Bi Sr Ca Research Articles

Surface morphology and electrical properties of BiSrCaCuO thin films deposited by reactive sputtering from multiple targets

Smooth, superconducting films of Bi 2Sr 2CaCu 2O 8+° have been prepared by reactive sputtering from elemental targets in the presence of ozone. The influence of substrate temperature, deposition rate, and ozone pressure on the resulting films are discussed. Films deposited on SrTiO 3 substrates are c-axis oriented and featureless for substrate temperatures below 710°C. Above this temperature, small inclusions of CuO appear. Films on MgO exhibit mixed a-axis and c-axis orientation below about 710°C, and inclusions of CuO above this temperature. The temperature at which this transition occurs increases with increasing deposition rate. T c increases and then decreases sharply with decreasing oxygen content. The oxygen partial pressure corresponding to the maximum T c of 77 K is well above the thermodynamic stability limit for Bi 2Sr 2CaCu 2O 8+°, suggesting that an optimum carrier concentration has been achieved for these films.

X-Ray absorption studies of bi-based superconductors

X-Ray absorption (XANES and EXAFS) spectroscopic techniques have been applied to study the local structural environment of host and dopant cations in Bi-based high-Tc superconductors; the materials studied include the following: BaBiO3, BaBi1 –xPbxO3, Ba1 –xKxBiO3, Bi2Sr2CuO6, Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10. XANES spectroscopy has been used to determine the valence state of Bi, Pb and Cu in these materials. In BaBiO3 the valence state of Bi is found to be close to IV both in the pure and doped material, and a disproportionation of BiIV into BiIII and BiV is not indicated by analyses of these data. The valence state of Pb is also found to be IV in BaBi1 –xPbxO3. In Bi-based cuprate superconductors, the formal valence states of Bi and Cu are slightly higher than their stoichiometric values of III and II. EXAFS spectroscopy has been used to examine the local structure around Bi, Ba, Pb, Sr, Ca and Cu cations in these systems. In the BaBiO3 system, doping with Pb and K results in a decrease in the metal–oxygen bond lengths. The results indicate that the local environment of all cations in the Bi–Sr–Ca–Cu–O system is characterised by substantial static disorder.

Photoemission studies of the BiSrCaCuO valence band

We review the photoemission literature relating to studies of the valence band region of the BiSrCaCuO (BiSCCO) family of high temperature superconductors. Through studies of the copper satellite and the low binding energy features, a picture has emerged of strongly correlated properties mixed with delocalized, band-like ones. We discuss the different aspects of this dual nature, as well as the superconducting properties as seen in photoemission results. In addition, we survey the experimental details of BiSCCO studies and discuss methods to ensure consistent, repeatable results.

Study of dielectric relaxation behaviour in Li-doped semiconducting BiSrCaCuO glasses

The temperature- (80–450 K) and frequency- (50−10 4 Hz) dependent dielectric constant, ϵ′, and loss, ϵ″, for Li-doped Bi 4Sr 3Ca 3− z Li z Cu 4O x ( z = 0.0, 0.1, 0.5 and 1.0) glasses, which become high T c superconducting oxides in their crystalline phases, have been measured. Unlike many other semiconducting transition metal oxide glasses, the temperature variations of ϵ′ and ϵ″ of the Li-doped glasses do not have a peak within the frequency range of the investigations. However, the imaginary ( M″) parts of the dielectric modulus, M ∗ , have peaks indicating Debye-type dielectric relaxation behaviour with a distribution of relaxation times. The relaxation behaviour is thermally activated.

High-T C superconducting glass-ceramics prepared by the melt-quenching methods in the bismuthate system with additives

High-T C superconducting glass-ceramics in the system BiSrCaCuO have been investigated with additives PbO and TeO 2 in the nominal composition Bi 2−x−yPb xTe ySr 2Ca 2Cu 3O z. Three alternative methods for synthesis of precursors have been studied. The free melt quenching technique and tellurium as additive stimulate the formation of high-T C phase.

Improvement of strain endurance of critical current of silver-sheathed superconducting tapes by reducing volume fraction of Bi(Pb)SrCaCuO oxide core

High T c Bi(Pb)SrCaCuO core/silver sheath superconducting composite tapes with various volume fractions of oxide core (0.05–0.5) were prepared by the powder-in-tube method. The influence of the volume fraction of the oxide core on the strain endurance of the critical current density at 0 T at 77 K was investigated. It was demonstrated that the strain endurance of the critical current can be raised by reducing the volume fraction of oxide, due to enhanced compressive residual strain in the oxide which is introduced during cooling from the heat treatment temperature to 77 K.

