Search

MgB2 multifilamentary wires and tapes were fabricated by the in situ powder-in-tube (PIT) process using acetone doped milled precursor powders. The critical current density of MgB2 is strongly enhanced by high energy milling and acetone doping. Furthermore, the liquid acetone can restrain the agglomeration of milled powder, which is beneficial to the fabrication of MgB2 multifilamentary wires. At 4.2 K and 10 T, the Jc of the 80 h doped Fe/Cu wire is 2.9 × 103 A cm-2, 20 times larger than that of the 1 h pure one (the '1 h' and '80 h', refer to ball milling times). The Jc of 80 h doped Fe/Monel tape is up to 9.2 × 103 A cm-2 at 4.2 K and 10 T. The Jc values of Nb barrier wires are comparable to those of the Fe barrier MgB2 wires. These results indicate that the addition of liquid C-containing material during high energy milling may be an effective way to get excellent properties of MgB2 for practical applications. [ABSTRACT FROM AUTHOR]

A comparative study of the addition of Pb to polycrystalline Ba0.6K0.4Fe2As2 has revealed that the Pb should be added after the Ba0.6K0.4Fe2As2 phase has formed. The uniformly dispersed Pb can ameliorate the Ba0.6K0.4Fe2As2 grain linkages. In contrast, pre-added Pb in the raw powder congregates to form large particles and has hardly any positive effect on the Jc enhancement. To conglutinate at the grain surface and avoid contamination of the Ba0.6K0.4Fe2As2, the material added should have a low melting point and inertia to the superconducting phase. This model also enables us to predict desirable dopants for enhancing superconducting properties of Ba0.6K0.4Fe2As2. [ABSTRACT FROM AUTHOR]

The effect of the hot pressing method on in situ powder-in-tube-processed MgB2/Fe tapes was studied. The tapes doped with and without nanosized C are hot-pressed at 100 MPa and 650-800°C for 1-5 h in a flowing Ar gas atmosphere. Compared to the non-hot-pressed tapes, the Jc-B property (at 4.2 K and 10 T) of pure and C-doped samples hot-pressed at 650°C for 5 h was enhanced by a factor of 2.5 and 1.4, respectively. For hot-pressed tapes, the optimized sintering temperature is below 700°C at 100 MPa. The enhancement of Jc performance for hot-pressed MgB2 samples is mainly attributed to the improved grain connectivity. [ABSTRACT FROM AUTHOR]

In this paper we report the superconductivity above 40 K in the electron doping single crystals Ca1-xRxFe2As2 (R D La, Ce, Pr). The x-ray diffraction patterns indicate high crystalline quality and c-axis orientation. The resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by partial replacement of Ca with rare-earth elements and the superconducting transition reaches as high as 46 K, which is higher than the value in electron doping FeAs-122 compounds formed by substituting Fe ions with transition metal, and even surpasses the highest value observed in hole doping systems with a transition temperature up to 38 K. The upper critical fields have been determined with the magnetic field along the ab-plane and c-axis, yielding an anisotropy of 2-3. Hall effect measurements indicate that the conduction in this material is dominated by electron-like charge carriers. Our results confirm the feasibility of inducing superconductivity in Ca122 compounds via electron doping using aliovalent rare-earth substitution into the alkaline earth site, which should add more ingredients to the underlying physics of the iron-based superconductors. [ABSTRACT FROM AUTHOR]

MgB2 bulks and tapes were fabricated using ball-milled precursor powders. The selected B powders have the &bgr;-rhombohedral crystalline structure as a main phase, with a small amount of MgO impurity. Milling time was tested from 0.5 to 120 h to get an optimized value. It is found that ball milling can facilitate MgB2 phase formation, refine the MgB2 grains and increase the defect density. Moreover, the active area, Bc2 and flux pinning are improved, which is of benefit to the enhancement of Jc performance. The 80 h milled MgB2 bulk sample has a Jc value of 2.8 × 104 A cm-2 at 5 K and 6 T, 41 times larger than that of the 0.5 h sample. At the same time, the transport Jc of the 80 h MgB2 tape reaches 2.3 × 103 A cm-2 at 4.2 K and 9 T, 32 times larger than that of the 0.5 h one. Due to the increase of impurities and the release of lattice deformation and strain, the Jc was decreased for the sample milled up to 120 h. [ABSTRACT FROM AUTHOR]

Fe-clad BaxK1-xFe2As2 superconducting tapes were fabricated by the ex situ powder-in-tube method combined with a short high-temperature annealing technique. The effects of annealing time and different dopants on the transport properties of BaxK1-xFe2As2 tapes were systematically studied. By co-doping with Ag and Pb, the transport critical current densities Jc of BaxK1-xFe2As2 tapes were significantly improved in the whole field region and the highest transport Jc was up to 1.4 × 10x A cm-2 (Ic D 100 A) at 4.2 K in self-field. It is proposed that the superior Jc values in the co-doped samples are due to the combined effects of Pb doping at low fields and Ag doping at high fields. [ABSTRACT FROM AUTHOR]

