Behavior Of Graphite Research Articles

A numerical approach for the determination of graphite deformation behaviour by using microtribological pressure tests

Most tribological systems use oil or grease to keep up the desired operation conditions, whereas unlubricated systems are relatively rare. Beside these three possibilities, tribological systems can also be lubricated by solid materials such as graphite. Especially in high temperatures environments, graphite has high potential to increase the lifetime of rolling bearings. For such applications, new coating and lubrication concepts for roller bearings are engineered. This process requires a detailed understanding of the system behaviour on micro and macro scale.This contribution investigates the influence of the surface topography and the material behaviour of graphite as coating in pressure tests. Numerical simulations in combination with microtribometer experiments allow to analyse the deformation behaviour of the graphite coating. A numerical parameter study is conducted to remodel microtribometer pressure experiments. The results of the experiments show that a graphite coating of thickness 3.6 μm compresses about 30% under a load of 402 mN in microtribometer experiments. The combination of experiments and simulations allows to determine a yield stress for a perfect elastic-plastic material model for further usage in finite element simulations.

Dynamic mechanical behavior of unfilled and graphite filled carbon epoxy composites

In the present study, the viscoelastic behavior of graphite filled and unfilled carbon epoxy composite is investigated through dynamic mechanical analysis. The investigation has been conducted using a three-point bend test (flexure mode) in the range of temperature from 30 to 200° C at one Hz frequency at arate of heating of 2° C/min.The viscoelastic characteristics including loss-modulus, storage modulus, damping factor, and composite glass transition temperature have been determined. It was found that the dynamic mechanical characteristics of the carbon fabric reinforced epoxy are highly dependent on the presence of graphite particulate composites. The storage modulus, glass-transition temperature, and loss modulus of filled graphite composites were higher than the unfilled ones. However, combining a graphite filler with carbon epoxy composite limits polymer molecules movement that leads to a decrease in tan δ.

Enhanced intercalation behaviors of edge-rich flakes-stacked graphite for Al-graphite dual-ion battery

Graphite has been almost proven to be the positive material with the highest voltage and superior cycle stability for Al-graphite dual-ion battery. However, the limited energy density of graphite as positive material still blocks the future practical application. It is of great significance to further improve the energy density of the battery to satisfy the growing demand. In this regard, an edge-rich flakes-stacked graphite prepared by intentionally eroding the outer dense graphitic layer of graphite paper is demonstrated to apply in Al-graphite dual-ion battery that the chloroaluminate anions can be directly transported from edge-rich graphene nanoflakes and diffused into interior graphitic layers, which will greatly shorten the diffusion pathway, thus revealing better rate capability. The edge-rich flakes-stacked graphite paper enables to deliver a reversible capacity of ~87 mAh g−1 at 50 mA g−1, significantly higher than that of the untreated graphite paper electrode. More importantly, owing to the ultrahigh areal mass loading advantage, the as-assembled batteries exhibit much enhanced areal capacity (1.2 mAh cm−2), which is distinctly the highest among the reported graphite-based electrodes.

Tribological Behaviour of MoS2 and Graphite Reinforced Aluminium Matrix Composites

Tribological behaviour of MoS2 and graphite reinforced aluminium matrix composites were investigated in this research. In the current scenario, applications in which either metal based alloys or composites are used, requires lubrication to avoid friction. In such cases, self-lubricating materials based aluminium composites can be fabricated to exhibit better tribological properties. These materials can be fabricated by reinforcing self-lubricating materials like graphite (Gr), molybdenum disulphide (MoS2) and so on within the aluminium matrix. In this tribological study, two different solid lubricants (Gr and MoS2) were taken at equal proportions to fabricate aluminium matrix composites individually through stir casting method. Pin on Disc apparatus was utilized to record the rate of wear of the composite specimen by considering the process parameters like sliding distance, load and counter-face disc hardness. Experiments were designed by Taguchi’s approach and analyzed by means of response values and analysis of variance (ANOVA). Results depicted that graphite based aluminium alloy composites possessed better wear resistance than MoS2 reinforced aluminium alloy composites.

