Melt-textured Growth Method Research Articles

Microstructural Nonuniformity of GdBCO Superconductor Bulk

High-temperature superconducting materials REBa2Cu3O7-δ(REBCO or RE123, RE is a rare earth), can widely be used in electric motors, nuclear magnetic resonance and other fields, because these can trap strong magnetic fields. The top-seeded melt-texture growth method is widely chosen in the preparation and performance research of various REBCO superconductor bulks, which can effectively suppress random nucleation and reduce weak connections between grains, etc. However, the superconductor bulks prepared by this method often have the problem of uniformed microstructure. A single-domain GdBCO superconductor bulk with NdBCO single-seed guidance and Y123 liquid source was successfully growth using the top-seeded melt-texture growth method, and the microscopic morphology of the samples were studied, which exhibits the microstructural nonuniformity of GdBCO superconductor bulk. The specimens at different positions, C1, C2 below the seed, P1, P2 below the cross pattern, of the bulk exhibit critical transition temperature, TC , higher than 94.5 K, with a maximum value of 95.9 K, demonstrating superior superconductivity. Cracks and pores are unevenly distributed in GdBCO superconductor bulk, and the pores mainly gather in the area below the seed and far from the upper surface. At the same distance from the upper surface, specimens below the cross pattern have higher self-field JC values (P1>C1, P2>C2) than specimens below the seed, which should be related to the nonuniformity of the pores.

The Performance of the Two-Seeded GdBCO Superconductor Bulk with the Buffer by the Modified TSMG Method.

The multiseeding technique is a method to grow large-sized REBa2Cu3O7-δ (REBCO, where RE is a rare earth element) high temperature superconducting bulks. However, due to the existence of grain boundaries between seed crystals, the superconducting properties of bulks are not always better than those of single grain bulks. In order to improve the superconducting properties caused by grain boundaries, we introduced buffer layers with a diameter of 6 mm in the growth of GdBCO bulks. Using the modified top-seeded melt texture growth method (TSMG), that is, YBa2Cu3O7-δ (Y123) as the liquid phase source, two GdBCO superconducting bulks with buffer layers with a diameter of 25 mm and a thickness of 12 mm were successfully prepared. The seed crystal arrangement of two GdBCO bulks with a distance of 12 mm were (100/100) and (110/110), respectively. The trapped field of the GdBCO superconductor bulks exhibited two peaks. The maximum peaks of superconductor bulk SA (100/100) were 0.30 T and 0.23 T, and the maximum peaks of superconductor bulk SB (110/110) were 0.35 T and 0.29 T. The critical transition temperature remained between 94 K and 96 K, with superior superconducting properties. The maximum JC, self-field of SA appeared in specimen b5, which was 4.5 × 104 A/cm2. Compared with SA, the JC value of SB had obvious advantages in a low magnetic field, medium magnetic field and high magnetic field. The maximum JC, self-field value appeared in specimen b2, which was 4.65 × 104 A/cm2. At the same time, it showed an obvious second peak effect, which was attributed to Gd/Ba substitution. Liquid phase source Y123 increased the concentration of the Gd solute dissolved from Gd211 particles, reduced the size of Gd211 particles and optimized JC. For SA and SB under the joint action of the buffer and the Y123 liquid source, except for the contribution of Gd211 particles to be the magnetic flux pinning center with the improvement of JC, the pores also played a positive role in improving the local JC. More residual melts and impurity phases were observed in SA than in SB, which had a negative impact on the superconducting properties. Thus, SB exhibited a better trapped field and JC.

Flux Pinning Properties of Single-Grain Bulk GdBCO Superconductors Processed by Different Thicknesses of Y123 Liquid Source.

