High-quality YBa2Cu3O7 films on 4-in. Wafers of sapphire, gallium arsenide, and silicon

Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa₂Cu₃O₇ with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al₂O₃, Si, and GaAs with buffer layers of CeO₂, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredT _c andj _c (77 K) were 87.5±0.2 K and >1×10⁶ A/cm², respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si₃N₄.This work was supported by the German Bundesminister für Forschung und Technologie.     

Novel ceramic substrates for high Tc superconductors

A group of complex perovskite oxides REBa₂NbO₆ (RE=La and Dy) have been synthesized and developed for their use as substrates for both YBa₂Cu₃O_7-δ and Bi(2223) superconductors. These materials have a complex cubic perovskite (A₂BB′O₆) structure with lattice constants,a=8·48?8·60 Å. REBa₂NbO₆ did not show any phase transition in the temperature range 30–1300°C. The thermal expansion coefficient, thermal diffusivity and thermal conductivity values of REBa₂NbO₆ are favourable for their use as substrates for highT _c superconductors. The dielectric constant and loss factor of REBa₂NbO₆ are in a range suitable for their use as substrates for microwave applications. Both YBa₂Cu₃O_7-δ and Bi(2223) superconductors did not show any detectable chemical reaction with REBa₂NbO₆ even under extreme processing conditions. Dip coated YBa₂Cu₃O_7-δ thick films on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 92 K and a current density of ～1·1×10⁴ A/cm² and Bi(2223) thick film on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 110 K and a current density of ～ 4×10³ A/cm² at 77 K and zero magnetic field. A laser ablated YBa₂Cu₃O_7-δ thin film deposited on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 90 K and a current density of ～5×10⁵ A/cm².     

Influence of deposition temperature on the superconductivity of Y1−x HoxBa2Cu3O7 −δfilms produced by dc-magnetron sputtering

Experimental results of research on the influence of deposition temperature (T _s) on crystal structure and superconductivity of Y_1?x HoxBa₂Cu₃O_{7 ?δ} (YHBCO) films deposited by dcmagnetron sputtering are reported. X-ray diffraction analysis showed that the films grew with preferential orientation of thec-axis normal to the substrate surface in the range of temperature 750–820°C. The single-crystal structure of the YHBCO films grown epitaxially at the optimal substrate temperatures of 820, 800, 760, and 750°C, respectively, have been established by rocking curves, Φ-scan, and electron channeling pattern (ECP). Typical values of the critical current density (A · cm^?2) at 77 K and 0.1 T field are 2.1×10⁵, 4×10⁵, 6.2×10⁵, and 3.1×10⁵ for thex=0, 0.2, 0.4, 0.7 films respectively, measured by a Quantum Design magnetrometer (H∥c).     

Pinning enhancement in MgB2 superconducting thin films by magnetic nanoparticles of Fe2O3

MgB₂ thin films were fabricated on r-plane Al₂O₃ ( ${1} \overline{{1}} {0} {2})$ substrates. First, deposition of boron was performed by rf magnetron sputtering on Al₂O₃ substrates and followed by a post-deposition annealing at 850 °C in magnesium vapour. In order to investigate the effect of Fe₂O₃ nanoparticles on the structural and magnetic properties of films, MgB₂ films were coated with different concentrations of Fe₂O₃ nanoparticles by spin coating process. The magnetic field dependence of the critical current density J _c was calculated from the M–H loops and magnetic field dependence of the pinning force density, f _p(b), was investigated for the films containing different concentrations of Fe₂O₃ nanoparticles. The critical current densities, J _c, in 3T magnetic field at 5 K were found to be around 2·7 × 10⁴ A/cm², 4·3 × 10⁴ A/cm², 1·3 × 10⁵ A/cm² and 5·2 × 10⁴ A/cm² for films with concentrations of 0, 25, 50 and 100% Fe₂O₃, respectively. It was found that the films coated with Fe₂O₃ nanoparticles have significantly enhanced the critical current density. It can be noted that especially the films coated by Fe₂O₃ became stronger in the magnetic field and at higher temperatures. It was believed that coated films indicated the presence of artificial pinning centres created by Fe₂O₃ nanoparticles. The results of AFM indicate that surface roughness of the films significantly decreased with increase in concentration of coating material.     

Fabrication of low cost YBCO coated conductors using Ag-clad Hastelloy substrates

Mechanically reinforced Ag-clad Hastelloy tapes were fabricated as the inexpensive substrates for coated conductors without any buffer layer. The clad substrate has good adhesion between Ag and Hastelloy layers. Superconducting YBa₂Cu₃O₇ (YBCO) films were directly deposited on Ag-clad Hastelloy substrates by chemical vapor deposition in high magnetic fields. YBCO films were highly oriented along the c-axis perpendicular to the substrate with a zero resistance transition temperature of 86.2 K and a transport J_c value of 10⁴ A/cm² at 77 K and zero magnetic field. These results indicate that the present work appears to be a promising way for the development of YBCO films for large-scale applications.     

