| 1. |
- Benavides, Vicente, et al.
(författare)
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Raman and electron microscopy study of C60 collapse/transformation to a nanoclustered graphene-based disordered carbon phase at high pressure/temperature
- 2015
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 252:11, s. 2626-2629
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Tidskriftsartikel (refereegranskat)abstract
- Transformation of C60 polymers to a superelastic hard carbon (nanoclustered graphene phase (NGP)) occurring in metal matrix at 5 GPa in a temperature interval of 1000–1100 K was studied by optical, scanning electron microscopy (SEM), and Raman spectroscopy. Raman spectral scan across the sample surface allowed us to identify different stages of the structural transformation. The SEM and Raman spectroscopy data testify for the NGP appearance at the defects concentration sites in the parent fullerite structure. We propose that the buckyballs collapse/formation of the NGP is governed by nucleation and growth (diffusive) mechanism unlike earlier discussed in the literature possibility of the martensitic-type (displacive) character of this transformation.
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| 2. |
- Benavides, Vicente, et al.
(författare)
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Raman spectroscopy and hardness study of C60 transformation into nanoclustered graphene phase at high pressure/high temperature
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, a study of the C60 – nanoclustered graphene phase (NGP) transformation via characterization of non-completely transformed carbon particles is presented. High-resolution (∽ 1 µm) Raman spectroscopy and nanoindentation measurements were performed on the same pre-selected sample area. The results evidence different steps of the transformation that allows establishing correspondence between the NGP/C60 ratio and the nanohardness: an abrupt increase in nanohardness from 2 GPa to 20 GPa for a stepwise NGP/C60 ratio change in the transformation zone. These results demonstrate that (I) at a micro-level (1 µm), the transformation of C60 into NGP does not occur simultaneously in the entire volume and (II) the residual C60 polymer is not desirable in superhard amorphous carbon materials. This work demonstrates importance of advanced experimental methodologies to characterization of disordered carbon phases.
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| 3. |
- Benavides, Vicente
(författare)
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Synthesis and characterization of nanocarbons as reinforced particles in metal composites
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work, several scientific problems related to high pressure–high temperature (HP–HT) synthesis of new materials using fullerite as a precursor were studied: first, the mechanism of the transformation of C60 crystal into a nano-clustered graphene phase (NGP) at a pressure of 8 GPa; and second, the effect of disorder introduced into C60 crystals by ball milling prior to HP–HT synthesis on the structure and properties of the NGP. A separate set of experiments was devoted to compression of C60 precursor at unexplored before pressure of 25 GPa and elevated temperatures in search for new type of disordered carbon-based materials.In the first study, Raman spectroscopy, HRSTEM-EELS, and indentation hardness demonstrate that, under pressure, C60 exhibits a path of transformation from polymerized C60 to NGP. This phase exhibits a short-range order and preferential orientation of nano-clusters of graphene assembled in a highly disordered carbon matrix. In our studies, we observe that the mechanism of C60 transformation into NGP could be understood in terms of nucleation and growth mechanism as opposed to the pseudomartensitic mechanism. Changes in Raman intensity of the Ag(2) C60 mode monitored in polished incompletely transformed carbon particles reveal different steps of transformation. Moreover, the polishing reveals the distribution of shear bands resulting from plastic deformation of the C60 monomer and following the direction of the <110> slip planes in FCC system.HRSTEM analysis reveals the presence of disorder as an intermediate state between the parent C60 and the nano-graphene units. EELS spectra show that C60 molecules in such state are present as monomers, and the intermediate phase is an sp2–sp3 disordered phase, in which the sp2 fraction is by up to 20% lower than that of graphene nanoclusters. The findings suggest that, after the collapse, the polymer structure breaks down with the formation of a disordered (sp2–sp3) carbon phase containing some fraction of residual C60 molecules. The graphene nanoclusters further nucleate and grow in the intermediate disordered phase. Thus, a nucleation and growth mechanism is proposed for the formation of NGP phase from C60 upon HP-HT action.For the second problem, highly disordered systems were obtained from ball-milled C60 through HP–HT demonstrating a promising technique to create hard (hardness > 30 GPa) disordered carbons at relatively low pressure (up to 8 GPa).The nanoarchitecture of NGP and disordered systems was studied using multi-wavelength Raman spectroscopy, HRSTEM, and indentation