Graphite Components Research Articles

Research on ring artifact reduction method for CT images of nuclear graphite components

Background The supporting structure of high-temperature gas-cooled reactors (HTGR) comprises over 3000 carbon/graphite components, necessitating computed tomography (CT) non-destructive testing before operational deployment as per reactor technical specifications. However, CT images are frequently marred by severe ring artifacts due to the response non-uniformity and non-linearity of detector units, which diminishes the ability to detect defects effectively. Methods To address this issue, we propose a physics-based ring artifacts reduction method for CT that employs pixel response correction. This method physically accounts for the cause of ring artifacts and leverages the prior knowledge of the detected object to enhance the accuracy of the detection process. Results Our proposed method achieved a notable reduction in ring artifacts, as evidenced by a 37.7% decrease in ring total variation (RTV) values compared to the originals, significantly enhancing image quality. It also surpassed traditional and machine learning methods in artifact reduction while maintaining image details. The lower RTV scores confirm our method's superior effectiveness in minimizing ring artifacts. Conclusion We believe that our research contributes to the enhancement of defect inspection performance in detection systems, which is crucial for ensuring the safety of reactors. The proposed method's effectiveness in mitigating ring artifacts while maintaining image quality highlights its potential impact on the reliability of non-destructive testing in the context of HTGR components.

A study on CT detection image generation based on decompound synthesize method

Background Nuclear graphite and carbon components are vital structural elements in the cores of high-temperature gas-cooled reactors(HTGR), serving crucial roles in neutron reflection, moderation, and insulation. The structural integrity and stable operation of these reactors heavily depend on the quality of these components. Helical Computed Tomography (CT) technology provides a method for detecting and intelligently identifying defects within these structures. However, the scarcity of defect datasets limits the performance of deep learning-based detection algorithms due to small sample sizes and class imbalance. Objective Given the limited number of actual CT reconstruction images of components and the sparse distribution of defects, this study aims to address the challenges of small sample sizes and class imbalance in defect detection model training by generating approximate CT reconstruction images to augment the defect detection training dataset. Methods We propose a novel CT detection image generation algorithm called the Decompound Synthesize Method (DSM), which decomposes the image generation process into three steps: model conversion, background generation, and defect synthesis. First, STL files of various industrial components are converted into voxel data, which undergo forward projection and image reconstruction to obtain corresponding CT images. Next, the Contour-CycleGAN model is employed to generate synthetic images that closely resemble actual CT images. Finally, defects are randomly sampled from an existing defect library and added to the images using the Copy-Adjust-Paste (CAP) method. These steps significantly expand the training dataset with images that closely mimic actual CT reconstructions. Results Experimental results validate the effectiveness of the proposed image generation method in defect detection tasks. Datasets generated using DSM exhibit greater similarity to actual CT images, and when combined with original data for training, these datasets enhance defect detection accuracy compared to using only the original images. Conclusion The DSM shows promise in addressing the challenges of small sample sizes and class imbalance. Future research can focus on further optimizing the generation algorithm and refining the model structure to enhance the performance and accuracy of defect detection models.

Three-Layer Electrorefining of Molten Zinc Held on Porous Carbon or Silica Sheets in Molten LiCl–KCl–ZnCl2

A three-layer electrolysis process utilizing porous holding materials with the impermeability to liquid metals due to the poor wettability but permeability to molten salt electrolytes was investigated, which enables to establish a three-layer configuration of metal(l)/electrolyte(l)/metal(l) by holding metals above the electrolyte regardless of their densities. Porous carbon or silica sheets were selected as the holding materials of liquid Zn, and upper electrode to hold liquid Zn was prepared. Electrorefining experiments was carried out in molten LiCl–KCl–ZnCl2 at 723 K. Three types of experiments were carried out focusing on anodic polarization, electrorefining with cathodic carbon, and three-layer electrolysis using liquid Zn as cathode.Anodic polarization experiments clarified that evolution of a large amount of Cl2 gas was observed from the graphite components of the upper electrode when a more positive potential than the Cl2/Cl− was applied. The use of insulating silica fiber sheets and holding material allowed for dissolution of Zn at higher current density without gas evolution. In the electrorefining experiments, synthesized crude zinc and ISP crude zinc were electrolyzed to obtain purified Zn at the cathode. The concentration of impurity elements in the cathode Zn was reduced by a factor of 8 to 200 compared to the value in the crude zinc, highlighting the purification effectiveness of the method. In the Zn(l)/electrolyte(l)/Zn(l) three-layer electrolysis, it was verified that the upper electrode was applicable as either an anode or cathode. Current efficiencies were close to 100% in all electrorefining and three-layer electrolysis experiments. From the results obtained, challenges associated with the three-layer electrolysis process were identified, and countermeasures to solve them were proposed.Acknowledgement: This work is conducted as a collaboration research of Laboratory of Design of Sustainable Materials and Processing, and Laboratory of Non-ferrous Extractive Metallurgy, Department of Materials Science and Engineering, Graduate School of Engineering, Kyoto University. Laboratory of Non-ferrous Extractive Metallurgy is an endowed chair by Mitsubishi Material Corp. We would like to acknowledge Prof. Hisaaki Fukushima at Kyushu University, Prof. Takashi Nakamura at Tohoku University, and Dr. Akihiro Kishimoto at Kyoto University for their valuable discussion. We also acknowledge Mitsui Mining & Smelting Co., Ltd., for supplying the prime western grade metal zinc (ISP crude zinc).

