Enhancing the pinning strengths in polycrystalline MgB2

The dependence of the critical current density, J_c, on the gamma irradiation dose in MgB₂ polycrystalline superconducting specimens has been studied. The MgB₂ samples were irradiated with 10 MR and 20 MR doses of γ‐rays. The critical current density, J_c was found to be enhanced almost at all temperatures after irradiation. The enhancement of J_c was found to be linear with the irradiation dose at all temperatures. We also report on the correlation between the superconducting transition temperature and the residual resistivity ratio. Most of our results agree with the Testardi and Mooij rules which suggest that the dominant pairing mechanism of MgB₂ is the phonon‐mediated interaction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magneto‐transport properties of polycrystalline YBa2(Cu1‐xMx)3O7‐δ (M = B and Mn)

The magnetic and transport properties of polycrystalline YBa₂ (Cu_1‐xM_x)₃ O_7‐δ (M = B and Mn) superconductor was investigated. Samples of YBa₂(Cu_1‐xB_x)₃O_7‐δ doped with several concentrations of boron B(x = 0.05 and 0.1) were investigated using magnetization measurements. A YBa₂(Cu_1‐xMn_x)₃O_7‐δ sample doped with Mn with concentration of x = 0.02 was investigated using current‐voltage (I‐V) measurements. Our results on the YBa₂(Cu_1‐xB_x)₃O_7‐δ samples reveal a considerable increase in the hysterisis width of the magnetization, M versus the applied magnetic field H with increasing boron concentration. The lower critical field was also found to be enhanced by boron doping. The critical current density, J_c was found to be significantly enhanced in the Mn‐doped sample. The enhancement of J_c was found to be more significant at the lower temperatures for all applied magnetic fields used (0 Oe, 300 Oe, and 500 Oe). Thus, chemical doping is suggested to enhance the vortex pinning forces in the YBCO samples. From the resistivity (R‐T) measurements, chemical doping of the samples was found to have no significant effect on the critical temperature, T_c. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ferromagnetic insulating and reentrant spin glass behavior in Mg doped La0.75Sr0.25MnO3 manganites

In this study, the effect of Mg substitution on structural, magnetic and electrical properties of La_0.75Sr_0.25Mn_1?xMg_xO₃ and La_0.75Sr_0.25?xMg_xMnO₃ (nominal compositions) samples are investigated by XRD, Ac susceptibility and electrical resistivity measurements. It is found that Mg does not replace La in the perovskite lattice. Also the results show that by increasing Mg doping levels, the paramagnetic–ferromagnetic and metal–insulator transition temperatures decrease. The reason for decreasing transition temperatures with increasing Mg concentration is, that the long-range FM order has been destroyed by the Mg, which is randomly occupying Mn site. This leads to the suppression of double-exchange interaction in the Mn³⁺–O–Mn⁴⁺ networks. Also the reentrant spin glass (RSG) state accompanied by FM transition, exists in high doped samples. The RSG state could be understood on the basis of double exchange ferromagnetic interaction in Mn³⁺–O–Mn⁴⁺ and super-exchange antiferromagnetic interaction in the Mn⁴⁺–O–Mn⁴⁺ networks.     

Microwave dielectric properties of La(1‐2x/3)Bax(Mg0.5Sn0.5)O3 ceramics

This study examined the potential applications of microwave dielectric properties of La_(1‐2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics in rectenna. The La_(1‐2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid‐state method with various sintering temperatures. An apparent density of 6.62 g/cm³, a dielectric constant of 20.3, a quality factor of 51,700 GHz, and a temperature coefficient of resonant frequency of ‐78.2 ppm/K were obtained for La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase Formation,Crystal Structure and Electrical Properties of La212 Type Compound Substituted by Silver or Praseodymium

The La212 type compounds substituted by silver or praseodymium are prepared by solid state reaction method. It is found that compounds La_2‐xSr_xCa_0.5Pr_0.5Cu₂O₆, La_1.6Pr_0.4Ca_1‐xSr_xCu₂O₆ and La_2‐xPr_xCa_0.5Sr_0.5Cu₂O₆ can be formed for x=0.4‐1.1, 0‐0.5 and 0‐1.5, respectively. A new member of La212 type compounds, La_2‐xAg_xCaCu₂O₆ is also prepared. Their structures are verified by Rietveld structure refinement to belong to the structure type of La212 cuprate oxide with space group I4/mmm. Their electrical properties are investigated. La_1.65Ag_0.35CaCu₂O₆ displays metal‐like behavior and its resistivity decreases with the decrease of temperature from 300K to 4.2K.     

