Non-superconducting Phase Research Articles

Superconducting properties of Y-Ba-Cu-O bulk with BaSnO3 addition

Dispersion of non-superconducting phase particles is commonly employed in melt textured REBa2Cu3O7-δ (RE123; RE: rare earth element) bulk superconductors in order to improve the critical current density. It has been clarified that the interfaces of RE2BaCuO5 (RE211) and RE123 matrix contribute to the flux pinning, and therefore, the critical current density can be enhanced due to the refinement of RE211 particles. In recent years additions of RE2Ba4CuMOy (M=Zr, Nb, Ta, Mo, U, Sn, and Re) particles have been investigated in an attempt to improve the pinning performance. While the common particle size is around 1-5 μm for the RE211, the size of RE2Ba4CuMOy particles is much finer and typically 50∼300 nm, implying that the pinning may be improved with the dispersion of RE2Ba4CuMOy particles. In this study, we have studied the effects of the BaSnO3 addition to Y2BaCuO5 (Y211) powders. It was found that two different compounds of Y2Ba4CuSnOy and Y2BaCuO5 phase particles were co-produced by adding BaSnO3 to Y211. These powders were added to the precursor and subjected to the melt-textured process to grow bulk Y-Ba-Cu-O superconductors of 20 mm diameter with the top-seed melt growth process. The finished bulk samples were observed with optical microscopy and subjected to trapped field measurements with Hall sensors. The trapped field measurements showed that BaSnO3 addition was effective in improving the field trapping ability.

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2−x La x CuO6+δ and Nd2−x Ce x CuO4−δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi2Sr2−xLaxCuO6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd2−xCexCuO4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO2 plane and superconductivity fluctuations in the insulating phase.

XRD spectra of new YBaCuO superconductors

XRD spectra of new YBaCuO superconductors were studied. There were 2 phases found in our samples, the superconducting phase and the non-superconducting phase. The more non-superconducting phase, the more anisotropy parameters were found. The amount of impurities have no effect on the value of c-axis which has a linear relation to the number of Cu-atoms. So the new formula of YBaCuO are the new types of superconductor in this family that have higher c-axis than the Y123.

Field Angle Dependence of Critical Current Density in YGdBCO Coated Conductor

The improvement of critical current density properties of REBCO coated conductors for applications in superconducting power devices is desired. It is known that the critical current density Jc of REBCO thin films made by the TFA-MOD method in a magnetic field is greatly improved by the introduction of BZO-nanoparticles of a non-superconducting phase as artificial pinning centers. In addition, the field angle anisotropy can also be reduced by isotropic flux pinning interactions of nanoparticles. However, a detailed report on the mechanism of flux pinning with a quantitative investigation in such coated conductors has not been given. In this study, the field angle dependence of the critical current density is measured for TFA-MOD-processed YGdBCO coated conductor with a superconducting layer of 0.65 μm thick, and the results are theoretically analysed to clarify the flux pinning mechanism.

The Effects of Sb Substitution on Structural Properties in YBa2Cu3O7 Superconductors

In this study, the effects of partial Sb3+ ion substitutions for the Y sites and the Cu sites on the superconducting properties of YBa2Cu3O y (Y123) ceramic superconductors were investigated. The samples were prepared by the conventional solid-state reaction method and the properties of the samples were investigated by means of X-ray diffraction, AC magnetic susceptibility measurements, scanning electron microscope, and energy dispersive X-ray analysis. The critical temperatures were determined to be in the range of 80–92 K for both Systems I and II. It was found that Sb-addition leads to the formation of the non-superconducting YBa2SbO6 phase, which has a negative effect on the critical temperature, since the highest critical temperature was measured for pure Y123. However, the increasing substitution level has a negligible effect on the X-ray diffraction analysis peak intensities of the superconducting phases. In addition, SEM images showed that Sb substitution decreases the grain size and modifies the microstructure development, which makes the samples denser.

