User:Surendra Karwasara

-: Table of Contents:-

Chapter -1 Introduction

Chapter -2 Classification of nanomaterials

Chapter -3 Methods

Chapter- 4 Aluminum synthesis

Chapter-5 Conclusion

Chapter-1

Introduction

This review is provided a detailed overview of the synthesis, properties and applications of nanoparticles (NPs) exist in different forms.

NPs are tiny materials having size ranges from 1 to 100 nm. They can be classified into different classes based on their properties, shapes or sizes.

The different groups include fullerenes, metal NPs, ceramic NPs, and polymeric NPs. NPs possess unique physical and chemical properties due to their high surface area and Nano-scale size.

Their optical properties are reported to be dependent on the size, which imparts different colors due to absorption in the visible region. Their reactivity, toughness and other properties are also dependent on their unique size, shape and structure.

Due to these characteristics, they are suitable candidates for various commercial and domestic applications, which include catalysis, imaging, medical applications, energy-based research, and environmental applications. Heavy metal NPs of lead, mercury and tin are reported to be so rigid and stable that their degradation is not easily achievable, which can lead to much environmental toxicity.

Nanoparticles (NPs; 1–100 nm in size) have a special place in Nano-science and nanotechnology, not only because of their particular properties resulting from their reduced dimensions, but also because they are promising building blocks for more complex nanostructures. This chapter gives an overview of NPs and their presence in our daily lives.

It provides examples of the use of NPs in nanotechnology to obtain different end-products in different sectors of economic activity. In addition, a classification of NPs based on their dimensions, morphology and chemical composition is presented. NP uniformity and agglomerations, with a special focus on super paramagnetic NPs and their Nano-composites, are discussed.

1.1. An overview of nanoparticles and nanotechnologies

'Nano' is a prefix used to describe 'one billionth', or 10−9, of something. The concept of nanotechnology was introduced by physics Nobel laureate Richard P Feynman in his famous lecture entitled 'There's plenty of room at the bottom' at the December 1959 meeting of the American Physical Society.

Since then, there have been many revolutionary developments in physics, chemistry and biology that have demonstrated Feynman's ideas of manipulating matter at the atomic scale.

In 1974, Norio Taniguchi (a professor at the Tokyo University of Science) invented the term 'nanotechnology' to describe extra-high precision and ultra-fine dimensions.

He introduced the 'top-down approach' by predicting improvements and miniaturization in integrated circuits, optoelectronic devices, mechanical devices and computer memory devices.

Approximately ten years later, K Eric Drexler introduced the 'bottom-up approach' when he discussed the creation of larger objects from their atomic and molecular components as the future of Nano-technology.

Nanotechnologies are now widely considered to have the potential to bring benefits in

Areas as diverse as drug development, water decontamination, information and communication technologies, and the production of stronger and lighter materials. Nanotechnologies involve the creation and manipulation of materials at the nanometer scale, either by scaling up from single groups of atoms or by refining or reducing bulk materials.

While the development of nanotechnologies is a modern multidisciplinary science involving the fields of physics, chemistry, biology and engineering, the production of nanoparticles (NPs), both in nature and by humans, dates from the pre-Christian erFor example, the Romans introduced metals with Nano metric dimensions in glass-making; the famous Lycurgus cup (currently exhibited at the British Museum), which displays a different color depending on whether it is illuminated externally (green) or internally (red), contains NPs of silver and gold.

In 1857, Faraday reported the synthesis of colloidal gold (and other metals such as Cu, Zn, Fe and Sn) and its interaction with light. For an overview and chronological table of nanotechnologies, see.

Another example of interest is the case of magnetic NPs. Krishnan, illustrated the role that magnetic materials play in biology and medicine. In the field of magnetic NPs, a noteworthy pioneering work was published by Blakemore in 1975, where biochemically precipitated magnetite (Fe3O4) was found in the tissues of various organisms including bacteria, algae, insects, birds and mammals.

Many of these organisms use biogenic magnetite to sense the Earth's magnetic field for orientation and navigation. For more details on the development of magnetic NP synthesis and its presence in biomedicine and biotechnology see.

Throughout the last century, the field of colloid science has developed enormously and has been used to produce many materials, including metals, oxides and organic products.

One of the first and most easily prepared magnetic colloidal systems was developed by Stephen Papell of the National Aeronautics and Space Administration in the early 1960s.

Papell's colloid consisted of finely divided particles of magnetite suspended in paraffin. To prevent particle–particle agglomeration or sedimentation, Papell added oleic acid as a dispersing agent. Subsequently, similar magnetic suspensions have also been synthesized with different nanometre sized particles of pure elements, such as iron, nickel and cobalt, in a wide range of carrier liquids.

Ordinary materials, when reduced to the Nano-scale, often exhibit novel and unpredictable characteristics such as extraordinary strength, chemical reactivity, electrical conductivity, super paramagnetic behavior and other characteristics that the same material does not possess at the micro- or macro-scale.

A huge range of nanomaterials is currently being produced at an industrial scale, while others are being produced at smaller scales as they are still under research and development (table).

Chapter - 2

Classification of nanomaterial

Typically, NPs are defined as an agglomeration of atoms and molecules in the range of 1–100 nm. They can be composed of one or more species of atoms (or molecules) and can exhibit a wide range of size-dependent properties.

Within this size range, NPs bridge the gap between small molecules and bulk materials in terms of energy states. NPs are generally classified based on their dimensionality, morphology, composition, uniformity and agglomeration.

2.1. Dimensionality

2.1.1. 1-D Nano-material

Materials with one dimension in the nanometre scale are typically thin films or surface coatings. Thin films have been developed and used for decades in various fields including electronics, information storage systems, chemical and biological sensors, fibre-optic systems, and magneto-optic and optical devices.

Thin films can be deposited by various methods and can be grown controllably at the atomic level (a monolayer).

     

           

The materials shown in figure2.1 (a) and (b) can be classified as 1-D nanomaterial,

2.1.2. 2-D Nano-materials

2D nanomaterials have two dimensions in the nanometre scale. These include for example, nanotubes, dendrimers, nanowires, fibres and fibrils.

Free particles with a large aspect ratio with dimensions in the Nano-scale range are also considered to be 2D nanomaterials. The properties of 2D systems are less well understood and their manufacturing capabilities are less advanced.

Iron nanorods shown in figure 2.1(d) can be classified as 2D nanomaterials.

2.1.3. 3-D Nano-materials

Materials that are Nano-scale in all three dimensions are considered to be 3D nanomaterials.

These include quantum dots or Nano-crystals, fullerenes, particles, precipitates and colloids.

Some 3D systems, such as natural nanomaterials and combustion products, metallic oxides, carbon black, titanium oxide (TiO2) and zinc oxide (ZnO) are well known, while others such as fullerenes, dendrimers and quantum dots represent the greatest challenges in terms of production and understanding of properties.

Cu NPs shown in figure 2.1(c) are classified as 3D nanomaterials

2.2. The morphology of NPs and Nano-composites

The morphological characteristics to be taken into account are the flatness, aspect ratio and spatial position of each element in the case of hybrid NPs (HNPs).

A general classification exists between high and low aspect ratio particles.

High aspect ratio NPs includes nanotubes and nanowires. Small aspect ratio morphologies include spherical, oval, cubic, prism, helical and pillar shapes.

Figure 2.2shows examples of different morphologies of NPs and Nano-composites. Transmission electron microscopy (TEM) images of mono dispersed Cu NPs, Fe nanorods and Cu core–Si shell NPs are shown in figure 2.2(a), (b) and (c), respectively.

