Graphite is widely used in lithium-ion batteries, nuclear systems, and semiconductor applications, where impurity content directly governs performance and application limits. Thermochemical purification is attractive due to its low cost and high efficiency; however, the phase evolution mechanisms of impurities in high-temperature carbon environments remain unclear, hindering targeted optimization of halogen purification. In this work, thermodynamic analysis combined with experimental validation was employed to elucidate the high-temperature phase transformation behavior of oxide and salt impurities in graphite. The results indicate that Mo, Nb, W, B, Ta, V, Cr, Si, Ti, Zr, Ba, and Ca are strong carbide-forming elements; Al, Fe, and Mn show weak carbide-forming tendencies; whereas Co, Ni, Mg, Na, Zn, and Cu mainly behave as non-carbide-forming elements. Based on this classification, the reactions of impurity oxides, carbides, and elemental species with chlorine radicals were systematically evaluated. For B, Si, and Al, halogen purification efficiency strongly depends on impurity occurrence state, where oxide species are difficult to remove directly, whereas conversion into carbides enables effective removal. In contrast, the purification behavior of Ni, Co, and Zr is only weakly affected by occurrence state. Further thermodynamic analysis reveals that, compared with Cl2 and CCl4, CHClF2 theoretically exhibits superior purification capability, enabling the direct removal of W and Si in both oxide and carbide forms without prior carburization. This work clarifies the relationship between impurity evolution and halogen purification behavior in graphite, and provides a theoretical basis for targeted impurity removal strategies.

Spherical natural flake graphite has long been a crucial anode material for lithium-ion batteries due to its high theoretical specific capacity, excellent electrochemical stability, and low production cost. Currently, over 90 % of the global supply of spherical graphite is produced in China, primarily through the cascaded air classifier mill process at an industrial scale. However, further improvements are necessary to enhance yield and optimize particle morphology. Despite its industrial importance, the underlying mechanisms of spheroidization remain insufficiently understood, hindering progress in improving product uniformity, controlling particle shape, and refining internal microstructure. This study systematically investigates the structural evolution of natural flake graphite throughout the cascaded air classifier mill spheroidization process, identifies a dominant mechanism involving the stacking of fragmented flakes onto primary flake substrates, and proposes a four-stage model comprising initial curling, aggregate, spheroidization onset, and densification. These findings provide fundamental insights into the formation of spherical graphite and lay a theoretical foundation for optimizing the cascaded air classifier mill production process.

Graphite, like many ceramics, exhibits brittle fracture behavior. In high-temperature gas-cooled reactors (HTRs), this brittleness makes fracture toughness a key determinant of the structural reliability and service life of nuclear graphite. Although extensive studies have clarified the fracture behavior of commercial nuclear graphites, strategies that regulate crack paths and improve crack resistance through structural design are still lacking, which limits improvements in reactor safety. In brittle ceramics, whisker toughening has been demonstrated to be highly effective. Inspired by this concept, we develop a fine-grained isostatically molded graphite reinforced with short-cut carbon fibers (SCF), and systematically evaluate the effects of fiber content and fiber–filler size matching on fracture toughness, flexural strength, and thermal conductivity. The results show that the introduction of fibers effectively improves the physical properties of graphite. In graphite prepared with 3.5 μm fillers, an 8 wt.% fiber addition increased the fracture toughness by 36.5% to 1.01 MPa·m1/2 and the flexural strength to 46.62 MPa, which are 16.1% and 18.9% higher than those of IG-11, respectively, while maintaining a thermal conductivity comparable to IG-11. Fractographic analysis and R-curve analysis reveal that the toughening behavior is governed by the size relationship between fibers and fillers. When the fiber length exceeds the filler size, crack bridging, fiber pull-out, and crack deflection dominate, leading to increases in both the initiation strain energy release rate (Ginit) and the critical strain energy release rate (GIc), thereby enhancing the fracture toughness of graphite. By contrast, when the fiber and filler sizes are comparable, fibers tend to split axially, resulting in reduced toughness. These findings not only demonstrate the feasibility of fiber-toughened graphite, but also establish fiber–filler size matching as a design principle for toughening, providing an effective pathway to develop safer and longer-lived nuclear graphite and a broadly applicable toughening strategy for other brittle carbon-based materials.

