CHAPTER TWO
LITERATURE REVIEW
2.1Composite Materials
A composite material is made by combining two or more materials oin most cases those that have very different properties that results in better properties than those of the individual components used alone (Campbell 2010). Notably, within the composite one can easily tell the different materials apart as they do not dissolve or blend into each other. The properties and performance of composites are far superior to those of the constituents. Composites consist of one or more discontinuous phases (reinforcement) embedded in a continuous phase (matrix). Examples are cemented carbides, rubber mixed with carbon black, wood among others.
Classification of Composite Materials
Composite materials may be classified Based on the type of matrix material and Based on the geometry of reinforcement. Based on the type of matrix material, a composite may be classifies into polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs) and carbon/carbon composites (C/Cs). Based on the geometry of reinforcement, a composite may be classified into, particulate reinforced composites, whisker/flakes reinforced composites and fiber reinforced composites.

Types of Matrix Material
Metal Matrix Composites (MMCs)
Metal matrix composites, at present though generating a wide interest in research fraternity, are not as widely in use as their plastic counterparts. High strength, fracture toughness and stiffness are offered by metal matrices than those offered by their polymer counterparts. They can withstand elevated temperature in corrosive environment than polymer composites. Most metals and alloys could be used as matrices and they require reinforcement materials which need to be stable over a range of temperature and non-reactive too. However the guiding aspect for the choice depends essentially on the matrix material. Light metals form the matrix for temperature application and the reinforcements in addition to the aforementioned reasons are characterized by high moduli.

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Most metals and alloys make good matrices. However, practically, the choices for low temperature applications are not many. Only light metals are responsive, with their low density proving an advantage. Titanium, Aluminium and magnesium are the popular matrix metals currently in vogue, which are particularly useful for aircraft applications. If metallic matrix materials have to offer high strength, they require high modulus reinforcements. The strength-to-weight ratios of resulting composites can be higher than most alloys.

The melting point, physical and mechanical properties of the composite at various temperatures determine the service temperature of composites. Most metals, ceramics and compounds can be used with matrices of low melting point alloys. The choice of reinforcements becomes more stunted with increase in the melting temperature of matrix materials.

Polymer Matrix Composites (PMCs)
Polymers make ideal materials as they can be processed easily, possess lightweight, and desirable mechanical properties. It follows, therefore, that high temperature resins are extensively used in aeronautical applications. There are two main kinds of polymers are thermosets and thermoplastics. Thermosets have qualities such as a well-bonded three-dimensional molecular structure after curing. They decompose instead of melting on hardening. Merely changing the basic composition of the resin is enough to alter the conditions suitably for curing and determine its other characteristics. They can be retained in a partially cured condition too over prolonged periods of time, rendering Thermosets very flexible. Thus, they are most suited as matrix bases for advanced conditions fiber reinforced composites. Thermosets find wide ranging applications in the chopped fiber composites form particularly when a premixed or moulding compound with fibers of specific quality and aspect ratio happens to be starting material as in epoxy, polymer and phenolic polyamide resins.
Thermoplastics have one- or two-dimensional molecular structure and they tend to at an elevated temperature and show exaggerated melting point. Another advantage is that the process of softening at elevated temperatures can reversed to regain its properties during cooling, facilitating applications of conventional compress techniques to mould the compounds. Resins reinforced with thermoplastics now comprised an emerging group of composites. The theme of most experiments in this area to improve the base properties of the resins and extract the greatest functional advantages from them in new avenues, including attempts to replace metals in die-casting processes. In crystalline thermoplastics, the reinforcement affects the morphology to a considerable extent, prompting the reinforcement to empower nucleation. Whenever crystalline or amorphous, these resins possess the facility to alter their creep over an extensive range of temperature. But this range includes the point at which the usage of resins is constrained, and the reinforcement in such systems can increase the failure load as well as creep resistance.

