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Isotropic polycrystalline graphite samples were irradiated with ~1 GeV 197Au and 238U ions of fluences up to 5 × 1013 ions/cm2. Beam-induced changes of thermophysical properties were characterized using frequency domain photothermal radiometry (PTR) and the underlying structural transformations were monitored by Raman spectroscopy. The ion range (~60 µm) was less than the sample thickness, therefore thermal diffusivity contributions of the irradiated as well as non-irradiated layer were considered when analyzing the PTR data. At the highest applied fluences, the thermal effusivity of the damaged layer degrades down to 20% of the pristine value and the corresponding calculated values of thermal conductivity decrease from 95 Wm−1K−1 for pristine material to 4 Wm−1K−1, a value characteristic for the glassy carbon allotrope. This technique provides quantitative data on thermal properties of ion-irradiated polycrystalline graphite and is very valuable for the prediction of lifetime expectancy in long-term applications in extreme radiation environments. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Nanocomposite microstructures dominating the anisotropic elastic properties in carbon fibers were studied, to construct a micromechanics model that can explain the anisotropic elastic properties of carbon fibers. Aluminum-based composites containing five types of carbon fibers were prepared, and their anisotropic elastic properties were measured using resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance. Then, all the independent elastic stiffness components of the carbon fibers were extracted from those of the composites using a composite model based on Eshelby's inclusion theory, Mori–Tanaka mean-field theory, and effective-medium approximation. Moreover, we newly developed a nanocomposite microstructure model that can fully reproduce all the anisotropic constants of carbon fibers exhibiting a wide variety of Young's moduli. In the developed model, the microstructures of carbon fibers were approximated as nanocomposites comprised of an amorphous carbon matrix and graphite-crystal inclusions that are aggregations of graphite nanocrystallites. Based on this nanocomposite microstructure model, the shape of the graphite-crystal inclusions and the elastic properties of the amorphous carbon matrix were analyzed, considering the volume fraction and orientation of the graphite nanocrystallites, as determined using X-ray diffraction. The analysis revealed that the shapes of the graphite-crystal inclusions are flat ellipsoids elongated along the fiber axis, and the aspect ratio of the graphite-crystal inclusions dominantly affects the anisotropy in the Young's modulus. This indicates that the graphite nanocrystallites are connected along the in-plane directions of the graphitic layers, and not the shape of nanocrystallites but their two-dimensional connectivity dominates the anisotropic elastic modulus in carbon fibers. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Two vastly different types of load-displacement responses observed in graphitic nanostructures under nano-compression are compared in terms of serration behaviors. Different from commonly encountered linear/nonlinear elastic deformation, a periodic serration behavior related to plastic flow is observed in amorphous carbon nanospheres. The true stress-strain relation exhibits a sole feature of type C serration, and comprehensive statistical, dynamical and fractal analyses further demonstrate a chaotic characteristic of dynamics for those serration events. When entering a quasi-steady flow stage, the elastic stress in each serration event could maintain a relatively stable level near ∼135 MPa, very close to the interlayer shear stress (ISS) of single crystalline graphite (∼140 MPa). This finding indicates the dependence of shear deformation on weak van der Waals interaction (elastic constant C 44), instead of other structural factors associated with high elastic constants of graphite cells. Based on the experimental results, a microscale ISS-driven shearing mechanism is proposed. The local flexibility induced by small graphene lamellas may facilitate interfacial slip between neighboring domains with commensurate contact. Such slip mode may be responsible for the mechanical serration phenomenon in graphitic materials. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

This work reports on a tribological study of magnetron-sputtered silicon-containing amorphous carbon thin films vs. their alumina counterparts. Temperature cycling during ball-on-disk tests in humid air revealed a decrease in the coefficient of friction from 0.3 to <0.02 beyond 240 ± 15 °C. Systematic variation of the environment confirmed oxygen to be responsible for the low friction. X-ray photoelectron spectroscopy of the wear tracks indicates oxidation of Si C bonds and formation of Si O C bonds, followed by further oxidation to SiO 2 above 450 °C. Ab initio molecular dynamics simulations of gas interactions with the a-C surface revealed dissociation of O 2 and the formation of oxides. Additional density functional theory calculations of Si incorporation into a graphene layer, resembling the surface of the film, showed preferential attraction of gaseous species (H, O, OH, H 2 O), to Si-sites as compared to C-sites. Hence, the temperature- and atmosphere-induced changes in friction coefficient can be understood based on correlative X-ray photoelectron spectroscopy and ab initio data: the formation of Si O C bonds stemming from a reaction of the as-deposited coating with atmosphere in the tribological contact is observed by theory and experiment. [ABSTRACT FROM AUTHOR]

