Showing papers in "Journal of Superconductivity and Novel Magnetism in 2012"

Structural, Magnetic, and Optical Properties of Zinc- and Copper-Substituted Nickel Ferrite Nanocrystals

Abstract: Ferrite nanocrystals are an interesting material due to their rich physical properties. Here we add non-magnetic dopants Zn and Cu to nickel ferrite nanocrystals, Ni1−x M x Fe2O4 (0≤x≤1, M=Cu, Zn), and study how relevant properties of the samples are modified accordingly. Basically, these dopings cause a rearrangement of Fe+3 ions into the two preexisting octahedral and tetrahedral sites. In fact, this, we show, induces pertinent magnetic properties of the doped samples. In the case of the Cu-doping, the Jahn–Teller effect also emerges, which we identify through the Fourier Transform Infra-Red Spectroscopy of the samples. Moreover, we show an increase in the lattice parameters of the doped samples, as well a superparamagnetic behavior for the doped samples is shown, while the Jahn–Teller effect precludes a similar behavior in the CuFe2O4 nanocrystals. The influences of Zn and Cu substitutions are investigated on the optical properties of nickel ferrite nanocrystals by photoluminescence measurement at room temperature.

Nanographene Magnetic Properties: A Monte Carlo Study

Abstract: The magnetic properties of an Ising antiferromagnetic model on a nanographene lattice which have spins that can take the values S=±3/2, ±1/2, are studied by Monte Carlo simulations. We only consider the nearest-neighbor interactions between the sites i and j. The zero field cooled and field cooled magnetizations, magnetic susceptibilities and hysteresis cycle are obtained for a nanographene structure with the effect of exchange interaction between sites i and j. The blocking temperature is also obtained for specific values of the external magnetic field.

Photoemission Spectroscopy of Ta2NiSe5

Abstract: We report temperature-dependent angle-resolved photoemission spectroscopy measurement of Ta2NiSe5 which shows a semiconductor-semiconductor structural phase transition at around 330 K. Characteristically, flat band at the top of the valence band is observed, which is ascribed to the excitonic insulator effect. The top valence band shifts to higher binding energy and its bandwidth increases as the temperature decreases. As the system exceeds the transition temperature, the flat feature of the valence band weakens though the exciton fluctuations remain finite.

The Influence of SnO2 Nano-Particles Addition on the Vickers Microhardness of (Bi, Pb)-2223 Superconducting Phase

Abstract: The Vickers microhardness of Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconducting phase with added SnO2 nano-particles was investigated. The concentration of SnO2 nano-particles x varied from 0.0 to 2.0 of the sample’s total mass. The experimental data of the Vickers microhardness were analyzed using Meyer’s law, the elastic/plastic deformation model, Hays–Kendall’s approach and the proportional specimen resistance model. The Vickers microhardness number Hv increased as x increased up to 0.4 wt%. The load dependence of Hv exhibited a normal indentation-size effect, Hv increased as the applied load increased. Moreover, this dependence was well fitted with the Hays–Kendall approach rather than the elastic/plastic deformation and proportional specimen resistance models. In addition, we calculated Young modulus, yield strength, fracture toughness and brittleness index as a function of both applied load and SnO2 nano-particle concentration.

Role of Cerium Addition on Structural and Superconducting Properties of Bi-2212 System

Abstract: This study examines the significant changes in the structural and superconducting properties of cerium (Ce) doped Bi-2212 superconductors via X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistance and transport critical current density (Jc) measurements. Ce concentration is varied from x=0.0 until 0.1 in a general stoichiometry of Bi1.8Sr2.0CexCa1.1Cu2.1Oy. Zero resistivity transition temperatures (\(T_{c}^{\mathrm{offset}}\)) of the samples produced by the conventional solid-state reaction method are deduced from the dc resistivity measurements. Furthermore, the phase fractions and lattice parameters are determined from XRD measurements when the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. The results show that \(T_{c}^{\mathrm{offset}}\) and Jc at self-field of the samples reduced gradually with the increase in the Ce addition. Maximum \(T_{c}^{\mathrm{offset}}\) of 79.7 K and Jc of 356.8 A⋅cm−2 at 77 K are obtained for pure sample as against 44.6 K and 18.7 A⋅cm−2, respectively, for the sample doped with 0.1 wt.% Ce. According to the refinement of cell parameters done by considering the structural modulation, the Ce doping is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample. As for SEM measurements, it is found that not only do the surface morphology and grain connectivity degrade but the grain size of the samples also decreases with the increase of the Ce addition. Moreover, EDX images indicate that the elements used for the preparation of samples distribute homogeneously and the Ce atoms enter into the crystal structure by replacing Cu atom. In addition, the variation of ΔTc (\(T_{c}^{\mathrm{onset}} -T_{c}^{\mathrm{offset}}\)) is investigated for the presence of impurities and weak links between superconducting grains of the samples. The possible reasons for the degradation in microstructural and superconducting properties are also interpreted.

