Huda M. Khdier*
| Kater Alnada Faris Husham | Wafaa M. Shali | Hayder A. Al-Atabi
© 2024 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
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This study investigates the enhanced properties of polymer-based nanocomposites achieved through the integration of nanometer-sized clay platelets and chemical compatibilizers. The objective is to analyze key interfacial effects, such as improved dispersion and adhesion, and their impact on mechanical, thermal, and barrier properties. Experimental evaluations show a 30-50% increase in tensile strength, and a 40% reduction in gas permeability, and thermal conductivity by up to 50% demonstrating the effectiveness of tailored nanocomposite formulations. Applications include lightweight automotive components with improved heat distortion resistance, scratch resistance, and low flammability, as well as sustainable packaging materials with superior gas barrier properties. This review concludes that nanocomposites offer transformative potential in molecular engineering, enabling advancements in both industrial and environmental contexts.
interfacial engineering, surface modification, mechanical properties, thermal conductivity
1.1 Background on nanocomposites
Nanocomposites represent a class of materials that are generated by incorporating two or more components, at least one in the nanoscale range. These materials have generated much interest owing to their properties of strength, flexibility, and conductivity, which improve appreciably with the addition of nanosized fillers like carbon nanotubes or nanoclays. Nanocomposites have become critical in modern applications such as aerospace, automotive, and electronics, addressing demands for lightweight, durable, and multifunctional materials.
1.2 Problem statement
One of the biggest challenges in making effective nanocomposites is controlling the interface between the nanosized filler and the main material, known as the matrix. The interfacial region plays a crucial role in how the composite behaves, particularly in transferring stress, conducting electricity, or managing heat. Poor control of this interface can lead to weak spots in the material, limiting the overall performance of the nanocomposite. Addressing these challenges is essential for realizing the full potential of nanocomposites.
1.3 Objectives of the study
This paper discusses how the interface of materials in nanocomposites influences their physical properties specifically, aims to
1.4 Scope of the research
This research focuses on the classification of nanocomposites, emphasizing the analysis of interfacial effects on mechanical, thermal, and electrical properties. The study will explore various fabrication techniques, such as melt mixing and solution processing, and their impact on the quality of interfacial bonding. Furthermore, the research includes a review of testing methodologies, such as tensile strength analysis and thermal conductivity measurement, to better understand the behavior of nanocomposites under different conditions.
2.1 History and evolution of nanocomposites
The development of nanocomposites is a relatively recent achievement in the field of material science, but the concept of combining materials to improve properties has ancient origins. Early civilizations, such as the Egyptians and Mesopotamians, used straw to reinforce mud bricks for construction, creating the earliest examples of composites. However, the modern field of nanocomposites began to emerge in the late 20th century with the rise of nanotechnology.
It was in the 1970s and 1980s when the properties of their traditional composite counterparts improved with the incorporation of nanoscale particles. One of the first few reinforcements used for enhancement in the mechanical properties of the developed material, namely strength and rigidity while staying light, were clay nanocomposites used within polymer matrices. Things took a turn for the better with the invention of carbon nanotubes in 1991 by Sumio Iijima as they empowered truly unique strengths in mechanical properties and electrical conductivity to the developed material resulting from CNTs. This sparked an incredible amount of research on how to integrate the nano-sized filler within the polymer matrix, as the nanocomposite material properties emerged.
Nanocomposite materials are, arguably, the forefront of the materials science field, and research efforts have focused on how to control and optimize the interface of the nanoscale filler and the matrix. The interface region is important as it allows the determination of properties of stress, heat transfer coefficients, and electrical properties. The developments in this regard have greatly expanded the applications of durable functional and improved-performance nanocomposite materials across multiple industries including automotive, aeronautical, electronics, and medicine [1].
2.2 Types of matrix and filler materials
Tables 1 and 2 summarize the characteristics and applications of different matrix materials and fillers commonly used in nanocomposites:
Table 1. Key characteristics and applications of matrices
|
Matrix Type |
Examples |
Key Characteristics |
Applications |
|
Carbon Nanotubes CNTs |
multi-walled CNTs (MWCNTs) |
They have the toughest tensile strengths, reaching over steel, and also the best electrical conductivity. |
electronic applications |
|
Graphene |
carbon oxide |
Exceptional mechanical strength, high thermal conductivity, and superior electrical properties. |
electronics, energy storage devices, and sensors |
|
Nanoclays |
laminated silicate material |
Barrier effect on properties such as gas permeability and fire resistance. |
automobiles, and at construction sites |
|
Metal Nanoparticles |
silver, gold, or copper |
high electrical, magnetic, and antimicrobial |
medical coatings and textiles |
Table 2. Key characteristics and applications of fillers [2]
|
Matrix type |
Examples |
Key Characteristics |
Applications |
|
Polymers [1, 3] |
polyethylene, polypropylene, and epoxy resins |
lightweight, flexible, good strength, and good resistance to chemical exposure |
industries and aerospace |
|
Metals [4] |
aluminum, magnesium, and titanium |
strength, thermal stability, and electrical conductivity |
high-performance applications, and aerospace |
|
Ceramics [4] |
silicon carbide, alumina |
high temperature stability and high resistance to wear and corrosion |
extreme operational environments such as turbines, heat exchangers, and tools |
Filler Materials: Fillers are the nanoscale materials that are added to the matrix to enhance specific properties. They are chosen based on the desired improvement in mechanical, thermal, or electrical performance [5].
