Graphene being an allotrope of carbon, is considered one of the best materials to be used in all sorts of industries throughout the world. The properties that graphene possesses are unique and highly authenticated in ways that can benefit the industries and humans as well in carrying out their daily life tasks with much ease. Graphene nanoplatelets are one of the types of graphene which are rich in their composition in their own unique way.
It has a wide range of properties which are a blend of all the good characteristics that a product can offer. Its applications as well bring forth a wide variety and are uniquely benefiting the industries. However, three of the best applications of graphene nanoplatelets are briefly explained in this article.
Graphene is carbon's allotrope that occurs as a 2-dimensional planar sheet. One can think of graphene as a single atomic graphite layer. Technically, graphene is non-metal. Its properties are similar to the characteristics of a semiconducting metal, and that's why graphene is usually known as a quasi-metal. As compared to the other non-metallic materials, graphene has many excellent characteristics.
There is covalent bonding of each carbon atom to 3 of the other atoms of carbon in a hexagonal array, with one free electron left per each carbon atom that is in a p-orbital which sits above the material's plane. In a graphene sheet, two pi-electrons are displayed by each hexagon, and they are delocalized and enable effective conduction of the electricity. Photons pass through unimpeded by the holes in the structure, giving rise to a high and increased thermal conductivity.
What are the characteristics of graphene?
In comparison with conventional materials, many excellent characteristics are possessed by graphene, which makes graphene a remarkable material to be used in electronic applications. Graphene's most significant and prevalent characteristic is its electrical conductivity. There is no electronic band-gap in graphene which means that there is no chance of it being switched off or on. The electrons function as massless relativistic particles, and a small overlap is possessed by the conduction and valence bands.
Graphene can display mobility of 1 X 104 cm2 V-1 s-1 and 1013 cm-2 of concentration of charge carriers at room temperature. This mobility can increase at low temperatures. An effect known as a half-integer Quantum Hall Effect (QHE) is also exhibited by graphene as the charge carriers function as the quasi-particles that are otherwise referred to as massless Dirac Fermions.
What is QHE?
The relationship of the charge carriers' velocity, density, and charge, is known as QHE. QHE takes place when you apply a magnetic field along the axis that is perpendicular to the conducting material's plane. The carrier's path turns curved under these conditions, which results in opposite charges getting accumulated at either of the material's ends.
Discrete band levels also referred to as Landau levels, were produced by the electron confinement because of graphene's two-dimensional nature. The charge carriers fill-up the Landau levels. Excellent mechanical, thermal, and optical characteristics are possessed by graphene. Despite the fact that the single sheet graphene is an extremely transparent material, but still, with the reflectance of less than 0.1%, each layer in thickness absorbs white light to up to 2.3%.
What is the suspended graphene sheet's role?
The amount of layers stacked on each other's top determines the increase in the linear absorbance. At room temperature, 3000-5000 W m-1 K-1 of thermal conductivity can be displayed by a suspended graphene sheet. Although even when it is attached to any other substrate, the thermal conductivity still won't drop to 600 W m-1 K-1. Phonons' Scattering at the interface causes this drop and impedes their movement, whereas the path of Phenon is uninterrupted in freestanding graphene.
Its thermal conductivity is still double the thermal conductivity of the copper. Graphene is undoubtedly in the list of the strongest materials that have ever been made; for instance, 42 N m-1 or more of stress can be withstood by a single-layer graphene sheet, with 1.0 TPa of Young modulus.
What are graphene's electronic characteristics?
The electron movement of graphene is extremely fluid as it has a delocalized pi-electron system across its entire surface. It is because of the overlapped pi-electrons that no band-gap is exhibited by the graphene system, allowing the electrons to move easily with no need of inputting the energy into the system. Graphene's electronic mobility is extremely high, and the electrons function as photons because of their ability to move. The electrons don't scatter even when moving sub-micrometer distances. According to various tests, graphene's electron mobility is 15,000 cm2V-1s-1, and it has the potential to produce more than 200,000 cm2V-1s-1.
What are graphene's thermal characteristics?
