Nano Composites and their Applications

ABSTRACT

Nanocomposites, a high performance material 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. In this unified overview the three types of matrix nanocomposites are presented underlining the need for these materials, their processing methods and some recent results on structure, properties and potential applications, perspectives including need for such materials in future space mission and other interesting applications together with market and safety aspects. Possible uses of natural materials such as clay based minerals, chrysotile and lignocellulosic fibers are highlighted. Being environmentally friendly, applications of nanocomposites offer new technology and business opportunities for several sectors of the aerospace, automotive, electronics and biotechnology industries.


                                                               INTRODUCTION
NANO COMPOSITES
Nanomaterials
Nanomaterials describe, in principle, materials of which a single unit is sized (in at least one dimension) between 1 and 1000 nanometres (10−9 meter) but is usually 1—100 nm (the usual definition of nanoscale    [1]).
Nanomaterials research takes a materials science-based approach to nanotechnology, leveraging advances in materials metrology and synthesis which have been developed in support of microfabrication research. Materials with structure at the nanoscale often have unique optical, electronic, or mechanical properties.
Nanocomposite is 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. In the broadest sense this definition can include porous media, colloids, gels and copolymers, but is more usually taken to mean the solid combination of a bulk
matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite will differ markedly from that of the component materials. Size limits for these effects have been proposed,[1] <5 nm for catalytic activity, <20 nm for making a hard magnetic material soft, <50 nm for refractive index changes, and <100 nm for achievingsuperparamagnetism, mechanical strengthening or restricting matrix dislocationmovement.
Nanocomposites are found in nature, for example in the structure of the abalone shell and bone. The use of nanoparticle-rich materials long predates the understanding of the physical and chemical nature of these materials. Jose-Yacaman et al. [2] investigated the origin of the depth of colour and the resistance to acids and bio-corrosion of Maya bluepaint, attributing it to a nanoparticle mechanism. From the mid-1950s nanoscale organo-clays have been used to control flow of polymer solutions (e.g. as paint viscosifiers) or the constitution of gels (e.g. as a thickening substance in cosmetics, keeping the preparations in homogeneous form). By the 1970s polymer/clay composites were the topic of textbooks,[3] although the term "nanocomposites" was not in common use.
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 material can be made up of particles (e.g. 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 an order of magnitude greater than for conventional composite materials. The matrix material properties are significantly affected in the vicinity of the reinforcement. Ajayan et al. [4] note that with polymer nanocomposites, properties related to local chemistry, degree of thermoset cure, polymer chain mobility, polymer chain conformation, degree of polymer chain ordering or crystallinity can all vary significantly and continuously from the interface with the reinforcement into the bulk of the matrix.
This large amount of reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composite. For example, adding carbon nanotubes improves the electrical and thermal conductivity. Other kinds of nanoparticulates may result in enhanced optical properties,dielectric properties, heat resistance or mechanical properties such as stiffness, strengthand resistance to wear and damage. In general, the nano reinforcement is dispersed into the matrix during processing. The percentage by weight (called mass fraction) of the nanoparticulates introduced can remain very low (on the order of 0.5% to 5%) due to the low filler percolation threshold, especially for the most commonly used non-spherical, high aspect ratio fillers (e.g. nanometer-thin platelets, such as clays, or nanometer-diameter cylinders, such as carbon nanotubes). The orientation and arrangement of asymmetric nanoparticles, thermal property mismatch at the interface, interface density per unit volume of nanocomposite, and polydispersity of nanoparticles significanlty affect the effective thermal conductivity of nanocomposites.[5]











NANO COMPOSITES CAN EXIST IN THREE OTHER FORMS :




Ceramic-matrix nanocomposites
In this group of composites the main part of the volume is occupied by a ceramic, i.e. a chemical compound from the group of oxides, nitrides, borides, silicides etc.. In most cases, ceramic-matrix nanocomposites encompass a metal as the second component. Ideally both components, the metallic one and the ceramic one, are finely dispersed in each other in order to elicit the particular nanoscopic properties. Nanocomposite from these combinations were demonstrated in improving their optical, electrical and magnetic properties [6] as well as tribological, corrosion-resistance and other protective properties.[7]
The binary phase diagram of the mixture should be considered in designing ceramic-metal nanocomposites and measures have to be taken to avoid a chemical reaction between both components. The last point mainly is of importance for the metallic component that may easily react with the ceramic and thereby lose its metallic character. This is not an easily obeyed constraint, because the preparation of the ceramic component generally requires high process temperatures. The most safe measure thus is to carefully choose immiscible metal and ceramic phases. A good example for such a combination is represented by the ceramic-metal composite of TiO2 and Cu, the mixtures of which were found immiscible over large areas in the Gibbs’ triangle of Cu-O-Ti.[8]
The concept of ceramic-matrix nanocomposites was also applied to thin films that are solid layers of a few nm to some tens of µm thickness deposited upon an underlying substrate and that play an important role in the functionalization of technical surfaces. Gas flow sputtering by the hollow cathode technique turned out as a rather effective technique for the preparation of nanocomposite layers. The process operates as a vacuum-baseddeposition technique and is associated with high deposition rates up to some µm/s and the growth of nanoparticles in the gas phase. Nanocomposite layers in the ceramics range of composition were prepared from TiO2 and Cu by the hollow cathode technique [9] that showed a high mechanical hardness, small coefficients of friction and a high resistance to corrosion.









Metal-matrix nanocomposites

Metal matrix nanocomposites can also be defined as reinforced metal matrix composites. This type of composites can be classified as continuous and non-continuous reinforced materials. One of the more important nanocomposites is Carbon nanotube metal matrix composites, which is an emerging new material that is being developed to take advantage of the high tensile strength and electrical conductivity of carbon nanotube materials. Critical to the realization of CNT-MMC possessing optimal properties in these areas are the development of synthetic techniques that are (a) economically producible, (b) provide for a homogeneous dispersion of nanotubes in the metallic matrix, and (c) lead to strong interfacial adhesion between the metallic matrix and the carbon nanotubes. In addition to carbon nanotube metal matrix composites, boron nitride reinforced metal matrix composites and carbon nitride metal matrix composites are the new research areas on metal matrix nanocomposites.[10]
A recent study, comparing the mechanical properties (Young's modulus, compressive yield strength, flexural modulus and flexural yield strength) of single- and multi-walled reinforced polymeric (polypropylene fumarate—PPF) nanocomposites to tungsten disulfide nanotubes reinforced PPF nanocomposites suggest that tungsten disulfide nanotubes reinforced PPF nanocomposites possess significantly higher mechanical properties and tungsten disulfide nanotubes are better reinforcing agents than carbon nanotubes.[11]Increases in the mechanical properties can be attributed to a uniform dispersion of inorganic nanotubes in the polymer matrix (compared to carbon nanotubes that exist as micron sized aggregates) and increased crosslinking density of the polymer in the presence of tungsten disulfide nanotubes (increase in crosslinking density leads to an increase in the mechanical properties). These results suggest that inorganic nanomaterials, in general, may be better reinforcing agents compared to carbon nanotubes.
Another kind of nanocomposite is the energetic nanocomposite, generally as a hybrid sol–gel with a silica base, which, when combined with metal oxides and nano-scale aluminum powder, can form superthermite materials.[12][13][14][15]







