Density Graphite Research Articles

Electrochemical behavior of pure graphite studied with a powder microelectrode

In this work, cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques have been used to study the lithiation/delithiation of graphite using powder microelectrode in a 1.2 M LiPF6 in EC:EMC electrolyte. The advantage of using the powder microelectrode is the possibility to easily study graphite without any additive and determine electrochemical characteristics of Li-insertion. The use of cyclic voltammetry at very low scan rates allows to estimate the diffusion coefficient of lithium inside graphite. The exchange current density of graphite for several state of charge (SoC) has also been determined by EIS measurements. Moreover, the powder microelectrode is a useful tool to study the formation of the solid electrolyte interphase (SEI) on graphite either by cyclic voltammetry or EIS.

OA074] Fano theory validation of the MCNP6.0 Monte Carlo code for medical applications

Purpose Fano theory states that charged particle fluence through a medium should be independent of mass density. Therefore, the validity of a Monte Carlo code depends on whether the transport of charged particles complies with this theory. In this work, in the framework of the EMPIR funded RTNORM project, the validity of the Fano theory in the Monte Carlo code MCNP6.0 is tested in a cavity. Methods The methodology used is based on the one performed for the code PENELOPE. Two concentrical cylinders of graphite are created, with the inner one having a density equal to the density of air, and the outer one with the natural density of graphite. A linear source of electrons with energy E 0 is created and the dose in the inner cylinder is determined. The width of the outer cylinder is equal to 1.4* R CSDA . In this setup, the value of: Q = Dose per particle E 0 1 + 2 ρ wall Δ z wall ρ gas Δ z gas should be equal to 1. Dose per particle is determined by means of the tally F6 of the MCNP6.0 code. Default as well as modified physics parameters are used to ascertain limits of the validity. Parameters such as EFAC, ESTEP are changed to verify the changes in the value of Q. Results Results show that if using default physics parameters in the MCNP6.0 code the Fano theory is not verified in the E 0 electron range of 50–200 keV. Conclusions MCNP6.0 has difficulty in tallying energy deposition accurately in a cavity if default physics parameters are used. In this energy range MCNP6.0 presents crossing-boundary artifacts that could probably be due to the use of the condensed-history electron transport Energy Indexing Algorithm (ITS algorithm).

Impact of new ICRU Report 90 recommendations on calculated correction factors for reference dosimetry

In 2016 the ICRU published a new report dealing with key data for ionizing radiation dosimetry (ICRU Report 90). New recommendations have been made for the mean excitation energies I for air, graphite and liquid water as well as for the graphite density to use when evaluating the density effect. In addition, the ICRU Report 90 discusses renormalized photoelectric cross sections, but refuses to give a recommendation on the use of renormalization factors. However, the Consultative Committee for Ionizing Radiation recommends to use renormalized photoeffect cross sections. Goal of the present work is to evaluate the impact of these new recommendations on clinical reference dosimetry for high energy photon and electron beams.The beam quality correction factor kQ was calculated by Monte Carlo simulations for compact and parallel plate ionization chambers. In case of photons seven phase space files from clinical accelerators and twelve spectra taken from literature from 4 MV to 24 MV and additionally a 60Co source were applied. As electron source thirteen electron spectra available in literature were used in the range of 4 MeV–21 MeV.The new ICRU recommendations have a small impact on Monte Carlo calculated kQ values for the chosen ionization chambers in the range of 0.1%–0.35% only—the difference increases for higher photon energies.The impact of the ICRU Report 90 recommendations on Monte Carlo calculated stopping power ratios sw,a, perturbation factors p and beam quality correction factors kQ was investigated and confirmed a decrese of sw,a by a fraction of a percent for photon and electron beams. This study indicates that the impact of the new ICRU recommendation is within 0.35%. The determined deviations should be taken into account, when widely published Monte Carlo calculated values are examined.

