Toyotaka Yuasa

Introduction of pinning centers into Tl‐(1223) phase of Tl–Sr–Ca–Cu–O systems

Pinning centers were successfully introduced into the (1223) phase of Tl–Sr–Ca–Cu–O system using its derivative (Tl/Pb)–(Sr/Ba)–Ca–Cu–O with the nominal composition of (0.5/0.5)−(1.6/0.4)–2–3. The bulk sample showed a strong flux pinning characteristics. The critical current density, Jc, was obtained from the magnetization measurement. The intragrain Jc was ∼43,000 A/cm2 at 77 K and 1 T. The transport Jc of a tape with the (1223) material in core was ∼500 A/cm2 at 77 K and 5 T.

Flux pinning in single Tl-layer 1223 superconductors

Abstract The presence of pinning centers in high temperature superconductors (HTSCs) is a prerequisite for obtaining a high critical current density ( J c ) in a magnetic field. We have discovered that, perhaps more generally, HTSCs having only a single layer between the superconducting copper layers (for example the YBaCuO or TlSrCaCuO systems) can possess effective pinning centers that are absent in the double layer cases. Several samples prepared by a partial melt process, with the nominal composition (Tl 0.5 Pb 0.5 ) 1 (Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O 9 , showed the high intragrain J c , by the magnetization method, greater than 2 × 10 4 A/cm 2 at 77 K and 1 T. This result suggests that more effective pinning centers exist in the (1223) phase of the modified TlSrCaCuO system (ref. [1]). We propose an explanation why both YBa 2 Cu 3 O 7 and (1223) of TSCCO contain useful pinning centers, whereas both Bi- and Tl-double-layer superconductors do not. Effective pinning, in this case greater for single layers rather than double layers, is related to the strength of the superconductive order-parameter in these layers intervening the CuO layers.

Fabrication of YBa/sub 2/Cu/sub 3/O/sub 7-x/ films on buffered metal tapes fired at low temperature by MOD method using trifluoroacetate salts

Metalorganic deposition (MOD) using trifluoroacetates is considered to be one of the most promising methods for coated conductors, because it enables us to obtain very high J/sub c/ films with low-cost facilities. We have optimized various processing conditions in this method and successfully obtained critical current density (J/sub c/) of 4.1 MA/cm/sup 2/ (77 K, 0 T) for the YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) film of 1800 /spl Aring/ in thickness on CeO/sub 2/ buffered YSZ single crystal, which was fired at 800/spl deg/C. In the case of YBCO films on buffered metallic substrates, it is difficult to fabricate under the same deposition condition. Then low temperature heat treatment is required for deposition on metal substrate. On the other hand, such heat treatment also permits non-reacted BaF/sub 2/ in fired film, that leads to low J/sub c/ of the film, Therefore it is the key to fabricate YBCO film with entire reaction of BaF/sub 2/ by the low temperature heat treatment. We have investigated chemical reactions in firing and established a firing profile to fabricate YBCO film even at 725/spl deg/C. With this profile, we can successfully fabricate the YBCO film at 725/spl deg/C on CeO/sub 2/ buffered YSZ single crystal which has J/sub c/ of 3.0 MA/cm/sub 2/ and the YBCO film on CeO/sub 2/ (sputtered)/YSZ (ion-beam-assisted deposition: IBAD) buffered Ni based alloy (hastelloy) tape which has J/sub c/ of 0.81 MA/cm/sup 2/.

Transport Critical Current Densities in Uniaxially and Biaxially Oriented Tl1(Ba0.8Sr0.2)2Ca2Cu3O9 Superconducting Films on Ag and SrTiO3 Substrates Prepared by a Spray Pyrolysis Method

Tl1(Ba0.8Sr0.2)2Ca2Cu3O9 films were prepared by a spray pyrolysis method on SrTiO3(100) single crystal substrates and on polycrystalline Ag tapes. The biaxially ( c- and a-axes) oriented Tl1(Ba0.8Sr0.2)2Ca2Cu3O9 film on SrTiO3 single crystal substrate showed very high performance of the transport J c, i.e. 2.8×105 A/cm2 at 77 K, 0 T and 5.0×104 A/cm2 at 77 K, 1 T (H//c- axis). In contrast, the c-axis oriented film was obtained on Ag tape with the J c of 8.0×104 A/cm2, at 77 K, 0 T, and of 6.9×103 A/cm2, at 77 K, 1 T (H//c- axis). In the c-axis oriented Tl1(Ba0.8Sr0.2)2Ca2Cu3O9 film, the weak link behavior remained, however the weak magnetic field dependence of the J c and the very high J c in the biaxially oriented film suggested that the weak link problem was eliminated. We concluded it would be necessary to get the biaxial orientation to produce practical wires of Tl-1223.

Flux pinning in TI-(1223) superconductor

Abstract A strongly pinned (TI/Pb)-(Sr/Ba)-CaCuO bulk superconductor of TI-(1223) phase with a single TI-O plane has been developed with a magnetic J c over 43 000 A cm −2 at 77 K and about 600 000 A cm −2 at 20 K and 1 T. Single crystals of TI-(1223) have been prepared and used for investigations of magnetization hysteresis anisotropy using SQUID. When the field was applied parallel to the c axis, a high field peak related to inhomogeneous stoichiometry and properties across the specimen was observed in the magnetization hysteresis loop around 1 T at 77 K. Ag-sheathed tapes with TI-(1223) core were fabricated by the drawing-rolling method. The transport J c was 25 000 A cm −2 in the absence of a magnetic field, and stayed - 500 A cm −2 up to 8 T at 77 K.

