Hall effect and thermoelectric studies on Pb and Ag-doped Bi2Sr2Ca2Cu3O x superconductors

Hall effect and thermoelectric power measurements at room temperature in the magnetic field of 18 kOe on Ag- and Pb-doped Bi₂Sr₂Ca₂Cu₃O _x samples indicate that the density of charge carriers decreases with increase in Ag concentration whereas the reverse is observed with Pb. From the reduced scattering factor, it is assumed that the scattering of the charge carriers occurs due to acoustic phonons in the normal state at room temperature.     

Preparationin situ and properties of thin films HTSC YBa2Cu3O x

We describe the technique ofin situ preparation of thin (d=0, 1-1μm) YBCO films by the RF magnetron sputtering of the ceramic target in the argon-oxygen atmosphere on the heated substrates, and report the preliminary results of investigation of the multilayer structure based on such films.     

High temperature superconductivity in orthorhombic LaBa2Cu3O7−δ

High temperature superconducting LaBa₂Cu₃O_7−δ has been prepared by ceramic and nitrate methods to understand the influence of preparation conditions on superconductivity. The characterization was carried out by X-ray diffraction and magnetic susceptibility measurements.T _c onset was observed at 88 K. Meissner effect has been observed above the liquid nitrogen temperature.     

Structural and Physical Properties of Nd Substituted Bismuth Cuprates Bi1.7 Pb0.3−x Nd x Sr2Ca3Cu4O12+y

BiPb-2234 bulk samples with nominal composition of the compound Bi_1.7Pb_0.3−x Nd _x Sr₂Ca₃Cu₄O_12+y (BSCCO) (0.025≤x≤0.10) have been prepared by the melt-quenching method. The effects of Nd substitution on the BSCCO system have been investigated by electrical resistance (R–T), scanning electron microscopy (SEM), X-ray diffraction (XRD) and magnetic hysteresis measurements. It has been the BSCCO (2212) low-T _c phase is formed for all the substitution levels, together with the BSCCO (2223) high-T _c phase. The results obtained suggest that with increasing Nd³⁺ doping for Pb²⁺ the (2223) phase existing in undoped BSCCO gradually transforms into the (2212) phase and hence all of the samples have a mixed phase formation. The R–T result of the samples show two-step resistance transition; first transition occurs at 100 K and second in an interval of 80–90 K, depending on the Nd concentration. We have found that the magnetization decreases with increasing temperature in agreement with the general characteristic of the high-T _c materials. The samples exhibit weak field dependence particularly after 2 T and changes on the magnetic hysteresis, M–H curve rather small compared to the conventional superconducting materials. The maximum critical current density, J _c, value was calculated to be 8.51×10⁵ at 4.2 K and J _c decreases with increasing temperature and the substitution level.      

Characterization and properties of antimony doped Bi1.6Pb0.3Sr2Ca3Cu4O x ceramics

The influence of Sb₂O₃ dopant on the superconductivity and the phase formation of the Bi(Pb)-Sr-Ca-Cu-O system was studied using differential thermal analysis/thermogravimetric analysis (DTA/TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), a.c. susceptibility and resistance measurements. It was found that the solubility of Sb³⁺ in the 110 K superconducting phase is limited; instead Sb³⁺ readily combines with other elements to form one or more unknown second phase(s), which melt at lower temperatures and accelerates the formation of the 110 K phase. Resistance measurements show the transition initiation temperature of the 110 K superconducting phase is unchanged but thatT _c (zero) was found to decrease from 107 K to 98 K. TEM examination generating lattice images from antimony-doped crystals, showed the structure to be inhomogeneous and to contain many defects.     

Oscillating magnetoresistance of (Bi,Pb)2Sr2Ca2Cu3O x ceramics doped with Mn

The magnetoresistance nearT _c(T _cR=0=100 K) has been measured on ceramic samples of (Bi, Pb)₂Sr₂Ca₂(Cu_0.98Mn_0.02)₃O_x (X-Ray diffractions and measurements by a SQUID magnetometer give almost single 2223 phase) in magnetic fields, up to 60 kOe using a dc four-probe technique. We observed periodic oscillations superimposed on a monotonie growth of the magnetoresistance with the amplitude decreasing with increase in magnetic field. Fourier transforms of the magnetoresistance data gives two sharply pronounced periods, 10.7 and 14.7 kOe. Apparently these oscillations are connected with inhomogeneous distributions of Mn in superconducting grains with phase slippage at grain boundaries.     

On the synthesis and structure of single-phase (Bi,Pb)2Ca2Sr2Cu3O10

We report an elegant method for the synthesis of single-phase Bi-2223 superconductor from a stoichiometric composition Bi_1.7Pb_0.3Ca₂Sr₂Cu₃O_y by a matrix reaction route. The superconducting transition temperatureT _c (R=0) of this single-phase compound is 120 K. The effect of Pb-content and sintering temperature on the formation and stability of Bi-2223 phase is described.     