Friction reduction and anti-wear mechanisms of surface-textured graphite under water lubrication

In this study, tribological behaviors of surface-textured graphite samples under water lubrication are analyzed by experiments and molecular dynamics simulations to reveal the underlying friction and anti-wear mechanism. Circular dimple-type textures are chosen for investigations. Ball-on-disc frictional tests are performed to investigate the influence of texture spacing on friction coefficient and wear degree. The sample with a texture spacing of 200 μm possesses the lowest friction coefficients and the smallest cross-sectional area of the wear mark. The roles of support strength of solid part among textures, water wettability and water film in determining the water-lubricated tribological behaviors are analyzed in detail. The water molecules trapped by the dimple-type textures can produce additional hydrostatic pressure, which is the most crucial factor. The results of this study can guide the design of water-lubricated graphite components with surface textures, such as mechanical seals and graphite bearings in nuclear main pumps and ships.

Seismic modeling and simulation of the graphite core in gas-cooled micro-reactor

To evaluate the structural safety of the graphite core in a gas-cooled micro-reactor and to assess its structural response under seismic loads, a study was conducted. By comparing the acceleration and velocity curves obtained from small-sized graphite block collision experiments and collision simulations, it was determined that the simulation results accurately represent the real collision behavior of graphite blocks. The collision stiffness and damping parameters were derived from these curves. Subsequently, simulations of graphite components in the core were performed to establish the stiffness and damping parameters of the graphite blocks, which were then incorporated into the core analysis calculations. To validate the accuracy of the core numerical model and simplify the vibration form, the core model was divided into in-plane and axial models. A full-core model calculation was then carried out to determine the forces between graphite components. The final results confirm that the graphite core adheres to the ASME design specifications under seismic loads.

New evidence for organic nature of carbonaceous substance in Archean banded iron fomation of the Kostomuksha greenstone belt, Karelian Craton, Russia

The paper considers the results of a study of particles of carbonaceous substance and sulfur isotopes of associated sulfides in metapelites of the Neoarchean banded iron formation (BIF) of the Kostomuksha greenstone belt of Karelia (Karelian Craton of the Fennoscandian Shield). According to the petrographic observations, the carbonaceous matter occurs within and between silicate minerals’ grains, inside sulfides or at the grain boundaries, separating sulfide crystals from biotite or amphibole. Scanning electron (SEM) and atomic force (AFM) microscopy revealed the several types of the carbonaceous material varying in structure and carbon content. Raman spectra approved both well-structured graphite and weakly structured graphite (kerogen) components of the carbonaceous substance. The isotopic composition of total organic carbon is typical for biogenic processes. The obtained δ13Corg value within the range of –27,9– –30,6‰ is consistent with carbon fixation by photo- or chemoautotrophs. The sulfur isotopy of the associated sulfides is marked by positive Δ33S anomaly (up to +0.94‰) and negative δ34S values (–2.06‰ ─ –4.1‰). Positive Δ33S values indicate sulfur genetic association with photochemical elemental sulfur (S8) from the atmosphere, while negative δ34S values reflect isotope fractionation in bacterial-mediated processes. Based on these observations, we believe that the initial carbonaceous substance has mainly organic origin.

