First-principle Studies Of Carbon-based And Carbon-like Superhard Materials

The search for superhard materials is motivated by the need of industrial applications, e.g., cutting tool and resistant coatings, which might make up for the limitations of the traditionally known superhard materials such as diamond and c-BN. Recent experimental synthesis and theoretical design have mostly focused on the carbon-based and carbon-like covalent compounds formed by light weight elements such as boron (B), carbon (C), nitrogen (N), and oxygen (O). The mechanical properties and hardness of those materials strongly depend on their dimension, size, composition, density and bonding types, which are important for investigating the origin of hardness and further providing the reasonable proof for predicting and designing superhard materials. In this thesis, we propose an empirical formula to build up a bridge between Vickers hardness and first-principles calculations, and discuss the structural parameters, mechanical, and electronical properties of carbon-related materials, including sp2and sp3mixed graphene monolith, crystalline P-BN and AlMgB14, amorphous glassy carbon, CNX and ternary B-C-N, and low-dimensional nanodiamond, nanowires, and new graphene allotrope.From a statistical manner, we collect and correlate experimental bulk (B), shear (G), Young’s modulus (E), and ductility (G/B) with Vickers hardness (Hv) for a number of covalent materials and fit quantitative Hv-G and Hv-E relationships in Chapter2. Using these experimental formulas and first-principles calculations, we further predict the microhardness of some novel potential hard/superhard covalent compounds (BC2N, AlMgB14, TiO2, ReC, and PtN2). None of them belong to superhard materials (Hv≥40GPa) except BC2N. The present empirical formula builds up a bridge between Vickers hardness and first-principles calculations that is useful to evaluate and design promising hard/sup erhard materials.One of great challenges in the field of graphene applications is to fabricate three dimensional graphene products which could inherit its excellent intrinsic properties and overcome its shortcomings. In Chapter3, we construct two classes of3-D graphene monoliths (GMs) with high surface area based on width-adjustable zigzag and armchair graphene nanoribbons as building blocks and sp3carbon linkers. Such design goes beyond previous widely used physical interaction and shows favorable cohesive energy and appreciable mechanical/dynamic stability. On account of their tailored motifs, wine rack pores and rigid sp3linkers, both two classes of GMs have high specific surface area, strong mechanical strengths, tunable band gap, and subtle negative or positive linear compressibility. By solving the zero band gap and dimensional problems of single layer nanosheet simultaneously, these new GM materials offer a viable strategy for realizing many promising applications, including semiconductor devices, energy storage, molecular sieves, sensitive pressure detectors, telecommunication line systems, and environment and biological field.Using first-principles calculations, we identify a new orthorhombic BN phase (namely, P-BN; space group:Pmn21), whose theoretical hardness and bulk modulus are403GPa and60.5GPa, respectively, comparable to those of c-BN. This P-BN phase, along with Bct-BN and Z-BN, is suggested as possible intermediate phases between h-BN and w-BN. For Al-Mg-B amorphous film, we discuss and explain the origin of hardness from both the theoretical and experimental sides. For the Al-Mg-B films along the AlMg isocontent line, nanoindentation test indicates that the hardness of films increases with increasing boron contents. Meanwhile, based on the electron density of states and Mulliken population analysis, the crystal hardness is primarily determined by the B12icosahedral skeleton, whereas the contributions of metal atoms manifest as the electron donor to boron atoms. These materials may be useful in the fields of mechanical, aerospace and military.Motivated by the superior hardness and Young’s modulus of diamond-like carbon (DLC) films, in Chapter5, we investigate the mechanical properties of sp2and sp3mixed glassy carbon, carbon nitride and ternary B-C-N films. We provide the theoretical picture of pressure-induced phase transfonnation in glassy carbon (GC) and correlation the hardness and bond types. Moreover, we predict a new crystalline carbon allotrope possessing R3symmetry (R3-carbon) using the stochastic quenching (SQ) method. The present results indicate that R3-carbon can be regarded as an allotrope similar to that of amorphous GC. A very small energy difference and the similarity of GC and the R3-carbon structures imply that small perturbations to this crystalline carbon allotrope may provide another possible pathway to amorphization of carbon besides quenching the liquid melt or gas by ultra-fast cooling. Based on ab initio molecular dynamics method, four amorphous CNX structures with different stoichiometries (CN0.47, CN0.67, CN0.92, and CN1.3) were generated within a100-atom supercell. Unfortunately, there is no superhard composition whose hardness can be comparable to those of diamond and P-C3N4. Characterizations of the pair correlation functions, bond length and the fraction of bond types of the amorphous carbon nitrides reveals that the N content in such structures plays a key role in determining the structural networks. Based on the structures of ta-C and random solution model, we present the distributions of mechanical properties and formation ability of amorphous BxCyNz solids on the ternary B-C-N phase diagram and predicted that on the phase area (B:15-30at.%; C:50-60at.%; N: 20-30at.%), B-C-N solids possess both excellent hardness and good formation ability. These theoretical results provide valuable guidance for intentionally synthesizing BxCyN7materials with desirable mechanical properties.In Chapter6, mechanical and electronic properties of selected low dimensional carbon-based materials under different dimensionals, pressure, and strain have been investigated by means of density functional theory calculations. The computed Young’s moduli of nanodiamonds are lower than the bulk value and increase with size, which can be fitted to an empirical function of diameter. For thinner diamond nanowires (area of cross section less than0.6tim2), the Young’s modulus and ideal strength of these diamond nanowires decrease with decreasing cross section and show anisotropic effects. Moreover, the band gap of diamond nanowires is sensitive to the size, crystallographic orientation and tensile strain, implying the possibility of a tunable gap. Then, from first-principles calculations, we predict a planar stable graphene allotrope composed of a periodic array of tetragonal and octagonal (4,8) carbon rings. The stability of this sheet is examined by the room-temperature molecular dynamics simulation and the electronic structure is studied using state-of-the-art calculations such as the hybrid density functional and the GW approach. We find a stable planar semiconducting carbon sheet with a band gap between0.43and1.01eV with excellent mechanical properties as good as graphene’s.