techniques. The Raman data treatment was carefully studied following the three-stage amorphization trajectory of amorphous carbon. The Raman model consists of G and D bands and data from semi-empirical models that include peak position, FWHM, and intensity ratio. A new approach proposed by the research team includes the presence of carbon pentagons (F band) and carbon heptagons as defects in the graphene clusters and are eventually present in the disordered carbon matrix as well. A peak deconvolution considering the G, D, F and heptagon bands is the model that allows building an empirical correlation between the Raman spectra features and hardness. Using peak deconvolution model based on G, D, F heptagon and sp3 carbon-derived bands allowed us to build an empirical correlation that can be used for a semi-quantitative estimation/prediction of hardness of an arbitrary disordered sp2 carbon-based system based on their spectroscopic (Raman) data.Finally, experiments on compressed C60 at 25 GPa, previously unexplored pressure, produce superhard 3D-C60 polymers at temperatures below 600 oC. As the temperature increases, sp3 carbon starts dominating the disordered structures. The synthesized materials are semiconductors exhibiting ultra-high hardness that in a particular case exceeds that of single crystalline diamond. UV-Raman spectroscopy reveals a high intensity of T band and a G band position typically observed in tetrahedral amorphous carbon (ta-C)-based thin films. The phase has a residual fraction of sp2 carbons, mainly linear chains and fused aromatic rings.In summary, the results demonstrate that a whole class of novel materials with outstanding physical properties - superelastic-hard and ultrahard semiconducting carbons - can be produced for demanding technological applications at HP-HT by using C60 as a precursor and tuning its microstructure.
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| 4. |
- Benavides, Vicente, et al.
(författare)
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Tuning structure and mechanical properties of nanoclustered graphene phase by controlled disorder of precursor C60 fullerite
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The paper compares the Raman spectroscopy, HRSTEM, and indentation hardness results of disordered systems synthesized by squeezing (8 GPa, 850 °C) C60 and ball-milled C60. Mechanical activation introduces substantial damage to the C60 crystal leading to the rupture of van der Waals interaction between the C60 units and a chemical reaction between the balls creating C60 dimers. The multi-wavelength Raman spectra of both, the compressed mechanical activated phase (MA-Phase) and without the mechanical activation phase (wMA phase) reveal that fused aromatic rings with a low fraction of nanographene clusters dominate the MA phase, whereas wMA phase is composed of nanographene clusters. Moreover, a Raman model is presented which introduces fullerene-like structures because of fivefold (F-band) and sevenfold carbon rings-like defects for the wMA phase and part of fused aromatic rings for the MA phase. HRSTEM-EELS data confirm that: nanographene clusters present in wMA (I) are smaller and not abundant in the MA phase (II). (III) EELS data reveal a higher fraction of sp3 bonds in the MA phase compared to that in wMA. The hardness of the MA Phase (37 GPa) is twice its value (18 GPa) in the wMA (IV). The extensive analysis of the Raman data yielded empirical dependences of Hardness vs ID/IG/Hardness vs ID/IF that can be useful for prediction of the hardness of sp2-dominant disordered carbon systems based on their spectroscopic data.
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| 5. |
- Chernogorova, Olga P., et al.
(författare)
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Indentation behaviour of superelastic hard carbon
- 2016
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Ingår i: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 96:32-34, s. 3451-3460
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Tidskriftsartikel (refereegranskat)abstract
- Superelastic hard carbon particles widely varying in structure andproperties have been studied by instrumented microindentationtechnique. The carbon particles up to 200 μm in size were producedby fullerene collapse upon high-pressure high-temperature treatmentof metal–fullerene powder mixture with simultaneous sintering ofmetal matrix composite materials (CM) reinforced by the particles.The structure and properties of the carbon particles were controlledby changing synthesis parameters and the state (composition andstructure) of the parent fullerite crystals. The specific features of theinstrumented indentation behaviour of the particles were studied asa function of their hardness. Mechanical properties of the particlestested at loads of up to 1970 mN exhibit an indentation size effect,which becomes more pronounced with increasing hardness of thecarbon particles. Upon holding at a constant load, the fullerenederivedcarbon particles undergo unrecoverable deformation, and theindentation creep CIT increases with increasing particle hardness. Anincrease in hardness of the reinforcing carbon particles substantiallyimproves the wear resistance of the CM and decreases their frictioncoefficient.