An insight into annealing mechanism of graphitized structures after irradiation

Graphite components are constantly subjected to a combined actions of annealing and irradiation due to high temperatures and thermal spike caused by irradiation when reactors are operating, resulting in complex microstructures along with matching changes in the material properties and dimensions. This study investigates the evolution process of initial irradiation defects at different annealing temperatures, which is difficult to captured by experiment. The findings indicate that 923K reactor-temperature annealing can also quickly restore isolated self-interstitial atoms and small-sized point defect clusters to intralayer locations on the nanosecond scale. However, multi-interlayer penetration damages and localized point defect aggregation from high-energy irradiation can lead to the disappearance of layered structural information, which allows these damage structures to develop into interlayer dislocations during annealing process. These interlayer dislocations further exacerbate the interlayer expansion of graphite crystal and may elucidate the mechanism behind volume expansion in nuclear graphite within reactors. Fully amorphized regions can also regain a layered structure approximately when guided by residual layered structures at 2000K, while chaotic regions or disordered layered structures form in the absence of guidance. These chaotic regions significantly exacerbate the volume expansion of graphite model, which may be related to the rapid volume increase observed in nuclear graphite components at the end of reactor life. The study provides insights into the transformation of initial irradiation defects into different types of defects under different temperatures and damage states, which serve as a critical foundation for assessing the evolution of irradiation defects in nuclear graphite for reactor applications and annealing studies.

3D analysis of crack propagation in nuclear graphite using DVC coupled with finite element analysis

The initiation and propagation of cracks within nuclear graphite components compromise the lifespan of advanced gas-cooled reactors and even lead to safety incidents. A thorough understanding of fracture properties in nuclear graphite is crucial for the effective management and design of graphite structural components. In this work, an in-situ test of a double cleavage drilled compression (DCDC) specimen was conducted using X-ray tomographic imaging to fully analyze the 3D crack propagation behavior in IG-11 nuclear graphite. “Subvolume Splitting” digital volume correlation (SS-DVC) was employed to accurately quantify the displacement fields surrounding the crack. The entire 3D crack propagation and crack surface around the crack front were precisely captured through 3D imaging processing of the gray level residual (GLR) field. Additionally, crack opening displacements (CODs) and stress intensity factors (SIFs) were determined using finite element (FE) analysis with extended finite element method (XFEM). Experimental results indicate that although mode I crack remains predominant throughout the whole process, complex crack propagation exists around the crack front. The utilization of irregular crack surfaces in FE analysis helps to obtain refined displacement fields and accurate SIFs of the crack front. This work provides comprehensive insights into crack propagation and reveals the influence of complex crack morphology on SIF calculations, facilitating accurate prediction of structural integrity and rational design of graphite components.

Lithium Redistribution Mechanism within Silicon-Graphite Electrodes: Multi-Method Approach and Method Validation

Li redistribution processes within Si-graphite composite (SiG) electrodes are analyzed using in situ and operando X-ray diffraction (XRD), ex situ light microscopy (LM), in situ optical microscopy of cross-sectioned full cells (CS-IOM), and 3D microstructure-resolved simulations of full cells. First, the lithiation behavior of graphite and SiG full cells (Si content 20.8 wt.-%) is analyzed. The results are used as validation of the methods (XRD, LM, CS-IOM, simulation). Second, the Li redistribution between the graphite component and Si component within SiG electrodes is investigated: By operando XRD measurements during charging in comparison with relaxed cells, a higher lithiation degree in the graphite component is found during charging compared to the relaxed state, indicating Li redistribution from graphite to Si during relaxation. The Li redistribution is directly observed by in situ and ex situ optical microscopy, where the golden LiC6 phase disappears during a 24 h relaxation period. The results are supported by simulations showing the variation in the Li concentration, not only in graphite but also within the Si component. Furthermore, all methods find that the Li redistribution is more pronounced at a higher C-rate of 0.5 C, suggesting a preference for graphite lithiation over Si lithiation.

Comparative Irradiated Dimensional Change Strain Analyses of Two Types of Graphite Components in a Thorium Molten Salt Reactor

Nuclear graphite plays a crucial role in thermal-spectrum thorium molten salt reactors (TMSRs) as both the neutron moderator and the construct for the coolant flowing channel. When subjected to irradiation and elevated temperatures, graphite components experience considerable deformation due to a combination of dimensional changes, thermal expansion, irradiation creep, elastic deformation, and changes in thermomechanical characteristics. The lifespan of the graphite component is a limiting factor in TMSR designs as it strongly correlates with the dimensional changes of the graphite. To evaluate the thermal and mechanical reactions of graphite component under TMSR core conditions, it is necessary to couple models of thermal-hydraulics, neutronics, and thermal-mechanics. This paper presents an enhanced methodology for analyzing the deformation of graphite components using the finite element method. Then, this method was applied to analyze a 10-year deformation history of a hexagonal prism assembly (HPA) and it was compared with the traditional hexagonal round channel assembly (RCA). The results demonstrate that the stress–strain field of both types of graphite components undergo significant variations with the increasing neutron fluence from irradiation. HPA graphite exhibits a slower deformation as compared to RCA graphite when subjected to identical operating conditions. In this case, HPA graphite has a lifespan of approximately 10 years, while RCA graphite lasts only 8.8 years.

