Melt-textured Growth Process Research Articles

A Practical Processing Method for the Fabrication of NdBCO Seed Crystals and High-Performance Single-Domain GdBCO Bulk Superconductors

NdBCO seed crystals and high-performance GdBCO bulk superconductors were prepared successfully by a modified top-seeded melt-textured growth (TSMG) process and by Gd + 011 top-seeded infiltration growth (TSIG) process. The modified TSMG process used a new composition powder (NCP, 0.9 Nd2O3 + 2.4 BaCuO2 + CuO) instead of the conventional composition powder (CCP, NdBa2Cu3O7-x + 0.4 Nd2BaCuO5), the GdBCO bulk superconductor seeded by the self-made NdBCO seed crystal. In the whole procedure above, only one precursor powder (BaCuO2) needs to be sintered, three kinds of powder and three pellets are required, so time and energy consumption were greatly reduced. The results of thermal analysis and X-ray diffraction (XRD) patterns revealed the reaction mechanism of the modified TSMG process. Superconductivity properties of the GdBCO sample were measured employing a levitation force measurement system and a vibrating-sample magnetometer (VSM); the maximum value of the levitation force of the GdBCO sample was 44.78 N (77 k, 0.5 T); the onset superconductivity transition temperature (Tc) was approximately 93.7 K; the transition width was 6.2 K, and the maximum value of critical current density (Jc) was 1.5 × 105 A∙cm−2 (0 T, 77 K). These results indicate that, by combing the modified TSMG process and Gd + 011 TSIG process, a practical processing method for preparing the NdBCO seed crystals and the high-performance single-domain GdBCO bulk superconductors was developed.

A Low-Cost Fabrication Technique for the Growth of Single-Domain GdBCO Bulk Superconductor from Raw Metal Oxides

A single-domain GdBCO bulk superconductor is usually fabricated using a mixture of pre-reacted precursor powders (including GdBa2Cu3O7−δ and Gd2BaCuO5) by the top-seeded melt texture growth (TSMTG) process. However, the synthesis of pure-phase precursor powder by the solid-state sintering reaction can not only consume time for weeks but also prolong the fabrication process and can lead to a low-efficiency and high-cost process. In this paper, we reported the successful growth of single-domain GdBCO bulk superconductors by the top-seeded metallic oxide melt growth (TS-MOMG) process in which the raw materials of Gd2O3, BaO, and CuO, the three different metallic oxides, are just involved to fabricate the single-domain GdBCO bulk superconductors. After detailed investigations in the morphology, levitation force, trapped magnetic field, and critical temperature of the grown samples with different ratios, it was found that the simplified precursor-phase source can be used to fabricate the single-domain GdBCO bulk superconductors more efficiently, which can combine the crystal growth procedure and greatly economize the preparation cost and the time to the most extent.

Levitation force of melt-textured YBCO superconductors under non-quasi-static situation

The superconducting levitation force of a simple superconductor-magnet system under non-quasi-static situation is investigated experimentally. Two yttrium barium copper oxide (YBCO) samples with different performances are chosen from two small batches of samples prepared by the top-seeded melt-textured growth process. The residual carbon content of the precursor powders of the two batches is different due to different heat treatment processes. During the experimental process for measuring the levitation force, the value of the relative speed between the YBCO sample and the permanent magnet is higher than that in conventional studies. The variation characteristics of the superconducting levitation force are analyzed and a crossing phenomenon in the force–displacement hysteresis curves is observed. The results indicate that the superconducting levitation force is different due to the different residual carbon contents. As residual carbon contents reduce, the crossing phenomenon is more obvious accordingly.

Fabrication and properties of single domain GdBCO superconducting rings by a buffer aided Gd+011 TSIG method

Single domain REBCO bulk superconductors can be fabricated by both the top-seeded melt texture growth process and the top-seeded infiltration and growth (TSIG) process. The buffer pellet technique used in these two methods has been demonstrated to act as an efficient seed and to aid the growth of the REBCO bulk superconductors. In this paper, we combined the Gd+011 TSIG method and buffer pellet technique together to fabricate single domain REBCO superconducting rings without any machining and obvious shrinkage of the final ring, which shows a satisfactory trapped field and very good magnetic shielding at the same time. It is found that (1) a single domain GdBCO ring, with inner diameter of 10 mm and outer diameter of 32 mm, has been fabricated by this method successfully. (2) The trapped field (0.32 T, 77 K) and maximum levitation force (80.1 N, 77 K) of the single domain GdBCO ring are nearly the same as the trapped field (0.33 T, 77 K) and maximum levitation force (80.2 N, 77 K) of the single domain GdBCO bulk with the same diameter and thickness, but the uniformity of trapped field is much better for the single domain GdBCO ring. (3) The single domain GdBCO ring shows very good magnetic shielding property, the magnetic flux density shielded by the superconducting ring is up to 0.40 T at 77 K, which is much higher than that of reported samples. The results indicate that the buffer aided Gd+011 TSIG method can provide an important way to fabricate high quality single domain REBCO superconducting rings directly.