Effect of Mn Addition on Structural and Superconducting Properties of (Bi, Pb)-2223 Superconducting Ceramics

Abstract: This study deals with the effect of Mn addition on the structural and superconducting properties of Bi1.8Pb0.4Sr2MnxCa2.2Cu3.0Oy ceramics with x=0,0.03,0.06,0.15,0.3 and 0.6 by means of X-ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), resistivity, and transport critical current density (Jc) measurements. Zero-resistivity transition temperatures (Tc) of the samples produced via the standard solid-state reaction method are estimated from the dc resistivity measurements. Moreover, the phase fraction and lattice parameters are determined from XRD measurements while the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. It is found that Tc values are obtained to decrease from 109 K to 85 K; likewise, Jc values are observed to reduce from 3200 A/cm2 to 125 A/cm2 with increasing Mn addition. According to the refinement of cell parameters done by considering the structural modulation, the Mn addition is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample (Mn0). SEM measurements show that not only the surface morphology and grain connectivity are seen to degrade but the grain sizes of the samples are found to decrease with the increase of the Mn addition as well. The EDX results reveal that the elements used for the preparation of samples distribute homogeneously and the Mn atoms enter into the crystal structure by replacing Sr and Cu atoms. The possible reasons for the obtained degradation in microstructural and superconducting properties are also interpreted.

Relationship between growth speed, microstructure, mechanical and electrical properties in Bi-2212/Ag textured composites

Abstract: Transport properties on Bi-2212 superconductors can be improved by grain orientation methods while mechanical ones can be tuned up by the addition of metallic silver. Both processes can be performed simultaneously using the Laser Floating Zone (LFZ) method. In this work, Bi2Sr2CaCu2Ox/3 wt.%Ag textured composite materials were prepared by a LFZ melting technique at different growth speeds (5, 15, 30, and 60 mm/h). In all cases, the observed microstructure after annealing shows the Bi-2212 phase as the major one. Although growth speed has no effect on the measured Tc values, a drastic change on the Jc values has been found. The best results, both mechanical and transport properties, have been obtained for samples grown at 15 mm/h.

Structural, Magnetic and Microwave Properties of Eu-doped Barium Hexaferrite Powders

Abstract: Eu-doped M-type barium ferrite powders (Ba1−x Eu x Fe12O19) with x=0.0, 0.1, 0.2 and 0.25 were prepared by sol-gel method. The synthesized samples are characterized by thermo gravimetric analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and vector network analyzer. All the synthesized samples have the nearly single-magnetoplumbite phase. The results show that the crystallite size of doped samples is smaller than pure one. The saturation magnetization of doped ferrites decreases by the increase in Eu doping, while the coercivity increases. Maximum coercivity achieved in this study is 6.12 KOe for x=0.25 sample. The enhancement of coercivity by Eu doping is mainly due to the higher magnetocrystalline anisotropy, which is attributed to the partial change of Fe+3 ion to Fe+2 ion. The maximum reflection loss (RL) of −43 dB at frequency range of 12–18 GHz for the x=0.1 sample was obtained. The increase in reflection loss at higher frequency suggests that the Eu-doped sample can be used for the application in microwave devices.

Structural, Optical and Magnetic Properties of Mn-doped CdS Diluted Magnetic Semiconductor Nanoparticles

Abstract: Diluted magnetic CdS:Mn nanoparticles were synthesized by the aqueous solution method with different manganese (Mn2+) concentrations (x=7–10 atom %) at room temperature in nitrogen atmosphere and capped with Thiogelycerol. The X-ray diffraction patterns of CdS nanoparticles with different Mn doping concentration indicated that samples have hexagonal structure at room temperature. Energy dispersive X-ray spectroscopy confirmed incorporative of Mn ions in CdS nanoparticles. UV-Visible spectroscopy is used to investigate optical absorption of Mn-doped CdS. From photoluminescence measurement it was found that the intensity of the luminescence spectra decreases by increasing Mn2+ dopant ions at high precursor concentration. Also, the room temperature ferromagnetic behavior of Mn-doped CdS nanoparticles is discussed by using hysteresis measurement results.

