Bi-oxide superconductors have a potential for practical applications in the scientific and engineering fields. But it is very difficult to synthesize a $Bi_2Sr_2Ca_2Cu_3O_x$ (Bi-2223) single phase in a short reaction time, because the formation rate of Bi-2223 phase is sluggish. To solve this problem, it is important to understand phase stability of Bi-oxide superconductors as a function of oxygen partial pressure and to utilize intermediate compounds in the Bi-Sr-Ca-Cu-O system as the precursors for synthesis of Bi-2223.
Bi-based superconductors such as $Bi_2Sr_2CuO_x$ (Bi-2201), $Bi_2Sr_2CaCu_2O_x$ (Bi-2212), $Bi_{1.7}Pb_{0.4}Sr_2Ca_2Cu_3O_x$ (Bi(Pb)-2223) and other intermediate compounds were prepared in air by solid-state reaction from the mixture of $Bi_2O_3$, $SrCO_3$, $CaCO_3$, CuO, $Pb_3O_4$ powders. The stabilities of Bi-based superconductors in the reduced oxygen partial pressure were investigated by oxygen coulometric titration method. The stability limits of Bi-2201, Bi-2212 and Bi-2223 at 820℃ were $4.0\times10^{-4}$, $1.4\times10^{-3}$, $2.0\times10^{-3}$ $Po_2$ atm, respectively. As the oxygen content was reduced, Bi-2223 was decomposed into a mixture of Bi-2201, $Ca_2CuO_3$ and $Cu_4O_3$ at 820℃. But the decomposed of Bi-2223 into Bi-2201 was proceeded through Bi-2212 at 780℃. Below this stability limit, all the samples completely melted. Since the stability limit of Bi-2223 was a little higher than that of Bi-2212, ti was impossible to synthesize Bi-2223 without the formation of Bi-2212 in reduced oxygen atmosphere.
Bi-2201, Bi-2212, $Ca_2CuO_3$,$(Ca_{0.91}Sr{0.09}CuO_2)$, $Ca_2PbO_4$, $Bi_2CaCuO_5$ and $Sr_2CaCu_2O_5$ were used as intermediate compounds for the formation Bi-2223 phase. Among these intermediate compounds, Bi-2201 and $(Ca_{0.91}Sr_{0.09})CuO_2$ are most interesting because they are almost on the straight line of Bi-2223, Bi-2212, Bi-2201 and $(Ca,Cu)O_2$ in the $Bi_2O_3-CaO-CuO-SrO$ quaternary phase diagram. Thus, mixture of Bi-2201 and $(Ca_{0.91}Sr_{0.09})CuO_2$-rich composition instead of the mixture with the exact Bi(Pb)-2223 overall composition. The formation of Bi-2223 was almost finished within 60 hours at 860℃ and 870℃. But Bi-2212 phase remained ever after long-time reaction. However, the merit of synthesis method using intermediate compounds was a shorter reaction time and a smaller amount of second phases compared to the conventional solid-state powder reaction method.
