HIGH-TEMPERATURE THERMAL-EXPANSION OF BIPBSRCACUO SUPERCONDUCTORS

The linear thermal expansion in the range 30-800 degrees C, X-ray diff raction and resistivity versus temperature of BiPbSrCaCuO samples depe nding 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 synthe sis process. X-ray patterns corresponding mainly to the Bi-2223 phase were observed for the quenched samples. The slowly cooled samples exhi bited patterns attributed mainly to the Bi-2212 phase and also the add itional reflections caused by the inclusions (PbO2, Bi2O3 and PbBi). T he coefficients of thermal expansion (in the range 30-800 degrees C) o f the quenched and slowly cooled samples were 6.2 x 10(-6) and 5.1 x 1 0(-6) K-1 respectively. The thermal expansion curve of the slowly cool ed samples showed some peculiarities at about 300 degrees C. The criti cal 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 inclusion s of the second phases) due to slow cooling of the samples at the sint ering process was attributed to the precipitation phenomenon on coolin g. The peculiarities on the thermal expansion curves around 300 degree s C of the slowly cooled samples were interpreted as a result of the i nfluence of inclusions.