Impurities In Graphite Research Articles

Electrochemical methods for the removal of impurities from thegraphite anode in spent ternary lithium-ion batteries

The use of lithium-ion batteries (LIBs) is becoming increasingly widespread, and a large number are reaching their end of life. The recycling and re-use of spent LIBs has attracted great attention. Because of the unchanged layer structure of the graphite anode in these batteries, their recycling does not require high-temperature graphitization, and only focuses on the removal of internal impurities. We used electrochemical treatment for the deep removal of internal metal impurities after the heat treatment, ultrasonic separation, and acid leaching of spent graphite. By comparing and analyzing the graphite in different recovery stages, it was found that the presence of organic impurities seriously affects the electrochemical performance. The presence of trace inorganic impurities such as Cu and Fe has little effect on the initial discharge specific capacity, but reduces the cycling stability of graphite. The content of the main metal impurities in the final recycled graphite was less than 20 mg/kg. The discharge specific capacity reached358.7 mAh/g at 0.1 C, and the capacity remained at 95.85% after 150 cycles. Compared with the reported methods for recycling spent graphite, this method can efficiently remove impurities in the graphite, solve the current problems of high acid and alkali consumption, incomplete impurity removal and high energy consumption. The recycled graphite anode has a good electrochemical performance. Our work provides a new recycling and regeneration path for spent LIB graphite anodes.

A facile approach for regeneration of graphite anodes from spent lithium-ion battery

Recycling of lithium-ion battery anodes has become a topic of interest due to the retirement of batteries. However, existing recycling methods have either been environmentally unfriendly or lacked high purity levels. Here, a simple and low-cost recycling method is developed. Firstly, organic matter in graphite is eliminated through heat treatment. The removal of metal impurities in graphite is then carried out by utilizing sulfuric acid (H2SO4) leaching. Lastly, potassium hydroxide (KOH) etching is utilized to restore the structural defects of graphite. This comprehensive process enhances the cycling stability and rate capability of graphite. The initial discharge capacity at 0.1 C can reach a specific capacity of 422.42 mAh/g, and the specific capacity of the discharge after 100 cycles is 365.97 mAh/g, which reaches the standard of commercial graphite (CG). The results demonstrate that the method effectively purifies and regenerates waste graphite (WG). This process can alleviate resource pressure while being suitable for industrial applications due to its simplicity and low cost.

Comprehensive analysis of ASTRA benchmark with MCU-HTR and Serpent codes

Key problems in the simulation of pebble-bed high-temperature gas-cooled reactors include approximations of TRISO fuel particles and pebble-bed models, choice of nuclear data libraries, and uncertainty in graphite impurity content. This study is focused on the investigation of these problems for the ASTRA critical facility simulation with MCU-HTR and Serpent Monte Carlo codes. This paper presents a comparative analysis of the ASTRA benchmark (IEU-COMP-THERM-008) through consistent code-to-code comparisons. Some studies were performed on the simplified models of the ASTRA facility. A special care was taken to develop similar models of the ASTRA facility for the calculations using MCU-HTR and Serpent. A good agreement in the multiplication factor (less than 160pcm) between the simulations of ASTRA critical configurations using MCU-HTR and Serpent with ENDF/B-VII.0 cross-section libraries was obtained. In addition, the reactivity impact of different graphite thermal scattering libraries included in ENDF/B-VIII.0 release on the calculated results for the ASTRA benchmark was examined. Graphite evaluations assuming 0, 10, and 30 % porosity were considered in Serpent simulations with ENDF/B-VIII.0 cross-section libraries. The spread of the calculated keff for the measured critical configurations having different core height was found to be approximately 200pcm for most selected models. Consequently, model’s ability to correctly capture the procedure of a reactor loading during startup was proven. However, a significant overestimation of the keff (1000–1500pcm) was observed when we used the equivalent boron content (EBC) in the reflectors graphite adjusted to reproduce the measured graphite absorption cross-section, the EBC in matrix graphite of 1 ppmwt., ENDFB/VIII.0 cross-sections, and 30 % or 10 % porous graphite thermal scattering libraries in the Serpent simulations.

