Direct Synthesis of a Low Temperature Stable Oxide, Ba_2Cu_3O_<5+d>, Using Barium Nitrate as a Flux

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Reaction of barium nitrate and CuO mixtures having the composition of molar ratio from 2 : 3 to excess BaO as BaO : CuO using a high-pressure-vessel produced Ba_2Cu_3O_<5+d> crystals easily at the temperature about 700ﾟC.It was difficult to purify the crystals by means of a chemical separation because the materials are easily altered by a moisture or a samll amount of water. Micro-Vickers hardness test for the sample separated mechanically from a flux showed the hardness is a similar value to that of calcite. The final objective of this study is to examine the direct synthesis of orthorhombic form of YBa_2Cu_3O_X superconductor by using a Ba (NO_3) _2 flux. In the reaction system of BaO-Y_2O_3 using a Ba (NO_3) _2 flux, no reaction products were not detected to preserve unreacted Y_2O_3, even in the specimen having excess barium nitrate.In the system of BaO-Y_2O_3-CuO,the composition mixture of barium nitrate, Y_2O_3, and CuO having cation ratio of 2 : 1 : 3 was heat-treated at 690ﾟC in the high-oressure-vessel. In the case of reactions that the reaction vessel was cooled after soaking at 690ﾟC for 40h not to release gases from the vessel and that the pressure in the vessel was kept at 0.5MPa at 690ﾟC to release gases, the main product was Y_2Cu_2O_5 and YBa_2Cu_3O_X was not detected. In the case of reaction that the pressure in the vessel was kept at 0.1MPa equal to the ambient pressure at 690ﾟC to decompose barium nitrate, YBa_2Cu_3O_X was able to form in addition of Ba_2Cu_3O_<5+d> and unreacted Y_2O_3 and Ba (NO_3) _2. TEM observation showed that no twined structure was not detected in the crystals of YBa_2Cu_3O_X, though a twined structure was observed in the crystals of specimen prepared by an ordinary method that the heat-treatment was conducted at 500ﾟC in oxygen atmosphere after YB_2Cu_3O_X was synthesized at 900ﾟC.It was infered that the orthorhmbic YBa_2Cu_3O_X superconductor was directly synthesized by the barium nitrate flux method.