The Role of Defects in Persistent Photoconductivity in YBa2Cu3O6 + x

In the persistent photoconductivity (PPC) phenomena, illumination of a material with light leads to a long-lived photoconductive state. During illumination of oxygen deficient YBCO near the insulator–metal transition, the illuminated material may even become superconducting. Accompanying the large relative changes in resistance are transport and structural changes as well as changes in the photoluminescence and infrared quenching spectrum. Infrared quenching data show that the magnitude of IR quenching saturates quickly as a function of visible photon dose, long before the PPC effect saturates. Our experimental and theoretical results have suggested that persistent photoconductivity (PPC) in YBCO may be explained by a model that incorporates both electrons trapped at defects (oxygen vacancies) and resulting structural rearrangement in the basal plane. A simple cellular automata model based on the idea that trapped electrons in the basal plane lead to structural rearrangements when illuminated with light predicts the correct time dependence of the photoexcitation process.     

Effect of additives on the morphology and superconductivity of YBa2Cu3O6+x ceramic materials

The effect of the addition of 0–15 wt.% alumina powder on the grain size, morphology and superconducting properties of sintered YBa₂Cu₃O_6+x was investigated as a function of alumina concentration in the composites. The results suggest that the grain size of YBa₂Cu₃O_6+x decreases with an increase in the additive concentration. Some surface erosion of the YBa₂Cu₃O_6+x grains was also noticeable in Al₂O₃/YBa₂Cu₃O_6+x composites. The results also indicate that the additive tends to stabilize the nonsuperconducting tetragonal phase of the YBa₂Cu₃O_6+x at the expense of the superconducting orthorhombic crystal phase.     

The Effect of Sb and Pb Doping on the Critical Temperature of the Bi1.6Pb x Sb y Sr2Ca2Cu3O z Superconductor

In this paper, the effect of Sb and Pb elements doping on the Bi_1.6Pb _x Sb _y Sr₂Ca₂Cu₃O _z (BPSSCCO) superconductor was studied. The BPSSCCO superconductor was synthesized by a solid state reaction method. The superconductivity properties such as the critical temperature, magnetic susceptibility, and room temperature resistivity have been investigated. In order to study the effect of Sb and Pb elements on the room temperature resistivity of the samples, the V–I curves were measured. The resistivity of the Sb doped samples at room temperature increases when the content of Sb is increased. It is also decreased by increasing the annealing time. The results showed that the optimum condition was x=0.35, y=0.05, with an annealing time of 270 h.     

Formation of high-T c Bi1.6Pb0.4Sr2Ca2(Cu0.9Ti0.1)3O x

Influences of sintering conditions onT _c are studied for Bi_1.6Pb_0.4Sr₂Ca₂(Cu_0.9Ti_0.1)₃O _x . HighT _c values are obtained at 1123±5 K sintering temperature and over 50 h sintering time. In order to explain theT _c changes, morphological discussion is attempted.     

Effect of Cd Addition on Superconducting Fluctuations and Mechanical Properties of Bi1.82Pb0.36Sr2Ca2Cd x Cu3O y System

A study was made of the effect of Cd additions on the superconducting and mechanical properties of Bi_1.82Pb_0.36Sr₂Ca₂ Cd_xCu₃O_y (x = 0.0, 0.15, 0.25, 0.35 and 0.55). Characterization of the Cd-samples using XRD, DTA, and SEM techniques, has confirmed that remarkably formation of low-T _c phase (2212) by the addition of Cd up to 0.35. High-resolution electrical resistivity ρ (T) data on the composition of Cd = 0.35 have been taken for investigating critically the superconducting fluctuations. Using the Aslamazov and Larkin (AL) and Lawrence and Doniach (LD) models of excess conductivity. Excess conductivity analysis shows that this composition (Cd = 0.35) is 2D in the temperature range 137.8–163.7 K and a 3D one below 137.8 K. Thus, a crossover from 2D to 3D is observed at 137.8 K. Sample microhardness and density are greatly improved by Cd-additions (0.35). This trend is probably due to the intercalation of cadmium between superconducting grains in compositions may provide a plastic-flow region that allows relaxation of undesirable stresses resulting from the grain anisotropy of superconductors.     

Effect of TaC Microadditions on the Microstructure and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + x Superconducting Ceramics

Information-theoretical approaches are used to assess the effect of nanocrystalline TaC microadditions on the thermodynamic conditions of microstructure evolution in (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 + x} superconducting ceramics. Optimization analysis of property–structure–composition relationships indicates that the introduction of TaC makes it possible to raise the midpoint transition temperature T _c of the ceramics by 4 K and their 77-K critical current density j _c by 1000 A/cm². It is shown using information interpretation of multifractal formalism that the optimal TaC content is about 0.14 wt %. At this doping level, the expected increase in T _c is 8 K, and that in j _c is 1300 A/cm². The crystallite and pore substructures in Bi-2223 ceramics are shown to be correlated.     

Effect of HfN Microadditions on the Microstructure and Superconducting Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + x Ceramics

The effect of ultrafine hafnium nitride additions (0.05–0.2 wt %) on the microstructure, phase composition, density, and superconducting properties (T _c and j _c) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 + x} ceramics is studied. The elemental composition of individual phases is determined, and the magnetic-field distribution in the ceramics is examined. The introduction of hafnium nitride increases the density of the ceramics and improves their microstructural stability at sintering temperatures between 840 and 850°. The low hafnium solubility in the superconductor offers the possibility of attaining higher magnetizations and 77-K critical current densities.