Properties Of Single Grain Research Articles

Space experiments using superconducting magnet technology

We report on a fast-track experiment on the International Space Station (ISS) with the Earth’s magnetic field, which has been recognized as a major task for a better understanding of our planet. For this purpose, YBCO bulk superconductors have been prepared, transported and stored on board the ISS. The properties of single grain superconducting bulk will be investigated before and after the ISS experiments. In parallel, the magnetic interaction between BEarth and our YBCO bulk was investigated. An external 3D Helmholtz coil cancels unwanted electromagnetic fields down to 5 μT. Highly sensitive flux-gate sensors monitored the magnetic configuration as the ceramic plate was cooled to cryogenic temperatures. Shielding and field concentration effects of the Earth’s magnetic field were observed and discussed. Thermally induced magnetic flux compression for future long-duration space missions was successfully tested, enabling future spacecraft shielding and braking functions.

Optimisation of stacked, bulk high temperature superconductors for trapped-field magnet applications

It is necessary to fabricate (RE)BCO bulk high temperature superconductors in the form of individual single grains in order to maximise the length scale over which current flows, and hence the trapped magnetic field. However, inherent difficulties in the grain growth process place limitations on the diameter and height of the single grain that may be achieved by existing melt processes. A practical approach to increase the height of the sample and the trapped field at its surface is by assembling individual single grains in a stack formation with their ab planes aligned parallel, primarily to avoid the expensive process of fabricating large, individual monoliths. The trapped fields observed at the top and bottom surfaces of a single grain sample are frequently different since both the superconducting and physical properties of single grain (RE)BCO samples are generally non-uniform. This leads to challenges in determining how to spatially arrange stacks of single grain samples to generate the largest and most uniform trapped field overall. In this study, we report the optimisation of two-stack configurations involving a total of 8 individual GdBCO/Ag single grains. The samples were arranged in four pairs and configured with different surfaces in contact in the assembly of the stack. The primary superconducting properties for trapped field and total flux distributions were measured at 77 K and compared for each stack arrangement. The initial results indicate that surfaces with inferior flux trapping properties (measured in terms of the overall trapped field value) of a two-sample stack should be positioned at the middle of the assembly to achieve the best overall trapped field and higher total flux at the external, and therefore, usable surface of the stack sample. A numerical modelling method that incorporates different Jc-B characteristics for the top and bottom layers of a single grain to take account of the variability in physical properties and spatial non-uniformity confirmed the optimised experimental arrangement of the stacked bulk samples. Furthermore, the optimisation of single grains of ring geometry to achieve a longer and wider uniform magnetic field zone inside the bore was also performed.

The effect of size and aspect ratio on the trapped field properties of single grain, Y–Ba–Cu–O bulk superconductors

Bulk, single grain (RE)Ba2Cu3O7−δ [(RE)BCO, where RE is a rare earth element or yttrium] high temperature superconductors exhibit significant potential for use in a variety of engineering applications due to their ability to trap large magnetic fields, which can be up to ten times greater than those generated by conventional, iron-based magnets. Limitations on the maximum size to which single grains can be grown, however, are a major obstacle to the further development of these materials. Indeed, multiple samples are often required to achieve the required superconducting properties in particular applications. The geometry of bulk (RE)BCO single grain samples plays an important role in determining the superconducting properties of a given technical arrangement. In order to gain a better understanding of the full application potential of bulk single grain superconductors, three relatively long, cylindrical YBCO single grains of different diameters were fabricated and their trapped field and total trapped flux measured at 77 K as a function of sample height. The effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established theoretical model. Conclusions based on the trapped field measurements on a variety of single grain samples are presented in this study and the possibilities of using assemblies of smaller samples for engineering devices, in particular, are discussed.

Synthesis of Y2BaCuO5 nano-whiskers by a solution blow spinning technique and their successful introduction into single-grain, YBCO bulk superconductors

