Recent Progress and Overview of Nanocomposites

Abstract

Nanocomposites are versatile materials because of possessing superior properties as compared to their parent materials. Due to their improved electrical, mechanical, thermomechanical, electronic, optoelectronic, thermal, and magnetic properties, these materials are receiving much attention from researchers all over the world. In every field, the focus of the research is to develop such materials which have low weight, superior strength, and enhanced performance as well as cost competitiveness in comparison to existing materials. The nanocomposite materials have been used in the fields of avionics, biomedical, auto industry, sports industry, oil/gas, construction, food industry, agriculture industry, and information technology. This chapter addresses the synthesis, unique properties, and diverse applications of nanocomposites in different fields.

1. Introduction

Composites consist of two materials; one of the materials is called the reinforcing phase (fibers, sheets, or particles), and is embedded in the other material called the matrix phase. The composite materials have more strength as compared to constituent materials. Wood is a common example of natural composites. Snails and oysters shells are also examples of natural composite materials. The history of the usage of nanocomposites is quite old. Egyptian have mixed straws with clay to form bricks. Mongols have used the composites in warfare and even in recent times, during World War II, the composite-based materials were used in military appliances, and in the modern era, a large number of composites are used in different fields [1].

The nanocomposite is a solid compound made up of several layers where at least one of the layers has one, two, or three dimensions with a nanometer size [2]. In nanocomposites, the atoms of the materials are arranged in the form of clusters or small grains. A solid multiphase having one of the 1, 2, or 3 phases less than 100 nm is called nanocomposites [3]. Nanoparticles, nanoclays, and nanofibers are examples of nanocomposites. Nanocomposites find their applications in the fields of medical and engineering.

Currently, scientists are facing the problem of data loss due to the overheating of imaging spectrographs in the Hubble Space Telescope. A probable solution is the use of a carbon nanotube that could dissipate the excess heat. The nanocomposites are lighter in weight and it is estimated that in near future the steel and aluminum-based products used in different engineering applications might be replaced with nanocomposites.

A Nanocomposite (NC) consists of several stages where, at least one, two, or three dimensions are in the nanometric range. Taking the size of objects up to the nanometer level creates optical connectors that are very important in the development of building material structures. Nanocomposites (NCs) provide opportunities on a completely new scale to solve hurdles ranging from the medical, pharma-industry to food packaging and from the electronics to energy-producing industries. Nanocomposites can be divided into three categories, which are as follows:

• Metal matrix nanocomposites (MMNCs).

• Ceramic matrix nanocomposites (CMNCs).

• Polymer matrix nanocomposites (PMNCs).

The aim of the development of nanocomposites is to designate which raw materials and processes are best suited to produce specific nanomaterials by studying their uses, benefits, and drawbacks. Transformation factor where the size of the material is made less than a certain level is known as “Sensitive size.” Taking the size of the material down to the nanometer level fabricates the interactions with phase interfaces that become necessary in the development of building material structures. The ratio within the surface area to the volume of a reinforced material is used in synthesizing nanocomposites has directly been related to the structure-property relationship. A material nanocomposite (10⁻⁹ m diameter) that is made from non-metallic, metallic, or polymeric constituents by a certain process offers added benefits to retain their primary features and overcome defects by expressing some new characteristics. Such materials present a multiphase crossover in the matrix as well as in reinforcing materials. Reinforced material is a dispersed surface in the form of composite materials, such as fiberglass and organic fibers, whereas, matrix material consists of a uniform state and contains metallic, non-metallic, and polymer-based materials [4]. Nanocomposites represent a new way to surpass the limitations posed by microcomposites and the monoliths which have become the objects of the future. NCs have the following advantages over other materials:

• The ratio between surface area and volume is high that allows a small fill size.

• Superior mechanical characteristics that are minimum wastage of power and initial resistance and possess high ductility.

• Having improved visual features (the particle size anchors the light transmission).

High robustness and effective impact are associated with nanoparticles along with the incorporation of composite matrix. But, insufficient understanding of structural materials and material to structural relationships, the need for easy particle breakdown and dispersion are the major hurdles in nanocomposite synthesis (see Figure 1).

Figure 1.

Broader classification and applications of nanotechnology.

2. Classification of nanocomposites

2.1 Metal matrix nanocomposites (MMCs)

Since the late nineteenth century, composite materials were widely applicable in many systems with greater efficacy [5]. MMCs are one of the largest groups of compounds that are often strengthened with clay. The combination of metals and ceramic structures offers a variety of applications. There have been different ways of making MMCs but, powdered metallurgy is considered a unique process. Indeed, powdered metallurgy (PM) has been considered an effective method that has transformed the material industry by allowing them to build complex structural elements that control the precise strength, flexibility of composite design, and produce highly soluble materials with desirable compositions [6]. It seems that there are two main factors that determine the mechanisms of action in MMCs, that is the desired features of a material and technical constraints. If manufacturing a special type of alloy, for example, it is imperative of establishing a hot phase that allows a phase transformation into a metallic wire form; however, the scientific possibility still exists to design it from a large billet but, technological hurdles would have not allowed it to do so. We would otherwise need to apply high strain and thermal modulus, which can eventually damage the required structure. Therefore, multistep processing channels are used to improve features and overcome technical barriers that are the backbone of the process. To elaborate further, two-step sintering (TSS) and multistep isothermal forging (MIF), and isothermal rolling (IR) as a good plastic deformation (SPD) process have been used.

The sorting of building materials to meet the desired mechanical requirements followed by fine grain sizes and high density is a challenge for MMCs, as it requires very high temperature and often requires to be aligned with mechanical stress in a standard environment, that is very costly for samples having large sizes. The process offered a constricted geometry of the sample matrix [7]. Such a challenge can be remedied by using pressure-less two-step sintering (TSS) where the exfoliated sintering has resulted in fine heat resistance-granular material that is highly pure and more stable. TSS is a robust and productive method for obtaining a good microstructure with theoretical density. The mechanism includes heating of the composite until it reaches a high temperature and gains 70–90% of theoretical density then returns to a lower temperature to obtain higher density with controlled grain growth. As the grain boundary responsible for grain growth, it has a higher enthalpy compared to the grain boundary distribution at a certain temperature, the latter accomplishes during the second-step sintering in mild thermal conditions, suppresses grain growth, and cause pores annihilation and full densification. In addition, the TSS can overcome the problems associated with performing cold compression/filtration and/or compaction of the water phase leading to the metal agglomeration, respectively. Many other studies have shown that MMCs obtained through different processes provide a higher initial temperature for many clay materials, such as zinc oxide, magnesium-niobium-doped yttrium oxide, barium titinate, and silicon carbide, as well as other compounds. Ceramic matrix materials include compounds that are titanium boride 40% by weight, titanium nitride, and aluminum oxide 10% by volume.

