Annealing Oxygen Pressure Research Articles

Deposition and Characterization of<tex>$hboxY_1-xhboxCa_xhboxBa_2hboxCu_3hboxO_7-delta$</tex>Epitaxial Thin Films

We report on the superconducting properties of epitaxial thin films Y/sub 1-x/Ca/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-y/ with 0/spl les/x/spl les/0.2, grown on SrTiO/sub 3/ [001] by Pulsed Laser Deposition (PLD) technique. The partial substitution of Ca/sup 2+/ at the Y/sup 3+/site in YBCO leads to a monotonic decrease of the zero resistance critical temperature, T/sub c/. The oxygen stoichiometry is varied by means of post-deposition thermal treatment varying the oxygen partial pressure. The dependence of T/sub c/ on annealing oxygen pressure can be attributed to an overdoping effect and/or to a cation disorder.

High oxygen-pressure annealing effects on the ferroelectric and structural properties of PbZr0.3Ti0.7O3 thin films

We report on the synthesis of polycrystalline thin films of PbZr0.3Ti0.7O3 (PZT) by the so-called chemical solution deposition technique. The thin films were deposited on Pt∕Ti∕SiO2∕Si substrates by the spin-coating method, and were heat treateded at 700°C in air and under several oxygen pressures (10<PO2<60bars). The structural, morphological, and ferroelectric properties were characterized by x-ray diffraction (XRD), infrared and Raman spectroscopy, atomic force microscopy (AFM), and polarization-electric-field hysteresis loop measurements. The XRD and the Raman spectroscopy results revealed that the films heat treated in air and at low oxygen pressures (PO2<40bars) are single phase. However, analysis of the data indicated a clear decreasing of the crystallization degree of the films with increasing oxygen pressure. AFM results showed the PZT films display a rosette structure embedded in a matrix, which comprises grains with an average grain size ranging from 60 to 120nm. Ferroelectric hysteresis loops’ measurements performed on these PZT films exhibited a clear decrease of the remnant polarization with increasing oxygen-pressure PO2. This study indicated some important effects of the high oxygen-pressure annealing on the physical properties of PZT thin films.

Processing and characterization of YBCO superconductors by top-seeded melt growth method in batch process

Single domain superconducting YBCO disks up to 30–50 mm in diameter have been prepared in batch process. A box furnace or a well-styled furnace was used in the processing. High oxygen pressure (up to 2 MPa) annealing of the samples was used in the final treatment. The levitation forces for 30 mm in diameter YBCO single domain disks of 1200 pieces were tested. It is shown that the proportion of the samples with above 10 N/cm 2 levitation force to the total is about 82%. The best value of the force is approximately 16 N/cm 2 under the conditions of 77 K and 0.5 T. An effective method of surface metallic coating on the bulk is also developed.

Phase evolution, structural and superconducting properties of Pb-freeBi2Sr2Ca2Cu3O10+δ single crystals

Sizable Bi2Sr2Ca2Cu3O10+δ (Bi-2223) single crystals were grown by the travelling solvent floating zone technique and subsequentlyannealed in O2 and/or Ar flow for 120–500 h in a temperature range of430–850 °C. The effect of annealing on the phase evolution as well as the structural andsuperconducting properties was studied using x-ray diffraction (XRD) andmagnetization measurements as well as ellipsometric measurements of the far-infraredc-axis conductivity. The results show that some of the as-grown Bi-2223crystals are nearly phase pure, while others contain a certain amount ofBi2Sr2CaCu2O8+δ (Bi-2212), Ca2CuO3 and CuO phases co-existing with the Bi-2223 phase. Annealing these multi-phase crystals inO2 flow at high temperature can lead to a phase transformation from Bi-2212 to Bi-2223,which can be understood by a layer-intercalation mechanism. For the samples consisting of Bi-2212 phase, nearly phase-pure Bi-2223 crystals can be obtained byhigh-temperature, long-time annealing. Annealing phase-pure crystals invarious atmospheres, temperatures and pressures (ranging from 1 to 550 bar)causes an alteration in oxygen content, resulting in a systematic change in thec-axis lattice parameters and superconducting transition temperatureTc. Interestingly,Tc is found to increasewith decreasing c-axis lattice parameters reaching K, but change very little with the further decrease ofc-axis lattice parameters, exhibiting a broad plateau on the plot ofTc versus c-axis lattice constant. The magnetization and ellipsometric measurements on high-oxygen-pressureannealed Bi-2223 crystals show much stronger Josephson coupling between theCuO2 layers,indicating a progressively higher hole doping upon increasing annealing oxygen pressure, althoughTc remains essentially unchanged. The anomalous saturation ofTc on the overdoped side is quite unique compared with the mono- or bi-layeredBi-based cuprates. It is possibly related to the significant difference in thehole doping between the crystallographically inequivalent inner and outerCuO2 planes in the multi-layered cuprate system.