2.3 Current applications of nanocomposites
Automotive Industry: Nanocomposites have revolutionized the automotive industry by providing lightweight materials that maintain high strength and durability. Polymer nanocomposites are used to produce body panels, engine components, and interior parts. By reducing the weight of these parts, nanocomposites help improve fuel efficiency and reduce emissions.
Nanocomposites also offer improved resistance to wear and corrosion, extending the lifespan of automotive components [6].
Aerospace Industry: In the aerospace industry, weight reduction is critical for improving fuel efficiency and increasing payload capacity. Nanocomposites allow for the production of lightweight yet strong materials used in the construction of aircraft frames, fuselages, and other structural components. The use of carbon nanotubes and graphene in aerospace nanocomposites provides high strength, excellent thermal conductivity, and resistance to fatigue, making these materials ideal for harsh operating conditions [7].
Electronics: The characteristics of nanomaterials can be utilized to improve electrical conductivity, thermal management, and flexibility of various electronic devices. CNT, graphene, and some of their nanocomposites are used specifically in applications such as sensors, transistors, and flexible displays. Nanocomposites improve the performance of batteries and supercapacitors specifically due to improving energy storage capacity and conductivity.
Medical Field: Nanocomposites have made significant advancements in the medical field, particularly in drug delivery systems and implants. Biocompatible nanocomposites can be engineered to release drugs at controlled rates, improving the effectiveness of treatments. Nanocomposite materials are also used in prosthetics and implants, providing strength and flexibility while being safe for human tissue [8].
Construction: Nanocomposites can be utilized to develop materials that are stronger, more resilient, and resistant to environmental forces in the construction industry. Once instance is that of nanocomposites with nano clay fillers for building materials to improve fire resistance and lower moisture permeability. Additionally, these coatings and paints continue to be utilized in a variety of coatings and paints to provide protection against corrosion and other forms of wear.
Further, it is known that interfacial effects are crucial in nanocomposites due to the fact that these interfacial properties whether one likes it or not influence the mechanics of the matrix with the nanoscale fillers through the transfer of mechanical load, heat, and electric charge. The interface can be termed as that area around the bond between the matrix and the filler material; the behaviour at this interface, however, is what sets nanocomposites apart from the use of old composites. In fact, all these effects are very important for the final performances of nanocomposites; therefore, all these effects have also to be controlled and tailored for better use in the assortment of offered applications. This section deals with the role of interfaces and interphase in engineering nanocomposites, their viscoelastic response under stress, and issues of nanocomposite design [9].
3.1 Role of interfacial bonding in nanocomposites
Within this study, interfacial bonding denotes the adhesion between the dispersed filler and the matrix material. The existence of strong interfacial bonds is significant because strong interfacial bonding leads to the meaningful development of mechanical properties in the nanocomposite system. During an applied load, the matrix needs to transfer stress to the filler, which typically possesses superior strength. If the interface is weak, subsequent matrix and filler separation results in poor mechanical performance.
Conversely, the effective transfer of load caused by strong interfacial bonding leads to significant improvements in properties such as tensile strength, modulus, and fracture toughness.
Moreover, since nanoscale fillers (e.g., CNTs, graphene, and nanofibers) from nanocomposites possess a large surface area to volume ratio, the importance of interfacial effects is amplified. Sustaining an effective interfacial bond is critical since the smaller the filler size, the relatively larger the contribution of the interface to the overall volume of the composite, superseding the properties of the bulk characteristics.
A number of different types of bonding are possible at the interface:
This is also the case for CNT-based polymer nanocomposites, which typically would need some sort of surface modification to achieve sufficient enhancement in bonding. Chemical functional groups on top of the functionalization of the surface itself, such as carboxyl (-COOH) or hydroxyl (-OH), may become more easily associated with the polymer matrix with improved interfacial bonding. The goal is to develop stronger associations between CNTs and polymer chains to further enable efficiency in the transferring of stress to improve mechanical properties.
Additional modifications of the surface, such as through plasma treatments or via the use of coupling agents, have also been made to the nanofiller surface characteristics to improve the compatibility with the matrix, thus providing vested improvements to the interfacial bonding.
3.2 Mechanisms of stress transfer across interfaces
The performance of nanocomposites, especially in terms of mechanical properties, relies heavily on the efficiency of stress transfer from the matrix to the filler. The filler, typically much stronger and stiffer than the matrix, reinforces the material by carrying a large portion of the load when stress is applied. However, for this reinforcement to be effective, the stress must be efficiently transferred across the interface [11].
Several mechanisms facilitate stress transfer across the interface:
3.3 Challenges in interfacial engineering
Despite the advantages of nanocomposites, engineering the interface between the matrix and filler poses significant challenges. Optimizing the interface is a delicate balance between enhancing adhesion and maintaining the intrinsic properties of the fillers and the matrix. Some of the key challenges include:
Addressing these challenges requires ongoing research into novel surface modification techniques, improved fabrication methods, and more efficient methods of controlling filler dispersion. Advances in nanocomposite manufacturing are critical to unlocking their full potential in various industries, including aerospace, automotive, electronics, and biomedical engineering.
