The Ideal Strength Study And Strategy Of Structure Determine Of Noval Superhard Materials

Firstly, with first principle calculation, we present a comprehensive study of a set of compounds made of the light elements and transition metals elements which are considered to be noval superhard materials. We calculat not only the structure parameter but also the ”strain-stress” relationship under a uniaxial normal compressive pressure beneath the indenter. For WB₄ which belongs to this class, we discuss the argument of the structure determination of it and calculat the property of a possible structure WB₃.Finally, we deliver a new strategy of structure determination which is designed by ourselves and based on first principle calculation and evolutionary crystal structure prediction.First of all, we introduce the materials which would be discussed in the following chapters. And then we explain the major calculation method used in this thesis.A) We study six hexagonal rhenium boride, carbide and nitride compounds（Re,Re3 N,Re2N,Re2 C,Re2B,Re B₂）. The six compounds are made of light elements and transition metal elements. The transition metal elements provide high valence electron density which leads to high bulk modulus while the light elements can provide strong covalent bonds which means high shear modulus. Unlike the light elements compounds, the light elements and transition metal elements compounds can be made with normal atmosphere pressure which can lower the cost of the products. While people are expecting such compounds to be noval superhard materials with low cost,the results of the hardness tests are far from satisfaction. While the hardness of Re B₂ is high（>45GPa） when the load of the indenter is low, the Vickers hardness drops downrapidly when the load gets higher and is lower than 20 GPa in the asymptotic-hardness region. The similar results are reported for the other materials and the hardness of such kinds of compounds are always lower than 30 GPa which doesn’t belong to the superhard materials category. In order to understand why the hardness decreases when the load force increases and the low hardness of these compounds, we comprehensively calculate the structure property, the ideal tensile strength, the ideal shear strength and the vickers shear strength. Moreover we observe and analysis the procedure of the collapse of the materials. According our calculation, the Vickers shear strengths of the six materials are all lower than 30 GPa and don’t belong to the superhard materials category. The Vickers shear strength which considers the pressure of the indenter are lower than pure shear ideal strength which doesn’t take the pressure into count by 30%. By analysising the changes of structure under various of deformation, we find that the uniaxial normal compressive pressure beneath the indenter can cause a large lateral volume expansion which accelerates the break of covalent bonds and weakens the strength of the materials.We believe that the lateral volume expansion caused by the normal compressive pressure is the reason why the hardness of Re B₂ decreases badly when the load force increases.Moreover, because the lateral volume expansion effect exists in all the six materials, we think the compounds made of rhenium cannot be superhard materials.B) We will study the WB₄ which is also made by transition metal elements and light elements. Among the materials designed by this conception, the WB₄ is one of the most widely studied materials. The high content of boron makes it easy to form a three dimensional covalent bond network which likely means a high hardness. Moreover tungsten element relatively commonly exists in the earth which can lower the cost of the product. According to these two reasons, numbers of experimental and theoritical groups are interested in the study of WB₄ and as expected, in the hardness test WB₄ exhibits the highest measured hardness（30GPa） compared to the other transition metal elements and light elements compounds including the Re B₂. While the structure provided by the experimental team has been proven to be unstable,an alternative WB₃ structural model is proposed to be the real structure. However it still remains unknown whether the WB₃ structure is the real structure of experimental example, which urges us to do more calculation work and compare the theoritical results with the experimen-tal ones. In the other hand, according to the study above, for the compounds made of rhenium, a lateral volume expansion caused by the normal compressive pressure of the indenter can reduce the hardness badly. It’s worth to analysis the crash procedure under various of defomation strain and figure out whether a similar phenomenon exists in the W B₃. In this study, we calculate the structure ideals shear strength and Vickers shear strength of the WB₃. Moreover, we calculate the normalized lattice constants ratio c/a with various of pressure in order to test if it can provide a possitive dependence which was observed in the experiment. According to our results, the normal pressure under the indenter can weaken the strength of WB₃ by 30% and the Vickers shear strength of the W B₃（22.3GPa） is lower than the Re B₂（27.6GPa） which is contrary to the experimental result. Moreover, we observed an negetive dependence of normalized c/a ratio with pressure which is again contrary to the experimental result. With these two evidences,we think the proposed WB₃ structure is incompatible with the properties of the experimentally synthesized WB₄ and that structural determination of the synthesized sample must be reopened for further study. Moreover, if we analysis the change of structures under the Vickers shear deformation, the lateral volume expansion caused by the normal pressure still exists in the WB₃ which, similar with Re B₂, will cause a early bond break and reduce the strength of the materials. According to the studies of rhenium and light elements compounds, ferrum boride compounds and chromium boride compounds, we find that the lateral volume expansion under the indenter is a common phenomenon for the transition metal elements and light elements compounds which make us believe that such kind of compounds cannot be superhard materials.C) We deliver a new strategy of crystal structure determination. The traditional method of structure determination is a hard work and often fails（see the WB₄）. With the quick development of material synthesis, a new strategy is needed to determine structure of huge amount of new synthesized materials. In this chapter we introduce a crystal structure determine software FXRD which is developed by ourselves. This package is based on the first principle calculation evolutionary crystal structure prediction. With the use of FXRD, from the hundreds of structure modes poposed by evolutionary crystal structure prediction software, we can find the structures whose XRD pattern match the experimental one. After this procedure, one can determine the structure of a newmade material without the previours acknowledge and experience. We consider FXRD as a bridge between the theoretical work and experimental work. We explain what is the definition of the difference between two XRD patterns in FXRD and demonstrate how to use this software. As an example, we use FXRD to find the structure of Re B₂ which is a well known structure. Moreover, we try to search the real structure of BC₂ N whose structure is unknown. After the usage of FXRD, we find a BC₂ N structure with space group P43212-BC₂ N whose calculated XRD pattern and ideal strength fit the experimental result very well.