Narrow Results

The corrosion performance of plasma sprayed alumina–40wt.% titania (A40T) coated high-density graphite was investigated in molten LiCl–KCl eutectic salt at $600^{\circ }$C for periods of 500, 1000, and 2000h under argon atmosphere. The microstructural and compositional investigations of corrosion-tested A40T coatings by scanning electron microscopy and energy dispersive X-ray spectroscopy showed that the samples exhibited better corrosion resistance up to 500h. The poor corrosion behavior and failure of A40T coating from graphite substrate in the long-term (1000 and 2000h) immersion was discussed in detail based on the microstructure and composition analysis. The phases existing in the as-sprayed coating ($\beta$-Al₂TiO₅, $\gamma$-Al₂O₃, R-TiO₂, and $\alpha$-Al₂O₃) remained in the detached coating after 1000h exposure to the molten salt. The reaction between molten salt and A40T coating was not evident from XRD analysis. A40T coatings with chromium carbide–nichrome (Cr₃C₂–NiCr) bond coat showed complete spallation of the top coating and oxidation of the bond coat as evident from the X-ray diffraction analysis. Detachment of the A40T coating occurred due to the penetration of the molten salt into the coating through the voids and pores present on the surface, which weakened the adhesion strength between the substrate and the A40T coating or top coat-bond coat interfaces.

The severe capacity loss of tin-based anodes restricts their use in lithium batteries (LIBs). To enhance tin anode cycle stability, tin-antimony double-discharge anodes were designed. Additionally, by ball milling, tin-antimony oxide, and flake graphite cycling-stable composites were created. The stacked flake graphite buffers tin volume rise and enhances electrical conductivity, cyclic specific capacity, and rate performance. After 400 cycles, the tin-antimony oxide-graphite anode shows good cycling specific capacity (480mAhg${}^{-1}$). Ball milling double-discharge anode material and graphite will inspire novel carbon composite anode materials.

A brief review is given on electronic and transport properties of carbon nanotubes mainly from a theoretical point of view. The topics include a description of electronic states in a tight-binding model and in an effective-mass or k · p scheme. Transport properties are discussed including absence of backward scattering except for scatterers with a potential range smaller than the lattice constant, its extension to multi-bands cases, and long-wavelength phonons and electron-phonon scattering.

In-plane magnetoresistance (MR) measurements performed on highly oriented pyrolytic graphite (HOPG) in Corbino-disk geometry revealed signatures of the electronic inhomogeneous state. In particular, the obtained results showed that besides of the Shubnikov-de-Haas (SdH) effect, the magnetic field B > 4 T triggers MR oscillations of a different origin that are spatially asymmetric. The possible coexistence of different electronic states in quantized magnetic field is discussed.

The wetting behavior of Ti-78Cu and Ti-50Cu alloys on graphite has been investigated by the sessile drop method in high vacuum. The contact angle of Ti-Cu alloys on graphite is influenced by the wetting temperature. The wetting of Ti-78Cu and Ti-50Cu alloys on graphite is chemical wetting. The microstructure and composition of the interfacial zone of the wetting samples were analyzed by SEM, EDX and XRD. Microstructure and phase analysis reveals that inter-diffusions and interfacial reactions take place in the wetting process. The reaction products include TiC and the intermetallic compounds composed of Ti and Cu. The inter-diffusions and interfacial reactions contribute to the interfacial bonding.

A zinc oxide–silver doped with graphite and nitrogen (ZnO/Ag doped with C/N) heterostructure was prepared by a simple method with a study of the photodegradation of methylene blue (MB) dye under sunlight irradiation. Results from X-ray diffraction (XRD) experiments show the presence of sharp peaks for ZnO, Ag, and C, with the wurtzite phase of ZnO. Energy-dispersive X-ray spectroscopy (EDX) results match the results of XRD and confirm the presence of Ag, Zn, C, O, and N. Field emission scanning electron microscope (FESEM) images confirm the presence of clusters of spherical nanoparticles (NPs). The optical properties results confirm the high-absorption ability in the visible area with an energy gap of 2.95eV. After 140min of exposure to sunlight radiation, the photocatalysis results show that the MB dye can be broken down very well by light.

