Twin Boundaries in Superconducting YBa2Cu3O7-x: Twin Boundary Energy and Its Dependence on Dopants, Additives, and Processing Parameters for Engineering Fine Twins.

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Effective pinning of magnetic fluxes is imperative of sustaining critical current. Otherwise a voltage drop is induced by moving magnetic flux lines and resistance develops. It is now recognized that flux-pinning by twin boundaries in YBa2Cu3O7-d (YBCO) exists but is highly dependent on the direction of flux-line motion. Their presence has helped the YBCO to exhibit higher critical current densities at high temperatures and high magnetic fields than those of the Bi, Hg and Tl cuprates despite their higher critical temperatures. It follows that a higher twin density leads to a larger flux-pinning force and a higher critical current density in magnetic fields and temperatures that are more attractive for applications. Yet very few efforts have been in engineering fine twins and twin domains to exploit their flux-pinning properties. Our project is to come up with methods to engineer fine twin structure. To identify these methods, we first need to know the value of the twin boundary energy and its dependence on a number of materials processing parameters. Information on twin boundary energy and its dependence on processing parameters allows the optimization of twin structure by controlling these processing conditions. This information can be exploited to produce the best twin morphology, which leads to maximum flux pinning and high critical current density. Specifically, equipped with the information, we can refine the twin structure in YBCO systematically. This allows the systematic investigation of critical current density (Jc) by magnetization and twin microstructure by microscopy in search for optimal twin structure for enhanced critical current density. Our proposed work will address the twin contribution to flux pinning and critical current density with the goal to obtain the optimal composition, additive, oxygenation and processing conditions for maximum flux pinning and highest Jc. This area of microstructure research is much ignored and can also provide excellent research training for graduate students.Impact: The major problem, for industrial applications of high temperature superconductors, is their low critical-current density (a measure of current that can be transmitted without electrical resistance) caused by poor current transmission at the grain boundaries and insufficient flux-pinning centers at application temperatures and magnetic fields. Twin boundaries in the high temperature superconductor YBa2Cu3O7-d (YBCO) are ubiquitous and have been proved to be effective pinning centers of magnetic fluxes in YBCO for (i) the practical range of operation at temperature ranging from -230 C to -190C and (ii) magnetic fields of the order of 20,000 times of the earth's magnetic field. New methods for engineering twin morphology will emerge from this proposed work that can be applied to YBCO coated tapes which can help to solve the long standing problem of low critical current density of the high temperature superconductors and allow board applications. Twinning as a material phenomenon affects important material properties in ferroelectrics, ferromagnetics, ferroelastics and intermetallic compounds with martensitic-transformation. As such, what we learn from twinning in YBCO, can be directly applied to these technologically important materials to optimize their properties.