The TEM images in figure 2.2(d) and (e) show a porous magnetite NP and magnetite cubes decorated with Ni Nano-crystals, respectively.

These NPs were designed and synthesized using the hydrothermal process for purification of histidine-tagged proteins.

Figure 2.2. TEM images of examples of NPs with different morphologies and compositions. (a) Mono dispersed Cu NPs, (b) Fe nanorods, (c) Cu–Si core–shell NPs, (d) porous Fe3O4 NPs, (e) Fe3O4 cubes decorated with Ni NPs, (f) porous silica spheres with γ-Fe2O3 NPs adsorbed on their surfaces and (g) γ-Fe2O3 NPs embedded in porous silica spheres. For more details about the preparation and characterization of these composites see.

With regard to Nano-composites, substantial progress has been made in recent years in developing technologies in the fields of magnetic microspheres, magnetic Nano-spheres and Ferro fluids.

Nano-spheres and microspheres containing a magnetic core embedded in a non-magnetic matrix are used in numerous biological applications.

They are used, for example, as carriers that can be targeted to a particular site by using an external magnetic field. In addition, the magnetic separation of organic compounds, proteins, nucleic acids and other biomolecules and cells from complex reaction mixtures is becoming the most suitable method for large scale production in bio-industrial purification and extraction processes.

For in vivo applications, it is imperative that well-defined biocompatible coatings surround the magnetic particles to prevent any aggregation and also to enable efficient protection of the body from toxicity.

However, for in vitro applications, biocompatible coatings are not essential; particles can be coated with non-toxic materials inert to chemical and biological media.

The particles employed in all these applications are mainly super-paramagnetic colloids with appropriate coatings, guaranteeing the stability and biocompatibility of the solutions.

Super-paramagnetic NPs exhibit magnetizations of magnitudes similar to those of ferromagnetic materials; however, they have neither coercivity nor remanance.

This behaviour, which is of quantum origin, is limited to Nano-crystals with sizes below the critical size .Conversely; most applications require super-paramagnetic colloidal dispersions with large magnetic responses.

Because the magnetization of a particle is proportional to its volume, the maximum magnetization that one can achieve is limited by the critical size of the super-paramagnetic transition, which depends on the material.

A well-established strategy to create super-paramagnetic particles with larger super-paramagnetic responses is using Nano-composites (see, for example, figure 2.2(f) and (g)).

These super-paramagnetic composites are typically made by embedding super-paramagnetic Nano-crystals in a non-magnetic matrix such as polystyrene or nano-vapours silica.

The resulting colloidal particles retain the super-paramagnetic response of their constituent Nano-crystals and show larger magnetization when an external magnetic field is applied. Furthermore, neither coercivity no remanance is observed at the working temperature.

However, in addition to the intrinsic super-paramagnetic behaviour of the constituent NPs, one must consider the interactions between the NPs inside the skeleton matrix due to their proximity and surface effects due to the coating; these can lead to changes in the overall magnetic response of the colloidal particle.

2.2.1. NP chemical composition

NPs can be composed of a single constituent material or be a composite of several materials. The NPs found in nature are often agglomerations of materials with various compositions, while pure single-composition materials can be easily synthesized using a variety of methods.

There are three main types of chemical ordering in HNPs (figure2.2.1) that describe the way in which the atoms of the elements are arranged within the same NP:

Mixed NPs can be either random or ordered (figure 2.2.1 (a)). Randomly mixed alloys correspond to solutions of solids, whereas ordered Nano alloys correspond to ordered arrangements of A and B atoms.

Core–shell NPs consist of a shell of one type of atom (B) surrounding a core of another type of atom (A) this pattern is generally denoted by A@B and is common for a large class of NPs. A subset of the core–shell category consists of multishell (or 'onion-like') NPs. These NPs have alternating A–B–A shells, or A–B–C in the case of ternary NPs as depicted in figure 2.2.1 (b).

Layered NPs are commonly referred to in the literature as Janus (or 'dumbbell-like') NPs. They consist of two types of NPs (A and B) sharing a common interface (figure 2.2.1 (c)). These types of NPs tend to minimize the number of bonds between elements A and B. This heterojunction structure facilitates phase separation.

Figure 2.2.1. Schematic images of binary NPs: a mixed structure (a), a core–shell structure (b) and a layered structure (c) of A and B elements.

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Figure 2.4. Schematic images of ternary NPs formed of elements A, B and C: multicore–shell morphology (the cores present a dumbbell-like morphology), (b) a core–multishell morphology and (c) an alloyed-core–shell morphology.

Because of the increasing need for multifunctional NPs, other complex structures of NPs such as multicore–shell structures in which the cores can present either 'dumbbell-like' or 'onion-like' structures have been reported in the literature.

2.2.2 NP uniformity and agglomeration

Based on their chemistry and electromagnetic properties, NPs can exist as dispersed aerosols, suspensions/colloids or in an agglomerate state. In fact, NPs tend to adhere to each other and to form agglomerates because of van der Waals forces that act over short distances, magnetic interactions, electrostatic forces present in the particles and adhesion forces related to the liquids adsorbed on their surfaces. Agglomeration due to Brownian motion is classified as 'coagulation'.

To avoid agglomeration, several processes include a post-synthesis stage to modify the particle surface by coating it with another organic or inorganic substance. In an agglomerate state, NPs may behave as larger particles, depending on the size of the agglomerate.

For example, magnetic NPs tend to cluster, forming an agglomerate state, unless their surfaces are coated with a non-magnetic material. Figure 1.6 illustrates the typical process of stabilization of γ-Fe2O3 NPs in an aqueous suspension. The molecules of the anionic surfactant sodium dodecyl sulphate (SDS) are adsorbed onto the surfaces of the Nano-crystals providing a negative charge in water. Therefore, the Nano-crystals in the solution repel each other electrostatically resulting in a stable colloidal suspension. For more details on this procedure see.

Figure 1.6. (a) An illustration of the stabilization process applied to γ-Fe2O3 NPs using SDS surfactant. (b) A TEM image of the precipitated Fe2O3 NPs without SDS. (c) A TEM image of SDS-modified Fe2O3 NPs (adapted from by permission of The Royal Society of Chemistry).

In the case of NPs deposited using vapour phase methods, the NPs are generally deposited on a solid substrate. The transfer of these NPs to a stable suspension is still under investigation.

For example, attempts were made to co-deposit NPs from the vapour phase with a beam of water vapour, methanol, or isopropanol onto a nitrogen-cooled substrate.

However, the stability of the resulting suspensions was not reported. Recently, a simple and environmentally friendly method for harvesting NPs was developed using polyvinylpyrrolidone (PVP) as a stabilizer.

PVP was selected as a non-toxic polymer with good wetting properties. Figure 1.7 shows the procedure used to harvest the NPs to a stable and homogeneous colloidal suspension.

Figure 1.7. (a) A schematic of the exfoliation procedure for the NPs. Step 1: multicore–shell NPs were deposited on a spin-coated PVP film on a glass substrate. Step 2: the NP/PVP/glass samples were immersed in methanol and sonicated for 15 minutes and then separated to remove excess PVP. Step 3: after washing the precipitated NPs with methanol, they were re-suspended in ultrapure water. (b) A dynamic light scattering histogram showing the size distribution of the HNPs. Reproduced from by permission of The Royal Society of Chemistry.

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2.2.3. NP characterization

Once NPs are synthesized, it is important to fully characterize and understand their structure. Over the years, many methods have been developed for this purpose.