Determination of Superconducting Parameters of GdBa2Cu3O7−δ Added with Nanosized Ferrite CoFe2O4 from Excess Conductivity Analysis

Abstract: Superconductor samples of the type (CoFe2O4) x GdBa2Cu3O7−δ , 0.0≤x≤0.1 wt.%, were synthesized by the conventional solid-state reaction technique and were characterized using X-ray powder diffraction (XRD) and scanning electron microscope (SEM). XRD analysis indicated that the orthorhombic structure of Gd-123 is not affected by nanosized ferrite CoFe2O4 addition, whereas the volume fraction of Gd-123 increased up to x=0.01 wt.%. Excess conductivity analysis of the investigated samples was analyzed as a function of temperature using the Aslamazov and Larkin (AL) model. It exhibited four different fluctuation regions, namely critical (cr), three-dimensional (3D), two-dimensional (2D), and short-wave (sw). The zero-temperature coherence length along c-axis, effective layer thickness of the two-dimensional system, and inter-layer coupling strength were estimated as functions of nanosized ferrite CoFe2O4 concentration. In addition, the thermodynamics, lower and upper critical magnetic fields, and critical current density were calculated from the Ginzburg number. It was found that the low concentration of nanosized ferrite CoFe2O4 addition up to x=0.01 wt.% improved the physical properties of Gd-123, while for x>0.01 wt.%, these properties were deteriorated.

Search for Superconductivity in Ultra-dense Deuterium D(−1) at Room Temperature: Depletion of D(−1) at Field Strength >0.05 T

Abstract: Ultra-dense deuterium D(−1) is expected to be both a superfluid and a superconductor as shown by recent theoretical research. Condensed D(−1) can be deposited on surfaces by a source which produces a stream of clusters. A magnetic field strongly influences the type of material formed. Very little of D(−1) and of the form D(1), which is strongly coupled to D(−1), exists on the magnet surface or within several mm from the magnet surface. Even the formation of D(−1) on the source emitter is strongly influenced by a magnetic field, with a critical field strength in the range 0.03–0.07 T. Higher excitation levels D(2) and D(3) dominate in a magnetic field. The excitation level D(2) is now observed for the first time. The removal of D(−1) and D(1) in strong magnetic fields is proposed to be due to a Meissner effect in long D(−1) clusters by large-orbit electron motion. The lifting of long D(−1) clusters above the magnet surface is slightly larger than expected, possibly due to the coupling to D(1). The previously reported oscillation between D(−1) and D(1) in an electric field is proposed to be due to destruction of D(−1).

A Study on Magnetoresistivity, Activation Energy, Irreversibility and Upper Critical Field of Slightly Mn Added Bi-2223 Superconductor Ceramics

Abstract: This study discusses the effect of Mn addition on the superconducting and physical properties in Bi1.8Pb0.4Sr2Mn x Ca2.2Cu3.0O y bulk superconductors with x=0,0.03,0.06,0.15,0.3, and 0.6 by means of the magnetoresistivity measurements. The magnetoresistivity of the samples prepared using the standard solid-state reaction method was measured for different values of the applied magnetic field strengths. The superconducting and physical properties of the samples such as the zero resistivity transition temperatures (T c ), irreversibility fields (μ 0 H irr ), and upper critical fields (μ 0 H c2) were deduced from the magnetoresistivity curves. Moreover, thermally activated flux creep model was studied for activation energy (U 0) values of the samples. According to the results of the measurements, not only were the T c and U 0 values of the samples found to decrease significantly but the μ 0 H irr and μ 0 H c2 values were also observed to reduce with the increase in the Mn addition, indicating that the doping degrades the physical and superconducting properties of the samples.

Synthesis and Characterization of Dendrimer-Encapsulated Iron and Iron-Oxide Nanoparticles

Abstract: In this paper, a series of iron (Fe) containing nanoparticles were prepared by employing PAMAM (Poly(amidoamine), dendrimers with different generations (G0-G3) as templates and sodium borohydride a ...

