Effect of Gd addition on the superconducting properties of (Nd-Sm-Eu)123 system

Bulk (Nd, Sm, Eu)_1-xGd_xBa₂Cu₃ O_y superconductors in which the Gd content x ranged from 0 to 0.25, were grown by the oxygen-controlled-melt-growth (OCMG) method. X-ray diffraction results show that the main phase is RE123 with a small amount of the second phase. SQUID measurements on the oxygenated samples showed that a small amount of Gd123 addition to (Nd, Sm, Eu)123 increased T_c and made the superconducting transition sharper. All the samples, except the Gd-free one, exhibited the secondary peak effect in the magnetization curves, which shows that the field-induced pinning is active, like the other RE123 fabricated by the OCMG process. The critical current density also increased with Gd addition, however, no systematic change was observed with the amount of Gd addition. A peak J_c value of 60,000 A/cm² was recorded with an applied field of 1.2 T (H//c) at 77 K. The results suggest that, in the (Nd, Sm, Eu)123 system the addition of Gd is effective in achieving high T_c and a large J_c.     

Effects of second phase and platinum additions on the microstructure and the critical current density of OCMG processed (Nd,Eu, Gd)–Ba–Cu–O

We have investigated the effects of second phase (Nd, Eu, Gd)₂BaCuO₅ (NEG211) and platinum addition on the superconducting properties for (Nd, Eu, Gd)Ba₂Cu₃O_Y (NEG123) composite prepared by the oxygen–controlled melt–growth (OCMG) process. The addition of NEG211 slightly decreased the critical temperature (T_c) and increased the critical current density (J_c), particularly in a low field region. The secondary peak effect, which is commonly observed in OCMG processed Nd123, was reduced with the NEG211 addition of >10%. Refinement of the NEG211 second phase was achieved by Pt addition, which led to a dramatic increase in both J_c values and the irreversibility field. Low field J_c varied as V_f/d, where V_f is the volume fraction and d is the mean diameter of NEG211, supporting the idea that the 211/123 matrix is responsible for the pinning-like melt–processed Y–Ba–Cu–O. However, in a high field region, due to the presence of additional pinning by field–induced pinning centers, such a relationship is not observed. Our results suggests that fine distribution of the second phase is effective in improving pinning properties in the (Nd, Eu, Gd)–Ba–Cu–O composite.     

Mixing Y(RE)BCO (RE=Nd,Sm) superconductors by crystal pulling method

Y_1−xRE_xBa₂Cu₃O_7−δ [Y(RE)BCO or REsBCO] superconductors were prepared by the crystal pulling method. The RE mixing content x in the crystal can be effectively controlled by the processing temperature. With an increase in processing temperature, the RE concentration in the liquid increased, which resulted in the higher RE substitution content in single crystals. The higher critical temperatures T_c above 92 K could be obtained by an optimized oxygenation treatment. The T_c value of Y(RE)BCO tends to be insensitive to the growth atmosphere of the oxygen partial pressure under a certain limit of the RE content. The Y_.727Nd_.273Ba₂Cu₃O_7−δ and the Y_.941Sm_.059Ba₂Cu₃O_7−δ samples show values of the critical current density J_c are about 2×10⁴ Acm⁻² at 1.2 T and 2.8×10⁴ Acm⁻² at 1.1 T for H//c, respectively, indicating that mixing REs 123 have obvious effects on J_c–H curves since RE ions are likely to substitute at both Y²⁺ and Ba²⁺ sites.     

Microstructure of high Jc NdBa2Cu3Ox/Nd4Ba2Cu2O10 composite superconductor

Single crystalline NdBa₂Cu₃O_x (Nd123) superconductors with dispersed Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were produced by the floating zone partial melting and solidification (FZPMS) method. The initial composition of the precursor material was Nd_1.8Ba_2.4Cu_3.4O_x with or without 0.1 wt% Pt addition. FZPMS was carried out in a low oxygen partial pressure atmosphere. Microstructure of quenched samples were investigated by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Furthermore, superconductive properties were measured by superconducting quantum interference device (SQUID). The results of SQUID measurements indicate that the critical temperature (T_c) of the samples with Pt addition so produced with the oxygen heat treatment (623 K for 300 h) was 94 K and the critical current density (J_c) was 2.3×10⁴ A/cm² at 77 K, 0.2 T.     

