscopically and

macroscopically identical to their SCF-CO2 dried counterparts. It was concluded that

dimensional stabilization gained by crosslinking enables the Aerogels to undergo the

surface tension forces of pentane and avoid intercapillary collapse. Once the

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hydrogel’s are subjected to pentane washing, they are also oven cured at 40◦C, this is

done to subject the hydrogel’s to the highest temperature they would encounter during

SCF-C02 drying process. This gives the microstructure a chance to solidify and gain

strength without any intercapillary forces acting on it. [5]

Their have also been other methods to replace the supercritical fluid drying

method. The Lawrence Livermore National Laboratory (LLNL) group has invented a

rapid supercritical extraction process, which is similar to injection moulding and is most

suitable for the production of monolithic and large pieces of aerogels. In this process

the sol is injected into a two-piece sealed high pressure steel mould which—after

injection—is rapidly heated. During the rapid heating phase the sol gels and ages.

Strains of the gel network upon heating are prevented by the mould. Once the liquid,

which is under high pressure, becomes a supercritical fluid, it is rapidly decompressed.

The whole process can be done in as little as 1 hour and large pieces of aerogels can

be made. One other means to prevent the gel collapse, quite similar to pentane drying

is done by Einarsrud et al [11], who aged their wet gels in an alkoxide/alcohol [O]

solution; they thus obtained addition of solid material to the necks and small pores of

the gel stiffens the microstructure considerably. The wet gels were then strong enough

to withstand the capillary pressures upon drying. Aerogels with densities as low as 240

kg/m3 have been made using this method. [11]

4. Fibre reinforced Aerogels

There hasn’t been much any research done to the best of the author’s

knowledge in fibre reinforced cross linked X-aerogels. As such a particular way of fibre

reinforcing which is effective with cross linker solution hasn’t been identified. It has

been shown that out of all reinforcing materials the greatest fibre reinforcement

provided in terms of overall mechanical strength was through Kevlar closely followed

by carbon and then glass. [12] But since aerogel is a silicon based material, a

prerequisite for the fibre reinforcement will be material which will readily form strong

bonds (covalent, van der waals) with the microstructure in aerogel. In that regard, glass

wool which is based on silica, readily forms bonds with aerogel and forms a type of

fiber matrix structure which significantly reinforces the microstructure. [7]

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Fiber matrix (see fig 8) plays a role of

compartments containing aerogels, which

supports aerogels and decreases the bulk size

of aerogel within aerogel–fiber matrix

composite. Flexible aerogel composite is

comprised of fiber matrices which have 90%

pores containing silica aerogels with more than

95% porosity. This fibre reinforcement process

mainly revolves around the surface treatment

method of the sol-gel process using TEOS (Tri-ethyl-orthosilicate) and Sodium Sillicate

[P] to synthesize the Aerogels. First the glass wool is immersed in Aerogels, after

which they are aged and surface treated using isopropylalcohol [Q], TMCS

(trimethylchlorosillicate) and hexane and then subjected with heat treatment. [7]

The results derived from those experiments showed that the microstructures of

aerogel composites like pore size and specific surface area were greatly affected by

precursor sol. Aerogel composites made of only colloidal [R] silica showed greater

density and lesser porosity, smaller pore size and specific surface area than those of

aerogels composites synthesized from colloidal and TEOS-based mixed sols. Aerogel

composites from mixed silica sols of colloidal silica and TEOS-based silica sols

showed smaller density, higher porosity, decreased thermal conductivity and greater

surface area. This showed that rugged flexible silica Aerogels could be made from a

colloidal silica sol and TEOS-based sol and dried at ambient pressure without the need

of SCF extraction. [7]

One other interesting approach shown to reinforce Aerogels has been through

Carbon nanotubes. The nanotubes fulfil one important aerogel precursor “they form

electrically percolating networks at very low volume fractions and elastic gels in

concentrated suspensions through van der Waals interaction mediated cross-linking”.

This allows carbon nanotubes to form physical bonds with the aerogel’s microstructure.

It is unclear as to what treating aerogels with poly vinyl alcohol (PVA) does to aerogel’s

microstructure but it was found that aerogels could support at least 8000 times its own

weight once treated with PVA and then reinforced using carbon nanotubes.

Furthermore it was also found that once reinforced with carbon nanotubes they can

also be reinforced further using polymeric fluids. However, the applications to these

carbon nanotubes aerogels remain mainly in exploiting its excellent electro-conductivity

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and as such in applications such as sensors, actuators, electrodes and thermoelectric

devices. [14]

Another way shown to reinforce aerogels has been with Ceramic Fibre Felts

[S]. It has been researched before that the aerogels can be strengthened by dispersing

particulates or short fibers in the silica sol, and casting the sol before gelation.

However problems were encountered when their was too much shrinkage in those

monoliths. It has been shown that dry ceramic fibre felt preforms can be used in the

synthesis process and weak silica aerogels can efficiently be reinforced with both silica

and carbon felt. Hydrophilic silica felt interacted more efficiently with silica aerogels

carrying a low proportion of hydrophobic groups, while a hydrophobic carbon felt

binded more effectively with Aerogels that carried a high proportion of hydrophobic

groups. It was also shown that these particles were shown to be very elastic and could

easily be recycled without significant loss of either mechanical integrity or catalytic

activity. [15]

5. Mechanical Testing

Mechanical strength is the most noticeable property enhancement through the

cross linking and therefore it’s important to characterize it and compare it with similar

materials. The characterization has been through dynamic mechanical analysis of

aerogels including compression testing and flexural testing [10]. Compressive

behaviour of X- Aerogels at high strain rates has also been examined. Table 1.0 from

ref [10] summarises typical values found when a compression test is done on cross

linked aerogels.

From the data collected it

was determined that the modulus

at room temperature is 611 MPa.

Since the samples have a low

density, typically a third (or less) of

the density of engineering

polymers such as polyethylene and polypropylene, it is concluded that cross-linked

silica aerogels have a comparable or higher specific modulus than engineering

polymers. It was also found that these values also varied considerably with

temperature. At elevated temperatures (210C) the samples shrank and the modulus

decreased steeply with higher temperatures. [10]

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The compressive strength data (see table 1) show that cross linking the aerogels has

increased failure strain by a factor of 13.5 and increased compressive strength by a

factor of 45 compared to normal aerogels. While the specific compressive yield

strength of these samples is found to be much lower than typical values of structural

materials, the specific compressive stress at ultimate failure is reasonably high and is

comparable to poly(methyl methacrylate) (PMMA)20 and Kevlar-49 epoxy composites.

[10]

The flexural test showed that the flexural modulus could be taken as 167 MPa

at room temperature for cross linked aerogels. This value found was doubled under

cryogenic temperatures. This test was also done at various temperatures and the link

between density and modulus showed that an effective way to increasing strength is

adding polymer to the underlying silica structure rather than adding more silica. [10]

One important variable

which is true with all the

mechanical tests on Aerogels is

that they become almost two or

three times stronger at cryogenic

temperatures. This is especially

true for Vanadium treated X-

Aerogels. Vanadium (VOx)

aerogels consist of entangled 100-

200 nm long 30-40 nm thick

wormlike substance. It can be

concluded that vanadium provides another type of fibre reinforcement (see Fig 9 A/B).

