high-T/sub C/ Superconducting Research Articles

Advances in HTS films for high power microwave applications

This paper describes recent work at DuPont to extend the high power microwave performance of YBa/sub 2/Cu/sub 3/O/sub 7/ and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ high-T/sub c/ superconducting (HTS) films. The YBa/sub 2/Cu/sub 3/O/sub 7/ films grown by in-situ off-axis magnetron sputtering using are produced by in-situ off-axis sputtering with radiant substrate heating. The Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films are produced by off-axis sputtering followed by annealing in oxygen and Tl/sub 2/O vapor. The microwave surface resistance and power handling of these films has been measured over a wide range of temperature (20 K to 100 K) using a HTS-sapphire resonator technique. At 70 K, the surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7/ films remains below 65 /spl mu//spl Omega/ (@5.56 GHz) with microwave surface current densities up to 5/spl times/10/sup 6/ A/cm/sup 2/. At 70 K, the surface resistance of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films remains below 200 /spl mu//spl Omega/ (@8.1 GHz) with microwave current densities up to 2/spl times/10/sup 6/ A/cm/sup 2/.

High-T/sub c/ superconductor-normal-superconductor junctions with polyimide-passivated ambient-temperature edge formation

Superconductor-Normal-Superconductor (SNS) edge Josephson Junctions (JJs) continue to be a focus of high-T/sub c/ superconductor (HTS) electronics development. The ability to form clean edges in HTS materials is essential for successful SNS edge JJs as well as crossovers and via contacts. We report a novel SNS JJ fabrication process in which a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///SrTiO/sub 3/ base electrode/insulator bilayer coated with a polyimide passivation layer is rotated during Ar ion milling through a reflowed photoresist mask to form omnidirectional edges. No sample cooling is used during milling. YBa/sub 2/Cu/sub 2.8/Co/sub 0.2/O/sub 7-/spl delta// normal layers and YLa/sub 0.05/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// counter-electrodes completed the SNS JJ's. Devices fabricated using this polyimide passivation layer have significantly smaller spreads in critical current and resistance than those processed without it.

Development of a 1 kWh-class module-type SMES-design study

The authors are planning to build a 1 kWh/1 MW (maximum stored energy/maximum power capability) module-type SMES (named ESK; experimental SMES of Kyushu Electric Power) as a first step towards the realization of practical SMESs for power line control. The main points of the design are those of: module-type coils for the development of SMES capacity scale-up; the choice of low loss stranded cables for reducing pulse operating loss; the choices of modified D shape coils and the reduction of stresses in the conductor-which become more serious in scaling-up and high-T/sub c/ superconductor (HTSC) current leads for covering weak points due to thermal loss in a module-type SMES which need many current leads. Some other points are also studied such as the design of the cooling system in which a single coil quench does not induce that of others, and harmonics suppression in the SMES power converter system.

Numerical evaluation of magnetization properties of a bulk HTSC with a hole

Field-cooled and pulsed field magnetizations of a bulk high T/sub c/ superconductor (HTSC) with a hole are evaluated by using thin plate multi layers numerical codes. Dynamic effects of the pulsed field magnetization is analyzed with the flux creep-flow model. Merits and demerits of the HTSC with and without a hole are discussed from their practical applications as a trapped field magnet.

Conceptual design of an electrical machine with both low and high T/sub c/ superconductors

The emergence of high T/sub c/ superconducting (HTS) wires with large critical current densities allows the design and construction of many applications, for example, rotating electrical machines. Nevertheless, the current "state of the art" of Bi "powder in tube" wires, the most advanced HTS conductor, is not adequate for an ac operation. We thus propose a fully superconducting synchronous machine with an NbTi stationary 4 K armature and a rotating Bi-2223 field winding at 20 or 40 K. Thanks to the armature cryogenic environment, the cooling of the Bi coils is very simple. The conceptual design of a typical machine is suggested and several electromagnetic designs are presented. The first one deals with a Bi field winding adapted to an existing NbTi armature of a tenth of kYA. Afterwards, large torque motors are theoretically designed comparing the high (at 20 and 40 K) and low T/sub c/ field winding solutions. The results are comparable for NbTi and Bi at 20 K. But with a Bi field winding operating at 40 K, the performances are degraded. Otherwise, at 4 or 20 K fully superconducting motors show high performances in terms of weight, volume, and efficiency. Nevertheless, the Bi field windings require a large amount of material indicating that improvements in critical current density are needed.

Minimal performances of high Tc wires for cost effective SMES compared with low Tc's

On the basis of a 22MJ/10MVA unit without stray field, the authors determine minimal performances for high T/sub c/ superconducting (HTS) wires, in order to obtain HTS superconducting magnetic energy storage (SMES) competitive compared with low T/sub c/ superconducting (LTS) ones. The cost equation mainly considers the wire volume, the fabrication process and losses. They then recommend HTS critical current densities and operating magnetic fields close to the present state-of-the-art for short samples. A 30% gain for HTS SMES compared with LTS one could be expected.

