Tae-Hyun Sung

Top-seeded melt growth of Y-Ba-Cu-O superconductor with multiseeding

Y-Ba-Cu-O superconductors were prepared by the top-seeded melt growth process with multiseeding technique. By using several seeds at the same time, large samples could be fabricated in a short time with simple heat treatment. However, the samples fabricated by the normal multiseeding technique show a rapid decrease of trapped magnetic field value across the grain boundaries because of the residual liquid layer. To remove the residual liquid layer, various types of multiseeding were newly suggested. The optimum result was obtained when the seeds were arranged without spacing. The individual grains were combined as a single domain and did not show any deterioration of magnetic properties at the boundary. The formation mechanism of a well-combined single domain by the multiseeding technique is discussed.

On measurement of optical band gap of chromium oxide films containing both amorphous and crystalline phases

Abstract A method to calculate the optical band-gap of thin films composed of both crystalline phase and amorphous phase is suggested. Chromium oxide films were taken for the optical band-gap measurement, and the results were compared with those obtained by the conventional method. There is discrepancy between the values calculated by two different methods. The discrepancy decreases as the deposition temperature increases to the point where crystalline peaks occur. It has been speculated that the simple Taucs relation for pure amorphous film would lead to an underestimation of the fundamental band-gap of the film composed of both amorphous and crystalline phases. A modified Taucs equation, δ=f1δc+(1−f1)δa, approach suggested by Krankenhagen et al. [J. Non-Cryst. Solids 198–200 (1996) 923] yields optical band-gap of the Cr2O3 film in the range of 4.7~5 eV when the deposition temperature was varied from room temperature to 300°C.

Multiseeding with (100)/(100) grain junctions in top-seeded melt growth processed YBCO superconductors

Multiseeding with (100)/(100) grain junctions of top-seeded melt growth (TSMG) processed YBCO superconductors was studied. Multiple seeding shortened the processing time for the fabrication of TSMG-processed YBCO superconductors. The relationship among the number of seeds, the levitation forces and the trapped magnetic fields of the TSMG-processed YBCO samples is reported. The characteristic of the (100)/(100) grain junction is discussed in terms of a wetting angle of a melt.

Effects of the seed distance on the characteristics of the (100)/(100) junctions of top-seeded melt growth processed YBCO superconductors using two seeds

Abstract The effects of the distance ( d ) between two Sm 1.8 Ba 2.4 Cu 3.4 O x seeds on the characteristics of the (100)/(100) junctions of top-seeded melt growth (TSMG) processed YBCO superconductors were investigated. The trapped magnetic field at the grain junction and the levitation force of the top surface decreased with increasing d value. The degradation of the properties was discussed in terms of the presence of the residual melt-forming phases (CuO and BaCuO 2 ) at the grain junction.

Design and characteristics of a superconductor bearing

A high-temperature superconductor (HTS) journal bearing was designed for a flywheel energy storage system. A rotor was supported at top and bottom by two HTS bearings. The rotor weight is 4 kg and the length and diameter of shaft were about 300 mm and 40 mm, respectively. Both the top and bottom bearings have two permanent magnet (PM) rings with an iron pole piece separating them. Static properties of the bearing were calculated by frozen image method and verified by experiments. The vertical stiffness of the bearing was 38.9 N/mm. The rotational drag and vibration were measured up to 8000 rpm. The loss coefficient was linear up to 8000 rpm and the final value was 7.5/spl times/E-6. The average of vibration was about 10 micrometer (zero/spl I.bar/peak) except the resonance speed range. Rotation tests were conducted over the same speed range at several chamber pressures. Chamber pressure of 0.4 mtorr is sufficiently low to minimize windage loss.

Dissolution of seed crystals during top-seeded melt growth of

During top-seeded melt growth processing of (Y123), dissolution of (Sm123) seed crystals was observed at temperatures below the melting point of the seed material. The dissolution was dependent on factors such as the seeding method (cold seeding or hot seeding), the holding temperature and the composition of the seed and compact. Applying the hot seeding method, lowering the seeding temperature and using (Sm211) excessive Sm123 seed and (Y211) excessive Y123 compact decreased the dissolution rate of the seed crystal, resulting in a well-grown Y123 single crystal. The dissolution behaviour of the Sm123 seed was explained in terms of the solubility of samarium in the liquid and the dissolution kinetics at the seed/liquid interfaces.

Pyroelectric Properties of Sol-Gel Derived Lanthanum Modified Lead Titanate Thin Films

Lanthanum-modified lead titanate (PLT) thin films were fabricated on Pt/Ti/SiO2/Si substrate using the sol-gel method and rapid thermal annealing technique. The pyroelectric coefficient, and the infrared responsivity and the detectivity of the films were measured using static and dynamic methods, respectively. The ferroelectric properties were also measured. The pyroelectric coefficient increased from 1.3 to 2.5 nC/cm2 K with increasing La content. However, these low coefficients of PLT films are due to their crystal quality and orientations. The infrared response properties of PLT films depended on their thermal time constants. τ t (thermal time constant) for the PLT films decreased with the La contents increasing. The effects of back surface etching on the responsivity and the detectivity were also discussed.

The piezoresponse force microscopy investigation of self-polarization alignment in poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films

We report the self-polarization alignment without external poling in spin-coated poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), thin films on transparent and flexible substrates. Piezoresponse force microscopy (PFM) allows the quantitative analysis of preferentially aligned polarization in the ferroelectric thin films. We found that as-received P(VDF-TrFE) thin films on transparent poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) electrodes showed stronger self-polarization alignment than those on indium-tin oxide (ITO) electrodes. The relative ratios for the aligned polarization per unit volume on PEDOT:PSS and ITO electrodes were 18.6% and 4%, respectively.

Spin-coated ultrathin poly(vinylidene fluoride-co-trifluoroethylene) films for flexible and transparent electronics

We demonstrate spin-cast ultrathin poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), films on flexible and transparent substrates. Importantly, the PEDOT:PSS electrode allowed for the superior piezoresponse value (17.14 ± 2.37 pm V−1) and the low coercive voltage (1.76 ± 0.79 V) of the 20 nm thick P(VDF-TrFE) films. The use of PEDOT:PSS allows for the coherent interface between PEDOT:PSS and P(VDF-TrFE), the strong adhesion at the interface and no interfacial interaction when compared to that of indium–tin oxide (ITO).

Effect of resolidification nature of SmBa2Cu3O7−y seeds on the growth mode of YBa2Cu3O7−y

Abstract SmBa 2 Cu 3 O 7− y (Sm123) seeds dissolved partly or completely during top-seeded melt growth (TSMG) processing of YBa 2 Cu 3 O 7− y . It made a solid solution of (Y 1− x Sm x )Ba 2 Cu 3 O 7− y and (Y 1− x Sm x ) 2 BaCuO 5 around the seed/compact interfaces. The dissolved parts resolidified during peritectic cooling and its resolidification mode affected the growth mode of Y123 grains significantly. The relationship between the growth mode of Y123 grains of TSMG-processed YBCO superconductors and the resolidification nature of Sm123 seeds is reported.