MgB2 Phase Research Articles

Effect of process variables on synthesis of MgB2by a high energy ball mill

The discovery of superconductivity of MgB2 in 2001, with a critical temperature of 39 K, offered the promise of important large-scale applications at around 20 K. Except than the other featured synthesis methods, mechanical activation performed by high energy ball mills, as bulk form synthesis or as a first step of wire and thin film productions, has considered as an effective alternative production route in recent years. The process of mechanical activation (MA) starts with mixing the powders in the right proportion and loading the powder mixture into the mill with the grinding media. The milled powder is then consolidated into a bulk shape and heat-treated to obtain desired microstructure and properties. Thus, the important components of the MA process are the raw materials, mill type and process variables. During the MA process, heavy deformation of particles occure. This is manifested by the presence of a variety of crystal defects such as dislocations, vacancies, stacking faults and increased number of particle boundaries. The presence of this defect structure enhances the diffusivity of solute hence the critical currents and magnetic flux pinning ability of MgB2 are improved. The aim of the present study is to determine the effects of process variables such as ball-to-powder mass ratio, size of balls, milling time, annealing temperature and contribution of process control agent (toluene) on the product size, morphology and conversion level of precursor powders to MgB2 after subsequent heat treatment. The morphological analyses of the samples were performed by a high vacuum electron microscope ZEISS SUPRA VP 50. The phase compositions of the samples were performed with an Rigaku-Rint 2200 diffractometer, with nickel filtered Cu Kα radiation and conversion level. The MgB2 phase wt % was calculated by the Rietveld refinement method. The obtained results were discussed according to the process variables to find out their affect on the structure of MgB2 and to determine the optimum synthesis condition. It was found that the ball-to-powder mass ratio of precursors mixed and the milling time remarkably affect the MgB2 phase wt % and product size.

Freeze drying technique to prepare doped nanosized B powder

ABSTRACTIn this study, we propose a new approach for the large-scale production of doped nanosized boron powder, which is also useful for manufacturing a wide choice of compounds. This work deals with synthesis of B with cryogenic freezing and lyophilization-based technique, evaluation of its structural properties, and study of superconducting properties of MgB2 wires manufactured with two types of B powder. This new process begins with dissolution of B2O3 precursor in hot water together with doping agents, which can be organic macromolecule or nanosized inorganic compound. This hot solution was then cryogenically frozen in liquid N2 and finally lyophilized. Nanostructured and doped boron powders were produced by magnesiothermic reaction of the lyophilized precursor (B2O3). After the structural and morphological characterization, these boron powders were used for preparation of MgB2 phase and MgB2-based conductors. Boron and MgB2 powders were morphologically analyzed by SEM technique to determine grain size distribution. Phase formation was studied and confirmed by XRD analysis. Short pieces (10 m long) of MgB2 superconducting wires were manufactured by ex situ PIT technique and characterized by magnetic and transport measurements. Results reported in this study confirm the suitability of the proposed method to produce nanostructured and doped boron powder. This nanosized boron can be used also in different fields where doping and nanosizing are fundamental and required characteristics.

Phase formation dynamics upon thermal explosion synthesis of magnesium diboride

Time-resolved X-ray diffraction was used to study the dynamics of phase formation in magnesium diboride produced by self-propagating high-temperature synthesis in the thermal explosion mode. The MgB2 phase was demonstrated to emerge without the formation of intermediate compounds. The effect of the mixture heating rate on the formation mechanism of the MgB2 phase was established. The presence of oxygen impurities has a significant impact on the kinetics and formation mechanism of MgB2. If the heating rate exceeds 150°/min, the oxide coating is not formed around the magnesium particles, which results in the solid-phase reaction of Mg+2B=MgB2 through a reaction diffusion mechanism. Moreover, the self-ignition temperature of the mixture is lower than the melting point for magnesium. Mechanical activation of the mixture leads to variations in the kinetics of MgB2 formation, significantly increases the period of simultaneous existence of Mg and MgB2, and reduces the temperature at which the reaction occurs.