Polymer-matrix nanocomposites
In the simplest case, appropriately adding nanoparticulates to a polymer matrix can enhance its performance, often dramatically, by simply capitalizing on the nature and properties of the nanoscale filler [16] (these materials are better described by the termnanofilled polymer composites [16]). This strategy is particularly effective in yielding high performance composites, when good dispersion of the filler is achieved and the properties of the nanoscale filler are substantially different or better than those of the matrix.
Nanoparticles such as graphene, carbon nanotubes, molybdenum disulfide and tungsten disulfide are being used as reinforcing agents to fabricate mechanically strong biodegradable polymeric nanocomposites for bone tissue engineering applications. The addition of these nanoparticles in the polymer matrix at low concentrations (~0.2 weight %) cause significant improvements in the compressive and flexural mechanical properties of polymeric nanocomposites.[17][18] Potentially, these nanocomposites may be used as a novel, mechanically strong, light weight composite as bone implants. The results suggest that mechanical reinforcement is dependent on the nanostructure morphology, defects, dispersion of nanomaterials in the polymer matrix, and the cross-linking density of the polymer. In general, two-dimensional nanostructures can reinforce the polymer better than one-dimensional nanostructures, and inorganic nanomaterials are better reinforcing agents than carbon based nanomaterials. In addition to mechanical properties, multi-walled carbon nanotubes based polymer nanocomposites have also been used for the enhancement of the electrical conductivity.[19]
Nanoscale dispersion of filler or controlled nanostructures in the composite can introduce new physical properties and novel behaviors that are absent in the unfilled matrices. This effectively changes the nature of the original matrix [16] (such composite materials can be better described by the term genuine nanocomposites or hybrids [16]). Some examples of such new properties are fire resistance or flame retardancy,[20] and accelerated biodegradability.
A range of polymeric nanocomposites are used for biomedical applications such as tissue engineering, drug delivery, cellular therapies.[21][22] Due to unique interactions between polymer and nanoparticles, a range of property combinations can be engineered to mimic native tissue structure and properties. A range of natural and synthetic polymers are used to design polymeric nanocomposites for biomedical applications including starch, cellulose, alginate, chitosan, collagen, gelatin, and fibrin, poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), poly(caprolactone) (PCL), poly(lactic-co-glycolic acid) (PLGA), and poly(glycerol sebacate) (PGS). A range of nanoparticles including ceramic, polymeric, metal oxide and carbon-based nanomaterials are incorporated within polymeric network to obtain desired property combinations.


Nanocomposites
The definition of nano-composite material has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed at the nanometer scale.
The general class of nanocomposite organic/inorganic materials is a fast growing area of research. Significant effort is focused on the ability to obtain control of the nanoscale structures via innovative synthetic approaches. The properties of nano-composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics.
This rapidly expanding field is generating many exciting new materials with novel properties. The latter can derive by combining properties from the parent constituents into a single material. There is also the possibility of new properties which are unknown in the parent constituent materials.
The inorganic components can be three-dimensional framework systems such as zeolites, two-dimensional layered materials such as clays, metal oxides, metal phosphates, chalcogenides, and even one-dimensional and zero-dimensional materials such as (Mo3Se3-)n chains and clusters. Experimental work has generally shown that virtually all types and classes of nanocomposite materials lead to new and improved properties when compared to their macrocomposite counterparts. Therefore, nanocomposites promise new applications in many fields such as mechanically reinforced lightweight components, non-linear optics, battery cathodes and ionics, nano-wires, sensors and other systems.^(23)
The general class of organic/inorganic nanocomposites may also be of relevance to issues of bio-ceramics and biomineralization in which in-situ growth and polymerization of biopolymer and inorganic matrix is occurring. Finally, lamellar nanocomposites represent an extreme case of a composite in which interface interactions between the two phases are maximized. Since the remarkable properties of conventional composites are mainly due to interface interactions, the materials dealt with here could provide good model systems in which such interactions can be studied in detail using conventional bulk sample (as opposed to surface) techniques. By judiciously engineering the polymer-host interactions, nanocomposites may be produced with a broad range of properties.
Inorganic layered materials exist in great variety. They possess well defined, ordered intralamellar space potentially accessible by foreign species. This ability enables them to act as matrices or hosts for polymers, yielding interesting hybrid nano-composite materials.
Lamellar nano-composites can be divided into two distinct classes, intercalated and exfoliated. In the former, the polymer chains alternate with the inorganic layers in a fixed compositional ratio and have a well defined number of polymer layers in the intralamellar space. In exfoliated nano-composites the number of polymer chains between the layers is almost continuously variable and the layers stand >100 Å apart. The intercalated nano-composites are also more compound-like because of the fixed polymer/layer ratio, and they are interesting for their electronic and charge transport properties. On the other hand, exfoliated nano-composites are more interesting for their superior mechanical properties.
Our work focuses on the lamellar class of intercalated organic/inorganic nanocomposites and namely those systems that exhibit electronic properties in at least one of the components. This subclass of nano-composites offers the possibility of obtaining well ordered systems some of which may lead to unusual electrical and mechanical properties. Selected members of this class may be amenable to direct structural characterization by standard crystallographic methods. An important issue in this area is the few structural details that are available, therefore, any system which can be subjected to such analysis is of interest. Nanocomposites also offer the possibility to combine diverse properties which are impossible within a single material, e.g. flexible mechanical properties and superconducting properties. This work is now in its infancy and we propose to carry out extensive investigations in the next funding period. Another exciting aspect is the possibility of creating heterostructures composed of different kinds of inorganic layers, which could lead to new multifuntional materials.
In the past we have focused our efforts on creating such materials with conjugated and saturated organic macromolecules. We have developed several general synthetic routes for inserting polymer chains into host structures and have designed many novel nanocomposites.^(23)
These include:
a. In-situ intercalative polymerization (ISIP) of a monomer using the host itself as the oxidant. The rationale behind intercalative polymerization is that host matrices with high electron affinity can oxidatively polymerize appropriate monomers in their interior.
b. Monomer intercalation followed by topotactic intralamellar solid state polymerization. This route creates conjugated polymers inside non-oxidizing hosts.^(24)
c. Direct precipitative encapsulation of polymer chains by colloidally dispersed single layers of a host. This approach gives access to a large variety of nano-composites with many kinds of polymers and hosts.




















