Graphite Components Research Articles

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

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

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.

The Study of Continuous Core Zoning to Extend the Graphite Component Irradiation Lifespan in Molten Salt Reactor

The Study of Continuous Core Zoning to Extend the Graphite Component Irradiation Lifespan in Molten Salt Reactor

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.

Evolution of the microstructure of superfine grain graphites under thermal oxidation

Nuclear graphite is used as a moderator and structural material in a number of Gen-IV reactor designs, such as the (Very) High Temperature Reactors. Chronic or acute thermal oxidation affects the mechanical properties of the graphite components and the impact on the structural integrity of the core depends on the evolution of the microstructure during oxidation.This study qualitatively examined the microstructure of four superfine grain graphites using polarised and fluorescent optical microscopy. These graphites had similar physical properties but notable differences in the oxidation rate at 700 °C. Using fluorescent microscopy, it was possible to distinguish between open and closed porosity in graphite, which is useful in understanding the progression of thermal oxidation and penetration depth. Based on the samples in this work, fine open and closed porosity may be linked to increased surface oxidation and relatively high oxidation rate, whereas high penetration depth may be linked to relatively low open porosity consisting mainly of large pores. Fairly large domains of coherent crystallite orientation, perhaps due to the presence of agglomerates, may also indicate lower oxidation rates at 700 °C.This paper demonstrated that fluorescent microscopy combined with image analysis, is a useful tool in understanding the progression of thermal oxidation and the corresponding effect on the microstructure.

Polymer composites based on aromatic polyamide and fillers of spherical and layered structure for friction units of high-performance equipment

Without the employment of new equipment, with units functioning at high levels of weights, sliding speeds and temperatures, the current growth of the sector is all but impossible. High levels of labour intensity have a detrimental impact on the dependability and durability of the product. Moving joints, which comprise friction units, frequently fail at the same time. It is vital to increase their dependability and durability. This can be accomplished by substituting more recent materials with higher-quality qualities for the friction pair materials. To create such materials, the researchers created an aromatic polyamide polymer that was filled with silicon- (silica gel, bentonite) and carbon-containing (technical carbon, graphite) components. It was determined that depending on their nature, structure and content in the polymer matrix, the addition of these fillers enables the reduction of the coefficient of friction and wear of the produced polymer composites by 1.5–2.5 and 20–40 times, respectively, during frictional interaction with steel. It was found that 10–15% by weight of the examined fillers in aromatic polyamide is the ideal concentration. It has been proven that polymer composites filled with layered materials (bentonite, graphite) perform better in terms of tribological qualities when in contact with steel than those filled with spherical elements (technical carbon, silica gel). The improvement of the tribological properties of the developed polymer composites in comparison to the original polymer was determined based on the microphotographs of steel surfaces and the study of their morphological (roughness) and physical (microhardness) properties. It was determined that the change in the friction nature was due to the creation of an antifriction coating on the metal friction surface from the products of tribochemical reactions occurring during friction.

Phase change materials designed from Tetra Pak waste and paraffin wax as unique thermal energy storage systems

Thermal energy storage systems (TES) based on shape-stabilized phase change materials (SSPCM) designed from recycled Tetra Pak (TP) waste, paraffin wax (PW), and expanded graphite (EG) were investigated in this study. This work represents the first study to explore TP waste composed of low-density polyethylene (LDPE)/aluminum (Al) components for energy storage applications. The LDPE part serves as a matrix conserving a material in a compact, solid shape after PW melting; PW acts as an active phase change component contributing to heat absorption/release through a phase change (from a solid to a liquid state, and vice versa) of its crystalline phase. EG serves as a filler that enhances the thermal conductivity and mechanical properties of materials.The focus was put on the optimization of the composition of SSPCM including PW, and EG, to check thermal, mechanical, and rheological properties which influence the future processability of such systems through extrusion, as well as to investigate the synergic effect of graphite and residual Al component on thermal conductivity and leakage of PW. There are two main demands on polymer/PW blends, namely well-separated melting peaks for both components at significantly different temperatures, and good compatibility between polymer and PW.The best performance of SSPCM investigated in this study was found for a mixture having the composition TP/PW/EG = 50/40/10 w/w/w. This mixture shows well-balanced properties, including appropriate heat storage and release parameters, thermal conductivity, thermal diffusivity, toughness and strength, and low leakage of PW from the material. This system can store 116.2 J/g of heat energy and release 93.8 J/g of heat energy. The determination of the heat energy storage and release was performed by the transient guarded hot plate technique. Tensile testing revealed that Young's modulus of the TP/PW/EG = 50/40/10 w/w/w composition was 924 ± 71 MPa and the stress at break was 8.2 ± 1.2 MPa, which are sufficient values from the applicability point of view. The composition stability of the prepared system was confirmed by rotational rheometry. The environmental relevance of these materials lies in the utilization of the waste, which has minimal usage, and after the hydropulping of Tetra Pak packaging, it accumulates in large volumes. This is the first study indicating that LDPE/Al recyclate is a cheap alternative for preparing TES materials, fulfilling all the requirements for such materials. This study indicates the potential of TP waste for the preparation of SSPCM using PW as a phase change component. The selection of PW with a specific melting point determines potential applications, including the building industry, thermal management of electronics, solar vapor generators for desalination, solar water heaters, battery/computer heat protection, etc.

