Calculated Absorption and Vibrational Circular Dichroism Spectra of Fundamental and Overtone Transitions for a Chiral HCCH Molecular Fragment in the Hypothesis of Coupled Dipoles

We establish the general behavior of absorption and vibrational circular dichroism spectra (VCD) of a chiral HCCH fragment lacking all symmetry elements; the study is limited to CH-stretching modes, and the Hamiltonian employed is written in terms of normal-mode coordinates and momenta and approximates two different Morse oscillators interacting through a harmonic coupling term; rotational strengths are evaluated within a hypothesis of coupled electric dipoles. Van Vleck contact transformations written in terms of raising and lowering operators are used to calculate spectra up to the manifold Dv = 4. Three transformations are necessary to obtain fourthorder terms in the relevant operators, namely, the electric and magnetic dipole moments. The dynamics of the system exhibits 1:1 resonance terms in addition to the Darling–Dennison coupling term. We discuss the importance of coupling between CH stretches with respect to differences in their local mechanical characteristics in determining the aspect of the absorption and VCD fundamental and overtone spectra of increasing quantum number

Quantitative structural and textural assessment of laminar pyrocarbons through Raman spectroscopy, electron diffraction and few other techniques

In pyrocarbon materials, the width of the Raman D band (FWHMD) is very sensitive to low energy structural defects (e.g., disorientations of the graphene layers). The correlation between the two parameters, FWHMD and OA (as derived from selected area electron diffraction: SAED), has allowed to differentiate various pyrocarbons unambiguously. Furthermore, the optical properties of pyrocarbons, i.e., the extinction angle, the optical phase shift and the ordinary and extraordinary reflectance, have been accurately determined at 550 nm by means of the extinction curves method. These results are completed by in-plane and out-of-plane dielectric constant measurements by angular resolved EELS. Moreover, the hybridization degree of the carbon atoms has been assessed by the same technique. About 80% of the carbon atoms of the pyrocarbons have a sp2 hybridization. The lack of pure sp2 carbon atoms, as compared to graphite, might be explained by the presence of sp3-like line defects.Comment: Xavier Bourrat : Present Address = ISTO - CNRS-Universit\'e d'Orl\'ean

Calidad del aire (COVS, MCOVS y MOHOS) en áreas urbanas e industriales de la región capital, provincia de Buenos Aires

Se presentan los resultados obtenidos durante la campaña de monitoreo de COVs (compuestos orgánicos volátiles) y MCOVs (metabolitos fúngicos volátiles) 2007-2008 en la región comprendida por los Partidos de La Plata, y Ensenada, comparando los resultados obtenidos en dos zonas con distinta fuente de contaminación y una zona control, y con los resultados de una campaña anterior realizada en el período 2000-2002.Facultad de Ingenierí

Synthesis of Mesoporous Silica@Co–Al Layered Double Hydroxide Spheres: Layer-by-Layer Method and Their Effects on the Flame Retardancy of Epoxy Resins

Hierarchical mesoporous silica@Co–Al layered double hydroxide (m-SiO2@Co–Al LDH) spheres were prepared through a layer-by-layer assembly process, in order to integrate their excellent physical and chemical functionalities. TEM results depicted that, due to the electrostatic potential difference between m-SiO2 and Co–Al LDH, the synthetic m-SiO2@Co–Al LDH hybrids exhibited that m-SiO2 spheres were packaged by the Co–Al LDH nanosheets. Subsequently, the m-SiO2@Co–Al LDH spheres were incorporated into epoxy resin (EP) to prepare specimens for investigation of their flame-retardant performance. Cone results indicated that m-SiO2@Co–Al LDH incorporated obviously improved fire retardant of EP. A plausible mechanism of fire retardant was hypothesized based on the analyses of thermal conductivity, char residues, and pyrolysis fragments. Labyrinth effect of m-SiO2 and formation of graphitized carbon char catalyzed by Co–Al LDH play pivotal roles in the flame retardance enhancement

Fabrication of Ce-doped MnO2 decorated graphene sheets for fire safety applications of epoxy composites: flame retardancy, smoke suppression and mechanism

Ce-doped MnO2–graphene hybrid sheets were fabricated by utilizing an electrostatic interaction between Ce-doped MnO2 and graphene sheets. The hybrid material was analyzed by a series of characterization methods. Subsequently, the Ce-doped MnO2–graphene hybrid sheet was introduced into an epoxy resin, and the fire hazard behaviors of the epoxy nanocomposite were investigated. The results from thermogravimetric analysis exhibited that the incorporation of 2.0 wt% of Ce-doped MnO2–graphene sheets clearly improved the thermal stability and char residue of the epoxy matrix. In addition, the addition of Ce–MnO2–graphene hybrid sheets imparted excellent flame retardant properties to an epoxy matrix, as shown by the dramatically reduced peak heat release rate and total heat release value obtained from a cone calorimeter. The results of thermogravimetric analysis/infrared spectrometry, cone calorimetry and steady state tube furnace tests showed that the amount of organic volatiles and toxic CO from epoxy decomposition were significantly suppressed after incorporating Ce–MnO2–graphene sheets, implying that this hybrid material has reduced fire hazards. A plausible flame-retardant mechanism was hypothesized on the basis of the characterization of char residues and direct pyrolysis-mass spectrometry analysis: during the combustion, Ce–MnO2, as a solid acid, results in the formation of pyrolysis products with lower carbon numbers. Graphene sheets play the role of a physical barrier that can absorb the degraded products, thereby extend their contact time with the metal oxides catalyst, and then promote their propagate on the graphene sheets; meanwhile pyrolysis fragments with lower carbon numbers can be easily catalyzed in the presence of Ce–MnO2. The notable reduction in the fire hazards was mainly attributed to the synergistic action between the physical barrier effect of graphene and the catalytic effect of Ce–MnO2

Probing the Nature of Defects in Graphene by Raman Spectroscopy

Raman Spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D' peak is maximum (~ 13) for sp3-defects, it decreases for vacancy-like defects (~ 7) and reaches a minimum for boundaries in graphite (~3.5).Comment: 14 pages, 4 figure

Spark plasma sintering of graphitized multi-walled carbon nanotube reinforced Ti6Al4V

Graphitized multi-walled carbon nanotubes (MWCNTGr) reinforced Ti6Al4V (Ti64) matrix composites prepared via the powder metallurgy route were synthesized by spark plasma sintering (SPS) technique. 1, 2 and 3 wt% MWCNTGr were dispersed in the Ti64 matrices by adapted high energy ball milling (HEBMA). Composite powder mixtures were sintered in vacuum at constant applied pressure, heating rate and isothermal holding time of 50 MPa, 100 °C/min and 5 min respectively. The sintering temperature was varied between 850 and 1000 °C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the as-received MWCNTs, MWCNTGr, admixed composite powders and the bulk sintered composites. MWCNTGr evolution during graphitization treatment, dispersion in Ti64 matrix and in the sintered composites was analyzed using the characteristic Raman peak intensity ratio (ID/IG). The relative density of the sintered MWCNTGr/Ti64 composites was enhanced with increased sintering temperature, but deteriorated with increased wt% MWCNTGr in the metal matrix. Vickers microhardness of the consolidated composites improved with increasing sintering temperature and weight fractions of MWCNTGr over that of the unreinforced matrix alloy. The formation of crystalline TiC interfacial product during composite powder processing and consolidation is also discussed

Quantifying defects in graphene via Raman spectroscopy at different excitation energies.

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity

Raman spectroscopy as a versatile tool for studying the properties of graphene.

Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene