Nd123 Matrix Research Articles

Mechanical properties of Nd–Ba–Cu–O bulk superconductors

We investigated the effects of Nd422 and Ag particles on the mechanical properties in Nd–Ba–Cu–O bulk superconductors. Both Nd422 and Ag particles were effective in decreasing the amount of microcracks running along the c direction. In the case of Nd422, however, excessive Nd422 addition enhanced the crack propagation, resulting in the degradation of mechanical strength. In the case of Ag addition, the beneficial effect of its ductile mechanical property was not observed. This was due to a relatively large size of Ag particles and low interfacial strength between Ag and Nd123 matrix. It was remarkable that the Weibull coefficient of the sample with Ag2O addition exceeded 13, which is reliable enough for practical engineering applications.

Correlation between the formation of growth bands and Nd210 addition in Nd/sub 1/Ba/sub 2/Cu/sub 3/O//sub 7-γ/ bulk samples

1 cm diameter single domain NdBaCuO pellets, obtained from powders of Nd/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (Nd123) with Nd/sub 2/BaO/sub 4/ (Nd210) addition, have been successfully prepared by top seeding melt textured growth technique. Distribution of Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ (Nd422) precipitates in the Nd123 matrix has been analyzed by means of polarized light optical microscopy and scanning electron microscopy. Relevant distribution inhomogeneity in the peripheral region of pellets with a high Nd210 addition have been revealed, as well as in regions of dendritic growth and around pores. Increasing difficulties in obtaining a complete growth of the single domain samples, in dependence of increasing Nd210 addition has been observed too. A qualitative growth model for high Nd concentration samples has been proposed to explain both these behaviors.

Study of (Nd,Sm)422 solid solution and its effect on the melt process of Nd123 bulk superconductors

We have studied the range of Nd–Sm solid solution in the (Nd 1− x ,Sm x ) 4−2 y Ba 2+2 y Cu 2− y O z system in the Nd123 matrix. X-ray diffraction studies for the sintered (Nd 1− x ,Sm x ) 4−2 y Ba 2+2 y Cu 2− y O z powders in air showed that the samples were a single phase 422 in the range of x=0–0.3, showing that Sm substituted on Nd site in Nd422. It was also found that Sm is soluble up to x=0.3 in (Nd 1− x ,Sm x ) 3.6Ba 2.4Cu 1.8O z . We then fabricated Nd123/Nd422 bulk superconductors by melt processing, in which Nd123 powder is mixed with 20 mol% (Nd 0.8,Sm 0.2) 3.6Ba 2.4Cu 1.8O z and 1 wt.% CeO 2. The sample exhibited high T c of 97 K and sharp superconducting transition when melt-processed under controlled oxygen partial pressure ( P O 2 ) of 0.01 atm.

Refinement of Nd-422 particles in Nd-Ba-Cu-O bulk superconductor

We have investigated the effect of oxygen partial pressure, the initial composition, and the addition of CeO/sub 2/ and/or Pt on the grain growth of Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ (Nd-422) in the Ba-Cu-O liquid. Logarithmic plots of the mean diameter of Nd-422 under various oxygen levels as a function of the holding time showed the power law behavior with the slope of 1/3, which suggested that the grain growth of Nd-422 could be explained in terms of the Ostwald ripening theory. A reduction of oxygen partial pressure, the shift of the initial composition toward Ba-rich direction, and a combined addition of Pt and CeO/sub 2/ could suppress the coarsening of Nd-422. Furthermore, a combination of these techniques was found to be effective in refining the Nd-422 in the Nd-123 matrix, which led to an enhancement of J/sub c/ values.

Fabrication and microstructure of large grainNd-Ba-Cu-O

Large, single grain Nd-Ba-Cu-O (NdBCO) has been successfullyfabricated using a top-seeded melt growth technique under reduced oxygenpartial pressure. We report a study of the grain growth rate within thetemperature range for which heterogeneous nucleation can be achieved in thissystem and investigate the variation in size, morphology and spatialdistribution of Nd-422 phase inclusions in the Nd-123 matrix with Nd-422 phaseinclusion concentration. The influence of Nd-422 phase particles on the growthrate, pores and macrocrack formation is also studied.

