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The enhanced performance of superconducting FeSe0.5Te0.5 materials with added micro-sized Pb and Sn particles is presented. A series of Pb- and Sn-added FeSe0.5Te0.5 (FeSe0.5Te0.5 + xPb + ySn; x = y = 0–0.1) bulks are fabricated by the solid-state reaction method and characterized through various measurements. A very small amount of Sn and Pb additions (x = y ≤ 0.02) enhance the transition temperature (Tconset) of pure FeSe0.5Te0.5 by ~1 K, sharpening the superconducting transition and improving the metallic nature in the normal state, whereas larger metal additions (x = y ≥ 0.03) reduce Tconset by broadening the superconducting transition. Microstructural analysis and transport studies suggest that at x = y > 0.02, Pb and Sn additions enhance the impurity phases, reduce the coupling between grains, and suppress the superconducting percolation, leading to a broad transition. FeSe0.5Te0.5 samples with 2 wt% of cometal additions show the best performance with their critical current density, Jc, and the pinning force, Fp, which might be attributable to providing effective flux pinning centres. Our study shows that the inclusion of a relatively small amount of Pb and Sn (x = y ≤ 0.02) works effectively for the enhancement of superconducting properties with an improvement of intergrain connections as well as better phase uniformity.

This article discusses the influence of MgB2 wire diameter on critical parameters, dominant pinning mechanism, and n value. Studies were conducted on multifilament MgB2 wires of the 0.84 mm and 0.64 mm diameter. The wires were annealed under 1 GPa isostatic pressure, between 650 and 730 °C. Studies show that the best results (Jc, Birr, n value, and point pinning mechanism) for a 0.64-mm wire were obtained with annealing temperature of 650 °C. However, the best results for wires with a 0.84-mm diameter were obtained with annealing temperature of 730 °C. This research was conducted for exploring the use of MgB2 wires in multi-section superconducting coils, e.g., main coils generating magnetic fields and compensation coils—increasing the homogeneity of the magnetic field. Our research indicates that the main coil made with larger diameter wires should be heated at higher temperature (730 °C). In contrast, compensating coils with smaller diameter wires should be heated at a lower temperature (650 °C).

This article presents the impact of MgB2 wire bending and diameter on transport critical current density and irreversible magnetic field of a resultant coil. Unreacted MgB2 wires 500 mm in length and 0.63 or 0.83 mm in diameter have been used in the fabrication of small diameter (14 mm) superconducting coils. The coils were subsequently annealed under isostatic pressure of 1 GPa for 15 min at 700 °C and 725 °C. Our results indicate that larger wire diameter, higher annealing temperature, and bending lead to slight reduction of critical current density and irreversible magnetic field in the coil.

The effect of cold high pressure densification (CHPD) on anisotropy of the critical current density (Jc) in « in situ » single core binary and alloyed MgB2 tapes has been determined as a function of temperatures at 4.2 K, 20 K and 25 K as well as at applied magnetic fields up to 19 T. The study includes binary and C4H6O5 (malic acid) doped MgB2 tapes before and after CHPD. It is remarkable that the CHPD process not only improved the Jc values, in particular at the higher magnetic fields, but also decreased the anisotropy ratio, Γ = Jc///Jc┴. In binary MgB2 tapes, the anisotropy factor Γ increases with higher aspect ratios, even after applying CHPD. In malic acid (C4H6O5) doped tapes, however, the application of CHPD leads only to small enhancements of Γ, even for higher aspect ratios. This is attributed to the higher carbon content in the MgB2 filaments, which in turn is a consequence of the reduced chemical reaction path in the densified filaments. At all applied field values, it was found that CHPD processed C4H6O5 doped tapes exhibit an almost isotropic behavior. This constitutes an advantage in view of industrial magnet applications using wires with square or slightly rectangular configuration.

Annealing undoped MgB2 wires under high isostatic pressure (HIP) increases transport critical current density (Jtc) by 10% at 4.2 K in range magnetic fields from 4 T to 12 T and significantly increases Jtc by 25% in range magnetic fields from 2 T to 4 T and does not increase Jtc above 4 T at 20 K. Further research shows that a large amount of 10% SiC admixture and thermal treatment under a high isostatic pressure of 1 GPa significantly increases the Jtc by 40% at 4.2 K in magnetic fields above 6 T and reduces Jtc by one order at 20 K in MgB2 wires. Additionally, our research showed that heat treatment under high isostatic pressure is more evident in wires with smaller diameters, as it greatly increases the density of MgB2 material and the number of connections between grains compared to MgB2 wires with larger diameters, but only during the Mg solid-state reaction. In addition, our study indicates that smaller wire diameters and high isostatic pressure do not lead to a higher density of MgB2 material and more connections between grains during the liquid-state Mg reaction.