Microstructural characterization of the CGB graphite grade from the molten salt reactor experiment

In the 1960s, the Molten Salt Reactor Experiment demonstrated the feasibility of molten salt reactors for civil applications. The Molten Salt Reactor Experiment used a nuclear graphite grade known as CGB in the core as the fast neutron moderator. For application in this purpose, CGB graphite underwent additional impregnation steps that are not typically used in conventional nuclear graphites. These additional impregnations significantly lowered the permeability and reduced the size of pore openings in CGB graphite, for the purpose of inhibiting molten salt ingression into the graphite. Although several publications report the mechanical properties and irradiation response of CGB graphite, little information has been published about the microstructure or sealant used for this graphite grade. Here the results of multiple advanced microscopy techniques are presented with the objective to investigate the unique microstructure and sealing technology of legacy material from the Molten Salt Reactor Experiment so it can be potentially readopted for modern reactors. The microstructural characterization shows that some pores were fully sealed by the impregnation whereas other regions contained foam-like materials that partially filled the void content. This manuscript also investigates and documents the nature and possible source of the sealant material. The pore sealing methods used for CGB are discussed and compared with other common technologies used to reduce the impregnation of molten salts into graphite components. Knowledge of CGB graphite's microstructure can enable development of new pore infiltration technologies to improve in operando behavior advancing the next generation of molten salt reactors.

Flexural Behavior of Graphite Tailings RC Beams in Chloride Environment

This paper investigates the mechanical properties of graphite tailings concrete beams under various curing environments. The results show that the concentration of graphite tailings has an important influence on the mechanical properties of concrete. Finally, the bending characteristics of the beam and the modified bearing capacity calculation formula are obtained, which provides a theoretical basis and the reference value for the construction of green building materials. Through research, the main innovations and main findings of this paper are as follows: (1) In the three kinds of concrete experimental environment, the fracture load and ultimate load of concrete beams with graphite tailings replacement rate of 20% are the largest. (2) In the three kinds of concrete test environment, when the concrete test beams under the same load, with the increase of graphite tailings replacement rate, the mid-span deflection of concrete beams showed a trend of decreasing first and then increasing. When the replacement rate of graphite tailings sand is 20%, the mid-span deflection of the test beam is the smallest. (3) Through experiments, it is found that the normal section bearing capacity of graphite tailings concrete beams after chloride salt erosion conforms to the plane section assumption. After chlorine salt erosion, the strain of the longitudinal tensile steel bar of concrete beams with different graphite tailings sand replacement rates is the same as that of ordinary concrete beams, and the slope of the strain curve is the same, showing a linear trend. The experimental results show that the replacement rate of graphite tailings sand has an insignificant effect on the strain of steel bars. (4) The Gauss function is used to fit the experimental value of normal section bearing capacity of concrete beam, and the calculation formula of normal section ultimate bearing capacity of graphite tailings sand concrete beam under chloride environment is established. The research results of this paper have significant reference value for the study of mechanical properties of graphite tailings sand concrete components.

Microstructural changes in nuclear graphite induced by thermal annealing

Irradiation-induced property change in nuclear graphite is particularly important when considering the in-service lifetimes of graphite components. In other works, it has been shown that annealing of irradiated graphite above the irradiation temperature may heal atomic-level defects and thus reverse some amount of physical property change. In this work, virgin nuclear graphite IG-110 was annealed at 2500 °C to observe any microstructural changes due solely to high-temperature thermal annealing. The results shown in this study suggest that fullerene-like defects, which are not observed in as-prepared nuclear graphites, will arise due to thermal annealing near graphitization temperature. Consequently, such defects may directly contribute to non -recoverable physical property change which has previously been observed in irradiated nuclear graphites. • Non-recoverable physical property changes to nuclear graphite. • High-temperature thermal annealing of nuclear graphite. • Microstructural changes induced solely by high-temperature thermal annealing. • Fullerene-like defects characterized via transmission electron microscopy.