However what is remarkable for Vanadium treated X-Aerogels (X-Vox) is that while

they possess similar mechanical strength at ambient temperature they get stronger by

nearly 4 times at cryogenic temperatures. They have been identified as potential

containers of cryogenic fuels like hydrogen or landing gear of space capsules on

cryogenic planetary surfaces. [16]

Another important thing to note when mechanically characterizing fibre

reinforced aerogels is that while some fibre-reinforced aerogels do show reduced

shrinkage during the production, which result in a lower matrix density; both hardness

and compressive strength tend to decrease with an increase in fibre reinforcement. [7]

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This is also true for in plane random fibre impregnated into any substance. The fracture

characteristic and work of fracture entirely depends upon degree of dispersion and

orientation. Wherever the fibres are held together in tightly packed bundles, they will

show higher failure strain while the local failure mechanism depends upon orientation

of each reinforcing unit. It can be predicted that the failure in transversely oriented

bundles will occur first while fibre fracture in bundles oriented normal and parallel to the

applied stress is deemed unlikely. Having said that, there is still no entirely satisfactory

theory to account for strength of in plane random fibre materials. [12]

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6. Applications

NASA has been using Aerogels for a variety of its experiments for many years. In the

stardust project, its unique porous structure has been utilized. It was used as a capture

media where it collected very small interstellar and cometary particles as they embedded

themselves in the porous aerogel. The galactic particles, ranging in size from 100 nm to

10 µm and travelling between 0.5 and 10 km/s, pass easily through gasses but vaporized

on impact with liquids or ordinary solids. Because aerogels have greater strength then air

but are far less dense than regular solids, they can slow the dust particles gradually and

capture them. Aerogels also preserve the tracks made by the particles, making it easy to

isolate each particle for study after the aerogels are returned to earth. [2]

If production costs can be reduced further, one of the larger potential

applications of silica aerogels lies in thermal insulation, especially in houses and offices

Silica aerogel has a thermal conductivity at room temperature of 20 mW/m•K, which is

lower than the thermal conductivity of air, 26 mW/m•K. The ultrashort path lengths of air

molecules in the aerogel pores inhibit conduction through the air, and the tiny width of

the silica filaments greatly limits conduction through the solid. These characteristics led

NASA to adopt aerogels as insulation material in space suits. [3]

Similarly, another application by NASA has been Aerogels use in the Pathfinder

Sojourner Mission on Mars (see fig 10). This time its lightweight and insulating properties

were used. Specifically its thermal insulation property was used to protect the sojourner.

It was calculated that the use of Aerogel not only produced better structural integrity and

thermal insulation performance but also reduced the weight by 2.5kg (a saving of 27%).

[2]

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Another interesting application is the use of aerogels as moulds for alloy

casting. Due to the excellent thermal insulation, a nearly one-dimensioned cooling and

solidification process is achieved. The high optical transparency of the aerogel moulds

allows monitoring of this process via IR- and video techniques. [17]

Opacified silica aerogels are most suitable as thermal superinsulants. The

integration of infrared absorbers, such as carbon black reduces the radioactive heat

transfer through the tenuous system efficiently. Additional evacuation leads to a

reduction of the thermal conductivity to about 0.003 W/mK for powdered aerogels. The

German car maker BMW is building aerogel superinsulated heat storage devices, which

are loaded with the waste heat of the engine. The storage can be used to defrost the

wind shield and preheat the engine on cold winter days. [17]

Carbon aerogels have been discovered relatively

more recently than silica aerogels, but they also have

significant properties and promising applications. One

such property is “extreme blackness”, caused by internal

scattering and absorption of light by the graphite

molecules that make up the carbon aerogel (see fig 11). In

a broad wavelength range from 0.25 to 14.3 µm, the

reflection coefficient of carbon aerogels is only 0.3%. This

makes the material ideal for some forms of solar-energy collectors. [3]

One of the most interesting and futuristic application found of Carbon aerogels

is the exploitation of its porous structure. Nitrogen doped carbon Xerogel’s are now

being investigated as future carriers of liquid hydrogen. It has so far been found that

these Xerogel’s have been able to satisfy the many pre-requisites of hydrogen carrying

capabilities such as volume, weight and realistic kinetics for charge and discharge of

hydrogen. [18]

Carbon aerogels also have a special property that of being a good electrical

conductors, which allows their use in supercapacitors. Because carbon aerogels have

huge surface areas per unit mass or volume and tiny pores, researchers have achieved

capacitances as high as 104 F/g and 77 F/cm3. Companies such as Cooper Electronic

Technologies (Boynton Beach, FL) are already producing aerogel supercapacitors.

Operated at up to 2.5 V, such supercapacitors can store energy at a density of 325

kJ/kg, about 70% that of the most advanced lithium-polymer batteries, now in

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development but years from commercialization. Power densities are more remarkable,

as high as 20 kW/kg, or 700 times more than the 30 W/kg of lithium ion batteries. At

present, small aerogel supercapacitors are used in electronic equipment, but in the

future, they could prove suitable for higher-voltage and higher-power applications, such

as electric vehicles.[3]

Commercial public wide stream use found for aerogels has been in high quality

tennis rackets developed by Dunlop (see Fig 12) and Babolat. They claim that the

aerogel’s “nanometer-sized molecular network delivers an unmatched strength to weight

ratio for enhanced stiffness and increased power”. Again the main quality gained by

Dunlop using aerogels in their racket is reduced weight, which they claim will offer more

control to the players and hence more power through the racket. [19] Another company

called Saloman, which makes snowboarding boots uses Aerogel as material which will

thermally insulate the boot and reduce the weight. [20]

In industrial mainstream terms, a company called Aspen Aerogels specialises in

making flexible, thin, durable thermal blankets made out of aerogels. They claim that

their products offer up to 8 times more effective solution than traditional insulating

materials. [21] Cabot Corporation meanwhile specializes in their own family of aerogels

dubbed the “nanogels”. They use it in architectural day lighting, in which their products

are not only more thermally insulating then glass but also offer high quality diffused light

and noise reduction. They also use its insulating properties in oil and gas pipelines,

industrial and cryogenic plants and vessels and in outdoor gear and apparel. They have

even found use of Aerogel in personal care products to “increase absorption, create

matte effects and control fragrance delivery in a variety of applications”. [22]

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Aims

The standard cross linking and ambient drying procedure from the literature is

to be characterized and specified so that it’s unambiguous. Different variables (see

Appendix B for variable matrix) in the standard procedure such as the list below are

changed:

• the cross linker type,

• cross linker concentration

• crosslinking solution quantity

• different types of agitation

• different rate of agitation

• cross linker washing time

• oven curing temperature

• oven curing time

• sealing during oven curing time and while drying at ambient conditions

• concentration of pentane for ambient drying

Furthermore a combination of the cross linking procedure and fibre

reinforcement procedure from the literature is also to be used to produce low density,

mechanically strong and flexible X-Aerogels. A number of issues which were faced by a

previous student [23] are to be addressed such as upgrading the storing and sealing

devices for the long production phase, better diffusion moulding, and stable ambient

drying method. Once the standard procedure is identified the samples are to be

mechanically characterized using various methods such as compression tests, Vickers

hardness test, 3 point flexure test and using the Scanning Electron Microscopy.