Results of research in the quantum capacitance $C_q$ of quasi-2D-crystals are presented. The detected extraordinary behavior of $C_q$ manifests itself in the existence of such energy ranges in which it is practically equal to zero. The causes for the existence of such ranges are: (a) dimensional quantization as a result of the nanoscale of the van der Waals gap, (b) a certain value of the width of the allowed zone in the plane of the layers, which is determined by the value of the effective mass. Intercalation and external electric field are effective factors capable of changing the position of ranges. Thus, in a system connected in a series of electrostatic and quantum capacitances, the resulting capacitance will significantly depend on the $C_q$ behavior. Comparative analyses of structures, chemical bonds, majority of coinciding characteristics of physical quantities in graphite, transition metal dichalcogenides (TMD), and layered crystals A₃B₆ allow us to assert that the obtained qualitative conclusions can be applied for each of them. Calculations of density of states (DOS) of quasi-2D crystals performed by authors within the framework of the improved Kohn–Sham density functional theory (DFT), namely the DFT taking into account the van der Waals forces in it, show a step-like form of DOS qualitatively similar to ours.

Hydrogenated amorphous carbon films (a-C:H) were deposited on p-type silicon (a-C:H/p-Si) and quartz substrates by excimer laser at room temperature using mixture ratios 1 to 9 and 3 to 7 of camphor to graphite by weight percentages. The presence of hydrogen in the a-C:H films has been confirmed by Fourier transform infrared spectroscopy (FTIR) measurements. The structure and optical properties of a-C:H films were respectively investigated by Raman scattering and UV-visible spectroscopy. The increase of sp³ sites in the a-C:H films has also been confirmed by the Raman spectra spectroscopy analysis. The increase of the optical band gap with higher camphor percentage in the target was believed to be due to the increase of the sp³ hybrid forms of carbon arising from camphor incorporation. The formation of a heterojunction between the a-C:H film and Si substrate was confirmed by current–voltage (I–V) measurement. The structure of a-C:H/p-Si cells deposited using mixture ratios 1 to 9 of camphor to graphite by weight percentages showed better photovoltaic characteristics with an open-circuit voltage of 400 mV and short-circuit current density of about 15 mA/cm² under AM 1.5 (100 mW/cm² at room temperature) illumination. The energy conversion efficiency and fill factor were found to be approximately 2.1% and 0.38, respectively. The carbon layer contributed to the energy conversion efficiency in the lower wavelength region has been proved by the quantum efficiency measurement.

We studied various ground state properties, e.g. cohesive energy, exfoliation energy, equilibrium lattice constants, elastic constant (C₃₃), compressibility, band structure, density of states and charge density of Graphite, Graphene and Hexagonal Boron Nitride (h-BN) using pseudopotential plane wave method. Most of the calculated physical quantities of graphite are found to be close to those of h-BN and these are in good agreement with available experimental data.

Friction and wear processes of AgCuX (G, CF and AlN) composites-CuAgV alloy friction pair and effects of different additive content in silver based composite on friction and wear behavior are studied in this paper. The microstructure of the brush wear surface is observed by SEM. The results show that when graphite content is up to 9 wt.%, Ag-Cu-CF-G composite exhibits the best wear properties; when the content of aluminum nitride is up to 0.5 wt.%, Ag-Cu-AlN-G composites has the most comprehensive performance. The wear loss of both composites arises with the increase of both pressure and speed, but when speed reaches a critical value, the increased amplitude of wear loss tends to be steady.

