Search

Nanofluid is a new fluid with special characteristics that can be used in various industrial heat transfer applications. The nanofluid was created using the two-step approach of dispersing nanoparticles into Jatropha oil. The experiments were conducted for four samples of mixture nanofluid at four different volume concentrations of 0.5%, 1.0%, 1.5%, and 2%. The nanoparticles were characterized by X-ray diffractometric analysis, X-ray fluorescence spectroscopy, and scanning electron microscopy. The greatest increase in thermal conductivity for CuO–Jatropha oil nanofluids was made possible by 0.5%, 1.0%, 1.5%, and 2% mixture was 4.5%, 6.01%, 7.6, and 9.1%. The study observed that a mixture achieved the highest thermal conductivity for nanofluid at 2%. At a mixture of 0.5%, the lowest value of thermal conductivity within the range under consideration, a “deeding” effect was seen. To determine the thermal conductivity of CuO–Jatropha oil nanofluids based on temperature, volume concentration, and nanoparticle mixture volume concentrations, the study suggested and compared four sample models. The effect of nanoparticles on the rise of thermal conductivity and viscosity was investigated at temperatures ranging from 298 to 323 K. The X-ray diffractometer is used to characterize nanoparticles. Viscosity and thermal conductivity have been tested in the lab. The findings show that the copper oxide nanoparticles improved the base fluid’s thermophysical abilities more than Jatropha oil alone. For the evaluation of the thermal conductivity and viscosity of nanofluid, new empirical correlations were presented based on experimental data. The thermal conductivity of the nanofluids increases as the temperature rises, while the viscosity of the nanofluids decreases.

Lead-free SnIn solders are promising for superconducting magnet applications. However, their superconducting properties are not as good as lead solders. In order to improve the superconducting performance of the Sn–In solders, researchers have investigated the superconducting properties of ternary systems such as Sn–In–Bi for solder joints. In this study, powders of Pb, Nb, AgCu and grapheme nano pellets in the ratios of 0.5–5 wt% have been added into SnIn (35:65) to investigate their microstructural, thermal and superconducting properties. The added materials enhance the superconducting properties. We find that even low Pb additions show a dramatic improvement in superconducting properties, with an increase in both T c and J c values of up to 6.35 K and 1.47 × 104 A m−2, respectively. This shows that much lower Pb content superconducting solders can be effective and could be used to replace the PbBi solder commonly used with the superconducting properties T c =8.4 K, H C2 = 1.77 T, H C = 0.0909 T.

The researchers wanted to see whether MWCNTs changed the physicothermal properties of solar thermal working fluids. Assessing thermal properties is vital for solar thermal efficiency. Lubricant contains silicone oil resurfaced. It contains 0.25, 0.5, 0.75, and 1.0% multiwalled carbon nanotubes. Before dispersion in thermic fluids, nanomaterials must be properly surface modified. Between 100°C and 300°C, a fluid’s thermal conductivity and specific heat physical characteristics like viscosity and density may be inferred from data collected between 50°C and 150°C. Thermal conductivity increases by 15% to 20% when carbon nanotubes are dispersed. The pressure drop is minimal at 0.5 percent weight fraction, demonstrating the suitability of nanofluids in closed loop systems. The characteristics are forecasted using feed-forward backpropagation method and GRNN, and the best of them is selected for prediction. In this research, hidden layer neurons and factors are examined.

Charcoal and firewood are the most common cooking fuels, despite the fact that they represent a variety of social and economic environmental difficulties in many affluent countries. Apart from the environmental implications of deforestation and resource loss, indoor air pollution caused by cooking with solid fuels causes 2 million fatalities each year. Biobriquettes are then utilized as a substitute for oils. Briquettes are also employed as carbon sources for cooking in various industries such as power plants, brick factories, and bakeries. Five specific binding materials were tested in this study for manual densification of cabbage waste, including beef tallow oil, starch, cassava binder, sodium silicate, and vinyl ester resin. The briquettes are made from a combination of 80% densified biological waste and 20% binder material. The sample density ranges from 545.564 to 591.278 kg/m3. The samples with the highest calorific value were beef tallow oil and vinyl ester resin, which had 5,357.26 and 5,800.79 kcal/kg respectively. The content of the samples were ashes, fixed carbon content, volatile material, and total humidity is 6.47% ± 1.13%, 18.43% ± 6%, 70.68% ± 6%, and 4.42% ± 4%, respectively. The sample with the most calorific value and densification is the one with the inorganic binder mixture.

The best J c performance (8 × 10 4 A·cm − 2 at 10 T, 4.2 K) has been obtained in our work among all the in-situ powder-in-tube (PIT) MgB 2 wires reported so far by tailoring their boron and magnesium precursors, as well as employing appropriate high pressure during the manufacturing process. Moreover, the second stage densification using the special HIP has been applied to commercial long-length wire. Our work suggests that the economical starting precursors and the combination of cold and hot densification techniques could represent a promising alternative for industrial and economical production of practical MgB 2 wires with excellent J c .