Graphene is an ideal material for the conduction of heat in the plane because of the graphene's repeating structure. Interplane conductivity is complicated, and other nanomaterials like CNTs are typically added for boosting interplane conductivity. Graphene's regular structure allows phonons' movement through the material along the surface with no impediment anywhere. Inter-plane and in-plane are the two types of thermal conductivity that are exhibited by graphene.
3000-5000 W m-1 K-1 is the single-layered sheet's in-plane conductivity, but due to the weak inter-plane Van der Waals forces, 6 W m-1 K-1 or low is the cross-plane conductivity. Despite not being measured directly, 2.6 μ J g-1 K-1 is the electronic gas' specific heat in graphene at 5 K according to the estimates.
What is graphene's mechanical strength?
With 1.3 x 1011 Pa of tensile strength, graphene is on the list of the strongest material that has ever been discovered. Graphene is very lightweight, i.e., 0.77 mgm-2, but it still has an un-imaginary strength. Graphene's mechanical strength stays unmatched, and it can majorly improve the strength of many of the composite materials.
What is graphene's flexibility?
The graphene molecule's repeating sp2 hybridized backbone allows the graphene to be flexible. They also offer enough stability and rigidity despite their rotation that the molecule can support other ions while withstanding changes. Graphene's flexibility makes it extremely desirable as it is a characteristic that many molecules want as not many of the molecules can be supportive and flexible at one time. With 0.5 TPa of Young's modulus, graphene has a spring constant of between 1-5 Nm-1 in terms of its elasticity.
What are Graphene nanoplatelets:
The revolutionary material of the 21st century is considered to be graphene. The thickness of a single graphite monolayer is 0.34 nm which is the thickness of one atom, while the lateral size of a single graphite monolayer can be a number of orders of magnitude larger. It has a complex synthesis process, and it can't be produced in mass yet. It is their potential of being produced on a large scale, exciting characteristics, and low cost that has made the graphene nanoplatelets (GNPs) become an alternative.Few graphite layers are possessed by graphene nanoplatelets, and their thickness varies from 0.7-100 nm.
What are the main characteristics of graphene nanoplatelets:
Graphene nanoplatelet's main characteristics are easy production, low cost, remarkable electrical and thermal conductivities, excellent mechanical characteristics, high aspect ratio with planar shape, and lightweight. Various applications as fillers of composites, neat coatings, and isolated materials are possessed by graphene nanoplatelets. Here, the main focus is on using graphene nanoplatelets as nano-fillers.
How are the graphene nanoplatelets synthesized?
Typically, micromechanical cleavage of bulk graphite is used to synthesize Graphene Nanoplatelets, and it can only synthesize graphene flakes in some quantity that are mixed in with the graphitic stacks. Mechanical cleavage is often used by large-scale GNP synthesis followed by a chemical reduction for the production of the final GNP product. Plasma exfoliation is another method for producing GNPs in bulk. Plasma exfoliation has many benefits, and one of its remarkable advantages is its capability of synthesizing and functionalizing graphene nanoplatelets for promoting dispersion in the host matrix in a processing, dry, single step.
What is the functioning of RF or Microwave Plasma Reactor?
A vacuum is applied in the Microwave Plasma Reactor or RF for eliminating the atmospheric contaminants along with the residual contaminants that are released during the plasma milling process. It is these advantages that make this material a remarkable choice for industrial applications of large scale with a broad range of the available functional groups like F, N2, NH2, COOH, and OH. As compared to CNTs, GNPs can be an ultimate cheaper material on a ton level because of the plasma purification or/and functionalization, capital equipment, and low price of the input materials, therefore making way for an increase in the industrial applications with the early adopters.
What are the future and current applications of GNP nanocomposites?
With different matrices like metals, concretes, polymers, and others, graphene nanoplatelets are broadly used as nanofillers. GNP's addition often improves the tribological and mechanical behavior, which increases the thermal conductivity and barrier characteristics, thus transforming insulating matrices into electrical conductors and functioning as a flame retardant. Getting a suitable GNP dispersion is not easy, and manufacturing these nanocomposites is one of those challenging tasks. There is a significant variation in the added GNP content as a function of the matrix's nature and the desired characteristics. Graphene nanoplatelets raised expectations, and those expectations and reasons significantly vary because of the great versatility of the graphene nanoplatelets, searching for diverse performances and thus diverse applications.
