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The selected article is focused on determining ‘the role of agglomeration in the conductivity of carbon nanotube composites near percolation. The study of agglomeration in nanocomposite materials comprising of carbon nanotubes is presented in detail. Agglomeration is similar to conductivity having a low level of concentration in carbon nanotube composites. The agglomeration spots increase with the concentration uniformly, whereas, the size of agglomerates tends to remain uniform with concentration and is dependent upon the proportion of seeded carbon nanotubes. Agglomeration can leave vacuums in samples favouring the bundles that are localised and are connected over carbon nanotubes branching out like hairs from the agglomerates. Agglomeration has an impact on the carrying of charge inside nanocomposites and for this purpose, simulations are performed. Agglomeration is harmful to carrying charges in such materials but for carrying the optimal charge having low levels of concentration, agglomerates are formed which has improved the carrying process. As the concentration increases, it is expected that the conductivity will not be dependent on agglomeration because the pathway of percolation is established.
Figure 1 Pathway of Percolation
Past studies considered that carbon nanotubes are more like filler for altering the properties (physical) of polymers and tiles to make them available for different applications. Carbon nanotubes are also considered sheets of graphene in which carbon atoms are arranged hexagonally. Studies that were related to experiments have described the importance of dispersion of carbon nanotubes and that the properties of carbon nanotubes should be displayed by composite materials. Carbon nanotubes have a large size; however, they are hollow from the inside which makes them lighter weighted. These features make the carbon nanotubes ideal for nanofillers for altering the properties (physical) of materials by increasing the strength, thermal and electrical conductivity of these materials and also by reducing their weight. Carbon nanotubes are also used for altering properties (electrical) of composite materials having different applications involving shielding of electromagnetic materials, reduction of electrostatic materials and sensing the strain for health monitoring. In percolation, the distribution of carbon nanotubes in a uniform manner provides an increased path of conductivity with less mobility. The article catered to the key issues that determine the role of agglomeration in the conductivity of carbon nanotube composites near percolation.
Article helped the general public in understanding the reason that carbon nanotube composites were not being produced for general use. The article carved a way forward for future research that can further cater to and finish this issue.
The thought process behind the selection of papers was based on the information and knowledge that the papers could add to the research. The selected papers were published by the field experts, thus the information and knowledge provided in the research are factual. Recent papers were selected to add recently updated knowledge and information to the research, the issue of nano-fillers agglomeration during dispersion has been occurring for ages, and the need for the practical production of nanocomposites is significant for the advancement of further technologies. The selected papers were published by different experts and consisted of several contributors, the papers had different contents and catered to different issues.
The selected papers provided significant value to the research with their distinct contents, different aspects of each paper were thoroughly observed and added to the research. The selected ten papers had different research topics, some catered to agglomeration while others to nanoparticles. Papers were selected based on the value they could add to the research, one such paper provided different reasons for agglomeration and offered remedies to resolve those reasons. The researcher intends to combine the gathered information and knowledge provided by these papers, the final research will contain a thorough analysis of each paper.
Atif and Inam have stated in their study that by adding nano-fillers to polymers, the performances of polymers can be improved extraordinarily. The chemical structure of nano-fillers additionally contains a large surface-to-volume ratio as well as a large chemical texture of the surface. Nano-fillers have high values of surface energies because of their high amount of surface area. When these nano-fillers of high energy values are added to polymers, strong interfacial interactions are produced. However, the production of polymer nanocomposites for practical use undergo several problems regarding its filler state since van der Waals forces are a factor and tend to agglomerate MLG and CNTs as described in the research done by Atif and Inam (2006). Further studies have been conducted on carbon nanotubes (CNTs), according to Tarlton et al. carbon nanotubes (CNTs) are considered to be filler material that alters the physical properties of various polymers. It has been established that carbon nanotubes (CNTs) are a factor in agglomeration with graphene and other composite materials. In the publication by Atif and Inam (2016) supported by the study of Tarlton et al. (2017), it has been explained that during the process of production of polymer nanocomposites, carbon nanotubes coagulate or agglomerate in bundles with one another due to the effect of Van Der Waals interactions. Therefore, the review will focus on the issue of agglomeration and whether it is the key issue preventing nanocomposites from general use.
