Graphite Crystals Research Articles

Curing mechanism, thermal and ablative properties of hexa-(4-amino-phenoxy) cyclotriphosphazene/benzoxazine blends

The demand for ablative material with high performance is eager in the aerospace industry. In this paper, hexa-(4-amino-phenoxy)-cyclotriphosphazene (CPA) was synthesized and then blended with bisphenol-A aniline benzoxazine (BA-a) firstly. The curing mechanism, thermal and ablative properties of cured blends were studied. The addition of CPA not only can decrease the curing temperature but can also increase the crosslink density of polybenzoxazine which is further beneficial for the improvement of thermal properties of cured blends. When the content of CPA is 5%, the Tg of the cured blend increases by 20 °C, and the char yield at 800 °C increases from 25.2% for pure polybenzoxazine to 49.2%. The oxyacetylene ablation tests showed that the ablation rate of cured blends could reach 0.047 mm s−1 while that of polybenzoxazine is 0.128 mm s−1. Further discussion based on SEM, XRD, Raman spectra showed that the CPA can effectively improve the graphitization degree of ablated samples and improve the integrity of graphite layer crystals. Moreover, the density functional theory (DFT) calculations and bond order analysis predict the bonding breakage sequence which is the C–N bonds between BA-a and CPA (N3–C55) > P–O bonds (P2–O12) > phosphazene ring, which means the phosphazene ring shows an effect after benzoxazine partly decomposed. The preparation method by adding CPA to benzoxazine to improve the ablative performance is facial and easy, which provides a new way of making novel ablative materials.

Mean inner potential of graphite measured by electron holography: Probing charge distribution and orbital diamagnetic susceptibility

The mean-inner-potential (MIP) of a crystal is the average electrostatic Coulomb potential within a crystal with respect to vacuum.We conducted off-axis electron holography experiments on highly-oriented-pyrolytic-graphite (HOPG) in a transmission-electron-microscope to measure the MIP from nanometer-scale volumes of Bernal graphite oriented with respect to the electron beam, along the principal-axis or directions in the basal plane. These MIP were related to mean orbital electron radii and diamagnetic susceptibilities in perpendicular planes.These measurements are interesting compared to magneto-transport measurements, which report large anisotropy. Along the principal-axis, the susceptibility is highest amongst diamagnetic materials. However, those measurements vary significantly depending on type, size, and thickness of samples, indicating that the signal is significantly extrinsic due to defect induced interfaces. Indeed, structural examination of HOPG show stacking-faults and planar rotations around the principal-axis, such that measuring intrinsic properties requires probing a volume of ∼102 × 102 × 102 nm3.Experiments on such individual graphite crystals with (0001) basal, or (1–100), (2-1-10) prismatic planes, resulted in MIP of 10.16 ± 0.40V, 11.37 ± 0.35V, 12.66 ± 0.41V, respectively. First-principles calculations confirm these anisotropic measurements with 11.72V, 13.65V, 14.56 V, respectively.These measured MIP enable to determine projected mean-radii of electron orbitals and volume susceptibilities at 0.704 ± 0.015 Å, (−1.99 ± 0.08) × 10−5; 0.744 ± 0.015 Å, (−2.23 ± 0.07) × 10−5; 0.785 ± 0.015 Å, (−2.48 ± 0.08) × 10−5.

Characterization of carbonized magnesium formed in a low energy plasma focus device

Carbon-ion deposition on magnesium substrate was performed using a low energy Mather-type plasma focus device. Deposition process was carried out at 0, 15, 30 and 45° angular position with respect to the device anode axis. X-ray diffraction (XRD) confirmed presence of Mg2C3, Mg (OH)2 and graphite crystals on the produced films. Field emission scanning electron microscope (FE-SEM) coupled to elemental mapping analysis from surface and cross section of the samples was performed. The analysis showed the formation of a multi-layer thin film, in which carbon nano particles (CNPs) covered the composite layer. The CNPs were formed by graphite crystals while the composite layer formed by combination of Mg2C3 and Mg (OH)2 crystals. The band gap energy of indirect transition (⁓1.75 eV) of Mg2C3 was extracted from optical measurements of UV–Visible spectrum. XRD and Raman spectroscopy were used to investigate the graphite crystal structure of CNPs at each deposition angle. The study indicates that the graphite crystal structure of the samples is consisted of nano-crystalline and reduced nano-crystalline graphite structures. X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental composition of the surface structure. The presence of Mg2C3 and Mg (OH)2 in composite layer, nano-crystalline, and reduced nano-crystalline graphite in CNPs was identified by high resolution transmission electron microscopy (HR-TEM). Potentio-dynamic polarization test was used to analyze the corrosion resistance of the prepared thin films with respect to un-deposited substrate. The findings indicate that the thickness and the size of Mg2C3 crystals have direct effect on corrosion resistance parameters.

