Melt-textured Materials Research Articles

Mapping spatial superconducting property variations in IG-processed bulk ternary (Gd0.33Y0.13Er0.53)Ba2Cu3Oy

To effectively utilize REBa2Cu3Oy material in various industrial applications, it is crucial to achieve high-quality material with consistent properties. Recent reports indicate that the Infiltration Growth (IG) process, although promising, demonstrates less uneven performance when compared to melt-textured materials. This study focuses on the recently developed single-grain ternary (Gd0.33Y0.13Er0.53)Ba2Cu3Oy material. A total of 36 small pieces were carefully selected for analysis, utilizing scanning electron microscopy (SEM) to examine microstructures, and superconducting transition temperature (Tc) and critical current density (Jc) at 77 K using a SQUID magnetometer. Magnetization results indicate consistent Tc values across the entire sample, while spatial variations in Jc values were observed. Specifically, Jc values no clear trend along the a/b-axis and decreased along the c-axis as the distance from the center increased, correlating with microstructural variations observed through scanning electron microscopy. Quenching experiments conducted on (Gd0.33Y0.13Er0.53)2BaCuO5 (RE211) pellets at 880°C and 1050°C revealed that the heating cycle had a significant impact on both particle and pore sizes in the RE211 secondary phase pellet. Larger pore sizes at the periphery were attributed to the fusion of RE211 secondary phase particles before Ba-Cu-O liquid infiltration. These findings emphasize the importance of further processing optimization to achieve uniform bulk materials for the IG process. Such optimization is crucial to produce uniform bulk superconducting material, essential for everyday applications.

Magnetic and Thermal Properties of HTS Bulk Magnet in the Pulsed-Field Magnetizing Process

In order to enhance the field-trapping ability of high Tc superconducting melt-textured bulk materials which act as quasi-permanent magnets when they capture the external magnetic fields, it is important to enhance the mechanical toughness of the materials to stand the stress induced by the magnetic repulsive force and thermal expansion. We adopted a dense Dy123-based bulk material with reduced void concentration in the experiment. Since the heat generations due to the flux motions in the samples results in the degradation of Jc, the time evolutions of the trapped magnetic fields and the temperature rises during and after the pulsed-field magnetizing processes were precisely measured at the same time to evaluate the penetrating flux motions and the heat generations in the sample. A single and couple of the magnetic pulsed fields with various intensities were successively applied to the sample at 30 K. The single magnetic field application exhibited a peak effect in the trapped-field behavior and tended to decline due to the heat generation. In the iterative pulsed-field application, the behaviors of the trapped fields and the temperature changes were found to be inverse between the first and the second pulsed-field applications. This implies that the flux penetration behavior into the sample magnet at the second field application is strongly restricted by the presence of former trapped fields which were formed by the first field applications.

Temperature dependence of the trapped field and mechanical properties of neutron irradiated and reinforced YBa/sub 2/Cu/sub 3/O//sub 7γ/ bulk superconductors

We report on the temperature dependence of the trapped field in neutron irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// bulk melt-textured materials. The field trapping capability of these materials at low temperatures is limited by their mechanical properties. We observe that samples without reinforcement usually break during activation at temperatures of around 50 K. Two reinforcement techniques were tested. Resin impregnation is found to be less effective than stainless steel bandaging to compensate for the magnetic stress generated during activation. The resin impregnated samples crack during activation at temperatures of around 45 K. Stainless steel bandaging improves the mechanical properties of the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// samples and trapped fields of up to 13.3 T were achieved at 33 K.

Effect of mesoscopic inhomogeneities on the critical current of bulk melt-textured YBCO

The downsizing 211-inclusions and an increase of their density leads to rise in mean critical current value in Y-based melt-textured material. Very often 211-inclusion are spread in the material volume non-homogeneous, with typical scale 50–100 μm. Therefore it is difficult to find the real correlation between local critical current and the inclusions distribution. We performed a study of a local critical current using modified magneto-optic technique on a melt-textured YBaCuO ceramic, found the areas with constant current and studied the real structure of the material in the areas, inclusions distribution and their sizes, by scanning electron microscopy and X-ray microanalysis. The estimation of a pinning in these places, by taking into account the amount of inclusions and the length of their boundaries, and comparison with the value of local critical current reveals a strait correlation between the density of inclusions and the current but shows remarkable quantitative disagreement.

