Top-seeded Melt Research Articles

MgO buffer-layer-induced texture growth of RE–Ba–Cu–O bulk

A novel cold-seeding approach of top-seeded melt growth of RE–Ba–Cu–O bulk was studied by employing a MgO crystal seed and a buffer pellet. The growth process is divided into two steps. The MgO seed is used to texture-grow the small RE–Ba–Cu–O pellet with high melting point (Tp) and the textured pellet induces the texture growth of the bulk at lower temperature. Undercooling and RE211 content of the pellet are adjusted to avoid the misorientation caused by lattice mismatch between MgO and the RE–Ba–Cu–O matrix. Bulk samples prepared with this method show good growth sections and superconducting performance. One of the promising advantages of this method is to process high Tp RE–Ba–Cu–O bulks with a cold-seeding method, for example Nd–Ba–Cu–O bulk.

Investigation of Microstructure and Superconducting Properties in the Fe–B Magnetic Particles Doped GdBa2Cu3O7-δ/Ag2O and GdBa2Cu3O7-δ/nano–Ag Superconducting Bulks

Abstract Fe–B magnetic particles doped GdBa2Cu3O7-δ superconducting bulks were fabricated by using the top-seeded melt growth method. 10 wt% Ag2O or equivalent 9.31 wt% nano–Ag particles (50 nm in diameter) were added into the precursors before the sample preparation. The Fe–B doped GdBa2Cu3O7-δ/nano–Ag showed poorer mechanical properties with an obviously porous/cracked structure than the GdBa2Cu3O7-δ/Ag2O one especially at the bottom part of the bulk. In the GdBa2Cu3O7-δ/Ag2O bulk, doped Fe element is mainly wrapped or attached by Ag particles. In contrast, the Fe element prefers to substitute on the Cu sites in the GdBa2Cu3O7-δ/nano–Ag bulk due to less protection from Ag particle. The microstructure and superconducting properties were investigated in these two series samples.

Oxygenation kinetics of YBCO-TSMG samples using the nanoindentation technique

Since hardness is a quantitative measure of bulk mechanical properties, it can be used to examine the kinetics of crack propagation inside YBa2Cu3O7−δ (YBCO or Y-123) ceramics. YBCO samples with a tetragonal phase were oxygenated at a range of temperatures (from 250 °C to 750 °C) for different times. For each oxygenation temperature and time, hardness was measured by the nanoindentation technique, to study the defects (macro-/microcracks and porosity) produced along the c-axis. These defects were visualized by optical microscopy. The main purpose of this study was to establish the oxygenation kinetics for YBCO samples, which were textured by the top-seeded melt growth technique. We studied the evolution of hardness perpendicular to the ab-plane, as measured by the nanoindentation technique at a maximum penetration depth of 150 nm. The results indicate that the nanoindentation technique can be used successfully to monitor oxygenation and to establish the kinetics of the process.

Enhanced Flux Pinning and Microstructural Study of Single-Domain Gd-Ba-Cu-O Bulk Superconductors With the Addition of Fe-Containing Alloy Particles

Introducing secondary-phase inclusions, especially magnetic particles, into the superconducting matrix is regard as an effective way to enhance the performance of high-temperature superconductors. The effect of Fe-B alloy additives on the microstructure and superconducting properties of top-seeded melt growth processed Gd-Ba-Cu-O bulk superconductors has been investigated systematically. Gd-Ba-Cu-O bulks 17 mm in diameter were melt-textured for various amounts of Fe-B additives in the range of 0-1.4 mol%. With the observations of scanning and transmission electron microscopy, the Fe-B alloy additives were found to decompose into Fe-containing particles with submicrometer in diameter. The refinement and homogeneous distribution of Gd2BaCuO5 inclusions seems to be improved remarkably. It leads to enhance flux pinning, flux trapping capability, and critical current density of Gd-Ba-Cu-O bulks, correspondingly. This phenomenon might be related to the magnetic interactions between the Fe-containing particles and the surrounding components, which may be the origin of the improvement of Gd2BaCuO5 nucleation process.

