Resistance-temperature Curves Research Articles

Preparation of 110 K BiPbSrCaCuO thin films by d.c. magnetron sputtering and their transport properties

Superconducting BiPbSrCaCuO thin films with a zero resistivity temperature of 110 K and a critical current density of 2733 A cm −2 (at 77.02 K) were deposited on MgO(100) single crystal substrates by d.c. magnetron sputtering. After deposition at 350–400°C and post annealing at 840°C for 1–5 h in air, with different cooling rates, thin films were obtained which were highly c-axis orientated, with zero resistivity temperatures, T c0, of 103–110 K. The thin film samples with a mixture of high (2223) and low (2212) T c phases show a low T c onset and sharp transition in the resistivity-temperature ( R- T) curves with relatively high current densities. The transport properties of the 100 K sample were studied. The temperature dependence of the critical current density exhibits weak-link characteristics. The mechanism of 2223 phase formation is also discussed.

Thermally activated dissipation and critical field H c2 in c-oriented high-T c Bi-Pb-Sr-Ca-Cu-O thin film

A set of resistivity-temperature (R-T) curves measured under various applied fields in a high-Tc Bi-Pb-Sr-Ca-Cu-O thin film which has a zero-resistance temperature Tc0 of 110 K is reported. The remarkable broadening of the transition width is discussed under the flux-creep model, considering the very short coherence length of this oxide superconductor. The resistivity is thermally activated, which is consistent with the Arrhenius law with a magnetic field and orientation-dependent activation energy U0(H,Θ). The U0(H,Θ) has a very high value of 381.6 meV under a field of 0.1 T parallel to the c axis. The upper critical field Hc2 determined from these R-T curves shows high values and the effect of flux creep to the Hc2(0) is examined by the irreversible behavior with the ‘‘giant’’ flux-creep model.

Effect of heat treatment and composition on the superconductivity in Pb-Sr-R-Ca-Cu-O oxide system

The effect of compositions, sintering temperatures and furnace atmospheres on the superconducting transition temperatures of several members of the series Pb2Sr2R1−x Ca x Cu3O z (0.0⩽x⩽0.7, R=Y, Gd) has been studied. The effect of partial replacement of Pb, Ca and Y by In has also been studied. The shapes of the resistivity-temperature curves and the zero resistance temperatures are found to be extremely sensitive to the synthesis parameters employed. Superconductivity is not observed in samples treated in oxidizing atmosphere or heated at very high temperatures. Samples sintered in flowing nitrogen showed incomplete transition in resistivity starting at about 75 K and extending down to 15 K. A new and comparatively simple synthesis procedure involving treatment of the samples in vacuum has been employed to obtain nearly single-phase materials showing metallic behaviour. Using this procedure, samples of the above composition, but containing no Ca(i.e. Pb2Sr2RCu3Oz, R=Y and Dy), are also found to be superconducting with zero resistance up to 48 K.

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa2Cu3O7(YBCO) on (100) LaAlO3 were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (Tc) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (Jc) at 77 K has an average value of 2×105 A cm−2, with a tendency for decreasingJc with increasing film thickness (0.2 to 0.8μm). Variations inJc are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJc. If the film composition is enriched in Y relative to Ba and Cu,Tc decreases by several degrees.

Current conduction and macroscopic quantum interference in BiSrCaCuO screen printed films

Electric current conduction properties of screen printed superconducting BiSrCaCuO films on alumina substrates were investigated. The film contains the low Tc 2212 phase. The temperature dependence of Hc2 and of critical current were determined from the magnetic field dependence of resistance-temperature curves and from current-voltage characteristics. The film cross section dependence of critical current density and change in the shape of current-voltage curves due to the cross section reduction of the film were observed. Quantum interference inherent in the film was demonstrated.

Electrical conduction and dielectric behaviour of the oxidic spinel Li0.5+0.5xCr0.3TixFe2.2?1.5XO4

The electrical conductivity of polycrystalline lithium-chromium ferrites substituted successively with Ti4+ was studied as a function of composition and temperature. The resistivity increased with composition and all the samples exhibited a change in slope in the resistivity-temperature curve. Two different regions with a large variation in activation energies were found. The dielectric constant and loss factor for different compositions were measured in the frequency range of 10kHz to 10 MHz. The dielectric permittivity showed a dispersion with frequency for all the samples. Peaks were observed for the samples with 0.4 ⩽ x ⩽ 0.8. The possible mechanisms involved are discussed.