Improvement of MTG Process for Preparation and its property of YBCO Superconductor

<p>To improve the superconductivity of high temperature YBCO superconductor, it was prepared and examined by MTG (melt-textured growth) process with use of SmBaCuO seed crystal for orientation, additives of Ag and 211 phase for current density and flux pinning, and heat treatment for oxygen absorption. SmBaCuO single crystals were prepared by MTG process with oxygen absorption and analyzed by XRD and Laue technique. It was estimated that optimum value of Y<sub>2</sub>BaCuO<sub>5</sub> (211 phase) contents in the textured YBCO superconductor as flux pinning center was about 20 wt.%. Ag contents have no influence on the critical temperature but have large influence on the critical current density of the textured YBCO. In the YBCO matrix in experiment, 10 wt.% of Y<sub>2</sub>BaCuO<sub>5</sub> was added, the final 211 content of YBCO made by MTG process could reach about 20 wt.% which was the optimum value for the critical current density. The Ag contents in textured YBCO and the effects of Ag on the superconductivity were also examined. A 5x5x2 mm<sup>3</sup> single crystal of SmBaCuO grown by the melted-condensed process was used to be the seed crystal in the preparation of YBCO. It was proved that the orientation of YBCO was the same as the orientation of the SmBaCuO seed. The effect of oxygen absorption of the bulk oriented YBCO was studied and the heat treatment of oxygen absorption would be in flowing oxygen, at 400 for about 24 hours. The critical current density of the textured YBCO prepared by the optimum technical conditions was about 7x10<sup>4</sup> A/cm<sup>2</sup> (77K, 0.01T).</p>

Influence of layered precursor pellets on the growth and properties of Y-Ba-Cu-O bulk superconductors by top-seeded melt-textured growth

It is well known that a fine and homogeneous distribution of Y2BaCuO5 (Y211) phase particles in single-grain Y-Ba-Cu-O (YBCO) bulk superconductors is essential for improving field-trapping ability. However, the size and concentration of Y211 phase particles in the fully melt-processed superconducting bulk increase significantly with the distance from the seed, which results in the accumulation of Y211 phase particles and the degradation of superconducting properties. In this paper, we report a new method of fabricating single-grain YBCO using layered precursor pellets. Using the top-seeded melt-textured growth process, single-grain YBCO bulk superconductors of about 22 mm in diameter and 9 mm in thickness were fabricated from layered precursor pellets and standard precursor pellets, respectively. The layered precursor pellets consist of precursor powders with 40 mol% Y211 at the top, 30 mol% Y211 in the middle and 20 mol% Y211 at the bottom of the whole pellets, while standard precursor pellets are prepared from precursor powders with only 40 mol% Y211. The growth morphology, microstructure and magnetic flux properties of the layered samples and standard samples were comparatively studied. The results proved that the layered precursor pellets allow a sufficient growth in the c-growth sector and a more uniform distribution of the Y211 phase in the matrix. The distribution of Y211 phase particles is qualitatively explained by the prevalent trapping/pushing theory. The trapped field at 77 K reaches 0.8 T, nearly 29% higher than the standard sample. The present results are very valuable for further improving the properties of YBCO bulk superconductors.

The fabrication process of a high performance and pure c-axis grown GdBCO bulk superconductor with the TSMT-IG technique

It is very important to choose a suitable seed crystal in the fabrication process of the (RE)BCO bulk superconductors by the top-seeded growth method, because a defective seed would result in the failure of the entire sample. Therefore, for optimizing the seed, a new approach is proposed in this paper for the fabrication process of Gd-Ba-Cu-O (GdBCO) bulk with promising superconducting properties. We developed the top-seeded melt texture and infiltration growth (TSMT-IG) process based on the modified top-seeded infiltration and growth (TSIG) process. In this modified infiltration growth, a mini pellet of thin melting texture (TMT) GdBCO was inserted between the NdBCO seed crystal and the solid-phase precursor pellet, resulting in the successful formation of well purified c-axis-oriented GdBCO crystals with a proper-sized TMT layer. The growth morphology, microstructure, side growth boundary, and superconducting properties of the GdBCO bulk superconductors with three different sizes of TMT pellets were studied in this paper. The results proved that the success rate for the fabrication of single-domain GdBCO bulk superconductors can be improved by the TSMT-IG process, which can also effectively inhibit the random nucleation phenomenon at some extent during the process of single-domain GdBCO bulk superconductor growth. In addition, the application of this new growth technology can help us not only to improve the growth performance of GdBCO samples with a proper size of the TMT mini pellet but also reduce the fabrication cost due to the use of only one precursor powder.