Microstructural and superconducting properties in single-domain Gd–Ba–Cu–O bulk superconductors within situ formed Fe3O4 ferrimagnetic particles

Introducing secondary-phase inclusions into the superconducting matrix isregarded as an effective way to enhance the flux pinning performance of hightemperature superconductors. The effect of Fe–B alloy additives on themicrostructure and superconducting properties of top-seeded melt growth processedGdBa2Cu3O7 − δ/Ag bulk superconductors has been investigated systematically.GdBa2Cu3O7 − δ/Ag bulks 17 mm in diameter were melt-textured for various amounts of Fe–B additives in therange of 0–1.4 mol%. By means of scanning and transmission electron microscopyobservations, the Fe–B alloy additives were found to decompose into ferrimagneticFe3O4, embedded inside or in the vicinity of silver particles. These ferrimagnetic particles may assistin reducing the Lorentz force exerted on the vortices and enhance the flux trapping capabilityand critical current density of Gd–Ba–Cu–O bulks. It was also found that the refinement ofGd2BaCuO5 and its homogeneous distribution was improved together with increase of the number ofGd2BaCuO5 particles when we increase the addition of Fe–B alloy particles. The amountadded introduces a systematic change in the particle distribution dependent onboth size and the numbers. The observed enhancement of the trapped fluxmay inherently include the magnetic interactions between the ferrimagneticFe3O4 with the surrounding compounds, which has a critical effect on the nucleation process ofGd2BaCuO5 inclusions.

Effects of calcium doping on the superconducting properties of top-seeded melt growth processed Y1.5Ba2−xCaxCu3Oy superconductors

The effect of calcium doping on the superconducting properties of top seeded melt growth (TSMG) processed Y1.5Ba2−xCaxCu3Oy superconductors was studied in terms of calcium content (Xca). YBa2−xCaxCu3O7−δ (Xca=0, 0.005, 0.01, 0.02, 0.04, 0.1, 0.3) powders were synthesized by the powder calcination method. YBa2−xCaxCu3O7−δ powders were mixed with 0.25mole Y2O3 powder and 1wt.% CeO2 as Y2BaCuO5 (Y211) refiner, and finally made into Y1.5Ba2−xCaxCu3Oy (Y1.5)+1wt.% CeO2 composition. The single Y123 growth on the top surface was observed up to Xca=0.1, while the multiple Y123 growth was observed at Xca⩾0.1. The superconducting transition temperature (Tc) and critical current density (Jc) of TSMG processed Y1.5 samples were inversely proportional to Xca. The Y211 size increased with increasing Xca due to the enhancement of Y211 coarsening by calcium doping. No Y211 refining effect by CeO2 was observed in the calcium doped samples. The Tc and Jc decrease by calcium doping are likely to be due to the calcium incorporation with the Y123 lattice and formation of coarse Y211 particles.

Enhancement of mechanical strength in Y–Ba–Cu–O bulk superconductor through liquid binder addition

We have studied the effects of the liquid binder (polyvinyl alcohol) addition (0–10wt%) on the mechanical properties of the green compacts and also on the superconducting properties of bulk Y–Ba–Cu–O superconductors of 20mm diameter produced with the top-seeded melt growth (TSMG) process. The mechanical properties of the green compacts with binder addition were characterized with the compression tests, which revealed that mechanical strength increased dramatically with increasing the amount of the binder addition. The binder-added green compacts were then subjected to the TSMG process and oxygen annealing. The trapped field measurements showed that we could produce single-grain bulk Y–Ba–Cu–O samples with binder additions up to 8wt% without any deterioration in the superconducting properties.

Effects of Y2O3 additions on the oxygen diffusion in top-seeded melt growth processed YBa2Cu3O7−y superconductors

To understand the effect of Y2BaCuO5 (Y211)/YBa2Cu3O7−y (Y123) interfaces on the oxygen diffusion in single grain YBa2Cu3O7−y superconductors, single grain Y123 superconductors with 0.05 and 0.3moles of Y2O3 additions were fabricated by a top-seeded melt growth (TSMG) process. Y123 compacts with Y2O3 additions were subjected to melt growth heating cycles with a cooling rate of 1°C/h through a peritectic temperature (1015°C) and then annealed at 450°C for 200h in flowing oxygen. The superconducting temperature (Tc) and critical current density (Jc) were estimated for the three different regions (top surface (s), intermediate (i) and center (c)) of samples. The amount of Y211/Y123 interface area in single grain Y123 superconductors was successfully controlled by Y2O3 additions. The Tc values of s regions were higher than those of i and c regions, which indicates the presence of more oxygen at the sample surfaces. In addition, the Tc values of i and c regions of the Y123 sample with 0.3mole Y2O3 addition were higher than those of the same regions of the Y123 sample with 0.05mole Y2O3 addition due to the promoted oxygen diffusion through Y211/Y123 interfaces and other related defects. In spite of the promoted oxygen diffusion by Y2O3 addition, the large Tc difference among the regions still existed, which suggests sluggish oxygen diffusion into single Y123 grains.