Graphite is one of the effective lubricant additives due to its excellent high-temperature endurance and self-lubricating properties. In this study, wear behavior of graphite used as sealing materials to cut off hot gas is evaluated at room and elevated temperature. Wear occurs on graphite seal due to the friction of driving shaft and graphite. Thus, a reciprocating wear test to evaluate the wear generated for the graphite by means of the relative motion between a shaft material and a graphite seal was carried out. The friction coefficient and specific wear rate for the changes of applied load and sliding speed were compared under different temperature conditions considering the actual operating environment. Through SEM observation of the worn surface, the lubricating film was observed and compared with test conditions.

In recent years, significant attention has been attracted to hyperbolic (or indefinite) materials resulting from their extraordinary optical properties, such as all-angle negative refraction. Here, tunable biaxial hyperbolic dispersion and negative refraction were found in graphite crystal. First principles were used to calculate the permittivity of graphite in $x$-, $y$- and $z$-directions, the results indicate that graphite crystals exhibit indefinite dielectric constant tensor in certain frequency range. By adjusting strain, the optical properties of materials changed, and even switched from hyperbolic to elliptical materials in some frequency. This work demonstrates the feasibility of achieving high-performance tunable biaxial natural hyperbolic materials which can be used in various ways.

A heteroepitaxial system of h-BN/monolayer graphene on Ni(111) has been investigated by means of vibrational spectroscopy, accompanied by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The system was prepared in an epitaxial manner on a Ni(111) surface by chemical vapor deposition (CVD). We found that phonon peaks in observed spectra showed typical features of Fuchs–Kliewer (FK) phonons. The vibrational spectra of h-BN films ranging in thickness from 0 to 8.7 monolayers have been compared with theoretical spectra based on a dielectric theory. Detailed analysis has revealed new types of phonons, of which the vibrational amplitudes are localized at edges of h-BN nanocrystals. In addition, we have observed subsidence phenomena of the graphene and h-BN layers into Ni substrate at elevated temperatures.

Heteroepitaxial multilayers, consisting of graphene/hexagonal boron nitride (h-BN) films on Ni(111), have been investigated by means of vibrational spectroscopy, low energy electron diffraction and Auger electron spectroscopy. Vibrational spectra of the heteroepitaxial film show the appearance of transverse optical (TO_⊥) modes with out-of-plane polarization at 100 meV of the monolayer h-BN, in contrast to the softening phonon at 90 meV on the pristine system of monolayer h-BN on Ni(111). The findings strongly suggest that the interfacial bonds between h-BN and Ni(111) weaken due to the formation of monolayer graphene.

The successful deposition of boron (B)-doped p-type (p-C:B) and phosphorous (P)-doped n-type (n-C:P) carbon (C) films, and fabrication of p-C:B on silicon (Si) substrate (p-C:B/n-Si) and n-C:P/p-Si cells by the technique of pulsed laser deposition (PLD) using graphite target is reported. The cells' performances are represented in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100 mW/cm², 25°C). The open circuit voltage (V_oc) and short circuit current density (J_sc) for p-C:B/n-Si are observed to vary from 230–250 mV and 1.5–2.2 mA/cm², respectively, and to vary from 215–265 mV and 7.5–10.5 mA/cm², respectively, for n-C:P/p-Si cells. The p-C:B/n-Si cell fabricated using the target with the amount of B by 3 Bwt% shows highest energy conversion efficiency, η = 0.20%, and fill factor, FF = 45%, while, the n-C:P/p-Si cell with the amount of P by 7 Pwt% shows highest energy conversion efficiency, η = 1.14%, and fill factor, FF = 41%. The quantum efficiencies (QE) of the p-C:B/n-Si and n-C:P/p-Si cells are observed to improve with Bwt% and Pwt%, respectively. The contributions of QE are suggested to be due to photon absorption by carbon layer in the lower wavelength region (below 750 nm) and Si substrates in the higher wavelength region. The dependence of B and P content on the electrical and optical properties of the deposited films, and the photovoltaic characteristics of the respective p-C:B/n-Si and n-C:P/p-Si heterojunction photovoltaic cells, are discussed.

