Kyu Jeong Song

Transport current loss and I/sub c/ 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.

High quality SmBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films on SrTiO/sub 3/ (100) substrates deposited by pulsed electron beam deposition

We for the first time report a successful fabrication of a high-J/sub C/ SmBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//(SmBCO) thin film (/spl sim/240 nm thickness) on SrTiO/sub 3/ (100) substrate by the pulsed electron beam deposition (PED) process. For this study, we systematically investigated the effect of processing parameters, including substrate temperature (Ts), ambient oxygen pressure (PO/sub 2/), and target-to-substrate distance, on superconducting properties, including critical temperature (T/sub C/) and critical current density (J/sub C/), of PED-processed SmBCO films. The highest J/sub C/ value of 1.37 MA/cm/sup 2/ at 77 K in self field was obtained from an optimally processed sample using Ts of 900/spl deg/C (corresponding to real substrate temperature of 810/spl deg/C), PO/sub 2/ of 15 mTorr, and target-to-substrate distance of 9 cm, evidencing that this process is a potential alternative to the PLD process.

Effect of W addition on the microstructure and properties of Ni-W substrates for coated conductors

We fabricated Ni and Ni-W alloys for use as substrates in YBCO coated conductor applications and evaluated the effects of the W addition on the texture, microstructure, and mechanical and magnetic properties of the substrate. Pure Ni and Ni-W (2, 3, and 5 at.%) alloys were prepared by plasma arc melting and then cold rolled and annealed in the temperature range of 600-1300/spl deg/C. The texture of the substrates was evaluated by pole-figure and orientation distribution function (ODF) analysis. The magnetic properties were also evaluated using the physical property measurement system (PPMS). It was observed that the Ni-W substrates had a stronger cube texture and a wider annealing temperature range in which the cube texture became stable than those of the pure Ni substrate. The full-width at half-maximums (FWHMs) of in-plane texture for the Ni-W substrate were 4.42/spl deg/-5.57/spl deg/ at an annealing temperature of 800/spl deg/C-1300/spl deg/C, while that of pure Ni was 9.5/spl deg/ at 800/spl deg/C. Therefore, it is considered that the addition of W enhances the formation of the cube texture and improves the texture stability at higher annealing temperatures. In addition, the Ni-W substrates had a smaller grain size and higher mechanical strength and hardness, as compared to those of the pure Ni substrate. These improvements are probably due to various strengthening mechanisms, such as solid solution hardening and/or grain size strengthening. PPMS analysis showed that the addition of W effectively reduced the saturation magnetization in an applied magnetic field, as well as the Curie temperature.

Design of HTS tape characteristics measurement system

High temperature superconductor (HTS) tapes are now commercially available for practical applications to apply various purposes. However HTS tapes show different electrical and mechanical characteristics, according to the manufacturers who are trying to apply various fabrication processes and treatments. From the viewpoint of an application, it is very important to investigate the properties of HTS tapes under mechanical stress, because the tapes will be wound with twisting and tension in applications such as magnets and cables. Considering of operating situations of applications, HTS tapes might be influenced not only mechanical deformation but also electromagnetic effects. Thus, we developed an equipment to measure the characteristics of HTS tapes, considering of these mechanical and electromagnetic conditions. This system can be used to measure the critical current and the AC loss in magnetic field with tension and twisting. A description of the design and some typical results are presented with discussions.