SiO2 Promoted CaO Diffusion to C Phase at 1500 and 1700 °C

To better understand the mass transfer behaviors in CaC2 production from CaO and coke, this paper studies the diffusion behaviors of CaO and graphite, with or without ash, at 1500 and 1700 °C. CaO and graphite are pressed into tablets and heated alone or in close contact. Physical and chemical changes in these tablets are analyzed by XRD and SEM+EDX. In some experiments, thin Mo wires are placed between the closely contacted CaO and graphite tablets to identify the diffusion direction. It is found that the diffusion between CaO and low-ash graphite is very limited. SiO2 in a high-ash graphite diffuses into CaO tablet and reacts with CaO to form Ca2SiO4, which then diffuses into the graphite tablet easily and leads to CaC2 formation at 1700 °C.

Synthesis of diamond composites via microwave sintering and the improvement of mechanical properties induced by in-situ decomposition of Ti3AlC2

Metal matrix diamond composites (MMDC) started from the pre-alloyed powders with the addition of different concentration of Ti3AlC2 MAX phase were prepared via microwave assisted sintering (MAS) method. The microstructure of samples was examined by SEM, EDS and XRD characterization methods. The mechanical properties including relative density, abrasive ratio, impact toughness, hardness, elastic modulus and bending strength were also analyzed to investigate the contribution of the additive of Ti3AlC2. The results demonstrate that Ti3AlC2 additive is beneficial for promoting the mechanical properties and the optimized Ti3AlC2 addition concentration is 25 wt %. The enhancement effect of the comprehensive mechanical properties could be due to the in-situ decomposition of Ti3AlC2 induced by diamond and the generated Al to stuff porosity and then conducive to densification of the sintered structure. Simultaneously, Al element reacts with diamond abrasive to produce Al4C3 transition layer on the diamond surface, which improves the bonding force between the diamond and the matrix for the MMDC sample. The advantages of microwave heating and the pre-alloying of raw powder can avoid the graphitization behavior of internal diamond abrasive owning to the relatively low sintering temperature. The potential sintering mechanism of the pre-alloyed diamond composites by the addition of Ti3AlC2 in the microwave field was also discussed.

A complex study of the dependence of the reduced graphite oxide electrochemical behavior on the annealing temperature and the type of electrolyte

In this work we investigate the influence of thermal treatment of reduced graphite oxide (RGO) on its functional composition and electrochemical performance. It is found that carboxyl, carbonyl, hydroxyl and epoxy groups are present on the RGO surface, witch when subject to thermal annealing in the temperature range 230–250°C can be controllably modified. In the process of thermal annealing, we show the formation of quinoid groups due to an increase in the number of defects. Decrease of the number of layers in RGO material and the quantity of oxygen-containing functional groups (OCFG) also occurs. With increase in annealing temperature, sequential removal of OCFG occurs as follows: carboxyl (250°C–600°C), hydroxyl (600°C–800°C), carbonyl and quinoid (700°C–1000°C). Electrochemical measurements over a wide range of pH values of the buffer electrolytes is possible to correlate the peaks in the cyclic voltammogram curves with the redox reactions of oxygen-containing functional groups as a function of applied potential. Peaks correlated with specific redox reactions which are identified as two-electron. The dependence of the specific capacities of materials on the electrolyte type has been studied. Highest capacitance was detected in 1M NaOH at a scan rate 2 mVs−1 and is equal to 210 Fg−1.

