| Research Abstract |
Phase diagram of Y-Ba-Cu-O four component and ite relating three component systems have been investigated under air of mixed gas of oxygen with argon. In order to determine experimentally the phase diagrams, powder X-ray diffraction, EMPA, microscop13EA\ : ic observation and thermal analysis were carried out. When the carbonates are used as starting materials and/or the specimen is calcined in natural air including carbonic acid gas, some ternary and quaternary oxycarbonates are easily formed. In the Y-Cu-O ternary system, there are five phases of Y_2O_3, CuO, Cu_2O, YCuO_2 and Y_2Cu_2O_5 and these phases compose the phase diagram depending on oxygen partial pressure. In the Ba-Cu-O ternary system, there are three ternary phases of BaCuO13EA\ : _2 and two polymorphisms of Ba_2CuO_3 under oxygen pressure around 1 atm. In the Ba-Y-O ternary system, it was found that two phases of Y_4Ba_4O_9 and Y_2BaO are the true oxide, but other phases reported till now are oxycarbonates. In the qua13EA\ : ternary system, five quaternary phases YBa_2Cu_3O_<6+x>, Y_2BaCuO_5, YBa_4Cu_3O_<8.5>, YBa_6Cu_3O_<10.5> and YBa_2Cu_4O_<7.5> are stable under air. Three isothermal sections at 1263, 1223 and 1173 K were determined and the phase relation has b13EA\ : een in detail investigated. Combining the results of htermal analysis, the whole sequence diagram for univariant reactions has been established in the Y-Ba-Cu-O quaternary system under oxygen partial pressure of 0.21 atm. Furthermore, two invariant reactions were confirmed to exist in the quaternary system from the oxygen partial pressure dependence of phase relations. Based on the knowledge mentioned above, the control of microstructure has been discussed to improve superconductivity.
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