A small quantum of shrinkage and the tendency of the shape to retain its original form are also to be accounted for. But reinforcements can change this condition too. The advantage of thermoplastics systems over thermosets are that there are no chemical reactions involved, which often result in the release of gases or heat. Manufacturing is limited by the time required for heating, shaping and cooling the structures. There are a few options to increase heat resistance in thermoplastics. Addition of fillers raises the heat resistance. But all thermoplastic composites tend loose their strength at elevated temperatures. However, their redeeming qualities like rigidity, toughness and ability to repudiate creep, place thermoplastics in the important composite materials bracket. They are used in automotive control panels, electronic products encasement etc.
Polyester resins on the other hand are quite easily accessible, cheap and find use in a wide range of fields. Liquid polyesters are stored at room temperature for months, sometimes for years and the mere addition of a catalyst can cure the matrix material within a short time.
Ceramics Matrix Composite
Ceramics can be described as solid materials which exhibit very strong ionic bonding in general and in few cases covalent bonding. High melting points, good corrosion resistance, stability at elevated temperatures and high compressive strength, render ceramic-based matrix materials a favourite for applications requiring a structural material that doesn’t give way at temperatures above 1500ºC. Naturally, ceramic matrices are the obvious choice for high temperature applications.

High modulus of elasticity and low tensile strain, which most ceramics posses, have combined to cause the failure of attempts to add reinforcements to obtain strength improvement. This is because at the stress levels at which ceramics rupture, there is insufficient elongation of the matrix which keeps composite from transferring an effective quantum of load to the reinforcement and the composite may fail unless the percentage of fiber volume is high enough. A material is reinforcement to utilize the higher tensile strength of the fiber, to produce an increase in load bearing capacity of the matrix. Addition of high-strength fiber to a weaker ceramic has not always been successful and often the resultant composite has proved to be weaker. The use of reinforcement with high modulus of elasticity may take care of the problem to some extent and presents pre-stressing of the fiber in the ceramic matrix is being increasingly resorted to as an option.

When ceramics have a higher thermal expansion coefficient than reinforcement materials, the resultant composite is unlikely to have a superior level of strength. In that case, the composite will develop strength within ceramic at the time of cooling resulting in micro cracks extending from fiber to fiber within the matrix. Micro cracking can result in a composite with tensile strength lower than that of the matrix.

Reinforcements
Reinforcements for the composites can be fibers, fabrics particles or whiskers. Fibers are essentially characterized by one very long axis with other two axes either often circular or near circular. Particles have no preferred orientation and so does their shape. Whiskers have a preferred shape but are small both in diameter and length as compared to fibers.
Reinforcing constituents in composites, as the word indicates, provide the strength that makes the composite what it is. But they also serve certain additional purposes of heat resistance or conduction, resistance to corrosion and provide rigidity. Reinforcement can be made to perform all or one of these functions as per the requirements.

A reinforcement that embellishes the matrix strength must be stronger and stiffer than the matrix and capable of changing failure mechanism to the advantage of the composite. This means that the ductility should be minimal or even nil the composite must behave as brittle as possible.

Types of Reinforcement
Particles
Microstructures of metal and ceramics composites, which show particles of one phase strewn in the other, are known as particle reinforced composites. Square, triangular and round shapes of reinforcement are known, but the dimensions of all their sides are observed to be more or less equal. The size and volume concentration of the dispersoid distinguishes it from dispersion hardened materials.

The dispersed size in particulate composites is of the order of a few microns and volume concentration is greater than 28%. The difference between particulate composite and dispersion strengthened ones is, thus, oblivious. The mechanism used to strengthen each of them is also different. The dispersed in the dispersion-strengthen materials reinforces the matrix alloy by arresting motion of dislocations and needs large forces to fracture the restriction created by dispersion. In particulate composites, the particles strengthen the system by the hydrostatic coercion of fillers in matrices and by their hardness relative to the matrix.

Three-dimensional reinforcement in composites offers isotropic properties, because of the three systematical orthogonal planes. Since it is not homogeneous, the material properties acquire sensitivity to the constituent properties, as well as the interfacial properties and geometric shapes of the array. The composite’s strength usually depends on the diameter of the particles, the inter-particle spacing, and the volume fraction of the reinforcement. The matrix properties influence the behavior of particulate composite too.