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Experimental

The Standard Procedure - for creating X-Aerogels by crosslinking hydrogels with a

polymer (diisocyanate) as set from literature [5]

“Tetramethoxysilane (TMOS), ammonium hydroxide and all solvents (HPLC grade) were

purchased from Aldrich. The diisocyanate crosslinker (Desmodour N3200) was donated

by Bayer Corporation. Silica hydrogel monoliths (∼1cm in diam.,∼4cm in length) were

prepared from TMOSvia a base-catalyzed route according to published procedures, let

stand (aged) for 48 h, were washed successively with ethanol (20 mL, 1×), acetone (4×,

8 h, 20 mL each time), a diisocyanate solution containing Desmodour N3200 in acetone

50% w/w (1×, 24 h, 20 mL), and were oven cured at ca. 53◦C for 48 h submerged in the

last isocyanate solution. At the end of the period, gels were washed with acetone (4×, 8

h, 20 mL each time) and were dried either with supercritical CO2, or were subjected to

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four washes with pentane (8 h, 20 mL each time) and dried in a 40◦C oven under

ambient pressure. The preparation procedure is summarized in Fig 13“ [5]

The procedure described in the previous page was set as standard and the experiments

started. As experienced by previous student [23] it was found that following this

procedure exactly didn’t produce desirable results and as such various variables were

changed until a procedure was found that produced X-Aerogels. In the following section

all the experiments which produced batches 1-11 are detailed. A standard procedure

which worked for the author is also detailed at the end of the experiments. Various

stages from that new standard procedure are referred during the experiments.

Materials Tetramethylorthosilicate (TMOS), Ammonium hydroxide (NH4OH) and

Pentane (C5H12), DuraSeal (roll size 5 in. × 150 ft) were purchased from Sigma-Aldrich

and used as received, Methanol (CH3OH), Ethanol (CH3CH2OH), deionised water,

Acetone (CH3COCH3). Desmodur N3200 was obtained courtesy of Bayer Corporation.

Knauf Space Blanket Glass Fibre wool was purchased from B&Q.

Equipment

The 14ml plastic narrow test-tubes

were used containers for later

stage washings and were

approximately 1cm in diameter

and 12cm long although they were

tapered towards the bottom. The

wider 45 ml test-tubes were also

12 cm long and were tapered

towards the bottom but 3 cm wide.

The glass test-tubes used for

moulding were 7cm long and 1 cm

wide. (See fig 14)

Experiments

The first batch was produced using the standard Sol-Gel process described in the

literature. Health and safety assessment was carried out, which examined potential

- 21 -

hazards and detailed safety measures needed to carry out these experiments. A Gantt

chart was also created to monitor the progress. (See Appendix A)

Batch 1 Oven Control

Solution A consisted of 15.2ml TMOS and 18ml of methanol, solution B was

18ml methanol, 6ml deionised water and 0.8ml ammonium hydroxide. It was left to be

magnetically stirred at fast agitation for 15 seconds and then poured into the plastic tube

moulds which were labelled 1 A-E. After successively washing it with ethanol and

acetone it was left in the crosslinking solution (50-50% acetone and diisocyanate). At

that stage it produced brown colour in the oven during the cross linking (stage 5). The

samples were then left in the oven at 55◦C to cure. An oven malfunction during this

phase led to the temperature going up to 150 °C. Th is melted everything including the

moulding plastic tubes and the Aerogels. It was found that the brown discolouration was

caused because of ethanol not washing the Hydrogel’s monoliths properly [6] because

part of it was inaccessible for washing. Thus the idea of moulding it with open sided test

tubes was introduced.

Batch 2 Effect of Open sided test tubes

The second batch was made using the open sided test tubes

(see Fig 15). Once the sol-gel process occurred and the 48 hour ageing

process occurred, the samples 2 A-E were pushed out of test-tubes into

bigger plastic moulds and left in ethanol to be washed. This process let

the liquid get round to all part of the gel and wash it properly. Brown

discolouration was not seen after that. Sample 2E, was left at ambient

conditions for too long when acetone ran out and replacement took 15

mins. This caused the sample to disintegrate, due to prolonged

exposure to moisture and hence increased capillary forces. The oven

malfunctioned again, this time temperature going up to 75°C and

damaging all the samples, turning them into Xerogels. Since this was

the second successive time the samples were damaged by the oven. It

was decided that the digitally controlled Special Operation Laboratory

ovens would be used from now on. Another idea of sealing the samples with DuraSeal

during curing (stage 5) was to be introduced from now on to prevent acetone fumes

escaping. It was also noted that the open sided test tubes idea was a success and will

now be used henceforward.

- 22 -

Batch 3 Effects of different types of agitation and preheat curing

Batch 3 was again produced using standard Leventis procedure [5]. But this time

few enhancements were introduced compared to batches 1& 2 to increase closeness to

the Leventis procedure. In the journal [6], the samples when washed were agitated

frequently. Because there was no means for timed frequent agitation, the

idea of constant agitation was introduced to see if it would make a

difference. Sample 3F was put in an ultrasonic bath for constant

agitation for an hour while sample 3G was put in wider test tubes under

constant magnetic stirrer. Samples 3 A-E were also cured at 30°C at the

ethanol/acetone washing stage to see if they would quicken the ageing

process. While this was a new innovation simply to see if it quickened

the whole procedure and it was not described in Leventis process. The

rest of the process though, was standard. It was found that after 24

hours sample 3F undergoing ultrasonic treatment structurally

disintegrated into a jelly and therefore discarded. The preheating curing process was

also discarded since there was almost 40-50% shrinkage and structurally the aerogels

were extremely weak. It was also thought that since the samples were very transparent

(the first indication of cross linking is samples going translucent) the heating had caused

the samples to micro structurally collapse and prevent cross linking (see fig 16). Sample

3G which was undergoing constant magnetic stirrer agitation was also found to have

been macro structurally damaged by the constant spinning magnetic bar. The constant

agitation idea as a whole was now discarded by either ultrasonic or magnetic stirrer

methods. Frequent timed agitation idea though was not totally discarded and thought of

to be put in future batches as an enhancement scope.

Batch 4: Effects of Aging time

Batch 4 was produced again using the standard sol-gel Leventis procedure

except for some forced/circumstantial periods of inactivity and therefore added ageing

times. The procedure was going on exactly as prescribed in the standard process until it

came to curing in the oven at elevated temperatures. Due to the unavailability of the

special operation lab’s digital oven the samples were made to wait for 10 extra days

under the cross linker solution. Under this extended period where the samples were kept

under the cross linker, sample 4D did a curious thing. The solution in the test-tube had

turned cloudy and the sample white. 4D (see Fig 17) had clearly cross linked, curiously

at room temperatures. After that aberration the standard procedure was again followed.

- 23 -

The final result was that 4D had shrunk less than the rest of the samples and found to be

much more environmentally stable and mechanically

stronger. The density was found to be approx 0.92 g/cm^3

which was almost double the suggested density in papers

so even though there was some success with cross linking

the sample was deemed too dense to be a true X-Aerogel.

Since the cross linking occurred with added time of the

sample in the cross linker, it was thought of as variable to

change in the next experiments. The cross linker solution

in which the 4D was produced was also examined; and it

was found that the solution left was denser as the acetone in the solution had been

evaporated during the crosslinking. Concentrations of desmodour/acetone were now

also thought off as possible parameters to change in the next batches.

