Papers by Costas Galiotis
Nanoscale, 2015
A new methodology is described here for the true uniaxial deformation of suspended monolayer grap... more A new methodology is described here for the true uniaxial deformation of suspended monolayer graphene.
Membranes
Successful ways of fully exploiting the excellent structural and multifunctional performance of g... more Successful ways of fully exploiting the excellent structural and multifunctional performance of graphene and related materials are of great scientific and technological interest. New opportunities are provided by the fabrication of a novel class of nanocomposites with a nanolaminate architecture. In this work, by using the iterative lift-off/float-on process combined with wet depositions, we incorporated cm-size graphene monolayers produced via Chemical Vapour Deposition into a poly (methyl methacrylate) (PMMA) matrix with a controlled, alternate-layered structure. The produced nanolaminate shows a significant improvement in mechanical properties, with enhanced stiffness, strength and toughness, with the addition of only 0.06 vol% of graphene. Furthermore, oxygen and carbon dioxide permeability measurements performed at different relative humidity levels, reveal that the addition of graphene leads to significant reduction of permeability, compared to neat PMMA. Overall, we demonstra...
Sensors, 2021
Recent findings have brought forward the potential of carbon nano-species, especially nanotubes a... more Recent findings have brought forward the potential of carbon nano-species, especially nanotubes and graphene, to impart exceptional multifunctional potential to cement, offering simultaneous enhancement of mechanical, fracture mechanical and electrical properties. While available knowledge on the topic is still limited, there is a complete absence of direct comparisons of the potential of the nano-species to improve strength and toughness and provide multifunctionality to the mortars. The study offers a comprehensive overview of these potentials, for mortars modified with pure graphene nanoplatelets and carbon nanotubes at consistent, directly comparable, concentrations up to 1.2 wt.%. Testing included flexure under pure bending moments, axial compression, electrical resistivity measurements and fracture tests under three point bending configuration; the latter were also independently assessed by acoustic emission. Differences in documented properties and optimal concentrations asso...
Polymers, 2021
In this study, Polyurea/Formaldehyde (PUF) microcapsules containing Dicyclopentadiene (DCPD) as a... more In this study, Polyurea/Formaldehyde (PUF) microcapsules containing Dicyclopentadiene (DCPD) as a healing substance were fabricated in situ and mixed at relatively low concentrations (<2 wt%) with a thermosetting polyurethane (PU) foam used in turn as the core of a sandwich structure. The shape memory (SM) effect depended on the combination of the behavior of the PU foam core and the shape memory polymer composite (SMPC) laminate skins. SMPC laminates were manufactured by moulding commercial carbon fiber-reinforced (CFR) prepregs with a SM polymer interlayer. At first, PU foam samples, with and without microcapsules, were mechanically tested. After, PU foam was inserted into the SMPC sandwich structure. Damage tests were carried out by compression and bending to deform and break the PU foam cells, and then assess the structure self-healing (SH) and recovery capabilities. Both SM and SH responses were rapid and thermally activated (120 °C). The CFR-SMPC skins and the PU foam core ...
Nanoscale Advances, 2019
In the present study, the stress transfer mechanism in graphene–polymer systems under tension is ... more In the present study, the stress transfer mechanism in graphene–polymer systems under tension is examined experimentally using the technique of laser Raman microscopy.
Advanced Composites Letters, 2010
The dielectric response of oxidized multi-walled carbon nanotube / epoxy resin composites, is inv... more The dielectric response of oxidized multi-walled carbon nanotube / epoxy resin composites, is investigated with respect to filler content concentration, over a wide temperature and frequency range. Specimens, below the percolation threshold, exhibit similar behaviour to that of the neat epoxy. Two relaxation modes are observed in the low temperature region, attributed to the re-arrangement of small parts of the polymer chain (γ-mode) and the reorientation of polar side groups (β-mode) respectively, where in the high temperature region the evolution of the α-mode is present. Direct current (DC) conduction follows the Vogel – Tamann - Fulcher equation as expected. The dielectric response of specimens, above the percolation threshold, follows the “Random Free Energy Barrier Model”. DC conductivity exhibits Arrhenius temperature dependence with two distinct regions. The activation energies of both regions were evaluated. The contact resistance between two adjacent carbon nanotubes was a...
Advanced Composites Letters, 2009
The current work reports on the tensile and fatigue behaviour of an autoclave-cured unidirectiona... more The current work reports on the tensile and fatigue behaviour of an autoclave-cured unidirectional carbon fibre reinforced epoxy-matrix laminate. The macromechanical properties of the composite are established under interrupted and uninterrupted, static and dynamic conditions at a maximum strain below the critical fatigue limit of the matrix material. The established S-N curve was used to calculate the endurance fatigue limit of the laminate as well as to record an increase in fatigue life of specimens tested at load levels lower than two standard deviations below the mean static strength of the material. The failure modes exhibited by the composite under all testing conditions were recorded and interpreted by means of the damage mechanisms that originate them. The results of interrupted testing showed that the combined effect of fatigue and residence at high stress levels for prolonged periods of time - conditions that simulate a realistic loading scheme- trigger premature fibre fa...
