The formation and growth behaviour of the Bi-2223 phase was investigated by controlling reaction pathway in precursors made with several intermediate compounds.
A comparative study of conventional process and new precursor process for the synthesis of Bi-2223 phase was conducted. When PbO was not added, there was no difference between two processes and 2223 phase was not formed in air at 850℃ for 100 h.
To enhance the reaction rate, $Bi_{0.82}Cu_{0.18}O_{x}$, the hyper-eutectic phase of $Bi_2O_3 + Bi_2CuO_4$ was used instead of PbO. It forms a liquid phase below 850℃. A mixture was made with $Bi_{0.82}Cu_{0.18}O_x$ and the intermediate compounds. The precursor of ideal composition, $Bi_2Sr_2Ca_2Cu_3O_x$ showed a small drop of electrical resistance at 110K, but Bi-2223 phase was not detected by X-ray diffraction analysis.
The possibility of direct formation of Bi-2223 phase was checked by sintering a mixture of $Bi_{0.82}Cu_{0.18}O_x$, $Sr_2CuO_3$, $Ca_2CuO_3$, CuO and PbO at 850℃ without calcination which was required at the conventional mixing process with $Bi_2O_3$, $SrCO_3$, $CaCO_3$, CuO and PbO. The new mixture was partially melted at 770℃ beacause of the eutectic liquid from $Bi_{0.82}Cu_{0.18}O_x$. It enhanced the formation of Bi-2212 phase with large grains instead of forming Bi-2223 phase. The new mixture also enhanced the tranformation rate from the Bi-2212 phase to Bi-2223 phase. It showed $T_c$ at 85K and the drop of elctrical resistance at 110K.
To optimize the new process, the content of Bi, Ca and Cu was controlled by the addition of $Bi_{0.82}Cu_{0.18}O_x$ and $Ca_2CuO_3$ at the fixed condition of 1.9Sr and 0.4Pb. The sample of $Bi_{2.0}Sr_{1.9}Ca_{2.2}Cu_{3.27}O_x$ exhibited zero resistance at 95K after sintering for 100 h. This means the possibility of zero resistance at 110K through the more control of composition and sintering temperature.
The Bi-2223 phase seemed to form both on the free surfaces and the grain boundaries of the Bi-2212 phase and grow along the ab planes of the Bi-2223 phase with the dissolution of the Bi-2212 phase, resulting in larger Bi-2223 platelets than those in the conventional process. And the grain size of Bi-2223 phase seemed to be dependent on the original grain size of Bi-2212 phase.
From the kinetic study with Johnson-Mehl-Avrami equation, diffusion controlled two-dimensional growth with zero nucleation rate was confirmed as the most probable model for the nucleation and growth of Bi-2223 phase from Bi-2212 phase.