The effect of Gd<sub>2</sub>Ba<sub>4</sub>CuNbO<sub>y</sub> doping on the properties of single domain GdBCO bulk superconductors

Single domain GdBCO bulk superconductors, with compositions of GdBa 2 Cu 3 O 7-δ (Gd123):Gd 2 BaCuO 5 (Gd211): Gd 2 Ba 4 CuNbO y (GdNb2411) =1:(0.4-x):x, (x=0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16), have been successfully fabricated by the top-seeding melt texture growth process (TSMTG). The effect of the different doping amounts of GdNb2411 on the growth morphology and the magnetic levitation force properties of single domain GdBCO bulk superconductors has been investigated. The results show that single domain GdBCO bulk samples can be fabricated within the scope of the doping, and the surface morphologies of the samples are closely related to the doping content x. It is of a smooth faced plane and four clear radius for the samples with x≤0.06 mol, but it is of a wrinkle surface for the samples with x≥0.08mol and the single domain area is observed to reduce with the increasing of x. The test results of the magnetic levitation force of the samples show that the levitation force of samples increases first and then decreases with the increasing of x, the largest levitation force of 25 N is obtained in the samples with x=0.06 mol. The results is very important for study in flux pinning of nanoparticles and further improvement in the properties of GdBCO bulk superconductors.

Electrical Properties of Lead-free Niobium Rich Piezoelectric (K0.95Li0.05) (Ta1-x Nb x ) O3 Single Crystals

The high quality (K0.95Li0.05) (Ta1-x Nb x ) O3 (KLTN), x = 0.60, single crystals have been successfully grown by the Top-seeded melt growth method (TSSG). Its structure, dielectric, pyroelectric and piezoelectric properties have been investigated. Based on the dielectric and pyroelectric properties measurements, rhombohedral to orthorhombic and orthorhombic to tetragonal transition temperatures are found to be 88 K, 195 K, respectively, and the Curie temperature was 445 K. From the pyroelectric curves, the integrated spontaneous polarization, ∼16 μC/cm2 and the figure of merits (FOMs) are calculated at room temperature. Resonance and anti-resonance method is used to measure the piezoelectric coefficients; thickness vibration mode (k t) 60.4% and the length-extensional vibration mode (k 31) 56.2%, for the KLTN single crystals are attractive for the piezoelectric and pyroelectric applications.

Enhanced flux pinning and formation ofBa4Y2CuMoOy in top-seeded meltgrowth processed YBa2Cu3O7 − d superconductors with Mo additions

The effect of Mo addition (0–10 wt%) on the superconductivity of top-seeded melt growth (TSMG)processed YBa2Cu3O7 − y (Y123) superconductors was studied. The low level Mo addition (≤1 wt%) led to a small decrease of the superconducting transition temperature (Tc) and increase of the critical current density (Jc).The Jc improvement induced by the low level Mo additions appeared as a peak effectat the intermediated magnetic fields and peak position shift to the lowermagnetic fields with increasing Mo content. The enhanced flux pinning causedby Mo additions seems to be attributed to the partial Cu substitution by Mo,YBa2(Cu1 − xMox)3O7 − d. The high level Mo additions (2–10 wt%), however, led to a largeJc decrease and broad superconducting transition due to the formation of low-Tc phases and the increased volume of the non-superconducting Mo-containing phase. Thesecond particle phase formed by the high level Mo additions was identified asBa4Y2CuMoOy (Mo4211) by x-ray diffraction (XRD) and scanning electron microscopy energy dispersivex-ray (SEM EDX) analysis.