Properties of MgB2 wires made by internal magnesium diffusion into different boron powders

Different boron powders were used for MgB2 wires manufactured by internal magnesium diffusion. The structure of the MgB2 core, critical temperature and critical currents of Cu/Ti sheathed wires differing only in boron powder were analyzed and compared. It was found that the particle size and purity of boron powders influence the creation of the MgB2 phase and, consequently, also considerably influence its superconducting properties. The highest critical current density in the low external field was measured for wire with a boron purity of 98.5% produced by Pavezyum. It was used also for stabilized multi-core MgB2 wire with high engineering current densities in low magnetic fields at 20 K, which may be attractive for some low field applications, e.g. high-power wind generators.

Improved transport critical current properties in glycerin-doped MgB2 wire using milled boron powder and a solid-state reaction of 600 °C

We adopted simultaneously three effects of milling, carbon (C) doping and solid-state reaction to improve critical current properties of MgB2 superconducting wire. The influences of heat-treatment temperature, including a conventional solid–liquid reaction of 650–1000 °C and a solid-state reaction of 600 °C below the melting point (650 °C) of Mg powder, were examined on the transport superconducting properties of in situ powder-in-tube (PIT) processed MgB2/Fe wires using ball-milled and glycerin-treated boron (B) powder. The aims of the mechanical milling and liquid glycerin treatment of the B powder were to reduce the grain size of the MgB2 and achieve homogeneous C incorporation into the MgB2, respectively. The superconducting properties of MgB2 wires heat-treated in the range of 650–1000 °C were investigated, and it was also investigated as to whether the C incorporation occurred even in a low-temperature solid-state process of 600 °C, thus resulting in an improvement of the superconducting properties by obtaining both high grain boundary density and C substitution effects. The MgB2 phase formation, actual C substitution amount, full width at half maximum (FWHM), critical temperature (Tc), magnetic field dependence of transport critical current density (Jc) and temperature dependence of the upper critical field (Hc2) were evaluated for glycerin-doped MgB2/Fe wires fabricated at different heat-treatment temperatures. The glycerin-doped MgB2 wire using milled B powder heat-treated at a solid-state of 600 °C showed the highest transport Jc values at 4.2 K over the entire applied field regime. It was revealed that the grain boundary density was higher and that the C substitution also occurred by a low temperature heat-treatment process, which led to a higher Jc. In addition, a solid–solid diffusion reaction with the pre-treated B powder resulted in poor crystallinity, which enhanced Hc2 and improved Jc.

Effect of deformation with Bridgman anvils on the structure, hardness, and critical current of a massive MgB2-based sample

The structure and properties of synthesized massive MgB2-based samples subjected to deformation with Bridgman anvils have been studied. Deformation results in the formation of fine-grained structure of the MgB2 phase, enhancement of interconnection of grains, complete disappearance of friable MgB2-phase areas, and abrupt increase in the microhardness.

Manufacturing process influence on superconducting properties of MgB2 wires prepared using laboratory made boron

Here we report a systematic study of the superconductive properties of mono-filamentary MgB2-based wires, manufactured with four different techniques. A detailed comparison of the influence of manufacturing technique and final heat treatment on superconducting properties has been given. The boron used was synthesized in laboratory following magnesiothermic reduction of boron oxide, purified thanks to several acid leaching and heat treated at high temperature, to enhance crystalline degree and remove impurities. MgB2 conductors were manufactured using the same B precursor through four different techniques (ex situ, in situ, the MgB4 or ‘mixed’ technique (half ex situ and half in situ), and reactive liquid infiltration (Rli)). Transport critical current density was measured on the best wire for each technique, considering the literature data in order to identify the corresponding best final heat treatment. Magnetic critical current density and critical temperature were investigated at different synthesis/sintering temperatures in order to evaluate their dependence to the applied final heat treatment and the data were compared. Critical current density was evaluated on short wire pieces by magnetic measurement at 5 K in a MPMS 5.5 T Quantum Design SQUID, while critical temperature was measured with a four probe system by drop of resistivity during the cooling process of the sample in a liquid helium dewar. A detailed morphological analysis is given, with void percentage evaluation and analysis of elemental Mg diffusion across the transversal cross section. X-ray diffraction was performed on MgB2 powder extracted removing each metal sheath, in order to investigate the influence of manufacturing process on the MgB2 phase. This study shows that despite the presence of a wide void within the superconducting core (due to the Mg diffusion) in the Rli sample, this manufacturing technique allows wires with higher Jc (105 A cm−2 at 3 T when heat treatment at 700 °C is applied) than other powder in tube techniques, while the highest Tc (39.2 K at the onset) can be reached by following the mixed technique, which also shows low dependence to heat treatment for both Tc and Jc. In order to establish the presence of some impurities in the lab-made B precursor energy dispersion spectroscopy analysis, inductively coupled plasma atomic emission spectrometry and X-ray analysis were performed on B.