The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 °C, after hot pressing at 200 °C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 °C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 °C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor. © 2017, Springer Science+Business Media New York.

Conventional homogeneous and microporous heterogeneous catalysts for quinoxalines production from diamines and diketones usually suffer from difficult separation or harsh reaction conditions. Here, we demonstrate the production of various substituted quinoxalines using ZnO nanoparticle-loaded, highly ordered, mesoporous silica KIT-6 materials as solid catalysts in room temperature. The results show that the KIT-6-130-10Zn sample (aged at 130 °C and containing 10 wt% ZnO) effectively produces quinoxalines up to the maximum of 99%. We propose that the enhanced performance of the ZnO-loaded KIT-6 materials resulted from the homogeneous distribution of ZnO nanoparticles, along with the KIT-6's high specific surface area and large pore sizes.

The capture and storage of CO2 have been suggested as an effective strategy to reduce the global emissions of greenhouse gases. Hence, in recent years, many studies have been carried out to develop highly efficient materials for capturing CO2. Until today, different types of porous materials, such as zeolites, porous carbons, N/ Bdoped porous carbons or metal-organic frameworks (MOFs), have been studied for CO2 capture. Herein, the CO2 capture performance of new hybrid materials, graphene-organic frameworks (GOFs) is described. The GOFs were synthesized under mild conditions through a solvothermal process using graphene oxide (GO) as a starting material and benzene 1,4-diboronic acid as an organic linker. Interestingly, the obtained GOF shows a high surface area (506 m(2)g(-1)) which is around 11 times higher than that of GO (46 m(2)g(-1)), indicating that the organic modification on the GO surface is an effective way of preparing a porous structure using GO. Our synthetic approach is quite simple, facile, and fast, compared with many other approaches reported previously. The synthesized GOF exhibits a very large CO2 capacity of 4.95 mmol g(-1) at 298K (1 bar), which is higher those of other porous materials or carbon-based materials, along with an excellent CO2/N-2 selectivity of 48.8.

This paper reports the fabrication, transport current testing, and finite element analysis of magnesium diboride (MgB2) racetrack coils made by a wind and react method at 4.2 K in self-field. This type of coil can be potentially appropriate for applications aimed at building block coils that can serve as wind turbine generator coils and correction coils for fusion magnets. We have fabricated racetrack coils using an in situ monofilament MgB2/Niobium/Monel wire; each coil had a wire length of 45 m and 80 turns. The best measured critical current of the coil was 134.9 A at 4.2 K and self-field of 0.55 T. In addition, the magnetic field and the Lorentz force contribution in the coil were estimated using finite element analysis.

The synthesis of a novel family of cyano-bridged trimetallic coordination polymers (CPs) with various compositions and shapes has been reported by changing the compositional ratios of Fe, Co, and Ni species in the reaction system. In order to efficiently control the nucleation rate and the crystal growth, trisodium citrate dihydrate plays an important role as a chelating agent. After the obtained cyano-bridged trimetallic CPs undergo thermal treatment in air at three different temperatures (250, 350, and 450 °C), nanoporous spinel metal oxides are successfully obtained. Interestingly, the obtained nanoporous metal oxides are composed of small crstalline grains, and the grains are oriented in the same direction, realizing pseudo-single crystals with nanopores. The resultant nanoporous spinel oxides feature interesting magnetic properties. Cyano-bridged multimetallic CPs with various sizes and shapes can provide a pathway toward functional nanoporous metal oxides that are not attainable from simple cyano-bridged CPs containing single metal ions.

In order to investigate the effect of laser irradiation on the superconducting properties of MgB2, a pellet-shaped sample was produced and cut into two pieces. The dot-shaped molten patterns, with the distance of 100 µm between them, were formed on the surface of one of the pieces by using an Nd:YVO4 laser. The grains of the irradiated sample were significantly grown due to the laser-induced sintering. The modified surface appeared to have dense and compacted grains as a result of increasing interconnectivity among the superconducting grains. The critical current densities at 5 K in a field of 3 T were found to be 5.38 × 104 and 8.96 × 104 A cm?2 for the reference and the irradiated samples, respectively. The increment of critical current density for the irradiated sample is presumably connected with the changing of pinning mechanism after the laser irradiation. For the irradiated sample which showed a higher value of critical current density at 5 K under high fields, ?? pinning was obtained as a dominant pinning mechanism, while the normal point pinning was dominant for the reference one. These results point out that the superconducting properties of bulk MgB2 can be changed and improved via laser irradiation in a short time. © 2016, Springer Science+Business Media New York.