New results on Magnetic Properties of Tin-Ferrite Nanoparticles

Abstract: This paper presents new physical properties of nanocrystalline SnFe2O4 ferrites, synthesized by co-precipitation method. Magnetization measurement indicates superparamagnetic behavior; the blocking temperature is about 300 K. We performed X-ray diffraction, infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), Energy Dispersive Spectroscopy (EDS), UV-visible measurement, superconducting quantum interference device (SQUID) and zero-field-cooled (ZFC)/field-cooled (FC) measurements. TEM images show the high crystallinity and grain size of ferrite nanocrystals. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is a partially inverse spinel. The obtained UV–vis data were used to calculate the energy band gap (3.82 eV) of nanocrystalline SnFe2O4. On the other hand the magnetic properties of the samples; saturation magnetization (M s ) and coercive field (H c ) were determined using a superconducting quantum interference device (SQUID).

Effect of Synthesis Parameters on the Properties of Superparamagnetic Iron Oxide Nanoparticles

Abstract: Iron oxide nanoparticles were coprecipitated in air medium using different sodium hydroxide (NaOH) concentrations, and their structural and magnetic properties were studied. It was observed that the precipitation of superparamagnetic iron oxide nanoparticles could be achieved above a critical NaOH concentration. This was followed by the investigation of the effect of the stirring rate on the structural and magnetic properties of the nanoparticles precipitated at 8.5 M NaOH and over. Morphological observation made by a transmission electron microscope (TEM) showed that the particle size of iron oxide nanoparticles was around 7.5 nm. Magnetization curves measured by a vibrating sample magnetometer showed zero coercivity indicating that the samples are superparamagnetic and the highest saturation magnetization (70.4 emu/g) was obtained at the stirring rate of 1100 rpm. The mean particle sizes of iron oxide nanoparticles calculated from the magnetization data are found to be consistent with the particle sizes obtained from the TEM images.

Hydrothermal Synthesis of SrFe12O19 and Its Characterization

Abstract: We have synthesized strontium hexaferrite particles in an alkaline medium using a hydrothermal process at 180 °C. Crystalline phase of samples were determined by XRD and spectroscopic, morphological, and magnetic investigation of the sample were FT-IR, SEM, and TG analysis, respectively. XRD analysis revealed few impurity phases in the as-made powder; upon calcinations, the material is converted to desired hexaferrite phase. As synthesized powder exhibits agglomerates with rather smooth facets, in the form of thick platelets. Upon calcination, all these structures were observed to transfer to rod-like structures. The As calcined sample has high specific saturation magnetization (Ms) values of 65 emu/g that is close to its theoretical value of 74.3 emu/g but the hydrothermally synthesized sample does not. This is in agreement with the observations from XRD analysis where few impurity phases observed in the as-made powder cause a weak magnetic response. Upon calcination, the material is converted to a desired hexaferrite phase with better magnetic properties.

Effect of Oleic Acid and Oleylamine Surfactants on the Size of FePt Nanoparticles

Abstract: FePt magnetic nanoparticles have been synthesized by superhydride reduction of FeCl2 and Pt(acac)2 at high temperature. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl2 and Pt(acac)2 in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 190 ∘C, followed by refluxing at 245 ∘C, led to monodisperse 3.5 nm FePt nanoparticles. The effect of oleylamine and oleic acid surfactants on the nucleation and growth of FePt nanoparticles were studied. The size of Pt was controlled by oleylamine surfactant in nucleation stage. To prevent sintering of the FePt nanoparticles, oleic acid surfactant was used in growth stage. The energy dispersive spectroscopy results revealed that the particle composition was first Fe11Pt89 in nucleation stage and after adding superhydride the composition changed to Fe63Pt37 in growth stage. The structural and magnetic measurements indicated that the L10 structure of FePt nanoparticles is formed after annealing and the coercivity of superlattice FePt nanoparticles increases to 7.5 kOe after heat treatments.

Synthesis and Characterization of Fe-Doped CdSe Nanoparticles as Dilute Magnetic Semiconductor

Abstract: Cd1−x Fe x Se (0≤x≤0.1) nanoparticles were synthesized by hydrothermal method. The solubility limit of Fe in CdSe nanoparticles was found to be less than 6 % as obtained from X-ray diffraction (XRD) study. With the increase in doping concentration at and above it, secondary phase of FeSe2 starts appearing. The presence of Cd, Se, and Fe has been confirmed by energy dispersive X-ray spectroscopy (EDS). The increase in band gap value has been confirmed by UV-visible spectra and the variation in emission intensity of photoluminescence (PL) measurements further indicates the incorporation of Fe in CdSe nanoparticles. Transmission electron microscopy (TEM) reveals the spherical nature of synthesized nanoparticles, and the particle size decreases with increasing Fe doping concentration. Fourier transform Infrared spectra (FTIR) confirm the capping of sodium dodecyl sulfate (SDS) surfactant on pure and Fe-doped CdSe nanoparticles. The synthesized nanoparticles show room-temperature ferromagnetic behavior, and the saturation magnetization value was found to increase with Fe doping concentration.

