金属Ti热学性质第一原理研究

First-principles study of thermal properties of metal Ti

应用基于密度泛函与密度泛函微扰理论的平面波赝势方法计算一组不同晶格常数下六角密堆积(hcp)结构金属Ti的声子谱及相应的静态总能,由此得到不同晶格常数下的自由能,由准谐近似及自由能极小判据得到自由能与温度的关系,进而计算热膨胀系数、定容摩尔热容及定压摩尔热容与温度的关系,对热膨胀系数及定容摩尔热容的第一原理计算值与德拜理论计算值进行比较。结果表明:295 K下声子谱理论值与实验值除在[001]方向上的光学纵模有少量偏差外,其余部分符合得很好;hcp结构金属Ti有一定程度的各向异性热膨胀,沿c轴方向与a轴方向的热膨胀系数比值为1.5左右;热膨胀系数、定容摩尔热容及定压摩尔热容第一原理计算值在较宽的温度范围内与已有的实验数据相符;热膨胀系数的德拜理论值仅在室温以下温区与实验结果相符;定容摩尔热容的第一原理计算值与德拜理论值在中温区有少量偏差,在低温及高温区非常接近。

A first-principles study of thermal properties of hexagonal close-packed （hcp） metal Ti using the pseudopotential plane-wave method in the framework of the density-functional theory and density-functional perturbation theory was reported. The phonon spectrum and static total energy were calculated at a set of different lattice parameters and free energies at different temperatures were obtained in the quasiharmonic approximation according to the criterion of free energy minimum. The curves of the coefficients of thermal expansion （CTE） and molar heat capacities at constant volume （pressure） to temperature were presented. The coefficients of thermal expansion and molar heat capacities obtained from first-principles approach and Debye theory were compared. The results show that the phonon spectrum at 295 K is in good agreement with available experimental data except the longitudinal optic mode in the [001 ]; the ratio of CTE along c axis and a axis is about 1.5; CTE and molar heat capacities obtained from first-principles approach are in good agreement with available experimental data in a wide range of temperature; CTE obtained from Debye theory only agrees with experiment below ambient temperature; molar heat capacities at constant volume calculated from first-principles approach and Debye theory have small discrepancy in medial temperature zone.