Improvement of Gd123 superconductor bulks with the additions of BaFe12O19

The flux pinning performance of the superconductors is important for the application of the Gd123 bulk superconductors. The study shows that to introduce the secondary phases into the Gd123 bulk matrix can enhance the flux pinning performance. In this article, by using top-seeding melt texture growth process method, single domain GdBa2Cu3O7-δ superconductor bulks doping with the different amounts of BaFe12O19 (0.0mol% to 0.8mol%) were successfully achieved. The property and micro-structure have also been investigated. The result shows that there is an obvious improvement on JC with 0.2mol% BaFe12O19 addition. The fine distribution and smaller size of Gd211 particles appear in the micro-structure which may result in the enhancement of JC. At the same time, BaFe12O19 may also form an effective pinning center to increase JC.

Performance enhancement of Gd–Ba–Cu–O high temperature superconducting bulks by BaHfO3 as pinning centers

Since the potential widespread applications of RE–Ba–Cu–O superconducting bulks, it is crucial to enhance their superconducting performances. Binary oxide HfO2 nanoparticles were added into Gd–Ba–Cu–O superconducting matrix before the melt textured growth process. It was found that the HfO2 particles reacted with surrounding matrix and consequently formed BaHfO3 inclusions maintaining nanoscale without significant ripening during the melt growth process. The superconducting transition temperature TC is almost not affected by HfO2 addition, and interestingly, the TC of 0.8mol% HfO2 doped sample increases slightly by about 0.1K in comparison with the undoped one. Meanwhile, the critical current density JC was improved over the whole magnetic field range. These BaHfO3 particles discovered near the liquid phase (Ba–Cu–O, Cu–O, etc.) should be the origin for the improvement of flux trapping capability and critical current density in this system.

Effect of BaFe12O19 nanoparticles doped on the properties of single domain GdBa2Cu3O7-δ high-temperature superconductors

The flux pinning performance of the superconductor is important for the applications of the GdBa2Cu3O7-δ superconductor bulk. To introduce the suitable secondary phase into the GdBa2Cu3O7-δ matrix is an important way to enhance the performance of flux pinning. By using top-seeded melt texture growth process, single domain GdBa2Cu3O7-δ superconductor bulks (20 mm in diameter) doping with different quantities of BaFe12O19 nano-particles (12O19(x=0, 0.2 mol%, 0.4 mol%, 0.8 mol%)+ 10 wt% Ag2O+ 0.5 wt% Pt. The effects of different quantities of BaFe12O19 nano-particles on superconducting properties and microstructure are also investigated. The result shows that the critical current density, JC, with 0.2 mol% BaFe12O19 additions reaches a maximum value in the zero field, which is about 5.5 × 104 A/cm2. And the critical current density JC, almost increases in the whole field compared with those of the undoped bulks. The microstructure and chemical composition of GdBa2Cu3O7-δ bulk with BaFe12O19 nano-particles are implemented by the SEM-EDS technique. It is found that BaFe12O19 nano-particles keeps a similar form to that of the precursor in the final superconductor bulk. The average size of Gd2BaCuO5 particles is reduced from 1.4 μm in the undoped bulk to 0.79 μm in the bulk with 0.2 mol% BaFe12O19 nano-particles. We suggest that BaFe12O19 nano-particles may form effective magnetic flux centers in the bulks, which affects the homogeneous distribution and refinement of Gd2BaCuO5 particles. Therefore, the improvements in the critical current density and the trapped field are observed in the GdBa2Cu3O7-δ bulk with low-level doped content. The superconducting transition temperature TC, can be maintained at around 92.5 K. However, with the addition of 0.4 mol% BaFe12O19 nano-particles, the critical current density and superconducting transition temperature decrease obviously. It indicates that the excessive addition of BaFe12O19 nano-particles may affect the superconductivity properties to reduce the critical current density, JC. The result indicates that the low-level content BaFe12O19 nano-particles can be an effective second phase for the improvement of the GdBa2Cu3O7-δ superconductor bulks, which is very important for the further enhancing the superconducting properties of GdBa2Cu3O7-δ bulks by introducing the flux pinning of nano-particles.