Abstract: We have, for the first time, fabricated chromium (Cr) carbide nanostructures on multiwalled carbon nanotubes (MWCNTs) in a MWCNT–copper (Cu) composite by adding small amounts of Cr. A single nanocrystal of carbide was epitaxially grown on the sidewall of chemically treated MWCNTs through the diffusion of solute Cr atoms to defects in the MWCNTs. Carbide has an island-shaped morphology with a maximum size of several hundred nanometers. The phase of the generated Cr carbide nanostructures was mostly Cr7C3, as determined by electron beam diffraction using the nanobeam diffraction mode of a transmission electron microscope. In particular, the Cr carbide nanostructures formed at radially unzipped sites were connected to the outermost wall and some inner walls of the MWCNTs. This is expected to increase the load-bearing capacity of the MWNCTs because the inner walls contribute to the load transfer, which is very effective for improving the mechanical and thermomechanical properties of the composites. [Copyright &y& Elsevier]

Abstract: The recyclability of carbon nanotube (CNT) sponges in terms of oil absorption was studied. The sorption capacities of these sponges are greater than 100gg–1 for many organics and oils with viscosities of 3–200cP, and they maintain sorption capacities of 20–40gg–1 after 10 cycles of absorption. About 98% of absorbed oil can be recovered by squeezing or converted to heat by directly burning the oil within the sponges. The sorption process has been described by a second-order kinetic model. The results demonstrate that CNT sponges are promising oil sorbents with good recyclability. [Copyright &y& Elsevier]

Abstract: Multi-walled carbon nanotube/cadmium sulfide hybrid heterostructures were easily synthesized by employing a thermal decomposition of thioacetamide as a sulfide-ion source in an aqueous regime. The resulting cadmium-sulfide phase is comprised of a zinc-blende structure of spherical polycrystalline nanoparticles (cadmium-sulfide nanoclusters) with the subunits of ca. 15nm, deposited on the nanotube surface. The formation of the cadmium-sulfide nanoparticles with zinc-blende structure (cubic crystal) suggests that the local concentrations of reacting ion species in the vicinity of the nanotube surface are different from those in the reaction solution. The cadmium-sulfide nanoparticles are comprised of a stoichiometrically ideal chemical-composition ratio (cadmium: sulfur=1:1.02) of cadmium and sulfur with the valence states of +2 and −2, respectively. The optical responses of the cadmium-sulfide phase for ultraviolet-visible light and photoluminescence spectroscopes show the proper size-effect and inherent optical properties of the cadmium-sulfide nanoparticles. [Copyright &y& Elsevier]

Abstract: Ultra-nanocrystalline diamond is a polycrystalline material with crystalline diamond grains in the nanometer size regime. We study the structure and mechanical properties of this material as a function of the average grain size, employing atomistic simulations. From the calculated elastic constants and the estimated hardness, we observe softening of the material as the size of its grains decreases. We attribute the observed softening to the enhanced fraction of interfacial atoms as the average grain size becomes smaller. We provide a fitting formula for the scaling of the cohesive energy and bulk modulus with respect to the average grain size. We find that both these properties scale as quadratic polynomials of the inverse grain size. Our formulas yield correct values for bulk diamond in the limit of large grain sizes. [Copyright &y& Elsevier]

Abstract: The paper presents a novel concept for the automated recognition and identification of different classes of porosity. This is applied to the porosity network in thermally oxidized nuclear graphite samples, which has been observed using computed X-ray tomography. The analysis is used to predict the dependence of thermal conductivity on thermal oxidation-induced weight loss. The model predictions, based on the Euler approach, are successfully compared with experimental measurements of thermal conductivity, obtained using the laser flash method. [Copyright &y& Elsevier]

Abstract: Cobalt nanoparticles encapsulated in graphitic shells were successfully synthesized using an advanced and cost-effective synthesis method. The graphite-encapsulated nanoparticles were synthesized using an electric plasma discharge generated in an ultrasonic cavitation field of liquid ethanol, followed by the separation, drying and annealing of particles. Prior to annealing, the core-shell structured nanoparticles were characterized by high-resolution transmission electron microscopy and X-ray diffraction, which revealed the presence of a Co3C and carbon supersaturation in the face-centered cubic β-Co phase in the core of carbon nanocapsules; some cores were discovered in an amorphous state. The majority of carbon nanocapsules were found to be spherical particles, less than 10nm in diameter. The annealed powder samples, in which cobalt carbides and amorphous inner cores transformed to crystallized α-Co and β-Co cores, showed coercivity values of 15.5kAm−1and saturation magnetization values of about 50Am2 kg−1, which is approximately 32% of the value of bulk cobalt particles. [Copyright &y& Elsevier]