ABSTRACT Fine‐grained nuclear graphite is a key material in high‐temperature gas‐cooled reactors (HTGRs). During air ingress accidents, core graphite components undergo severe oxidation, threatening structural integrity. Therefore, understanding the oxidation behavior of nuclear graphite is essential for reactor safety. The influence of oxidation involves multiple factors, including temperature, sample size, oxidant, impurities, filler type and size, etc. The size of the filler particles plays a crucial role in this study. Five ultrafine‐ and superfine‐grained nuclear graphite samples (5.9–34.4 μm) are manufactured using identical raw materials and manufacturing processes. Isothermal oxidation tests conducted at 650°C–750°C are used to study the oxidation behavior. Additionally, comprehensive characterization is performed to analyze the crystal structure, surface morphology, and nanoscale to microscale pore structure of the samples. Results indicate that oxidation behavior cannot be predicted solely based on filler grain size. Reactive site concentration, characterized by active surface area, dominates the chemical reaction kinetics, whereas pore tortuosity, quantified by the structural parameter Ψ, plays a key role in regulating oxidant diffusion. These findings clarify the dual role of microstructure in oxidation mechanisms and establish a theoretical and experimental basis for the design of high‐performance nuclear graphite capable of long‐term service in high‐temperature gas‐cooled reactors.

Graphite is widely used as a neutron moderator, reflector, and structural component in graphite-moderated reactors. Fast neutron irradiation causes graphite crystals to expand along the c axis and contract along the a axis. Historical measurements based on bulk highly oriented pyrolytic graphite (HOPG) were potentially biased by the presence of Mrozowski microcracks, which accommodate c-axis expansion and thus underestimate true crystal strain. In this work, we establish the intrinsic anisotropic dimensional change of graphite crystals under irradiation by employing mechanically exfoliated, crack-free graphite nanosheets subjected to 100 keV He+ ion irradiation over 200-750 degrees C and doses up to 2 dpa. High-resolution atomic force microscopy was used to quantify thickness and in-plane dimensional variations before and after irradiation, enabling direct determination of c-axis expansion and a-axis contraction. The results reveal significantly larger dimensional change at low doses (<0.2 dpa) compared with historical neutron-irradiation data of bulk HOPG reported by B. T. Kelly, demonstrating that microcrack accommodation substantially suppresses the apparent strain in bulk specimens. The intrinsic c-axis strain follows a concave power-law dependence on dose, while temperature strongly reduces both c-axis expansion and a-axis contraction due to enhanced defect mobility and recombination above similar to 300 degrees C. The calculated crystal volume change indicates a net volumetric swelling below 2 dpa, with a maximum at 200 degrees C, consistent with defect-cluster-controlled growth. These results provide benchmark data for intrinsic crystal-level irradiation growth of graphite and offer a refined physical basis for multiscale modelling of dimensional change in nuclear graphite.

Phase change materials (PCMs) are attractive for thermal regulation, yet their practical use is hindered by low thermal conductivity (TC) and inefficient filler utilization arising from interfacial thermal resistance. Existing carbon-filler strategies usually improve heat transport only along preferred directions, resulting in strong anisotropy and low thermal-conductivity enhancement efficiency per filler fraction (TCEF). Here, we report a precursor-mediated interfacial engineering strategy coupled with confined mesophase pitch foaming to construct a hierarchically integrated graphitic architecture with multidirectional heat pathways. Low-temperature carbonized fibers with retained surface activity serve as conductive backbones and confinement frameworks, offering two key advantages: enhanced surface activity that improves fiber dispersion and interfacial coupling, and synchronized shrinkage with pitch-derived foam carbon that reduces mismatch and strengthens fiber–foam compatibility. The resulting architectures provide two complementary thermal-transport modes. SC500–3000 achieves an ultrahigh in-plane TC of 71.33 W·m−1·K−1 at only 24.09 vol% filler, corresponding to a 296-fold enhancement over paraffin and a TCEF of 1230%, making it suitable for directionally enhanced heat transport. Meanwhile, SC700–3000 delivers balanced multidirectional transport with in-plane and through-plane TC values of 35.92 and 21.3 W·m−1·K−1, respectively, yielding a low anisotropy ratio of 1.69 for volumetric thermal-buffering applications. This work demonstrates a precursor-level interfacial activation strategy integrated with confined directional graphitic foaming as a powerful strategy to achieve tunable multidirectional thermal transport in PCM systems.

