Greenessen Theory of graphite thermal expansion the thermal expansion of solids is an increase in the average distance between neighboring atoms in solids. The potential energy of the neighboring two atoms in the crystal is the function of the distance r between the nuclei. The potential energy curve u-r is an asymmetrical curve, the atomic distance increases with the increase of the vibration energy, because the resonance does not change the average distance between atoms, strictly speaking, the distance should be increased with the increase of the non harmonic motion. This is the starting point of Greenessen (E. Grtinecsen) Thermal expansion theory. Graphite Material

In a certain vibrational energy, the distance of the two atoms is changed near the equilibrium position, because of the asymmetry of the potential energy curve, the mean distance r is greater than the equilibrium distance r0, and the average distance is greater under higher vibrational energy. The vibration energy increases with the increase of temperature, and the average distance between the two atoms increases with the increase of temperature, resulting in swelling. When the crystal expands, the elastic energy increases and the lattice vibration frequency decreases. Greenessen constant γ is usually introduced to describe the change of frequency w with volume V.Graphite Material

The thermal expansion of polycrystalline graphite material is determined by the linear expansion coefficient αa, αc, micro-crystal orientation parameter R and micro-pore properties, and the body integral ratio of the micro-crystalline graphite. Multi-crystalline inks have cylindrical symmetry with the axis of formation. In the study of carbon materials, the symmetry axis of graphite is generally taken as the z axis of the axis. The orientation parameters of polycrystalline graphite materials are usually expressed along the orientation parameter Roz of the symmetric axis. The direction parallel to the symmetric axis is called the Z-direction or the parallel direction, and the line expansion coefficient in this direction is recorded as Α∥; the direction perpendicular to the symmetric axis is called the x direction or the vertical direction, and the line expansion coefficient in this direction is recorded as Α⊥. This point, for molded graphite products, often with the cis-grain and cross grain direction of confusion, worth noticing.Graphite Material