Dopamine-grafted heparin as an additive to the commercialized carboxymethyl cellulose/styrene-butadiene rubber binder for practical use of SiOx/graphite composite anode

Graphite is used commercially as the active material in lithium ion batteries, frequently as part of a graphite/SiOx composite. Graphite is used in conjunction with SiOx to overcome the limited energy density of graphite, and to lessen the adverse effects of volume expansion of Si. However, electrodes based on graphite/SiOx composites can be made with only 3–5 wt % SiOx because of the increased failure of electrodes with higher SiOx contents. Here, we developed a new polymer binder, by combining dopamine-grafted heparin with the commercial binder carboxymethyl cellulose (CMC)/styrene butadiene rubber (SBR), in order to more effectively hold the SiOx particles together and prevent disintegration of the electrode during charging and discharging. The crosslinking using acid-base interactions between heparin and CMC and the ion-conducting sulfonate group in heparin, together with the strong adhesion properties of dopamine, yielded better physical properties for the dopamine-heparin-containing CMC/SBR-based electrodes than for the commercial CMC/SBR-based electrodes, and hence yielded excellent cell performance with a retention of 73.5% of the original capacity, a Coulombic efficiency of 99.7% at 150 cycles, and a high capacity of 200 mAh g−1 even at 20 C. Furthermore, a full cell test using the proposed electrode material showed stable cell performance with 89% retention at the 150th cycle.

Electrochemical Characterization of High Energy Density Graphite Electrodes Made by Freeze-Casting

Despite extensive and continuous research to find alternatives, the Li-ion battery community still relies on graphite to manufacture most negative electrodes. Although graphite has many advantages, its relatively low volumetric energy density remains a limiting factor for many heavy-duty applications, such as electric vehicles (EVs) and grid storage. With the aim of increasing cells level energy density and decreasing cost by building thicker electrodes, freeze-casting, a shaping technique able to produce low-tortuosity structures by using ice crystals as a pore-forming agent, is used here to produce porous graphite anodes. The electrochemical performance is assessed using galvanostatic constant current (GCC) charge–discharge as well as hybrid pulse power characterization (HPPC) techniques. The obtained GCC discharge capacities of ∼18, ∼14, and ∼7 mAh/cm2 at C/10, C/5, and 1C, respectively, show a 5-fold enhancement compared with conventional composite electrodes. Finally, our freeze-cast electrodes also ...

An entirely enclosed scanning tunnelling microscope capable of being fully immersed in liquid helium.

We present an ultrahigh-vacuum (UHV)-sealed high-stability scanning tunnelling microscope (STM) that can be entirely immersed in liquid helium and readily used in a commercial Dewar or superconducting magnet. The STM head features a horizontal microscanner that can become standalone and ultrastable when the coarse approach inertial motor retracts. Low voltage is enough to operate the STM even at low temperature owing to the powerful motor. It is housed in a tubular chamber of 49 mm outer diameter, which can be pumped via a detachable valve (DV), UHV-sealed and remain sealed after the DV is removed. The entire so-sealed chamber can then be inserted into liquid helium, where in situ sample cleavage is done via vacuum bellow. This allows sample cleavage and STM measurements to take place in better UHV with higher cooling power. Quality atomic resolution images of graphite and charge density wave on 1T-TiSe2 taken in ambient and 14 K conditions, respectively, are presented.

Enhanced specific surface area and thermal conductivity in ultrathin graphite foams grown by chemical vapor deposition on sintered nickel powder templates

The specific surface area and effective thermal conductivity of ultrathin graphite foams (UGFs) grown by chemical vapor deposition (CVD) have been limited by the large pore sizes of reticulated nickel foam templates. Here we show that nickel templates formed by sintering of low-cost nickel powder can reduce pore size and increase specific surface area. Accordingly, the effective density of a free-stranding graphite foam grown on the high-surface area templates can be increased by an order of magnitude relative to one grown on a reticulated nickel foam template. This increase is accomplished without increasing the thickness of the CVD graphite or decreasing the structural quality of the graphite. Based on experiments with nickel powder with and without the reticulated nickel template present, we found that both the particle size and sintering temperature influenced template morphology. Because of a 68-times increase in the volumetric specific surface area of a graphite foam synthesized on a sacrificial sintered powder template compared to a UGF grown on a reticulated template, we have been able to increase the relative graphite density and effective thermal conductivity from 0.87 ± 0.09% to 8.4 ± 0.3% and from 2.1 ± 0.3 Wm−1K−1 to 16.3 ± 1.1 Wm−1K−1, respectively.