In the publication by Rishi et al. (2019), the size of agglomerate was found to be reduced as described through the onion peel theory. The images of Scanning Electron Microscopy (SEM) exhibit that agglomerates are shed rotten from the surface of the agglomerates in a way like the peeling of an onion. However, a model was proposed by Coran and Donnet that shows a disparity in the rate of dispersion in mixed time. In this model, the fillers that were not dispersed were found to be equal to the number of agglomerates having sizes of more than five micrometres as demonstrated by Rishi et al. (2019). To measure the strength of agglomerates, a wide range of models are present in the literature. In the publication by Deng et al. (2016), the Weiler model provides maximum power to the overall size of elements as it has been accounted for overall scattering for main elements, whereas, in comparison with the Kendall theory, the power dependent on the theory was minute because of the impact of defects within agglomerates as shown in figure 1.
Figure 2 Size of nanoparticles (adapted from Deng et al. 2016)
This model had the capability of measuring the strength of agglomerates in terms of dispersion by observing the contacts that were broken inside the agglomerates. Additionally, further adding to the insights provided by Deng et al. (2016), in the research of Weiler et al. (2010), the forces that act on the agglomerate's surface area are having scarce partners for contact in comparison with the inner ones. In the publication by Zare (2016), the theory of agglomeration for nanoparticles explained the method of reinforcement of nanocomposites through the agglomeration of Nanoparticles as compared to the forming bonds among Nanoparticles and polymers. Additionally, further adding to the insights provided by Zare (2016), in the research of Zare (2015), the Kelly-Tyson theory states that the maximum stress in carbon nanotubes is higher than its failure stress, which causes the carbon nanotubes to break.
Defining Nanocomposites and Agglomeration
Nanocomposites are double-phased dense elements, which means that one phase has one or two dimensions that are less than a hundred nanometres. The Nanocomposites identify several characteristics of extensive nature, for example, bulk solidity, high thermal stabilisation, and reduced penetrability having a low concentration of nanofiller. In the research done by Zare (2016), it was observed that Nanocomposites exhibit several revenues, for example, accessible, fabricated easily, having optical, electrochemical and magnetic characteristics of high level and little cost. Moreover, the agglomeration has produced several faults and focuses on stress in Nanocomposites that play a part in the deterioration of the characteristics of the sample. Alongside agglomeration, the structure of nanofillers has played a significant part in the total mechanical behaviour of Nanocomposites. Nanoparticles of large size in nanocomposites reduce the agglomeration of nanoparticles as described by the study by Zare (2016). The nanofiller substance increases the level of agglomeration of nanocomposites. Furthermore, the actual volume of nanoparticles in polymer nanocomposites has been decreased through agglomeration. Agglomeration involves both nanofiller and agglomerate diameters. When the non-filler substance is added and the size of the nanofiller is reduced, the level of agglomeration is increased.
Agglomerations are considered the musters for Nanoparticles, the aggregation is regarded as the solid or thick constituent part of collections. During the nanocomposite procedure, an agglomerate of small size tends to cause a high agglomeration effect. Agglomerations are reliant on materials that are lightly collective and are shattered through mechanical forces. In the publication by Zare (2016), the agglomeration has prevented the properties and quality of nanoparticles, for example, the stress force, stress methods and stress regularity. The agglomeration was produced through mutual forces between nanoparticles, these forces are also known as Van Der Waal forces.
Properties of nanoparticle agglomerates
Agglomerates have a greater influence on several properties of Nanoparticles, for example, the conduction of electricity. In the research done by Deng et al. (2016), it was observed that due to the electromagnetic response of nanoparticle agglomerates, they display interesting electrical properties. An imperative stricture that defines structural properties for agglomerates was connected to the hydrodynamic area of agglomerates. In this way, the transport properties of agglomerates are symbolised. Additionally, further adding to the insights provided by Deng et al. (2016), the research of Yang et al. (2012) observed that nanoparticles are considered for research work due to typical electrical, photo-electronic and optical properties as compared to conservative state substances. Furthermore, the interphase properties are a region of polymer near nanoparticles having dissimilar mechanical and chemical properties from nanoparticles and polymer. Additionally, further adding to the insights provided by Zare (2016), the research of Odegard, Clancy and Gates (2005), an effective interface model was brought into use to deduce the properties of elasticity of composites with Nanoparticles possessing a boundary of similar shape (spherical) as an active particle. The model consists of models and fixed size in the section near-spherical surface known as interphase.