Critical current density and related microstructures of TFA-MOD YBCO coated conductors

Abstract Metalorganic deposition process using trifluoroacetate (TFA-MOD) processed YBCO film on LaAlO 3 and IBAD (YSZ/Hastelloy) substrate exhibited high critical current density, J c , at 77.3 K. The obtained J c values at 0 T were 7 and 1.7 MA/cm 2 for LaAlO 3 and CeO 2 buffered IBAD, respectively. High J c values over 10 5 A/cm 2 were maintained up to 4 T ( B ∥ c -axis) for the LaAlO 3 substrate and up to 3 T ( B ∥ c -axis) for the CeO 2 buffered IBAD substrate. These results imply that TFA-MOD with simple fabrication process is very promising for the practical YBCO coated conductor.

Microstructure of TFA-MOD superconductor film on metal tape

Superconductor film of YBa2Cu3O7� d having high-Jc values was formed on a CeO2/YSZ buffered metal tape by the metal-organic deposition using trifluoroacetates (TFA-MOD) method. Transmission electron microscopy (TEM) confirmed the presence of microstructures of multi-layered structure of YBCO/CeO2/YSZ/Ni-based alloy. The thickness of each thin film YBCO, CeO2 and YSZ, was estimated to be 0.2, 0.5 and 1.3 lm, respectively. Both the film of YSZ on polycrystalline and the film of CeO2 on the YSZ film were composed of columnar grains aligned h 100 i direction. The columnar structure of the CeO2 film was strongly correlated to that of the structure of YSZ film. While highly oriented YBCO film was formed on the CeO2 film, it was not dense and had a rough surface. TEM-EDS revealed copper oxide in the film of YBCO and secondary phase produced from YBCO and CeO2 (BaCeO3) between the two layers. 2002 Elsevier Science B.V. All rights reserved.

Biaxially Oriented Tl-1223 Film With High Jc on {100} Textured Ag Tape

Biaxial crystal orientation on metal substrate is requested to achiev high Jc superconducting wires of Tl-1223 operating in liquid nitrogen. We were successful to obtained textured Ag tapes, as substrates for Tl-1223 films. About 90% of Ag grains were aligned in {100} direction in the Ag tapes. Tl-1223 films prepared by a spray pyrolysis method on the textured Ag tapes have biaxial crystal orientations with the Jc of 10,000 A/cm2 at 77K, 1T. The magnetic field dependence of the films was much improved, suggesting that the grain boundary weal-link problem was overcome.

Carbon expelling scheme and required conditions for obtaining high-J/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-x/ film by metalorganic deposition using trifluoroacetates

High critical current density (J/sub c/) YBa/sub 2/Cu/sub 3/O/sub 7-x/ films were routinely prepared by metalorganic deposition using trifluoroacetates (TFA-MOD). Although the growth scheme of the YBa/sub 2/Cu/sub 3/O/sub 7-x/ films has been studied, we still have two or more unsolved problems during heat-treatments. The first problem is chemical reaction, including the carbon-expelling scheme during the calcining process. Considering Gibbs free energy changes in the related chemical reactions and electronegativity of atoms, we explain the expelling scheme. During the calcining process metal-oxygen bonds remain after the decomposition. The fluorine partially attacks and replaces oxygen linked to Y and Ba. Consequently, these metal compounds become an amorphous matrix of metal oxyfluorides. Only CuO grows to be nanocrystalline. These results are consistent with the present precursor films derived from highly purified coating solution by the Solvent-Into-Gel method. The second problem is clarification of the required conditions to obtain high J/sub c/ YBCO film by TFA-MOD. Transmission electron microscopy revealed the existence of isolated Ba and Y oxides besides CuO grains within the YBa/sub 2/Cu/sub 3/O/sub 7-x/ layer, which are derived from the normally purified coating solution. Optimal calcining processing, which certainly suppresses only the CuO grains, never improves the J/sub c/ of the films containing Ba and Y oxides. Thus the required condition for high J/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-x/ films is a combination of a highly purified coating solution and an optimized calcining process. With that combination, we prepared 210 - 230 nm thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ films on CeO/sub 2/ buffered YSZ for which the J/sub c/s were 10.5 - 11.4 MA/cm/sup 2/ (77 K, 0 T) by four-probe transport measurement.

Magnetic Field Dependence of Critical Current Density of Polycrystalline Tl-2223 Wire

The magnetic field dependence of critical current density Jc for polycrystalline Tl-2223 tape-shaped wire was examined in terms of a model regarding a sample as a bulk which consists of an anisotropic superconductor with an incomplete crystal alignment. We assumed that the critical current density in each grain would be governed only by the perpendicular component of magnetic field to the a-b plane, especially in a high magnetic field. In a low field, we also assumed that the grain boundary weak link would be dominant in terms of Jc. It was found that the magnetic field dependency of Jc in high fields was well described by the present calculation in spite of an inconsistency in low magnetic fields. It was predicted that a practical Jc value of more than 10,000 A/cm2 in a magnetic field above 5T might be possible with a crystal alignment factor of standard deviation σ below 0.5, assuming the crystal alignment in the wire to be of Gaussian distribution.