Burcu Saner Okan

Nano-engineered design and manufacturing of high-performance epoxy matrix composites with carbon fiber/selectively integrated graphene as multi-scale reinforcements

Three different architectural designs are developed for manufacturing advanced multi-scale reinforced epoxy based composites in which graphene sheets and carbon fibers are utilized as nano- and micro-scale reinforcements, respectively. In the first design, electrospraying technique as an efficient and up-scaleable method is employed for the selective deposition of graphene sheets onto the surface of carbon fabric mats. Controlled and uniform dispersion of graphene sheets on the surface of carbon fabric mats enhances the interfacial strength between the epoxy matrix and carbon fibers and increases the efficiency of load transfer between matrix and reinforcing fibers. In the second design, graphene sheets are directly dispersed into the hardener-epoxy mixture to produce carbon fiber/epoxy composites with graphene reinforced matrix. In the third design, the combination of the first and the second arrangements is employed to obtain a multi-scale hybrid composite with superior mechanical properties. The effect of graphene sheets as an interface modifier and as a matrix reinforcement as well as the synergetic effect due to the combination of both arrangements are investigated in details by conducting various physical–chemical characterization techniques. Graphene/carbon fiber/epoxy composites in all three different arrangements of graphene sheets show enhancement in in-plane and out of plane mechanical performances. In the hybrid composite structure in which graphene sheets are used as both interface modifier and matrix reinforcing agent, remarkable improvements are observed in the work of fracture by about 55% and the flexural strength by about 51% as well as notable enhancement on other mechanical properties.

Design and fabrication of multi-walled hollow nanofibers by triaxial electrospinning as reinforcing agents in nanocomposites

Multi-walled triaxial hollow fibers with two different outer wall materials are fabricated by core-sheath electrospinning process and integrated into epoxy matrix with or without primary glass fiber reinforcement to produce composites with enhanced mechanical properties. The morphologies of multi-walled hollow fibers are tailored by controlling the materials and processing parameters such as polymer and solvent types. The triaxial hollow fiber fabrication is achieved through using a nozzle containing concentric tubes, which allows for the transport of different fluids to the tip of the nozzle under the applied high voltage. In comparison to uniaxial electrospun fibers, the hollowness of electrospun fibers enables one to manufacture new reinforcing agents that can improve the specific strength of composites. It is shown that the mechanical properties of epoxy matrix composite incorporated with electrospun fibers as primary fiber reinforcement can be significantly tailored by properly selecting the wall materials, diameters, and the amount of electrospun fibers. We have also presented that triaxial electrospun hollow fibers as co-reinforcement in the glass fiber-laminated epoxy matrix composites enhance the flexural modulus by 6.5%, flexural strength by 14%, the onset of first layer of glass fabric failure strain by 12.5%, and final failure strain by 20%.

Design and fabrication of hollow and filled graphene-based polymeric spheres via core–shell electrospraying

Two dimensional graphene oxide sheets are converted into three dimensional (3D) hollow and filled microspheres by using three different carrying polymers through one-step core–shell electrospraying technique without applying any post treatments. Electrospraying process prevents the aggregations and crumbling of graphene sheets by constructing 3D interconnected framework, and provides homogeneous dispersion of graphene sheets in polymer solution under electric field, and allows the polymer chains to crawl into graphene layers forming intercalated structure. The proper polymer concentration and solution viscosity are determined by using Mark–Houwink–Sakurada equation to produce an ideal graphene based polymeric sphere structure via electrospraying. Graphene based polymeric spheres with controllable hollowness are successfully fabricated by changing core solvents. The connectivity of graphene sheets in polymeric shell is improved by increasing carbon networks after carbonization process. Morphology, shrinkage behaviour and structural properties of spheres are evaluated by tailoring polymer type, polymer concentration, graphene amount, flow rate and applied voltage.

An Improved Technique for the Exfoliation of Graphene Nanosheets and Utilization of their Nanocomposites as Fuel Cell Electrodes

Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional 2D honeycomb lattice. The graphene sheets in graphite interact with each other through van der Waals forces to form layered structure. The first graphene sheets were obtained by extracting monolayer sheets from the three-dimensional graphite using a technique called micromechanical cleavage in 2004 [. There are numerous attempts in the literature to produce monolayer graphene sheets by the treatment of graphite. The first work was conducted by Brodie in 1859 and GO was prepared by repeated treatment of Ceylon graphite with an oxidation mixture consisting of potassium chlorate and fuming nitric acid [. Then, in 1898, Staudenmaier produced graphite oxide (GO) by the oxidation of graphite in concentrated sulfuric acid and nitric acid with potassium chlorate [. However, this method was time consuming and hazardous. Hummers and Offeman found a rapid and safer method for the preparation of GO and in this method graphite was oxidized in water free mixture of sulfuric acid, sodium nitrate and potassium permanganate [.

Size and Dispersion Control of Pt Nanoparticles Grown Upon Graphite-Derived Nanosheets

Graphite oxide (GO) nanosheets, graphene nanosheets (GNS), and nanocomposites comprising of GO or GNS coated with polypyrrole (PPy) were prepared and assessed for their ability to influence the surface deposition and growth of Pt nanoparticles. GO was obtained from graphite via oxidation and exfoliation, and GNS was obtained from GO in a subsequent reduction. Both GO and GNS were coated with PPy via in situ polymerization of pyrrole (Py), forming surface-enhanced materials. Scanning electron microscope, energy-dispersive x-ray, transmission electron microscopy, electron energy loss spectroscopy, Raman, and atomic force microscope findings showed that the Pt nanoparticle loading, agglomeration size, aggregate morphology, and surface dispersion varied according to the nanosheet surface, nanocomposite type, and Py/nanosheet feed ratio. Surface oxygen functionalization along GO, GNS, and their nanocomposites influenced the loading, dispersivity, and morphology of nanoparticle agglomerates. PPy/GO nanocomposites yielded an improved nanoagglomerate surface dispersion and loading compared to samples. The PPy-coated substrates offered a greater intrinsic propensity for redox processes, resulting in higher Pt loadings. Additionally, these nanocomposites provided more surface reduction sites compared to bare nanosheets, and the additional sites contributed toward forming smaller, more homogeneously dispersed Pt nanoparticle agglomerates. Bringing together the electrical properties of PPy and physicomechanical traits of carbon nanosheets, it follows to reason that the nanocomposites produced, particularly GO-based nanocomposites, offer promise as a nanoparticle support material for use in catalysis, electrocatalysis, and hydrogen storage.