Influence of simultaneous doping of Tl and Bi on microstructure and critical current density of HgBa2Ca2Cu3O8+δ

It has been found that simultaneous doping of Tl and Bi at Hg site of HgBa₂Ca₂Cu₃O_8+δ HTSC phase induces interesting microstructural variants in the form of long period polytypoid (LPP) like structure embodying native defect substructures. It has been observed that the as synthesized (HgTl_0.2−xBi_x)Ba₂Ca₂Cu₃O_8+δ (with x = 0.05, 0.10, 0.15) phases have superconducting onset transition temperature (T_c) of about ∼133 K. However, the intragrain critical current density (J_c) for the various materials with different compositions varies significantly. It varies from 6.2 × 10⁶ A/cm² to 2.9 × 10⁶ A/cm² at 5 K and 1 T for (HgTl_0.10Bi₁₀)Ba₂Ca₂Cu₃O_8+δ and (HgTl_0.15Bi_0.05)Ba₂Ca₂Cu₃O_8+δ HTSC phase respectively. A correlation between the intragrain J_c and the defect substructures has been found to be present. This correlation has been described and disscussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the effects of Ga3+ and La3+ ions in MgCu ferrite: Humidity‐sensitive electrical conduction

Pure and gallium or lanthanum substituted MgCu ferrites, Mg_0.5Cu_0.5Fe_2‐xM_xO₄ (with x = 0 or 0.2 and M = La or Ga) have been prepared by solid state reaction. Sintering experiments were carried out at different temperatures between 900 and 1100°C. The phase composition and lattice parameters were determined by XRD, while the effect of Ga and La substitutions on the granular structure was studied by SEM. Experimental results revealed that the densification behaviour and some physical properties are in close relation with the structural changes of pure ferrite caused by the presence of La and Ga substitutions. The gallium containing compound, Mg_0.5Cu_0.5Fe_1.8Ga_0.2O₄, is monophasic and contains a great number of pores, whereas the lanthanum containing compound, Mg_0.5Cu_0.5Fe_1.8La_0.2O₄, is biphasic and exhibits a high density. The humidity characteristics show that the gallium ion enhances the humidity sensitivity of the MgCu ferrite sintered at 1000°C. This interesting effect is promising for the future of the Ga‐substituted MgCu ferrite to be used as sensitive material for fabrication of ceramic humidity sensors. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Particularities of Infinite‐Layer Compounds Sr1–xLnxCuO2–δ (Ln=La,Pr, Nd,Eu)

The superconducting phases with general formula Sr_1–xLn_xCuO_2–δ with Ln=La (Tc=24…40K); Pr (Tc=30…44 K); Nd (Tc=8K…40 K); Eu (Tc…40 K) and non‐superconducting phases of CaCuO₂– type structure were prepared by high‐pressure synthesis. The products were examined by X‐ray powder diffraction. For the determination of composition Rietveld method was used. Structure investigations have shown that non‐superconducting phases have oxygen vacancies. With in the Cu formal charge range from 1.81 to 1.93 the infinite layer cuprates exhibit superconductivity. Coexistence of both superconducting and non‐superconducting phases of the CaCuO₂–type structure in samples with Ln=Nd and Eu was found.     

Role of Silver on Superconducting Properties of BPSCCO (2223) System

The role of silver for the superconducting properties of the Bi Pb Sr Ca Cu O (2223) system have been investigated systematically. Samples with various concentrations of Ag were prepared by the matrix method. D.C. resistance results showed that T_c(0) varied between 100–109 K for various compositions of Ag. J_c measured at LNT from I–V data shows an increase with silver addition. J_c decreased from 90 Amp/cm² at zero field to 16 Amp/cm² at 100 Oe. The T_c(0) obtained form the susceptibility data agreed with those obtained from resistance measurements. X-ray diffraction results showed that the dominant phase in BPSCCO is the high-temperature (2223) phase and the same is improved in silver added BPSCCO samples. The scanning electron micrographs indicated that silver is precipated along the grain boundaries, separating the superconducting grains. These results have showed that silver addition does not destroy the superconductivity and at the same time enhances the critical current density.     

