Synthesis of Bi1.5Pb0.5Sr2Ca2Cu3Oy via sol-gel method using different acetate-derived precursors

Sol-gel method have been used to convert metal carbonates, oxides andnitrates into corresponding acetate precursors for gel formation.About 90% volume of high T_C superconducting phase (2223)has been obtained for oxide-carbonate system. This value iscomparable to that obtained from pure acetates (91% volume).However, the nitrates-'derived sample has a high-T_Cphase of 88% due to poor conversion of nitrates to acetates asobserved by the infrared spectra. The dominance of high-T_Cphase was observed in all systems as evidenced in the single steptransition of R-T curves. Hence the percentage of thehigh-T_C phase dictates the value of T_C(R = 0)as 104 K, 103 K and 99 K, for oxide-carbonate, pure metal acetatesand nitrates system respectively.     

Evolution of 2223 phase in the Bi2Pb0.6Sr2Ca2Cu3.1Oy system

The role of cationic ratios (Sr/Ca and Bi/Pb) in the evolution of high-T _c phase (2223) in samples prepared under different sintering conditions, starting with a composition of Bi₂Pb_0.6Sr₂Ca₂Cu_3.1O_y, has been investigated by employing energy-dispersive X-ray analysis, scanning electron microscopy and X-ray diffraction. As manifested by the observed decrease in Sr/Ca ratio, an increase in sintering temperature from 822 to 852 °C increases the disorder in Sr-O and Ca layers. The observed increase in the volume fraction of 2223 phase and the contraction inc-axis parameter have been explained on the basis of the observed decrease in Sr/Ca ratio. It thus appears that the disorder caused by the intersubstitution of Ca and Sr in SrO and Ca layers and partial replacement of Bi by Pb in the structure promote the evolution and growth of 2223 phase.     

Metal-insulator transition and possible superconductivity in Pb2.2Cu0.8Sr3.1La1.5Cu1.5Oy with hexagonal structure

A metal-insulator transition and possible superconductivity has been found in Pb_2.2Cu_0.8Sr_3.1La_1.5 Cu_1.5O_y. The structure of this compound belongs to the hexagonal symmetry with the lattice parameter a=10.11 ?, c=3.56 ? and space group P6ˉ2 m. In this structure, there are not the traditional square CuO₂ planes as known in most cuprate superconductors. The parent compound of this phase is Pb₃Sr₅CuO₁₂(3501) which is an isolator. By replacing Pb and Sr with Cu and La, respectively, a metal-insulator transition and strong structural modulations occur in this material. Received: 27 July 1999 / Reviewed and accepted: 10 August 1999     

Effect of cobalt on the high T c phase of the Bi1.6Pb0.4Sr2Ca2Cu3Oy system

A.c. susceptibility, X-ray diffraction (XRD), SEM and porosity studies have been performed on Bi_1.6Pb_0.4Sr₂Ca₂Cu_3–x Co _x O _y (x=0, 0.05, 0.1) superconductors. XRD and SEM results have shown that (i) the compound with x=0 contains mixed phases of 2223 and 2212, (ii) the increase in concentration of cobalt (from x=0 to 0.1) helps the compound to attain a structure of strong 2201 and a small amount of Ca₂CuO₃ impurity phase. The onset temperatures of the diamagnetic signal of these superconductor samples with x=0, 0.05, and 0.1 as observed from a.c. susceptibility measurements are 106, 60 and 50 K, respectively. The highest onset temperature, 50 K, observed in the sample with x=0.1 rather than the usually reported value of 20 K associated with 2201 phase in bismuth oxide compounds, may be due to the presence of Ca₂CuO₃ impurity phases. SEM and porosity results show that the cobalt helps to increase the grain and pore sizes.     

Appropriate thermal procedure for the preparation of high-T c phase of Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor

The high-T _c phase of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y has been synthesized using an appropriate thermal procedure. D.c. electrical resistivity and X-ray diffraction studies have been done. The measurements show that a slow cooling process is necessary for a better control of the thermal process. The variation in T _c as a function of annealing time is also reported. Indexed X-ray diffraction patterns indicate the formation of high- as well as low-T _c phases.     

Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3O x

Samples of the series Bi_1·9−x Pb _x Sb_0·1Sr₂Ca₂Cu₃O _y withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withT _c=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-planeρ _ab (H, θ,T), which depends on the angleθ between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values ofρ _ab (H, θ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter γ≡(m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence ofρ _ab (H,θ,T).     

Neutron structural study of the Bi-monolayer compound (Bi0.5Cu0.5)Sr2(Y0.8Cu0.2)Cu2O7 + δ—the Role of excess oxygen in superconductivity

Comparative neutron structural investigations are made on nonsuperconducting (slow-cooled) and superconducting (liquid-nitrogen quenched;T _c ^onset=70 K andR=0 at 20 K) samples of (Bi_0.5Cu_0.5)Sr₂(Y_0.8Cu_0.2)Cu₂O_{7 + δ} in order to examine the role of excess oxygen on the superconducting behavior of this “1212”-phase compound. Analysis of refined structural parameters shows that the two main factors which influence the superconductivity in (Bi, Cu)-1212 are: (i) the extent of occupancy of excess oxygen at O(5), the 2(e) site located in between the two CuO₂ pyramidal layers, and (ii) the length of the apical Cu(2)-O(2) bond. The (Bi, Cu)-O monolayer plays the role of the “charge reservoir” quite effectively as reflected by the substantial increase in the length of the apical (bridging) Cu(2)-O(2) bond due to depletion of excess oxygen in this layer. Surprisingly, vacancies at as many as 10% of the O(1) site belonging to the CuO₂ planes do not appear to disrupt the flow of current in the Cu(3d)-O(2p) planar network in the superconducting state. Relevant features of the structure of the (Bi, Cu)-1212 phase, in particular the role of excess oxygen and its occupancy at different sites, are discussed in the light of the available data on the isostructural (Pb, Cu)-1212 phase and the (La, Sr, Ca)₃-Cu₂O_{6 + δ} phase superconductors.     

Modulated microwave absorption in PbBi2Sr2Ca2Cu3Oy powders and possible flux melting

Temperature-swept magnetically modulated microwave absorption is reported in powdered PbBi₂Sr₂Ca₂Cu₃O _y with a range of particle sizes and in fields up to 700 mT. In low fields a signal of positive sign is observed originating from weak link regions. In higher fields two other signal appear. One is very weak and almost independent of field and is attributed to the bulk intrinsic transition atT _c≈112 K. The second broader and stronger negative peak moves rapidly to low temperatures with increasing field and is attributed to melting of the flux lattice.     

Superconductivity in (Bi,Pb)2Ca2Sr2Cu3−xVxOy

The superconducting properties of V-doped (Bi,Pb)2Sr2Ca2Cu3Oy (2223) samples have been investigated by means of X-ray diffraction and electrical resistivity measurements. The volume fraction of the 2223 phase decreases with a decrease in the c lattice parameter. The onset of superconductivity in the samples remains almost constant while the temperature at which the resistivity becomes zero decreases with increasing dopant content. The electrical resistivity data suggest that the substitution of copper by vanadium suppresses the superconductivity in (Bi,Pb)2Sr2Ca2Cu3–xVxOy by destroying the phase coherence and by pair-breaking effects. The suppression of superconductivity in the system could also be due to a decrease in the carrier concentration.     

Structural, superconducting and mechanical properties of molybdenum substituted Bi1.8Sr2Ca1.1Cu2.1Oy

The effect of MoO₃ addition on the properties of Bi-2212 superconducting ceramic samples prepared by solid state reaction method has been investigated. Mo content was varied from 0 to 0.25 on a general stoichiometric formula Bi_1.8Sr₂Mo_xCa_1.1Cu_2.1O_y. Electrical resistivity showed that transition temperature width increased directly with the Mo amount. XRD data have shown that MoO₃ addition in the Bi_1.8Sr₂Mo_xCa_1.1Cu_2.1O_y precursor reduces the amount of Bi-2212 phase. In addition, J_c values of the samples, calculated from the hysteresis loops using the Bean’s model, decreased with increasing Mo substitution. Vickers microhardness measurements show that samples are very sensitive to Mo content and applied load. In addition, various models like Meyer’s Law and Young’s Modulus equations have been used to better explain the mechanical properties of samples.     