Transmission Electron Microscope (TEM):-

TEM is a very powerful technique for the characterization of NP size, composition and crystalline structure. When an electron beam interacts with a sample, the electrons can be transmitted, scattered, backscattered or diffracted.

TEM uses the transmitted electron signal to form an image of the sample. The transmitted electron beam is dependent on the sample thickness; for thin samples (a few nanometres), the transmitted electrons pass through without significant energy loss.

STEM differs from TEM by focusing the electron beam into a narrow spot that is scanned over the sample in a raster.

Because the attenuation of the electrons depends significantly on the density and thickness of the sample, the transmitted electron beam forms a 2D image of the sample.

In hybrid samples, STEM imaging allows the identification of different components based on intensity variation.

This intensity variation is related to the difference in the atomic numbers of each component (Z-contrast).

In addition, the rastering of the beam across the sample makes it possible to couple STEM with other characterization methods such as EELS, allowing direct correlation of image and quantitative data thus obtain details regarding the chemical composition of NPs.

General TEM analysis does not have sufficient resolution to determine the crystallinity of a nanomaterial.

However, high-resolution TEM (HRTEM) can be successfully employed for the characterization of the crystallinity of a sample with atomic resolution, as well as for providing information regarding electron diffraction analysis.

      Figure 1.8. The transmission electron microscope used for the characterization of the NPs

XPS is a surface-sensitive quantitative spectroscopic technique that measures the elemental composition, chemical states and electronic states of the elements within the material.

XPS spectra are obtained by irradiating a material with a beam of x-rays while simultaneously measuring the kinetic energy of the electrons that escape from the top 0 to 10 nm of the material being analysed. XPS requires high vacuum (P ~ 10−8 mbar) or ultra-high vacuum (P < 10−9 mbar) conditions.

However, when used to analyse NPs, the importance of the coverage of the NPs must be kept in mind: high coverage leads to high-quality spectra.

On the other hand, to quantify the composition of the NPs, an inert transfer of the sample to the analysis chamber is necessary to avoid contamination and oxidation of the NP surface.

The system used to analyse the NPs presented in this book is shown in figure 1.9.

Figure 1.9. The XPS system used to analyse the NPs presented in chapters 3 and 4 of this book.

CHAPTER - 3

Introduction to Synthesis of Nanomaterials

Materials scientists are conducting research to develop novel materials with better properties, more functionality and lower cost than the existing one. Several physical, chemical methods have been developed to enhance the performance of nanomaterials displaying improved properties with the aim to have a better control over the particle size, distribution [1].

3.1 Methods to Synthesis of Nanomaterials

In general, top-down and bottom-up are the two main approaches for nanomaterials synthesis.

a. Top-down: size reduction from bulk materials.

b. Bottom-up: material synthesis from atomic level.

Top-down routes are included in the typical solid –state processing of the materials. This route is based with the bulk material and makes it smaller, thus breaking up larger particles by the use of physical processes like crushing, milling or grinding.

Usually this route is not suitable for preparing uniformly shaped materials, and it is very difficult to realize very small particles even with high energy consumption.

The biggest problem with top-down approach is the imperfection of the surface structure. Such imperfection would have a significant impact on physical properties and surface chemistry of nanostructures and nanomaterials.

It is well known that the conventional top-down technique can cause significant crystallographic damage to the processed patterns.

Bottom –up approach refers to the build-up of a material from the bottom: atom-by-atom, molecule-by-molecule or cluster-by-cluster. This route is more often used for preparing most of the Nano-scale materials with the ability to generate a uniform size, shape and distribution.

It effectively covers chemical synthesis and precisely controlled the reaction to inhibit further particle growth. Although the bottom-up approach is nothing new, it plays an important role in the fabrication and processing of nanostructures and nanomaterials.

Synthesis of nanoparticles to have a better control over particles size distribution, morphology, purity, quantity and quality, by employing environment friendly economical processes has always been a challenge for the researchers [2]. The choice of synthesis technique can be a key factor in determining the effectiveness of the photovoltaic as studies. There are many methods of synthesizing titanium dioxide, such as hydrothermal, [3, 4] combustion synthesis, [5] gas-phase methods, [6, 7] microwave synthesis and sol-gel processing [8]. This research focuses on sol-gel processing and characterization techniques which was discussed in great detail.

3.1.1 Hydrothermal Synthesis

Hydrothermal synthesis is typically carried out in a pressurised vessel called an autoclave with the reaction in aqueous solution. The temperature in the autoclave can be raised above the boiling point of water, reaching the pressure of vapour saturation.

Hydrothermal synthesis is widely used for the preparation of TiO2 nanoparticles which can easily be obtained through hydrothermal treatment of peptised precipitates of a titanium precursor with water [9].

The hydrothermal method can be useful to control grain size, particle morphology, crystalline phase and surface chemistry through regulation of the solution composition, reaction temperature, pressure, solvent properties, additives and aging time [10].

3.1.2 Solvothermal Method

The Solvothermal method is identical to the hydrothermal method except that a variety of solvents other than water can be used for this process.

This method has been found to be a versatile route for the synthesis of a wide variety of nanoparticles with narrow size distributions, particularly when organic solvents with high boiling points are chosen.

The solve thermal method normally has better control of the size and shape distributions and the crystallinity than the hydrothermal method, and has been employed to synthesize TiO2 nanoparticles and nanorods with/without the aid of surfactants.

3.1.3 Chemical Vapour Deposition (CVD)

Chemical vapour deposition (CVD) is one of the most common processes used to coat almost any metallic or ceramic compound, including elements, metals and their alloys and intermetallic compounds.

This process is often used in the semiconductor industry to produce high-purity, high-performance thin films.

In a typical CVD process, the substrate is exposed to volatile precursors, which react and/or decompose on the substrate surface to produce the desired film.

Frequently, volatile by products that are produced are removed by gas flow through the reaction chamber. The quality of the deposited materials strongly depends on the reaction temperature, the reaction rate, and the concentration of the precursors [11].

Cao et al. prepared Sn4+-doped TiO2 nanoparticles films by the CVD method and found that more surface defects were

Present on the surface due to doping with Sn [12]. Gracia et al. synthesized M (Cr, V, Fe, Co)-doped TiO2 by CVD and found that TiO2 crystallized into the anatase or rutile structures depending on the type and amount of cation present in the synthesis process. Moreover, upon annealing, partial segregation of the cations in the form of M2On was observed [13]. The advantages of this method include the uniform coating of the nanoparticles or Nano film. However, this process has limitations including the higher temperatures required, and it is difficult to scale up [14].

Chemical vapour deposition is a classical deposition method to produce high quality and high-performance solid thin film materials, such as carbon nanotube, graphene, diamond, and metal.

Therefore, this method was widely employed to grow carbon materials on the surface of clay minerals by chemical reaction and/or decomposition of volatile precursors.

Bakandritsos et al. successfully realized the growth of multi walled carbon nanotubes on the surface of montmorillonite via chemical vapour deposition of acetylene after loading different amounts and types of metal salt catalyst precursors.

It was found that the anion of metal salt controlled the hydrolysis during metal deposition on the surface of montmorillonite and induced the transition to metal carbides during carbon deposition.

Manikandan et al. systemically investigated the growth characteristics of carbon nanotubes on montmorillonite-supported iron catalysts by acetylene decomposition at various experimental conditions.

The results indicated selectivity toward carbon nanotubes increased with increases in the growth temperature. At lower temperatures, carbon deposits were mostly amorphous.

