纳米晶超硬-超韧过渡金属B、N化合物中纳米力学的研究

Designing new superhard-ultratough materials with novel/improved performances has been the constant endeavors of the physics and material science communities. Recently, polycrystalline diamond of nanostructures and cubic boron nitride polycrystalline bulks with “ultrafine nanotwins” have been synthesized under high pressure and temperature conditions. Both the nano-structured materials are colorless with much enhanced hardness, thermal stability, yield strength, and fracture toughness compared to that of natural diamonds which provides an effective route for superhard-ultratough material strengthening...Transition metal nitrides/borides have recently attracted considerable interest because of their unique mechanical, electronic and chemical properties for technological applications. Unfortunately, these materials do not even approach the hardness of cubic boron nitride, owing to the partially ionic and the metallic bondings. We propose to improve the mechanical performance of the transition metal nitrides/borides by producing nano scale/ nano-structure grains. The keys to this success are based on ball-milling and high-pressure/high-temperature reactions in which the pressure could suppress the grain growth to micron scale. Neutron Pair Distribution Function（NPDF）will be an effective way to analyze the short-range/ long range disorder of the structure in nano grains, since the B, N elements are light thus weak X-ray scatters. “Diamond Anvil Cell (DAC)” and “Deformation-Dia” incorporated with synchrotron X ray diffraction will be used to derive the equation of state and the constitutive law of the single phase nano crystalline materials. The after deformed samples will be further examined by TEM characterizations. By analyzing the relations of nano structure-mechanical performances-micro defects characterization, the nano mechanism in the non-metallic superhard materials will be further developed and better understood.

寻找硬度达到甚至超过传统金刚石，并且具有高断裂韧性、抗腐蚀、能够导电的新型超硬-超韧材料是长期以来物理和材料科学领域中的重要课题。近年来，人们利用纳米尺寸以及纳米形貌的强化作用，成功合成了硬度比传统金刚石还高的纳米金刚石、立方BN，为结构功能材料的设计开辟了新的思路。过渡金属硼化物和氮化物是超硬材料体系中的重要成员，具有高强度、高稳定性、非绝缘性等重要性质。本项目中，我们主要以纳米过渡金属硼化物和氮化物为研究对象，通过高能球磨以及二级推进压机在高温高压条件下反应，合成具有纳米尺寸/形貌的超硬-超韧纳米晶；通过中子衍射/PDF分析等手段，精修得到纳米尺寸调制的晶格结构信息；利用金刚石对顶砧、deformation-dia型大腔体压机并结合同步辐射X射线衍射技术研究样品的弹性-塑性形变机理；利用透射电子显微镜技术分析微观形变机制，建立结构-性质的基本物理学模型，进一步完善纳米力学理论。

硬质与超硬材料的探索一直是凝聚态物理领域一个重要的研究方向，同时在实际的工业生产中也具有巨大的应用前景。传统的超硬材料诸如金刚石、立方氮化硼等，通常由轻元素（B-C-N-O）以共价键的形式组成，这种强B-C-N-O共价键有着非常良好的方向性，既能够抵抗各向同性的压缩，也能够抵抗不同方向的剪切，因而表现出极高的力学强度。然而，传统的超硬材料的缺陷也非常突出：金刚石容易发生石墨化，而立方氮化硼的合成需要异常苛刻的温度和压力条件。另外，纯共价键合形式往往决定了其电绝性或宽带半导体，而工业上许多条件下要求材料在具有超硬力学特性的同时也要具有类似金属的导电特性；另外，除了低电导特性之外，在诸多领域人们也需要超硬-高透光材料以及可调节能带半导体材料。在本项目中，我们以过渡金属B、N化合物为研究对象，通过高温高压的方式合成新型硬质/超硬材料，并通过纳米化等方式进一步提高其力学性能。另外，我们还对过渡金属轻元素化合物的综合电、磁物性进行了研究，发现了几种既具有优良力学特性的材料，同时也发现了其优良的导电性能，甚至超导特性，磁性以及半导体特性等等，发掘了其更深层次的物理机制，为我们全面认识过渡金属轻元素化合物体系，构建统一的结构-力学-电学-磁学-光学模型打下基础，进一步拓展了此类材料的应用空间。