2.2 Ceramic matrix nanocomposites (CMNCs)

Ceramic matrix nanocomposites (CMNCs) are added with solitary or multiple layers of ceramics to strengthen the crack resistance, heat absorption, and chemical resistance. Whereas, the main flaws of ceramics are their stiffness and less durability that keep them away from being used for industrial references. The limitation has been overcome by the production of ceramic-matrix (CMNCs) nanocomposites. The CMNC model incorporates a matrix in which energy dispelling components (fiber, platelets, or particulates) are added to CMNCs to reduce stiffness and increase crack resistance [2]. Raw materials for CMNC matrixes include alumina, SiC, SiN, etc. Generally, all reinforcements of the nanocomposites are of nanometric sizes. Iron and other metal powders: TiO₂, silica, clay are used for amorphous reinforcements. The most common reinforcements are clays and silicates, due to their low particle sizes and well-studied chemical interactions. The addition of clays and silica layers even in small amounts modify matrix structures. Many different approaches are designed for CMNCs integrations. Recently modified techniques are single source-precursor technology that is based on melt spinning of mixed raw materials followed by pyrolysis of the nanofibers. Some established mechanisms are PM; polymeric monomer method, spray-pyrolysis, and vapor methodologies (CVD and PVD) [8]. Chemical methods are the sol-gel process, colloidal method, and rain synthesis [9].

Mixtures of metal amalgamation and mechanical milling are widely used to process a promising program but these methods require an accurate measurement of powdered concentrations to produce a system in a metastable state and then there are a few steps that make strong semifinal products. Various combinations of metallic reactants are another method of directly producing the metallic bulk, for example, the Mg system is a hybrid matrix that includes fusion welding and composite casting requiring metal in the form of a liquid or roll cladding/bonding of solid-state welding. In all of these ways, metals present a diffusion-bound under relatively moderate pressure at a higher temperature for a long time; so, it is not possible to produce a good microstructures. In the process, the metal disk is pressed in high pressure environment with simultaneous torsion straining and processing, which is usually carried out at room temperature (RT), the process is equally effective even on hard as well as on amorphous substances, such as Mg alloys. Also, processed metals usually show improvements in physical and mechanical properties through the use of critical grain refinement and deep introduction of point and line disorders (see Tables 1–3).

Class	Matrix/Reinforcements	Properties	Reference
Ceramic/Matrix	Al2O3/NdAlO3	Improved Photoluminescence	[10, 11]
	Al2O3/Mo	Improved strength and toughness	[12, 13]
	Al2O3/W	—	[11]
	Al2O3/ZrO2	—	[14]
	Al2O3/SiC	—	[15]
	Si3N4/SiC	—	[16, 17]
	MoSi2/ZrO2	—	[18]
	B4C/TiB2	—	[19]
	MgO/SiC	—	[20]
	Al2O3/TiO2		[13]
	Al2O3/CNT		[18, 21]

Table 1.

Examples of ceramic-based nanocomposites.

Matrix/reinforcement	Ag/Au	Improved catalytic activity	[22]
	Ag-Pd	Improved catalytic activity	[22]
	Au-Pd	Improved catalytic activity	[22]
	Cu/Nb	Improved microhardness	[23]
	Al/AlN	Higher compression resistance	[24]
	Al/SiC	Improved hardness	[25]
	CNT/Sb	Improvement in Li intercalation properties	[26]
	Cu/Al2O3	Improved microhardness	[27]
	CNT/Fe3O4	Improved electrical conductivity	[28]
	Fe-Cr/Al2O3	—	[21]; [29]
	Co/Cr	—	[12]; [19]
	Fe/MgO	—	[30]
	Mg/CNT	—	[31]
	Al/CNT	—	[32]

Table 2.

Examples of metal-based nanocomposites.

Class	Matrix/Reinforcements	Properties	Reference
Polymers/metal oxide	PANI/WO3		[33]
	PPANI/CeO2	Thermally stable material	[34]
	PANI/ Sm2O3, La2O3	—	[35]
	PANI/La-Nd	Electromagnetic interference	[36]
	PANI/ Ce-TiO2	sensor	[37]
	PANI/Nd2O3:Al2O3	Dielectric constant
	PPY/Sm2O3	Supercapacitor	[38, 39]
	PPY/Y2O3	Semiconductor	[30]
	PPY/Nb2O5	—	[38]
	PPY/CeO2	Sensor	[40]
	PPY/RuO2	Supercapacitor	[41]
	PPY/RuO2	Supercapacitor	[42]
	Polyindole (PIN) TiO2	Semiconductor	[43]
	PEO/La2O3	Semiconductor and Solid Polymer Electrolyte (SPE)	[44]
	PVA/Ho/Gd	Optical display	[45]
	PVDF/La2O3	Thermally stable material	[46]
Polymers/NPs	Poly (styrene sulfonate) Co3O4 nanoparticles	Humidity Sensing	[47]
	Polypyrrole (PPy) Titanium dioxide (TiO2)	Gas sensor Applications	[48]
	Polyaniline (PANI) Halloysite	Supercapacitor Applications	[49]
	Poly (allylamine hydrochloride) Graphene Oxide	Enhanced Mechanical Properties	[50]
	poly-L-lysine Graphene Oxide	Bio-scaffold coating	[50]
	Gelatin Graphene	Cellular imaging / Drug Delivery	[47]
	Sodium alginate Graphene	Oxide Tissue Engineering	[51]
	poly(lactic-co-glycolic acid) Graphene Oxide	Enhance Mechanical and Thermal properties	[52]
	Polyurethane (PU) Graphene Oxide	Enhance Mechanical and Thermal properties	[53]
	Poly(propylene fumarate) Graphene Oxide	Tissue Engineering	[54]
	polyvinylidene difluoride (PVDF) Graphene/ZnO	Temperature Sensing Applications	[55]

Table 3.

Examples of polymer-based nanocomposites.

PANI = polyanialine, PPY = polypyrrole, PVA = Polyvinyl Alcohol, PEO = Poly(ethylene oxide), PVDF = Polyvinylidene fluoride.

When fine-grained ceramic or other solid particles are embedded in a “soft” metal matrix to form metal matrix compounds (MMCs), the elements of the matrix materials can be greatly improved and strengthened. The strengthening of the mechanism for MMCs has been tested by many researchers. It has been thought that desired characteristics of composite metal structures with nano-sized ceramic particles (1.0–100 nm), called MMNCs, can be greatly improved even in these lowest volume conditions. Currently, mechanical mixing (e.g., high-power ball milling) for metal and ceramic powders is generally used to study the characteristics of MMNCs. Mixing ceramic particles with nanosize is energy as well as a time-consuming and costly procedure. Exfoliation, like the liquid-phase process, is best known for its ability to produce products with complex shapes. It will be desirable to synthesize MMNCs parts that are not as heavy as cast with the distribution of good reinforcement and integrity of the structure. However, there are ceramic particles with nanosize that put forth several problems that is very difficult to disperse the same is true for liquid metals because of their unwetting nature, the metal matrix having large surface to volume ratios, which facilitates agglomeration and cluster formation. Powerful ultrasonic waves have been proven very helpful in the context that they produce important indirect effects of liquids, namely transient cavitation as well as acoustic radiation. Acoustic cavitation covers the formation, growth, folding, and collapse of small object bubbles, which produce momentary (in microseconds) small “hot spots” that can attain temperatures (5000°C), pressures (1000 atm), and temperature rise and drops of 10¹⁰ K/s. The combination of impact with higher temperatures can also create improvements in the wetting between liquids and particles, thus facilitating the preparation of diffused compounds with effective microparticles.

It is thought that strong cavitation of the microscale transient, as well as macroscopic dispersion, may effectively disperse nanoparticles into soluble alloys and improve their wetting, thus making them more productive in performance as highly castable, light-weighted MMNCs.

