Solidification of Bi₂Sr₂CaCu₂O_y and Bi₂Sr_1.75Ca_0.25CuO_y

Abstract

The solidification processes for the compositions Bi 2 Sr 2 CaCu 2 O y (2212) and Bi 2 Sr 1.75 Ca 0.25 CuO y (2201) were determined as a function of oxygen partial pressure. During solidification in argon, the superconducting phases were generally not observed to form for either composition. The solidification processes for the compositions Bi2Sr2CaCu20y (2212) and Bi2Sri.75Cao.25CuOy (2201) were determined as a function of oxygen partial pressure. During solidification in argon, the superconducting phases were generally not observed to form for either composition. In both cases, the solidus is lowered to approximately 750 °C. Solidification of Bi2Sri 75Ca0.25CuOy in Ar resulted in a divorced eutectic structure of Bi2Sr2_A:Cax0>, (22JC) and Cu2O while solidification of Bi2Sr2CaCu20>, in Ar resulted in a divorced eutectic structure of Bi2Sr3_xCa;c03, (23x) and Cu2O. Solidification of Bi2Sri.75Cao.25CuOy in O2 resulted in large grains of 2201 interspersed with small regions containing the eutectic structure of 22* and CuO/Cu2O. Solidification of Bi2Sr2CaCu2Oy in partial pressures of 1%, 20%, and 100% oxygen resulted in multiphase samples consisting of 2212, 2201, some alkaline-earth cuprates, and both divorced eutectic structures found during solidification in Ar. For both compositions, these latter structures can be attributed to oxygen deficiencies present in the melt regardless of the overpressure of oxygen. These eutectic structures are unstable and convert into the superconducting phases during subsequent anneals in oxygen. The formation process of the 2212 phase during solidification from the melt was determined to proceed through an intermediate state involving the 2201 phase.

The effect of Yb addition in Bi2Sr2Ca1 xY bxCu2Oypartial melted thick films

“…For doped compounds, the process is further complicated by additional elements in the Bi-Sr-Ca-Cu-O system which may cause the growth of additional phases during partial melting. Partial melting and solidification of Bi 2 Sr 2 CaCu 2 O y strongly depends on processing parameters such as maximum melting temperature, time of reaction and solidification rate [13]. The partial oxygen pressure also affects the reaction kinetics during partial melting and phase growth [14,15].…”

From imperfect to perfect Bi2Sr2CaCu2Ox (Bi–2212) grains

“…13 Little information is available about the amount and distribution of the intergrowths within the grains. In general, their amount was found to be increased in samples of Bi-rich starting compositions.13 Furthermore, they are frequently found at twin boundaries, and their insertion is thought to be a possible mechanism for solid-state diffusion parallel to the c-axis.9 Lang et aln reported that the alternate arrangement of the half-cells of 2201 and 2212 is slightly preferred; only few identical half-cells are nearest neighbors.…”

Phase decomposition of single crystals annealed in vacuum

“…There have been some results about the phase decomposition of Bi 2 Sr 2 CaCu 2 O y polycrystal samples [8][9][10] but single crystals can be used more readily to clarify some of this issue since they have less impure phase, crystal grain and other crystal defects. As pointed out by some authors of our group [11,12] the Bi 2 Sr 2 CaCu 2 O y crystal is ready to decompose when annealed in air at temperatures above 450 • C and the segregated phases, including Bi oxides, Bi x Sr y Ca z oxide and Bi 2 Sr 2 CuO x (Bi2201), are located on the surface of the annealed crystal sheets.…”