Structural Design Of Several New Superhard Materials

Superhard material possesses great mechanical property, which is suitable to beused as the tool in manufacturing or producing other materials. For example, it playsan irreplaceable part in geological drilling, cutting and grinding, production ofprecision instruments as well as the modern advanced sciences. Diamond and cubicboron nitride are the two major superhard materials used in industry. Unfortunatelythe thermochemical stability of diamond is poor in that when the temperature exceeds750°C, it is easily to be converted into graphite or react with iron. Thus it is not goodfor ironware manufacturing under high temperature. Compared with diamond, thecubic boron nitride possesses great thermochemical inertness, but with a low hardness(around66GPa). Therefore it is an urgent and crucial problem in this field to find anew superhard material with both high hardness and high thermochemical inertness.However there still exist many difficulties in synthesizing superhard material byexperiment. As the synthesizing temperature and pressure is hard to control, therequirements are extremely strict. It is time consuming and needs a great deal of rawmaterials to conduct the experiment. Thus lots of manpower and material resourceshave to be invested to explore the synthesizing conditions, which makes theexperiment in high cost, small success probability, low effectiveness and hard torealize large scale industrial manufacture. But fortunately with the development of thetheories and computer technology, the first-principles calculation method has been a significant method in condensed matter physics and material science to resolvevarious problems. The recently developed structure prediction method brings a newperspective extremely expanding the research range of this field. Thus we cancalculate the conditions required in synthesizing the superhard materials, such as thepressure range of the chemical compounds, to provide guidance to the synthesizingexperiments and explore the existing synthesized compounds to calculate and analyzeits relevant properties. Thus material structural prediction searching designtechnology is very necessary in assisting the synthesizing experiments.There are two potential candidates in superhard material: light elements and itscompounds, and the compound of transition metal and light elements. The allotropeand compounds of light elements such as boron, carbon, nitrogen and oxygen arenumerous and the bonding types are complicated, so it is easily to form covalentcompounds with short bond length, high bond density, low ionicity and high atomicarrangement density. The transition metal is of great cohesive energy, high chargedensity at critical point of bonding, great bulk modulus. But unfortunately it is of lowhardness. If some light element added, the transition metal would form a strongcovalent bond with these light elements so that it can be converted into material withhigher hardness.The first part of this paper is conducted under this background, aiming to take thesuperhard materials as motivation target, through a set of effective methods insearching the crystal structure of new superhard materials, to search for the potentialsuperhard structure with low energy and high hardness in the triad composed of boronnitrogen and oxygen. It is noteworthy that with this searching method only thechemical constituent and ambient pressure need to be given to get the distributioncurve of the hardness of crystal structure to energy. We choose the B3NO compoundof isoelectronic with diamond, through the hardness prediction procedures finding anumber of crystal structures and hence drew out the distribution curve of hardness to energy. Find out the B3NO (oI20and oP20) superhard structure with high hardnessand low energy as the important candidate for theory establishment and synthesizingexperiment. Further calculation indicates that these two structures have stabledynamics, with a hardness of more than40GPa and both are semiconductor. Thisfunctional orientation motivates the improvement of correctness of locating thestructure in structure searching and expands people’s understanding of the functionalorientation such as superhard material orientation. Thus it can be used as significantreference for studying other functional materials.The second part of the work is to explore another type of superhard material——structure of compounds of the transition metal and light elements. WN has excellentchemical, mechanical and thermal properties, and it is one of the superhard materialsthat are relatively cheap and easy to be synthesized. Recent experimental work of atungsten nitride synthesis aroused the research boom. A lot of theoretical orexperimental researches made a number of tungsten nitride compound with manytypes of proportions, but the most stable tungsten nitride ratio has not beendetermined. Here, we searched for a lot of tungsten nitrides by CALYPSO structureprediction and AIRSS methods to search the most thermodynamically stablestructures. Our calculations show that all these three structures have a great elasticitymodulus, and hardness calculations show that their hardness is similar with quartz,even better than quartz and other materials. Thermodynamic stability calculationsshow the three structures are kinetically stable, while the structure given by theexperiment is dynamically unstable. By comparing the experimental and simulativeXRD of cP6-WN and hP4-WN, the synthesis of compound are likely to be thesestructures.The third part is to design the superhard material structure. As we all know, inaddition to searching for the crystal structure by structure prediction method,designing the structure of superhard materials is also the focus of researches. Apply the existing superhard crystal structure is one of the most common methods ofsuperhard materials design. There will be a new structure in experimental synthesizedcompound apart from the existing structures. So just apply the conventional structurelimits the design development. As a candidate of one of the superhard materials thecompound of transition metal and light elements can not accurately determine thelocation of the light elements just by XRD because of light elements’ complex anddiverse bonding with small scattering cross section, poor quality of heavy metals andlight elements, too differentiated atomic radius. Even if new materials are synthesizedin experiments, its true element ratio and crystal structure will be controversial for along time. As we all know, the crystal structure is the fundamental factor indetermining material properties, and also the foundation to understand the mechanismof the macroscopic and microscopic properties of materials. Here we adopted themethod of moving along the diagonal lattice of carbon atoms of platinum finding outthe low-pressure structure in the synthesized carbonized platinum compounds.Through calculating the elastic constants, hardness and tensile shear simulationstructures we found carbonized platinum as hard materials. By studying thecompound tensile shear curve we found under the tensile shear force, the structuralchanges in bond angles led to fluctuations in the structure of the tensile shear curve,which enhances people’s understanding of the process of tensile shear.