2. PROCESSING OF NANOCOMPOSITES
2.1. Raw materials
As with microcomposites, CMNC matrix materials include Al2O3, SiC, SiN, etc., while metal matrices employed in MMNC are mainly Al, Mg, Pb, Sn, W and Fe, and a whole range of polymers, e.g. vinyl polymers, condensation polymers, polyolefins, speciality polymers (including a variety of biodegradable molecules) are used in PMNC. In general, it is the reinforcement that is in the nanorange size in these materials. Both synthetic and natural crystalline reinforcements have been used, such as Fe and other metal powders, clays, silica, TiO2 and other metal oxides, although clays and layered silicates are the most common. This is so due to their availability with very low particle sizes and well-known intercalation chemistry,  in addition to generating improved properties even when they are used at very low concentrations25)  .  Most of these reinforcements are prepared by known techniques: chemical, mechanical (e.g. ball milling), vapour deposition, etc.; details of these may be found in many of the references given in the following sections.
Similarly, CNTs are prepared mostly by chemical/vapour deposition methods and details are available elsewhere319-327. A bibliometric analysis of CNTs made in 2000335 revealed that about 49% of the patents filed between 1992-1999 were related to the processing of CNTs and about 14% to their structure, properties and models. CNTs consist of graphene cylinders and are available in two varieties, as single walled (SWCNT) and multi walled (MWCNT), with about 70% yield in the case of SWCNT317. While SWCNTs are single graphene cylinders, MWCNTs consist of two or more concentric cylindrical sheets of graphene around a central hollow core. Both types exhibit physical characteristics of solids, either metallic or semiconducting in nature, with microcrystallinity and very high aspect ratios of 103.
Surface modifications of reinforcements are carried out to give homogeneous distribution with less agglomeration, and to improve interfacial bonding between the matrix and the nanosized reinforcements. Details on these can be found in the references given for each type of nanocomposites in later Sections. In the case of CNTs, use of surfactants, oxidation or chemical functionalization of surfaces are some of the techniques employed27. Chemical methods may be more effective, particularly for polymer and ceramic matrices. Physical blending and in situ polymerization are used for improving dispersion in the case of CNT-reinforced polymer composites, while alignment of CNTs could be achieved by techniques such as ex-situ techniques (filtration, template and plasma-enhanced chemical vapour deposition, force field-inducements, etc.)33.
2.2. Processing methods
Despite their nano dimensions, most of the processing techniques of the three types of nanocomposites remain almost the same as in microcomposites. This is also true even for CNT-reinforced composites. Details on these techniques are given below.
2.2.1. Ceramic Matrix Nanocomposites (CMNC)
Many methods have been described for the preparation of ceramic matrix nanocomposites 20,38,53-112. The most common methodologies, as used for microcomposites, are Conventional powder method; Polymer precursor route; Spray pyrolysis; Vapour techniques (CVD and PVD) and Chemical methods, which include the sol-gel process, colloidal and precipitation approaches and the template synthesis. While Table 3a lists systems prepared by some of these methods, Table 3b shows their advantages and limitations. Scheme 1a depicts the conventional powder method and Scheme 1b illustrates the polymer precursor route used in the synthesis of an Al2O3/SiC nanocomposite.



A large variety of parameters affecting the sol-gel process, such as type of solvent, timing, pH, precursor, water/metal ratio, etc., allow a versatile control of structural and chemical properties of the final oxide materials61. Regarding the processing of carbon nanotubes (CNT)-reinforced ceramic nanocomposites, many approaches have been described20,74. Several of these are listed in Table 3c.
2.2.2. Metal Matrix Nanocomposites (MMNC)
The most common techniques for the processing of metal matrix nanocomposites are96Spray pyrolysis; Liquid metal infiltration; Rapid solidification; Vapour techniques (PVD, CVD); Electrodeposition and Chemical methods, which include colloidal and sol-gel processes. Table 4a lists various systems prepared by these methods and Table 4b shows their advantages and limitations. Only two reports are found, for example, on Fe-based nanocomposites prepared by solidification techniques. The first one, by Branagan121, is called "devitrified nanocomposite steel". This was obtained by quenching the metallic glass obtained from a Fe-based alloy, followed by devitrifying the glass precursor through heat treatment above its crystallization temperature. This resulted in a material showing a crystalline multi-phase microstructure. The formation of nanophases was explained by the high nucleation frequency within the limited time for growth of grains before impingement. In order to explain the very high hardness of these Fe-based nanocomposites, Branagan and Tang studied96novel nanostructures obtained in bulk Fe alloys by designing alloy compositions with different amounts of W and C to get maximum solubility. Difficulties have been encountered in preparing composites with very fine particles due to their induced agglomeration and non-homogeneous distribution. Use of ultrasound helped to improve the wettability between the matrix and the particles.
A number of CNT-reinforced MMCs have been synthesised by different techniques87, since the first report in 200297). Some of these techniques are listed in Table 4c.
2.2.3. Polymer Matrix Nanocomposites (PMNC)
Many methods have been described for the preparation of polymer nanocomposites, including layered materials and those containing CNTs 98. The most important ones are i) Intercalation of the polymer or pre-polymer from solution; ii) In-situ intercalative polymerization; iii) Melt intercalation (Figure 1); iv) Direct mixture of polymer and particulates; v) Template synthesis; vi) In-situ polymerization; and vii) Sol-gel process. Publications dealing with various methods for the incorporation of nanodispersoids into conducting polymers are also available19,201,234; the most prominent one is probably the incorporation of inorganic building blocks in organic polymers.



Table 5a shows the procedures adopted in some of these processes, while their advantages and limitations are listed in Table 5b.
Intercalative processes employed for the preparation of polymer-based nanocomposites, including those containing layered silicates, are shown in Scheme 2. It may be noted that, in this method, a range of nanocomposites with structures from intercalated to exfoliated can be obtained, depending on the degree of penetration of the polymer chains into the silicate galleries. As a result, this procedure has become standard for the preparation of polymer-layered silicate combinations.



The preparation of CNT-reinforced polymer nanocomposites is generally performed by different methods, including direct mixing, solution mixing, melt-mixing and in-situ polymerisation. These, as applicable to various systems187,202, are listed in Table 5c.
Similarly, different processing techniques, mostly chemical and electrochemical methods, have been employed for the preparation of conducting polymer nanocomposites19. Table 6a summarizes these methods, while a relevant categorization of these nanocomposites is presented in Table 6b.
In the case of nanocomposites containing layered reinforcements, depending on the nature of the components (layered silicate, organic cation and polymer matrix), the method of preparation and the strength of interfacial interactions between the polymer matrix and the layered silicate (modified or not), three different types of PLS nanocomposites may be obtained, as illustrated inFigure 218.



When the polymer is unable to intercalate between the silicate sheets, a phase-separated composite (Figure 2) is obtained, whose properties stay in the same range as that of traditional microcomposites. On the other hand, in intercalated nanocomposites, the insertion of a polymer matrix into the layered silicate structure occurs in a crystallographically regular fashion, regardless of the clay to polymer ratio. A well ordered multilayer morphology built up with alternating polymeric and inorganic layers is generated. Normally, only a few molecular layers of polymer can be intercalated in these materials.
The in-situ method can be used with mineral/vegetal fibres, with the possibility to attach the polymer to the grafted surface through linking groups (coupling agents), which optimize the interface bonding and, consequently, the mechanical properties176.
Despite the successful use of these different methods for the preparation of polymer-based nanocomposites, information on various factors is still lacking, such as i) the use of an appropriate method for a specific matrix-reinforcement combination or ii) the maximum amount of reinforcements to give optimum property combinations and lower the cost of the processes, etc. Therefore, it is still necessary to look into these aspects including use of simulation and modelling techniques.
Structure and Properties
The structure of nanocomposites usually consists of the matrix material containing the nanosized reinforcement components in the form of particles, whiskers, fibres, nanotubes, etc.93. Different investigators have employed various equipments and techniques for the characterization of nanocomposites, including atomic force microscopy (AFM), scanning tunnelling microscopy (STM), Fourier transformed infrared spectroscopy (FTIR), X ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), scanning and transmission electron microscopy (SEM/TEM), etc. For example, AFM is a powerful tool to study the surface even down to the nanometre scale, as evident from the work of Veith et al.303,304. Simultaneous small angle X ray scattering (SAXS) and X ray diffractometry (XRD) studies have been recently used for quantitative characterization of nanostructures and crystallite structures in some nanocomposites34,307,308. In addition, theoretical calculations/simulations have been worked out to predict strength properties, including stress/strain curves41,5
Ceramic matrix nanocomposites
Ceramics are usually brittle and easily fractured as consequence of crack propagation. There have been attempts to make ceramics suitable for engineering applications through the incorporation of a ductile metal phase or another ceramic into the matrix. This leads to improved mechanical properties such as hardness and fracture toughness, which occur as a result of the relationship between the different phases, matrix and reinforcements, at the phase boundaries throughout the material. The surface area/volume ratio of the reinforcement materials is of fundamental importance in the understanding of the structure-property relationship in CMNCs. We shall therefore first discuss these improvements in some ceramic-based nanocomposites and relate them with the observed morphologies.
3.1.1. Ceramic matrix-discontinuous reinforcement nanocomposite systems
Table 8 shows examples of ceramic nanocomposites and of the observed improvements in their properties compared to the respective monolithic materials. Table 9 compares the mechanical properties of the Al2O3/SiC system and its microcomposite counterpart39,103-105.






