正交系超硬材料bc-BC_xN的第一性原理研究

发布时间：2018-05-07 15:28

本文选题：第一性原理 + 超硬材料　；参考：《西南大学》2015年硕士论文

【摘要】：超硬材料以其优异的高硬度、高弹性模量、良好的耐磨性和不可压缩性,在工程机械、切削加工、航空航天、矿物开采、耐磨涂层以及其它工业领域有着广泛的应用。众所周知此前已知的应用最广泛的最硬和次硬材料分别是金刚石(维氏硬度Hv=96GPa,下同)和立方氮化硼(c-BN,Hv=63 GPa)。然而金刚石较低的热稳定性和较差的化学稳定性很大程度上限制了它的应用空间,而热稳定性相对较高的立方氮化硼(c-BN),虽然是硬度仅次于金刚石的物质,却远小于金刚石。因此工业和技术的需要促使人们继续寻找硬度高而且稳定性也很好的新型超硬材料,B-C-N系化合物结合金刚石的高硬度和c-BN的热稳定性有望成为新型超硬材料而受到广泛关注。本文基于Luo等研究的bc-结构的超硬BC2N材料的基础上,设计和构建了bc-BCN和bc-BC4N,并利用基于密度泛函理论的第一性原理的方法,研究了这几种超硬材料(包括bc-BCN,bc-BC2N,和bc-BC4N,文中统称为BCxN,或者BCN,BC2N,和BC4N)的性质(弹性常数,弹性模量,弹性各向异性,硬度,理想强度和高温下的最小热导率,电子结构和光学性质等),同时还计算了金刚石和c-BN的结果并进行了对比。系统的分析了三种正交系bc-结构的BCxN的物理性能,希望能够为BCxN在实验上的研究和工业上的应用提供些理论参考。本文第三章给出了BCN、BC2N和BC4N的晶体结构,并绘制和分析电荷密度图和电荷差分密度图。发现围绕着N原子的电荷密度最高,B原子周围的电荷密度则最低,电荷差分密度图显示电荷从N原子转移到邻近的C-C键,而B-N键周围的电荷则倾向于转向N原子。对BCxN的力学性质研究表明,BCN、BC2N和BC4N都是偏脆性的材料,其中BCN有相对较好的延展性,屈服强度也较好,其次是BC2N不过BC4N的弹性模量最高,且硬度也是最高的,达到68GPa (LDA方法计算,下同)。BC2N次之,硬度为65.2 GPa,BCN的硬度值最小。此外对BCxN的理想强度进行了分析,发现BC4N剪切应力值最小的方向是(110)[001],应力值为63GPa,与c-BN的理想强度值相差无几。力学各向异性研究表明,BCN、BC2N和BC4N都是弹性各向异性的,三者的各向异性程度随着BC4N→BC2N→BCN的顺序增加,因此综合分析认为BC4N力学性质最佳。高温下最小热导率的研究表明,硬度值较高的晶体结构,其热导率也相对较高。数据显示BCN的热导率最低,而BC4N的热导率值最大。且沿着BCN、BC2N和BC4N晶体材料的不同晶向,它们的热导率都是各向异性的。在第六章我们研究了BCxN的电子结构和光学性质,发现BCN、BC2N和BC2N的能隙都非常小,分别是0.819eV,0.595eV和0.340eV,且导带的最低点和价带的最高点都不在同一布里渊区对称点,属于间接带隙半导体。BCxN的光学性质非常相近,在可见光区,三种晶体的吸收系数非常小,反射率也很低,折射率在2.0～2.5之间,且BC4N的折射率要相对较高一点,其次是BC2N。
[Abstract]:Super hard materials have been widely used in engineering machinery, cutting, aerospace, mineral mining, wear-resistant coatings and other industrial fields due to their excellent high hardness, high elastic modulus, good wear resistance and incompressibility. It is well known that the most widely used hard and subhard materials are diamond (Vickers hardness Hvn 96GPA) and cubic boron nitride (CBN) c-BN Hvn 63 GPA. However, the low thermal stability and poor chemical stability of diamond limit its application space to a great extent, and the cubic boron nitride (CBN), which has relatively high thermal stability, is much less than diamond in hardness, although it is second only to diamond in hardness. Therefore, the need of industry and technology urges people to continue to look for new superhard materials with high hardness and good stability. The high hardness of B-C-N system compounds and the thermal stability of c-BN are expected to become new superhard materials and attract wide attention. In this paper, we design and construct bc-BCN and bc-BC4N based on bc- structure superhard BC2N materials studied by Luo et al, and use the first principle method based on density functional theory. The properties (elastic constant, modulus of elasticity, elastic anisotropy, hardness, ideal strength and minimum thermal conductivity at high temperature) of these superhard materials (including bc-BCN-bc-BC2N and bc-BC4N, collectively known as BCxN, or BCN-BC2N, and BC4N) have been studied. The electronic structure and optical properties of diamond are also calculated and compared with those of c-BN. The physical properties of BCxN with three orthogonal systems are systematically analyzed. It is hoped that this paper can provide some theoretical references for the experimental research and industrial application of BCxN. In the third chapter, the crystal structures of BCN-BC2N and BC4N are given, and the charge-density diagram and charge-differential density diagram are drawn and analyzed. It is found that the charge density around the N atom is the highest and the charge density around the B atom is the lowest. The charge differential density diagram shows that the charge transfers from the N atom to the adjacent C-C bond, while the charge around the B-N bond tends to shift to the N atom. The mechanical properties of BCxN show that both BCN-BC2N and BC4N are brittle materials, among which BCN has relatively better ductility and better yield strength, and then BC2N has the highest modulus of elasticity and the highest hardness, which is in accordance with the 68GPa method. The hardness of 65.2 GPA BCN was the lowest. In addition, the ideal strength of BCxN is analyzed. It is found that the direction of minimum shear stress of BC4N is 110) [001], and the stress value is 63 GPA, which is similar to the ideal strength of c-BN. The study of mechanical anisotropy shows that both BCN-BC2N and BC4N are elastic anisotropy, and their anisotropy increases with the order of BC4N BC2N BCN. Therefore, the comprehensive analysis shows that the mechanical properties of BC4N are the best. The study of the minimum thermal conductivity at high temperature shows that the thermal conductivity of the crystal structure with higher hardness is also relatively high. The data show that BCN has the lowest thermal conductivity, while BC4N has the highest thermal conductivity. The thermal conductivities of BCN-BC2N and BC4N crystal materials are anisotropic. In chapter 6, we study the electronic structure and optical properties of BCxN. It is found that the band gaps of BCN-BC2N and BC2N are very small, which are 0.819 EV ~ 0.595eV and 0.340 EV, respectively, and the lowest point of conduction band and the highest point of valence band are not symmetrical in the same Brillouin zone. The optical properties of the indirect band-gap semiconductor. BCxN are very similar. In the visible region, the absorption coefficient of the three crystals is very small, the reflectivity is very low, the refractive index is between 2.0 and 2.5, and the refractive index of BC4N is relatively high, followed by BC2N.
【学位授予单位】：西南大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ163

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