The bulk processing of 2223 BSCCO powders I. Densification and mechanical response

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Abstract
The anisotropic mechanical properties of densified BSCCO (Bi-Sr-Ca-Cu-O) powders are of paramount importance during thermo-mechanical processing of superconducting tapes and wires. Maximum current transport requires both high relative density and a high degree of alignment of the single crystal superconducting planes parallel to the plane of the tape. This is also a configuration that causes high stresses during compressive (i.e. densifying) processing modes. These high stresses can lead to cracking, and thus degrade the density, and eventually the conductive properties of the tape. The current work develops a micromechanically based material model for such densified powders. The deformation mechanisms of interest are crystallographic glide and porosity evolution; thus the model takes the form of a porous, elastic viscoplastic polycrystal material theory. This has been achieved by coupling the modified Taylor type polycrystal model of Schoenfeld, Ahzi and Asaro in a generic way to yield surface type flow theories. The porosity model of Fleck, Kuhn and McMeeing is used to describe the evolution of porosity with deformation. Compaction experiments on 2223 BSCCO (Bi2Cr2Ca2Cu3Ox) powder is done in a confined channel die environment so as to simulate the plane strain tape rolling environment. The model is calibrated and compared to these experimental results, and then employed to resolve the effects of initial texture and confinement pressure on the densification and ultimate formability of the powder. In Part II of this work, the current model will be applied in order to resolve states of stress and textural alignment in the BSCCO conductor during tape rolling, and hence improve the current state of the art in tape manufacturing.