It can be seen from these tables that there is a significant improvement in the strength of the nanocomposite compared with its micro counterpart. The fracture strength, as an example, is noticeably higher because of the higher interfacial interaction between the particles in nanocomposites. Besides, Al2O3-5 to 15% SiC systems exhibited90 superficial grooves of plastic deformation compared to the intergranular fracture observed in monolithic materials. There was no time-dependant wear transition for these composites even at loads of 20-100 N, but pre-transition wear rates of 1-2 x 10-8 mm/Nm were observed for both the monolithic and composite materials. The specific wear rate decreased with sliding distance. This enhancement of properties observed in ceramic nanocomposites can also be illustrated by the Si3N4/SiC system
.


Metal matrix nanocomposites
3.2.1. Metal - discontinuous reinforcement systems
Table 12 illustrates examples of some metal matrix nanocomposites and their respective properties.



The α -Fe/Fe23C6/Fe3B system provides a good example of how unique properties may arise from metal nanocomposites. Table 13 shows the measured hardness values (GPa) of the ingot and ribbon samples prepared from this system122. Vickers hardness values of these two forms of the alloy produced by Branagan and Tang122 were found to be 10.3 and 11 GPa in the as-solidified condition. The ribbon variety showed increased hardness with increasing heat treatment temperature, showing a maximum of 16.2 GPa at 973 K [higher than any existing commercial steel and hard alloys] and there after decreasing to 10.5 GPa at 1123 K. This can be compared to the decreasing trend of the ingot type (8 and 6.6 GPa at 873 and 973 K respectively).



The Al/SiC system also illustrates the advantages of metal nanocomposites compared to their micro counterparts145-147. Figure 12a shows plots of Vickers hardness vs. SiC content, while Figure 12b presents plots of Young's and shear modulus as a function of SiC content. There is a linear increase in hardness with increasing volume fraction of the harder phase (SiC) until the maximum value of 2.6 GPa for the sample that contains 10 vol. % of SiC. The values of Young's and shear modulus increase significantly with increasing SiC content, suggesting the formation of a nanocomposite material containing a brittle phase (SiC) embedded in the ductile Al matrix. Table 14 shows some mechanical properties of both nano- and microcomposites of Al/SiC.


Polymer matrix nanocomposites
Structure-property correlations in polymer nanocomposites have been extensively dealt with in a recent book292, which describes the mechanical properties of polymers based on nanostructure and morphology. Table 15 presents examples of these properties.
3.3.1. Polymer matrix - discontinuous reinforcement (non-layered) nanocomposites
The reinforcing materials employed in the production of polymer nanocomposites can be classified according to their dimensions** 18. For example, when the three dimensions are in the nanometre scale, they are called isodimensional nanoparticles. Examples include spherical silica, metal particles and semiconductor nanoclusters219. The second kind of reinforcement is formed by nanotubes or whiskers, which contain two dimensions in the nanometre scale and one larger, forming an elongated structure. Carbon nanotubes and cellulose whiskers, extensively studied as reinforcing nanofillers, can be included in this second category. The third type of reinforcement is characterized by only one dimension in the nanometre range220-222. In this group, the filler contains sheets one to a few nanometres thick and hundreds to thousands nanometres long. This family is called polymer-layered nanocomposites2,18,26. These materials are obtained by intercalation of the polymer (or a monomer subsequently polymerized) inside the galleries of the layered host. Many synthetic and natural crystalline hosts that are able, under specific conditions, to intercalate a polymer have been described. Examples include graphite, metal chalcogenides (MoS2, for example), clays, layered silicate (montmorillonite, hectorite, saponite, fluoromica, fluorohectorite, vermiculite and kaolinite) and layered double hydroxides. Nanocomposites based on clay and layered silicates have been widely investigated due to the availability of clay starting materials and their well-known intercalation chemistry as mentioned earlier18,50,51,223.
Figure 20 shows the variation in quasi-static fracture toughness as a function of the volume percentage of TiO2 in the polyester/TiO2 nanocomposite. The addition of TiO2 particles has a great effect on fracture toughness. At loadings of 1, 2 and 3 vol. %, an increase of 57, 42 and 41%, respectively, can be observed, when compared with that of the original polyester. However, at 4 vol. % TiO2, toughness (0.55 MPam1/2) decreased approximately to the value given by the polyester matrix (0.54 MPam1/2).This variation can be explained in terms of nanocomposite structure, as illustrated in Figure 21.






It can be observed that the specimens containing 1, 2 and 3 vol. % TiO2 show excellent particle dispersion. Conversely, considerable agglomeration was present in the specimens containing 4 vol. % TiO2. Authors have assigned the initial increase in fracture toughness, followed by the precipitous decline observed at 4 vol. %, to the level of dispersion of nanoparticles within the matrix and to the weak bonding between the titania particles and the polyester180.
The structure of polymer nanocomposites reinforced with isodimensional particles is similar to that of ceramic and metal nanocomposites. In this type, the reinforcement material is distributed all over the polymer matrix, as illustrated in Figure 22. The microstructure of some particle-reinforced polymer nanocomposites is shown in Figure 23. Figure 23a shows the TEM micrograph of polyacrylic acid/silver nanocomposite, showing spherical Ag particles of 10-80 nm size183. Secondary aggregates are formed due to the low viscosity nature of the composite solution. Figure 23b shows the TEM micrograph of an oxidised poly (4-vinylpyridine) homopolymer-Fe2O3 nanocomposite showing ultrafine and crystalline spherical particles of the dispersoids in the 20-200 nm range223. In this case, the particles retain the nanosize dimension due to the protective action of the polymer layer.