The anisotropic morphology of silver particles in Y-123/Y-24Nb1/Ag nanocomposite bulk high-temperature superconductors

Y2Ba4CuNbO12 (Y-24Nb1) and silver (Ag) are recognized as potential candidates for improving both flux pinning and the mechanical properties of bulk rare earth (RE)–Ba–Cu–O [(RE)BCO] high-temperature superconductors (HTS). Recent attempts to add Ag2O to superconducting Y-123/Y2Ba4CuNbO12 composites, however, have produced a highly anisotropic morphology of Ag particles in samples grown by top-seeded melt growth (TSMG). This morphology has been attributed to strong particle pushing effects due to the presence of Y-24Nb1 nanoparticles in the composite microstructure. An investigation of the formation of anisotropic Ag particles in the YBCO bulk microstructure indicates that these pushing effects generate different morphological microstructural zones in the composite. These include a zone free of inclusions other than acicular Ag particles, a zone of segregated additives (i.e., Y-24Nb1, Y-211, and Ag), and a zone containing fine Ag and other particles distributed uniformly throughout the local microstructure. The particle pushing/trapping theory has been used to explain these extraordinary features of the distribution of Ag inclusions. The superconducting and mechanical properties of samples containing very fine silver inclusions are also discussed briefly.

Joining of textured YBCO with YBCO added Ag 2O additive

The engineering applications of high temperature superconducting (HTS) single domain YBCO block require the material with larger size and more complex shape. However, traditional top-seeded melt growth (TSMG) method cannot meet this requirement. In this paper, solder technology of YBCO blocks using Ag 2O-doped YBCO slice has been performed in air with a rapid cooling rate, and the oxygen treatment techniques with a shorter time were used in the process. Microstructure analysis and superconducting characterization of the joint were investigated. The results show that a high quality joint is obtained without any macro cracks. Furthermore, the grains in the joint are well oriented as same as that of the base YBCO block. However, some defects such as voids and Cu–Ba–O phase segregations are discovered in the middle of the joint. Much finer Y211 (Y 2BaCuO 5) grains are attained in the soldering zone compared with that in the base YBCO. The trapped field measurement on bonded YBCO blocks shows a single maximum, indicating good superconducting properties in the joint.

Axial and radial ac susceptibility measurements on melt-processed single-grainY–Ba–Cu–O discs

The field amplitude and frequency dependent complex ac susceptibilityχ(Hm, f) of four Y–Ba–Cu–O discs made by a top-seeded melt growth technique has beenmeasured at 77 K with the ac field applied parallel to either the disc axis (thec axis of the grain)or its radius (the ab plane of the grain). The results are interpreted by considering the anisotropic nature of the critical-currentdensity, Jc. The axial susceptibility is dominated byJcab, c for current withinthe ab plane andfield parallel to the c axis, whereas the radial susceptibility is dominated byJcc, ab for current along thec axis and field within theab plane. It is concludedthat the ratio of Jcab, c to Jcc, ab in a thin, round surface layer ranges between 2 and 5 for discs ofdifferent thicknesses, and both are much smaller than the value ofJcab, ab associated with intrinsic pinning. In addition, the measured susceptibilities of thicker graindiscs suggest more granular behavior, indicating the existence of microcracks within thesample microstructure.

Transmission Electron Microscopy and Atomic Force Microscopy Observation of Air-Processed GdBa2Cu3O7-δ Superconductors Doped with Metal Oxide Nanoparticles (Metal = Zr, Zn, and Sn)

Single-domain, c-axis-oriented 30-mm-diameter GdBa2Cu3O7-δ (Gd123) melt-textured bulk superconductors have been successfully grown by the top-seeded melt growth method from precursors of Gd123, Gd2BaCuO5, and Ag2O in air with doping of nanosized ZrO2, ZnO, or SnO2. Transmission electron microscopy (TEM) unveils a large amount of BaZrO3 or BaSnO3 particles with an average diameter of approximately 50 nm respectively embedded in ZrO2 or SnO2 doped samples, while no Zn-rich nanoparticles are observed in the ZnO-doped samples. The critical temperature Tc is almost unchanged up to a doping amount of 10 mol % for ZrO2- or SnO2-doped Gd123 melt-textured bulks, while ZnO-doped Gd123 becomes non-superconductive at this doping level. By atomic force microscopy (AFM), nanostripes with a wavelength of 15 nm are observed in the ZnO-doped Gd123 sample. Nanoscale particles in the grown Gd123 single domain, together with the micro-defects induced by nanoparticle doping, can account for the enhanced superconducting properties.