Preparation of stoichiometric BiSrCaCuO superconducting thin films by RF diode sputtering

The chemical composition of sputter-Deposited BiSrCaCuO thin films was investigated as a function of system geometry and some controllable parameters. Films prepared using conventional configuration were bismuth deficient owing to the selective resputtering by energetic oxygen particles released at the target surface. Exploiting an unusual configuration, which included both an off-axial substrate position and a grounded plate in front of the target, the film stoichiometry reproduced that of the target. The fundamental properties of prepared films are also considered in the paper. In particular, the crystal structure and the superconductivity of in-situ grown stoichiometric films are discussed.

Physical and constructional principles for high-Tc superconducting magnets

Abstract The development of high temperature superconductors offers on the one hand a new possibility for building magnets due to their extremely high upper critical magnetic field, but represents on the other hand rigorous limitations because of their relatively low critical current and rigid and brittle mechanical properties. Among the practicable methods of magnet manufacture the simplest one is to use ceramic spirals with conventional electrical contacts to convey the current. As a second possibility, solenoids of traditional type can be realized, based on cut, electrically contacting rings. Both of these types can be supplied by conventional d.c. electrical treatment. These magnets can, however, create fields of only low intensity and the enormous power dissipation in the contact resistivities causes almost unsolvable cooling problems. In order to avoid these difficulties we have elaborated a new procedure for building high-Tc superconducting magnets, which seems to be more useful from the point of view of applications. They are constructed of individual rings and energized by a contactless external magnetic induction method. Subseqently, the persistent self current provides a magnetic field for every ring. Stacking the rings on top of each other allows the magnetic field strength of the solenoid-like arrangement to be increased, although the small critical current and the thickness of the rings limit its value. Further improvements may be looked for using thick film- or thin layer systems, the high critical current densities of which guarantee a radical increase in the magnetic field. Among the superconducting materials, mainly the YBaCuO and Bi(Pb)SrCaCuO compounds are worthy of consideration.

A new pulse method for determining transport critical current of superconducting films

Abstract A new pulse method has been elaborated to solve the problems associated with the measurement of transport critical current in high-Tc superconductors. This process results in higher sensitivity and accuracy compared with conventional techniques. The essence of the determination of the critical current is to measure the harmonics of the voltage drop caused by applied pulses of short duration and low repetition rate. In this paper the principle of the measuring procedure is described and, as an example, the results for superconducting Bi(Pb)SrCaCuO thick films with high critical current densities up to 5000 – 7000 A/cm2 at 77 K are presented. The results of measurements produced by this method are compared with those from more conventional techniques.

Role of Ag additions in the microstructural development, strain tolerance, and critical current density of Ag-sheathed BSCCO superconducting tapes

Ag-sheathed tapes of Bi–Sr–Ca–Cu–O (BSCCO) and BSCCO–Ag superconducting cores were made by a powder-in-tube technique and subjected to repeated cycles of pressing and heat treatments. These thermomechanical treatments resulted in enhanced texturing and grain growth that improved the critical current density (Jc). Additions of Ag to the BSCCO core further increased texturing and brought an additional improvement in Jc. Strain tolerance of the tapes was evaluated by measuring Jc before and after application of a predetermined level of tensile strain in a uniaxial mode. The fraction of Jc retained after the strain application was higher in the BSCCO–Ag composite tapes than in the monolithic BSCCO. For a 1.2% applied strain, 90% of the initial Jc was retained in the BSCCO–Ag composite tape, compared to only 40% in the monolithic BSCCO tapes. The higher strain tolerance of the BSCCO–Ag tapes may be related to improved mechanical properties (strength, flexibility, and fracture toughness) and grain connectivity due to Ag addition.

Single-phase region of the 2212-BiSrCaCuO superconductor

The phase relationships and solid-solution region of the Bi 2Sr 2CaCu 2O 8+δ (2212) superconductor were investigated at 850°C under air atmosphere in a cross-section of the system BiSrCaCu(O) defined by the formula Bi 2+ y Sr 3− y− x Ca x Cu 2O 8+δ. The system was studied by means of XRD and EMA and an approximate position of the 2212 single-phase region is delimited by the values of y=0.1 and 0.8≤ x≤1.3. The single-phase region determined may be divided into two subregions, Sr-rich in the range 0.8≤ x≤1.0 and Ca-rich in the range 1.1≤ x≤1.3, these differing in the number and character of intergrowths. The highest superconducting transition temperature ( T c) was found for x=0.8.