Then they were replaced by crystalline carbon with increases in the temperature, and larger amounts of graphitic carbon structure were observed at 750°C rather than at 850°C due to the deactivation of acid sites on montmorillonite.

When the growth experiments were carried out at the same carbon-feed ratio, the excessive carbon supply tended to deposit as amorphous carbon with the catalysts containing low concentration of iron oxide. On the contrary, either carbon nanotubes or graphitic carbon was formed at the catalysts, containing significant higher amounts of iron oxide.

Under similar conditions, multi walled carbon nanotubes or single-walled carbon nanotubes were also anchored on the surface of kaolinite, nontronite, and sepiolite.

In addition, carbon nanotubes also can be confined to grow in the interlayer spacing of expandable clay minerals. For example, sponge-like exfoliated carbon nanotube/vermiculite hybrids with different contents of carbon nanotube were prepared by intercalating aligned carbon nanotube arrays into natural vermiculite interlayers for oil adsorption.

However, the chemical vapour deposition method has many dis-advantages; for example, the precursors need to be volatile at near-room temperatures. Most of them are toxic, expensive, and dangerous accompanied with the generation of some hazardous matters, such as CO and HF. Therefore, it is hard to prepare carbon/clay minerals Nano-composites on a large scale by chemical vapour deposition.

3.1.4 Thermal Decomposition and Pulsed Laser Ablation

Pure and doped metal nanomaterials can be synthesized via decomposing metal alkoxide and salts by applying high energy using heat or electricity.

However, the properties of the produced nanomaterials strongly depend on the precursor concentrations, the flow rate of the precursors and the environment. Kim et al. synthesized TiO2 nanoparticles with a diameter less than 30 nm via the thermal decomposition of titanium alkoxide or TiCl4 at 1200°C

Liang et al produced TiO2 nanoparticles with a diameter ranging from 3 to 8 nm by pulsed laser ablation of a titanium target immersed in an aqueous solution of surfactant or deionized water.

Nagaveni et al prepared W, V, Ce, Zr, Fe, and Cu ion-doped anatase TiO2 nanoparticles by a solution combustion method and found that the solid solution formation was limited to a narrow range of concentrations of the dopant ions.

However, the drawbacks of these methods are high cost and low yield, and difficulty in controlling the morphology of the synthesized nanomaterials.

3.1.5 Templating

The synthesis of nanostructure materials using the template method has become extremely popular during the last decade. In order to construct materials with a similar morphology of known characterized materials (templates); this method utilizes the morphological properties with reactive deposition or dissolution. Therefore, it is possible to prepare numerous new materials with a regular and controlled morphology on the Nano and micro-scale by simply adjusting the morphology of the template material. A variety of templates have been studied for synthesizing Titania nanomaterials [18, 19]. This method has some disadvantages including the complicated synthetic procedures and, in most cases, templates need to be removed, normally by calcinations, leading to an increase in the cost of the materials and the possibility of contamination.

3.1.6 Combustion

Combustion synthesis leads to highly crystalline particles with large surface areas. The process involves a rapid heating of a solution containing redox groups. During combustion, the temperature reaches approximately 650°C for one or two minutes making the material crystalline. Since the time is so short, the transition from anatase to rutile is inhibited.

3.1.7 Gas Phase Methods

Gas phase methods are ideal for the production of thin films. Gas phase can be carried out chemically or physically. Chemical Vapour Deposition (CVD) is a widely used industrial technique that can coat large areas in a short space of time.

During the procedure, titanium dioxide is formed from a chemical reaction or decomposition of a precursor in the gas phase.

Physical vapour deposition (PVD) is another thin film deposition technique. Films are formed from the gas phase but without a chemical transition from precursor to product.

For TiO2 thin films, a focused beam of electrons heats the titanium dioxide material. The electrons are produced from a tungsten wire heated by a current. This is known as Electron beam (E-beam) evaporation.

Titanium dioxide films deposited with E-beam evaporation have superior characteristics over CVD grown films such as, smoothness, conductivity, presence of contaminations and crystallinity.

Reduced TiO2 powder (heated at 900°C in a hydrogen atmosphere) is necessary for the required conductance needed to focus an electron beam on the TiO2.

3.1.8 Microwave Synthesis

Various TiO2 materials have been synthesised using microwave radiation. Microwave techniques eliminate the use of high temperature calcination for extended periods of time and allow for fast, reproducible synthesis of crystalline TiO2 nanomaterials.

Corradi et al prepared colloidal TiO2 nanoparticle suspensions within 5 minutes using microwave radiation.

High quality rutile rods were developed combining hydrothermal and microwave synthesis, while TiO2 hollow, open ended nanotubes were synthesised through reacting anatase and rutile crystals in NaOH solution.

3.1.9 Conventional Sol-Gel Method

The sol-gel method is a versatile process used for synthesizing various oxide materials.

This synthetic method generally allows control of the texture, the chemical, and the morphological properties of the solid.

This method also has several advantages over other methods, such as allowing impregnation or coprecipitation, which can be used to introduce dopants.

The major advantages of the sol-gel technique includes molecular scale mixing, high purity of the precursors, and homogeneity of the sol-gel products with a high purity of physical, morphological, and chemical properties.

In a typical sol-gel process, a colloidal suspension, or a sol, is formed from the hydrolysis and polymerization reactions of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxide.

A general flowchart for a complete sol-gel process is shown in this Figure

           Figure 3.1 Sol-Gels and Drying Flowchart

Any factor that affects either or both of these reactions is likely to impact the properties of the gel.

These factors, generally referred to as sol-gel parameters, includes type of precursor, type of solvent, water content, acid or base content, precursor concentration, and temperature. These parameters affect the structure of the initial gel and, in turn, the properties of the material at all subsequent processing steps.

After gelation, the wet gel can be optionally aged in its mother liquor, or in another solvent, and washed.

The time between the formation of a gel and its drying, known as aging, is also an important parameter.

A gel is not static during aging but can continue to undergo hydrolysis and condensation.

Table 3.1 Important Parameters in the Various Steps of a Sol-Gel process

This table showed a summary of the key steps in a sol-gel process which includes the aim of each step along with experimental parameters that can be manipulated.

Furthermore, syneresis, which is the expulsion of solvent due to gel shrinkage, and coarsening, which is the dissolution and reprecipitation of particles, can occur.

These phenomena can affect both the chemical and structural properties of the gel after its initial formation. Then it must be dried to remove the solvent.

3.2 Aerogel

One important parameter that affects a sol-gel product is the drying condition. Due to the surface tension of the liquid, a capillary pressure gradient is present in the pore walls and this may be able to collapse most of the pore volume when solvent is removed.

One convenient way to avoid pore collapse is to remove the liquid from the pores above the critical temperature (Tc) and critical pressure (Pc) of the fluid, namely, supercritical drying.

Under supercritical conditions, there is no longer a distinction between the liquid and vapour phases: the densities become equal; there is no liquid-vapour interface and no capillary pressure.

This type of drying prevents the formation a liquid-vapour meniscus which recedes during the emptying of the pores in the wet gels. The resulting dried gel, called an aerogel, has a pore volume similar to that of the wet gel.

3.3 Xerogel

Conventional evaporative drying induces capillary pressure associated with the liquid vapour interface within a pore, causing shrinkage of the gel network.

In a sample with a distribution of pore sizes, the resultant differential capillary pressure often collapses the porous network during drying. The dried sample often has low surface area and pore volume.

3.4 Cryogel

Another way of avoiding the presence of liquid-vapour interface is to freeze the pore liquid and sublime the resulting solid under vacuum.