YBa2SnO5·5, a novel ceramic substrate for YBCO and BiSCCO superconductors

YBa₂SnO_5·5 has been synthesized and sintered as single phase material for its use as substrate for both YBCO and BiSCCO superconductors. YBa₂SnO_5·5 has a complex cubic perovskite (A₂BB’O₆) structure with the lattice constanta = 8·430 Å. The dielectric constant and loss factor of YBa₂SnO_5·5 are in a range suitable for its use as substrate for microwave applications. YBa₂SnO_5·5 is found to be chemically compatible with both YBCO and BiSCCO superconductors. The thick film of YBCO screen printed on polycrystalline YBa₂SnO_5·5 substrate gave aT _c(0) of 92 K and a critical current density (J _c) of 4 × 10⁴ A/cm² at 77 K. A screen printed BiSCCO thick film on YBa₂SnO_5·5 substrate gaveT _c(0) = 110 K and current density 3 × 10³ A/cm² at 77 K.     

Proton-induced reduction ofR s,Jc, andT c in YBa2Cu3O7−δ thin films

We have explored the effect of 2-MeVH⁺ irradiation on the superconducting transport properties of thin films of YBa₂Cu₃O_7?δ [T _c, J_c(B=0; 77 K, 4.2 K), andR _s(36 GHz;T)]. The inductively measured critical temperatureT _c changed slowly and uniformly (～2 K per 10¹⁶/cm²) for fluences less than ～3×10¹⁶/cm². Beginning at ～3–4×10¹⁶/cm², the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwaveT _c (as defined by the sharp transition inR _s as a function of temperature) resembled the low-frequency inductiveT _c measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance (～6–10 mΩ) was not affected for fluences up to 5×10¹⁶/cm². We have interpreted the sudden and reproducible reduction in the microwaveT _c transition as a sensitive indicator of disruption in the copper-oxygen chain sublattice and compared the proton-induced change to that observed in oxygen gettering studies of bulk materials.     

Synthesis of the LnNb7O12 (Ln = La, Ce, Pr) Discrete-cluster compounds

The new compounds CeNb₇O₁₂ and PrNb₇O₁₂, isostructural with LaNb₇O₁₂, have been synthesized and characterized by x-ray diffraction. The structures of the LnNb₇O₁₂ (Ln = La, Ce, Pr) compounds have space group P2₁/c (Z = 4), with lattice parameters a = 10.764(2), 10.743(2), 10.738(4) Å; b = 9.192(2), 9.146(2), 9.137(3) Å; c = 10.313(2), 10.304(2), 10.303(3) Å; and β = 104.25(2)°, 104.31(2)°, and 104.21(3)°, respectively. The reduction in the lattice parameters of LnNb₇O₁₂ in going from La to Pr correlates with the ionic radii of Ln: r(La³⁺) > r(Ce³⁺) > r(Pr³⁺). In the Nd(Sm)-Nb-O systems, phases of stoichiometry LnNb₇O₁₂ have not been obtained.     

Preparation and properties of Tl2Ba2CaCu2O8 thin films

Anex situ process has been developed to produce thin superconducting Tl₂Ba₂CaCu₂O₈ films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103–107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO₃, LaAlO₃, and Y-ZrO₂ substrates. A critical current density,J _c, of 2×10⁶ A/cm² at 77 K was measured on LaAlO₃. Film morphology was studied by SEM, AFM, and STM.     

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB₂ thin films were deposited on MgO (100) substrate and r-plane Al₂O₃ $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB₂ films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J _c was calculated from M–H loops and also the magnetic field dependence of F _p was compared for the different temperature ranges from 5 to 25 K. The critical current density J _c was found to be around 1.0×10⁶ A/cm² and 1.7×10⁶ A/cm² in zero field at 5 K for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al₂O₃ in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.     

Effect of Substrates on Superconducting Properties of MgB2 Thick Films Fabricated by Spraying Method

MgB₂ thick films have been prepared on Al₂O₃, MgO and SrTiO₃ ceramic substrates using the spray method with two different heat-treatment cycles, 925???C with 10?min (Group?A) and 610???C with 24?h (Group?B). The structural/microstructural (XRD, SEM) and transport (R?CT, MR?CT) properties of the films prepared were investigated. XRD analysis showed that the films in Groups?A and?B consisted mainly of MgB₂, but the peaks originating from the substrates were also observed in films fabricated on the Al₂O₃ substrate. From scanning electron microscopy, it was seen that the surface of MgB₂ thick films prepared is not sufficiently homogeneous. Some cracks and heaps with different sizes were observed. The best electrical results were obtained for films in Group?A on Al₂O₃ substrate. The T _c value for films in Group?A was found to be 36.1?K, 36?K and 35?K, for Al₂O₃, MgO and SrTiO₃ substrates, respectively. No superconducting state was reached for films in Group?B on the SrTiO₃ substrate. In all the films which showed a superconducting state, the magnetic field strongly affected T _c . A?tail in the resistance curves was observed with increasing magnetic field for films in Group?A on the MgO and SrTiO₃ substrates. At magnetic fields above 1?T for films in Group?A on the SrTiO₃ substrate and above 2?T for films in Group?A on the MgO substrate, the zero-resistance temperature, T ₀, was not obtained.     