Nanotechnology, 2018
The mechanical integrity of composite materials depends primarily on the interface strength and t... more The mechanical integrity of composite materials depends primarily on the interface strength and the defect density of the reinforcement which is the provider of enhanced strength and stiffness. In the case of graphene/ polymer nanocomposites which are characterized by an extremely large interface region, any defects in the inclusion (such as folds, cracks, holes etc.) will have a detrimental effect to the internal strain distribution and the resulting mechanical performance. This conventional wisdom, however, can be challenged if the defect size is reduced beyond the critical size for crack formation to the level of atomic vacancies. In that case, there should be no practical effect on crack propagation and depending on the nature of the vacancies the interface strength may be in fact increase. In this work we employed argon ion (Ar +) bombardment and subsequent exposure to hydrogen (H2) to induce (as revealed by X-ray & Ultraviolet photoelectron spectroscopy (XPS/UPS) and Raman spectroscopy) passivated atomic single vacancies to CVD graphene. The modified graphene was subsequently transferred to PMMA bars and the morphology, wettability and the interface adhesion of the CVD graphene/PMMA system were investigated with Atomic Force Microscopy technique and Raman analysis. The results obtained showed clearly an overall improved mechanical behavior of graphene/polymer interface, since an increase as well a more uniform shift distribution with strain is observed. This paves the way for interface engineering in graphene/polymer systems which, in pristine condition, suffer from premature graphene slippage and subsequent failure.
ACS Applied Materials & Interfaces, 2017
Multilayered graphitic materials are not suitable as load-bearers due to their inherent weak inte... more Multilayered graphitic materials are not suitable as load-bearers due to their inherent weak interlayer bonding (for example, graphite is a solid lubricant in certain applications). This situation is largely improved when two-dimensional (2-D) materials such as a monolayer (SLG) graphene are employed. The downside in these cases is the presence of thermally or mechanically induced wrinkles which are ubiquitous in 2-D materials. Here we set out to examine the effect of extensive large wavelength/ amplitude wrinkling on the stress transfer capabilities of exfoliated simply-supported graphene flakes. Contrary to common belief we present clear evidence that this type of "corrugation" enhances the load bearing capacity of few-layer graphene as compared to 'flat' specimens. This effect is the result of the significant increase of the graphene/polymer interfacial shear stress per increment of applied strain due to wrinkling and paves the way for designing affordable graphene composites with highly improved stress-transfer efficiency.
ACS applied materials & interfaces, Jan 5, 2016
The mechanical behaviour of a prototype touch panel display which consists of two layers of CVD g... more The mechanical behaviour of a prototype touch panel display which consists of two layers of CVD graphene embedded into PET films is investigated in tension and under contact-stress dynamic loading. In both cases, laser Raman spectroscopy was employed to assess the stress transfer efficiency of the embedded graphene layers. The tensile behaviour was found to be governed by the "island-like" microstructure of the CVD graphene and the stress transfer efficiency was dependent on the size of graphene "islands" but also on the yielding behaviour of PET at relatively high strains. Finally, the fatigue tests, which simulate real operation conditions, showed that the maximum temperature gradient developed at the point of "finger" contact after 80000 cycles does not exceed the glass transition temperature of the PET matrix. The effect of these results on future product development and the design of new graphene-based displays is discussed.
Scientific Reports, 2015
Thin membranes, such as monolayer graphene of monoatomic thickness, are bound to exhibit lateral ... more Thin membranes, such as monolayer graphene of monoatomic thickness, are bound to exhibit lateral buckling under uniaxial tensile loading that impairs its mechanical behaviour. In this work, we have developed an experimental device to subject 2D materials to controlled equibiaxial strain on supported beams that can be flexed up or down to subject the material to either compression or tension, respectively. Using strain gauges in tandem with Raman spectroscopy measurements, we monitor the G and 2D phonon properties of graphene under biaxial strain and thus extract important information about the uptake of stress under these conditions. The experimental shift over strain for the G and 2D Raman peaks were found to be in the range of 62.3 ± 5 cm–1/% and 148.2 ± 6 cm–1/%, respectively, for monolayer but also bilayer graphenes. The corresponding Grüneisen parameters for the G and 2D peaks were found to be between 1.97 ± 0.15 and 2.86 ± 0.12, respectively. These values agree reasonably well...
