F. H. Chen

Effect of Ca2PbO4 additions on the formation of the 110 K phase in Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting ceramics

The growth of the Bi2Sr2Ca2Cu3Ox phase in the high Tc superconducting Bi‐Pb‐Sr‐Ca‐Cu‐O system through reacting Bi2Sr2CaCu2Ox with Ca‐ or Cu‐rich intermediate phases, Ca2CuO3, CuO, and Ca2PbO4, has been studied. X‐ray diffraction and electrical and diamagnetic analyses revealed that the interaction and interdiffusion of these intermediate phase led to the formation of a high percentage of the 2223 phase and a sharp single‐step superconducting transition at Tc(R = 0) = 108 K. Also, Ca2PbO4 proved essential for accelerating the growth rate of the 110 K phase from the 85 K phase.

Synthesis of high-Tc superconducting Bi-Pb Sr-Ca-Cu-O ceramics prepared by an ultrastructure processing via the oxalate route

The oxalate precursor yielded by a sol-gel processing can be pyrolysed and oxidized at 840 ° C for 40h into the desired high-Tc superconductor in the lead-doped Bi-Sr-Ca-Cu-O system withTc(onset) = 140 K andTc(zero) = 104K. The mechanisms of the sol-gel reaction were monitored by analysis of Fourier transformation infrared (FTIR) spectroscopy. Moreover, identification of the various phases existing on the specimens after firing at selected temperatures, was made by analysis by FTIR spectroscopy and X-ray diffractometry. Based on the experiment results, the mechanisms of forming Bi2Sr2CaCu2O8+δ (phase 2212) might be identified as the reaction of Bi2Sr2CuO6+δ, Ca2CuO3 and CuO at 800 ° C. Furthermore, Bi2Sr2Ca2Cu3O10+δ (phase 2223) might be synthesized by the reaction of phase 2212, Ca2CuO3 and CuO at 840 ° C. Ca2PbO4 found in lead-doped materials may catalyse the formation reaction of Ca2CuO3 at 650 ° C and play a role of a flux to fuse Bi2Sr2CaCu2O8+δ, CuO and residual Ca2CuO3 and then improve the further synthesis of Bi2Sr2Ca2Cu3O10+δ at 840 δ C.

Preparation of HIGH-Jc superconducting oxide in the Bi-Sr-Ca-Cu-O system by oxalate gel processing

Abstract The high- T c superconducting oxide in the Bi-Sr-Ca-Cu-O system, with superconducting transition temperature at about 80 K, has been fabricated by the sol-gel method using oxalic acid and nitrates as starting materials. Compared to the conventional solid-state reaction method, this chemical technique offers less fabricating time and lower sintering temperature to get a homogeneous superconductor. Thermal analysis, infrared spectra and X-ray diffraction data indicate that the mechanisms of decomposition and phase transformation of specimens prepared at various temperatures are greatly different.

Growth parameter effect in superconducting YBa2Cu4O8 thin films by d.c. magnetron sputtering

High- Tc YBa2Cu4O8 (124) thin films have been made by d.c. magnetron sputtering deposition on (100) MgO substrates. The effect of several processing variables, including the ratio of oxygen to argon, total pressure, and substrate temperature, on the superconducting properties of the thin films, were systematically investigated. The as-prepared films annealed in flowing oxygen at 800°C for 4 h under ambient pressure obtained nearly phase-pure 124 and exhibited superconducting onset transition at 75 K.

Double resonant behavior of microstrip ring resonator in Tl‐based high Tc superconducting films

High Tc Tl–Ba–Ca–Cu–O superconducting thin films have been made by the dc magnetron sputtering followed by postannealing. In this study, a Tl‐based microstrip ring resonator was fabricated by using standard microelectronic photolithographic patterning and wet chemical etching. Experimental results indicated that these ring resonators exhibit the best unloaded Q of 3269 at its resonant frequency of 3.35 GHz and some other samples display the double resonant behavior at 77 K. The frequency difference of the double resonant peaks depends on the input microwave power, and the splitting is ascribed to spatial variations in the film microstructure. The physical properties of the films are about 1 μm thick, as high as 105 K transition temperature, and greater than 105 A/cm2 current density at zero magnetic field and 77 K.

Preparation of YBa2Cu4O8 Superconducting Bulk at Ambient Oxygen Pressure via Oxalate Gel Route

The superconducting bulk YBa2Cu4O8 compound was prepared by a modified sol-gel method via an oxalate route. In this route, metal nitrates were used as the starting ingredients, added with and without Na(NO3), respectively. The 124 bulk could be straightly produced by sintering the as-pelletized bulk in flowing oxygen at 800°C for 1–5 days. The obtained bulk YBa2Cu4O8 compound showed a nearly phase-pure 124, examined by X-ray diffraction, and a Tc(zero) of 80K by resistivity measurement. Ac and DC magnetic susceptibilities were also shown the identical onset temperature of superconducting state. Results reveal that the addition of Na(NO3) could prominently speed up the formation of the 124 phase.

Copper valency of superconducting phases in the Pb-doped Bi-Sr-Ca-Cu-O system

Abstract Pb-doped Bi-Sr-Ca-Cu high- T c superconducting oxides have been prepared by a modified sol-gel process via the EDTA route. Upon decomposing the X-ray photoemission spectrum of the Cu 2p 3 2 core level of the superconducting oxide into three components Cu 1+ , Cu 2+ and Cu 3+ , we have found an obvious trend: the corresponding [Cu 1+ ]/[Cu 2+ ] ratio increased as more 110 K phase was formed in the sample. According to this trend, a defect model was proposed; it can reasonably explain some previous experimental results, such as a reduced oxygen atmosphere sintering, which effectively facilitates the formation of the 110 K phase, and the coexistence of copper and oxygen vacancies in Bi-system superconductors, observed by high-resolution transmission electron microscopy. On the other hand, the dimerization of holes in oxygen sites was also suggested to be a possible mechanism responsible for the increase in the ratio of [Cu 1+ ]/[Cu 2+ ].