Multi-seeded Melt Research Articles

Pulsed field magnetization of 0°–0° and 45°–45° bridge-seeded Y–Ba–Cu–O bulk superconductors

Large, single-grain (RE)BCO (where RE = rare earth or Y) bulk superconductors with complicated geometries are required for a variety of potential applications, such as rotating machines, magnetic bearings and magnetic separation. As a consequence, the top multi-seeded melt growth process has been studied over many years in an attempt to deliver large, single grains for practical applications. Among these techniques, the so-called bridge-seeding produces the best alignment of two seeds during melt processing of multi-seed samples. In this paper, the trapped field performance and magnetic flux dynamics of two bridge-seeded, multi-seed samples magnetized by pulsed field magnetization are analyzed: one with a 45°–45° and another with a 0°–0° bridge seed. Based on an analysis of the flux penetration across the seeds and in-between the seeds of the 45°–45° multi-seed sample, an estimated Jc distribution over the ab-plane is determined, which provides the basis for further analysis via numerical simulation. A three-dimensional finite-element model, developed to qualitatively reproduce and interpret the experimental results, was employed to investigate the influence of the length of the bridge seed for such multi-seed samples. The simulation results agree well with the observed experimental results, in that the multi-seed sample’s particular inhomogeneous Jc distribution acts to distort the trapped field profile from a traditional conical Bean’s profile, which is determined by the length and direction of the bridge seed on the bulk surface.

Multi-seeded growth of melt processed Gd–Ba–Cu–O bulk superconductors using different arrangements of thin film seeds

Gd–Ba–Cu–O (GdBCO) bulk superconductors of diameter of 42mm have been fabricated via a multi-seeded melt growth (MSMG) method in air using well-oriented Nd–Ba–Cu–O thin film seeds. The effect of seed orientation on the growth process and the formation of grain boundaries were investigated systematically by using two thin film seeds arranged with crystallographic orientations of (100)/(100) and (110)/(110), respectively. The multi-seeded process was then firstly extended to four thin film seeds arranged asymmetrically [(110)/(110)], which involved placing the seeds in pairs, with two adjacent seeds positioned relatively close to each other and the other two at a greater separation. This modified technique is beneficial to obtain a large size GdBCO bulk in a shorter time as well as clean grain boundaries by rejecting the excess melt liquid thoroughly along the growth front. More interestingly, a domain of non-equilibrium shape is formed during the MSMG process, which leads potentially to the growth of much larger samples. The growth mode of the enhancement in growth rate is interpreted by the presence of a high index crystallographic (110) face to the initial growth front.

212 Results of single arm, multi-center, two-stage phase II clinical study of thalidomide in Taiwanese patients with myelodysplastic syndrome (MDS)

Gd–Ba–Cu–O (GdBCO) bulk superconductors of diameter of 42 mm have been fabricated via a multi-seeded melt growth (MSMG) method in air using well-oriented Nd–Ba–Cu–O thin film seeds. The effect of seed orientation on the growth process and the formation of grain boundaries were investigated systematically by using two thin film seeds arranged with crystallographic orientations of (100)/(100) and (110)/(110), respectively. The multi-seeded process was then firstly extended to four thin film seeds arranged asymmetrically [(110)/(110)], which involved placing the seeds in pairs, with two adjacent seeds positioned relatively close to each other and the other two at a greater separation. This modified technique is beneficial to obtain a large size GdBCO bulk in a shorter time as well as clean grain boundaries by rejecting the excess melt liquid thoroughly along the growth front. More interestingly, a domain of non-equilibrium shape is formed during the MSMG process, which leads potentially to the growth of much larger samples. The growth mode of the enhancement in growth rate is interpreted by the presence of a high index crystallographic (110) face to the initial growth front.

Multiseeded melt growth of bulk Y–Ba–Cu–O using thin film seeds

Y–Ba–Cu–O (YBCO) and Sm–Ba–Cu–O thin films have been used for the first time as heterogeneous seeds to multiseed successfully the melt growth of bulk YBCO in a multiseeded melt growth process. The use of thin film seeds, which may be prepared with highly controlled orientation (i.e., with a well-defined a-b plane and precisely known a-direction), is based on their superheating properties and reduces significantly contamination of the bulk sample by the seed material. A variety of grain boundaries were obtained by varying the angle between the seeds. Microstructural studies indicate that the extent of residual melt deposited at the grain boundary decreases with increasing grain boundary contact angle. It is established that the growth front proceeds continuously at the (110)/(110) grain boundary without trapping liquid, which leads to the formation of a clean grain boundary.

