首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 0 毫秒
1.
Compact swelling in Pb-doped Bi-Sr-Ca-Cu-O superconductor has been studied by observing the effects of the size of calcined powders, volatilization of materials, and sintering of high- T c (2223) powders. The bulk density increases at the early stage of sintering, for about 20 h, and then decreases. Densification occurs when the low- T c (2212) phase and a liquid phase exist, whereas dedensification occurs with the formation of the 2223 phase regardless of the presence of the liquid. Gas evolution from specimens does not appear to be responsible for compact swelling. Compact swelling is explained by anisotropic growth of thin, platelike 2223 grains in random orientation. When 2223 grains grow in a preferred direction, compact swelling is suppressed.  相似文献   

2.
The Ba-doped superconducting (Bi,Pb)2Sr2- x Ba x Ca2Cu3O y and (Bi,Pb)2Sr2Ca2- x Ba x Cu3O y (0 ≦ x ≦ 1.0) were prepared by using a melt-quenching method, and the effect of Ba additions on the glass-forming ability and the crystalline phase was examined. The glass-forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO3 as well as CaO was observed in the melt-quenched sample of (Bi,Pb)2SrBaCa2Cu3O y . BaPbO3 crystals were precipitated in all glass-ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high- T c phase, and (Bi,Pb)2Sr1.4Ba0.6Ca2Cu3O y glass-ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a T c of 103 K, although BaPbO3 and the low- T c phase were still largely present.  相似文献   

3.
Superconducting coupling nature at grain boundaries in Bi2Sr2CaCu2O x glass-ceramics consisting mainly of the low- T c phase was first examined by measuring superconducting properties and temperature or ac field dependence of ac complex susceptibility. It was found from the ac loss peaks that superconducting coupling at grain boundaries was basically characterized by three types of weak links. The weak-link behaviors at grain boundaries depended strongly on cooling conditions after annealing and annealing time and temperature. Particularly, it was found that the weak links at grain boundaries were improved by prolonged annealing at 840°C. The furnace-cooled glass-ceramics obtained by annealing at 820° or 840°C for about 200 h exhibited a critical transport current density (77 K, zero magnetic field) of about 200 A/cm2.  相似文献   

4.
Ba2YCu3O7 ceramics doped with either Pr or U at 0.000 17 to 1.7 wt% levels have been prepared. For each sample J c (magn) has been measured with a vibrating sample magnetometer. No improvement in J c was found for either dopant and it is concluded that neither provides the clusters necessary to produce suitable pinning sites.  相似文献   

5.
Ag2O-doped superconducting Bi2Sr2Ca1Cu2O x ceramics were prepared by a melt-quenching–reheating method. It is found that the Ag2O-doped, as-cast specimens exhibit superconductivity ( T c= around 80 K) by heat treatment at temperatures around 800°C even in an evacuated and sealed silica glass tube, while the undoped specimens do not and vaporize by the corresponding heat treatment. Conversion of the Ag2O-doped, as-cast specimens into superconducting ceramics when heated in an evacuated vessel is explained in terms of the oxygen donor of Ag2O in the specimen. This finding enables us to fabricate a desired shape of superconducting Bi2Sr2Ca1Cu2O x ceramics sealed in metals or glasses. The addition of Ag2O to Bi2Sr2Ca1Cu2O x melt, however, had deleterious influences on the superconducting properties ( T c and J c) of the resultant ceramics when obtained by heat treatment in air.  相似文献   

6.
The effects of annealing on the microstructure and phase chemistry of directionally solidified Bi2Sr2CaCu2O8 (2212) were studied. Boules grown at fast growth rates are multiphase, contain no 2212, and exhibit some preferred orientation; Sr-deficient 2201 exhibits a (220) texture. These samples, however, do show a significant amount of 2212 after annealing, with (220) texture similar to the texture of the preexisting 2201 phase in the as-grown boules. In contrast, 2212 grown directly from the melt at slow growth rates has (200) texture. Boules grown at moderate growth rates are multiphase and contain 2212 as one of the phases; these boules show an increase in 2212 volume fraction after annealing. Microstructural observations in both cases indicate solidstate formation of 2212 occurs topotactically on or within the preexisting 2201 phase. Boules grown at fast growth rates are oxygen-deficient, since they show an increase in mass with annealing, whereas boules grown at moderate growth rates show no appreciable mass increase with annealing. An estimate of the Cu(I) concentration in the liquid zone during crystal growth at fast growth rates was obtained from gravimetric analysis.  相似文献   

