Effects of Mg-Coating and Boron Type on Electromechanical Properties of IMD MgB2/Fe Wires

Journal of Superconductivity and Novel Magnetism - In this study, mechanical and electrical characterization measurements were performed on non-bent and gradually bent IMD MgB2/Fe superconducting...     

Frequency and Voltage Dependence of Dielectric Loss of MgB2 Composites at Different Temperatures

The dielectric loss (ε″) properties of MgB₂ composites were investigated by using the conductance–voltage (G/w–V) measurements in the wide frequency and applied bias voltage range at four different temperature levels. Experimental results show that both G/w and ε″ are found strong functions of frequency and applied bias voltage for each temperature level. These changes in G/w and ε″ are considerably high especially at low frequencies and temperatures. The values of ε″ decrease exponentially with increasing frequency until 100 kHz and then become almost constant. Such behavior of G/w and ε″ shows that interfacial polarization is more effective especially at low frequencies. In addition, the current–voltage (I–V) characteristics were also evaluated for 100, 200, and 300 K, and the I–V curves for each temperature show linear behavior. Results indicate that the G/w–V measurements may be a useful tool to characterize the dielectric properties and conductivity of MgB₂.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Time-Dependent Diffusion Coefficient of Fe in MgB<Subscript>2</Subscript> Superconductors

The iron (Fe) diffusion in superconducting MgB₂ bulk samples has been studied for sintering time durations of 15 min, 30 min, 1 h, 2 h, and 4 h at 900^°C. Fe coating bulk polycrstalline superconducting MgB₂ samples for Fe coating were prepared by pelletizing and used in the diffusion experiments with initial sintering at 800^°C for 1 h. A thin layer of Fe was coated on MgB₂ pellets by evaporation in vacuum. Effects of Fe diffusion on the structural, electrical, and superconducting properties of MgB₂ have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), energy-dispersive X-ray spectroscopy (EDS), and resistivity measurements. Fe diffused samples have slightly increased critical transition temperatures and have larger lattice parameter c values, in comparison with bare samples. Fe diffusion coefficients were calculated from depth profiles of c parameter and room temperature resistivity values. Depth profiles were obtained by successive removal of thin layers from Fe diffused surfaces of the samples. Our results have shown that the Fe diffusion coefficient decreases with increasing sintering time and resistivity measurements can be utilized for determination of diffusion coefficient.     

Formation of High-Field Pinning Centers in Superconducting MgB<Subscript>2</Subscript> Wires by Using High Hot Isostatic Pressure Process

This paper demonstrates the effects of hot isostatic pressure (HIP) on the structure and transport critical parameters of in situ MgB₂ wires without a barrier. Our results show that only HIP and nano-boron allow the formation of more high-field pinning centers, which lead to the increase in critical current density (J _c) at high applied magnetic fields. Nano-boron and annealing at a low pressure increase the J _c in the low magnetic field. This indicates that nano-particles create more high-field pinning centers. In addition, the results show that nano-boron improves the connection between the grains. Scanning electron microscope results show that HIP increases the reaction rate between Mg and B, density, and homogeneity of the MgB₂ material. Additionally, HIP allows to create a structure with small grains and voids and eliminates the significance of the number of voids. High isostatic pressure allows to obtain high J _c of 10 A/mm² (at 4.2 K) in 10 T and increases irreversible magnetic field (B _irr) and upper critical field (B _c2). Measurements show that these wires have high critical temperature of 37 K.     

Monofilamentary In Situ Fe/MgB<Subscript>2</Subscript> Superconducting Wires Fabricated by Pellet-in-Tube Method

Thin monofilamentary Fe/MgB₂ superconducting wires without barriers are investigated by means of electrical transport measurements and surface and structural analysis methods. Small diameter wires are fabricated by pellet-in-tube method (PeIT) to obtain a high uniform initial filling density and heat treated as a function of various sintering temperatures and times. The results are discussed in terms of the grain connectivity, Fe₂B phase formation, and the relation between wire diameter and sintering conditions. We suggest that PeIT has a crucial importance to achieve homogeneous initial filling density, which leads to the fabrication of uniform long-length MgB₂ wires.     