Levitation force and trapped magnetic field of multi-grain YBCO bulk superconductors

A levitation force (LF), attractive force (AF) and trapped magnetic field (TMF) at 77 K of the multi-seeded melt processed (MSMG) bulk samples (single grain to five grain) were studied. The LF and TMF values of the MSMG-processed samples, cooled by both field cooling and zero-field cooling method, were smaller than that of a single grain sample, depending on the number of a grain. The trapped magnetic field analysis showed the magnetic fields were easily penetrated through the grain boundaries of multi-grain samples, because of the weak connection of (1 0 0)/(1 0 0) junctions. The poorly connected grain boundaries are thought to be the cause for the low LF and TMF values of the multi-grain samples.

Preparation of high-quality HTS rings for application in the magnetic bearing of cryotanks and pinning in grain boundaries

Seeded melt growth of YBCO high-temperature superconductors is one of the most promising preparation techniques to obtain high-quality HTS tiles for application, e.g. in magnetic bearings. Semi-finished HTSL products of complex shapes have to be developed by different seeding and multi-seeding techniques. To obtain large hollow cylinders designed for application in the magnetic bearing of a cryotank a modified multi-seeded melt growth (MSMG) process was employed. This cryotank will be mounted for testing in a vehicle of a major German car manufacturer. The MSMG process introduces grain boundaries into the HTS tiles. For transport current investigations of [0 0 1]-tilt grain boundaries in melt textured YBCO a series of MSMG bicrystals have been prepared. They exhibit a dependence of the critical current density on misorientation angle which is much weaker than the one observed in thin-film bicrystals. The bulk samples have dimensions larger than the magnetic penetration depth along the grain boundary. Thus, flux pinning has to be taken into account. Different contributions to the longitudinal pinning force have to be considered: vortices at grain boundaries can be pinned by magnetic interaction with Abrikosov vortices in the banks, by defects in the grain boundary itself or by defects which are located next to the grain boundary.

Study of structural properties and morphology of multi-seeded NdBaCuO bars

We report on the morphological and structural properties of multi-seeded melt textured NdBa 2Cu 3O 7− δ bars. In particular, by means of pole figures X-ray diffraction analysis, we have studied the misorientation angles between the crystallographic axes of adjacent domains, induced by the single-crystal MgO seeds. Polarised light microscopy has been used to study the precipitate particles distribution and their segregation mechanism. We focused the attention on the domains boundary regions, where the growth dynamics is expected to differ from single-domain's one, and a different precipitates distribution may occur. The absence of secondary phases and precipitate deposits in this region has been observed. Connections of mechanical fragility with the preferential formation of cracks along the crystallographic axes' directions have also been investigated.

A study on the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors

We studied the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors. The use of the multi-seeded melt growth process can reduce the processing time remarkably compared with the single-seeded melt growth process, but the levitation forces and the trapped magnetic field value decreased as the number of seeds increased due to the formation of weakly linked grain boundaries at the grain junctions. The degradation of the trapped magnetic field was attributed to the presence of the residual liquid at the grain boundary. To minimize the degradation of the magnetic properties, various processing parameters such as seed distance and grain orientation were tested.

Influence of the seed distance on the microstructure and thesuperconducting properties of grain boundaries in a multi-seeded melt growthmonolith

The influence of the seed distance (dS) on the properties ofgrain boundaries (GBs) has been studied in order to obtain a precipitate-freeGB. On the top of one monolith, two distances between two seeds were chosen,i.e. 4 and 11 mm. For dS≃11 mm, some copper-rich phases and largepores can be observed throughout the GB. In contrast, for dS≃4 mm,neither residual copper-rich phases nor pores are observed within the GB. Thegrowth direction for the two samples seems to be parallel or nearly parallelto the GB. It reveals that the absence of misorientation between the twodomains cannot prevent the formation of a residual copper phase layerthroughout the GB; only a shorter seed distance will do this. However,element-mapping measurements revealed the occurrence of macrosegregation ofthe Y-211 particles on top of the monolith and centred on the GB. This tendsto vanish towards the bottom of the sample and only a thin layer remains.Magneto-optical measurements performed on the sample with dS≃4 mm,revealed that the critical current density through the GB differs according tothe growth directions.

Multi-seeded melt growth processed YBCO superconductors

Effects of processing variables on the top surface properties (the levitation force and trapped magnetic field) of the multi-seeded melt growth process were studied. The variables were the number of seeds, seed distance, initial composition of a YBCO compact, variety of the grain junctions. The processing time was reduced by providing many growth sites for 123 grains. However, the properties were decreased as the number of seeds increased due to the poor connection of the grain junctions. We summarize the ways to minimize the degradation of the magnetic and levitation properties at the grain junctions.

