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.