Toshiya Doi

Critical current density of MgB2 thin film with pinning centres introduced by deposition in oxygen atmosphere

Critical current densities Jc of MgB2 thin films have been measured; these were prepared by an electron beam evaporation method without any post-annealing. In order to improve the characteristics of Jc in high magnetic fields, a MgB2 thin film was deposited in an O2 atmosphere. The values of Jc of the O2-doped film have exceeded those of the non-doped film in magnetic fields applied perpendicular and also parallel to the c-axis. The peak of Jc caused by the c-axis correlated pinning centres has been observed in the angular dependence of Jc. The upper critical field Bc2 and the vortex glass–liquid transition field Bg have been estimated from temperature dependences of resistivity curves in both directions of the magnetic fields perpendicular and parallel to the c-axis. It is found that Bc2 and Bg are increased by the introduction of O2 in both the directions of the applied field. This fact indicates that effective pinning centres have been introduced by the deposition of the MgB2 thin film in an O2 atmosphere.

As-grown superconducting MgB/sub 2/ films prepared by electron beam deposition

As-grown superconducting MgB/sub 2/ thin films were prepared on Al/sub 2/O/sub 3/ [001] single crystals by electron beam deposition technique by changing the substrate temperature, the flux rate ratio of Mg/B and the basal pressure before the deposition starting. Higher Mg deposition rate and lower basal pressure gave a lower normal state resistivity of the MgB/sub 2/ thin films and gave a higher critical temperature. Magnetic hysteresis of the MgB/sub 2/ film prepared under lower basal pressure was smaller than that of the film prepared under higher basal pressure. Although the critical temperature of the MgB/sub 2/ film prepared under higher basal pressure became lower, the critical current density was enhanced. Higher basal pressure introduced some pinning centers in to MgB/sub 2/ films. These results suggest that precision control of the atmosphere during the MgB/sub 2/ deposition is very important for obtaining the high critical current density.

Relationship between microstructure and Jc property in MgB2/α-Al2O3 film fabricated by in situ electron beam evaporation

A transmission electron microscopy (TEM) study has been carried out on an MgB2/?-Al2O3 film that exhibits the typical property of critical current density (Jc) under magnetic fields. The MgB2 layer of 300?nm in thickness was grown on a (001)?-Al2O3 substrate using an in situ electron beam evaporation method. Jc of the film takes significantly high values when the applied magnetic field is perpendicular to the film surface. The MgB2 layer consists of fine columnar MgB2 crystals 20?30?nm in size. The columnar MgB2 crystals grow almost perpendicular to the substrate surface and have no crystallographic orientation relationship with the ?-Al2O3 substrate because of an amorphous layer formed first on the substrate. A high density of columnar grain boundaries within the MgB2 layer may be effective for the enhancement of the flux-pinning under the perpendicular magnetic field.

Thermal stability of nanometer-sized NiO and Sm-doped ceria powders

A fine NiO powder and a Sm-doped ceria powder with a composition of Ce 0 . 8 Sm 0 , 2 O 1 . 9 were synthesized by heating the oxalate precursors at 300-1200 °C in air to produce a cermet (anode material) for solid oxide fuel cell. A 0.2 M Ni(NO 3 ) 2 solution and a 0.2 M Ce(NO 3 ) 3 -Sm(NO 3 ) 3 solution were mixed with 0.4 M oxalate solution, respectively, to produce the oxalate precursors. Only the cubic phase of Ce 0 . 8 Sm 0 . 2 O 1 . 9 was formed in the calcined powders from the Sm-doped cerium oxalate. However, the mixed phases of NiO and Ni were produced in the NiO precursor after the calcination at 300-600 °C. At higher temperatures, only NiO was detected. The primary particle sizes, which were determined from the Brunauer-Emmett-Teller analysis surface areas, were 60 nm for NiO and 10 nm for Ru/Sm-doped ceria (SDC) after the heat treatment at 400 °C. The oxalate precursors of SDC and NiO provided 433 and 259 kJ/mol of the activation energy, respectively, for sintering/grain growth in the temperature range from 600 to 1200 °C. As-produced SDC precursor formed platelike secondary particles of 0.5-2-μm length by the heating at 800 °C. Heating of Ni oxalate at 800 °C produced isotropic fine NiO secondary particles of 0.5-2-μm sizes.

Flux Pumping Effect of HTS Films in a Traveling Magnetic Field

This paper reports the possibility of High Temperature Superconductor (HTS) magnetic flux pump for the compensation of slight current decay in persistent mode HTS magnet. As a first step, the linear type magnetic flux pump (LTMFP) is fabricated with the use of the wound BSCCO tapes, and then the preliminary pumping test is carried out by using NbTi load coil in liquid helium. Further, the current transport property of the produced MgB2 thin films is measured, and then the fabrication of the LTMFP has been completed for the purpose of the higher performance as well as higher temperature (e.g., 20 K) operation.

