The phase transformation of Bi-Pb-Sr-Ca-Cu-O bulk materials are rapidly melted and solidified by a CO2 laser

The phase transformation of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O_y bulk materials rapidly melted and solidified by a CO₂ laser with the scanning speed of 40 mm/s were investigated. Results of x-ray diffraction pattern, scanning electron microscopy and energy-dispersive x-ray analysis showed the decomposition of the high-T_c phase in the laser irradiated region. Nonsuperconducting phases such as CaO and (Sr_1−xCa_x)CuO_y were found to be in the melting zone. On the other hand, (Sr_1−xCa_x)CuO_y and 2212 phase were also found in the heat-affected zone. When the irradiated samples were treated with 835‡C for 72 h in air, the laser treated region changed into the high-T_c as a major phase, in addition to the low-T_c phase and nonsuperconducting phase. However, the high-T_c phases are piled up randomly. The transport critical-current density of the laser treated samples after annealing is lower than that of the original sintered one, i.e. at 77K and zero magnetic field.     

On the role of Bi2Sr2Ca1Cu2O8 and Bi2Sr2Cu1O6 on the weak links and critical currents in (Bi,Pb)2Sr2Ca2Cu2O10/Ag superconducting tapes

Silver-sheated (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconducting tapes with different Bi₂Sr₂CaCu₂O₈ (Bi2212) and Bi₂Sr₂Cu₁O₆ (Bi2201) concentrations, were prepared by using a two-step sintering processing and by varying cooling rates in the fabrication of the superconductors. The effect of residual Bi2212 and Bi2201 phases on weak links and critical currents of the Bi2223/Ag tapes was investigated. It was found that residual Bi2201 caused weak links at grain boundaries and limited the current-carrying capacity of the tapes. Comparatively, the residual Bi2212 phase had much less influence on both weak links and critical currents. Elimination of Bi2201 by sintering tapes at a low temperature in the final thermal cycle, or by cooling the tapes slowly, increased critical current by a factor of two. Flux pinning property was also improved by removing the residual Bi2201 phase.     

Fabrication of Bi(Pb)-Sr-Ca-Cu-O superconducting Thin films by rapid thermal annealing

Superconducting Bi(Pb)-Sr-Ca-Cu-O thin films were successfully fabricated on MgO substrates by dc sputtering and rapid thermal annealing. No furnace annealing was used in the process. The superconducting thin films, deposited by dc sputtering for 8 hr and rapid thermally annealed at 845°C for 60 sec, showed a superconducting transition onset temperature of 110 K and a zero resistance transition temperature of 86 K. Highly oriented films with the c-axis normal to the substrate surface were obtained. To our knowledge, this is the first time that Bi(Pb)-Sr-Ca-Cu-O superconducting thin films have been fabricated using rapid thermal annealing.     

Powder processing of high temperature ceramic superconductor Tl2Ba2Ca2Cu3O10

We have developed a technique to produce high quality Tl₂Ba₂Ca₂Cu₃O₁₀ powders used for making superconducting wire, tape, lead, shield, and other large scale bulk applications. Starting with T1₂O₃, BaO₂, CaO, and CuO, we mix and grind these chemicals with a machine ball mill and then press the ground mixture into pellets. The pellets are sintered at about 895‡C for at least 30 h in an oxygen atmosphere. The sintered material is mainly the Tl₂Ba₂Ca₂Cu₃O₁₀ compound. To get more homogeneous superconductor powders, we pulverize the sintered material and use a magnetic superconducting material selector to separate and grade the material. Finally, the top grade material has a phase purity of <98% and a T_c(r < 0) of 123–126K.     

Effect of mechanical deformation on the mass density of Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10 wire and tape

The mass densities of Ag-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ wire and tape varies during the mechanical deformation process, which is one of the steps of the oxide-powder-in-tube technique used to fabricate the composite superconductor. Results show that the rolling has a more significant effect on densifying the tape core, whereas the drawing process can only densify the core to about 75% of the theoretical density. SEM observations of the rolled samples also reveal very dense morphology, consistent with the mass density calculations. SEM observation also shows that with increasing the deformation extent, the average grain size is reduced. It is proposed that although the rolling densifies the tape core significantly, it also destroys the crystallinity of the superconducting phases and results in the formation of an amorphous phase. Since the textured Bi-2223 phase forms by the epitaxial growth on the textured Bi-2212 seed crystals, the deformation induced texture is critical. Appropriate deformation extent is necessary, since too high an extent of deformation may change the well-aligned grains into amorphous phase. The formation of the amorphous phase is harmful to the texturing formation of the Bi-2223 phase, which finally leads to critical current degrading.     

