Graphite Damage Research Articles

Shock-induced phase transition and damage in nano-polycrystalline graphite affected by grain boundaries

Dynamic structural response of nano-polycrystalline graphite under shock compression is investigated using molecular dynamics (MD) simulations. Hugoniot data shows that the structural transition is activated at shock pressure P∼30 GPa (experimental range, 20–50 GPa), resulting in the formation and extension of hexagonal diamond nuclei along grain boundaries, embedded incoherently among thin-graphite grains. As P increases from 130 GPa, the structure starts to liquefy, accompanied by a decrease in shear stress τ from approximately 5.3 GPa, and completely liquefies at P∼250 GPa (melting pressure of graphite, 180–280 GPa) and τ ∼ 0 GPa. In ultrahigh-pressure region, a two-wave structure is generated consisting of an elastic shock wave and a phase transition wave, and when the piston velocity exceeds 5.2 km/s, the latter wave can catch up with the elastic one, eventually becoming a single over-driven wave. During the relaxation of compressed nano-polycrystalline graphite, void nucleation inside the sample induces the initiation of visible cracks when piston velocity is higher than 1 km/s. At low piston velocities, the cracks propagate gradually along grain boundaries due to shear-slip effects. While at high piston velocities, direct spall of the nano-polycrystalline graphite makes it into multiple fragments by ultrahigh strain rate tensile forces. This study provides a useful guide to the structural transition and dynamic damage evolution of nano-polycrystalline graphite under shock compression.

Evolution of radiation-induced damage in nuclear graphite – A comparative structural and microstructural study

Graphite, as a material for high-temperature gas-cooled reactors (HTGR), will be exposed to harsh environment. The stability of graphite structure under irradiation is of a key importance for efficiency, reliability and security of the Generation IV nuclear reactors. Three types of nuclear grade graphite were subjected to irradiation in this research – two commercially manufactured (IG-110 and NBG-17) and the laboratory's in-home material (NCBJ). The samples were exposed to 150 keV Ar+ and He+ ions bombardment at 400 °C with fluences ranging from 1E12 to 2E17 ion/cm2 in order to simulate in-reactor conditions. For analysis of the level of structure damage, type of created defects and crystallite size changes under ion irradiation ex-situ Raman spectroscopy was used. The methodology of spectra fitting was developed. Furthermore, SEM observation of irradiated materials was performed. Results showed structural degradation of materials by the means of amorphisation: slight at a low fluence level, rising rapidly at higher irradiation values. Furthermore, stronger structural disorder was found in the materials irradiated with heavier Ar+ ions than with lighter He+. Microstructural evolution of the nuclear graphites aligned with the structural deterioration in its stepwise character.

High-temperature behaviour and interfacial damage of CGI: 3D numerical modelling

Superior mechanical and thermal properties, high wear resistance and a competitive price of compacted graphite iron (CGI) have made it an integral part of industry worldwide. In its applications in automotive engines, high-temperature environments cause thermal expansion that can result in emergence of interfacial damage in CGI. Although graphite-matrix interfacial damage is considered the main damage mechanism that can lead to total fracture of CGI, extensive research on CGI has not yet fully investigated this phenomenon at the microscale, especially under pure thermal loading. This paper focuses on the high-temperature performance of CGI and the onset of damage in graphite in thermal cycles. Three-dimensional numerical models are developed, with a single graphite inclusion embedded in a unit cell of the metallic matrix. Elastoplastic behaviour is considered for both phases in simulations. The effects of morphology and orientation of graphite inclusions on a response of an entire unit cell to thermal loading are investigated. Also, the influence of periodic and fully-fixed boundary conditions on the damage behaviour of CGI is discussed. The results can give a better understanding of the fracture mechanisms of CGI exposed to elevated temperatures.

A novel method for quantifying irradiation damage in nuclear graphite using Raman spectroscopy

Raman spectroscopy has long been used in studying irradiation damage in carbon/graphite materials. It is however unclear if the measurements from different types of materials are directly comparable. Further, decoupling the contribution from irradiation temperature and fluence to the total damage in nuclear graphite possessing irradiation damage gradient is currently not readily feasible as it requires a complete set of test reactor irradiation experiments over the relevant temperature and fluence range. A novel methodology has therefore been proposed and developed to quantify total irradiation damage evolution at crystal level based on graphite Raman G-band position shift. Specifically, G-band positions derived from a large number of spectra collected from the fractured surface of microfine-grained POCO ZXF-5Q graphite possessing proton irradiation damage gradient were plotted with open literature Raman data on HOPG, BEPO (AXGP graphite), PCEA and IG-110 graphite as a function of dpa. The total damage level within 2σ beam radius in this POCO graphite was estimated to be equivalent to ∼2–5 dpa at ∼350–370 °C. Derived G-band positions were then mapped to the three-stage amorphization trajectory model indicating beam centre area has entered the second stage, i.e., transitioning from nanocrystalline graphite into amorphous carbon. G-band position relative shift (ΔG) curve with a ‘turn-around’ peak as a function of total damage can be used for in-service lifetime prediction. The developed methodology has the potential to ‘unify’ total damage levels across different grades of nuclear graphite caused by ion, neutron and proton irradiation at different temperatures.