Aqueous electrochemical delithiation of cathode materials as a strategy to selectively recover lithium from waste lithium-ion batteries

Lithium recovery from end-of-life Li-ion batteries (LIBs) through pyro- and hydrometallurgical recycling processes involves several refining stages, with high consumption of reagents and energy. A competitive technological alternative is the electrochemical oxidation of the cathode materials, whereby lithium can be deintercalated and transferred to an electrolyte solution without the aid of chemical extracting compounds. This article investigates the potential to selectively recover Li from LIB cathode materials by direct electrochemical extraction in aqueous solutions. The process allowed to recovering up to 98% of Li from high-purity commercial cathode materials (LiMn2O4, LiCoO2, and LiNi1/3Mn1/3Co1/3O2) with a faradaic efficiency of 98% and negligible co-extraction of Co, Ni, and Mn. The process was then applied to recover Li from the real waste LIBs black mass obtained by the physical treatment of electric vehicle battery packs. This black mass contained graphite, conductive carbon, and metal impurities from current collectors and steel cases, which significantly influenced the evolution and performances of Li electrochemical extraction. Particularly, due to concomitant oxidation of impurities, lithium extraction yields and faradaic efficiencies were lower than those obtained with high-purity cathode materials. Copper oxidation was found to occur within the voltage range investigated, but it could not quantitatively explain the reduced Li extraction performances. In fact, a detailed investigation revealed that above 1.3 V vs. Ag/AgCl, conductive carbon can be oxidized, contributing to the decreased Li extraction. Based on the reported experimental results, guidelines were provided that quantitatively enable the extraction of Li from the black mass, while preventing the simultaneous oxidation of impurities and, consequently, reducing the energy consumption of the proposed Li recovery method.

Leaching kinetics of impurity removal from aphanitic graphite by HCl leaching

ABSTRACT As a systematic study, this investigation explored the effects of various leaching conditions (hydrochloric (HCl) acid concentration, solid–liquid ratio, temperature, reaction time, and stirring speed on graphite impurity removal rate) to model the ash removal leaching kinetics and evaluated their mechanisms by analytical analyses. The single-factor results indicated that the optimal impurity removal rate (α) was 14.64% at 8 mol/L HCl concentration, 0.1 g/mL solid–liquid ratio, 343 K acid leaching temperature, 120 min reaction time, and 400 rpm stirring speed. Iron-bearing minerals were efferently removed using HCl. The kinetic process analyses indicated that the aphanitic graphite acid leaching process for the ore fitted well with the unreacted shrinkage core model (diffusion reaction). The reaction’s apparent activation energy (E a) was 23.43 kJ/mol, and the leaching process’s frequency factor (A) was 15.17. The kinetic equation could be considered as (1 − 3(1−α))2/3+2(1−α) = 15.17e–23.43/(RT) t. From the results, it was concluded that the HCl acid leaching process belongs to diffusion control.

Growth of 2-inch diamond films on 4H–SiC substrate by microwave plasma CVD for enhanced thermal performance

We report a novel method of producing composite substrates by growing diamond films on a 4H–SiC substrate by microwave plasma chemical vapor deposition to enhance the thermal management for thermal diffusion applications in GaN-based RF power devices. Morphology, grain orientation and structure of the diamond films were characterized by scanning electron microscopy, electron backscatter diffraction, X-ray diffraction and Raman spectroscopy. The diamond films were uniform and compact, without voids and gaps observed via cross-sectional view of microscopy, and without any graphite phase and impurity peak. Thermal conductivity (TC) of the diamond-SiC composite substrates was simulated and calculated. Results indicate that the TC of the composite substrates is up to 512.02 ± 6.37 W m−1 K−1 when the SiC thickness is 500 μm, which is 1.53 ± 0.02 times of the pure SiC substrate, and this high TC increases to 1171.13 ± 67.23 W m−1 K−1 after the 4H–SiC substrate is thinned down to 50 μm. The coefficient of thermal expansion (CTE) of the composite substrates can be engineered by changing the thickness of SiC substrate.

Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite

The diffusion, penetration and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated into a bilayer. Our computational results show that at a sufficiently high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking configuration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking configuration, in turn, has a significant effect on cluster diffusion. We therefore find different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking configuration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite.