Phase relations in the system ln-CulnS2

By DTA and x-ray diffraction the phase relations in the pseudobinary system In-CuInS₂ have been investigated. CuInS₂ hs a melting point of 1090&#x00B0; C and within this system there is a broad region of liquid immiscibility. A four phase invariant reaction exists at 633&#x00B0; C which is of the form: L₂ = L₁+ CuInS₂ + InS.     

Aspects of the current understanding of the supercurrent transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed Tapes—the “railway-switch” model

The “railway-switch” model describes the superconducting current transport in (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries (“railway-switch”) constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flükiger,Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary “powderin-tube” samples [J_c(T < 77K, B < 0 T) < 20..30 kA/cm²] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density j_c ^c along the c axis (or the even lower critical current density j_c ^t across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ silver-sheathed tapes for applications. On leave from Consorzio INFM, Universitá di Genova, Italy.     

First time in-situ investigation of 2223 formation measured on (Bi,Pb)2Sr2Ca2Cu3O10+x thick films on Ag tape prepared by the screen printing technique

Detailed investigations were performed on the reaction kinetics of the superconducting BPSCCO 2223 phase in order to be able to improve tape processing and critical current density J_c of 2223 layers on Ag tape made by the organic binder method. The J_c values presently vary between 6–12 × 10³A/cm² (77 K, O T). To our knowledge, it is the first time that the formation of the 2223 has directly been observed employing in-situ high temperature X-ray diffraction (HTXRD). This measurement was carried out on screen printed samples of BPSCCO on Ag foil heated in a specially designed closed crucible preventing the evaporation of lead. The main findings are that the 2223 phase arises from the 2212 phase at 820–835°C within a short timescale of about 2–3 h. These results were correlated with measurements on a Simultaneous Thermal Analysis (STA) showing endothermic reactions which indicate the starting of partial melting at about 825–836°C. According to the literature we found that the reaction kinetics are controlled by diffusion processes under the presence of partial melt.     

Electrical and infrared study of Bi2Sr2Ca1Cu2O8 in semiconducting,superconducting ceramic and superconducting glass ceramic state

We have studied the electrical and infrared properties of Bi₂Sr₂Ca₁Cu₂O₈ compound in three states. Electrical and IR measurements show that the pure powder state sample is a semiconductor, the ceramic Bi₂Sr₂Ca₁Cu₂O₈ sample prepared after annealing at 820°C for 240 hours shows a T_c of 85 K, whereas Bi₂Sr₂Ca₁Cu₂O₈ sample prepared through glassy route, i.e. melting at 1250°C and annealing at 820°C for 240 hours shows a drop of T_c by 5 K. The infrared spectra of superconducting ceramic and glass ceramic states in the available frequency range of measurement reveals the presence of three phonons. Since the vibrational mode around 595 cm^?1 is due to CuO₂ layers and as the CuO₂ layers are responsible for T_c in the ceramic superconductors, any change in these layers will affect the T_c. The shifting of the 595 cm^?1 mode towards lower frequencies in the glass ceramic due to different preparation process, indicates that there is a change in CuO₂ layers resulting in a change of T_c, which is confirmed by Four probe dc measurements.     

Internal friction of superconducting YBa2Cu3O6+x wires at 200 kHz under in situ heat treatment

Using piezoelectric LiNbO₃ crystal oscillator method at 200 kHz, we have measured internal friction and modulus of YBa₂Cu₃O_6+x short wires with 1 mm-diameter and about 5mm length under in situ cyclic heating-cooling treatment at temperatures of 300–1020K. Polycrystalline YBa₂Cu₃O_6+x wires show anelastic behavior as well as orthorhombic-to-tetragonal (O-T) phase transition within present temperature range. There appears a strong internal friction peak below the O-T transition temperature, corresponding to the atomic site relaxation between oxygen and a vacancy in the Cu-O plane of oxygen defective YBa₂Cu₃O_6+x. A novel peak is observed after the cyclic heat treatments at around 700K. This relaxation is attributed to the hopping movement of oxygen in the defective local structure of YBa₂Cu₃O_6+x.     