USES AND APPLICATIONS OF NANO COMPOSITES
A nanocomposite is a matrix to which nanoparticles have been added to improve a particular property of the material. The properties of nanocomposites have caused researchers and companies to consider using this material in several fields.
A survey of  the applications of nanocomposites:
The following survey of nanocomposite applications introduces you to many of the uses being explored, including:
Producing batteries with greater power output.
 Researchers have developed a method to make anodes for lithium ion batteries from a composite formed with silicon nanospheres and carbon nanoparticles. The anodes made of the silicon-carbon nanocomposite make closer contact with the lithium electrolyte, which allows faster charging or discharging of power.
Speeding up the healing process for broken bones.
Researchers have shown that growth of replacement bone is speeded up when a nanotube-polymer nanocomposite is placed as a kind of scaffold which guides growth of replacement bone. The researchers are conducting studies to better understand how thisnanocomposite increases bone growth.
Producing structural components with a high strength-to-weight ratio.  For example an epoxy containing carbon nanotubes can be used to produce nanotube-polymer composite windmill blades. This results in a strong but lightweight blade, which makes longer windmill blades practical. These longer blades increase the amount of electricity generated by each windmill.
Using graphene to make composites with even higher strength-to-weight ratios. Researchers have found that adding graphene to epoxy composites may result in stronger/stiffer components than epoxy composites using a similar weight of carbon nanotubes. Graphene appears to bond better to the polymers in the epoxy, allowing a more effective coupling of the graphene into the structure of the composite. This property could result in the manufacture of components with higher strength-to-weight ratios for such uses as windmill blades or aircraft components.

Making lightweight sensors with nanocomposites. A polymer-nanotube nanocomposite conducts electricity; how well it conducts depends upon the spacing of the nanotubes. This property allows patches of polymer-nanotube nanocomposite to act as stress sensors on windmill blades. When strong wind gusts bend the blades the nanocomposite will also bend. Bending changes the nanocomposite sensor's electrical conductance, causing an alarm to be sounded. This alarm would allow the windmill to be shut down before excessive damage occurs.
Using nanocomposites to make flexible batteries. A nanocomposite of cellulous materials and nanotubes could be used to make a conductive paper. When this conductive paper is soaked in an electrolyte, aflexible battery is formed.
Making tumors easier to see and remove.Researchers are attempting to join magnetic nanoparticles and fluorescent nanoparticles in a nanocomposite particle that is both magnetic and fluorescent. The magnetic property of thenanocomposite particle makes the tumor more visible during an MRI procedure  done prior to surgery. The fluorescent property of the nanocomposite particle could help the surgeon to better see the tumor while operating.
Applications of Nanocomposites2,16-35,258,305,310,337-378
From the foregoing, it becomes evident that nanocomposites may provide many benefits such as enhanced properties, reduction of solid wastes [lower gauge thickness films and lower reinforcement usage] and improved manufacturing capability, particularly for packaging applications. Tables 21 to 23 present potential applications of ceramic-, metal- and polymer-based nanocomposites, respectively. As it can be observed, the promising applications of nanocomposite systems are numerous, comprising both the generation of new materials and the performance enhancement of known devices such as fuel cells, sensors and coatings. Although the use of nanocomposites in industry is not yet large, their massive switching from research to industry has already started and is expected to be extensive in the next few years.
For instance, the (Al1-xTixN)/α-Si3N4 super hard nanocomposite, which has been developed by the Czech company SHM Ltd. as a tribological coating for tools, is suitable for hard and dry cutting operations such as drilling, turning and milling, and is reported to be now industrialized337,339. In this case, a novel method, which employs vacuum arc coating with a rotating cathode, is used for commercial production. This super hard (Al1-xTix)N/α-Si3N4 possess high tensile strength, in the range of 10-110 GPa, and a lifetime 2-4 times higher than that of the materials currently employed as wear resistant coatings.
Similarly, one of the leading application areas is the automotive sector, with striking impact due to improved functionalities such as ecology, safety, comfort, etc. Details on the commercial usage of nanocomposites in automotives and future developments in this sector (including CNT-based nanocomposites) are now available362. For instance, there are reports on the current use of a number of nanocoatings in different parts of Audi, Evobus and Diamler Chrysler automobiles, as well as ongoing trials on fuel cells, porous filters (foams) and energy conversion components, which include nanoTiO2-containing paints. Additionally, light weight bodies made of metal- or polymer-based nanocomposites with suitable reinforcements are reported to exhibit low density and very high strength (e.g. carbon Bucky fibers, with strength of 150 GPa and weight ≈ 1/5th of steel). Also, two-phase heterogeneous nanodielectrics, generally termed dielectric nanocomposites, have wide applications in electric and electronic industries338.
Metal and ceramic nanocomposites are expected to generate a great impact over a wide variety of industries, including the aerospace, electronic and military305, while polymer nanocomposites major impacts will probably appear in battery cathodes6,342, microelectronics343, nonlinear optics344, sensors345, etc. Improved properties include significant enhancements in fracture strength (about 2 times) and toughness (about one half time); no time dependent wear transitions even at very low loads; higher high temperature strength and creep resistance; increased hardness with increasing heat treatment temperature; hardness values higher than those of existing commercial steel and alloys; possibility of synthesis of inexpensive materials; and significant increase in Young's modulus [about 105%], shear modulus and fracture strength (almost 3 times compared to microcomposites). These are brought out mainly by the nanosize reinforcements used, which result in an appropriate morphology for the products.Tables 21 and 22 summarize the possible developments associated with these materials in catalysts, sensors, structural materials, electronic, optical, magnetic, mechanical and energy conversion devices suggested by researchers in the field.



CNT-ceramic composites, on their turn, are reported340 to be potential candidates for aerospace and sports goods, composite mirrors and automotive spares requiring electrostatic painting. Such materials have also been reported341 to be useful for flat panel displays, gas storage devices, toxic gas sensors, Li+ batteries, robust but lightweight parts and conducting paints. One example is the Al2O3-CNT composite, which shows high contact damage resistance without a corresponding increase in toughness and hardness. It is reported92 to be a candidate for engineering and biomedical applications.
Despite these possibilities, there are only limited examples of industrial use of nanocomposite, mainly due to the challenges in processing and the cost involved, particularly for non-structural applications. In fact, one recent review371 deals with various methods for the preparation of super hard coatings with merits and demerits of each method. However, the intense research in both metal- and ceramic-based nanocomposites suggests that the days are not far off when they will be actually in use. The cost factor may be a particularly serious problem for general engineering applications, while this may not be the case for specialized applications in electronics, aerospace, biomedical and other sectors, since the advantages might far outweigh costs and concerns in these sectors.
On the other hand, polymer-based nanocomposites are in the forefront of applications due to their more advanced development status compared to metal and ceramic counterparts, in addition to their unique properties. These include 2-3 fold strength property increase, even with low reinforcement content (1-4 wt. (%)) [e.g. 102.7% in Young's modulus] with complete elimination of voids/holes; gas barrier properties (about 200,000 times over oriented PP and about 2000 times that of Nylon-6 with tenfold requirements of expensive organic modifiers) 372, biodegradation and reduced flammability [about 60% reduction of heat release rate], etc. In addition, a good possibility of enhancing the shelf life of the existing MRE packaging and trays used in the UGR-H&S polymeric materials has also been reported350. This is due to the limitations of existing MRE packings, which do not meet the US military standards such as minimum shelf life of 6 and 12 months at about 322 K (120 °F) and 299 K (80 °F) respectively. In this case, nanocomposites, which exhibit better gas barrier properties, can provide a longer shelf life. Such packaging, with different matrices and reinforcements, as well as different processing conditions, is being field tested by the US army since 2002 to arrive at an optimum combination. This is expected to reduce cost by 10-30% (nearly US$ 1-3 million) compared to the presently used materials, in addition to better performance.
Various types of polymer-based nanocomposites, containing insulating, semiconducting or metallic nanoparticles, have been developed to meet the requirements of specific applications. Recently, some PLS nanocomposites have become commercially available18, being applied237 as ablatives and as high performance biodegradable composites265,267,280,343,346, as well as in electronic and food packaging industries346,347. These include Nylon-6 (e.g. Durethan LDPU60 by Bayer Food Packages)18 and polypropylene for packaging and injection-molded articles, semi-crystalline nylon for ultra-high barrier containers and fuel systems, epoxy electrocoating primers and high voltage insulation, unsaturated polyester for watercraft lay-ups and outdoor advertising panels, and polyolefin fire-retardant cables, electrical enclosures and housings. Table 24 shows some examples of commercially available polymer nanocomposites. As an example, Nylon-6/surface-modified montmorillonite 2 wt. (%) nanocomposites are currently available from two commercial sources, Honeywell Engineered Polymers & Solutions and Bayer AG. Some of the products made from nanocomposites are shown in Figure 42.