Carbonaceous mesophase, a discotic nematic liquid crystal, is formed during the liquid-phase carbonization of coal-tar pitch, petroleum pitch, and polycyclic aromatic hydrocarbons. The microstructure established during the brief plastic stage decisively governs the ultimate structure and properties of graphitizing carbons. Around the mesophase transition temperature, aromatic molecules undergo thermal polycondensation to yield anisotropic mesophase spherules. Subsequent coalescence introduces defects such as O- and X-type disclinations, ultimately producing bulk mesophase with complex optical textures. However, the detailed coalescence pathways and defect transformations remain poorly understood. In this study, polarized optical microscopy with a modified sensitive-tint technique and in situ hot-stage imaging was employed to investigate the morphological and defect evolution of the mesophase. Results revealed a five-stage coalescence process: mesophase spherules, primary coalesced spherules, multiple-coalesced spherules, irregular coalesced bodies, and bulk mesophase. Early coalescence proceeds through multiple pathways, and the resulting defects depend on the angle between the polar axes of contacting spherules, predominantly forming lamellar-folded regions. Progressive coalescence drives sequential defect evolution from lamellar-folded regions to O- and X-type disclinations, disclination dissociation, and ultimately U- and Y-type disclinations. These findings clarify the mechanisms of mesophase growth and coalescence and provide guidance for the structural design of high-performance carbon materials.

With the development of high-temperature gas-cooled reactors, the coefficient of thermal expansion (CTE) of nuclear-grade graphite plays an increasingly important role in reactor design. A lower CTE enhances both the integrity of the graphite core structure and reactor efficiency. In this paper, we present a new isotropic graphite grade with a low CTE, utilizing spheroidized natural flake graphite (SFG) as a filler material. The ultralow isotropic CTE of 2.6-2.9 x 10-6 K- 1 in the SFG-based graphite, owing to the ability of the slit-shaped pores within the SFG particles to accommodate cross-plane thermal expansion. To enhance the baking performance of the SFG-based graphite, hybrid fillers of SFG/coke or SFG/microcrystalline graphite (MG) were used to prevent cracking of the green bodies. In particular, the addition of MG prevents cracking without changing the low CTE value of the SFG-based graphite. This research contributes to the development of new graphite materials with low CTE that can be used in nuclear engineering, the semiconductor industry, and other high-temperature environments.

Effective thermal management is critical for high‐performance electronics facing challenges of heat flux and interfacial conduction. Traditional carbon fiber‐based thermal interface materials (TIMs) and phase change thermal interface materials (PCTIMs) often fail to simultaneously achieve high through‐plane thermal conductivity (TC) and mechanical compliancedue to limitations in fiber alignment and material anisotropy. This study presents a paradigm‐shifting design of helical carbon fiber composite (HCFC) addressing these limitations. The helical architecture inherently aligns fibers along the z ‐axis, enabling enhanced through‐plane TC (2.97 W m − 1 K − 1 ) without complex alignment processes. HCFCs exhibit superior rebound rates exceeding 80%, ensuring reliable thermal contact under compression. Integrated into a paraffin wax matrix, the HCFC‐based PCTIM achieves a significantly improved through‐plane TC of 3.72 W m − 1 K − 1 , high latent heat (150 J g − 1 ), and minimal leakage (1.55 wt%). The helical structure enhances phase change material accommodation while sustaining performance across pressure/temperature variations. Leveraging cost‐effective Polyacrylonitrile‐based carbon fibers, this design surpasses traditional TIMs in both thermal and mechanical properties. This work establishes HCFC‐based TIMs/PCTIMs as scalable solutions for advanced thermal management in electronic and energy systems, addressing the growing demand for high‐performance materials.

Surface anchoring occurs when planar molecules of carbonaceous mesophase interact with a foreign substrate. This interaction can result in either face-on or edge-on surface anchoring, depending on the substrate's nature. However, our study delves beyond conventional surface anchoring phenomena, revealing a remarkable epitaxial growth mechanism between carbonaceous mesophase and needle coke (NC) surfaces. Here, we observe the replication of planar liquid crystal molecules onto the basal planes of coke, leading to epitaxial growth. Following carbonization, this growth results in a fused NC/binder interface. Our investigation extends throughout graphitization stages. Optical texture analyses demonstrate coherent orientation between the mesophase-derived binder and the coke substrate, maintained up to 2500 degrees C. Moreover, observations of intragranular crack morphology validate the strength of the fused filler/binder interface. Such findings underscore the pivotal role of epitaxial growth in forming interface structure and mechanical robustness in graphite. This study elucidates the fundamental role of epitaxial growth in shaping the interfacial structure of carbonaceous mesophase on coke surfaces, offering insights into graphite's mechanical properties and paving the way for advanced carbon material design.

