Study On Microstructure And Superconducting Properties Of MgB₂ Wires Fabricated By Two-Step PIT

Japanese scientists have found that the critical temperature of MgB2 is up to 39 K in early March 2001, So the MgB2 superconductor could be applied in the temperature region of 20～30K obtained by using the cooler without complex and expensive liquid helium conditions. MgB2 has a very simple chemical composition, crystal structure and grain boundary which can carry high current. At the same time, MgB2 of coherence length is longer than the structure of perovskite-type oxide superconductor. It means that MgB2 is easier to introduce effective flux pinning centers to improve the superconductivity. So MgB2 superconductors have great potential for applications in superconducting power, electronic devices and magnetic resonance imaging (MRI).There are two main methods for preparation of superconducting wires:one is an ex-situ process the other is an in-situ process. In the former process preparation is simple, but it leads to the micro-cracks difficult to bridge during the heat treatment process. In the latter porcess melting Mg and the B reacting into a phase can be effectively bridge into the micro-cracks. At the same time, it is found that the chemical doping can be easily introduced and the critical current density of MgB2 does not decrease rapidly with increasing magnetic field. However it is observed that MgB2 core wires have a large number of holes without pressure by SEM. The density of MgB2 superconductors is only about 1.5g/cm3. Based on the above two methods of preparation MgB2 superconductors of the advantages and disadvantages. In this paper MgB2 wires were fabricated by two-step PIT. In the first step heat treatment system were improved to 900℃unacted on metal package, so it was more vulnerable to introduce doping elements; Because of the powder fully broken after sintering particles were refined to reduce the drawing process of a "sausage" effect. It was observed that mixed powders were very dense. For example the density of the bulks of MgB2 have improved about 2.0 g/cm3 made by two-step reaction. Thus by two-step process could enhance effective the area of carrying current and the capacity of critical current density. In this paper MgB2/Nb/Cu superconducting wires fabricated by two-step PIT. In order to reveal the dopants on Mg-B systemthe phase formation behavior, detailed study of a typical SiC, TiC-doped. The first Mg and B powders mixed by Mg:B=1:4 with x=0 (5wt%,8wt%, 10wt%,12wt%) SiC (TiC) nanoparticles by mixed-grinding pressing bulks, then heated for one hour at 800℃or half an hour at 900℃under ultra-high purity Ar. The second broken the sintered bulks, mixed-grinding the powders adding different quality of Mg to Mg:B=1:2. The third powders were filled into the Nb/Cu pipes then drawed toΦ1.0mm after the heat treatment at 650℃,700℃,750℃,800℃,850℃for 2 hours. Finally obtained MgB2 superconducting wires with differernt dopings at different temperatures. the phase structure, microstructure and chemical composition were investigated by X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM). Critical temperature(Tc) at different magnetic fields were investigated by Physical Property Measurement System (PPMS). Hysteresis loop (M-H) were investigated by Magnetic Property Measurement System:Superconducting quantum interference devic (SQUID) then calculated the critical current densities(Jc), flux pinning force (Fp), the upper critical field (Hc2), the irreversible field (Hirr). It showed the doping elements could introduce effective flux pinning center and increase the critical current density of MgB2 wires at high temperature and high magnetic field. Synthetically comparison the effects of different dopings and heat treatment process on the properties and microstructure of MgB2 supeconductor materials, on the basis of what giving the best preparation of process parameters of two-step PIT. The Hirr value.of MgB2/Nb/Cu wires with SiC doping reached 5T at 12K, The Jc value reached 104A/cm2 at 15K,1.5T. So two-step PIT was available mean of making a certain length and stability of superconducting properties of MgB2/Nb/Cu wires.