Most CMNCs have low fracture resistance and are brittle. In addition to the discovery of ceramic coated CMNCs and silicon carbide (SiC), the modern focus is on the construction of ceramic-based nanocomposites with improved properties. Carbon nanotubes agglomeration increases the material’s toughness by energy quenching through elastic modulus in the deformation stage. However, the design complexities have put a limit on the syntheses of these nanomaterials. The main drawback has been the nonuniformity of carbon nanotubes (CNTs) in the matrix suspension. The deformation of CMNCs has often been associated with high thermal and reactive environments that occur during the production of CMNCs. Nevertheless, there is sizeable progress in the field of nanocomposites but still, these are just preliminary steps to develop nanocomposites, a significant amount of exploration and effort is further required to ultrafine these manufacturing techniques. For example, a team from the University of California, Davis, has developed alumina ceramic by combining a single wall of carbon nanotubes (SWCNTs) with Al₂O₃ nanopowders using PM method. The resulting nanocomposite had advanced thermal, electronic, and mechanical characteristics. The highly potent anisotropic nanocomposite has a thermal ratio of 3:1 in an aligned plane. Electrical conductivity was far better than pure alumina matrix. Most importantly, the fracture strength was thrice higher than alumina with the crack resistance, heat absorption, and shock resistance capacity. Recently, at Tohoku University, a research group has synthesized a sophisticated CMNC on alumina ceramic through multi-walled carbon nanotubes. This process has reduced the phase separation that has resulted in a nanocompound with more uniformity in its structural phase. The addition of 0.9% acid-contained MWCNTs produced a component with a crack capacity of 5.90–0.27 MPa m1/2, greater than pure alumina NC (3–5 MPa m1/2) and a stronger bending capacity of 27%. A Chinese group of Qingdao University of Science and Technology has reported the MWCNT/zircona CMNCs produced by spark-sintering process had 18% higher fracture strength as compared to pure zircona. Another study by US Nano Labs has prepared a high-density boron carbide (B₄C) containing CMNCs. This composite was produced by the hot pressure-sintering process. However, none of these techniques have produced significant fracture toughness and heat dispelling properties, such as those in SiC-fiber-reinforced composite.

2.3 Polymer nanocomposites (PMNCs)

While nanotechnology still presents a picture of the future, nanocomposites set an example for realistic and rapidly booming applications. For instance, Geoff Ogilvy won the 2006 US Open golf tournament by using a nanomaterial-reinforced polymer-based club. Nanocomposites include materials that is CNTs, mineral materials, metals, and other fillers that can greatly improve composite structures. They attract a lot of awareness and some have been commercially available, having abilities to offer all kinds of uniqueness. Polymer-based products are the best-selling categories of NCs and covered global revenue of approximately, 223 million US dollars, in the year 2009. The nanomaterial’s inclusions to the polymeric materials can enhance polymer characteristics that is robustness and strength, Young’s modulus, impact endurance and scratch proofing, heat absorption, chemical defiance with electrical insulation and thermal adherence, stability toward the thermal shocks. Currently, minerals compounds and CNTs-based materials are more widely used than NPs. One of the premier commercial systems for PMNCs was used by Toyota, which has used nanoclay with nylon-6 PMNC [56] in their engine component showed an excellent result. In the late 1980s, Toyota Central Research Labs partnered with Ube Industries, a Japanese supplier of fossils, to cement a new 6-nylon composite coated with layers of montmorillonite (naturally occurring silicate clay). The component of this clay has enhanced Toyota’s new model’s performance which subsequently found its uses in a time belt cover, benefiting from improved temperature adherence and size stability. Since then, few car manufacturers have used nanocomposites of clay material in auto parts, such as rocker box coverings, body panels; the latter is 60% lighter and is more fracture-proof than regular automotive parts. The cargo bed for the 2005 GM model Hummer used approximately 3-kilogram of molded parts of nanoclay/polypropylene nanocomposite in its trim, mid-bridge, canvas panel, and box protectors. Polymer barrier technology was also benefited from these material NCs. Nylon/nanoclay composite is also applied for beverage bottles and in the food packaging industries. The addition of clay can significantly reduce gas/vapor infiltration, as clay platelets and thus prevent mobility, leading to significant improvements in shelf life. CNT-based nanocomposites are gaining increased industrial use from sports and leisure to technology, automotive, and defense motives. CNTs are attractive because of their excellent physical properties that often surpass many highly advanced materials and are now embedded in many polymeric NCs. Many automotive systems are sprayed with electrostatic paints. Plastic body panels need to be carefully processed for the paint to work properly. CNT is being applied as an alternative to carbon black, an expensive primer. The extra edge is being low CNT loading is needed to acquire the required conduction for the polymer to retain half of its actual length than 3–4% length saved when using carbon black. Importantly it is ensured that a panel must maintain its strength at a critical decrease of temperatures and never breaks. In addition, CNTs are so minute and used for such a low load that the higher end of class “A” is available in obtained NCs. The high-power output of CNT-nanocomposites is also utilized in the electronic industry, mainly to reduce the chances of damage caused by electrostatic accumulation or emissions. The PMNCs have found their applications in integrated circuits (IC). Joint Electronic Engineering Council trays wafer carriers and IC test that burn sockets because of high potential differences, combined with these materials having superior thermophysical properties to avoid the disaster. An example of a substance used in the industry is the Plasticyl range of CNTs/thermoplastic and nanocomposites, produced by the Nanocyl component as a precursor. Other benefits of PMNCs are seawater-cooled intercoolers on large diesel engines and in the power stations, where PMNCs will offer a robust substitution to copper-containing alloy, thermal rescue systems from fire hydrants and flue gases, working under 3008 celsius, whereas commercially used MMNCs systems lose their robustness in the chemical management as well as in processing industries where fissile environments prevail [56]. Demonstrating their strength and toughness, these materials have found applications from being used in baseball bats, bicycle frames, and power boats to military boats and aircraft. The leading company in nanocomposite technology is Applied Nanotech Holdings.

However, adding PMNCs, especially CNTs, in a resin or other matrices is not an easy task. Problems, such as segmentation, merging, poor disintegration, and poor adherence to host, should be overcome during integration. Some companies have developed methods that are specific for certain NP. For example, Zyvex uses new technologies based on solid composite polymers, in which large interactions within the polymer core and the nanotube surface occur with noncovalent (“aromatic”) interactions. Although these interactions are much weaker with fragile bonds than covalent interactions, their total impact strengthens the composite leading to stable systems. Similarly, dispersing nanoparticles of clay onto polymers requires special techniques, most commonly involving solution, in situ polymerization, and intercalation using clay with appropriate treatment. Some manufacturers now produce CNTs based on chemical incorporation on compounds while others offer PMNCs masterbatches, which usually contain 10–20% polymer composite by weight. A variety of polymers, including acrylonitrile, poly styrene, butadiene, polybutylene, polycarbonate, polystyrene, terephthalate, and polyamide. Similarly, masterbatches containing scattered nanoclays are available commercially. As CNTs have excellent properties overall that often they overshadow many highly advanced material compounds. Therefore, the chain resources are easily existing now to produce composite materials having more adapted and refined characteristics.

