Structural Design And Physical Properties Of Novel Superhard Twinned Boron-based Materials

Title:Structural Design and Physical Properties of Novel SuperhardTwinned Boron-based MaterialsAuthor:Nan MinMajor:Condensed Matter PhysicsSupervisor:Prof.Quan LiSuperhard materials with Vickers hardness exceeding 40 GPa are widely used in industrial applications.Single-crystal cubic diamond is a typical traditional superhard material with ultrahigh hardness（60–120 GPa）.However,it exhibits poor thermal stability and chemical inertness for cutting ferrous metals.Another traditional superhard material is cubic boron nitride（c-BN）,which exhibits better thermal and chemical stability than diamond,but its Vickers hardness remains limited at 46–66 GPa,far lower than that of diamond.Therefore,seeking new superhard materials with high hardness and stability is an unremitting pursuit for the scientists.The candidate system for superhard materials includes light-element covalent materials and transition-metal light-element materials.High resistance to deformation of structures stem from the three-dimensional strong covalent network formed by light elements B,C,and N,which is the main reason of high hardness in these materials.Transition metals with high bulk modulus can provide electrons for the system and resist compression.However,the hardness of the materials in this system are still not hard enough.Recent studies have highlighted the importance of improved lattice-matching twin boundaries in enhancing the stability and mechanical properties of ceramics.Experimentally synthesized nanotwinned diamond and nanotwinned c-BN exhibited unprecedented hardness,stimulating interest in exploration of twin-strengthening mechanisms and constructing superhard twinned structures through theoretical calculations.Here,we systematically solve the problems in constructing twinned structures with complex structural units and complex bonding by self-developed method.A systematic twinned structure designing and theoretical simulation study are conducted on candidate systems of boron-based superhard materials,achieving the following results:1.Introducing twin boundaries is an effective way to improve the mechanical properties of materials.Designing twinned structures and searching for the laws of their physical properties is the hot topic in the field of material physics.However,the twinned structures obtained by simple combining method have a lot of problems such as the presence of non-coherent twin boundaries,mismatched bonding near the twin boundaries,and non-unique bonding at twin boundaries.This work developed a twinned structure designing program to solve the problems in crystal plane selection in anisotropic structures,thickness differences caused by periodicity,and bonding diversity caused by various splicing ways with mirroring,rotation,and translation methods.This software has improved the efficiency in constructing twinned structures,enriched the type of the twinned structures,and laid the foundation for future research on multifunctional new twinned structures.2.The similarities between the covalent bond networks and excellent mechanical properties of diamond and c-BN have spurred interest in superhard B-C-N materials with all-sp³ strong covalent bonds.Such materials are expected to possess dual advantages of improved mechanical properties and chemical inertness.Extensive efforts have been invested in synthesizing and characterizing diverse B-C-N compounds.The most intriguing material among them is c-BC₂N synthesized under high-temperature and-pressure with an experimental hardness of 62-76 GPa,surpassing that of c-BN.This work used the systematic method to construct twinned structures of BC₂N.The newly identified nanotwinned BC₂N variant（τ-BC₂N）exhibits better energetic stability and higher mechanical strength than single-crystal BC₂N,attributed to the optimized bonding behavior and crystal orientations induced via twinning.The simulated X-ray diffraction patterns ofτ-BC₂N match those reported in previous experimental data,indicating thatτ-BC₂N is an ideal candidate structure of superhard BC₂N material prepared in previous experiments.The nanotwinned BC₂N displays superior Vickers hardness of～140 GPa,surpassing that of diamond.The computationally tailored approach and results obtained by twin-strengthening strategies offer powerful insights for the rational design and functional optimization of nanotwinned B-C-N compounds containing intricate multiatomic constituents and structures.3.The distinctive electron deficiency and unusual multi-center bonding situations of boron give rise to fascinating chemical complexity and imaginative structural polymorphism.Previous experimental studies have reported Vickers hardness values of30-58 GPa forγ-B₂₈,positioning it as the second hardest elemental solid after diamond.This work used the systematic method to construct the new twinnedγ*-boron based on the well-known hardest elemental boron,γ-B₂₈.Notably,the newly propoundedγ*-boron phases exhibit considerably close energy levels withγ-B₂₈ at ambient conditions.The simulated X-ray diffraction patterns of stable twinned structure present excellent agreement with experimental data,indicating the existence of twinnedγ*-boron in previous experiments.First-principles calculations reveal a 7.5%increase in the ideal Vickers shear strength ofγ*-boron compared withγ-B₂₈,attributed to diverse bond responses within the twinned slabs.The evaluated hardness of nanotwinnedγ*-B reaches 59 GPa in consideration of the size hardening effect.This work designs new polymorphs of boron with improved mechanical properties and expands the knowledge about twinned structures of boron.This work used a self-developed twinned structure designing method and first principles calculations,designing nanotwinned structures and simulating physical property in B,C,and N elements and compounds.This method targeting structural units effectively increases the density of twinned structures,releasing intrinsic stresses,improving the stability and mechanical properties of the structures.This work provides new insights in designing ideal twinned structures and seeking new superhard materials.