This research was funded by the Institute of Low Temperature and Structure Research, PAS, the Institute of High Pressure Physics, PAS and Ministry of National Defense Republic of Poland Program-Research Grant MUT Project 13-995. The wire samples of this work were fabricated with financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) research project with grant numbers 217 M665. Our results show that a lower density of unreacted Mg + B material during an Mg solid-state synthesis reaction leads to a significant reduction in the quantity of the superconducting phase and lowers the homogeneity of the superconducting material. It also significantly reduces the irreversible magnetic field (B-irr), critical temperature (T-c), upper magnetic field (B-c2), engineered critical current density (J(ec)), and n-value, despite high isostatic pressure (HIP) treatment and the use of nanoboron in the sample. Our measurements show that samples with large boron grains with an 8% higher density of unreacted Mg + B material allow better critical parameters to be achieved. Studies have shown that the density of unreacted material has little effect on B-irr, T-c, B-c2, J(ec), and the n-value for an Mg liquid-state synthesis reaction. The results show that the critical parameters during an Mg liquid-state synthesis reaction depend mainly on grain size. Nanoboron grains allow for the highest B-irr, T-c, B-c2, J(ec), and n-values. Scanning electron microscopy (SEM) images taken from the longitudinal sections of the wires show that the samples annealed under low isostatic pressure have a highly heterogeneous structure. High isostatic pressure heat treatment greatly improves the homogeneity of MgB2. Institute of Low Temperature and Structure Research, PAS; Ministry of National Defense Republic of Poland Program-Research Grant MUT Project [13-995]; Scientific and Technological Research Council of Turkey (TUBITAK) research project [217 M665]; Institute of High Pressure Physics, PAS

Our results show that a lower density of unreacted Mg + B material during an Mg solid-state synthesis reaction leads to a significant reduction in the quantity of the superconducting phase and lowers the homogeneity of the superconducting material. It also significantly reduces the irreversible magnetic field (Birr), critical temperature (Tc), upper magnetic field (Bc2), engineered critical current density (Jec), and n-value, despite high isostatic pressure (HIP) treatment and the use of nanoboron in the sample. Our measurements show that samples with large boron grains with an 8% higher density of unreacted Mg + B material allow better critical parameters to be achieved. Studies have shown that the density of unreacted material has little effect on Birr, Tc, Bc2, Jec, and the n-value for an Mg liquid-state synthesis reaction. The results show that the critical parameters during an Mg liquid-state synthesis reaction depend mainly on grain size. Nanoboron grains allow for the highest Birr, Tc, Bc2, Jec, and n-values. Scanning electron microscopy (SEM) images taken from the longitudinal sections of the wires show that the samples annealed under low isostatic pressure have a highly heterogeneous structure. High isostatic pressure heat treatment greatly improves the homogeneity of MgB2.

The single phase lanthanum orthoniobate with tetragonal structure has been synthesized by the means of mechanosynthesis method. The studies have shown the crystal structure of La0.98Ca0.02NbO4 depends on the synthesis stage. The samples were predominantly in the tetragonal phase with a trace amount of the monoclinic phase. The SEM studies of morphology and microstructure have shown nanocrystallinity of the materials. The Raman studies showed the significant difference between the Raman spectra of nano- and microcrystalline La0.98Ca0.02NbO4. A decrease of Debye temperature, shown a change of thermal properties due to the presence of nano-sized particles in La0.98Ca0.02NbO4. Electrical characterization results suggest the protons are predominant charge carries in the material for the whole measured range of temperature and shows the total conductivity value of 4.0 × 10−5 S cm−1 at 600 °C in wet air. The calculated activation energies were equal to (0.81 ± 0.04) eV under dry and (0.73 ± 0.06) eV under humidified conditions. That indicates that nanostructuring was successfully stabilized the high-temperature tetragonal phase.

In this work, we investigated the influence of isostatic pressure (of 0.1 MPa and 1.1 GPa) during heat treatment, doping, size and shape of crystalline boron grains, and annealing time at low annealing temperature (570 °C) on the formation of high-field pinning centers in MgB2 wires. The results indicate that high isostatic pressure (1.1 GPa) at low annealing temperature (570 ° C) and annealing time of 120 min significantly increases the density of high-field pinning centers in MgB2 wires with small and large boron grains. Transport measurements show that an increase of annealing time from 120 to 210 min at 1.1 GPa slightly decreases the critical current density (Jc) and irreversibility field (Birr) in MgB2 wires with large boron grains, suggesting that longer annealing time weakly affects the density of high-field pinning centers. On the other hand, for MgB2 wires with small boron grains, the annealing time of 210 min significantly reduces $J_{\mathrm {c}}$ and $B_{\text {irr}}$ . This indicates that the longer annealing time at 1.1 GPa significantly reduces the density of high-field pinning centers. Our studies indicate that dislocations created by the hot isostatic pressure process significantly increase $B_{\text {irr}}$ in the temperature range from 5 to 12 K. On the other hand, strains due to the shrinkage of MgB2 material increase $B_{\text {irr}}$ in the temperature range from 12 to 34 K. The results show that small grains of crystalline boron and high isostatic pressure lead to a high density of dislocations and strains during transformation to the MgB2 phase.