Erratum

“(110)-oriented Bi–Sr–Ca–Cu–O superconducting thin films prepared by metalorganic chemical vapor deposition” [J. Mater. Res. 8, 978 (1993)]

The effect of post-annealing on laser-deposited superconducting Bi–Sr–Ca–Cu–O thin films

Superconducting Bi–Sr–Ca–Cu–O thin films were obtained from post-annealing partially crystallized and amorphous films grown on MgO(100) by pulsed laser deposition. The substrate temperature investigated was in the range of 350–750 °C, over a range of pressure 0.1 to 100 mTorr. The as-deposited films were annealed in 7.5 vol.% O2/Ar or in air at 800–865 °C from several minutes to a few hours. Unlike the pure Bi2Sr2CaCu2O8+δ (2212) phase (Tc = 80 K) which is easily formed after a long continuous period of post-annealing at a temperature below 830 °C, the formation of (Bi, Pb)2Sr2Ca2Cu3O10+δ (2223) phase from the as-deposited amorphous films requires repetitive annealing cycles of short duration in air at 850 °C to simultaneously achieve good crystal quality, small surface roughness, and sharp diamagnetic transition (Tc = 110 K). After annealing, the temperature is lowered down to ∼650 °C by quenching in air and then a slow-cooling step is employed. This procedure was found to enhance the volume fraction of the 2223 phase as compared with a direct slow-cooling process. The trade-off between annealing temperature and time was observed to affect the phase formation and the smoothness of the annealed films. To optimize the post-annealing conditions, Rutherford backscattering spectrometry, x-ray diffraction, and scanning electron microscopy were systematically used to examine the composition, structure, and morphology of the films, respectively.

Flux pinning, granularity and the irreversibility line of the high-Tc superconductor HgBa2Cu04+x

THE recent discoveries of superconductivity at 94 K in the single-CuO2-layer compound HgBa2CuO4+x (Hg-1201)1 and at 133 K in its two- and three-copper-layer analogues2 (Hg-1212 and Hg-1223) have renewed interest in the search for new high-transition-temperature (high-Tc) superconductors. But whatever the Tc, high values of intergrain critical current density are required for large-scale applications; thus, the material must have good electromagnetic connectivity between grains (it must not be electromagnetically granular) and it should be able to maintain its current-carrying capacity in high magnetic fields (it should have a high irreversibility field, H*(T)). Putilin et al.1 surmised that the small CuO2 layer spacing of Hg-1201 might yield a high H*(T). We report here that H*(T) is indeed significantly higher than for the two- and three-copper-layer Bi–Sr–Ca–Cu–O compounds (Bi-2212 and Bi-2223), but lower than for YBa2Cu3O7 (Y-123). The low-temperature flux pinning is also strong. Like Y-123, however, Hg-1201 is electromagnetically granular. A high degree of grain alignment will probably be necessary to remove this granularity; by analogy with the bismuth compounds, this is likely to be easier if the two-mercury-layer counterparts of Bi-22XY (Hg-2201, -2212 or -2223) can be synthesized.

Glass microstructure and initial crystallization of Pb 0.32Bi 1.68Sr 1.75Ca 2Cu 3O x

Microstructure and phase development of as-quenched and annealed samples of the glass of composition Pb 0.32Bi 1.68Sr 1.75Ca 2Cu 3O x have been studied. Evidence for liquid-liquid phase separation during quenching was found The microstructure of this glass during the early stage of crystallization is similar to that found for Bi 2Sr 2CaCu 2O x (2212) glass. The first phase that crystallizes on annealing at low temperatures appears to be Cu 2O. In further heat treatments the sequential crystallization of the R-phase of (Pb yBi 2− y )Sr 2CuO x (‘2201’) composition and the Pb yBi 2−ySr 2CaCu 2O x (‘2212’) phase was found, in agreement with other studies. Phase separation determines the initial crystal phase and the development of the superconducting phases. Phase separation observed in this glass points to the possibility that immiscibility may be a common feature of a wide range of Bi(or PbBi)SrCaCuO compositions.

Thermodynamic stability and superconductivity of the Bi–Sr–Ca(Y)–Cu–Li–O system

In order to understand the effects of the thermodynamic stability on superconductivity, the thermodynamic stability of the Bi2Sr2CaCu2-xLixOy system and the Bi2Sr2YCu2-xLixOy system was studied. It is found that the broad ranges (0 ≤ x ≤ 0.6) of solid solution may be thermodynamically stable at high temperatures for the Bi2Sr2CaCu2-xLixOy), system and the Bi2Sr2YCu2-xLixOy), system. At equilibrium at low temperatures, however, solid solubility is eliminated, and only the end member compounds and possibly intermediate compounds of fixed composition remain for 0.3 ≤ x ≤ 0.6. It suggests that Li can destroy the thermodynamic stability. Superconductivity is improved by Li-doping, which may be related to the thermodynamic instability.