In the research done by Zare (2016), the structural and mechanical properties, for example, permeability, order and strength of agglomerates are described as difficult because the imaging devices of high resolution are not available and there is a lack of techniques for the experiment. Permeability was the ratio of the volume of vacuum to volume engaged by packed particles, it described the packing properties of nanoparticles. Nanoparticles have effective parameters and performance measured by thermal and mechanical properties.
Agglomeration is a key issue preventing nanocomposites from becoming general use
Nanoparticles should be used in such a manner that they form a combination with additional specific materials so that the desired result can be achieved, for example, materials where the efficiency of agglomeration can be reduced. In the research done by Xuyan et al. (2018), if a dynamic solid of conductive nature such as carbon nanotubes is synthesised in the morphological solid of the nanosheets, there is an opportunity for the prevention of not only the agglomeration of Nanosheets and also for the conductivity of a dynamic solid. Additionally, the research of Xuyan et al. (2018) states that the three dimensions of nanocomposites are supportive and the Nanocomposites are not inclined to agglomeration. The publication of Xuyan et al. (2018), exhibits that Nanocomposites are less inclined to agglomeration and active surface area can be sustained over a long period. In the research done by Tarlton et al. (2017), it was observed that agglomeration is harmful to the thickening impact of Nanoparticles in Nanocomposites. In the publication by Atif and Inam (2016), it was stated that agglomeration could be evaded through organic solvents, choosing appropriate diffusion or methods of production and making the fillers functional.
In the research done by Atif and Inam (2016), one of the reasons that prevent the production of enhanced polymer nanocomposites is that the high-performing polymers are only achieved if the dispersion of the filler is uniform, otherwise, agglomeration will take place. The dispersion state of fillers plays a major role in preventing the production of polymer nanocomposites because the Van der Waals forces are in factor and cause agglomeration between MLG and CNTs. Additionally, the high thermal conductivity of carbon nanotubes and multilayer graphene produced conductive (electrical and thermal) polymer nanocomposites. In the publication by Atif and Inam (2016), past studies identified that nanocomposites of carbon nanotubes and multilayer graphene can be used as a support for the growth of cells and transplant resources for flawed bones of humans. The issue needed a solution before including carbon nanotubes and multi-layer graphene polymer nanocomposite in human beings. The main factor on which the polymer nanocomposite properties were dependent is the bond strength (interfacial). Nanocomposites are manufactured at low temperatures for increasing the level of the configuration of carbon nanotube. Nanocomposites were found to have damping capacity at a higher range of temperature as described in the study by Atif and Inam (2016). In the research done by Atif and Inam (2016), multi-layer Graphene has the capability of improving the flexural and mechanical properties of nanocomposites.
MLG has a coiled structure and tends to undergo wrinkling when compressed (in-plane) or shear. Wrinkling changes the shape of thin and flexible material as stated in the study of Atif and Inam (2016), and shown in figure 2.
Figure 3 Wringkling of multi-layer Graphene: a) typical wrinkling pattern b) magnified view of wrinkles (adapted from Atif and Inam 2016)
In the publication by Xuyan et al. (2018), when wrinkling occurs in MLG, MLG’s shape is not allowed to be regained as the strain energy stored within MLG is not sufficient. If sufficient elastic strain energy is not stored within MLG, irreversible wrinkling occurs which can only be altered through external effects.