Our research show that the higher packing density of the unreacted material (Mg +2B) by 8%, especially for Mg in the solid state, allows to eliminate the large number of large voids (20 mu m), improves the homogeneity and density of the MgB2 superconducting material, allows to obtain a greater number and longer lengths of connections between MgB2 grains, eliminates the formation of the intermetallic phase (iron borides), allows to obtain MgB2 grains of a uniform size and shape, increase irreversible magnetic field (Birr), critical temperature (T-c) and upper magnetic field (B-c2). Research show that spherical grains significantly reduce the number of inter-grain connections in the material with low and high density of unreacted material and allows to obtain the more superconducting phase. Research shows that MgB2 grains with a plate-like and rectangular shape allow to obtain more connections between the grains. Studies show that thermal treatment under high iso-static pressure does not allow to obtain the large amount of superconducting phase for the unreacted material of low density. Moreover, studies show that the high packing density of unreacted material and thermal treatment under high isostatic pressures (0.8 GPa and 1.1 GPa) allow to obtain a large amount of superconducting phase even for Mg in the solid state. Additionally, studies points that heat treatment under medium isostatic pressure (0.3 GPa) significantly reduce B-irr and B-c2. Our results are important for long superconducting wires made by using the powder-in-tube (PIT) technique. Because they indicate that the higher packing density of the unreacted material (Mg +2B) will allow to obtain a the superconducting material with greater homogeneity and density, and improve the critical parameters e.g. coils. (C) 2021 Elsevier B.V. All rights reserved.

We report the influences of the high hot isostatic pressure (HHIP) on Ba(Fe 0.92 ,Co 0.08 ) 2 As 2 superconducting bulks materials. Magnetic measurements showed that this material has a critical temperature ( T c ) of 26 K. The HHIP process increases the density of flux pinning centers without altering the dominant flux pinning mechanisms and without decreasing of T c . Structural studies have shown that the HHIP process increases the density of the material, reduces the size of voids, and thus allows to obtain homogeneous distribution of the components. Additionally, the HHIP process removes cracks in polycrystalline Ba(Fe 0.92 ,Co 0.08 ) 2 As 2 , creates more superconducting phase and forms less amount of Ba precipitates.

A series of MgB2 samples doped with C, RE2O3 oxides (RE = Ce, Nd), and C and RE2O3oxides was prepared by the hot isostatic pressing method at high pressure: 1 GPa under argon gas. Based on X-ray diffraction and magnetization measurements it was found that the RE2O3 is not incorporated into MgB2. A secondary REB4 phase is present after the synthesis. No decrease in the critical temperature (Tc) is observed after the RE2O3 doping. The Tc of C and RE2O3 doped MgB2 is decreased by 3 K. The doping decreases the diamagnetic response in comparison to pure MgB2 phase. The highest critical current density (Jc) was 3.4 × 105 A/cm2 at 4.2 K and 0.5 T for the bulk material, and the irreversibility field for the Nd2O3 doped sample was located at high field at T = 30 K. The results indicate that inside grains of pinning centers increase Jc from 4.2 K to 25 K and decrease Jc at 30 K. On the other hand, pinning centers between grains (precipitate) increase Jc at 30 K and decrease Jc from 4.2 K to 25 K in the middle and high magnetic fields. Our research shows that carbon-encapsulated boron creates a point pinning centers.

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.

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 effects of heat treatment temperature, time and isostatic pressure on the grain connectivity and superconducting properties of Mg11B2 wires prepared using nano-amorphous isotopic boron powder (11B) have been investigated. The article presents detailed studies of the Mg11B2 material structure of the wires using scanning electron microscope (SEM). Heat treatment at low and high temperature for different annealing time and isostatic pressure does not create an internal macro-defect that disrupts connectivity in the Mg11B2 superconducting wires. Our study shows that dense longitudinal superconducting layers are formed at high annealing temperature of 800 °C for 60 min. Formation of dense longitudinal superconducting layers under high isostatic pressure (HIP) allow improved grain connectivity and high density of pinning centers and as a result, high engineering critical current density, Jec, at 4.2 K has been achieved. Also, high upper critical field (Bc2) and high irreversible magnetic field (Birr) were obtained using HIP process while superconducting transition temperature (Tc) was not significantly affected. Better grain connectivity obtained using high annealing temperature and long annealing time under low isotopic pressure leads to Jec enhancement at high magnetic fields at 20 K. Our results show that the annealing temperature of 800 °C for 60 min under isostatic pressure of 1 GPa yields the required Jec of 25 A/mm2 and 50 A/mm2 for ITER applications in magnetic fields of 8 T and 7 T, respectively. The promising results obtained in this work could pave the way for designing low activation superconducting Mg11B2 wires to be considered for next generation fusion magnets.