Structural, Electrical and Optical Properties of Graphite Films are Drawn with Pencils of Different Hardness

The paper presents the results of studying the structural, optical and electrical properties of thin films of graphite depending on the hardness of the rods (2H, H, HB, B and 2B) obtained by the "Pencil-on-semiconductor" method. Such studies are of great importance for the further development of highly efficient devices based on heterojunctions for electronics and optoelectronics. Typical images of the surface formed by reflected electrons (BSE) were obtained using a scanning electron microscope and shown at three magnifications (100x, 500x and 1000x). Since the cores of the studied pencils consist of mixtures of clay and graphite, a more detailed analysis of the elements that make up the cores was conducted. EDS analysis showed that the main components of the studied rods are purified graphite powder, as well as O, Al and Si, which are part of kaolin whose formula is H4Al2Si2O9, or Al2O3 • 2SiO2 • 2H2O - the main component of ordinary clay. The elemental composition of the microvolume of the studied samples was also determined. Regardless of the error that occurs when determining the composition of C and O (~ 12%), it can be argued that there is still a regularity between the graphite content and the hardness of the pencil. That is, the higher the graphite content, the softer the rod. The thickness of the graphite films was measured using the MII-4 interferometer according to the standard method. The average thickness of all investigated films was ~ 150 nm since the thickness of the films obtained by this method is mainly determined by the roughness of the surface of the salt substrate. Drawn graphite films have a higher resistivity than bulk samples (pencil rods) from which they were made. The resistance of the films increases with an increase in the hardness of pencils, due to an increase in the number of clay impurities in graphite, which is a dielectric. It was found that an increase in stick hardness leads to an increase in transmission.

Determination of Impurities in Graphite Using Proton Induced Gamma Ray Emission, Total Reflection X-ray Fluorescence and Instrumental Neutron Activation Analysis

Determination of impurities in graphite is very important for its quality control, as their presence even at trace level can affect the performance of graphite in various applications. Graphite with equivalent boron content (EBC) less than 5 mg kg-1 is considered as nuclear grade. Elements with high neutron absorption cross section (boron and rare earths) contribute significantly to EBC. Non-destructive method is preferred as there is no sample processing and probability of loss of volatile elements while digestion. Proton Induced Gamma Ray Emission (PIGE), Instrumental Neutron Activation Analysis (INAA) were utilized for the non- destructive determination of impurities in both nuclear and commercial grade graphite. Low Z elements like Li, B, F, Na, Al and Si were detected in graphite by PIGE whereas Na, K, Sc, Cr, Mn, Fe, Co, Zn, Rb, Zr, Sb, Cs, La, Ce, Nd, Sm, Eu, Tb, Yb, Hf, Ta, Th were determined using INAA. Few elements like Ca, Ti, V, Ni, Sr and Pb remained undetected by both the non-destructive techniques. These elements were determined by Total Reflection X-ray Fluorescence (TXRF) after digestion of the graphite samples by dry ashing. Combinations of these techniques were utilized to get maximum information regarding the impurities present in graphite.

Determination of nuclear graphite impurities by prompt gamma activation analysis to support decommissioning operations

Elemental composition of non-irradiated nuclear grade graphite was determined by Prompt Gamma Activation Analysis (PGAA) to quantitively assess some neutron activation precursors. The knowledge of these data is paramount for an accurate radiological characterization of the material before decommissioning of graphite-moderated nuclear reactors. Bulk results of most of the nuclides were consistent with benchmark Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) and literature data. In the case of 14N and 35Cl, depth distribution profiles were observed and quantified for the first time. These outcomes could shine a light on 14C and 36Cl depth distribution profiles and fractional release during leaching experiments.

Synthesis of Submicron Boron Carbide by the Non-Vacuum Method with Indirect Supply of the Thermal Energy of a DC Arc Discharge

The results of experimental studies demonstrating the possibility of obtaining boron-carbide powder by treating a mixture of carbon and amorphous boron containing a small amount of boron oxide with DC arc discharge plasma are presented. A feature of the applied nonvacuum electric-arc method for the synthesis of boron carbide is its implementation in an open air environment due to achievement of the effect of screening the reaction zone with gaseous oxide and carbon dioxide. In this case, the initial reagents are heated to the required temperatures by a DC arc discharge plasma burning in the immediate vicinity of the initial-material mixture in the cavity of the graphite crucible. The resulting material contains particles of boron carbide with a size in the submicron range, which are characterized by the “shell–core” type structure. The scheme of the discharge circuit of a laboratory reactor with horizontal arrangement of electrodes and an indirect supply of thermal energy to the initial reagents described in the paper makes it possible to significantly increase the content of the boron-carbide phase and reduce the content of the graphite impurity phase.

