Powder In Tube Method Research Articles

Ex situ fabrication of MgB2/Al tapes with high critical current density

We have considerably improved the critical current density,Jc, in appliedmagnetic fields, B, of Al-sheathed MgB2 tape. Our MgB2/Al tape,made with MgB2 powder obtained from the core of an in situ powder-in-tube (PIT) processed tape, was fabricatedvia an ex situ PIT method without heat treatment and additive compounds. The highestJc obtained at 10 Tand 4.2 K was 880 A cm−2. This value exceeds that of conventional ex situ fabricatedMgB2/Al tapes made from commercialMgB2 powder. The improvementin the Jc-B performance ofthis ex situ fabricated MgB2 tape is considered to be due to a significant improvement in the magnetic irreversibility field of theMgB2 cores.Both the MgB2 and the Al sheath show a low density and a low induced radioactivity.Our experimental data obtained using Al as the sheath material of theMgB2 tape indicate the great potential of the tape for specific applications, such as in fusionreactors and on-board magnets.

Variation of vortex structure characteristics of Bi-2223/Ag superconducting tapes with respect to applied magnetic field direction

A study of the effects of external magnetic field orientation on the vortex characteristics was performed on a Bi-2223/Ag superconducting tape prepared by powder in tube method incorporating two-axial rolling and eccentric rolling processes. The data used for the analysis were obtained by magnetoresistance measurements in external magnetic field ranging from 0 to 16 T, at field orientations of 0°, 25°, 45°, 60°, and 90° with respect to the broad face of the tape. It is shown that the H irr and H TAFF phase boundaries tend to shift to lower temperature region with increasing field angle. On the other hand, the H FF curves remain roughly unaffected, leading to broadening of the unpinned liquid phase region between the H TAFF and H FF curves. Further analysis of the H irr( T) data for θ = 90° on the basis of Kim’s model of pancake depinning has yielded a whole set of vortex parameters and H c2 ( θ).

Graphite coating effects on the critical current characteristics of MgB2/Cu wires fabricated by an in situ PIT method

In order to improve the critical current characteristics of Cu sheathed MgB2 wires, we studied the carbon doping and coating effects on JC of wires prepared by an in situ powder-in-tube (PIT) method. Raw materials of Mg and B powders were mixed with a certain amount of MgB2 powder to increase the core density of wires in the heat treated samples. The pellets of these mixed powders were inserted into Cu sheaths and manufactured into wires by rotary swaging, two-axial rolling and drawing under cold-working. The wires were finally heat treated at 600°C for 2h in Ar gas atmosphere. The critical current IC of short samples of graphite 5at.% doped MgB2/Cu with diameter of 1.0mm reached 548A (JC=440kA/cm2) at 4.2K in self-field. The reaction of Mg with Cu decreased for graphite coating samples compared with non-coating ones. We obtained high performance MgB2/Cu wires by doping of diamond and graphite powder in the in situ PIT method. We found that the graphite coating on the inside surface of Cu sheath was effective for suppressing the reaction of Mg with Cu sheath. We also fabricated solenoid coils by wind and react method heat treated under the above conditions. The IC value was 230A at 4.2K for a coil produced by using a 16m wire. This coil generated a magnetic field of 0.42T at liquid He temperature.

Transport Alternating Current Losses of BSCCO and YBCO Tapes

After the discovery of HTS (High Temperature Superconductor), many types of HTS conductors have been developed. Several BSCCO tapes fabricated by PIT (Powder In Tube) method became commercially available, and the transport current loss characteristics of BSCCO tapes have been investigated by many research groups. The transport current loss characteristics of YBCO tapes, however, are not well studied. This is because measuring transport current requires tape samples over a certain length (15 cm for our own system). Many groups are currently focusing their effort on manufacturing long YBCO tape conductors, and new world records of length and current carrying capability are replacing the old ones in an increasingly fast rate. This paper deals with transport current losses of commercially available BSCCO tapes and YBCO tapes made at Korea Electrotechnology Research Institute (KERI) of Korea. YBCO tapes show relatively high transport current losses. This can be explained by the fact that the YBCO tape does not consist of filaments. Measurement results of the tapes are presented with analyses and discussions.