Numerical analysis of magnetization of a bulk high T/sub c/ superconductor

Magnetization of a bulk high T/sub c/ superconductor (HTSC) is evaluated with the numerical simulation codes. The field-cooled magnetization of the HTSC is analyzed by using the critical state model. The pulse magnetization process is also analyzed by using the flux creep-flow model. The numerical solutions are discussed to clarify the time dependent effects in the reported experimental results on the pulse magnetization.

Electromagnetics in high-T/sub c/ superconductors

The behavior of electromagnetic fields in high-T/sub c/ superconductors (HTSs) is studied in order to examine their effects in classical electromagnetic boundary value problems. It is shown that an HTS can not be simply treated as a low loss conductor and boundary conditions of HTSs cannot be considered as perfect conducting boundaries like conventional treatments. The electromagnetics of HTS are investigated in terms of complex conductivity, surface impedance with applied magnetic fields, and computational electrodynamics using the proposed model.

Impact of a normally conducting surface layer on attenuation in high-T/sub c/ superconducting microstrip transmission lines

A two-dimensional (2-D) model is adopted to characterize high-T/sub c/ superconducting (HTS) microstrip transmission lines with a normally conducting surface layer. The model is verified by comparison with a full-wave analysis calculation for identical all-superconducting lines and by calculation of the current distribution and propagation properties for a normal conductor version of the microstrip. Estimates are obtained for the transmission line attenuation constant versus thickness and conductivity of the resistive surface layer. The results suggest that in situ deposition of a normal metal overlayer on the HTS film surface may be a viable technique to secure the long-term performance of microwave devices.

Circuit design for a high-T/sub c/ Josephson sampler

We propose a high-speed and high-sensitivity sampler circuit using high T/sub c/ superconducting (HTS) Josephson junctions. This circuit consists of five non-latching junctions, superconducting lines and a superconducting ground-plane. A main feature of this circuit is a non-latching comparator junction whose switching generates a circulating current in a superconducting loop. Computer simulation and prototype layout were performed to study its ability and feasibility.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A high gain YBCO antenna array with integrated feed and balun

A supergain antenna array of closely spaced printed dipoles made of thick film YBCO was designed and experimentally tested. An optimization of the directive gain with respect to interelement spacing, dipole length, and other circuit parameters, subject to constraints in array size, was performed by combining the spectral domain method of moments and transmission line theory. The design procedure accounts far losses associated with the strip material, both at the feed and the radiating dipoles, and power dissipation in the substrate. A practical design of a sixteen element array is demonstrated, with an integrated feeding network and a balun, also made of high-T/sub c/ superconductors (HTS). Performance is compared against simulated data. The superconducting array has gain significantly higher than that of a similar supercooled silver array albeit at the expense of transmission bandwidth. It is shown that supergain is practically achievable by miniature, printed HTS dipole arrays.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Numerical analyses of trapped field magnet and stable levitation region of HTSC

Stable levitation with a permanent magnet and a bulk high T/sub c/ superconductor (HTSC) is examined numerically by using the critical state model and the frozen field model. Differences between a permanent magnet and a trapped field magnet are discussed in terms of the levitation force property. The stable levitation region of the HTSC on a ring magnet and on a solenoid coil are calculated using numerical methods. The obtained results are discussed from the point of view of differences in magnetic field configuration.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Supergain printed arrays of closely spaced dipoles made of thick film high-Tc superconductors

Closely spaced dipoles in an array configuration, if properly excited, may result in end-fire superdirectivity. Albeit at the expense of transmission bandwidth and radiation efficiency. Efficiency of superdirective arrays can be considerably improved if a high-T/sub c/ superconductor (HTS) is used in the realisation of the radiating elements and the feeding and matching network, resulting in practical supergain radiators. Basic feeding configurations, which in printed dipole arrays may induce superdirective excitation, an explored and characterised in terms of their efficiency and bandwidth. Superdirective and supergain array designs, operating at frequencies around 1 GHz, made of thick film YBa/sub 2/Cu/sub 3/O/sub 7-x/ on yttria stabilised zirconia (YSZ) substrates are demonstrated. Their measured end-fire gains exceeded that of similar cryogenically cooled thick film Ag arrays by 3-6 dB.< >

Degradation of the holmium—barium—copper oxide superconductor, HoBa2Cu3O6.92, under the action of H2O and D2O: a study by the radiothermoluminescence method

Information about the existence of impurities and defects on the surfaces of high-T{sub c} superconductors (HTSC) can be obtained by radiothermoluminescence (RTL). The degradation of HTSC materials results from their interaction with water and CO{sub 2}. It has been established that the intensity of RTL is very insignificant for freshly prepared HTSC or for samples stored in a dry atmosphere. However, a large increase in RTL intensity occurs if specimens are in contact with water. Barium hydroxide, carbonate, and oxides make the largest contribution to RTL of superconducting ceramics with degraded surfaces. The RTL curves for Ba(OH){sub 2} and BaCO{sub 3} differ slightly from each other, which makes it difficult to estimate the contribution of each of them to the luminescence observed.