Simulation-based studies on graphite absorption properties for ASTRA critical experiments

This work is focused on the improvement of the graphite impurity content characterization for the ASTRA critical facility. The equivalent boron content for the graphite reflectors of the ASTRA facility reported in the benchmark specifications was estimated using the results of the exponential experiments performed by graphite manufacturer. Because raw measured data are not available for these experiments, we tried to reassess the equivalent boron content for the ASTRA graphite reflectors using the results of analogous exponential experiments performed earlier on the empty RBMK critical facility at the National Research Center «Kurchatov Institute» and further simulation of the experiments utilizing the Monte Carlo code Serpent with ENDF/B-VII.0, ENDF/B-VII.1, and ENDF/B-VIII.0 cross-section libraries. The same grade graphite was used for manufacturing the graphite blocks for the RBMK critical facility and the ASTRA critical facility. The Monte Carlo simulations confirmed the accuracy of the exponential measurements performed for RBMK graphite characterization. It was also shown that the simulation must reproduce the graphite absorption cross-section estimated using the exponential experiments for the agreement between the measured and calculated distribution of thermal neutron flux. Therefore, the equivalent boron content should be assessed individually for each cross-section library used in the simulation. In case of the ASTRA facility the carbon capture cross section of 3.36mb at 0.0253 eV (ENDF/B-VII.0) was used to deduce the equivalent boron content of 1 ppmwt from the measured results. Thermal capture cross-section of carbon was significantly increased in the ENDF/B-VII.1 and other recent cross-section libraries. Therefore, when calculating the ASTRA benchmark with these cross-section libraries the equivalent boron content reported in the benchmark specification should be reassessed.

Stacking-Mediated Diffusion of Ruthenium Nanoclusters in Graphite

The diffusion, penetration and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated into a bilayer. Our computational results show that at a sufficiently high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking configuration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking configuration, in turn, has a significant effect on cluster diffusion. We therefore find different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking configuration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite.

A direct dc arc emission spectrometry determination of chlorine impurities in nuclear grade graphite

In the present study, the direct current arc atomic emission spectrometry (dc arc AES) method was proposed for direct determination of chlorine impurities in nuclear-grade graphites and graphite foils in the range of 1‐–100 ppm in order to meet nuclear power industry requirements. The procedure is based on near-infrared (NIR) AES approach for determination of non-metal impurities, contrary to the most common vacuum ultraviolet AES approach. Carbon dc arc in argon atmosphere was used as radiation source. This radiation source employs standard extra high purity 6 mm diameter standard shaped graphite electrodes and standard vertical spectroanalytic stand, but employs several improvements as well: special protective fused quartz tube, and specially built fiber optic refocuser. Registration of spectra was performed via a novel compact tunable spectrometer with linear complementary metal oxide semiconductor array (CMOS) detector. As the work in near-infrared spectral range is hampered by spectral interference factors, measures were taken to counter them. Due to specifics of the radiation source, the excitability of chlorine was studied both in air and argon in presence of carbon, as well as in its absence. Artificial standards for calibration of the spectrometer were prepared by mixing extra high purity NaCl and graphite powder and evaluated by two independent approaches: indirectly by dc arc AES determination of Na, and directly by water extraction and photometrical determination of Cl. The characteristics of evaporation and excitation in dc arc of chlorine from both standards and real nuclear graphite samples were studied due to the differences in the nature of chlorine source in them. The detection limit of was determined 0.8 μg/g. The whole cycle of measurement, including sample preparation is about 10 min.

Impact of graphite impurities on the structure and optical properties of the sodium borate oxide glass

Impact of graphite impurities on the structure and optical properties of the sodium borate oxide glass

Quantitative Determination of Trace Heavy Metals and Selected Rock-Forming Elements in Porous Carbon Materials by the X-ray Fluorescence Method.

A new X-ray fluorescence (XRF) method is proposed for sample preparation and impurity quantification for elements heavier than sodium in carbon materials. The analysis is suitable for various materials including amorphous ones, such as petroleum cokes, with an impurity content higher than 0.01%. We compared a new method with the regular additive method to measure impurities in electrode graphite and petroleum coke. The XRF-based method provides the same sensitivity and accuracy and much greater reproducibility of the analysis results for variations in the sample mass, its density, and coverage by exciting X-ray radiation. The method does not require changes in the instrument software and is easily implemented on commercial analytical equipment.

