Katsumi Ohata

Partial oxygen pressure effects on the morphology of Y‐Ba‐Cu‐O thin films in laser deposition process

The characterization of Y‐Ba‐Cu‐O thin films prepared by 193‐nm‐ArF laser deposition at partial oxygen pressures ranging wide from 0.01 mTorr to 5 Torr has been investigated. These films have been grown on (100) SrTiO3 substrates held at 700 °C. Surface morphology, superconducting transition temperature and crystalline orientation strongly depend on a partial oxygen pressure during deposition. The smooth and highly crystal‐oriented films with the superconducting transition temperature above 80 K and transport critical current densities of 1.0×105 A/cm2 (without magnetic field at 77 K) have been obtained at 10 to 50 mTorr of oxygen.

Superconductivity and crystallinity of Ba2Y1Cu3O7−δ thin films prepared by pulsed laser deposition with substrate bias voltage

Substrate bias voltages were found to be significantly effective in preparing high‐quality laser‐deposited superconducting Ba2Y1Cu3O7−δ films at reduced substrate temperatures. The zero‐resistance temperature of the biased films, positive bias being more effective than negative, decreased very slightly when the substrate temperature was lowered, whereas that of the unbiased films decreased considerably. In addition, the surface morphology and c‐axis orientation have been improved by applying substrate bias voltages. Bias voltages within ±500 V hardly affect the composition of the resulting films so that stoichiometric films have been obtained from a stoichiometric target. A time‐resolved optical observation revealed that a short time emission, probably being oxygen plasma, occurred in a few μs after the laser pulse impingement. The improvement in crystallinity of the resulting films is attributed to this emission. The velocity of emissive species in the plume was determined to be 6×105 cm/s. Under positiv...

Generation of 23.4 T using two Bi-2212 insert coils

Development of a 1 GHz superconducting NMR magnet is in progress at the Tsukuba Magnet Laboratory (TML) of the National Research Institute for Metals (NRIM). This magnet will contain a BSCCO inner coil, which should generate a central field of 23.5 T in a backup field of 21.1 T. In order to accomplish this targeted field, we fabricated two Bi-2212 double-pancake coils (Coil A and Coil B). They were installed in the high-field superconducting magnet system at the TML/NRIM. Their performance was measured in a backup field of 18 T. Coil A was made of 20 double-pancakes wound with Ag sheathed Bi-2212 tape conductors. Ag-Mg tape was co-wound for mechanical support. Its winding was 147 mm in outer diameter and 220 mm in height. It generated a central field of 21.4 T in a clear bore of 61 mm. Coil B was located inside Coil A. Its 6 double-pancakes were wound with Bi-2212 tape conductors reinforced with Ag-Mg-Ni alloy sheath. The outer diameter and height of the winding were 48 mm and 63 mm, respectively. Coil B generated the highest field of 23.4 T in a backup field of 21.4 T. This study confirmed that the present performance of the Bi-2212 coils had already satisfied the required conditions for the inner coil of the 1 GHz NMR magnet from the viewpoint of high-field generation.

Effect of an Oxygen Pressure on Crystal Growth of a-Axis Oriented YBa2Cu3O7-x Films Prepared by Pulsed Laser Deposition

Abstract The effect of an oxygen partial pressure on the growth of a-axis oriented YBa2Cu3O7−x films has been investigated by cross-section transmission electron microscopy and scanning electron microscopy. The samples were deposited on SrTiO3 (100) substrates at 700°C and oxygen partial pressure of 20, 50 and 100 mTorr. Under 20 mTorr, a-axis and c-axis oriented domains equally grow over the whole substrate, resulting in the alternate structure of both with a separation of about 150 A in TEM images. The alternate structure continues from the substrate to the film surface. At 50 and 100 mTorr, the a-axis oriented domain becomes dominant above a thickness of 40 and 100 A, respectively. A disordered layer is observed between the a-axis oriented domain and substrate for 50 mTorr and the c-axis oriented domain grows on the substrate for 100 mTorr. The surface morphology is improved with decreasing the c-axis oriented domain.

