The High Pressure And High Temperature Synthesis And Physical Properties Of Nb-C Compounds

The strong covalent bonds between transition metal atoms and light element atoms with high valence electron density are the results of unique electronic properties of transition metals.The compounds formed by transition metal and light elements are generally considered to be potential superhard materials.Up to now,several transition metal light element(TMLE)compounds have been studied by theoretical and experimental methods.Strong hybridization between transition metal d orbitals and light element p orbitals in TMLEs is of vital importance to its hardness.Meanwhile,TMLEs possess many properties,such as superconductivity,catalysis and magnetic etc.It provides an ideal platform to search for high hardness multi-functional materials.Rock-salt structure niobium carbide is investigated in this thesis.High pressure and high temperature method was used to fabricate niobium carbide.The crystal structure,microstructure,mechanical,superconductivity and oxidation ability of niobium carbide are characterized and the conclusions are listed below.1.Niobium carbide was fabricated under the condition of 5.0 GPa,2300 K,15 min with the ratio of niobium to carbide of 1:0.6 via high pressure and high temperature solid reaction.The characterization of SEM,TEM and density measurement demonstrate that niobium carbide is of high relative density(?98.0%).The above results declare that high pressure and high temperature method is an effective method to fabricate high relative density bulk specimens.2.The Vickers hardness characterization of niobium carbide yields an asymptotic Vickers hardness value of 21.7 GPa.The hardness of niobium carbide approaches and even exceeding some other tradition high hardness materials,which may be useful in the areas of cutting and grinding.The hardness of niobium carbide is consistent with high light element concentration TMLEs and niobium carbide is rock-salt structure.Those results illustrate that the bonds between niobium and carbide are of strong covalent feature because there is strong hybridization between transition metal d orbitals and light element p orbitals.Besides,high hardness and even superhard materials can be obtained by selecting proper transition metals and light elements.3.The measurements of resistivity and magnetic susceptibility under low temperature demonstrated that niobium carbide exhibits second-type superconductivity below 11.8 K.Thermogravimetry measurements give an oxidation temperature of 550 ? showing strong oxidation ability.These properties demonstrate that Nb C is a potential high hardness superconducting material.4.The calculations of electronic properties of Nb C by first-principles method indicated that Nb C possessed high bulk modulus,high Bader of chemical bonds and high ELF.It demonstrated that Nb-C was a strong covalent bond,which elucidated its high hardness.In addition,the strong covalent bonds lead to high density of states at the Fermi energy level and high phonon frequency,which have strong correlations between electrons to form Cooper pairs.Therefore Nb C has high superconducting transition temperature.