Whiskers
Single crystals grown with nearly zero defects are termed whiskers. They are usually discontinuous and short fibers of different cross sections made from several materials like graphite, silicon carbide, copper, iron etc. Typical lengths are in 3 to 55 N.M. ranges. Whiskers differ from particles in that, whiskers have a definite length to width ratio greater than one. Whiskers can have extraordinary strengths up to 7000 MPa.

Whiskers were grown quite incidentally in laboratories for the first time, while nature has some geological structures that can be described as whiskers. Initially, their usefulness was overlooked as they were dismissed as incidental by-products of other structure. However, study on crystal structures and growth in metals sparked off an interest in them, and also the study of defects that affect the strength of materials, they came to be incorporated in composites using several methods, including power metallurgy and slip-casting techniques.

Metal-whisker combination, strengthening the system at high temperatures, has been demonstrated at the laboratory level. But whiskers are fine, small sized materials not easy to handle and this comes in the way of incorporating them into engineering materials to come out with a superior quality composite system.

Ceramic material’s whiskers have high moduli, useful strengths and low densities. Specific strength and specific modulus are very high and this makes ceramic whiskers suitable for low weight structure composites. They also resist temperature, mechanical damage and oxidation more responsively than metallic whiskers, which are denser than ceramic whiskers. However, they are not commercially viable because they are damaged while handling.

Fibres
Fibers are the important class of reinforcements, as they satisfy the desired conditions and transfer strength to the matrix constituent influencing and enhancing their properties as desired. Glass fibers are the earliest known fibers used to reinforce materials. Ceramic and metal fibers were subsequently found out and put to extensive use, to render composites stiffer more resistant to heat. Fibers fall short of ideal performance due to several factors. The performance of a fiber composite is judged by its length, shape, orientation, and composition of the fibers and the mechanical properties of the matrix.

The orientation of the fiber in the matrix is an indication of the strength of the composite and the strength is greatest along the longitudinal directional of fiber. This doesn’t mean the longitudinal fibers can take the same quantum of load irrespective of the direction in which it is applied. Optimum performance from longitudinal fibers can be obtained if the load is applied along its direction. The slightest shift in the angle of loading may drastically reduce the strength of the composite.

There are several methods of random fiber orientations, which in a two-dimensional one, yield composites with one-third the strength of a unidirectional fiber-stressed composite, in the direction of fibers. In a 3-dimension, it would result in a composite with a comparable ratio, about less than one-fifth. In very strong matrices, moduli and strengths have not been observed. Application of the strength of the composites with such matrices and several orientations is also possible. The longitudinal strength can be calculated on the basis of the assumption that fibers have been reduced to their effective strength on approximation value in composites with strong matrices and non-longitudinally orientated fibers.

It goes without saying that fiber composites may be constructed with either continuous or short fibers. Experience has shown that continuous fibers (or filaments) exhibit better orientation, although it does not reflect in their performance. Fibers have a high aspect ratio, i.e., their lengths being several times greater than their effective diameters. This is the reason why filaments are manufactured using continuous process. This finished filaments.

Short-length fibers incorporated by the open- or close-mould process are found to be less efficient, although the input costs are considerably lower than filament winding. Most fibers in use currently are solids which are easy to produce and handle, having a circular cross-section, although a few non-conventional shaped and hollow fibers show signs of capabilities that can improve the mechanical qualities of the composites. Given the fact that the vast difference in length and effective diameter of the fiber are assets to a fiber composite, it follows that greater strength in the fiber can be achieved by smaller diameters due to minimization or total elimination of surface of surface defects. After flat-thin filaments came into vogue, fibers rectangular cross sections have provided new options for applications in high strength structures. Owing to their shapes, these fibers provide perfect packing, while hollow fibers show better structural efficiency in composites that are desired for their stiffness and compressive strengths. In hollow fibers, the transverse compressive strength is lower than that of a solid fiber composite whenever the hollow portion is more than half the total fiber diameter. However, they are not easy to handle and fabricate.