Monte Carlo modeling and simulations of AHTR fuel assembly to support V&V of FHR core physics methods

The Fluoride Salt Cooled High Temperature Reactor (FHR) is a liquid salt-cooled reactor utilizing TRISO-based fuel. Its low operating pressure and large thermal margin offer attractive safety. However, its complex core design, in particular the double heterogeneity introduced by TRISO particles, as well as the presence of several moderating materials, require careful verification and validation (V&V) of the core physics modeling and simulation (M&S) methods. Due to their ability to accurately simulate complex geometries and utilize continuous-energy nuclear data, Monte Carlo methods are used in this study to evaluate core physics phenomena and provide reference solutions for verification of lower fidelity methods. This work is focused on the FHR design with hexagonal fuel elements consisting of fuel plates (“planks”), i.e., the Advanced High-Temperature Reactor (AHTR) developed by ORNL. The pebble-bed FHR design, PB-FHR, is not directly considered in this work, although the results of this study are relevant for PB-FHR as well. The SERPENT Monte Carlo code is selected and utilized to perform core physics analyses of an FHR fuel assembly. The issues investigated include detailed fission distribution, quantification of certain approximations, study of neutron spectra in different material regions, and impact of graphite density, temperature, and scattering matrix data. This study contributes to V&V of modeling and simulation methods for FHR.

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Personal cooling devices reduce energy loads by allowing buildings to operate with elevated setpoint temperatures, without compromising on the occupant comfort. One such novel technology called the Roving Comforter (RoCo) uses a compact R134a based vapor compression system for cooling. Following its cooling operation, during which waste heat from the condensing refrigerant is stored in a phase change material (PCM), a two-phase loop thermosiphon is used to discharge (solidify) the PCM to enable its next operation. The transient operation of this thermosiphon is the focus of the present article. Use of a PCM as the storage medium provides high energy density due to the ability to store thermal energy as latent heat during the phase transition; however, the discharge rate is limited by the low thermal conductivity of the PCM. Insertion of a graphite foam within the PCM can increase the rate of discharge and decrease the downtime of the cooling device. Since graphite enhancement involves a tradeoff between improving the discharge time at the expense of PCM volumetric latent heat, the impact of graphite foam density on the PCM discharge rate is investigated by using a Modelica-based transient model of the thermosiphon. The semi-empirical model, which uses relevant heat transfer coefficient and pressure drop correlations for both refrigerant and airside heat transfer, captures the complex phenomena involving simultaneous phase change of the refrigerant and the PCM. The graphite enhanced PCM selected from this analysis results in a 51% reduction in the discharge time with addition of only 5% to the thermal storage weight, without compromising the required cooling time.

Improving Li-ion battery charge rate acceptance through highly ordered hierarchical electrode design

In Li-ion technology, increasing electrode loading (thickness) is one approach to improve performance; however, this approach typically compromises power density and safety. To achieve the goal of decoupling energy and power density, a novel electrode architecture is proposed. The electrode design enhances uniform ionic current, especially in thick electrodes. A highly ordered and hierarchical (HOH) graphite anode concept was designed, fabricated, and tested for efficacy. The HOH electrodes consisted of ordered arrays of macro-scale line-of-sight linear channels made through laser ablation. SEM and Raman spectroscopy demonstrated that laser ablation is a feasible approach to fabricate HOH electrodes without affecting the graphite anode chemistry, respectively. A 65–120% improvement in charge rate acceptance (5.5 mAh/cm2) was achieved in the HOH electrodes compared to conventional electrodes. A restricted diffusion direct current polarization test determined that the HOH design improved ionic flow throughout porous electrodes. Altogether, the results of this study suggest that improved charge rate acceptance can be achieved by engineering electrode porosity to mitigate the effects of concentration polarization in high energy density graphite anodes. These findings can facilitate the development of higher energy and power density Li-ion batteries, while improving resilience against Li plating under severe charge conditions.

Feasible preparation and improved properties of Ag‐graphite composite coating for switch contact by cyanide‐free electrodeposition

A cyanide‐free electrodeposition system was employed to prepare Ag‐graphite composite coatings on Cu substrate for switch contact. The effects of graphite concentration, current density, stirring speed, and plating temperature on the thickness and quality of the composite coating were investigated. The feasibility of cyanide‐free electrodeposition for Ag‐graphite composite was confirmed and the optimized conditions were determined. The growth mechanism of the composite coating was also analyzed. The properties of the composite coating were also investigated compared with Cu substrate and Ag coating. The results show that the Ag‐graphite composite coating has the lowest friction coefficient, moderate corrosion resistance, and relatively low contact resistance. The present results indicate that Ag‐graphite composite coating prepared by cyanide‐free electrodeposition is suitable for disconnector coating.