Nanocomposites are materials based on a polymer, metal, or ceramic matrix with fillers with dimensions on the nanometer scale, which have exhibited improvements in mechanical, thermal, electrical, or barrier characteristics compared to traditional composites. Fillers can be present in the 1 to 100 nanometer dimensions and can include nanoparticles, nanotubes, nanofibers, or nanoclays. The small scale and large surface area of nanofillers significantly alter the interface, i.e., the matrix: filler interface, which is a critical aspect of composite properties than dimension fillers of larger scales. This chapter addresses the classification of nanocomposites by composition, manufacturing processes, and types of resultant morphology and interfaces.
4.1 Classification of nanocomposites by composition
Nanocomposites can be classified based on the composition of the matrix and filler materials. The matrix typically provides the bulk of the material, while the nanofiller enhances specific properties [14].
Nanocomposites with a Polymer Matrix: The polymer matrix-mentioned nanocomposites are the most common form of nanocomposite, in that the matrix is made of a polymer while the fillers are made of nanoparticles in the forms of silica, nanoclays, carbon nanotubes (CNT) or graphene. Polymer nanocomposites have numerous applications in industries like automotive, aerospace, and electronics due to being lightweight, flexible, and for more enhanced properties like mechanical strength and electrical conductivity. Typical applications use carbon nanotubes to improve electrical conductivity and mechanical properties in CNT-reinforced polymer nanocomposites serving electronics and structural material purposes.
Metal Matrix Nanocomposites (MMNCs): In metal matrix nanocomposites (MMNCs), reinforcement of nanofillers such as carbon nanotubes, silicon carbide (SiC), or alumina (Al2O3) are incorporated into a matrix metal of aluminum, magnesium or titanium. MMNC applications can be found specifically with high strength-to-weight applications, along with wear resistance and high temperature sustainability. For instance, nanocomposite aluminum with reinforcement using SiC demonstrated excellent thermal conduction and improvement in mechanical properties for heat exchanger and engine applications.
Ceramic Matrix Nanocomposites: A ceramic matrix nanocomposite consists of a ceramic based matrix, based on alumina or zirconia, assembled with nanoparticles, or carbon nanotubes, as reinforcement. Ceramic properties increase thermal stability, corrosion resistance, and mechanical properties. Due to ceramic body properties, there has been much interest in ceramic matrix nanocomposites, used in high temperature sectors. A few examples are turbine blades cutting tools to biomedical implants. An example is CNT-reinforced zirconia nanocomposites used in dental implants because of their toughness and biocompatibility.
Hybrid Nanocomposites: Hybrid nanocomposites are made up of more than one type of nanofiller, and/or a combination of matrices. Hybrid uses the more than one component synergies to founder performance properties. An example of hybrid polymer composites.
4.2 Classification by manufacturing techniques
The manufacturing process of nanocomposites plays a crucial role in determining the dispersion of nanofillers and the properties of the final composite. Nanocomposites can be classified based on the techniques used for their fabrication:
The mixing of solution is a method by which the nanofiller and matrix are dissolved in a solvent and then evaporated to yield the nanocomposite. This method is very given for polymer nanocomposites particularly if Layered silicates: that is, nanoclays are being employed as nanofiller. The advantages of this method are; good filler dispersion and the disadvantages of this strategy is the time that it will take to get rid of the solvent and the possible hazardous nature to the environment.
Melt Mixing: This is the method by which the nanofiller is mixed together with the polymer matrix melted state. In melt mixing the primary thermoplastic polymers are focused on wherein a nanofiller is mixed with the polymer during the extrusion or injection mold process. This one is preferred in an industry setting due to easier case of upscaling, as uniform filler dispersion is difficult to obtain especially if the filler has a higher aspect ratio such as CNTs.
In-Situ Polymerization: The nanofiller is dispersed in a monomer solution which is then polymerized. This method produces conditions that are good for the interfacial bonding between the filler and polymer matrix as the polymer chains will grow around the dispersed nanofillers. More of a general perspective, in-situ polymerization has been applied to polymer/clay nanocomposites and polymer/CNT nanocomposites.
Powder Metallurgy: Powder metallurgy is simply to create a metal matrix nanocomposite. The metal powders and the nanofillers are mixed, compacted at high pressure, and thereafter sintered. Powder metallurgy will control exquisitable the composition and structure of a composite, although it is very expensive and time consuming.
Sol-Gel Process: This is one of the most used methods to prepare ceramic. The matrix material has to be synthesized first as a sol: that is; a colloidal suspension, and then the fillers are added to the sol as the final step before it is [15].
4.3 Composite morphology and interface types
Nanocomposites Morphology: Types of formed interfaces definitively play a role in determining performance. Morphology is dictated to a large extent through the distribution, alignment, and dispersion of the nanofillers, and the nature of the interface (e.g. filler-matrix) is, of course, also key.
Particle-Based Nanocomposites: Describe materials using particle-like nanofiller (i.e. nanoparticles or nanoclays) homogenously dispersed in the matrix. Particle-based nanocomposites have the interface being the relationship between filler-matrix through covalent bonds or van der Waals forces.
Agglomeration of Nanoparticles is a considerable concern here, with agglomeration leading to poor dispersion, and subsequently, poor performance.
Fiber-Based Nanocomposites: For fiber-reinforced nanocomposite, fillering of nanofibers or nanotubes is used. Carbon nanotube (CNT) reinforced nanocomposites serve as a prime example of this materials class. The interface becomes the important issue in fiber-based nanocomposites due to stress transfer between the matrix and the nanofibers, and nanofiber alignment can also play an important role in affecting the mechanical and electrical properties of the composite. Surface functionalization is usually used to enhance bonding in nanofiber interfaces.