Carbon‐based insulation materials exhibit remarkable potential for use in thermal protection systems (TPS) in extreme environments such as hypersonic vehicles and deep‐space missions. This is attributed to their ultralight structure, exceptional thermal insulation properties, and outstanding high‐temperature stability. Nevertheless, traditional carbon aerogels frequently experience significant volume shrinkage during fabrication, which makes it challenging to optimize their structural and thermal performance. Inspired by the performance enhancement induced by hollow fiber structures, a carbide‐derived carbon (CDC) strategy was employed in this study to fabricate a hollow carbon fiber‐based porous insulation material (CF‐H); carbon fiber felt (CF) was used as the structural template. The CDC strategy combined the template method with a conformal transformation mechanism to achieve minimal volume shrinkage (10.22%) and high porosity (98.84%). The hollow fiber framework reduced density (19 mg·cm −3 ), minimized heat transfer, and provided low thermal conductivity (0.09553 W·m −1 ·K −1 at 300 °C). Moreover, CF‐H retained the needle‐punched architecture of the CF template, thereby exhibiting excellent elasticity under mechanical stress. In conclusion, applying the CDC strategy to develop lightweight, high‐performance carbon‐based insulation materials offers a novel perspective on design and development of TPS insulation for application in extreme aerospace conditions.

Graphene oxide (GO) was incorporated into refined low quinoline insoluble coal tar pitch (RLQICTP), and the pyrolysis and carbonization products were generated using liquid-phase dispersion and co-carbonization method. The results indicate that the controllable transformation of the mesophase progresses from the flow domain anisotropy to an oriented flow domain texture, and then to a mosaic texture, by adding GO content. In the initial stages of the reaction, a low content of GO serves as a nucleating agent, facilitating the formation of mesophase spheres. Conversely, an excessive content of GO in the later stages impedes the aggregation of these spheres, leading to the development of island texture that subsequently evolve into mosaic texture. When 0.06 wt% GO is added, the domains texture with good orientation is formed, resulting in the highest Optical Texture Index (OTI) value. While 1.00 wt% GO is added, the mosaic texture is formed resulting in the lowest OTI value. At the same time, an appropriate amount of GO facilitates the formation of a more ordered carbon layer structure and promotes easier graphitization. This work explores the structural evolution and carbon formation mechanisms during pyrolysis and carbonization, providing both a theoretical basis and technical support for the preparation of high-performance carbon precursor materials.

Nuclear graphite is widely used in high-temperature gas-cooled reactors due to its excellent neutron moderation capabilities and mechanical stability at elevated temperatures. However, graphite components in these reactors are exposed to complex environments and diverse loads, including dynamic forces potentially arising from earthquakes, aircraft impacts, and internal explosions. This study investigates the fracture behavior of nuclear-grade graphite, specifically SIAMC fine-grained graphite and IG-11 graphite, under dynamic loading by examining their fracture morphology. Fracture surface roughness of graphite discs was measured, and three-dimensional surface reconstructions were conducted. Results show a significant decrease in surface roughness with increasing strain rates, indicating a trend toward a flatter fracture surface driven by changes in crack propagation paths. Furthermore, the microscopic crack growth mechanism of fine-grained nuclear graphite under dynamic loading is elucidated, enhancing our understanding of its fracture behavior under dynamic conditions.

Point defects typically reduce the thermal conductivity ( κ ) of a crystal due to increased scattering of heat‐carrying phonons, a mechanism that is well understood and widely used to enhance or impede heat transfer in the material for different applications. Here an opposite effect is reported where the introduction of point defects in graphite with energetic particle irradiation increases its cross‐plane κ by nearly a factor of two, from 10.8 to 18.9 W m K −1 at room temperature. Integrated differential phase contrast imaging with scanning transmission electron microscopy revealed the creation of spiro interstitials in graphite by the irradiation. The enhancement in κ is attributed to a remarkable mechanism that works to the benefit of phonon propagation in both the harmonic and anharmonic terms: these spiro interstitial defects covalently bridge neighboring basal planes, simultaneously enhancing acoustic phonon group velocity and reducing phonon–phonon scattering in the graphite structure. The enhancement of κ reveals an unconventional role of lattice defects in heat conduction, i.e., easing the propagation of heat‐carrying phonons rather than impeding them in layered materials, inspiring their applications for thermal management in heavily radiative environments.