2.4 Manufacturing of Composites
The manufacturing processes of composite materials are
Open Mold Processes
Close Mold Processes
Filament WindingPultrusion Processes
A. Open Mold Processes
This shaping process uses a single positive or negative mold surface to produce laminated structures. The starting materials (resins, fibers, mats, and woven roving) are applied to the mold in layers, building up to the desired thickness. This is followed by curing and part removal. Common resins are unsaturated polyesters and epoxies, using fiberglass as the reinforcement. The open mold processes are;
Hand Lay-Up
Spray-Up
Vacuum Bagging
Automated Tape Laying Machine
Hand Lay-Up
Manual lay-up involves cutting the reinforcement material to size using a variety of hand and power-operated devices. These cut pieces are then impregnated with wet matrix material, and laid over a mold surface that has been coated with a release agent and then typically a resin gel-coat. The impregnated reinforcement material is then hand-rolled to ensure uniform distribution and to remove trapped air. More reinforcement material is added until the required part thickness has been built-up. Manual lay-up can also be performed using pre-impregnated reinforcement material, called ‘prepreg’. The use of prepreg material eliminates separate handling of the reinforcement and resin, and can improve part quality by providing more consistent control of reinforcement and resin contents. Prepreg must be kept refrigerated prior to use, however, to prevent premature curing.

Hand Lay-Up Method (AE681 Composite Materials)
Spray-Up
In spray-up, resin is sprayed onto a prepared mold surface using a specially designed spray gun. This gun simultaneously chops continuous reinforcement into suitable lengths as it sprays the resin. Liquid resin and chopped fibers are sprayed onto an open mold to build successive laminations. Attempt to mechanize application of resin-fiber layers and reduce lay-up time. Applications are lightly loaded structural panels, e.g. caravan bodies, truck fairings, bath tubes, small boats, etc. The deposited materials are left to cure under standard atmospheric conditions.

Spray-Up Method(AE681 Composite Materials)
Vacuum Bagging
The vacuum–bag process was developed for making a variety of components, including relatively large parts with complex shapes. Applications are large cruising boats, racecar components, etc. It uses atmospheric pressure to suck air from under vacuum bag, to compact composite layers down and make a high quality laminate. Layers from bottom include: mold, mold release, composite, peel-ply, breather cloth, vacuum bag, also need vacuum valve, sealing tape.

Vacuum Bagging(AE681 Composite materials)
Automated Lay-Up
Automated tape-laying machines operate by dispensing a prepreg tape onto an open mold following a programmed path. Typical machine consists of overhead gantry to which the dispensing head is attached. The gantry permits x-y-z travel of the head, for positioning and following a defined continuous path.

B. Close Mold Process
This process involves the usage of molds consisting of two sections that open and close each other in a molding cycle. Tooling cost is more than twice the cost of a comparable open mold due to the more complex equipment required in these processes. There are three classes based on their counterparts in conventional plastic molding:
a. Compression molding
b. Transfer molding
c. Injection molding
Compression Molding
In this method, a charge is placed in lower mold section, and the sections are brought together under pressure, causing charge to take the shape of the cavity. Mold halves are heated to cure TS polymer. When molding is sufficiently cured, the mold is opened and part is removed. Several shaping processes for PMCs based on compression molding. The differences are mostly in the form of the starting materials
Transfer Molding
For transfer molding, a charge of thermosetting resin with short fibers is placed in a pot or chamber, heated, and squeezed by ram action into one or more mold cavities. The mold is heated to cure the resin. Name of the process derives from the fact that the fluid polymer is transferred from a pot into a mold.

Injection Molding
Injection molding is noted for low cost production of plastic parts in large quantities. Although most closely associated with thermoplastics, the process can also be adapted to thermosets. Processes of interest in the context of PMCs:
Conventional injection molding
C. Filament Winding
Filament winding refers to wrapping a narrow fiber tow or band of tows of resin impregnated fiber around a mandrel of the shape to be produced. When the mandrel is removed, a hollow shape is the result. Uses for filament winding include pipe, tubing, pressure vessels, tanks and items of similar shape. Filament winding is typically applied using either hoop or helical winding. In hoop winding, the tow is almost perpendicular to the axis of the rotating mandrel. Each mandrel rotation advances the material-delivery supporting carriage one band width, butting the edge of one band next to the previous band. In helical winding, material is deposited in a helical path in one direction, then turns around on end and returns in a helical path in the opposite direction. Filament winding mandrels may be metallic or non-metallic and designed to either collapse to facilitate part removal or may be dissolvable after curing.

