层状晶体负热膨胀系数探讨

Discussion of the Negative Thermal Expansion Coefficient of the Laminar Crystal

根据热力学和统计物理理论 ,应用德拜模型 ,求出石墨晶体的自由能。在此基础上导出石墨晶体热膨胀系数随温度变化的关系式。结果表明 :对石墨晶体 ,除T =0K外 ,在温度为 15K和 4 2K附近 ,热膨胀系数为零 ;在 15～ 4 2K的温度范围内 ,它为负值 ;在较高温度范围内 ,它近似为常数。理论计算与实验结果相符。出现负热膨胀现象的原因在于层状晶体中 ,平面层中原子横向声振动对自由能有较大统计权 ,某些温度范围内 ,横向声振动会导致垂直平面层方向原子间距离随温度的变化情况与平面层方向的情况相反 ,出现膜效应。

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.