The performance of critical current density of GdBa2Cu3O7−δ (GdBCO or Gd123) superconductor bulk has an important influence on its practical applications. In this work, four single-domain GdBCO superconductor bulks were successfully processed by the modified top-seeded melt-texture growth method. The addition of a YBa2Cu3O7−δ (Y123) liquid source with different thicknesses, 0 mm (S0), 3 mm (S3), 5 mm (S5), 7 mm (S7), was introduced to study the influence on the superconducting properties. GdBCO bulk with the addition of the Y123 liquid source with a 3-mm thickness shows the best superconducting properties. The addition of the Y123 liquid source results in a decrease in the Gd3+ ion concentration required for Gd123 growth; thus, Gd2BaCuO5 (Gd211) particles in the liquid source need a larger self-decomposition to diffuse Gd3+ ions to Gd123 growth front, which refines the size and leads to a homogenous distribution of the Gd211 particles in the bulks. Thus, the more pinning centers of fined Gd211 particles improve the superconducting properties of GdBCO bulk. With increases in the thickness of Y123 liquid source to 5 mm and 7 mm, high RE3+ (Gd3+ and Y3+) concentration can coarsen Gd211 particles and fuse with Gd211 liquid source. The superconducting properties apparently drop. Therefore, the addition of a Y123 liquid source with a suitable thickness is a positive modification to obtain high-performance GdBCO bulk.

Attraction and repulsion forces in melt-textured and sintered YBCO-superconductors: a comparative study

In this paper, we measure the attraction (suspension) and repulsion (levitation) forces produced by the interaction between a permanent magnet and different bulk superconductors. The measurements of the interaction force HTS-PM were carried out with a relatively simple technique developed by us, which is reproducible, reliable and low cost. The obtained results were analyzed with the Bean’s critical-state model assuming a uniform magnetic field applied to the superconductor. Two superconducting samples of YBa2Cu3O7-δ (YBCO) prepared by solid-state reaction method and by the melt-textured growth method (MTG) were used. Both samples presented a different hysteresis behaviour in the field cooling (FC) and zero field cooling (ZFC) regimes. Levitation and suspension phenomena were observed in the MTG sample; however, the sintered sample (S) with Oxygen deficiencies (δ > 0.15) displayed a slight levitation force but did not show a suspension force, the latter one attributed to a more efficient magnetic flux pinning. Additionally, the critical current density of both samples was determined from the maximum gap of the force (ΔF) in the FC regime. The obtained values were between 43.00 A/cm2 and 2,758 A/cm2 for the sintered and MTG samples, respectively. These values show a remarkable difference between sintered and MTG samples like that observed from magnetization measurements, which indicate that attraction and repulsion force measurements could provide a rapid and reliable characterization method of polycrystalline superconducting samples.

LEVITATION AND SUSPENSION FORCES MEASUREMENT SYSTEM FOR HIGH TC SUPERCONDUCTORS

The interaction forces, attraction (suspension) and repulsion (levitation) between a permanent magnet and diferent bulk Yttrium-based superconductors were carried out with an automatic system developed in our group, which is reproducible, reliable and low cost. Two superconducting samples of YBa2Cu3O7-δ (YBCO) prepared by solid-state reaction method (S) and by the melt-textured growth method (MTG) were used. Both samples were characterized by XRD technique and presented the characteristic peaks and intensity relations of the YBCO-123 superconducting phase. Oxygen deficiencies δ > 0,15 and δ < 0,10 for the S and MTG samples respectively, were observed. Both samples displayed diferent hysteresis behavior in the Force (F) vs. Distance (z) measurements when they were field cooled (FC) and zero field cooled (ZFC). This behavior agrees with hysteresis loops observed in magnetization measurements. Both levitation and suspension phenomena were observed in the MTG sample with large hysteresis loops of the force. On the contrary, the S sample with small hysteresis loops did not show a suspension force, only displayed a slight levitation force.