Technological contributions in the areas of gas barrier, reinforcement and flame retardancy have also been extensively exploited355,356. For example, heat-resistant polymer nanocomposites are used to make fire fighter protective clothing and lightweight components suitable to work in situations of high temperature and stress. This includes hoods of automobiles and skins of jet aircrafts, as opposed to heavier and costlier metal alloys. They can also replace corrosion-prone metals in the building of bridges and other large structures with potentially lighter and stronger capabilities357, 358. Also, unsaturated polyester (UPE) nanocomposites can be employed in fibre-reinforced products used in marine, transportation and construction industries359-361. Currently, UPE/fibreglass nanocomposites, whose formulations are available from Polymeric Supply, Inc., are being used in boat accessories that are stronger and less prone to colour fading362.
Regarding the variety of applications of polymer nanocomposites, prominent impacts over the automotive industry can be highlighted, including their use in tyres, fuel systems, gas separation membranes in fuel cells and seat textiles, mirror housings on various vehicle types, door handles, engine covers, intake manifolds and timing belt covers363,364, with some of these already being exploited. For example, a thermoplastic nanocomposite containing nanoflake reinforcements (trade name Basell TPO-Nano) is being employed for the development of stiff and light exterior parts, like the step-assists by GM348. Also, porous polymer nanocomposites can be employed for the development of pollution filters365. Other promising technological application in the horizon is in air bag sensors, where nano-optical platelets are kept inside the polymer outer layer for transmitting signals at speed of light gaining milliseconds to bring down the level of possible impact injuries373. Finally, polymer/inorganic nanocomposites with improved conductivity, permeability, water management and interfacial resistance at the electrode are natural candidates for the replacement of traditional Nafion PEM in fuel cells, and are currently under trial349.
Improvements in the mechanical properties of polymer nanocomposites have also resulted in their many general/industrial applications. These include impellers and blades for vacuum cleaners, power tool housings, and mower hoods and covers for portable electronic equipment, such as mobile phones and pagers366. Another example is the use of polymer nanocomposites in glues for the manufacturing of pressure moulds in the ceramic industry.
The development of environmentally friendly, non-foil and better packaging materials can reduce the amount of solid waste, improve package manufacturing capabilities, and reduce the overall logistics burden to users. In this context, the incorporation of nanoclay particles into thermoplastic resins has shown to be highly effective to improve barrier properties and package survivability351. Such excellent barrier characteristics have resulted in considerable interest in clay nanocomposites in food packaging applications, both flexible and rigid. Specific examples include packaging for processed meats, cheese, confectionery, cereals and boil-in-the-bag foods, also extrusion-coating applications in association with paperboard for fruit juice and dairy products, together with co-extrusion processes for the manufacture of beer and carbonated drinks bottles367. The use of nanocomposite formulations would be expected to enhance considerably the shelf life of many types of food. Honeywell industries have also been active in developing a combined active/passive oxygen barrier system for polyamide-6 materials368. It is mentioned here that Triton Systems and the US Army are conducting further work on barrier performance in a joint investigation, as mentioned earlier. They are trying to develop a non-refrigerated packaging system capable of maintaining food freshness for three years. Polymer/clay nanocomposites are currently showing considerable promise for this application.
The reduction of solvent transmission is another interesting aspect of polymer/clay nanocomposites. A study conducted by the UBE Industries has revealed347 significant reductions in fuel transmission through polyamide-6/66 polymers by incorporation of a nanoclay filler. As a result, these materials are very attractive for the development of improved fuel tanks and fuel line components for cars. In addition, the reduced fuel transmission means significant cost reductions. The presence of filler incorporation at nanolevels has also been shown to have significant effects on the transparency and haze characteristics of films369 in comparison to conventionally filled polymers. The ability to minimize the extent to which water is absorbed can be a major advantage for polymer materials that are degraded in moist environments370.
Finally, CNT-polymer composites are reported28 to be potential candidates for data storage media, photovoltaic cells and photo diodes, optical limiting devices, drums for printers, etc.








 

                         Conclusions
In conclusion, new technologies require materials showing novel properties and/or improved performance compared to conventionally processed components. In this context, nanocomposites are suitable materials to meet the emerging demands arising from scientific and technologic advances. Processing methods for different types of nanocomposites (CMNC, MMNC and PMNC) are available, but some of these pose challenges thus giving opportunities for researchers to overcome the problems being encluntered with nanosize materials. They offer improved performance over monolithic and microcomposite counterparts and are consequently suitable candidates to overcome the limitations of many currently existing materials and devices. A number of applications already exists, while many potentials are possible for these materials, which open new vistas for the future. In view of their unique properties such as very high mechanical properties even at low loading of reinforcements, gas barrier and flame related properties, many potential applications and hence the market for these materials have been projected in various sectors. Thus all the three types of nanocomposites provide opportunities and rewards creating new world wide interest in these new materials.



