In this study we synthesize a novel mesoporous titanium oxide (TiO2) through calcination of a bioinspired supramolecular assembly of polymeric micelles, based on the thymine-functionalized homopolymer, poly(4-vinylbenzyl triazolylmethyl-methylthymine), complexed with Ti(OH)(2)(2+) ions. We characterize thin films of these structures using wide-angle X-ray diffraction, small-angle X-ray scattering, nitrogen adsorption/desorption isotherms, transmission electron microscope, and scanning electron microscope. This approach is simple, effective, and inexpensive when compared with the synthesis of diblock or triblock copolymers typically required to prepare soft templates. This new mesoporous material exhibits superior photocatalytic activity for the degradation of the organic dye methylene blue almost three times higher than that of bulk TiO2.

A single-phase current-limiting transformer has been developed based on flux transfer. The transformer consists of two independent iron cores, one primary coil, two separate secondary coils, and superconducting windings/loops. Two iron cores are in a mechanical contact and clamped. Superconducting windings/loops act as coupling/decoupling mechanism between two iron cores. The transformer has been made operational via a switchgear of the applied dc current for a potential use in the current limitation when the transition between the superconducting and normal (quench) states can be triggered. DC and ac currents can be applied simultaneously in fault and no-fault conditions to mimic the realistic cases of use. With decoupling ac energy transfer, we propose that conventional switching off may be replaced with the approach described.

In this study, the role of laser irradiation on the upper critical field (H c2), the irreversibility field (H irr) and the activation energy (U 0) of bulk MgB2 superconductor was first time investigated with the aid of magnetoresistivity measurements conducted at different applied magnetic fields in a range of 0–15 kOe as a function of temperature from 25 to 40 K. For this aim, a disk shaped MgB2 superconductor was produced and cut into two pieces. One of the pieces was irradiated under high vacuum conditions by using an Nd:YVO4 laser. The obtained results showed an increase in the critical temperature (T c ) by about 1 K after irradiation. Also, the values of H c2(0) were respectively found to be around 143 and 151 kOe while the values of H irr(0) were about 122 and 130 kOe for the reference and the irradiated samples. Using the thermally activated flux flow model the dependence of U 0 on magnetic field was determined. The maximum U 0 values of 0.43 and 0.83 eV in zero magnetic field were respectively obtained for the reference and the irradiated samples. Also, the magnetic field dependence of the critical current density J c (B) values at different temperatures were slightly increased after the irradiation. These results point out that the superconducting properties of bulk MgB2 can be improved via laser irradiation.

It is crucial to improve the magnetic levitation, guidance and stiffness performances of the HTS Maglev systems for real-scale applications. In this study, the magnetic levitation force, guidance force and magnetic stiffness values were determined for single and multi-seeded YBCOs on different PMGs in ZFC regime and different cooling heights (CHs) after the magnetic flux distributions simulation of the using PMGs. Implemented study showed that the additional downward PMs make an enhancing contribution on the magnetic levitation force, guidance force and magnetic stiffness of both single and multi-seeded YBCOs. It was also determined that both the force and stiffness ratios of multi-seeded YBCO to single-seeded YBCO increase with increasing CH, indicating that the multi-seeded YBCO has much more interaction domain than single-seeded at higher gaps. It is considered that the results obtained in this study are very useful in technological applications of Maglev and bearing systems, because there is no detailed study on the magnetic stiffness, levitation and guidance force comparison to clarify electromagnetic behavior of single and multi-seeded YBCOs for different HTS-PMG arrangements. (C) 2015 Elsevier B.V. All rights reserved.

We fabricated malic acid $$(\hbox {C}_{4}\hbox {H}_{6}\hbox {O}_{5})$$ -added $$\hbox {MgB}_{2}$$ bulks by wet mixing and “Two-step solid state reaction method”. The effects of adding malic acid on $${ T}_{\mathrm{c}}$$ , $${ J}_{\mathrm{c}}-{ H}$$ behaviour and lateral levitation force $$({ F}_{\mathrm{x}})$$ features of bulk $$\hbox {MgB}_{2}$$ have been investigated. A systematic decrease in the critical temperature $${ T}_{\mathrm{c}}$$ with increasing adding level confirms the substitution of C at the B site of $$\hbox {MgB}_{2}$$ . While the 4 wt% sample showed the best $${ J}_{\mathrm{c}}$$ of $$3.7\times 10^{4}$$ $$\hbox {A/cm}^{2}$$ at 4 T and 5 K, 15 wt% sample showed uncompetitive lower critical current density $$({ J}_{\mathrm{c}})$$ , which ascribes the poor connectivity due to the excessive unsubstituted C distribution at grain boundaries and the presence of high MgO amount. At 24 and 28 K, the 4 and 6 wt% malic-acid-added samples exhibit a higher lateral force than pure sample. Based on the observed values of M–H, $${ J}_{\mathrm{c}}$$ (H) and lateral levitation force $$({ F}_{\mathrm{x}})$$ , it can be concluded that the 4 wt% malic-acid-added sample is the best of the studied samples.