Microstructure and growth of joins in melt-textured YBa2Cu3O7−δ

Joining of melt-textured YBa2Cu3O7-δ (Y123) grains has been achieved without use of an external agent. The technique uses barium-cuprate liquid phase released from platelet boundaries to mediate the growth of Y123 at the interface between two grains. The epitaxial nature and high quality of the growth was determined by optical and transmission electron microscopy. The composition of Ba–Cu–O phases found in some parts of the joins was determined by electron probe microanalysis. A clean low-angle join was found to consist of a grain boundary with dislocation networks and facets. Transport critical current measurements on this type of join revealed strongly coupled behavior. The technique shows promise for the joining of melt-textured material for power engineering applications.

Magnetothermal instabilities in cylindrical melt-textured Y-Ba-Cu-O

The magnetothermal flux jump instabilities of the critical state in cylindrical melt-textured Y-Ba-Cu-O samples are investigated during the magnetization process. The flux jumps result in a drastic reduction of the magnetization accompanied by a strong heating of the sample. We determine the first flux jump field as function of the parameters, which influence the stability, such as the heat transfer coefficient between the surface of the sample and the surrounding, the sweep rate of the external field and the temperature of the sample. The comparison of the thermal and magnetic diffusivity of the melt-textured material indicates isothermal behavior of the flux jump scenario, supporting a dynamic stability criterion.

Surface nanostructuring and damage morphologies along 2 GeV gold-ion-implanted tracks

Abstract Good-quality melt-textured YBa2Cu3O7-x materials with Y2BaCuO5 non-superconducting inclusions were irradiated with 2GeV gold ions in order to produce a surface layer with columnar defects. The finaL goal is to gain a basie understanding of the role of the surface columnar defects in determining those magnetic properties of the materials that are related to the vortex confinement. The particie energy and the sample thickness were chosen in such a way that the percentage of irradiated volume is about 10%. In the paper the intrinsic structure and the damage morphology were analysed by means of transmission electron microscopy studies in both cross-section and plan-view geometry. As a result, the columnar defect density, size and shape were obtained. The irradiation produces parallel heavily damaged tracks whose diameter is between 3 and 6nm. The overlapping of some columnar defects was highlighted and had to be taken into account to understand the superconducting properties of irradiated samples. Th...

Zn doping of YBa 2Cu 3O 7 in melt textured materials: peak effect and high trapped fields

The previously introduced modified melt crystallization process (MMCP) has been applied to prepare single grain YBCO bulk material with Zn partially substituting for Cu. Hole doping was controlled by an appropriate oxidizing treatment of the as-grown bulk. A field induced pinning was indicated by a well pronounced peak of the critical current density j c in the j c vs. H relationship for the maximal oxidized (overdoped) material containing Zn, whereas pure overdoped YBCO shows no peak effect. The peak effect for Zn-doped YBCO appearing for T=77 K at a relatively high field of about 3 T can be attributed to pair breaking by locally induced magnetic moments due to in plane Zn for Cu substitution. Besides high quality of the bulk YBCO, the peak effect is the reason for the trapped field as large as 1.12 T at 77 K in the cylinder of only 25 mm in diameter.

Defects and self-organized vortices in melt-textured YBa2Cu3O7-x

This paper presents the structural as well as the magnetic properties of high-quality YBa2Cu3O7-x melt-textured material, before and after 2 GeV Au ion irradiation. The irradiation affects a surface layer of about 10% of the whole sample thickness. The core of the paper consists in the investigation of the behaviour of the critical current densities Jc and the pinning forces Fp as a function of the magnetic field. The pinning forces are analysed using a modified Dew-Hughes model. Particular emphasis is given to the influence of different kinds of defects, evaluated at 65 and 75 K, before and after irradiation. The low-current regime is also investigated by means of the irreversibility line evaluation. It is shown that at higher temperature the main properties of the irradiated samples are determined by the competition and/or correlation between surface columnar defects and intrinsic defects, distributed inside the full volume. The vortices trapped in the surface layer by the columnar defects seem to contribute to organize the matching phenomena between the defect lattice and the vortex lattice, at fields near and above the dose-equivalent field.

