High Temperature And High Pressure Synthesis And Property Studies On Molybdenum Carbides/borides

Transition metal light element(TMLE)compounds tend to manifest high hardness and plentiful functional properties.Therefore,there might be plentiful high hardness multi-functional materials to be harvest.Superconducting materials have important applications in industrial power systems,magnetic systems,medical equipments,military gears,etc.In particular,superconducting materials must bear excellent mechanical performance,high compression resistance,outstanding wear resistance,strong oxidation resistance and terrific chemical stability to be used in extreme conditions.It is of great scientific significance,irreplaceable technical importance and promising prospects to develop high hardness superconducting materials.Superconducting materials in TMLE compounds are concentrated in light element(LE)compounds of niobium and molybdenum.LE compounds of molybdenum show more excellent mechanical properties.Compared with metal alloys,copper-based and iron-based superconductors,LE compounds of molybdenum are lighter and cheaper,and they are appropriate to the development trends of low-cost and lightweight superconducting materials.According to the phase diagrams,the obscure phase boundaries of molybdenum carbides/borides make it difficult to synthesize a single phase.Therefore,it is necessary to study the electrical and mechanical properties systematically.HPHT method is effective to synthesize the single phase of molybdenum carbides/borides and to adjust bonding characteristics and electronic structures for optimizing the electrical and mechanical properties.It provides important guidance for the development of new hard superconducting materials with exploring the relationship between structure and hardness or conductivity/superconductivity.In this work,we systematically studied the potential high hardness superconducting molybdenum carbides/borides.The obtained innovative results are as following:1.Two extremely compact and easy-to-be-characterized massive molybdenum carbides,Mo₃C₂(space group:P6₃/mmc)and Fe₂N-Mo₂C(space group:Pbcn),were synthesized with HPHT method.Their conductivity is comparable to copper and platinum.The hardness far exceeds that of traditional alloy superconductors.It explains that the covalent octahedral reinforcement units are extremely beneficial to the hardness of superconducting materials.The measured room temperature resistivities are on the orders of 10^-8??m and 10^-7??m respectively,which are equivalent to the commonly used wire materials as copper and platinum.Electrical and magnetic measurements showed that their T_c are 8.9 K and 7.5 K,respectively.The upper critical magnetic field is 7.0 T,which is comparable to the widely used Nb-Zr alloy.Carbon atoms are interstitial in the molybdenum metal lattice to form spatial octahedral structures.The vibrations of molybdenum atoms and carbon atoms promote electro-acoustic coupling and help the formation of superconducting Cooper pairs.Vickers hardness values of Mo₃C₂ and Mo₂C are 13.5 GPa and 13.4 GPa respectively from the hardness measurements,which are much higher than the hardness value of about 2.8 GPa of traditional Nb Zr superconducting materials.XPS measurements results and first-principles calculations confirmed that the Mo-C octahedrons are composed of covalent and ionic chemical bonds to form reinforcing units,which enhance the compression resistance of the material and improve the hardness of these two molybdenum carbides.The above results indicate that The molybdenum carbides are stronger and lighter than traditional binary metal alloy superconductor materials,which provide suitable materials for the application of superconducting materials in extreme conditions.Mo₃C₂ and Mo₂C are high hardness superconducting materials.2.The di-molybdenum boride Mo₂B(space group:I4/mcm)with Al₂Cu structure was synthesized for the first time by HPHT method.Its superconductivity was discovered.Normal resistivity is comparable to that of manganese-copper alloy.The boron pinning structure is proposed for the first time.The formation of excellent electrical conductivity was explored,and the improvement of mechanical properties by boron pinning structure was explained.The hardness is higher than the orthorhombic MoB and hexagonal MoB₂.A new relationship between boride structure caused by changes in boron concentration and hardness was proposed.The measured resistivity at room temperature is in the order of 10^-7??m,and its conductivity comes from high electronic density of states,which are provided by the pinned Mo atoms.From electricity and magnetism measurements,the obtained T_c is 6.0 K,which is higher than many known molybdenum borides.Its Vickers hardness value is 16.5 GPa from hardness measurements.Analysis of its electronic structure and chemical bond properties showed that B-B bonds along the c-axis have strong covalent effect and pinning Mo and B atomic layers to prevent interlayer slippage,which improves the hardness.This study shows that compared with high-boron molybdenum borides,the low-boron phase of Mo₂B has better conductivity,higher superconducting transition temperature and good hardness,indicating that Mo₂B is a potential high hardness multifunctional material.In the boron-containing compounds,the induced formation of pinning structure can improve hardness and other physical properties.3.The uniform and dense single phase molybdenum borocarbide Mo₂BC(space group:Cmcm)was synthesized firstly by HPHT method.Its superconductivity was discovered by characterization.Normal resistivity is comparable to that of nickel-chromium alloy.The boron chain-coupled octahedral structure is proposed for the first time.It explains that two-dimensional(2D)boron zigzag chains enhance directional covalence,and the octahedrons help to combine the two-dimensional boron zigzag chains to reduce the anisotropy difference in compression resistance,thereby increasing the hardness.It is proposed that the construction of covalent three-dimensional structure to avoid 2D slippage structure is beneficial to mechanical properties.The superconducting zero-resistance and the Meissner characteristic were obtained,and the T_c is 7.0 K.It is a type-?superconductor from hysteresis phenomenon.The resistivity order at room temperature is 10^-6??m.The vickers hardness is 18.2 GPa from hardness measurement,which is much higher than that of?-MoB with 2D boron chains.It is clarified that three-dimensional structures formed by 2D boron chains and octahedrons is beneficial to the hardness.This study shows that Mo₂BC is a high-hardness material with superconductivity and electrical conductivity.In the process of material's design and synthesis,the mechanical properties of materials can be improved by constructing three-dimensional structure with two-dimensional boron chains.The properties of four compounds were compared in this paper.It is found that appropriate electronic localization provided by LE"tie"can make a material bears good conductivity,superconductivity and high hardness at the same time.The crystal structure was used as a variable to study the hardness difference of the four compounds and summarize the rules.The research of chemical bonds and electronic structures illustrate that selecting appropriate light elements such as boron,and adjusting concentration of LE to form light element skeletons can enhance the electronic localization and strong covalent interaction in TMLE compounds.Adjusting the crystal structure to form three-dimensional configurations can avoid or compensate for weak mechanical properties caused by 2D slippage structure to enhance the hardness of superconducting materials.The relationship between electrical conductivity and T_c was studied.It is noted that the T_c of three binary molybdenum carbides/borides are positively correlated with electrical conductivity.The excellent conductivity comes from the high valence electron density where Mo atoms dominates the Fermi surface.The work of this paper is helpful to enhance the understanding of high-hardness conductor/superconducting materials and develop novel high hardness multi-functional materials.