In this method, the gel liquid is first frozen and thereafter dried by sublimation.

Therefore, the formation of a liquid-vapour meniscus is prevented. The materials obtained are then also termed cryogels. Their surface area and mesopore volume tend to be smaller than those of aerogels, although they remain significant.

However, freeze-drying does not permit the preparation of monolithic gels.

The reason is that the growing crystals reject the gel network, pushing it out of the way until it is stretched to the breaking point. It is this phenomenon that allows gels to be used as hosts for crystal growth: the gel is so effectively excluded that crystals nucleated in the pore liquid are not contaminated with the gel phase; the crystals can grow up to a size of a few millimetres before the strain is so great that macroscopic fractures appear in the gel.

Nevertheless, the gel network may eventually be destroyed by the nucleation and growth of solvent crystals, which tend to produce very large pores.

To attenuate this event, a rapid freeze process known as flash freezing has been developed. It is also important that the solvent has a low expansion coefficient and a high pressure of sublimation.

3.5 Applications of sol-gel method

Applications for sol-gel process derive from the various special shapes obtained directly from the gel state (monoliths, films, fibres, and monosized powders) combined with compositional and microstructural control and low processing temperatures.

Compared with other methods, such as the solid-state method, the advantages of using sol-gel process include.

The use of synthetic chemicals rather than minerals enables high purity materials to be synthesized.

It involves the use of liquid solutions as mixtures of raw materials. Since the mixing is with low viscosity liquids, homogenization can be achieved at a molecular level in a short time.

Since the precursors are well mixed in the solutions, they are likely to be equally well-mixed at the molecular level when the gel is formed; thus on heating the gel, chemical reaction will be easy and at a low temperature.

Changing physical characteristics such as pore size distribution and pore volume can be achieved.

Incorporating multiple components in a single step can be achieved.

Producing different physical forms of samples is manageable.

4.3.2.4 Inert Gas Condensation

Inert gas condensation (IGC) is a process in which vaporized material is rapidly cooled into solid phase by interactions with a condenser gas. In 1930 Pfund synthesized bismuth nanoparticles by evaporating bismuth over tungsten wire.

However, a contemporary version of the IGC process was developed by Gleiter's group to synthesize Fe Nano-particles.

In this process, the vaporized material is obtained by resistive heating of a refractory metal boat or crucible which acts as the evaporation source.

The material to be evaporated is supplied to the source either by directly placing it on the source before the evaporation or by a variety of continuous loading mechanisms during evaporation, e.g. wire insertion.

Interactions between the vaporized material with inert gas molecules and chamber walls provide the rapid cooling. Upon cooling, vaporized material supersaturates and nucleation starts.

Nuclei are taken away from the growth region by either a convective or a forced flow. Finally, the particles are collected on a cold surface that is in the form of a solid cold finger or a filter with porous surface that can trap the particles.

Important parameters controlling particle size Gas pressure, evaporation temperature, inert gas type, and cooling rate are the main parameters that control the particle size and shape in the IGC process.

Studies show that pressure is the most important parameter that affects the particle size as compared to all other parameters.

The total gas pressure in the chamber regulates the diffusion rate of the vaporized material from the growth region and, therefore, the particle size.

Increasing vapour pressure by increasing flux of the evaporated species in the growth region forces more coalescence which results in the formation of larger particles.

Evaporation temperature also affects particle size, albeit in a smaller way.

Since the kinetic energy of the particles gets larger with the increase of the evaporation temperature, the nuclei possess excess energy and coalesce after collisions. Evaporation temperature is also connected with the vapour pressure.

Lower evaporation temperature corresponds to lower vapour pressure, yielding smaller particles. Inert gas type also plays an important role on the particle size.

It is the size of the inert gas molecules that changes the dynamics. Since massive gas particles can block the escape of the evaporated particles from the growth region more efficiently than the less massive ones, the resulting particle size is smaller for inert gas with lower molar (atomic) mass, for instance helium.

Thermal conductivity of the inert gas also has a secondary effect. Helium is usually chosen owing to its highest thermal conductivity amongst all the noble gases and provides a higher cooling rate.

Particle growth processes-

Particle growth in the IGC process can be classified into three stages: nucleation, coalescent coagulation, and agglomeration.

These three stages are depicted .Initially; the temperature in the vicinity of the evaporation source is high to obtain a reasonable vapour pressure of the evaporant.

As the evaporant moves away from the source, the temperature decreases, causing super saturation of the vaporized material leading to the homogeneous nucleation in the gas phase.

At high super saturation, a large amount of small particles is formed upon rapid nucleation of the vapours.

Nucleation usually starts very fast and continues at a very high rate. The nucleation process reduces the super saturation and slows down further nucleation.

Particles subsequently grow by Brownian coagulation as they are removed from the evaporation region to the particle collection region. Particles coalesce quickly after coagulation.

Further decrease in temperature slows down the coalescence rate and agglomerate growth takes over.

Figure 4.5. Schematic diagram of the processes that contribute to particle growth in IGC.

Particles produced by the IGC technique can form agglomerates by either hard or soft agglomeration depending on the material properties, the atmosphere, and the temperature–time history.

Hard agglomeration is a result of strong bonding of primary particles via neck formation, whereas soft agglomeration results from the weak bonding of primary particles due to van der Waals’ forces.

Although soft agglomerates can be separated into their constituent primary particles, separation of hard agglomerates is extremely difficult. Details on particle growth mechanism in IGC are reported by Flagan and Lunden.

A schematic diagram of the IGC system is shown in Figure 4.6. It is composed of two main sections. The left side, where there are two ports for two different evaporation sources, is for the production of vaporized material, and the right side is for the collection of the particles. Evacuation of the system can be maintained by mechanical and turbo pumps connected in series.

Figure 4.6. Schematic diagram of the IGC system.

1: Evaporation boat; 2: stainless steel filter; 3: hopper for the collection of particles; 4: wire feeding unit; 5: power supply; 6: inert gas cylinder; 7: turbo pump; 8: roots blower; 9: mechanical pump; 10: gas circulation line.

Pressure readings are taken by thermocouple and cold cathode gauges. Inert gas circulation is realized by a roots blower. A suitable power supply is used for the resistive evaporation.

The maximum temperature that can be reached depends on the evaporation source and the power supply.

In order to maintain continuous particle production, a wire feeding mechanism is used to provide material to the hot boat. Monometallic wires, like Fe and Ag, or bimetallic wire in the desired ratio for alloys like CuAg or NiFe, can be evaporated.

Ward et al. reported the preparation of Mn nanoparticles from evaporation of Mn powder by IGC.

In the IGC technique the collection of the particles is done by condensing the agglomerates either on a liquid nitrogen cooled surface or on a cylindrical stainless steel filter that has micro-meter-size pores on its surface, placed in the collection chamber.

Circulating inert gas passes through the filter while leaving the agglomerates on the filter surface.

Since the particles reaching the filter are agglomerates of micro-meter size, they are easily captured by the pores.

Particles accumulated on the filter surface are lifted off from the filter surface by occasionally reversing the inert gas flow direction.

Reverse flow knocks the particles off the filter surface. The displaced particles fall down into the hopper underneath the collection chamber.

There are minor variations in the IGC processes depending on what type of material is being evaporated.

For example, powder evaporation requires a specific evaporator, whereas wire evaporation can be done on a continuous basis, as shown in Figure 4.6. Selection of the evaporation source also depends on the materials being evaporated.

Lithography

The main technology to realize a very tiny feature size for Nano-components is lithography. Optical lithography is the key technology to be utilized today and it is expected to be relevant beyond 70 and 100 nm with the use of 157-nm wavelength and 193-nm wavelength tools, respectively.

