Melt-solidified Bulks Research Articles

Improved Superconducting Properties of YBCO Melt-Solidified Bulks by Addition of Ba–Cu–O–Cl

In order to improve critical current properties for REBCO melt-solidified bulks, dilute addition of Ba 2 Cu 3 O 4 Cl 2 up to 5.0 mol% against starting Y123 powder was attempted in the synthesis of YBCO bulk samples. The addition of Ba 2 Cu 3 O 4 Cl 2 enhanced J c at 77 K and the homogeneity of J c in the bulk. Microstructural observation by TEM revealed that stacking faults parallel to the ab-planes were confirmed in the Y123 matrix of a Ba 2 Cu 3 O 4 Cl 2 -free bulk, whereas they were hardly observed in a 0.5 mol% Ba 2 Cu 3 O 4 Cl 2 -added one. It was suggested that the addition of Ba 2 Cu 3 O 4 Cl 2 can enhance crystallinity of the Y123 superconducting phase and improve homogeneity of YBCO bulks.

Systematic change of flux pinning in (Dy,RE)123 and (Y,RE)123 melt-solidified bulks with unit cell orthorhombicity

RE-mixing is one of the most effective ways to enhance critical current density Jc of RE123 melt-solidified bulks; however, the intrinsic effects of RE-mixing on Jc have not yet been well clarified. Through synthesis and analysis of eight types of melt-solidified bulks containing one or two RE elements, the effects of RE-mixing on Jc-H characteristics were systematically investigated. It was revealed that RE-mixed bulks show higher Jc than RE123 bulks accompanied by the secondary peaks of Jc in a high field region. Analyses on the relationship between Jc and orthorhombicity suggested that the RE substitution level for Ba sites can be controlled by RE-mixing to enhance Jc in magnetic fields. Combined with the results of magnetic measurements at 40 and 60 K up to 15 T, RE-mixed bulks originating from Y123 powders were found to be high potential materials for high field generating applications.

Synthesis of RE123 Melt-Solidified Bulks under Low Pressure Pure Oxygen

Abstract In general, the superconducting properties of REBa2Cu3O y (RE123: RE= rare earth elements) melt-solidified bulks systematically degrade with an increase in distance from the seed crystal. Our previous study indicated that partial substitution of RE for the barium-site causes the deterioration. Although the crystal growth under reducing atmospheres is effective for suppression of the substitution level of RE for the barium-site, a lot of pores formed by remaining inert gas under reducing atmospheres, resulting in poor mechanical properties. We have attempted to develop RE123 bulks by a new method, melt-solidification under low pressure pure oxygen atmosphere, to suppress substitution of RE for barium and to eliminate pores. The Y123 bulks without pores were successfully fabricated by the new method. The Y123 bulks showed apparently higher J c below 2 T at 77 K than the conventional Y123 bulks melt-solidified in air.

New Approaches for Enhancement of $J_{\rm c}$ for RE123 Melt-Solidified Bulks

New two methods to fabricate Y123 melt-solidified bulks with excellent superconducting properties have been developed. One is the new crystal growth method from the seed crystal which was put its c-axis parallel to the top surface and the other is addition of high-temperature annealing process in moderately reducing atmosphere after the crystal growth. The former method increased high-Jc regions and the latter enhanced Jc particularly at positions far from the seed crystals and at low temperatures through suppression of yttrium substitution for barium in the present study. The field trapping property was also improved by introduction of the reductive annealing process prior to the oxygen annealing at low temperatures.

Improved Superconducting Properties of Y123 Melt-solidified Bulks through Controlling Crystal Growth Direction and Cation Stoichiometry

Two new approaches to fabricate Y123 melt-solidified bulks with excellent superconducting properties have been developed. One is a new method of crystal growth from a seed crystal with its c-axis parallel to the top surface, and the other is a high-temperature annealing process under a moderately reducing atmosphere after crystal growth. In the present study, the former method increased the area of high-Jc regions, and the latter enhanced Jc, particularly at positions far from the seed crystal at low temperatures through suppression of yttrium substitution for barium. The field-trapping property was also improved through introduction of a reductive annealing process prior to oxygen annealing at low temperatures.