7.
Dense large-grain ceramic samples of Ba2YCu2Ox have been oxidized at 400° to 500°C in 10 to 30 MPa of oxygen for 1 to 10 d. The high-pressure treatment has increased the equilibrium content x slightly above 7 but there is no concomitant increase in Tc. At x = 7.03 the superconducting transition, measured by ac susceptibility, has an onset temperature of ∼91 K and ΔT∼4 K and no change in lattice parameter is observed. High oxygen pressure increases the rate of oxidation.  相似文献   

8.
(Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) precursor powders with large and small amounts of Ca2PbO4 phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca2PbO4 on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature ( T m,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca2PbO4. Tapes with a small amount of Ca2PbO4 had a higher T m,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca2PbO4. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current-carrying capacity of the tapes were significantly improved by reducing the Ca2PbO4 content of the precursor powders. These results are of practical significance for the commercialization of Bi-based high-temperature superconductors.  相似文献   

9.
The microstructure of partial-melt-processed YBa2Cu3O x /HfO2 has been studied by transmission electron microscopy. A characteristic spherulitic microstructure is formed in the system. A model for the growth mechanism has been proposed. The critical heterogeneous nucleation of the YBa2Cu3O x phase appears to occur from the melt in an epitaxially controlled manner on CuO particles. Subsequent growth of YBa2Cu3O x platelets from the nucleus region is repeatedly interrupted by the nucleation of hafnium-rich phases in the liquid at the solid/liquid interface in a manner that again appears to be epitaxially controlled and that promotes the splay of the c orientation of the YBaCuO grain.  相似文献   

10.
The cation solubility limits of the n = 2 and n = 3 superconducting phases in the Bi2(Sr x Ca1 − x ) n +1Cu n O y system were established along tie lines with compatible phases via electron probe microanalysis on bi- (or poly-) phasic samples prepared at 860°C. Pb additions (15 mol% of the Bi content) were used to facilitate formation of the n = 3 phase. In each case football-shaped volumes in composition space were established as the solubility limits which bordered on the nominal compositions 2212 or 2223 (Bi:Sr:Ca:Cu) with the long axis parallel to the Sr-Ca side of the quaternary (i.e., Sr-to-Ca intersolubility) but also extending toward Bi and Cu. This means that, for the most part, the superconducting phases are alkaline-earth deficient relative to the ideal 2212 and 2223 compositions. The Pb content in the 2223 phase is typically 10% of the Bi content. T c variations could be correlated with variations in Sr or (Sr + Bi) content and the length of the c -axis parameter.  相似文献   

11.
Submicrometer powder of singlephase YBa2Cu3Ox (YBC), prepared from nitrate salts, were free-sintered and sinter-forged at 1223 K. The levitation force on magnets exerted by the superconductor was found to depend on the thickness of the superconductor, the shear strain imposed during sinter-forging, and the time and temperature of the high-pressure oxygen anneal after sinter-forging. The first result is direct proof of the type II nature of the YBC superconductor. In the second experiment a shear strain of 300% increased the levitation force by a factor of 3. The third result confirmed that YBC transforms to the tetragonal structure at 1223 K, which must be restored to the orthorhombic structure by annealing at lower temperature. Optimum results were obtained when the specimens were annealed in pressurized oxygen (3 MPa) for 50 h at 673 K.  相似文献   