Effects of High-Concentration Cu and Sn on the Nucleation and Growth Behavior of Graphite on Rare-Earth Compounds During the Solidification of Cast Iron

Effects of High-Concentration Cu and Sn on the Nucleation and Growth Behavior of Graphite on Rare-Earth Compounds During the Solidification of Cast Iron

Stress Graphitization Behavior of c/c Composites Fabricated from Milled Short Pitch-Based Carbon Fibers and their Electrical Properties

Stress Graphitization Behavior of c/c Composites Fabricated from Milled Short Pitch-Based Carbon Fibers and their Electrical Properties

Pyropermittivity and pyroelectret behavior of graphite

This work reports the pyropermittivity (effect of temperature on the relative electric permittivity κ) and pyroelectret behavior (effect of temperature on the electret’s inherent DC electric field E, with an electret meaning a permanent electric dipole) of graphite, namely isotropic (polycrystalline) graphite with DC resistivity ρ (1.16 ± 0.08) × 10−5 Ω.m, κ 530 ± 18 at 2 kHz, and E 1.30 × 10−5 V/m at room temperature. These effects are to be distinguished from the pyroelectric effect. They pertain to the dielectric behavior, which stems from the carrier-atom interaction. They are relevant to temperature sensing and the conversion of thermal energy to electrical energy. Pyropermittivity allows temperature sensing by capacitance measurement. Pyroelectret allows temperature sensing by electric field measurement. The material is a true electret, i.e., an electret that does not require poling. Upon heating (20–70 °C), κ increases by 32% (positive pyropermittivity), ρ increases by 70% (positive pyroresistivity, reflecting the metallic character), and E increases by 1900% (positive pyroelectret). As a result, the electric power volumetric/gravimetric density due to the electret (important for energy conversion) increases by 24000%. The increase in ρ upon heating promotes the dielectric behavior. All effects are reversible upon cooling. The pyroelectret coefficient is (1.53 ± 0.04) × 10−14 C/(m2.K) – first report for materials in general.

Role of grain boundaries in the dielectric behavior of graphite

Electric-polarization-causing carrier-atom interaction occurs in graphite at grain boundaries, as indicated by the electric permittivity (the main dielectric property) being linearly related to the reciprocal of the grain size. A smaller grain size increases the permittivity by increasing the fraction of carriers that participate in the polarization and by facilitating the carrier hopping from one grain boundary to an adjacent one during polarization. The linearity further means that the strength of the carrier-atom interaction is independent of the grain size. Specifically, the relative permittivity (2 kHz) of isotropic graphite increases from 380 to 5100 with decreasing grain size (coke raw material particle size) from 12 to 1 μm. The fraction of carriers that participate in the polarization (≤7.8 × 10−11) also decreases with increasing grain size, but is less sensitive to and less well-correlated with the grain size than the permittivity. Extrapolation of the abovementioned linear curve to infinite grain size gives relative permittivity 69 (corresponding to the fraction of carriers that participate 1 × 10−11). These two values are much smaller than those for finite grain sizes, indicating that the permittivity and participating carriers are essentially completely due to the carrier-atom interaction at the grain boundaries. The grain size influences the resistivity slightly.

Energy dissipation during fracturing process of nuclear graphite based on cohesive crack model

General consensus in the literature is that the fracturing behavior of nuclear graphite shows a certain degree of nonlinearity due to the presence of a fracture process zone (FPZ) ahead of the crack tip. Thus, it is necessary to take the influence of the FPZ into account when investigating the fracture properties of nuclear graphite. Besides, the energy-based approach has been widely used to characterize the fracturing process in the FPZ. In consideration of all these factors, this paper examines the dissipated energy during the entire fracturing process of local graphite (NG-CT-01) found in China based on the cohesive crack model. The calculation process for estimating the average dissipated energy of the crack growth is obtained through three-point bending tests, which are performed on center-notched graphite beams. Electronic speckle pattern interferometry (ESPI) is used to obtain the full-field deformation of the beams. The experimental and analytical results are then compared, and a satisfactory agreement between the two results are found, which validates the reliability and accuracy of the formulas. The tension softening curves of graphite are plotted and different cohesive laws (bilinear, tri-linear and exponential) are used to determine the average amount of energy consumed during crack extension. The tri-linear softening model is used to evaluate the relationship between the energy dissipation of the fracturing process and the fracture mechanism in the FPZ of the graphite. Furthermore, the energy dissipated during the entire fracturing process of the graphite beam is examined based on three stages: initiation of cracking, propagation of stable cracking and unstable fracturing.