Copper sheath MgB 2 wires fabricated by an in situ PIT method

We investigated the applicability of Cu as sheath materials for MgB 2 wires prepared by in situ PIT (powder-in-tube) method in comparison with stainless steal. Since the critical current density of MgB 2 increases with TiH 2 doping, we prepared TiH 2 doped MgB 2 Cu sheath wires 40 m in length and ∅ 1.0 mm or 0.5 × 1.0 mm 2 in cross section by rotary swaging, drawing, and two-axial rolling under cold working. We then annealed the samples at 600–850 °C for 1–2 h in Ar gas atmosphere. The critical current of TiH 2 (6%) doped MgB 2/Cu short sample annealed at 650 °C reached 208 A ( J c=230 kA/cm 2) at 4.2 K and self-field. We also fabricated several coils using these wires. The I c value was ∼100 A at 4.2 K for a coil prepared with use of a 5 m length wire. These results suggest that it is possible to fabricate Cu sheathed MgB 2 wires with good performance by using the in situ PIT method.

Transport Current Loss and<tex>$rm I_rm c$</tex>Degradation of HTS Tapes Under Mechanical Load

Performance of an HTS (high temperature superconductor) tape is evaluated by measuring the critical current I/sub c/, AC loss and mechanical properties of the short samples. However, in coils or magnets for applications, the HTS tapes are under mechanical load caused by winding tension, bending and sometimes twisting. Also thermal contraction by the cooling changes the tension. The mechanical load reduces the critical current of the superconducting wire, and the I/sub c/ degradation affects the AC loss of the wire. We measured the I/sub c/ degradations and AC losses (transport current loss) of commercial Bi-2223 tapes processed by a PIT (powder in tube) method under tension. The I/sub c/ and AC loss measurements are done in the same time with the same voltage taps on the tape conductors. The measurement results and discussions on the relationship between I/sub c/ degradation and AC loss are presented.

Fabrication of MgB 2 superconducting wire by in situ PIT method

We have fabricated the MgB 2 superconducting wire with a copper sheath by using an in situ synthesis powder in tube (PIT) method. The diameter of MgB 2 wire was reduced down to 0.5 mm. MgB 2 coil with 27 mm∅ inner diameter and 3.5 m length was made using this wire. In the first current raising process, the critical current ( I c) of the coil was 76 A at 4.2 K in self-field which corresponded to J c≈170,000 A/cm 2 and J e≈40,000 A/cm 2 was achieved in magnetic field of H≈0.14 T. In the second run, the coil did not quench at the maximum current of 200 A, which corresponded to J c≈440,000 A/cm 2, J e≈100,000 A/cm 2 and H≈0.37 T.

Supercondcuting properties in MgB2/Fe wires prepared by PIT method

The MgB2 formation was analyzed physically and chemically and the MgB2/Fe wires were fabricated by powder in tube (PIT) technology. The microstructure of MgB2 in wires was studied by the scanning electrical microscope (SEM), which shows a good connection of grains and the size of MgB2 grain is 1–3 μm. The results ofJc measured by the standard four probes method show thatJc value reaches 6.1×104 A/cm2 at 20 K in self field.

Characterizing superconducting BSSCO tapes and bulk joints

For many potential applications, long lengths of superconducting tapes and bulk materials with high current capacities are needed. Producing long lengths of superconducting bulk and tapes by joining is considered a challenge. Superconducting monofilament Bi(2223) tapes were fabricated using the powder in tube method with groove and flat rolling. The tapes were rolled to a final thickness of 250 /spl mu/m. The tapes and bulk materials were processed using a combination of pressing and sintering. The average critical currents for the tapes and bulk materials were 70 and 180 A at 77 K and self field respectively. Superconducting tapes and bulk specimens were joined using lap and butt configurations and thermomechanically processed to make superconducting joints. The maximum joint efficiencies for the lap and butt joints were in the range of 30% to 100%. The phase changes and microstructural development in the joint region were investigated using hardness measurements, XRD and SEM.

Microstructure and high critical current of powder-in-tube MgB2

We report dc transport and magnetization measurements of the critical current density, Jc, and microstructural analyses of MgB2 wires fabricated by the powder-in-tube method (PIT) using commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as a sheath material. We identify a weak-link behavior in the as-drawn wire associated with the presence of microcracks. By appropriate heat treatments, we can increase Jc by more than one order of magnitude due to a recrystallization process promoted by the excess Mg, which results in the elimination of most of our microcracks. In contrast, inappropriate annealing conditions result in a deterioration of connectivity due to an inhomogeneous loss of Mg. Grain size and porosity play only a secondary role in determining Jc in PIT MgB2.

