CeO2-buffered Sapphire Substrates Research Articles

Effect of inductors to mitigate the hot-spot problem in parallel-connected superconducting thin-film fault current limiting elements

We have been developing superconducting thin-film fault current limiter (FCL) elements, in which high-resistivity Au–Ag alloy shunt layers are used to protect YBa2Cu3O7 (YBCO) thin films deposited on CeO2-buffered sapphire substrates. The high resistance of the thin films enables the element to withstand high electric fields of more than 40 Vpeak cm−1 during the current-limiting period after quenching, thus greatly reducing the amount of YBCO thin film needed and, consequently, the cost of an FCL. We have succeeded in fabricating and testing 500 V/200 A FCL modules using two 20 cm long YBCO films connected in parallel. In the present study, we performed extensive switching experiments on FCL elements, in which two YBCO films are connected in parallel to achieve higher rated currents, and confirmed the previously observed phenomenon that the hot-spot problem causing film damage just after quench initiation becomes more severe when the total critical current of the thin films is higher. We have investigated the origin of this phenomenon and found that a rapid current transfer from the first-quenched film with lower critical current to the other film causes higher current in the secondly-quenched film that sometimes leads to hot spots. It is demonstrated that the serious hot-spot problem can be mitigated by the use of inductors when the high-resistance FCL elements are connected in parallel. Based on these findings we propose an appropriate architecture of a high electric-field superconducting thin-film FCL that can be used in a real power grid.

Fabrication and Properties of Double-Side Tl2Ba2CaCu2O8 Thin Film on CeO2 Buffered Sapphire Substrate

The double-side Tl2 Ba2 CaCu2O8 (Tl-2212) superconducting thin films were fabricated on CeO2 buffered sapphire substrates. The reactive magnetron sputtering technique was used to grow CeO2 buffer thin films on sapphire substrates. Making use of the metal cerium as a sputtering source, the depositing rate is much higher compared with the CeO2 target. The Tl-2212 thin films on CeO2 buffered sapphire substrates were fabricated by a dc magnetron sputtering and post-annealing process. The x-ray diffraction indicates that the thin film is pure Tl-2212 phase with the c-axis perpendicular to the substrate surfaces, and epitaxially grown on the CeO2 buffered sapphire. The critical transition temperature Tc is around 106K, the critical current density Jc is around 3.5MA/cm2 at 77K, and the microwave surface resistance Rs at 77K and 10 GHz of the film is as low as 390 μΩ.

Preparation of large-size Y123 films on CeO2-buffered sapphire substrates by MOD using a low-cost vacuum technique

High-Jc YBa2Cu3O7 (Y123) films with good homogeneity have been obtained on CeO2-buffered sapphire substrates by metal organic deposition (MOD) using a low-cost vacuum technique; only nitrogen gas and air instead of a high-purity gas mixture of argon and oxygen were supplied in the high-temperature heat treatment. A 2cm×12cm rectangular Y123 film was prepared in a large-size tube furnace with uniform temperature zone. The total pressure and oxygen partial pressure in the furnace were controlled by the flow rates of nitrogen and air and by evacuation speed of a rotary pump. The apparent inductive-Jc values, except those at a few positions near the film edges, were within 3.0±0.3MA/cm2, i.e., the fluctuation of Jc was within ±10% of the average. Comparison of the spatial distributions of Ic, film thickness and XRD intensity of Y123 suggests that the crystallinity of Y123 and Jc values at local points are uniform over the film and the thickness distribution affects the local Ic values, or apparent Jc distribution.

Vortex State Microwave Resistivity in Tl-2212 Thin Films

We present measurements of the field induced changes in the 47 GHz complex resistivity, Δρ~(H, T), in Tl2Ba2CaCu2O8+x (TBCCO) thin films with T c ≃ 105 K, prepared on CeO2 buffered sapphire substrates. At low fields (μ0 H < 10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level Δρ~(H, T exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property Δρ~(H, T = Δρ~[H/H * (T)], where the scaling field H ∗ (T) maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.

Microwave Response of Perfect YBa2Cu3O7−x Thin Films Deposited on CeO2-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z s(T), of epitaxial YBCO thin films deposited on CeO2-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R s(T) at low temperatures obeys the exponential law: R s(T) = R res+R 0⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R s(T) behavior.

