Iman Maartense

Flux Pinning of Y-Ba-Cu-O Films Doped With

The superconducting properties of YBa₂Cu₃O_7-x (YBCO or 123) thin films doped with BaZrO₃ (BZO) nanoparticles by multilayer and single-target methods were studied and compared. Thin films of 123 + BZO were processed by pulsed laser deposition on LaAlO₃ and SrTiO₃ and single crystal substrates. Multilayer (BZO_0.6nm-1.4 _nm/123 ₁₅ _nm) structures were grown by alternating deposition from 123 and BZO targets, and BZO additions of 0-2 vol% were deposited using (123_1-xBZO_x) single-targets. The multilayer and single-target methods of BZO addition caused significant differences of superconducting transition temperatures (T_c) measured by AC susceptibility, and critical current densities (J_c) measured by both magnetic and transport methods as a function of temperature (T), applied magnetic field (H) and angle of H field incidence (thetas) . Single-target films had almost linear decrease T_c of and self-field J_c with BZO vol% addition, and compared to multilayer films had lower J_c (77 K, H < 4 T) however had improved high-field properties for J_c (77 K, H > 4T. Multilayer films had almost no decrease of T_c and self-field J_c for high BZO additions up to 10 vol% and very strong peak of J_c (H// ab in - plane) and constant and high J_c (H, 0 < thetas < 80 deg) . Single-target BZO-2 vol% films had slightly enhanced J_c(H//c-axis), consistent with results by other authors.

{\rm BaZrO}_{3}

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.

Nanoparticles by Multilayer and Single Target Methods

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. >

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

AC susceptibility and X-ray diffraction techniques have been used to investigate the superconductive and structural properties in the system Bi2-xPbxSr2Ca2Cu3Oy with x=0, 0.2, 0.4 and 0.6, and sintered in air at Ts=860, 870 and 880 degrees C for 40 h. The best bulk superconductive properties (zero resistance below 103 K) were obtained with x=0.4 and Ts=860 degrees C. Tc of the granular 2223 phase remained constant at 107 K, independent of x and Ts. A higher Ts increases the proportion of the high-Tc (2223) phase relative to the low-Tc (2212) phase. However, the occurrence of partial melting is detrimental to both the intergranular coupling and the bulk-Tc values of these ceramic samples and may also lead to the formation of two coexisting low-Tc, 2212 subphases, when x=0.6.

Principles of application of high temperature superconductors to electromagnetic launch technology

Thin films of (Y1?xEux)Ba2Cu3O7?? with x = 0?1 were grown by pulsed laser deposition on single crystal substrates, to determine the effect of Y and Eu substitution on the microstructural and superconducting properties. The film critical transition temperature (Tc), critical current density (Jc) and surface roughness were found to be strongly dependent on the substrate choice, with the best properties achieved on CeO2?YSZ substrates. Substrates with varying lattice mismatch from ?2% to +1% were studied, including LaAlO3, SrTiO3, CeO2-buffer-coated Zr0.905Y0.095O2 (YSZ), and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT). With increasing Eu substitution from x = 0 to 1, the Tc on CeO2?YSZ substrates increased steadily from 89 to 93?K. The value of Jc was measured by magnetic methods at 65 and 77?K, and by transport methods at 77?K for selected samples and different angles of orientation Happl(?) = 0??90?. As Eu substitution was increased from x = 0 to ???0.2, Jc at 77?K increased strongly more than two-fold for ?T and for H<0.5?T for all orientations 0????90?; but, it decreased more than two-fold for ?T. The Jc(H) properties at both low and high fields correlated well with microstructural features observed by scanning electron microscopy. The low-field was enhanced when the grain size was reduced to ~100?nm size, and the self-field and high-field Jc(H) appeared to be strongly affected by the film density and porosity. A controlled dense-island structure was noted for x = 0.75, on CeO2?YSZ substrate.

Effect of sintering temperature on the superconductive properties of Bi(Pb)-Sr-Ca-Cu-O ceramics

A near-field evanescent microwave microscope based on a coaxial transmission line resonator with a tungsten tip protruding through an end-wall aperture is used to measure local dielectric properties of thin film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in superconducting state below critical temperature T/sub c/=91 K at T=79.4 K and in normal state at room temperature (T=298 K). The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission lines resonant frequency. The shift of the resonators frequency is measured as a function of tip-sample separation and associated change in quality factor (Q) image scans of the thin film is obtained. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film deposited on LaAlO/sub 3/ substrate.

Microstructural and superconducting properties of (Y1−xEux)Ba2Cu3O7−δ thin films: x = 0–1

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.

Characterization of local dielectric properties of superconductor YBa2Cu3O7-δ using evanescent microwave microscopy

Nanoparticle dispersions of various phases were added to YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO or 123) thin films by multilayer pulsed laser deposition, to determine their effect on flux pinning. The different pinning materials examined include Y/sub 2/BaCuO/sub 5/ (Y211 or green-phase), La/sub 2/BaCuO/sub 5/ (La211 or brown-phase), Y/sub 2/O/sub 3/, CeO/sub 2/, and MgO, with lattice constant mismatches varying from 0.5% to 12% with respect to YBCO. Y211 and Y/sub 2/O/sub 3/ provided significant pinning increases at temperatures of 65 K and 77 K, however other phases provided enhancements only at 65 K (for CeO/sub 2/ and La211) for limited range of applied field strengths. An interesting correlation between T/sub c/ transition widths and pinning strengths was observed. The additions produced markedly different nanoparticle and film microstructures, as well as superconducting properties.

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

Thin superconducting films of YBa2Cu3O7−x were deposited onto (100) SrTiO3 substrates at 750 °C in 100 and 200 mTorr of O2, by ArF excimer laser ablation. The as‐deposited films were strongly oriented with the c axis perpendicular to the surface when they were cooled to room temperature in oxygen. The highest values of transport critical current density (Jc≳105 A/cm2 below 80 K) were observed in films cooled in flowing O2. We find that, in all films, Jc follows a power law (1−T/Tc)n with n≊2, near the critical temperature, Tc, when Tc is defined as the temperature at which Jc=0. It appears that this behavior is governed by the short coherence lengths in this high‐Tc material, as suggested by G. Deutscher and K. A. Muller [Phys. Rev. Lett. 59, 1745 (1987)], and not by the specific nature of the large‐scale structure of the films.

Addition of alternate phase nanoparticle dispersions to enhance flux pinning of Y-Ba-Cu-O thin films

Abstract Pulsed-laser deposition (PLD) is a very powerful and rapid deposition technique, which can produce exceptional-quality thin films. Although PLD has tremendous versatility and potential, PLD capabilities are still constrained by a lack of process control. At the Air Force Research Laboratory on-site at Wright-Patterson AFB, we are developing in-situ/real-time control methodologies for PLD and other thin-film deposition processes. We have identified appropriate sensors for closed-loop feedback control and utilized them with YBa2Cu3O7-x (YBCO) to identify critical process control parameters. Control instrumentation has recently been improved by the addition of moveable fixed-position plume-emission sensors. The reproducibility of YBCO film quality increased significantly when process control was applied to PLD processing. PLD process control techniques were applied during simulated and actual BSTO depositions. A comparison of deposition control with the controlling sensor at different distances from the substrate heater position is discussed. Further process refinement using time-resolved spectral components of the PLD plume on subsequent film quality is also discussed.