Influence of Silicon on Nucleation and Growth Process of Spherical Graphite in Ni-C Alloys Solidification Structure

Plasma etching, SEM and quantitative metallographic analysis technology were used to study the influence of Si on nucleation and growth process of spherical graphite in Ni-C alloys solidification microstructure. Pure Si, Ni-Si alloys and Fe-Si alloys were added into Ni-C melts respectively, and the solid microstructures were compared. The density, diameter size and roundness of spherical graphite were studied in this work to reveal the influence of Si on the morphology of graphite. The Ni-C alloy was etched by plasma etch, and the morphologies and composition of graphite nucleus were observed by scanning electron microscope (SEM) and electron energy spectrum analysis (EDS). The results suggested that the high Si melt zone in melt strongly promoted the enrichment and carbide of graphite, which both promoted the nucleation and growth of graphite. The change in Si content had a large effect on the roundness of the graphite, the higher the Si content, the better the roundness of the graphite.

Molten Salt Corrosion Resistance of Yttria Stabilized Zirconia Coating with Silicon Carbide Interlayer on High Density Graphite

The high temperature corrosion behaviour of yttria stabilized zirconia (YSZ) coated high density graphite (HDG) with and without SiC interlayer was evaluated in LiCl–KCl molten salt at 600 °C. The YSZ coated HDG samples without interlayer were tested for 50, 250, 1000 and 2000 h and the YSZ coated HDG samples with SiC interlayer were studied for 50, 500, 2000 and 3500 h durations. Characterisation of the corrosion tested samples revealed no delamination or failure of the YSZ coating in both the cases, and insignificant weight gain was observed in all the tested samples in the molten salt. The lamellar morphology of the plasma spray coating was intact even after exposure to molten salt for long duration, implying no corrosion attack of the coating and HDG in the molten salt. SEM cross-section examination confirmed neither coating degradation, nor penetration of molten salt into the HDG substrate. X-ray diffraction and Laser Raman spectroscopic analyses ascertained that no phase change occurred in YSZ after the long exposure in molten salt. Present study confirmed the phase stability and durability of YSZ coating in providing corrosion resistance and protection to HDG for high-temperature molten salt applications.

Effect of graphite target power density on tribological properties of graphite-like carbon films

In order to improve the tribological performance, a series of graphite-like carbon (GLC) films with different graphite target power densities were prepared by magnetron sputtering. The valence bond and microstructure of films were characterized by AFM, TEM, XPS and Raman spectra. The variation of mechanical and tribological properties with graphite target power density was analyzed. The results showed that with the increase of graphite target power density, the deposition rate and the ratio of sp2 bond increased obviously. The hardness firstly increased and then decreased with the increase of graphite target power density, whilst the friction coefficient and the specific wear rate increased slightly after a decrease with the increasing graphite target power density. The friction coefficient and the specific wear rate were the lowest when the graphite target power density was 23.3 W/cm2.

Experimental investigation of density of pyrolytic graphite up to melting point

This article contains data on the measurement of the density of pyrolytic graphite with an initial density of 2.18 g/cm3 in the temperature range 3000–4800 K under isobaric conditions at a static external pressure of 1 kbar. The features of the experimental technique, representing a certain complexity due to intense sublimation of graphite at high temperatures, are also described.

Исследование пористой структуры графита для силицирования

This article presents the results of a study of the porous structure and physicomechanical properties of graphite intended for impregnation with molten silicon. The study of the porous structure of graphite has been carried out by the method of mercury porosimetry, based on the property of mercury not to moisten solid carbonaceous bodies and to fill the pores only at the corresponding hydrostatic pressure. Each pressure value corresponds to a certain pore diameter. The analysis of mechanical strength under compression, of apparent density, total, open and closed porosity of industrial graphite for siliconizing has shown unevenness of its properties and porous structure. Graphite for siliconizing is characterized by high density and microcracking. The analysis of mercury porogram of graphite samples has confirmed the unevenness of its porous structure. The average pore diameter of graphite samples widely varies from 3 to 21 micrometers. All graphite samples are distinguished by a large number of pores of less than one micrometer, into which the molten silicon penetrateswith difficulty. Of the nine graphite samples studied, five samples are distinguished by a low (2–5 %) content of large pores over 100 micrometers; in four graphite samples the number of pores over 100 micrometers is quite significant and varies within 10 ... 20 %. According to the research results, the presence of large pores in graphite also reduces the efficiency of impregnation and impairs the properties of siliconized graphites. Determination of the physicochemical properties of graphite impregnated with molten silicon has shown that, despite the non-uniformity of properties and porosity of the initial graphite, sufficiently high apparent density of siliconized graphite is reached, which is 2.24–2.27 g/cm 3 and meets the requirements. However, the uneven porosity of graphite leads to decreasing content of silicon carbide. Only in five samples of the siliconized graphite the silicon carbide content has been reached at the minimum level of requirements of 22.23 %. In four samples the content of silicon carbide is below the permissible value and equals 20 %. The results allow us to conclude that it is necessary to reduce the density of graphite for siliconizing and reduce the number of pores less than 1 micrometer and more than 100 micrometers.

