Current progress in ternary LREBa2Cu3Oy materials and their application

The J_c and H_irr values at 77 K of ternary light-rare-earth compounds, LREBa₂Cu₃O_y “LRE-123”, are usually high enough to serve in various applications. Several sources of vortex pinning can be in these composites tailored to fit the needs of the particular application. The list comprises LRE/Ba solid solution, oxygen vacancies, large particles of secondary phases, twin planes, nanoscale lamellas, etc. By means of the latter defects one can achieve a very high irreversibility field. Refinement of secondary phase particles and the optimal choice of their amount enhance the electromagnetic performance in a broad temperature range, up vicinity of T_c, allowing levitation at liquid oxygen, 90.2 K. An optimum content of MoO₃ doubles the self-field super-current at 77 K, H||c-axis. Altogether, the pinning tailoring in ternary LRE-123 materials provides a flexible and reliable way to fit the electromagnetic performance with the needs of sophisticated high-temperature and high-magnetic-field applications.     

The Effects of Chemical Doping and Hydrostatic Pressure on T c of Y1−y Ca y Ba2Cu3O x and NdBa2Cu3O x Single Crystals

Westudied T _c of Y Ba ₂ Cu ₃ O _x (Y123), Y _0.89 Ca _0.11 Ba ₂ Cu ₃ O _x (YCa123) and NdBa ₂ Cu ₃ O _x (Nd123) single crystals with various oxygen contents x. Compared to T _c (x) of Y123 the T _c (x) curves of YCa123 are shifted to lower oxygen contents and the maximum transition temperature T _c,max decreases with increasing Ca content whereas in Nd123 T _c (x) is shifted to higher oxygen contents and T _c,max is increased. According to the universal parabolic T _c (n _h ) behavior the differences in T _c (x) of Y123, YCa123 and Nd123 can be ascribed to different hole concentrations n _h in the CuO ₂ planes caused by doping via changes in chemistry or structure. In order to study the influence of structural changes on T _c we examined the hydrostatic pressure effect dT _c /dp (p 0.6GPa). In the underdoped region, at n _h 0.11, the examined compounds show a peak in dT _c /dp which is very pronounced for systems with well ordered CuO chains. As this peak occurs at the same n _h in all investigated systems it is not caused by oxygen ordering, but its origins might be found in a strong influence of lattice deformations on the electronic structure.     

Enhanced Superconducting Properties of Air-Processed GdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript> Single Domains with BaCO<Subscript>3</Subscript>/BaCuO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript> Addition

Single domain GdBa₂Cu_7-δ (Gd123) bulk superconductors were fabricated in air by top-seeding melt-texture growth. Performance of the air-processed Gd123 was successfully enhanced by addition of both BaCO₃ and BaCuO_2−x , which suppress the formation of Gd_1+x Ba_2−x Cu₃O_7-δ solid solutions. The optimum doping amount ranges from 0.05 to 0.15, M BaCO₃ and 0.05 to 0.1, M BaCuO_2−x per molar Gd123. The distribution of the second phase particles was observed by scanning electron microscopy. A narrow band formed by Gd₂BaCuO₅ particle concentration appeared around the seeding zone in both a–b plane and c-growth sector in Gd123 single grain. Trapped magnetic field density reached 0.67, T for sample with 24 mm in diameter and 8, mm in thickness and a high critical current density J _c up to 91,200, A/cm² was achieved at 77, K under self-field.     

The High-Temperature Superconductor EuBa2Cu3O6 + x: Role of Thermal History on Microstructure and Superconducting Properties

We present here combined resistivity and micro-Raman spectroscopy measurements for the superconductor EuBa₂Cu₃O_{6 + x} (Eu-123, EuBCO). We found an important dependence of microstructure and critical temperature on preparation temperature. We also show a strong orientation effect and a T _c of 93 K in samples prepared at high temperature relative to those prepared at lower temperature, which exhibit lower T _c, broad transitions, and no grain orientation. In addition, we show the influence of annealing conditions on superconducting properties by preparing samples with the same oxygen content (x = 0.38) but with various T and P(O₂). The peculiar behavior of Eu-123 with temperature by considering a different hole injection sequence for Eu-123 with respect to Y-123 is explained. Micro-Raman measurements seem to support this hypothesis.     