Crystal growth and transport properties of spin ladder compounds La 4+4 nCu 8+2 nO 14+8 n ( n=2, 3)

Here we summarize recent progress on the study of spin ladder compounds, La 2Cu 2O 5 (four-leg) and La 8Cu 7O 19 (five-leg), n=2 and 3 members of the homologous series La 4+4 n Cu 8+2 n O 14+8 n . Following the successful crystal growth of La 2Cu 2O 5, we grew a pure-phase five-leg system La 8Cu 7O 19 by a modified slow cooling method. The room-temperature resistivity of the as-grown La 8Cu 7O 19 crystal was 1.9 Ω cm, which is much lower than that of the La 2Cu 2O 5 crystals (≈1 .7×10 3 Ω cm). A significant drop in the resistivity (11.0 m Ω cm at 300 K) was observed upon hole doping by means of high oxygen-pressure annealing, although no transition was observed from the semiconducting behavior. In addition, a new crystal growth method to enhance hole doping level of La 2Cu 2O 5 is shown.

Trial of hole-doping to 4-and 5-leg ladder compounds

The electrical resistivity and magnetic susceptibility measurements have been made for the spin- 1 2 4- and 5-leg ladder compounds, La 8Cu 8O 20 and La 8Cu 7O 19, respectively. Hole-doping has been attempted by means of high oxygen pressure annealing and substitution of Li +1 for Cu +2, La 8Cu 8− x Li x O 20. For La 8Cu 8O 20 and La 8Cu 7O 19, the electrical resistivities exhibit semiconductive behaviors, and the magnetic susceptibilities have a broad peak at around 150K. For the annealed samples under high oxygen pressure, the electrical resistivity becomes more conductive than that for as-sintered samples. The Li-substituted samples La 8Cu 8− x Li x O 20 with x=0.4, 0.8 and 1.2 also shows the semiconductive behavior, but it becomes more conductive with increasing the Li concentration x. The magnetic susceptibility shows the broad peak at around 150K for as-sintered sample, and spin-glass like behavior was observed at around 10K. This transition is shifted to low temperature with increasing Li concentration.

Influence of oxygen background pressure on the structure and properties of epitaxial SrTiO3/La0.35Nd0.35Sr0.3MnO3 heterostructures grown by pulsed laser deposition

Epitaxial La0.35Nd0.35Sr0.3MnO3 (LNSMO) films and SrTiO3 (STO)/LNSMO heterostructures have been grown on LaAlO3 substrates by pulsed laser deposition. The effect of oxygen content on structure and properties of both the LNSMO and STO/LNSMO films was investigated through x-ray diffraction, atomic force microscopy, and resistivity-temperature measurements. It is found that the out-of-plane lattice constant and the metallic-semiconducting transition temperature of the LNSMO films are greatly influenced by the oxygen pressure during deposition, but, insensitive to the in situ annealing oxygen pressure ranging from 2×10−6 to 10 Torr after the deposition. For the STO/LNSMO heterostructures, oxygen out-diffusion from the LNSMO layer is evidenced when the top STO is deposited at an oxygen pressure of less than 5×10−4 Torr and temperatures higher than 500 °C. Our results strongly suggest that at the surface of as-grown LNSMO films a native passivation layer for oxygen diffusion may exist, and this layer could be damaged after depositing the STO film at reduced oxygen pressures.