Synthesis and superconductivity of MgB2/Fe–Se–Te composites

MgB2/FeSe0.75Te0.25 composites were synthesized to see the effect of the co-existence of superconducting phases on the superconductivity. MgB2 powders were mixed with pre-alloyed FeSe0.75Te0.25 or elemental powder mixture, followed by sintering at 400°C. The resulting materials were superconducting MgB2/FeSe0.75Te0.25 composites without impurity phase when using pre-alloyed FeSe0.75Te0.25 as a starting powder. When FeSe0.75Te0.25 was prepared by in situ reaction from the elemental powder mixture of Fe–Se–Te, on the other hand, non-superconducting impurity phases partly formed after sintering. No double superconducting transition was observed in MgB2/FeSe0.75Te0.25 composites in spite of coexistence of two superconducting phases.

Structure of vacancy-ordered single-crystalline superconducting potassium iron selenide

With single crystal X-ray diffraction studies, we compare the structures of three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2, K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a ({\sqrt}5 x {\sqrt}5 x 1) super cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250K of a = b = 8.729(2) {\AA} and c = 14.120(3) {\AA}, a = b = 8.7186(12) {\AA} and c = 14.0853(19) {\AA} and at 295 K, a = b = 8.8617(16) {\AA} and c = 15.304(3) {\AA} for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty, whilst the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2) % to 85.3(3) %. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se - F - Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors distinguishes themselves from the structure of the non-superconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site as well as lower alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity.

Effect of Carbon Substitution on the Superconducting Properties of Nanocarbon-, Nanodiamond- and Nano-SiC-Doped MgB2

A comparative study on the effect of doping of nanocarbon (C), nanodiamond, and the well-established nano-SiC in MgB2 is carried out under identical doping levels (C: 2.6 and 5.2 wt%) and processing conditions (800°C, 2 h). The bulk samples are prepared via powder-in-sealed-tube method. The XRD analysis shows no reacted or unreacted phases in the case of lightly nano-C-doped sample while all other doped samples show unreacted or/and reacted phases. The a-axis shrinkage is observed for all doped samples, among which nano-C-doped samples show maximum shrinkage, an evidence for C substitution at B sites. The C substitution is also confirmed by the analysis of full-width at half-maximum values for (100), (101), (002), and (110) peaks which are broadened. The C substitution together with the presence of nonsuperconducting phases decreases the TC to a small extent. The JC(H) is significantly enhanced for all doped samples compared with the pure sample among which MgB1.9C0.1 (nano-C) exhibits the best performance in JC enhancement up to a field of 8 T.

The Effect of Sintering Temperature Variation on the Superconducting Properties of ErBa2Cu3O7−δ Superconductor Prepared via Coprecipitation Method

The effect of heat treatment on the superconducting properties of ErBa2Cu3O7−δ (ErBCO) ceramic materials has been studied. The nano-metal oxalate precursor was prepared using coprecipitation (COP) method. The prepared materials were subjected to calcination process at 900 °C for 12 h and then sintered under oxygen environment for 15 h at 920 °C, 930 °C, 940 °C, and 950 °C, respectively. All samples showed a metallic behavior and single-step transition in the R–T curves. The best zero critical current, TC(R=0)=91.4 K, was for the sample sintered at 920 °C. XRD data showed single phase of an orthorhombic structure. As the sintering temperature increases, the formation of nonsuperconducting phases (impurities) was observed when the samples sintered above 920 °C. The formation of nano-oxalate powders via COP method is a very efficient procedure to produce high-quality superconductors with less processing temperature required.

Enhanced flux pinning and formation ofBa4Y2CuMoOy in top-seeded meltgrowth processed YBa2Cu3O7 − d superconductors with Mo additions

The effect of Mo addition (0–10 wt%) on the superconductivity of top-seeded melt growth (TSMG)processed YBa2Cu3O7 − y (Y123) superconductors was studied. The low level Mo addition (≤1 wt%) led to a small decrease of the superconducting transition temperature (Tc) and increase of the critical current density (Jc).The Jc improvement induced by the low level Mo additions appeared as a peak effectat the intermediated magnetic fields and peak position shift to the lowermagnetic fields with increasing Mo content. The enhanced flux pinning causedby Mo additions seems to be attributed to the partial Cu substitution by Mo,YBa2(Cu1 − xMox)3O7 − d. The high level Mo additions (2–10 wt%), however, led to a largeJc decrease and broad superconducting transition due to the formation of low-Tc phases and the increased volume of the non-superconducting Mo-containing phase. Thesecond particle phase formed by the high level Mo additions was identified asBa4Y2CuMoOy (Mo4211) by x-ray diffraction (XRD) and scanning electron microscopy energy dispersivex-ray (SEM EDX) analysis.