Scaling of Pinning Force Density in (Bi,Pb)-2223 Added MgB2 Superconductors

Addition of different amount of superconducting Bi1.8Pb0.26Sr2Ca2Cu3O10+x on the superconducting properties of polycrystalline MgB2 is investigated. Samples are synthesized using solid-state reaction under argon atmosphere at 775 ∘C. X-ray diffraction (XRD) data shows that major reflection belongs to MgB2 phase along with small amount of MgO. Traces of 2223 phase are seen for MgB2 with higher content of 2223. Absence of significant change in peak positions and in lattice parameters indicates no substitution effect in the present system. Unreacted 2223 phase is present as isolated grains, or clusters at the grain boundaries. The superconducting transition temperature for MgB2 is only marginally influenced with the addition of 2223. Field dependence of critical current density (Jc), however, displays significant enhancement (∼ 2 times) for 1 wt % 2223 added MgB2 pellets. The field dependence of Jc is explained well using collective pinning model. Further, the pinning force density of both pure and 2223 added MgB2 displays excellent scaling with respect to the field (hn), corresponding to which Fp drops to half of its maximum value. The observed scaling of normalized pinning force density (Fp/ Fpmax) is described well in terms of an expression derived based on Jc expression proposed by collective pinning model in small bundle region.

Control of core structure in MgB2 wire through tailoring boron powder

A common fabrication process for MgB2 wire, namely, the in situ powder-in-tube process, forms numerous voids within the wire core, and void formation cannot be completely avoided. The orientation is, however, known to be aligned more or less along the current-flow direction when ductile coarse magnesium powder is used as a precursor, and further tailoring approaches could open up the way to improving the transport critical current density. Herein, we have used boron powders with different particle sizes, in combination with the coarse magnesium powder, and evaluated their size effects on the phase composition, microstructure, and transport properties. A mixture of the coarse magnesium powder with large-sized boron powder in the wire core, after cold working and sintering, forms a granular morphology. In contrast, an aligned core appears during the reduction process for wire which is prepared by using fine boron powder. The sintering process, especially at a low temperature, where magnesium evaporation hardly occurs, yields an aligned structure, mainly consisting of MgB2 phase, along the wire direction. These findings demonstrate that the initial size of the starting materials is critical for the tailored structure.

Liquid-Phase Shock-Assisted Consolidation of Superconducting MgB2 Composites

An original two-stage liquid-phase hot explosive compaction (HEC) procedure of Mg-B precursors above 900 ∘C provides the formation of superconductivity MgB2 phase in the whole volume of billets with maximal T c = 38.5 K without any further sintering. The liquid-phase HEC strongly increases the solid-state reaction rate similar to photostimulation, but in this case, due to the high penetrating capability of shock waves in a whole volume of cylindrical billets and consolidation of MgB2 precursors near to theoretical density allows one to produce bulk, long-body cylindrical samples important for a number practical applications.

Increased critical current density of in-situ MgB2 wires by Mg deficiency

In this contribution the influence of the Mg:B-stoichiometry in the precursor on the structural and electrical properties of bulks and wires prepared with mechanically alloyed powder is discussed. The Mg to B ratio in the powder was varied in the range of x=0.7 to 1.1 (MgxB2). Analysing the properties of bulks and wires prepared with such precursor powders it was found that Mg-deficiency can increase the amount of MgB2 phase, the MgB2 diffraction peak width, the irreversibility field and the critical current density considerably.

