2. Experimental methods
A method of fabrication of graphene-carbon nanotube stacks includes the
steps of depositing a first graphene layer on a metal foil, transferring
the first graphene layer to a current collector, depositing a first
layer of a catalytic metal on the first graphene layer, alternately
depositing graphene and catalytic metal layers one upon the other so as
to form a stack of alternating graphene and catalytic metal layers on
the first graphene and catalytic metal layers, transforming the
catalytic metal layers into arrays of metal nanoparticles by thermal
breakdown of the catalytic metal layers, and precipitating carbon
nanotube outward from the metal nanoparticles. The carbon nanotubes are
precipitated in a single execution of the precipitating carbon nanotube
outward from the metal nanoparticles step, resulting in simultaneous
growth of the carbon nanotubes and expansion of the graphene-carbon
nanotube stack. The catalytic metal is a transition metal, for example,
nickel. The graphene layers are formed by a chemical vapor deposition
process. The carbon nanotubes are formed by a chemical vapor deposition
process. The catalytic metal layers are formed by a physical vapor
deposition process.
The carbon nanotubes may be any length, diameter, or chirality as
produced by any of the various production methods [45, 46]. The
chirality of the carbon nanotubes is such that the carbon nanotubes are
metallic, semi-metallic, semiconducting or combinations thereof [47,
48]. Carbon nanotubes may include, but are not limited to,
single-walled carbon nanotubes, double-walled carbon nanotubes,
multi-walled carbon nanotubes, shortened carbon nanotubes, oxidized
carbon nanotubes, functionalized carbon nanotubes, purified carbon
nanotubes, metalized carbon nanotubes and combinations thereof. The
carbon nanotubes may be pristine or functionalized. Functionalized
carbon nanotubes, as used herein, refer to any of the carbon nanotubes
types bearing chemical modification, physical modification or
combination thereof. Such modifications can involve the carbon nanotube
ends, sidewalls, or both. Illustrative chemical modifications of carbon
nanotubes include, for example, covalent bonding and ionic bonding.
Illustrative physical modifications include, for example, chemisorption,
intercalation, surfactant interactions, polymer wrapping, solvation, and
combinations thereof. Unfunctionalized carbon nanotubes are typically
isolated as aggregates referred to as ropes or bundles, which are held
together through van der Waals forces. The carbon nanotube aggregates
are not easily dispersed or solubilized. Chemical modifications,
physical modifications, or both can provide individualized carbon
nanotubes through disruption of the van der Waals forces between the
carbon nanotubes. As a result of disrupting van der Waals forces,
individualized carbon nanotubes may be dispersed or solubilized.
Unfunctionalized carbon nanotubes may be used as-prepared from any of
the various production methods, or they may be further purified.
Purification of carbon nanotubes typically refers to, for example,
removal of metallic impurities, removal of non-nanotube carbonaceous
impurities, or both from the carbon nanotubes. Illustrative carbon
nanotube purification methods include, for example, oxidation using
oxidizing acids, oxidation by heating in air, filtration and
chromatographic separation. Oxidative purification methods remove
non-nanotube carbonaceous impurities in the form of carbon dioxide.
Oxidative purification of carbon nanotubes using oxidizing acids further
results in the formation of oxidized, functionalized carbon nanotubes,
wherein the closed ends of the carbon nanotube structure are oxidatively
opened and terminated with a plurality of carboxylic acid groups.
Oxidative purification methods using an oxidizing acid further result in
removal of metallic impurities in a solution phase. Depending on the
length of time oxidative purification using oxidizing acids is
performed, further reaction of the oxidized, functionalized carbon
nanotubes results in shortening of the carbon nanotubes, which are again
terminated on their open ends by a plurality of carboxylic acid groups.
The carboxylic acid groups in both oxidized, functionalized carbon
nanotubes and shortened carbon nanotubes may be further reacted to form
other types of functionalized carbon nanotubes. For example, the
carboxylic acids groups may be reacted to form esters or amides, or they
may be reacted in condensation polymerization reactions to form polymers
having the carbon nanotubes bound to the polymer chains. Condensation
polymers include, for example, polyesters and polyamides.
Functionalized graphene-carbon nanotubes may also be incorporated into
polymers using standard polymerization techniques [49, 50]. The
functionalized graphene-carbon nanotubes may be dispersed in the polymer
and not covalently bound to the polymer chains [51, 52].
