Phase Transition In Graphite Research Articles

Introducing surface adsorption lithium storage mechanism to enhance safety of graphite

The thermal stability of lithiated graphite plays an important role in the thermal safety of lithium-ion batteries (LIBs). However, a safer graphite anode usually leads to a lower energy density. This article starts from multiple hazardous factors of lithiated graphite when thermal runaway occurs, considering the impact of defects on the electrochemical performance and thermal safety of graphite electrodes. In this study, turbostratic graphite was prepared as the anode for LIBs. The stacking of graphite was altered to form narrow pores and spherical aggregates, and introducing defects. By varying the ball-milling time, the structural changes of different ball-milled graphites were analyzed. The impact mechanism of defect structure on electrode electrochemical performance and safety performance was investigated. The results show that the spherical structure and large specific surface area are beneficial to increase the active sites and delay the self-heating of the battery. The formation of defects and pores increases the adsorption sites, which makes the surface adsorption of Li+ dominant and improves the ion diffusion. In addition, the dominance of surface adsorption and desorption effectively suppresses the problem of graphite flake peeling and intensified thermal runaway caused by graphite phase transition and lithium evolution heat release at high temperature. The thermal safety test was carried out by accelerated calorimeter (ARC), and its thermodynamics and reaction kinetics were quantitatively analyzed. It was proved that the introduction of surface adsorption lithium storage method successfully delayed the thermal runaway of the battery, realizing the balance between high electrochemical performance and high safety.

Preparation of nanodiamonds based on phase transformation of vertical sheet under atmospheric pressure

A basic and important way to prepare diamond is to make graphite experience the phase transformation under the high-pressure high-temperature (HPHT) condition. However, this method needs stringent equipment and high investment cost. Recently, we proposed a method to prepare the diamond by phase transformation of graphite at atmospheric pressure with monodispersed Ta atoms. It is found that a phase transformation happens to H atoms under atmospheric pressure, but the role of O atoms has not been investigated. Here, we use tantalum wires as Ta source and heat the filaments to prepare vertical graphene containing Ta atoms in hot filament chemical vapor deposition (HFCVD) system. And then the vertical graphene layers are annealed in oxygen-containing environment, and nanodiamonds are obtained by phase transformation from the vertical graphene under atmospheric pressure. The results show that the sample morphologies are the same as the untreated vertical graphene’s, when the annealed ambient air pressure is at 10 Pa and 50 Pa with oxygen atom content of 1.96% and 2.04%, respectively; TEM tests reveal TaC and graphite but no diamond in these samples . Nanodiamond grains with the size range of 2–4 nm are observed in the amorphous carbon region of samples annealed at 100 Pa and 500 Pa air pressure with oxygen atom content increasing to 2.77% and 3.11%, respectively, indicating that oxidation facilitates the phase change from Ta-containing vertical graphene to diamond at atmospheric pressure. When the air pressure of the annealing environment rises to 1000 Pa with the oxygen atom content of 3.54%, the sample is extensively oxidized and the graphite structure is severely damaged,which means that a large number of oxygen atoms tend to disrupt the graphite structure rather than promote the phase change into diamond. These results supply a way to prepare nanodiamond and show the effect of O atoms in the graphite phase transition at atmospheric pressure.

Alkali metals induced stacking phase transition of graphite

It is well known that alkali metals can intercalate into graphite to form graphite intercalation compounds but the structural change of graphite induced by alkali metals has been paid less attention. Herein, we report that alkali metals (Li, Na, K) induce the phase transition from ABC-stacking (or 3R phase) to AB-stacking (or 2H phase) of graphite. Specifically, for the graphite powder containing ∼45% 3R phase in a mixed phase, the temperature needed to ensure a complete stacking phase transition is 600 °C, 500 °C or 350 °C, for Li, Na or K, respectively, much lower than that needed in bare annealing (1800 °C) while without these alkali metals. Simulations based on density functional theory (DFT) suggest that the charge transfer from alkali metals to graphene distinctly lowers the energy barrier of transition, explaining the different phase transition temperatures for these alkali metals.

An Improved Experiment for Measuring Lithium Concentration-Dependent Material Properties of Graphite Composite Electrodes.