Effects of thermal history during LEC growth on behavior of excess arsenic in semi-insulating GaAs

Abstract The effects of the thermal history during the LEC growth of a semi-insulating GaAs crystal are discussed from the viewpoint of changes of the behavior of the excess arsenic dissolved in the GaAs crystal. Near the melting point, solid-solution of excess arsenic is observed in the crystal. In the B2O3 layer, both the formation of EL2 and the generation of scattering centers occur, and the two phenomena are competitive. In the ambient gas, the homogeneous nucleation of arsenic occurs.

Deposition Pressure Effects on the Laser Plume of YBa2Cu3O7-δ

The control of the preferential orientation in oxide superconducting films, which is generally done by deposition temperatures, was achieved by choosing the oxygen pressures during the deposition. Time-resolved optical observations of the laser plume were carried out at the same time with the film deposition. Upon increasing the oxygen pressure, the velocities of the emissive species ablated from the target were decreased. The kinetic energy effects of the ablated species on the film orientation are discussed, and the orientation change can be qualitatively understood by a surface migration model as well as the case of the deposition temperature dependence.

Effect of ODS-Cu Composition for Mechanical-Electromagnetic Property of Bronze-Processed

The oxide dispersion strengthened copper (ODS-Cu) was composed to Cu stabilizer in Nb3Sn wires with/without Ta-reinforced filaments to improve the mechanical-electromagnetic property. The wires are 1-mm diameter and heat treated at 650 C for 200 hours. The critical current as a function of the uniaxial tensile strain/stress was explored at 4.2 K, in magnetic fields up to 18 T. The stress/strain sensitivity of the critical current is discussed from a large stress/strain application point of view.

{\rm Nb}_{3}{\rm Sn}

A new Bi-2212/Ag round-shaped wire with tape-shaped multifilaments has been successfully developed. The wire includes 126-960 tape-shaped filaments with triple rotation symmetry, having a good crystal alignment in each filament. We refer to the new wire as ROSATwire, (ROtation-Symmetric Arranged Tape-in-tube wire). Since the ROSATwire structure yields complete symmetrical arrangement of the tape-shaped filaments, it eliminates the need for a rolling machine, but allows us to use a drawing or extrusion machine. We found that the present wire fabrication process markedly improves not only productivity and lowers cost, but also enhances the transport J/sub c/ of the Bi-21212/Ag wire. The I/sub c/ and J/sub c/ reached >340 A and 1000 A/mm/sup 2/ at 28 T and 4 K.

Superconducting Wire

Over 2 km-long ROSAT wires with round-shaped cross-section have been successfully fabricated to a prototype magnet having the same dimensions as the actual insert magnet of a 1 GHz-NMR spectrometer. 400 m-long ROSAT wires with rectangular shaped cross-section have also been manufactured without any degradation of the current carrying capacity through wire processing. Another prototype magnet has been fabricated with the rectangular ROSAT wire. For each magnet, the present study demonstrates the feasibility of processing both 1 m-long Bi-2212/Ag leads and ROSAT wire-NbTi wire joints at each end of the leads to the magnet construction.

A new symmetrical arrangement of tape-shaped multifilaments for Bi-2212/Ag round-shaped wire

Abstract We have developed a new type of Bi-2212/Ag round wire named rotation symmetric arranged tape-in-tube wire (ROSATwire). It displays excellent superconducting properties at low temperature. Our next step is to realize longer length ROSATwires for practical use. By combining the fabrication process of base structure ROSATwires with standard multi-filament wires, we have established a process for the fabrication of several-hundred meter class wires on a laboratory scale. 254 and 379 m long wires with Ag or Ag–Mg–Ni sheathing 1320 filaments showed an I c of over 400 A, an overall J c ( J e ) of 160 A/mm 2 , and a J c of 700 A/mm 2 . 2 km long Bi-2212/Ag round ROSATwires were also successfully fabricated due to a redesign of the wire structure.