We have studied the nanoscale structure and unoccupied electronic states in FeSe1-xTex by a combined analysis of Se K, Te L1 and Fe K-edges X-ray absorption measurements. Extended X-ray absorption fine structure (EXAFS) results show that iron-chalcogen (Fe-Se and Fe-Te) distances in ternary FeSe1-xTex are similar to those measured for binary FeSe and FeTe. The local Fe-Se/Te distances determined by different absorption edges fit well in the characteristic Z-plot of random alloys, providing unambiguous support to the inhomogeneous nanoscale structure of the ternary FeSe1-xTex system. X-ray absorption near-edge structure (XANES) spectra reveal a gradual evolution of the unoccupied valence electronic states as a function of Te-substitution in FeSe1-xTex. The Fe 3d-Se 4p/Te 5p hybridization is found to decrease with Te-substitution, accompanied by an increase in unoccupied Se 4p states and a decrease in unoccupied Te 5p states. The results are discussed in the frame of local inhomogeneity in the FeSe1-xTex system driven by random alloying of Se/Te atoms.

Superconducting wires are widely used for fabricating magnetic coils in fusion reactors. Superconducting magnet system represents a key determinant of the thermal efficiency and the construction/operating costs of such a reactor. In consideration of the stability of 11B against fast neutron irradiation and its lower induced radioactivation properties, MgB2 superconductor with 11B serving as the boron source is an alternative candidate for use in fusion reactors with a severe high neutron flux environment. In the present work, the glycine-doped Mg11B2 bulk superconductor was synthesized from isotopic 11B powder to enhance the high field properties. The critical current density was enhanced (103 A·cm−2 at 20 K and 5 T) over the entire field in contrast with the sample prepared from natural boron.

The effects of malic acid addition (from 0 to 15 wt% of the total MgB 2) on the levitation force properties of bulk MgB2 have been investigated. All samples were prepared from magnesium powder, amorphous boron powder, malic acid (C4H6O5) and toluene (C7H8) by using two-step solid state reaction method. Vertical and lateral levitation force measurements that are under both zero-field-cooled (ZFC) and field-cooled (FC) regimes were carried out at different temperatures of 24, 28 and 32 K for samples with various adding level. It was found that the reasonable malic acid adding has a positive impact on the levitation properties. At 24 K and 28 K, the 4 wt% and 6 wt% malic acid added samples exhibits a higher levitation force than pure sample. In the case of the optimally additive 4 wt% sample, the maximum levitation force corresponds to 18.60 N, whereas the pure sample shows 16.95 N at 24 K for ZFC regime. In this study the enhancing effect of malic acid adding on the levitation force properties of MgB2 has been first time investigated and reported. © 2014 Elsevier B.V. All rights reserved., 113T008 110T622, The authors are grateful to Prof. Dr. Neriman Demirbaş for great interest and to Elvan Coşkun for the levitation force measurements. This work was supported by Turkish Science and Technology Council-TUBITAK under the Project No. 113T008 . Magnetic levitation force measurements at low temperatures were performed at solid state research laboratory in Recep Tayyip Erdogan University using the system designed by the project supported by the Scientific and Technological Research Council of Turkey (TUBITAK) , with Project No. 110T622 .

Sintered Nd-Fe-B magnets are some of the strongest magnetic materials showing magnetic properties. Blending addition is frequently performed in order to improve the magnetic properties of these materials. In this work, high-melting point Ni and low-melting point Cu were added to Nd15Fe77B8 ingot by blending. As a result of structural analysis, it was observed that 0.2 wt% Cu + Ni addition improved the structure of the material; on the other hand, excessive addition resulted in porosity. It was also observed that 0.2 wt% addition improved the magnetic properties, and Curie temperature increased with increasing addition ratio.

The levitation properties of MgB $$_{2}$$ prepared by hot press at 200 $$^{\circ }$$ C and then pellet/closed tube method has been investigated. The vertical and lateral levitation forces ( $$F_{z}$$ and $$F_{x})$$ on a cylindrical NdFeB permanent magnet (PM) below a disk-shaped bulk MgB $$_{2}$$ were measured during the vertical and lateral traverses of the PM in different cooling heights (CHs) and measurement heights (MHs) at temperatures of 20, 25 and 30 K to investigate the effect of the initial CH and MH on the levitation performance of MgB $$_{2}$$ . For temperatures below 30 K, it was observed that $$F_{z}$$ increases with increasing CH. However, a minute variation in $$F_{z}$$ and a big hysteresis loop are observed at 30 K. From the lateral traverses, it was obtained that the $$F_{z}$$ with attractive character increases with decreasing MH and the hysteresis effect increases for small MHs due to the increment of the magnetic field intensity which the sample feels with decreasing MH. In addition, it was seen that the character of $$F_{x}$$ varies depending on both MH and measurement temperature. The higher hysteresis obtained in $$F_{x}$$ than in $$F_{z}$$ during lateral traverses implies that the motion of the flux lines in MgB $$_{2}$$ is especially in lateral rather than vertical direction. Finally in this study, it was shown that the levitation performance of MgB $$_{2}$$ depends not only on the measurement temperature but also on the CH and MH conditions. These results can be useful for optimizing the levitation performance of MgB $$_{2}$$ superconductors for potential applications.

