Microstructure and thermal expansion behavior of natural microcrystalline graphite

Abstract 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.