Nanostripes in GdBa2Cu3Ox high-Tc superconductors

High-T_c superconductors with light rare earth (LRE) elements instead of Y exhibit nanoscale stripe structures on the surface as observed by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) scans. Within the GdBa₂Cu₃O_x (GdBCO) system exhibiting relatively high critical current densities, nanoclusters arranged in a stripe-like fashion are observed in undoped material, while adding of nanoparticles (ZnO₂, ZrO₂) leads to the formation of nanostripes as observed in other LRE superconductors. The nanostripes in doped GdBCO exhibit periodicties between 20 and 50 nm and corresponding step heights of 0.3–0.8 nm. Using polarized light microscopy and electron backscatter diffraction (EBSD) analysis, we determined the direction of the nanostripes with respect to the known twin structure.     

The Comparisons Between Y-123 and Y-124 Superconductor Substituted with Ca at the Cu-Site

The comparison between YBa₂Cu_3?x Ca _x O _δ and YBa₂Cu_4?x Ca _x O₈ superconductors substituted with Ca at the Cu-site was investigated. The concentration of Ca varied from x=0.00 to x=0.15. Resistivity and current density measurement (without magnetic field) were measured using four-probe method. The samples were characterized using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). From the resistivity measurement, the critical temperature (T _c?zero) in x=0.00 was 84 K for Y-123 and 83 K for Y-124 superconductor system. As the Ca concentration increased to x=0.15, both superconductor systems showed a decrease in value whereby 71 K for Y-123 and 74 K for Y-124. This was a result of the decrease in the hole concentration. Critical current density (J _c ) decreased with the further increment of Ca concentration owing to grain connectivity and an increase of porosity. At 50 K, J _c decreased from 3.9790 A/cm² at x=0.00 to 3.5184 A/cm² at x=0.15 for Y-123, and from 3.6209 A/cm² at x=0.00 to 0.5243 A/cm² at x=0.15 for Y-124. The crystallographic crystal structure showed that both Y-123 and Y-124 superconductor systems exhibited an orthorhombic form. FESEM microscopy showed that the Y-123 sample had less porosity compared to Y-124 samples and the resulting Y-123 sample had a higher J _c compared to the Y-124 sample.     

Microstructural and superconductive studies of Ag-YBa2Cu3O7−x composites

Composites of Ag-YBa₂Cu₃O_7?x were synthesized and carefully characterized for crystal structure and microstructure by X-ray diffraction, optical microscopy, SEM, TEM, and EDAX techniques in order to investigate the effect of Ag additions on the superconducting properties of 123 compounds. The a.c. susceptibility data show thatT _c (onset) of 123+Ag composites vary between 90.2 and 91.8 K. TheJ _c values we measured for 123 material without silver were in the range of earlier reported values for the pure 123 material. Whereas there is a relatively small increase in the critical current for the YBCO/Ag₂O ratio of 3, the variation of the grain size of the composites shows that theseJ _c changes are due to slight variations in the grain size rather than any dramatic effect of Ag inclusions as speculated earlier.     

Microstructural and superconductive studies of Ag-YBa2Cu3O7?x composites

Composites of Ag-YBa₂Cu₃O_7–x were synthesized and carefully characterized for crystal structure and microstructure by X-ray diffraction, optical microscopy, SEM, TEM, and EDAX techniques in order to investigate the effect of Ag additions on the superconducting properties of 123 compounds. The a.c. susceptibility data show thatT _c (onset) of 123+Ag composites vary between 90.2 and 91.8 K. TheJ _c values we measured for 123 material without silver were in the range of earlier reported values for the pure 123 material. Whereas there is a relatively small increase in the critical current for the YBCO/Ag₂O ratio of 3, the variation of the grain size of the composites shows that theseJ _c changes are due to slight variations in the grain size rather than any dramatic effect of Ag inclusions as speculated earlier.     