Slotline ring resonator test method for high-T/sub c/ superconducting films

A novel method of testing high-T/sub c/ superconducting (HTS) films by means of a slotline ring-like resonator is presented. This method does not require any metallic contact with the HTS films and ensures that the tests have quick turnaround times. Small samples can be tested very easily. The resonator circuit used has been studied analytically using Sonnet Inc.'s em software. The parameters of the HTS films are obtained by fitting the theoretical results to the experimental ones. YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films on LaAlO/sub 3/ have been tested and the theoretical and experimental results are presented and compared. The results match very well. This method is useful for quick screening HTS films since no patterning of the film is required. >

Resistive fault current limiters with HTSC-measurements and simulation

The results reported concern polycrystalline Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub x/ and Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ bulk material. The transition between superconducting and normal-conducting state of YBCO and BSCCO high-T/sub C/ superconductors (HTSCs) was investigated experimentally. By using these results, simulations of the behavior of HTSC current limiters in electrical power networks were carried out. Assuming a homogeneous conductor, the self-quenching initialized by exceeding the critical current is calculated for conductors with different critical current densities. Questions of inhomogeneity along the conductor, current pulse triggering, and transient overvoltages are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nanosecond inductive energy storage pulsed power system using YBCO superconductor opening switches

An inductive energy storage pulsed power system (IESPPS) using high T/sub c/ superconductor (HTS) opening switches, made of YBCO films, was successfully demonstrated. Nanosecond electrical pulses with a pulse compression ratio of 10 and a power gain of more than 40 were produced.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-T/sub c/-superconductor-sapphire microwave resonator with extremely high Q-values up to 90 K

A theoretical analysis was performed for an extremely high-Q resonator formed by a sapphire rod sandwiched by a pair of high-T/sub c/ superconductor (HTS) films. A number of these HTS-sapphire-HTS resonators at C-band and Ka-band were designed, fabricated, and tested. At 5.552 GHz, Q/sub 0/ reached 2*10/sup 6/ at 90 K, 3*10/sup 6/ at 80 K and 1.4*10/sup 7/ at 4.2 K with circulating power up to 500 kW. Formulas for calculating the resonant frequency and Q-value derived from the theoretical analysis were verified by experimental data with good agreement. Three different thin-film HTS materials-Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/, YBa/sub 2/Cu/sub 3/O/sub 7- delta / and Tl/sub 0.5/Pb/sub 0.5/Sr/sub 2/CaCu/sub 2/O/sub 7/-were tested. The sensitivity of the high Q-value and the parasitic coupling to the case modes are discussed. Applications such as frequency stabilized oscillators, filters, and HTS film characterization are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microstrip resonators and filters using high-T/sub c/ superconducting thin films on LaAlO/sub 3/

Very low microwave losses have been measured in YBa/sub 2/Cu/sub 3/O/sub 7/ linear resonators, ring resonators, and bandpass filters. The stoichiometric 1-2-3 compound was deposited on LaAlO/sub 3/ substrates, patterned microwave circuits, and overcoated with a passivating LaAlO/sub 3/ layer. HTS (high T/sub c/ superconductor) linear microstrip resonators demonstrated Q values greater than 1200 at 10 GHz, corresponding to surface resistances less than 300 mu Omega . Identical silver resonators showed Q values of 60 and surface resistance of 15 m Omega . The high-frequency transition temperature for these HTS films was greater than 83 K. Q values of 240 were measured in ring resonators at 15 GHz. Finally, the authors designed, fabricated, and tested a two-pole, Chebyshev narrow-bandwidth bandpass filter. The HTS filter was designed to be a 1% bandwidth with 0.2-dB ripple. Insertion loss was 2 dB at 4 K, rising to 3 dB at 77 K. The authors observed a temperature dependence in the filter center frequency which they attribute to kinetic inductance effects. The proposed HTS filters outperform similar cryogenic silver filters, indicating that practical levels of HTS performance have been achieved.

As-deposited high T/sub c/ superconducting thin films on ITO-coated glass

High-T/sub c/ superconducting (HTSC) thin films deposited on a glass substrate coated with an oxide buffer layer have been studied without subsequent postanneal. The structure (HTSC/ITO/glass) came to be composed entirely of oxide materials. The deposition of the superconducting films was carried out by either excimer laser ablation or sputtering below the substrate temperature of 750 degrees C. The ITO buffer layer inhibits interdiffusion between YBaCuO and SiO2 and does not cause any cracks. The YBaCuO thin films were grown on ITO-coated glass oriented with the c-axis. The zero-resistivity transition temperature (T/sub c/) reached 80 K. ITO is considered a unique candidate for the stabilizer because of its transparence and electrically conductive properties. The structure of the superconducting film on ITO should be useful in industrial fabrication of superconducting plates, coils, and optoelectronic devices.