Characteristics Measurements of HTS Tape with Parallel HTS Tapes

Many high temperature superconducting (HTS) tape manufactures make an effort to reduce the transport current loss of HTS tapes. The knowledge of critical current and self-field in an HTS tape is very useful to compute the transport current losses. The spatial distribution and magnitude of self-field are variable due to the neighboring materials. In this paper, the critical currents and the transport current losses of BSCCO and YBCO tapes with paralleled magnetic material (Ni tape) and/or diamagnetic material (BSCCO tape) are experimentally investigated to improve the AC loss properties. The critical currents of HTS tapes with paralleled Ni tape are slightly decreased and the transport current losses are markedly increased. However, the critical currents and transport current losses of HTS tapes with paralleled BSCCO tape have not current carrying are more improved than single HTS tape

Effect of Trivalent Bi Doping on the Seebeck Coefficient and Electrical Resistivity of Ca3Co4O9

We present the effects of trivalent Bi doping on the microstructure and thermoelectric (TE) properties of Ca3Co4O9 (Ca-349). Specimens were prepared by spark plasma sintering (SPS). The lattice parameters of the conducting [CoO2] layer and insulating [Ca2CoO3] layer of Ca-349 were determined using Rietveld refinements of x-ray diffraction (XRD) data. Partial substitution of Bi for Ca did not lead to any change in the misfit ratio of the conducting versus insulating layers. XRD and transmission electron microscopy (TEM)/energy-dispersive spectrometry (EDS) results show the presence of Bi2O3 phase in the grain-boundary region. The Seebeck coefficient (S) increased with increasing Bi content due to the decrease in the hole carrier concentration after Bi doping. As the amount of Bi was increased, the electrical resistivity (ρ) initially decreased but then increased with further addition of Bi. Addition of small amount of Bi led to large decrease in electrical resistivity due to the increased amount of Bi2O3 phase in the grain-boundary region. With further increase of Bi addition, the electrical resistivity increased due to the decrease in the hole carrier concentration. Optimum Bi doping not significantly affecting the hole carrier concentration is an effective approach for increasing the Seebeck coefficient and decreasing the electrical resistivity of Ca-349.

MICROSTRUCTURAL ANALYSIS OF CORE-SHELL STRUCTURED MgB2 SUPERCONDUCTING FIBERS FABRICATED BY HYBRID PHYSICAL CHEMICAL VAPOR DEPOSITION

We have deposited MgB2 thin films on the carbon fiber by hybrid physical–chemical vapor deposition (HPCVD) method which is known as the most effective technique for depositing MgB2 thin films without pore formation. By adopting carbon fiber as substrate and gas phase reaction between diborane (B2H6) gas and Mg vapor for MgB2 formation, core-shell structured MgB2/C composite fiber could be synthesized. The high quality of the prepared MgB2 fibers was confirmed with scanning electron microscopy (SEM), and the critical temperature (Tc) of the sample deposited at 590°C and 12 sccm flow rate of B2H6 was measured at ~40 K. The microstructures of MgB2 layer was characterized using transmission electron microscopy (TEM). The bright-field TEM images showed MgB2 with the grain size of several nanometers. The MgB2 thin film showed highly dense microstructure without pore and well-connected to the carbon fiber interface. High-resolution TEM (HRTEM) images and scanning transmission electron microscopy (STEM) revealed the presence of interface phase between MgB2 thin films and carbon fiber.

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.

A Noncontact T c Evaluation Technique Using a Hall Probe Array

A noncontact superconducting transition temperature (<italic>T</italic>_c) measurement system using a Hall probe array was devised to measure <italic>T</italic>_c of high-temperature superconductor (HTS) coated conductors (CCs) without physical contact to the surface, thereby avoiding contact with electrodes, which can cause damage to the conductor surface. The variations of the Hall voltage (<italic>V</italic>) distribution, which comes from the distribution of magnetic field on the surface of the CC, under a fixed applied field at different temperatures were used to estimate <italic>T</italic>_c. The measured <italic>T</italic>_c of the HTS GdBCO CC made by SuNAM was approximately 93.2 K by the noncontact <italic>T</italic>_c method, as compared to approximately 93.5 K by the conventional four-probe-contact method in a commercial PPMS. Thus, the <italic> T</italic>_c obtained by this system is comparable with that from the transport four-probe method. In addition, the noncontact <italic>T</italic>_c measurement system devised and used in this study is simple because it does not need comparison and conversion with values measured by contact methods.