General Principles of Sol-Gel Processing

8.1.1. General Principles of Sol-Gel Processing

The sol-gel process refers broadly to room temperature 'solution routes' for preparing mainly oxide materials. The process involves the hydrolysis and polymerization of metal alkoxide precursors of silica, titania, zirconia as well as other oxides. The solutions of precursors are reacted to form irrevers- ible gels that dry shrink to rigid oxide forms [1]. The inter-disciplinary approach is followed in the sol- gel process (see the extensive work done by Klein on sol-gel in ref. [2]).

There are ‘new materials’, which are built up from the molecular level like [3] : Use : (Environmentally Acceptable Precursors) ↓ Assemble to Size Approximating Device : (Chemical and Physical Transformations) ↓ Model Cooperative Properties The concept of sol-gel processing is actually akin to the processing of the ‘nano-structure’. First

of all, this process starts with a nanometer-sized molecular unit, and it goes through the reactions, which are also on the nanometer scale. Since these ‘molecular’ sizes are susceptible to be scattered by light, the scale of the sol-gel process or rather the progression can be probed by the ‘light scattering’ (LS) tech- nique. Obviously, some more sensitive techniques like small angle neutron scattering technique can also

be employed for this purpose, as described for the nano particles of magnetite in the section 5.7. The concept behind the sol-gel process is that a ‘combination of chemical reactions’ turns a

‘homogeneous solution of reactants’ into an → ‘infinite molecular weight oxide polymer’. This poly- meric unit is a 3-dimensional structural skeleton, which is surrounded by the ‘inter-connected pores’. From an ideal point of view, this polymeric unit is isotropic, homogeneous, and obviously uniform in the nano-domain. Moreover, it can exactly replicate its mould and the miniaturization of all the features is possible without distortion. The gels contain ‘pores’, and the nano-phase porosity and the nano- structure of the gels are both scientifically and technologically important.

8.1.1.1. Precursor Alkoxides

Initially, the sol-gel process involves a ‘homogeneous solution’ as precursors. The alkoxides are the organo-metallic precursors for silica, alumina, zirconia, titania, etc. One of the most common sys-

271 tems consists of “Tetraethyl Orthosilicate” (TEOS), alchol and water. This solution can react to an

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extent, where the molecular structure can no longer be reversed. This particular point can be considered as a ‘critical point’, which is known as “Sol-Gel Transition Point”. In the whole structural constitution, the “Gel” is an elastic solid filling the same volume as the solution.

It is important to know about the “Common Alkoxides” for ‘Sol-Gel Processing’, which are :

1. TEOS

2. Trimethyl Borate

3. Aluminium Sec-Butoxide

4. Titanium Iso-Propoxide

5. Zirconium Iso-Propoxide The last one is discussed in some details in the section 3.7.2. Now, the ‘chemical reactions’

should be described, which are the most fundamental in the entitre process for the preparation of nano materials through ‘Sol-Gel Route’.

8.1.1.2. Chemical Reactions in Solution

1. Non-Aqueous Process

The rate of chemical reaction depends on the following : (a) pH, (b) Concentration, and (c) Solu- tion . In case of preparation of the ‘alumina powder’ from aluminium sec-butoxide, the following reac- tions occur :

Al(OC 4 H 9 ) 3 +H 2 O → Al(OC 4 H 9 ) 2 (OH) + C 4 H 9 OH

2Al(OC 4 H 9 ) 2 (OH) → 2AlO(OH) + yC 4 H 9 OH

2Al(OC 4 H 9 ) 2 (OH) + 2H 2 O → Al(OH) 3 + 2C 4 H 9 OH

(4) Normally, a catalyst is used to start the reactions and control the pH of the solution. The series of

AlOOH or Al(OH) 3 → Al 2 O 3 + zH 2 O

reactions that occur are as follows :

1. The first reaction is the ‘hydrolysis’ to make the solution active,

2. The process (1) is followed by ‘condensation-polymerization’,

3. These reactions go along with further hydrolysis. The ‘molecular weight’ of the oxide polymer is increased by these reactions, which result in

either the mono-hydroxide AlOOH (boehmite) or the tri-hydroxide Al(OH) 3 (bayerite-4). When one desires to make the ‘transparent activated alumina gels’ from ASB, there is one well-studied composi- tion for achieving the desired property, which involves an addition of little excess of water in the initial solution, e.g. 100 ml water/ASB. There is a need of the hydrolysis process to take place at slightly above 800°C in order to preferentially promote the formation of boehmite, rather than bayerite. The majority of the transition metal (TM) oxides have been prepared by non-aqueous sol-gel methods. Since these TM oxides can be fruitfully applied in certain optical devices, this technique assumes a special impor- tance, and they also have a variety of applications.