Optimization of the properties of codoped single-domain Y–Ba–Cu–O bulk superconductors

The intrinsic effects of nanoscopic MnO 2 powders addition combined with Fe cation substitutions for copper sites on the microstructure and superconductive properties of YBa 2Cu 3O 7− δ (Y123) melt-solidified bulks have been investigated. On the one hand, an increase in Y 2BaCuO 5 (Y211) particle pushing, leading to an inhomogeneous bulk microstructure, is caused by increasing MnO 2 content due to increased net interfacial energy, Δ σ 0; and, on the other hand, an addition of MnO 2 powders is effective in enhancing both the δT c- type and δl-type pinning. It also shows that the Fe addition helps to optimize the high magnetic field performance and Y211 particle distribution in textured pellets. Further, this experiment suggests that a combination of the element substitution and the nanoscopic particle is a beneficial way to optimize the microstructure and superconductive properties of single-domain bulk superconductors.

Effect of doping on the superconductive properties of GdBa2Cu3O7−δ single-domain superconductors fabricated in air

Abstract The intrinsic effects of BaCuO 2 , Ga 2 O 3 and nanoscopic Gd 2 BaCuO 5 (Gd211) powders addition on the superconductive properties and microstructures of GdBa 2 Cu 3 O 7− δ (Gd123) melt-solidified bulks fabricated in air have been investigated. The differential scanning calorimetry (DSC) measurements show that addition of Ga 2 O 3 does not change the peritectic temperature of the mixtures. The microstructures of the Ga 2 O 3 -doped samples are similar to those observed in gallium-free samples. It is found that the magnetic critical current density ( J c ) appears to be very sensitive to the change in carrier density near the optimal doping state. Interestingly, only when the impurity content and the oxygenation condition are both suitable then the evident secondary magnetization peak (SMP) appears. Finally, by proper oxygen control, a remarkable increase in the J c at 2 T has been further induced due to an enhanced δ T c pinning. As a result, the high-performance air-processed Gd123 single-domain samples without a post-annealing in Ar have been prepared. Further, this experiment suggests that carrier density and impurity concentration play a crucial role for SMP at 77.3 K.

Field Trapping Properties of $c$-Growth Region in RE123 Melt-Solidified Bulks

We have fabricated Ag-free and Ag-added Dy123 cylindrical melt-solidified bulks with large <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -growth region. Samples cut from <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -growth region of each bulk exhibited high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> up to high magnetic fields and excellent field trapping properties. On the contrary, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and field trapping properties in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -growth region were suppressed by subgrain boundaries. Magnetic flux penetration along the subgrain boundaries was confirmed in magneto-optical images of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -growth region. Both <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and field trapping properties of Dy123 bulks were improved by Ag-addition due to low level Ag substitution for Cu site in the CuO chain.

3-Dimensional Grain Orientation of RE-Ba-Cu-O Superconductors Using a Modulated Oval Magnetic Field

Tri-axial orientation of twin-free powders of RE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15-delta</sub> (RE247, RE = Y, Dy, Ho, Er) was attempted by modulated rotating magnetic fields. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis orientation was almost completely achieved for all the four RE247 compounds, whereas the degrees of in-plane orientation depended on RE element. Especially for Y247, the degrees of in-plane and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis orientation were determined to be ~ 2.5degand < 2deg, respectively, by the phi-scanning and rocking curve measurements. This excellent in-plane orientation is comparable or superior to those for coated conductors ( ~ 4deg) and melt-solidified bulks ( ~ 8deg) of REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-delta</sub> . Magnetic field dependence of Lotgering factors revealed that the degrees of in-plane orientation were sensitive to the applied magnetic field. The required magnetic field for the tri-axial orientation is determined by the in-plane magnetic anisotropy rather than the magnetic anisotropy between in-plane and out-of-plane directions.

Excellent Critical Current Properties of Dilute Sr-Doped Dy123 Melt-Solidified Bulks at Low Temperatures

We have previously reported that dilute impurity doping technique universally enhances the critical current density of RE123 superconductors. These doping effects on the J c properties depend on substituted cation sites. In this paper, we report the flux pinning properties of the dilute Sr- or Co-doped Dy123 melt-solidified bulks in a wide temperature range. It was found that the dilute Sr-doping effectively enhance the J c for Dy123 melt-solidified bulks with low level Dy/Ba substitution. For the dilute Sr-doped Dy123 samples, the temperature coefficient of J c in 30 kOe was found to be approximately two times larger than for undoped materials above 40 K, meaning that it is an effective method for developing high-J c RE123 materials operating at low temperatures.

True effects of microstructure and oxygen contents on flux-pinning properties of Y123 melt-solidified bulks

We have investigated the relationship between microstructure and pinning properties of Y123 melt-solidified bulks having systematically controlled oxygen contents. The subgrain structure was developed along crystal growth direction in the a-growth region, while it was not observed in the c-growth region. The formation of such subgrains could be explained by an elastic property of the edge dislocations and RE123 crystal growth process. Through evaluations on the second peak effects of J c– B curves and irreversibility fields, it was found that Y123/Y211 interfaces do not act as the predominant pinning centers under high fields against the reported theoretical predictions. Furthermore, our results suggested that the precipitation of Y211 particles decreases J c in high fields. The resulting pinning properties of Y123 melt-solidified bulks were discussed in relationship to the subgrain structure.