Melt processing for strong flux pinning in RE-Ba-Cu-O (RE: Nd,Sm, Eu,Gd) superconductors

Factors affecting a characteristic flux pinning in light rare earth (RE)-Ba-Cu-O (RE: Nd, Sm, Eu, Gd) superconductors fabricated by the oxygen-controlled-melt-growth (OCMG) process have been investigated through a comparative study. At 77K and for the applied field parallel to the c-axis of the sample (H//c), the flux pinning of all OCMG-processed REBa₂Cu₃O (RE123) samples studied was very sensitive to the oxygen partial pressure (P_O2) controlled during the melt growth and thus, with lowering P_O2, the peak field (B_pk) in the M-H loops shifted to a high field and the irreversibility line (IL) shifted to a high H-T region. For a Ndl23 sample, as the oxygen annealing temperature increased above 300‡C, both B_pk and IL were systematically depressed. However, B_pk for all systems was insensitive to the amount of the second phase (Nd₄Ba₂Cu₂O₁₀ (Nd422) and RE₂BaCuO₅ (RE211) for the other) inclusion in the superconducting RE123 matrix, supporting that the characteristic flux pinning is due to the superconducting matrix.     

Superconducting Properties of (M x /YBa2Cu3O7−δy )N Multilayer Films with Variable Layer Thickness x

The superconducting properties of (M _x /YBa₂Cu₃O_7−δy )_N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y₂BaCuO₅ (211), Y₂O₃, BaZrO₃, CeO₂, SmBa₂Cu₃O_7−δ (Sm123), brown-phase La₂BaCuO₅ (La211), and MgO. Multilayer (M _x /YBa₂ Cu₃O_7−δy )_N structures were grown by pulsed laser deposition onto SrTiO₃ or LaAlO₃ single-crystal substrates by alternate ablation of separate YBa₂Cu₃O_7−δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T _c), critical current density as a function of applied magnetic field J _c(H), self-field J _c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO₃ at 65 to 77 K, Y₂O₃ at 65 K, and CeO₂ for H < 0.5 T. BaZrO₃ had a noticeably different epitaxy forming smaller size nanoparticles ∼8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 × 10¹¹ nanoparticles cm⁻². To optimize flux pinning and J _c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J _c unexpectedly increased with increasing x layer thickness.     

Y2BaCuO5 particle coarsening during melt processing of YBa2Cu3O7−x

Quenching experiments were carried out to study the coarsening phenomena of Y₂BaCuO₅ (211) particles in liquid (Ba₃Cu₅O₈) which form as a result of the peritectic decomposition of YBa₂Cu₃O_7−x (123) above 1010°C. The morphology of 211 particles was observed to become more acicular as the time of hold increased at 1040°C, showing an anisotropic growth rate of the 211 particles in the liquid. Also, the coarsening rate was found to be increased in the presence of excess added liquid phase (Ba₃Cu₅O₈) in the 123. Externally added 211 particles were found to act as heterogeneous nucleation sites for the peritectically formed 211. The observed cored 211 morphology development in the Er 211 doped, melt textured, Y 123 pellets was explained on the basis of ripening of 211 particles. Reduced coarsening of 211 particles was observed in the presence of 0.5 wt.% Pt doping.     

Studies of the La1+xBa2−xCu3Oz prepared from highly homogeneous precursors

Single phase materials of La_1+xBa_2−xCu₃O_z with x=0.0,…,0.98, were synthesized from highly homogeneous precursors, prepared by spray-drying or the polymerized complex method. These samples were characterized using X-ray diffraction (XRD), Raman spectroscopy and AC susceptibility measurements. The effects of composition, processing time, temperature and oxygen partial pressure, on the formation and decomposition of La_1+xBa_2−xCu₃O_z prepared by different methods, were investigated. We found that: T_c gradually decreased with the increase of La content; orthorhombic distortions appear at x<0.25 and x>0.7; and single phase stoichiometric La-123 ceramic samples, obtained at low oxygen partial pressure and quenched, decomposed into La-123ss (x=0.2) and BaCuO₂ after annealing at a rather low temperature, below 500°C.     

The effect of increased humidity in air on the surface properties of C-axis oriented Nd1Ba2Cu3Oy thin films

Laser ablated c-axis oriented Nd₁Ba₂Cu₃O_y (Nd123) thin films were exposed intentionally to highly humid air. The resulting surfaces were investigated by an ultrahigh vacuum scanning tunneling microscope system, an atomic force microscope system, and the standard four-probe resistance measurements. It was found that an increased humidity in air could degrade the originally stable surfaces of Nd123 thin films and change the surface morphology and electronic properties of the Nd123 thin films remarkably. A mechanism of the degradation is proposed to explain the observed results. Our study indicates that moisture in air is one of the major sources responsible for the surface degradation of 123-based high temperature superconductors upon exposure to air.     