Topography changes and microstructural evolution of nuclear graphite (IG-110) induced by Xe26+ irradiation

AbstractThe microstructure of nuclear graphite, a key material in nuclear reactors, is affected by the high-flux irradiation. The damage to the graphite by irradiation is important for reactor safety. To understand the damage of nuclear graphite by irradiation, IG-110 nuclear graphite, a representative nuclear graphite, was chosen to investigate the change in morphology and microstructure caused by 7 MeV Xe26+ irradiation with peak damage doses of 0.1-5.0 displacements per atom (dpa) for samples of a size of 40.0 mm×40.0 mm×2.0 mm. The topography and microstructure of IG-110 were characterized by SEM, AFM, grazing incidence XRD, Raman spectroscopy and nano-indentation. Results indicate that after 7 MeV Xe26+ irradiation at a dose of 0. 11 dpa, a ridge-like structure appears on the surface of the IG-110 graphite, mainly in the binder region, and the surface roughness increases slightly. With a further increase of the irradiation dose, the ridge-like structure also appears in the filler region. At a dose of 0.55 dpa, pore shrinkage increases accompanied by pore closure, and the surface roughness also increases. The changes in topography and microstructure caused by irradiation are attributed to the expansion of graphite along the c-axis direction. With increasing irradiation dose the defect density and the degree of in-plane disorder in the graphene sheets increases, while the modulus and hardness of the graphite first increase and then decrease. Their increase is caused by dislocation pinning and closure of fine pores, while their decrease is attributed to an increase in porosity and the generation of an amorphous structure.

Large-Scale Modeling of Damage and Failure of Nuclear Graphite Moderated Reactor

Abstract The UK Advanced Gas-Cooled reactors (AGRs) have cores made of graphite bricks with dual functions: as structural elements of the core, providing space for and separating fuel and control rods; and as moderator of the nuclear reaction. Nuclear graphite is a quasi-brittle material, where the dominant mechanism for failure is cracking. While cracking of isolated bricks is expected due to operation-induced changes in graphite microstructure and stress fields, these could be tolerated as far as the overall structural function of the core is maintained. Assessment of the whole core behavior has been previously done with whole scale models where bricks have been considered as rigid body elements connected by elastic-brittle springs. This approach does not allow for the realistic assessment of the stresses in the bricks and associated brick cracking. Reported here are results from an ongoing project, which addresses this shortcoming. The proposed model uses deformable bricks with appropriate interactions, allowing for physically realistic whole core analysis. The results are focused on the damage that a graphite moderated reactor develops during a life cycle, how this affects the behavior of the whole core, and how changes in bricks' behavior impacts the core integrity. The proposed methodology is a major step toward high-fidelity assessment of AGRs' fitness for service, required for supporting continuous safe operation and life-extension decisions.

Graphite sheets in study of radiation dosimetry and associated investigations of damage

Present work builds upon prior investigations concerning the novel use of graphite-rich polymer pencil-lead for passive radiation dosimetry. Working with photon-mediated interactions at levels of dose familiar in radiotherapy, exploratory investigations have now been made using graphite produced commercially in the form of 50 μm thick sheets. Focusing on the relationship between absorbed radiation energy and induced material changes, investigations have been made of thermo- and photoluminescence dose dependence, also of alterations in Raman spectroscopic features. Photoluminescence studies have focused on the degree of structural order of the samples when exposed to incident MeV energy gamma-radiation, supported by crystallite size evaluations. The results are consistent and evident of structural alterations, radiation-driven thermal annealing also being observed. The results, supportive of previous TL, Raman and photoluminescence studies, are readily understood to arise from irradiation changes occurring at the microscopic level. Notwithstanding the non-linearities observed in the conduct of Raman and photoluminescence studies there is clear potential for applications in use of the defect-dependent methods herein, providing sensitive detection of radiation damage in graphite and from it dose determination. Most specifically, the readily available thin graphite sheets can provide the basis of a low-cost yet highly effective system for studies of radiation-driven changes in carbon (and/or carbon based composites), also as a dosimetric probe of skin dose, its atomic number closely matching with the effective atomic number of soft tissues.

