The High Pressure And High Temperature Synthesis And Physical Properties Investigation Of Cr-B Compounds

The high valence electron density in transition metals?TM?is responsible for the high value of bulk modulus under compression,while the introduction of light elements such as boron can significantly increase the resistance to plastic shear due to the formation of strong covalent metal-boron and boron-boron bonding networks.Consequently,the transition metal borides can be considered as a kind of potential superhard materials.Transition metal borides have a wide application for their incompressibility,high hardness,chemical inertness,good electrical and thermal conductivity.In the thesis,polycrystalline Cr B,Cr₃B₄ and Cr B₂ are synthesized by different HPHT conditions.Their crystal structures have been confirmed by X-ray diffraction and refined by Rietveld method.The density,morphology,oxidation resistance,hardness and magnetism of the samples have been characterized.Combined with the analysis of the crystal structures,it can be found that the concentration of B has a great influence on the material properties.The substructure unit of B atoms in the Cr-B compounds has experienced the following changes with increasing boron concentration: a zigzag chain of B shaped in Cr B;a zigzag chain of polymer boron shaped in Cr₃B₄;boron layered structure in Cr B₂ which is similar to graphene and three-dimensional cage-like structure in Cr B4.The hardness test shows that the hardness of Cr-B compounds has been greatly improved compared with chromium,and the hardness value can be qualified as hard material.Additionally,the hardness of chromium borides shows a change?first increasing then decreasing,and increasing again?,which indicates that the hardness of TM boride is related not only with concentration of B element,but also with the formed framework structure of B atoms.The magnetic measurements indicate that the different substructure unit of B atoms in chromium borides leads to the changes of the magnetic properties of Cr B,Cr₃B₄ and Cr B₂,from paramagnetism to super-paramagnetism and toanti-ferromagnetism,respectively.The TG-DAT analysis reveals that Cr B,Cr₃B₄ and Cr B₂ possess better heat resistance?>1000 ??than diamond and tungsten carbide which are widely used in industrial application.For better understanding the structural stability of Cr₃B₄,the high pressure X-ray diffraction is carried out.The incompressibility of unit cell and the axial compression of lattice parameters have been analyzed.The orthorhombic structured Cr₃B₄ remains stable up to 25 GPa,indicating that the structure has strong stability below 25 GPa.Because of the strong covalent zigzagging chain of B in ortho-Cr₃B₄,there is strong incompressibility in c-axis of Cr₃B₄.Although the hardness of Cr₃B₄ does not reach the standard of hard material or super-hard material?> 40 GPa?,its hardness has been greatly improved by introducing the light element B forming the zigzagging chains.