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.