Polycrystalline Ba0.6K0.44Fe2As2.2 was fabricated using a two-step solid reaction method by two different sintering processes. Although different microstructures were observed between these samples, similar superconducting properties were obtained. It seems that the sequence for high temperature with short annealing time and low temperature with long sintering time has little effect on the superconducting properties of the Ba0.6K0.44Fe2As2.2 sample. But, for the Ag-added samples, the Jc value was significantly affected by the sintering process, indicating a different mechanism to pure Ba0.6K0.44Fe2As2.2. [ABSTRACT FROM AUTHOR]

SmFeAsO1-xFx tapes were prepared using three kinds of starting materials. This showed that the starting materials have an obvious effect on the impurity phases in the final superconducting tapes. Compared with the other samples, the samples fabricated with SmAs, FeO, Fe2As, and SmF3 have the smallest arsenide impurity phases and voids. As a result, these samples possess much denser structures and better grain connectivities. Moreover, among the three kinds of sample fabricated in this work, this kind of sample has the highest zero resistivity temperature, ∼40 K, and the largest critical current density, ∼4600 A cm-2, in self-field at 4.2 K. This is the highest Jc value reported so far for SmFeAsO1-xFx wires and tapes. [ABSTRACT FROM AUTHOR]

In this paper, we report a systematic investigation of the crystal structure and superconducting properties of iron-based superconductors Eu1-xNaxFe2As2 (x = 0-0.5). X-ray diffraction patterns indicate that the compounds form the ThCr2Si2-type structure with space group I4/mmm. The systematic evolution of the lattice constant demonstrates that the Eu ions are successfully replaced by Na. The use of alkali metal substitution into the Eu site allows us to suppress the magnetic/structural phase transition in the parent compounds and superconductivity reaches as high as 35K with a doping level of x = 0.5. In addition, single crystals of Eu1-xNaxFe2As2 (x = 0, 0.5) have been successfully synthesized using the self-flux method. The upper critical fields have been determined with the magnetic field along the ab-plane and the c-axis, yielding an anisotropy of 1.7. The high upper critical fields and the low superconducting anisotropy of the Na-doped EuFe2As2 compounds indicate a potential for applications such as the generation of high magnetic fields. [ABSTRACT FROM AUTHOR]

MgB2 tapes were fabricated by the in situ powder-in-tube method using C2H4O3-treated B powder as a source material. It is found that the Jc values of MgB2 tapes are significantly improved by treating B powder with C2H4O3. At 4.2 K, the Jc of the C2H4O3-treated sample sintered at 800 degC reaches 5 x 103 A cm [?] 2 at 12 T. The C2H4O3-treated B improves not only the upper critical field through enhancing intraband scattering but also the flux pinning by increasing grain boundaries and C substitution effects. A larger amount of intragranular structural defects and grain boundaries were found in the C2H4O3-treated MgB2 matrix. Benefiting from the good crystallinity, the C2H4O3-treated tape sintered at 800 degC shows a bigger active area fraction and a higher Jc than the sample sintered at 700 degC. These results suggest that treating B powder with C2H4O3 is an effective way to enhance the critical current density of MgB2 superconductors. [ABSTRACT FROM AUTHOR]

We report the effect of yttrium acetate addition on in situ powder-in-tube processed MgB2/Fe tapes. The amount of yttrium acetate was varied from 0 to 20 wt% and sintering was performed at 700-850 degC for 1 h in a flowing Ar atmosphere. We found that a significant Jc-B enhancement was easily achieved in high field by yttrium acetate doping. At 4.2 K, the transport Jc for the best yttrium-acetate-added tapes (10 wt%) reached almost 104 A cm [?] 2 at 12 T. The improvement of Jc performance in the yttrium-acetate-doped samples can be attributed to the combination of the carbon substitution for B and stronger flux pinning. [ABSTRACT FROM AUTHOR]

Polycrystalline Ba0.6K0.44Fe2As2.2 was fabricated using a two-step solid reaction method by two different sintering processes. Although different microstructures were observed between these samples, similar superconducting properties were obtained. It seems that the sequence for high temperature with short annealing time and low temperature with long sintering time has little effect on the superconducting properties of the Ba0.6K0.44Fe2As2.2 sample. But, for the Ag-added samples, the Jc value was significantly affected by the sintering process, indicating a different mechanism to pure Ba0.6K0.44Fe2As2.2. [ABSTRACT FROM AUTHOR]

In this paper, we report a systematic investigation of the crystal structure and superconducting properties of iron-based superconductors Eu1-xNaxFe2As2 (x = 0-0.5). X-ray diffraction patterns indicate that the compounds form the ThCr2Si2-type structure with space group I4/mmm. The systematic evolution of the lattice constant demonstrates that the Eu ions are successfully replaced by Na. The use of alkali metal substitution into the Eu site allows us to suppress the magnetic/structural phase transition in the parent compounds and superconductivity reaches as high as 35K with a doping level of x = 0.5. In addition, single crystals of Eu1-xNaxFe2As2 (x = 0, 0.5) have been successfully synthesized using the self-flux method. The upper critical fields have been determined with the magnetic field along the ab-plane and the c-axis, yielding an anisotropy of 1.7. The high upper critical fields and the low superconducting anisotropy of the Na-doped EuFe2As2 compounds indicate a potential for applications such as the generation of high magnetic fields. [ABSTRACT FROM AUTHOR]