Nonlinear elastic response of pan based carbon fiber to tensile loading and relations to microstructure

In this study, wide- and small-angle scattering (WAXS, SAXS) are used to analyze the structure of polyacrylonitrile (PAN) carbon fiber. Wide angle scattering is used to identify lattice parameters such as d-spacings, stacking height of crystalline planes (Lc), average crystallize size (Lc), and various other features (void content, degree of graphitization). Small angle scattering results show orientation and size distributions of pores/amorphous regions. Several analysis techniques were applied to small angle scattering results to ensure accuracy. XRD Data collected is correlated to single fiber mechanical properties and a fiber model is developed. Fiber modulus is heavily related to crystallite dimensions and orientation as well as pore structure. Authors hypothesize that the non-linear elasticity observed in the fiber direction of carbon fibers arises from elastic planar motion within turbostratic carbon. As crystalline regions grow and expand, the lattice structure improves lowering the barriers for (002) quasi-graphene plane motion. This miniscule elastic planar motion can lead to a large nonlinear response to tensile loading especially for high crystallinity carbon fibers and underlying deformation mechanics is addressed considering transverse-isotropy of turbostatic graphite crystals.

Insights on the deep carbon cycle from the electrical conductivity of carbon-bearing aqueous fluids

The dehydration and decarbonation in the subducting slab are intricately related and the knowledge of the physical properties of the resulting C–H–O fluid is crucial to interpret the petrological, geochemical, and geophysical processes associated with subduction zones. In this study, we investigate the C–H–O fluid released during the progressive devolatilization of carbonate-bearing serpentine-polymorph chrysotile, with in situ electrical conductivity measurements at high pressures and temperatures. The C–H–O fluid produced by carbonated chrysotile exhibits high electrical conductivity compared to carbon-free aqueous fluids and can be an excellent indicator of the migration of carbon in subduction zones. The crystallization of diamond and graphite indicates that the oxidized C–H–O fluids are responsible for the recycling of carbon in the wedge mantle. The carbonate and chrysotile bearing assemblages stabilize dolomite during the devolatilization process. This unique dolomite forming mechanism in chrysotile in subduction slabs may facilitate the transport of carbon into the deep mantle.

Analysis of Activation Process of Carbon Black Based on Structural Parameters Obtained by XRD Analysis

In the present study, carbon black activated by CO2 gas was examined through XRD analysis, especially with regard to changes in its structural parameters. Based on the results, its activation process was thoroughly analyzed. The activation process was controlled by isothermally activating the carbon black inside a reaction tube through which CO2 gas flowed. With this approach, the degree of activation was varied as desired. At an early stage of the activation process, the amorphous fraction on the carbon black surface was preferentially activated, and later the less-developed crystalline carbon (LDCC) region inside the carbon black particles started to be activated. The latter process was attributable to the formation of pores inside the carbon black particles. As the activation process proceeded further, the more-developed crystalline carbon (MDCC) region started to be activated, thereby causing the pores inside the carbon black particles to grow larger. At the last stage of the activation process, La was found to be decreased to about 40 Å. This implied that the edges of the graphite crystals had been activated, thus causing the internal pores to grow and coalesce into larger pores. Activated conductive Super-P with enhanced pore properties is expected to have wide applications.

Macrochondrules in Some Chondrites: 1. Structural-Mineralogical Characteristics

The results of structural, mineralogical and chemical study of rare structural units of chondrites, macrochondrules and their fragments, found in five chondrites of different chemical groups and petrological types (Allende CV3, Krymka LL3.1, Saratov L4, "Velyka Balka" L4-5, Château-Renard L6), are given. Most of them are generally similar to ordinary chondrules and previously studied macrochondrules. They have a radial and porphyritic texture, consist of olivine and pyroxene, and are covered by a silicate rim, which is fine-grained in the macrochondrules of unequilibrated chondrites and coarse-grained in equilibrated ones. Only two macrochondrules among studied one’s are extraordinary and indicate specific conditions for their formation in the protoplanetary nebula. The first one, separated directly from the Allende chondrite, is characterized by the presence of a thin amorphous shell with a porous structure and with unambiguous sculpture features of instantaneous melting and solidification of its surface silicate layer. The second one, studied in a polished section of the Krymka meteorite, is characterized by a zonal structure and the presence of graphite grains and possibly bitumen inclusions. According to the SiO2/MgO ratio, its fine-grained silicate rim with rare graphite crystals and possibly bitumen inclusions corresponds to the fine-grained rims of ordinary chondrules, but is different from the carbonaceous material of meteorites.

