Bi-2223 Silver-sheathed Tape Research Articles

Present status of the development of superconducting power cable

In this paper, we focus on our R&D activities in the field of high-temperature superconducting (HTS) power transmission cable, and summarize its present status. Five- to seven-meter cable models and 50-m-long conductors were developed using silver-sheathed Bi-2223 tape wires. A 30-m single-phase cable prototype having a closed cooling cycle was also constructed. Numerical and experimental approaches were carried out to reduce AC losses in these conductors. Based on these activities, we have started a new project to develop a 100-m, 66 kV/114 MVA three-phase power transmission cable system prototype and conduct long-term loading tests. In this project, we aim at certifying its manufacturing ability and its practicability, and also extracting the necessary information for development of a field-level HTS power cable system.

Heat transfer characteristics of liquid nitrogen for cyclic heating

Bubble formation and the critical heat flux to transit to boiling state for cyclic heat inputs have been investigated using a platinum - 0.5at% cobalt (Pt-Co) wire and a silver sheathed Bi-2223 tape, in order to evaluate the cooling stability for superconducting AC power apparatus. Sinusoidal cyclic heat inputs ranging from 2 to 150 Hz were applied to a horizontal thin wire immersed in a pool of saturated liquid nitrogen under normal atmospheric pressure. A high-speed video camera with a frame speed of 1000 frames/s has been used to observe bubble formation around the wire. The bubble frequency and the critical heat flux to transit to boiling state have been measured for each frequency. As a result, it has been found that the bubble frequency and the critical heat flux have depended on the heating frequency. The same experiment has been performed on a silver sheathed Bi-2223 tape, however, the mechanism of bubble formation is different. The dependence of the critical heat flux on the heating frequency has been also observed.

Ｂｉ‐２２２３銀シーステープ線材の超低電界領域のＥ‐Ｊ特性の評価

E-J characteristics in a Bi-2223 silver-sheathed tape wire were estimated in the range of a very low electric field by analyzing a relaxation of the magnetization in a magnet field parallel to the c-axis measured by using a SQUID magnetometer. The scaling of the E-J characteristics was examined, and the result was compared with the theoretical analysis based on the flux creep-flow model in which the distribution of flux-pinning strength was considered. It was found that the experimental result on the E-J characteristics was approximately explained by the theory, though a good agreement was not obtained for the scaling parameters.

Distribution of flux pinning strength in a superconducting Bi-2223 silver-sheathed tape

The distribution of the flux pinning strength was estimated from a measured current–voltage curve for a monofilamentary Bi-2223 silver-sheathed tape in a magnetic field parallel to the c-axis. It is found that the distribution can be fitted well to the Weibull function of a very wide distribution. This wide distribution seems to be caused by the low critical current density at weak links. The obtained Weibull function can be approximated fairly well by a simpler distribution function with which the scaling of current–voltage curves is analyzed using the flux creep–flow model. The obtained scaling parameters agree well with the experimental results.

AC susceptibility of a monofilament Bi-2223/Ag superconducting tape in a perpendicular field

Measurements and calculations are presented of the real ( χ′) and imaginary ( χ″) parts of the AC susceptibility of a monofilament silver-sheathed Bi-2223 tape as a function of temperature T=4–130 K, frequency ω/2 π=5 Hz–5 kHz and ac magnetic field amplitude μ 0 H m=0.02–7 mT. The susceptibilities consist of a hysteretic component due to induced currents in the superconductor core and an eddy current component due to induced eddy currents in the silver sheath. At high temperatures (>70 K) the low frequency limit is used to rigorously calculate the hysteretic and eddy current susceptibilities while at low temperatures the susceptibility is approximately described as being due to eddy currents flowing along the edges of the tape. The measured χ″ and hence AC loss at low frequencies (<50 Hz) and high temperatures is dominated by the hysteretic loss which varies with amplitude but is essentially independent of frequency. At higher frequencies and low temperatures, the eddy current loss of the silver sheath becomes dominant and increases with increasing frequency.

AC transport current loss of assembled conductor of HTS tapes

We study experimentally and theoretically the AC transport current loss characteristics of a tape in multiple tapes assembled in single layer and subject to external field produced by transport currents of adjacent tapes. We measured the AC transport current losses of a Bi2223 silver-sheathed tape in a single layer arrangement of three tapes using our newly developed potential leads arrangement to avoid spurious loss components caused by the magnetization in the adjacent tapes. In the paper, the influence of the external AC field produced by adjacent tapes on the loss characteristics is studied based on the experimental results and theoretical analysis.

AC-induced DC voltage in HTS coil

The interplay between AC and DC currents in a High- T c Superconducting (HTS) coil, made of multifilamentary silver-sheathed Bi-2223 tape, was investigated. We observed that the application of a small sinusoidal current in the frequency range of 50–500 Hz into the coil, while it is already carrying a DC current in the range of 16–22.5 A, caused an increase in the coil DC voltage. The DC voltage increment due to the AC signal is found to increase linearly with frequency and quadratically with amplitude. The DC voltage increment increases as the coil current grows towards its critical value of 22.2 A. This result may be important in some power applications such as fault current limiters (FCL) and superconducting magnet energy storage (SMES) based on HTS coils.