Enhanced superconductivity in Nd1+xBa2-xCu3O7+δby low oxygen partial pressure annealing

Previous work has shown that unlike YBa₂ Cu₃ O_7-δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution Nd_1+xBa_2-x Cu₃O_7+δ (Nd123ss) for 0.04≤ × ≤0.6.^1–3) An earlier paper showed that although the superconducting properties decrease nonlinearly for increasing x, T_c can be varied by increasing the annealing temperature without changing the low temperature oxygen soak.² The changes in microstructure and T_c with increasing x are analogous with Y123 with increasing δ except that the total hole concentration remained constant. T_c was modeled in terms of oxygen disorder resulting from Nd³⁺ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in T_c as a function of x and processing conditions can be explained by the number of fourfold coordinated coppers on the chain sites. In this paper, the model has been further substantiated by processing in 1% O₂. The annealing in a reduced oxygen partial pressure followed by a 450°C oxygen soak resulted in a marked increase in T_c compared to the 100% PO₂ anneal. The low PO₂ anneal favors pairing of Nd³⁺ substituting for Ba²⁺ to conserve oxygen ions, resulting in fewer disrupted fourfold-coordinated coppers thus increasing charge transfer from the planes to the chains.     

Understanding the phase relations and cation disorder in LRE1+xBa2− xCu3O7+δ

Unlike Y123 which forms only a stochiometric compound, the light rare earth elements (LRE) form a solid solution LRE_1+xBa_2-xCu₃O_7+δ (LRE123ss), with increasing substitution of the LRE³⁺ for the Ba²⁺ with increasing ionic radii of the LRE. Charge balance is maintained by increasing oxygen occupation on the anti-chain sites. The range of solubility is partially controlled by the oxygen partial pressure (PO₂). The peritetic decomposition temperature also increases with increasing ionic radii. At doping levels of 0 > × > 0.1, there is an increase in T_c when the high temperature annealing (T ∼ 940‡C) is performed in low PO₂ (> 0.1 bar). The maximum T_c occurs at a doping level of ∼x < 0.05 for Nd and Gd. When annealing is performed in 1 bar PO₂, there is a gradual decrease in T_c with increasing x. These phenomenon can be understood in terms of the number and distribution of the LRE which substitute on the Ba site.     

The influence of the phase equilibria on the critical temperatures Tc of the high-Tc Bi−Sr−Ca−Cu−O and Y−Ba−Cu−O compounds

The phase equilibria of YBa₂Cu₃O_7−d (123 phase) strongly influence its oxygen content and consequently its critical temperature T_c. With increasing content of BaCuO₂ or CuO, T_c decreases. Whereas, if Y₂BaCuO₅ (211 phase) is in equilibrium with the 123 phase, the oxygen content and T_c of which is high. Bi_2+x(Sr,Ca)₃Cu₂O_8+d exhibits an extended single phase region and its T_c is strongly affected by its chemical composition. In addition, the oxygen content of the phase is a function of the temperature and influences the T_c of the phase.     

Processing and physical properties of single phase Tl2Ba2CaCu2O8 and (Tl0.5Pb0.5)Sr2−xBaxCa2Cu3O9 superconductors

Single phase Tl₂Ba₂CaCu₂O₈ (2212) superconductors with T_c of 97 K were obtained by a two-step synthesis. Prolonged annealing at 860°C in the second step of the synthesis resulted in a higher T_c, at the expense of the growth of an impurity trilayer 2223 phase in the form of stacking faults in the 2212 phase. Nearly single phase (Tl_0.5Pb_0.5)Sr_2−xBa_xCa₂Cu₃O₉) (1223) samples were obtained in a single air-sintering step. Replacement of Sr by Ba is necessary to grow the 1223 phase. Samples with x=0.75 displayed T_c as high as 117 K with a superconducting volume fraction over 50%.     

An alternative method to introduce fine Y2BaCuO5 precipitates into YBa2Cu3Ox with enhanced flux pinning

We have examined the effect of fine Y₂BaCuO₂ (211) precipitates on the flux pinning in textured YBa₂Cu₃O_x (123). We have used an alternative method to introduce fine Y₂BaCuO₅ (211) precipitates in YBa₂Cu₃O_x (123) without melting at high temperatures and without subsequent quenching as in the quench melt growth (QMG) process. Instead of generating the liquid by heating above 1200°C as in the QMG method, we obtain the liquid composition by a solid-state route at much lower temperatures. The microstructure achieved by using this alternative method is similar to that achieved by the quench-melt-growth process. We show that the fine 211 precipitates produced by this process can effectively enhance flux pinning.     

Microstructure and properties of Tl0.75Bi0.25Sr1.6Ba0.4Ca2Cu3Ox tapes

Ag-clad Tl_0.75Bi_0.25Sr_1.6Ba_0.4Ca₂Cu₃O_x tapes were fabricated by a powder-in-tube technique. The starting powders consisted of Bi₂Sr₂CaCu₂O_x plus simple oxides. The tapes were heated in flowing O₂ at 835–865 °C for 7.5–10 h. Room-temperature pressing at 1.0–1.5 GPa produced tapes with denser, more phase-pure cores, and transport critical current density J_c at 77 K in self-field that was increased by a factor of ≈2. The maximum J_c value of 6 × 10³ A/cm² was obtained with heating at 840 − 850 °C for ≈ 10 h, with three intermediate pressing steps. The cores of the best tapes were still rather porous and contained significant concentrations of nonsuperconducting phases.     