Preparation of high-Jc Y123 films on CeO2-buffered sapphire substrates by MOD using a low-cost vacuum technique

High-Jc Y123 films were obtained on CeO2-buffered sapphire substrates by metal organic deposition using a low-cost vacuum technique. The total pressure (Ptotal) and oxygen partial pressure (pO2) in a tube furnace were controlled by evacuation with a rotary pump and by flowing gas mixtures of O2 and an inert gas (N2 or Ar). The effects of Ptotal and inlet gas flow rates (Vgas) on the growth of Y123 films were investigated for films heat-treated at 750°C. From the results of X-ray diffraction and monitored pO2, it was found that the pO2 became lower by residual carbon in the prefired films when heat treatments were carried out at a higher Ptotal and/or using a slower Vgas, and that there contained an a-axis orientation in Y123 films at a lower Ptotal. High critical-current density (Jc) Y123 films were obtained by heat treatment at Ptotal=104Pa and Vgas=200ml/min. An average Jc of the film was 3.4MA/cm2 and the fluctuation of Jc was within ±15% of the average.

Microstructural observations of epitaxial Y123 films on CeO2-buffered sapphire by metal organic deposition

Epitaxial YBa2Cu3O7 (Y123) films were obtained on CeO2-buffered sapphire substrates by a fluorine-free MOD process. A 170-nm-thick homogeneous film revealed a high critical current density (Jc > 4 MA/cm2) at 77 K, whereas a 320-nm-thick film had a low Jc of 0.7 MA/cm2. Both of the films have neither a-axis orientation nor BaCeO3 phase by x-ray diffraction. In this article, the microstructures of these Y123 films were observed by crosssectional transmission electron microscopy. The epitaxial c-axis layers were observed successively from the interface with CeO2 buffer to the surface. Selected area diffraction (SAD) exhibited that the orientation relationship among the Y123, CeO2-buffer and sapphire substrate was: Y123(001)[110] ∥ CeO2(100)[100] ∥ Al2O3(-1012)[1-210]. The impurity phase on the surface was revealed to be a CuYO2 compound by energy dispersive X-ray spectroscopy and SAD. The Y123 grains on the surface of the 320-nm-thick film were somewhat inclined to the epitaxial Y123 layer; this is one of the reasons for degraded Jc in the thicker films.

Fabrication of Double-Sided YBa2Cu3O7 Films on CeO2-Buffered Sapphire Substrates by MOD Process

Double-sided YBa 2 Cu 3 O 7 (YBCO) films were successfully prepared on 50-mm-diameter CeO 2 -buffered sapphire substrates by metalorganic deposition (MOD) process using an acetylacetonate coating solution. Mapping analysis of superconducting current densities (J c ) at 77.3 K revealed that J c values of the double-sided films indicated in excess of 2 MA/cm 2 in the center parts with a small decrease of J c at the outer side of the specimens. The J c values of one side (A) are higher than those of the other side (B). Microwave surface resistance (R s ) of sides A and B of the film exhibited 0.57 and 0.60 mΩ, respectively, at 70 K (12 GHz). The difference in the R s values should be attributed to the slight difference in the J c values, which arose from the surface morphology of the CeO 2 buffer layer and heat treatment conditions during the firing process in MOD.

Microwave Properties and Critical Current Density of YBCO Thin Films Deposited on CeO2-Buffered Sapphire Substrates Using Sputtering Targets of Different Compositions

The effect of the composition of a YBCO film on the microwave properties deposited on sapphire <R> substrates with a CeO2 buffer layer was investigated. CeO2 and YBCO films were deposited by RF magnetron sputtering with targets of different compositions. Y-rich crack-free YBCO films with a thickness of 0.5 µm, deposited onto a 25-nm-thick CeO2 film, showed good crystallinity and a low microwave surface resistance. Microwave surface resistances of 0.09 mΩ at 20 K and 0.7 mΩ at 77 K were obtained at a frequency of 12 GHz, and a critical current density of 0.1 MA/cm2 at 77 K was estimated for the 0.5-µm-thick YBa1.5Cu2.5Oy film.