Brief summary of our former work on high hydrogen pressure syntheses of novel hydrides and studies of their properties is supplemented with new results. Syntheses and properties of a number of hydrides (unstable, metastable or stable in ambient conditions) derived under high hydrogen pressure from intermetallic compounds, like MeT 2 , MeNi 5 , Me 7 T 3 , Y 6 Mn 23 and YMn 12 (where Me = zirconium, yttrium or rare earth; T = transition metal) are presented. Stabilization of ZrFe 2 H 4 due to surface phenomena was revealed. Unusual role of manganese in hydride forming processes is pointed out. Hydrogen induced phase transitions, suppression of magnetism, antiferromagnetic-ferromagnetic and metal-insulator or semimetal-metal transitions are described. Equations of state (EOS) of hydrides submitted to hydrostatic pressures up to 30 GPa are presented and discussed.

Here we report a new versatile technique to manufacture MgB2 massive samples, called reverse advance internal magnesium diffusion (r-AIMI). The idea focuses on the goal of obtaining dense bulk or wire samples depending on synthesis conditions. In respect to the traditional AIMI procedure, in which a central Mg rod is covered with a B corona, here a Mg tube is filled with B powder and clad in a Ti external sheath, which is quite similar to the traditional (powder in tube) technique. After cold deformation, during which several intermediate low temperature heat treatments are necessary in order to relax the Ti sheath and Mg tube, samples are reacted at high temperature and ambient pressure to form a dense MgB2 core. The MgB2 phase results are totally disconnected from metallic sheath, and can be easily extracted and characterized. Critical current density measurements show values exceeding 106 A cm−2 below 1.5 T at 20 K. In the last part of the paper, we show the effect of final heat-treatment performed under high pressure to eliminate the present void and connect the external sheath to the internal MgB2 core and so permitting the electric transfer necessary for power applications of wires.

In this article, the significant impact of a lamellar (layered) structure and a high isostatic pressure on the normal state resistance (R-n), critical temperature (T-c), irreversible magnetic field (B-irr), upper magnetic field (B-c2) and critical current densities (J(c)) at 4.2 K and 20 K was presented. Our research showed that annealing at temperatures in the range of 630 degrees C-680 degrees C (above the melting point of Mg) at atmospheric pressure (0.1 MPa) did not create a lamellar (layered) structure. This led to low T-c, J(c) and B-irr, and high R-n. The analysis, made by using scanning electron microscopy (SEM), showed that the annealing temperature increased up to 700 degrees C under a pressure of 0.1 MPa, which created a lamellar structure. This led to significant growth of T-c, J(c) and B-irr and a slight increase of R-n. Moreover, the measurements showed that annealing at temperatures from 630 degrees C to 700 degrees C did not change the B-c2 value. In comparison to pressureless heat treatment, annealing under the high isostatic pressure of 1.1 GPa obtained a lamellar structure with layers of lower thickness and higher density. This led to significant increases in J(c) and B-irr and a visible reduction of R-n. SEM analysis showed that the increase of isostatic pressure up to 0.3 GPa created a lamellar structure with thicker layers and lower density. This microstructure led to lower J(c) and B-irr and significantly higher R-n. On the other hand, the SEM analysis showed that annealing under 0.8 GPa did not cause the formation of a layered structure, and as a result, it led to significant reductions in B-irr and J(c) (4.2 K and 20 K) and higher R-n. The increase of the isostatic pressure from 0.1 MPa to 1.1 GPa did not affect T-c (B = 0 T) and B-c2. The results indicated that the layered structure obtained a high density of pinning centers, which were particularly effective at higher magnetic fields. J(c) of 100 A/mm(2) in 8 T at 4.2 K was obtained in in situ undoped MgB2 wires after annealing at 700 degrees C for 40 min under an isostatic pressure of 1.1 GPa. (C) 2018 Elsevier B.V. All rights reserved.

Superconducting coils are designed for the generation of specified magnetic fields (low, intermediate, or high). To function in different field regimes, it is important to analyze the pinning mechanics and the types of pinning centers in the superconductor in a precisely defined magnetic field. Correct identification of pinning centers would allow for the formulation of approaches to increase the critical current density and critical magnetic field, ultimately making it possible to reduce the cost of producing superconducting coils (with smaller amounts of wire needed). This paper will present a method to analyze pinning centers in MgB2 superconductor wires in low, intermediate, and high magnetic fields at 4.2 K, 20 K, 25 K, and 30 K. This method allows us to elucidate the influences of doping, isostatic pressure, and reduction of the wire diameter on individual pinning centers in a specific magnetic field. In this article, we also show a method to analyze the types of pinning centers and their impact on the engineering critical current density (Jec). This method not only allows for the identification of the types of pinning centers, but also identifies which type allows for a shift in the Jec criterion of 100 A/mm2 to enable higher magnetic fields.