The effect of impregnation by resin and CNT compound on the mechanical and magnetic properties of YBCO superconductor

Bulk YBCO superconductor was manufactured with the top-seeded melt grown method. The artificial hole with 0.5 mm and 0.7 mm parallel to the c-axis was mechanically drilled. The YBCO bulks containing artificial holes were impregnated by the mixture of resin and CNT (carbon nano tube), respectively. In order to estimate the influence of the hole and impregnation by CNT and resin, the magnetic and mechanical properties were measured and compared with before and after reinforcement. It was confirmed that the maximum magnetic flux density of the sample without any holes was 0.243 T, while the maximum magnetic flux density of sample with 0.5 mm and 0.7 mm holes in diameter was around 0.244 T and 0.208 T, respectively. As compared with the results before and after reinforcement, the trapped field value taken on the perforated YBCO is somewhat higher than the ones after reinforcement.

Critical-current density of melt-grown single-grain Y–Ba–Cu–O disks determined by acsusceptibility measurements

The field amplitude and frequency dependent complex ac susceptibilityχ(Hm,f) of three Y–Ba–Cu–O disks made by a top-seeded melt growthtechnique has been measured at 77 K with the ac field applied along thec-axis of the samples (parallel to their thickness). A procedure based on theBean model has been developed to calculate the critical-current densityJc near the surface of the sample from the measuredχ(Hm) for the case where the maximum imaginary componentχ′′ is not reached.

The homogeneity of levitation force in single domain YBCO bulk

The pellet homogeneity of levitation force versus the position in comparison to the seed or to the top surface has been studied in the entire volume of a single domain YBa 2Cu 3O 7− δ bulk sample processed by the top-seeded melt texturing growth (TSMTG). It is found that the levitation forces increase and peak at a depth of 3 mm from the top of the sample at liquid nitrogen temperature. In other words, the second disk has the largest levitation force density. The phenomenon can be interpreted by the interaction between the microcracks or pores produced by crystal growth and the oxygenation. We propose a model in which Y211 particles distribution leading to microcracks and pores reduces the effective induced shielding current loops (ISCL) and increases the perimeters of ISCL. This corresponds to a decrease in the grain size and results in greatly reduced levitation forces of the bottom of the bulk. From the research, we know that the density of the YBCO bulk is also an important parameter for the levitation properties. The result is very attractive and useful for the fundamental studies and fabrication of TSMTG YBa 2Cu 3O 7− δ bulk.

The effects of artificial holes with different diameters on the surface magnetic properties in YBCO superconductor

Bulk YBCO superconductor was manufactured with the top-seeded melt grown method. In order to elucidate the effects of defects in YBCO, the melt-textured superconductor was arbitrary drilled by mechanical tool on the surface along c-axis. Typical artificial hole diameters were 0.5, 0.7 and 1.0mm, respectively. Based on the measurements of magnetic flux mapping and levitation force, we conclude that the magnetic field trapping capacity decreased systematically with increasing the diameters of hole. The maximum magnetic flux density of the sample without any holes was 0.05T, while the maximum magnetic flux density of sample with 1.0mm hole in diameter was very low as around 0.023T.

The collective flux pinning force in(Sm0.5Eu0.5)Ba2Cu3O7−δ superconductors

We have measured the isothermal magnetization loops of top-seeded melt processed(Sm0.5Eu0.5)Ba2Cu3O7−δ+10 mol% (SmEu)BaCuO5+10 wt% Ag crystals in a magnetic field up to 8 T parallel to thec-axis from 50 to 87 K using a vibrating sample magnetometer. All of the isothermalJc(H) curves show a second peak. The scaling of theFp–H curves indicates thatthe flux pinning force Fp comes from the collective pinning of two kinds of pinning centres,Fp = FpH+FpL. One(FpH) is theΔTc-typepinning centre originating from the RE–Ba and RE–RE substitution responsible for the second peak; theother (FpL) is the normal pinning contributed by the surface and point pins. TheHc2–T diagram implies two critical temperatures. The crossover occurs at about 88.2 K.