3. Synthesis of nanocomposite materials

3.1 Synthesis through melt intercalation and template synthesis methods (sol-gel technology)

Melt-intercalative polymerization has been attributed as a process in which in situ sheets polymerization occurs. It is based on the release of embedded silicate inside an aqueous precursor in which polymerization is initiated thermally, by irradiation, by diffusion of an appropriate initiator, or by natural raw material. Initially, nylon-6 montmorillonite nanocomposite was synthesized by this technique, then its production facilitated the synthesis of other thermosetplastics and nanocomposites. In situ polymerization is the simplest form of thermosetclay NCs. Disadvantages of the method are; fast reaction rates and reliability on clay incorporation, swelling of clay, measurement of the dispersed layer monomer of clay, and oligomer is formed when polymerization is yet to be completed. The method is free from the solvent presence and does not require a matrix and contains layered silica in a fused state. Conventionally, used methodologies include injection molding or extrusion in which thermoplastic NCs are mechanically mixed with organophilic clay at high temperatures. In this step, the polymer gets exfoliated to form the desired nanocomposite. This method is very simple to prepare thermoplastic nanocomposites. In the event of failure of in situ polymerization or formation of improper polymers, this method has been used.

3.2 Template synthesis (sol-gel technology)

An aqueous solution or a gel containing material and silicate block has been utilized during this process. During nucleation processing, the inorganic host crystals grow and are adsorbed within the layered surface. The sol-gel method can enhance the elimination between silicate layers through the single-step process in the absence of important oniumions, with some disadvantages. First, in the composition of clay compounds, the clay mineral requires high amounts of heat energy, which decomposes polymers matrix. Also, the negative tendency has been found while merging during silicate growth. Sol-gel process has commonly been employed for the generation of dual-layer nanocomposites, but with very little variation in concentrated silicates [2]. The natural features of the matrix structure have allowed it to be the most widely used synthetic material.

The main advantages of the method are as follows:

• Limitations of spray pyrolysis and efficacy in producing ultrafine grains.

• More uniform and well-oriented nanopowders with the larger surface area are used in multicomponent systems.

• Production of large-scale uniform, nano-sized particles.

• The sol-gel process is quite versatile, processing at low temperature, better homogeneity, rigorous stoichiometrically controlled, and renders pure products.

• The composite is porous having a low wear-resistivity and is weakly bonded.

• Include rapid solidification process (RSP), effective, reliable, and simple.

• High strength, high specific modulus, a combination of high performance, and low density of reinforced fibers making a low modulus.

• In heavily denser fiber glasses, the specific elastic modulus of the fiber-glass resins is slightly lower than MNCs.

• High shock absorbance, unlike classic materials, where fracture propagations cause breakages, nanocomposites have low matrix toughness with interfacial de-bonding and fiber splitting.

• There are only a few numbers of nanofibers, which are observed with fracture-effect on load transfer through the matrix, on intact nanofibers. When nanocomposite material is loaded for short time bearing capacity has not been affected, even encompassing defects in the desired nanocomposite.

The drawback with metal-metal nanocomposites is agglomeration and nonhomogeneous composition. The preparation of high-quality polymer nanocomposite materials using appropriate processing methods is essential to achieve high NC performance. Unique processing methods have been designed for the preparation of polymer nanocomposites. One universal method of preparation for all nanocomposites is not possible due to the structural and chemical differences of NCs and the different types of materials used. Each process requires specific processing conditions depending on the synthesis method, the type of nanoscale filler, and the required structures. In general, processing different technologies does not produce the same results.

3.3 Layered filled/nanocomposites (LFNCs)

Since silicate clay is hydrophilic, it is not suitable for mixing and blending with many compounds. In addition, the electrostatic forces cause the solid accumulation of platelets of clay. The neighboring platelets can share counter-ions, resulting in stacked platelets. It does not work with untreated clay to form nanocomposites because most of the clay matter is trapped internally and shows an interaction with layered nanocomposites. So, the clay must be processed well, before it can be utilized to prepare a nanocomposite material. The ion-exchange method is commonly used to obtain molded clays which make it more compatible with organic nanocomposites. After that, the clay can be mixed with different materials to get the desired product. Toyota has begun extensive research on nanocomposites molding and done a lot of work on loaded nanocomposites. There are four main processes that are used for the synthesis of polymer composites. These processes are as follows:

1. Intercalation of polymer or pre-polymer from solution.

2. In situ intercalative polymerization.

3. Direct melt intercalation.

4. Template synthesis for layered silicate/polymer nanocomposites.

The silicate layers are hardened and the polymer is melted to further processing stages. The concentrated silicate is swollen in a solvent, for example, chloroform, toluene, or water. Thereafter, the surface silicate and polymer solutions are mixed, the polymer chains bind and the solution inside the silicate layers evaporates. The composite structure remains the same during solvent removal, resulting in nanocomposite being deposited between the moving layers. Because of their excellent properties, that often used in materials, that are embedded in many nanocomposites. For example, amino acids convert montmorillonite (MMT) which is degraded by caprolactam monomer at 100 degrees centigrade and initiate its ring opening to detect MMT/nylon-6 nanocomposites. Ammonium cation of amino acids prefers the separation of caprolactam. The number of carbon atoms in the amino acid moiety greatly affects the flammability, which indicates that the concentration of caprolactam monomer is higher.

In the second process, embedded silicate begins to swell in an aqueous monomer mixture to form a polymer solution between the coated clay layers. Although the methods of interlamellar polymerization are best known for using concentrated silicates. Polymer nanocomposites are receiving a lot of attention due to the nanocomposite activity of MMT/nylon-6. In addition, two-step in situ polymerization was used to prepare MMT/polymer nanocomposite. These two steps include the preparation of the treated MMT solution and the mixing of polymers, respectively.

In the third process, melt intercalation occurs directly and composite silicates are combined with molten state NPs without the requirement of solvent. The polymer mixture is drawn by cutting over the softening area of the polymer suspension. The expanded chains of the polymer penetrate the intermediate layers of silicate from the melting of the polymer mass during the shrinkage. Fourthly, the process enforces as polymer suspension behaves as a template to form layered clay material. Silicon-based polymer materials are made from in situ hydrothermal crystallization, where a colloidal matrix of polymer gel and silicon-based NCs are being synthesized. This technique is being radially used to assemble nanocomposites with a double layer where silicates are formed in a solid solution consisting of building blocks of silicate and polymer precursor. This process is most suitable for water-soluble polymers, such as hydroxypropylmethylcellulose (HPMC), poly (dimethyldi-allylammonium) (PDDA), poly (vinyl-pyrrolidone) (PVPyr), and poly (aniline) (PANI)).

Polymerization techniques are well known for using concentrated silicates. Polymer nanocomposites are receiving high admiration due to nanocomposite induction. The in situ polymerization process could also be used to prepare nanocomposites loaded with graphene oxide. Natural graphite flakes are efficient and wearable structures with a carbon axis at the normal lattice. Because there are no active ionic groups of natural graphite flakes present, it is difficult to combine monomers onto graphite components to form graphite/polymer NCs with ion-exchange interactions. However, the dispersion of graphite has many holes with a diameter of 2–10 μm. Firstly, the graphitic dispersion starts interacting with a polymer solution with the help of the sonication technique and then polymers get embedded in the dispersed graphite holes and the solvent is released, thereafter. Graphene and composites are then obtained through heat transfer or immersion process. In addition, an electrostatic bonding process has been reported for the preparation of graphene/polymer nanocomposites [29]. First, polystyrene (PS) latex was synthesized using hexadecyl trimethyl ammonium bromide (cationic surfactant), creating favorable conditions in the areas of PS micelles. In addition, CNT/polymer resin (such as epoxy) nanocomposites could be prepared by means of thermal compression. The thin layer of the nanotube network is first detected by multiple nanotube dispersing steps and suspension filters, the large CNT sheets are then processed as a permeable resin. These large sheets are assembled to form solid nanocomposites for thermal mechanics.