Batch 5: Effects of Curing times and concentration

Batch 5 was produced using standard procedure but varying new parameters

such as curing times and concentrations of desmodour/acetone as well as due to a

shortage of time because of the approaching Xmas holidays a fast method advocated by

Zhang [2] The faster method is standard except for only 1 washing of acetone is advised

after the initial ethanol washing, saving 3 days. The aerogels after the quick wash by

acetone and ethanol, were left in the varying concentration of acetone/desmodour at

room temperature for 4 days and then cured at 55-60 °C in the oven for 5 days. There

was no time left for the 4 pentane washing of ambient drying phase , so they were all left

in pentane after the cross linking phase for the entire Christmas break period/exam

period (20 days). When they were checked after the holidays, all samples had cracked

and shrunk except for sample 5A which had a distinct white colour, was very strong and

had clearly cross linked. It was found to have shrunk by 40% though with a density of

1.15 g/cm^3, almost three times the density suggested in the papers. With the cross

linking success of 5A it was now clearly established that higher concentration (66-34 w/w

of desmodour/acetone) and longer curing times were clearly a factor. Another thing that

was different with 5A was that a wider test-tube then normal was used for it, this led to

the conclusion that quantity of the cross linker was also a factor. The high density was

attributed to the fact that all 4 pentane washings and heat treatment weren’t completed

and that led to the water being trapped into the pores and increasing the density.

- 24 -

Batch 6/7: Effects of test-tube sizes, temperature/time to age/cure, fibre reinforcement

Batch 6 was produced using standard procedure but with different parameters

changed such as test-tube sizes (see fig 14), desmodour/acetone concentration, time to

age/cure, temperature at which to age/cure. Meanwhile batch 7 was also produced at the

same time but using glass wool to create the fibre reinforced X-Aerogel batch. Zhang’s

fast procedure which only uses 1 acetone washing was also used. Unfortunately the

special operations oven broke this time, to elevate the temperatures to 200C to destroy

most of the sampled by melting the test-tubes onto the aerogels. 6B was an exception

which was kept out of oven for a longer diffusion time with cross linker. 6B was then

found to have cross linked at a higher concentrated solution in a wider test-tube at

normal prescribed temperature. The density was also found to be lower this time to 0.75

g/ cm3. The concentration (66-34 w/w of desmodour/acetone), temperature (53C) and

the use of wider test-tube were now set as the normal parameter to be used from now

on. The density was still higher than normal and that was the issue to be addressed from

now on. In previous research, [1] it is advocated that using more alcohol washes

reduces density as it produces stronger microstructure which helps flush out the excess

water from the hydrogels during the ambient drying phase. It was now decided that

Zhang’s fast procedure was to be discarded and normal 4 acetone washes to be

introduced back to see if they reduced the density.

Batch 8: Effects of frequent timed agitations and different cross linker on X-Aerogels

Batch 8 was produced with the set normal standards taken from the successful

6B with some added different parameter changes such as frequent agitation using a

timed adaptor on magnetic stirrer as well using a different cross linker type (N3300 see

Fig 18). The frequent agitation was introduced to cut back production time since it helps

samples crosslink faster by depositing layers of cross linker on the mesoporous structure

during the timed agitation as well as making the microstructure stronger by applying

those different layers on the structure. [4] The different cross linker variable was

- 25 -

introduced to see if a different chain from N3200 (diisocyanate) to N3300 (tri-isocyanate)

would produce lower density Aerogels. All the samples had cross

linked and found to have a density between 0.65 and 0.68 g/ cm3.

It was concluded that using the N3300 cross linker which has a

different chain then the N3200 produced lower density samples

along with using frequent agitation variable. Both the variable

change were deemed as success (see fig 19) and to be used from

now on as normal. Since a set frequent agitation time wasn’t

advised anywhere and the one used in this batch was a random

guess 15 min agitation every 2 hour for 24 hours, it was decided

that another agitation time should be used in the next batch just to

see if it enhances the samples.

Batch 9: Effects of frequent timed agitations and different cross linker on fibre reinforced

X-Aerogels

The same variables which were applied to batch 8 were also applied to batch 9

which was put into production at the same time as batch 8 and which was to be

produced using glass wool to create the fibre reinforced

X-Aerogel batch. It was hard to homogenously

distribute the glass wool along the samples, so the final

samples once dried under ambient condition were

found to have fibre reinforced in some parts and just

being normal X-Aerogel in other. It was noted that the

samples with glass wool around them had shown

absolutely no shrinkage at all, and were found to be

with really low density (approx 0.4 to 0.5 g/ cm3) which

is exactly the same as described in the papers. The

samples also didn’t appear to be flexible at all as it was

hoped for, as the cross linker was quite stiff all round

the glass wool. Another important thing to notice was

that sample hadn’t stayed in their mould at all, and had taken its shape in a smooth

zigzag cylinder (see fig 20). It was thought that this was due to the non-homogenous

distribution of glass wool. The glass wool only being applied in some parts made the

structure heavier in that particular part and let the cylinder bent outwards due to the

added weight and lose its perfect cylinder structure.

- 26 -

Batch 10: Effects of increased rate of agitation

Batch 10 was produced using the variables taken as the standard from the

previous batches excluding the rate of agitation. This time it was thought to speed up the

agitation process by having 15 minutes agitation every hour for 24

hours. This did a curious thing, after the agitation process was done

and the samples had undergone 53C heat treatment in the oven the

cross linker solution turned a shade of yellow. But after the samples

were taken out of it and put under acetone, the samples showed no

yellow discolouration and after the ambient drying process turned

white. A ramped up rate of agitation also seemed to have done the

trick in terms of a macro-structurally good sample (see Fig 21). The

samples showed less than 5% shrinkage and had a density of around

0.66 g/cm^3. The sample was also deemed good enough to be able

to proceed for mechanical testing and all 4 test samples from batch

10 were sent for various tests. It was now decided that only a lower density was now

desirable and therefore the latest batch will try to incorporate that.

Batch 11: Effects of using higher concentration Pentane

Batch 11 was produced using all

the set variables gained from the

previous batches including the ramped

up rate of agitation in last batch. The only

variable to change in this batch now was

the quality of pentane. The pentane used

in all the previous batches was 98.1%

pure, while the pentane used in literature

[6] is 99.8% pure HPLC grade pentane.

Ambient drying phase is what determines

the density of the sample, specifically

there is a clear link between the size of

the average pore diameters of the samples and surface tension of the ambient liquid

under which the samples are kept. The lower the surface tension the larger the average

diameter of the pore size and hence lower the density (see fig 22). Using the new

pentane which has a lower surface tension then the old pentane brought the density

- 27 -

down to 0.62 g/ cm3 which was a 7% improvement on previous densities. Overall,

compared to the density described in the papers it was 92% accurate. [5] It was also

found that using a ramped up rate of agitation this time produced macroscopically weak

samples (hairline cracks around monoliths) and performed poorly in mechanical tests. It

was decided that, the increased rate of agitation should be put under review and as a

variable change for next batches.

New standard procedure – for the creation of X-Aerogel which are cross linked with a

polymer. Includes variables changes which were looked at in different experiments

detailed above.

Stage 1

Solution A is created which consists of 15.2ml TMOS and 18ml of methanol. In the Sol-

Gel process, the silica precursor Silica Alkoxide which is found Tetramethylorthosilicate

TMOS is essential for the solution (SOL) to form.

Soultion B is created and consists of 18ml methanol, 6ml deionised water and 0.8ml

ammonium hydroxide (catalyst)

Stage 2

Solution A and B are mixed together on magnetic stirrer on fast setting for 3 mins. (any

longer and the gel will form in the mixing beaker). Once they are mixed together properly

they are poured for moulding into the two way test-tubes which are sealed on both sides

by DuraSeal. Once sealed, they are then left to age for 48hours at ambient temperature.