2. Aqueous Process

The sol-gel processing route also involves the use of ‘Aqueous Colloidal Sols’. This sol contains nanometer-sized particles, and hence it is quite natural to include such ‘sols’ as precursor materials, which admit that the mechanism for attaining the ‘sol-gel transition’ is quite different. The aggregation

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or agglomeration of the ‘sol particles’ is normally caused by changing the pH or the concentration in sols, like the well-known Ludox.

Then, there is a ‘gelation’ process. The sols can be gelled in such a way that the ‘oxide skeleton structure’ is a continuous linkages of the sol particles. But, there are some discrete features that make up this ‘skeleton structure’, which corresponds to the sol dimensions. The other features are obviously the “pores” within the ‘secondary particles’. There is a difference between the chemical and structural aspects, i.e. between the non-aqueous alkoxide precursors and the aqueous sol precursors, which are usually blurred at a later stages of the sol-gel process.

8.1.1.3. The Process Details

1. Mixing

The ‘mixing’ is a first step for the single alkoxide, multiple alkoxide, and colloidal sol processes. Since the building blocks are nanometer in size, the particle size is smaller than the wavelength of the visible light.

2. Gelling

The ‘gelling’ is determined approximately as the time when the solution shows no flow. This is known as the ‘gelation time’, i.e. the ‘time to gel’. Here, the viscosity plays an important role for the transition from a viscous liquid to a rigid structure.

3. Shape Forming

The sol-gel process can be used to make the ‘bulk materials’, which are done by casting and moulding . This process can also be used to make a micro porous ‘preform’, which is near ‘net shape’. This preform is called ‘Monolithic’, which has its obvious reference to its continuity. The ‘monolithic gels’ can be formed from an alkoxide solution or from a colloidal sol. However, there is a difference between two types of monolithic gels, i.e. between the colloidal gels and the alkoxide gels. The main difference between them lies in their structures of the ‘pores’. The colloidal gels have larger pores between the particles, whereas the alkoxide gels have smaller pores, which are less than 10 nm in size. Obviously, by considering all the above points, the making of the ‘monolithic gels’ is quite a challeng- ing problem in the sol-gel process. There are other areas of importance. These are “thin-films”, “high aspect ratio fibre”, etc. for optical transmission, and many more.

4. Drying

After having selected a particular geometry or shape and accordingly, designing the right chemi- cal formulations, we have to take several operational steps further, which are generally common to the monolithic gels, the thin-films and the fibres. All these gels must be or have to be ‘dried’. So, the ‘drying’ is an important step in the whole operation. The thin-films and the fibers can be dried quickly in air due to their smaller dimensions, since it has been found that all the observed ‘shrinkages’ are taken up along the thin dimension in the case of thin-films, and not in the ‘plane of the substrate' if the thick- ness is below 1 micron.

However, for monolithic gels due to their thicker dimensions, the drying is more difficult. We would like to obtain a gel with a nano-structur and at the same time, it has also to be ‘crack-free’ gel. Accordingly, there are mainly two treatments of drying, which are in use. The first possibility consists of forming an ‘aerogel’, which is dried in an ‘autoclave’ by hypercritical technique. Then, the solvent is removed above its critical points. The result in aerogel is about 10% dense and shows no appreciable shrinkage. The second option is more simpler and it consists of a ‘xerogel’, which are dried by natural

273 evaporation. The xerogels are 60% dense and undergoes a reduction of volume by about 35-65% during

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the drying process. By going through the process of chemical reactions, gelation, and drying, the processed gel

materials have several characteristics of the corresponding ceramic oxide, but they are more porous than the corresponding ceramic material, which are obtained by other processing routes. It is already known that both the water and the solvent escape through the “inter-connected” pores that remain open at the surface until the gel is fired at around 600°C or higher depending on the chemical composition.

