Preparation And Superconductivity Of High Density And Purity MgB₂

MgB₂ superconductor has a broad application prospect because of its unique excellent performance,and it is expected to replace the traditional superconductor materials in some fields.Density and purity play an important role in the preparation of high-performance MgB₂ superconductor,which is also the key to improve the critical current density J_c.In this paper,hot pressing and diffusion method are used to obtain MgB₂ superconductor with high density and purity.Different additives are introduced to improve the J_c and flux pinning force.MgB₂ bulk materials are prepared by hot pressing.The effects of different sintering pressure and temperature on the micro morphology,crystal structure and superconductivity of MgB₂ bulk materials are studied.It is found that the existence of holes is restrained.The density and purity of MgB₂ bulk are improved compared with those prepared by traditional in-situ method,and the J_c is also improved.Based on the preparation of bulk materials by hot pressing,MgB₂ wires are prepared by different sintering temperature of hot pressing.The effects of pressure in the sintering process promote the densification of wire.At lower sintering temperature,finer grains are formed,and more grain boundaries are provided to pinning force.The results show that the grain boundary pinning is the main pinning center.The J_c performance is also improved under the external magnetic field.In the process of hot pressing sintering,the effects of MgB₄ addition on the microstructure and superconductivity of MgB₂ superconductor bulk are studied.It is found that the addition of MgB₄ does not affect the lattice parameter and superconducting transition temperature T_c of MgB₂but promote densification.In the whole external magnetic field,the addition of MgB₄ results in the formation of finer MgB₂ grains compared with the sample without addition.Grain boundary pinning as the main pinning center increases the pinning force under the magnetic field,thus the J_c is improved.And the best J_c value is obtained when the addition amount is 3wt%.MgB₂ bulk materials are prepared by diffusion method after ultrasonic pretreatment of precursor powder.It is found that the crystal structure and the T_c of MgB₂ are almost not affected.But the morpology of the sample after ultrasonic pretreatment is flatter and denser,and the J_c is also increased.The increase of J_c is due to the denser structure of the sample after boron powder pretreatment,which increases the effective current carrying area and the grain connectivity.By the means of the pretreatment of boron powder,the effects of graphene addition on the superconductivity of MgB₂ bulk prepared by diffusion method are studied.It is found that the morphology of all samples is compact and uniform,and no obvious holes appear.With the increase of graphene content,the change of lattice parameter of a-axis and c-axis is very small,indicating that the lattice is damaged a little.And the T_c is slightly decreased.The J_c value of MgB₂ sample is higher than that of the sample without graphene addition.The increase of J_c is related to the flux pinning force.According to the reduced pinning force curve,it is found that grain boundary pinning is the main pinning mechanism.