Vapour cooled high current leads utilizing Bi-2223/Ag tapes

A novel high current lead, utilizing silver-sheathed Bi-2223 tapes, has been developed and tested, which allows addition or removal of individual, vapour cooled tapes, thus adjusting the current capacity and optimizing the heat leak per current. Minimum heat leak is obtained by tapering the lead, using more tapes at the high temperature end. To date, the leads have been tested using applied currents up to 900 A DC. However, they are designed for a current capacity of several kA.

Microstructural study of Bi2223/Ag tapes made using a two-stage sintering procedure

Silver-sheathed Bi2223 tapes were made using the 'powder in tube' technique and the effect on the tape properties of using a two-step procedure in the final thermal cycle was investigated. A sintering time of 50 h at followed by a second step at for 50 h yields a critical current density almost doubled compared to the one-step process (50 h at ). Comparative measurements and detailed microstructural analysis lead to the conclusion that the improvement is caused by the removal of Bi2201 in the low-temperature step. We also demonstrate that our two-step process is essentially equivalent to a slow-cooling procedure which has been utilized by other groups.

Effect of thermal cycles on thermal expansion of silver-sheathed Bi2223 tape at 10–310 K

The linear thermal expansion coefficient, α, of silver-sheathed multifilamentary Bi2223 ((Bi, Pb) 2Sr 2Ca 2Cu 3O 10±x) tapes was measured over several thermal cycles of 10–310 K using an optical dilatometer. The tapes showed anomalous temperature dependence of α. At the end of the first thermal cycle, a large spontaneous expansion of 0.12% was observed in their longitudinal direction of the tape. After the first thermal cycle, the α– T curves of the tapes showed a repeated hysteresis behavior. This temperature dependence of α resulted from the silver sheath yielding due to internal stress between it and the Bi2223 filaments.

Effects of precursor powder particle size on critical current density and microstructure of Bi-2223/Ag tapes

The effect of particle size distribution on the nature of the precursor powder and the subsequent phase formation and critical current density of silver sheathed Bi-2223 tapes has been studied. Three precursor powders with different particle size distributions were obtained by grinding the calcined powder for different times. The effects of precursor powder particle size were identified by a combination of XRD, SEM and EDX analysis. The results indicate that the phase formation, microstructure evolution and critical current density are sensitive to the precursor particle size distribution. It has been shown that a small particle size promotes the formation of the Bi-2223 phase, and that coarse powder leads to Ca 2CuO 3 and Ca 2PbO 4 remaining in the final tape. It has been found that too fine a powder may cause the formation of large alkaline–earth cuprate grains [(Sr,Ca) 14Cu 14O 41 and (Ca,Sr) 2CuO 3]. The critical current density tends to increase when the amount and size of non-superconducting phase decrease in the final tapes

Bending of silver-sheathed (Bi,Pb)-2223 tapes investigated by magneto-optical flux visualization

The effect of bending on silver-sheathed Bi-2223 tape is investigated by means of magneto-optical (MO) visualization of flux distributions. One single piece of rolled tape is used throughout all experiments starting from as-prepared, and subsequently the tape is bent to 4 mm diameter and then down to 1 mm diameter in 1 mm steps. Flux patterns are visualized using an intact tape, i.e. the visualization is done through the silver sheath after removing the bending strain. It is shown that bending of the tape causes a series of cracks running perpendicular to the tape axis (rolling direction). This crack pattern remains practically unchanged down to 1 mm bending diameter. The flux penetration changes from nearly homogeneous (as-prepared) to entirely crack-governed. Vortices are found to penetrate the tape preferentially along the cracks and from there into the remaining bulk. In bent samples, a very homogeneous flux distribution is observed when the external field is decreased after having reached the full-penetration field. A further decrease of the field forces vortices to leave the sample along the cracks, and trapped vortices remain within the Bi-2223 grains (grain clusters). With each step of bending, the values for the first flux penetration and full-penetration field are decreased. The observations of flux patterns at 1 mm bending diameter clearly demonstrate that intergranular currents are still flowing in the sample. Our observations demonstrate the capability of the MO technique for non-destructive testing of superconducting tapes.

Determining factors of the critical current density of silver-sheathed Bi2223 tapes

The factors determining the critical current density, Jc, of the Bi2223 tapes are discussed, which include the intragrain and intergrain properties of the tape.