Electron beam lithography (EBL)-

Electron beam lithography (EBL) is a powerful technique for creating nanostructures that are too small to fabricate with conventional photolithography. State of the art EBL systems can achieve resolutions of a few nanometres.

The technique works by moving a highly focussed electron beam over a sample to write out a pattern designed with suitable CAD tools.

The pattern is recorded in an electron sensitive film (or resist) deposited on the sample before exposure by spin coating. The electron beam induces a change in the molecular structure and solubility of the resist film. Following exposure to the electron beam, the resist is developed in a suitable solvent to selectively dissolve either the exposed or unexposed areas of the resist

Electron beam lithography refers to a direct writing lithographic process that uses a focused beam of electrons to form patterns by material modification, material deposition (additive), or material removal (subtractive).

In the material modification mode the electrons have an energy that is sufficient to cause a chemical or structural modification of a surface, in material deposition electrons are used to induce the deposition of a volatile compound on a surface, and in the removal mode the e-beam is energetic enough to remove sections of material from a substrate.

Electron lithography offers higher patterning resolution because of the shorter wavelength associated with the 10–100 keV electrons involved.

There are two basic ways to scan an electron beam – raster scanning and vector scanning. In raster scanning, the image is partitioned in pixels that are printed in a left-to-right/top-to-bottom sequence.

The beam sweeps the substrate horizontally left to right at a steady rate, turning on when surface exposure is needed and turning off when exposure is not required.

After one line is finished, the beam blanks and then moves back to the left where it starts sweeping the next line.

In raster scanning, the beam scans the whole surface even in those areas where no features are present.

In vector scanning, the image is partitioned in features (vectors). The electron beam is directed only to the specific positions where features are present, and hops from feature to feature. In vector scanning, time is saved because the beam is able to move in any direction and does not scan the whole surface.

Chapter- 4

Aluminium synthesis

Nanotechnology plays an increasingly crucial role in many key technologies of the new millennium. The application of Nano scale materials and structures, usually ranging from 1 to 100 nm, is an emerging area of Nano science and nanotechnology.

Nanoparticles show unique properties compared to the bulk metals therefore a lot of research work has been reported for the synthesis and applications of metal nanoparticles.

Some of these metals (particularly aluminum) are previously widely used in energetic material formulations.

Aluminium nanoparticles are of interest to a variety of fields including pyrotechnic, propellant, and explosive industries. Aluminium powder has been added to a range of these compositions to increase their performance through raising reaction energies, flame temperatures, and increasing blast rates.

Nanoparticles of aluminium are more favourable because of their high enthalpy of combustion and rapid kinetics which increase these reaction properties even further.

It is known that Nano-sized aluminum particles is a new energetic material with very high reactivity because of large specific surface area, and is expected to be applied to a next generation propellant in the field of aerospace applications.

The reactivity of aluminum nanoparticle depends on the particle diameter. It is reported that 30-50 nm aluminum nanoparticles are most sensitive.

Physical and chemical properties of aluminum (Al) and especially its nanoparticles, are favorable enough to make them applicable in a variety of applications such as alloy powder metallurgy parts for automobiles and aircrafts, heat shielding coatings of aircrafts, corrosion, resistant, conductive and heat reflecting paints, conductive and decorative plastics, soldering and termite welding, pyrotechnics and military applications (rocket fuel, igniter, smokes, and tracers).

Nano scale Al particles are also studied as high-capacity hydrogen storage materials.

There has been an extraordinary growth in nanoscience and technology in recent years, mainly due to both the development of new techniques to synthesize nanomaterials and the accessibility of tools for the classification and manipulation of nanoparticles.

Production of nanoparticles requires understanding of the fundamentals of nanoscale chemistry and physics, and know-how to commercialize them. Broadly speaking, there are two approaches to nanoparticle production: top-down and bottom-up.

The former makes a material decrease its size from large to nanoscale, whereas the latter produces nanomaterials by starting from the atomic level.

Generally, metal nanoparticles can be prepared and stabilized by chemical, physical and biological methods; the chemical approach, such as chemical reduction, electrochemical techniques, photochemical reduction and pyrolysis and physical methods, such as Arc-discharge and physical vapor condensation (PVC) is used.

Living organisms have huge potential for the production of nanoparticles/nanodevices of wide applications.

Nevertheless high purity powders and nanopowders of active metals such as Al are not easily synthesized in as much as their rapid oxidation occurs easily.

The small sizes of aluminum nanoparticles make them particularly susceptible to excessive oxidation while being stored prior to use.

Typically, the thickness of an oxide coating on an aluminum particle ranges from 1.7 to 6.0 nm, irrespective of the size of the particle. If the passivating coatings also have an affinity for the binder material, then mixing problems can be resolved as well.

Studies have shown that the size, morphology, stability and properties (chemical and physical) of the metal nanoparticles are strongly influenced by the experimental conditions, the kinetics of interaction of metal ions with reducing agents, and adsorption processes of stabilizing agent with metal nanoparticles.

Hence, the design of a synthesis method in which the size, morphology, stability and properties are controlled has become a major field of interest.

It attempts to present an overview of Al nanoparticles preparation by various methods.

4.1 Al nanoparticles Synthesis

Synthesis of nanomaterials by a simple, low cost and in high yield has been a great challenge since the very early development of nanoscience.

The techniques for synthesizing aluminum nanoparticles can be divided into solid-phase, liquid-phase and gas-phaseprocesses.

The solid-phase techniques include mechanical ball milling and mechanochemical, the liquid-phase techniques include laser ablation, exploding wire, solution reduction, and decomposition process, whereas the gas-phase processes include gas evaporation, exploding wire, and laser ablation process.

4.2 Solid-Phase Synthesis

4.2.1 Mechanical Ball Milling

Mechanical milling as a solid state synthesis usually performed using ball milling equipments that generally divided to “low energy” and “high energy” category based on the value of induced the mechanical energy to the powder mixture.

The objective of milling is to reduce the particle size and blending of particles in new phases.

The different type of ball milling can be used for synthesis of nanomaterials in which balls impact upon the powder charge.

High-energy ball milling is a convenient way to produce nanosized powders. It is the most common method reported in the literature for the synthesis of intermetallic nanoparticles.

Before a mechanical milling is started, powder(s) is loaded together with several heavy balls (steel or tungsten carbide) in a container.

By vigorously shaking or high-speed rotation, a high mechanical energy is applied on the powders because of collision with heavy balls. As in the figure shown in below-

.

A rock tumbler Ball milling

Mechanical ball milling has been used to blend aluminum with magnesium and carbon in order to alter its chemical properties and combustion behavior. The studies to make blends with magnesium used particles tens of micrometers in size.

4.2.2 Mechanochemical Synthesis

Mechanochemistry is the coupling of mechanical and chemical phenomena on a molecular scale and includes mechanical breakage and chemical behavior of mechanically-stressed solids.

Mechanochemical synthesis differs from standard ball milling. A standard ball milling process under inert atmosphere results in a moderate reduction of powder particle size and eventually the formation of nanosized grains within micron-sized particles.

The mechanochemical method involves the initiation of a solid-state displacement reaction during the ball milling process which can result in nanosized particles (down to ∼5 nm in size) embedded within larger by-product phase particles.

In mechanochemical processes that utilize to change the chemical composition of precursors, the high energy ball milling equipments is generally used.