Analysis of indentation size effect (ISE) behavior in low-load Vickers microhardness testing of (Sm123)1−x(Nd123)x superconductor system

Indentation size effect (ISE) for (Sm123)_1?x(Nd123)_x superconducting samples which were fabricated by the solid state reaction technique for values of x = 0.00, 0.05, 0.10, 0.20, and 0.30 was investigated by analyzing the theoretical models. When the experimental data of a number of single crystals which have the different crystal structure and different chemical bonding inside the polycrystallined samples were analyzed with the ISE models, the sample encountering with resistance and elastic deformation was observed as well as plastic deformation. The microhardness values on different surfaces of materials were calculated by using Meyer Law, proportional specimen resistance model, modified proportional specimen resistance model, elastic/plastic deformation model and the Hays–Kendall (HK) approach. The results showed that the HK approach was determined as the most successful model. Furthermore, X-ray powder diffraction and scanning electron microscope measurements were analyzed for superconducting properties of (Sm123)_1?x(Nd123)_x superconductor system. The results showed that microhardness values at the minimum load and averaged plateau region of load increased with increase of Nd123 concentration. Nd123 content can be used as to be estimated the microhardness value of (Sm123)_1?x(Nd123)_x superconducting sample in the range of 0.878–2.717 GPa. The control of the microhardness value by using Nd123 content in (Sm123)_1?x(Nd123)_x superconducting structure can be useful in technological applications in superconductivity industry.     

Further Increase of T c in Y-Ba-Cu-O Superconductors

This paper presents the synthesis and characterization of three new Y-based high temperature-superconducting compounds having relatively higher transition temperature T _c compared with that of the Y-123, Y-124, and Y-247. The electrical resistivity measurements indicate the onset critical temperatures as high as 98 K. Atomic compositions of these new Y-Ba-Cu-O superconductors are Y₂Ba₃Cu_5.2O _x , Y₂Ba₅Cu₉O _x , and YBa₄Cu₅O _x . The X-ray analysis has shown that they have a similar crystalline structure as Y-123 phases.     

Stoichiometry Analysis and Normal-State Properties of SmBa2Cu3−x Ru x O7−δ Superconducting Phase

The electrical resistivity of normal and superconducting states for SmBa₂Cu_3?x Ru _x O_7?δ (Sm-123) phase with 0.00 ≤ x ≤ 0.50, prepared by the conventional solid-state reaction technique, was studied. X-ray powder diffraction (XRD), scanning electron microscope (SEM), particle-induced X-ray emission (PIXE), Rutherford backscattering spectroscopy (RBS) and electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution in Sm-123 phase. Both the phase formation and superconducting transition temperature T _c enhance up to x = 0.05. For x > 0.05, suppression of both the phase formation and T _c is observed and the superconductivity is completely destroyed around x = 0.50. The normal-state electrical resistivity was analyzed by the two- and three-dimensional variable range hopping (2D-VRH and 3D-VRH) and Coulomb gap CG. The dominant mechanism for Sm-123 phase is CG with x ≤ 0.20 while is 3D-VRH for x ≥ 0.30.     

Influence of fluorine doping on the transport properties of GdBa2Cu3O7-X

Undoped and fluorine-doped materials of GdBa₂Cu₃O_7-XF_x (with x = O, 0.05, 0.1 and 0.2) were prepared at high temperature (950° C) by solid state reaction followed by oxygen treatment. X-ray diffractograms of the undoped sample showed single orthorhombic phase. Resistivity measurements showed thatT _c(0) decreases with fluorine concentration. Thermopower measurements indicated a peak just before the onset of the superconducting transition. The reasons for the decrease ofT _c and the anomalous behaviour of thermopower have been explained.     