Growth anisotropy effect of bulk high temperature superconductors on the levitation performance in the applied magnetic field

Growth anisotropies of bulk high temperature superconductors (HTSCs) fabricated by a top-seeded melt texture growth process, that is, different pinning effect in the growth sectors (GSs) and growth sector boundaries (GSBs), possess effect on the macro flux trapping and levitation performance of bulk HTSCs. Previous work (Physics Procedia, 36 (2012) 1043) has found that the bulk HTSC array with aligned GSB pattern (AGSBP) exhibits better capability for levitation and suppression of levitation force decay above a permanent magnet guideway (PMG) compared with misaligned GSB pattern (MGSBP). In this paper, we further examine this growth anisotropy effect on the maglev performance of a double-layer bulk HTSC. In contrast to reported trapped flux cases (Supercond. Sci. Technol. 19 (2006) S466), the two superposed bulk HTSCs with same AGSBP with PMG are found to show better maglev performance. These series of results are helpful and support a new way for the performance optimization of present HTS maglev systems.

Progress in Processing of Thin Wall HTc Bulk Superconducting Cryomagnets

We report on our progress in the preparation of single domain of high temperature superconducting (HTS) bulk YBCO and GdBCO cryomagnets with multiple holes. Three mains steps have been explored and discussed in this study: (i) Gd2BaCuO5 (Gd211) and Y2BaCuO5 (Y211) and thin wall preforms shaping by the extrusion technique. (ii) GdBa2Cu3O3 (Gd123) YBa2Cu3O3 (Y123) single domain obtained from Y211 using the Seed Infiltration Growth (SIG) process. (iii) Annealing under high oxygenation pressure in order to improve the superconducting properties. The thin wall samples with larger specific areas offer some advantages as better thermal exchange, shorter oxygen diffusion paths, possibility to reinforce the material to overcome the mechanically stresses during magnetization. The process and the microstructural features at various stages of processing, with particular emphasis on the characteristics of Gd211 and Y211 inclusions are discussed. Properties like the magnetic trapped field at different temperatures, the superconducting transition temperatures, and critical current densities of the resulting composites are presented and compared with their values in samples processed by other variants of the melt textured growth process.

Microstructure and superconducting properties in air-properties GdBa2Cu3O7-δ superconductor with the additives of nano particles

It is regarded as an effective method to improve the flux pinning performance by the additives of the secondary phase inclusions in nano sizes into high temperature superconductor bulks. We prepared the single domain superconductor GdBa2Gi3O7-δ bulks with variable additions of (ZrO2+SnO2+ZnO) nano-particles in air by using top seed melt-textured growth process. The effect of nano-particle additions on superconductivity properties has been investigated. An enhancement of the critical current JC in low and intermediate field at 77K and trapped field was discovered by the additions of the nano-particles. At the same time, the superconductor transition temperature, TC, slightly decreases from 93.5K to 91.5K The microstructure measurements show that the nano-particle inclusions enhance with the increase of the content of nano-particles, which may illuminate the JC enhancement of the specimens.

Enhancement of flux pinning in GdBa2Cu3O7−y bulks prepared by nanoparticle-powder-assisted method

High quality GdBa2Cu3O7−y (Gd123) textured bulks with Nd2BaCuO5 (Nd211) nanoparticle precipitations have been fabricated by a nanoparticle-powders-assisted MTG (melt-textured-growth) technique. The high density nanoscale flux pinning sites were introduced into Gd123 by mixing Nd211 nanoparticle powders (about 20–50 nm) with Gd123 nano-precursors before the MTG process. Microstructural analyses reveal that a large number of Nd211 nanoparticles with a size around 50–150 nm were inserted in the Gd123 matrix, forming a kind of superconducting nanocomposites. The critical current density at 77 K is systematically increased and the flux pinning behavior is significantly improved. The scaling behavior of the flux pinning force shows a magnetic field dependent feature with a peak located at hp≈0.4. This may be the fingerprint of melt-textured 123 compounds, which cannot be interpreted by the simple superposition of different types of elementary pinning sources.

Enhanced JC in air-processed GdBa2Cu3O7−δ superconductor bulk grown by the additions of nano-particles

Single domain superconductor GdBa2Cu3O7−δ bulks with variable additions of (ZnO+ZrO2+SnO2) nano-particles was prepared in air by using top seed melt-textured growth process. The effect of nano-particle additions on superconductivity properties has been investigated. An enhancement of the critical current JC in low and intermediate field at 77K and trapped field was discovered by the additions of the nano-particles. At the same time, the superconductor transition temperature, TC, slightly decreases from 93.5K to 91.5K. The experimental results show that the accumulation of the nano-particle inclusions may improve JC of the specimens.