Novel tetrasubstituted metal-free (2), zinc (II) (3), cobalt (II) (4), and copper (II) (5) phthalocyanines bearing 2-phenylethanolate groups on the peripheral positions were synthesized by the cyclotetramerization reaction of the phthalonitrile derivative 1. The new compounds were characterized by a combination of IR, 1H NMR, 13C NMR, UV-Vis, elemental analysis, and MS spectra data. The thermal stabilities of the phthalocyanine compounds were determined by thermogravimetric analysis. For metal-free (2) and zinc (3) phthalocyanines, photochemical (photodegradation and singlet oxygen quantum yields) and photophysical (fluorescence quantum yields and fluorescence lifetimes) properties were analyzed in dimethylsulfoxide (DMSO). The cobalt (4) and copper (5) phthalocyanines were not evaluated for this purpose because of their paramagnetic behavior. The metal-free (2) and zinc (3) phthalocyanines' fluorescence quenching behaviors were also investigated. The new phthalocyanine compounds' fluorescence emissions were effectively quenched by 1,4-benzoquinone in DMSO. The thin films of the cobalt phthalocyanine compound (4) were grown by electron beam evaporation technique. Crystalline CoPc (4) thin films were investigated by X-ray diffraction spectroscopy. Surface morphology of the CoPc (4) thin films was characterized by SEM.

WOS:000335666900002 Nanoindentation technique was used to characterize the mechanical properties of Gd-deposited bulk YBaCuO superconductors fabricated by solid-state reaction method. In order to determine the hardness and reduced modulus of the samples, load-displacement data were analysed by using the Oliver-Pharr method. The hardness values exhibited significant peak load-dependence especially at lower peak loads, while the reduced modulus values were found to be nearly constant at studied loading range. In order to find true hardness of the samples, the peak load-dependency of hardness was analysed by using Meyer's law, minimum resistance model, elastic/plastic deformation model, energy balance model, Nix-Gao model and Mukhopadhyay approach. Of the aforementioned models, energy balance model and Mukhopadhyay approach were found to be the most effective models to explain the load-dependency of hardness.

In this study, single-crystal Y123 samples were grown by a cold top-seeding method by using Nd123 seed, and the effect of Y2O3 buffer layer was investigated. The upper critical magnetic field and coherence length were established as 110 T and 17.3 Ǻ, respectively. The dependence of the effective activation energy U of the flux pinning on the magnetic field and temperature of the sample were determined using the Arrhenius activation energy law from the resistivity curves. It was found that the deduced value of the activation energy for a Y123 sample is in good agreement with the corresponding values in YBCO samples. The maximum activation energy value was approximately 0.9 eV in the zero magnetic field. In order to examine the homogeneity of the pinning properties of different layers, rectangular specimens were cut from the sample. AC susceptibility measurement was performed, and it was found that the shifting of the peak temperature (Tp) with an AC magnetic field is small, indicating good pinning properties. The normalized pinning force density versus the reduced field was examined at different temperatures to determine the pinning mechanism. It was found that normal core-type pinning was effective, and in low fields, pinning was only due to Y211 particles.

Nanoporous carbon materials are a versatile source of carbons that would be useful in applications ranging from electronics to electrochemical energy storage. Here, we focus on nanoporous carbon materials prepared by direct carbonization of zeolitic imidazolate frameworks (ZIF-8) towards supercapacitor applications. Several types of nanoporous carbons have been prepared by varying the applied carbonization temperature. The symmetric devices assembled using nanoporous carbon electrodes were tested for their optimal performance in the electrolyte of sulfuric acid solution. We demonstrate the effects of various factors (e.g., surface area, nitrogen content, degree of graphitization, and relative percentage of micropores) on the performance.

The capture and storage of CO

Cobalt sulfide/sulfur doped carbon composites (Co9S8/S-C) were synthesized by calcining a rationally designed sulfur-containing cobalt coordination complex in an inert atmosphere. From the detailed transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses, the electrocatalytically active Co9S8 nanoparticles were clearly obtained and combined with the thin sulfur doped carbon layers. Electrochemical data showed that Co9S8/S-C had a good activity and long-term stability in catalyzing oxygen evolution reaction in alkaline electrolyte, even better than the traditional RuO2 electrocatalyst. The excellent electrocatalytic activity of Co9S8/S-C was mainly attributed to the synergistic effect between the Co9S8 catalyst which contributed to the oxygen evolution reaction and the sulfur doped carbon layer which facilitated the adsorption of reactants, prevented the Co9S8 particles from aggregating and served as the electrically conductive binder between each component.