Ｉｎ−ｓｉｔｕＰＩＴ法によるＭｇＢ２／Ｃｕ線材の作製

We investigated the applicability of a Cu sheath to MgB2 wires that were prepared utilizing the in-situ synthesis PIT (powder-in-tube) method and compared it with that of stainless steel. Since the critical current density of MgB2 increases with TiH2 doping, we prepared TiH2-doped Cu sheath MgB2 wires 40 m in length and φ1.0 mm or 0.5x1.0 mm2 in cross-sectional dimension utilizing rotary swaging, drawing, and two-axial rolling at room temperature. We then annealed the samples at 600∼850°C for 1∼5hr in an Ar gas atmosphere. The critical current of the TiH2 (6%)-doped MgB2/Cu short sample annealed at 650°C reached 139A(JC = 132kA/cm2) at 4.2 K and ambient field. We also investigated the effect of repeated rolling and annealing on the critical current density of MgB2/SUS wires and confirmed that the enhancement of JC is caused by the densification of MgB2 cores. Finally, we concluded that Cu-sheathed MgB2 wires produced utilizing the in-situ PIT method are promising for practical use.

ＭｇＢ２超伝導材料の現状と展望

Recent studies on MgB2 superconductors are reviewed. MgB2 single crystals show Bc2 anisotropy factors of 2.6—4.5. The Bc2 of a single crystal at 4.2 K is ∼15 T when the field is parallel to the ab plane. MgB2 bulks, wires and tapes show a higher Bc2 than single crystals. This Bc2 enhancement can be explained by the introduction of defects that decrease the coherence length. MgB2 thin films show a much higher Bc2 than bulk materials. The highest Bc2, 49 T at 0 K, was reported for highly resistive MgB2 thin film. One of the effective pinning centers in MgB2 seems to be grain boundary. Effective pinning centers are also introduced by nanometer-size impurity additions such as SiC. SiC addition significantly increased Jc values in the high field regions, and hence Birr. Most tapes and wires are now fabricated by a powder-in-tube (PIT) method. The ex-situ PIT method employs MgB2-reacted powder and hard sheath materials such as stainless steel. The Jc of ex-situ tapes sensitively depends on the packing density of the MgB2 core. A maximum Jc of ∼4.5×105 A/cm2 is obtained at 4.2 K and zero fields. Ex-situ processed tapes show Jc anisotropy with respect to the field orientation due to the grain alignment. The in-situ PIT method employs a powder mixture of Mg and B. High-energy ball milling and mechanical alloying of the starting powder mixture enhanced the Jc values. The replacement of MgH2 with Mg is effective for improving the reactivity and enhancing Jc values. The Jc at 4.2 K and zero magnetic field reached 2—3×106 A/cm2. The highest Jc, 2.5×104 A/cm2, was obtained at 10 T and 4.2 K when SiC was added to in-situ processed tape prepared with MgH2+B powder. This Jc is comparable to that of a commercial Nb-Ti conductor.

High critical current density in Ti-doped MgB2/Ta/Cu tape by powder-in-tube process

Ti-doped MgB2/Cu tapes with Ta as a buffer layer were prepared through the in situ powder in tube method by using Mg, Ti, and B powders. The phase compositions, microstructure features, and superconducting properties were investigated by x-ray diffraction, scanning electron microscope, and superconducting quantum interference device magnetometer. It is found that TiB2 phase was formed in Ti-doped MgB2 tape. Magnetization measurement results show that the critical transition temperature of MgB2/Ta/Cu tape with Ti doping is around 38 K. The irreversibility field Hirr and critical current density Jc can be greatly enhanced by Ti doping. Hirr of the Mg0.9Ti0.1B2 tape is as high as 7.4 T at 10 K. The high critical current density Jc of 1.5×106 A/cm2 (10 K, self field) and 9.3×105 A/cm2 (20 K, self field) are obtained in the Mg0.9Ti0.1B2 tape. In addition, a suitable amount of Ti doping can lead to a high density and fine grain size of MgB2, which may be the reason for high Jc in Ti-doped MgB2 tapes.

Improvement of critical current density in the Cu/MgB 2 and Ag/MgB 2 superconducting wires using the fast formation method

The powder in tube method has been used to fabricate Ag- and Cu-clad MgB 2 wires using an in situ reaction method. The effects of short time sintering on the critical current densities of Ag- and Cu-clad MgB 2 wires were studied. All the samples were examined using X-ray diffraction, scanning electron microscope, and magnetization measurements. For Ag-clad wire J c is improved by more than two times after the short time sintering process. J c values of 1.2×10 5 A/cm 2 in zero field and above 10 4 A/cm 2 in 2 T at 20 K have been achieved for Ag-clad MgB 2 wire which is only sintered for a few minutes at 800 °C. However, a remarkable degree of reaction has been found between the superconducting cores and the sheath materials, leading to the formation of Cu 2Mg and Ag 3Mg for Cu- and Ag-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 result shows that iron is still the best sheath material because of the lack of reaction between Fe and the superconducting MgB 2 material.