Study on mechanical properties of graphite particulates reinforced aluminium matrix composite fabricated by stir casting technique

The lineage of applied materials science is always in demand for light weight and highly performing materials. Such materials would find their applications in aircraft, structural, non-structural and automobile industries, etc. The present research study focuses on the production of aluminium (AA6351) matrix composites reinforced in different mass fractions of graphite particulates by using stir casting method. The mass fraction of reinforcement was varied from 0% to 12% in stages of 4%. Hardness and tensile strength of the composite were investigated. The microstructures of the produced composites were examined by scanning electron micrographic test. The SEM images revealed the non-homogeneous distribution of graphite (Gr) particles in the matrix and this may be due to low density of graphite. The test results revealed that the mechanical properties of the composite decrease with increase in the mass fraction of graphite particle content, this may be due to poor interfacial bonding between the reinforcement and the matrix. The brittle nature of the reinforcing particles (graphite) plays a vital role in decreasing the mechanical properties because the graphite as a soft reinforcement is brittle in nature and so it enhanced the brittleness in the AMCs.

Carbon-rich shungite as a natural resource for efficient Li-ion battery electrodes

A wide variety of synthetic carbon allotropes embrace tremendous potential for energy storage applications. There have been number of reports on the synthesis of carbonaceous anode materials with lithium (Li) storage capacity larger than the theoretical limit for graphite (372 mAh/g, corresponding to LiC6). However, besides the performance, available resources and cost efficiency are obstacles that could hinder their exploitation. Here we present carbon-rich Precambrian rock “shungite” as a natural resource for a Li-ion battery anode. It is found that upon structural modification the energy density of fully lithiated “shungite” can exceed the density of graphite, becoming comparable to non-graphitizable (“hard”) carbon. High-resolution transmission electron microscopy (HRTEM) studies of modified “shungite” suggest that it consist of spatially arranged fractals of bended, curved, mono- or stacked graphene layers. By analyzing the features of 7Li Nuclear Magnetic Resonance (NMR) spectra of fully lithiated “shungite” we conclude that the enhanced storage capacity is mainly due to the presence of open edge few layered graphene flakes. We thus suggest carbon-rich “shungite” as an alternative and effective natural resource for Li-ion battery electrodes.

Monte Carlo Analysis of the Battery-Type High Temperature Gas Cooled Reactor

Abstract The paper presents a neutronic analysis of the battery-type 20 MWth high-temperature gas cooled reactor. The developed reactor model is based on the publicly available data being an ‘early design’ variant of the U-battery. The investigated core is a battery type small modular reactor, graphite moderated, uranium fueled, prismatic, helium cooled high-temperature gas cooled reactor with graphite reflector. The two core alternative designs were investigated. The first has a central reflector and 30×4 prismatic fuel blocks and the second has no central reflector and 37×4 blocks. The SERPENT Monte Carlo reactor physics computer code, with ENDF and JEFF nuclear data libraries, was applied. Several nuclear design static criticality calculations were performed and compared with available reference results. The analysis covered the single assembly models and full core simulations for two geometry models: homogenous and heterogenous (explicit). A sensitivity analysis of the reflector graphite density was performed. An acceptable agreement between calculations and reference design was obtained. All calculations were performed for the fresh core state.

Dielectric Polarization of a High-Energy Density Graphite Anode and Its Physicochemical Effect on Li-Ion Batteries

The high energy density graphite anode for the commercial LIBs has critical problems on Li+-ion kinetics due to decreases both in electrode porosity and electrolyte permeability. To overcome issues, interfaces of graphite particles in the anode are polarized using poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) with the high dielectric constant (e = 8.4), high solubility with lithium salt, and ability to trap a large amount of liquid electrolyte. The PVDF-HFP treatment promoted electrolyte permeability into the graphite electrode with a high mass loading of 13.8 mg cm–2 and a density of 1.7 g cc–1 (a current density over 5 mA cm–2) which particularly leads to an improvement of capacity retention from 77% of a bare electrode to 95% over 40 cycles. These achievements were attributed not only to the enhancement of the lithium-ion kinetics but also to the stable formation of a solid electrolyte interface (SEI) layer on the graphite surface.