A new perspective on density and strength loss profiles at the surface of thermally oxidized nuclear graphite

Oxidation of graphite components could influence their designed life in a high-temperature nuclear reactor. The oxidized regions could potentially lower the allowed stress capacity. The American Society of Mechanical Engineers rules for the design and construction of graphite-moderated reactors recommend that subsurface regions that might become excessively damaged by oxidation during reactor operation be identified and excluded from geometry and stress calculations. Identification of oxidation-affected regions is possible, in principle, through complex modeling exercises of reactor behavior during hypothetical accident scenarios coupled with graphite oxidation models, but this procedure may not have the precision needed for informed decisions. This paper proposes an alternate method, based on interpretation of a series of well-designed oxidation experiments, which could augment the designer's tools. The procedure is illustrated by data on oxidation by air of several graphite grades (NBG-18, PCEA, IG-110, R4-650) that are corroborated with independent literature information, when available. The Wichner model for graphite oxidation used for this analysis provides conservative results that could be quickly implemented in the design process.

Carbon contamination of elemental beta-boron promoted a stable boron carbide by spark plasma sintering

The effect of carbon contamination on the elemental beta boron (β-B) from the graphite components of spark plasma sintering (SPS) was investigated. The X-ray diffraction, analytical electron microscopy (AEM) and Raman spectroscopy witnessed boron carbide (B13+xC2-x where x=0.05–0.35), when SPS temperature was above 1750 °C. The degree of carbon contamination became severe with increasing SPS temperature to 1800 °C and 1850 °C and formed a single-phase rhombohedral boron carbide with about 12 wt% of carbon. Our observations indicate that the carbon gets kinetically trapped into β-B to form boron carbide with a primitive unit cell consisting mainly of icosahedral B11C and three-atom chain of CBB configurations. After 6 months of natural aging, it has been revealed that boron carbide formed at and below sintering temperatures of 1800 °C is not a thermodynamically stable phase, and transformed reversibly to β-B structure. X-ray photoelectron spectroscopy ascertained that there are no longer B-C bonds and the existence of more CC bonds in a 6 months aged specimen, suggesting carbon is no longer strongly bonded to boron, but precipitates as graphitic structure. Further examination using atom probe tomography and AEM confirmed the existence of carbon nanoclusters within grains and high concentration of carbon segregating onto grain boundaries. The phase transformation of boron carbide to mostly boron resulted in a significant loss of hardness and modulus in a bulk SPS specimen. This study underscores the importance of solid solution and local bonding environment in icosahedral boron-rich solids for phase stability and sustained properties.

Oxidation Mechanism and Surface Characterization of Pyrolytic Graphite in Simulated Air for Pyrochemical Reprocessing Application

Accidental air ingress into the containment chamber of pyrochemical reprocessing of spent nuclear fuels can cause severe oxidation of graphite components. Pyrolytic graphite (PyG) coating on conventional high-density graphite (HDG) is proposed for protection against oxidation, molten salt, and molten metal corrosion. The aim of the present study is to evaluate the thermal stability and kinetic parameters and propose a suitable oxidation mechanism of PyG under simulated air using thermogravimetry analysis (TGA). In dynamic oxidation, a linear increase in the oxidation onset, peak, and burnout temperature is observed for PyG synthesized at increasing pyrolysis temperature from 1200 to 2400°C except for 1400°C. The isothermal oxidation study showed a similar trend i.e., a decrease in the linear oxidation rate constant value for any given isotherm with increasing pyrolysis. The Arrhenius plot showed two-stage oxidation regimes operating between 600 to 1000°C. In regime-I, between 600 to ∼800°C controlled by surface chemical reaction kinetics, the activation energy is measured between 100-200 kJ.mol−1. The graphite-oxygen reaction initiates and propagates preferentially along the prismatic planes at the sample edges, where the dangling bonds of stacked graphite layer structures are exposed. In regime-II operating above ∼800°C, the surface reaction rate is infinitely faster, and the diffusional mass transport becomes a rate-limiting factor. In this regime, the graphite-oxygen initiates by pore formation on defective reactive sites on the bulk surface and propagates by pore-widening and pore-coalescence mechanism. The observed differences in the oxidation rates for different pyrolysis PyG attributed to its intrinsic material properties are investigated. This work provides new insights into the oxidation mechanism of the PyG during accidental air ingress in a pyrochemical reprocessing plant.