Alternately, the functionalized graphene-carbon nanotubes may be
dispersed in the polymer and covalently bound to the polymer chains. For
example, amino-functionalized graphene-carbon nanotubes may react with
epoxy resins through their amino groups. Amino-functionalized
graphene-carbon nanotubes are formed by peroxide-mediated introduction
of carboxylic acid groups on sidewalls of pristine graphene-carbon
nanotubes, followed by amide-functionalization using a diamine.
Similarly, fluorinated graphene-carbon nanotubes may react with amino
groups of epoxies curing agents to displace fluorine and form a
cross-linked epoxy polymer covalently bound to the graphene-carbon
nanotubes. Fluorinated graphene-carbon nanotubes are prepared by direct
sidewall fluorination of graphene-carbon nanotubes using elemental
fluorine. The particular type of functionalized graphene-carbon
nanotubes utilized in the various cases herein may be varied across a
wide range of functionality. For example, desired solubility or
reactivity properties of the functionalized graphene-carbon nanotubes
will dictate the choice of functionalized graphene-carbon nanotube type
utilized in the various cases herein. The process comprises the steps:
providing a porous mat comprising graphene-carbon nanotubes having an
average longest dimension in the range of 2 micron to 2000 microns,
wherein at least a portion of the graphene-carbon nanotubes are
entangled; contacting the mat with one or more condensation polymer
precursors, and optionally a catalyst; polymerizing the one or more
polymer precursors in the presence of the mat at a temperature in the
range of about 180 °C to about 360 °C to form a nonporous
fiber-reinforced polymer composite comprising a mat of graphene-carbon
nanotubes embedded in a condensation polymer produced from the polymer
precursors, wherein the graphene-carbon nanotubes are present in the
composite in an amount ranging from about 0.08 weight percent to about
80 weight percent, based on the weight of the graphene-carbon nanotubes
and the condensation polymer; and curing the graphene-carbon nanotube
fiber-reinforced polymer composite.
The graphene-carbon nanotube fiber-reinforced polymer composites utilize
nanotechnology enhancements to provide advantageous durability and
structural stability improvements over conventional fiber-reinforced
polymer composites not containing graphene or carbon nanotubes. In
particular, the graphene-carbon nanotube fiber-reinforced polymer
composites provide increased resistance to tension-tension and
tension-compression fatigue failure compared to conventional
fiber-reinforced polymer composites. Inclusion of graphene-carbon
nanotubes at the fiber-matrix interface in graphene-carbon nanotube
fiber-reinforced polymer composites provides advantageous resistance to
polymer matrix cracking, longitudinal cracking along the fiber-matrix
interface, and fiber delamination, all of which are dominant failure
mechanisms in conventional fiber-reinforced polymer composites. Thus,
the graphene-carbon nanotube fiber-reinforced polymer composites provide
a nanotechnology solution to mitigating the evolution of failure
mechanisms and extending failure lifetimes under fatigue loading. The
graphene-carbon nanotube fiber-reinforced polymer composites include a
fiber component, a polymer matrix component, and a quantity of
graphene-carbon nanotubes. The polymer matrix component and the fiber
component form a fiber-matrix interface. The quantity of graphene-carbon
nanotubes coats at least a portion of the fiber component. The
fiber-matrix interface further includes the portion of graphene-carbon
nanotubes.
Normal fatigue crack progression is suppressed at the fiber-matrix
interface where graphene-carbon nanotubes are present. Since fatigue
crack progression leads to fiber-matrix longitudinal delamination, the
graphene-carbon nanotubes enhance fatigue lifetime under both
quasi-static and cyclical fatigue loading conditions. Controlled
laboratory testing conditions are used to evaluate the benefits of
graphene-carbon nanotube fiber-reinforced polymer composites over
conventional fiber-reinforced polymer composites not containing graphene
or carbon nanotubes coating the fiber component. As an initial test of
the graphene-carbon nanotube fiber-reinforced polymer composites, the
tensile strength and tensile stiffness of graphene-carbon nanotube
fiber-reinforced polymer composites and fiber-reinforced polymer
composites are evaluated and compared. Testing is conducted by ASTM
testing methods ASTM D3039 and ASTM D3039M-17. Graphene-carbon nanotube
fiber-reinforced polymer composites utilized in the tensile strength and
tensile stiffness studies contain about 0.2 to about 0.8 weight percent
graphene-carbon nanotubes coating the carbon fibers. Both tensile
stiffness and tensile strength are improved in the graphene-carbon
nanotube fiber-reinforced polymer composites, particularly at higher
weight percentages of graphene-carbon nanotubes. The improvement for
both mechanical properties vary depending on the quantity of
graphene-carbon nanotubes used to coat the carbon fibers.