Magnetic Characterizations of Cobalt Oxide Nanoparticles

Abstract: Cobalt oxide (Co3O4) nanoparticles were synthesized by a novel microwave-assisted decomposition reaction of the cobalt nitrate hexahydrate, Co(NO3)2⋅6H2O. While most of the traditional methods for the preparation of Co3O4 are at relatively high temperature, microwave-assisted decomposition was adapted to have better control in the production of Co3O4 nanoparticles. The temperature dependence of the magnetic properties for the Co3O4 was investigated by vibrating sample magnetometer (VSM) and electron spin resonance (ESR) techniques. VSM and ESR measurements have shown a phase transition occurring at around 31 K, as the antiferromagnetic transition temperature for the bulk Co3O4 crystal exhibits almost the same value. The average particle size of the sample at around the transition temperature is estimated as 2.015 nm. The title compound was characterized and identified by an x-ray powder diffraction (XRD).

Magnetic and Microstructure Properties of Ni-Doped ZnO Films and Powder by Sol–Gel Process

Abstract: Zn1−xNixO precursor solutions with a different ratio (x=0.01–0.03) were prepared by sol–gel synthesis using Zn and Ni based alkoxide. Reel-to-reel sol-dip coating method was used to grow Zn1−xNixO films. Zn1−xNixO powders and films, annealed at various temperatures, were tried to observe the doping ratio and temperature effects on magnetic and microstructure properties. The surface morphologies of all samples were characterized by ESEM and EDS. The crystal structures of the ZnNiO powders were characterized using 2θ–θ x-ray diffraction (XRD). The alterations of the lattice parameters have been obtained by the TREOR program. ESR spectra of Zn1−xNixO powder samples were collected at room temperature. The magnetization and microstructure of the powders and films with different dopant ratios, temperature, and the time of annealing process are presented.

Theoretical Investigations of Hysteresis Loops of Ferroelectric or Ferrielectric Nanotubes with Core/Shell Morphology

Abstract: In this paper, the hysteresis behavior of the nanotubes, consisting of a ferroelectric core of spin-1/2 surrounded by a ferroelectric shell of spin-1/2 with ferro- or anti-ferroelectric interfacial coupling is studied by using the transverse Ising model (TIM). Based on a probability distribution method, the effective field theory has been used to examine the effects of the interfacial coupling constant, the transverse field, and the temperature on the hysteresis loops of the nanotubes. A number of characteristic behaviors have been found, such as the existence of triple hysteresis loops for appropriate values of the system parameters. The remanent polarization and the coercive field, as functions of the temperature, are examined.

The Effect of PbSe Addition on the Mechanical Properties of Bi-2212 Superconductors

Abstract: The effects of PbSe addition and heat treatment on the structure and mechanical properties of Bi-2212 superconductors are analyzed. Glass ceramic method is used to produce PbSe added samples. The mechanical properties are derived using Vickers microhardness measurement results. Well known analysis methods like Meyer’s law, PSR (Proportional Sample Resistance) Model, EDP (Elastic/Plastic Deformation Model), Hays-Kendall approach and IIC (Indentation-Induced Cracking Model) are used to calculate load independent microhardness values of the samples. HK approach produces successful results for the samples A, E, F, G and H indicating ISE (Indentation Size Effect) behavior. The IIC model successfully explains RISE (Reverse Indentation Size Effect) behavior for the samples B, C and D. RISE behavior is observed only for the samples where the plastic deformation is dominant. ISE behavior is observed for the samples where both elastic and plastic deformation are produced. The results of XRD and SEM analysis show improvement with PbSe addition in the crystal structures and surface morphologies of superconducting samples.

