Hongli Suo

Large transport critical currents in dense Fe- and Ni-clad MgB2 superconducting tapes

We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3×105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ∼1 MA/cm2, corresponding to a critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2.

Superconducting properties of MgB2 tapes and wires

Abstract An overview of the current state of development of MgB2 wires and tapes prepared by several techniques is presented, based on the known literature data and our own results. We focus on the powder-in-tube processing method, using pre-reacted MgB2 powders surrounded by an Fe sheath (ex situ processing route). The study of the effect of the initial MgB2 particle size shows that after reducing the initial particles by ball milling to sizes of the order of 3 μm, the degree of texturing and the upper critical field, Bc2, remain unchanged, while both the critical current density, Jc, and the irreversibility field, Birr, show a considerable increase, followed by a decrease after longer milling times. After various recrystallization times and temperatures we show that a critical amount of impurities introduced during the particle reduction process is responsible for the observed maximum of Jc and Birr. Our analysis indicates that this is a grain boundary effect, thus pointing the way for further improvement of the transport properties. A preferential orientation along the c-axis has been observed in Fe/MgB2 tapes by X-ray diffraction. An anisotropy ratio of 1.3 for both Bc2 and Birr was found. The Fe/MgB2 tapes exhibit a very high n factor, which opens up the possibility for a persistent mode operation at 4.2 K and moderate fields. Transport Jc values above 104 A/cm2 are obtained in monofilamentary Fe/MgB2 tapes at 4.2 K and 6.5 T and at 25 K and 2.25 T. Multifilamentary tapes were found to exhibit lower Jc values due to the presently used deformation process yielding a lower density with respect to monofilaments. In all measured tapes, quenching was observed at the lowest applied fields. Improvement of the thermal stability of MgB2 tapes will be one of the major challenges in future developments.

Improved transport critical current and irreversibility fields in mono- and multifilamentary Fe/MgB2 tapes and wires using fine powders

Mono- and multifilamentary Fe/MgB2 tapes and wires with high transport critical current densities have been prepared by the powder-in-tube technique using fine powders. The influence of the initial MgB2 grain size on critical current density, upper critical and irreversibility fields has been studied. After reducing the MgB2 grains to micrometer size by ball milling, the critical current density, Jc, was enhanced, while the upper critical field, μ0Hc2, remained unchanged. The anisotropy ratio between the upper critical fields parallel and perpendicular to the tape surface was determined to be 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive Jc values, measured at various temperatures between 4.2 and 25 K. On monofilamentary tapes, Jc values close to 105 A cm−2 were measured at 25 K/1 T, while Jc values ≈106 A cm−2 were extrapolated for 4.2 K/0 T. Fe/MgB2 tapes exhibit high exponential n factors for the resistive transition: n values of 60 and 30 were found at 4 T and 6 T, respectively. Multifilamentary wires (with seven filaments) show slightly lower Jc values, 1.1 × 105 A cm−2 at 4.2 K/2 T. The improvement of thermal and mechanical stability of MgB2/Fe tapes and wires appears clearly as a challenge for future developments.

Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes

Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube process. After annealing, we measured transport jc values up to 1.1 × 105 A cm−2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with seven filaments fabricated by two-axial rolling, and up to 5 × 104 A cm−2 at 4.2 K in 1 T in a MgB2/Fe tape with seven filaments. For higher currents these multifilamentary wires and tapes quenched due to the insufficient thermal stability of the filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.

Preparation and characterization of (100) cube textured Ag substrates for in-plane oriented HTS tapes

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

Transport properties and exponential n-values of Fe/MgB2 tapes with various MgB2 particle sizes

Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2 powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on Bc2, while the superconducting properties of the individual MgB2 grains are essentially unchanged. Reducing the particle size causes an enhancement of Birr from 14 to 16 T, while Jc has considerably increased at high fields, its slope Jc(B) being reduced. At 4.2 K, values of 5.3 × 104 and 1.2 × 103 A/cm2 were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high exponential n factors were measured: n=148, 89 and 17 at 3.5, 5 and 10 T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed.

{110}⟨110⟩ textured Ag ribbons for biaxially aligned YBa2Cu3O7-x coated conductor tapes

Texture development in pure Ag was investigated in order to achieve pure {110}011 textured Ag ribbons that can be used as a substrate for YBa2Cu3O7-x superconducting coated tapes without a buffer layer. Silver ingots were rolled by different percentages and to different thicknesses. They were then annealed at different temperatures to get a stable recrystallization texture. The textures were measured using x-ray pole figures and orientation distribution function (ODF) analysis. The rolled texture is found to be {110}112, with a {110}011 component for high deformation rates. As the recrystallization temperature is increased from 100 to 800 °C, the broad {110}112 as-rolled texture is reduced and the {110}110 texture is enhanced. A small amount of a secondary texture originating from twinning is evident in fully recrystallized samples. No change in the {110}110 recrystallization texture was observed after several weeks at room temperature and after 1 h at 900 °C, which is promising for coated conductor processing.

Stable (110) textured Ag ribbons for biaxially aligned YBa2Cu3O7-d coated tapes

Texture development in pure Ag was investigated in order to obtain {110}uvw textured Ag ribbons that can be used as a substrate for YBa2Cu3O7 superconducting coated tapes without any buffer layer. The starting material was 99.95% pure commercial Ag foils. A 20% deformation reduction was used at each step of the cold rolling process after which an optimal annealing was achieved at 800 °C for 4 h in a primary vacuum. This process leads to large-grain ribbons with the {110}112 orientation. A sharp and pure texture is obtained with a FWHM smaller than 6° in the three x-ray pole figures that were used to calculate the orientation distribution function. Crystallographic orientation maps achieved by electron back-scattering diffraction show a misorientation from the normal direction of less than 4°. A twinning mechanism is used to explain the formation of the {110}112 texture. The stability of the {110}112 texture is confirmed by further annealings up to 900 °C, a practical temperature for coated tapes preparation. Lengths up to 25 cm with a uniform {110}112 texture have been obtained.

Quantitative study of the inhomogeneous distribution of phases in Fe-sheathed ex situ?MgB2 tapes

Quantitative phase analyses have been performed on Fe-sheathed ex situ MgB2 tapes after the deformation process and after annealing at 700, 850 and 920 °C in vacuum. The techniques used were x-ray diffraction, differential thermal analysis and scanning electron microscopy. All superconducting cores were found to contain certain amounts of MgO, the largest amounts being observed near the sheath, and after annealing at the highest temperature. Small amounts of MgB4 were detected in the inner part of the core, presumably formed by the decomposition of MgB2. At the MgB2/Fe interface, a reaction layer was observed, which was identified as the boride Fe2B. In addition, tapes annealed at 700 °C revealed the presence of the metastable phase Fe23B6. The interaction of the MgB2 filament with the Fe sheath and the presence of trapped oxygen limit the possibilities of the current tape fabrication techniques.

Superconductivity in high-pressure synthesized pure and doped MgB2 compounds

Dense pure and doped (Mg1 − xAx)B2 samples with A = Na, Ca, Cu, Ag, Zn, and Al were synthesized at high pressure–high temperature in a multi-anvils press (3.5–6 GPa, 900–1000°C) for 0 < x ≤ 0.20. They were studied by X-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by AC susceptibility, magnetization, and transport measurements. Only Al is really substituted on the Mg site. The other elements form secondary phases with B or Mg. No large effect is observed on the superconducting properties Tc, jc critical current, Hirr, and Hc2.