金属碳化物力学和热力学性质第一性原理研究
发布时间：2018-12-17 16:30
【摘要】：二元金属碳化物有着丰富的晶格结构形式和优异的物理化学性能,因而在物理学、化学、材料科学及地球科学中有着巨大的应用价值。利用基于密度泛函理论(Density functional theory)的第一性原理计算方法,本论文研究了稀土金属碳化物Yb-C系统和碱土金属碳化物Ca-C系统的力学和热力学性质。论文主要研究了两方面的问题:一是研究了Habburd U值对计算Yb-C系统(其中包括Fm-3m相的YbC、I-43d相的Yb_2C_3、I4/mmm相的YbC_2和P63/mmc相的YbC_6)力学和热力学性质的影响;二是研究了压强对碱土金属碳化物Ca-C系统(其中包括C2/m相的Ca_2C_3、C2/m相和Pnma相的Ca_2C)力学性质的影响。4f电子的关联效应对于稀土金属碳化物的电子结构和热力学性质有重要的作用。本论文中,我们运用了GGA+U的方法,计算了4f电子在不同Hubbard U值修正下,YbC、Yb_2C_3、YbC_2和YbC_6的力学和热力学性质。结果表明,四种镱碳化物的晶格常数明显地伴随着U值的增大而增大,相反的,它们的弹性常数多数随着Hubbard U值的增大而减小。采用VRH近似(Voigt-Reuss-Hill approximation),我们计算了四种镱碳化物的体弹模量B、杨氏模量E和剪切模量G,并发现它们的值与Hubbard U值也表现出明显的相关性。YbC_6的B、E和G的值在四种碳化物中最大而呈现出质地硬和脆的力学特性,而YbC_2则呈现出质地软和易延展的力学特性。利用各向异性指数和因子,我们分析了四种镱碳化物的力学各向异性。声子谱计算发现,只有YbC_2和YbC_6是动力学稳定相,这与实验的结论是一致的。压强能改变材料中原子之间的成键特性从而带来材料物理性能的改变。本论文中,我们研究了不同压强下C2/m相的Ca_2C_3(C2/m-Ca_2C_3)、C2/m相的Ca_2C(C2/m-Ca_2C)和Pnma相的Ca_2C(Pnma-Ca_2C)的力学性质。它们的弹性常数C11、C22和C_33的数值伴随着压强呈现出增长趋势,且0到30 GPa下的C2/m-Ca_2C_3、0到7.5 GPa下的C/2m-Ca_2C和7.5到30 GPa下的Pnma-Ca_2C都符合Born-Huang晶格动力学稳定标准。它们的体弹模量B、杨氏模量E和剪切模量G都随着压强的升高而升高。各向异性百分比分析表明三种钙碳结构的各向异性随着压强的升高而增大。此外,C2/m-Ca_2C的弹性模量B和E以及各向异性百分比AB在5至7 GPa压强范围内增长趋势几乎消失,这预示着相变的产生。我们的研究表明,4f电子的强关联效应对镱碳稀土金属化合物的力学和热力学性质有着显著的影响;我们关于钙碳碱土金属碳化物压强下力学性质的研究,完善了对于新近发现的钙碳相的力学性质的认知。
[Abstract]:Binary metal carbides have great application value in physics, chemistry, material science and earth science because of their rich lattice structure and excellent physical and chemical properties. Based on density functional theory (Density functional theory), the mechanical and thermodynamic properties of rare earth metal carbide Yb-C system and alkali earth metal carbide Ca-C system are studied in this paper. This paper mainly studies two problems: first, we study the Habburd U-value pair calculation of Yb-C system (including the Yb_2C_3, of YbC,I-43d phase of Fm-3m phase). The influence of YbC_2 of I4/mmm phase and YbC_6 of P63/mmc phase on mechanical and thermodynamic properties; The second is to study the pressure pair alkali earth metal carbides Ca-C system (including C 2 / m phase Ca_2C_3,). The correlation effect of 4f electrons plays an important role in the electronic structure and thermodynamic properties of rare earth metal carbides. In this paper, we use the GGA U method to calculate the mechanical and thermodynamic properties of YbC,Yb_2C_3,YbC_2 and YbC_6 for 4f electrons modified by different Hubbard U values. The results show that the lattice constants of the four ytterbium carbides increase obviously with the increase of U value. On the contrary, the elastic constants of the four ytterbium carbides decrease with the increase of Hubbard U value. The VRH approximation (Voigt-Reuss-Hill approximation),) is used to calculate the bulk elastic modulus (B), Young's modulus (E) and shear modulus (G) of four ytterbium carbides, and it is found that their values are also significantly correlated with the Hubbard U value. The values of E and G are the largest among the four carbides, showing the mechanical properties of hardness and brittleness, while the mechanical properties of YbC_2 are soft and extensible. Using anisotropy exponents and factors, we analyze the mechanical anisotropy of four ytterbium carbides. The calculation of phonon spectra shows that only YbC_2 and YbC_6 are kinetic stable phase, which is consistent with the experimental results. The pressure energy changes the bonding properties between atoms in the material, which leads to the change of the material physical properties. In this paper, we study the mechanical properties of Ca_2C_3 (C2/m-Ca_2C_3), Ca_2C (C2/m-Ca_2C) of C _ 2 / m phase and Ca_2C (Pnma-Ca_2C) of Pnma phase at different pressures. Their elastic constants C _ 11 C _ 22 and C _ S _ 33 show an increasing trend along with the pressure. Moreover, the C/2m-Ca_2C of C _ 2 / m -Ca _ 2C _ (2) C _ (2) C _ ( Their bulk elastic modulus B, Young's modulus E and shear modulus G all increase with the increase of pressure. The anisotropy percentage analysis shows that the anisotropy of the three calcium carbon structures increases with the increase of pressure. In addition, the elastic modulus B and E of C2/m-Ca_2C and the anisotropic percentage AB almost disappear in the range of 5 to 7 GPa pressure, which indicates the generation of phase transition. Our results show that the strong correlation effect of 4f electrons has a significant effect on the mechanical and thermodynamic properties of ytterbium carbon-rare earth metal compounds. Our study on the mechanical properties of calcium carbon alkali earth carbide under pressure has improved the understanding of the mechanical properties of the recently discovered calcium carbon phase.
