Textured Bulk Research Articles

Controllable synthesis: Bi2S3 nanostructure powders and highly textured polycrystals

Uniform Bi2S3 nanostructured powders with a preferential [001] orientation, including nanotubes, nanorods, and nanowires, were synthesized by hydrothermal synthesis using Bi(NO3)3 and Na2S as raw materials, CO(NH2)2 as a pH modifier and ethanediol (C2H6O2) as solvent. The growth mechanism of the Bi2S3 nanostructure in hydrothermal process was investigated. The precursor nanostructures of Bi2S3 single crystal powders were maintained in the Bi2S3 bulk after spark plasma sintering (SPS) at 673 K for 5 min, thereby the highest texture degree F(h k 0) = 0.82 was reached for the sample sintered by using nanotubes as precursor powders, which is much higher than 0.18 for the sample sintered by using the powder prepared by mechanical alloying. The present result suggests that direct sintering of the orientated single crystal powders by applying SPS technique is easy to provide a highly textured Bi2S3 bulk, which benefits include enhanced electrical transport property.

Thermoelectric conversion via laser-induced voltage in highly textured polycrystalline NaxCoO2 ceramic

We have studied and analyzed the laser-induced voltage effect in highly c-axis-oriented polycrystalline NaxCoO2. The textured and layered stacking NaxCoO2 (x ∼ 0.7) bulks were prepared by a solid-state reaction process. Under the irradiation on Na0.67CoO2 bulk surface with pulsed laser (λ = 248 nm), the induced voltage signals were observed on the inclined surface with rise time 30 ns–43 ns and peak voltage 200 mV–500 mV; the voltage peak values show a linear dependence of laser energy densities. The crystal grains orientation plays a critical role in voltage peak value whether in film or texture bulk. The transverse voltage signal brings the information of thermoelectric anisotropy. In NaxCoO2 band structure, the Fermi surface is different in the ab plane and along the c axis, leading to anisotropy of Seebeck coefficient. Additionally, the artificial structure of the inclined surface for highly textured bulk enables us to obtain a transverse voltage on inclined surface. These results demonstrated the layered textured bulk has potential applications in waste-heat conversion via transverse thermoelectric effect.

Size Effect of Insulating Nano-Inclusions in Y–Ba–Cu–O Bulk Superconductors Fabricated by Seeded Infiltration Growth

We have conducted an experimental study on the effect of the pinning centers size on the superconducting properties in textured YBCO samples. In this work, we investigated the effectiveness of the addition of an insulating inclusion on the superconducting and magnetic properties of textured YBCO bulk materials. The purpose of this study was to ascertain whether the size of artificial pinning centers is able to affect the superconducting properties of our samples. To this effect insulating nano-pinning centers with two different particle size distributions of about 20 nm and 130 nm have been successfully incorporated within YBCO matrix by slight doping with nano-particle alumina dispersions. The microstructure and superconducting properties were investigated by scanning electron microscopy (SEM) and SQUID. The results indicate that slight inclusions of nano-alumina can enhance the flux-pinning capability of samples. This study underlines the dependence of the pinning force on the size of the pinning centers.

Li and Ce doping of melt-textured YBCO: Improved jc at medium fields

The top seeded melt textured growth method was applied to a Ce doped YBCO material with a partial substitution of Cu with Li. The amount of Li in the samples varied from 0.25 to 0.05 at% relative to the copper atoms. The change of T c and j c compared to a reference sample was measured using SQUID magnetometer. Field induced pinning was indicated by elevated j c in the j c( B) curve. The peak effect at 77 K appeared in the moderate field region around 0.5 T. Optimal lithium content was determined to be at 0.05 at%. Final Li content in textured bulk material was verified with atomic absorption.

Dependency of Flux Jump on Temperature and Field-Sweep Rate for Textured (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7-δ Bulk Superconductor with Gd-211 Particles

Anisotropy flux jumps in the mixed state of a textured (Nd0.33Eu0.33Gd0.33)Ba2Cu3O7-δ bulk superconductor with Gd-211 doping particles have been studied by means of magnetization measurements in the model of the magnetic field paralleling and perpendicular to the c axis. A typical anisotropy flux jump was observed at a temperature ranging from 2.0 to 3.0 K. Under the magnetic field perpendicular to c axis, no flux jump was found at whole temperature range until the sweep rate of 200 Oe/sec. For the magnetic field paralleling to the c axis, the number of flux jumps decreased with the increase of temperature, and the third quadrant of the M–H curve is the most flux-instability quadrant. The magnetic field sweep rate dependences of flux jumps were also studied and the influence of flux creep on flux jumps was also discussed.