The mechanochemical synthesis process has been used in the past to synthesize a broad range of metal nanoparticles (e.g. Ag, Co, Cr, Cu…) as well as other compounds such as oxides and sulphides.

Particle size control can be gained by adjusting factors such as: the volume fraction of the by-product phase formed during milling, milling time, milling collision energy (ball-to-powder mass ratio and ball size), milling temperature, and the use of process control agents.

In 2009, a mechanochemical synthesis process has been used to synthesise aluminium nanoparticles.

The aluminium is synthesised via a solid state chemical reaction which is initiated inside a ball mill at room temperature between either lithium (Li) or sodium (Na) metal which act as reducing agents with unreduced aluminium chloride (AlCl3). The Al nanoparticles were ∼25–100 nm in dimensions as measured by TEM.

4.3 Liquid-Phase Synthesis

4.3.1 Laser ablation

Pulsed laser ablative deposition (PLD) is an attractive synthetic method owing to its ability to produce nanoparticles with a narrow size distribution and a low level of impurities.

Aluminum nanoparticles with diameters of tens to 500 nm of various shapes can be prepared by irradiating an aluminum foil with 50 fs pulses of a 0.8 μm wavelength laser beam.

Three main steps contribute in laser ablation synthesis method and formation of nanoparticles from a target immersed in liquid. Laser pulse, first, heats up the target surface to the boiling point, and thus, plasma plume containing vapor atoms of target is generated.

Then, plasma expands adiabatically; and finally, nanoparticles will be generated when condensation occurs. Synthesis parameters such as laser wavelength, laser energy, pulse width, liquid media type, and ablation time can notably affect the product characteristics.

In 2010, Aluminum nanoparticles were synthesized by pulsed laser ablation of Al targets in ethanol, acetone, and ethylene glycol.

Comparison between ethanol and acetone clarified that acetone medium leads to finer nanoparticles (mean diameter of 30 nm) with narrower size distribution (from 10 to 100 nm).

Hur et al. report the synthesis of Mg-Al and Zn-Al-layered double hydroxides using the laser ablation in the liquid technique.

Average diameters of these structures were about 500 nm and the thickness of a single layer was approximately about 6.0 nm.

                      TEM view of nanoparticles generated by ablation of bulk Al target  

                          In ethenoyle

In 2009, aluminium nanoparticles were produced by pulsed laser ablation of a sample of pure aluminium situated in distilled water.

They provides the possibility to generate a large variety of nanoparticles that are free of both surfaceactive substances and counter-ions.

The sample was irradiated by the focused output of the third harmonics of pulsed nanosecond Nd: YAG laser operating at 10 Hz frequency. The typical thickness of the liquid above the target was 10 mm.

Stratakis et al. reported on Generation of Al nanoparticles via ablation of bulk Al in liquids with short laser pulses.

The colloidal nanoparticles solutions obtained with fs pulses exhibit a yellow coloration and show an increased optical absorption between 300 and 400 nm, tentatively assigned to the plasmon resonance of nanosized Al.

Generated Al nanoparticles exhibit minimal oxide cladding and were pretty stable as they became slowly oxidized by air oxygen. The average size of Al nanoparticles formed lies between 10 and 60 nm, depending on the experimental conditions.

This is illustrated in Fig. where the NPs generated using 150 ps pulses have a core-shell structure with metallic core. On the other hand presence of an oxide layer is well seen on TEM images of NPs produced by ablation with ps pulses without using anaerobic conditions.

In 2012, it was investigated the viability of laser ablation of W and Al metal targets immersed in acetone and water, respectively, as a technique to produce metal nanoparticles. The setup used for the synthesis of metal particles by laser ablation of a metal target in liquid medium is presented in Figure.

Figure   Schematic representation of the       experimental setup (laser ablation)

Particles size distributions peak were relatively narrow, peaking at 50 – 70 nm.

One problem with this method is that in long ablation times, the ablation rate decreases.

It occurs when high concentrations of nanoparticles in produced colloidal solution blocks the laser path, and thus, a part of laser energy is absorbed by formerly synthesized nanoparticles instead of the target surface.

4.3.2  Exploding Wire

Electro-explosion of metal wires has only recently been seriously applied to make aluminium nanoparticles.

In electro-explosion, a brief but powerful current pulse creates an electromagnetic field around the wire that holds it together while it is superheated to tens of thousands of degrees.

When the current ceases, the electromagnetic field disappears and the wire fragments into Nano sized particles.

The shapes and sizes of the resulting particles depend on many factors, such as the shape and size of the wire, the voltage, and the nature of the electrical pulse.

Sent et al. described a process for the production of nanoparticles of Cu, Ag, Fe and Al which involves exploding their respective wires, triggered by large current densities in the wires.

The explosion was carried out in a dense medium, typically water or some heavy alcohol where the particles remain suspended and is collected in the following manner.

An initial centrifuge of the suspension at 5000 RPM separates the fluid from the solid mass. While the former is rejected, the solid mass is dispersed in electronic grade acetone.

4.3.3 Solution Reduction

In this method, it is used from a reducing agent for the reduction of Al+3 ions in solution.

The chemical reduction route is simple and not time consuming. It has also an immense potential to scale up when required to meet mass manufacturing needs.

Treatment of aluminium chloride with lithium aluminium hydride in methylene at 164 °C affords aluminium nanoparticles. The nanoparticle aggregates made by this method were 110 – 210 nm in diameter.

This method proved to be inconvenient for scale up and, even after being washed, the nanoparticles still contained measurable levels of carbon, oxygen, and chlorine.

In 2012, Al nanoparticles were synthesized by solution reduction process successfully. They used from benzildiethylenetriamine as a reducing agent in methanol, ethanol, water, acetonitrile, cyclohexane and dimethylsulphoxide.

The best results obtained by the ethanol for the synthesis of Al nanoparticles in the range of 4–13 nm.

The influences of parameters on the size of Al nanoparticles were studied and the referential process parameters were obtained [2].Also Al nanoparticles (5–8 nm size) have been synthesized by using NaBH4 or LiAlH4,

In another study, it reported the synthesis of Al/Au bimetallic nanoparticles in water solution. They used from Al+3 and Au+3 metal salts and reducing solution contain sodium citrate, tannic acid, and sodium carbonate.

In 2012, the synthesis of aluminium nanoparticles in a polypropylene (PP) matrix by a sol–gel process in the melt was investigated.

Their work confirms that it is possible to produce inorganic nanoparticles in a polymeric matrix by reaction in the molten state without solvents.

Generally in chemical reduction method, reducing agent is a chemical solution such as benzildiethylenetriamine, lithium aluminium hydride and etc.

4.4 Decomposition Process

The chemical route based on thermal and/or catalytic decomposition of alane in the presence of a surface passivation agent for particle protection and stabilization has been identified as being particularly promising.

The passivation agent for Al nanoparticles could be a metal coating or organic molecules such as per fluorinated carboxylic acids, which could also serve as an oxidant source under energetic conditions.

This method has generally yielded Al particles of 50-200 nm in average sizes, though smaller particles have been obtained recently in sonochemical environment with oleic acid as the surface passivation agent.

Li et al. reported the use of Nano scale cavities in per fluorinated ionomer membrane as templates for the facile synthesis of small Al nanoparticles (diameters on the order of 10 nm) via catalytic decomposition of an alane precursor.

Aluminium nanoparticles can be obtained on a larger scale by decomposing isolated samples of the alane amine adduct H3Al (NMe2Et) in methylene at 164 °C.

The aluminium nanoparticles aggregates synthesized by this method were with diameters of 44 – 82 nm.