In this ageing process the real Sol-Gel process occurs. The TMOS reacts with deionised

water to form Silicon Oxide and Methanol.

Si(OCH3)4 (liq.) + 2H2O (liq.) = SiO2 (solid) + 4HOCH3 (liq.)

The solution (SOL) is created by various reactants that are undergoing hydrolysis and

polycondensation reactions forming silica particles in the solution. While the GEL is

created as the condensation reactions continue and more and more deposits of silica

are formed and the particles can join together with covalent bond to form a three

dimensional structure. The SOL-GEL phase essentially produces a rigid substance

called a Hydrogel. It consists of solid and liquid parts. 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.

- 28 -

Stage 3

Once the ageing process is done the hydrogels are pushed out of the open sided test

tubes into a wide plastic test tube and washed with ethanol (1X, 24 hours). Since the

hydrogels are quite fragile at this point they are washed extremely carefully with ethanol

and stirred very gently. This is done to get rid of the catalyst Ammonium Hydroxide.

Stage 4

After washing with ethanol they are washed with acetone (4X, 24 hours) to get rid of

excess methanol, ammonium hydroxide and ethanol. The standard Leventis procedure

says 4 washing every 8 hours should be done. While care is taken to be as close to 8

hour washing procedure as possible, it is just the way the Leventis group decided to do

its experiments, another group does it at 12 hours. There is no scientific reasoning

behind it (i.e. a specific half-life and microstructure changing precisely at 8 hours). Due

to lab access issues, it was decided to do the washes every once every 24 hours and it

was found that it worked just as well.

Stage 5

Diisocyanate/Acetone (66-34%) solution is prepared by thoroughly mixing together on a

magnetic stirrer at the fastest setting for 15-20 mins. Then the hydrogel monoliths are

submerged under the diisocyanate solution. At first the hydrogels float because they

have a lower density then cross linker solution but as the solution infiltrates through the

pores it submerges under the solution. Frequent agitation is required at this stage.

Experiments have been done with agitation occurring every 2 hours for 15 mins using a

timer. The samples are agitated thus for 24 hours using the lowest setting of magnetic

stirrer and smallest magnetic bar (8mm) to preserve its macroscopic structure. Sealing is

not important at this stage.

Stage 6

The monoliths which are still submerged under the cross linking solution are now

transferred to an oven and cured at 55 C. It is at that specific temperature because

acetone boils at this point and is evaporated off. Leaving behind pure diisocyanate which

reacts with silicon oxide, to create polyurea chains to widen the neck. It is left in the oven

for 72 hours. If the cross linking has occurred the solution turns yellowish the samples

initially transparent become translucent. Sealing using DuraSeal is extremely important

at this stage because a correct balance must be stuck between letting the acetone fume

- 29 -

off and still keeping the samples underneath the solution while making it last for 72

hours.

Stage 7

Once crosslinked the remaining diisocyanate is washed off using acetone (4X, 24

hours). They are sealed using test-tubes at this point.

Stage 8

The monoliths are then subjected to washings of HPLC pentane (4X, 8 hours). This is

the substitute process for supercritical drying and is shown to be equally effective.

Stage 9

The monoliths are now left in the oven for 48 hours at 40C. This is the highest

temperature they would have faced if they had gone through the SCF-CO2 process.

- 30 -

Mechanical Testing - Methodology

Once batch 10 had produced mechanically test capable samples, a slot was

booked in the materials laboratory and the samples underwent various tests. The first

test to undergo was the compression test.

Sample sizes and strain rate were set

from various literature which had had

undertaken similar tests. [10] Figure 23

shows the test setting. 23A shows the

sample intact and ready to undergo the

test while 23B shows the sample

compressed and having undergone the

test. Before the test was undertaken a few

steps were taken such as polishing both

the cylinder surfaces so that both the surfaces were flat. This was important so that when

the samples are undergoing compression, the sample don’t slip out. A protective

transparent sheet of glass was also put on both sides of the tests since the exact

brittleness of the samples wasn’t known and the sample may break and fly in all

directions.

The compression machine was connected to the computer and the data was

analyzed using Zwick Roell software. A curve showing pressure and percentage

compression was produced through the software for analysis. The test was repeated

with similar length specimens as well as differing length specimen to get various values.

Vickers hardness test was done using Wickers Wolpert

Vickers video flar system (Fig 24) The way this system

works is that a weight is chosen and it indents a diamond

mark (Fig 24A) into the substance. By observing and

calculating the diagonals between the diamond mark, we

can calculate the exact hardness using a formula. Since the

samples were not completely opaque, they had to be gold

coated so that the surface reflected light and the readings

off the indenter could be taken and hardness calculated.

Different weights and different samples were used to see if

the value of hardness calculated was consistent.

- 31 -

Three way flexure tests were done using the setting seen in figure 25. Smallest

radii of points were used since a standard small monolith was used. The strain rate (0.5

mm/min), which was the same as the compression tests, was used. Protective

measures such as transparent sheet in front of the test were put again for safety

reasons.

The data produced was analyzed using the Zwick-Roell software, to get flexural modulii.

Scanning Electron Microscopy tests were also done using XL30 ESEM electron

microscope in the Electrical department on various samples including non cross linked

aerogels and cross linked X-Aerogels for contrast. The SEM was set at 20kV to acquire

good quality images, this however kept damaging the samples even after gold coating as

electrons kept damaging the surface and the surface kept changing. It was therefore

decided to take the setting down to a setting of 10kV so that images could be taken with

minimal electron damage. Even with such a low setting though there was some damage

but some clear images were taken but nowhere near the magnification to the one

described in the literature (200nm). [5]

- 32 -

Results and Discussion

The last procedure that was used to produce batch 11 was very different from

the one used in batch 1. It was an evolving process which proved that it was not possible

to directly copy the standard Leventis procedure as it was highly ambiguous and a lot of

research was needed to set every constant and parameter exactly right. There were a lot

of other problems encountered along the various experiments such as workday timings,

holiday’s in-between experiments, faulty ovens which compounded the problems. But

even after all those problems it was possible to set the standard procedure right and

produce mechanically test capable samples.

From batch 2 it was found that it was impossible to crosslink and properly wash

the samples without inventing a system which let the washing take place all round the

sample. Hence a two way test-tube sealed properly on both sides where the SOL-GEL

process could occur and the samples pushed out into a wider test-tube where the liquids

could get round to all parts of the sample was invented (stage 3). After this innovation,

liquids’ not washing the samples properly was not an issue anymore. Consequently, all

the variable change was then directed onto cross linking the samples properly. It was

found that N3300 which has a different cross linking chain reacts more readily and

quickly with the hydrogels then N3200 and was hence on used as the standard (stage 5).

Furthermore concentration (66/34 % w/w of desmodour/acetone) and quantity (40 ml)

were also identified as crucial variables (stage 5). It was concluded after various

experiments that a higher concentration solution of cross linker with acetone reacts more

quickly and readily with the hydrogels. Furthermore with the use of wider test-tubes it

was also found that quantity of the cross linker was also a factor and hydrogels only

reacted with the cross linker with higher quantities of cross linker available during oven

curing stage.