3.4 Synthesis of chitosan nanocomposites (Ch-NCs)

To date, there have been several reported studies on the integration of Ch-NCs using a variety of integrated approaches. Researchers have developed several novel methods for the synthesis of Ch-NCs. Such methods include emulsion droplet coalescence, micellar modification, ionic gelation, precipitation, sieving, and spray drying. These methods have been used in the integration of chitosan-based materials which are used for drug delivery and other biomedical applications. However, the use of nanocomposites for agro-applications is still very limited. This can only happen if nanocomposite sources are economical and consistent. To ascertain the desirable characteristics, chitosan has been used for nanocomposite synthesis. As per the literature, ionic gelation methods and spray suspension methods have been considered as the most suitable synthetic methods for the production of large Ch-NCs. The mechanism of ionic gelation has been discussed. In this system, well-charged amino groups are combined with the less well-gelled tripolyphosphate (TPP). TPP is an anionic cross-linker that binds to the chitosan molecule and converts it into nanoparticles. TPP is nontoxic, so it is used in the production of chitosan-based nanomaterials (ChNMs). The plant response to nanocomposites used depends on a number of factors, including particle size, size distribution index, higher zeta capacity, and component nature. Nanocomposites and their activities with naturally occurring materials have introduced the environmentally friendly pollution-free method to deal with many challenges. Manufactured nanocomposites can be used as a foliar application, seed growth, and in soil mixing.

Chitosan-based nanomaterials are very extensively tested on plants to identify various factors, such as antimicrobial, adhesive, antioxidant. Chitosan can be used as a single ingredient or combined with other substances, such as copper (Cu), zinc (Zn), and silver (Ag), to synthesize the material of interest. Chitosan exhibits a strong metal bonding due to the availability of free amine groups throughout the polymeric spine of chitosan. The Zn^{+ 2} and Cu^{+ 2} have an important role in plant growth and germination; therefore, researchers have focused more on these two metal ions, by combining them with chitosan substrate.

3.5 Chitosan-Zn nanocomposites (Ch-ZnNCs)

The researchers have incorporated a variety of plant micronutrients onto NCs, including zinc. Zinc (Zn) was named as an integral part of the plant micronutrient in 1869. The addition of Zn to plants was intended to ensure its continued availability and increased efficiency. Zinc also protects plants from different environmental hazards (sun, water, etc.). Interactions between Zn-chitosan molecules have been demonstrated by using analytical methods, such as FT-IR and X-ray diffraction. The amino moiety in chitosan has shown two different styles, which are as follows:

1. There was only a solitary amino moiety that showed any type of bonding with chitosan in a pendant pattern.

2. Bridging pattern was found in metal ions when two or more amino groups got embedded in a metal chunk.

In 2018, Ch-Zn nanocomposite was prepared by the incorporation of low-molecularized chitosan molecules by iron-containing organosol. In a standard test, Zn granules (0.5 g), toluene (120 ml), and chitosan (4 g) were used. The synthesized NC was also tested for the physicochemical parameters by using different analytical methods, such as; SEM, TEM, and XRF. The nanocomposite, when combined with iron, has shown excellent antifungal activity against Rhizoctonia solani. Du, Niu, in 2009, loaded Zn²⁺ granules into chitosan solution to improve antibacterial activity [48]. In summary, ZnSO₄ solution was obtained by adding 0.3%, w/v Zn-granules to chitosan-solution (dissolved in 1% (v/v) acetic acid and TPP (1% w/v). The results of the study showed that the increased concentration of Zn²⁺ significantly improved the potential zeta of nanocomposite which led to an increase in the combined antibacterial activity. Chitosan zinc oxide nanocomposites (Ch-ZnO) were also tested for antifungal activity against Fusarium wilt (created by Fusarium oxysporum F.sp. Ciceri in chickpea). In addition to reducing recorded disease ~40% after using Ch-ZnO; and contributed to the increased growth of chickpeas.

3.6 Chitosan-Cu nanocomposites (Ch-CuNCs)

Copper (Cu) is one of the most important nutrients in plants. Although excessive use of Cu is harmful to all plants, Cu is allowed for organic farming. There are several reported copper-based fungicides. Various formulas are designed for the successful absorption of Cu by plants. It acts as an elicitor in plant cells to accelerate enzymatic activity. In other compounds, chitosan-copper nanocomposites (Ch-CuNCs) have also been tested for their antifungal activity in tomato inhibitors; Alternaria solani and Fusarium oxysporum. TEM, SEM-EDS, AAS-TEM, and SEM micrographs successfully demonstrated the inclusion of Cu in the chitosan matrix. Ch-CuNCs inhibits 70.5% and 73.5% mycelia growth and 61.5 and 83.0% algae growth rate in Alternaria solicit and Fusarium oxysporum, respectively. Plant lesion control was demonstrated when a significant decrease was observed in nano formulation-treated plants. The percentage efficacy of disease control (PEDC) success rate for Ch-Cu was recorded as 87.7%. In another report, Ch-Cu nanocomposite has shown significant antifungal activity against Sclerotium rolfsii and Rhizoctonia solani. The synthesized compound shows a means diameter of around 2–3 nm and is shown to be evenly distributed in nanocomposite uniforms. The results showed excellent results for the prevention of tested fungal diseases. Nanoparticles synthesized in acetone, produced a much higher degree of inhibition compared to those inferred by using toluene solvent. Jaiswal et al. synthesized Ch-CuNCs by adding copper sulfate to a chitosan solution followed by the incorporation of NaOH. The size of the copper particles produced was recorded as 700–750 nm. The solution is applied to plants referred to as a fungicide. The results revealed an important protective effect built against fungal pathogens. Chitosan-Cu nanocomposite has also been shown to be an important growth promoter in a variety of plants that performs Ch-Cu nano formulation and is combined with maize seedlings. Nano formulation has shown promising effects on plant growth by reducing the activity of α-amylase and protease enzyme and increasing the amount of protein content in seed germination (see Figure 2).

Figure 2.

Flow chart diagram for the synthesis of nanocomposites.

4. Properties

4.1 Electrical properties

The addition of CNTs to composite materials has had a significant impact on improving the conductive properties of nanocomposites. It is reported that the addition of CNTs improves the mechanistic and thermal characteristics of nanocomposites. Multiscale strengthening with NPs greatly improves enthalpy and electronic efficiency in related NCs. Conventional filters, such as carbon and glass filters are a viable solution in developing a combination of multiple functions. The doping of carbon black and nanotubes has led to the improved electronic operations of polymer film and organic sheets. Electrical conductivity depends on the concentration or the amount of filling material applied to nanopolymers. Semicrystalline polyamino compounds have shown better electrical performance than noncrystalline polycarbonate. When polymer films are applied to an organic sheet in nanofibers agglomeration, it causes an increase in electrical activity. The insulator material such as polycarbonate can be made conductive by adding it to nanocomposite material of varying compositions. The cheapest plastic known is made from nanocomposite material has both mechanical and optical features and is gaining future use. By assembling the right amount of CNTs in plastic, it becomes an electrical device. Cheap plastic is used to make optical discs, used in high-performance air-conditioning products and to shield these from electrical transformations and pulsation that cause the failure of the product. By changing the number of CNTs in polycarbonate, the performance of nanocomposite is also improved. The mixing of conductive particles to the polymer has a reasonable impact on the dielectric properties of composites. By the advent of electrical devices (capacitors, resistors, and others) on printed circuit boards, advanced compounds of nanocomposites have prevailing properties, such as:

• The integrated capacitors can be functionalized to produce large capacitance.