Current and angular-dependent transport in the mixed state of Bi 2Sr 2CaCu 2O 8+δ

A phenomenology describing recent transport measurements in high quality BiSrCaCuO films and crystals in which the angle θ of the applied field relative to the c-axis is varied is illustrated with fits to experimental results. The data show no evidence for conventional flux-flow; the model features phase-slip in a network of Josephson junctions. At low temperatures, low current densities, and for large fields, the resistivity varies approximately as ϱ ps (H,T,θ)= ϱ n [ I 0 (γ 1 3 (H,T,θ) 2)] 2 with γ≈ A T T c (T|Hcos(θ)|) . The 1 3 power is characteristic of a “stretched exponential” response.

Growth and characterization of laser-deposited superconducting Bi–Sr–Ca–Cu–O thin films

Superconducting Bi–Sr–Ca–Cu–O thin films with thickness 400–3500 Å have been deposited on (100) MgO and (100) LaAlO3 by a pulsed excimer XeCl laser (λ=308 nm, τ=20 ns, and energy density=4–5 J/cm2). The oxygen pressure during deposition and the substrate temperature are in the range of 0.1–100 mTorr and 350–750 °C, respectively. The as-deposited films were annealed either inside (in situ) or outside (ex situ) the deposition chamber to improve the crystallinity and the superconducting transition of the films. The formation of 2212 and 2223 phases was controlled by tailoring the deposition and annealing parameters. The corresponding transition temperatures (Tc’s) of these phases in the ex situ films measured by zero-field cooled diamagnetic transition at 10 Oe are 80 and 110 K; however, the in situ mirrorlike shiny films with 2223 phase only exhibit Tc=81 K (or 98 K in an ac field of 0.1 Oe at 125 Hz). These films were also characterized by scanning electron microscopy, transmission electron microscopy, superconducting quantum interference device magnetometer, and ac susceptometer to study their morphology, microstructure, and magnetic properties. The critical magnetization current density (Jc) of the ex situ films is of the order of 107–108 and 105–106 A/cm2 for H⊥c axis and H∥c axis, respectively, at 7 K up to 5 T. Jc of the in situ films is two orders of magnitude lower than that of the ex situ films, and further enhancement of Jc appears promising if better crystallinity of the in situ films can be achieved. A qualitative phase diagram with respect to the development of 2212 and 2223 phases is given and discussed.

Growth of 2122-phase of BiCaSrCuO superconducting films by LPE process and possibility of its growth in microgravity environment

Abstract The absence of buoyancy driven thermal instabilities in space offers an ideal environment for the growth of defect-free high temperature superconducting thin films. Terrestrially processed thin film specimens as well as bulk single crystals, especially when grown by a melt method like the liquid phase epitaxy (LPE), suffer computational inhomogeneities which are introduced by the thermal instabilities caused by the earth's gravitational force. This leads to the nucleation of various types of defects in the material which adversely influence its fundamental physical, chemical, structural and mechanical properties. This drastically inhibits its technological usefulness. High quality epitaxial layers of the 2122-phase of BiCaSrCu--oxide (BCSCO) having single crystalline structure have been grown by the optimization of the LPE process. The films were grown on twin free single crystals substrates of NdGaO3 from supersaturated solutions of KCl and BCSCO. Of the commonly used substrates for the growth of high-Tc films, NdGaO3 has a unique advantage of having a very small lattice mismatch, of about 0.4%, with the 2122-phase of BCSCO. The temperature regime of 850 to 830 °C is found to be the most suitable range for the formation of epitaxial layers of the 2122-phase on NdGaO3 substrates. X-ray diffraction, SEM and TEM studies reveal the excellent epitaxial and single crystalline nature of the films. Compositional analysis is carried out by the Rutherford Back Scattering (RBS) technique. Also, these films exhibit 100% reflectivity in the far infrared regime of 2 to 25 microns. The standard 4-point probe resistivity measurement shows the onset of superconductivity at 90 K and the zero resistance at 83 K. These LPE experiments, if conducted in the microgravity environment of space, should allow us to grow superior films of the 80 K phase of BCSCO with no or minimal defects. The results obtained on space grown samples will not only be of immense value for understanding the physical mechanisms of this unique phenomenon rather it will also help in introducing improvements in the growth technology on earth to produce high quality samples for various electronic and photonic applications.

Structural transformations and metastable phases produced by mechanical deformations in the Bi–Sr–Ca–Cu–O superconducting system

The structural transformations occurring during the mechanical alloying of mixtures of elemental oxides in the Bi–Sr–Ca–Cu–O system have been studied. Metastable cubic structures with complete chemical disorder and amorphous phases can be synthesized by strong mechanical deformations. With proper thermal treatments, these metastable phases transform into the high Tc superconducting structures. Discussions on the mechanisms of formation and on the thermal stability of the various metastable phases are presented.