These nanocomposites have unlimited future and current applications, from materials with improved thermal and mechanical behavior up to new functional materials, like adsorbents, energy harvesting, new electronic devices, sensors, etc. There are various particular examples of the broad versatility of the nanocomposites reinforced with graphene nanoplatelets, and their summarization is written here. Manufactured composite's behavior and characteristics are affected by the dispersion of graphene nanoplatelets together with the attained exfoliation degree. It is this reason that makes this study of theirs extremely important in the production of these materials.
A new technique, known as Terahertz time-domain spectroscopy (THz-TDS), provides knowledge regarding the dispersion of graphene and analyzes the material's dielectric behavior. It also allows the investigation of the polymer nanocomposite's electronic quality.
What is their mechanical behavior?
The lifetime, chemical resistance, and mechanical behavior of the GNPs increase with the addition of the GNPs to the polymer matrices. When added, GNPs also lessen their capability of absorbing water, which increases their resistance against the aggressive humid environments. GNPs barrier characteristics are related to the production of tortuous paths for the water molecules, and they markedly depend on their lateral size, thickness, and geometry.
All of this is because of their spatial arrangement that causes the modification of the GNP's effective specific surface area. Combined with the improved thermal conductivity by the addition of GNP, this makes these potential nanocomposite materials for the storage and conversion of solar energy. Freeze-thaw (F-T) resistance is also improved by adding GNPs to the concrete.
The addition of liquids to water and grease or oils lubricants is an example of the reinforcement of liquids. Improved thermal conductivity is displayed by the water-based graphene composite, whereas with the addition in the volume of graphene, its melting and freezing time lessens. This water's behavior is exciting enough to store the electrical energy in the batteries or as the storage for compressed air.
Moreover, the addition of only GNP is not enough at times as it needs the GNP to combine with the other fillers for attaining the desired behavior. Also, they used to look for a synergic effect between both of the nanofillers in these cases, and the same is the case with the addition of the GNPs in the liquid lubricants. Its friction and wear characteristics are enhanced by the combined usage of the two additives, titanium dioxide nanopowders, and GNP. One should know that graphene nanoplatelets are enough for reducing the grease lubricant's friction as a filler, with no additive.
The versatility of graphene:
Graphene is capable of being utilized in various kinds of applications, and that is why since the last decade, graphene nanoplatelets have been known as the most excellent nanofillers that can bring a revolution in society, offering society some developments and devices.
What are the three best application areas of graphene nanoplatelets?
1. Graphene Nanoplatelets for Stretchable Electronics and Strain Sensors:
The field of stretchable and wearable electronics is inclining towards rare material arrangements for fabricating soft sensors and electrical devices and for designing circuitries on deformable organisms/structures and curvilinear. When it comes to building the compliant nanocomposites for electronics, one favorable approach is the junction of an elastomeric and pliable material with a conductive nanomaterial. When deformation (compressed, wrapped, or stretched) of such nanocomposites occurs, the recorded capacitance and conductivity fluctuations get exploited by the sensor technologies.
Devices like tactile, pressure, and strain sensors, use these variations as a feedback mechanism. The electrical characteristics of the material changes when the conductive nanofillers that are inside the matrix are more connected or separated apart from external stimuli typically, which are often mechanical. In past years, there were investigations and productions of the stretchable and flexible GnP-based sensors by following those approaches.
2. Wearable Electronics Based on Graphene Nanoplatelets:
In comparison with the common fabrics, various applications and functionalities are possessed by smart textiles that are fibrous materials. In 2018, there were predictions that US$ 20.6 billion would be reached by the wearable device market in that year. These garments' major promising potential is their electrical conductivity as many fields like deformable, wearable, and flexible electronics are going after it, the emerging Internet of Things especially.
