Melt-textured Growth Research Articles

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.

Introducing Nanosized Pinning Centers Into Bulk Gd–Ba–Cu–O by Infiltration Method

Chemical doping is considered as an economic and effective method to enhance the flux pinning properties of high-temperature superconducting materials. However, it is difficult to realize a large doping amount in textured grown bulk superconductors since the doped particles are prone to accumulate and affect the melt-textured growth of the bulk superconductors. In the present work, we introduced nanosized $\hbox{ZrO}_{2}$ particles into the Gd–Ba–Cu–O samples through the external liquid source by infiltration method. A large doping amount of $\hbox{ZrO}_{2}$ particles up to 20 mol% has been realized in finely grown Gd–Ba–Cu– O samples. Improvement in the trapped field has been found for low-level doped samples. The localized microstructure and distribution of the doped particles have been observed by scanning electron microscopy and discussed in this paper.

Role of microtubule acetylation in T cell activation and downstream function (IRC8P.450)

Abstract Microtubules (MTs) play a number of important roles in antigen-mediated T cell activation and effector functions. Tubulin can undergo a number of post-translational modifications including acetylation of alpha tubulin. This acetylation affects MT growth dynamics and ability of motor proteins to traffic along them. We investigated the role of this acetylation in T cell function by using a mouse bearing deletion of the gene for alpha tubulin acetyltransferase 1 (aTAT1), the main acetyltransferase tubulin. Absence of aTAT1 leads to complete loss of acetylated alpha-tubulin. We found that CD4+ T cells from an aTAT1 deficient mouse showed more proliferation than controls upon stimulation with intermediate levels of antigen. We observed no difference in the level of calcium influx between WT and KO CD4+ T cells upon antigen stimulation. An important function of microtubules in CD8+ T cells is trafficking of cytolytic vesicles. Given that acetylation can affect the movement of motor proteins, we tested whether aTAT1 KO CD8+ and Natural Killer (NK) showed altered ability to kill target cells. These findings indicate that acetylation of alpha-tubulin is important in regulating T cell effector responses.

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 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.

Superconducting Properties of Melt-Textured Growth YBa2Cu3O7−x Materials with Y211 Addition

Effects of Y211 phase addition into YBa2Cu3 O7−x bulk superconductors were investigated in order to provide high-quality grain structure for yttrium barium cop- per oxide (YBCO) bulk material. Various molar amounts of Y211 were added into Y123, and it was expected to affect the formation of the grains and to increase the crit- ical current density (Jc) and onset transition temperature (Tc) of YBCO. The samples were fabricated by using melt- textured growth (MTG) method and characterized in terms of ac magnetic susceptibility and superconducting quantum interference device (SQUID) magnetization measurements. A simple algorithm was used to calculate the critical cur- rent densities of the samples according to Bean's critical state model. According to addition of Y211 phases into pure Y123, bulk samples with different molar amounts showed that the highest critical temperature and critical current den- sity were obtained with a molar amount of n = 0.125 Y211-added sample as 94 K and 126.000 A/cm 2 , respec- tively, under the 1-T applied magnetic field at a temperature of 10 K.

Flux pinning properties of Gd–Ba–Cu–O trapped field magnets grown by a modified top-seeded melt growth

We report a modified top-seeded melt-growth technology for processing high performance Gd–Ba–Cu–O trapped field magnets (TFMs) by providing an additional liquid source during the melt-textured growth. The enriched liquid source allows sufficient growth in the a−b plane with an increased growth rate, and depresses the accumulation of Gd2BaCuO5 (Gd211) particles, which largely suppresses the formation of sub-grain boundaries in the a-growth sectors. An average trapped magnet field of 1.2 T is achieved for the 32 mm Gd–Ba–Cu–O TFMs at 77 K. These Gd–Bd–Cu–O TFMs exhibit superior in-field flux trapping performance, thanks to the significant second peak effect of the critical current density (Jc). The additional liquid source intensifies the Gd–Ba substitution, and therefore increases the in-field Jc, especially at the temperature range of 30–70 K, which is meaningful for practical applications.

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.

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.