Plasma Deposition of Ti-C Based Coating on AISI 420 Steel Using Titanium Cathodic Cage and Graphite Lid

This work investigates the plasma deposition of Ti-C based coatings on AISI 420 steel using a cathodic cage technique with titanium and graphite components. The study examines two treatment conditions: cathodic potential and floating potential samples, with the objective of evaluating their influence on the coating properties. The results indicate that the cathodic potential treatment results in the formation of thicker coatings with enhanced microhardness, thereby improving wear resistance. Although both treatments demonstrated satisfactory wear resistance, they negatively impacted the corrosion resistance of the steel. X-ray diffraction analysis revealed the formation of TiC, Fe3C, Cr7C3 and Cr23C6 phases in the treated samples, which contribute to the improved microhardness and wear resistance. Wear tests confirmed reduced wear volume in the treated samples. The cathodic potential treatment exhibited greater microhardness and an extensive carburized layer, but the corrosion and wear resistance were similar in both conditions.

Methods for Characterization of Heterogeneous Aging in Large Lithium-Ion Batteries

Lithium-ion batteries have become the predominant storage solution for electrified vehicles and therefore research is moving forward at high speed. The high demand for improvements in extension of the battery life requires effective methods for the detection and understanding of aging phenomena. Recent high energy lithium-ion batteries teardown analyses, performed at KTH [1], have revealed a considerable extent of heterogeneity in the degradation phenomena on multiple levels. These phenomena are expected to strongly affect the rate of ageing of cells, and in turn contribute to an early end-of-life of the system.This presentation will focus on the description of the characterization methods that will be used for future studies on the uneven distribution of cell ageing. Testing has been performed on multiple configurations of silicon – graphite composite electrodes where, given the large volume expansion of the silicon during operation, has additional variables in the degradation mechanisms.Material characterization has been performed in the form of cross section SEM, and NMR, following a methodology developed at KTH to quantify Li-plating in different areas of the jellyroll [2], but extending it to Si. On the other hand, electrochemical measurements have been performed with small lab cells under controlled conditions. In parallel a physics-based model for describing the behaviour of such cells is being developed. The model is currently able to interpret quasi-OCV experimental data and derive the behaviour of each active material involved during these processes.[1] Smith, Alexander J., et al. "Localized lithium plating under mild cycling conditions in high-energy lithium-ion batteries." Journal of Power Sources 573 (2023): 233118.[2] Fang, Yuan, et al. "Quantifying lithium lost to plating and formation of the solid-electrolyte interphase in graphite and commercial battery components." Applied Materials Today 28 (2022): 101527.

Preliminary analysis of the irradiation deformation of a typical molten salt reactor graphite component

Nuclear graphite commonly serves as the moderator and constructs the coolant flowing channel in liquid fuel molten salt reactors. Under irradiation and high temperature, graphite components will experience obvious deformation due to irradiation creep, thermal expansion, dimensional change, elastic deformation, and thermo-mechanical properties change. The lifespan of graphite components is a limiting parameter of liquid fuel molten salt reactors, determining by the deformation of the graphite components. An improved approach based on the finite element method for graphite deformation analysis was proposed in this paper. Then this method was utilized to analyze a 10-year deformation history of a typical graphite component. The findings show that the stress-strain field of the graphite component significantly varies with the increasing irradiated neutron fluence. The dimensional change plays a primary role in influencing the deformation of the graphite components, determining the overall deformation trend. The irradiation creep plays a secondary role in influencing the deformation, alleviating the deformation of the graphite components to prolong the graphite lifespan. The improved method, which considers various factors like irradiation creep, thermal expansion, dimensional change, and elastic deformation, tends to provide more accurate results compared to the traditional method that focuses solely on the dimensional changes. Over 10 years, the contraction deformation of graphite components results in a reduction in graphite volume of up to 3.87%. The space for graphite contraction is filled with molten salt, resulting in a maximum increase of 22.49% in fuel volume. The alteration in volume of both graphite and fuel leads to a reduction in reactivity of up to 840 pcm.