Flux pinning in largeNdBa2Cu3O7−δgrains fabricated by seeded-melt growth

The magnetization of top seeded-melt processed ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (Nd-123) containing 15 mol % ${\mathrm{Nd}}_{4}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10}$ (Nd-422) has been investigated by vibrating sample magnetometry over a wide temperature $(50\mathrm{K}lTl94\mathrm{K})$ and field $(0l{\ensuremath{\mu}}_{0}Hl12\mathrm{T})$ range. The origin of the peak in the M-H loops and high irreversibility field values observed in this system are discussed on the basis of these data. The normalized pinning force determined from the hysteresis width is observed to scale as a function of reduced magnetic field ${(h=H/H}_{\mathrm{irr}})$ and exhibits a peak at ${h}_{\mathrm{max}}=0.33,$ suggesting that normal defects dominate the pinning mechanism in this material. This is in contrast with other recent reports of ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}},$ which suggest that the peak effect is associated with a local variation in superconducting transition temperature due to the formation of a distributed solid-solution phase in the Nd-123 matrix.

Microstructural and elastic behaviour of OCMG-processed NdBaCuO materials

A systematic investigation of an HTSC material viz; Nd 1.8Ba 2.4Cu 3.4O y superconductor prepared by the oxygen controlled melt growth (OCMG) process has been undertaken by studying its microstructure and elastic behaviour. The results are compared with those obtained on samples prepared by melt powder melt growth (MPMG) processed YBa 2Cu 3O y material. The X-ray diffraction studies of the new material clearly indicate the predominant structure of NdBa 2Cu 3O y. The microstructural investigations performed by means of optical and scanning electron microscopy confirm the presence of a second phase formed due to the uniform distribution of Nd 4Ba 2Cu 2O 10 (Nd-422) particles of size 1–12 μm size in the Nd-123 matrix. Further, magnetization measurements also show that the sample exhibits a pronounced secondary peak effect along with a critical current density value of 36 000 A/cm 2 at T=77 K in a field of 1.6 T applied parallel to the c-axis. Finally, the ultrasonic velocity measurements reveal that the elastic anomalies generally observed among most of the RE-123 materials, especially in the temperature region of 100–250 K are totally absent in the case of Nd-123 of the present investigation.

Microstructure and properties of oxygen controlled melt textured NdBaCuO superconductive ceramics

Nd 1+ x Ba 2− x Cu 3O 7− δ (123) bulk superconductors have been synthesised by the oxygen controlled melt growth method. Unlike the YBaCuO system, platinum doping or Nd 4Ba 2Cu 2O 10 (422) rich compositions do not refine the peritectic ‘422’ secondary phase, but the latter improves the microstructural quality. Low oxygen partial pressure and high purity precursors are necessary to achieve in a reproducible manner high T c and J c (up to 56 000 A/cm 2 in 0 T and 30 000 A/cm 2 under 1.5 T). The fishtail effect observed at 77 K is compared with the one observed in the YBCO system, and is discussed in terms of oxygen deficiency, Nd–Ba substitution, defects and vortex lattice. Our TEM observations did not evidence any Nd–Ba substituted clusters in the Nd123 matrix; however, some diffuse streaks, observed in the [100] zone ED pattern, support the fact that the source of the peak effect in this system is mainly due to oxygen disorder and low stability of the orthorhombic phase.

Effect of a systematic variation of Nd 4Ba 2Cu 2O 10 content on the microstructure of melt processed NdBa 2Cu 3O 7−δ superconductor

NdBa 2Cu 3O 7− δ superconductor is melt processed in flowing argon gas of high purity, with systematically increasing Nd 4Ba 2Cu 2O 10 (Nd-422) content. The changes in the morphology and size of the Nd-422 inclusions in the Nd-123 matrix are studied and compared with those observed elsewhere in the Y-123 system. An explanation for these variations is obtained by studying the microstructural features at the stage of incongruent melting. Microstructural features such as width of the platelets, the nature of interplatelet gaps, etc., are also studied as a function of Nd-422 content.

Effect of a systematic variation of Nd 4Ba 2Cu 2O 10 content on the magnetic properties of melt processed NdBa 2Cu 3O 7− δ superconductor

The effect of a systematic variation of Nd-422 phase content on magnetic properties of melt textured Nd-123 has been investigated. Precisely estimated microstructural parameters such as volume fraction, morphology of Nd-422 particles, and the interface area between the Nd-422 particles and Nd-123 matrix, are correlated to the measured critical current densities. It is found that in the low field region and at higher temperatures, the interfacial pinning by Nd-422 particles is dominant. An anticlockwise tilt observed in the magnetization hysteresis loops at 77 K has been explained by a critical state model in combination with the standard theory of paramagnetism for Nd 3+ ions with a Brillouin function.