Fabrication of HTS Bearings With Ton Load Performance

The considerable development effort of HTS magnetic bearings with respect to heavy loads is demonstrated. Radial type 200 mm bearing in an optimized design was developed for implementation in a new 5 kWh superconducting flywheel energy storage system. A prototype bearing, consisting of a modular assembled melt textured YBCO bulk 200 mm hollow cylinder with permanent magnet rotor is presented. Axial rotor displacement causes pinning forces of 8 and 10 kN axially at a temperature of 78 K and 71 K, respectively. Maximum radial forces are 4.7 kN at sub- cooled LN2. Fabrication technology incorporates the construction and engineering of a low-weight, nonmagnetic G-10 bearing cryostat. Ultra-low heat transfer structural elements were designed and constructed to reduce the thermal loss through the support to less than 2 Watt under 1 ton load.

Preparation of textured zinc oxide ceramics by extrusion and spark plasma sintering

Since texture was confirmed to improve the properties of ZnO ceramics in various applications, an extrusion–spark plasma sintering (SPS) process was utilised to obtain a textured bulk ZnO ceramics. Rod-like, anisotropic ZnO particles were utilised as the original powder. The particles were aligned in such a position where c axes of the particles were parallel to the extrusion direction after the extrusion process. By spark plasma sintering method, a textured, near full dense homogeneous bulk ZnO ceramics with fine grains was obtained, sintering at relatively low temperature and within short time. Since the orientation was only controlled in one particular direction, i.e. c axis direction, it was a fibre texture that was achieved in this bulk ZnO. The extrusion–SPS process was confirmed to be an effective technique to induce texture in bulk ZnO ceramics.

Effect of Surface and Growth Conditions for Formation of Textured Polycrystalline GaN Crystals by Reactive N2 Plasma

The surface and growth conditions for the formation of textured polycrystalline bulk gallium nitride (GaN) crystals under nitrogen (N2) reactive plasma were investigated. Langmuir probe measurements were used to determine the electron density values near the Ga surface at different target locations and ranged from (6–16)×1010/cm2. Optical spectroscopy measurements showed reactive gallium (Ga) and nitrogen species present in the plasma. When the target was bombarded with highly energetic ions, peaks of sputtered Ga at 403 and 417 nm wavelengths were observed in the optical spectroscopy measurements. The top Ga surface showed increased roughening and the resulting crystals were of poor crystalline quality, with fresh Ga leftovers. Smoother crystals were formed when the Ga target was shielded from the direct bombardment of highly energetic N2 plasma, with no sputtered Ga peaks observed during the bombardment. X-ray diffraction (XRD) analysis results revealed well-oriented crystals at the (0002) reflection of GaN. This proved that the shielding of highly energetic ions to decrease roughening of the surface and allowing more neutrals for the reaction were effective in forming well-oriented bulk GaN crystals.

反応性テンプレート粒成長法による電子セラミックスの配向制御

The design and fabrication of textured bulk polycrystals have been one of the key issues for ceramists in their effort to extract the best performances from functional materials through powder processing. Reactive-templated grain growth (RTGG) is a processing method in which reactive template particles are mixed with complementary reactants and aligned, and the product is formed in-situ during heat-treatment, preserving the orientation of the template. RTGG has been applied to simple perovskite-type materials, yielding highly textured polycrystals with enhanced piezoelectric properties. The proposed method has also been extended to layer-structured materials, including bismuth layer-structured ferroelectrics and both p- and n-type oxide thermoelectrics with complex compositions. The optimum combination of microstructural and compositional designs is expected to produce outstanding materials with superlative performances.

Influence of texture on electrical properties of ZnO ceramics prepared by extrusion and spark plasma sintering

An extrusion-spark plasma sintering (SPS) process was successfully utilized to obtain a textured, near full dense homogeneous bulk ZnO ceramics. Rod-like, anisotropic ZnO particles were extruded and sintered, and highly c-axis-preferred orientation was obtained in the bulk ZnO, which can be called as fiber texture. The textured bulk ZnO showed more low-angle grain boundaries compared with that without texture. It was found that the highly preferred orientation decreased the electrical resistivity obviously in the bulk ZnO at room temperature. The observed high proportion of low-angle grain boundaries due to the texture resulted in a quite lower resistivity in grain boundaries at room temperature, which was the main reason for the reduction in electrical resistivity of the textured bulk ZnO ceramics.