Critical current in silver sheathed Bi-2223 tapes

Silver sheathed Bi-2223 tapes have been investigated by transport and magnetization measurements in the temperature range between 4.2 K and 77 K for magnetic fields up to 12 T. The superconducting current through the tape can be explained by assuming a network of platelike grains coupled by weak links. At high temperatures the critical current is governed by flux creep of pancake vortices within the grains. At temperatures below 40 K this intragrain critical current exceeds the intergrain Josephson current leading to a weak link limitation in the tape. In this picture the differences between critical transport and magnetization currents can be explained by a reduction of the geometrical length scale of current loops. An improvement of the weak link current density is observed in multifilamentary wires and tapes with AgCu sheath material.

Electromagnetic interfilament coupling of silver-sheathed Bi-2223 multifilamentary tapes in transverse AC magnetic fields

To discuss the electromagnetic interfilament coupling of silver-sheathed multifilamentary Bi-2223 (HTS) tapes, AC losses of HTS tapes with various twist pitches and sample lengths in transverse magnetic fields were measured at 77 K by a magnetization method. Both hysteresis loss and eddy current loss in fields perpendicular to the HTS tape surface were approximately one order of magnitude greater than those in parallel fields. While AC losses in parallel fields were extremely reduced with a decrease in the twist pitch or the sample length, those in perpendicular fields scarcely decreased. AC losses of the untwisted HTS tape, which were proportional to the frequency in a perpendicular field of 29 mT, were slightly smaller below 70 Hz and larger above 150 Hz than those of the HTS tape twisted at a pitch of 8.6 mm. It is concluded that the coupling loss of untwisted HTS tapes is much smaller than the eddy current loss at commercial frequencies because of complete electromagnetic interfilament coupling.

Scaling of current-voltage curves in superconducting Bi-2223 silver-sheathed tape wires

The current-voltage characteristics are measured for two Bi-2223 tape specimens with different critical current densities at various temperatures under the magnetic field parallel to the c-axis. It is found that the current-voltage curves are scaled on two master curves by normalizing as predicted in the vortex glass-liquid transition theory. However, the obtained dynamic critical indices increased, and the static critical indices decreased appreciably with increasing magnetic field. These experimental results are compared with the numerical calculation based on the flux creep and flow model, taking the distribution of the flux pinning strength into account. Agreement is obtained between the experimental and theoretical results not only on the scaling curve but also on the critical indices and the transition line. The static critical index showing a divergence of the correlation length near the transition temperature can also be explained from the temperature dependence of the pinning correlation length. These results suggest that the scaling behavior is not correlated to the phase transition of fluxoids assumed in the vortex glass-liquid transition theory but seems to be correlated to a kind of transition governed by the flux pinning.

Inhomogeneous Current Distribution in Ag/Bi2223 Tape

In order to investigate the inhomogeneous current distribution in the silver sheathed Bi2223 tapes, a scanning Hall probe measurement was carried out. The z component of self magnetic field was converted to the local current density, of which spatial change comprised two characteristic features. The inhomogeneity of longitudinal current distribution along the current flow was suggested to be attributed to the positional change of oxide layer thickness due to the work instability. Transverse current distribution is caused from the degree of misalignment of Bi2223 layered grains as well as the distribution of non-superconducting phases.

Ag/Bi2223テープ材における電流密度の不均質分布

In order to investigate the inhomogeneous current distribution in the silver sheathed Bi2223 tapes, a scanning Hall probe measurement was carried out. The z component of self magnetic field was converted to the local current density, of which the spatial change comprised two characteristic features. The inhomogeneity of longitudinal current distribution along the current flow was suggested to be attributed to the positional change of oxide layer thickness due to the work instability. Transverse current distribution is caused from the degree of misalignment of Bi2223 layered grains as well as the distribution of non-superconducting phases.

Self-field AC losses and critical currents in multi-tube Ag - Bi-2223 conductors

The purpose of this work was to investigate the influence of different technological treatments of silver sheathed Bi-2223 tapes on the critical current density and the AC transport losses. The tapes were produced using the `tube-in-tube' technique, by including a silver rod in the centre of the superconducting powder during packing of the silver tube. The aim of the process is to increase the silver to superconductor surface area and thus also the alignment at the centre of the conductor ceramic core. AC transport losses were measured by means of an electrical method using sinusoidally varying currents in the frequency range 30 - 180 Hz. In this range the power losses are hysteretic. The measured variation in losses from those predicted by a critical state model is attributed to the complex geometry of superconducting regions existing in these tapes.

The effect of weak links on the critical current in (Bi,Pb)2Sr2Ca2Cu3O10 silver sheathed tapes

Silver sheathed Bi-2223 tapes with critical current densites of about 15 kA/cm2 (77 K, B=0) have been investigated by transport and magnetization measurements. The superconducting current through the tape can be explained by assuming a network of platelike grains coupled by weak links. At high temperatures the critical current is governed by flux creep of pancake vortices within the grains. At temperatures below 40 K the critical current is found to be limited by weak links. This model explains the temperature and field dependence of the critical transport current. Differences between the critical transport and magnetization current observed at medium and low temperatures can be explained by a reduction of the geometrical length scale of current loops.