The ability of single grain Y-Ba-Cu-O (YBCO) bulk superconductors to trap large magnetic fields is due generally to the presence of embedded, non-superconducting Y2BaCuO5 (Y-211) phase particles, in optimum amount, in the superconducting YBa2Cu3O7-δ (Y-123) phase matrix, which increases flux pinning and, hence, critical current density of the bulk material. The presence of smaller particles of Y-211 could be expected to improve further the superconducting properties of single grain, bulk YBCO samples, although the smallest Y-211 particles that have been engineered to date within the bulk microstructure are around 800 nm in size. It is extremely challenging to reduce the Y-211 particle size any further due to problems of particle agglomeration that originate, inevitably, from the effects of surface energy and reactivity at relatively high processing temperatures. In this work, we report a novel approach to the fabrication of Y-211 pinning centres in bulk YBCO superconductors in the form of nano-whiskers manufactured by a solution blow spinning technique. The Y-211 nano-whiskers, of which 62% were smaller than 500 nm, were added to the YBCO precursor powders to produce a single grain bulk sample by the buffer-aided top-seeded melt growth (BA-TSMG) processing technique. The resulting YBCO single grain, of diameter 20 mm, was able to trap a magnetic field of 0.63 T at 77 K with an associated critical current density of 3.9 × 104 A/cm2 in self-field. The results of this study demonstrate clearly that the use of Y-211 nano-whiskers is a promising route for enhancing flux pinning in bulk YBCO single-grains, which is potentially significant for the development of high field engineering applications.

Comparison of the effects of platinum and CeO2 on the properties of single grain, Sm–Ba–Cu–O bulk superconductors

SmBa2Cu3O7−δ (Sm-123) is a light-rare-earth barium-cuprate (LRE-BCO) high-temperature superconductor (HTS) with significant potential for high field industrial applications. We report the fabrication of large, single grain bulk [Sm–Ba–Cu–O (SmBCO)] superconductors containing 1 wt% CeO2 and 0.1 wt% Pt using a top-seeded melt growth process. The performance of the SmBCO bulk superconductors containing the different dopants was evaluated based on an analysis of their superconducting properties, including critical transition temperature, Tc and critical current density, Jc , and on sample microstructure. We find that both CeO2 and Pt dopants refine the size of Sm2BaCuO5 (Sm-211) particles trapped in the Sm-123 superconducting phase matrix, which act as effective flux pinning centres, although the addition of CeO2 results in broadly improved superconducting performance of the fully processed bulk single grain. However, 1 wt% CeO2 is significantly cheaper than 0.1 wt% Pt, which has clear economic benefits for use in medium to large scale production processes for these technologically important materials. Finally, the use of CeO2 results generally in the formation of finer Sm-211 particles and to the generation of fewer macro-cracks and Sm-211 free regions in the sample microstructure.

A new seeding technique for the reliable fabrication of large, SmBCO single grains containing silver using top seeded melt growth

Silver (Ag) is an established additive for improving the mechanical properties of single grain, (RE)Ba2Cu3O7−δ [(RE)BCO, RE = Sm, Gd and Y] bulk superconductors. The presence of Ag in the (RE)BCO bulk composition, however, typically reduces the melting temperature of the single crystal seed in the top seeded melt growth (TSMG) process, which complicates significantly the controlled nucleation and subsequent epitaxial growth of a single grain, which is essential for high field engineering applications. The reduced reliability of the seeding process in the presence of Ag is particularly acute for the SmBCO system, since the melting temperature of SmBCO is very close to that of the generic NdBCO(MgO) seed. SmBCO has a high superconducting transition temperature, Tc, and exhibits the most pronounced ‘peak’ effect at higher magnetic field of all materials in the family of (RE)BCO bulk superconductors and, therefore, has the greatest potential for use in practical applications (compared to GdBCO and YBCO, in particular). Development of an effective seeding process, therefore, is one of the major challenges of the TSMG process for the growth of large, high quantity single grain superconductors. In this paper, we report a novel technique that involves introducing a buffer layer between the seed crystal and the precursor pellet, primarily to inhibit the diffusion of Ag from the green body to the seed during melt processing in order to prevent the melting of the seed. The success rate of the seeding process using this technique is 100% for relatively small batches of samples. The superconducting properties, critical temperature, Tc, critical current density, Jc and trapped fields, of the single grains fabricated using the buffers are reported and the microstructures in the vicinity of the buffer of single grains fabricated by the modified technique are analysed to understand further the effects of buffers on the growth process of these technologically important materials.

The processing and properties of single grain Y–Ba–Cu–O fabricated from graded precursor powders

The preparation of single grain, Y–Ba–Cu–O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa2Cu3O7−δ (Y-123) and Y2BaCuO5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, Tc and Jc, of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Improving the superconducting properties of single grain Sm–Ba–Cu–O bulk superconductors fabricated in air by increased control of Sm/Ba substitution effects

The extreme sensitivity of Sm/Ba at high temperature in air becomes an obstacle to the fabrication of SmBCO single grains that exhibit stable and reliable superconducting properties. In this research, the superconducting properties of SmBCO single grains fabricated by top seeded melt growth (TSMG) from different batches of commercial SmBa2Cu3O7−d (Sm-123) precursor powder using different processing atmospheres (air and 0.1% O2 in Ar), different processing methods (isothermal growth and continuous cooling) and different amounts of BaO2 content to suppress Sm/Ba substitution in air have been investigated in an attempt to understand fully the TSMG process for this system. As a result, based on extensive data, a novel and simple, low temperature post-annealing approach is proposed specifically to overcome the sensitivity of Tc to Sm/Ba substitution in order to simplify the fabrication of SmBCO and to increase its reliability with a view to the practical processing of these materials. Initial processing trials have been performed successfully to demonstrate the viability of the novel post-annealing process.