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| 6. |
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| 7. |
- Navarro Prado, F., et al.
(författare)
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Nanofiber-Structured TiO2 Nanocrystals as a Scattering Layer in Dye-Sensitized Solar Cells
- 2017
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Ingår i: ECS Journal of Solid State Science and Technology. - : Electrochemical Society. - 2162-8769 .- 2162-8777. ; 6:4
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Tidskriftsartikel (refereegranskat)abstract
- We developed a scattering layer composed of TiO2 nanocrystals assembled into a densely packed three-dimensional network of nanofibers to localize light within a photoanode used in dye sensitized solar cells (DSSCs). The electro-netting approach was applied to obtain polyamide 6 nanofibers with bi-modal diameter distribution, followed by solvothermal synthesis for the coating of TiO2 nanocrystals on the polymer template. The resulting nanofiber-structured scattering layer (NFSL) is composed of TiO2 nanofibers (200-300 nm in diameter) supporting an ultrathin nanofiber network (diameters within 10-50 nm) and exhibits strong light scattering in the visible range (400 to 700 nm). This NFSL was applied on top of a transparent active TiO2 layer (TL) forming the photoanode in DSSCs. The performance of the bi-layered photoanode was compared to its analogue, fabricated with commercial scattering layers containing different sizes of nanoparticles. The DSSCs assembled with the NFSL showed an 18% enhancement in power conversion efficiency (PCE) compared to that of DSSCs whose photoanode contained only a TL. This enhancement factor was improved up to 31% when the bi-layered structure was post-treated with TiCl4. The PCE improvement was mainly associated with the light harvesting efficiency within the photoanode because of scattering from the NFSL and increased dye adsorption due to the addition of this top layer. These conclusions were inferred from diffuse reflectance behavior, dye loading measurements, external quantum efficiency and electrochemical properties. Our work demonstrates a promising approach without the requirement of time consuming and complicated procedures for the fabrication of a densely packed 3D nanofiber network scattering layer for diverse energy conversion devices and photocatalytic applications
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| 8. |
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| 9. |
- Yuan, Mingzhi, et al.
(författare)
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Fragmentation and structural transitions of few-layer graphene under high shear stress
- 2021
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 118:21
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Tidskriftsartikel (refereegranskat)abstract
- A key factor that determines the mechanical and electrical performance of graphene-based materials and devices is how graphene behaves under extreme conditions, yet the response of few-layer graphene to high shear stress has not been investigated experimentally. Here we applied high pressure and shear to graphene powder using a rotational diamond anvil cell and studied the recovered sample with multiple means of characterization. Sustaining high pressure and shear, graphene breaks into nanometer-long clusters with generation of large number of defects. At a certain stress level, it transforms to amorphous state and carbon onions. The reduction of infrared reflectivity in the severely sheared phase indicates the decrease in conductivity. Our results unveil the shear sensitive nature of graphene, point out the effects of shear on its physical properties, and provide a potential method to manipulate this promising material.
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| 10. |
- Zhang, Shuangshuang, et al.
(författare)
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Discovery of carbon-based strongest and hardest amorphous material
- 2022
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Ingår i: National Science Review. - : Oxford University Press. - 2095-5138 .- 2053-714X. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Carbon is one of the most fascinating elements due to its structurally diverse allotropic forms stemming from its bonding varieties (sp, sp2, and sp3). Exploring new forms of carbon has always been the eternal theme of scientific research. Herein, we report the amorphous (AM) carbon materials with high fraction of sp3 bonding recovered from compression of fullerene C60 under high pressure and high temperature previously unexplored. Analysis of photoluminescence and absorption spectra demonstrates that they are semiconducting with a bandgap range of 1.5–2.2 eV, comparable to that of widely used amorphous silicon. Comprehensive mechanical tests demonstrate that the synthesized AM-III carbon is the hardest and strongest amorphous material known so far, which can scratch diamond crystal and approach its strength. The produced AM carbon materials combine outstanding mechanical and electronic properties, and may potentially be used in photovoltaic applications that require ultrahigh strength and wear resistance.
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