109 research outputs found
Production and mechanical characterization of graphene micro-ribbons
Patterning of graphene into micro- and nano-ribbons allows for the tunability
in emerging fields such as flexible electronic and optoelectronic devices, and
is gaining interest for the production of more efficient reinforcement for
composite materials. In this work we fabricate micro-ribbons from CVD graphene
by combining UV photolithography and dry etching oxygen plasma treatments.
Raman spectral imaging confirms the effectiveness of the patterning procedure,
which is suitable for large-area patterning of graphene on wafer-scale, and
confirms that the quality of graphene remains unaltered. The produced
micro-ribbons were finally transferred and embedded into a polymeric matrix and
the mechanical response was investigated by in-situ mechanical investigation
combining Raman spectroscopy and tensile/compressive tests
Failure Processes in Embedded Monolayer Graphene under Axial Compression
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.Comment: 28 pages. Manuscript 20 pages, 8 figures. Supporting information 10
pages, 6 figure
Interfacial Stress Transfer in a Graphene Monolayer Nanocomposite
Graphene is one of the stiffest known materials, with a Young's modulus of 1
TPa, making it an ideal candidate for use as a reinforcement in
high-performance composites. However, being a one-atom thick crystalline
material, graphene poses several fundamental questions: (1) can decades of
research on carbon-based composites be applied to such an ultimately-thin
crystalline material? (2) is continuum mechanics used traditionally with
composites still valid at the atomic level? (3) how does the matrix interact
with the graphene crystals and what kind of theoretical description is
appropriate? We have demonstrated unambiguously that stress transfer takes
place from the polymer matrix to monolayer graphene, showing that the graphene
acts as a reinforcing phase. We have also modeled the behavior using shear-lag
theory, showing that graphene monolayer nanocomposites can be analyzed using
continuum mechanics. Additionally, we have been able to monitor stress transfer
efficiency and breakdown of the graphene/polymer interface
Failure Processes in Embedded Monolayer Graphene under Axial Compression
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 up to -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
Compression Behavior of Single-layer Graphene
Central to most applications involving monolayer graphene is its mechanical
response under various stress states. To date most of the work reported is of
theoretical nature and refers to tension and compression loading of model
graphene. Most of the experimental work is indeed limited to bending of single
flakes in air and the stretching of flakes up to typically ~1% using plastic
substrates. Recently we have shown that by employing a cantilever beam we can
subject single graphene into various degrees of axial compression. Here we
extend this work much further by measuring in detail both stress uptake and
compression buckling strain in single flakes of different geometries. In all
cases the mechanical response is monitored by simultaneous Raman measurements
through the shift of either the G or 2D phonons of graphene. In spite of the
infinitely small thickness of the monolayers, the results show that graphene
embedded in plastic beams exhibit remarkable compression buckling strains. For
large length (l)-to-width (w) ratios (> 0.2) the buckling strain is of the
order of -0.5% to -0.6%. However, for l/w <0.2 no failure is observed for
strains even higher than -1%. Calculations based on classical Euler analysis
show that the buckling strain enhancement provided by the polymer lateral
support is more than six orders of magnitude compared to suspended graphene in
air
Strain-engineered graphene grown on hexagonal boron nitride by molecular beam epitaxy
Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene
Graphene and related materials in hierarchical fiber composites: Production techniques and key industrial benefits
Fiber-reinforced composites (FRC) are nowadays one of the most widely used class of high-tech materials. In particular, sporting goods, cars and the wings and fuselages of airplanes are made of carbon fiber reinforced composites (CFRC). CFRC are mature commercial products, but are still challenging materials. Their mechanical and electrical properties are very good along the fiber axis, but can be very poor perpendicular to it; interfacial interactions have to be tailored for specific applications to avoid crack propagation– and delamination; fiber production includes high-temperature treatments of adverse environmental impact, leading to high costs. Recent research work shows that the performance of CFRC can be improved by addition of graphene or related 2-dimensional materials (GRM). Graphene is a promising additive for CFRC because: 1) Its all-carbon aromatic structure is similar to the one of carbon fiber (CF). 2) Its 2-dimensional shape, high aspect ratio, high flexibility and mechanical strength allow it to be used as a coating on the surface of fiber, or as a mechanical/electrical connection between different fiber layers. 3) Its tunable surface chemistry allows its interaction to be enhanced with either the fiber or the polymer matrix used in the composite and 4) in contrast to carbon fibers or nanotubes, it is easily produced on a large scale at room temperature, without metal catalysts. Here, we summarize the key strategic advantages that could be obtained in this way, and some of the recent results that have been obtained in this field within the Graphene Flagship project and worldwide
Nanostructured Heteroarm Star Block Terpolymers via an Extension of the "In-Out'' Polymerization Route
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