TEM observations of melt-textured Nd123 superconductors

Transmission electron microscopy (TEM) observations have been conducted on melt-textured Nd123 crystals. Some structural defects in the Nd123 matrix and Nd123/Nd422 interfaces and the detailed microstructure of crystal boundaries are investigated. Two styles of stacking faults resulting from the insert atomic layer and the disappearance of the insert atomic layer have been observed. The origin of these stacking faults is discussed. A new lattice matching state has been discovered in the low-angle Nd422/Nd123 interfaces. Nd123 lattices near Nd422/Nd123 interfaces change their c-axes in order to minimize the misorientation between the c-axis of Nd123 lattice and that of Nd422 lattice. Rich edge dislocations in the Nd123 matrix are induced by the Nd123 crystal lattice adjustment near Nd422/Nd123 interfaces. The whole pinning effect of Nd422 precipitate is predicted to depend on the competition of two pinning sites due to low-angle Nd422/Nd123 interfaces with alleviated stress and rich edge dislocations associated with the Nd123 lattice regulation, respectively.

Size reduction of Nd 4Ba 2Cu 2O 10 second phase particles in NdBa 2Cu 3O 7 − y melt-textured at reduced oxygen partial pressure

Melt-textured NdBa 2Cu 3O 7 − y (Nd123) superconductor containing fine Nd 4Ba 2Cu 2O 10 (Nd422) particles was prepared by using Nd 1.8Ba 2.4Cu 3.4O x (Nd1.8) powder attrition-milled with 1 wt% CeO 2 addition. The effect of the CeO 2 addition on the size reduction of the Nd422 was remarkable and similar as those observed for Y 2BaCuO 5 (Y211)/Sm 2BaCuO 5 (Sm211) in the Y-Ba-Cu-O/Sm-Ba-CuO system. The Nd422 particles were finely dispersed within the melt-textured Nd123 matrix with a narrow size distribution of a few microns. Superconducting transition temperatures ( T c) of the Nd123 and the Nd1.8 samples melt-textured in an oxygen partial pressure of P( O 2) = 0.001 atm were as high as 95 K.

Field-induced pinning effect of Nd(Ba 1− xNd x) 2Cu 3O 7−δ single crystals grown by the traveling-solvent floating-zone method

We have studied the superconducting properties of Nd(Ba 1− x Nd x ) 2Cu 3O 7−δ (Nd123, x ≈ 0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O 2 in Ar atmosphere. The enhancement of the magnetization with increasing field is observed in the hysteresis ( M- H) loop in fields both parallel and perpendicular to the c-axis of the Nd123 single crystals as well as in the bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The composition variation of Ba/Nd is observed in the matrix of Nd123 crystals by an analytical TEM equipped with a cold field-emission gun. It turns out that the enhancement is due to the field-induced pinning effect ascribed to the weak superconducting NdBa substitution regions in the Nd123 matrix.

酸化物超伝導体の合成と特性 ＴＳＦＺ法で作ユしたＮｄ（Ｂａ，Ｎｄ）２Ｃｕ３Ｏ７−σ単結晶の超伝導特性

We have studied the superconducting properties of Nd(Ba1-xNdx)2Cu3O7-δ (Nd123, x≈0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M-B) loop is observed in the Nd123 single crystals as well as in the Nd 123 bulk crystals prepared by the oxygen-controlledmelt-growth (OCMG) method. The critical current density (Jc) of the Nd123 single crystals is 70, 600 A/cm2 in 1.0T at 77K for the applied field perpendicular to the ab-plane. The existence of Nd and Ba composition variation in the matrix of Nd123 crystals is observed by an analytical TEM equipped with a cold field-emission gun. The high Jc value of the Nd123 single crystals in the applied field is explained by the field-induced pinning centers caused by the Nd-Ba substitutions in the Nd123 matrix.

High critical-current density of Nd(Ba,Nd)2Cu3O7−δ single crystals

We have studied the superconducting properties of Nd(Ba1−xNdx)2Cu3O7−δ (Nd123, x≊0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M–B) loop is observed in the Nd123 single crystals as well as in the Nd123 bulk crystals prepared by the oxygen-controlled melt growth (OCMG) method. The critical current density (Jc) of the Nd123 single crystals is 70 600 A/cm2 in 1.0 T at 77 K for the applied field perpendicular to the a-b plane. Uniform flux density distribution with the same shape as the sample is observed in the field perpendicular to the a-b plane by the magneto-optical flux-density observation. Finely dispersed white regions in the dark-field image due to Nd substitutions for Ba are observed in the Nd123 matrix by the transmission electron microscopy. The high Jc value of the Nd123 single crystals in the applied field is explained by the field-induced pinning centers caused by the Nd–Ba substitutions in the Nd123 matrix.