Study on incompatible mechanism in chemical mechanical polishing of the novel graphite/diamond composite

The novel graphite/diamond composite (Gradia) possesses semiconducting instinct characteristics with controllable resistivity. Surface finishing plays a crucial role in using Gradia. However, controlled removal of the Gradia surface is challenging because physical and chemical properties differ between the graphite and diamond components. The impact of incompatibility on the material removal of Gradia is unclear. This study employed chemical mechanical polishing (CMP) to finish the Gradia surface with K2FeO4 oxidant. First, the sub-nano-scale roughness of the Gradia surface was attained by applying optimized polishing parameters. Then, the polished subsurface was examined. The incompatible graphite and diamond lattice distortion was the primary damaged mechanism rather than amorphization. The chemical reaction in CMP was analyzed with X-ray photoelectron spectroscopy and reactive molecular dynamics simulation. The carbon atoms formed a variety of chemical groups with the oxidizing of hydroxyl. The chemical activity of the graphite phase was superior to the diamond phase, leading to more types of reaction and a higher percentage of products. In the material removal stage, experimental observations and simulation analyses revealed that the diamond of sp3-carbon transformed to sp2-carbon, softened, and subsequently removed together with the graphite by abrasive scratching.

Seismic response prediction using intensity measures: Graphite nuclear reactor core model case study

Seismic response analyses of structures have conventionally used the peak ground acceleration or spectral acceleration as an intensity measure to estimate the engineering demand parameters. An extensive shaking table test program was carried out on a quarter-sized advanced gas-cooled reactor (AGR) core model to investigate the global dynamic behavior of the system with degraded graphite components while subjected to seismic excitation. Evaluation of the most widely considered intensity measures, with respect to their capability for predicting the seismic response of an AGR core–like structure, is performed. Twenty intensity measures of 16 distinct seismic input motions are formulated and correlated, with experimental measurements describing the dynamic response of the reactor core model. Linear correlations are constructed for each intensity measure to statistically determine the best metric for predicting the seismic response of the AGR core model, and statistical analysis indicates that the acceleration spectrum intensity (ASI) is best suited to characterize and describe the structural demand of an AGR core-like structure when subjected to seismic loading. A response prediction tool is developed, based on empirically derived linear correlations, to estimate column distortions and determine the critical input motion for further experimental and numerical studies. Statistical analysis indicates that predicted column distortions, compared against direct experimental displacements, are significant, repeatable, and accurate.

Optimizing the potential of intercalation on anode for long-cycle 420 Wh/kg Li-ion batteries

Utilization of novel cathode and anode materials is expected to achieve ultrahigh energy density of Li-ion batteries, but it usually suffers from a large capacity fade in full cells. Here we propose a strategy towards optimizing reversibility of intercalation/deintercalation in anode via regulating the negative/positive capacity ratio (N/P) ratios in full cells with Li-rich layered oxide cathode and SiOx@graphite anode. Compared with high N/P ratio, the cell with an optimal N/P ratio of 0.90 displays the best cycling performance. Some comprehensive characterizations such as in-situ pressure and electrochemical measurements reveal that the lower N/P ratio could reduce the polarization on this composite anode to maximize the utilization of the graphite component and minimize the irreversible pressure growth. On the basis of unraveled failure mechanisms, a 21 Ah multilayer pouch cell with low N/P ratio delivers the energy density up to 420 Wh kg−1 based on total mass of the cell and the capacity retention retains 82% after 300 cycles at 0.2 C. This work highlights the strong coupling between cell parameters and electrochemical performances, providing a new insight towards the dominant factor for irreversible Li-ion behavior during cycling besides the structural degradation of materials for high-energy-density Li-ion batteries.

Unusual material removal characteristics of the novel graphite/diamond composite in mechanical polishing

Novel graphite/diamond composite (Gradia) is ultrahard, tough and semiconductive. Surface finishing of Gradia is difficult because of the disparity in mechanical properties between diamond and graphite components. In this work, material removal characteristics of Gradia in mechanical polishing were studied with experiments and molecular dynamic simulations. First, the Gradia surface was well-finished. Then, microstructural investigation showed the main mechanism of phase transformation was amorphization during polishing. The graphite and diamond phases of Gradia underwent interconversion under the mechanical action. In addition, initial and diamond-transformed graphitic phases experienced distortion, twist and reconstruction, producing graphite film. Finally, a Gradia tool was created and demonstrated a lower wear rate than single-crystal diamond tool, suggesting an unusual self-lubricating characteristic with abrasion-induced generation of graphite.