Layered Nanocomposites: This category refers to materials which are made by dispersing layered nanofiller (e.g. nanoclays) into the matrix. Layered fillers contain different morphologies, intercalated morphologies (matrix chains inserted in-between the filler layers), or exfoliated morphologies (individual filler layers separated from the rest of the stacked unit and/or diffusely homogenously dispersed throughout the matrix). Exfoliated morphology is conceptually preferred (vs. intercalated morphology) in order to enhance mechanical, thermal and barrier properties as a result of high filler-matrix area of interaction.
Core-Shell Nanocomposites: In core-shell nanocomposite, a nanofiller is also enveloped by a shell with a different composition or combinations of compositions [16].
The involvement of the interfacial region between the nanomaterial and the matrix is critical in dictating the performance of the nanocomposite system. Strong interfacial bondens allow for efficient stress transfer from the matrix to the reinforcement phase, thereby improving mechanical properties such as tensile strength, strain, and toughness, while weak interfacial bondens can create failure mode at lower than anticipated external stresses, compromising the potential opportunities for the nanocomposite to be used and effectively tested in real-world applications.
5.1 Mechanical properties (Stress, strain, and toughness)
Mechanical properties such as stress, strain, and toughness are directly influenced by the interfacial characteristics between the nanofillers and the matrix. Proper dispersion of nanomaterials and robust interfacial bonding can significantly improve these properties by optimizing load transfer and minimizing the formation of voids and cracks at the interface [17].
5.1.1 Influence on tensile strength
Tensile strength is defined as the maximum stress that material can withstand while being stretched, before failing. The interfacial strength between the matrix and the nanomaterial has a profound impact on tensile strength of nanocomposites. A strong interface promotes efficient load transfer of tensile stress and allows the composite to take a higher load 50% more without breaking. If appropriately dispersed and later well-bonded to the matrix, nanomaterials, with a structure such as CNTs, graphene, silica nanoparticles, etc., can also have a significant improvement in tensile strength due to their high-aspect-ratio and high surface area structures [18].
Fatigue resistance is the property of a material to withstand repeated loading without fracturing the material in a given lifetime. When evaluating improvements in fatigue resistance, properties associated with the interface are the critical factor since it increases service life under dynamic loading by 20-30%. Fatigue failure normally initiates at weak locations in a material, such as interfaces that have poor bonding, where cracks can initiate and propagate.
The well-bonded interface of the nanocomposites significantly reduces the stress concentration around any defect, in addition, fatigue resistance may be realized on performances by nanofillers, such as CNT or graphene, bridging of microcracks that may be initiated under cyclic loading.
Interfacial interactions, relative to chemical modifications and surface treatments, were noted to improvement of nanocomposite cyclic fatigue life considerably. A strong interface inhibits crack growth that derives from cyclic stresses with a consideration to overall service life of a material under dynamic loading.
5.2 Thermal properties
Thermal properties in nanocomposites are heavily influenced by the interfacial interaction between the matrix and nanofillers. These interactions determine the composite's ability to resist heat, conduct thermal energy, or provide insulation. Nanomaterials can greatly enhance or inhibit thermal behavior, depending on how they are incorporated into the matrix. For example, Graphene and boron nitride nanofillers improve thermal conductivity by up to 50%
Heat Resistance in Various Matrices: Refers to materials that retain their mechanical properties at elevated temperatures, with nanofillers like carbon nanotubes and graphene improving thermal stability. This is due to reduced thermal expansion and enhanced matrix reinforcement, making them ideal for applications in the aerospace, automotive, and electronics industries [19].
Thermal Conductivity and Insulation: In nanocomposites, thermal conductivity is determined by the kind of nanofillers added and their distribution within the matrix. The addition of nanofillers with high thermal conductivity, such as graphene and boron nitride, can significantly enhance the heat-conduction capability in composites by up to 50%. However, these nanofillers can only act fully provided that there is good interfacial bonding, which will guarantee efficiency in thermal transport across the interface.
Contrarily, some nanocomposites are designed for thermal insulation; by using low thermal conductivity filler. While poor interfacial bonding cannot help in reducing heat flow through the material and results in better insulation properties, by tuning the interfacial interaction, manufacturers can optimize the thermal behavior for applications that require either high thermal conductivity or insulation.
5.3 Electrical properties
The electrical properties of nanocomposites are significantly affected by the different filler types and quality of interaction between filler and matrix. Conductive fillers, for instance, carbon nanotubes or metal nanoparticles, enhance the electrical conductivity in nanocomposites, while insulating fillers reduce conductivity depending on the application.
5.3.1 Conductivity
In general, electric conductivity in nanocomposites is a result of electron transport through the nanofillers. The conductivity derives from the interfacial connection of the fillers to the matrix. This is possibly from a percolation effect when a continuous network of conductive fillers is embedded within an insulating matrix of polymer and allows electron flow through the nanocomposite.
While strong interfacial bonding provides a means to improve electron transfer between nanofillers and matrix entities and can enhance conductivity, weak or incompatible interfaces may act as an electron barrier and decrease overall conductivity in nanocomposites. This surface functionalization typically improves ligamenting and interfacial bonding, enabling improved electron mobility, and increasing conductivity [20].