Effect of tungsten doping on laser ablation and graphitization of diamond-like nanocomposite films

Laser surface microstructuring of diamond-like nanocomposite (DLN) films is a promising technique to improve their properties. For industrial applications, UV excimer or IR lasers with high pulse repetition rate are commonly used. In this paper, we investigate the influence of metal (tungsten) doping of DLN film on the ablation and graphitization behavior of the films using KrF laser (λ = 248 nm, τ = 20 ns) and Yb:Yag laser (λ = 1030 nm, τ = 8 ps). The response is found to be sensitive to the laser wavelength and pulse duration. For UV ns-pulses, the ablation threshold is insensitive to W-doping, showing only a slight increase for the highest tungsten concentration of 27 at.%, while the ablation rate decreases. Conversely, an increase of tungsten content in the case of the irradiation by IR ps-pulses induces a drop in the ablation threshold and an increase in the ablation rate. The structural modification of the films is analyzed by Raman spectroscopy, which also allows us to estimate the thickness of a residual graphitized layer on the surface after laser irradiation. It is found that laser-induced graphitization is stronger for doped DLN films after multipulse irradiation.

Improvement and validation of RELAP5/SCDAP code on evaluating the graphite behavior during oxidation process

RELAP5/SCDAP code has been improved by incorporating relevant chemical reaction models to evaluate the graphite oxidation behaviour in High Temperature Gas-cooled Reactor (HTGR). The developed code was validated against three individual experiments with different emphasis. It could be concluded that the developed Burn-off factor (M(B)) correlation showed a better performance on fitting the experimental data than the model proposed from the previous researches. Besides, the variation of the fractions of each gas species after the chemical reactions show a good agreement with the experimental results. Moreover, the temperature transient during the graphite oxidation was also reasonably reproduced with acceptable deviation. All in all, after the improvement, RELAP5/SCDAP code has a better capacity in evaluating the graphite behavior during the oxidation conditions, which improves its reliability on HTGR safety analysis.

Ion-beam-assisted characterization of quinoline-insoluble particles in nuclear graphite

The irradiation behavior of graphite is essential for its applications in nuclear industry. However, the differences between graphite's behaviors are not well understood because of the very limited knowledge of microstructural differences between graphites. One typical structure, quinoline insoluble (QI) particle, was investigated using IG-110 and NBG-18 graphite. After irradiation, the QI particles on the polished surface were proved to become hillocks and can be easily identified by using a scanning electron microscope (SEM). Thus a method combining ion irradiation and SEM characterization was proposed to study the distribution and concentration of QI particles in graphite. During irradiation, the QI particles were found to evolve into densified spheres which have weak bonding with the surrounding graphite structures, indicating that the densification of QI particles does not contribute obviously to graphite dimensional shrinkage. A much higher concentration of QI particles in NBG-18 than IG-110 was characterized and is suggested to be responsible for the smaller maximum dimensional shrinkage of NBG-18 than IG-110 during irradiation.

Effects of B and Ti addition and heat treatment temperature on graphitization behavior of Fe-0.55C-2.3Si steel

We determined the effects of alloying elements and heat treatment temperature on the graphitization behavior of a medium-carbon high-silicon steel (Fe-0.55C-2.3Si, wt%). Trace amounts (<150 ppm) of B and Ti were individually added to base steel and graphitization heat treatment was applied to the three compositions (base steel, B-bearing steel, and Ti-bearing steel) at 700 and 750 °C. The grain size of the B-bearing steel was similar to that of the base steel, whereas that of the Ti-bearing steel was ∼37% lesser, which can be attributed to grain boundary pinning effects induced by TiN particles. For all three steels, cementite in pearlite decomposed during heat treatment and graphite formed mostly along the grain boundaries where carbon diffusion readily occurred. The graphitization rate follows the order of Ti-bearing steel > B-bearing steel > base steel, which is attributed to the numerous grain boundaries in the Ti-bearing steel and presence of BN particles in the B-bearing steel because the grain boundaries and BN particles act as major graphite nucleation sites. A higher abundance of nucleation sites for graphitization in the initial microstructure leads to higher graphite density but reduced average graphite size. For all three steels, higher heat treatment temperature promoted both cementite decomposition and graphite formation by accelerated carbon diffusion, which reduced the graphitization completion time, decreased the graphite number density, and increased the graphite size. Fine and uniformly distributed graphite forms within a short time (1 h) in the Ti-bearing steel treated at 750 °C, which significantly decreased its strength and improved its ductility.