Fabrication and superconducting properties of MgB2 composite wiresby the PIT method

Dense MgB2/Cu wires with Ta as a buffer layer were successfully fabricated by the powder-in-tube (PIT) method. The microstructure was investigated by optical microscopy. Magnetization measurements were carried out by using a superconducting quantum interference device (SQUID) magnetometer at magnetic fields up to 7 T from 5 K to 35 K. The transition temperature of the MgB2 wire is around 38.4 K and the irreversibility field is 6.6 T at 5 K. The critical current density as high as 105 A cm−2 (5 K, self-field) and 104 A cm−2 (20 K, 1 T) has been obtained. The results suggest that the powder-in-tube (PIT) process is promising in preparing high-quality MgB2 wires.

Fabrication and transport AC losses of (Bi,Pb)2223 multifilamentary tapes with resistive barriers

The Ag-sheathed (Bi,Pb)-2223 multifilamentary tapes with different filament distribution and resistive barriers have been fabricated by powder-in-tube (PIT) method. A rectangular deformation process by using a four-roller machine was applied to fabricate the multifilamentary wires. Bi-2201 and Ag-Cu alloy sheets were introduced into the tape as resistive layers to decrease the coupling between filaments. The transport AC losses were measured on these tapes at 77 K with power frequency between 50 Hz and 1000 Hz The results indicated that the behaviors of transport AC losses are influenced by both filament distribution and resistive barrier. The introduction of resistive barriers to divide the tape into several thin strips is effective to reduce the transport AC losses. For the prepared 39 filament tapes, when the resistive layers are configured parallel to the tape surface, the transport AC losses at 50 Hz measured at 77 K are greatly reduced compared to those of 37 filament tape without barriers fabricated by a conventional PIT method.

RADIO FREQUENCY BEHAVIOUR OF PIT-PROCESSED BI(2223) TAPES

We have studied the response of long superconducting (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10 based conductors fabricated by the Oxide Powder In Tube method when exposed to a radio-frequency magnetic field excitation. In particular, we have characterized their properties when employed as a part L-C resonators typical of pick-up coil devices in Magnetic Resonance Imaging. In these conditions, Bi(2223) conductors cooled down to 77 K have shown superior properties compared to already optimized copper based devices: tri-dimensional superconducting pick-up coils conceived for a resonance frequency between 6 and 9 MHz present an improvement of the quality factor Q by more than an order of magnitude, and an increase of the signal-to-noise ratio by a factor larger than three. This promising result opens new perspectives for Bi-based conductors in the field of the detection of weak high frequency signals.

Effect of gamma irradiation on the transport critical current density of Tl2Ba2Ca2Cu3O10 tapes

Specimens of Tl2Ba2Ca2Cu3O10 tapes were prepared using the powder in tube method. The voltage-current (V - I ) characteristics were used to study the temperature and magnetic field dependence of the critical current density Jc(T,B). Obtained results confirmed the high sensitivity of the superconducting tape to the applied magnetic field and are explained based on the weak links and Josephson junctions between the grains. The temperature dependence of Jc was also derived and discussed in terms of the thermally activated flux flow (TAFF). The effect of gamma irradiation on the critical current density Jc(T,B) was also studied up to a gamma dose of 400 MR. Gamma irradiation was found to increase Jc at relatively low temperature and magnetic field ranges. Therefore, gamma irradiation is a useful method to improve the superconductivity of the Tl2Ba2Ca2Cu3O10 tapes.

The influence of heating temperature on the superconducting properties of silver-gold sheathed Bi-2223 tapes destined for power applications

For power engineering applications using HTS conductors, long-length Ag-Au sheathed Bi-2223 tapes are being developed. By a continuous fabrication process (rolling), tapes with J/sub c/ up to 20 000 A cm/sup -2/ (77 K, 0 T) have fabricated using the powder in tube method. The tapes were heat treated in the temperature range of 805-835/spl deg/C in an atmosphere of 8% oxygen in nitrogen. The influence of temperature and time on the evolution of the superconducting properties is investigated by means of electric transport current measurements (I/sub c/ and its distribution). Phase development at different processing stages is examined by XRD. The microstructure and phase assemblage at different heating temperatures are characterized with SEM/EDX. The critical current distribution in the tapes is studied by analyzing the second differential of the V-I curves and the effects of cracks and secondary phases are discussed.

Texture hardening in PIT method and its effect on texture formation during annealing

In present study the influence of several factors on the texture of Bi-based superconductor prepared by powder in tube (PIT) method has been studied. The grain growth of 2212 and 2223 phases during the heat-treatment process leads to obvious improvement of their texture degrees. The texture of mother phase 2212 is well beneficial to improving these texture degrees. These results are analyzed based on the mechanism of the minimum interface energy texture formation that was raised in the early works.