In another study, pure Mg-Al alloys and Ni nanoparticles prepared by thermal decomposition on bipyridyl complex of metals.

In 2009, the synthesis of aluminium nanoparticles was investigated systematically using dimethyl ethylamine alane and 1-methylpyrrolidine alane as precursors and molecules with one or a pair of carboxylic acid groups as surface passivation agents.

Meziani et al. found that the passivation agent played dual roles of trapping aluminium particles to remain them Nano-scale during the alane decomposition and protecting the aluminium nanoparticles from surface oxidation.

4.5 Gas-Phase Synthesis

4.5.1 Gas Evaporation-

The most common method to synthesize aluminium nanoparticles is the evaporation of aluminium from the molten state into a chamber filled with an inert gas, where the gaseous metal condenses.

The purity of the aluminium starting material, and the type and purity of the inert gas atmosphere, strongly influence the properties of the aluminium nanoparticles obtained.

A modified inert gas evaporation method called cryomelting can also be used to make aluminum nanoparticles. In the cryomelting process, the evaporated metal is rapidly condensed in region cooled to about 70 K.

This method can produce 20 – 500 nm aluminum nanoparticles in which 60% of the particles are smaller 70 nm in size, as observed by TEM.

In another research, the thermal behavior of aluminum nanoparticles prepared by inert gas condensation process was investigated.

Also Al nanoparticles were prepared by the inert gas condensation method by Fernández et al. It was found the presence of an alumina overlayer of approximately 4 nm covering the aluminium nanoparticles (23 nm in diameter).

In 2010, a novel electromagnetic levitational gas condensation (ELGC) system was designed and manufactured for the synthesis of aluminum nanoparticles. It was found that the best argon flow rate for the synthesis of aluminum nanoparticles was found about 10-15 lit/min.

4.5.2 Exploding Wire-

Wire explosion is basically a top-down approach to produce metallic Nano-powders.

A pulsed discharge system is used to supply a high power pulsed current to a thin metal wire and lead to the wire explosion. Large amount of heat from Joule heating will be dissipated in the wire to melt, evaporate and subsequently ionize it.

Plasma formed during the process expands due to its high temperature and high density.

This plasma will be rapidly cooled during expansion when it interacts with the surrounding gas and nanoparticles will be formed through nucleation process.

Yap et al. (2008) reported exploding wire discharge for synthesis of Al nanoparticles. The Al nanoparticles were less than 100 nm in dimensions as measured by SEM images.

The schematic of the wire explosion chamber is shown in fig. In another study, Nano aluminium particles were produced by wire explosion process (WEP) in nitrogen, argon and helium atmospheres.

The relationship between size of the particle generated in the explosion process and the type of inert gas/pressure was analysed.

It is realized that energy deposited to the conductor and duration of current flow have major impact on particles produced by this process [39]. In 2012, an experimental device based on the electrical explosion of metallic wires for the nanopowders production and collection was designed and built.

Also, aluminum nanopowders were produced by electrical exploding aluminum wire and collected by the microporous membrane filter successfully under different experimental conditions.

Also Sindhu et al. proposed a modelling of the nanoparticles formation in the wire explosion process.

It was found that the plasma formed during the explosion plays a major role in the particle formation, and the modelling studies confirm that particle formation is not an instantaneous process but requires a certain time period to form stable sizes and shapes.

4.6 Laser Ablation

As mentioned above, pulsed laser ablation (PLA) is an attractive synthetic method owing to its ability to produce nanoparticles with a narrow size distribution.

In the view of gas dynamics, the PLA process can be classified into (i) evaporation of the target material and (ii) hydrodynamic expansion of the ablated plume into the ambient gas.

In nanoparticle formation, the following stages must be considered: (i) homogeneous nucleation, where vapor atoms produced by laser ablation have been supersaturated, and (ii) particle growth, where the critical nuclei are growing, capturing atoms on their surfaces, and making the transition into large particles.

Aluminum nanoparticles were synthesized using laser ablation method in argon gas as ambient gas by Yamamoto et al.

They found that it is possible to control the particle size synthesized by controlling the ambient gas temperature.

In addition, Al nanoparticles generated by laser ablation can be coated with carbon by introducing ethylene to the argon quench flow. The resulting nanoparticles have an average mobility diameter of 80 nm.

In 2011, the process of nanoparticle generation during nanosecond and picosecond laser ablation of various metals (Ni, Al, W and stainless steel) in ambient air and argon gas was investigated.

It was found that the size distribution and number concentration of generated metal particles during the laser ablation in ambient air differed from those in argon gas medium.

The number concentrations of generated nanoparticles during the laser ablation in argon gas, compared to the produced nanoparticle concentrations in ambient air, were up to 100 times higher.

Chapter 5

Conclusions

Al nanoparticles are one of the most important nanoparticles because of their applications.

These nanoparticles have many important applications that include: pyrotechnic, propellant, explosive industries, rocket fuel, igniter, smokes, tracers, alloy powder metallurgy parts for automobiles and aircrafts, heat shielding coatings of aircrafts, corrosion, resistant, conductive and heat reflecting paints, conductive and decorative plastics, soldering and termite welding.

Application of Al nanoparticles in these fields is dependent on the ability to synthesize particles with different chemical composition, shape, size, and monodispersity.

Generally, there are various methods to synthesize Al nanoparticles. The techniques for synthesizing aluminum nanoparticles can be divided into solid-phase, liquid-phase and gas-phaseprocesses.

Nevertheless high purity powders and nanopowders of active metals such as Al are not easily synthesized in as much as their rapid oxidation occurs easily.

The small sizes of aluminum nanoparticles make them particularly susceptible to excessive oxidation while being stored prior to use.

As a result, much attention has been devoted to modifying the aluminum nanoparticles in order passivate the surface against the formation of an oxide overlayer, and thereby obtain longer shelf lives and better burn properties.

Chemical vapor deposition of micro/nanostructured coatings involves the chemical reactions of gaseous reactants on the surface of a heated substrate.

This bottom-up manufacturing process can provide highly pure coatings with precise control at atomic or Nano-metric scale. It can fabricate monolayers, multilayers, Nano composite layers, as well as nanostructured and also functionally graded layers at relatively low processing temperatures.

This project reviews this powerful technique for fabrication of micro/nanostructured coatings from its principles to applications and also the future view of this useful technique.

The discussions include a lot of examples and images from published for better understanding.

This project deals with the synthesis of Nano-particles, and the synthesis and fabrication of Nano-composites—metal, ceramic, and polymeric. Various methods used to synthesize nanoparticles, such as coprecipitation, hydrothermal synthesis, inert gas condensation, ion sputtering scattering, microemulsion, microwave, pulse laser ablation, sol-gel, sonochemical, spark discharge, template synthesis, and biological synthesis, will be described.

The synthesis of metal Nano composites includes spray pyrolysis, liquid infiltration, the rapid solidification process, high-energy ball milling, chemical vapor deposition, physical vapor deposition, and chemical processes—sol-gel and colloidal.

The synthesis of ceramic nanocomposites includes the powder process, polymer precursor process, and the sol-gel process.

Finally, the fabrication of polymer nanocomposites includes intercalation, in situ intercalative polymerization, melt intercalation, template synthesis, mixing, in situ polymerization, and the sol-gel process.

The bottom-up technique involves molecular fabrication and self-assembly processes—assembling a larger whole beginning with very minute building blocks like molecules and atoms.

This “bottom-up” method is regarded as the path to upcoming processes and products, integrating chemistry, physics, biomimetic, novel engineering, information technology, and metrology and characterization methods.