Crystal growth and characterization of the 4-leg spin ladder compound La 2Cu 2O 5

Crystallization of La 2Cu 2O 5 has been investigated in the La 2O 3–CuO system for various starting compositions both in air and oxygen atmospheres. In air, we find that there exists a new region of crystallization of La 8Cu 7O 19 phase, just above that of La 2Cu 2O 5. However, this region vanishes completely in flowing-oxygen atmosphere. By utilizing this, large single crystals of the 4-leg spin-ladder compound La 2Cu 2O 5 have been successfully grown from CuO flux by the slow cooling method. Room-temperature resistivity parallel to the ladder direction (‖ b) of the as-grown crystals is estimated to be 1.7×10 3 Ω cm. A significant decrease in the resistivity (798 mΩ cm at 300 K) is observed upon hole doping by means of high oxygen pressure annealing (approximately 400 atm) using a hot isostatic pressing furnace. However, no transition was observed from the semiconducting behavior.

Peak effect and oxygen ordering in YBa 2Cu 3O 7− δ single crystals

We have investigated the second magnetization peak in pure YBa 2Cu 3O 7− δ single crystals with various oxygen contents (6.91<7− δ<6.97) and degrees of oxygen vacancy ordering, as achieved by low (1 bar) and high (100 bar) oxygen pressure annealing. Although the position of the peak changes drastically with oxygen stoichiometry, no dependence on the distribution of oxygen vacancies has been found for temperatures below 70 K. For T>70 K, however, ordering effects become important as demonstrated by the disappearance of the peak for the high pressure annealed samples. These results suggest that while at low temperatures, pinning of the vortex system by clusters or a more homogeneous distribution of oxygen vacancies is similar, at elevated T, the former are much stronger pinning sites leading to larger hysteresis and the presence of the peak.

Influence of annealing oxygen pressure on in-plane epitaxy and twinning in (110)YBa2Cu3O7−δthin films

The effects of annealing oxygen partial pressure on the in-plane epitaxy and twin formation in (110)-oriented ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) thin films grown on (110) ${\mathrm{SrTiO}}_{3}$ substrates by pulsed laser deposition have been studied by high-resolution x-ray diffraction, using diffraction-space mapping with a four-circle diffractometer. These films are found to have a structure in which the crystalline a and b axes are tilted about $0.5\ifmmode^\circ\else\textdegree\fi{}$ away from those of the substrates, while keeping the (110) planes parallel to each other. A decrease of in situ annealing oxygen pressure induces a reduction in twinning, which is finally suppressed in the film annealed at an oxygen pressure of 600 mTorr. Our results suggest that twin formation during a tetragonal-to-orthorhombic transformation is controlled by competition between the uniaxial force exerted at the interface by the vicinal substrate surface and the thermodynamic force originating from the difference in the lattice parameters for the a and b axes of orthorhombic YBCO.

High pressure oxygenation in melt textured NdBa/sub 2/Cu/sub 3/O/sub 7/: identification of pinning mechanisms

A competing effect inducing both an enhancement of interface pinning and aging of the critical currents has been established in NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ (Nd123/422) melt-textured ceramic composites upon high oxygen pressure annealing. It is suggested that Nd123/422 interfaces are enriched in antisite defects, which are properly oxygenated only upon high PO/sub 2/ annealing. Additional evidence for the enhanced oxygenation is gained from the observation of a consequent decrease of the field induced anomaly at intermediate fields.

Growth of biaxially oriented conductive LaNiO 3 buffer layers on textured Ni tapes for high- Tc-coated conductors

Electrically conductive LaNiO 3 buffer layers have been grown on textured nickel tapes by sputter deposition in a reducing atmosphere. These conductive buffered Ni tapes can be employed as long flexible substrates for biaxially aligned high-temperature superconductors, and can electrically stabilize the superconductor during transient loss of superconductivity. Deposition temperatures of about 400°C result in epitaxial growth of LaNiO 3 films, while higher deposition temperatures may cause the formation of La 2NiO 4 films. X-ray θ–2 θ, θ, and φ scans reveal that the LaNiO 3 films have good out-of-plane and in-plane orientations of 5° and 9–10°, respectively. Subsequent oxygen anneals at 750°C and 850°C change neither the biaxial orientations of the film nor cause any cracks or other surface defects. The resistivity of the LaNiO 3 films can be further reduced by increasing the annealing temperature and oxygen pressure during post-annealing. Highly textured YBa 2Cu 3O 7 (YBCO) films were grown on the LaNiO 3-buffered Ni by pulsed laser deposition (PLD).