In the research done by Zare (2016), it has been established that the aggregation/agglomeration of nanoparticles occurs in every nanocomposite. When nanoparticles are in the process of production or are being incorporated into the polymer, aggregation/agglomeration is formed. It has been recognised that Van der Waals forces or chemical bonds play a factor in direct mutual attraction between nanoparticles by causing aggregation/agglomeration. Aggregation/agglomeration can be avoided by particle coating using capping agents, separating them by charging the filler surface using electrostatic revulsions, and by the application of a coupling agent or compatibiliser. Additionally, if optimal parameters are used in the process of production, aggregates can be broken effectively. It has been stated in the research done by Zare (2016) and *Zare (2017), that the aggregation/agglomeration level in nanocomposites can be increased by reducing the filler size and adding non-filler content. A high concentration of nanofillers in polymer nanocomposites is harmful because the agglomerates produce several defects in the samples that are responsible for worsening the properties of polymer nanocomposites. When the nanoparticles are concentrated efficiently in polymer nanocomposites, the size of polymer nanocomposites is reduced through the agglomeration of nanoparticles.
In the research done by *Calisi et al. (2013), Agglomeration can be prevented and controlled by tailoring the dispersion state of nano-fillers. Certain processes can tailor the state of nano-fillers and thus avoid the factor of agglomeration in nanocomposites as described by Atif and Inam (2016) and *Calisi et al. (2013).
In the research done by *Fujisawa et al. (2013), the dispersion state of fillers is improved by using dispersant solvents. The dispersion state of fillers is improved by lowering the viscosity of the polymer matrix. The dispersant solvent eases the dispersion process because of its vital characteristics. It exists in the low viscous state and thus can lower the viscosity of the polymer matrix, which eventually helps in the process of dispersion as described by Atif and Inam (2016) and *Fujisawa et al. (2013). Few cases reported that the use of organic solvents in polymers has affected their mechanical properties. However, the mechanical properties were not affected by the usage of organic solvents having a low boiling point. In the publication by Atif and Inam (2016), it was stated that organic solvents have advantages and disadvantages regarding the properties of polymers.
Agglomeration can be prevented with different methods of dispersion. In the research done by *Calisi et al. (2013), it is stated that different approaches can be used to disperse nano-fillers in the polymer. The external force can be applied to nano-fillers to disentangle them, after which the disentangled dispersed nano-fillers can be enclosed in the polymer matrix as described by *Calisi et al. (2013). The enclosing is done to prevent the nano-fillers from re-aggregating and to yield metastable dispersion. The forces that are applied to the nano-fillers can be mechanical stirring or sonication. The other approach most persistently used is the disentangling of nano-fillers via dispersing them in a suitable solvent. However, graphene sheets are first separated and then dissolved for graphene-based nano-fillers, which eventually results in polymer solutions as described by Atif and Inam (2016) and *Calisi et al. (2013).
In the research done by *Calisi et al. (2013) and Atif and Inam (2016), in the process of sonication, high-frequency sound waves are used in agitating the nano-fillers in a solution. A Sonication device, as shown in figure 3, is used for sonication processes that are of high energy and uniformly disperse the MLG and CNTs in the polymer matrix.
Figure 4 Sonication device (adapted from Atif and Inam 2016)
Calendering is another method which uses shear force to disperse nano-fillers in the polymer as shown in figure 4.
Figure 5 A) Calendering mill and B) its working principle (adapted from Atif and Inam 2016)
Calendering mill is a specially designed three-roll mill and consists of gaps between its rolls. During calendaring, the roll gaps in the calendar produce a great force that disperses, mixes and homogenises viscous materials. Ball Milling is also used for the dispersion of MLG and CNT, high-quality ball mills are used in reducing the size of particles.
Figure 6 A) shear mixer and B) extruder (adapted from Atif and Inam 2016)
Figure 7 Schematic of the shearing device (adapted from Atif and Inam 2016)
High-shear mixing and extrusion is another but more common method with which high numbers of CNTs are dispersed uniformly, twin screws rotating at high speed create a shear flow to disperse CNT agglomerates as shown in figure 6.