Resistance and Current Density Studies on Plasma Sprayed Coatings of Bi(Pb)?Ca?Sr?Cu?O Superconducting Materials

Plasma spraying is a potential technique for forming flexible tapes from brittle high T_c oxides. It is possible to obtain superconducting Bi(Pb) Ca Sr Cu O coating by suitable heat treatment schedule after spraying. In an effort to get maximum transport current densities (J_c) of the coating, the content of lead and sintering time have been optimised. A J_c value of 200 Amp/cm² is obtained in Bi_1.4Pb_0.6 · Ca₂Sr_1.9Cu₃O_y specimen coated on silver sprayed Fe[(Ag)/Fe] substrate. Remarkable improvement in J_c values up to 694 Amp/cm² is obtained in the same specimen coated on Ca₂Sr_1.9Cu₃O_y sprayed Fe[Ca₂Sr_1.9Cu₃O_y)/Fe] substrate. The observed decrease in J_c(B) curves with increase in magnetic field shows the presence of weak coupling between the grains.     

Studies on electrical conduction behavior of La1‐3xCaxBaxSrxMnO3 synthesized by chemical route

In the manganite La_1‐xM_xMnO₃ (M = Ca, Ba, Sr) the doping concentration introduces a mixed valency (Mn³⁺, Mn⁴⁺) which governs the magnetic and electrical properties of the compound. The perovskite oxides La_1‐3xCa_xBa_xSr_xMnO₃ (x = 0.00, 0.05, 0.10) were prepared by chemical method. Single‐phase formation is confirmed by XRD studies. The electrical behavior of compositions with x = 0.00, 0.05 and 0.10 in the system La_1‐3xCa_xBa_xSr_xMnO₃ was studied in the temperature range 300‐420 K. It is observed that conductivity decreases with increasing temperature as well as dopants concentration. Metallic behavior of these compositions decreases with increasing dopants concentration (x). The microstructures of these samples have been characterized using scanning electron microscopy (SEM). (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Twin Structure of (La,Nd)GaO3 Solid Solutions

For La_1‐xNd_xGaO₃ crystals the La‐Nd substitution leads to decrease of spontaneous strains and for composition with x≈0.32 the six possible twin states of orthorhombic phase (m3mFmmm) may be degenerated in three twin states inhered in m3mF4/mmm species when a distorted perovskite pseudocell becomes tetragonal. The {110} and {112} reflection twins and axial twins with compositional planes close to (211) and (21‐1) (S‐walls) were identified in La_1‐xNd_xGaO₃ (x=0.07, 0.12, 0.20) solid solutions crystals. All observed twins are typical for crystals with GdFeO₃ type perovskite‐like structure. It has been shown that for x≤0.2 and x≥0.5 orientations of S‐walls weakly depend on La/Nd ratio, whereas in the range of 0.2<x<0.5 they depend strongly on the solid solution composition. The tilt angle between two twin states across twin boundary in La_1‐xNd_xGaO₃ (x<0.6) solid solutions is smaller than that between two twins in pure LaGaO₃ or NdGaO₃.     

X-ray diffraction studies of the structure of LaA1Ga metallic glasses

The LaLa and LaA1 partial atomic distribution functions have been determined for glassy La_1?xA1_x from X-ray diffraction studies of isomorphous alloys of La_1?xA1_x, La_1?x(A1Ga), and La_{1 ?x}Ga_x for x = 0.20, 0.24 and 0.28. The atomic short range order of these La based metallic glasses is quite different from that of typical amorphous transition metal-metalloid alloys and from dense random packing models. A relatively short and well defined LaA1 nearest neighbor distance suggests some covalency in the bonds between unlike atoms and possible chemical ordering in the alloys.     

Magnetoresistance,voltage‐current characteristics,and Hall effect measurements of bulk MgB2 superconductors

Bulk MgB₂ samples were prepared from the commercially available powder (Alfa‐Aesar). One of the samples was used in measuring the transport properties by the DC four‐probe technique while the other was used in measuring Hall effect using the van‐der‐Pauw configuration. From the transport measurements, we noticed that the R‐T curves shift to lower temperatures under applied magnetic field without any observed enlargement of the transition width. T_c gradually decreases from 37 K at zero field to 32 K at B = 1.4 T. Our V‐I data were found to obey a power law expression V ∝ I^{β(T, B)}. The change of β with temperature and magnetic field was shown and discussed. R_H is positive under positive applied magnetic field. The 1/ R_H linear dependency on T, usually observed in high temperature superconductors, could not be observed in our case. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of Pb‐ion irradiation on melt‐textured YBa2Cu3Ox crystals