Magnetization, magnetic susceptibility, and effective magnetic moment of the Fe3+ ions in Bi2Fe4O9

Bi₂Fe₄O₉ with an orthorhombic structure and lattice parameters a = 7.9595 Å, b = 8.4297 Å, c = 5.9912 Å, and V = 401.987 ų has been prepared by solid-state reactions method. Its molar magnetic susceptibility measured as a function of temperature in the range 5–950 K indicates that Bi₂Fe₄O₉ is an antiferromagnet with a Néel temperature of 258 K. In the range 280–750 K, its molar magnetic susceptibility exhibits Curie-Weiss behavior, which allowed us to determine the Weiss constant (Θ = ?1468 K) of this material and the effective magnetic moment of the Fe³⁺ ions $\left( {\mu _{eff}^{Fe^{3 + } } = 6.37\mu _B } \right)$ . Magnetization versus magnetic field data show no magnetic hysteresis, indicating that the Bi₂Fe₄O₉ sample studied exhibits no weak ferromagnetism.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7?δ (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (3∶1) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Synthesis,crystal structure refinement,electrical and magnetic properties of BaV13O18 and SrV13O18

Polycrystalline samples of BaV₁₃O₁₈ and SrV₁₃O₁₈ were prepared by solid-state reaction of BaCO₃, SrCO₃, V₂O₅ and V at 1773–2073 K in flowing Ar. The crystal structures of BaV₁₃O₁₈ (R-3, a_h=12.6293(10) Å, c_h=7.0121(4) Å) and SrV₁₃O₁₈ (a_h=12.5491(7) Å, c_h=6.9878(3) Å) were refined by the Rietveld method using X-ray diffraction data. BaV₁₃O₁₈ exhibited semiconducting behavior with electrical resistivity from 5.8×10⁻³ to 2.7×10⁻³ Ω cm at 100–300 K. Electrical resistivity of SrV₁₃O₁₈ ranged from 1.5×10⁻³ to 1.8×10⁻³ Ω cm, and it increased slightly up to around 250 K and decreased above 250 K with increasing temperature. Negative Seebeck coefficients of both compounds at 100–300 K indicated that electron was the dominant carrier. BaV₁₃O₁₈ and SrV₁₃O₁₈ showed paramagnetism with the effective magnetic moment of 0.11μ_B and 0.15μ_B, respectively, at 10–100 K.     

3D like vortex behavior and the thermally activated flux flow mechanism in (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting films

Approximately 1 μm thick high quality epitaxial c-axis oriented (Hg_0.8Re_0.2)Ba₂Ca₂Cu₃O_x superconducting films have been prepared on MgO (100) substrates using spraying technique and post-Hg-Vapor annealing. The effect of the heating temperature–time combinations and the filling factor of Hg (ff_Hg) on the physical, electrical and magnetic properties of the thick films have been investigated. The XRD investigations showed that the a–b plane of HgRe-1223 phase align parallel to the substrate surface. The best T _c and T _zero were found to be 130.6 and 127.2 K, respectively. The superconducting transition of the films has been measured under applied magnetic field up to 6 T. The results obtained suggested that dissipative resistivity can be explained by thermally activated flux motion below critical temperature under applied magnetic field. The temperature and field dependences of the activation energy in the thermally activated flux flow region have also been investigated. The calculated values of m and α values were found to be 1.42–1.49 and 0.498–0.518 respectively and suggesting a 3D like behavior and the thermally activated flux flow mechanism for all films fabricated. Magnetic properties of the films up to 6 T have also been investigated. The calculated value of critical current density, J _c, was found to be 4.7 × 10⁶ A/cm² at 10 K for the optimally treated films.     

Effects of deposition temperature on phase purity, orientation, microstructure and property of cobalt substituted bismuth ferrite thin films

Multiferroic BiFe_0.95Co_0.05O₃ thin films were fabricated on Pt/Ti/SiO₂/Si substrates at various temperatures by pulsed laser deposition. It was found the deposition temperature had great effects on phase purity, orientation, microstructure and multiferroic properties of these films. The optimized deposition temperature was close to 600?°C. Polarization–electric field (P–E) and magnetization–magnetic field (M–H) hysteresis loops at room temperature were observed simultaneously in the films fabricated at 600?°C. The remnant polarization, coercive electric field (P _r , E _c ) and the remnant magnetization, coercive magnetic field (M _r , H _c ) of the films deposited at 600?°C were (0.95?μC/cm², 31?kV/cm) and (0.59?emu/cm³, 130 Oe), respectively. These results might have implications for further investigations on high quality BiFe_0.95Co_0.05O₃ multiferroic films.     

