Investigation of mechanical, thermal and electrical properties of graphene reinforced carbon/epoxy multiscale composites

Graphene is one of the amazing discoveries in the twenty-first century for its extraordinary properties. It possesses superlative physico-mechanical, thermal, and electrical properties. At present, graphene is used in so many applications such as aerospace, automotive, healthcare, protective clothing, and so on. The discovery of graphene gave materials research and nanotechnology a huge boost and added a whole new dimension. It is 300 times stronger than steel, harder than diamonds, flexible, translucent, and a better conductor than copper (by about 1,000x). Graphene has the potential to change everything from computers to energy storage if it lives up to its hype.

Effects of Graphene:

Incorporation of GNPs (Graphene Nanoplatelets) with composite materials has a significant contribution to the thermal and electrical properties. Compounds with different GNPs content (0–8 wt%) were mixed and found that thermal conductivity of the 8 wt.% GNPs/epoxy composites was 1.181 W/m/K, which is increased by 627% compared with those of the neat epoxy. Furthermore, it has been found that the thermal stability and electrical property also have a certain degree of improvement. [1]

Graphene nanoplatelets (GNPs) with various surface functional groups have been found to be potentially capable of enhancing the interlaminar fracture toughness of E-glass/epoxy composites. The interlaminar fracture toughness was increased significantly under mode I fracture, but not as much for the modes II and III. [2]

Graphene layer

The GNP (Graphene nanoplatelets)/NCA (nanocarbon aerogels) hybrids (1 wt%) with different mixing ratios were dispersed in epoxy resin to prepare GNP/NCA/epoxy nanocomposites. It has been found that mechanical properties such as the tensile strength, flexural strength, and impact strength of the nanocomposites containing 1 wt% of the GNP/NCA hybrids, were superior to those of the nanocomposites containing 1wt% of a single carbon nanomaterial and pure epoxy resin. Furthermore, the growing cracks in the interlaminations of the CFRP laminate undergo crack deflection by GNPs and crack pinning by NCAs, effectively suppressing crack growth. [3]

Graphene and nanocomposites

Deformations and cracks generated in the structure can produce changes in the electrical conductivity of the material. So changes in the electrical conductivity can be used to measure the composites delamination and crack growth. In the glass fibre/epoxy/graphene nanoparticles composites, graphene nanoparticles form conductive networks in the epoxy resin with the auto-detection capability of deformation and damage. Furthermore, it has been found that electrical resistance increased as the crack grew in the mode-I, mode-II fracture toughness tests.

Laser-induced graphene (LIG) has been formed directly on the surface of aramid fabrics to improve the interlaminar strength of the composite and introduce multifunctionality. It was possible to form uniform graphene microstructures whose morphology can be controlled using induction power and pulsing density. Laser-induced graphene coatings improved the short beam strength (by 70%) and Mode I fracture toughness (by 12%) of aramid fibre-reinforced composites. Furthermore, induced graphene didn’t change the chemical composition of the treated aramid fabric surface rather it maintained the specific strength, damping properties of the aramid fabric and introduced surface conductivity as well. [5]

High-quality continuous monolayer graphene films with millimetre-sized domain sizes have been synthesized by using an oxygen-assisted CVD technique. The surface layer formed by bonded oxygen in Cu (Cu2O layer) used in this method, provided a higher priority for graphene nucleation, an increased growth rates of graphene domains, and cleaning the undesirable carbon and newly born defects during the growth. This method demonstrated significant improvement to the resultant graphene such as a largely decreased nucleation density from ~106 to ~101 nuclei/cm2, an increased average domain size from ~0.004 to ~1.5 mm and film coverage from 64% to 100%. [6]

There are some remarkable methods utilized for the incorporation of graphene into the CFRP composites like (a) electrospraying on the surface of carbon fibres as interface modifiers (b) dispersion into the epoxy resin to improve the matrix properties and (c) combination of (a) & (b) at a time. The careful and tailored integration of graphene sheets to FRP composites, it is possible to develop materials with the desired multi-functionality. When the graphene was used as both an interface modifier and matrix reinforcement simultaneously, CFRP composite structure gain multi-functionality while preserving their mechanical integrity. Furthermore, these CFRPs structures manifest a significant enhancement (240%) in electrical conductivity. [7]