Novel Triaxial Raman Scanning Platform for Evaluating Integrity of Graphite Electrodes in Li-Ion Batteries

The quality of Li-ion batteries is highly related to the crystallinity of the graphite used in their electrodes. Therefore, the development of two-dimensional detection equipment for evaluating the integrity of graphite electrodes is critical. In this study, a triaxial scanning platform was developed for Raman spectroscopy. Two-dimensional Raman mapping of the defects (D band) and stacking (2D band) of graphite electrodes was performed to determine atomic-level variations caused by charging. Through Raman mapping, the effects of constant-current-constant-voltage (CC-CV) and CC-reFLEX fast charging were compared. The graphite damage from CC-reFLEX fast charging was less extensive than that from CC-CV charging. Furthermore, investigations using the triaxial platform revealed that the graphite on a curved electrode exhibited disordered stacking after several charging-discharging cycles. This phenomenon creates safety concerns.

Recent Advances in Stability of Carbon‐Based Anodes for Potassium‐Ion Batteries

AbstractThe commercialization of potassium‐ion batteries requires promising anodes with excellent reversible capacity and cycle stability. Graphite anodes have attracted extensive research owing to the formation of stable intercalation compounds, which also helps to focus attention on low‐cost carbon‐based anodes. However, the slow diffusion of potassium ion (K+) and structural damage of graphite caused by large size may lead to capacity decay. Herein, the modification strategies have been summarized, including microstructure regulation, heteroatom doping and building stable solid electrolyte interphase. And the opportunities and challenges faced by these improvement methods are proposed. Meanwhile, carbon‐based materials used as hosts or conductive carriers to improve the performance of other anodes are also summarized.

Fullerene-like defects in high-temperature neutron-irradiated nuclear graphite

Irradiation-induced defect evolution in graphite is particularly important for its application in graphite-moderated nuclear reactors. The evolution of defects directly influences macroscopically observed property changes in irradiated nuclear graphite which, in turn, can govern the lifetime of graphite components. This article reports novel defect structures and the irradiation response of microstructural features occurring in high-temperature irradiated nuclear graphite IG-110. High resolution transmission electron microscopy (HRTEM) was used to characterize specimens neutron-irradiated at a high temperature (≥800 °C) at doses of 1.73 and 3.56 atomic displacements per atom (dpa). Concentric shelled and fullerene-like defects were found to result in swelling along the c-axis and contraction along the a/b-axis of crystallites. Furthermore, such defects are shown to occur within, and partially fill, Mrozowski cracks prior to turnaround dose. In addition, in situ TEM under similar irradiation conditions was used to capture the real-time dynamic evolution of defects, providing unambiguous analysis of the evolution of the graphite structures during irradiation. Results suggest the mainstream theory for radiation damage in nuclear graphite (which assumes additional basal plane formation as the sole reason) to be an incorrect interpretation of defect evolution contributing to irradiation-induced property changes at higher temperatures.

Graphitization damage on seamless steel tube of pressurized closed-loop of steam boiler

The pressurized closed-loop of steam boiler has been in operation for five years has experienced leakage problems at the bottom of the reducer and at the center of the tube on weldment. The metallographic analysis found that the leakage was due to the degradation of microstructure as a result of long-term overheating. The degraded microstructure of the tube boiler material dominated by graphitization. The graphitization normally applicable at 450°C, but with high-pressure water flow (90 bar) in the combination of internal stress on the tube material due to the welding process as well as the formation of the reducer, making the boiler operating temperature is sufficient to produce graphitization. The leakage occurred when the chain and accumulated graphitization happen because the cavities that covered the graphite facilitates the cracking propagation due to the presence of stresses.

Inverse identification of graphite damage properties under complex stress states

As a key material in high-temperature gas-cooled reactors (HTGR), nuclear graphite is a type of quasi-brittle material with complex damage mechanisms. However, it is difficult to characterise the damage properties of nuclear graphite under complex stress states using conventional testing approaches or inverse methods. In this study, a hybrid identification method based on 8-node quadrilateral element digital image correlation (Q8-DIC), a double iterative finite element model updating (FEMU) technique, and artificial neural networks (ANNs) is presented to characterise the damage properties of IG11 graphite material under complex stress states. First, this method is verified using simulated tests on a ring under diametrical compression, then applied to actual mechanical testing of graphite material to evaluate its damage properties. Finally, factors affecting the evolution of damage are discussed. The results indicate that the first principal strain has the most significant effect on the damage evolution of the graphite material.