Growth of Ge, Al and Sb on highly oriented pyrolytic graphite (HOPG) was systematically investigated using in situ scanning tunneling microscopy (STM). At room temperature (RT), three dimensional (3D) clusters of all three elements nucleate and grow at the step edges and defect sites of HOPG. The clusters of Al and Ge form chains, while Sb islands are mostly isolated. With further deposition at RT, Al clusters grow and coarsen into faceted islands with craters on the top (111) facets, whereas ramified single- and double-layer cluster islands are observed for Ge. When deposited or annealed at T ≥ 175°C, Ge forms crystallites but with randomly oriented facets. As spherical Sb islands grow beyond certain size, (111) facets appear on the top. Additionally, crystalline 2D films and 1D nanorods are observed for Sb deposited at RT. At T ≈ 100°C and higher flux, only the 2D and 1D Sb islands are formed. These different growth behaviors reflect the unique nature in which the atoms (molecules), clusters and crystallites of each element interact with HOPG surface and with each other.

Germanium was deposited onto highly oriented pyrolytic graphite (HOPG) with and without antimony in ultra-high vacuum. The surface morphology was analyzed using in situ scanning tunneling microscopy (STM) at room temperature (RT). The film grows exclusively in 3D island mode and was affected significantly by substrate defects. At initial stage, nucleation of cluster occurred at step edges and defect sites. Later, we found various types of Ge nanostructures on HOPG in different deposition conditions and stages, including cluster chains, cluster islands, nanowires, and double layer ramified islands at RT. Compact Ge islands were observed when depositing at a substrate temperature of 450 K or after an annealing at 600 K following RT deposition. In addition, the pre-deposited Sb on graphite enhances the sticking probability and suppresses the surface diffusion of Ge atoms, resulting in a significant increase in Ge cluster island density on HOPG terraces.

Tunneling effect of graphite is studied by atomic force microscopy. The tunneling current in the gap between a conductive tip and a newly cleaved graphite surface at a given voltage keeps almost constant when the gap-distance varies within 1 μm, which cannot be explained by the commonly accepted well model of the potential outside a conductor. Thus modification of the well model is made and a cup model, with a certain wall thickness of the cup, is proposed in the present work.

The Fe–Al–X(Cu, Ni, Ti) composites reinforced with graphite particles were fabricated employing the powder metallurgy process for slide bearing applications. In all samples, graphite particles of 6 wt.% were added. Elemental powders were mixed to specific compositions with a powder lubricant, and then the mixed powders were compacted at 500 MPa. The green compacts were heated to 450°C to remove the lubricant and sintered at 1200°C for 1 h. The sintering was conducted in vacuum. The sintering of binary Fe–Al–graphite system showed low density and growth in dimension. Complex addition of Cu, Ni and Ti improved the sinterability of green compacts due to occurrence of partial liquid phases. Hardness of sintered samples strongly depended on dimensional change during sintering. The samples of high hardness showed better wear resistance properties.

In order to analyze the adsorption capacities of different solid substrates, we present a multi-step method to separately study the isotherm at different pressure ranges (steps). The method is based on simple gas isotherm measurements (nitrogen, methane, carbon dioxide, argon, and oxygen) and is tested to describe the adsorption process and characterize a graphitized surface (GCB) and two different granular activated carbons (GAC). The GCB isotherms are described as a sum of Fowler-Guggenheim-Langmuir shifted curves; isotherm behaviors are quite similar at different temperatures, but change below a certain threshold. In GAC the first steps show the same adsorption characteristics at low pressures (Dubinin's description), but this behavior changes at higher pressure regimes, which allows one to elucidate how heterogeneous the surfaces are or how strong the interactions between adsorbed molecules are for this marginal adsorption to occur. We tested different approaches (from BET multilayer to Aranovich) and found quite different features. We finally conclude that if the description of the adsorption on complex substrates, such as those presented here, is carried using only one model, e. g. Dubinin in case of GACs, the resulting characteristics of the adsorbent would be very biased.