The effect of oxygen partial pressure during heat treatment on the microstructure of dip-coated Bi-2212/Ag and Ag alloy tapes

To study the phase relations in the Bi-2212 and Yb 2 O 3 system, Bi 2 Sr 2 Ca 1−x Yb x Cu 2 O y thick films are prepared by partial melt processing via an intermediate reaction between Bi-2212 and Yb 2 O 3 . When Bi-2212 and Yb 2 O 3 are partially melted and then slowly cooled, solid solutions of Bi 2 Sr 2 Ca 1−x Yb x Cu 2 O y form by reactions between liquid and solid phases which contain Yb. Following these reactions, Ca is partially replaced in Bi-2212 matrix and participates in the formation of secondary phases, such as Bi-free, (Ca, Sr)O x and CaO. Variation of the Bi-2212-Yb 2 O 3 ratios and processing parameters changes the balance between the phases and leads to different Yb:Ca ratios in the Bi-2212 matrix of processed thick films. When the partial melting process is optimized for each sample to minimize the growth of secondary phases, x = 0.42-0.46 for the samples prepared at pO 2 = 0.01 atm, x = 0.24-0.29 for the samples prepared at pO 2 = 0.21 atm, x = 0.18-0.23 for the samples prepared at pO 2 = 0.99 atm are obtained regardless to the starting compositions.It is found that superconducting properties of Bi 2 Sr 2 Ca 1−x Yb x Cu 2 O y thick films strongly depend on the processing conditions, because the conditions result in different Yb content in the Bi-2212 matrix and the volume fraction of the secondary phases. The highest T c (0) of 77, 90 and 91 K were obtained for the samples processed at 0.01, 0.21 and 0.99 atm of O 2 , respectively.

Section: Superconducting Propertiesmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

The effect of Yb addition in Bi₂Sr₂Ca_1 xY b_xCu₂O_ypartial melted thick films

Ilyushechkin

Yamashita

Boskovic

2004

“…The morphology and chemical reactivity of the present phases when Bi2212 begins to form have significant impact on the electrical properties of the wire as they are critical for forming the final microstructure. Presently, PMP is performed in flowing oxygen (P O2 = 1 atm) because it results in the highest J c [18,19].…”

Section: Introductionmentioning

confidence: 99%

Influencing factors on the electrical transport properties of split-melt processed Bi₂Sr₂CaCu₂O_xround wires

Liu

Schwartz

2012

Here we study the effects of varying split-melt processing (SMP) heat treatment temperatures on multifilamentary Bi 2 Sr 2 CaCu 2 O x (Bi2212) wires to understand the factors that influence the electrical transport properties. Sharp threshold behaviors result in an order-of-magnitude difference in critical current density and distinct difference in magnetization behavior. Quantitative image analysis of SEM micrographs shows a strong correlation between the content of 2-4 µm interfilamentary bridges and electrical performance. Avrami analysis of re-solidification indicates that Bi2212 grain nucleation is heterogeneous and site-saturation limited, with a transition in the reaction mechanism corresponding to the depletion of liquid. During the first SMP heat treatment, oxygen is fully recovered, accelerating Bi2201 and/or Bi2212 phase nucleation. Large, flat Bi2201 grains form from the Bi2201 liquid at the start of the second heat treatment and subsequently act as Bi2212 grain nucleation sites. During cooling from high temperature, Bi2212 grains form via a solid/liquid reaction that is enhanced by the liquid, that facilitates second phase dissolution and cation diffusion. At lower processing temperatures, and when the liquid is depleted, Bi2212 grain growth is from a slow solid-state reaction.

“…Nevertheless, all of them have been obtained in polycrystalline systems, which are significantly influenced by the presence of highly defective regions like grain boundaries. In fact, Fujii et al [10][11][12] proved that such boundaries can show the presence of amorphous areas, whose features are controlled by the oxygen partial pressure during the crystallization process. On the other hand, the measurements by Prouteau et al [13] have shown that the grain boundaries are characterized by extended non-stoichiometric zones, depleted in oxygen, which has also been confirmed from an electronic point of view by the Josephson junction experiments on YBCO [6].…”

Section: Introductionmentioning

confidence: 99%

Photoconductivity experiments on superconducting Bi₂Sr₂CaCu₂O_8+xwhiskers

Palumbo

Cagliero

Agostino

et al. 2007

In the present paper we report the results of photoconductivity experiments carried out in nearly optimally doped, single-phase Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) whiskers. Samples with both straight and bent shapes have been investigated by means of the four-probe and two-probe configurations in dark and in illuminated conditions. Experimental data invariably show the absence of a persistent photoconductivity (PPC) effect. Such observations, combined with the very high crystal quality of the whiskers, which corresponds to the absence of defects acting as electron traps, can be explained in the framework of the electron-hole pair excitation model (Kudinov et al 1990 Phys. Lett. A 151 358). In contrast, experimental data show a reversible increase of the electrical resistance under illumination, which is fully consistent with a local heating effect due to the thermal load induced by the light. Such resistance increase is especially large for samples measured in the two-probe configuration, suggesting a possible supplementary effect due to an interaction between the light and the c-axis resistivity or the contact resistance.