Melt processing and microstructural analysis of GdBa2Cu3Oy with varying amounts of Gd2BaCuO5 phase

Bulk samples of GdBa2Cu3Oy (Gd-123) with varying amount of Gd2BaCuO5 (Gd-211) phase are melt processed. The processing conditions and the resulting macro and microstructures were discussed. The Tc of the material is found to be sensitive to the processing atmosphere, and the percentage of 211 in the precursor powder. Pure Gd-123 samples behaved differently as compared to the compositions with excess 211 phase. The amount of porosity and the average size of pores in the samples decreases with the increasing amount of Gd-211 in the precursor powder. The densification of the melt textured material is ascribed to the densification process of the precursors prior to the texturing stage. Irrespective of the Gd-211 content macro-cracks within domains were observed in parallel to the intrinsic micro-cracks. At the melting stage refinement of the Gd-211 particles originating from the decomposition of Gd-123 was observed with the increasing amounts of external Gd-211 in the liquid. The quantitative analysis of the microstructural features, i.e., the residual Gd-211, platelet width, domain sizes of the resulting melt textured materials were presented. Addition of Ag resulted in mild refining of 211 particles.

Using melt-textured YBCO for superconducting electromotors

High-temperature melt-textured material prepared in a batch process with reproducible material parameters and properties can be used successfully to construct electromotors with an increased energy and force density. A production process with high efficiency and moderate costs is the basis for the technical application of bulk melt-textured yttrium-barium-copper-oxide. The increased production rate also demands adequate characterization techniques that have to provide the material parameters needed for the specific application. The most suitable characterization technique has to be chosen for routine characterization. In this article, measurements were performed to simulate magnetic processes in electromotors. Functional models running in liquid nitrogen were constructed.

Melt texturing and thermomechanical processing of (Bi,Pb)-1212 superconductors

The conditions of melt texturing of bulk (Bi,Pb)-1212 were improved by adaption of (i) the chemical composition of the precursor, (ii) the temperature/time schedule and (iii) the method of post-treatment. It is shown that especially an increase of the Ca and Pb content is suitable and well textured materials are obtained. In melt textured materials the best values are T c, χ =94 K and j c, χ =7.8×10 5 A cm −2 (5 K, 0 T). However, thermomechanical processing has failed to induce a pronounced grain alignment in (Bi,Pb)-1212 PIT tapes and the superconducting properties are still dominated by weak intergrain superconducting couplings.

Directional solidification by appropriate chemically active single crystal seed: An alternative way of generating large superconducting 123 single domain

A Dy2O3 single crystal has been used as a seed for the growth of isothermally melt-textured Dy-123 material. The nucleation-controlled step has been observed to be related to the heterogeneous nucleation of 211 particles at the surface of the dysprosium oxide single crystal. The subsequent growth mode seems to be controlled by a high concentration gradient of dysprosium in the liquid phase. This leads to a directional solidification process of the 123 phase. The size of the 211 particles seems to decrease as the distance from the dysprosium oxide single crystal increases.

Melt processing of (Nd, Y)BaCuO and (Sm, Y)BaCuO composites

Solid solutions of and high-temperature superconducting materials were synthesized. The (Nd, Y)BaCuO composite samples melt textured in air atmosphere show high critical current densities of about . A double exchange of both and by (or by , respectively) was observed in these materials. The results indicate vacancies on barium sites to exist additionally as defects in the substituted 123 phase of the melt textured materials. The microstructure reveals (Nd, Y)-211 or (Sm, Y)-211 inclusions with different Nd,Y and Sm,Y contents, respectively.

Pattern formation in isothermally non-seeded melt-textured with 15 wt% addition

Optically polarized light micrographs of isothermally melt-textured compounds (with 15 wt% addition) are presented to show evidence for pattern formation of particles in grains. Different types of pattern were observed and were understood as arising from the crystal growth mechanism of 123 melt-textured materials. In an appendix, the case of patterns in magnetically textured samples is discussed.