制备大尺寸单畴YBCO超导块材的新方法

The top seed melt-textured growth method (TSMTG) is a most effective method for the preparation of large size single domain YBCO bulk superconductors, but suffers from sample shrinkage and serious sample deformation problems during the TSMTG process; the top-seeded infiltration growth method (TSIG) was invented to overcome these problems. However, three different powders, comprising Y2BaCuO5 (Y211), YBa2Cu3O y (Y123), and BaCuO2, must be prepared before fabrication of single domain YBCO bulk superconductors by the traditional TSIG method. This process is complicated, time-consuming, energy-consuming, and leads to atmospheric pollution, and it is also impossible to control the density and the distribution of the Y211 particles in the samples. To solve these important technical problems, we have invented a new TSIG method whereby a mixture of Y2O3 + 1.2 BaCuO2 powder is used as a new solid phase to replace the traditional solid-phase Y211, and a mixture of Y2O3, BaCuO2 and CuO powder is used as a new liquid phase rather than the traditional (Y123 + 3 BaCuO2 + 2 CuO) phase. Using this new solid phase and liquid phase, single domain YBCO bulk superconductors with diameters of 59 mm and 93 mm have been successfully prepared by the proposed TSIG process. We found that: (1) the crystal growth rate of the single domain YBCO bulks prepared by the new TSIG process is approximately 1.7 times higher than that of similar samples prepared by the traditional TSIG method; (2) the levitation force of single domain YBCO bulks prepared by the new TSIG process is 1.65 times higher than that of samples prepared by the traditional TSIG method; (3) the Y211 particles prepared by the new TSIG process are much smaller in size and have a more reasonable distribution than in the samples prepared by the traditional TSIG method, and this can effectively improve the flux pinning force of the sample. These results indicate that the new TSIG process that was developed in our laboratory is a very important and highly practical method for the fabrication of low-cost, large size, high-quality single domain rare earth BCO (REBCO)-type bulk superconductors.

Effect of Sn-doping on the Superconducting Properties of HoBa2Cu3O y , Obtained by the MTG Method

Samples with nominal compositions HoBa2Cu3Oy and (Ho0.97Sn0.03)Ba2Cu3Oy are obtained by the melt-texture-growth (MTG) method and their superconducting properties are investigated. It is shown that the Sn-doping increases the irreversibility field and the upper critical field of the pristine compound. Both the undoped and Sn-doped samples undergo a vortex glass-vortex liquid transition. The observed phenomena are discussed.

Remarkable peak effect in Jc( H, T) of Y–Ba–Cu–O bulk by using infiltration growth (IG) method

This study presented the superior superconductivity of Y–Ba–Cu–O (YBCO) bulk materials grown by using modified infiltration growth (IG) method with a small Sm–Ba–Cu–O crystal as seed. Compare to the bulks grown by melt-textured growth (MTG) method, IG bulk is denser and with finer second phase (Y 2Ba 1Cu 1O 5, Y211). In addition, remarkable peak effect was observed of the IG YBCO bulk material. Peak effect in J c( H, T) is an distinctive property of RE–Ba–Cu–O (RE = rare earth element, e.g. Sm, Nd) superconductors. However, it is found that peak effect is unusual in Y–Ba–Cu–O material. In this study, magnetism measurements also show peak effect in J c( H, T), especially at low temperatures. At 50 K, the peak position of J c( H, T) was about 5 Tesla. J c value of peak, J c(peak, 50 K), is comparable to that of self-field, J c(0, 50 K). On the other hand, it is noted that this effect is spatial-dependent, i.e. it only can be found in particular part of bulk. To interpret the mechanism of peak effect and the spatial-dependence, different microstructure analysis, which includes scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray absorption spectrum (XAS) were used. The results indicated that the composition is inhomogeneous throughout the bulk. Diffusion of Sm from seed to YBCO bulk was observed, which is believed to occur during high temperature, and thus changed the superconductivity. The inhomogeneity of microstructure was correlated to the spatial-dependent of peak effect. Finally, this study also demonstrates a possible way to introduce peak effect in YBCO materials and thus enhance the applications in high field regions.

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.

Growth and investigation of melt-textured SmBCO in air for preparation of Sm123 seedcrystals

For an efficient preparation of SmBCO seeds in high quantity and quality weinvestigated two different ways to manufacture Sm123 grains. Sm123 bulks weregrown by the top-seeded melt-textured growth method. Using MgO seedssingle-grained Sm123 samples with a diameter of 18 mm and a thickness of 10 mmwere obtained. Disc-shaped, large-sized SmBCO grains were prepared along thea–b cleavage plane of the bulks. Subsequently, the material was carefully separated into a large quantity ofabout 2 × 2 × 2 mm3 sized seeds with well-known crystallographic orientation. The results imply the feasibility ofa production of larger SmBCO bulks in a batch process. In a second approach, Sm123 barswere melt-processed by a modified Bridgman method in a vertical furnace. Long barsamples were prepared with typical sizes of 7 mm in diameter and 140 mm in length. Thesintered bars were pulled up across a hot zone of a maximum temperature of1090 °C and a longitudinalthermal gradient of 15 K cm−1. For the translation of the samples different constant pulling rates from 0.3 to5.5 mm h−1 were tested. The microstructure of the samples was investigated by means of polarizedoptical microscopy.