Study on purification of flaky graphite by argon arc plasma torch

High purity graphite with purity above 99.9%, as an industrial raw material, plays an important role in the high-tech field. The existing physical and chemical methods of graphite purification technology has high cost serious damage to equipment and environment by acid and alkali, and complex processes. Thus the development of an excellent and effective graphite purification technology has become a research hotspot in recent years at home and abroad. A purification method of flaky graphite by arc plasma is established in this paper. The characteristics of high temperature which can be produced quickly by using arc plasma is used to treat the flaky graphite samples with a purity of 94.18% from Jixi City of Heilongjiang Province, under high temperature. The results show that the optimal discharge parameters are air flow rate 25 L/min, current 400 A and power 10 kW. At this point, the surface temperature of the arc plasma is up to 3350 ℃. Scanning electron microscope is used to compare the microstructure of graphite samples before and after arc processing to find the characteristics of shredding and breaking of graphite samples. The graphite purity and impurities are analysed according to national standard chemical analysis method of GB/T 3521 2008. After arc treatment, the purity of graphite is increased to 99.21%.

Simulation of the HTR-10 Operation History With the PANGU Code

The 10 MW High Temperature Gas-cooled Reactor-Test Module (HTR-10) is the first High Temperature Gas-cooled Reactor (HTGR) in China, which was operated from January 2003 to May 2007. The HTR-10 operation history provides very important data for the validation of HTGR codes. In this paper, the HTR-10 operation history is simulated with the PANGU code, which has been recently developed for HTGR reactor physics analysis and design. Models and parameters are constructed based on the measured data of the actual conditions. The simulation results agree well with the measurements in all steady-state power periods. The discrepancy of keff is generally below 0.5%, and the discrepancy of coolant outlet temperature is generally below 5°C. It is also figured out that the burnup of graphite impurities has considerable influence on the keff at the end of the operation history, which can cause over 1.5% discrepancy when neglecting the burnup of graphite impurities. By this work, the PANGU code’s applicability in actual HTGR fuel cycle simulations is demonstrated.

Diesel Soot as a Supercapacitor Electrode Material

Diesel soot (DS) collected directly from the exhaust tailpipe of the diesel-powered vehicle was explored for its performance as an electrochemical supercapacitor electrode material. X-ray diffraction and Raman spectroscopy evidenced the existence of graphite carbon form in DS along with low amount of defects in the form of disordered graphite, amorphous carbon, ionic, and polyene impurities. The carbon (C, ∼94 at.%) and oxygen (O, ∼5 at.%) elements were identified using the X-ray photoelectron spectroscopy technique on the DS surface. The morphology of DS powder consisted of nanoparticles with nearly spherical-shaped morphology as observed under the scanning electron microscope. Further, the transmission electron microscope showed the chain type interconnectivity of DS nanoparticles. The average surface area, average pore radius, and total pore volume of DS were measured to be 88 m2 g−1, 1.62 nm, and 0.17 cc g−1, respectively. Cyclic voltammetry and galvanostatic charge-discharge studies were performed to explore the electrochemical behavior of DS slurry-coated electrode. The supercapacitor behavior of DS was studied using various aqueous and organic electrolytes. Higher values of specific capacitance (Csp) were achieved for DS in the case of aqueous electrolytes in comparison to organic electrolytes. The observed highest specific capacitance (Csp) value was 36.77 F g−1 for DS at the current density value of 0.25 A g−1 using 0.5 M H2SO4 electrolyte. DS retains nearly 78.10% of its initial capacitance even after the execution of 5000 cycles, clearly indicating the excellent durability and stability of the material. Thus, DS could be a promising supercapacitor electrode material.

Investigation of radioactive reactor graphite by gamma-spectrometric method

In this research radioactive contamination of graphite of the 4th block of the Leningrad NPP was determined using the method of gamma-spectrometric analysis. The obtained results of the specific activity of isotopes were compared with the contamination of the graphite masonry of industrial uranium-graphite reactors. It was found that in comparison with graphite of industrial uranium-graphite reactors, the LAES-4 masonry has less contamination with decay products and actinoids, which indicates that, unlike industrial reactors, there were no incidents of fuel Rod destruction in power plants. But at the same time, it was noted that the RBMK masonry has an increased content of60Co. It was determined that this is due to the increased content of iron and cobalt impurities in unradiated graphite for RBMK. This research is important in terms of confirming the view that spent graphite from each specific uranium-graphite reactor has important features that need to be taken into account when planning decommissioning measures.