Melt-textured Samples Research Articles

Use of the ac inductive method to assess the quality of bulk melt textured superconductors

Through a measurement of flux profiles, the inductive method of Campbell has not only been used for determining the bulk critical current density of large sized samples but has also been extended to study the coupling strength of the domains in melt textured NdBa 2Cu 3O 7− δ superconductors. An analysis of the profiles measured for the slow (1°C/h) and fast cooled (40°C/h) samples through peritectic temperature, T p clearly showed that the domain boundaries in the fast cooled samples are decoupled in the presence of external dc fields. The observed features of flux profiles are correlated to the microstructural features of the samples. It is proposed that this technique can be used routinely to assess the quality of large size melt textured samples for high current applications like magnetic bearings or quasi permanent magnets.

Measurements of misorientation angles over a large distance in strongly coupled grain boundaries

Grain boundaries in polycrystalline melt-textured YBCO samples fabricated by the liquid phase removal method have been analyzed by transmission electron microscopy (TEM). These samples contain a number of grains separated by high-angle grain boundaries with lengths up to several millimeters and J/sub c/ values up to 18000 A/cm/sup 2/ at 77 K. The results of TEM studies show that the high misorientation angles of these boundaries are maintained over a large distance by certain intra-grain features such as platelet boundaries and low-angle grain boundaries. Measurements on adjacent platelets as well as at locations inside single grains show that the presence of these boundaries maintains the misorientation of the high-angle grain boundary in these samples within 6 degrees. This indicates that high-angle grain boundaries in these samples do not dissociate into low-angle grain boundaries and the high superconducting current is carried by the entire length of the boundary.

Performing Effective YBCO Grain Alignment and Growth by Silver and Spurious Phase Addition

The effect of silver and the so-called green phase, Y 2 BaCuO 5 (211), on YBa 2 Cu 3 O x (123) melt textured samples has been investigated to check whether better grain alignment is possible in shorter times. We demonstrate that the addition of silver to YBCO always led to enhanced texturing in short times, while not disturbing the typical microstructure of the composite 123 plus 211, that is believed essential for high critical currents in these high Tc superconductors.

Harmonic Analysis of the AC Magnetic Response on Directionally Solidified YBaCuO Samples

The transport properties and the dynamics of the flux lines have been investigated on YBaCuO melt textured by measuring the AC susceptibility harmonics as function of the temperature, at different frequencies and amplitudes of the AC magnetic field. The interpretation of the behavior of the experimental curves has been done by their comparison with the results obtained by critical state pictures and by simulations of the diffusion processes of the magnetic field which determine the AC response in presence of different regimes of flux dynamics. The temperature dependencies of the third harmonics of the AC susceptibility at different frequencies have been used in particular to determine what is the flux dynamic regime governing the magnetic response of melt textured samples in the different experimental conditions. A good qualitative agreement has been found between the experimental curves and the ones simulated by assuming that dissipation processes are a sequence of independent flux creep and flux flow events within a collective pinning framework.

Rf impedance of sintered BSCCO and melt-textured YBCO superconductors

The authors present the temperature and field dependence of radio frequency response of sintered BSCCO 110-K phase and melt-textured YBCO with Ag and 211 inclusions. The rf resistance and reactance measurements are carried out using a marginal oscillator operating at 11.9 MHz. Melt-textured samples are found to exhibit insignificant low-field losses and hence are potential candidates for applications at radio frequencies. The field induced loss in sintered BSCCO is described by invoking two kinds of weak links. The rf penetration depth of the materials measured is found to be much larger than the London penetration depth and represents the effective screen depth. More importantly, the authors observed an anomaly in the penetration depth of BSCCO which is explained in terms of an interplay between intragranular and Josephson penetration depths.

Reconstruction of critical current density profiles from AC loss measurements

A method of reconstruction of critical current profiles from measurements of a hysteretic AC loss is proposed. As an example, we have applied this approach to study the degradation in the bulk melt-textured HTS sample with a resonance oscillations technique developed earlier for granular HTS samples and now modified to study HTS bulks. The critical current density profile was determined after the degradation in the thin undersurface layer of about 60 μm.

Critical current anisotropy in YBCO superconducting samples

The critical current densities J c ab and J c c in YBCO superconductors are shown to be simultaneously determined by means of measurements of a rectangular sample response to an external low-frequency ac magnetic field. A method is based on the correlation of the critical current density and ac magnetic susceptibility and takes into account for both shielding currents J c ab and J c c . The magnetic field dependencies of J c ab and J c c have been measured for melt-textured YBCO samples. The anisotropy parameter J c ab / J c c is found to increase with the dc magnetic field H.