Microstructure and Superconducting Properties of Single Grains of Y-Ba-Cu-O Containing Y-2411(M) and ${\rm Y}_{2}{\rm O}_{3}$

Y-Ba-Cu-O (YBCO) single grains have the potential to generate large trapped magnetic fields for engineering applications, and research on the processing and properties of this material has attracted interest world-wide over the past 20 years. In particular, the introduction of flux pinning centers to the large grain microstructure to improve its current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Jc</i> , and hence trapped field, has been investigated extensively. Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> CuMO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> [Y-2411(M)], where M=Nb , Ta, Mo, W, Ru, Zr, Bi and Ag, has been discovered recently to form very effective flux pinning centers due primarily to its ability to form nano-size inclusions in the superconducting phase matrix. However, the addition of the Y-2411(M) phase to the precursor composition complicates the melt-processing of single grains. The addition of Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> to the precursor composition, however, broadens the growth window of single YBCO grains containing Y-2411 (M). We report an investigation of the microstructures and superconducting properties of single grains of this composition grown by top seeded melt growth (TSMG).

The effect of Ag and Y-24W1 addition on the microstructure and superconducting properties of single grain Y–Ba–Cu–O

We have reported recently the fabrication of high performance, nano-composite Y–Ba–Cu–O (YBCO) bulk superconductors containing nano-sized Y 2Ba 4CuMO (Y-2411, where M is typically a transition metal element) phase inclusions that contribute effectively to flux pinning. However, these studies have revealed that the field trapping capability of YBCO bulk superconductors containing the Y-2411 phase is limited due to the presence of micro- and macro-cracks in the bulk sample microstructure. Here, we report the fabrication of nano-composite YBCO bulk superconductors containing Ag and W-based Y-2411 (i.e. Y 2Ba 4CuWO y ) secondary phase inclusions (SPI's) in an attempt to investigate the influence of Ag content on the formation and propagation of a/ b and c- axis micro- and/or macro-cracks in the Y-123 matrix. The crack density for samples containing Ag is observed to be significantly lower than for samples fabricated without Ag. A detailed study of the microstructures and superconducting properties of Ag-doped, nano-composite bulk YBCO is presented.

Superconducting properties of Gd-Ba-Cu-O single grains processed from a new, Ba-rich precursor compound

Gd-Ba-Cu-O (GdBCO) single grains have been previously melt-processed successfully in air using a generic Mg-Nd-Ba-Cu-O (Mg-NdBCO) seed crystal. Previous research has revealed that the addition of a small amount of BaO2 to the precursor powders prior to melt processing can suppress the formation of Gd/Ba solid solution, and lead to a significant improvement in superconducting properties of the single grains. Research into the effects of a higher Ba content on single grain growth, however, has been limited by the relatively small grain size in the earlier studies. This has been addressed by developing Ba-rich precursor compounds Gd-163 and Gd-143, fabricated specifically to enable the presence of greater concentrations of Ba during the melt process. In this study, we propose a new processing route for the fabrication of high performance GdBCO single grain bulk superconductors in air by enriching the precursor powder with these new Ba rich compounds. The influence of the addition of the new compounds on the microstructures and superconducting properties of GdBCO single grains is reported.

Processing and properties of single grain (RE)–Ba–Cu–O bulk superconductors

Recent advances in the processing of bulk, single grain (RE)BCO high temperature superconductors (HTS) fabricated by top seeded melt growth (TSMG) are described, including the refinement of (RE)2BaCuO5 (RE-211) second phase inclusions in the bulk superconducting (RE)Ba2Cu3O7−δ (RE-123) matrix by mechanical milling, the generation of new nano-scale pinning centres based on the Y2Ba4CuMOy phase (2411), where M=[Nb, Ta, Mo, W, Re, Zr, Hf], and the creation of MILD pinning sites by heavy ion irradiation. Details of a practical melt processing technique for the fabrication of light rare earth (LRE) bulk materials in air, in particular, based on the development of a generic seed crystal are described. Record Jc’s have been reported for GdBCO and (NEG)BCO, and trapped fields as high as 3T and 17T have been achieved in bulk (RE)BCO grains at 77 and 29K. Techniques employed to characterise bulk materials, such as Hall probe mapping, magnetoscan and EBSD are outlined. Finally, details of emerging applications of single grain (RE)BCO materials in motors and generators, magnetic bearings, flywheels and trapped flux devices are described.