Magnetic Behavior of Light Rare Earth Ions in (Nd,Eu,Gd)-123 Superconductors

The paramagnetic light-rare-earth (LRE) ions significantly participate on the total measured magnetic signal of LRE-123 superconductors. At low temperatures, most of the paramagnetic LRE ions become magnetically ordered. The ordering depends on the ions distance in the given compound. We investigated magnetic behavior of LRE ions in the (Nd_0.33Eu_0.38Gd_0.28)Ba₂Cu₃O _y superconductor in a broad temperature and magnetic field range with the aim to find thermodynamic characteristics of the compound. The thermodynamic reversible magnetization and the associated thermodynamic characteristics were obtained.     

Synthesis and crystallographic study of Pb-Sr hydroxyapatite solid solutions by high temperature mixing method under hydrothermal conditions

The solid solutions in the system of Pb and Sr hydroxyapatite, Sr_10−xPb_xHAp (x = 0-10), were successfully synthesized by high-temperature mixing method (HTMM) at 160 °C for 12 h under hydrothermal conditions. The samples were characterized by X-ray diffraction, chemical analysis and electron microscopic observation, and the site of the metal ions in the solid solutions was analyzed with the Rietveld method. The lattice constants, both a and c, of the solid solutions varied linearly with Pb content. It was found that Pb ions in the solid solutions preferentially occupied the M(2) site in the apatite structure. HTMM gives Sr-Pb HAp solid solutions much better crystallization. However, due to the formation of intermediate compound of Pb₃O₂(OH)₂ in the Pb(NO₃)₂·4H₂O solution before mixing with (NH₄)₂HPO₄ solution at 160 °C, HTMM causes the decrease of crystallization of the samples with high Pb content.     

Dielectric and thermal properties of xPbTiO3-(1 − x)SrTiO3 Polycrystals

SrTiO₃ and PbTiO₃ perovskites are combined to form the xPbTiO₃-(1 – x)SrTiO₃ (PST) solid solution. In this work, a study of its dielectric and thermal properties is reported as a function of PbTiO₃ content. The dielectric properties of the xPbTiO₃-(1 – x)SrTiO₃ solid solution are determined through a thermoelectric analysis technique and hysteresis measurements. Such measurements made at room temperature for all compositions show the influence of one component upon the other resulting in a response to the electric field that involves a strained lattice behavior. A limiting case of antiferroelectric-like behavior is observed for x = 0.5. The thermal properties such as the specific heat capacity (c) and thermal diffusivity () were determined using a photoacoustic technique (PA) and the temperature relaxation method (TRM). The thermal conductivity was calculated from the results obtained for c and .     

Compositional effects on the properties of (1-x)BaTiO3-xBi0.5Na0.5TiO3 ceramics

The compositional effects on the crystal structure, phase transition, dielectric, ferroelectric and piezoelectric properties of the (1-x)BaTiO₃-x(Bi_0.5Na_0.5)TiO₃ solid solution ceramics were investigated. After sintering at 1200 °C for 2 h, the ceramics with different content of (Bi_0.5Na_0.5)TiO₃ (BNT) formed single-phase solid solutions with perovskite structure. The lattice constant c/a ratio of the solid solutions decreased as BNT content increased except that with 20 mol% BNT, which had the largest value of c/a ratio for all samples. The second phase transition corresponding to BaTiO₃ (BT) at about 5 °C shifted to low temperature and gradually disappeared as BNT increased. A new secondary phase transition appeared at 32 °C and 64 °C, respectively, when BNT content was 5 and 14 mol%. The Curie temperature, T _c, shifted in the temperature range between 112 °C and 166 °C, and the remanent polarization, P _r, decreased whereas the coercive field, E _c, increased as the BNT content increased. The relatively high value of piezoelectric coefficient d₃₃ together with a relatively high Curie temperature and low loss tangent and stable dielectric properties were obtained when the addition of BNT was 20 mol%.     