• The greater compatibility was found by industrial PCB fabrication with other composite materials.

• Robust process.

• Abandoning of leaded materials.

• The higher number of life cycles.

• Greater reliability and when it is required to increase dielectric properties of NCs then it has been added with ferroelectric ceramics material that has more permeation to work with that is BaTiO₃.

4.2 Organic conducting material nanocomposites (OMNCs)

They have a wide variety of properties with some important features, such as ease of processing, recycling, cost-effectiveness, and sustainability. The nonstandard semiconductors possess better electronic features, such as high thermal conductivity, high throughput, and high electrical conductivity. The inorganic NPs semiconductors materials have better luminescent and image processing properties. By combining both the polymers NPs and inorganic semiconductors form a hybrid nanocomposite that became the dominant candidate for photovoltaic cells. A variety of materials are used to form NCs. High performance has been achieved by mixing the CNTs to titina and P₃HT with the ability of power conservation. The addition of CNTs improves the thermomechanical characteristics of nanocomposites. Multistage strengthening with nanoparticles enhances electronic efficiency. The carbo-glass amalgamation is a viable solution for developing a composite having multiple functions. The addition of carbon allotropes has led to the improved electrical conduction of polymer and organic sheets. There are many factors that affect a hybrid system and require research to improve these systems. Overall, the nanotubes array and the nanorod-based hybrid system work better. Second, the interaction between organic and inorganic elements determines the effectiveness of cost sharing as well. Additionally, the alignment of power levels in the interface is one of the most important aspects of hybrid systems. Therefore, greater caution is required during the selection of these genotypes. Some guidelines should be followed to improve these systems. The correct combination of inorganics and metal semiconductors should also be taken into account. So,

• In the process of charge separation, a nanostructure with a large interface should be used.

• Good contact between inorganic and biological elements should occur.

The nanostructure network greatly assists in hybrid systems, TiO₂ has been employed as among the most widely used nanomaterial in our daily lives.

4.3 Multifunctional properties

Multifunctional materials, such as nanocomposites, are mostly applied as active sensors that perform multiple functions. Au (Pt) doped with α Fe₂O₃, nanospindles work in many ways to combine co-oxidation, and gas sensing devices. Catalytic activity is measured in a stainless steel bed reactor, while a gas measuring device performs gas sensor testing. It was found that the activity of various NPs led to higher performance in both functions compared to α Fe₂O₃. The reason for the improved effect is due to the active Au-NPs that act as a catalyst for sensitive reactions and also exhibit high efficiency in low-temperature co-oxidation. In 2010, thick and dense oxidized nanorod was produced in a row to form a strong fabric with good resistance to washing and pressure cycles [3]. Polymeric materials attract a lot of attention because of their advanced properties and functional performance in various industries. In structural features, thermoset polymers are very important in fields, such as automotive and aviation. In addition, the high strength of the thermoset polymers makes them compatible with their metal counterparts and creates a multi-layered environment. Recently, advances in nanotechnology introduced many innovative features in NCs. These benefits include an increase in strength, lightness, and durability. Nanocomposites are receiving a lot of attention because of the advanced mechanistic properties that have improved their stability.

There are different materials that are used in the production of thermoset nanocomposites. One of the most extensively used material components is carbon nanoparticles (CNPs) and nanoclays. One of the major hurdles is the dispersing of NPs on the matter substrate during the synthesis process. Nanocomposites contain 10–12% nanoclays with greater strength and durability than nanocalcium products. In nanoclay components, nanoparticles are extracted and synthesized. This improves the mechanical and physical performance of the filler and matrix optical connector which is very helpful in eliminating stress by improving the mechanical properties of nanocomposites. High-pressure mixing is better than direct mixing that can induce clay breakage. Titanate conversion is used for better spreading of nanoparticles. Due to the excellent mechanical properties, the requirement for low filling load, reinforcement strength, less weight, and corrosive environment of nanoparticles are found in some materials, such as cellulose, which is an ideal ingredient for the development of enviro-friendly polymers. Many researchers have focused on high-quality performance, extraction, and mechanical performance of filling polymer matrix in varying proportions. There are some challenges to the formation of nanocomposites, such as low dispersion of natural solvents, agglomeration, inclination, and hydrophilic nature. Due to growing environmental concerns, regulations have placed a great deal of interest in developing enviro-safe materials. Natural fibers have many advantages over synthetic due to their eco-friendly nature, but working with natural fibers, we cannot find the same strength that we can get from synthetic fibers.

Cellulose nanocrystals have been used in systematic and geometrically modeled cellulose fillers in a variety of useful products. It has been found that it will improve the mechanistic and thermal range of nanocomposites. Structure toughness with NPs affects the heat resistivity and electronic mobility in related NCs. As compared to traditional fillers, woody cellulose offers multidimensional combinations of variable functions. Microcrystalline cellulose as a colloidal matrix found in water with high solid concentrations, such as Celish (Trade name from Daicel Corporation) which provides 10% cellulose slurry and nanofibers. Solid-liquid crystals are used in a variety of optical applications. Researchers have successfully developed optically transparent wood cellulose nanocomposites with a small young’s modulus and low thermal increase. In addition, they have successfully applied an electroluminescent to flexible transparent cellulose nanocomposites resulting in a low coefficient of thermal expansion. To prevent the scattering of ionic diffusivity, cellulose whiskers (less than 10% concentrate) can be used in low-density electrolyte polymers having applications in lithium batteries. Low-density loudspeaker membranes with high Young’s modulus can be made from melamine-formaldehyde and micro-fibrillated cellulose. Electrospun cellulose nanofibers are used as an affective membrane that allows the purification and penetration of molecules based on physical or chemical characteristics instead of the weight or size of a molecule. Cellulose nanofibers are an excellent part of biological systems due to their load-bearing properties, low toxicity, excellent mechanical properties, biodegradability decay, and biocompatibility. Cellulose nanocomposites can be obtained from soft pulp from wood by mechanical fibrillation process. Mixing of the mixture can also be done using nanowhiskers and semiconducting polymers. NCs are very useful in producing stable materials with improved performance and mechanical properties. Scientists are trying to modify thermoset NCs to use Polyoles of vegetable oil-based chemicals instead of bio-based resins to stabilize and reduce dependence on petroleum-based resins. According to a recent study, nanocomposites can also be made from environmentally friendly vegetable oils.

4.4 Thermomechanical properties

Previously, thermoplastic materials were used with nanocellulose materials, showing the advantage of high crack resistance and recycling. The strength and durability of nanocomposites are greatly enhanced by the use of nanocellulose on a thermoplastic composite particulate or composite-based dispersion with the benefits of nanocellulose in resin interaction and the limited surface area of cellulose fibers. Fiber impacts on the mechanical/thermal properties of biocomposites based on carbon nanofibers (CNFs). It has been found that up to 40% of fiber content laminar increase in fiber modulus was observed using phenolic resins. With the inclusion of CNF in epoxy resins, a significant increase in the glass transition temperature was found. With 5% epoxy film of CNFs at 30°C, modulus showed an increase from 2.6 GPa to 3.1 GPa. In addition to the changing temperature of the glass, a significant increase has been reported. The mechanical properties have been drastically improved by adding up to 2% CNFs by weight while continuous addition of CNFs reduces mechanical and thermal conversion features due to agglomeration. Increased reinforcement of CNFs around bamboo fibers in the poly-lactic acid (PLA) matrix has been found to bind CNFs and improves fracture resistance that prevents fracture growth. But when, the CNFs did not gain weight of 1%, the cause of the fractured impact was increased by 200%. It has been investigated that the processing of CNFs could lead to safer, lightweight compounds with different properties, such as barriers and open spaces with multiple applications in electronics, sensors, energy storage, packaging, medicine, and automotive production. Nanocellulose films are also used to induce the barrier properties to the resulting composites. In addition, high-porosity aerogels, ease out the gas outflows and due to their hydrophobic properties, moisture absorbance has also been aided. Nanocellulose integration offers a wide range of applications that include weapon systems and flexible display devices. Highly effective NCs are possible using CNFs.