Influence of trap states on dynamic properties of single grain silicon thin film transistors

The transient properties of single grain–thin film transistors (SG-TFTs) with high electron mobility have been studied. Overshoot current induced by trap states has been observed in most of the devices. A method of ac measurements has been used to investigate the trap processes. Both transient and ac measurements show that the response of some SG-TFTs with high field effect mobility is dominated by a single trap level. Bias stressing on SG-TFT can induce more trap states and thus change the ac response of the device.

Magnetic properties of single grain R–Mg–Cd primitive icosahedral quasicrystals (R=Y, Gd, Tb or Dy)

The growth and characterization of single grains of the primitive icosahedral quasicrystal R–Mg–Cd (R=Y, Gd, Tb or Dy) are described. From the thermodynamic properties, it is confirmed that the system is a spin glass exhibiting the characteristic spin-freezing transition of such systems. There is no evidence for two distinct freezing transitions previously observed for polygrain samples. The bulk magnetic properties and the effect of crystalline electric fields on the spin-freezing temperature are found to be very similar to those of the face-centred icosahedral quasicrystal R–Mg–Zn.

Study on the electrical properties of single grain boundaries in BaTiO3 ceramics

In the article, the electrical properties of single grains and single grain boundaries in donor doped BaTiO3 ceramics have been investigated. The results show that the grains have no positive temperature coefficient of resistance effect (PTCR effect) if the influence of electrode was neglected. The results also show that different grain boundaries have various PTCR effect. The characteristics of current-voltage do not follow the Heywang model and accord with our modified model. The maximum barrier heights of single grain boundaries are also deduced experimentally.

Investigations into the thermoluminescence properties of single quartz grains using an imaging photon detector

The TL properties of single grains of quartz from a series of sediment samples have been measured. A distribution of both natural TL and the TL measured after a laboratory beta dose (second glow TL) was obtained. When the ratio of the natural TL to the second glow TL was plotted, it was observed that as well as a number of grains which were bright in both natural and second glow TL, there were also many grains which had a significantly higher natural TL as compared with the second glow TL. The grains were then examined in reflected and transmitted light with an optical microscope, in an attempt to explain grain-to-grain behaviour in terms of any difference in physical properties and the presence of inclusions within individual grains.

Delectric properties of single grain in PLZT ferroelectric ceramics

Abstract The hot pressed and grain-grown transparent ceramics with maximum grain size 80∼100 μm provides the possibility of investigating the dielectric properties of single grain from ceramic specimens. The ion milling technique was employed to separate the grains. The dielectric permittivities of single grain of PLZT have been measured. It is found that the range of dielectric constant of single grains in unpoled state is larger than that of the bulk ceramic. Poling in both single grains and bulk ceramics reduces the dielectric constant in the poling direction, and increases it in the direction perpendicular to the poling field. The anisotropy of dielectric permittivity for PLZT 8/65/35 is calculated from the single grain data.

Studies on microstructure and microproperties of PLZT ceramics

The microstructure of PLZT transparent ferroelectric ceramics were studied by high-resolution electron microscopy (HREM) and electron diffraction. The following experimental results were obtained: 1. (1) The structure of micro-domains in the cubic α phase is exactly the same as that of the ferroelectric β phase after poling. The ‘fringe’-shaped micro-regions in the HREM micrograph of 7·9/70/30 PLZT is the result of lattice overlapping of the orthorhombic micro-domains and the cubic α phase matrix. 2. (2) A 2 × d 111 superlattice along the 〈111〉 directiion was observed in 7·9/70/30 PLZT ceramics. It is probably the result of gliding or rotation on the {111} plane due to self-regulation of the structure in the presence of point defects. 3. (3) The lattice image micrographs taken near the grain boundary show that the grain boundaries in PLZT ceramics are very fine. Most of their widths are less than 6Å; some are only 2–3 Å. However, in some places the grain boundaries are broadened, and may reach several tens of ångströms. The structure of broadened grain boundaries appears amorphous. In addition, using specially constructed instruments, the electrical and electrooptical properties of single grains in PLZT ceramics were measured and compared with the properties of the ceramic materials.