Electrical properties of NbTe 4 and TaTe 4

Electrical properties of 1 3 - filled charge-density wave systems, NbTe 4 and TaTe 4, are studied. For both systems, the resistivity is isotropic in the tetragonal a— c plane and increases with increasing temperature. For TaTe 4 no anomalous structure is found in the resistivity-temperature curve because it is commensurate in the temperature range studied. For NbTe 4 changes in the slope of the resistivity-temperature curve corresponding to the incommensurate-discommensurate and the discommensurate-commensurate transitions are observed. Changes are observed in the temperature derivative of the thermoelectric power at these transitions.

Superconducting superlattices: Verification of two-dimensional nature in high T c Bi2Sr2(Ca1−xYx)Cu2O8 superconductors

An approach to evaluate the two-dimensional nature in a high Tc superconductor is examined on the superconducting superlattice prepared by stacking of superconductor and nonsuperconductor layers. The Bi2Sr2(Ca1−xYx)1Cu2O8-based superlattices have been formed by stacking two different unit cell layers containing different yttrium concentrations. One unit layer contains lower yttrium concentration (x=0.15) to be a superconductor, and the other contains higher concentration (x=0.5) to be a semiconductor, and these layers are periodically stacked with appropriate ratios. These superlattices exhibit the same resistance-temperature curve as the original superconducting films with x=0.15. This behavior can be explained by the strong two-dimensional character in this Bi2Sr2(Ca1−xYx)Cu2O8 high Tc superconductor.

Li substitution in the Bi 2Sr 2Ca 1Cu 2O 8 superconductor

The T c value of the Bi 2Sr 2Ca 1Cu 2O 8 superconductor (2212-phase or BSCCO) rises with the substitution of Li for Cu. For the nominal composition of Bi:Sr:Ca:Cu:Li = 2.2:1.8:1.05:1.45:0.7, almost single 2212-phase is obtained by sintering at low temperature, 740°C, followed by quenching to room temperature. The samples shows a T c,zero of 91 K in the resistance-temperature ( R- T) curve and a T c,on of 92 K in the Meissner effect. Both Ar annealing and O 2 annealing make the R- T curve semiconductive. The results of X-ray diffraction measurements, electron microscopy and elemental analysis on this sample indicate that the Li ion occupies the Cu site.

Oxygen Nonstoichiometry and valences of bismuth and copper in Bi2.00Sr1.88Ca1.00Cu2.14Oy

The oxygen nonstoichiometry of Bi2.00Sr1.88Ca1.00Cu2.14Oy was determined as a function of temperature and oxygen partial pressure. It was found that the shape of y−log P02 curves above 800°C is quite different from that below 800°C. Above 800°C, the y−log P02 relation resembled those of other kinds of high Tc superconducting oxides such as Ba2YCu3O7−δ and (La1−xSrx)2CuO4−δ. However, below 800°C y slightly depended on the temperature but did not depend on the oxygen partial pressure. It was found by chemical analysis on quenched samples that the copper valence was 2.07 irrespective of the annealing condition, and the bismuth valence was nearly constant when the annealing temprature was above 800°C but increased with the decrease of the annealing temperature below 800°C. The tailing of resistivity-temperature curves in the vacinity of the critical temperature seemed to be due to the existence of Bi5+ and neutral oxygen species.

Investigation of the coupling mechanism between superconducting grains in high- Tc superconductors

Electric field generation due to transport current was investigated experimentally in high- T c YBa 2Cu 3O 7−δ ceramic superconductors. The external magnetic field and transport current density dependence of resistivity-temperature characteristics, as well as the external magnetic field and temperature dependence of current-voltage characteristics, were investigated in the temperature range from 4.5 K up to T c and in magnetic fields of 0 to 6 T. The separation of intragrain and intergrain (grain boundary) contributions to the electric field was performed. The upper critical field of superconducting grains was calculated from the magnetic field dependence of resistivity-temperature curves. A power law dependence of voltage on ( I- I c) was found in up to 5 mV/cm electric fields. An exponential temperature dependence was also found.