Graphene Oxide (GO) can be deposited or incorporated with carbon fibres (CF) by various bonding types like van der Waals forces, zwitterionic interactions and covalent bonds. Characterization of functional groups, surface elements, contents of introduced GO, surface structures, morphologies and wettability of GO/CF revealed that incorporation by covalent bonding is the most effective way to improve the interfacial properties. Covalently bonded GO/CF composites showed an enhancement in the flexural strength (by 28.7%), interlaminar shear strength (by 22.7%) and interfacial shear strength (by 50.6%) as compared with the pristine CF. [8]

Three-roll mill dispersion technique was found to be advantageous over sonication and mechanical mixing method. The three-roll dispersion was effective, repeatable and potentially scalable to disperse graphene into epoxy to increase the electrical conductivity. The weight percentage of graphene used ranged from 0.5 to 5.0 and the percolation threshold of graphene was found to be 1.0 wt%. Through-the-thickness or volume electrical conductivity increased by nine log cycles, thermal conductivity doubled and fracture toughness increased by one-third for 1.0 wt% addition of graphene to epoxy. On top of that, both tensile and flexural moduli showed a marginal increase but the ultimate strength and fracture strain decreased. [9]

Although the incorporation of GO (graphene oxide) has disadvantages of increasing void content in the composites, properties of the hybrid composites were enhanced due to the improved fibre-matrix adhesion and the mechanical interlocking of the epoxy with GO. Mechanical properties of the GO (0.2 wt.%)/CF/epoxy composites showed an improvement in the tensile strength (34%), Young’s modulus (20%), toughness (83%), flexural strength (55%) and flexural modulus (31%) as compared to the CF/epoxy composite. Furthermore, the dynamic mechanical analysis of the composites resulted in increased storage modulus (56%), loss modulus (114%) and damping capacity, tan∂ (22%) at its glass transition temperature. [10]

Thermally Reduced Graphene Oxide (TRGO) possesses more wrinkled structure compared to the GO surface. The wrinkled surface texture of TRGO enhanced mechanical interlocking with epoxy resin, thereby improving the load transfer capability of the CF/epoxy composite. The good adhesion between CF and TRGO modified epoxy resin also increased the ILSS (67%), in-plane fracture toughness (62%) and impact toughness (93%) up to a certain limit of TRGO content (0.2 wt%) present in the prepared composites. However, the excess incorporation of TRGO (more than 0.2 wt%) contrarily agglomerates in the epoxy resin and makes micro-voids in the CF/epoxy composites. The micro-voids reduced the mechanical properties of the TRGO/CF/epoxy composites. [11]

Continuous and multi-layered graphene has been produced by “etching-precipitation” method. In this method, a large number (≥10) of the graphene layer was possibly controlled. First Fe-C films of various carbon concentration and thickness were prepared by reactive spraying. Then multi-layered graphene with resistivity down to ~240 μΩ cm and IG/ID ratio of 8–16 was achieved by removing Fe from the Fe-C films at moderate temperatures of 600–650 °C. [12]

Graphene oxide (GO) was successfully coated onto carbon fibre surfaces via ultrasonically assisted electrophoretic deposition. Deposition of graphene oxide introduced some polar groups to carbon fibre surfaces and changed the surface morphologies of carbon fibres. Carbon fibre functionalization has improved the ILSS (55% from 36.7 to 56.9 MPa) of GO coated carbon fibre/epoxy resin composites. [13]

Polyetherimide (PEI) and graphene oxide (GO) complex size coating have improved the interfacial interactions and bonding of CF/PEEK composites. The surface morphologies of CF has been changed with different amount of GO and changed the composite performances accordingly. Different amount of both PEI and GO has significant influences on the composite interfacial performance. Carbon fibres when coated with PEI only showed an increase of interfacial shear strength (IFSS) from 43.4 MPa to 49.4 MPa but increased to 63.4 MPa when coated with both PEI & GO. Furthermore, PEI + GO coating reduced the damping area of CF by about 50% and the ILSS value of CF/PEEK composites increased by 12% from 92.5 MPa to 103.5 MPa. [14]