REFERENCES
1. Kamigaito, O (1991). "What can be improved by nanometer composites?". J. Jpn. Soc. Powder Powder Metall 38 (3): 315–21. doi:10.2497/jjspm.38.315. in Kelly, A, Concise encyclopedia of composites materials, Elsevier Science Ltd, 1994
2. Jose-Yacaman, M.; Rendon, L.; Arenas, J.; Serra Puche, M. C. (1996). "Maya Blue Paint: An Ancient Nanostructured Material". Science 273 (5272): 223–5.doi:10.1126/science.273.5272.223. PMID 8662502.
3. B.K.G. Theng "Formation and Properties of Clay Polymer Complexes", Elsevier, NY 1979; ISBN 978-0-444-41706-0
4. P.M. Ajayan, L.S. Schadler, P.V. Braun (2003).Nanocomposite science and technology. Wiley. ISBN 3-527-30359-6.
5. Tian, Zhiting; Hu, Han; Sun, Ying (2013). "A molecular dynamics study of effective thermal conductivity in nanocomposites". Int. J. Heat Mass Transfer 61: 577–582.doi:10.1016/j.ijheatmasstransfer.2013.02.023.
6. F. E. Kruis, H. Fissan and A. Peled (1998). "Synthesis of nanoparticles in the gas phase for electronic, optical and magnetic applications – a review". J. Aerosol Sci. 29 (5–6): 511–535. doi:10.1016/S0021-8502(97)10032-5.
7. S. Zhang, D. Sun, Y. Fu and H. Du (2003). "Recent advances of superhard nanocomposite coatings: a review". Surf. Coat. Technol. 167 (2–3): 113–119.doi:10.1016/S0257-8972(02)00903-9.
8. G. Effenberg, F. Aldinger and P. Rogl (2001). Ternary Alloys. A Comprehensive Compendium of Evaluated Constitutional Data and Phase Diagrams. Materials Science-International Services.
9. M. Birkholz, U. Albers, and T. Jung (2004)."Nanocomposite layers of ceramic oxides and metals prepared by reactive gas-flow sputtering" (PDF). Surf. Coat. Technol. 179 (2–3): 279–285. doi:10.1016/S0257-8972(03)00865-X.
10. S. R. Bakshi, D. Lahiri, and A. Argawal, Carbon nanotube reinforced metal matrix composites - A Review, International Materials Reviews, vol. 55, (2010),http://web.eng.fiu.edu/agarwala/PDF/2010/12.pdf
11. Lalwani, G; Henslee, AM; Farshid, B; Parmar, P; Lin, L; Qin, YX; Kasper, FK; Mikos, AG; Sitharaman, B (September 2013). "Tungsten disulfide nanotubes reinforced biodegradable polymers for bone tissue engineering".Acta biomaterialia 9 (9): 8365–73.doi:10.1016/j.actbio.2013.05.018. PMC: 3732565.PMID 23727293.
12. Gash, AE. "Making nanostructured pyrotechnics in a Beaker" (pdf). Retrieved 2008-09-28.
13. Gash, AE. "Energetic nanocomposites with sol-gel chemistry: synthesis, safety, and characterization, LLNL UCRL-JC-146739" (pdf). Retrieved 2008-09-28.
14. Ryan, Kevin R.; Gourley, James R.; Jones, Steven E. (2008). "Environmental anomalies at the World Trade Center: evidence for energetic materials". The Environmentalist 29: 56–63. doi:10.1007/s10669-008-9182-4.
15. Janeta, Mateusz; John, Łukasz; Ejfler, Jolanta; Szafert, Sławomir (2014-11-24). "High-Yield Synthesis of Amido-Functionalized Polyoctahedral Oligomeric Silsesquioxanes by Using Acyl Chlorides". Chemistry – A European Journal 20 (48): 15966–15974.doi:10.1002/chem.201404153. ISSN 1521-3765.
16. b c d Manias, Evangelos (2007). "Nanocomposites: Stiffer by design". Nature Materials 6 (1): 9–11.doi:10.1038/nmat1812. PMID 17199118.
17. Lalwani, Gaurav; Henslee, Allan M.; Farshid, Behzad; Lin, Liangjun; Kasper, F. Kurtis; Yi-, Yi-Xian; Qin, Xian; Mikos, Antonios G.; Sitharaman, Balaji (2013). "Two-dimensional nanostructure-reinforced biodegradable polymeric nanocomposites for bone tissue engineering". Biomacromolecules 14 (3): 900–909.doi:10.1021/bm301995s. PMC: 3601907.PMID 23405887.
18. Lalwani, Gaurav; Henslee, A. M.; Farshid, B; Parmar, P; Lin, L; Qin, Y. X.; Kasper, F. K.; Mikos, A. G.; Sitharaman, B (September 2013). "Tungsten disulfide nanotubes reinforced biodegradable polymers for bone tissue engineering". Acta Biomaterialia 9 (9): 8365–8373. doi:10.1016/j.actbio.2013.05.018.PMC: 3732565. PMID 23727293
19. Singh, BP; Singh, Deepankar; Mathur, R. B.; Dhami, T. L. (2008). "Influence of Surface Modified MWCNTs on the Mechanical, Electrical and Thermal Properties of Polyimide Nanocomposites". Nanoscale Research Letters 3 (11): 444–453. doi:10.1007/s11671-008-9179-4.
20. "Flame Retardant Polymer Nanocomposites" A. B. Morgan, C. A. Wilkie (eds.), Wiley, 2007; ISBN 978-0-471-73426-0
21. Gaharwar, Akhilesh K.; Peppas, Nicholas A.; Khademhosseini, Ali (March 2014). "Nanocomposite hydrogels for biomedical applications". Biotechnology and Bioengineering 111 (3): 441–453.doi:10.1002/bit.25160. PMC: 3924876.PMID 24264728.
22. Carrow, James K.; Gaharwar, Akhilesh K. (November 2014). "Bioinspired Polymeric Nanocomposites for Regenerative Medicine". Macromolecular Chemistry and Physics 216: n/a–n/a.
23. "Conductive Polymer Bronzes. Intercalated Polyaniline in V2O5 Xerogels" M.G. Kanatzidis, C.-G. Wu, H.O. Marcy, C.R. Kannewurf J. Am. Chem. Soc., 1989, 111, 4139-4141
24. "Conductive-Polymer Intercalation in Layered V2O5 Xerogels. Intercalated Polypyrrole" C.-G. Wu, M.G. Kanatzidis, H.O. Marcy, D.C. DeGroot, C.R. Kannewurf Polym. Mat. Sci. Eng. 1989, 61, 969-973.
25. "Synthesis, Structure and Reactions of Poly(ethylene-oxide)V2O5 Intercalative Nano-composites" Y.-J. Liu, J. L. Schindler, D. C. DeGroot, C. R. Kannewurf, W. Hirpo, M. G. Kanatzidis, Chem. Mater. , 1996, 8, 525-534.
26. Ray SS, Okamoto M. Polymer - layered silicate nanocomposites: a review from preparation to processing. Progress in Polymer Science. 2003; 28(11):1539-1641.      
27. Andrews R, Weisenberger MC. Carbon nanotube polymer composites. Current Opinion in Solid State and Materials Science. 2004; 8(1):31-37.      
28. Wang C, Guo ZX, Fu S, Wu W, Zhu D. Polymers containing fullerene or carbon nanotube structures.Progress in Polymer Science. 2004; 29(11):1079-1141.      
29. Pandey JK, Reddy KR, Kumar AP, Singh RP. An overview on the degradability of polymer nanocomposites. Polymer Degradation and Stability. 2005; 88(2):234-250.      
30. Thostenson ET, Li C, Chou TW. Nanocomposites in context. Composites Science and Technology. 2005; 65(3-4):491-516.      
31. Jordan J, Jacob KI, Tannenbaum R, Sharaf MA, Jasiuk I. Experimental trends in polymer nanocomposites: a review. Materials Science and Engineering: A. 2005; 393(1-2):1-11.      
32. Choi SM, Awaji H. Nanocomposites: a new material design concept. Science and Technology of Advanced Materials. 2005; 6(1):2-10.      
33. . Xie XL, Mai YW, Zhou XP. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Materials Science and Engineering: R. 2005; 49(4):89-112.      
34. . Ray SS, Bousmina M. Biodegradable polymers and their layered silicate nanocomposites: in greening the 21st century materials world.Progress in Matererials Science. 2005; 50(8):962-1079.         [ Links ]
35. Pandey JK, Kumar AP, Misra M, Mohanty AK, Drzal LT, Singh RP. Recent advances in biodegradable nanocomposites. Journal of Nanoscience and Nanotechnology. 2005; 5(4):497-526.      