Shock-wave-induced degradation of superconductivity and localized lattice deffects in explosively fabricated YBa 2Cu 3O 7

X-ray diffraction studies for explosively fabricated YBa 2Cu 3O 7 at pressures ranging from 4 to 19 GPa show characteristic orthorhombic split peaks at 2 θ ≅ 32° and 58° to be altered systematically with increasing pressure, and the midpoint values, R(T c) R 0 = 1 2 , of the resistance-temperature curves decrease with increasing pressure, indicating a broadening of the superconducting transition. Moreover, the T c onset (at 91K) does not charge between 4 and 8 GPa peak pressure in contrast to R-T curves corresponding to oxygen loss. No oxygen loss has been measured up to peak pressures of 13 GPa. To achieve full recovery (and a sharp transition) in these materials, post-fabrication oxygen processing at temperatures in excess of 900°C is required and is believed to be due to a unique localized lattice defect formed during fabrication. This defect has been observed by high-resolution transmission electron microscopy and has been presumably created by some form of defect clustering or “precipitation” in the shock front.

Effect of precipitates on the electrical resistivity-temperature curves in an aged Ti-51.8 at % Ni shape memory alloy

Etude du processus de precipitation de la phase Ti 3 Ni 4 et de son influence sur les temperatures de transformation de phase de la matrice dans un alliage Ti-51,8% at. Ni apres vieillissement

Thermoanalytical characterization (DTA and TG) of RBa 2Cu 3O 7-x (R  Y, Gd, Nd) superconductors

Differential thermal analysis (DTA) and thermogravimetric (TG) analysis were used to characterize RBa 2Cu 3O 7-x (R  Y, Gd, Nd) superconductors. Even if the resistivity-temperature curves are similar for the three compounds, DTA shows that the behavior of these materials during synthesis and sintering is different. Of the three systems, the Nd-Ba-Cu-O system is the most favorable for the formation of the 123 superconductor because secondary phases are present in smaller quantities. TG curves suggest the presence of a transition from the orthorhombic phase to the tetragonal phase in a nitrogen atmosphere at about 550–600°C for the three perovskites. The energy required to insert oxygen atoms in the crystal structure is smaller for the neodymium and gadolinium compounds than for the yttrium compound (Gd < Nd < Y). There is also a proportional relation between the ionic radius of the rare earth element and the onset melting temperature.

Structure and superconductivity of variously prepared Tl-Ca-Ba-Cu-O compounds

Starting materials, compositions and heat treatments were varied to study the influence of preparation conditions on the structure and superconductivity of Tl-Ca-Ba-Cu-O compounds. Twenty-four different groups of samples were synthesized, and their phases, structures, superconducting parameters and inter-relations were investigated. Most of the samples are superconducting above 100 K. The critical temperatures occupy a rather wide interval of 61–121 K. A few specimens have no superconductivity down to liquid helium temperatures. The number, type, quantity and proportion of phases produced strongly depend on the chemical quality and mixture of the starting materials and on the heat treatment. We searched for all known phases which may be responsible for superconductivity. It has been observed that better superconductivity generally occurs for the more highly crystalline structures. In our experiments, the presence of non-reacted barium cuprates in the samples does not obstruct the establishment of superconductivity, but Tl 2O 3, CaO, BaO, CuO and other impurities disturb and limit the quality of superconductivity. The superconducting parameters are also affected by the oxygen stoichiometry. The resistivity-temperature curves of some samples indicate the presence of a two-phase system. A change with time of the microstructure of some samples was observed, and was reflected in macroscopic change of the superconducting parameters. This is supported by NMR investigations. Scanning tunneling microscopy indicates an oriented surface structure of the samples.

Ｂｉ‐Ｓｒ‐Ｃａ‐Ｃｕ‐Ｏ系超伝導体の高ＴＣ相生成に及ぼす“ｉｎｅｒｔ ｐａｉｒ”元素酸化物の添加効果

Specimens of M2.2Sr2.0Ca2.0Cu3.zOy [M2.2=Bi1.8Pb0.4-x Inx and Bi1.8Pb0.2M'0.2 (M'=Sn, Sb and Y)] were sintered at 855°C for 40-96hr after decarbonation in air atomosphere and their resistance-temperature curves were compared in order to investigate effects of "inert pair" elements on the formation of the high-Tc phase. In low level of In content (x-value) below 0.18, the stabilization of the high-Tc phase by incorporation of Pb remained almost unaffected. The step wise (shoulder-like) drop in resistance curves due to the coexistence of high and low- Tc phases was observed on specimens of In0.2, Sn0.2 and Sb0.2, showing that lighter inert pair elements have poorer tendency to stabilize the high-Tc phase. The specimen of Y0.2 showed disappearance of the high-Tc phase as well as in specimens of In0.4 without Pb.