Investigations of the zero-field ( a,b)-plane conductivity of YBa 2Cu 3O 7−δ near the critical temperature

The resistive transition R( T) and the current–voltage characteristics ( I–V) in the range 10 −9–10 −6 V and close to the critical temperature ( T c) were measured in a single-grain of melt-textured YBa 2Cu 3O 7−δ (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the ( a,b) planes and in zero external magnetic field using a SQUID picovoltmeter. In the case of the melt-textured sample, we measured the I–V curves also at higher levels of dissipation. For T< T c, we interpreted the lower part of the I–V curves in the framework of the Jensen–Minnhagen model of current-induced unbinding of thermally-created vortex–antivortex pairs, which leads to V∝ I( I− I c1) a−1 . Our results confirm a recently proposed current–temperature ( I–T) phase diagram by S.W. Pierson. In addition, we determined for the first time the dependence of the characteristic (unbinding) current I c1 with t=1− T/ T c for temperatures very close to T c and low transport currents.

Growth and physical properties of melt textured (Gd,Y)Ba 2Cu 3O x

Melt textured samples grown from mixtures of GdBa 2Cu 3O x with Y 2BaCuO x , and from GdBa 2Cu 3O x with Gd 2BaCuO x , were successfully grown in air using the top seeding method employing NdBa 2Cu 3O x seed crystals. For GdBa 2Cu 3O x /Y 2BaCuO x , the superconducting transition temperature is 91 K (diamagnetic onset) and the critical current at 77 K is 25 000 A/cm 2 at zero applied field and above 10 000 A/cm 2 at 2 T. The irreversibility field at 77 K is about 6 T. The green phase particles are small and well dispersed, even without the addition of Pt. However, samples grown in air from the GdBa 2Cu 3O x /Gd 2BaCuO x mix had T c's of 75–80 K, fairly low critical currents and showed large green phase inclusions.

Influence of Nd 4Ba 2Cu 2O 10 phase inclusions on flux penetration in melt-textured NdBa 2Cu 3O 7− δ superconductors

NdBa 2Cu 3O 7− δ superconductor is processed with varying Nd 4Ba 2Cu 2O 10 (Nd-422) content, thereby varying systematically its microstructure. The samples are then investigated by studying the flux penetration into the samples as a function of magnetic field using a flat-band lock-in technique. Unlike in sintered samples, the flux profiles in the melt-processed samples are found to be linear, whose slopes increase with increasing Nd-422 content. The slope of the profile gives a direct, model-independent measure of the critical current density in the materials, and the specimens measured can be large enough to include several millimeter-sized domains. The critical current densities thus obtained are compared with those obtained using SQUID magnetometer, wherein the samples have to be much smaller and mostly single domain. The extent of flux penetration, reversible penetration depth, and the slope changes in the flux profiles at high fields are correlated to the microstructural features of the melt-textured samples.

Microstructure and flux dynamics in YBa 2Cu 3O 7 plastically deformed at room temperature

The influence of dislocations on pinning of vortices has been investigated in melt-textured YBa 2Cu 3O 7− x and YBa 2Cu 3O 7− x ceramics. The dislocations, introduced by plastic deformation at room temperature in the orthorhombic phase, are characterized using transmission electron microscopy. dc and ac magnetic measurements revealed that the plastic deformation of melt-textured samples induced a decrease of the activation energy for flux motion. This effect has been related in a preceding paper to the microcracks created in important density along the (001) planes during the deformation process. The comparison of these results with those obtained on a ceramic material confirms this interpretation.

Homogeneity of top-seeded single-grained YBCO superconductors

The homogeneity of J c( H,T) and microstructure of top-seeded melt-textured (TSMT) YBCO samples has been investigated systematically. A series of ten samples was cut from one single-grain sample at different distances and orientations from the seed, SQUID magnetization measurements of these ten samples have shown that J c(0 T, 75 K) varied from 1.02 × 10 4 to 2.49 × 10 4 A m −2 with an average of 1.80 ± 0.45 × 10 4 A cm −2. A second TSMT sample was sliced into four single-grained samples parallel to the a-b plane and the distribution of trapped magnetic field mapped for each slice. The sample 3–6 mm below the seed exhibits the largest single peak with the highest trapped field. For samples far away from the seed, very complicate patterns and various field intensities were observed. It is believed that inhomogeneities within the single-grained region of the sample may account for reports that TSMT samples larger than about 3 cm diameter do not exhibit further improvements in their trapped magnetic field and associated levitation force.

Influence of disorder on the vortex-lattice melting in twinned oxygen-deficient ortho-IIYBa2Cu3Oy(y≈6.6)

We investigated with ac susceptibility the mixed state of twinned, deoxygenated single crystals and highly $c$-axis-oriented melt-textured samples of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ with oxygen stoichometry corresponding to that of the orthorhombic II phase. In the single crystal we found, below ${H}^{*}\ensuremath{\approx}700$ Oe and over almost four orders of magnitude in frequency, a frequency-independent ``irreversibility line'' (IL). Complementry dc magnetization measurements seem to disregard an IL arising from geometric effects when $H\ensuremath{\Vert}c$, and they suggest a possible meltinglike transition of the flux-line lattice below ${H}^{*}$. The reduced value of ${H}^{*}$ is associated with the increased anisotropy induced by the oxygen depletion, and an upper limit of the anisotropy parameter is estimated as ${\ensuremath{\gamma}}_{\mathrm{max}}\ensuremath{\approx}147$. However, in the twinned melt-textured sample having a major grade of disorder due to the presence of micrometric nonsuperconducting precipitates of ${\mathrm{Y}}_{2}{\mathrm{BaCuO}}_{5}$, the IL is frequency dependent for all applied magnetic fields, suggesting a cancellation of ${H}^{*}$ in the sufficiently disordered specimen.