T(x) phase diagram of the Cu2Se-Al2Se3 system

Twenty alloys of various compositions in the Cu₂Se-Al₂Se₃ system were prepared and investigated. The T(x) phase diagram of the Cu₂Se-Al₂Se₃ system was obtained from x-ray diffraction, differential thermal analysis, and microstructure investigation for the first time. The homogeneity region of the CuAlSe₂ semiconducting compound was established. Previous articles by this author were presented with the spelling of his name as B.V. Korzun.     

Dislocations and flux pinning in YBa2Cu3O7

The dislocation structures of bulk textured and epitaxial thin film YBa₂Cu₃O₇ superconductors are examined. Correlations between increases in flux pinning and dislocation densities are noted. A model for flux pinning by individual dislocations is presented. This gives a treatment of strain induced effects and effects of normal state region interactions. It is shown that the values of pinning predicted are in line with experimental observations.     

Fabrication and Characterization of Bi(Pb)-Sr-Ca-Cu-O/Ag2O Superconducting Tubes

Superconducting magnetic shields of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O_y with or without Ag₂O superconductor were fabricated in tubular form with one end closed by using a cold isostatic pressing method. It was found that the addition of Ag₂O in the BPSCCO samples resulted in no significant effect on T_c when sample were treated at 835‡C in air. However, the critical current density and the shielding magnetic flux density (at 77K) were found to be decreased, compared to the pure BPSCCO sample. The correlations of superconducting properties with micro-structure of these materials are discussed in this paper.     

Visualization of Dopant Oxygen Atoms in a Bi2Sr2CaCu2O8+δ Superconductor

A proper amount of excess oxygen plays a significant role in hole‐doped cuprate high‐T_c superconductivity. Here, the dopant oxygen in Bi₂Sr₂CaCu₂O_8+δ is directly imaged via integrated differential phase contrast combined with state‐of‐the‐art scanning transmission electron microscopy. The location of dopant oxygen is observed to be consistent with the position inferred from local strain analysis of the incommensurate structure. The influence of dopant oxygen on the local atomic lattice and electronic structure is further explored using first‐principle calculations. The dopant oxygen atoms not only aggravate the distortions of the local atomic arrangement but also alter the electronic states by transferring charge from the BiO planes to the CuO₂ planes. The underlying mechanism of charge transfer is resolved. The results may also be applicable to other oxygen‐doped cuprates with high‐T_c superconductivity.     

Fabrication of Bi2Sr2CaCu2O x and Bi2Sr2Ca2Cu3O x Superconductor Thick Films by Using Cu-Free Precursors on Ni Substrates

We studied the formation of Bi₂Sr₂CaCu₂O _x (Bi-2212) and Bi₂Sr₂Ca₂Cu₃O _x (Bi-2223) thick films in a heat treatment process of the Ni-sheathed Bi-Sr-Ca-Cu-O (BSCCO) system. Cu was electrodeposited initially on the Ni substrates (Cu/Ni). Well-oriented Bi-2212 superconductor thick films were formed successfully on Ni tapes by liquid reaction between Cu-free precursors and Cu/Ni tapes. However, only a small amount of Bi-2223 was formed. Thick films were prepared by screen-printing with Bi₂O₃, SrCO₃, and CaCO₃ powders on Cu/Ni tapes and heat treating them. Heat treatment was performed in the temperature range of 750–850°C in a tube furnace for several minutes to hours. The phases and the microstructures of the high temperature superconductor thick films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electrical properties were examined by the standard four-probe method. At the heat treatment temperature, the specimens were in a partially molten state during reaction between the oxidized copper layer and the screen-printed precursors on the Cu/Ni tapes.     

The influence of the phase equilibria on the critical temperatures Tc of the high-Tc Bi−Sr−Ca−Cu−O and Y−Ba−Cu−O compounds

The phase equilibria of YBa₂Cu₃O_7−d (123 phase) strongly influence its oxygen content and consequently its critical temperature T_c. With increasing content of BaCuO₂ or CuO, T_c decreases. Whereas, if Y₂BaCuO₅ (211 phase) is in equilibrium with the 123 phase, the oxygen content and T_c of which is high. Bi_2+x(Sr,Ca)₃Cu₂O_8+d exhibits an extended single phase region and its T_c is strongly affected by its chemical composition. In addition, the oxygen content of the phase is a function of the temperature and influences the T_c of the phase.     

