(Cu,Tl)Ba₂Ca₃Cu₄O_x compositions: II. Heating rate applied to synthesis of superconducting ceramics

Superconducting samples with a starting composition Cu_0.5Tl_xBa₂Ca₃Cu₄O₁₂ (x = 1–0.35) were prepared in Au tubes from oxides by using different heating rates. The optimum heating rate was 0.11 °C min⁻¹ applied between 860 and 880 °C. These samples show the highest content of 1234 phase and the lowest of Ba–Cu–O phases as well as the highest critical temperatures. The highest values were T_c = 119.2 K and T_c0 = 115.1 K. All samples show J_c values around 6 × 10⁵ A cm⁻², at 60 K and 0.5 T. In the rest of the samples, heated by using lower or higher heating rates, a high concentration of 1223 and/or 1245 phases is detected. The content of the 1234 phase and of Ba(Ca,Tl)–Cu–O residual liquid phase, in the samples synthesized for the optimum heating rate, depends on the Tl content in the starting mixture. The influence of the oxygen content in the starting mixture on phase composition is not as strong as of Tl, but it has a major role in establishing the superconducting behaviour of the 1234 grains. Some arguments that suggest the dependence of the transport properties of the non-superconducting matrix on oxygen content will be discussed. In our technological arrangement the determined optimum heating rate can be applied to the synthesis of other single-layered superconducting phases, e.g. 1212 phase from a Cu_0.25Tl_0.75Ba₂CaCu₂O_8.115 starting composition.