A New Cobaltite with a Tubular Structure: Bi3.7Sr11.4Co8O28-δ, the n = 2 Member of the Series [Bi2Sr2CoO6]n[Sr8Co6O16-δ

A cobaltite, Bi{sub 3.7}Sr{sub 11.4}Co{sub 8}O{sub 28{minus}{delta}}, with a tubular structure derived from the 2201-type has been synthesized for the first time. This oxide represents, like the homologous manganese phase, the n = 2 member of the large structural [Bi{sub 2}Sr{sub 2}MO{sub 6}]{sub n}[Sr{sub 8}M{sub 6}O{sub 16{+-}{delta}}] family (M = Cu, Co, Mn). Although its structure is similar to the Mn phase, it crystallizes in a different space group, Ammm (or Amm2), with a = 5.5232(2) {angstrom}, b = 23.465(1) {angstrom}, and c = 23.462(1) {angstrom}. The HREM study confirms that the structure consists of the stacking along b (or c) of 2201-type slices with single cobalt-deficient perovskite slices [Sr{sub 8}Co{sub 6}O{sub 6-{delta}}]. But the important difference with the Mn phase concerns the nature of the Co{sub 4} pillars, which are at the intersection of two [Sr{sub 8}Co{sub 6}O{sub 16{minus}{delta}}] slices. They form four corner-sharing CoO{sub 4} tetrahedra instead of MnO{sub 6} octahedra and MnO{sub 5} pyramids. This structural difference means that the Mn and Co phases do not form a complete solid solution. The tubular series Bi{sub 4{minus}y}Sr{sub 12{minus}z}Mn{sub 8{minus}x}Co{sub x}O{sub 28{+-}{delta}} could only be obtained for 0 {le} x {le} 2.5. The magnetic properties of this oxide aremore » very complex and are very sensitive to the oxygen content. In particular, after oxygen pressure annealing, they are characteristic of spin-state transitions with temperature close to 50 and 450 K, similar to those observed in the LaCoO{sub 3} compound.« less

Enhanced interface pinning and aging by high pressure oxygenation of melt textured Nd 1+ xBa 2− xCu 3O 7− δ

A competing effect inducing both an enhancement of interface pinning and aging of the critical currents has been established in NdBa 2Cu 3O 7− δ /Nd 4Ba 2Cu 2O 10 (Nd123/422) melt-textured ceramic composites upon high oxygen pressure annealing. We demonstrate that, when high pO 2 treatments are properly chosen, Nd123/422 interfaces become the dominant flux pinning centre, strongly increasing the critical currents at low and intermediate magnetic fields. It is suggested that Nd123/422 interfaces are enriched in antisite defects, which are properly oxygenated only upon high pO 2 annealing. Additional evidence for the enhanced oxygenation is gained from the observation of a consequent decrease of the field induced anomaly at intermediate fields. A second consequence of high pressure oxygenation is the appearance of an aging effect in the critical currents and irreversibility line. This is the first time that aging is described in Nd123/422 ceramic composites.

Sol–Gel Synthesis and Simultaneous Oxidation ofLn2Ba2Cu2Ti2O11−δ

The synthesis of several substituted and unsubstituted derivatives of the series of layered perovskitesLn2Ba2Cu2Ti2O11by a sol–gel method is reported. Reduced synthesis temperatures were attained for La, Nd (750°C), and Eu (900°C) derivatives, whereas Tb derivatives could not be obtained by this method. The products were characterized by small particle sizes of 30–100 nm (compared to ∼800 nm for samples prepared by the classical ceramic method). Furthermore,in situoxidation of the samples took place leading to oxidized products, such as La2Ba2Cu2.15Ti2O11, which earlier could have been obtained only by high oxygen pressure (5 kbar) annealing.