Inhomogeneous oxygen interchange during annealing and cooling of textured bulk Bi2Sr2CaCu2O8+δ superconductors

The optimized annealing of bulk Bi2Sr2CaCu2O8+δ (Bi-2212) materials textured using the laser floating zone technique requires a two-step process, consisting of a first step at 870 °C and a second step at 801 °C. In this paper, we present an analysis of the changes induced within the samples along this thermal treatment and the subsequent cooling to room temperature. During the initial step the cationic diffusion takes place, while the oxygen content is adjusted during the second step, at lower temperature. The evolution of the superconducting properties reveals that the oxygen diffusion into the sample is not homogeneous and that the different oxygen contents along the annealing are also a consequence of a strong interchange between the central and the external sample regions. It has been shown that, at the different stages of the thermal process, the samples present different degrees of homogeneity in their superconducting properties, this being caused by the improvement of one region to the detriment of the other.

Effects of thermal exchanges on the YBCO transition and the superconductivity recovery

The influence of the current slope (dI/dt) and thermal exchanges on the magneto-thermal transition of textured bulk YBCO are analyzed within the framework of the resistive Fault Current Limiter (FCL). This study was carried out at 90.5 K both in pressurized liquid nitrogen, in vacuum and when the YBCO sample is in contact with alumina plates. The superconductor U(I) response is governed by a power law, depends on dI/dt and is influenced by the thermal surroundings. These phenomena can be justified if the dissipated energy during the transition is concentrated in a small volume of the conductor. The second part of this paper deals with the thermal recovery of bulk materials. Even in the case of film boiling where the exchanges are poor, a thermal gradient due to the weak thermal conductivity of HTc materials appears in the section of the conductor. This point and its consequences are described and discussed by using a finite element software.

Very efficient light emission from bulk crystalline silicon

Due to its indirect bandstructure, bulk crystalline silicon is generally regarded as a poor light emitter. In contrast to this common perception, we report here on surprisingly large external photoluminescence quantum efficiencies of textured bulk crystalline silicon wafers of up to 10.2% at T=130 K and of 6.1% at room temperature. Using a theoretical model to calculate the escape probability for internally generated photons, we can conclude from these experimental figures that the radiative recombination probability or internal luminescence quantum efficiency exceeds 20% at room temperature.

Giant magnetoresistance in bulk metallic multilayers prepared by cold deformation

A cyclic mechanical deformation process combining hydrostatic extrusion and rolling has been used to prepare strongly textured bulk multilayer samples with individual layer thicknesses on the nanometer scale. CIP-MR values in the range 8%-1% were measured in both Fe-Ag and FeNi-Ag-CoFe samples in the temperature range 5 K to 300 K. Room-temperature CPP-MR values of 6% were measured in the FeNi-Ag-CoFe sample.

Microstructure of bulk PMP YBCO with submicrometre Y2BaCuO5particles

The microstructure of bulk YBCO processed by the powder melting processusing a submicrometre Y2BaCuO5 precursor in combination with the addition of CeO2was characterized by scanning electron microscopy, transmissionelectron microscopy and high resolution electron microscopy. The mean diameterof the Y2BaCuO5 precursor was less than 500 nm. As a result, themean diameter of the Y2BaCuO5 particles entrapped in the textured bulk YBCO isabout 500 nm. Two types of Y2BaCuO5 were found within the bulk YBCO: onewith one or two nanometric Y2O3 cores; the other, however, hasno cores. The refinement mechanism for Y2BaCuO5 particles related to theemployment of a submicrometre Y2BaCuO5 precursor and the minor addition ofCeO2 is discussed. In addition, a high density of twins with an averagespacing of about 60-80 nm was observed. Moreover, a twin domain boundarystructure where domains with different orientation twins meet one another wasfound. The formation mechanisms for this high density of twins and the twindomain boundary structure are also discussed.