The levitation and pinning force properties of nanocrystalline MgB2 superconductors prepared by a mechanical alloying technique with varying milling duration have been investigated. Levitation force measurements under zero-field-cooled regime were carried out at different temperatures (16, 25 and 30 K) as a function of the gap between bottom surface of the superconductor and the top surface of a cylindrical Nd-Fe-B permanent magnet whose diameter and height are 30 mm. An increase in the levitation force was observed due to the decrease in particle size caused by increasing the milling time. It was determined the levitation force of the sample milled for 72 h was higher than that milled for 24 h at the same measurement temperature. This is indicative of an enhancing flux pinning capability at the grain boundaries of the MgB2. Also, it was observed that the pinning properties of the superconductor shifted from surface pinning to normal point pinning above Bmax when the milling time was increased from 24 to 72 h. © 2009 Elsevier B.V. All rights reserved.

In this study, two kinds of melt-powder-melt-growth (MPMG) YBCO sample grown on a buffer layer of Y2O3 addition were fabricated. The compacted powders were located on a crucible with Y2O3 powder freely poured and a buffer layer of pressed Y2O3. AC susceptibility measurements of the samples as a function of temperature was reported for several different AC magnetic-field amplitudes (H ac ) in the presence of static bias magnetic field (H b ) directed along H ac . The loss peaks are found to shift towards lower temperatures as the AC field strength is increased. The frequency effect on the AC susceptibility was also measured. As the frequency increases, the peak temperature shifts to higher temperature. This effect can be interpreted in terms of flux creep.

MgB2 thin films were deposited on MgO (100) substrate and r-plane Al2O3 $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB2 films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J c was calculated from M–H loops and also the magnetic field dependence of F p was compared for the different temperature ranges from 5 to 25 K. The critical current density J c was found to be around 1.0×106 A/cm2 and 1.7×106 A/cm2 in zero field at 5 K for MgB2 films deposited on MgO and r-plane Al2O3 substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al2O3 in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB2 films deposited on MgO and r-plane Al2O3 substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.

MgB2 film with a thickness of about 600 nm was deposited on the MgO (100) single crystal substrate using a “two-step” synthesis technique. First, deposition of boron thin film was carried out by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850 °C in magnesium vapor. The upper critical field Hc2 has been estimated from temperature dependences of resistivity curves in both directions of the magnetic fields perpendicular and parallel to the c-axis. Resistivity measurements of the film were performed using a standard four-probe method under different magnetic fields up to 70 kOe in zero fields cooling regime. The upper critical magnetic field Hc2(0) at T=0 K for 90 % of Rn was calculated by the extrapolation Hc2(T) to the temperature T=0 K. The results showed that Hc2∥ab(0) and Hc2∥c(0) was found to be around 22 T and 18 T, respectively. Using extracted data, the zero-temperature coherence lengths and field anisotropy ratio were calculated. In order to determine the activation energy of thermally activated flux flow of the film, Arrhenius law was taken into account.

MgB2 thin films were fabricated on MgO (100) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850 °C in magnesium vapor. In order to investigate the effect of FeO nanoparticles on magnetic properties of MgB2 thin films, the films were coated with different concentrations of FeO nanoparticles by spin coating process. The magnetic field dependence of the critical current density $J_{\mathrm{c}}$ was calculated from the M–H loops and also magnetic field dependence of the pinning force density $f_{\mathrm{p}}(b)$ was determined for the films containing different concentrations of FeO nanoparticles. The values of the critical current density $J_{\mathrm{c}}$ in zero field at 5 K was found to be around 1×106 A/cm2 for pure MgB2 film, 1.4×106 for MgB2 film coated with 25 %, 7.2×105 for MgB2 film coated with 33 %, 9.1×105 for MgB2 film coated with 50 % and 1.1×106 A/cm2 for MgB2 film coated with 100 %. It was found that the film coated with 25 % FeO nanoparticles has slightly enhanced critical current density and it can be noted that especially the film coated with 25 % FeO became stronger in the magnetic field. The films coated with FeO were successfully produced and they indicated the presence of artificial pinning centers created by FeO nanoparticles. The superconducting transition temperature of the film coated with 25 % FeO nanoparticles was determined by moment–temperature (M–T) measurement to be 34 K which is 4 K higher than that of the pure film.

MgB2 thin films were fabricated on MgO (1 0 0) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post-deposition annealing at 850 °C in magnesium vapor. In order to investigate the effect of Fe2O3 nanoparticles on the structural and magnetic properties of films, MgB2 films were coated with different concentrations of Fe2O3 nanoparticles by spin coating process. The magnetic field dependence of the critical current density Jc was calculated from the M–H loops and also magnetic field dependence of the pinning force density fp(b) was investigated for the films containing different concentrations of Fe2O3 nanoparticles. The critical current density Jc was found to be around 1.8 × 106 A/cm2 and 1.3 × 106 A/cm2 for the films with the concentration of 50% and 33% Fe2O3, respectively. It was found that the films coated with Fe2O3 nanoparticles have slightly enhanced critical current density.