Bi-Sr-Ca-Cu-O high T C superconductors (abstract)

A recent paper by Maeda et al.1 reported the discovery of a new 120 K superconductor based on a rare-earth–free ceramic material in the bismuth-strontium-calcium-copper-oxide system. We have confirmed the existence of the 120-K phase in the oxide system. The resistivity-temperature curve showed a fairly well-defined double-dip shape with the first dip starting at ∼120 K and the second at ∼95 K. The transition temperature, TC(R=0), was 84 K. This material appears to contain two superconducting phases with different TC’s. A pellet made from this superconductor levitates well at 77 K (liquid-nitrogen temperature) above a neodynium-iron-boron magnet with a surface magnetic field of ∼3000 Oe, thus indicating a nontrivial critical field in this material. The critical current density and its magnetic field dependence will be reported. The magnetic and mechanical behavior of the new superconductor will also be discussed.

A study of thermal relaxation and crystallization of an Fe 81B 13.5Si 3.5C 2 amorphous ribbon

As-melt-spun and heat-treated Fe 81B 13.5Si 3.5C 2 amorphous alloy ribbons have been studied through differential scanning calorimetry (DSC), thermomagnetic analysis (TMA) and electrical resistance change with temperature. TMA showed that the value of T c of the as-melt-spun ribbon (354 °C) increased by about 6 °C owing to thermal relaxation. The change in slope of the resistance-temperature trace at about 170 °C corresponded to exothermic reactions due to structural relaxation recorded by DSC. Crystallization starts in the temperature range 380–400 °C and proceeds through multiple steps as indicated by the occurrence of multiple DSC peaks as well as breaks in the electrical resistance-temperature curves.

Radiation Effects in High Tc Superconductors for Space Applications

YBa2Cu3O7 is one of the new class of superconductors having a critical temperature (Tc) above 90 K. Devices using these materials would require minimal refrigeration to operate in space. It is important, therefore, to determine the effect of natural space radiation on these materials. Preliminary experiments have been performed to study displacement damage and ionization effects in 100?m thick films of YBa2Cu3O7. Samples of these films were irradiated at room temperature with 1) 56 MeV electrons up to a total fluence of 1.6×1015 cm-2, 2) 6 MeV protons up to a total fluence of 5.0×1013 cm-2, and 3) 63 MeV protons up to a total fluence of 1.09×1014 cm-2. It is concluded from the electron irradiations that the major effect of ionization is to increase the normal resistivity of the material. Although Tc(onset) was not much affected by either the electron or the proton irradiations, Tc(completion) varied substantially with proton fluence and at the higher fluences there was evidence of a non-superconducting region even at 4 K. It appears from the shape of the resistivity-temperature curve that high energy proton irradiation induces a partially superconducting multi-phase mosaic. Comparing worst case space environments to the radiations described above, it is concluded that this high Tc material could be made to remain superconducting for extended space missions with only moderate shielding.

Digital temperature compensation of a thermistor flowmeter

The temperature compensation of a thermistor flowmeter is accomplished digitally, thus simplifying the compensation problem by replacing required hardware with software. The resistance-temperature curve of the thermistor is a least-squares fit to a logarithmic function whose coefficients are stored in computer memory. Signals representing the temperature of the heated thermistor and the ambient temperature are digitised and converted to temperature values based on the stored resistance-temperature curve. In this case the signals are obtained from a single thermistor switched between the high and low overheat modes. The thermistor temperature T, the ambient temperature Ta, and the electrical power input are used to form the dissipation factor (K=P/(T-Ta)) which calibrates the flowmeter when plotted against velocity. Results from calibration experiments on a thermistor flowmeter designed for use in human coronary arteries are presented to demonstrate the viability of the method.