Melt‐textured YBa₂Cu₃O_x crystals have been irradiated along the c‐axis with ²⁰⁸Pb⁵⁶⁺ ions corresponding to dose matching fields, B_Φ = 0.5 T and B_Φ = 2.0 T. Magnetization measurements were conducted along the ab plane of the samples. The strength of pinning sites was investigated by measuring magnetization hysterisis and the saturation remanent magnetization M_R at several temperatures. We have found that the pinning strength was considerably enhanced after irradiation at both doses. Interestingly, the pinning strength at a Pb‐ion irradiation which corresponds to the dose matching field B_Φ = 0.5 T, was found to be significantly larger than that at the dose matching field B_Φ = 2.0 T at all temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and electrical properties of Mg‐doped ZnO nanoparticles

Mg_xZn_1‐xO (x=0.01‐0.3) nanoparticles were synthesized by the sol‐gel technique using solutions of Mg and Zn based organometalic compounds. The electrical properties of Mg doped zinc oxide (ZnO) were studied within wide temperature range from 300 to 500 K under the N₂ gas flow (flow rate: 20 sccm) and in the frequency range from 40 Hz to 1 MHz for ac electrical measurements. The dc conductivities and the activation energies were found to be in the range of 10^‐9‐10^‐6 S/cm at the room temperature and 0.26‐0.86 eV respectively depending on doping rate of these samples. The ac conductivity was well represented by the power law Aω^s. The conduction mechanism for all doped ZnO could be related to correlated barrier hopping (CBH) model. The complex impedance plots (Nyquist plot) showed the data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the nanoparticle grains. The crystal structures of the Mg_xZn_1‐xO nanoparticles were characterized using X‐ray diffraction. The calculated average particle sizes values of Zn_1‐xMg_xO samples are found between 29.72 and 22.43 nm using the Sherrer equation. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterization of superconducting MgB2 rods

Superconducting Cu/Fe/MgB₂ and Steel/Cu/MgB₂ rods were prepared through the ex‐situ modified PIT technique with explosive consolidation method. The phase composition, microstructural features were investigated by X‐ray diffraction and scanning electron microscope. The AC susceptibility for a core samples was measured by the AC susceptometry. Measurements of the magnetization hysteresis loops were also carried out. It is found that no impurity phase was formed in MgB₂ rods. In addition, it was found that grains in both Cu/Fe/MgB₂ and Steel/Cu/MgB₂ rods are densely packed. The magnetic measurements show that the critical temperature of Cu/Fe/MgB₂ and Steel/Cu/MgB₂ rods is 37.7 K and 37.6 K, respectively. The high critical current density of 9.22 × 10⁵ A/cm² (4.1 K, 0.2 T) was obtained for Steel/Cu/MgB₂ rod. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some structural considerations on the perovskite‐type A1‐ySryCo1‐xFexO3‐δ solid solution series

Solid solution series of La_1‐ySr_yCo_1‐xFe_xO_3‐δ were extensively studied in the past as cathode materials for solid oxide fuel cells. However, the crystal structure behavior of La_1‐ySr_yCo_1‐xFe_xO_3‐δ solid solution series when La‐ions are replaced with another rare‐earth ion or metallic alkaline earth metal is at present not fully understood. Here we report X‐ray powder diffraction measurements performed on samples of the Sm_0.8Sr_0.2Co_1‐xFe_xO_3‐δ solid solution series. This study demonstrates that the average A‐cation radius, as well as the Fe content (x), affects the structural modification of the A_1‐ySr_yCo_1‐xFe_xO_3+δ solid solution series significantly. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Substitution effects on ferroelectric,leakage current and anti‐fatigue characteristic of Bi4‐xSbxTi3O12 thin films

Bi_4‐xSb_xTi₃O₁₂ (BSTO) (x = 0, 0.03, 0.04, 0.05, 0.06 and 0.07) thin films have been fabricated on Pt/Ti/SiO₂/Si substrates by sol‐gel method. The effects of various Sb³⁺ content on microstructure and ferroelectric properties of systems are investigated. XRD show that Bi_4‐xSb_xTi₃O₁₂ (x≠0) thin films prefer (117) orientation. The substitution Sb³⁺ for Bi³⁺ reduces the grain size of the film surface. Compared to the BTO (x = 0) film, Bi_4‐xSb_xTi₃O₁₂ films display exciting electric properties. Especially when x = 0.04, the film Bi_3.96Sb_0.04Ti₃O₁₂ has achieved the max 2Pr value of 87μC/cm². This film also has a better anti‐fatigue characteristic, which can be up to 10¹⁰ switching cycles without fatigue. The leakage current density improved with J = 8×10⁻⁸ A/cm².