Hybrid open-frameworks: hydrothermal synthesis,structure determinations and magnetic properties of MIL-29, two copper diphosphonates (CuII(H2O))2{O3P–X–PO3} with X=C2H4, CH2–C6H4–CH2

Two copper diphosphonates were hydrothermally synthesized using ethylenediphosphonic and p-xylenediphosphonic acids, synthesized according the Arbuzov method. The structure of (Cu^II(H₂O))₂{O₃P–CH₂–CH₂–PO₃} already described from powder data, was solved from single crystal data. Its symmetry is monoclinic (space group P2₁/c (no. 14)) with lattice parameters a=8.0670(4) Å, b=7.5889(4) Å, c=7.3997(4) Å, β=116.281(2)°, V=406.18(4) ų, Z=2. The structure of (Cu^II(H₂O))₂{O₃P–CH₂–(C₆H₄)–CH₂–PO₃} was solved by powder X-ray diffraction (space group P2₁/c (no. 14)) with lattice parameters a=10.812(1) Å, b=7.577(1) Å, c=7.412(1) Å, β=92.34(1)°, V=606.7(2) ų, Z=2. Both structures are built up from identical inorganic layers covalently linked by the organic chains which act as pillars. The inorganic layers contain dimers of edge-sharing CuO₄(H₂O) square pyramids linked by the PO₃C tetrahedra. Both compounds are antiferromagnetic at 4(1) K.     

Synthesis and Characterization of MgB2 Thin Films Prepared by?2.4?Mhz Ultrasonic Spray Pyrolysis System

Nano-sized B and Mg powders have been successfully deposited onto single crystal MgO(100) and Al₂O₃(001) substrates using 2.4 MHz ultrasonic spray pyrolysis system and an appropriate solution to obtain thin films of MgB₂ superconductors. After an in-situ heating process, ??600?C1000 nm thick superconductor films were obtained. The microstructure, electrical, and magnetic properties were characterized by means of particle size analyzer, XRD, SEM?CEDX, R?CT, and M?CH analysis. The effect of particle concentration in the solution, spraying time, and heating temperature on the quality of the MgB₂ films were discussed. The best T _c and T _zero results were obtained to be 39.5 and 37.4 K, respectively, for the film deposited on the Al₂O₃(001) substrates. Magnetic properties of the MgB₂ films were investigated at 3 different temperatures and up to 5 T. Symmetric hysteresis loops for all temperature and field cases were obtained and maximum $J_{c}^{\mathrm{mag}}$ value was calculated to be 4.0×10⁶ A?cm^?2 at 10 K for 0 T for the film deposited on the Al₂O₃(001) substrates. The results obtained were found to be highly dependant on the particle concentration in the solution, heating temperature of substrates and carrier gas flow rate during spraying.     

Crystal Structure and Superconductivity of MgB2 Thin Films Grown on Various Oxide Substrates

The crystal structure and superconductivity of MgB₂ thin films grown on various oxide substrates were investigated by X-ray diffraction and resistance measurement. The films were prepared by a two-step method, in which precursors B films were annealed in Mg vapor at 900^○C. The X-ray diffraction shows that the MgB₂ films grown on C–AL₂O₃, R–AL₂O₃, and MgO (001) are c-axis oriented while the films grown on SrTiO₃ (001), LaAlO₃ (001), and ZrO₂ (001) are aligned with the (101) direction normal to the substrate planes. All the grown films show superconductivity and their transition temperature varies with the substrates in the range of 34–39 K. We think that the transition temperature variation is probably due to the lattice matching between the film and the substrate, as well as the interdiffusion at the film/substrate interface. The experimental results suggest that if there is no severe interdiffusion at the film/substrate interface in the high temperature annealing process, more substrates could be used for the growth of MgB₂ films using the two-step method.     

Liquid phase epitaxy processing for high temperature superconductor tapes

Liquid phase epitaxy has been used to grow high-temperature superconductor thick films for high J_c applications. Growth on single crystal substrates has so far produced very encouraging results. YBa₂Cu₃O_7−δ films with thickness up to 10 μm can be grown at a typical rate of 1 μm/min, with excellent in-plane texture and 100% density. T_C values of 91 K and J_c values over 1×10⁶ A/cm³ (77 K, 0 T) have been achieved. More effort is necessary to grow REBa₂Cu₃O_7−δ (RE=rare earth) thick films on metallic substrates.