5.3.2 Effect of filler-matrix interaction on conductivity
The other big variable that changes not only the general conductivity but also the frequency and stability of electron transfer is the interaction between fillers and matrix. The stronger filler-matrix interaction tends to stabilize conduction pathways, while poor bonding decreases electrical performance because it causes discontinuities inside the conducting pathways. So far, numerous filler surface treatments and functionalizations have been applied in polymeric nanocomposites aiming at the improvement of these interactions and development of more effective electron transport networks. Besides, the degree of dispersion of fillers and its distribution in a matrix is one determining step toward obtaining a potential level of conductivity. To that effect, well-dispersed nanofillers with good interfacial contact normally result in higher conductivity than agglomerated fillers, which create isolated conductive islands [20].
5.4 Barrier properties
In nanocomposites, barrier properties mean the prevention of gases, liquids, or chemicals to permeate through the materials. These properties are one of the very basic features of packaging, protection, and thermal insulation applications. The performance of such barriers will be decided by the interaction between matrix-nanofillers.
5.4.1 Gas and liquid permeability
A composite, upon the addition of nanofillers, exhibits improved barrier properties against gases and liquids due to a tortuous path created and a reduction in the molecular diffusion rate through the materials. The important thing in this aspect is the interfacial interaction between the filler and the matrix; strong bonding minimizes voids or defects that could allow permeation.
Nanocomposites with nanofillers such as clay or graphene that are well dispersed and strongly bonded can potentially reduce the permeability of gases and liquids by 40–50%. This is because nanofillers act as physical barriers to force the molecules to extend their journey more tortuous to pass through the composite material. Improved interfacial bonding serves to ensure that the nanofillers fully reinforce the matrix; hence, this improves the barrier performance [10].
5.4.2 Effects on corrosion resistance
Another reason for improved barrier properties, including corrosion resistance, is good interfacial bonding. In the case of metal-based nanocomposites, the nanofiller probably acts as protective layers that shield the matrix from contact with corrosive media up to 30%. A strong enough interface from the nanofiller toward the metal matrix ensures the protective barrier remains intact under mechanical or chemical attack [21].
5.5 Advantages, disadvantages, and limitations of interfacial engineering
Advantages [22]:
Disadvantages [22]:
Limitations [22]:
A proper understanding of the matrix material and nanofillers is crucial to achieving the desired properties in nanocomposites. Then, within this section, we will present an overview of the materials and methods employed for the preparation of nanocomposites. The subsequent subsections represent a critical review of matrix materials and nanofillers used in this work.
6.1 Detailed review of materials used
In principle, a nanocomposite is made up of two major components: the matrix, which is normally the continuous phase, and the nanofillers, which serve as the reinforcement phase. The interaction of these two components will, to a large degree, determine the characteristics of the composite material. Which kind of matrix-polymer, metal, or ceramics- and which kind of nanofiller-carbon nanotube, nanoclays, etc.-should be chosen depends on the requirements for application.
6.1.1 Matrix materials (Polymer, metal, ceramics)
Polymer Matrix: The most used nanocomposites are of the polymer matrix type, due to versatility, light weight, and economics. Normally, the polymer matrix usually provides the bulk properties of the composite, while the nanofillers provide improvements in selected properties such as strength, thermal stability, or electrical conductivity. Commonly used polymers in nanocomposites include epoxy, polyethylene, polypropylene, and polystyrene. The role of the very important polymer/nanofiller interface will be discussed later.
Among the effective ways to improve interfacial bonding in polymer matrices, there is surface functionalization of nanofillers, providing better compatibility and effective load transfer between polymer and nanomaterial. Several works have demonstrated that reinforcement with carbon nanotubes or graphene significantly improved the tensile strength, thermal stability, and electrical conductivity with respect to conventional composite matrices.
Applications of the metal matrix nanocomposites have been used where high mechanical strength, electrical conduction, and resistance at high temperature are required. Aluminum, magnesium, and titanium are the commonly used metal matrices due to their low density and desirable mechanical properties. Addition of nanofillers based on ceramic particles or carbon-based materials can significantly enhance strength, stiffness, and thermal properties of the metal matrix.
Such composites' performance principally depends on the quality of the interfacial bonding between the metal matrix and the nanofillers. Poor interfacial bonding may lead to either early-stage failure or low mechanical strength [22].
Ceramic: Ceramic matrix nanocomposites have very good high-temperature resistance, wear resistance, and thermal stability; therefore, they can be applied under extreme conditions, including in aerospace and defense industries. A critical point is that ceramic matrices are usually brittle, while the addition of nanofillers aims at their toughening and strength enhancement. Usual ceramic matrices are SiC and Al2O3; very often, the reinforcement is provided by the addition of nanoscale ceramic particles or carbon nanotubes to improve mechanical properties, resistance against cracking. The main goal of ceramic nanocomposites is to ensure an effective interface between the matrix and the filler, because poor interfacial bonding can initiate cracks under stress.
6.1.2 Nanofiller types (Carbon Nanotubes, Nanoclays, etc.)
Carbon Nanotubes (CNTs): Considering different nanofillers, the carbon nanotubes became one of the most fashionable ones due to their outstanding mechanical, thermal, and electrical properties.