Effects of grain boundaries on the levitation force and trapped magnetic field of the multi-seeded melt growth processed YBCO superconductors

Various YBCO grain boundary junctions were prepared by a top-seeded melt growth process using two seeds and the grain boundary properties were investigated. The results of the levitation forces and trapped magnetic fields for the top surfaces of the samples showed that the (1 1 0)/(1 1 0) grain junction was more strongly connected than other grain boundaries. We discussed the connectivity of the grain boundaries in terms of the wetting angle of a melt at the boundaries.

Processing variables of the multi-seeded melt growth process of YBCO superconductors: the properties of YBCO samples with various grain junctions

Two-grain YBCO samples with various grain junctions were prepared by a multi-seeded melt growth process and the levitation force and the trapped magnetic field of the samples were examined. The trapped magnetic field was dropped at all grain junctions and the degree of the deterioration depended on their crystallographic planes of the grains. The levitation forces of the samples with top surfaces normal to the c-axis (e.g., (1 0 0)/(1 0 0) and (1 1 0) (1 1 0) junctions) were higher than that of the samples with top surface partly and completely parallel to the c-axis (e.g., (1 0 0)/(0 0 1) and (0 0 1)/(0 0 1) junctions).

Study of morphological and structural properties of multi-seeded NdBaCuO samples

In this work, we report on the morphological and structural properties of multi-seeded melt textured NdBaCuO bars, using MgO seeds. We have characterised the samples by x-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. In particular by these techniques we analysed the characteristics of boundaries joining adjacent domains. For 2 centimetres spacings between adjacent seeds, good texturing and absence of Nd422-rich quenched liquid deposits or other spurious phases near the boundaries have been observed. On the other side a serious mechanical fragility, sensibly enhanced along the boundary regions, is found. The superconducting properties, such as critical temperature and critical current density have been investigated by means of magnetic measurements.

Characterization of natural and artificial low-angle boundaries in YBCO TSMG samples

Since several years, technical applications of bulk HTS YBCO superconductors are of growing interest. However, shapes of HTS tiles needed for complex applications require the joining of two or more single-domain monoliths. The welding technique leads to the formation of a low-angle grain boundary and then requires a high connectivity between two adjacent single-domain materials. This study was devoted to the characterization of the microstructure and the superconducting properties of natural and artificial low-angle grain boundaries. The natural grain boundary formed during the multi-seeded melt growth (MSMG) process exhibits a non-uniform microstructure during the growth. An energy dispersive spectroscopy (EDS) analysis carried out across the grain boundaries always reveals a depletion of copper and an accumulation of yttrium. Mechanical joining of two single-domain monoliths leads to the formation of an artificial grain boundary. This kind of joining was performed either without or with a welding agent, i.e., YbBa 2Cu 3O 7− x (Yb-123). The first method leads to a good connectivity between welded Y-123 platelets. In contrast, the control of the mechanical welding process with a welding agent is more difficult. At 940°C, Yb-123 decomposes into Yb 2BaCuO 5, BaCuO 2 and CuO. This decomposition deteriorates the superconducting properties between two adjacent domains.

Macrosegregation of Y 2BaCuO 5 particles in top-seeded melt textured monoliths

Y 2BaCuO 5 (Y-211) particles distributed in the textured matrix play an important role for the superconducting properties. The Top-Seeded Melt Growth (TSMG) process always leads to a macrosegregation of Y-211. For TSMG and Multi-Seeded Melt Growth (MSMG) monoliths, the characterization of the distribution of the Y-211 particles was performed over slices in the (001) plane and (001) direction, respectively. As soon as the growth commences, a zone with a low content of less than 10 mol% of Y-211 appears. The low content of Y-211 can mainly be explained by the pushing/trapping model. This model relates the radii of the smallest trapped Y-211 particles to the growth rate. Furthermore, when the undercooling Δ T reaches 25°C, the growth front stops to push the particles and starts to trap them.

Multi-seeded melt crystallization of YBCO bulk material for cryogenic applications

YBCO bulk materials were crystallized by a multi-seeded melt growth process. Up to four biaxial oriented Sm-123 seeds were placed on the top of rectangular shaped YBCO bars (up to ). The oriented grown single grains are joined by partially coherent grain boundaries with an improved current transport capability and high levitation forces with overall values of 55-65 N. Segregation of nonsuperconducting rest melt and microcracks limit the current transport across the grain boundary.