Chemical (Sr,Co)-doping effect on critical current density for Dy123 melt-solidified bulks

The dilute impurity doping for CuO-chain site was found to largely improve the critical current properties of both Y123 single crystals and Y123 melt-solidified bulks in our previous study. In addition, dilute Sr-doping to Ba site is also effective for enhancement of J c without serious decrease in T c as in the case of dilute Zn doping for CuO 2-plane site. In the present study, we have attempted further enhancement of flux pinning force of the Dy123 melt-solidified bulk, which is essentially more promising materials than Y123, by impurity doping for Cu site in the CuO-chain and for Ba site. Although the (Sr,Co)-co-doped Dy123 melt-solidified bulks showed systematically suppressed T c with their doping levels, high T c's well exceeding 90 K were maintained. All the Sr-doped or Co-doped Dy123 bulks exhibited higher J c than the undoped one. More improved J c properties were achieved by (Sr,Co)-co-doping, suggesting that pinning potential at local regions around doped impurities become deeper by Co-doping, resulting in stronger point-defect-like pinning sites. In addition, more detailed studies on the dilute Sr-doping and the dilute (Sr,Co)-Co-doping were carried out for Y123 single crystals in order to clarify the difference between their doping effects on the J c properties. The vortex transition field, H *, to the disordered state in the M– H loops for the Co-doped Y123 single crystal was located at the lower field than that of the Sr-doped one, meaning that the Co ions strongly affect the vortex system compared with the Sr ions.

Enhanced flux pinning properties of RE123 melt-solidified bulks by dilute impurity doping

In our previous study, dilute impurity doping to various cation sites of the Y123, particularly to the Cu site in the Cu-O chain, was found to dramatically enhance Jc of single crystals and its melt-solidified bulks. In order to clarify the essential role of dilute doping for improved pinning, TEM observation was performed for Co or Ga doped Y123 melt-solidified bulks. Density of twins did not change by the Co and Ga doping. The local compositional analysis by TEM-EDS indicated that the doped Co homogeneously dispersed in the Y123 matrix. These suggested generation of point-defect-like pinning centers by dilute doping for Cu site of the CuO chain. In addition, further enhancement of Jc was attempted for Dy123 melt-solidified bulks by the same method. The Co-doped Dy123 melt-solidified bulks also exhibited improved Jc, particularly for samples grown under higher cooling rate during crystal growth. A high Jc of ~60 kA cm-2 under 10~20 kOe at 77 K was achieved, when a cooling rate of 0.9°Ch-1 was applied. Systematic decrease in Jc with an increase of the cooling rate observed for undoped Dy123 bulks suggested that the high cooling rate suppressed partial Dy substitution for the Ba, resulting in a decrease of effective pinning sites. Therefore, the high Jc observed for the Co-doped Dy123 bulk grown under a high cooling rate can be explained by generation of effective pinning sites around Co ions in the strong superconducting matrix of Dy123 with low substitution level of Dy at the Ba site.

Novel functions of silver on superconducting properties for RE123 melt-solidified bulks

The intrinsic effects of Ag-addition on crystal structure and superconducting properties of Y123 melt-solidified bulks have been investigated. Increased c-axis length and decreases of maximum Tc and carrier density caused by Ag-addition suggested the incorporation of Ag into the Y123 matrix, in which Ag+ was considered to substitute for Cu ion of Cu-O chain. It was also found that Ag-addition enhanced Jc properties all applied fields at 77 K. The distinct second peak of Jc observed for the Ag-added bulk suggested that the Ag-incorporated regions acted as effective pinning centers under disordered vortex lattice.

Effects of Residual Carbon on Superconducting Properties of Y123 Melt-Solidified Bulks

Effects of residual carbon on the carrier-doping level and resulting superconducting properties were systematically investigated for Y123 melt-solidified bulks containing different amounts of carbon. The dependence of Tc on annealing temperature in flowing oxygen was almost identical among each sample from underdoped to optimally doped state and all the samples exhibited highest Tc when they were annealed at ~500degC. On the other hand, the carbon-free samples showed apparently lower Tc than those of the carbon containing samples in the overdoped state. Especially, a sample fabricated in air showed the quite small decrease in Tc by overdoping, suggesting that residual carbon suppresses effective carrier doping to be heavily overdoped state for Y123 melt-solidified bulks. Therefore, Y123 melt solidified bulk with less residual carbon exhibited higher Hirr and Jc properties at the overdoped state.