Variation of Mechanical Properties of Cr Doped Bi-2212 Superconductors

This study shows the influence of Cr inclusions on the mechanical properties of Bi_1.8Sr_2.0Cr _x Ca_1.1Cu_2.1O _y (Bi-2212) superconducting samples (x=0, 0.1, 0.3, 0.5, 0.7, and 1) prepared by conventional solid-state reaction route with the aid of the microhardness (H _v) measurements. Moreover, some characteristics such as Vickers microhardness, Young’s (elastic) modulus (E) and yield strength (Y), being responsible for the potential technological and industrial applications, are theoretically evaluated from the microhardness curves belonging to the samples and compared with each other. It is found that the load dependent microhardness values decrease nonlinearly as the applied load enhances until 2 N beyond, which the curves shift to the saturation region, confirming that all the samples exhibit the indentation size effect (ISE) nature. Further, the elastic modulus and yield strength values observed reduce with the enhancement of the applied load and Cr inclusions in the Bi-2212 matrix. The experimental findings are also analyzed by Meyer’s law, proportional sample resistance model (PSR), modified proportional sample resistance model (MPRS), elastic/plastic deformation model (EPD), and Hays–Kendall (HK) approach. According to the results obtained, the load independent microhardness values calculated by EPD, PSR, and MPSR models are far from the values of the plateau region; however, the HK approach is the most suitable model for the microhardness calculations of the samples prepared in this study.     

Synthesis and properties of β type Bi(III)2?2xDy(II)2xO3 ? xx solid solution

In the present work, tetragonal β type Bi(III)_{2 − 2x}Dy(II)_2xO_{3 − x}■_x (■:,empty oxygen site) solid solutions have been synthesized in the ranges 0.06 ≤ x ≤ 0.08 at 750°C and 0.05 ≤ x ≤ 0.09 at 800°C by doping small amounts of Dy₂O₃ into monoclinic α -Bi₂O₃. X-ray powder diffraction (XRD) has been used for characterisation of the doped Dy₂O₃ powders as well as for calculation of the unit cell parameters. The polymorphic phase transitions, crystallographic properties, particle sizes, the scanning electron microscope (SEM) microprobe analysis and Dy₂O₃ content dependence of the lattice parameters of the observed β-phase has been reported. The obtained solid solutions had nonstoichiometric character and nonstoichiometry increases with the increasing amount of Dy₂O₃ addition. The experimental results suggested that oxygen vacancies are present in the crystal structure of Dy₂O₃ doped β type solid solutions.     

Role of annealing temperature on microstructural and electro-optical properties of ITO films produced by sputtering

This study probes the effect of annealing temperature on electrical, optical and microstructural properties of indium tin oxide (ITO) films deposited onto soda lime glass substrates by conventional direct current (DC) magnetron reactive sputtering technique at 100 watt using an ITO ceramic target (In₂O₃:SnO₂, 90:10 wt%) in argon atmosphere at room temperature. The films obtained are exposed to the calcination process at different temperature up to 700 °C. X–ray diffractometer (XRD), ultra violet-visible spectrometer (UV–vis) and atomic force microscopy (AFM) measurements are performed to characterize the samples. Moreover, phase purity, surface morphology, optical and photocatalytic properties of the films are compared with each other. The results obtained show that all the properties depend strongly on the annealing temperature. XRD results indicate that all the samples produced contain the In₂O₃ phase only and exhibit the polycrystalline and cubic bixbite structure with more intensity of diffraction lines with increasing the annealing temperature until 400 °C; in fact the strongest intensity of (222) peak is obtained for the sample annealed at 400 °C, meaning that the sample has the greatest ratio I ₂₂₂/I ₄₀₀ and the maximum grain size (54 nm). As for the AFM results, the sample prepared at 400 °C has the best microstructure with the lower surface roughness. Additionally, the transmittance measurements illustrate that the amplitude of interference oscillation is in the range from 78 (for the film annealed at 400 °C) to 93 % (for the film annealed at 100 °C). The refractive index, packing density, porosity and optical band gap of the ITO thin films are also evaluated from the transmittance spectra. According to the results, the film annealed at 400 °C obtains the better optical properties due to the high refractive index while the film produced at 100 °C exhibits much better photoactivity than the other films as a result of the large optical energy band gap.     

Synthesis and characterization of β type solid solution in the binary system of Bi2O3-Eu2O3

We have investigated Bi₂O₃-Eu₂O₃ binary system by doping with Eu₂O₃ in the composition range from 1 to 10 mole% via solid state reactions and succeeded to stabilize β-Bi₂O₃ phase which is metastable when pure. Stability of β-Bi₂O₃ polymorph was influenced by heat treatment temperature. Tetragonal type solid solution was obtained in 3–6 mole% addition range when annealed at 750°C and the range was 2–7 mole% when annealed at 800°C. We have also carried out investigations on lattice parameters, microstructural properties and elemental compositions of this β type solid solution for each doping ratio. Lattice parameters increased with amount of Eu₂O₃ addition. Our experimental observations strongly suggested that oxygen deficiency type non-stoichiometry is present in doped β type solid solutions.