Infiltration-growth processing of superconductor

Melt processing of (Sm-123) is done by infiltrating liquid phases into an Sm-211 preform and allowing the growth of Sm-123 to take place. The melt-textured samples are found to be free of macrodefects, and the microstructure contains uniformly distributed fine-sized Sm-211 particles. The behaviour of the critical current density of the samples in high fields is reported here.

Comparative study of magnetic properties between melt-textured and single crystalline Y–Ba–Cu–O

We studied magnetic properties of a melt-textured Y–Ba–Cu–O with Y 2BaCuO 5 inclusions and a single crystalline YBa 2Cu 3O y grown by a crystal pulling technique by changing oxygen annealing conditions with the emphasis placed on the effect of doping level on flux pinning. Magnetization hysteresis loop measurements at 20 K and 77 K showed that the critical currents of the melt-textured sample were three times as large as those of a single crystal, which confirms the contribution of Y 2BaCuO 5 to pinning enhancement. In order to enhance flux pinning, it is important to introduce effective pinning centers in the good matrix with sufficiently large oxygen content.

Microstructure change during oxygen annealing and the effect on the levitation force of melt-textured Y - Ba - Cu - O superconductors

The influence of oxygen annealing on the levitation force was studied for melt-textured multidomain YBCO samples cooled with and without magnetic field (field cooled (FC) and zero-field cooled (ZFC)). For both the FC and ZFC samples the repulsive force rapidly increased with increasing oxygen annealing time, reaching a maximum value at a certain annealing time and then slightly decreasing when the samples were further annealed. The attractive force of the FC samples followed a similar pattern to that of the repulsive force while the attractive force of the ZFC samples was small and nearly constant regardless of annealing time. The variation of the levitation force with oxygen annealing time is explained in terms of the microstructure developed during oxygen annealing.

Dependence of the Glassy Exponent on Field and Temperature in Textured YBCO Specimen

The glassy exponent Μ of a melt-textured YBa2Cu3O7-x sample is derived from measurements of the magnetic hysteresis loops with various applied field sweep rates and at different temperatures and magnetic fields. Thus, the dependences of the exponent Μ on temperature and field are obtained. Furthermore, we report for the first time the crossover-line of the vortex creep regime from a single vortex to a small bundle.

Influence of Irradiation-Induced Correlated and Random Disorder on Flux Pinning in Bulk YBCO Melt-Textured Samples

YBCO melt textured samples were studied after Au ion irradiation of two opposite surface layers of the samples and after proton irradiation. The scaling behavior of the pinning force was studied in order to get information about mechanisms responsible for pinning in the studied samples. Correlated disorder introduced bysurface ion irradiation is effective at high reduced temperatures and leads to the occurrence of a kink in the irreversibility line and to the shift of this line toward higher fields and temperatures. Enhancement of critical current density,increasing with temperature and field, was observed. Random disorder due to proton irradiation does not change the irreversibility line and only provides critical current enhancementdecreasing with temperature and field.

High irreversibility fields and current densities inNdBa2Cu3O7−δsingle crystals and melt-textured samples

${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals and melt-textured samples were prepared in reduced atmospheres starting from the Nd-poor side of the phase diagram in order to minimize the Nd/Ba substitution and the second phase content. Current density $j$ and irreversibility field ${B}_{\mathrm{irr}}$ were measured as a function of temperature and angle between the $c$ axis and applied field direction in samples with different impurity and oxygen contents. Current density vs field shows a pronounced peak effect which results from small clusters of oxygen vacancies, whereas a second peak is related to the twin structure. The current density at 77 K is for fields up to 4 T above $4\ifmmode\times\else\texttimes\fi{}{10}^{4}A/{\mathrm{cm}}^{2}$ and reaches values of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{5}A/{\mathrm{cm}}^{2}$ in the peak at 2.1 T. The irreversibility fields parallel to the $c$ axis reach 13.4 and 11.8 T at 77 K in pure and homogeneous single crystals and melt-textured samples, respectively. These extremely high values, which are caused by the twin structure, decrease to 10 and 8 T, respectively, if the current is optimized. The correlation and interference between $j$ and ${B}_{\mathrm{irr}}$ in the presence of weakly and strongly interacting defects such as small clusters of oxygen vacancies and Nd-422 precipitates, respectively, as well as the influence of the twin structure are analyzed and discussed.

Numerical calculations of the magnetic behaviour of a superconductor sample with finite size

Previously many research groups have modelled the magnetic response for ideal superconductor samples, but recently Brandt [1] has suggested a method of numerical calculations for a realistic type II superconductor with finite dimensions. These numerical calculations were used to compute the theoretical dynamic magnetic response of a melt-textured (solid - liquid melt growth) sample. The results were compared with actual magnetization data and it was found that such method works well for samples of high-temperature superconductors with finite dimensions.