Low percolation concentration for zero thermoelectric power in Y1Ba2Cu3O7−x

Effect of oxygen deficiency on the thermoelectric power and the resistive transition is studied by changing the cooling rate of the sintered specimens of R₁Ba₂Cu₃O_7−x where R=Y, Gd and Sm. Slow-cooled (60°C/h) specimens are superconducting and show positive TEP with a broad peak aboveT _c . It is a decreasing function of temperature at higher temperatures. Pair fluctuation aboveT _c is thought to be responsible for this peak. Highly O₂-deficient sample of Y₁₂₃ has a finite resistivity atT _c and yet shows a zero TEP. It is suggested that the percolation concentration of the superconducting phase for zero TEP should be much smaller than that required for the electrical conduction.     

Rising Tc in Bi and Cu co-doped BaTiO3

The solid solution (Ba_1−2xBi_2x)(Cu_xTi_1− x)O₃ (0.0 < x ≤ 0.10) was prepared by conventional high temperature reaction. In the region of x ≤ 0.040 single phases of tetragonal perovskite-type compounds were obtained and the c-axis increased up to x = 0.015 and the maximum of the a/c ratio was 1.0123 at x = 0.010. In x > 0.040 a small amount of an unknown impurity phase appeared in addition to a tetragonal perovskite-type phase. The Curie temperature Tc increased to 140 °C at x = 0.010. Rising Tc was confirmed by temperature dependence of the dielectric constants and the endothermic peaks observed in DTA curves. This is the first example for rising Tc in the solid solution based on BaTiO₃ except for doping of Pb²⁺ ion.     

Nanostructured Ce1−xZrxO2 solid solutions produced by mechanochemical processing

The nanostructured Ce_1−xZr_xO₂ solid solutions (x ≤ 0.2) have been successfully synthesized from CeCl₃–ZrCl₄–NaOH mixtures by mechanochemical processing as a gradual transformation, involving in-situ CeO₂ and amorphous ZrO₂ formation as intermediates. Solid solutions type-Ce_1−xZr_xO₂ along with NaCl as diluent were obtained at different milling times, with a final composition of Ce_0.8Zr_0.2O₂ after 5 h and 15 h under high and low energetic milling conditions, respectively. The NaCl formed during the mechanochemical reaction, which is eliminated by washing after calcination of the as-milled sample, allows to obtain a Ce_0.8Zr_0.2O₂ solid solution with high surface area and nanometric grains. The nanostructured Ce_0.8Zr_0.2O₂ solid solution shows good thermal stability after prolonged heating at 600 °C. However, the addition of an extra amount of diluent during mechanochemical reaction evidences a detrimental effect, avoiding the formation of solid solution or deteriorating the textural/microstructuctural characteristics of the obtained Ce_1−xZr_xO₂ solid solution. Removal of NaCl previous to calcination improves notably the textural/microstructural characteristics of the Ce_0.8Zr_0.2O₂ solid solution.     

Isotope Studies of the CMR Compounds La1−xCaxMnO3+δ

Oxygen isotope effect studies of the ferromagnetic Curie temperature T _c of La_1?x Ca _x MnO₃ are presented. The isotope exponent α₀=?ΔlnT _c/Δlnm ₀ changes from 0.4 to 0.14 in the range 0.2<x<0.43. The isotope exponent decreases strongly with increasing tolerance factor, or decreasing lattice distortion. Above Tc the conductivity is characteristic of small polarons. Raman scattering shows a prominent peak at 230 cm?1. The peak width could be related to site-dependent Jahn–Teller distortions above T _c, becoming significantly smaller at and below T _c. IR reflectivity data show a much larger zero frequency IR conductivity than dc conductivity. The IR peaks are independent of temperature between 150 and 295 K.