Structural Phase of Y358 Superconductor Comparison with Y123

Abstract: We analyze an experiment in which two distinct superconducting phases YBa2Cu3O7−δ (Y123) and Y3Ba5Cu8O18−δ (Y358) coexisted. This experiment enabled us to characterize the recently introduced Y358 phase in contrast to the Y123 phase, thus to resolve some discrepancies reported in associated properties of Y358. Specifically, our experiment indicates the transition temperature $T_{\mathrm{C}}^{\mathrm{mid}}=105~\mathrm{K}$ and 94 K for Y358 and Y123, respectively, and that Y358 has five CuO2 planes and three CuO chains, with Pmm2 symmetry and lattice parameter (a,b,c)=(3.845,3.894,31.093) A, in agreement with density functional theory predictions for this specific structure.

Superstripes and Complexity in High-Temperature Superconductors

Abstract: While for many years the lattice, electronic and magnetic complexity of high-temperature superconductors (HTS) has been considered responsible for hindering the search of the mechanism of HTS, now the complexity of HTS is proposed to be essential for the quantum mechanism raising the superconducting critical temperature. The complexity is shown by the lattice heterogeneous architecture: heterostructures at atomic limit; (b) electronic heterogeneity: multiple components in the normal phase; (c) superconducting heterogeneity: multiple superconducting gaps in different points of the real space and of the momentum space. The complex phase separation forms an unconventional granular superconductor in a landscape of nanoscale superconducting striped droplets, which is called the “superstripes” scenario. The interplay and competition between magnetic orbital charge and lattice fluctuations seems to be essential for the quantum mechanism that suppresses thermal decoherence effects at an optimum inhomogeneity.

Modeling and Simulation of Resistive Superconducting Fault-Current Limiters

Abstract: Superconducting fault-current limiters (SFCL) offer ideal performance in electrical power system. The design of SFCL has to be both flexible, to allow an easy adaptation to the specific requirements of each particular application, and a high quality standard with reproducible properties. Up to now no simulation model of SFCL has been validated or introduced in the Library of MATLAB software. In this paper a simulation model for a novel re- sistive type superconducting fault-current limiter is pro- posed. This model includes the electric field-current den- sity (E-J ) characteristics of High-Temperature Supercon- ductors (HTS). A graphical interface using Graphical User Interface (GUI) of MATLAB is developed in order to ease the operation of the proposed model. This one facilitates the introduction or the parameter modification of materials candidate to a SFCL model. Thus, the operation characteris- tics and limitation behavior of SFCL have been investigated. The developed model accurately predicted the current-time waveforms achievable with typical limiters, and improved standard of understanding concerning the fault-current lim- itation mechanisms.

Investigation of Structural and Superconducting Properties of Cr Added Bi-2212 Superconducting Ceramics

Abstract: This study reports the effect of Cr addition on the structural and superconducting properties of Bi1.8Sr2.0CrxCa1.1Cu2.1Oy superconductor with x=0, 0.1, 0.3, 0.5 0.7, and 1 by means of X-ray analysis (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and resistivity measurements. The samples studied in this work were prepared using the standard solid-state reaction method. Zero resistivity transition temperatures (Tc) are qualitatively estimated from the dc resistivity measurements. The zero resistivity transition temperatures are obtained to decrease from 81 K to 53 K with the increase in Cr addition. Moreover, the phase and lattice parameters were determined from XRD measurements. Based on the refinement of cell parameters done by considering the structural modulation, the Cr addition was confirmed by an increase of the lattice parameter a and decrease of the cell parameter a of the samples in comparison with that of the undoped sample (Cr0). As for the microstructure and element composition analyses on the surface of the samples produced, SEM and EDS measurements were investigated. According to the measurements, not only were the grain sizes of the samples noted to decrease, but also the surface morphology and grain connectivity were obtained to degrade with increase in the Cr addition. The possible reasons for the observed degradation in microstructural and superconducting properties due to Cr addition were also discussed.

Fabrication of the New Y 3 Ba 5 Cu 8 O y Superconductor Using Melt–Powder–Melt–Growth Method and Comparison with YBa 2 Cu 3 O 7−x

Abstract: YBaCuO superconducting sample with nominal composition in the stoichiometric ratio of 3:5:8 was fabricated by using the Melt–Powder–Melt–Growth (MPMG) process. Microstructural property of the sample was examined by X-rays diffraction (XRD) pattern and also observed at micrographs taken by both a polarized optical microscope and a scanning electron microscope (SEM) with energy dispersion analysis of X-rays (EDAX). Identification of chemical composition of the sample was investigated by the SEM-EDAX. Electric and magnetic properties were performed with a standard four-probe dc technique and magnetic levitation force measurements, respectively. In addition, all of these properties were compared with a standard MPMG YBa2Cu3O7−x (Y123) superconductor. According to the obtained results, it was determined that empirical resulting formula of the sample was Y3Ba4.89Cu8.17Oy (Y358) and also the magnetic levitation force of the Y358 was 169.8 mN under zero-field-cooled (ZFC) regime at 77 K while that of the Y123 was 99.3 mN.