The in situ curvature measurement of bilayer beam electrodes is widely used to measure the lithium concentration-dependent material properties of lithium-ion battery electrodes, and further understand the mechano-electrochemical coupling behaviors during electrochemical cycling. The application of this method relies on the basic assumption that lithium is uniformly distributed along the length and thickness of the curved active composite layer. However, when the electrode undergoes large bending deformation, the distribution of lithium concentration in the electrolyte and active composite layer challenges the reliability of the experimental measurements. In this paper, an improved experiment for simultaneously measuring the partial molar volume and the elastic modulus of the graphite composite electrode is proposed. The distance between the two electrodes in the optical electrochemical cell is designed and graphite composite electrodes with four different thickness ratios are measured. The quantitative experimental data indicate that the improved experiment can better satisfy the basic assumptions. The partial molar volume and the elastic modulus of the graphite composite electrode evolve nonlinearly with the increase of lithium concentration, which are related to the phase transition of graphite and also affected by the other components in the composite active layer. This improved experiment is valuable for the reliable characterization of the Li concentration-dependent material properties in commercial electrodes, and developing next-generation lithium batteries with more stable structures and longer lifetimes.

Shock-wave polymorphic transition in porous graphite

For the polymorphic transformation of porous graphite in the shock, the previously proposed model linking the process of graphite phase transition with a change in the elastic energy of a substance has been tested. It is shown that the model plausibly describes the experimental results outside the transition zone in a fairly wide range of changes in the porosity of samples with their different initial structure. It is discussed how the model under consideration changes the currently existing ideas about the thermodynamics of the polymorphic transition of matter in a shock wave. Keywords: polymorphism, shock wave, porosity, graphite, diamond, phase transition.

Wide Critical Fluctuations of the Field-Induced Phase Transition in Graphite.

In the immediate vicinity of the critical temperature (T_{c}) of a phase transition, there are fluctuations of the order parameter that reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to a high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20%) reduction in the amplitude of the magnetic field (from 33 to 27T) leads to a threefold decrease of T_{c} and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T_{c}. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the 3DXY universality class as in the case of the ^{4}He superfluid transition.

Ударно-волновой полиморфный переход в пористом графите

For the polymorphic transformation of porous graphite in the shock, the previously proposed model linking the process of graphite phase transition with a change in the elastic energy of a substance has been tested. It is shown that the model plausibly describes the experimental results outside the transition zone in a fairly wide range of changes in the porosity of samples with their different initial structure. It is discussed how the model under consideration changes the currently existing ideas about the thermodynamics of the polymorphic transition of matter in a shock wave.

Tribological properties improvement of H-DLC films through reconstruction of microstructure and surface morphology by low-energy helium ion irradiation

The hydrogenated diamond-like carbon (H-DLC) films were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique and implanted with 60 keV He ion (He+) over a wide dose ranges. A two-steep model for the evolution of tribological properties of H-DLC films after irradiation has been proposed and discussed. Firstly, at the He+ doses range from 0 to 3 × 1016 cm−2, irradiation treatment induced structural transformation of graphitization in the H-DLC films modification layer due to the increase of sp2/sp3 ratio, which led to the upgradation of tribological properties owing to the lubrication effect of graphite. The decreases of the root mean square roughness (Sq) led to a decrease in the resistance to the relative movement of the friction pair. Therefore, the reduction of friction coefficient and wear rate of H-DLC was contributed to be the combined effect of graphite phase transition and low Sq value. As the dose was increased to 1 × 1017 cm−2, the degradation of tribological properties was evidenced, which could be ascribed to the formation of irradiation-induced He bubbles and holes on the surface of films. In addition, when the irradiation dose increased from 1 × 1016 cm−2 to 3 × 1016 cm−2 and then to 1 × 1017 cm−2, the variation of H/Ef value also indicated that the anti-wear properties of H-DLC film first increased and then decreased. Based on the experimental results, the H-DLC film that irradiated by the He+ at the dose of 3 × 1016 cm−2 shows the best tribological properties with low friction coefficient and high anti-wear properties while maintaining agreeable mechanical properties.

Detonation synthesis of carbon nano-onions via liquid carbon condensation

Transit through the carbon liquid phase has significant consequences for the subsequent formation of solid nanocarbon detonation products. We report dynamic measurements of liquid carbon condensation and solidification into nano-onions over ∽200 ns by analysis of time-resolved, small-angle X-ray scattering data acquired during detonation of a hydrogen-free explosive, DNTF (3,4-bis(3-nitrofurazan-4-yl)furoxan). Further, thermochemical modeling predicts a direct liquid to solid graphite phase transition for DNTF products ~200 ns post-detonation. Solid detonation products were collected and characterized by high-resolution electron microscopy to confirm the abundance of carbon nano-onions with an average diameter of ∽10 nm, matching the dynamic measurements. We analyze other carbon-rich explosives by similar methods to systematically explore different regions of the carbon phase diagram traversed during detonation. Our results suggest a potential pathway to the efficient production of carbon nano-onions, while offering insight into the phase transformation kinetics of liquid carbon under extreme pressures and temperatures.