Based on structure, carbon nanotubes could be SWCNTs and MWCNTs. They have high tensile strength with low density and excellent thermal conductivity; therefore, they can serve as reinforcement phases in both polymeric and metallic matrices. CNTs improve the electrical conductivity of the host material due to its excellent charge transport properties.
However, due to the high surface energy and tendencies of agglomeration, only one of the maCNTs disperses properly in the matrix material. Functionalization on the surface and such modification techniques is done for increasing the interaction between CNTs-matrix, hence allowing better stress transfer and improvement in the properties [23].
Nanoclays: Nanoclays are a type of layered silicate material that finds extensive application as nanofillers in polymer nanocomposites. These nanomaterials have exhibited enhanced mechanical, thermal, and barrier properties of the composite. Specifically, nanoclays have proved to enhance the properties of gas and moisture barriers; hence, they have found several applications in packaging and automotive uses. When dispersed in a polymer matrix, nanoclays create a tortuous path for gas molecules, therefore reducing permeability.
The varieties of nanoclays that have been in use include iontmorillonite, bentonite, and kaolinite. All such nanoclays provide some advantages in regard to cost-performance. Exfoliation and intercalation of these nanoclays into the matrix are crucial to achieving the desired properties for composite improvement [18].
Other Nanofillers Graphene, Metal Oxides, etc. Graphene is a carbon-based nanofiller that has recently gained considerable interest owing to its exceptionally high mechanical strength, electrical conductivity, and large surface area. Graphene, therefore, when incorporated into nanocomposites, enhances the material not only mechanically but also thermally and electrically. Like CNTs, graphene needs proper dispersion in the matrix in order to derive optimum benefit. In this regard, metal oxide nanoparticles, inclusive but not limited to titanium dioxide (TiO₂) xide, have been widely applied as nanofillers in reinforcement to UV resistance, photocatalytic activity, and mechanical strength in polymer nanocomposites. Normally, these nanomaterials are used in coatings and sunscreens among other applications due to their UV protection and durability [8].
6.2 Fabrication techniques
The fabrication techniques employed to produce nanocomposites will be determinant in what the final properties of nanocomposites are. All the techniques will be chosen in terms of the matrix material characteristics and nanofiller, as well as the properties desired in the final composite. The following section provides an overview of three common techniques: solution blending, in situ polymerization, and melt intercalation.
6.2.1 Solution blending
In the broader sense, solution blending is one of the most straightforward and widely used techniques for nanocomposite fabrication, especially when considering polymer-based systems. In such a technique, the polymer matrix and nanofiller are dissolved or dispersed in a common solvent. In such a case, the nanocomposite is formed once the complete evaporation of the solvent in a homogeneous solution has taken place [24].
Process: Firstly, the polymer must be dissolved in an appropriate solvent; for instance, chloroform or toluene. Secondly, the nanofillers are incorporated into the solution of the polymer by methods like sonication and/or high-shear mixing, which may involve carbon nanotubes or nanoclays. Lastly, it is cast onto a substrate or mold, after which the solvent evaporates, usually in an oven or under a vacuum to remove residual solvent.
The nanocomposite film or material is recovered after evaporation of the solvent.
Advantages: The method provides relatively easy dispersion of nanofillers within the matrix. Solution blending can realize good homogeneity and is applicable for the low-temperature processing of sensitive nanomaterials. The technique is rather useful for those polymers that cannot be easily melt-processed [18].
Challenges: The selection of an appropriate solvent is of prime importance, as it has to dissolve the polymer and the nanofillers efficiently without degradation. Sometimes, residual solvent is left in the final product, which can alter some of its properties.
Applications: This technique finds applications in thin films, coatings, and membranes where highly controlled composite structures are required.
6.2.2 In situ polymerization
Another popular method involves in-situ polymerization, which widely occurs in the preparation of nanocomposites. In this technique, nanofillers are usually dispersed in monomers that polymerize to form a continuous polymer matrix in which the nanomaterials are embedded.
Process: Above all, the nanofillers must be mixed with some monomer precursors such as graphene or carbon nanotubes. The addition of a thermal or UV catalyst starts the reaction of polymerization; hence, it initiates the polymerization. In the course of polymerization, nanofillers become included in the monomers' growing chains, thus evenly dispersed in the final material.
Advantages: This methodology gives strong interfacial bonding between the matrix and nanofillers because, during polymerization, the polymer is formed around the nanofiller. It has excellent nanofiller dispersion and compatibility with the matrix polymer.
In situ polymerization technique may in turn give better morphology control of the nanocomposites and such a technique applies to most kinds of polymers.
Challenges: This has to be done by judicious control of the conditions of polymerization-temperature and pressure and use of solvent or catalysts to prevent agglomeration of nanofillers. This could perturb the polymerization process of some nanofillers and alter the properties of the final composite.
Applications: In situ polymerization has seen wide applications in preparing structural materials, adhesives, and coatings with functional interfacial interactions between the matrix and nanofillers.
6.2.3 Melt intercalation
The melt intercalation is one of the most conventional fabrication techniques for polymer nanocomposites without involving any kind of solvent. It is especially effective for those polymers which can be processed at high temperatures.
Process: Melt intercalation has generally been performed at high temperatures using an extruder or mixer where the molten state is a general state in which the mixing of the polymer and nanofillers takes place. Nanofillers are usually either mechanically mixed to achieve intercalation or dispersion in a polymer matrix. The fillers used are usually layered silicates, most of which are nanoclays. The melting and flowing process allows the polymer chains to penetrate the layers of the nanofillers, thus forming a nanocomposite. The material is allowed to cool following mixing, hence solidifying the nanocomposite.