Genesis of Graphite in Metapelites in the North-Western Border of the Lypniazhka Massif (the Inhul Domain of the Ukrainian Shield)

This study presents results of an investigation of metamorphic rocks of the Inhul-Inhulets series located in the northwestern border of the Lypniazhka granite-migmatite massif (Inhul domain, the Ukrainian Shield). The rocks were studied petrographically and mineralogically and carbon isotope, Raman spectroscopic and microprobe measurements were made. Graphite and calcite were given special attention. Metapelites and quartz-rich graphite-biotite-garnet rocks were investigated. The former consist of biotite, graphite-biotite, amphibole-bearing graphite-biotite gneisses. Graphite in them is evenly distributed through the rock groundmass. The δС13 values of graphite lie between -39.4‰ and -33.6‰ (relative to PDB). The graphite is considered to be of biogenic origin. Quartz-rich graphite-biotite-garnet rocks are less common, but they also contain graphite. The latter occurs as inclusions in the major minerals either forming clusters. Its δС13 values fall between -28.45‰ and -22.2‰ (relative to PDB). Based on the Raman spectra, carbon from the gneisses has an ordered graphite structure. The temperature of graphite crystallization was estimated to be between 554 and 630°С and corresponds to the amphibolite facies.

Creating graphene and graphite coatings on metal field electron emitters to improve their characteristics

The growth of graphene and graphite structures during the catalytic dissociation of benzene molecules on the surface of iridium and rhenium field electron emitters was studied using field electron and desorption microscopy. The emission and operational characteristics of the manufacture emitters were studied. The carbon coating increases the resistance of emitters to adsorption of gas molecules and ion bombardment. The grown graphite crystal allows you to get a beam of electrons localized in a narrow angle when emitting from the angle of the crystal. Metal emitters with a carbon coating in relatively poor vacuum conditions have greater stability and longer lifetime compared to metal emitters.

Mirror to measure small angle x-ray scattering signal in high energy density experiments.

Small angle x-ray scattering (SAXS) is a well established technique to detect nanometer scale structures in matter. In a typical setup, this diagnostic uses a detector with a direct line of sight to the scattering target. However, in the harsh environment of high intensity laser interaction, intense secondary radiation and high-energy particles are generated. Such a setup would therefore suffer a significant increase of noise due to this background, which could eventually prevent such measurements. In this paper, we present a novel tool consisting of a mosaic graphite crystal that works as a mirror for the SAXS signal and allows us to position the detector behind appropriate shielding. This paper studies the performance of this mirror both by experiment at the European XFEL (X-Ray Free-Electron Laser Facility) laboratory and by simulations.

Memory Effect in the Spatial Series Based on Diamond and Graphite Crystals.

To study the relation between the structure of a compound and its properties is one of the fundamental trends in chemistry and materials science. A classic example is the well-known influence of the structures of diamond and graphite on their physicochemical properties, in particular, hardness. However, some other properties of these allotropic modifications of carbon, e.g., fractal properties, are poorly understood. In this work, the spatial series (interatomic distance histograms) calculated using the crystal structures of diamond and graphite are investigated. Hurst exponents H are estimated using detrended fluctuation analysis and power spectral density. The values of H are found to be 0.27–0.32 and 0.37–0.42 for diamond and graphite, respectively. The calculated data suggest that the spatial series have long memory with a negative correlation between the terms of the series; that is, they are antipersistent.

Design and performance characterisation of the HAPG von Hámos Spectrometer at the High Energy Density Instrument of the European XFEL

The von Hámos spectrometer setup at the HED instrument of the European XFEL is described in detail. The spectrometer is designed to be operated primarily between 5 and 15 keV to complement the operating photon energy range of the HED instrument. Four Highly Annealed Pyrolitic Graphite (HAPG) crystals are characterised with thicknesses of 40 μm or 100 μm and radius-of-curvature 50 mm or 80 mm, in conjunction with either an ePix100 or Jungfrau detector. The achieved resolution with the 50 mm crystals, operated between 6.5 and 9 keV, matches that reported previously: ∼8 eV for a thickness of 40 μm, whereas, with an 80 mm crystal of thickness 40 μm, the resolution exceeds that expected. Namely, a resolution of 2 eV is demonstrated between 5–6 keV implying a resolving power of 2800. Therefore, we posit that flatter HAPG crystals, with their high reflectivity and improved resolving power, are a powerful tool for hard x-ray scattering and emission experiments allowing unprecedented measurements of collective scattering in a single shot.