5. Densification

The objective of any ceramic material is to obtain a dense or pore-free material by the various routes of sintering. The gel is obviously no exception in this regard. The ultimate aim of the ‘sol-gel process’ is to obtain a dense, i.e. pore-free, oxide material, and hence the final stage of processing is the ‘sintering’. The smaller particles sizes in the nao range giving rise to a ‘high surface area’ of the gel is considered to be an advantage in terms of a ‘higher driving force’ for the sintering process. Therefore, the sintering process is expected to occur at lower temperatures than that required in the conventional powder compacts. This is the final step of the whole operation, which is independent of the fact that whether the compact was prepared by an aqueous or non-aqueous route. The main point lies in a ‘driv- ing force’, i.e. the rate of diffusion of different atomic species, which makes the particles grow to a certain extent and which eventually removes the porosity in the final product, or a material is formed that is similar to conventionally processed material, which did not go through the sol-gel processing route [2] (See section 2.2.3 for sintering of ceramics).

The presence of lower number of pores added with the high-purity uniform nano-structure be- come the ‘hallmark’ of the sol-gel process. However, there are still some challenges in each of the above mentioned processes. Apart from being cost-effective, very often the yield of the ‘actual’ material for further processing to different shapes is not too high. Moreover, in sol-gel route in particular, there are problems of segregation, contamination and unusual pore formation [3]. Still, the challenge of the sol- gel process is to exploit the “nano-structure” aspects of the process to derive the “real benefits” [1 - 3].

It should be pointed out here that even after proper drying and densification, the sintered gel still contains OH ions, which are considered impurities for optical fibre communication in terms of increas- ing the losses in dB/Km. Even for the preparation of nano-structured alumina, zirconia or any other material by the sol-gel route for some hi-tech applications, the presence of these OH ions is detrimental. In order to assess the OH content in the gel materials, it is better to do a study of the vibrational spectra of ‘metal cation-OH’ vibrations in the IR spectra [1]. But the analysis is complicated for determining the concentration of OH ions, since in the majority of cases, the deconvolution of the IR spectra were not done. In the gel glasses in the boro-silicate system, a detailed deconvolution of the IR spectra was done by computer in order to estimate even the concentrations of B-OH and Si-OH from the respective vibra- tional bands, which are distinctly different for each case of vibrations [4]. So, that sums up this sub- section on sol-gel process.

Does the inherent ‘nano-structure’ give us any new material, which can make wonders in the newer fields of electronics and communication ?

8.1.1.4. Behaviour of Some Gels

The literature on gels and their behaviour is so extensive that it is simply impossible to cover even the most important ones. So, only a few examples of the behaviour of some gels will be given here. Some of these gels are not really ordinary, since they have some extra-ordinary applications and a set of

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excellent properties. Some of these ‘gels’ will be described here to highlight the issue of the superiority of producing nano-materials through the sol-gel route.

1. Gallium based Nano-Materials

Some unique Ga-based nano-materials have been prepared by Sinha et al.[5]. The films were dip coated on the quartz substrates. The annealing of the films was carried out between 300°C to 1100°C in air atmosphere for one hour using a tubular furnace. The films were characterized by XRD, SEM, AFM, UV-VIS and PL techniques. The XRD spectra revealed that the films started to crystallize as α-GaO(OH) with orthorhombic structure, when annealed at 300°C. On increasing the annealing temperature, this

structure collapsed and began to transform into α-Ga 2 O 3 . A pure α-Ga 2 O 3 film having rhombohedral structure was obtained at 500°C [5].

A further increase of the annealing temperature indicated destruction of α-Ga 2 O 3 crystal lattice, and the appearance of β-Ga 2 O 3 structure. At 700°C and onwards, a pure β-phase with monoclinic struc- ture was observed. It was claimed that the phase pure α-GaO(OH) and α-Ga 2 O 3 thin films have been prepared for the first time to evaluate their optical properties. The SEM and AFM studies revealed that all the films were ‘crack free’ and very ‘smooth’. The films with different structures showed both al- lowed and direct semiconducting transitions. The highest semiconducting band gap (5.27eV) was found

for α-GaO(OH) among all the phases examined in this study whereas that for α-Ga 2 O 3 was intermediate between α-GaO(OH) and β-Ga 2 O 3 phases. Both strong green and blue emissions with low FWHM were observed in phototo-luminescence study, and these gel-based materials have a great future [5].

2. Dye-Doped Gel Glasses

Many applications of the sol-gel process were focused on the optical properties. For example, the optical behavior of organically dye-doped gel-glasses that were incorporated into the porosity of sol-gel glasses without deterioration of their photo-physical properties. For example, the spectral behavior and chemical stability of dye-doped gel-glasses are quite interesting. The chosen molecular structure of these dyes, i.e. specific Dye Lasers as molecular probes, were used for the study of the surface properties of the porous cage where these molecules were entrapped, in terms of homogeneity, polarity and viscos- ity and dye stabilization. The feed-back from steady-state dynamics and polarization spectroscopy stud- ies is crucial to establish the preparation of the optimal conditions. In particular, ‘Organically Modified Ceramics’ were successfully used for specific preparations [6-8].