Abstract: The detailed evolution of the graphitized microstructure of carbon steel during annealing has been studied by thin-foil transmission electron microscopy, enabled by the refined dispersion of small graphite nodules produced. The graphitization annealing of 0.38wt.% carbon steel in the quenched martensitic state resulted in the formation of graphite nodules, of the order of 2–5μm in diameter, after 2–4h. Two types of graphite nodules were observed to form: one possesses an irregular morphology and exhibits a coring particle (either a nitride or an oxide), whilst the other is more regularly spheroidal and apparently without a nucleating particle at its core. Analytical electron microscopy has been employed to characterize the microstructure of the graphite nodules formed, in order to provide a better understanding of their nucleation and growth, in particular, the mechanism of formation of the spheroidal graphite nodules without an apparent nucleating particle. For these more spherical nodules, the results showed that the outer mantles had a strong graphitic character, whilst the core regions exhibited a more amorphous carbon structure. It is deduced from the available experimental evidence that the nucleation sites most likely to result in this structure are dissolving cementite particles in the martensite. [Copyright &y& Elsevier]

Abstract: This paper reports an investigation of the influence of initial stress on the flexural vibration of an individual multi-wall carbon nanotube with simply supported ends, based on a laminated elastic beam model considering the van der Waals force interaction between two adjacent nanotubes. The results obtained show that the influence of initial stress in carbon nanotubes on their natural frequency is obvious, but the influence of initial stress in carbon nanotubes on the intertube resonant frequency of multi-wall carbon nanotubes is not obvious, especially for large aspect ratios. The influence of initial stress in carbon nanotubes on their flexural vibration modes is dependent on the tension or compression forms of the initial stress. This investigation on the influences of initial stress in multi-wall carbon nanotubes on natural frequency and intertube resonant frequency may be used as a useful reference for the application and design of nano-oscillators, nano-drive devices, nano-sensors and actuators in which multi-wall carbon nanotubes act as basic elements. [Copyright &y& Elsevier]

Polycrystalline thin films show transverse isotropy (or hexagonal symmetry) and show five independent elastic constants because of columnar structure, texture, residual stress, and local incohesive bonds (or microcracks). In this paper, we developed an advanced method for measuring the anisotropic elastic constants of thin films using resonance ultrasound spectroscopy and determined the elastic constants of chemical-vapor-deposition diamond thin films. Mechanical resonance frequencies of a film/substrate layer specimen were measured, from which elastic constants of the thin film were determined by an inverse calculation. Mode identification for observed resonance frequencies is the key for their successful determination. We achieved this by measuring the displacement distribution on the vibrating-specimen surface. Determined elastic constants are smaller than those of bulk diamond, and they are elastically anisotropic between in-plane and out-of-plane directions. We attribute these compliant and anisotropic thin films to the incohesive bond at grain boundaries. A micromechanics model consistently explains the observation. [Copyright &y& Elsevier]

Nanocomposites obtained by mechanical milling of graphite and magnesium with organic additives (benzene, cyclohexene or cyclohexane) have been studied with the aim of preparing novel hydrogen storage materials. The organic additives were very important in the milling processes to determine the characteristics of the resulting carbon nanocomposites. The mechanical milling with high energy resulted in the generation of large amounts of dangling carbon bonds in graphite with simultaneous decomposition of graphite structure. Such dangling bonds of carbon acted as sites to take up hydrogen. It has been proved by temperature programmed desorption (TPD) and neutron diffraction measurements that the hydrogen taken up in the nanocomposites exists in at least two states; in the form of C–H bond formation in the graphite component and in the form of hydride in the magnesium component. The relative amounts of two types of hydrogen strongly depended upon differences in additives used (benzene, cyclohexene or cyclohexane). When C6D6 besides C6H6 was used as additive, the hydrogen taken up was discussed from the standpoint of isotope effects. Upon addition of titanium tetraisopropoxide, the features of hydrogen uptake by the nanocomposites changed completely. [Copyright &y& Elsevier]

The deformation micromechanics of flake and spheroidal graphitic cast irons have been studied for the first time using Raman spectroscopy. It has been found that the Raman band peak positions of the graphite shift to a lower wavenumber during tensile deformation, and to a higher wavenumber in compression. The elastic properties of the cast irons have been investigated using various theoretical models. It has been shown for flake iron that the Young’s modulus is in close agreement with the models for disk-shaped inclusions, and with models for spherical inclusions for the spheroidal iron. In the case of the graphite stress at 1% strain, the same models underestimate the stress reached in the graphite possibly due to breakdown of the interface between the graphite particles and the iron matrix. It has been shown further that Raman spectroscopy can follow the elastic recovery of the cast iron and can pinpoint the onset of permanent plastic deformation by comparing the peak positions of the Raman before and after unloading. [ABSTRACT FROM AUTHOR]