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.

Interaction mediated by size differences between Y 2BaCuO 5 and CeO 2 particles in melt-textured YBCO superconductors

The Refining of Y 2BaCuO 5 (Y211) is crucial in the fabrication of YBa 2Cu 3O x (Y123) bulk superconductors with a high critical current density ( J c ) for power engineering applications. In this work, the influence of an addition of Y211 and CeO 2 of different sizes on the Y211 refinement and superconducting properties of melt-textured Y123 superconductors is investigated. Nano-size Y211 and nano-size CeO 2 were prepared by attrition-milling (250 rpm and 2 h). Using mixed powders consisting of 85 wt.% Y123, 15 wt.% Y211, and 1 wt.% CeO 2, Y123 quasi-single crystals were fabricated via a top seed melt-textured growth process. The interaction between Y211 and CeO 2 of different particle sizes was examined in terms of the size, uniformity and density of the Y211 particles that existed in the melt-textured samples. The reaction between the nano-size Y211 and the nano-size CeO 2 was more violent due to the more frequent collisions compared to the other reactions (micro-size Y211/nano-size CeO 2 or micro-size Y211/micro-size CeO 2), resulting in the most effective for refinement of Y211 particles and the best J c value of 4.2 × 10 4 (A/cm 2) (at 77 K and a zero-field).

Enhanced performance in bulk superconductor GdBa2Cu3O7- with additions of -Fe2O3 particles

By using a top seeding melt-textured growth process, single domain GdBa2Cu3O7−δ superconductor bulks with variable additions of oxide particles α-Fe2O3 were prepared in air. The effect of the α-Fe2O3 additions on the superconductivity properties was investigated. An increase of the critical current JC in low and moderate fields at 77 K was observed by adding tiny amounts of the α-Fe2O3 particles. The superconductor properties were not suppressed after the addition of the particles, with the superconducting transition temperature around 93.5 K. The present report showed that the α-Fe2O3 oxide particles may be embedded in the GdBa2Cu3O7−δ matrix to form the pinning center to enhance the critical current and the trapping field. With increasing α-Fe2O3 particles, however, the superconductor transition temperature shifts to 93 K and the critical current decreases.

Enhanced Jc in air-processed GdBa2Cu3O7−δ superconductor bulk grown by the additions of two Nd2BaCuO5 seeds

By using top seeding melt-textured growth process, single domain GdBa2Cu3O7−δ bulk superconductor has been fabricated successfully in growth with the additions of two Nd2BaCuO5 (Nd123) seeds. Both microstructure and superconducting properties were investigated from the seed position to the boundary of the sample. The experiments showed the enhanced superconducting properties such as the critical current density (Jc) and the irreversibility (Birr) in the edge of the domain and the part between two seeds. The additions of two Nd123 seeds accumulate the richer Gd2BaCuO5 inclusion particles and improve the Jc at the edge of single domain, which is confirmed by scanning electron microscopy.

Enhanced Growth Rate of a YBCO Bulk in the Melt-Textured Process Under 1 atm Oxygen Pressure

By the employment of 1 atm oxygen pressure as the growth atmosphere, we report a success in the enhanced growth rate of the melt-textured (MT) YBCO bulk. A well-oriented c -axis single-domain bulk with a size of 42 mm in diameter was fully grown at a high average growth rate of about 0.32 mm/h, which is 1.5 times larger than that in air. It was found that the increased oxygen partial pressure has a profound influence on the MT growth (MTG) for YBCO. The enhanced growth rate can be interpreted by both increased solubility and diffusion coefficient of yttrium in the Ba-Cu-O liquid under high oxygen partial pressure. After oxygenation, single-domain samples were determined to have a sharp superconducting transition temperature at 91.7 K, indicating that the superconductivity of crystals grown under high oxygen pressure does not degrade. Additionally, due to the rapid growth, the trapping effect of Y211 particles becomes predominant. As a result, a relatively more homogeneous distribution of Y211 particles was observed in the Y123 matrix. Finally, the most important advantage of this technique is that the enhanced growth rate of Y123 gives less opportunity from self-nucleation in the liquid, which takes place beyond the growth window and the embryonic period. Thus, this oxygen-increased MTG process is very promising for producing large-sized YBCO bulks with good superconducting properties.