新型超硬材料的微观设计

发布时间：2018-04-02 13:24

  本文选题：第一性原理　切入点：过渡金属硼化物　出处：《上海海洋大学》2017年硕士论文

【摘要】：超硬材料具有硬度高、熔点高、耐磨损和化学惰性良好等优异性质,在众多前沿机械领域广泛用做表面涂层、金属钻头和切割刀具等。由于金刚石在高温下会与氧、铁等发生反应,故限制了其广泛的应用,突破现有超硬材料的局限,寻找更为稳定的新型超硬材料已成为一种必然的趋势。目前,最主流的方法就是将硼、氧、碳、氮等轻的元素加入到过渡金属中,形成类似于金刚石中的共价键。本文通过以密度泛函理论为框架的第一性原理计算,以过渡金属硼化物(WB_x、Mo B_x和Mn B_4)为对象,对其力学特性进行系统研究,并阐述了力学特性的微观机制,为设计新型超硬材料提供有力依据。(1)根据W-B和Mo-B体系结构的结构特点,提出了TMB_3多型体这一结构,即由相同的AH单元根据不同的堆垛顺序构成的。我们列举了15种最小周期的结构相,并给出了详细的原子位置坐标、空间群以及堆垛序。根据第一性原理,我们计算了这15种结构的形成能,得出TMB_3多型体可视为2H和3R结构的组合,并且TMB_3多型体是稳定的。最后,我们计算了15种结构的体变模量、剪切模量和理论硬度,发现其力学特性几乎不受晶胞内金属原子层数的影响。此外,TMB_3多型体由于结构无序和声子折叠等因素显示出了极低的晶格热导率。由此,我们进一步提出可以合成多型体共存的多相固溶物来提高外在硬度,达到设计新型超硬材料的目的。(2)通过第一性原理对W-B和Mo-B体系的结构、能量和动力学特性进行了全面地计算。研究结果表明,h P3-TMB_2相在能量和动力学上是不稳定的,对其计算出的结构参数、力学特性以及XRD图谱都明显偏离了实验结果。相比之下,我们提出的TMB_3多型体在能量和动力学上更稳定,并且多型体的力学性能都很好的与实验数据吻合。因此总结出长期以来所认为的h P3-TMB_2相事实上是对一类在轴向有着纳尺度有序但长程无序的TMB_3多型体。更重要的是,我们展示了这种结构和组成上的改变是通过减轻理想h P3结构中强反键态相互作用的结果。因此,本工作不仅解决了一些长期以来存在的关于W-B和Mo-B体系晶体结构和力学特性的疑惑,而且还证实了一类多型体的存在。我们的研究结论为开发功能性的超硬纳米复合材料提供了新的思路。(3)通过第一性原理计算全面地研究了Mn B_4的三种磁态(NM、FM和AFM)的三种结构(m S10、o P10和m P20)的相稳定性、力学特性和电子结构。通过计算否定了m S10结构是Mn B_4的结构相,但m P20和o P10结构是可行的。更重要的是,我们发现了Mn B_4有着伴随磁结构与电子的相变,在438K(824K)时经受了从低对称性的NM m P20绝缘体到高对称性的FM(AFM)o P10金属的转变。根据Peierls扭曲,通过打破结构简并性来使NM m P20相稳定,而根据Stoner机制,通过解除自旋简并来使FM(AFM)o P10相稳定。因此,这种独特的由绝缘体向金属的转化,以及高刚度、高硬度等特性使得Mn B_4在技术应用方面潜力巨大,比如在极端条件下的热电开关和场效应晶体管。
[Abstract]:Super hard materials with high hardness, high melting point, abrasion resistance and excellent properties such as good chemical inertness, widely used as a surface coating in many advanced machinery, metal drilling and cutting tools. The diamond in high temperature and oxygen, iron reaction, which limits its application widely, break through the limitations of the existing super hard materials, looking for a more stable new superhard materials has become an inevitable trend. At present, most mainstream is boron, oxygen, carbon, nitrogen and other light elements added to the transition metal, the formation of a covalent bond is similar to that of diamond in. Through the calculation based on density functional theory as the first principle frame, with transition metal boride (WB_x, Mo B_x and Mn B_4) as the object, the systematic study on the mechanical properties, and discusses the microscopic mechanism of the mechanical properties, provide a strong basis for the design of new superhard materials. (1) according to W-B and M The structure characteristics of o-B system structure, puts forward the structure of TMB_3 POLYTYPOIDS, AH from the same unit according to the different composition of the stacking sequence. We list structure of 15 minimum period, and gives the detailed atomic coordinates, space group and stacking sequence. According to the principle, we calculate the formation of these 15 kinds of structure can be obtained, TMB_3 polytypoid can be considered as a combination of 2H and 3R structure, and the TMB_3 polytype is stable. Finally, we calculate the structure of the 15 kinds of bulk modulus, shear modulus and hardness theory, found that the mechanical properties of almost not affected by the metal atoms in the cell the number of TMB_3 POLYTYPOIDS. In addition, due to the disordered structure and phonon folding factors such as showing the lattice extremely low thermal conductivity. Therefore, we further propose a synthesis polytype coexistence of multiphase solids to improve external hardness, achieve the design of a new type of super Hard material. (2) the structure of the first principle of the W-B and Mo-B system, energy and kinetic characteristics of the calculation. The results show that the H phase in P3-TMB_2 energy and dynamics is unstable, the calculated structural parameters on its mechanical properties, and deviated from the XRD map the results of the experiment. In contrast, we propose TMB_3 polytypoid more stable in energy and dynamics, and the mechanical properties of polytypes are good agreement with the experimental data. It summarizes that the long h P3-TMB_2 phase is in fact of a class in the axial direction with TMB_3 nano scale ordered but long rangedisorder polytypes. More importantly, we show that changes in structure and composition is through reducing the ideal h P3 structure of strong anti bonding state interaction results. Therefore, this work not only solves some long existing in the 鍏充簬W-B鍜孧o-B浣撶郴鏅朵綋缁撴瀯鍜屽姏瀛︾壒鎬х殑鐤戞儜,鑰屼笖杩樿瘉瀹炰簡涓，

本文编号：1700596