【学位授予单位】：烟台大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O611.3;O641.1
，
本文编号：2384449
[Abstract]:Binary metal carbides have great application value in physics, chemistry, material science and earth science because of their rich lattice structure and excellent physical and chemical properties. Based on density functional theory (Density functional theory), the mechanical and thermodynamic properties of rare earth metal carbide Yb-C system and alkali earth metal carbide Ca-C system are studied in this paper. This paper mainly studies two problems: first, we study the Habburd U-value pair calculation of Yb-C system (including the Yb_2C_3, of YbC,I-43d phase of Fm-3m phase). The influence of YbC_2 of I4/mmm phase and YbC_6 of P63/mmc phase on mechanical and thermodynamic properties; The second is to study the pressure pair alkali earth metal carbides Ca-C system (including C 2 / m phase Ca_2C_3,). The correlation effect of 4f electrons plays an important role in the electronic structure and thermodynamic properties of rare earth metal carbides. In this paper, we use the GGA U method to calculate the mechanical and thermodynamic properties of YbC,Yb_2C_3,YbC_2 and YbC_6 for 4f electrons modified by different Hubbard U values. The results show that the lattice constants of the four ytterbium carbides increase obviously with the increase of U value. On the contrary, the elastic constants of the four ytterbium carbides decrease with the increase of Hubbard U value. The VRH approximation (Voigt-Reuss-Hill approximation),) is used to calculate the bulk elastic modulus (B), Young's modulus (E) and shear modulus (G) of four ytterbium carbides, and it is found that their values are also significantly correlated with the Hubbard U value. The values of E and G are the largest among the four carbides, showing the mechanical properties of hardness and brittleness, while the mechanical properties of YbC_2 are soft and extensible. Using anisotropy exponents and factors, we analyze the mechanical anisotropy of four ytterbium carbides. The calculation of phonon spectra shows that only YbC_2 and YbC_6 are kinetic stable phase, which is consistent with the experimental results. The pressure energy changes the bonding properties between atoms in the material, which leads to the change of the material physical properties. In this paper, we study the mechanical properties of Ca_2C_3 (C2/m-Ca_2C_3), Ca_2C (C2/m-Ca_2C) of C _ 2 / m phase and Ca_2C (Pnma-Ca_2C) of Pnma phase at different pressures. Their elastic constants C _ 11 C _ 22 and C _ S _ 33 show an increasing trend along with the pressure. Moreover, the C/2m-Ca_2C of C _ 2 / m -Ca _ 2C _ (2) C _ (2) C _ ( Their bulk elastic modulus B, Young's modulus E and shear modulus G all increase with the increase of pressure. The anisotropy percentage analysis shows that the anisotropy of the three calcium carbon structures increases with the increase of pressure. In addition, the elastic modulus B and E of C2/m-Ca_2C and the anisotropic percentage AB almost disappear in the range of 5 to 7 GPa pressure, which indicates the generation of phase transition. Our results show that the strong correlation effect of 4f electrons has a significant effect on the mechanical and thermodynamic properties of ytterbium carbon-rare earth metal compounds. Our study on the mechanical properties of calcium carbon alkali earth carbide under pressure has improved the understanding of the mechanical properties of the recently discovered calcium carbon phase.
【学位授予单位】：烟台大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O611.3;O641.1
，
本文编号：2384449
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