Synthesis and transport properties of oxygen controlled melt growth textured(MixedRE)Ba2Cu3Oy (RE = Nd,Sm, Gd, Eu)

((RE)0.33Eu0.33Gd0.33)Ba2Cu3Oy bulksuperconductors (RE = Sm, Nd) were textured by the top seeded melt textured growth method in a reducedoxygen atmosphere, so-called oxygen controlled melt growth (OCMG). Largesingle domains of diameter 16 mm were successfully grown. Their microstructureand their magnetic and transport properties were investigated and compared.The resistive transition was found around 88 K, which is not the optimalTc of 95 K that could be obtained on a small re-oxygenated sample cutout from the bulk. Transport measurements were also performed onthese samples at various temperature from 77 to 88 K. In the case ofNd0.33Eu0.33Gd0.33Ba2Cu3Oy (NEG) composition, the angular dependence of the critical current densities along the(a,b) planes as a function of magnetic field was also measured.Transport critical current densities higher than50 000 A cm−2 arereached at 77 K and up to 2 T, corresponding to the nominal critical currents of 90 A injected through sectionsless than 0.25 mm2 reproducibly. This confirms the high quality of the single domain obtained with a wellcontrolled process, evidencing that mixing of rare earth is a very promising composition fortransport and high-field applications.

Superconducting properties of textured (LRE I0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y (LRE I=Nd, Sm) processed by OCMG

((LRE) 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y bulk superconductor (LRE=Sm, Nd) were textured by the top seeded melt textured growth method in a reduced oxygen atmosphere. Their physical properties and their microstructure were studied. (Sm 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y textured material exhibited a critical temperature of 94 K and critical current densities at 77 K of 50 000 A cm −2 under self field and 20 000 A cm −2 under 2 T. In the case of (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y , an optimal critical temperature of 95 K is reached. Moreover, high critical current density under self field of 60 000 A cm −2 is obtained. This last result has been confirmed by transport measurements. Indeed, critical current densities at 77 K higher than 50 000 A cm −2 have been measured when the applied field was below 2 T.

Field sweeping rate dependent tail of magnetization curve

SmBa 1.9Cu 3O y samples were fabricated by melt-textured growth method. The tail of magnetization hysteresis loop (magnetization curve) was observed. The effect of the magnetic field sweeping rate on the tail was studied. It is found that the faster the sweeping rate the sharper the tail. The experimental results could not be confirmed by numerical simulation, which was based on the model of flux creep with the surface barrier and lower critical field.

Properties of the (Sm0.33Eu0.33Gd0.33)Ba2Cu3Oy superconductor prepared by different processes in air

Bars and pellets of the (Sm0.33Eu0.33Gd0.33)Ba2Cu3Ox superconductor were processed in air, using the floating zone method and the top-seeded melt-textured growth method, respectively. The samples were prepared using different experimental conditions, i.e. maximal processing temperature, translation rate or cooling rate. Their physical properties and their microstructure were studied. All the samples exhibit a satisfying superconducting transition whereas the critical current density greatly depends on the processing parameters. The sample prepared by the floating zone method at 1070 °C with a translation rate of 2 mm h−1 exhibits a very high Jc value of 70000 A cm−2 in the self-field and more than 30000 A cm−2 at 1.7 T. The pellet processed at 1080 °C with a cooling rate of 2 °C h−1 has a high Jc reaching about 56000 A cm−2 in the self-field and more than 32000 A cm−2 under 1.5 T and still 20000 A cm−2 at 2.5 T. This rare-earth mixture is promising for the application of this material under a high magnetic field.

Second peak effect in SmBa 2Cu 3O y

Two SmBa 2Cu 3O y samples with Y211 (Sm1 and Sm2) were fabricated by the melt-textured growth method to study the effect of heat treatment and argon atmosphere on the second peak effect (SP). Magnetization–temperature ( M– T) measurement at 10 G field cooling shows that the sample Sm1 has a transition temperature T c=80 K but the Sm2 has higher T c (=90 K) and higher irreversibility fields than Sm1. Magnetization versus field ( M– B curves) at various temperatures T in fields up to 7 T was measured. The SP of the two samples was observed and compared over a range of temperatures. Discussion of flux pinning and the effect of Ar and 211 is made.