Advantages: Because it does not involve the use of a solvent, melt intercalation is an environmentally friendly and economical process. The process is compatible with existing polymer processing machinery such as extruders and injection molders and hence is easy to scale. Suitable technique for high-temperature polymers and allows very good mechanical reinforcement due to good filler dispersion
Challenges: The high processing temperature may degrade temperature-sensitive nanofillers or polymers. Sometimes, the nanofillers require uniform dispersion, which is a very cumbersome task to conduct, particularly in the case of fillers which have a tendency to be in an agglomerated state.
Applications: This is one of the widely used methods in the automotive, packaging, and electronic industries where high-performance material is required.
6.3 Characterization methods
6.3.1 Microscopy
Transmission Electron Microscopy (TEM):
Principle: It is based upon the principle of the transmission of the electron beam through an ultra-thin specimen. The electrons are transmitted as a result of their interaction with the sample, which produces high-resolution images [20].
Other applications involve internal structure nanocomposite studies. Transmission Electron Microscopy is powerful and efficient in visualizing the size, shape, and distribution of nanofillers at atomic levels. It allows for observation of the crystallinity of materials and the nature of interfaces.
Its advantages are that it provides high spatial resolution down to a few nanometers, thus enabling a closer look at nano-structural details, which are of high relevance for the understanding of how interfacial properties do control overall material behavior.
Scanning Electron Microscopy (SEM):
Principle: The method of scanning electron microscopy depends upon focused beams of electrons for scanning the surface of the sample. The details in the images develop from secondary electrons emitted from the surface.
Other examples of applications include: SEM in morphology and topography studies on nanocomposites, taking into account several factors such as the distribution of filler, agglomeration, or surface fractures; it can also be applied to elemental analysis via EDX coupled with SEM.
The advantages are that it yields three-dimensional-like images of the sample's surface and analyses larger areas than TEM, which is useful for assessing bulk properties.
Atomic Force Microscopy (AFM):
Principle: Scanning of a sharp tip attached to a cantilever over the sample surface, and measurement of the forces between the tip and the surface, creates topographical maps.
Applications: AFM represents one of the key techniques for the investigation of nanoscale surface roughness, mechanical properties, and morphology. AFM is employed to provide information on how surface characteristics influence adhesion and the mechanical behavior of the polymer nanocomposites.
Advantages: It provides high-resolution imaging up to a few nanometres. Moreover, it operates either in air, liquid, or vacuum conditions; hence, combining advantages for many types of nanocomposite studies.
6.3.2 Spectroscopy
Fourier Transform Infrared Spectroscopy (FTIR):
Principle: In FTIR spectroscopy, infrared radiation passes through the sample, where absorption is measured against wavelength; this technique yields information on molecular vibrations.
Applications: Identification of functional groups in nanocomposites using FTIR is important; it would allow researchers to establish how the matrix interacts with the fillers. It can detect chemical changes due to nanofiller surface treatment.
Advantages: Nondestructive and small sample sizes can be analyzed by FTIR. In addition, FTIR can be used to monitor the change in chemical composition due to processing or aging.
X-ray Photoelectron Spectroscopy (XPS):
Principle: XPS is a surface chemical analysis technique wherein a material is irradiated with X-rays and the kinetic energy of emitted photoelectrons is measured.
Applications: XPS is an indispensable technique in determining the elemental composition, chemical state, and bonding environment of elements at the surface of nanocomposites. It can be used to establish changes in the surface chemistry following modifications or treatments.
Advantages include quantitative information about surface composition as deep as 10 nm, making it an excellent tool to understand how surface interactions may affect overall material properties, as shown in Table 3.
Table 3. Characterization method as technique, advantages, disadvantages
|
Technique |
Advantages |
Disadvantages |
|
Melt Mixing Simple |
Scalable, suitable for thermoplastics |
Filler aggregation, potential degradation |
|
Solution Mixing |
Better filler dispersion, suitable for various polymers |
Solvent recovery, limited scalability |
|
In Situ Polymerization |
Strong interfacial bonding, uniform dispersion |
Complex, time-consuming |
|
TEM |
High-resolution imaging of interfaces |
Expensive, requires ultra-thin samples |
|
SEM |
Surface analysis, larger area coverage |
Lower resolution than TEM |
|
AFM |
High-resolution surface roughness and mechanical data |
Limited to surface studies |
|
FTIR |
Non-destructive, functional group identification |
Limited depth information |
|
XPS |
Surface chemical analysis, quantitative data |
Expensive, shallow penetration depth (~10 nm) |
6.4 Testing procedures
6.4.1 Mechanical testing
Tensile testing:
Objective: To assess how materials respond to uniaxial tension, yielding key properties such as tensile strength, elongation, and Young's modulus.
Method: A standardized specimen (typically dog-bone shaped) is subjected to a controlled tensile load until failure. The force and elongation are recorded to generate a stress-strain curve [20].
Applications: Essential for determining the suitability of nanocomposites in load-bearing applications, such as in construction or automotive components.
Flexural testing:
Objective: To evaluate the material’s resistance to bending, providing insights into its flexural strength and stiffness.
