Formation characteristics and phase transition of high-Tc superconducting phases have been investigated in Fe-doped YBaCuO and Pb-doped BiSrCaCuO systems by X-ray diffraction analysis, optical microscopy, scanning electron microscopy, transmission electron microscopy and electrical resistivity measurement.
The phase transition from orthorhombic to tetragonal in $YBa_2(Cu_{1-x}Fe_x)_3O_{7-y}$ superconductor is induced by replacing Cu with Fe. Amixture of the orthorhombic and tetragonal phases exists in specimens containing Fe of about x=0.03. In orthorhombic platelike grains are observed striations which are related to the formation of twins in the grains. The phase transition can be easily identified by the disappearance of the striations in platelike grains under polarized light. As Fe content increases, the twin spacing in orthorhombic phase becomes narrow. In the tetragonal phase at x ＞ 0.03, the fine tweed structure is observed instead of the twin structure of the orthorhombic phase. The c parameter does not change even though the phase change occurs, implying that the tetragonal phase in Fe-doped specimen contains more oxygen than that in undoped specimen. The zero resistance temperature $(T_c)$ and oxygen content decrease with Fe content. Nevertheless, the specimen containing Fe up to 2 at% shows high $T_c$ of 84K, suggesting that the superconductivity in $YBa_2(Cu_{1-x}Fe_x)_3O_{7-y}$ is more related to the oxygen content than the crystal structure. The formation characteristics of high-$T_c$ superconducting phase in the Pb-doped BiSrCaCuO system has been studied. During calcination, the low-$T_c$ phase with Tc=80K forms together with other second phases such as $PbCa_2O_4$, $Sr_{3-x}Ca_xCu_5O_y$ and a small amount of the high-$T_c$ phase. The low-$T_c$ phase is slowly transformed into the high-$T_c$ phase during sintering by reactions with Ca and Cu containing second phases.
The formation reaction of high-$T_c$ phase with $T_c$=107K appears to be most fastest at about 855℃. At this temperature the specimen contains small amount of a liquid formed by the melting of $PbCa_2O_4$. The Liquid seems to enhance the mass transport for the formation of the high-$T_c$ phase. High-$T_c$ single phase can be obtained by prolonged sintering of about 300h.
The formation kinetics of the high-$T_c$ phase has been investigated by varying Bi/Pb ratio. Substitution of 30% Pb for Bi is found to be most effective for the formation of the high-$T_c$ phase. As the Pb/Bi ratio increases, the lattice constant of the C axes of the low-$T_c$ and the high-$T_c$ phases increases. The increase in c axesappears to be caused by the Pb substitution for Bi site.
Specimens of $Bi_{1.4}Pb_{0.6}Sr_{4-x}Ca_xCu_{3.6}O_y$ have been prepared by sintering at 805℃~870℃. The crystal structure and zero resistance temperature alter with the Sr/Ca ratio. For 0≥x≥0.5, the c parameter of the unit cell is 24Å and no superconductivity is observed down to 77K. For 1≥x≥1.5, the c parameter increases to 31Å and $T_c$ reaches to 81K. As the Ca content increases to x=2 and 2.5, the formation of high-$T_c$ phase with c=37Å is observed. The variation of the c axes and the zero resistance temperature is thought to be related to the mono, double and triple Cu-O layer in the unit cell.
Compact swelling is observed during sintering of the Pb-doped BiSrCaCuO oxide. The apparent density increases at the early stage of sintering of about 20h, and then decreases. The densification occurs when the low-$T_c$ phase and a liquid phase exist while the de-densification occurs with the formation of the high-$T_c$ phase irrespective of the presence of the liquid phase. The maximum relative density of about 80% indicates that the gas evolution from the specimen during sintering does not affect the compact swelling. The compact swelling is explained by the anisotropic growth of thin platelike high-$T_c$ grains.