We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB2 wires. These defects reduce the critical current of MgB2 wire. Detection and removal of these defects will allow us to produce MgB2 wires with ex-situ MgB2 and Nb barriers that will have improved critical current density. Manufacturing of MgB2 wires with new ex-situ MgB2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep.

We report a study on the structural and transport properties of long in situ MgB2/Fe wires that are prepared by use of amorphous boron and nano amorphous boron powders with 50-50% weight ratio. The wire samples are fabricated by means of a standard Powder-in-Tube (PIT) method. Transport measurements are applied under high magnetic fields, of up to 9 T, obtained in a Bitter magnet. We find that use of a mixture of the amorphous boron, and amorphous nano boron precursor powders at equal amounts is very promising way to fabricate long wires without any degradation in transport engineering Ice values in the presence of low and moderate magnetic fields.(C) 2017 Elsevier B.V. All rights reserved.

The effect of wire diameter reduction on the critical current density of pristine MgB2 wire was studied. Wires were treated by a hot isostatic pressing method at 570 °C and at pressures of up to 1.1 GPa. It was found that the wire diameter reduction induces an increase of up to 70% in the mass density of the superconducting cores. This feature leads to increases in critical current, critical current density, and pinning force density. The magnitude and field dependence of the critical current density are related to both grain connectivity and structural defects, which act as effective pinning centers. High field transport properties were obtained without doping of the MgB2 phase. A critical current density jc of 3500 A mm−2 was reached at 4 K, 6 T for the best sample, which was a five-fold increase compared to MgB2 samples synthesized at ambient pressure.

Hot axial and hot isostatic pressing was applied for single-core MgB 2 /Ti tapes. Differences in transport current density, n -exponents and critical current anisotropy are discussed and related to the grain connectivity influenced by pressing. The magnetic Hall probe scanning measurements allowed observing the isolated regions for axially hot pressed sample attributed to the longitudinally oriented cracks introduced by pressing. The highest current densities were measured for the tape subjected to hot isostatic pressing due to improved connectivity.

Novel magneto-resistive memory effect is demonstrated, which appears in lithographically patterned Y-shaped, three-terminal nanostructures, consisting of three nanowires joined in one point, formed in the epitaxial layer of a ferromagnetic (Ga,Mn)As semiconductor. The effect, in which the zero-field resistance of a pair of nanowires depends on the direction of the previously applied magnetic field, results from a rearrangement of magnetic domain walls, contributing an extra resistance, between different pairs of nanowires in the structure. Two-state behaviour of the nanostructure resistance provides its usefulness for applications in non-volatile memory devices.

In this paper the results of critical current measurements for MgB2 wires and tapes in iron sheath with and without nano-SiC doping are presented. We focus on power-in-tube processing technique, using both in situ and ex situ methods. In situ MgB2 wires and tapes were fabricated from MgH2 and B or Mg and B powders. The methods such as hydrostatic extrusion and rolling were used. The samples were annealed under high Ar gas pressure (hot isostatic pressing) at 750◦C and 1.0 GPa for 40 min. It was found that critical current of MgB2/Fe superconducting wire or tape with nanoSiC dopant increased in higher magnetic field values in comparison to pure MgB2. A significant difference of Jc in tapes made by in situ way from MgH2 and Mg were found.

We show transport results of the critical temperature (T-c), irreversible magnetic field (B-irr), upper critical field (B-c2), critical current density (J(c)) and the pinning force (F-p) for undoped MgB2 wires without barriers. The results of SEM show that the hot isostatic pressure process increases the density and uniformity of MgB2 materials. Hot isostatic pressure allows for obtaining small grains and increasing the number of connections between the grains, accelerated reaction rate and limits the diffusion of Fe atoms into MgB2 material from the adjacent sheath. The pressure of 1.1 GPa increases T-c quite significantly in high magnetic field range of 6-12 T, improves the B-irr and B-c2 and increase the J(c) at 4.2 K and 20 K of about three times. Our results may be important for this field since many MgB2 wires are made from big grains of boron greater than 1 mu m. Moreover, our research can be important for the production of MgB2 wires for the International Thermonuclear Experimental Reactor with boron B-11. (C) 2016 Elsevier B.V. All rights reserved.