Many efforts have been made to prepare metal-reinforced material nanocomposites which have structural significance and greater toughness compared to their contemporary counterparts. But they still exhibit larger differences while analyzing their physicomechanical properties. The strengthening process includes metal oxide scattering, stiffening, preventing premature solidification, load transfer, and difference in coefficient of thermal expansion in MMNCs. MMNCs combine metal components with ceramic precursors to having enhanced mechanical properties and toughness.

4.5 Biological properties

Nanocomposites can be a part of the living organisms present in this diverse biosphere. The materials used must be structurally, biologically, physically, chemically, and mechanically compatible with the surrounding tissue. Since the mechanical properties are mainly affected by the elastic modulus, the transfer of load, durability, and higher strengths are of particular interest. Metals, polymers, and ceramic composites are approved for the synthesis of the required materials. Some examples of polymer filling compounds are given below:

• The bone fractures can be repaired by using these fillers as external fixators with the help of epoxide carbon fiber composite.

• Bone fixing screws and as replacement of bone plates in different body parts.

• The bone joints can be replaced by using these fillers, for example., carbon fibers (PEEK) are applied as total hip replacement material.

4.6 Optical properties

The impact of particle treatment on nanocomposite substances is very important. The in situ sensitization of alumina nano-sized particles dispersed in Methyl methacrylate resin and successive polymerization incorporates a better level of optical transmission at near-infrared exposure than untreated Al₂O₃. Experiments have found that the gold nanoparticles (AuNPs) when incorporated on polyethylene oxide/polyvinyl pyrrolidone (PEO/PVP) components show an increasing trend of AuNPs to combinations of visual parameters tha increases urbach strength, and optical power gap is also raised.

4.7 Magnetic properties

There are two classes of NCs compounds exhibiting magnetic properties, one containing metal NPs and the other ferrite NPs. Basically, lack of hysteresis, shows 18 increased superparamagnetic activity in ferrite NPs. Nanocomposite containing 2.8% concentration of ferrites was found to have no hysteresis at room temperature and was clearly visible. They also found that nanocomposites containing γ-Fe₂O₃ NPs in the electromagnetic polymer matrix were also free of hysteresis. Nanoparticles from nickel oxide synthesis in Polyvinyl cinnamate also show magnetic properties. They found a ferromagnetic state in nickel nanocomposites. Additionally, weight gain, magnetic response, and hysteresis values are obtained with the incorporation of nickel oxide NPs on the nanocomposite material.

5. Applications

5.1 Applications as structural materials

Thermoset polymers composites and nanocomposites are very important in today’s ultratech world. Items, such as seafood, automobiles, aircraft, are examples of nanocomposites applications. Most importantly, the improved structural modification, NCs of special strength make them compatible with metal materials incorporated in various locations. These materials are easy to process and therefore, have a broad range of applications. With the advent of nanotechnology, NCs offer numerous benefits as synthetic nanomaterials, such as stability, lightweight, and sustainability. Nanocomposites are receiving a lot of attention because of their advanced mechanical properties that have increased the reliability of these materials. Different materials are used in the production of different types of nanocomposites. The most widely used compounds are carbon nanoparticles loaded on nanoclays. Dispersion of NPs is one of the major hurdles faced by researchers recently. Nanocomposites contain 10–12% of nanoclays that are more potent and stronger than nanocalcium compounds.

The nanoscale size significantly improves physicochemical and chemical interactions. The morphology found in nanocomposites can change the phase, important for the development of various structures. Mixing and aero treatment are two important factors that improve the performance of a given NC. The variety of combinations between matrix, synthetic additives, and nanofillers allows for a wide range of materials used in fire reactions, electronic structures, optical performance, mechanical and thermal properties. Improvement of filling quality greatly improves the distribution of such nanocomposites through multiple applications. The impact of nanoparticles on the mechanical properties of polymer composites is an important factor to consider. The addition of nanofiller significantly reduced the resistance coefficient compared to pristine epoxy. Besides, 1% by weight of nanofillers showed better results than 3% by weight of nanofillers, which was unexpected. It may be due to agglomeration particles leading to poor dispersion in the epoxy matrix.

5.2 Applications of nanoclays

Nanoclays improve the mechanical and physical characteristics of the filler and matrix optical interface, which is very helpful in eliminating pressures by improving the mechanical properties of nanocomposites. Due to the impressive mechanical properties, requirements, such as low filling, stiffness, low weight, and biodegradability of nanoparticle materials, make them ideal for enviro-compliant material development. Many researchers have focused on high-quality optimization, extraction, and machine performance by filling the matrix polymer with varying degrees. There are some challenges to the formation of NCs namely, severe dispersion of organo-solvents, agglomeration, and the presence of a hydrophilic environment.

5.3 Applications of nanocrystal of naturally occurring materials

Due to the growing environmental concerns, it has indeed been emphasized that natural fibers have many advantages over synthetics because of their eco-friendly nature. However, when we use natural fibers, we cannot get more stiffness than synthetic fibers. Cellulose nanocrystals have been used as structural and geometric models of “cellulose” in various functional products. In addition, the active incorporation of electroluminescent compounds in naturally found nanocomposites results in a lower coefficient of thermal expansion. To prevent ionic diffusivity, cellulose whiskers (less than 10%) can be applied to low-density electrolyte polymers that are used in lithium-ion batteries.

5.4 Applications in coating materials

Organic and inorganic composites based on alkoxy-lanes and alko-oxides have great uses in hard-coated eye glass lenses. The addition of NPs to epoxy silanes acts as a linker between organic and inorganic moieties that greatly enhance abrasion control without affecting the transparency of the glass material. Nanocomposites have also been developed for low surface free energy coatings. To add up in the mechanical properties, nanotubular materials are extensively used that gave strength to NCs where light weight and hardness are required for the resultant NCs. The use of nonlinear optics, including optical sensor protection from high-intensity laser beams, flat panel displays, electromechanical actuators, light-emitting diodes, field-effect transistors, supercapacitors, and optical limiters are some applications of CNTs. Nanocomposites have provided significant advances over conventional NCs in tropical, mechanical, electrical, and barrier properties. Furthermore, it maintains transparency and reduces the flammability of the polymer matrix.

5.5 Applications in industries

The industrial applications include:

• Automotives: Use of nanocomposite materials in gas tanks, bumpers, interior, and exterior panels of automotive.

• Construction industry: Use of composites in building materials and sectional or structural panels.

• Aerospace: Highly efficient and heat resistant material panels of NPs are operating in the aerospace industries.

• Electrical and electronics: The uses of NCs in electrical switchboards and electronic components of various devices are increasing every day.

• Food Packaging: NCs are also used in containers and wrapping films during the food-packaging process.