Structure and Superconducting Properties of Cd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy

A new Cd-containing superconductor with nominal composition ofCd_0.8Ba₂(Y_0.7Ca_0.4)Cu_3.5O _y was synthesized and investigated. The obtained Cd and Ca-doped 123 phase exhibits an orthorhombic (T _c=80 K) or tetragonal (T _c=65 K) modification depending on the reaction atmosphere. It was shown that the combined Cd and Ca substitution facilitates the 123 phase formation. The results of the EDX analysis, as well as the comparison of the obtained lattice parameters with those of undoped, Cd-doped, and Ca-doped 123 have shown that both Cd and Ca enter the 123 phase and form a new Cd–Ba–Y–Ca–Cu–O superconducting compound.     

Influence of heat treatment conditions on the phase composition and superconducting properties of Bi1.6Pb0.4Sr1.98K0.02Ca2Cu3F0.8Oy ceramic

The influence of synthesis temperature and time on the properties and formation of superconducting phases in Bi_1.6Pb_0.4Sr_1.98K_0.02Ca₂Cu₃F_0.8O_y samples has been studied by x-ray phase analysis and by measuring the electrical resistance. It is found that the ratio of the 2223 and 2212 phases formed during synthesis at 845 °C for 240 h remains almost constant in the range 845–855 °C, regardless of the additional synthesis time. Synthesis at higher temperatures leads to breakdown of the 2223 phase and enhances the content of the 2212 phase and impurity phases. The highest values T _c(R=0)=112.8 K were obtained for samples synthesized at 845 °C for 240 h. Pis’ma Zh. Tekh. Fiz. 23, 30–34 (July 26, 1997)     

Synthesis and properties of Bi1.6Pb2-xSr2-xCa2Cu3Oy

Bi_1.6Pb_0.4Sr_2-xCa₂Cu₃O_y (x = 0, 0.2, 0.4) high-T_c materials were prepared from oxide-carbonate mixtures and presynthesized mixed oxides. The 2223 superconducting phase was found to be formed most rapidly in thex = 0.4 sample if the Pb-and Ca-containing starting reagents were separated in the initial stage of synthesis. The highest superconducting transition temperature, T_c (R = 0) = 104.3 K, was attained in the stoichiometric 2223 material. The materials were characterized by electrical resistivity, magnetic susceptibility, and Hall effect measurements. The Hall data were used to evaluate carrier concentration and mobility. The 77-K resistivity of the materials was measured as a function of magnetic field.     

脉冲激光沉积Ba0.5Sr0.5TiO3、Pb0.5Ba0.5TiO3和Pb0.5Sr0.5TiO3薄膜及其介电性质

采用脉冲激光沉积法,在Pt／Ti／SiO2／Si基底上分剐制备厚度为350nm的Ba0.5Sr0.5TiO3(BST)、Pb0.5Ba0.5TiO3(PBT)和Pb0.5Sr0.5TiO3(PST)薄膜并研究了它们的介电性质。XRD显示,在相同的制备条件下三者具有不同的择优取向,PST具有(110)择优取向,PBT具有(111)择优取向,而BST则是混合取向。SEM显示三者样品表面均匀致密,颗粒尺寸大约在50nm至150nm之间。PST与BST、PBT相比有更高的介电常数,在频率为10kHz时,分别为874、334和355,而损耗都较低,分别为0．0378、0．0316和0．0423,同时PST漏电流也是最小的。测量薄膜的C-V特性扣铁电性能表明室温下BST呈现的是顺电相,PST和PBT则呈铁电相。本文也测量了薄膜在不同频率下的介电温度特性,BST、PBT和PST均表现出频率弥散现象,即随着频率的降低．居里温度降低而介电常数会升高。并测得BST和PST的居里温度分剐为-75和150℃。而PBT的居里温度在250℃以上。本文研究表明：与BST相比较,PBT的介电常数与之相近,漏电流较大;而PST具有高介电常数,较小的漏电流和较大的电容-电压调谐度,在相关半导体器件中的应用将有很大的潜力。     

Sc Addition to (Bi,Pb)2Sr2Ca2Cu3Ox Freeze-Dried Ceramic

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc₂O₃ was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis, X-ray diffraction (XRD), Scanning electron microscopy (SEM), and χ′(T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.