It was also found in the oven curing stage that the temperatures at which the

cross linking had to occur must remain at or around 53C due to acetone’s boiling

temperatures mark (stage 6). Furthermore the curing time was also a factor; longer

exposure at elevated temperatures of cross linker to the hydrogels gave it a better

chance to react properly (stage 6). This gave rise to temperature and evaporation

management required during the curing phase to ensure the samples were always

submerged. It was found that if the acetone boiled off too quickly and left the sample half

in the cross linker liquid and half into the ambient air, the part of the sample exposed to

- 33 -

ambient air turned quickly into a xerogel and shrank substantially, making it micro-

structurally weak and rendering it incapable to turn into X-Aerogel. It was therefore

crucial to leave the temperature just under or at 53C to prevent such thing happening. At

the same time it was also important that evaporation occurred and the acetone

evaporated slowly during the 72 hour curing period so that the cross linker could react to

the hydrogels. For this to happen while keeping temperatures stable was a prerequisite,

it was also important to seal the test-tubes properly with Dura-Seal and screw tops in

such a way that while it allowed the fumes to escape and evaporation to occur it did so in

a slow manner so that it lasted for the entire 72 hours. If the samples were sealed too

tightly, it was found that their was fume build-up and that increased the pressure inside

the test-tubes. All these constraints gave rise to temperature and evaporation

management to keep samples under the solution at all times and to prevent the

shrinkage.

After further experiments it was also concluded that the frequent agitation was

required and it produced both micro-structurally and macro-structurally better samples. A

faster rate was decided as the standard since it gave better results. Furthermore the

higher grade lower surface tension pentane was also deemed as a success since it gave

lower density samples.

Fibre reinforced samples while strong and possessing low-density were macro-

structurally not good enough to go under testing. It was concluded that a more careful

process must be drawn where a more homogenous distribution of glass wool is possible

so that cross linking and shrinkage is linear. Since the samples showed no obvious sign

of being flexible and were indeed quite stiff, it was concluded more glass wool in the

hydrogel’s structure might increase the flexibility as seen in literature [7]. However due to

a lack of time that particular test batch wasn’t possible.

Mechanical test results

Compression Test

We can see from the table 2.0 that while the compressive yield strength

remains relatively similar the compressive stress at final failure varies drastically for all 4

different samples. This can be directly linked to the failure strain. The sample which has

failed at the lowest strain also has the lowest compressive stress at final failure and vice

versa the sample which has the highest failure strain percentage also has the highest

- 34 -

compressive stress at final failure. The compression yield strength was calculated using

the tangent leaving the curve in the force/distance graph (Fig 26).

Table 2.0

This is due to the way the specimens were cut before the test was taken place.

The specimens were cut using a hacksaw which produced random dents around the

circumference of the monoliths. These random dents proved to be high stress points due

to their jaggedness and when the specimens were compressed at such a high strain the

specimen cracked and fell away at those various high stress points and failed at those

particular points. Because of the way the hacksaw was used the stress points produced

were absolutely random and hence all the specimens failed at varying failure strains. The

specimen which produced the highest failure strain and hence the highest compressive

stress at final failure had the most smooth edges and therefore was able to strain that

much.

Sample No.

Density (g/cm3)

Compressive Yield Strength (MPa)

Compressive Stress at final failure (MPa)

Failure Strain (%)

1 0.66 13.2 67.1 42.85 2 0.67 16.7 119 53.33 3 0.66 12.4 91.6 50.34 4 0.68 13.3 198 79.4

- 35 -

To compare the values between the tests done here and values expected from

literature it is found that compressive yield strength is almost triple of what it should be.

This could be attributed to the fact that these samples were denser then the one’s used

in literature.[10] The only compressive stress at final failure which is comparable is the

highest one, that of 198 MPa. It is 94% accurate and importantly occurs at within 98.2%

of the final strain experienced by the literature samples. (See fig 27 and 28). The curves

are slightly different due to the varying strain rates.

- 36 -

Vickers Hardness Test

Table 3.0

The Vickers hardness test values were found as in table 2.0 and calculated using this

equation where f is calculated using weight and gravity and d is ithe

indentation distance. As such a test had never been done on X-Aerogels before a base

value to which compare it wasn’t found.

3 way Flexure test

The highest flexural modulli found during 3 way Flexure test was 33Mpa which was 20%

of the expected value of 167 MPa found in literature [10]. The results were a bit flawed

since the monoliths subjected to these tests had several hairline cracks in them. These

samples were from batch 11 which were subjected to higher agitation rate, which may

have led to produce such macroscopically flawed samples. More experiments will be

needed on future batch samples to characterize the true flexural modulii of the samples.

Materials properties comparison

Materials Density Compressive

Strength(MPa) Vickers

Hardness Flexural

Strength(MPa) Price

(GBP/kg) PET (30% PAN Carbon

Fiber) 1.43 160 40 260 5.05

PET (35-45% Glass Fiber and Mica)

1.58 155 35 200 5.5

PE (Low/Medium Density, Branched Homopolymer)

0.92 15 3 28 1.09

PET (40% Glass Fiber. Flame Retarded)

1.76 130 32 200 1.5

PE (Crosslinked, Molding) 1 25 5.7 35 2.1 PP (Homopolymer, high

flow) 0.89 40 10 45 0.9

PP (Homopolymer, 40% glass and mineral)

1.23 60 15 90 1.3

Kevlar 49 Aramid Fiber 1.45 250 28 2250 70.8 Bisphenol molding

compound (Glass Fiber) 1.8 125 15.1 150 2.5

Bamboo 0.7 70 no data 35 1.23 Table 4.0

Sample No.

Indentation distance (µm)

Weight (g) Vickers Hardness

1 200 100 4.7 2 203 100 4.41 3 159 50 3.9 4 149 50 4.2

- 37 -

After all the mechanical characterization it was important compare the aerogel created to

other materials. Cambridge material selector is one such software which uses the data

given by the user to find materials which possess similar properties.(see table 4.0 for a

data on selection of similar materials). See Appendix C for Density Vs Compression

strength graph showing various relevant materials. By using density specific

compression and density specific hardness as a search criterion it was found that X-

Aerogel while extremely unique in itself most closely matched to natural Bamboo

(transverse), a silicon polymer based material called BiSphenol A and engineering

polymers polyethylene and polypropylene. Bamboo and BiSpehnol A while matching

were still more dense then the X-Aerogel by almost 40% and had almost 50% less

compression stress. Furthermore, BioSphenol is banned in most countries as it poses a

health risk. The engineering polymer polyethylene and propylene along with their glass

fibre reinforced counterparts had double or triple the density of X-Aerogels. The

compressive strength of all those materials were comparable to that of X-Aerogels

except for Kevlar. Vickers hardness measurement was only comparable with low density

polyethylene and cross linked polyethylene for X-Aerogel as it was found to be lower

then most other materials. As a true flexural modulus for the samples wasn’t found, a

true verdict on comparison with other materials cannot be made. However the flexural

modulii found in literature is comparable to glass cross linked polyethylene and

BioSphenol.

SEM Analysis

Figure 29.shows the SEM analysis done on a X-Aerogel. Due to the electron damage it

wasn’t possible to go at the level of magnification of the literature (200nm) [5]. However

from what we can see at 200-500 µm (levels of magnification, detailed at the bottom of

each image), it is unclear as to whether the samples have shown any porosity. From a

close look at Fig 29B we can see that there are some chains as well as some dark

areas. These could be the cross linker N3300 chains, and the dark spots mesopores,

however this can’t be certain until an even closer look at the sample is done at 100-200

nm range.

- 38 -

- 39 -

Fig 30 shows the SEM analysis of a non cross linked Aerogel for contrast and

comparison purposes. We can clearly see that even 100 µm microscopic level there

seems to be no chains and because it’s a mechanically weaker sample there is more

damage done to it then the X-aerogel sample.