Microcrack-free thick YBCO/CeO 2/Al 2O 3 films prepared by a large-area pulsed laser deposition system

High-quality YBa 2Cu 3O 7− δ (YBCO) films were deposited on CeO 2-buffered sapphire substrates using a large-area pulsed laser deposition system. Interplay of parameters such as thickness and target-to-substrate distance on the surface morphology evolution and superconducting properties of the films were examined. In particular, YBCO films with thicknesses up to ∼1.8 μm were successfully deposited with no observable microcracks on the surface. Characterization of the films revealed the following unique features: (1) a porous morphology, consisting of interconnected islands of varied heights and deep holes; (2) higher normal-state resistivity value, which is attributed to the presence of various defects in the film; and (3) Y-rich composition. These factors are considered to contribute to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Using this system, a 2 ″ YBCO film with thickness of ∼370 nm and no observable microcracks was deposited. A microwave surface resistance value of R s∼1.4 mΩ was obtained at 77.3 K at a resonant frequency of 10.7 GHz. Mapping of the critical current density ( J c) by inductive measurement showed values of ∼1.4–1.6 MA/cm 2 at 77.3 K.

Self-assembled CeO2 buffer layers on R-cut sapphire for high-current-density YBa2Cu3O7−δ films

Experimental evidence of a cooperative self-assembly process was obtained where high-temperature O2 annealing (1000 °C) induced mass transport of CeO2 deposited on R-cut sapphire substrates to form a surface reconstruction on the substrate. When the CeO2 film exceeded a critical thickness (∼10 nm), an atomically flat surface with a high density of nanodots was preferred by the reconstruction (revealed by scanning atomic force microscopy) and the CeO2 layers had high lattice and surface perfection (revealed by x-ray diffraction). YBa2Cu3O7−δ films grown on such CeO2-buffered sapphire substrates had a high transition temperature (Tc ρ = 0 > 90 K) and a high critical current density (Jc > 3.0 × 106 A cm−2 at 77.3 K and a zero applied magnetic field).

Thickness and composition dependence of microwave properties of YBCO thin films on CeO 2-buffered sapphire substrates

We investigated the influence of the crystallinity and the composition of YBCO film on the formation of cracks in layers deposited on sapphire 〈R〉 substrates with CeO 2 buffer layers. The CeO 2 films and YBCO films were deposited using RF magnetron sputtering. Y-rich YBCO films, 0.5–1 μm thick, deposited onto a 300 Å thick CeO 2 film showed good crystallinity and low microwave surface resistance. In high magnification SEM observation, cracks were not observed for films up to 1 μm in thickness.

TlBaCaCuO-(2212) thin films on lanthanum aluminate and sapphiresubstrates for microwave filters

High-quality Tl2Ba2CaCu2O8 films for microwave filterapplication are prepared on both sides of 2 inch lanthanum aluminatesubstrates as well as on double-sided CeO2 coated r-cut sapphiresubstrates. The films have thicknesses as high as 1 µm in the case oflanthanum aluminate substrates. On CeO2 buffered sapphire substrates itis possible to prepare crack-free superconducting films as thick as 400 nm.For both substrate materials we reach high critical current densities ofJc>2×106 A cm-2 at 77 K. The microwave surfaceresistances Rs were measured to fall short of 100 µΩ at 5.6 GHzand 77 K in the low-field case, comparable to the best films made ofYBa2Cu3O7-x material. The films were able to tolerate highmicrowave power levels corresponding to magnetic field amplitudes ofBRF>2 mT. The critical temperature exceeds 102 K. This excellentperformance is maintained even after several thermal cycles. The films wereused to produce four-pole band-pass filters for the C-band which wereoperated without any detectable loss in their transfer characteristics upto temperatures of about 85 K. This is the first time that large area (2inch) double-sided Tl2Ba2CaCu2O8 films have been prepared onsapphire with excellent critical current density in conjunction with lowsurface resistance.