Tribological Behavior of Copper and Graphite of Layered Friction Materials

Copper and graphite are the main components of copper-based brake materials commonly used in high-speed trains. Their interaction influences the tribological performance of copper-based brake materials. In this article, with copper and graphite as the materials, three kinds of layered samples were prepared as pins and tested against a disc made of H13 steel. Friction tests were conducted under the condition of 0.8–2.35 m/s and pressure of 0.5 MPa using a pin-on-disc tribometer. Results showed that the friction coefficient decreased with an increase in the angle between boundary orientation and the friction direction in the range of 0°–90°. When the angle was 90°, the interaction between copper and graphite was the strongest. Graphite provided positions for friction film accumulation. Copper and its friction film covered and protected the graphite and boundary and reduced the friction coefficient.

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.

4D synchrotron X-ray microtomography of fracture in nuclear graphite after neutron irradiation and radiolytic oxidation

Herein, the first study is presented using 4D synchrotron X-ray microtomography to capture all stages of crack development in neutron irradiated and radiolytically oxidised nuclear graphite. Employing a novel loading setup, specimens of Gilsocarbon graphite, both unirradiated and irradiated at 301 °C to 19.7 × 1020 neutrons/cm2 (∼2.6 displacements/atom (dpa)), were loaded to generate a crack. All stages of the fracture process were then captured using synchrotron X-ray imaging. Reconstructed tomographic images and 3D segmented crack volumes have been used to observe and analyse the irradiation-induced evolution of the graphite microstructure as well as corresponding changes in the crack initiation, propagation, and arrest behaviour of graphite after neutron irradiation. Close examination of the applied stress-strain curves highlights the suppression of micro-crack-based damage accumulation in irradiated graphite. Moreover, as well as the crack-bridging and deflection mechanisms characteristic of unirradiated graphite, crack arrest in the irradiated graphite is shown to be significantly influenced by crack tip blunting. This change is associated with the growth of the open pore structure of graphite, specifically the enlargement and increased frequency of macro-pores, resulting from the simultaneous radiolytic oxidation of the graphite microstructure during neutron irradiation.

Spectral manifestations of coal metamorphism: Insights from coal microstructural framework

The present study investigates the microstructural framework of coals from the Raniganj and the Jharia Basins as well as from the Himalayan fold-thrust belts of Sikkim, India, by vitrinite reflectance, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The mean random vitrinite reflectance data may imply that the coal samples from the Raniganj and the Jharia Basins range from the bituminous D to B, and would have experienced the burial heating corresponding to ~92 °C to 143 °C unlike the anthracite A samples that might have undergone the hydrothermal metamorphism at ~333 °C to 367 °C. The Raman spectra exhibit preferential removal of sp3 hybridized moieties from the anthracite samples at anchizonal metamorphic stage due to thermo-stress degradation effect induced by the Himalayan orogeny. Further, this study records the reduction of net dipole moment change due to the expulsion of functional groups from the aromatic rings making them infrared inactive. Additionally, the C1s spectra reveal the thinning of CC peaks and the lowering of the CO peak intensity with an increase in coal rank. This study, hence, documents the microstructural transformations in coal that can be used for structural maneuvering to control the liquefaction, gasification, pyrolysis, combustion as well as synthetic graphitization behavior.