The surface topography and microstructure change of densified nanopore nuclear graphite impregnated with polyimide and irradiated by xenon ions

The densified isostatic graphite (D-IG-110) was obtained from isostatic graphite (IG-110) by impregnating with polyimide, which reduced the median pore diameter of IG-110 from 2.060 μm to 0.065 μm. The D-IG-110 samples were irradiated with 7 MeV Xe26+. The surface morphology and microstructure changes of samples were characterized by scanning electron microscopy, grazing incidence X-ray diffraction and Raman spectroscopy, respectively. The initial graphitization degree of D-IG-110 is slightly reduced due to the amorphous structure of polyimide-derived carbon (polyimide-C). Graphite crystals undergo lattice expansion along the c-axis under irradiation. Compared with IG-110, D-IG-110 samples have a lower crystallite size change rate along the C-axis, and the polyimide-C in D-IG-110 can as a buffer for this expansion. The polyimide-C shrunk after irradiation at lower surface irradiation dose (0.02 dpa) caused by irradiation-induced graphitization, and turned to expand with the further increase in dose. The D-IG-110 samples irradiated at lower dose, the lower shrinkage rate of crystallite sizes along a-axis in the region containing polyimide-C than IG-110 also indicates the irradiation-induced graphitization, and polyimide-C also slows down the rapid increase in surface residual stress.

Elastic instability in graphite single crystal under dynamic triaxial compression: Effect of strain-rate on the resulting microstructure

We focus on the behavior of graphite under triaxial loading at a constant strain-rate using large scale molecular dynamics simulations. Buckling patterns (chevrons) in graphite nucleate from an elastic instability strongly related to the material anisotropy and subsequently grow until the first diamond nuclei appear. We show that the phase transition completely inhibits the growth of chevrons in buckled graphite, the diamond grain size being determined by the size of chevrons at the onset of nucleation. Cubic-diamond clusters nucleate within chevrons of buckled graphite and grow until the parent phase is entirely transformed. This phenomenon leads to nano-structured diamond polycrystals, with orientations of interfaces given by those of the buckled material right before the nucleation process. The buckled graphite microstructure is shown to strongly influence the final microstructure/size of nano-diamonds.

Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene.

Liquid- phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e., how thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here, we demonstrate that during ultrasonic LPE the transition from graphite flakes to graphene takes place in three distinct stages. First, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zigzag directions. Second, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to optimize the lateral dimensions, thickness, and yield of graphene and other 2D materials in large-scale LPE for various applications.

Graphite thermal expansion coefficient measured by in-situ x-ray diffraction

Precision temperature measurement of a nano system with high sensitivity and fast response is still a challenge. The marvelous thermal and mechanical properties of graphite will allow the creation of superior nanoscale temperature sensors. In-situ x-ray diffraction was employed to determine the graphite hexagonal crystal lattice dimensions and the coefficient of thermal expansion based on the calculation of its interatomic distance. The energy of graphite was mapped over the first Brillouin zone in the temperature range of 50 °C–1200 °C at intervals of 50 °C. Energy-based comparative studies between the quantum free electron approach obtained by an inelastic scattering and an harmonic oscillator are introduced by the principal quantum number associated with the excitation level. The hexagonal lattice constants, interlayer distance and interatomic distance of graphite crystals are investigated analytically with consideration given to their temperature dependence and the carbon peak (002), where the 2θ value decreases slightly with increasing temperature. The coefficient of thermal expansion of graphite-based interatomic distance is negative and tends toward zero with increasing temperature, which is in very good agreement with experiments. Moreover, the energy probability distributions enclosed by reciprocal lattice vectors of the hexagonal lattice are defined and interpreted based on lattice dimensions with varying temperature. Linear changes of the temperature-driven unit cell lattice dimensions and analysis of the kinetic energy of the electron in graphite may both be utilised for the advanced temperature interpretation model and preliminary design of a precise nanothermometer.