Ion beam studies of Tl-based superconducting films prepared fromfluorides

The elemental composition and depth profiles ofBa-Ca-Cu-(O, F) precursor and Tl-Ba-Ca-Cu-O (TBCCO)superconducting films were studied by ion beam analysis, such asRutherford backscattering spectrometry (RBS) and resonantnuclear reaction (RNR). A series of 150-200 nm thick BCC(O, F)212 and 223 precursor films was prepared by sequential thermalevaporation of BaF2, Cu and CaF2 componentsonto single-crystalline MgO and/or CeO2 bufferedsapphire substrates. The BaF2 and CaF2components are chemically more stable under atmosphericconditions than those of BaO and CaO oxides. Fluorine waspartially removed from some of the as-evaporated BCC(O, F)precursor films by means of an ex situ annealing invacuum and dry oxygen at 720 °C. RBS and RNRmeasurements were performed on these films. Both types ofprecursor, i.e. the as-evaporated as well as the defluorinatedones, were then thallinated in a single-zone reaction chamber at860 °C for 30 minutes using a crude TBCCO pellet as asource of thallous oxide Tl2O. The synthesizedsuperconducting TBCCO films were characterized by resistanceversus temperature measurements, x-ray diffraction, RBS and alsoRNR analysis using the resonance at a proton energy of 340 keVof the 19F(p,αγ)16O nuclearreaction. Mainly the Tl-2212 phase was observed in the superconductingTBCCO films, with Tc values up to 106 K. From the RBSinvestigations we found that films with higher Tc have aslight excess of Tl, Ca and Cu components. While the RNR studyrevealed the presence of fluorine in both types of precursorfilm, no fluorine was observed in the superconducting ones(within the limit of sensitivity of our method).

Superconducting properties of YBa2Cu3O7 step-edge Josephsonjunctions prepared on CeO2-buffered sapphire substrates

Step-edge Josephson junctions in the c-axis oriented YBa2Cu3O7 films were fabricated on the CeO2-bufferedsapphire substrates. The step angles were controlled in the wide range of20-75° by an Ar ion milling technique. The I-V curves ofjunctions fabricated on the 35° step with the film thickness to thestep height ratio of about 0.8 exhibited a resistively shunted junction-likebehaviour with an ICRN product of about 250 µV and a criticalcurrent density of about 2×104 A cm-2 at 77 K. Thecritical currents of the step-edge junctions increased approximately linearlywith decreasing temperature but the normal resistance remained almostconstant. All the samples of the step-edge Josephson junctions satisfied ascaling behaviour of ICRN∝(JC)0.5, and a dc SQUID made out ofthose junctions exhibited a typical voltage modulation in an applied magneticfield.

Preparation of regular arrays of antidots in YBa2Cu3O7 thin films and observation of vortex lattice matching effects

Optimized sputtered YBa2Cu3O7 (YBCO) thin films on CeO2-buffered sapphire substrates are patterned into square lattices of submicrometer holes (antidots) with diameters of 250–450 nm and lattice parameters of d=500–1000 nm without deterioration of superconducting properties. In the mixed state, matching effects between the Abrikosov vortex lattice and the artificial antidot lattice are observed. These effects are in the form of peaks or cusps in the critical current density recorded as a function of magnetic induction at integers n and specific rationals k/l of the matching field Bm=(Φ0/d2). The experimental results are discussed in the context of existing theories. The existence of multiquanta vortices confined by the holes in YBCO films are considered.

Observation of and noise reduction by vortex lattice matching in YBa2Cu3O7 thin films and rf SQUIDs with regular arrays of antidots

Abstract We report on the detection of matching effects between antidot and Abrikosov vortex lattices for fields far below the matching field Bm using rf SQUIDs and demonstrate, that antidots can strongly reduce the low-frequency l/f noise in active devices in unshielded environment. Square lattices of submicron holes (antidots) with diameters of 250–450 nm and lattice parameters ranging from 0.5–5 μm are patterned into optimized sputtered YBa2Cu3O7 thin films on CeO2-buffered sapphire substrates without deterioration of superconducting properties. A special experimental set-up consisting of an antidot lattice in flip-chip configuration with a bicrystal rf-SQUID has been used for the measurements. Thus, two different matching conditions are exposed upon the vortex lattice defined by the vortex–antidot interaction and the vortex–grain boundary interaction at the grain boundary in the washer. Vortices within the vicinity of the grain boundary of the washer can either be trapped (reduction of l/f noise) or exposed to a kind of double potential (increase of l/f noise) depending upon the magnitude of the applied magnetic induction and the geometrical arrangement between antidot array and grain boundary in the washer. Matching effects between vortex and antidot lattice could be observed in the form of minima in the noise spectrum for magnetic inductions much smaller then the matching field, e.g. for B*=(1/18)2Bm. The maxima in the low-frequency noise are a result of the existence of two matching conditions, i.e. caused by the presence of the grain boundary in the SQUID washer. The experiments demonstrate that thermally activated hopping of vortices can strongly be reduced by antidots in the superconducting layer. The resulting reduction of the low frequency excess noise bears a great potential for applications of active high-Tc superconducting devices if other matching conditions are avoided, e.g. by using step-edge type rf-SQUIDs.