• Oils and gas pipelines: Rusting on these pipelines is a major problem for steel products that reduce the life of infrastructure that is implanted. There is a need for more attention to find a solution to this problem. Often, corrosion and failure are not considered in the construction and installation of plumbing systems because one cannot measure the damage of corrosion due to unforeseen environmental events. The multiple issues of physical damage, corrosion, and natural phenomenon cause sudden failures. The use of NCs has major importance in the production and repairing of damaged pipeline structures. Nanocomposites provide excellent benefits in the production of pipes and overlay toward repairing rusty steel pipelines. The cost and performance of fiber-reinforced nanocomposite as an alternative to steel pipes is recently gaining importance. Therefore, nanocomposite structures are designed according to an engineering perspective. Each component provides a certain function and interaction that aids the structure and creates a significant difference in their performance.

  The reinforced materials add up some important advantages to the plumbing and pipeline systems are as follows:

• The nanocomposites exhibit anisotropic properties that provide strength to extraordinary collapse and burst due to pressure increase.

• These nano-derived pipes have increased load-carrying ability, high compressibility, and have greater tensile strength.

• The use of welding or joining these nano pipes to a long distance is not required.

• Very few of these materials are required to be replaced. Therefore, the replacement ratio is very less than metallic pipes.

• They are highly corrosion resistant.

• They fulfill all the standards set for gas and oil pipelines.

5.5.1 Applications in automobile industry and auto parts

Nanocomposites are well-suited for automotive parts due to their tolerance to breakage during harsh damages. Nanocomposites loaded with supports are useful to give strength to the different parts of a material where high efficacy is required. Because of the high pollution concerns, car manufacturers are working on the development of such technologies that can effectively control the problem. Therefore, nanocomposites are used to deal with this problem. Nanocomposites have improved barriers, heat resistance, impact adherence, and mechanical properties than conventional compounds. Therefore, the development of structural compounds having properties, such as biodegradability and recycling is a challenging goal. Such compounds are extensively used in automotive body parts. Industries are deeply concerned about the following factors using NCs:

• Aesthetical perspectives.

• Ability to recycle the used raw materials.

• Reduction of heavily weighted products.

• High performance with greater precision.

Nanotechnology is a driving force that has brought many changes in different industries at the level of component development, material selection, and system execution.

Nanocomposites are also used in:

• Development of sophisticated computer chips and capacitors with the help of thin films.

• It is used as a polymer electrolyte in different battery systems.

• Structural NCs are used in fuel tanks.

• Nanocomposites are used in the automotive industry.

• The blades and high-performance impellers are derived by using NCs.

• They are used in gas-oxygen barrier systems, thereby reducing the delamination/cracking of composite materials.

• Fiber-reinforced nanocomposites have excellent mechanical advantages, but the problem of delamination appears due to different mismatches.

5.6 Biomedical applications

Carbon nanotubes (CNTs) are well investigated for their importance in health-related features, especially those which are used in medicine. CNTs provide strength for metal matrix compounds, composite matrix compounds, and ceramic matrix compounds. They also improve mechanical properties, in vitro cell testing, and biocompatibility (In vivo). Composites having flexible mechanical properties could be used in tissue engineering, genetic engineering, medicine, scaffolding, implants, and as fillers of various compounds to expand their mechanical properties. The modified materials, such as plocaprolactum, are used in tissue engineering because of their robustness and adaptability in living tissues, it is the material of choice in the field of biomedical engineering. Strengthened compounds of CNTs not only serve as a bone marrow transplant, but their use in medical systems undoubtedly gave opportunities for advancement for the next generation in the fields of, biologically designed organisms, such as tissue engineering, cell therapy, drug delivery, and diagnostic equipment production. Magnetic sponges based on local, flexible, and readily available NC units are excellent for medical usage.

5.7 The application in insulative materials

The addition of nanocomposite to inorganic compounds improves their physical properties and gave the number of applications based on inorganic composites within the material. Nanocomposites are composed of carbon nanotubes as filler material, are used in the electronics industry. Their electrical insulation range and thermal properties have made them suitable to be used in an area where insulative electrical characteristics are required. The challenges of agglomerations are one of the biggest problems when adding nanomaterials to composite materials. Due to agglomerations, nanomaterials cannot be dispersed evenly, which is a major obstacle to the commercial application of nanocomposites. When agglomeration occurs during mass production then the uniform structures are more difficult to achieve. Some other applications are as follows:

• Thermoelectric devices with quite flexible properties.

• When combined with organonano they are used for dying.

• Inorganic NCs show photoelectric characters and are used in photocells.

• Polyethylene glycol NCs are used for effective drug delivery systems.

• Polyethers Ag-based nanocomposites are used in antimicrobial and biomedical remedies.

• Tissue engineering technique includes inorganic nanocomposites of calcium phosphate and poly lactic acid.

• MMNCs, such as cobalt oxide nanocomposites, are applied for humidity sensing in meteorology.

• Reduced graphene oxide NCs have applications in energy-harvesting devices.

• The doped alumina-zinc nanocomposites are used to increase the dielectric constant and also increase the conductivity of the composite.

• PMMA doped iron oxide composites are used for electromagnetic uses and also have shielding capacity.

• Cellulose loaded with copper and ZnO have shown significant UV resistance, therefore, they are used as UV-protection devices.

5.8 Agro-applications

The use of agro-biomass is a promising and continuous process of producing naturally occurring NCs, where agro-biomass is used simultaneously. In this regard, lignin-derived agro-biomass is an economical resource for the production of functional biomaterials that are compatible and sustainable. In the case of metal oxide nanocomposites (MONCs), the use of lignin-based MONCs should be extended to dynamic fields, for example, ultraviolet (UV) protection, photocatalysis, and antimicrobial agents. The development of lignocellulosic biomass as raw material should be a viable option for the development of UV protective materials from an industrial point of view.

Today different integration methods for the production of nanocomposites (CMNCs, MMNCs, and PMNCs) are available, but limitations and barriers also exist, which require the exploration of new techniques and engineered methodologies. These NCs offer improved performance in addition to their monolithic and microcomposite counterparts and are well-adapted partners to overpower the constraints of many existing materials and devices. Today the use of nanocomposites is taking place rapidly, but still, their full potential has not been attained. Making such highly stress-tolerant equipment, low-load reinforcements, gas barrier, and flame-related adherence create potential applications and marketplaces for these items. So, these nanocomposites discussed have the potential to create a new material age in the future.

6. Conclusion

Nanocomposites are versatile materials and are the current focus of research across the world. Different methods, for example, sol-gel, electrospinning, precipitation, melt mixing, solution mixing, in situ polymerization, in situ interactive polymerization, intercalation, melt intercalation, and template synthesis, are used for the synthesis of nanocomposites. The nanocomposites have unique electrical, thermal, and mechanical properties and hence are materials of choice in a variety of fields, for example, automotive industry, aerospace, semiconductor, electronics, and biomedical applications. Due to their light weight and broader mechanical strength, in the future, these materials will replace the metals and their alloys in different fields. Progress is going on for silicon-carbon nanocomposites for the manufacturing of lithium-ion batteries with greater performance output, nanotube-polymer composites for faster bone healing, polymers-based windmill materials for better performance, development of lightweight and efficient sensors, and development of magneto-fluorescent materials for better tumor imaging. Polymer nanocomposites are the material of choice for packaging companies in near future. The future challenge is to synthesize such materials that bear better material properties and price compatibility in comparison to existing materials.