Filament Winding (AE681 Composite materials)
D. Pultrusion
Pultrusion is a continuous process used primarily to produce long, straight shapes of constant cross-section. Pultrusion is similar to extrusion except that the composite material is pulled, rather than pushed, through a die. Pultrusions are produced using continuous reinforcing fibers called ‘roving’ that provide longitudinal reinforcement, and transverse reinforcement in the form of mat or cloth materials. These reinforcements are resin impregnated by drawing through a resin wet-out station; and generally shaped within a guiding, or preforming, system. They are then subsequently shaped and cured through a preheated die or set of dies.

Pultrusion Process (AE681 Composite materials)
Overview of Epoxy Resin
Epoxy resin belongs to the principal polymer under the term thermosetting resins which covers a wide range of cross-linking polymers including unsaturated polyester resins, phenol-formaldehyde resins, and amino resins. Thermosetting polymers form an infusible and insoluble mass on heating, due to the formation of a covalently cross-linked and thermally stable network structure. They are generally amorphous and possess various desirable properties such as high tensile strength and modulus, easy processing, good thermal and chemical resistance, and dimensional stability. The term epoxy resin is applied to both prepolymers and to cured resins; the former is characterized by a three-membered ring known as the epoxy.

Epoxy resins are among the most important thermosetting polymers and are used extensively as adhesives in many applications including aerospace. The recent development of epoxy resin-based renewable organic materials has attracted a lot of attention. It is also noteworthy that, cardanol-based novolac resins have been investigated for use as modifiers and curing agents for commercial epoxy resins improving toughness and other mechanical properties. The mechanical properties of epoxy resin are affected by the ratio of curing agent to epoxy resin; this mixing ratio has very important practical implications. Recently, composite materials have been developed for applications in severe environments, such as cryogenic and high temperatures, water repellency, corrosion resistance and ultraviolet radiation epoxide, oxirane or ethoxyline group (Pyeong et al., 2017)
.

2.4 Nanoparticles
Nanoparticles (from Greek nanos – dwarf) are organic or inorganic solid particles. The dimension of nanoparticles is not defined in a uniform manner. Nanoparticles are generally defined as particulate matter with at least one dimension that is less than 100 nm. This definition puts them in a similar size domain as that of ultrafine particles (air borne particulates) and places them as a sub-set of colloidal particles (SCENIHR 2005). A nanoparticle is the most fundamental component in the fabrication of a nanostructure, and is far smaller than the world of everyday objects that are described by Newton ‘ s laws of motion, but bigger than an atom or a simple molecule that are governed by quantum mechanics. In general, the size of a nanoparticle spans the range between 1 and 100 nm. Metallic nanoparticles have different physical and chemical properties from bulk metals (e.g., lower melting points, higher specific surface areas, specific optical properties, mechanical strengths, and specific magnetizations), properties that might prove attractive in various industrial applications. However, how a nanoparticle is viewed and is defined depends very much on the specific application (Satoshi and Nick, 2013).

2.4.1 Classification of Nanoparticles
There are various ways to classify nanoparticles. Nanoparticles are classified based on one, two and three dimensions (Hett, 2004).

2.4.1.1 One Dimension Nanoparticles
One dimensional system, such as thin film or manufactured surfaces, has been used for decades in electronics, chemistry and engineering. Production of thin films (sizes1-100nm) or monolayer is now common place in the field of solar cells or catalysis. This thin films are using in different technological applications, including information storage systems, chemical and biological sensors, fibre-optic systems, magneto-optic and optical device.

2.4.1.2 Two Dimension Nanoparticles (Carbon Nanotubes)
Carbon nanotubes are hexagonal network of carbon atoms, 1 nm in diameter and 100 nm in length, as a layer of graphite rolled up into cylinder. CNTs are of two types, single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) .The small dimensions of carbon nanotubes, combined with their remarkable physical, mechanical and electrical properties, make them unique materials (Kohler et al., 2004). They display metallic or semi conductive properties, depending on how the carbon leaf is wound on itself. The current density that nanotubes can carry is extremely high and can reach one billion amperes per square meter making it a superconductor. The mechanical strength of carbon nanotubes is sixty times greater than the best steels. Carbon nanotubes have a great capacity for molecular absorption and offering a three dimensional configuration. Moreover they are chemically and chemically very stable.