The Effect of High-energy Ball Milling on the Microstructure and Properties of Ti-doped MgB2 Bulks and Wires

MgB2 bulks were prepared by high-energy ball milling of Mg, B and Ti powders at ambient temperatures. The mixed powders were ball-milled for 0-10h using a ball-to-powder mass ratio of 10 and sintered at 750°C for 2h. The phase and microstructure of MgB2 were characterized by means of X-ray diffraction (XRD) and Scanning electron microscope (SEM). XRD results reveal the appearance of a small amount of MgO impurity, and the relative percentage composition of MgO phase is gradually increased with prolonged the milling time. It proved that the sample milled 5h has the best homogeneity in grain size. The milled 5h powders were further processed to wires. The wire samples were fabricated by the in-situ powder-in-tube (PIT) method. The mechanical properties were analyzed by stress-strain measurements at room temperature and the superconducting behaviors are investigated by electrical tests at 4.2K. The critical engineering current Ic reaches above 330 A (Jce exceeding 2.2×104 A/cm2) at 4.2K and 2 T.

Growth of Stable Bilayer CrO 2 /MgB 2 Films by Pulsed Laser Deposition

We have used the pulsed laser deposition (PLD) technique followed by post-annealing process with magnesium vapor at 700∘C for 30 min to grow a high-quality CrO2/MgB2 bilayer prepared on Al2O3 (1102) single crystal. The X-ray diffraction patterns showed high-quality orientated growth of MgB2 phase. The obtained films were approximately 300 nm thick and had lateral dimensions of 5×5 mm2. SQUID magnetometry, magneto-optical Faraday microscopy, and magneto-transport techniques were used in order to measure the critical current density (JC) that was found to be 7– 9×106 A/cm2 at 15 K. Transition temperature (TC) for the films was ∼37 K. The obtained magnetic hysteresis loops at low temperatures exhibited asymmetry with respect to H = 0. This can be explained in terms of pinning of flux lines in the films due to magnetic interaction with the ferromagnetic CrO2 layer.

Improved superconducting properties in Ti-doped MgB2 prepared by two-step reaction method and high-energy ball milling

High Jc Ti-doped MgB2 disks were synthesized by two-step reaction method and high-energy ball milling. In the first step, the sample with a nominal composition of Mg0.8Ti0.2B4 was sintered at 900°C for 2h. In the second step, an appropriate amount of Mg was then added to get the stoichiometry of Mg0.9Ti0.1B2, which was milled for 5h and sintered at 750°C for 2h. The phase, microstructure and superconductivity of MgB2 were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and superconducting quantum interference device (SQUID). The critical current density reaches 6.9×104Acm−2 at 20K and 1T. The investigation showed the efficient combination between two-step reaction method and high-energy ball milling is effective to obtain high density and fine grain size bulk MgB2 with high Jc performance.

Effect of (Bi, Pb)-2223 addition on thermal transport of superconducting MgB2 pellets

Polycrystalline MgB2 pellets containing 0, 3, 7, and 10wt.% of (Bi, Pb)-2223 {Bi1.8Pb0.26Sr2Ca2Cu3O10+x} superconducting powder has been synthesized by solid state reaction. Micro-structural observations confirm a well developed MgB2 phase with randomly oriented hexagonal grains. XRD data shows absence of substitution effect and presence of unreacted 2223 as isolated grains or, clusters is indicated in FESEM. In addition, transition temperature (Tc) of MgB2 remains practically unaffected with 2223 addition in MgB2. Temperature dependence of thermal conductivity of the present set of samples does not show any anomalous features at/below Tc. However, the magnitude of thermal conductivity decreases substantially with addition of 2223. Ideal Wiedemann–Franz’s law fails to provide acceptable estimation of the electronic part of the total thermal conductivity. Two band analysis of the electronic thermal conductivity in superconducting state reveals dominance of π band contribution. Analysis also confirms that addition of 2223 in MgB2 introduce extra defects/disorder that cause a substantial decrease in π band gap, as well as, intraband relaxation time of pure MgB2. On the other hand, σ band gap of MgB2 is not much affected due to addition of 2223. Lattice thermal conductivity analyzed using Callaway’s expression indicates that it is mainly governed by phonon scattering from boundary, point defects and sheet-like faults. However, dominant role is played by phonon scattering from sheet like faults, which decides the lattice thermal conductivity of both pure and 2223 added MgB2.