ACS nano, Jan 20, 2015
The deformation of monolayer graphene, produced by chemical vapor deposition (CVD), on a polyeste... more The deformation of monolayer graphene, produced by chemical vapor deposition (CVD), on a polyester film substrate has been investigated through the use of Raman spectroscopy. It has been found that the microstructure of the CVD graphene consists of a hexagonal array of islands of flat monolayer graphene separated by wrinkled material. During deformation, it was found that the rate of shift of the Raman 2D band wavenumber per unit strain was less than 25% of that of flat flakes of mechanically exfoliated graphene, whereas the rate of band broadening per unit strain was about 75% of that of the exfoliated material. This unusual deformation behavior has been modeled in terms of mechanically isolated graphene islands separated by the graphene wrinkles, with the strain distribution in each graphene island determined using shear lag analysis. The effect of the size and position of the Raman laser beam spot has also been incorporated in the model. The predictions fit well with the behavior...
ACS Applied Materials & Interfaces, 2015
The stress transfer mechanism from a polymer substrate to a nanoinclusion, such as a graphene fla... more The stress transfer mechanism from a polymer substrate to a nanoinclusion, such as a graphene flake, is of extreme interest for the production of effective nanocomposites. Previous work conducted mainly at the micron scale has shown that the intrinsic mechanism of stress transfer is shear at the interface. However, since the interfacial shear takes its maximum value at the very edge of the nanoinclusion it is of extreme interest to assess the effect of edge integrity upon axial stress transfer at the submicron scale. Here, we conduct a detailed Raman line mapping near the edges of a monolayer graphene flake that is simply supported onto an epoxy-based photoresist (SU8)/poly(methyl methacrylate) matrix at steps as small as 100 nm. We show for the first time that the distribution of axial strain (stress) along the flake deviates somewhat from the classical shear-lag prediction for a region of ∼2 μm from the edge. This behavior is mainly attributed to the presence of residual stresses, unintentional doping, and/or edge effects (deviation from the equilibrium values of bond lengths and angles, as well as different edge chiralities). By considering a simple balance of shear-to-normal stresses at the interface we are able to directly convert the strain (stress) gradient to values of interfacial shear stress for all the applied tensile levels without assuming classical shear-lag behavior. For large flakes a maximum value of interfacial shear stress of 0.4 MPa is obtained prior to flake slipping.
Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial com... more Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of-0.60 % corresponding to a yield stress of-6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1-2 nm. These results were compared with DFT computations performed on analogue coronene/ PMMA oligomers and a reasonable agreement was obtained.
Nature Communications, 2011
Carbon fibres are a significant volume fraction of modern structural airframes. Embedded into pol... more Carbon fibres are a significant volume fraction of modern structural airframes. Embedded into polymer matrices, they provide significant strength and stiffness gains by unit weight compared with competing structural materials. Here we use the Raman G peak to assess the response of carbon fibres to the application of strain, with reference to the response of graphene itself. our data highlight the predominance of the in-plane graphene properties in all graphitic structures examined. A universal master plot relating the G peak strain sensitivity to tensile modulus of all types of carbon fibres, as well as graphene, is presented. We derive a universal value ofaverage-phonon shift rate with axial stress of around − 5ω 0 − 1 (cm − 1 mPa − 1), where ω 0 is the G peak position at zero stress for both graphene and carbon fibre with annular morphology. The use of this for stress measurements in a variety of applications is discussed.
Nanoscale carbon-based materials such as carbon nanotubes (CNTs) have raised much interest during... more Nanoscale carbon-based materials such as carbon nanotubes (CNTs) have raised much interest during the recent years due to their extraordinary mechanical, thermal and electrical properties [1]. Buckypapers are nano-porous, self-standing thin sheets of carbon nanotubes and are made by randomly distributed CNTs [2]. Their average thickness could be in the range of 30-200μm. The chemical modification of such CNTs pre-forms is required in order to couple effectively the unique CNT mechanical/electrical properties with those of the host polymer. Our work is focused on the fabrication of CNT-based thin sheets and their nanocomposites using automated aerospace processing. Four different modification strategies of multi wall CNTs (MWCNTs) were used for treating their graphitic surface, namely hydlochloric acid, nitric acid, ammonium hydroxide/hydrogen peroxide mixture and sulfuric acid/hydrogen peroxide mixture. CNT buckypapers were fabricated following a two step procedure; initially, the modified CNTs are dispersed in aqueous media under tip sonication and then the stable suspensions are filtrated through membrane filters and dried under vacuum (Fig. 1).
Small, 2009
(UK) [ ÃÃ ] CG would like to thank Prof. N. Melanitis (HNA, Greece) for useful discussions during... more (UK) [ ÃÃ ] CG would like to thank Prof. N. Melanitis (HNA, Greece) for useful discussions during the preparation of this manuscript. FORTH/ICE-HT acknowledge financial support from the Marie-Curie Transfer of Knowledge program CNTCOMP [Contract No.: MTKD-CT-2005-029876]. GT gratefully acknowledges FORTH/ICE-HT for a scholarship and ACF, KN, and AKG thank the Royal Society and the European Research Council for financial support.
Progress in Polymer Science, 2010
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit:
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Papers by Costas Galiotis