Properties of MgB/sub 2/ films with very high transport critical current densities

Magnesium diboride, MgB/sub 2/, thin films are fabricated through two different kinds of process. One is an in-situ process by using electron beam deposition and the other is an ex-situ process by using the combination of pulsed laser deposition and heat treatment. The critical current density, J/sub c/, is investigated as a function of external magnetic field in the range of 0-7 T and/or temperatures ranging from 4.2 K to the critical temperature by using dc 4-probe transport method. The in-situ processed film shows very high J/sub c/, e.g., 7.1/spl times/10/sup 6/, 1.2/spl times/10/sup 6/, and 1.4/spl times/10/sup 5/ A/cm/sup 2/ in 0, 4, and 7 T (perpendicular fields), respectively. Angular dependence of J/sub c/ is much different between the two kinds of films. J/sub c/-angle (magnetic field) curves of the in-situ processed film show two peaks; one is around the field perpendicular to the film surface and the other is around the parallel field. On the contrary, the curves of the ex-situ processed film have only one peak around the parallel field. Microstructure analyses show that the in-situ film has columnar grains aligned perpendicular to the film surface and that the ex-situ one has granular grains with random orientations. These results indicate that the grain boundaries between columnar grains act as effective pinning centers and enhance J/sub c/ in the perpendicular fields.

Fabrication of YBa2Cu3O7 thin film on cube-textured Cu tape

We achieved a biaxially oriented YBa2Cu3O7 film on a {100} ⟨001⟩ textured Cu tape with Jc of 1.9×106u2002Au2009cm−2 at 77 K. A Ni overlayer was deposited by electroplating technique on the {100} ⟨001⟩ textured Cu tape to improve oxidation resistance of Cu substrate, and then YBa2Cu3O7 was deposited on the CeO2/YSZ/CeO2 buffered Ni-electroplated Cu tape. We confirmed that the Ni-electroplated Cu tape lost its ferromagnetism after 700u2009°C at 1 h heat treatment. Our rolling assisted biaxially textured substrate (RABiTS) type coated conductor can be used for ac applications and is thought to be more cost feasible than the standard RABiTS architecture using Ni-alloy tapes.

High-temperature and high-field performance of MgB2 films with Jc of 106 A cm−2 (4.2 K, 4 T)

MgB2 thin films were fabricated on sapphire substrate by using electron beam deposition and without any post-treatment. The films have columnar grains aligned perpendicular to the film surface. Superconducting properties were investigated in magnetic fields, B, in the range 0–28 T and/or at temperatures, T, ranging from 4.2 K to the critical temperature, Tc, by using the DC four-probe transport method. Tc-values of the films are 33.12–33.90 K in self-field. The films show very high Jc at 4.2 K, e.g., 4.7–7.1 × 106 A cm−2 in self-field, and 0.8–1.1 and 0.3–0.4 × 106 A cm−2 in 4 T of the perpendicular and the parallel fields, respectively. At 4.2 K, the films carry 0.4–1.0 and 0.8 × 105 A cm−2 even in a perpendicular field of 8 T and the parallel fields, respectively. Jc exceeds 105 A cm−2 at 27 K in 1 T and 15–17 K even in 5 T of the perpendicular fields. The upper critical fields, Bc2 and irreversibility fields, Birr, at 4.2 K are 27.6–27.9 T and 16.7–19.2 T in the parallel fields, and 17.5–20.4 T and 10.7–13.4 T in the perpendicular fields, respectively. The strong pinning at grain boundaries between columnar grains is confirmed at high temperatures and below 7–8 T at 4.2 K.

Preparation of Y1Ba2Cu3Ox superconducting tape formed on silver substrate by chemical vapor deposition technique

Abstract Ag(1xa01xa00) single crystalline substrate and Ag{1xa01xa00} 〈1 1 0〉 textured tape were investigated as one of the functional substrate, on which YBCO film with high J c property could be formed directly. J c of YBCO film deposited directly on Ag(1xa01xa00) substrate was 10 4 A/cm 2 orders of magnitude, which was as same as one on Ag non-textured substrate. It was considered that J c was limited by the interfacial reaction between YBCO and Ag, especially Cu diffusion into Ag substrate during the YBCO deposition. However, J c was improved up to 10 5 A/cm 2 orders of magnitude by the pre-deposition of Cu-rich YBCO layer or Cu on Ag(1xa01xa00) substrate and Ag{1xa01xa00} 〈1 1 0〉 tape. This result means it is important to control the interfacial reaction between YBCO and Ag to obtain high J c by the direct deposition of YBCO film on Ag substrate.

Biaxially oriented NdBa/sub 2/Cu/sub 3/O/sub 7/ films prepared on {100} textured Ag tapes without any buffer layers

We successfully obtained biaxially oriented NdBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films with the T/sub c/ of 89 K on {100} textured Ag tapes without any buffer layers. The lower substrate temperature seems to be favorable for the better biaxial orientation of the NdBa/sub 2/Cu/sub 3/O/sub 7/ grains; the film deposited at 650/spl deg/C had an excellent biaxial crystal orientation, while on the textured Ag tape heated at 720/spl deg/C, NdBa/sub 2/Cu/sub 3/O/sub 7/ phase grew in two orientations, 45/spl deg/ apart.