Nanotechnology propels miniaturization, which allows the fabrication of electronic components functioning on a single fiber. Although, the incorporation of conductive nanoparticles on/inside fibrous networks more often targets textile's direct functionalization. The obtained results were remarkable in applications related to electrochemical sensors, supercapacitors, and generators when such kind of approach was used.
However, more research is needed for creating wearable conductive materials that, even under ambient conditions (air, sunlight, etc.) and mechanical stress, possess stable electronic performance. Wearable electronics particularly need the production of a new class of materials that are washable, foldable, and flexible and maintain electrical conductivity at a satisfactory level at the same time. Polymeric nanocomposite materials are best for conductive wearable technologies because of the easy process of manufacturing, polymer's intrinsic mechanical characteristics, and accessibility to the large spectra of characteristics with diverse nanoparticles.
The promising methods here are (1) transferring of CVD-graphene films on textiles, (2) using graphene-based materials for the production of conductive fibers, and (3) fabric's functionalization with Graphene Oxide (GO). In comparison with metal functionalization, improvements are shown by all of these approaches.
Although one can identify some of the limitations, that is;
(1) Graphene's chemical vapor deposition is costly, and film's transferring procedure is very difficult.
(2) Excellent electrical characteristics are possessed by Graphene freestanding fibers, but they have difficulty in adapting to the current industry of garments.
(3) Steps of reduction are needed by graphene oxide, and the obtained sheet resistance is usually in thousands of Ω/sq (high).
Another potential procedure was recently reported for imparting electrical conductivity to the fabrics that were industrially produced through GNPs-based functionalization. One benefit of this method is that it can be adapted by numerous commercially available materials, for instance, polyesters and cotton. GNPs are also scalable as they are already made in quantities (hundreds of tons) that are appropriate for the textiles market.
3. Advanced Reinforced Graphene Nanoplatelet-Based Bio-Nanocomposites:
In the advanced materials field, a prominent role is played by graphene-based reinforced nanocomposites. Enhanced plastic-based structural materials of the new generation are made because of the excellent mechanical characteristics of GO, GnPs, and single-layer graphene. These 2-dimensional carbon-based fillers, for instance, are exciting and interesting to realize next-generation wind turbine designs, elements of structure in aerospace, automotive lightweight components, anti-corrosion coatings, concrete, and sporting goods. As compared to single-layer graphene, GnP powder is typically already formed cheaply and at larger scales, and it has relatively gained more interest in the composite market.
Selection as nanofillers:
As compared to GOs, GnPs have lower concentrations of the defect and are stronger; that is why they are already chosen as the nanofillers to toughen the polymeric materials. With clay's multi-flakes structure, GnPs integrate carbon nanotube's mechanical features that can impart excellent structural characteristic enhancements.
As compared to these nanofillers, GnPs perform very better when it comes to improving the nanocomposite's mechanical characteristics like their resistance to crack growth and fatigue, fracture energy, fracture toughness, elastic modulus, and tensile strength because of their higher compatibility with the polymers matrix. The flake-like filler's homogeneous dispersion in the polymer matrix is a very major need for attaining an effective reinforcement. These conditions are important, but they are not enough.
The other important things to consider are the alignment, flake dimensions (lateral thickness and size), and chemical interaction with the matrix. Also, Papageorgiou et al., in their latest paper, investigated the mutual interaction between the hosting matrix and the graphene-based materials in detail. The polymer matrix determines the fillers' modulus. When the matrix is stiffer, and the concentrations of filler are considerably low, then the GNPs modulus is particularly larger, which suggests that the nanoflake modulus does depend on the matrix.
Biopolymers play very little role in polluting the environment. They are biodegradable. Maybe the will be an alternative for synthetic, oil-based, polluting polymers in the future. Incorporation of nano-sized reinforcements in the bio-polymeric matrixes can enhance these bioplastic's performance deficiency and attain characteristics that would be similar to the characteristics of the traditional long-lasting ones.
Graphene nanoplatelets being one of the types of graphene, present a wide range of properties and characteristics which enhance the productivity of these materials and add up to their efficacy. Nonetheless, excellent applications are also playing a huge role in maintaining and enhancing the productivity and usage of these materials.