Microstructural study of the effect of an excess of Y2BaCuO5 and BaSnO3 doping on the texturing process of YBa2Cu3O7−x bulk superconductors

Melt texture process of YBCO leads to a YBa₂Cu₃O_7−x matrix where Y₂BaCuO₅ particles are observed. The Y₂BaCuO₅ inclusions size and distribution depend upon several parameters: YBa₂Cu₃O_7-x grain size in the presintered, heating rate, dopants. The influence of an excess of Y₂BaCuO₅ and/or BaSnO₃ on these Y₂BaCuO₅ particles are observed in the liquid phase and in the texture domain. According to the dopant used, two kinds of coarsening of Y₂BaCuO₅ can be observed: an isotropic and an anisotropic. The control of the distribution of Y₂BaCuO₅ particle size is of primary interest to improve the efficiency of the MTG process. In particular large cooling rate (5°C/h) during the texture formation could be used by adding Y₂BaCuO₅ + BaSnO₃ to YBa₂Cu₃O_7−x composition.     

Ternary phase relations of the chalcopyrite compound CuGaSe2

The ternary Cu-Ga-Se phase diagram has been determined by DTA and x-ray analysis. In addition to two ternary solid solutions based on the binary compounds Cu_2.xSe and Ga₂Se₃, only one ternary phase, the chalcopyrite CuGaSe₂ (peritectic m.p. 1030°C), was encountered. The liquidus contains two regions of liquid immiscibility, one which extends from the Cu-rich (Cu, Se) binary liquid immiscibility to the Ga-rich (Ga, Se) binary immiscibility, and the other which is a minor extension of the Se-rich (Cu, Se) binary liquid immiscibility. The liquidus maxima include those at the binary boundaries: Cu (1087°C), Cu₆₇Se₃₃ (1148°C), GaSe (960°C); and a ternary liquidus maximum at Cu₁₉Ga_28.5Se_52.5 (1112°C), the maximum melting point of a solid solution based on the defect-zincblende phase of Ga₂Se₃. The primary phase fields are identified and the crystal growth of CuGaSe₂ solid solutions from nonstoichiometric melts is discussed, especially the most satisfactory Bridgman growth from the Cu₂Se-CuGaSe₂ join. Subsolidus phase relations are also proposed for the Cu-Ga-Se system, and probable similaritics in all I-III-VI ternary diagrams are suggested.     

Humidity sensitivity of Sr(Sn,Ti)O3 ceramics

The SrSn_1-x Ti _x O₃ system humidity sensors have been investigated in this study. The electrical conductivity of these sensors respond to the presence of moisture. The sensitivity of these humidity sensors depend on a number of factors including the material composition and the microstructure of the ceramic body. In this system, SrSn_0.5Ti_0.5O₃ was found to be quick in response to abrupt change in humidity, and the response does not change due to ambient temperature change. The response time to the humidity change is estimated to be within 15 sec.     

Characterization of the texturing of YBa2Cu3O7−δ in the presence of liquid phase additives

Grain growth and texturing of YBa₂Cu₃O_7−δ is greatly influenced by the presence of liquid phase additives during sintering. Oxides such as TiO₂, SiO₂, Bi₂O₃, and Pr₆O₁₁ were incorporated into the liquid phase during the sintering of YBa₂Cu₃O_7−δ (123) by use of grain boundary diffusion couples and the microstructure was analyzed using scanning electron microscopy/electron dispersive spectroscopy. Exaggerated grain growth and domain formation was observed in bulk specimens. Differential thermal analysis and real time dynamic x-ray diffraction were used to determine reaction sequencing. The ability and extent of domain formation was determined for 123 samples coupled with impurity oxides to be a function of sintering temperature (940–980°) and oxygen partial pressure. Enhanced texturing was observed at low PO₂ atmospheres. The addition of Bi₂O₃ and TiO₂ was shown to degrade dc magnetic susceptibility of 123 whereas SiO₂ and Pr₆O₁₁ enhanced it. The domain formation and texturing takes place in the bulk for 123 at temperatures of 980°C or below (i.e. well below the peritectic decomposition temperature) by the interaction of an impurity doped liquid phase followed by a precipitation and exaggerated grain growth.