Figure 4. Effect of the weight fraction of the graphene-carbon nanotube hybrid material on the hardness for the fiber-reinforced polymer composite.
The low-resolution scanning electron micrographs of the graphene-carbon nanotube hybrid material are illustrated in Figure 5 for the production of fiber-reinforced polymer composites. Graphene is the term for a modification of carbon having a two-dimensional structure in which each carbon atom is surrounded by three further carbon atoms so as to form a honeycomb-like pattern. In this respect, graphene may be regarded as a single graphite layer. However, the term graphene also includes thin stacks of single graphite layers which owing to their small thickness have physical properties which differ substantially from those of graphite bulk material. Each graphene platelet has a length and a width parallel to the graphite plane and a thickness perpendicular to the graphite plane. The largest dimension is here referred to as the length, the smallest dimension as the thickness and the last dimension as the width. The carbon nanotubes and the graphene platelets are advantageously dispersed separately or together in an aqueous medium and the dispersions obtained are subsequently combined. The dispersing step can be carried out with the aid of ultrasound and jet dispersers. Material property refers to the response of a material to an external stimulus [61, 62]. Non-limiting examples of material properties include mechanical properties, electrical properties, magnetic properties, thermal properties, chemical properties, and acoustical properties. Mechanical properties refer to the response of a material to an applied load or force [63, 64]. Non-limiting examples of mechanical properties include Young’s modulus, specific modulus, strength, for example, tensile, compressive, shear, yield, bearing, and creep, ductility, Poisson’s ratio, hardness, impact toughness, resilience, fatigue limit, and fracture toughness. Thermal properties refer to a material’s response to applied heat. Non-limiting examples include thermal conductivity, thermal diffusivity, coefficient of thermal expansion, emissivity, specific heat, melting point, glass transition temperature, boiling point, flash point, triple point, heat of vaporization, heat of fusion, pyrophoricity, autoignition temperature, and vapor pressure. Electrical properties refer to the response of a material to an applied electric or electromagnetic field. Non-limiting examples include electrical conductivity, electrical resistivity, permittivity, dielectric constant, dielectric strength, and piezoelectric constant. Composite or composite material refer to a material composed of two or more materials, where each material possesses a distinct phase at a length scale of interest and a distinct interface is present between each of the two or more materials [65, 66]. Reinforced composite refers to a composite including at least two phases, a matrix phase that is continuous and that surrounds at least a portion of a dispersed phase [67, 68]. The composite is formed from a free mixture of graphene, carbon nanotube, and porous carbon. The graphene self-aligns in a plurality of sheets approximately parallel to a substrate upon which the mixture is deposited, while at least a portion of the carbon nanotubes are aligned at a defined angle to the graphene sheets. The carbon nanotubes exhibit no preferred orientation and are approximately random. Concurrently, the plurality of graphene sheets is oriented approximately horizontally, that is approximately parallel to the substrate surface. Depositing a layer of graphene over the cleaned layer of carbon nanotube film to form a carbon nanotube-graphene hybrid film includes transferring chemical vapor deposition grown graphene using several known transfer processes, such as polymer assisted transfer. The graphene films can also be directly obtained from bulk graphite through a scotch tape transfer process. Graphene can also be deposited through solution in the form of dissolved graphene oxide. This can be accomplished through spraying the solution or spinning graphene oxide flakes suspending in a solvent over the substrate containing carbon nanotubes, and graphene oxide flakes can later be reduced to graphene through gas or solution phase reducing treatments. The polymer interacts with solvents. The combination of the Van der Waals inhibition and polymer-solvent interaction causes the wrapped carbon nanotubes to be much more readily suspended at high concentrations in solvents. This enables creation of high-concentration carbon nanotube solutions and suspensions, which in turn substantially enables manipulation of carbon nanotubes into the bulk material. The novel electrical properties are isotropic in compositions where the carbon nanotubes are essentially randomly oriented with one another, such as in an electrically-insulating matrix.