A Modified Phenomenological Force Model for Charge Process of Lithium-Ion Battery

Lithium-ion batteries have been widely used as an important countermeasure to alleviate the energy crisis and environmental pollution, especially in automobile field. However, batteries still suffer from safety issues which limits wider application and sometimes arises public concern. A robust battery management system (BMS) is critical to detect possible safety problem of the battery pack. Many researches have been tried different ways to improve the accuracy and robustness of BMS by introducing mechanical signals like force and swelling. Most models reported in articles related SOC or concentration of lithium ions in electrodes with swelling or force through a look-up table. However, it’s not clear what C rate is appropriate for the look up table. Also most of the models are validated only for discharge process. Actually, the charge process is also important since there have been various thermal runaway accidents of automobiles during charging. In this paper, a modified phenomenological force model is proposed. The deviation of the predicted force and detected ones is shown. Then the effectiveness of utilizing swelling-SOC look-up table at different C-rates is discussed. Finally, it’s pointed out that lithium plating is responsible for the force error at end of charge, which implies that the force signal has the potential to detect lithium plating which is critical for battery aging and safety issues. The phenomenological force model is built as followed: firstly, swelling at free state and force at constrained state of cells at different C rates (1/6C, 1/3C, 1C, CCCV protocol) of commercial 30.5Ah NCM/C pouch cells is obtained separately. Then force and swelling is plotted together through their relation with SOC, as can be seen is Figure 1 (a) and (b). Finally, cells are regarded as linear springs and a specific experiment is chosen to identify the stiffness as 0.0151kN/µm for force at 1C and 0.0130kN/µm for force at 1/3C. The pure thermal swelling is investigated by 2C pulse test (charge and discharge continuously for 1440s). It’s found that the cell only expands for 1.5µm when temperature rises 8°C and thus thermal swelling for this types of cells will be ignored in further analysis. For force at 1/3C charging, by substituting swelling at 1/6C with 1/3C, RMSE can be reduced from 0.06713kN to 0.02607kN (accuracy of force sensor is 0.045kN). Similar result can be seen for other situation. Such differences originate from middle SOC which relates to the phase transition of graphite. At high SOC which corresponds to the end of Figure(a), force experiences a downward trend for force at 1C, which can be seen for discharge process or low C rates. This can be explained by the mechanism shown in Figure 1 (c) and (d). For low C rates, graphite particles can absorb all the li-ions in time. Thus, no extra li-ions will accumulate outside those particles. However, for high C rates charging, the number of Li-ions intercalate into a graphite particle per second is much higher. Here we assume that green line in Figure 1(c) indicates the capacity that a graphite particle can absorb per second, which is driven by voltage. Thus for stage ①, Li concentration will increase monotonously and precipitate out at certain point, which occupy larger volume. During constant voltage period (stage ②), as current drops, the precipitated Li will be dissolved and intercalated back to graphite particles which will reduce overall volume of cells. This downward trend can’t be seen for discharge process because during Li-ions will migrate from graphite anode to NCM cathode and won’t accumulated outside graphite particles. Moreover, the volume change of NCM cathode is much smaller. To conclude, it’s found that by utilizing swelling detected at same C rate as force, the nonlinear effect at middle SOC range arisen from phase transition of graphite can be eliminated. The downward trend of force at high C rates charge process is attributed to Li plating. Further work will focus on further exploiting force signal to identify Li plating and assess safety issue of cells. Figure 1. A phenomenological force model. (a) Force vs swelling for different C rates during charge, 1/6C-1/3C means swelling is detected at 1/6 and force is detected at 1/3C; (b) Force vs swelling for different C rates during discharge; (c) Force and current during charge; (d) mechanism of Li plating and absorbing for high C rate during charge. Figure 1

Effect of dispersion corrections on ab initio predictions of graphite and diamond properties under pressure