Rare-earth-modified Sr0.5Ba0.5Nb2O6, ferroelectric crystals and their applications as infrared detectors

Rare-earth-modified ferroelectric crystals with the formula (Sr_1−xBa_x)1− 3y/2 R_yNb2₂O₆, where R = La, Nd, Sm, Gd, and Lu, have Been prepared and studied. When R = La, Nd, x ≃ .5 and y = 0.02, the modified material, at room temperature, exhibited twice the pyroelectric coefficient and four times the dielectric constant of the unmodified Sr_1−xBa_xNb₂O₆ (x ≃ .5). Curie temperatures decreased, dielectric constants increased, while loss factor and detector signal-to-noise ratios remained nearly the same with the addition of rare earth doping. The calculated response based on the measured properties agree with the measured response of actual detectors. These properties suggested that the modified SBN are good materials for small element or array pyroelectric infrared detector applications.     

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

The structural and superconducting properties of (Gd_1−x−yCa_yHf_x)Ba₂Cu₃O_z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd_1−xHf_x)Ba₂Cu₃O_z lowers the oxygen content and decreases T_c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd_0.85−yCa_yHf_0.15)Ba₂Cu₃O_z provides proper matching between the ionic radius and valence of Gd³⁺ (0.94 Å) and the average ionic radius and valence of Hf⁴⁺ (0.78 Å) and Ca²⁺ (0.99 Å). As the Ca content increases, the T_c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R_1−xHf_x)Ba₂Cu₃O_z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T_c.     

Effects of shock-induced defect density on flux pinning in melt-textured YBa2Cu3O7−δ

To introduce a high density of homogeneously distributed defects in YBa₂Cu₃O_7−δ (Y123), melt textured samples were shock-compacted at pressures up to 12.6 GPa (126 Kbar) at orientations favorable to slip along the basal planes.¹ Shock compressing melt-textured Y123 results in a nearly uniform detect density which is two to three orders of magnitude higher than in unshocked melt-textured material. However, the intergranular critical current density in bulk samples (J_c ^b) decreases by two orders of magnitude in the as-shocked state. This decrease in J_c ^b is attributed to microfractures. However, if the shocked disk is annealed in O₂ then ground, sieved, and magnetically aligned, J_c for H τ c-axis (J_c ^ab) is enhanced two to three times over the unshocked value. This indicates that the increase in dislocations density does increase flux-pinning.     

Y-123 Films on technical substrates

For high-current applications, homogeneous well-textured high-temperature superconducting Y₁Ba₂Cu₃O_7−δ (Y-123) films on technical ceramic or metallic substrates are required. The ion-beam-assisted deposition (IBAD) of yttria-stabilized zirconia (YSZ) buffer layers, which serve as a template for Y-123, was extended to large-area substrates as well as differently shaped long substrates. TEM, as well as XRD investigations, was performed to observe the relevant growth mechanisms. The influence of nucleation, growth selection, and homoepitaxial growth on the development or the maintenance of a preferential orientation of the YSZ buffer is discussed. Y-123 films were prepared by a modified pulsed-laser-deposition (PLD) technique based on a quasi-equilibrium substrate heating and variable azimuth scanning of the target. The method allows a PLD coating of long pieces of tapes and tubular substrates because no direct mechanical contact of a substrate with the heater is required. Critical current densities, J_c, above 10⁶ A cm⁻² were achieved on polycrystalline Ni tapes as well as on polycrystalline YSZ sheets at 77 K and self fields. The dependence of J_c on magnetic field resembles that of Y-123 deposited on single-crystalline substrates and demonstrates the absence of weak links due to grain boundaries. The thickness dependence of J_c is interpreted in terms of a nucleation layer appearing in the early stage of film growth. A growth model is proposed which seems to be in agreement with the experimental observations. The measured strain tolerance of the films does not impede their application.     

Microstructure study and hyper frequency electromagnetic characterization of novel hexagonal compounds

The physical characteristics and preparation of novel Z-type hexagonal compound, which have iron deficient composition of Ba₃Co_2(0.8−x)Cu_0.4Zn_2xFe_23.5O₄₁ (x≤0.30), was investigated. The results show that Zn has little effect on the morphology of compact and homogeneous ferrites with average 10-15 μm of lathy grain size, but make the formation temperature range narrow. In the range of Zn incorporation, the hexagonal cell lattice parameters (a and c) of ferrites reserve stable. However, the key magnetic and electrical properties, such as initial permeability, quality factor, resistivity, dielectric constant and magnetic hysteresis as well as Curie temperature were also characterized. The mechanisms involving in generating these variations were also discussed in this paper.