Sm123 and Sm211 were prepared by melt-powder-melt-growth and solid-state-reaction techniques, respectively, to have the nominal composition of (Sm123) 0.75 (Sm211) 0.25. After the preparation of this composition, the Fe-B magnetic powder (MP) was added to the composition for 0.000, 0.010, 0.015, 0.025, 0.050 wt.% to investigate the effect of MP addition on the levitation force density (LFD). Additionally, different permanent magnets (PM) were used as magnetic-field source to investigate the effect of magnetic-field gradient on the LFD of MP added superconductors. Our findings indicate that the MP addition acts as a flux pinning centre in the sample and enhances the LFD up to 0.025 wt.% adding amount, and the optimum flux gradient which produces a screening current in the sample occurs when the B/d (magnetic-field intensity/ diameter of PM) ratio is equal to 0.060. These results imply that the experimental data can be useful for fabricating process of superconducting samples with larger MLF values, and designing of superconducting magnets, flywheel energy storage and maglev systems. © 2012 Springer Science+Business Media, LLC.

In this work, single-crystal samples 20 mm in diameter were grown by a cold top-seeding method. In order to study the effect of an Y2O3 buffer layer, a compacted precursor was located in a crucible on a buffer layer of freely poured Y2O3 powder. The YBCO bulk samples were carefully prepared in the same chemical composition. In order to examine the homogeneity of the samples, rectangular specimens were cut from a sample. The structural orientation of the specimens was measured by a X-ray diffractrometer (XRD). The microstructure was observed with a Scanning Electron Microscope (SEM). The magnetic properties were measured using a Vibrating Sample Magnetometer (VSM) at 77 K with the applied magnetic field parallel to the c-axis. As a result only (00l) peaks were observed in X-ray diffraction data, indicating that all specimens are highly oriented with the c-axis perpendicular to the top surface. Also the cut samples exhibited very fine Y211 inclusion distributed in the bulk sample observed in SEM micrographs. The critical current density J c (A/cm2) has been estimated by the extended Bean model. The maximum value of the critical current density at 77 K is 5.2×104 A/cm2 in a self-field. The trapped magnetic field measurements of the samples were performed by using Hall probe scanning device with static field of 0.5 T at 77 K. The single-domain sample exhibits a trapped field of 1140 G at 77 K.

In this work, the trapped magnetic field was calculated for square bulk superconductors using the sand-pile model and Biot–Savart law. The numerical simulation of the trapped magnetic field was performed varying the sample dimensions, distance between the sample surface and observation point. In addition the magnetic field characteristics of superconducting bulk arrangement in an array was determined. The trapped magnetic field was found to be depending on the sample dimensions and distance between the sample and observation point. It is observed that, when the distance from the surface is larger, the value of the trapped magnetic field decreases, but the variation of the trapped magnetic field becomes small due to averaging the distribution of the trapped magnetic field.

Flux pinning is a very significant mechanism to improve critical current density in MgB2 for many applications, such as developing the performance of wires and tapes. In this study, we have done a systematic study of SiC inclusions in the main matrix of MgB2. Sample production in the form of MgB2-x (SiC) (x) was carried out by using solid state reaction method (x=0,0.01,0.02,0.04,0.06,0.10) in the argon atmosphere. The structural and magnetic properties of the prepared samples were investigated. Structural characterization of samples was performed by the X-ray powder diffraction (XRD) technique and the Scanning Electron Microscopy (SEM) measurements. Magnetic properties were determined by M-H loops, magnetization-temperature (M-T), and AC susceptibility measurements. The T (c) of the samples was determined by the M-T measurements while the J (c) is determined through the M-H measurements. SiC inclusions up to some ratios (x=0.06) increase the critical current density by up to about 50%, while the critical temperature T (c) remains unchanged in compatible with the literature.

The flame-quench-melt-growth (FQMG) method was used for the preparation of bulk YBa2Cu3O7−x (Y123). Both the superconducting and nonsuperconducting regions changing with thickness of superconducting sample were determined by X-ray diffraction (XRD) patterns and also observed at micrographs taken by the polarized optical microscope. It was observed that superconducting forming ratio which was examined for different thicknesses of the sample ranging from 6.54 to 1.58 mm strongly enhanced. This means that a region near the top surface of the sample contains more superconducting phase than the other regions. These observations would be helpful in designing and manufacturing the practical applications of YBCO systems.