Pore structure evolution of IG-110 graphite during argon ion irradiation at 600 °C

Irradiation-induced pore structure evolution of graphite is especially important for its applications in molten salt reactors. If irradiation expands the pores, more salt will intrude into graphite components, which is undesired. In this study, multi-energy argon ions were used instead of neutrons to introduce displacement damage in IG-110 graphite at 600 °C, in order to investigate pore structure evolution of graphite during irradiation. About 71% of the pores shrank after irradiation to 21 dpa, which was proved to be a result of c-axis swelling of graphite crystallites. In contrast, only 12% of the pores expanded. The total area of gas-escape pores gradually decreased to 89% at 21 dpa. At the turnaround dose and beyond, extensively distributed submicron pores were observed, indicating an increase in newborn porosity. Combining the evolution of original pores and newborn pores, a model depicting the overall porosity’s evolution during irradiation is given in this study. Because the original porosity decreases continuously with dose and newborn pores are small in sizes, this study strongly suggests that irradiation will not facilitate salt intrusion into graphite.

Irradiation damage in nuclear graphite at the atomic scale

Microstructures of nuclear graphite have been investigated based on the analysis of both X-ray diffraction patterns and High Resolution Transmission Electron Microscopy images (HRTEM) from experiments and from molecular dynamics simulations. One filler particle – composed of almost aligned crystallites separated by Mrozowski cracks – and the whole sequence of single crystal graphite irradiated from its virgin state up to highly damaged (64.5% of defects) have been simulated. Simulations are assessed against experiments based on the analysis of its microstructures extracted with Bragg's law and Scherrer's equation applied to simulated X-ray diffraction patterns. In particular, the evolution of cell parameters with defect concentration generated by irradiation agrees well with experimental observations. Simulated HRTEM images show many features observed in experimental images in both virgin and irradiated French nuclear graphite. Some of these features can be linked unequivocally to defined atomistic configurations. Basal grain boundaries (GBs), Mrozowski cracks, graphene sheets and their folding belong to this category. Conversely, some patterns in simulated HRTEM cannot be related to a unique atomistic configuration and might eventually give rise to misleading interpretation. This is evidenced for edge dislocations in virgin nuclear graphite as well as for residues of graphene layers in highly damaged graphite.

Quantitative and qualitative study on the solid electrolyte interface formed by 2-(5H) furanone: A novel additive for propylene carbonate-based lithium-ion battery electrolytes

To improve the poor solid electrolyte interface (SEI) formed with propylene carbonate (PC)-based electrolyte on a graphite electrode, 2(5H)-furanone was assessed as a novel electrolyte additive together with fluoroethylene carbonate (FEC). The mass change during the formation of the SEI was monitored with different 2(5H)-furanone to FEC ratio using an electrochemical quartz crystal microbalance. The single addition of FEC was insufficient to prevent the decomposition of the electrolyte and damage of graphite electrode, due to FEC's more negative reduction potential compared to PC, leading to serious irreversible charge loss during the 1st charge-discharge process. On the other hand, 2(5H)-Furanone suppressed the graphite damage effectively, which resulted in relatively higher discharge capacity. However, for the electrolyte containing only 2(5H)-furanone, minor charge loss due to electrolyte reduction continued even after the SEI formation. This led to SEI with excessive amount and resistance, which resulted in a severe capacity drop at rapid discharge rates. The mixed additive of 2(5H)-furanone and FEC suppressed both the damage of the graphite electrode and residual reduction currents. The LIBs with this co-additive system exhibited much lower film resistance associated with the SEI and showed superior electrochemical performances.

The regularities of high-fluence ion-induced graphitization of diamond

The process of high-fluence ion-induced graphitization and the influence of irradiation by 10–30 keV Ar+, Ne+, N+, N2+ and C+ ions at temperatures from 30 to 720 °C on the conductivity and microstructure of polycrystalline diamond surface layer were experimentally studied. The increase of diamond temperature during irradiation leads to the ion-induced graphitization at Тir > Tgr ≈ 200 °C. It has been found that Tgr practically coincides with the corresponding temperatures of dynamic annealing of radiation damage in graphite. The Raman spectra indicate that irradiation with neon and argon ions at temperatures of the diamond more than Tir > 500 °C leads to the formation of nanocrystalline graphite layer that increases resistivity of the irradiated layer. This effect is not observed under irradiation by nitrogen ions. As for C+ ion irradiation the synthesis of diamond with a thin graphite-like layer formation on the surface has been observed.