In the research done by *Gu et al. (2016), functionalisation is another process that solves dispersion-related problems by modifying the surface of particles. Further added by Atif and Inam, the surface of graphene needs modification to disperse in the polymer matrix, alpine has a very smooth surface which creates weak interfacial bonds with polymer because of the properties of polymer nanocomposites, the properties depend heavily on the strength of its interfacial bonds. Different functionalisation methods help disperse MLG and CNTs in the polymer matrix. Furthermore, the method of oxidation is used for modifying the surface of CNTs, different treatments are used for successfully performing the oxidation method. Superacids are another method with which modification of MLG and CNTs can be achieved, strong acids can be used in dissolving and dispersing MLG and CNTs in large numbers as described by Atif and Inam (2016) and *Gu et al. (2016). The use of nitric acid and sulphuric acid as remedies resulted in the functionalisation of carbon nanotubes and multi-layer graphene, and also reduced the size of fillers.
Figure 8 Functionalisation process (adapted from Atif and Inam 2016)
The functionalisation reduced the clipping force of nanotubes to 15 per cent and the mechanical properties of nanocomposites got affected.
In the research done by Atif and Inam (2016), it was observed that the state of dispersion of MLG and CNTs in the polymer matrix is also improved by adding hybrid nano-fillers. Hybrid fillers used as synergistic effects caused an enhancement in the state of dispersion of fillers. Further added by Atif and Inam (2016), the physical properties of hybrid nanocomposites are improved when hybrid nano-fillers create synergistic effects. Physical synergistic effects take place during dispersion because of the multiphase hybrid structure of Titania. Moreover, further synergistic effects are found during dispersion when nanoclay is used as described by Atif and Inam (2016). Therefore, the enhancement in the state of dispersion is dependent upon synergistic effects among the nanofillers such as carbon nanotubes and Titania.
This review revolves around the theoretical framework, the concept of nanoparticles and agglomeration and the properties of nanoparticle agglomerates. Part of the review discusses agglomeration as a key issue preventing nanocomposites from becoming of general use. Furthermore, this review discusses the factors affecting nanocomposites from becoming general use, Nanoparticles aggregation/agglomeration in polymer particulate nanocomposites and Reasons and remedies for the agglomeration. The theoretical framework comprises of different models explaining the concept of nanoparticles, nanocomposites and agglomerates. The concept of nanoparticles was described with properties which included the mechanical, thermal and electrical properties of nanoparticles agglomerates. Reasons and remedies for the agglomeration describe the nature of organic solvents, methods of dispersion for carbon nanotubes and multi-layer graphene, the functionalisation process for carbon nanotubes and multi-layer Graphene and the concept of Hybrid nanofillers.
Nanocomposites are double-phased dense materials, they have extensive characteristics, and exhibit significant revenues. However, the production of nanocomposites is not practical because agglomeration occurs between nano-fillers during dispersion, furthermore, the issue of nonproduction of nanocomposites is because of the structure of nano-fillers. Nanocomposites are produced when nano-fillers, MLG and CNTs are dispersed, however, during dispersion, MLG and CNTs were agglomerating because of the physical properties of carbon nanotubes (CNTs) were altering the polymer matrix and agglomerating with MLG. However, dispersing nano-fillers in large quantities has decreased the chances of agglomeration to some extent.
The properties of nano-fillers are influenced by agglomerates. Nano-fillers are good conductors of electricity, and thus they agglomerate due to electromagnetic responses. Furthermore, the interphase properties of the neighbouring region of the polymer have different mechanical and chemical properties. The optical, electrical and photo-electronic properties of nano-fillers were not adjusting with the interphase properties.
The agglomeration can be controlled and prevented through various methods, these methods include changing the dispersion state of nano-fillers. Organic solvents can be used to improve the dispersion state of fillers, these solvents can lower the viscosity of the polymer matrix, and thus the dispersion process can be eased. Other methods include applying force to disentangle nano-fillers and then enclosing them in the polymer matrix, and modifying the nanofillers surface to make the dispersion process feasible. Furthermore, the addition of hybrid nano-fillers forms hybrid nanocomposites, these nanocomposites are created when hybrid-nano-fillers are combined with nano-fillers, such as MLG and CNTs. Synergistic effects occur during dispersion when nano-fillers are combined with hybrid nano-fillers.
Significant progress has been made over the years in solving nano-fillers agglomeration, however, further researches are due to make the production of nanocomposites practical. Further insights are to be given on solving agglomeration when nanocomposite production starts, and significant changes will occur towards the further advancement of similar technologies.
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