Effect of low level RE mixing on critical current properties of RE123 melt-solidified bulks

Effects of low level Tb doping on the critical current properties of Dy123 melt-solidified bulks were systematically studied. The Tb-doped bulks showed high J c characteristics compared to the undoped bulks. The microstructural observation, X-ray diffraction and local compositional analyses revealed that fine BaTbO 3 precipitates were dispersed in the Dy123 matrix. In the a-growth region, J c in low fields was largely improved to be ∼10 5 A/cm 2, suggesting that those particles acted as effective pinning centers. A sample cut from just below the seed in the c-growth region of the Tb-doped bulk showed enhanced second peak effect in J c– H curves. This is probably due to the local fluctuation of RE composition induced by Tb doping.

Enhanced flux pinning properties of RE123 crystals by dilute impurity doping for Cu–O chain

In order to improve the critical current properties of Y123 melt-solidified bulks, low level doping of impurities, such as Co, Fe and Ga, to the Cu site in Cu–O chain was attempted. All these doped samples showed high T c above 90 K. Largely enhanced secondary peaks were observed in their magnetization hysteresis loops at 77 K and J c reached approximately 50 kA cm −2 under 10–20 kOe, while undoped sample exhibited poor J c performance without showing the peak effect. This result suggests that introduction of local disorders in the Cu–O chain by dilute impurity doping is more excellent way to improve critical current properties of the RE123 system than direct impurity substitution for Cu in the Cu–O 2 plane.

ＲＥ１２３溶融凝固バルクの高臨界電流特性化に対する微量置換効果

Effects of RE mixing and dilute impurity doping for the CuO chain on the critical current properties for RE123 melt-solidified bulks were investigated. Low level Lu or Tb mixing to the RE site of Y123 or Dy123 melt-solidified bulks showed high Jc characteristics compared to the undoped bulks. Especially in the a-growth region of Tb mixed bulks, Jc in the low magnetic field was largely improved up to ∼ 105 A cm-2 at 77 K. Through microstructural observation, X-ray diffraction and local compositional analysis, the formation of finely dispersed BaTbO3 precipitates was confirmed in the Dy123 matrix, suggesting that these precipitates acted as effective pinning centers. The addition of fine BaTbO3 powder was also effective for enhancement of Jc. Furthermore, Tb4O7 added Dy123 melt-solidified bulks exhibited remarkably improved Jc due to the generation of fine BaTbO3 precipitates with 0.1 ∼ 0.2 μm in size in the Dy123 matrix without decreasing Tc. This result means Tb4O7 is a more effective additive than CeO2, which slightly decreases Tc. On the other hand, effects of dilute impurity doping to the CuO chain of Y123 melt-solidified bulks on their flux pinning properties were also studied. All the Fe-, Co-, and Ga-doped samples maintained high Tc above 90 K and exhibited dramatically improved Jc characteristics accompanying huge second peaks in Jc-H curves. This result suggests that the introduction of local disorders in the CuO chain by dilute impurity doping is more excellent way to improve the critical current properties of the RE123 system than direct impurity substitution for Cu in the CuO2 plane, which always decreases Tc.

Enhanced flux pinning properties of YBa2Cu3Oy by dilute impurity doping for CuO chain

Effects of dilute impurity doping to the CuO chain of Y123 on the flux pinning properties were studied using single crystals and melt-solidified bulks. Cobalt-doped Y123 single crystals and iron-, cobalt- or gallium-doped Y123 melt-solidified bulks exhibited dramatically improved Jc-H characteristics accompanying huge secondary peaks in Jc-H curves. In field Jc of these samples are apparently higher than that of a low level zinc-doped bulk. These results strongly suggest that introduction of local disorders in the CuO chain by dilute impurity doping is a more promising way to improve Jc of RE123 materials than impurity doping to the CuO2 plane.

Improvement of Flux Pinning Properties of RE123 Materials by Chemical Doping

Improvement of flux pinning properties of RE123 single crystals and melt-solidified bulks was attempted by dilute doping of impurity elements to various cation sites except the Cu at the CuO2 plane. All the examined dilute doping for RE123, such as Sr to Ba-site, Lu to Y-site and Co to Cu-site in the CuO-chain, was found to increase critical current density, Jc, particularly under magnetic fields accompanying enhanced second peak effect. These results suggest that the dilute doping is universally effective for improving the critical current properties of the RE123 system independent of the doped cation sites.