The phase constitution of synthesized Tl-Ba-Ca-Cu-O superconductors in terms of starting chemical composition, thermal reaction condition, and reaction kinetics has been investigated by differential thermal analysis and thermogravimetric analysis techniques. Especially, much effort has been taken to seek an optimal processing procedure to obtain the high superconducting temperature phase. Such a procedure encompasses a thermal decomposition reaction of $BaCO_3$ in the presence of excess $CaCO_3$ and CuO, which has led to the formation of $BaCuO_2$ and $Ca_2CuO_3$. Then the subsequent $Tl_2O_3$ reaction with the $BaCuO_2/Ca_2CuO_3$ mixture has been investigated. Especially, the evaporation behavior of thallium oxide was closely investigated for the several heattreatment procedure. The characteristic microstructure ascribed to the starting chemical compositions and the sintering temperatures was observed, and its morphology and superconductiong properties has been studied in relation with high and low Tc phases.
To identify phases from the mixed phases in Tl-Ba-Ca-Cu-O, X-ray peak profiles have been simulated for $(TlO)_mBa_2Ca_{n-1}Cu_nO_{2n+2+1.5m}$ phases with m = 1-2 and n= 1-4. They agree well with the measured X-ray diffraction patterns. With these results, peaks by the single Tl-O layered phases like $TlBa_2Ca_3Cu_4O_x$ (1234), $TlBa_2CaCu_2O_x$ (1212), $TiBa_2Ca_2Cu_3O_x$ (1223) phases were identified on an XRD pattern of highly disordered samples. In addition unpublished XRD patterns of $TlBa_2Ca_3Cu_4O_x$ (2234) and $TlBa_2Ca_4Cu_5O_x$ (1245) phases were obtained.
For the measurement of a bulk superconducting property as a function of temperature varying from liquid nitrogen temperature to room temperature, an ac susceptibility measurement system by the mutual inductance method was built in a HP4194A Impedance/Gain-phase analyzer. Complex ac susceptibility was measured also as a function of frequency from 1 kHz to 300 kHz at various field amplitudes for a sintered Tl-Ba-Ca-Cu-O superconductor. Complex ac susceptibility measurement is very useful to investigate the coupling phenomena in the high Tc ceramic superconductor. The coupling phenomena dependent on the microstructure and on the composition was investigated. Also, a frequency dependence behavior of the peaked imaginary part of susceptibility as a function of temperature was observed. From the frequency dependence of peak shift in the χ"-T plot, grain boundary flux creep activation energy was obtained approximately 4.0 eV in 1.3 Oe in the $Tl_2Ba_2Ca_3Cu_4O_x$ sample sinterd at 900°C for 60 min.
The harmonics generation phenomena was observed in the 2212 low Tc phase, 2223 high Tc phase and mixed phases of both. This conforms that the ceramic superconductor is composed of an array of weakly coupled Josephson junction.
Transport critical current density, Jc, was measured for Tl-Ba-Ca-Cu-O ceramic superconductors. It is dependent on the starting chemical composition and heat treatment condition which results in varied phases and microstructures. When thallium oxide was added in excess, it expedites weak grain junction and Jc decreased. Jc in Tl-Ba-Ca-Cu-O system reached up to 700 A/$cm^2$ for the 2234 and 2223 compositions sintered at 900°C. A hysteretic Jc changes under applied magnetic field up to 0.9 Tesla has been observed.