3. Glass Dispersed Liquid Crystals

The ‘glass dispersed liquid crystal’ (GDLC) films, which are prepared by organic doping of Sol- Gel matrices, may be used as electro-optical devices. The films scatter light according to the number of droplets and the relative refractive indices of the LC and the silica matrix. The LCs are ‘birefringent’,

i.e. they show elasto-optic effect. Therefore, their refractive index depends on the LC orientation and the optical angle of incidence. If the film is coated with transparent electrodes, and an electric field is applied, a reorientation of the LC director in the droplet occurs, producing a variation of the LC refrac- tive index as ‘seen’ by the incoming light. If the refractive index of the sol-gel substrate matches the new LC index, the material changes from an ‘opaque scattering state’ to a ‘transparent state’. This feature can be used for preparing devices for visual presentation, i.e., the ‘displays’ [9].

The unaltered GDLCs switch from white opaque to colorless transparent states. If these materi- als are used for the ‘displays’, the color needs to be incorporated for many applications. The direct-view, backlighted passive displays, usually include color filters located between the backlight system and the electro-optical material. In GDLCs, the color may be included in the sol-gel matrix or in the liquid crystal itself, allowing the preparation of GDLC color displays [9].

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4. Synthesis of Glass-Metal Nano-Composite

The glass-metal nano-composites incorporating ultrafine particles of iron, nickel, cobalt and manganese, respectively, in a silica glass matrix have been prepared by heat treatment of a gel derived from a sol containing silicon tetra-ethoxide and a suitable metal organic compound. The metal particles in all the nano-composites are isolated and spherical-shaped with diameters ranging from 3 nm to

10 nm. The films of these nano-composites with thickness of the order of a few micrometres have been prepared on glass slides by a simple dip-and-pull technique. The optical absorption spectra of the nano- composite films have been measured over the wavelength range 200 nm to 2000 nm. The ‘effective medium’ theories, due to Maxwell-Garnett and Bruggeman, respectively, have been used to calculate the optical absorption of these materials theoretically. The Maxwell-Garnett theory gives results, which are in better agreement with experimental data than those obtained from Bruggeman formalism. The filling factor ‘f ’ as estimated from the least-squares fit of the experimental results with the Maxwell- Garnett theory has a value in the range 1 to 4% [10].

5. Metal-Silica and Metal Oxide-Silica Nanocomposites

The nano-composites based on sol-gel processing of Me-SiO 2 and MeO-SiO 2 are quite interest- ing. Some modifications of the sol-gel process were investigated with the aim of modifying the charac- teristics of the nano-composites so that their properties might be modulated continuously. In absence of strong interactions among oxide nano particles and silica matrix, the number and therefore the size of the nano particles should not depend on the presence of links between the metal precursor and the silica matrix, which act as the nucleation sites. On the other hand, every cavity inside the matrix network constitutes a nucleation site, which affects the maximum size of the forming nano particles [11].

In order to improve the preparation process, one has to play on the factors determining the char- acteristics of the pores in the matrix. Therefore, attempts were mainly focused on varying those param- eters, which affect the preliminary steps of the gelation process. The tests were performed in order to find the best preparation conditions, which allow the tailoring of the particle size in the final nano- composite.

In the case of Fe 2 O 3 -SiO 2 system, efforts have been also put into finding the conditions, which allow to obtain final nano-composites in which the iron oxide is in the form of pure maghemite The influence of performing mechanical treatments of the gels, of performing the drying step under supercritical conditions, of the gelation temperature and time, and of varying the amount and type of the solvent werer tested [11].

The first attempt, i.e. mechanical treatment, dealt with the use of mechanical milling of the dried gels [12]. The ball milling was adopted in order to promote the fragmentation of the silica network that, in principle, should favour a better control of the microstructure of the composite allowing a redistribu- tion of nano-sized particles [13]. A nano-composite with 28% iron oxide was treated at 300°C. The

XRD spectra showed a sharpening of Fe 2 O 3 reflections, which increases with milling times. The TEM observations indicated that the nano particle size distribution is more homogeneous after milling. These results suggested that the ball milling induces the crystallization of Fe 2 O 3 nano particles by mechanical activation. The fine crushing of the particles results in an increasing amount of the number of small pores and in a decrease of particle size which is responsible for the better stability of Fe 2 O 3 towards the transition.