Method: A beam specimen is subjected to a bending load at its midpoint until it fails. The load and deflection are measured to calculate flexural strength and modulus.
Applications: Crucial for materials used in beams, panels, or other structures where bending loads are prevalent.
6.4.2 Thermal testing
Thermogravimetric Analysis (TGA):
Objective: To measure changes in a material's mass as a function of temperature, providing insights into thermal stability and decomposition behavior.
Method: A sample is heated in a controlled atmosphere, and the weight loss is monitored continuously as the temperature increases. This data helps identify thermal events such as decomposition or oxidation.
Applications: Important for assessing the thermal stability of nanocomposites in high-temperature applications, such as aerospace or automotive components.
Differential Scanning Calorimetry (DSC):
Objective: To analyze thermal transitions in materials, such as melting, crystallization, and glass transition temperatures.
Method: The sample and a reference material are heated or cooled at a constant rate. The heat flow into the sample is measured, revealing endothermic (melting) and exothermic (crystallization) transitions.
Applications: Vital for understanding the thermal behavior of nanocomposites in various applications, helping to optimize processing conditions and predict performance under thermal stress [21].
6.4.3 Electrical testing
Objective: To evaluate the electrical properties of nanocomposites, such as conductivity and dielectric characteristics, which are essential for electronic applications.
Method: Various techniques can be employed, including:
Applications: Critical for developing conductive nanocomposites for applications in sensors, conductive films, and electronic devices.
The interfacial region between the matrix and the nanofillers becomes very critical in defining the properties of the resulting nanocomposites. Strong interfacial bonding has been shown to result in enhanced load transfer, reflected as an improvement in facilitating efficient load transfer, thermal conduction, and electron mobility, significantly enhancing the mechanical, thermal, and electrical properties. This section compares the performance of nanocomposites with and without interfacial modification, supported by specific case studies, and discusses the feasibility of interfacial engineering for industrial applications [20].
Case Study 1: Carbon Nanotube (CNT)-Polymer Composites
In CNT-reinforced polymers, surface functionalization of CNTs with carboxyl or hydroxyl groups improves adhesion to the polymer matrix.
Treated Systems: Functionalized CNT-polymer composites exhibit a 50% improvement in tensile strength and a 20% increase in fatigue life compared to untreated systems
Untreated Systems: CNTs tend to agglomerate due to van der Waals forces, leading to poor dispersion and weak stress transfer, significantly limiting mechanical performance [20].
Case Study 2: Nanoclay-Epoxy Composites
Nanoclay-based nanocomposites demonstrate similar trends:
Treated Systems: Exfoliated nanoclays dispersed with surface modifiers show a 30% reduction in gas permeability and a 40% improvement in thermal stability due to better interaction with the matrix.
Untreated Systems: Aggregated nanoclays fail to form effective barriers, resulting in weaker mechanical and barrier properties [21].
7.1 Scalability and economic feasibility scalability
Challenges: Achieving uniform dispersion and functionalization at industrial scales remains complex, especially for high-volume production.
Solutions: Advances in automated processes, such as spray drying for uniform coating and inline dispersion techniques, have made scalability more feasible for certain applications.
Economic Feasibility:
7.2 Correlation between interface and overall properties
From all the discussions made above, it will be very easy to establish a direct relation between interfacial properties and nanocomposites properties. The effectively designed interface has consequences not only in mechanical reinforcement but also in the improvement of thermal and heat conductivity and electrical conductivity. As an example, the increase in interfacial interactions decreases the probability of filler aggregation, which acts like a defect and reduces the mechanical strength of the composite material. Furthermore, regarding the tailored interfaces, the thermal stability can be increased and degradation retarded due to the better protection of filler materials by the matrix. Electrical properties are interface-dependent too: good connectivity of conductive fillers increases conductivity [22].
7.3 Performance prediction based on interface modification
Performance prediction of nanocomposites is based on the modification of interfaces, considering the degree of interaction between filler and matrix and associated material response. The interface can be optimized using various modifications such as chemical treatment and functionalization of nanofillers, which could provide enhanced mechanical, thermal, and electrical performance. For example, functionalization of fillers with appropriate chemical groups may form covalent bonds with the polymer matrix, which is very effective in enhancing the interface. Thus, performance models and simulations that make use of interfacial adhesion, filler orientation, and distribution result in the prediction of the overall behavior. Guiding and controlling of the future design and engineering of such materials is therefore aimed at achieving this through processes [25, 26].
8.1 Summary of key findings
8.2 Limitations of the study
8.3 Environmental and safety impacts
The environmental and safety implications of nanocomposites are increasingly significant:
Production Challenges: Surface functionalization processes and solvent-based preparation techniques often use hazardous chemicals, increasing environmental risks.
Disposal and Recycling: The presence of nanomaterials complicates recycling processes and can lead to long-term environmental persistence, with potential toxicity.
8.4 Recommendations for sustainable practices
To address these concerns, the following strategies are recommended:
Eco-Friendly Processes: Develop green functionalization methods and solvent-free preparation techniques to reduce environmental impact.
Biodegradable Matrices: Incorporate biopolymers as matrix materials to facilitate recycling and reduce long-term environmental persistence.
Lifecycle Assessments: Conduct lifecycle studies to assess the environmental and safety impacts of nanocomposite production, use, and disposal.
8.5 Recommendations for future research
Future research should cover the following:
8.6 Possible future industrial uses
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