D.C. Knoll

Synthesis and processing of (Hg,Pb)1Ba2Ca2Cu3Oy superconductors

Abstract Synthesis and processing of bulk (Hg,Pb)Ba2Ca2Cu3Oy (HgPb1223) superconductor using commercial BaCaCuO precursor powders is reported. HgPb1223 is shown to form over a wide range of temperatures, 750–880°C. The microstructure and intergrain connectivity of samples depends strongly on heat treatment and Hg-pressure during synthesis. Use of an external Hg-source during the reaction improves the density of the HgPb1223 products. The chemical nature of the external Hg-source changes the Hg-pressure and influences melting characteristics and grain growth. CaHgO2 is shown to be a particularly effective external Hg-source for producing dense HgPb1223 materials with improved grain growth and texture. Microstructure of HgPb1223 synthesized using CaHgO2 as the external Hg-source is characterized by large colonies of well-aligned superconducting grains. As-synthesized HgPb1223 samples were superconducting with Tc values in the range 132–134 K without post annealing in oxygen or argon. Phase purity and microstructural characteristics are correlated with magnetization measurements.

Quench studies on a layer-wound Bi2Sr2CaCu2Ox/AgX coil at 4.2 K

To evaluate the controlled quench behavior of high temperature superconducting (HTS) coils, particularly when using HTS coils in a hybrid configuration as an insert in a low temperature superconducting magnet, a layer-wound solenoid using Bi2Sr2CaCu2O wire was instrumented with several strip heaters to generate quenches in the axial and azimuthal directions. An array of distributed voltage taps and thermocouples were used to monitor the quench signals. Minimum quench energies (MQE) and quench propagation velocities (NZPVs) were determined. Results show that quench energies were moderate. NZPVs were slow but quench reaction times were of the same order as reaction times obtained at low quench energy densities in Nb3Sn coils.

AC loss measurement with a phase difference between current and applied magnetic field

Total AC losses in multi-filamentary, silver-sheathed Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (Bi-2223) tape were measured as functions of transport current, applied magnetic field and frequency by electromagnetic and calorimetric methods. Results of both methods agree well with each other. The calorimetric method was employed to investigate the effect of the phase difference between transport current and applied field, /spl Delta//spl phi/, on total AC loss. Total loss changes significantly with /spl Delta//spl phi/ when applied magnetic field is perpendicular to the surface of the sample. The effect of /spl Delta//spl phi/ on total loss in parallel magnetic applied field is smaller.

Electromagnetic and calorimetric measurements for AC losses of a YBa2Cu3O7−δ coated conductor with Ni-alloy substrate

The AC losses of a rolling-assisted-biaxially-textured-substrate (RABiTS™) processed YBa2Cu3O7−δ (YBCO) coated conductor (Goyal et al 1996 Appl. Supercond. 4 403–27) with Ni-alloy substrate and Cu stabilizer were measured at 77 K by both calorimetric (CM) and electromagnetic (EM) methods. In the CM method, improvements in the measurement were observed when a Cernox temperature sensor was used to measure the temperature rise on the sample instead of a differential thermocouple. The effect of heat transferred from current leads was taken into account to improve the accuracy of the CM results. In the EM method, the magnetization losses of the sample in a perpendicular applied field were measured by an in-plane pick-up coil. The calculation of the calibration factor C of the pick-up coil is discussed. Good agreement between AC loss results obtained from the CM and EM methods confirmed the validity of our measurements and calculations.

Dielectric design validation of a helium gas cooled superconducting DC power cable

Successful validation of the dielectric design of high temperature superconducting DC cables cooled with pressurized helium gas circulation is reported. The dielectric design features were developed based on the tests of several short sample cables. The features were adapted for the dielectric design of the cable and termination components such as high voltage bushings. A high voltage DC withstand test followed by AC partial discharge measurements on a 30-m long superconducting DC cable cooled with helium gas operating at 50-60 K showed that the dielectric design is suitable for the cable system.