Comparison of Critical Current Density in Undoped, Glycine-doped, and Cu-and-Glycine-Co-doped MgB2 Synthetized from nm-Boron and μm-Boron

Nano-boron (800 nm) and μm-boron (25 μm) precursor were used to synthetize glycine-doped, Cu-and-glycine-co-doped, and undoped MgB2 samples at 800 ∘C. The C substitution level caused by glycine doping, the MgO content, and the full width at half maximum of the (101) peak for MgB2 phase were compared to evaluate the critical current density (J c) of the six samples. The undoped sample from the nm-boron powder showed enhanced J c over the entire field in contrast with those from 25-um boron, since the excess MgO in nm-boron prepared sample serves as effective pinning centers. On the contrary, due to the reduced MgO pinning centers as well as the increase of the grain size, the glycine-doped nm-boron sample only enhanced the J c performance in the high-field region (H>4.5 T), while the low-field J c values showed a considerable decrease. For the Cu-and-glycine-co-doped sample, the J c performance is nearly without regard to the size of the boron precursor as the high-field J c of the nm-B sample decreased a little, while the low-field J c remained at the same level as that of the μm-B sample.

Mechanism of the formation and specific features of the structure of massive samples of compound MgB2

Using different methods, it has been revealed that two MgB2 phases with the same hexagonal lattice, which differ in the contents of Mg and B (in the limits of the homogeneity range), as well as in the concentration of impurity oxygen and in the microstructure, are formed. The regions that correspond to these two phases of MgB2 have relatively large sizes (100–500 μm) and, alternatingly, fill the entire volume of the sample. It is assumed that the two-phase state of MgB2 is due to the specific features of the mechanism of its formation (when synthesizing at 800–1000°C), which includes the stage of the melting of magnesium, the dissolution of solid boron in the melt to a concentration that corresponds to the composition of the MgB2 compound, and the subsequent crystallization of the MgB2 compound in the melt with the formation of a dendrite-like structure, which is accompanied by an appropriate redistribution of the main components and impurities.

Preparation and characterisation of Ru doped MgB2

Samples with Mg1−xRuxB2 nominal stoichiometry were prepared by sintering at 800°C. The critical transition temperature decreases up to a substitution level of x≈0.015. A maximum solubility limit slightly in excess of 1at.% Ru for Mg is confirmed by energy dispersive spectroscopy measurements. From and beyond this limit, an unidentified phase appears in the X-ray diffraction patterns. Interestingly, the lattice parameters of the MgB2 phase are constant up to x=0.015, but start to decrease for higher Ru contents. This feature could be related to the fact that a Mg–Ru impurity phase results in a Mg-deficient (Mg,Ru)B2 matrix.

Effect of graphene oxide doping on superconducting properties of bulk MgB2

In the present paper we report the effect of graphene oxide (GO) doping on the structural and superconducting properties of MgB2. Bulk polycrystalline samples have been synthesized via a solid state reaction route with compositions MgB2 + x wt% of GO (x = 0, 1, 2, 3, 5, 7 and 10) by sintering at ∼850 ° C in a reducing atmosphere of Ar/H2 (9:1). The x-ray diffraction results confirm the formation of the MgB2 phase in all samples, together with traces of a MgO impurity phase. The XRD data results also show substitution of carbon for boron, but in the present case the actual amount of carbon substituting for boron is very small as compared to other carbon sources. A substantial improvement in the critical current density, Jc(H), has been observed in the entire magnetic field range (0–8 T) for samples x = 1, 2 and 3 as compared to the undoped sample. In addition to Jc(H), marginal improvements in the upper critical field (Hc2) and the irreversibility field (Hirr) have been observed for the doped samples x = 1, 2 and 3 with respect to pristine MgB2. Furthermore, a curious result of the present investigation is that there is no change in the superconducting transition temperature (Tc) up to a doping level of 10 wt%. The possible mechanisms of flux pinning and correlations between the observed superconducting properties and structural characteristics of the samples have been described and discussed in this paper.

Preparation and Characterization of Mg1−x B2 Bulk Samples and Cu/Nb Sheathed Wires with Low Grade Amorphous Boron Powder

MgB2 bulk and wire samples were prepared using cheap, low grade amorphous boron powders. Based on chemical analysis performed on the starting reagents, three nominal stoichiometries were studied. It was found that the structural and superconducting properties of the bulk samples were not affected by the composition, but that residual Mg was left in the wires for the nominal MgB2 composition. In contrast, slightly Mg-deficient compositions were free from residual Mg and exhibited higher critical current densities. The MgB2 phase formation kinetics was not influenced by the variations in the nominal powder composition.