Bernd Utz

Switching behavior of YBCO thin film conductors in resistive fault current limiters

The quenching process of YBCO thin film conductors designed for resistive fault current limiters has been analyzed by electrical and optical measurements. The influence of the switching voltage as well as the critical current and normal resistance of the thin film conductors on the quench propagation has been studied in detail. The experimental results show that homogeneous quenching can be achieved. The YBCO parameters can not be varied independently of the shunt layer thickness.

Test of a 1 kA superconducting fault current limiter for DC applications

A low voltage fault current limiter (FCL) having a nominal current of 1 kA has been set up using switching elements based on YBCO thin films fabricated by reactive thermal co-evaporation. The films showed critical current densities exceeding 1 MA/cm/sup 2/ @ 77 K. After patterning and contacting the films, the elements have been assembled in a closed cryostat for operation in a liquid nitrogen bath. The FCL model was successfully tested using prospective fault currents from 25 kA to 150 kA. The electric data show a peak current of 2.7 kA within 1 ms and a limitation to approximately nominal current within 5 ms. Due to this fast response of the YBCO switching elements, FCL coupled grids can be instantaneously decoupled during a fault, leaving the faultless part of the grid practically unimpaired.

Continuous YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// film deposition by optically controlled reactive thermal co-evaporation

We built a thermal co-evaporation system to deposit YBCO films on large areas in a continuous process which requires some basic modifications to the established standard co-evaporation setup. An optical rate control system based on atomic absorption spectroscopy has been developed to measure the individual metal fluxes, in addition the metal sources can be reloaded in-situ for long times. The performance of these new components is discussed and results of first YBCO films grown on small samples are shown.

Ba/sub 1-x/K/sub x/BiO/sub 3/ epitaxy on various substrate materials

We have fabricated Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) films by thermal co-evaporation of the metals K, Ba and Bi. To improve the epitaxy a high temperature BaBiO/sub 3/ seed layer is deposited prior to the BKBO film which results in T/sub c/ values up to 24 K on standard substrates. As BKBO grows well on perovskites we deposited thin PrBa/sub 2/Cu/sub 3/O/sub 7/ (PBCO) layers by thermal co-evaporation on MgO, CeO/sub 2/ buffered sapphire, YSZ buffered Si and YSZ substrates. BKBO films on these systems are epitaxial and show the same T/sub c/ values as on standard substrates. Thus, by using a PBCO buffer layer technical substrates are available for BKBO thin films.<<ETX>>

Investigation of buffer layers for YBCO coated conductors

Abstract Biaxially aligned, heteropitaxial oxide buffer layers and superconducting YBa 2 Cu 3 O 7−x (YBCO) thin films were deposited with PLD on Ni-based cube textured substrates. Critical current transport measurements in zero field showed j c values to 0.7 MA/cm 2 . FWHM of x-ray φ-scans of the buffer and the YBCO were about 9° and 11°, respectively. The oxide buffer was a bilayer, composed of CeO 2 , Y 2 O 3 , Y 2 O 3 or YSZ in various combinations and thickness. Since the Ni-diffusion barrier efficiency strongly depends on the buffer morphology detailed SEM and FIB (focused ion beam) studies of the different buffer systems were carried out. They revealed a columnar but dense growth characteristics in the buffer bilayers but also deficiencies at the substrate surface. These observations indicate that a smooth substrate surface without significant impurities is essential for YBCO coated conductors.

Very large area YBa2 Cu3O7-δ film deposition

Large area deposition of YBa2Cu3O7-(delta ) is desirable for cost-effective production of thin superconducting films at larger scale. Our technique of thermal co-evaporation should be particularly well suited for this goal because it is intrinsically homogenous. We have achieved large area deposition by using a rotating disk heater with an oxygen pocket. This arrangement allows for intermittent metal deposition and oxidation scheme with which we fabricate separated zones.Here we present a improved version of this deposition scheme with which we fabricate high quality YBCO films on an area which is 9 inches in diameter. The area is used for simultaneous deposition on smaller wafers, e.g. 12 wafers of 2 inches, or for large sapphire wafers of 8 inches.

Very large area YBa2 Cu3O7-delta film deposition

Large area deposition of YBa2Cu3O7-(delta ) is desirable for cost-effective production of thin superconducting films at larger scale. Our technique of thermal co-evaporation should be particularly well suited for this goal because it is intrinsically homogenous. We have achieved large area deposition by using a rotating disk heater with an oxygen pocket. This arrangement allows for intermittent metal deposition and oxidation scheme with which we fabricate separated zones.Here we present a improved version of this deposition scheme with which we fabricate high quality YBCO films on an area which is 9 inches in diameter. The area is used for simultaneous deposition on smaller wafers, e.g. 12 wafers of 2 inches, or for large sapphire wafers of 8 inches.

Very large area YBa 2 Cu 3 O 7-δ film deposition

Large area deposition of YBa2Cu3O7-(delta ) is desirable for cost-effective production of thin superconducting films at larger scale. Our technique of thermal co-evaporation should be particularly well suited for this goal because it is intrinsically homogenous. We have achieved large area deposition by using a rotating disk heater with an oxygen pocket. This arrangement allows for intermittent metal deposition and oxidation scheme with which we fabricate separated zones.Here we present a improved version of this deposition scheme with which we fabricate high quality YBCO films on an area which is 9 inches in diameter. The area is used for simultaneous deposition on smaller wafers, e.g. 12 wafers of 2 inches, or for large sapphire wafers of 8 inches.