High Tc superconductor powder preparation by emulsion drying method and study on oxygen behavior in $(Bi,Pb)_2Sr_2Ca_2Cu_3O_y$

Abstract

The superconducting powders in $YBa_2Cu_3O_{7-x}$ and Bi-Pb-Sr-Ca-CuO system were easily prepared from water in oil type emulsion by loading each cation into the aqueous phase. In $YBa_2Cu_3O_{7-x}$ system, the superconducting orthorhombic phase was formed by calcining at $750\,^\circ\!C$ for 10h in $O_2$. The size of the superconducting pahse powders was submicron(less than $0.5 \mu m$). The density of the sintered specimen using this powders was about 95\% of the theoretical density and the resistance sharply decreases at about 90K. In Bi-Pb-Sr-Ca-Cu-O system the low Tc phase ($(Bi,Pb)_2Sr_2Ca_1Cu_2O_y$) was formed by calcining at $800\,^\circ\!C$ for 10h in an oxygen partial pressure of 1/20atm. The shape of calcined powder is thin plate of which size is about $25 \mu m$ and thickness is smaller than $0.55 \mu m$. It was obsrved that the high Tc phase ($(Bi,Pb)_2Sr_2Ca_2Cu_2O_y$) was formed by sintering at $850\,^\circ\!C$ for 30h in oxygen pressure of 1/20 atm without intermediary grinding. The above sintered sample exhibited superconductivity with a Tc(zero)=107K Then, a homogeneous $(Bi,Pb)_2Sr_2Ca_2Cu_3O_y$ phase was prepared using the emulsion drying method to study the influence of oxygen content variation on properties. The samples annealed in lower and higher oxygen partial pressure than $Po_2=10^{-2}$ atm show Tc shifte to the lower temperautres compared with the sample annealed in $Po_2=10^{-2}$ atem which shows the highest Tc, 107K. The resistivity curve also shows a minimum value from room to superconducting transition temperatures in sample annealed in $Po_2=10^{-2}$ atm.XPS measurement was carried out to study the behavior of oxygen and the change of chemical environments of each ion in the high Tc phase dependent on the annealing atmosphere changes becoming the origin of these phenomena. The tendency of increasing the binding energies of Bi $4f_{7/2}$ and Cu $2p_{3/2}$ with increasing the oxygen partial pressures during the heat treatments mea...