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

有效的磁通钉扎是维持临界电流的必要条件。否则，移动磁通线会引起电压降，并产生电阻。 现在人们认识到，在YBa_2Cu_3O_（7-d）（YBCO）中存在由孪晶界引起的磁通钉扎，但这种钉扎高度依赖于磁通线运动的方向。 它们的存在有助于YBCO在高温和高磁场下表现出比Bi，Hg和Tl铜酸盐更高的临界电流密度，尽管它们的临界温度更高。 因此，较高的孪晶密度导致较大的磁通钉扎力和较高的临界电流密度，在磁场和温度下，这对应用更有吸引力。 然而，很少有努力已经在工程精细孪晶和孪晶畴，以利用其磁通钉扎性能。 我们的项目是提出设计精细孪晶结构的方法。为了确定这些方法，我们首先需要知道孪晶边界能的值及其对一些材料加工参数的依赖性。孪晶界面能及其对加工参数的依赖性的信息允许通过控制这些加工条件来优化孪晶结构。这些信息可以用来产生最好的孪晶形态，从而产生最大的磁通钉扎和高临界电流密度。具体地说，我们可以利用这些信息系统地细化YBCO中的孪晶结构。这使得系统的调查临界电流密度（Jc）的磁化和孪晶显微镜在寻找最佳的孪晶结构，提高临界电流密度。 我们提出的工作将解决双贡献磁通钉扎和临界电流密度的目标，以获得最佳的组合物，添加剂，氧化和加工条件，最大的磁通钉扎和最高的Jc。微观结构研究的这一领域被忽视，也可以为研究生提供优秀的研究培训。影响：对于高温超导体的工业应用来说，是它们的低临界电流密度（测量可以在没有电阻的情况下传输的电流）由晶界处的不良电流传输和不足的磁通量引起-钉扎中心位于应用温度和磁场处。 高温超导体YBa_2Cu_3 O_（7-d）（YBCO）中的孪晶晶界是普遍存在的，并且已经被证明是YBCO中磁通量的有效钉扎中心，适用于-230 ℃到-190 ℃的实际工作温度范围和20，000倍地球磁场的磁场。工程孪晶形态的新方法将出现从这个拟议的工作，可以应用到YBCO涂层磁带，可以帮助解决长期存在的问题，低临界电流密度的高温超导体，并允许板应用。孪晶作为一种材料现象，影响着铁电体、铁磁体、铁弹体和马氏体相变金属间化合物的重要材料性质。 因此，我们从YBCO中的孪生中学到的知识可以直接应用于这些技术上重要的材料，以优化其性能。

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.