YBCO-Buffered NdBCO Film with Higher Thermal Stability in Seeding REBCO Growth and Recycling Failed Bulk YBCO Superconductors

In this work, we report a strengthened super- heating effect caused by a buffering YBa2Cu3Oy (YBCO or Y123) layer in the Nd1+xBa2−xCu3O7−y (NdBCO or Nd123) thin film with MgO substrate (i.e., NdBCO/YBCO/ MgO thin film). In the cold-seeding melt-textured (MT) growth, the NdBCO/YBCO/MgO film presented an even higher superheating level, about 20 °C higher than that of nonbuffered NdBCO film (i.e., NdBCO/MgO film). Pole figure (X-ray Φ-scan) measurement reveals that the Nd- BCO/YBCO/MgO film has better in-plane alignment than the NdBCO/MgO film, which may be the main origin of the enhanced thermal stability. Using this NdBCO/YBCO/MgO film as the seed and under a maximum processing temper- ature (Tmax) up to 1120 °C, we succeeded in growing vari- ous RE1+xBa2−xCu3O7−y (REBCO, RE = Gd, Sm, Nd, RE refers to rare elements) bulk superconductors and recycling the failed REBCO (RE = Y) bulks in a simple and feasible 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.

YBCO-buffered NdBCO film with higher thermal stability in seeding REBCO growth

In this work, for the first time, we report a strengthened superheating effect caused by a buffering YBa2Cu3Oy (Y123 or YBCO) layer in a Nd1+xBa2−xCu3O7−y (Nd123 or NdBCO) thin film with a MgO substrate (i.e., NdBCO/YBCO/MgO thin film). In cold-seeding melt-textured (MT) growth the NdBCO/YBCO/MgO film presented an even higher superheating level, about 20 °C higher than that of non-buffered NdBCO film (i.e., NdBCO/MgO film). Using this NdBCO/YBCO/MgO film as the seed and under a maximum processing temperature (Tmax) of up to 1120 °C, we succeeded in growing various RE1+xBa2−xCu3O7−y (REBCO, RE = Gd, Sm, Nd, RE refers to rare elements) bulk superconductors. Pole figure (x-ray Φ-scan) measurement reveals that the NdBCO/YBCO/MgO film has better in-plane alignment than NdBCO/MgO film, indicating that the induced intermediate layer improves the crystallinity of the NdBCO film, which could be the main origin of the enhanced thermal stability. In short, possessing higher thermal stability and enduring a higher Tmax in the MT process, the NdBCO/YBCO/MgO film is beneficial to the growth of bulk superconductors in two aspects: (1) broad application for REBCO materials of high peritectic temperature (Tp); (2) effective suppression against heterogeneous nucleation, which is of great assistance in growing large and high-performance REBCO crystals.

A Tethering Mechanism Controls the Processivity and Kinetochore-Microtubule Plus-End Enrichment of the Kinesin-8 Kif18A

Metaphase chromosome positioning depends on Kif18A, a kinesin-8 that accumulates at and suppresses the dynamics of K-MT plus ends. By engineering Kif18A mutants that suppress MT dynamics but fail to concentrate at K-MT plus ends, we identify a mechanism that allows Kif18A to accumulate at K-MT plus ends to a level required to suppress chromosome movements. Enrichment of Kif18A at K-MT plus ends depends on its C-terminal tail domain, while the ability of Kif18A to suppress MT growth is conferred by the N-terminal motor domain. The Kif18A tail contains a second MT-binding domain that diffuses along the MT lattice, suggesting that it tethers the motor to the MT track. Consistently, the tail enhances Kif18A processivity and is crucial for it to accumulate at K-MT plus ends. The heightened processivity of Kif18A, conferred by its tail domain, thus promotes concentration of Kif18A at K-MT plus ends, where it suppresses their dynamics to control chromosome movements.

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).

Properties of thermally activated flux motion in MTG-GdBCO measured by AC susceptibility

Thermally activated flux motion in melt texture growth Gd-Ba-Cu-O bulk has been studied by ac susceptibility measurements. The irreversibility line Birr(T) and the intrinsic pinning energy U(T, B) of the bulk have been extrapolated from the measurements, which can be written as Birr(T) = BO(1-T/Tc)n where 80T < BO < 98T and 1.4 < n < 1.48, and U(T,B)=U0(1−t)1.5B−0.8 where t is the reduced temperature T/Tc and B is magnetic field, respectively. Also experiments reveal that peak temperature Tp of the out of phase component χ′′ max(T) shifts towards low temperature region with the decreasing of the ac field. All these results suggest that the thermally activated flux motion process might be based on the Bean's critical state model, rather than thermally assisted flux-flow (TAFF) process.

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.