Graphitization of a Polycrystalline Diamond under High-Fluence Irradiation with Noble Gas and Nitrogen Ions

To analyze the process of the ion-induced graphitization of a polycrystalline diamond, the surfacelayer conductivity and microstructure are studied experimentally after high-fluence irradiation with Ne+, Ar+, N+, and ions with energies of 20–30 keV at irradiation and heat-treatment temperatures ranging from 30 to 720°R in vacuum. After irradiation with argon ions at room temperature and subsequent heat treatment, the resistivity ϱ of a modified layer decreases exponentially with increasing treatment temperature T ht and reaches the graphite value ϱ at Tht = 700°R. Such a temperature T ht is insufficient for surface-layer graphitization by nitrogen ions. The increase in the diamond temperature under irradiation leads to a decrease in the ion-induced thermal graphitization temperature T g by several hundred degrees. It is found that the temperature T g is almost coincident with the corresponding temperature Ta of the dynamic annealing of radiation-induced damage in graphite. Analysis of the irradiated layer using Raman spectroscopy reveals the heterogeneous structure of the modified layer containing graphite and amorphous phases, the ratio between which correlates with the layer resistivity. Under argon-ion irradiation at diamond temperatures of 500°R or more, an increase in ϱ of the irradiated layer is observed, which is related to the formation of nanocrystalline graphite. This effect is not observed under nitrogen-ion irradiation.

On the damage and fracture of nuclear graphite at multiple length-scales

Gilsocarbon graphite, as a neutron moderator and load-bearing component in the core of the UK fleet of Advanced Gas-Cooled Reactors, possesses complex microstructural features including defects/pores over a range of length-scales from nanometres to millimetres in size. As a consequence, this material exhibits different characteristics when specimens of different length-scale are deformed. In this work, the deformation and fracture of this material have been characterised using in situ methods for specimens of micrometre size (meso-scale) and the results are then compared with those measured one length-scale smaller, and those at the macro-scale. At the micro-scale, sampling a volume of material (2 × 2 × 10 μm) excludes micro- and macro-size pores, the strength was measured to be as high as 1000 MPa (an elastic modulus of about 67 GPa). When the specimen size is increased by one order of magnitude to the meso-scale, the strength is reduced to about 100 MPa (an elastic modulus of about 20 GPa) due to the inclusion of micro-size pores. For larger engineering-size specimens that include millimetre-size pores, the strength of the material averages about 20 MPa (an elastic modulus of about 11 GPa). This trend in the data is discussed and considered in the context of selecting the appropriate data for relevant multi-scale modelling.

A time-dependent atomistic reconstruction of severe irradiation damage and associated property changes in nuclear graphite

Detailed knowledge regarding the nature of and mechanisms causing neutron irradiation damage in graphite remains a scientific and technological challenge, particularly at high irradiation doses. Using electrons as a surrogate for neutron irradiation, we develop a time-dependent atomistic reconstruction strategy fed by a time series of high-resolution transmission electron microscopy (HRTEM) images, to monitor damage propagation in a graphite grain up to a dose of about one displacement per atom (i.e. well beyond the conventional irradiation simulations based on molecular dynamics). The reduction in crystalline order and the development of interlayer bonding observed in the models with increasing irradiation time induce significant modifications of the elastic constants and thermal conductivity. Homogenizing these properties to the case of isotropic polycrystalline graphite we are able to reproduce the increase in Young's modulus and decrease in thermal conductivity observed experimentally for reactor graphites with increasing dose. Further validation of the models is provided via a comparison of simulated and experimental data from irradiated material such as: HRTEM images, carbon K-edge electron energy loss spectra, dose rate and stored energies.

Development of a brazed diamond wire for slicing single-crystal SiC ingots

Fixed abrasive wires, electroplated wire saws, and resinoid wire saws are widely used to slice brittle materials. However, poor wear resistance becomes a crucial issue when these tools are used in hard brittle materials, such as single-crystal silicon carbide (SiC). This problem was addressed in this study through the development of brazed diamond wires. The production method and the slicing performance of brazed diamond wire were systematically investigated. A complete wire manufacturing line was developed to fabricate brazed diamond wire. Effect of brazing temperature on diamond grain graphitization damage was experimentally researched. The results indicated that the diamond grain graphitization damage was weakened or avoided with the optimized brazing temperature and holding time. Effect of brazing temperature on core wire mechanical property was evaluated by tensile strength, breaking twist strength, and repeated bending strength. Long brazed diamond wires were produced with optimized produce parameters for various brazing alloys. Sawing experiments on single-crystal SiC reveal that the slicing ability and slicing efficiency of the self-made brazed wire saws were better than those of commercial electroplated wire saws.