Graphite serves as moderator and structural material in various type of graphite-based reactors. The presence of impurities within the graphite matrix can lead to adverse consequences, including diminished fuel efficiency, heightened radionuclide production, and catalytic oxidation. High-purity graphite materials also find essential applications in diverse industries, such as semiconductor manufacturing and chemical analysis. In this study, we conducted an investigation into IG-11 graphite, which has an ash content of 347 ppm, and its purified version, IG-110 nuclear grade graphite, which has an ash content of 12.7 ppm, to understand the purification mechanism of polycrystalline graphite. The analysis of Time-of-flight secondary ion mass spectrometry spectra revealed that metallic impurities in IG-11 were primarily segregated within graphite porosities or appeared as discrete point inclusions, rather than uniformly distributed throughout the graphite matrix. IG-110 demonstrated a significant reduction in impurities such as Na, K, Ca, and Al compared to IG-11, but Ti was still present within its porosity. Notably, impurities residing on the walls of these porosities can accelerate graphite oxidation. The establishment of the theory on impurity segregation in porosities help to develop innovative graphite purification techniques that produce graphite with even greater purity and stronger oxidation resistance.

Matrix graphite is used as a structural material, thermal conductor, moderator, and secondary fission product barrier for fuel elements in high-temperature gas-cooled reactors (HTRs). Due to its high graphitization degree and compressibility, natural flake graphite (NFG) is used as the main filler in traditional A3-3 matrix graphite, whereas artificial graphite (AG), with a lower graphitization degree than NFG, serves as an additive for toughness and gas permeability. Matrix graphite could be improved in terms of thermal conductivity, oxidation resistance, and irradiation performance by increasing the degree of graphitization. However, reports on the development of new matrix graphite formulations are scarce. In this study, MG-20 matrix graphite was prepared by mixing 60 wt % NFG, 20 wt% natural microcrystalline graphite (MG), and 20 wt% phenolic resin. Due to the high graphitization degree (higher than AG) and low coefficient of thermal expansion (CTE) of MG, MG-20 exhibited higher thermal conductivity (similar to 6%) and lower CTE (similar to 2.4%) than A3-3. Thus, MG-20 with higher graphitization degree and better thermal properties than A3-3 could improve the performance of HTR fuel elements in the future.

Artificial graphite, a key material in numerous industries, is typically produced through processes involving coaltar pitch and coke filler particles. The baking process, integral to its production, influences the final product's structural integrity. Despite extensive research on coal-tar pitch pyrolysis, there is a gap in understanding the carbonization behavior within the green body, as the coal-tar pitch is carbonized in confined space and shows location-dependent pyrolysis behavior. This study investigates the location-dependent pyrolysis behavior of green bodies by introducing a novel parameter, baking degree (eta). Experimental and simulation studies reveal delayed pyrolysis behavior within the central part of the green body, confirmed through examination of porosity development. Such delayed pyrolysis is not caused by temperature gradient, but should be attributed to the kinetic factors associated with carbonization behavior in the confined space. Introducing the eta helps to grasp more precisely the pyrolysis process of the binder in the green body. By optimizing process parameters to minimize the difference in eta between the center and surface of the green bodies during the baking process, the delayed carbonization of binders within the green bodies can be effectively reduced, thereby enhancing the performance of the artificial graphite.

High-purity rayon-based carbon fiber (CFRay) felt holds significant promise for applications in thermal protection systems and semiconductor crystal growth furnaces. This study employs halogen gas purification and hightemperature purification techniques in the fabrication of high-purity CFRay felt. Additionally, the influence of the purification process on the thermal insulation and oxidation resistance of CFRay felt is investigated. The glow discharge mass spectrometer analysis reveals that purification at a lower temperature of 2300 degrees C achieves a purity level of 6.9 ppm, surpassing both purified CFRay felt without halogen at 2300 degrees C (56.1 ppm) and hightemperature treated CFRay felt at 3000 degrees C (10.4 ppm). Thermal insulation and dynamic-static oxidation resistance tests show that the properties of CFRay felt are concurrently affected by the heat treatment temperature. Compared to high-temperature purification, halogen gas purification operates at lower temperatures, preventing the graphitization of CFRay, thereby ensuring high purity while enhancing thermal insulation and demonstrating relatively superior oxidation resistance. This study offers novel insights into the preparation and performance adjustment of high-purity CFRay felt.