In this study, two kinds of melt-processed YBCO samples (Y1050-1 and Y1050-2) grown on a buffer layer of Y2O3 addition were fabricated. The compacted powders were located on a crucible with Y2O3 powder freely poured and a buffer layer of pressed Y2O3 to avoid liquid to spread on the furnace plate. The superconducting transition temperature was determined by a standard four-point method at temperatures between 40 and 100 K. The transition temperatures of Y1050-1 and Y1050-2 were found to be 91.6 and 92.9 K at zero magnetic fields, respectively. The magnetization measurements were performed using a vibrating sample magnetometer (VSM) on the same system at 5, 40, and 77 K. The critical current densities for Y1050-1 and Y1050-2 samples were determined to be 6.5×103 and 5.6×103 A/cm2 at 77 K in zero magnetic fields, respectively. The trapped magnetic field measurements of the samples were performed by using a Hall probe scanning device and a Fe–Nd–B permanent magnet at 77 K. It was found that the trapped magnetic field of the Y1050-1 sample is higher than that of the Y1050-2 sample.

In this study, two kinds of melt-processed YBCO samples with Y2O3 addition were fabricated and their microstructures were defined by XRD analysis and polarized light optical microscopy. The mechanical properties of these compounds have been investigated by measuring the Vickers hardness. The superconducting transition temperature was determined by inductive measurements for oxygenated and non-oxygenated samples. The compacted powders were located on a crucible with a buffer layer of Y2O3 and a crucible with Y2O3 powder freely poured to avoid liquid to spread on the furnace plate. It was found that the sample located on crucible with freely poured Y2O3 powder has shown almost single crystal and indicated a very sharp transition curve.

In this work, the trapped magnetic field was calculated using the sand-pile model and Biot–Savart law. The numerical simulation of the trapped magnetic field was performed varying the sample shape and dimensions, distance between the sample surface and observation point. In addition, the magnetic field characteristics of superconducting bulk arrangement in an array was determined. The trapped magnetic field was found to be depending on the sample shape and dimensions and distance between the sample and observation point. It is observed that, when the distance from the surface is larger, the value of the trapped magnetic field decreases, but the variation of the trapped magnetic field becomes small due to averaging the distribution of the trapped magnetic field.

Anomalous ferromagnetic behaviour of the Y₂O₃ and CuO components in YBa₂Cu₃O[subscript y] (Y123) was observed after severe reduction of particle size. The particle size reduction was performed in two ways. First, Y₂O₃, BaCO₃ and CuO were ground for different times to reduce the particle size to the nanoscale. It was found that the superconducting phase could not be obtained without heat treatment, but a weak ferromagnetic behaviour of the mixture was observed at liquid and room temperatures. Second, preheated Y123 powders were subjected to grinding for various times followed by a second sintering process. It was found that the superconducting transition temperature, T[subscript c], was strongly suppressed by increasing grinding time, and ferromagnetic behaviour at both room and liquid nitrogen temperatures was also observed. In order to understand which compound is responsible for this weak ferromagnetism, the Y₂O₃, BaCO₃ and CuO powders were separately ground and M-H loops taken at 290 K. The results indicated that both Y₂O₃ and CuO powders showed weak ferromagnetism, but BaCO₃ showed anti ferromagnetism. It is thought that energy transfer to the powder particles by grinding results in severe plastic deformation of the particles to produce dislocations, vacancies and atomic disorder. We discuss then the observations in the context of recent studies of the magnetic properties of nanoscale oxide materials.

A series of Co-doped ZnO films were fabricated by a spray pyrolysis method. The X-ray diffraction patterns of the films showed that the undoped and Co-doped ZnO films exhibit wurtzite crystal structure. Surface morphology of the films obtained by scanning electron microscopy reveals that pure ZnO film has the hexagonal shaped microrods. The introduction of Co content in the structure did not affect the surface morphology of the films significantly. Magnetic hysteresis loops were observed at room temperature, indicating that ferromagnetism can be realized with Co doping into ZnO. The values of remnant magnetization and coercive field slightly decreased when the concentration of Co was increased beyond 1 at.%. The calculated band gap value of undoped and 1 at.% Co doped samples was found to be around 3.08 eV. The band gap energy increases upon 3 at.% Co concentration followed by a decrease for further increase in Co concentration toward 5 at.%. Published by Elsevier B.V.

The resistance transitions of Gd-diffused polycrystalline Y123 samples have been measured as a function of temperature and magnetic field. It is deduced from experimental results that the activation energy increases when the diffusion annealing temperature increases and also the activation energy of Gd-diffused samples are larger than that undoped Y123 sample. The magnetic field dependence of U follows a power law, i.e. U ∝ H − α , with the values of α are 0.42, 0.56 and 0.61, respectively, for pure Y123, and Gd-diffused Y123 at 800 and 900 °C. The changing of α values from 0.42 to 0.61 imply that the thermally activated dissipative resistivity is directly associated with pinning mechanism changes from intrinsic planar defects pinning to extrinsic point defects pinning because of large Gd content into Y123 sample by the means of diffusion annealing temperature increasing. In this paper, the possible reasons responsible for the effect of Gd-diffusion on the pinning behaviors are discussed.