Chemical versus electrochemical doping of layered complex perovskites Ln 2Ba 2Cu 2Ti 2O 11 (Ln  La, Eu, Tb)

Complex perovskites Ln 2Ba 2Cu 2Ti 2O 11 (Ln  La, Nd, Eu, Tb) present the topology typical of high- T c superconducting cuprates. Doping of this series has been attempted and several cationic substitutions accomplished. Under regular reaction conditions oxygen contents readjust leaving the oxides effectively undoped. This can be overcome in certain cases by using stronger oxidizing conditions like high oxygen pressure annealing. We have also performed electrochemical oxidation of these compounds in a molten salt electrolyte. The oxygenation of Ln 2Ba 2Cu 2+ x Ti 2− x O 11− y (Ln  La, Eu) and Ln 2Ba 2Cu 2Ti 2O 11 (Ln  Tb) by means of these methods is described, and a comparison of high-pressure versus electrochemical results made.

Upper‐Phase Stability Limit of Bi‐2212

The upper stability line for Bi2Sr2CaCu2O8+α (Bi‐2212) was determined over the temperature range 540°–800°C and the oxygen partial pressure range 1.5 ×101‐7.4× 10‐3 atm using coulometric titration and high oxygen pressure annealing. The decomposition products resulting from Bi‐2212 oxidation are CuO, two alkaline‐earth bismuthates, and the Raveau phase, Bi11Sr9Cu5Oy. The main alkaline‐earth bismuthate is a known phase of cation composition Bi:Sr:Ca = 2.00:2.48:1.19 (Bi:Sr:Ca 9:11:5). The other alkaline‐earth bismuthate is a minor phase of cation composition Bi:Sr:Ca = 2.00:0.53:0.39. The kinetics for Bi‐2212 decomposition are very slow; e.g., at 700°C in air, the reaction is incomplete after 7 days.

Enhancement of T c in Y 1−xPr xCu 2.8Re 0.2O 6+z (z ∼ 1; x=0,0.75) due to the elevated oxygen pressure annealing

We report on the annealing study of the Y 1−xPr xSr 2Cu 2.8Re 0.2O 6+z (z ∼ 1; x = 0,0.75) samples under elevated oxygen pressures (200 bar). For the samples prepared at atmospheric pressures T c = 42 K for x=0 and < 5 K for x=0,75. After annealing for x=0, T c increased from 42 to 55 K and superconductivity was observed at 14 K for the x=0.75 sample. The data may be explained due to a reduction in oxygen chain defects.

Pulsed excimer laser ablation of (Pb,La)TiO3 thin films for dynamic random access memory devices

Polycrystalline (Pb,La)TiO3 thin films were deposited by pulsed excimer laser ablation on Si and Pt coated Si substrates. The crystallinity of the films showed dependence on the annealing temperature and oxygen pressure during ablation. Similar trends reflected in the dielectric behavior in terms of varying dielectric permittivity between 430–620 as the ablation pressure varied between 1 and 100 mTorr. Films exhibited the presence of ferroelectricity in terms of hysteresis in capacitance-voltage characteristic in a metal-ferroelectric-metal configuration. The charge storage density estimated from the polarization measurements was about 10 μC/cm2, while a leakage current density of 10−8 A/cm2 was observed at an applied field of 100 kV/cm.

Annealing of Excimer-Laser-Ablated BaTiO3 Thin Films.

Thin films of BaTiO3 have been deposited on quartz substrates by the excimer laser ablation method. The effects of substrate temperature, oxygen pressure and post-deposition annealing were investigated. At substrate temperatures above 300°C, randomly oriented crystalline BaTiO3 films with a tetragonal structure were obtained, and the crystallinity was independent of oxygen pressure during the deposition. The film with a-axis preferred orientation perpendicular to the substrate surface was obtained by annealing at 1000°C for 60 minutes.