Experimental and numerical studies of the effect of phase difference between transport current and perpendicular applied magnetic field on total ac loss in ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tape

The effect of phase difference between transport current and perpendicular applied magnetic field, Δφ, on total ac loss in multifilamentary, silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) superconducting tapes was calculated numerically using Brandt’s equation [Phys. Rev. B 54, 4246 (1996)] and the results were compared with experimental results obtained by the calorimetric method. In the numerical calculations, the superconducting transition was modeled by E-J characteristics represented by a power law. The dependence of critical current density on the n value was taken into account. The dependence of critical current density and n value on applied magnetic field was obtained from experimental data. It was observed that Δφ has a significant effect on the loss behavior of Bi-2223 tapes and depends strongly on the relative values of transport current and applied magnetic field. The effects of Δφ on the individual components of transport loss and magnetization loss were also calculated. Good agreement betwee...

Growth of Hg/sub 0.9/Re/sub 0.1/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ on a metallic substrate

The large-scale application of any high temperature superconductor will require a metallic sheath to satisfy the mechanical and thermal requirements of superconducting magnets. Thus, an important step in the development of a high temperature superconductor is to study the properties of the superconductor in contact with a metallic surface. Due to the highly corrosive atmosphere necessary for the synthesis of Hg/sub 0.9/Re/sub 0.1/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ ((Hg,Re)1223) the selection of a suitable metal substrate is critical. In this study, we investigate the synthesis of(Hg,Re)1223 samples on Ag, AgHg, Au, and Pt substrates. The samples were prepared by mixing precursor powders in a dry methanol solution which was then pipette dropped or centrifuge coated onto the metal substrates. The samples were encapsulated in quartz and annealed. Microstructural analysis of phase growth and grain alignment at the superconductor/metal interface were carried out on multiple length scales via x-ray diffraction, scanning electron microscopy, and x-ray microanalysis. Magnetic characterization has been carried out in a SQUID magnetometer.

Temperature Dependence of Total AC Loss in High-Temperature Superconducting Tapes

A versatile experimental facility was designed and set up to measure transport ac losses, magnetization ac losses, and total ac losses in high-temperature superconductors at variable temperatures. Several sets of measurements were carried out in the temperature range of 35 K to 100 K. Sample temperature during the measurements could be controlled within plusmn0.5 K of set temperature. Temperature dependence of transport losses reflects variation of critical current density of the tapes with temperature. Temperature dependence of magnetization losses exhibits an interesting behavior with a peak, whose position shifts to lower temperatures as the magnetic field is increased. Experimental data of ac losses at various temperatures are compared with those calculated using numerical methods. Generally, the simulated results reproduce well the experimental data.

Temperature dependence of critical currents and ac transport losses in (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Oy tapes

The critical currents and self-field ac losses of YBa2Cu3Oy coated conductors and (Bi, Pb)2Sr2Ca2Cu3Ox tapes were measured at several temperatures between 45 K and the respective critical temperature. The temperature dependence of ac losses was measured at 50 Hz using the lock-in method for a transport current. The frequency dependence of ac loss was measured at 55 K for transport current frequencies from 25 to 400 Hz. The results show that variation of ac transport loss as a function of normalized critical current is nearly the same at all temperatures in the measured temperature range for both YBa2Cu3Oy and (Bi, Pb)2Sr2Ca2Cu3Ox tapes. The temperature dependence of the loss factor, however, is different for (Bi, Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Oy tapes because of the ferromagnetic loss in the NiW-based coated conductor substrate. Similarities and differences in the temperature and frequency dependence of ac transport losses between (Bi, Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Oy tapes are discussed.

Variable Temperature Total AC Loss and Stability Characterization Facility

The design of a versatile ac loss and stability characterization facility for high temperature superconducting materials suitable for variable temperature measurements is described. A non-metallic vessel inside a transverse field double-helix magnet acts as the measurement chamber. A cryocooler cools the samples to a target measurement temperature between 35 and 80 K. The facility is suitable for measurements on samples as long as 15 cm with ac transport current and ac background magnetic field, both at variable frequency. The facility allows for sample rotation to vary its orientation with respect to the magnetic field. Initial temperature measurements showed that the sample could be maintained at uniform temperature to plusmn0.5 K. AC loss measurements performed on a precursor setup, identical in concept, confirmed the suitability of the double-helix magnet design and cryocooler based cooling arrangement.