We have synthesized single crystals of the misfit layer cobaltate [Sr2TlO3][CoO2]1.77 and determined its crystal structure by the means of X-ray single-crystal diffraction (XRSD) and transmission electron microscopy. The synthesis took place by spontaneous crystallization from an oxide melt at high gaseous pressure. The structure is described by two monoclinic subsystems, which share the unit cell parameters, a=4.933(5) A, c=11.470(12) A, and β=98.00(10)°, but are incommensurate in the direction of b-axes: b1=5.009(8) A and b2=2.830(8) A, where b1 is for the subsystem [Sr2TlO3] and b2 for [CoO2]. The layer sequence in the structure consists of CdI2-type structured (CoO2)∞ and three puckered rock-salt (RS) layers, two of which are (SrO)∞ and one (TlO)∞. The solved structure confirms that the thermoelectric compounds [Ca2CoO3][CoO2]1.62 (Co-121) and [Sr2TlO3][CoO2]1.77 (Tl-121) are isostructural.

Low-temperature charge-carrier transport in simple magnetoresistive nanodevices, consisted of narrow constrictions of submicron width in the epitaxial layer of a ferromagnetic (Ga,Mn)As semiconductor, has been investigated and correlated with magnetic properties of the layer. The devices containing constrictions; revealed abrupt jumps of a reduced resistance that appeared when the sweeping magnetic field crossed the regions of the coercive field of the layer magnetization. In contrast, the non-constricted reference device displayed abrupt jumps of an enhanced resistance at the same values of magnetic field. We interpret the both features, whose positions on the magnetic-field scale reflect the hysteresis loop of magnetization, as manifestation of domain wall contribution to the (Ga,Mn)As layer resistance. The negative contribution of a domain wall to the resistance in the constricted device results most likely from the suppression of the weak localization effects by a domain wall located at the constriction. (Less)

Ferromagnetic n-(Eu,Gd)Te/p-PbTe semiconductor heterostructures were grown by molecular beam epitaxy on BaF 2 (1 1 1) monocrystalline substrates. The epitaxial growth of the layers was controlled in situ by reflection high-energy electron diffraction. X-ray diffraction and atomic force microscopy techniques revealed high structural and morphological quality of deposited layers. Gd content up to 5 at.% in (Eu,Gd)Te layers was estimated by energy dispersive X-ray fluorescence analysis. Hall effect measurements in (Eu,Gd)Te layers showed very high electron concentration of n ≈ 10 20 cm −3 present due to Gd doping. Magnetic characterization of (Eu,Gd)Te/PbTe heterostructures was done using SQUID magnetometry and revealed ferromagnetic transition in n-(Eu,Gd)Te with the Curie temperature T C = 11–15 K. Both in-plane resistance of (Eu,Gd)Te layer and (normal to the plane) (Eu,Gd)Te–PbTe junction resistance measurements were carried out as a function of temperature and magnetic field showing changes in charge transport processes at temperatures close to the Curie temperature.

We fabricated a simple magnetoresistive microdevice formed by a narrow constriction of submicron width in the epitaxial film of a ferromagnetic (Ga,Mn)As semiconductor, and investigated magnetic properties of the film and the low-temperature charge-carrier transport through the constriction. We have revealed sharp jumps of a lowered conductance in a non-constricted sample and jumps of an enhanced conductance in the constricted one, which appeared when the sweeping magnetic field crossed the regions of the coercive field of the film. We argue that the both features result from a contribution of a magnetic domain wall to the conductance. While the spin-orbit interaction can be responsible for the negative contribution of a domain wall to the conductance, presumably the suppression of the weak localization effects by a domain wall located in the constriction results in the positive contribution to the conductance.

The presented compound, Sr2TaMnO6, has a weak, disordered magnetic structure. The metal oxide was prepared under high isostatic oxygen pressure. The doubling of the perovskite structure was proven with electron diffraction and powder neutron diffraction. Combining neutron- and X-ray diffraction data, the room-temperature structure was modelled with the Rietveld method. Both octahedral positions are partially occupied by Mn and Ta, but with different Mn/Ta ratios. AC- and DC-magnetic measurements indicate a magnetic transition at about 17 K and the AC-magnetic susceptibility, both real and imaginary part, is frequency dependent, suggesting that the material has a spin-glass feature. The magnetic spins freeze during a wide temperature range and a possible explanation is a competative situation between the double exchange (ferromagnetism) and the super-exchange (anti-ferromagnetism).

