超硬材料晶体结构设计方法的优化与应用

Superhard materials are widely used as polishing, drilling, and cutting tools. The search for new superhard materials is the requirement of modern industrial development and hot research topic of material science. Variety of reactants and different temperature and pressure conditions are usually needed to experimental synthesis of new superhard materials.In this process large amount of manpower and material resources will be wasted. Therefore it is necessary to carried out theoretical design to provide useful trails for experimental synthesis. Previously, theoretical design of superhard materials usually adopts structure substitution method or structure prediction method that based on looking for global energy minimum solution of structure. These methods cannot ensure that the obtained structure has superhard property. Therefore, previously we have developed the inverse design method in which the theoretical hardness was used as fitness function to search the hardest structure. The method can predicted a large number of superhard structures only by given the external pressure and chemical composition. It is more efficiency than the previous traditional methods. In order to guide experimental synthesis and increase the adaptability of the new method, this project intends to optimize and develop the above method in the following aspects: (1) create more universal hardness calculation methods for structural predictions, such as introduce “anisotropy” and “metallicity” in hardness calculation, (2) consider the energy stability in the superhard structure predictions to design the structure with both low energy and high hardness, (3) develop multifunctional superhard materials prediction method by using multi-objective optimization algorithm. Finally, we will apply the newly developed method to design new superhard materials.

超硬材料作为研磨、钻探、切割工具应用广泛，探索新型超硬材料是工业发展的需求，也是材料科学领域的研究热点。实验合成需要用各种反应物在不同条件下不断尝试，耗费人力物力。因此需要理论设计为实验提供有利指导，以往理论设计采用结构替代法或寻找全局能量最低解的结构预测法，不能保证所获结构硬度高，为此，我们利用粒子群优化算法，初步建立了以“硬度”为导向搜索全局最硬结构的逆向设计法。给定压力和化学组分，该方法便能一次获得大量超硬结构，效率高、目标性强。但考虑对实验的指导意义及加强方法的普适性，本项目拟对前期方法进行优化和拓展：（1）研发更适宜结构预测的具有普适性的硬度计算方法，如在硬度计算中考虑“各向异性”及“金属性”修正等；（2）引入能量稳定性判据，设计低能量高硬度的结构；（3）利用多目标优化算法，发展集多种功能于一身的新型超硬材料。最后将新方法应用在轻元素化合物等重要超硬材料体系中，设计新型超硬材料。

超硬材料应用广泛，理论和实验研发新型超硬材料具有重要的科学和现实意义。以硬度为导向的超硬材料晶体结构理论设计方法在超硬材料设计上展现出效率高、目标性强等优点。本项目对前期发展的基于CALYPSO的超硬材料晶体结构设计方法进行优化及适当拓展，引入了“金属性”、“能量”、‘配位数’等多目标，使其便于多功能超硬材料的设计。同时选取几种典型轻元素体系，利用该方法设计了若干新型超硬材料，并研究了其硬度机理。预测了B4C高压下的超硬结构，其以独特的硼隧道骨架内嵌之字形碳链构成，其理论硬度为55GPa，且具有金属性。设计了两种动力学稳定，较菱方类B4C结构热力学焓更低的金属性超硬B4N，这两种新B4N结构由富硼材料独特的B12二十面体结构单元构成，其具有高于40GPa的理论硬度。提出了两种新型半导体特性的超硬B2CN高压结构，这两种结构与金刚石结构类似，各原子均为四面体成键，焓差计算表明两个新结构可以从Pmma相在高压下相变得到，二者具有半导体性质，这一发现打破了缺电子B2CN应具有金属性的传统认识，研究发现这种非金属性可能与其内部B-C四面体单元扭曲导致的三中心键密切相关。研究了tI12碳的理想强度及在不同理想应变条件下的键演化模式，揭示了其高弹性低强度的机理。研究了bc8碳的理想强度，发现了bc8碳的理想剪切强度在常压下低于金刚石，但200 GPa压力以上则高于金刚石，并揭示了其内在根源。