Discussion of the Negative Thermal Expansion Coefficient of the Laminar Crystal

The variation of the thermal expansion coefficient of the graphite crystal is derived. According to the theory of temperature with the thermodynamic and statistical physical, the free energy of the graphite crystals is given with the De-Bye Model, and relationship between the thermal expansion coefficient of the graphite crystal and the temperature range is probed. The results are showed: the thermal expansion coefficient is equal to zero if T =0 K or T is on the verge of the 15 K or 42 K; negative if the temperature is in the range of 15 to 42 K; constant if the temperature is higher. The theoretical calculations accord with the experimental results. From this, In the layered crystals the reason that there is the negative expansion is that the statistical weight of the transverse acoustic vibration for the atoms in the plane layer is very large. In some temperature ranges, the variation of distance between atoms in the direction perpendicular to the plane layer with the temperature resulted in by the transverse acoustic vibration reverses with that in the plane layer. Then the film effect is resulted in.

Discussion of the Negative Thermal Expansion Coefficient of the Laminar Crystal

The variation of the thermal expansion coefficient of the graphite crystal is derived. According to the theory of temperature with the thermodynamic and statistical physical, the free energy of the graphite crystals is given with the De-Bye Model, and relationship between the thermal expansion coefficient of the graphite crystal and the temperature range is probed. The results are showed: the thermal expansion coefficient is equal to zero if T =0 K or T is on the verge of the 15 K or 42 K; negative if the temperature is in the range of 15 to 42 K; constant if the temperature is higher. The theoretical calculations accord with the experimental results. From this, In the layered crystals the reason that there is the negative expansion is that the statistical weight of the transverse acoustic vibration for the atoms in the plane layer is very large. In some temperature ranges, the variation of distance between atoms in the direction perpendicular to the plane layer with the temperature resulted in by the transverse acoustic vibration reverses with that in the plane layer. Then the film effect is resulted in.