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

在700 ℃左右的高压容器中，将摩尔比为2：3的硝酸钡和CuO混合物与过量的BaO按BaO：CuO反应，容易生成Ba_2Cu_3O_（5+d）晶体。从熔剂中机械分离出的试样的显微维氏硬度测试表明，其硬度值与方解石的硬度值相近。本研究的最后一个目的是研究用Ba（NO_3）_2助熔剂直接合成正交相YBa_2Cu_3O_x超导体。在BaO-Y_2 O_3-CuO体系中，当BaNO_3：CuO = 2：1：1时，Y_2O_3与BaO-Y_2 O_3反应生成的产物中没有未反应的Y_2O_3，而当BaNO_3：CuO = 2：1时，Y_2O_3与BaO-Y_2 O_3反应生成的产物中没有未反应的Y_2O_3。3在高温高压容器中进行690 ℃热处理。在690 ℃保温40 h后冷却不放气和在690 ℃保持0.5MPa压力放气的反应中，主要产物为Y_2Cu_2O_5，未检测到YBa_2Cu_3O_X。在690 ℃下保持容器内压力为0.1MPa与常压相当的条件下分解硝酸锌，除了Ba_2Cu_3O_（5+d）和未反应的Y_2O_3和Ba（NO_3）_2外，还能生成YBa_2Cu_3O_X。透射电镜观察表明，YBa_2Cu_3O_X晶体中没有观察到层状结构，在900 ℃合成YB_2Cu_3O_X后，再在500 ℃氧气氛中热处理，所得样品的晶体中虽然观察到了斜方结构，但推测YBa_2Cu_3O_X是正交晶系的。采用硝酸锌助熔剂法直接合成了X超导体。

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.