Use of Sm-123 + Sm-211 Mixed-Powder Buffers to Assist the Growth of SmBCO and ZrO<inf>2</inf>-doped SmBCO Single Grain, Bulk Superconductors

In this paper single domain, bulk SmBCO samples have been fabricated successfully in air in a top-seeded melt growth (TSMG) process using a conventional chamber furnace, in addition to samples doped with ZrO2. In order to improve the reliability of seeding, Sm-123+ Sm-211 mixed-powder buffers were used to increase the success of the SmBCO single grain growth process. SmBCO single grains of diameter as large as 20 mm and 10 mm in thickness with and without ZrO2 were fabricated successfully using Sm-123 + Sm2BaCuO5 (Sm-211) mixed-powder buffer layers. The geometric configurations of the buffers were also optimized as part of this study to further increase the success of the single grain growth process. Superconducting properties of Jc and Tc of the specimens under the buffer layer have also been investigated.This is the accepted manuscript. The final version is available from IEEE at http://dx.doi.org/10.1109/TASC.2014.2381855

Design, fabrication, and characterization of engineered materials in the microwave and millimeter wave regime

In this paper we present the study of a two-dimensional square-lattice photonic crystal with all-angle negative refraction in its first band. Using this photonic crystal, we designed and fabricated a flat lens functioning as a cylindrical lens, by increasing the vertical dimension of the photonic crystal. Two-dimensional finite-difference time-domain simulation validated negative imaging and sub-wavelength resolution. To perform the experiment, a microwave imaging system was built based on a vector network analyzer. Field distributions were acquired by scanning the imaging plane and object plane. The experiment demonstrated negative refraction imaging in both amplitude and phase, and verified sub-wavelength resolution

The successful incorporation of Ag into single grain, Y-Ba-Cu-O bulk superconductors

The use of RE-Ba-Cu-O [(RE)BCO] bulk superconductors, where RE=Y, Gd, Sm, in practical applications is, at least in part, limited by their mechanical properties and brittle nature, in particular. Alloying these materials with silver, however, produces a significant improvement in strength without any detrimental impact on their superconducting properties. Unfortunately, the top seeded melt growth (TSMG) technique, used routinely to process bulk (RE)BCO superconductors in the form of large, single grains required for practical applications, is complex and has a large number of inter-related variables, so the addition of silver increases the complexity of the growth process even further. This can make successful growth of this system extremely challenging. Here we report measurements of the growth rate of YBCO-Ag fabricated using a new growth technique consisting of continuous cooling and isothermal hold (CCIH) process. The resulting data form the basis of a model that has been used to derive suitable heating profiles for the successful single grain growth of YBCO-Ag bulk superconductors of up to 26 mm in diameter. The microstructure and distribution of silver within these samples have been studied in detail. The maximum trapped field at the top surface of the bulk YBCO-Ag samples has been found to be comparable to that of standard YBCO processed without Ag. The YBCO-Ag samples also exhibit a much more uniform trapped field profile compared to that of YBCO

Growth rate of YBCO-Ag superconducting single grains

The large scale use of (RE)Ba₂Cu₃O₇ bulk superconductors, where RE=Y, Gd, Sm, is, in part, limited by the relatively poor mechanical properties of these inherently brittle ceramic materials. It is reported that alloying of (RE)Ba₂Cu₃O₇ with silver enables a significant improvement in the mechanical strength of bulk, single grain samples without any detrimental effect on their superconducting properties. However, due to the complexity and number of inter-related variables involved in the top seeded melt growth (TSMG) process, the growth of large single grains is difficult and the addition of silver makes it even more difficult to achieve successful growth reliably. The key processing variables in the TSMG process include the times and temperatures of the stages within the heating profile, which can be derived from the growth rate during the growth process. To date, the growth rate of the YBa₂Cu₃O₇-Ag system has not been reported in detail and it is this lacuna that we have sought to address. In this work we measure the growth rate of the YBCO-Ag system using a method based on continuous cooling and isothermal holding (CCIH). We have determined the growth rate by measuring the side length of the crystallised region for a number of samples for specified isothermal hold temperatures and periods. This has enabled the growth rate to be modelled and from this an optimized heating profile for the successful growth of YBCO-Ag single grains to be derived

Effect of Y-211 particle size on the growth of single grain Y-Ba-Cu-O bulk superconductors

The engineering of fine Y2Ba1Cu1O5 (Y-211) inclusions of average particle size 1−2 μm within the continuous, superconducting YBa2Cu3O7−δ (Y-123) phase matrix of single-grain, bulk high temperature Y–Ba–Cu–O (YBCO) superconductors is fundamental to achieving high critical current density in these materials. However, bulk samples fabricated using fine Y-211 precursor powders generally suffer from incomplete growth of the Y-123 phase compared to samples fabricated with coarser Y-211 particles of average particle size >5 μm in the precursor powder. In this study, the effects of Y-211 particle size on processing parameters during growth of large single YBCO grains are reported. Chemical analysis using EDX of cross-sections of single YBCO grains indicates that the loss of liquid phase during melt processing is greater when finer Y-211 precursor powder is employed, which accounts for the observed growth limitations and low sample yield. Specific modifications to the single grain growth process have been made based on the results of this investigation to reduce liquid loss and to enable the use of fine Y-211 precursor powder without compromising complete single grain growth or yield.Yunhua Shi would thank the financial support from EPSRC (grant ref. EP/K02910X/1), United Kingdom.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0022024814008045#

Comparison of the superconducting properties of Y-Ba-Cu-O and Y-Ba-Cu-O-Ag bulk superconductors

The widespread use of RE-Ba-Cu-O [(RE)BCO] bulk superconductors, where RE=Y, Gd or Sm, in practical applications requires large single grains that exhibit uniform superconducting properties. Until recently, however, it was difficult to grow successfully YBCO-Ag bulk materials in the required single grain form, due primarily to the relative complexity of the top seeded melt growth process (TSMG) and the introduction of an alloying element (Ag) to the precursor composition. In most cases, alloying elements are used to improve the mechanical properties of the bulk superconductor whilst, at the same time, aim to cause minimal detrimental effect on the superconducting properties of the fully processed sample. In this work we investigate the effect of the addition of silver to YBCO on the superconducting properties of the bulk single grain, including trapped field, T c and J c , and on the sample microstructure

A robust seeding technique for the growth of single grain (RE)BCO and (RE)BCO-Ag bulk superconductors

Bulk, single grains of RE-Ba-Cu-O [(RE)BCO] high temperature superconductors have significant potential for a wide range of applications, including trapped field magnets, energy storage flywheels, superconducting mixers and magnetic separators. One of the main challenges in the production of these materials by the so-called top-seeded melt growth (TSMG) technique is the reliable seeding of large, single grains, which are required for high field applications. A chemically aggressive liquid phase comprising of BaCuO2 and CuO is generated during the single grain growth process, which comes into direct contact with the seed crystal either instantaneously or via infiltration through a buffer pellet, if employed in the process. This can cause either partial or complete melting of the seed, leading subsequently to growth failure. Here, the underlying mechanisms of seed crystal melting and the role of seed porosity in the single grain growth process are investigated. We identify seed porosity as a key limitation in the reliable and successful fabrication of large grain (RE)BCO bulk superconductors for the first time, and propose the use of Mg-doped NdBCO generic seeds fabricated via the infiltration growth (IG) technique to reduce the effects of seed porosity on the melt growth process. Finally, we demonstrate that the use of such seeds leads to better resistance to melting during the single grain growth process, and therefore to a more reliable fabrication technique