There are several approaches to the description of van der Waals (vdW) forces within density functional theory. While they are generally found to improve the structural and energetic properties of those materials dominated by weak dispersion forces, it is not known how they behave when the material is subject to an external pressure. This could be an issue when considering the pressure-induced structural phase transitions, which are currently attracting great attention following the discovery of an ultrahard phase formed by the compression of graphite at room temperature. In order to model this transition, the functional must be capable of simultaneously describing both strong covalent bonds and weak dispersion interactions as an isotropic pressure is applied. Here, we report on the ability of several dispersion-correction functionals to describe the energetic, structural, and elastic properties of graphite and diamond, when subjected to an isotropic pressure. Almost all of the tested vdW corrections provide an improved description of both graphite and diamond compared to the local density approximation. The relative error does not change significantly as pressure is applied, and in some cases even decreases. We therefore conclude that the use of dispersion-corrected exchange-correlation functionals, which have been neglected to date, will improve the accuracy and reliability of theoretical investigations into the pressure-induced phase transition of graphite.

Tunable magnetoresistance in thin-film graphite field-effect transistor by gate voltage

Magnetic-field-induced semimetal-insulator phase transition in graphite has regained attention, although its mechanism is not fully understood. Recently, a study performed under the pulsed magnetic field discovered that this phase transition depends on thickness even in a relatively thick system of the order of 100 nm and suggested that the electronic state in the insulating phase has an order along the stacking direction. Here we report the thickness dependence observed under dc magnetic fields, which nicely reproduces the previous results obtained under the pulsed magnetic field. In order to look into the critical condition to control the phase transition, the effect of electrostatic gating is also studied in a field-effect transistor structure since it will introduce a spatial modulation along the stacking direction. Magnetoresistance, measured up to 35 T, is prominently enhanced by the gate voltage in spite of the fact that the underlying electronic state is not largely changed owing to the charge-screening effect. On the other hand, the critical magnetic field of the semimetal-insulator transition is found to be insensitive to gate voltages, whereas its thickness dependence is fairly confirmed. By applying positive gate voltages, a prominent oscillation pattern, periodic in magnetic field, becomes apparent, the origin of which is not clear at this stage. Although electrostatic control of the phase transition is not realized in this study, the findings of gate-voltage tunability will help determine the electronic state in the quantum limit in graphite.

Reducing the Metal Content in PCD Polycrystalline Diamond Layer by Chemical and Electrochemical Etching

This article is devoted to investigating polycrystalline diamond compacts (PDCs), which find broad application in drilling, tool-and-die, and building branches of industry. They are a complex composition of diamond and cermet phases. The diamond phase consists of diamond grains of various granulometric compositions and shapes and forms a strong hard skeleton. A cermet phase serves as a binder. The presence of catalyst metals in a diamond layer of bilayer PDC composite materials lowers their operational properties, because the difference in thermal expansion coefficients between diamond grains and catalyst can lead to material cracking in the cutting process, while a high temperature when fabricating the diamond tool and its operation in the cutting zone can lead to the reverse diamond–graphite phase transition. In order to increase wear-resistance characteristics of diamond PCD composites formed using catalyst metals (cobalt and tungsten), etching of metals from the surface of the tool working zone is performed by two methods, notably, electrochemical and chemical. Electrochemical etching is performed in sulfuric acid with various current modes and concentration, and chemical etching is performed in a mixture of hydrochloric and nitric acids and a mixture of fluoric and nitric acids. The distribution of the chemical composition over the depth of PCD samples after etching is performed using scanning electron microscopy. It is established that electrochemical etching is more active kinetically, while chemical etching is promising for industrial application. Abrasive tests of PCD samples before and after etching show the absence of a noticeable effect of both electrochemical and chemical etching of their abrasive ability.

Thickness-dependent phase transition in graphite under high magnetic field

Various electronic phases emerge when applying high magnetic fields in graphite. However, the origin of a semimetal-insulator transition at $B \simeq 30\; \textrm{T}$ is still not clear, while an exotic density-wave state is theoretically proposed. In order to identify the electronic state of the insulator phase, we investigate the phase transition in thin-film graphite samples that were fabricated on silicon substrate by a mechanical exfoliation method. The critical magnetic fields of the semimetal-insulator transition in thin-film graphite shift to higher magnetic fields, accompanied by a reduction in temperature dependence. These results can be qualitatively reproduced by a density-wave model by introducing a quantum size effect. Our findings establish the electronic state of the insulator phase as a density-wave state standing along the out-of-plane direction, and help determine the electronic states in other high-magnetic-field phases.

