Charles E. Oberly

Hysteretic loss reduction in striated YBCO

Abstract Magnetization vs. applied field measurements ( M–H loops) were taken on short samples of YBa 2 Cu 3 O 7− δ (YBCO) thin films which were divided into narrow filaments. The YBCO was deposited using pulsed laser deposition onto single-crystal LaAlO 3 substrates, with a range of film thicknesses from 0.25 to 0.33 μm. Using a YAG laser, the thin films were patterned into linear striations by removing strips of the superconductor by laser ablation. The resulting striated filamentary structure serves to reduce the effective width of the YBCO films and hence the hysteresis loss in the superconducting samples. The magnetization measurements were taken over the temperature range of 4.2–77 K in applied fields of 0–17 kOe using a vibrating sample magnetometer. The measured hysteresis losses show a highly linear relationship between superconductor filament width and hysteresis loss as anticipated. However, the laser ablation process did result in the redeposition of YBCO along the edges of individual filaments. Degradation of T c and J c due to the ablation process is discussed.

AC loss characteristics of multifilamentary YBCO coated conductors

One of the methods to reduce magnetization loss of YBCO coated conductors in a perpendicular magnetic field is subdividing the YBCO layer into filaments and twisting them as a whole. A 10 mm wide multifilamentary YBCO coated conductor with 200 /spl mu/m wide filaments was prepared by striation using the laser ablation technique. The number of filaments was 40. The sample length was varied from 100 mm to 25 mm, and their magnetization loss was measured at various frequencies. The measured magnetization losses were scaled using sample length, frequency, and field amplitude. This clarifies the magnetization loss characteristics of multifilamentary YBCO coated conductors and obtains empirical expressions for the magnetization loss. The measured loss was compared with the loss calculated numerically using a two dimensional FEM model. The experimentally confirmed effect of striation to reduce the magnetization loss was compared with theoretical predictions.

Measurement of transport critical current of Y-Ba-Cu-O using an inductive method

An inductive, contactless procedure discussed by E.A. Harris et al. (Cryogenics, vol.28, p.685-7, 1988) was used to measure two samples of Y-Ba-Cu-O one of which was conventionally sintered, the other prepared using a melt-processing technique. When made into ring form and placed as a tertiary winding on a transformer, the sample could be driven to the normal state and a critical current density determined. It was shown that the melt-processed material has a very large critical current density when compared with the conventionally sintered material, and it exhibits an instantaneous transition to the fully normal state at a well-defined current level. It was found that rapid degradation will occur in melt-processed Y-Ba-Cu-O if it is exposed to condensation after removal from the low-temperature environment. The critical current density invariably decayed during repeated experimentation from a level difficult to achieve with the equipment at hand, to a level not much above the non-melt-processed Y-Ba-Cu-O. The higher levels of measured critical current density could be restored with reoxygenation.

Lightweight superconducting generators for mobile military platforms

Mobile military platforms impose severe restrictions on size, weight and efficiency of megawatt class power components. Severe shock, vibration and tilt requirements are additional limitations not required by commercial utility power systems. Many military power system restrictions and requirements can be met by application of superconductor technology only if the cryogenic containment and cooling are relatively transparent to the system. Advantages and limitations for military systems of various superconducting generator architectures are discussed. Current and future technology directions for high temperature superconducting generators are assessed

Development of nickel alloy substrates for Y-Ba-Cu-O coated conductor applications

Fabrication of long-length, textured substrates constitute a critical step in the successful application of coated High Temperature Superconductors (HTS). Substrate materials stronger than nickel are needed for robust applications, while substrates with non-magnetic characteristics are preferred for AC applications. The present work is thus focused on development of texture in high strength, non-magnetic substrate materials. As the development of cube texture is easier in medium to high stacking fault energy materials, binary alloys based on nickel were evaluated for the present application. High purity alloys were melted and hot/cold worked to obtain thin tapes. The development of texture in these alloys as a function of processing parameters was studied by X-ray diffraction and metallographic techniques. Orientation Imaging Microscopy (OIM) was used to quantify the extent of texture development in these substrates. Results to date on the development of texture by thermo-mechanical processing of these alloys are presented.

Principles of application of high temperature superconductors to electromagnetic launch technology

A review is presented of advances in the performance of bulk high-temperature superconductors (HTSC) which permit conductor and magnet development at practical magnetic fields to be pursued for high-current applications such as electromagnetic launchers (EMLs). While early hopes for a superconductor critical temperature (T/sub c/) approaching room temperature have not been fulfilled, numerous HTSC with T/sub c/ between 60 K and 125 K exist which can be successfully processed. Some of these HTSC are well enough understood that small conductors and coils may be fabricated for operation near 20 K. Numerous physics, magnetic flux mechanics, materials processing, and structural support issues remain for resolution before large-scale coils made of HTSC can be operated at high energy storage density at temperature well above 20 K. Properties and materials processing of HTSC and their relation to EML applications technology are described. >

YBa2Cu3O7−x–Ag thick films deposited by pulsed laser ablation

Abstract Ag-doped YBa 2 Cu 3 O 7− x films, with thickness ranging from 0.06 to 2.5 μ m, were deposited by pulsed laser ablation onto (100) LaAlO 3 single-crystal substrates. The target was YBa 2 Cu 3 O 7− x with 5 wt.% Ag addition. The presence of Ag in the films in concentrations of ∼1 at.% was detected by X-ray fluorescence and secondary ion mass spectrometry (SIMS) analysis. Biaxial alignment of the films was indicated by φ scans with full-width-half-maximum (FWHM) spread of 1–2° for various thicknesses. Utilizing a standard deposition process, most films showed a critical transition temperature ( T c ) >90 K as measured by the ac susceptibility technique. Film critical current densities ( J c ) on the order of 10 6 A/cm 2 were measured at 77 K with a four-probe technique on a 100- μ m-wide patterned microbridge.

Conceptual approach to the ultimate low AC loss YBCO Superconductor

Wide tapes of YBCO coated conductors make minimization of ac loss very difficult. Subdivision of the tape into narrow filaments does not provide complete transposition when twisted and current sharing is not equalized due to inductance differences. For ac power applications operating near 400 Hz for military aircraft power systems, equal inductance filaments and short twist pitch are very important to loss minimization. A fully transposed YBCO tape that approximates a Rutherford cable has been conceived to minimize ac loss at high frequency. Retention of optimal texture of YBCO in edge turnaround regions and filament crossovers in the Rutherford tape is essential.

Bi-based high temperature superconducting tapes by cold rolling method

Bi-based high-temperature superconductor doped by lead with and without silver addition was used to produce tape approximately 1.2-cm wide and up to 20-cm long. Powder was calcined at 800 degrees C, 810 degrees C, and 835 degrees C for 50 h each time and loaded into silver tubes with o.d.=6.35 mm and i.d.=4.35 mm. Tapes with nominal thicknesses of 0.5 mm, 0.35 mm, and 0.2 mm were produced by the cold rolling technique. Direct transport current, DC magnetization, and AC susceptibility measurements were performed to characterize the superconducting properties of the tapes. The authors observed a significant increase in the magnetization critical current density with increasing number of calcinations and decreasing thickness of the tape, which is equivalent to the introduction of deformations into the system, thus increasing the number of pinning centers. A silver addition (3 wt.%) increases connectivity between grains and creates much cleaner grain boundaries.