36. Awaji H, Choi SM., Yagi E. Mechanisms of toughening and strengthening in cermaic-based nanocomposites. Mechanics of Materials 2002; 34(7):411-422.         [ Links ]
37. Niihara K. New design concept of strctural ceramics-ceramic nanocomposite. Journal of the Ceramic Society of Japan (Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi). 1991; 99(6):974-982.      
38. . Nakahira A, Niihara K. Strctural ceramics-ceramic nanocomposites by sintering method: roles of nano-size particles. Journal of the Ceramic Society of Japan. 1992; 100(4):448-453.      
39. . Ferroni LP, Pezzotti G, Isshiki T, Kleebe HJ. Determination of amorphous interfacial phases in Al2O3/SiC nanocomposites by computer-aided high-resolution electron microscopy. Acta Materialia. 2001; 49(11):2109-2113.         [ Links ]
40. She J, Inoue T, Suzuki M, Sodeoka S, Ueno K. Mechanical properties and fracture behavior of fibrous Al2O3/Sic ceramics.Journal of European Ceramic Society. 2000; 20(12):1877-1881.      
41. Tjong SC, Wang GS. High-cycle fatigue properties of Al-based composites reinforced with in situ TiB2 and Al2O3 particulates.Materials Science and Engineering: A. 2004; 386(1-2):48-53.      
42. Athawale AA, Bhagwat SV, Katre PP, Chandwadkar AJ, Karandikar P. Aniline as a stabilizer for metal nanoparticles. Materials Letters. 2003; 57(24-25):3889-3894.      
43. Akita H, Hattori T. Studies on molecular composite. I. Processing of molecular composites using a precursor polymer for poly (P-Phenylene benzobisthiazole). Journal of Polymer Science: Part B: Polymer Physics. 1999; 37(3):189-197.      
44. Akita H, Kobayashi H. Studies on molecular composite. III. Nano composites consisting of poly (P-phenylene benzobisthiazole) and thermoplastic polyamide. Journal of European Ceramic Society. 1999; 37(3):209-218.      
45. Akita H, Kobayashi H, Hattori T, Kagawa K. Studies on molecular composite. II. Processing of molecular composites using copolymers consisting of a precursor of poly (P-phenylene benzobisthiazole) and aromatic polyamide. Journal of European Ceramic Society. 1999; 37(3):199-207.      
46. Chang JH, An YU. Nanocomposites of polyurethane with various organoclays: thermomechanical properties, morphology, and gas permeability. Journal of European Ceramic Society. 2002; 40(7):670-677.      
47. Zavyalov SA, Pivkina AN, Schoonman J. Formation and characterization of metal-polymer nanostructured composites. Solid State Ionics. 2002; 147(3-4):415-419.      
48. Thompson CM, Herring HM, Gates TS, Connel JW. Preparation and characterization of metal oxide/polyimide nanocomposites.Composites Science and Technology. 2003; 63(11):1591-1598.         [ Links
49. Liu TX, Phang IY, Shen L, Chow SY, Zhange WD. Morphology and mechanical properties of multiwalled carbon nanotubes reinforced nylon-6 composites. Macromolecules. 2004; 37(19):7214-7222.      
50. Theng BKG. The chemistry of clay-organic reactions. New York: Wiley; 1974.      
51. . Ogawa M, Kuroda K. Preparation of inorganic composites through intercalation of organoammoniumions into layered silicates.Bulletin of the Chemical Society of Japan. 1997; 70(11):2593-2618.         [ Links ]
52. Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Karauchi T, Kamigaito O. Mechanical properties of nylon-6-clay hybrid.Journal of Materials Research. 1993; 8(5):1185-1189.         [ Links
53. Stearns LC, Zhao J, Martin P. Harmer Processing and microstructure development in Al2O3-SiC 'nanocomposites'. Journal of European Ceramic Society. 1992; 10(3):473-477.         [ Links ]
54. Borsa CE, Brook RJ. Fabrication of Al2O3 /SiC nanocomposites using a polymeric precursor for SiC. In: Hausner H, Messing GL, Horano S, editors. Proceedings of the International Conference of Ceramic Processing Science and Technology; Sept. 11-14, 1994, Friedrichshafen. Westerville, Germany: The American Ceramic Society; 1995. p. 653-658. (Ceramic Transactions. Vol. 51)        [ Links ]
55. Riedel R, Strecker K, Petzow G. In situ polysilane-derived silicon-carbide particulates dispersed in silicon nitride composite.Journal of the American Ceramic Society. 1989; 72(11):2071-2077.         [ Links ]
56. Riedel R, Seher M, Becker G. Sintering of amorphous polymer-derived Si, N and C containing composite powders. Journal of European Ceramic Society. 1989; 5(2):113-122.         [ Links ]
57. Riedel R, Seher M, Mayer J, Szabo D. Polymer-derived Si-based bulk ceramics. Part I: Preparation, processing and properties.Journal of European Ceramic Society. 1995; 15(8):703-715.         [ Links ]
58. Livage J. Sol-gel processes. Current Opinon in Solid State and Materials Science. 1997; 2(2):132-136.         [ Links ]
59. Vorotilov KA, Yanovskaya MI, Turevskaya EP, Sigov AS. Sol-gel derived ferroelectric thin films: avenues for control of microstructural and electric properties. Journal of Sol-Gel Science and Technology. 1999; 16(2):109-118.         [ Links ]
60. Hench LL, West JK. The sol-gel process. Chemical Review. 1990; 90(1):33-72.         [ Links ]
61. . Ennas G, Mei A, Musinu A, Piccaluga G, Pinna G, Solinas S. Sol-gel preparation and characterization of Ni-SiO2 nanocomposites.Journal of Non-Crystalline Solids. 1998; 232-234:587-593.         [ Links ]
62. Sen S, Choudharya RNP, Pramanik P. Synthesis and characterization of nanostructured ferroelectric compounds. Materials Letters. 2004; 58(27-28):3486-3490.         [ Links ]
63. Viart N, Richard-Plouet M, Muller D, Pourroy G. Synthesis and characterization of Co/ZnO nanocomposites: towards new perspectives offered by metal/piezoelectric composite materials. Thin Solid Films. 2003; 437(1-2):1-9.         [ Links
64. Kundu TK, Mukherjee M, Chakravorty D, Sinha TP. Growth of nano-alpha-Fe2O3 in a titania matrix by the sol gel route. Journal of Matererials Science. 1998; 33(7):1759-1763.         [ Links ]
65. Baiju K, Sibu CP, Rajesh K, Pillai PK, Mukundan P, Warrier KGK, Wunderlich W. An aqueous sol-gel route to synthesize nanosized lanthana doped titania having an increased anatase phase stability for photocatalytic application. Materials Chemistry and Physics. 2005; 90(1):123-127.         [ Links ]
66. Ananthakumar S, Prabhakaran AK, Hareesh US, Manoharan P, Warrier KGK. Gel casting process For Al2O3-SiC nanocomposites and its creep characteristics. Materials Chemistry and Physics. 2004; 85(1):151-157.         [ Links ]
67. Sivakumar S, Sibu CP, Mukundan P, Pillai PK, Warrier KGK. Nanoporous titania-alumina mixed oxides: an alkoxide free sol-gel synthesis. Materials Letters. 2004; 58(21):2664-2669.         [ Links
68. . Warrier KGK, Anilkumar GM. Densification of mullite-SiC nanocomposite sol-gel precursors by pressureless sintering. Materials Chemistry and Physics. 2001; 67(1-3):263-266.         [