In situ nano-SiC doped MgB2 wires were fabricated from MgH2 and B powders. Hydrostatic extrusion, followed by rotary swaging and two-axial rolling, were applied as the forming processes. The critical current Jc of MgB2 wires, made from MgH2 and B powders, was significantly improved by nano-SiC doping. Nano-SiC doping substantially increased the upper critical (irreversibility) field Bc 2 above 20 T. The maximum Jc values were measured for samples having 6 at.% SiC in low field and for those having 12 at.% SiC in high field, above 10 T. During the final sintering at 670 °C, the SiC decomposed and formed an Si-rich layer at the inner circumference of the Fe sheath. The composition of the core of SiC doped wires is more inhomogeneous in comparison to undoped ones, with MgO, Mg2Si and probably Mg2SiO4 as the major segregated phases. Strong segregation of Si within the MgB2 core was also observed. The highest Tc−mid = 39.3 K was measured for undoped wire. For the optimal SiC doping amount ~6 at.%, at high field, there was no difference in Jc between hydrostatically extruded and hydrostatically extruded plus two-axially rolled wire. This can be attributed to the beneficial effect of hydrostatic extrusion, which causes higher density of the core in comparison to traditional deformation processes.

In this contribution, we present the results on the preparation and the characterization of MgB2 thick films supported on metal/Al2O3 and metal/BN substrates via electrophoretic deposition technique (EPD) under the electric field of 120–180 V/cm. We followed two different routes: deposition of MgB2 powder, both pure and SiC doped and deposition of boron powder. EPD was followed by a high T/high p treatment in a furnace in saturated Mg atmosphere for sinterization or annealing. The structural and morphological properties of MgB2 thick films were monitored by X-ray diffraction and scanning electron microscopy: the diffraction patterns were characteristic of the hexagonal MgB2 phase and the surfaces resulted homogeneous with grain dimensions up to 1 µm. The good superconducting properties, together with the great adherence to the substrate, possibly make these films important in view of shielding and power applications. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Both threading and misfit dislocations in partially relaxed InGaAs/GaAs heterostructures with a small lattice-mismatch have been investigated by means of chemical etching and atomic force microscopy (AFM). An anisotropic etching in HF–H2SO4–H2O2 based aqueous solution was successfully used to determine the polarity of the crystallographic surfaces and an ultrasonic-vibration aided etching in CrO3–HF–H2O based solution was employed to reveal threading dislocations on the heterostructure surfaces. AFM measurements of surface morphology of the structures revealed a well-resolved cross-hatch pattern, which reproduced the network of underlying misfit dislocations, and, in addition, the outcrops of threading dislocations on the surface in a form of characteristic craters. Analysis of the results allowed drawing a conclusion on the mechanism of misfit strain relaxation in the investigated heterostructures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied narrow (submicron) constrictions in the layers of ferromagnetic semiconductor (Ga, Mn)As. We have demonstrated a contribution of the quantum localization effects to the magnetoresistance of the constricted samples. We have also found a negative contribution of a domain wall trapped in the constriction to the resistance, due presumably to the erasing of the localization effects by the domain wall.

From the beginning of the Mg–B system investigations, pressure was an important factor in procedures obtaining the bulk superconductors for application purposes: at hot isostatic pressing (HIP-ing), annealing under inert or active gas medium, i.e. argon with hydrogen and nitrogen, or for extrusion methods for wires manufacturing. MgB 2 reveals interesting properties. We present the current state of our technological efforts in improving the microstructure and superconducting properties of this material, with use of the high gas pressure and high temperature treatment of samples crystallized in crucibles made of BN and SiC. As the precursor we mainly use the commercial MgB 2 powder. The results of microstructure investigations and XRD analyses are presented.

A superconducting Hg0.7Re0.3Ba2Ca2Cu3O8+δ thick film on a polycrystalline LaAlO3 (LAO) substrate was obtained by a two-step method by screen-printing a Re0.3Ba2Ca2Cu3Ox precursor coating on the substrate followed by high-pressure gas firing in Hg vapour. Morphological observations have revealed that the adherent to the ceramic substrate film exhibited highly porous microstructure composed of plate-like crystallites characterized by a superconducting transition temperature of about 110 K. The major phases identified were a high-Tc (Hg,Re)-1223 phase and a low-Tc (Hg,Re)-1212 phase. Non-superconducting secondary impurity phases were also identified. Energy dispersion X-ray spectroscopy analyses at the substrate/film boundary showed a thin continuous layer of BaLa2O4 compound, the presence of which confirmed chemical interaction between the LAO substrate and (Hg,Re)–Ba–Ca–Cu–O film.

In order to search for novel giant-magnetoresistance systems, we fabricated and investigated narrow constrictions in the layers of the ferromagnetic semiconductor (Ga,Mn)As. We found that constrictions a few hundred nanometers wide, tailored by means of the electron-beam lithography and wet etching, were not conducting at liquid helium temperatures unless illuminated, probably due to the trapping action of surface states appearing on an extra surface area denuded by the etching. To avoid this, we used selective implantation of oxygen ions into the ferromagnetic layer to tailor the constrictions. We have shown that such an implantation inactivates Mn acceptors in the layer and destroys ferromagnetism. We propose an application of oxygen ion implantation as a method of fabricating microcircuits in future spin electronics based on Mn-containing III-V semiconductor compounds.