Flux pinning energy of YBa 1.9Na 0.1Cu 3O 7− δ

Na-doped YBa 1.9Na 0.1Cu 3O y +40 mol%Y211 (YBNCO) and Na-free YBa 2Cu 3O y (YBCO) samples were fabricated by the melt-textured growth (MTG) method to study the effect of doped Na ion on flux pinning. The effective pinning energy ( U( T, H dc, J)), irreversibility line ( H irr( T)) and critical current density ( j c( H dc)), were determined by measurement of ac susceptibility (acs) as well as hysteresis loop measurements for the samples, where T, H dc and J are temperature, dc magnetic field and current density, respectively. It is found that H irr( T) was shifted to lower temperatures while J c( H dc) and U( T, H dc, J) of YBNCO became smaller than that of YBCO. Hence, the doping Na ion plays a negative rule on enhancing flux pinning in YBCO. The appearance of the second peak in J c( H dc) relation and the enhancement of anisotropy in YBNCO is a further support to the conclusion.

Influence of process on flux pinning for MTG SmBa 2Cu 3O y

Two SmBa 2Cu 3O y samples (Sm1 and Sm3) with different processes were fabricated by the melt-textured growth method to study the effect of heat treatment and argon atmosphere on flux pinning. Magnetic measurement shows that Sm1 is of lower transition temperature, lower irreversibility field and more pronounced second peak effect. But Sm3 has higher T c, higher H irr and less pronounced SP. It was found that the flux pinning of SM1 is stranger than that of SM3 in wide range of temperature. It evidences that the irreversibility field and critical current density are independent of each other. Discussion of flux pinning and the effect of Ar and 211 is made.

Effect of Na doping on flux pinning of YBa 1.9Na 0.1Cu 3O 7− δ

We have prepared Na-doped YBa 2Cu 3O y (YBa 1.9Na 0.1Cu 3O y +40 mol% Y211) (YBNCO) and Na-free YBa 2Cu 3O y (YBCO) samples by the melt-textured growth method to study the effect of doped Na ion on flux pinning. The ac susceptibility curves as well as the hysteresis loops were measured for the samples. Then the effective pinning energy ( U( T, H dc, J)), irreversibility line ( H irr( T)) and critical current density ( j c( H dc)) were determined, where T, H dc and J are temperature, dc magnetic field and current density, respectively. We found that, with Na doping, the H irr( T) line shifted to lower temperatures while the J c( H dc) and U( T, H dc, J) became smaller. Hence Na ions play a negative role in the flux pinning of YBCO. The appearance of the second peak in the J c( H dc) curves and the enhancement of anisotropy in YBNCO further support this finding.

Effective activation energy U( T, H) in Na-doped MTG-YBCO crystals

We have measured the resistivity of Na-doped melt textured growth method MTG-YBCO ( YBa 2−x Na x Cu 3 O y+40 mol% Y211 , x=0.0, 0.1, 0.2) crystals as a function of temperature T and magnetic field H up to 7.5 T. The results show that for the pure MTG-YBCO ( x=0.0) crystal the effective activation energy U follows U∝(1−T/T c ) ln H for H‖ ab and U∝(1− T/ T c) H −0.82 for H‖ c. With certain Na addition, U follows U∝(1− T/ T c) m H − α and there is a crossover from the exponent α≈0.5 at lower field to α≈1 at high field and a value of m≈2 for H‖ ab. While for the configuration H‖ c, we get large values of m≈2.5 and α≈1.75. Our results suggest that the flux pinning behaviors become more complicated with Na addition. The possible reasons responsible for the effect of Na addition on the flux pinning behaviors are discussed.

Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

The unidirectional solidification method was used to prepare well textured YBa 2Cu 3O 7− δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient ( G) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect ( a– b planes are not aligned with the bar axis), high values of self field J c (higher than 20 kA/cm 2) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases ( R>2 mm/h), the measured J c values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of J c is well correlated with the different microstructure.