ChemInform Abstract: Bi-Sr-Ca-Cu-O High-Tc Superconductors Produced by Using a Rapid Thermal Melt Processing Technique.

Bi–Sr–Ca–Cu–O high-Tc superconductors have been fabricated using a rapid thermal heat treatment technique. The conventionally sintered pellets were rapidly heat treated for 2 min at 1200 °C with a heating rate of 1000 °C min–1 and then annealed. The rapidly heat-treated samples exhibited non-superconducting crystalline phases which became superconducting upon annealing in air. A Tc(zero) of 105 K was obtained for the sample annealed at 861 °C for 240 h. 30% Pb was substituted for Bi in order to stabilize the high-Tc(110 K) phase. Correlations between the processing conditions and the resulting properties of sintered and rapidly heat-treated samples have been studied. Based on low-field magnetization data, the low critical field, Hc1, of heat-treated samples was estimated to be 14% higher than that of the as-sintered samples. The bulk density increased by 10% after thermal heat treatments. In the rapidly heat-treated samples, 95% of the phases was estimated to be the high Tc(2223) phase. X-Ray analysis data showed that the undesirable impurity phases were eliminated after heat treatment. Additionally, the effect of the magnetic field on the electrical resistance has been studied as a function of temperature, and Hall effect measurements have been carried out down to the superconducting state from pre-onset conditions to essentially zero resistance.

ChemInform Abstract: Superconducting and Non-Superconducting (Down to 4.2 K) Phases of MA′A′ ′2Cu2Ox Compounds with Perovskite-Like Structure

ChemInform Abstract: Superconducting and Non-Superconducting (Down to 4.2 K) Phases of MA′A′ ′2Cu2Ox Compounds with Perovskite-Like Structure

Preparation of Y0.5[Yb1−x Nd x ]0.5Ba2Cu3O z Superconductors Using Nitrate Routes

The chemical solution nitrate route was used to prepare Y0.5[Yb1−xNdx]0.5Ba2Cu3Oz superconducting phase with x=0.1, 0.2 and 0.3. Different calcination temperatures from 870 to 900 °C were used. Both XRD and DSC techniques were utilized in monitoring the Y0.5[Yb1−xNdx]0.5Ba2Cu3Oz superconducting phase and the non-superconducting impurity phases such as BaCuO5, CuO and RE2BaCuO5. These two techniques were used to study the effect of impurity phases on the decomposition temperature of Y0.5[Yb1−xNdx]0.5Ba2Cu3Oz superconducting phase. The value of x, the Nd amount, and the calcination temperature both influenced the type and number of impurity phases, which resulted in changing the eutectic and peritectic reaction temperatures.

Influence of Mg deficiency on crystal structure and superconducting properties inMgB2single crystals

The effects of high-temperature vacuum-annealing induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 C, the average Mg content in the MgB2 crystals systematically decreased, while Tc remains essentially unchanged and the superconducting transition slightly broadens from 0.55 K to 1.3 K. The reduction of the superconducting volume fraction was noticeable already after annealing at 875 C. Samples annealed at 975 C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(VMg)xB2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence Tc, in agreement with observations. The significant reduction of the superconducting volume fraction without noticeable Tc reduction indicates the coexistence, within the same crystal, of superconductive and non-superconductive electronic phases, associated with regions rich and poor in Mg vacancies.