D-wave superconductivity of the hole-doped(SrCa)10Cu17O29ladder compound

We performed tunneling experiments on single crystals of hole-doped two-leg ladders $(\mathrm{SrCa}{)}_{10}{\mathrm{Cu}}_{17}{\mathrm{O}}_{29}$ with ${T}_{c}\ensuremath{\cong}75$ K and, for reference purposes, on textured bulk samples of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Bi2212). Point-contact and break-junction (Bi2212 only) junction configurations were used. The point-contact junctions, both superconductor--insulator--normal-metal and superconductor--normal-metal, were using Ag counterelectrodes. In all experiments the tunneling direction was along ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ planes (for the ladders) and ${\mathrm{CuO}}_{2}$ planes (for Bi2212). The recorded tunneling conductance spectra were reproducible with sharp, well-defined energy-gap structures. All junctions exhibited a pronounced zero-bias conductance peak disappearing at the critical temperature. Experimental results are consistent with the formation of zero-energy Andreev bound states at the surface of the superconductors and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave symmetry of the order parameter $\ensuremath{\Delta}$ in both compounds.

HTS thick film components for fault current limiter applications

Fault current limiters are potentially one of the first applications of HTS materials in the power engineering field, and both inductive and resistive devices are being actively developed by many groups world-wide. A wide range of HTS materials is being considered for these devices including biaxially textured planar YBCO films, BSCCO silver clad wires, and textured bulk BSCCO and YBCO. Each type of device has its own particular requirements and restrictions with respect to acceptable component geometries, but the important physical property requirements of the materials appear to be similar in both cases. ALSTOM Research and Technology Centre is building a model current limiter based on an inductive design requiring superconducting components in the form of thick film materials on ceramic substrates. In this paper the design requirements for the current limiter are considered and the development and characterisation of the YBCO thick film materials for the device are reported. In particular, the property and performance issues concerning the scale-up of component sizes required for the demonstrator device are discussed. Preliminary results of the behaviour of the thick film components in an inductive FCL environment are also reported.

Role of the twin boundary for the occurrence of peak effect in Y-Ba-Cu-O superconducting bulks melt grown under varied oxygen pressure

A novel melt-growth technique, i.e. melt growth under varied oxygen pressure (MGVOP) in which the oxygen partial pressure is increased to solidify the initial melt, has been employed to prepare melt-textured superconducting bulks of the Y-Ba-Cu-O system. The obtained textured bulk samples consisted of two phases, i.e. superconducting and non-superconducting (Y-211). The critical current density of the samples reached as high as 40 kA (at 65 K) at a high magnetic field of T to exhibit a peak in the versus relation, i.e. a peak effect. To elucidate the causes of the peak effect and the high values of at high magnetic fields, submicron structures of the MGVOP-processed samples were examined by transmission electron microscopy (TEM), and the distributions of both Y-211 particles and twin boundaries were studied. In order to collect sufficient data for statistical analyses, specimens for `large-area TEM observations' were prepared using a focused ion beam thinning technique such that the TEM-observable area of the specimen was as wide as . Statistical analyses of the TEM images yielded the volume fraction of Y-211 phase, the average size of Y-211 grains and the inter-twin-boundary distance for each sample. It was concluded that a sample that had a higher twin-boundary density possessed a higher value at the peak in the versus characteristics, while fine Y-211 particles were likely to contribute to the value only at low magnetic fields.

Microstructure and properties of textured bulk phase Tl(Bi)-1223 superconductors

Bi-doped Tl-1223 superconductors of the composition Tl n− x Bi x Ba 0.2Sr 1.8Ca 2Cu 3O y ( n=0.96−1.12 and x=0.25, 0.3, 0.35, 0.4) and Tl n−0.25 Bi 0.25Ba 0.4Sr 1.6Ca 2Cu 3O y ( n=0.97−1.04) were prepared from oxidic Ba–Sr–Ca–Cu precursor material. Upon calcination of the metal nitrates, the respective amounts of Tl 2O 3 and Bi 2O 3 were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantly of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. T c(0)-values ranged between 114 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.8 kA cm −2 at 77 K were measured for the as-sintered samples. Texturing was obtained by grinding, repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 4.8 kA cm −2 by the thermo-mechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.

Flux creep determined ac magnetic response in type-II superconductors

The ac magnetic response in type-II superconductors was analyzed on the basis of flux creep theory. A model was proposed to describe the variation of current density in alternating field, which simultaneously took into account the sweeping applied field and the flux creep inside the sample. The frequency dependent ac susceptibility of a textured YBa2Cu3O7 bulk sample was measured and explained in this model.