Future Plans

Even with the success of establishing an unambiguous standard procedure

there are a number of enhancements that could be tried to make better samples. There

is some research done on silica nanoparticles to make them super hydrophobic [24]. If

we could incorporate this method, there would be a chance to not only create

mechanically stronger but potentially super hydrophobic samples. The super

hydrophobicity is done by treating the nanoparticles with Vinyltriethoxylsilane (VTEOS)

during the production stage. Obviously more experiments would need to be done before

this could be incorporated but it’s a variable to introduce in the future.

Another potential idea which could make the samples mechanically stronger is

a matrix cross linker. In the standard procedure N3300 is currently being used as the

sole cross linker. We can attempt to crosslink the hydrogels by a matrix cross linker

which contains a mixture of different chains of N3200 and N3300 and acetone. Because

the chains in both the cross linkers are different, potentially different chains in the

mesoporous structure side by side could make a stronger sample.

Epoxy cross linking with ambient drying instead of SCF drying is another idea

that could also be attempted. It is quite a straightforward process and similar to standard

diisocyanate cross linking procedure. The hydrogels need to be surface modified with

APTES and instead of cross linking with Desmodour they could be cross linked with Tri-

functional epoxides and instead of then super critically fluid drying they could be dried at

ambient temperature using the HPLC pentane.

Another idea which is easy to try but was never attempted during the

experiments due to health and safety reasons was the idea of curing final samples at

230C for two hours. Some research published has tried this with success, as residual un-

reacted cross linker left in pores is forced to react with each other at elevated

temperature and this might provide means to provide further strength without increasing

density [6]. The reason why this particular idea wasn’t attempted in the experiments was

that it was never possible to move the large furnace into the fume hood. Since such an

experiment involving diisocyanate cross linker which can give off dangerous fumes

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including cyanide fumes at elevated temperatures had never been performed, it was

decided that it was very important to conduct this particular experiment under complete

safety of a fume hood.

Moulding is another issue which needs to be addressed. It is proposed that in

stage 2 where SOL-GEL process occurs and moulding is done in the two-way glass test-

tubes. Teflon non-stick test tubes could be used so that samples are pushed out with

minimal of force and hence suffer minimal damage. The Teflon tubes could be in any

shape required so that X-Aerogel can be produced in the required shape. It is desirable

for the ultimate aim of this project that large sheets of X-Aerogel be produced. We could

conceivably create a large hollow rectangular/square block mould using Teflon spray and

any other stiff material. The large sheets could be acquired once the whole X-Aerogel

standard process is done and the sheets could be cut off in the desired thickness from

the large block structure.

One idea that was suggested as a future idea for this project by previous

student was the idea of automatic washing concept. This idea was a given a serious

thought and looked at for feasibility. While it is true that if an automatic washing concept

was generated it would cut production time by almost 50-60%, it was decided that more

sophisticated equipment will be required for the idea to be brought into reality. The

samples need a vigorous washing in acetone after ethanol wash and cross linking, and

it would be difficult to replicate that in an automatic environment even with the use of a

shaker. Consequently, it was decided that such an idea would be unfeasible with lots of

complications and it was decided to solely concentrate on different variable changes in

this project.

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Costs

Once standardizing the method it is important to calculate the price of how much each

sample will cost to make in terms of materials. In that regard, a list has been made to

include all the substances that are used, including the new expensive HPLC pentane to

get a price per unit volume (£/cm3). It has been shown in the literature that we should

expect shrinkage at most of 2-3% [7], and even though our samples have shown higher

shrinkage then that at around 7-8% at times, we can still calculate the average volume of

the sample and by that calculate a price. Our mould was 5 cm long and 1cm in diameter

giving us a volume of 3.93cm3. All the quantities taken are on the upper side especially

the washer solvents since their use is dependent on the particular batch.

Material Quantity Cost (£)

Tetramethylorthosilicate 2.6 ml 0.47 Methanol 6 ml 0.06

Ammonium Hydroxide 0.1333 ml 0.003

Ethanol 15 ml 0.44

Acetone 150ml 2.21

N3300 Di-isocyanate 20g 0.105

HPLC Pentane 60ml 3.62

Total 6.908

Table 5.0 Considering that our sample volume is 3.93 cm3 and unit cost 6.908 it gives us an

approximate price per unit volume of 1.75 £/ cm3.

There is potential to decrease this price calculated by reusing some solvents like

pentane and acetone. However some research must be done to clearly establish that

reusing solvents doesn’t microscopically/macroscopically undermine the samples.

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Conclusion

With the production of mechanical characterization capable X-Aerogel samples

and with mechanical test results which show results similar to the ones in published

literature it is safe to say that the new standard procedure including the changed

variables has been a success. However the same cannot be said for the Fibre

Reinforced X-Aerogels, as it was difficult to get conformity and homogenous distribution

along the samples and produce a mechanically test capable sample. More experiments

need to be done to get glass wool distribution right and gets more flexibility.

While there was some success with mechanical testing, the SEM results were not as

great as we would have liked as it was not possible to go into the same level of

magnification as in the literature (200-500 nm range). It would have been ideal if we

could get access to that level of magnification or analyse future samples in Brunauer-

Emmett-Teller (BET) surface area analysis through nitrogen adsorption porosimetry; so

that we can comment about the microstructure and give a verdict on the porosity. The

automatic washing concept would be a good idea to implement for future ideas if enough

sophisticated equipment incorporating all the requirements can be put together.

Furthermore it will also be interesting to see the ideas listed in the future plans to get into

action and see if they provide any sort of enhancement.

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Glossary of Chemistry Terms [26, 13]

A. When water reacts with another substance and as a result the oxygen in water makes

a bond with the substance.

B. A chemical condensation leading to the formation of a polymer by the linking together

of molecules of a monomer and the releasing of water or a similar simple substance.

C covalent bond is a form of chemical bonding that is characterized by the sharing of

pairs of electrons between atoms, or between atoms and other covalent bonds. In short,

attraction-to-repulsion stability that forms between atoms when they share electrons is

known as covalent bonding.

D. Van der Waals' forces include all intermolecular forces that act between electrically

neutral molecules.

E. A network of polymer chains that are water-insoluble

F. One step base catalysed gels involves preparing two solutions; the first with the silicon

alkoxide in alcohol and the second with water, alcohol and a base as a catalyst. These

are mixed and once mixed the hydrolysis and polycondensation reactions begin. The sol

would then be poured into appropriate moulds and allowed to gel. [23]

G. Two step acid-base catalysed gels preparation begins with the silicon alkoxide being

mixed with alcohol, water and an acid. This is then be refluxed for four hours with stirring

as the temperature is controlled. Some of the solvent is then distilled off until the solution

reaches a certain temperature and then allowed to cool to room temperature. More silica

alkoxide is then added and further refluxing for four hours before a second distillation

removing more solvent. This is then cooled to room temperature and then acetonitrile

added to make up the stock solution for the gels. That is further mixed with a solution of

acetonitrile, water and a base catalyst so that the SOL can begin reactions and gelation

can occur. [23]

H. A mesoporous material is one with pore diameters between 2 and 50nm, they have

vast surface areas, which make them useful as catalysts.]

I. Isocyanate is the term given to a molecule that contains a functional group consisting

of one nitrogen, one carbon and one oxygen atom arranged: –N=C=O.