10.1007/s11455-008-1002-1

A new method is proposed for growing Y1Ba2Cu3O7−δ (YBCO) rods and special growth regimes are described which provide decreasing losses in these rods in the superconducting state. It is shown that the thermocapillary mechanism of liquid phase transfer during the melting and crystallization of YBCO is responsible for the formation of a nonsuperconducting phase at the boundaries between crystal blocks. Results of the measurement of current-voltage characteristics of YBCO rods in the superconducting state are presented.

On the non-superconducting state in the phase diagram of NaxCoO2·yH2O

For the understanding of the superconducting state of NaxCoO2·yH2O (x∼0.3), we have carried out various kinds of experimental studies. Among results of these studies, we show here data of the specific heats, frequency-dependent ac-magnetic susceptibility and 59Co-NQR and 2D NMR and argue what electronic state is realized in the nonsuperconducting phase, which appears in the T–3νQ phase diagram of this system, νQ being the Co-NQR quadrupole frequency. The transition observed in this phase at ∼7K is not due to a magnetic ordering but possibly due to a charge ordering or the CDW formation. Slightly below this transition temperature, the system exhibit glassy behavior. The results indicate that the superconducting phase cannot necessarily be considered to be located near the magnetic phase.

Studies on the superconducting state of NaxCoO2·yH2O – Overview

Results of experimental studies carried out to clarify the superconducting state of NaxCoO2·yH2O (x∼0.3; y∼1.3) are presented. Various means such as NMR, neutron magnetic scattering, and specific-heats and magnetic-susceptibility measurements have been applied to both poly- and single-crystalline samples to obtain internally consistent data. Effects of non-magnetic impurities and oxygen isotope substitution on the superconducting transition temperature Tc have also been studied. Data obtained for the mother system NaxCoO2 have also presented useful information on the superconducting state of NaxCoO2·yH2O. In the course of these studies, we have shown by measuring Knight shifts of 59Co and 23Na of aligned crytalline samples that the Cooper pairs are in the singlet state. Neutron inelastic scattering measurements on large single crystals have presented the firm evidence for the disappearance of the low energy ferromagnetic excitation with decreasing T, excluding the triplet Cooper pairing in NaxCoO2·yH2O consistently with the Knight shift. In the Tc–νQ phase diagram, it has been confirmed that there is a nonsuperconducting phase, which divides the superconducting region into two, νQ being the nuclear quadrupole frequency. The possible different symmetries between the two superconducting phases suggested by this characteristic νQ dependence shown in the phase diagram, has been carefully examined mainly by specific heat measurements and NMR. However, any difference between the pair states of these phases has not been found. The appearance of the nonsuperconducting phase should be considered due to an instability which appears within a single superconducting phase.

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

Abstract The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived x Nb 2 O 5 −(100− x )TiO 2 coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.

Search for new iron oxypnictide superconductors by using high-pressure synthesis technique

Here we report on the invention of the Co-doped induced superconductivity in PrFe 1− x Co x AsO ( x = 0 and 0.075). Polycrystalline samples were prepared by high-pressure synthesis method. The PrFe 1− x Co x AsO ( x = 0 and 0.075) crystallizes with the ZrCuSiAs type crystal structure (P4/nmm). The PrFeAsO is a parent compound with non-superconducting phase which can be transformed to superconductor by replacement 7.5% of Fe by Co. The magnetic susceptibility measurement shows the onset of superconducting transition temperature at 15 K.

Structure and electrical characterization of Li 1− xNbO 2 superconductor with Tc = 14–17 K

The crystal structure and electrical properties of Li 1− x NbO 2 have been investigated using XRD and electrical measurements. Li 1− x NbO 2 crystallizes in a hexagonal layered structure. A superconducting transition with T c = 14–17 K was observed. However, this transition significantly depends on the Li content in compound. It implies that the superconductivity in Li 1− x NbO 2 is induced by Li-removal from the near stoichiometric non-superconducting phase. Li 1− x NbO 2 shows a metallic characterization for Li content being more than 0.90 and a superconducting behavior for Li content in the range of 0.50–0.60, respectively. The lattice parameters and unit cell volume of Li 1− x NbO 2 compound increase with decreasing Li content from 0.90 to 0.50. And the upper critical field H c2(0) is 12.1 ± 0.5 T.