Theory of Strains in Auxetic Materials

Abstract: This paper is dedicated to Prof. Jacques Friedel, an inspirational scientist and a great man. His excellence and clear vision led to significant advances in theoretical physics, which spilled into material science and technologi- cal applications. His fundamental theoretical work on com- monplace materials has become classic. We can think of no better tribute to Friedel than to apply a fundamental analysis in his spirit to a peculiar class of materials—auxetic materi- als. Auxetic materials, or negative-Poisson'-ratio materials, are important technologically and fascinating theoretically. When loaded by external stresses, their internal strains are governed by correlated motion of internal structural degrees of freedom. The modelling of such materials is mainly based on ordered structures, despite the existence of auxetic be- haviour in disordered structures and the advantage in manu- facturing disordered structures for most applications. We de- scribe here a first-principles expression for strains in disor- dered such materials, based on insight from a family of 'iso- auxetic' structures. These are structures, consisting of inter- nal structural elements, which we name 'auxetons', whose inter-element forces can be computed from statics alone. Iso-auxetic structures make it possible not only to identify the mechanisms that give rise to auxeticity, but also to write down the explicit dependence of the strain rate on the local structure, which is valid to all auxetic materials. It is argued that stresses give rise to strains via two mechanisms: aux- eton rotations and auxeton expansion/contraction. The for- mer depends on the stress via a local fabric tensor, which we

Structural, Magnetic and Electrical Properties of Cu Substituted Mn–Zn Soft Nanoferrites

Abstract: Soft nanoferrites of nominal composition Mn0.5CuxZn0.5−xFe2O4 with 0.0≤x≤0.35 were prepared by chemical co-precipitation method. The formation of single phase spinel structure with different compositions, sizes and macrostructure were confirmed by X-ray diffraction patterns and scanning electron microscopic (SEM) measurements. The lattice parameter decreased with increase in Cu2+ content. The crystallite size of the powder samples varied from 14 to 27 nm. The theoretical density increased with increase in Cu2+ content. Room temperature saturation magnetization was measured as a function of copper content. The saturation magnetization (Ms) and Bohr magneton (μB) increases up to x=0.25 due to increased A–B interactions in the AB2O4 type spinel nanoferrites. Dielectric permittivity, dielectric loss tangent and complex impedance plots were studied in the frequency range 20 Hz–5 MHz. Loss peak occurs for all the studied compositions and shifts towards low frequency with increased Cu2+ content. Complex impedance spectroscopic studies confirmed that conduction in the samples is due to grain boundaries. The high values of DC electrical resistivity support this result.

Effect of Ce Addition on the Magnetoresistivity, Irreversibility Field, Upper Critical Field and Activation Energies of Bi-2212 Superconducting Ceramics

Abstract: This study aims to analyze the effect of Ce addition on the microstructural, superconducting and physical properties of Bi1.8Sr2.0Ce x Ca1.1Cu2.1O y ceramics with x=0, 0.001, 0.003, 0.005, 0.01, 0.03, 0.05 and 0.1 via X-Ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-Ray (EDX) and magnetoresistivity measurements. The ceramics produced in this work are prepared using the standard solid-state reaction method. The zero resistivity transition temperatures (T c), activation energies (U 0), irreversibility fields (μ 0 H irr) and upper critical fields (μ 0 H c2) are determined from the resistivity versus temperature (R–T) curves under dc magnetic fields up to 7 T. The results show that T c and U 0 values of the samples are found to decrease dramatically with the increase in the Ce-content and applied magnetic field. Moreover, XRD results indicate that all the samples contain the Bi-2212 phase only and exhibit the polycrystalline superconducting phase with less intensity of diffraction lines with the increase of the Ce addition. As for the results of SEM images, the texturing, crystallinity, grain size distribution, layered grain growth and grain connectivity are observed to degrade with the increase of the Ce doping. Besides, the irreversibility fields and upper critical fields are found to degrade as Ce doping increases. Penetration depths (λ) and coherence lengths (ξ) are also discussed.