The unavailability of high quality precursor is encouraging researchers to seek effective ways to fabricate magnesium diboride (MgB 2 ) wire. Herein, cost-effective amorphous boron powder produced through a diborane (B 2 H 6 ) gas process is investigated for the possibility of further industrial application. A thin carbon layer to encapsulate the boron particles is simultaneously deposited by pyrolysis of hydrocarbon. We found that the carbon-encapsulated amorphous boron has a high upper critical field due to impurity scattering, and thereby, enhanced high-field critical current density.

Single-core MgB2/Cu and MgB2/Fe/Cu wires and tapes have been made by the powder-in-tube method with the use of commercial MgB2 powder (Alfa Aesar) involving hydrostatic extrusion and two-axial rolling processes. High-pressure post-treatments such as cold isostatic pressing, double hydrostatic extrusion and uniaxial pressing were applied. The grain connectivity, Ic(B) curves, pinning in MgB2 grains, upper critical field, deformation homogeneity, compact density and preferable alignment crystallites for the [001] (c-axis) direction were all improved and enhanced by application of various high-pressure treatments for the MgB2 compound and composites. Hydrostatically extruded MgB2 wires have been shown to be better than rolled wires and are very promising for possible application in windings.

Single-core MgB2 wires and tapes have been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar). Composites have been made using the two-axial rolling process in Cu and/or Fe/Cu sheaths. Alternative deformations by wire drawing, rotary swaging and cold isostatic pressing have been applied to PIT wires and tapes. Current–voltage characteristics and transport current densities in the self-field and in the external field were measured. It was found that the grain connectivity of ex situ MgB2 is affected by the applied sheath and the mode of deformation. Two-axial rolling has generated the highest powder density resulting in the best grain connectivity. The highest transport current densities of 8700 A cm−2 and 55 830 A cm−2 were measured for Cu and Fe/Cu sheathed square wires, respectively. Cold isostatic pressing at 1.5 GPa has increased current density and n-exponent, which suggests an improvement in grain connectivity. It was found that the external pressure improves the inter-grain connectivity but decreases the pinning in MgB2 cores.

Differential thermal analysis shows many interesting properties that allow for prompt finding the place (p,T) and characteristics (endothermic, exothermic) of a phase transition or chemical reaction. In application to reactive sintering under active gas atmosphere at high pressure despite of numerous technical problems encountered during DTA measurements we found its unique possibilities. That allows for quantitative estimation of nitrogen involved in reaction of phase transition from the hexagonal phase to the cubic phase of MoN, and vice versa, in high gas pressure condition. DTA high gas pressure measurement system has the maximum operate temperature 2000°Cat pressure 2 GPa.

In this article, we report the results of a study that shows the existence of pinning centers inside grains and between grains in NbTi wires. We accurately show the ranges of magnetic fields in which the individual pinning centers operate. The pinning centers inside grains are activated in high magnetic fields above 6 T. We show the range of magnetic fields in which individual defects, dislocations, precipitates inside grains and substitutions in the crystal lattice can operate. We show the existence of a new kind of high field pinning center, which operates in high magnetic fields from 8 to ∼9.5 T. We indicate that dislocations create pinning centers in the range of magnetic fields from 6 to 8 T. In addition, our measurements suggest that the peak effect (increased critical current density (J ) near the upper critical field (B )) could be attributed to martensitic (needle-shaped) α′-Ti inclusions inside grains. These centers are very important because they work very effectively in magnetic fields above 9.5-10 T. We also show that the α-Ti precipitates (between grains) with a thickness similar to the coherence length create pinning centers which work very effectively in magnetic fields from 3 to 6 T. In magnetic fields below 3 T, they act very efficiently in grain boundaries. The measurements indicate that the pinning centers created by dislocations only can be tested by transport measurements. This indicates that dislocations do not increase the magnetic critical current density (J ). Cold drawing improves pinning centers at grain boundaries and increases the dislocation density, and cold-drawing pinning centers are responsible for the peak effect.

The fabrication method of superconducting thin films of compositions HgBa2Ca2Cu3O8+δ (Hg-1223) and Tl2Ba2CuOy (2201) on single-crystalline SrTiO3 and LaAlO3 substrates is reported. The highest obtained Tc was 134 K and Jc over 106 A cm–2 at 77 K. High pressure DTA(HP-DTA) was applied to grow mercury- and thallium-based high-temperature superconducting crystals and thin films, to identify melting points of particular phases within these oxide systems and determine suitable processing conditions. The DTA system operates at the: maximum temperature of 1200°C, volume up to 5 cm3, working pressure up to 1.5 GPa and at a working atmosphere — inert gas with up to 25% oxygen.