12.
Sintering YBa2Cu3O7- x bulk forms at 1050°C followed by annealing at 980°C causes the development of a thick oriented surface layer (Lotgering factor = 0.7). The thickness of the layer depends on the thermal treatment, which is a two-step sintering process. Firing at 1050°C for 2.5 h followed by 30 h at 980°C leads to the development of a 0.1-mm-thick surface layer, with clear indication that longer annealing would result in a thicker film. Some orientation develops during un-axial compaction of the powders. Lotgering orientation factor calculation from X-ray diffraction analysis. SEM, and TEM were used to characterize the microstructure of these samples. T c was similar to that of conventionally processed high-density samples, between 83 and 87 K. Some thermal treatments resulted in samples that displayed high resistivity above T c , possibly caused by segregation of Cu to the grain boundaries.  相似文献   

13.
Retrograde densification of pelletized calcines and glasses having an approximate (Bi,Pb)2Sr2Ca2Cu3O10 starting stoichiometry and sintered at ∼850°C can be described by first-order rate equations. Retrograde densification in the calcine precursors was largely due to the development of open pores, and was approximately proportional to the concentration of the (Bi,Pb)2Sr2CaCu3O10 phase. In the glasses, retrograde densification is mainly caused by porosity accompanying the growth of a needlelike Sr─Ca─Cu─O phase, together with (Bi,Pb)2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2CaCu2O8.  相似文献   

14.
Bi2Sr2CaCu2O8 was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.  相似文献   

15.
High-resolution transmission electron microscopy and optical diffractograms have revealed that chemically vapor deposited films of superconducting YBa2Cu3O x react to form an interaction layer with single-crystal yttriastabilized zirconia. The approximately 5 nm thick interlayer was identified as BaZrO3. Zirconium was also found to diffuse through the entire YBa2Cu3O x film.  相似文献   

16.
Sintered bulk samples of YBa2Cu3Ox of 95% to 99% theoretical density have been consolidated by a one-step densification process. Superconducting transitions determined by dc magnetization experiments are above 90 K. The metallurgical state of the material, both before and after consolidation, is characterized by X-ray diffraction and electron microscopy analysis as well as density and resistivity measurements. Improvement in critical current density (Jc) in the consolidated material is related to changes in the microstructure. Particular attention is focused on the granular nature of the compound and the influence of the consolidation process on the weak-link structure.  相似文献   

17.
Samples of nearly theoretical density are produced by post hot isostatic pressing (post-HIP) of presintered YBa2Cu3O7-x using argon as the pressure medium. The hot isostatically pressed samples show improved values for critical current density at 77 K. Experimental aspects of post-HIP are described, and microstructural development of YBa2Cu3O7-x during sintering and post-HIP is discussed.  相似文献   

18.
Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O, (2223) tapes were made by the oxide-powder-in-tube method. Tapes were heat-treated isothermally at several different temperatures in 7.5% O2/Ar, then quenched into oil to retain the phase assemblages at the reaction temperatures. 2223 formed between ∼810° and ∼837°C. The Avrami equation was applied to describe the kinetics of 2223 formation from a mixture of Bi2Sr2CaCu2O x and nonsuperconducting phases, mainly Ca2PbO4 and CuO. The calculated Avrami exponent, n ∼ 1, indicated that the kinetics in this system could be described as a diffusion-controlled, two-dimensional nucleation and growth process. The apparent activation energy for forming 2223 was ∼2900 kJ/mol from ∼817° to ∼825°C and ∼890 kj/mol from ∼825° to ∼837°C. A temperature-time-transformation diagram was constructed based on the kinetic data; it describes the transformational behavior of this particular system.  相似文献   

19.
The successful preparation of single-phase Hg-doped (2223) using a nitrate decomposition/solid state reaction method is described in this paper. The effects of Hg doping on the formation of single-phase (2223) have been investigated by X-ray and synchrotron radiation diffraction. When compared with Pb-doped system, the use of Hg as a dopant has resulted in the reduction of both the final sintering temperature and the annealing time of the single-pure (2223) material, without any obvious deterioration of the superconducting properties.  相似文献   

20.
Pronounced textures have been found in hot-extruded and hot-pressed YBa2Cu3Ox powders. The basal plane is preferentially oriented normal to the maximum compression axis in hot deformation. Thus, the extrusion texture of these ceramics should facilitate maximum current-Carrying capability: along the extrusion axis, suggesting hot extrusion or drawing as a favored method of producing superconducting wires.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号