2.4.1.3 Three Dimension Nanoparticles
The three dimension nanoparticles include Dendrimers, Quantum Dots, Fullerenes (Carbon 60), (QDs).
Fullerenes are spherical cages containing from 28 to more than 100 carbon atoms, contain C60. This is a hollow ball composed of interconnected carbon pentagons and hexagons, resembling a soccer ball. Fullerenes are class of materials displaying unique physical properties. They can be subjected to extreme pressure and regain their original shape when the pressure is released. These molecules do not combine with each other, thus giving them major potential for application as lubricants. They have interesting electrical properties and it has been suggested to use them in the electronic field, ranging from data storage to production of solar cells. Fullerenes are offering potential application in the rich area of nanoelectronics. Since fullerenes are empty structures with dimensions similar to several biological active molecules, they can be filled with different substances and find potential medical application (Tomalia, 2004).

Dendrimers represents a new class of controlled-structure polymers with nanometric dimensions. Dendrimers used in drug delivery and imaging are usually 10 to 100 nm in diameter with multiple functional groups on their surface, rendering them ideal carriers for targeted drug delivery (Wiener et al., 1994). The structure and function of dendrimers has been well studied. Contemporary dendrimers can be highly specialized, encapsulating functional molecules (i.e., therapeutic or diagnostic agents) inside their core (Li et al., 2007). They are considered to be basic elements for large-scale synthesis of organic and inorganic nanostructures with dimensions of 1 to 100 nm (Tomalia et al., 2004). They are compatible with organic structure such as DNA and can also be fabricated to metallic nanostructure and nanotubes or to possess an encapsulation capacity (Fu et al., 2007). Dendrimers have different reactive surface groupings (nanostructure) and compatible with organic structure such as DNA so their prolific use is particularly in the medical and biomedical fields.The pharmaceutical applications of dendrimers include nonsteroidal anti-inflammatory formulations, antimicrobial and antiviral drugs, anticancer agents, pro-drugs, and screening
agents for high-throughput drug discovery(Cheng Y et al., 2008). Dendrimers may be toxic because of their ability to disrupt cell membranes as a result of a positive charge on their surface (Mecke et al., 2004).

Quantum dots can have anything from a single electron to a collection of several thousands. The size, shape and number of electrons can be precisely controlled. They have been developed in a form of semiconductors, insulators, metals, magnetic materials or metallic oxides. It can be used for optical and optoelectronic devices, quantum computing, and information storage. Colour coded quantum dots are used for fast DNA Testing. Quantum ots (QDs) refer to the quantum confinement of electrons and hole carriers at dimensions smaller than the Bohr radiuos. QD nanocrystals are generally composed of atoms from groups II and VI (that is CdSe, CdS, and CdTe) or II and V (such as In P) at their core. A shell (that is ZnS and CdS) can be further introduce to prevent the surface quenching of excitons in the emissive core and hence increase the photostability and quantum yield of emission (Goldberg M et al., 2007). QDs also provide enough surface area to attach therapeutic agents for simultaneous drug delivery and in vivo imaging, as well as for tissue engineering (Larson DR et al., 2003).

2.5 Synthesis of Nanoparticles
The methods of synthesis of nanoparticles are well known for a long time as compared to the other nanomaterials. For the synthesis of nanoparticles, the processing conditions need to be controlled in such a manner that the resulting nanoparticles have the following characteristics: (i) identical size of all particles, (ii) identical shape, (iii) identical chemical composition and crystal structure, and (iii) individually dispersed with no agglomeration.

