Year

2004

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting & Electronic Materials - Faculty of Engineering

Abstract

The work in this thesis concentrates on the fabrication and characterization of MgB2 superconducting bulk wire and tape. An overview of the research on MgB2 superconductor during the last three years is also provided. High transport and magnetic critical current density values above 105 A/cm2 have been obtained for metal-clad wires and tapes. Fe-clad MgB2 tapes were fabricated using a powder-in-tube technique. The tape shows a sharp transition with a transition width ΔTc of 0.2 K and a Tc0 of 37.5 K. An high transport critical current value of 1.7 × 104 A/cm2 for both 29.5 K in 1 Tesla and 33 K in zero applied field has been achieved. The effects of sintering time and temperature on the formation and critical current densities of Feclad MgB2 wires is also investigated. MgB2 wires were sintered for different periods of time at predetermined temperatures. In contrast to the common practice of sintering for several hours, results show that there is no need for prolonged heat treatment in the fabrication of Fe/MgB2 wires. A total time in the furnace of several minutes is enough to form nearly pure MgB2. Jc of 4.5×105 A/cm2 in zero field and above 105 A/cm2 in 2 T at 15 K has been achieved for Fe/MgB2 wires sintered for a short time. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB2 wires. Ag and Cu clad MgB2 wires were also fabricated using an in-situ reaction method. The effects of a shorter than usual sintering on the critical current densities of Ag and Cu clad MgB2 wires were studied. For Ag clad wire Jc is improved by more than two times after the short period sintering process. Jc values of 1.2×105 A/cm2 in zero field and above 104 A/cm2 in 2 T at 20 K have been achieved for Ag clad MgB2 wire which is only sintered for a few minutes at 800 oC. However, a remarkable degree of reaction has been found between the superconducting cores and the sheath materials, leading to the formation of Cu2Mg and Ag3Mg for copper and silver clad wires, respectively. The results show that the short sintering causes less reaction between the magnesium and the sheath materials and markedly improves the critical current density. Our results also show that iron is still the best sheath material for MgB2 superconductor wire and tape. Sixteen-filament stainless steel/Fe/MgB2 wires were fabricated by the powder-in-tube method followed by groove rolling. Magnetic critical current densities of 3.4×105 A/cm2 in 0.5 T and about 1.9×105 A/cm2 in 1 T at 5 K were achieved. Results on transport Jc of solenoid coils up to 100 turns fabricated with Cu-sheathed MgB2 wires using a windreaction in-situ technique are reported. Despite the low density of the single core and some reaction between the Mg and the Cu-sheath, our results demonstrate that the decrease in transport Jc with increasing length of MgB2 wires is insignificant. Solenoid coils with diameters as small as 10 mm can be readily fabricated using a wind-reaction in-situ technique. The Jc of coils is essentially the same as for straight wires. Jc values of 133,000 A/cm2 and 125,000 A/cm2 at 4 K and self field have been achieved for small coil wound using Cu-sheathed tape and Cu-sheathed wire respectively. The results indicate that the MgB2 wires have potential for large scale applications. The effect of chemical doping on the superconductivity and critical current density of MgB2 superconductor is investigated. Enhancements in the Jc field performance as well as the irreversibility field were obtained due to chemical doping with both C and SiC nano-particles. Doping MgB2-x(SiC)x/2 with x = 0, 0.2 and 0.3 and a 10 wt% nano-SiC doped MgB2 sample, led to slight decrease in Tc and significantly enhanced Hc2, Hirr and Jc at high magnetic fields. Compared to the non-doped sample, Jc for the 10 wt% doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K, which is considered to be a benchmark operating temperature for MgB2, the best Jc for the doped sample was 2.4×105 A/cm2 at 2 T, which is comparable to Jc of the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was 36,000 A/cm2, which is an order of magnitude higher than for the Fe/MgB2 tape. Our results show that there are two distinguishable but closely related mechanisms: increase of Hc2 and improvement of flux pinning that control the performance of Jc(H) in the samples. SiC-doping introduced many nano-scale precipitates and disorders at B and Mg sites, provoking a high resistivity of ρ (40K) = 300 μΩ-cm (RRR = 1.75) for the SiC-doped sample, leading to significant enhancement of both Hc2 and Hirr with only minor effects on Tc. EELS and TEM analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10 m and an extensive domain structure of 2-4nm domains in the doped sample which serve as strong pinning centers. The effect of nano-SiC doping on the critical current density and flux pinning of Fe/MgB2 wires is also investigated. The depression of Tc with increasing SiC doping level remained rather small. High level SiC doping resulted in a substantial enhancement in the Jc(H) performance. The transport Jc for all the wires is comparable to the magnetic Jc at higher fields despite the low density of the samples. The transport Ic for the 10 wt% doped Fe/MgB2 wire reached 675 A at 24 K and 1 T (Jc = 140,000 A/cm2) and 500 A at 20 K and 2T (Jc = 103,000A/cm2). The transport Jc for the 10wt% SiC doped MgB2 wire is 30 times higher than for the undoped wire. SiC doped MgB2 polycrystalline samples were fabricated using different grain sizes (20 nm, 100 nm, and 37 μm) of SiC and different doping levels (0, 8, 10, 12, 15 wt %) in order to investigate the effect of the particle size of the starting SiC powder on the properties of samples. Results show that grain sizes of the starting precursors of SiC have a strong effect on the critical current density and its field dependence. The smaller the SiC grains are, the better the Jc field performance is. Significant enhancement of Jc and the irreversibility field Hirr were revealed for all the SiC doped MgB2 with additions up to 15 wt%. A Jc as high as 20,000 A/cm2 in 8 T at 5 K was achieved for the sample doped with 10 wt% SiC with a grain size of 20 nm. Results indicate that the nano-inclusions and substitution inside MgB2 are responsible for the enhancement of flux pinning. Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4 nano-particle carbon powder were prepared using an in-situ reaction method under well-controlled conditions to limit the extent of C substitution. It was found that both the a-axis lattice parameter and the Tc decreased monotonically with increasing doping level. However, for the sample doped with the highest nominal composition of x=0.4 the Tc dropped only 2.7 K. The nano-C doped samples showed an improved field dependence of the Jc compared with the undoped sample over a wide temperature range. The nhancement by C-doping is not as strong as for nano-SiC doped MgB2. X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2 particles. A study of ac susceptibility, magnetic shielding and the sample size effect is presented in Chapter 6. Systematic ac susceptibility measurements were performed on MgB2 bulk samples. It is shown that the flux creep activation energy is a nonlinear function of the current density U , indicating a nonlogarithmic relaxation of the current density in this material. The dependence of the activation energy on the magnetic field is determined to be a power law (J ) ∝ J −0.2 U(B) ∝ B−1.33 , showing a steep decline in the activation energy with magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in MgB2. Magnetic shielding was investigated by means of transport critical current measurements for Fe-sheathed MgB2 round wires. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6 T at 32 K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6 T. This was well beyond the maximum field for effective shielding of 0.2 T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least 3 times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between the field and the current showed that the transport current does not flow straight across the wire, but meanders between the grains. The effect of sample size on the critical current density and the flux pinning of pure and SiC doped MgB2 bulk samples has been investigated. At high fields a systematic degradation of magnetic Jc and Hirr was observed as the sample size decreased. However, Jc remarkably increased on decreasing the sample volume at low magnetic fields below 1 T. The SiC doped samples show less sample size effect than the pure samples, indicating a larger n-factor and therefore a stronger pinning effect due to SiC doping.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.