Dendrite formation in the Bi2Sr2CaCu2O8-Ag system: reactions between Ag and oxide melt

The reactions between Ag and oxide melt in the Bi2Sr2CaCU2O8-Ag system have been investigated. When an Ag sheath reacts with the oxide melt, two kinds of dendritic phases are formed near the Ag sheath. One dendrite with the c omposition of Ag0.98CU0.02 was large, 1-100 mum, and more or less spherical in overall shape. The other dendrite with the composition of approximately Ag0.24Cu0.76 was small, 0.1-1 Irm, and typically dendritic. The small dend rites formed always along the Bi2Sr2CaCu2O8 superconducting plates. When th e sample was heat treated at 70 degreesC in air, the dendrite shape changed from smooth to faceted, and the color from white to yellowish red for the large dendrite and from white to dark green for the small one. These observ ations imply that the dendrites were metallic at high temperature but oxidi zed during the heat treatment at 70 degreesC. The formation of the two kind s of dendrites is attributed to the separation of the Bi melt into metallic and oxide liquids due to a miscibility gap. The formation of the Ag-Cu den dritic phases also seems to be responsible for the inhomogeneous microstruc ture, which is commonly observed between an Ag sheath and the central ceram ic region, because the dendrite formation must cause inhomogeneous liquid c omposition in the sample. (C) 2000 Elsevier Science B.V, All rights reserve d.