Effects of shock-induced defect density on flux pinning in melt-textured YBa2Cu3O7−δ

To introduce a high density of homogeneously distributed defects in YBa2Cu3O7−δ (Y123), melt textured samples were shock-compacted at pressures up to 12.6 GPa (126 Kbar) at orientations favorable to slip along the basal planes.1 Shock compressing melt-textured Y123 results in a nearly uniform detect density which is two to three orders of magnitude higher than in unshocked melt-textured material. However, the intergranular critical current density in bulk samples (Jcb) decreases by two orders of magnitude in the as-shocked state. This decrease in Jcb is attributed to microfractures. However, if the shocked disk is annealed in O2 then ground, sieved, and magnetically aligned, Jc for H τ c-axis (Jcab) is enhanced two to three times over the unshocked value. This indicates that the increase in dislocations density does increase flux-pinning.

Effects of shock-induced defects and subsequent heat treatment on flux pinning in melt-textured YBa 2Cu 3O 7−δ

Shock compression of melt-textured YBa 2Cu 3O 7−δ (Y123) results in a nearly uniform defect density which is two to three orders of magnitude higher than unshocked melt-textured material. The crystallographic alignment of the sample relative to the shock wave is critical because slip is limited to the basal plane in Y123. In the experiments the specimens were cut so that their effective c-axis was about 30° from the shock direction. Specimens were recovered intact from high shock pressures. The superconducting order parameter is decreased in the as-shocked state due to disorder of the crystalline lattice, predominantly of the Cu-O chains. A 450°C anneal in oxygen decreases the random atomic disorder, while retaining a large defect density. The defects remaining after a 450°C anneal enhance the intragranular critical current density by a factor of two to three over the unshocked initial value, depending on temperature and applied magnetic field. This shock process could be scaled to larger sizes using chemical explosives. These results show that shock-induced defects enhance J c's in oxygen-annealed melt-textured Y123. The maximum enhancement is expected to occur for shock pressures of about 5 GPa. Determinations of how the degree of mosaic spread and the size and distribution of Y 2BaCuO 5 affect the distribution of defects remain to be investigated.

Transport properties across high-angle bicrystals of melt-textured YBa/sub 2/Cu/sub 3/O/sub 6+x/

The transport properties of eight individual bicrystals in melt-textured YBa/sub 2/Cu/sub 3/O/sub 6+x/ were investigated in low and high fields at 4 K and 77 K. Bicrystals were cut from samples produced by the melt texture process with 10% and 20% added Y/sub 2/BaCuO/sub 5/. Misorientations between the crystals were determined by pole figure analysis, backscatter channeling patterns and light microscopy. The bicrystals exhibited a wide range of misorientations in both the ab planes and c-axis direction and all boundaries were high angle. Contrary to many reported expectations for melt-textured materials, all of the grain boundaries were weakly coupled. However, the strength of the coupling varied with misorientation angle and a weaker field dependence of J/sub c/ was observed in the samples with lower misorientation angle and lower grain boundary resistivity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A possible replacement of single crystal by melt-textured material in the study of YBa2Cu3O7?y

Improved superconducting properties were observed in melt-textured YBa2Cu3O7-y ceramic material. The inductive current density has a small field dependence up to 5.5 T over a wide temperature range. The field and temperature dependences of the magnetic properties are similar to the behavior found in single crystals. Values of the intragrain critical current density in melt-texture samples have the same order of magnitude as in single crystals. The reduced porosity and improved grain alignment help to probe intrinsic physical properties that depend on crystallographic orientation. Thus experiments similar to those involving single crystals are possible. The modified Bean critical state model applied to single crystals can also be used in these melt-textured samples. Thus, it is possible to use melt-textured samples in some basic research which originally required the use of single crystals.

A √2 × √2 surface reconstruction of (001) cleaved GdBa 2Cu 3O 7 − δ

The atomic structure at the (001) surface of GdBa 2 Cu 3 O 7 − δ obtained by cleavage of melt-textured material is determined with high resolution electron microscopy. It was observed that in a majority of the clean (001) surfaces, the surface layer is a CuO 2 layer neighboring the Gd-vacancy layer of the bulk (bulk-Gd-CuO 2 -vacuum). The preference for this termination at the CuO 2 layer does not depend on δ . For this termination a √2 × √2 surface reconstruction occurs. For the second most occurring interruption: bulk-CuO 1 − δ -BaO-vacuum the surface relaxation is rather small. The surface reconstruction and surface relaxations can explain the absence of a superconducting energy gap in the photoemission spectra.