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Any organic compound containing this group may be referred to as an isocyanate,

J. An abbreviation for "by weight or by Volume," to describe the concentration of a

substance in a mixture or solution. Ex. 2% w/w means that the mass of the substance is

2% of the total mass of the solution or mixture or 30% v/v means that the volume of the

substance is 30% of the total volume of the solution or mixture.

K. Polyurea is a type elastomer that is derived from the reaction product of an isocyanate

component and a synthetic resin blend component through step-growth polymerization.

L. One of a class of strongly basic substances derived from ammonia by replacement of

one or more hydrogen atoms by a basic atom or radical.

M. Tetrahydrofuran (THF) is a colorless, water-miscible organic liquid with low-

viscosity at "room" (standard) temperature and pressure (and across a further range of

temperaturesIt is one of the most polar of the organic functional class of ethers, and has

a relatively low freeze point, and so is a commonly used modern organic chemical

laboratory solvent across a range of temperatures.

N. Meniscus - The point at which a liquid's curved side’s touch it's container's edge.

O. An ionic compound formed by removal of hydrogen ions from the hydroxyl group in

an alcohol using reactive metals

P. Sodium Sillicate is the common name for a compound sodium metasilicate, Na2SiO3,

also known as water glass or liquid glass.

Q. Isopropanol, isopropyl alcohol, or 2-propanol (CH3)2CHOH, is a colorless liquid that is

miscible with water.

R. A colloidal suspension is a substance in which one material is dispersed evenly

throughout another.

S. Ceramic fiber felts are produced from corresponding blowing fiber (ST, HA, HP, HZ)

with vacuum formed technology. Ceramic fiber felts not only possess the typical function

of ceramic fiber, but also have hard texture, excellent toughness and intensity, and

excellent fire resistance and heat preservation.

- 45 -

References

[1] “History of Silica Aerogels” Microstructured Materials Group [Online] Arlon Hunt

Michael Ayers 2009 http://eetd.lbl.gov/ECS/Aerogels/aerogels.htm

[2] “Aerogel: Catching comet dust” NASA Jet propulsion Laboratory [Online] 2005

http://stardust.jpl.nasa.gov/tech/aerogel.html

[3] “Less is more with Aerogels” Eric J Lerner [Online] 2005

http://physicist.org/tip/INPHFA/vol-10/iss-5/p26.html

[4] “Nanoengineering Strong Silica Aerogels” Nicholas Leventis,* Chariklia Sotiriou-

Leventis,* Guohui Zhang, and Abdel-Monem M. Rawashd eh. 2002 Nano Letters Vol

2, pp 957-960

[5] “ Nanoengineered Silica-Polymer Composite Aerogels with No Need

for Supercritical Fluid Drying” Nicholas Leventis , Anna Palczer and Linda

Mccorkle 2005 Journal of Sol Gel Science and Technology Vol 35. Pp 99-105

[6] “Cross-linking Amine-Modified Silica Aerogels with Epoxies:

Mechanically Strong Lightweight Porous Materials” Mary Ann B. Meador, Eve F.

Fabrizio, Faysal Ilhan, Amala Dass, Guohui 2005 Chem Mater Vol 17 pp.1085-1098

[7] “Synthesis and pore analysis of aerogel–glass fibre composites by ambient drying

method “Chang-Yeoul Kim, Jong-Kyu Lee, Byung-Ik Kim 2007 Colloids and surfaces

A. In press

[8]”Frozen Smoke” National Geographic Today [Online] Bijal P. Trivedi 2002

http://news.nationalgeographic.com/news/2002/05/0508_020508_TVaerogel.html

[9] “Isocyanate-crosslinked silica aerogel monoliths: preparation

and characterization” Guohui Zhang, Amala Dass, Abdel-Monem M. Rawashdeh,

Jeffery Thomas, Joseph A. Counsil, Chariklia Sotiri ou-Leventis, Eve F. Fabrizio,

Faysal Ilhan, Plousia Vassilaras, Daniel A. Scheima n, Linda McCorkle, Anna Palcz

2004 Journal of Non-Crystalline Solids p.152–164

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[10]”Chemical, Physical, and Mechanical Characterization of Isocyanate

Cross-linked Amine-Modified Silica Aerogels” Atul Katti, Nilesh Shimpi, Samit Roy,

Hongbing Lu, Eve F. Fabrizio 2006 Chem Mater Vol 18 pp.285-296

[11] “Aerogels—Recent Progress in Production Techniques and Novel Applications”

J. Fricke and A. Emmerling 1998 Journal of Sol-Gel Science and Technology 13, pp

299–303

[12] “An introduction to composite materials” Derek Hull 1980 Cambridge Solid State

Science Services pp 156.208-209

[13] “ Aerogels” Wikipedia [Online] 2008 http://en.wikipedia.org/wiki/Aerogel

[14] “Carbon Nanotube Aerogels” Mateusz B. Bryning, Daniel E. Milkie, Mohammad

F. Islam, Lawrence A. Hough, James M. Kikkawa, and Arjun G. Yodh 2007

Advanced Materials pp. 661-664

[15] “ Shaping and Mechanical Reinforcement of Silica Aerogel Biocatalysts with

Ceramic Fiber Felts” Karout, P. Buisson, A. Perrard and A.C. Pierre 2005 Journal of

Sol-Gel Science and Technology 36, 163–171

[16] “Polymer nanoencapsulated mesoporous vanadia with unusual ductility at cryogenic

temperatures “ N. Leventis, C. Sotiriou-Leventis S. Mulika A. Dass a J. Schnobrich,

A. Hobbs, E. F. Fabrizio 2008 Journal of Material Chemistry

[17] “Aerogels—Recent Progress in Production Techniques and Novel Applications”

J. Fricke and A. Emmerling 1998 Journal of Sol-Gel Science and Technology 13, pp

299–303

[18 ] “Hydrogen adsorption on nitrogen-doped carbon xerogels” Kyung Yeon Kanga, Burtrand I. Leea,b, Jae Sung Leea 2009 Carbon 47 pp 1171–1180

[19] Dunlop Sport 2009 [Online] Aerogel racket information.

http://www.dunlopsports.com/Dunlop-Tennis/Technology/Aerogel1/

[20] Edge Riders 2009 [Online] Description of Salomon Snowboard boots

http://www.edgeriders.com/product_info.php?products_id=3224

- 47 -

[21] Aspen Aerogels 2009 [Online] Industrial cases of their aerogel applications

http://www.aerogel.com/markets/cases.html

[22] Cabot Corporation 2009 [Online] Nanogel Applications

http://www.cabot-corp.com/Aerogel

[23] “ Production of a novel X-Aerogel for ballistics” Euan Trousdale 2008 University of

Glasgow Mechanical Engineering Project Reports pp 1-33

[24] “Super-hydrophobicity of silica nanoparticles modified with vinyl groups”

Longjian Xue, Jian Li, Jun Fu, Yanchun Han 2009 Colloids and Surfaces A:

Physicochem. Eng. Aspects 338 pp15–19

[25] Tohoku University Japan 2009 [Online] Supercritical fluid diagram

http://www.che.tohoku.ac.jp/%7Escf/about/about%20sc%20fluid-E.htm

[26] Chemistry definitions Chemicool Dictionary, David D. Hsu Massachusetts Institute

of Technology.

Appendix

A. Gantt Chart

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B. Variable Matrix - This variable matrix details various settings changed during

experiments. This through process helped identify a standard process.

- 49 -

C. Density Vs Compression strength graph from Cambridge Engineering Materials

Selector.

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