Моделирование фазовых переходов графитов в алмазоподобные фазы

AbstractThe method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I 4_1/ amd to tetragonal LA 10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.

Shock loading of graphite between water layers: Numerical experiments

A series of numerical experiments on shock loading of graphite between water layers is realized. A simple model of the phase transition of graphite to diamond is formulated. The general scheme of the computational experiment is based on mechanical and thermal interactions of different substances (graphite, diamond, water) subjected to impact by a massive steel flyer in a cylindrical channel. The process of graphite-to-diamond transformation is traced out. The important problem of retaining the formed diamond sample and some favorable conditions to solve this question are discussed.

Simulation of the phase transition of graphite to the diamond-like LA3 phase

The phase transition of graphite to a diamond-like LA3 phase is simulated by the methods of the density functional theory (DFT). The calculations are performed in the local density approximation (LDA) and the generalized gradient approximation (GGA). It is found that the structural transformation must occur at a pressure of 60 or 74 GPa according to calculations based on the DFT–LDA and DFT–GGA, respectively. The height of the potential barrier separating the structural state corresponding to the LA3 phase from the state corresponding to graphite exceeds 0.13 eV/atom. This indicates the possibility of stable existence of the diamond-like LA3 phase under standard conditions.

Surface morphology and structural types of natural impact apographitic diamonds

External and internal morphologies of natural impact apographitic diamonds (paramorphoses) have been studied. The (0001) surface morphology of the paramorphoses reflects their phase composition and the structural relationship of its constituting phases. Growth and etch figures together with the elements of crystal symmetry of lonsdaleite and diamond are developed on these surfaces. The crystal size of lonsdaleite is up to 100 nm, and that of diamond is up to 300 nm. Two types of structural relations between graphite, lonsdaleite, and diamond in the paramorphoses are observed: the first type (black, black-gray, colorless and yellowish paramorphoses): the (0001) graphite face is parallel to the (100) lonsdaleite face and parallel to (111) diamond; the second type (milky-white paramorphoses): the (0001) graphite is parallel to the (100) lonsdaleite and parallel to the (112) diamond. The first type of the paramorphoses contains lonsdaleite, diamond, graphite or diamond, lonsdaleite, the second type of the paramorphoses contains predominantly diamond. The direct phase transition of graphite → lonsdaleite and/or graphite →diamond occurred in the paramorphoses of the first type. A successive phase transition graphite → lonsdaleite → diamond was observed in the paramorphoses of the second type. The structure of the paramorphoses of this type shows characteristic features of recrystallization.

A review on the structure of cold-compressed graphite phase

The room-temperature structural phase transition of graphite at elevated hydrostatic pressure has drawn considerable scientific interest for its fundamental importance in condensed matter physics and materials science over the past few decades. A pressure-induced phase transition has been demonstrated in previous experiments by the measurement of electrical resistance, optical spectrum, X-ray diffraction spectrum, inelastic X-ray scattering and Raman spectroscopy. However, the nature of the cold-compressed graphite phase has been puzzling the experts and pioneers in the field of high pressure research due to some inherent factors, until recently a monoclinic structure, i.e. [Formula: see text]-carbon, stands out from the other structure candidates and successfully accounts for the crystal structure of the cold-compressed graphite phase both in theory and experiment that eventually putting an end to this long-lasting controversial issue. This paper reviews the recent progress on the pressure-induced phase transitions of graphite at room temperature especially for the theoretical investigations. The review will focus on the recent proposed novel carbon allotropes as candidate structures of the cold-compressed graphite phase by using different crystal structure prediction methods. The history of structure determination of cold-compressed graphite phase is discussed.

Size-dependent phase transition of graphite to superhard graphite under high pressure at room temperature

In situ electrical resistivity measurement of graphite compressed in two different pressure cycles in one single experiment at room temperature in the pressure range of 0-34 GPa has been reported. The electrical results indicate that less superhard graphite formed in the second cycle than in the first one. Our experiments identify the phase transition of the graphite at 15.1 and 17.9 GPa for the first and second pressure cycles, respectively, and explain why the superhard post-graphite cannot indent diamond films. The scanning electron microscopy of the graphite powders before and after pressure annealing helps to support the conclusion that both the phase transitions and the formation of superhard post-graphite are sensitive to the grain size of the initial graphite sample under high pressure.