High-temperature thermal expansion of BiPbSrCaCuO superconductors

The linear thermal expansion in the range 30-800 degrees C, X-ray diffraction and resistivity versus temperature of BiPbSrCaCuO samples depending on the cooling rate (at the end of sintering) were investigated. Two types of samples were prepared: quenched ones (about 1000 degrees C/min) and slowly cooled ones (3 degrees C/min) in the thermal synthesis process. X-ray patterns corresponding mainly to the Bi-2223 phase were observed for the quenched samples. The slowly cooled samples exhibited patterns attributed mainly to the Bi-2212 phase and also the additional reflections caused by the inclusions (PbO2, Bi2O3 and PbBi). The coefficients of thermal expansion (in the range 30-800 degrees C) of the quenched and slowly cooled samples were 6.2 x 10(-6) and 5.1 x 10(-6) K-1 respectively. The thermal expansion curve of the slowly cooled samples showed some peculiarities at about 300 degrees C. The critical temperature of the quenched sample (108 K) is higher than that for the slowly cooled sample (92 K). The degradation of the Bi-2223 phase (in the form of increasing the content of Bi-2212 phase and inclusions of the second phases) due to slow cooling of the samples at the sintering process was attributed to the precipitation phenomenon on cooling. The peculiarities on the thermal expansion curves around 300 degrees C of the slowly cooled samples were interpreted as a result of the influence of inclusions.