Intrinsic Capacitance of Molybdenum Disulfide.

The metallic, 1T polymorph of molybdenum disulfide (MoS2) is promising for next-generation supercapacitors due to its high theoretical surface area and density which lead to high volumetric capacitance. Despite this, there are few fundamental works examining the double-layer charging mechanisms at the MoS2/electrolyte interface. This study examines the potential-dependent and frequency-dependent area-specific double-layer capacitance (Ca) of the 1T and 2H polymorphs of MoS2 in aqueous and organic electrolytes. Furthermore, we investigate restacking effects and possible intercalation-like mechanisms in multilayer films. To minimize the uncertainties associated with porous electrodes, we carry out measurements using effectively nonporous monolayers of MoS2 and contrast their behavior with reduced graphene oxide deposited layer-by-layer on atomically flat graphite single crystals using a modified, barrier-free Langmuir-Blodgett method. The metallic 1T polymorph of MoS2 (Ca,1T = 14.9 μF/cm2) is shown to have over 10-fold the capacitance of the semiconducting 2H polymorph (Ca,2H = 1.35 μF/cm2) near the open circuit potential and under negative polarization in aqueous electrolyte. However, under positive polarization the capacitance is significantly reduced and behaves similarly to the 2H polymorph. The capacitance of 1T MoS2 scales with layer number, even at high frequency, suggesting easy and rapid ion penetration between the restacked sheets. This model system allows us to determine capacitance limits for MoS2 and suggest strategies to increase the energy density of devices made from this promising material.

Tunable biaxial hyperbolic dispersion and negative refraction in graphite

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 [Formula: see text]-, [Formula: see text]- and [Formula: see text]-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.

Carbon-oxygen isotope records of Precambrian metamorphic rocks in the Yeongnam massif area, South Korea

Carbon isotope thermometry between calcite and graphite has been applied to Precambrian granulite facies marbles in the Northeast Yeongnam massif, Gyeongbuk Province Korea. 11 pairs of calcite-graphite samples indicate the peak metamorphic temperature of 752 53 °C which well matches with the temperature estimated with a petrological study by other researchers. Graphite from this study mainly shows highly lustrous coarse grains and fine crystalline graphite crystals on coarse grains. Thus it well presents the peak metamorphic temperature. Fine graphite crystals on coarse graphite grains are possibly originated from the later stage overgrowth but their high lustrousness may represent prograde metamorphism after formation, which is related to later igneous activity which is related to the Jurassic granite intrusion and near the granite the highest temperature, 835 °C, was obtained. With almost no shift in δ13C and only locally shifted δ18O in the calcite samples of the study area, CO2-depleted fluid may have affected the marble at the last stage of metamorphism by the Jurassic granite intrusion. The δ13C and δ18O values of the calcite in the study area show relatively higher values than other Precambrian calcite. It would be interesting to compare the C–O isotope values with those from the Cathaysia Craton of the South China Block which is believed to be correlated to Yeongnam massif.

Hopper-skeletal and hemispherical crystallization of graphite in iron–carbon–silicon alloys

The growth of graphite during solidification and solid-state transformation of Fe–C–Si alloys continues to be the subject of extensive research. This is particularly valid for spheroidal graphite, as control of graphite shape is vital to achieving the level of performance required in contemporary applications of these casting alloys. For irons solidifying in the stable system, growth of spheroidal graphite typically starts in the liquid before the eutectic reaction, and then continues through carbon diffusion from the liquid to the austenite through an austenite shell, to finally being completed through growth in solid state because of decreased carbon solubility during cooling or iron carbide decomposition if metastable solidification also occurs. The process may be complicated by partial or even complete metastable solidification. This last scenario is typical for industrial malleable iron. To further understand graphite growth mechanisms, irons of similar composition but with four level of Mg (<0.01–0.047%) were rapidly solidified in copper molds to produce samples with metastable eutectic. The samples were annealed at 950 °C for 4–40 min to promote graphite growth, and then quenched in water. Optical and scanning electron microscopy on polished and deep etched specimens was used to study the growth kinetics of nucleation and growth of graphite. Sequential nucleation of graphite on complex Mg sulfides that have nucleated on Mg oxides was documented. Unique features such as honeycomb structure typical for hopper skeletal-type crystals analogous to ice crystals, and hemispherical growth of graphite aggregates in Fe–C–Si alloys were observed and documented. The experimental findings are additional support of the multi-mechanism theory of graphite growth in iron alloys.