Designing New Superhard Metal Borides

创造和掌握越来越难的材料是一项至少与化学本身一样古老的奋进。硬质材料对工具、技术及其所维持的社会的重要性是不言而喻的，因此通常的考古实践是通过所使用的工具资源来命名史前人类时代，即石器时代、青铜时代等。考虑到机械加工行业对全球经济的贡献，很明显，新一代切削刀具可能在全球范围内产生重大影响;超硬材料的世界市场预计将在未来5年内达到200亿美元以上。不幸的是，超硬材料很少，而那些超硬材料（如金刚石）需要在极端压力和高温下合成，因此对于许多应用来说过于昂贵。在材料研究部固态和材料化学计划的支持下，研究团队正在开发下一代超硬材料，而不是在环境压力下生产-超硬金属硼化物。培训下一代材料化学家，并将该项目的兴奋带给公众，将产生更广泛的影响。首席研究员担任加州大学洛杉矶分校美国化学学会学生会员的导师。这个位置已成功地用于达到许多初中和高中学生通过访问他们的学校，培养对化学的热爱本科生和K-12学生。共同首席研究员正在积极开发新的方式来教科学与高中教师通过她的位置作为外展主任为加州纳米系统研究所。参与这项提案的每一位研究生都帮助开展外展项目，并指导本科生。本科生将协助研究生进行研究，从而增加未来材料化学研究生的储备。技术摘要十年前，主要研究人员提出，可以在大气压下合成的新超硬材料（不像金刚石或立方氮化硼）可以通过将共价键合到高价电子密度金属中来进行成分设计。共价键阻止剪切，电子密度增加了不可压缩性。过渡金属硼化物通过提供高价电子密度、多个硼-硼共价键和独特的晶体结构来克服这些参数。这个想法首先用二硼化锇（不可压缩）证明，然后用二硼化锇和四硼化钨（两者都是超硬和不可压缩的）证明。在材料研究部固态和材料化学项目的支持下，研究团队通过关注结构和键合图案来改进和扩展这些材料。超硬设计工具包可以通过研究新的四硼化钨固溶体的机械性能来扩大，以形成十二硼化物型结构，通过掺杂铁来增加电子密度，并通过检查钨的较低硼化物，其通过包含0-D，1-D和2-D硼结构来补充四硼化钨的3-D硼网络。这类化合物可作为理解B-B和B-金属键的性质如何影响硬度的模型系统。每种成分都可以使用高压径向衍射来研究，以获得关于失效和滑动的晶格平面特定信息。最终的目标是第三个设计参数的发展，明确阐明的结构和键合图案，最应该改善宏观力学性能的基础上微观键合。通过这种方式，主要研究人员希望发现额外的基本设计规则，这些规则可用于合理合成越来越复杂的超硬但易于合成的材料家族的新成员。

Non-Technical AbstractThe creation of, and mastery over, ever-harder materials is an endeavor at least as old as chemistry itself. The importance of hard materials to tools, technology, and the societies they sustain is so self-evident that it is common archaeological practice to name pre-historic human eras by the tooling resources employed, i.e. Stone Age, Bronze Age, etc. In the modern age, superhard materials are needed for high-performance cutting tools, especially for preventing tool damage when cutting advanced ceramics and superalloys. Considering the contributions of the machining industries to the global economy, it is clear that a new generation of cutting tools could have a significant impact worldwide; the world market for superhard materials is projected to reach over $20 billion within the next 5 years. Unfortunately, very few superhard materials exist and those that do (such as diamond) need to be synthesized under extreme pressure and heat, thus making them too expensive for many applications. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, the research team is developing the next generation of superhard materials than can be produced at ambient pressure - superhard metal borides. Training the next generation of materials chemists and bringing the excitement of this project to the general public will provide broader impact. The Principal Investigator serves as the faculty advisor to the Student Members of the American Chemical Society at UCLA. This position has been successfully used to reach many middle and high school students through visits to their schools, fostering a love for chemistry for both undergraduates and K-12 students. The co-Principal Investigator is actively developing new ways to teach science with high school teachers through her position as the Director of Outreach for the California NanoSystems Institute. Each graduate student involved in this proposal helps with outreach projects and mentors undergraduates. The undergraduates will assist the graduate students in research, thereby increasing the future graduate student pool in materials chemistry.Technical AbstractTen years ago, the Principal Investigators suggested that new superhard materials that could be synthesized at atmospheric pressure (unlike diamond or cubic BN) could be compositionally designed by incorporating covalent bonding into high valence electron density metals. The covalent bonds prevent shear and the electron density adds incompressibility. Transition metal borides exemplify these parameters by providing high valence electron density, multiple boron-boron covalent bonds, and unique crystallographic structures. This idea was first demonstrated with osmium diboride (incompressible), and then with rhenium diboride and tungsten tetraboride (both of which are superhard and incompressible). With support from the Solid State and Materials Chemistry program in the Division of Materials Research, the research team refines and expands on these materials by focusing on structure and bonding motifs. The superhard design toolkit can be enlarged by studying the mechanical properties of new tungsten tetraboride solid-solutions to form dodecaboride-type structures, increasing electron density through doping with iron, and by examining the lower borides of tungsten, which complement the 3-D boron network of tungsten tetraboride by containing 0-D, 1-D, and 2-D boron structures. This family of compounds serves as a model system for understanding how the nature of B-B and B-metal bonds affect hardness. Each composition can be studied using high-pressure radial diffraction to obtain lattice plane specific information about failure and slip. The eventual goal is the development of a third design parameter to explicitly elucidate the structures and bonding motifs that should most improve macroscopic mechanical properties based on microscopic bonding. In this way, the Principal Investigators hope to uncover additional fundamental design rules that can be used to rationally synthesize new members of an increasingly complex family of ultra-hard yet easy to synthesize materials.