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Microcrystalline graphite (MG) is a major form of natural graphite and an important raw material with various applications in advanced technologies, such as lithium-ion battery anodes and isotropic graphite filler particles. MG is known to be an aggregate of graphite crystallites with a lateral dimension of less than 1 μm. However, the microstructure of MG remains unclear. In this study, we report the porous structure of a purified Chinese MG with an apparent density of 1.3–1.4 g/cm3. MG typically contains many microslits that are approximately 1–2 μm in length and up to 200 nm in width. The microslits provide MG with a stronger ability to accommodate c-axis thermal expansion than Mrozowski cracks, resulting in a low coefficient of thermal expansion (CTE) of approximately 2 × 10−6/K. This unique thermal expansion behavior makes MG a promising filler material for developing artificial graphite with a low CTE, and enables the modulation of the CTE of nuclear graphite.

Natural graphite such as flake graphite and microcrystalline graphite have important applications in nuclear engineering, especially in high-temperature gas-cooled reactors. Owing to requirements for low ash content and total equivalent boron content, thermal and gas purification is necessary for producing natural graphite powder with nuclear-grade purity. In this study, representative Chinese flake graphite and microcrystalline graphite were purified thermally or with halogens at graphitization temperatures. By thermal purification at 3000 °C, natural flake graphite and microcrystalline graphite containing at least 99.9% carbon was achieved. The metallic impurities remaining after thermal purification were mainly elements that formed carbides with extraordinarily high melting/boiling points, such as B, Ti, Ta, V, W, and Mo. Gas purification with halogens further increased the purity of natural flake graphite and microcrystalline graphite to nuclear grade, that is, the ash content and total equivalent boron content were less than 50 ppm and 0.01 ppm, respectively. Meanwhile, an investigation of the purification mechanism helped to understand the chemistry and thermodynamics of thermal purification of natural graphite.

The material properties of nuclear graphite are defined by the as-manufactured graphite texture and their evolution due to irradiation and oxidation in reactors. Thus an understanding of the graphite microstructure and texture is essential in simulating and the predicting material property changes in nuclear graphite. The texture of nuclear graphite involves the preferred orientation of graphite crystallites and the domain structure. This study developed a technique based on combined micro-Raman spectroscopy mapping and atomic force microscope, and demonstrate the application of this technique to texture characterization of GCMB Gilsocarbon graphite. A crystallite orientation analysis approach was proposed to describe the local arrangement of graphite crystallites. For comparison, the texture characterization results are present for Gilsonite coke filler, and different parts of binder phase, respectively. The domain orientation within Gilsonite coke filler is irregular, but statistically follows the spherical particles. The major binder phase shows less ordered texture, however, certain parts of the binder phase shows very ordered planar crystallite orientation, and even larger lateral crystal size than that of filler particle. The clear description of texture provides the bases for future simulations and investigation of the properties and irradiation performance of Gilsocarbon graphite.

Mesocarbon microbeads (MCMBs) are attractive precursors for high-density, high-strength polycrystalline graphite due to their self-sintering capability. They have the shape of a standard sphere and stack in a completely random orientation during compaction, thereby leading to an isotropic texture. In this paper, MCMB-based graphite shows better isotropic property than commercial nuclear graphite, as demonstrated by a coefficient of thermal expansion-based isotropy ratio of 1.00. It was found that variations in apparent density within block are approximately 2–5% in graphitized artifacts. The variation of flexural strength within block is small, revealing excellent homogeneity of MCMB-based graphite. Density distribution measurement indicates that artifacts produced in the same batch also have low variation from block to block. Furthermore, the flexural strength of this material is greatly influenced by heat treatment temperature and mean particle size. The fracture mechanism is also shown by a detail examination of fracture surfaces. These results may help to gain a better understanding of MCMB-based graphite.

Graphite materials are used in high-temperature gas-cooled reactor (HTR) as moderator and reflector. Nuclear graphite is currently manufactured using coke as the filler; however, improvements in the performance of nuclear graphite are anticipated in order to enhance the safety and lifetime of HTRs. Natural microcrystalline graphite (MG), which is a metamorphic product of coal, is an emerging candidate as filler material. This paper proposes the approach of preparing isotropic graphite using MG fillers. The characterization results of MG ores and purified powder indicate a polycrystalline and near-isotropic structure of MG particles. Thus, near-isotropic graphite with an isotropy ratio of 1.10–1.15 can be easily obtained via cold isostatic pressing. The thermal diffusivity of MG-based green body is much higher than that of coke-based one, therefore facilitates the baking step. The advantages of MG-based graphite also include a high degree of graphitization, and a low coefficient of thermal expansion, both of which are highly beneficial to nuclear applications. As a result, MG exhibits a large potential for the application in isotropic nuclear graphite.

We demonstrate the potential use of asphalt emulsions as a binder for the preparation of polycrystalline graphite. An emulsion mixing method was adopted to create the natural microcrystalline graphite and asphalt binder mixture, and graphite samples with a bulk density of 1.7 g/cm 3 and a flexural strength of ca. 22 MPa were prepared. This approach is advantageous because of the uniformity of the asphalt coating, and it also reduces the use of coal tar pitch.