Different amount of graphene oxide (GO) contents incorporated with the polymer matrix material influence the performances of the CFRPs. The incorporation of 0.10 wt% GO into the epoxy resin enhanced the interlaminar shear strength (ILSS) to 96.14 MPa for laminates that are almost 8.05% higher as compared with the control composites. Thus the introduction of GO has improved the resin toughness and adhesion between CF and epoxy resin. Furthermore, the glass transition temperature has also been increased (5ºC) with the addition of GO. [15]

Modified epoxy resin by hydrazine reduced graphene oxide (rGO) showed remarkable performance enhancement of CF/epoxy composites. Different amount of rGO was tried but 0.2 wt% loading was found to be best. At 0.2 wt% level, ILSS, impact strength, and critical stress intensity factor (KIC) were enhanced by∼(84, 100, and 33) %, respectively. rGO has been found to be compatibly dispersed in the epoxy matrix and showed better fracture toughness properties at 0.2wt% loading as compared with 0.4wt% loading. Thus rGO can be used to arrest matrix fracture and fibre fracture as well. [16]

Modified carbon fibre composites with GO coatings have improved the fibre-resin adhesion and thus upgraded interfacial properties such as surface roughness, surface area and surface energies. ILSS properties have been improved significantly by 47% with GOs coated CFs, 44% with thermally reduced graphene oxide (TrGOs)/CFs and 41% with reduced exfoliated graphene oxide (rEGOs)/CFs. They also improved the Flexural and Mode I fracture properties of the modified composites. In addition, electrical conductivity has been found to be enhanced by 127% in the through the thickness direction for TrGOs-CFs. [17]

Direct solid-state pyrolytic conversion of sodium carboxylates (i.e., sodium gluconate and sodium citrate) in the presence of Na2CO3 yields (almost 51.4%) monolayer graphene in gram-scale (7.37 g) that is comparable to that of the CVD process. This direct solid-state pyrolytic conversion strategy overcomes the drawbacks of traditional exfoliation and chemical vapour deposition methods successfully. [18]

Graphene sheets (GSs) could be grown vertically on the CNFs carbonized in NH3 and thus prepared the 3DGFs in a thermal CVD. These 3DGFs (3D Graphene fibres) possess a unique structure with fibrous shape, nanoscale pores, exposed single-layered graphene edges and high performances in multiple aspects as compared with the existing graphene materials. This new strategy can be widely used to grow the vertical GSs on many other substrates by pre-depositing a polymer over layer and then carbonizing in NH3 using the thermal CVD method. [19]

Small amount of 2D graphene can be added with the matrix materials and it is challenging to get well-dispersed CVD-grown graphene into polymer matrices at high graphene contents with preferential orientations. With the growing advancement in material science, a simple technique was developed to significantly improve the maximum amount of CVD-grown graphene allowed in the epoxy matrix with proper dispersion. Simultaneous achievements of high-quality graphene, uniform dispersion, seamless interconnection network, and high graphene contents with preferential orientations has been demonstrated. Consequently, exceptional electrical conductivity (50 S cm-1) and thermal conductivity (8.8 Wm-1K-1) could be achieved. Besides, the high filler loading of 8.3 wt% also gives rise to the remarkable fracture toughness of 2.18 MPa √m, well over 100 % enhancement over the neat epoxy. [20]

Vertical graphene can be directly grown on the carbon fibres using plasma-enhanced CVD method at comparatively low temperature (400℃). The height of vertical graphene can be controlled via plasma density and further plasma density can be controlled by adjusting the distance between the substrate and plasma centre. Growing graphene does not deteriorate the mechanical strength of the carbon fibres rather improves the interfacial shear strength between the carbon fibre and the epoxy, with the maximum increase in IFSS of ~118.7% at a vertical graphene height of ~4.2 μm. [21]

A new method of in situ growth of a graphene-related structure on the surface of carbon fibres has been experimented that showed desizing is essential for the growth of the graphene-related layer on the surface of the carbon fibres. In addition, the tensile property of the carbon fibre have not been affected too much after size removal from the carbon fibre, which have reduced 7 and 5% respectively. Though graphene-related layer could be coated more characterisation methods need to be applied to characterise the bonding between the generated coatings and the carbon fibre. More importantly, properties of modified carbon fibre reinforced composites should be justified. [22]


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