Nanoparticles can be synthesized by both top-down or bottom-up approaches. Two well-known top-down approaches are milling (or attrition) and thermal cycling. Attrition produces nanoparticles of a wide range of diameter ranging from 20 nm to several hundred nanometers. The shape of the particles varies as well. They may contain impurities from the milling medium. The nanoparticles made by this process are usually used in the fabrication of nanocomposites and bulk materials having nano grains where perfections in size and shape, and presence of impurities do not matter significantly. Moreover, some of the defects can get annealed during the sintering process. A bulk material having very small thermal conductivity but a large coefficient of thermal expansion may be subjected to repeated thermal cycling to produce very fine particles. However, this technique is difficult to design and the control of particle size and shape is difficult.

The bottom-up methods are more popular than the top-down methods. There are several bottom-up methods such as homogeneous and heterogeneous nucleation processes, microemulsion based synthesis, aerosol synthesis, spray pyrolysis and template-based synthesis.

2.6 Properties of NanopaticlesThe outstanding importance of nanoparticles and nano structured systems can be ascribed to :1. Particle size
Bioavailability: in water non soluble substances can be transported as nanoparticles in an organism of human beings (application in life sciences)
2. Large specific surface area
Strong surface area effects (e.g. reactivity, high energy of surface area, adsorption, higher solubility, lower melting point etc.)
3. Change of electronic properties
Quantum effects of particles ; 10 nm, importance for electronic and optoelectronic application
2.6.1 Characterization of Nanoparticles
Characterization of nanoparticles is based on the size, morphology and surface charge, using such advanced microscopic techniques as atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Properties such as the size distribution, average particle diameter, charge affect the physical stability and the in vivo distribution of the nanoparticles. Properties like surface morphology, size and overall shape are determined by electron microscopy techniques. Features like physical stability and redispersibility of the polymer dispersion as well as their in vivo performance are affected by the surface charge of the nanoparticles.

2.7 NanocompositesA nanocomposite is a composite material, in which one of the components has at least one dimension that is nanoscopic in size that is around 109 m (Shivani, 2015). Nanocomposite can also be defined as a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm), or structures having nano-scale repeat distances between the different phases that make up the material (Kamigaito, 1994).The general idea behind the addition of the nanoscale second phase is to create a synergy between the various constituents, such that novel properties capable of meeting or exceeding design expectations can be achieved. Nanocomposites are high performance material that exhibit unusual property combinations and unique design possibilities. With an estimated annual growth rate of about 25% and fastest demand to be in engineering plastics and elastomers, their potential is so striking that they are useful in several areas ranging from packaging to biomedical applications (Pedro et al., 2009). According to Charles (2014), in mechanical terms, nanocomposites differ from conventional composite materials due to the exceptionally high surface to volume ratio of the reinforcing phase and/or its exceptionally high aspect ratio. The reinforcing materials can be made up of particles (minerals), sheets (e.g. exfoliated clay stacks) or fibres (e.g. carbon nanotubes or electrospun fibres). The area of the interface between the matrix and reinforcement phase(s) is typically in order of magnitude greater than for conventional composite materials.

2.8Antimicrobial Metals
Metal can be extremely toxic to most bacteria and yeast at exceptionally low concentrations Because of this biocide activity; some particular metals have been used as antimicrobial agents since ancient times. For instance, vessels made of Cu and Ag have been used for water disinfection and food preservation since the time of the Persian kings. This practice was later adopted by the Phoenicians, Greeks, Romans and Egyptians. Despite this long evidence, today the specific mechanisms explaining the toxicity of metals are not yet fully elucidated as several variables are involved. However, depending on the metal property, the biocide behavior can be triggered by: (a) the metal reduction potential and (b) the metal donor atom selectivity and/or speciation (Antonella Piozzi, 2015).
Due to this effect, metals are being reduced to their nanoscale to achieve an increase in the surface area to volume so as to inhibit more effective means in application. Copper and silver have been found to be a useful metal in antimicrobial applications whereby they resist the growth of some bacteria. The reduction to nanoscale of copper and silver have proven to be more effective in this application which brings about various research work on this synthesis of this notable metals in the nanoscale.

Mechanism of Antimicrobial Metals (Antonella, 2015)
A summary of the main mechanisms behind the antimicrobial behavior of metal as separated according to the specific metal property responsible for this action: (a) reduction potential and (b) donor atom selectivity and/or speciation.