Complete flux jumping in nano-structured MgB2 superconductors prepared by mechanical alloying

High density nano-crystalline MgB₂ bulk superconductors with induced pinning centres were prepared from elemental precursors by a sequence of ball milling, heat treatment, and final pressing. The XRD results revealed the main phase was MgB₂ with a minor component of MgO. The magnetic moment versus temperature indicated that the transition temperature of MgB₂ samples was around 34 K, which is less than the typical transition temperature of commercial powders and also the transition temperature strongly depended on the milling time. It is well known that introduction of defects, grain boundaries and impurities act as effective flux pinning centres in MgB₂ and results in increased critical current density, J_c and decreased the transition temperature, T_c. The magnetization measurements were performed using VSM at 10 K, 20 K and 30 K, and the M–H curves indicated a complete flux jump effect, which is a severe problem for the application of superconductors. It was determined that a noticeable amount of heating (0.3 K jumps at 10 K) occurs at these jumps. In addition, it was found that the sweeping rate of magnetic field and the size of bulk sample are very effective to promote the flux jumping and whereas a low sweeping rate (12 Oe/s) avoids these “avalanches”, consistent with a kind of supercritical phenomenon: going slower allows the field gradients to stay close enough to equilibrium so that the avalanche effect is not triggered. In contrast, the sweeping rate of 100 Oe/s leads to numerous jumps.     

Synthesis and crystal structure of Ba3Si4C2

SiC powder prepared by the Na flux method at 1023 K for 24 h and Ba were used as starting materials for synthesis of tribarium tetrasilicide acetylenide, Ba₃Si₄C₂. Single crystals of the compound were obtained by heating the starting materials with Na at 1123 K for 1 h and by cooling to 573 K at a cooling rate of −5.5 K/h. The single crystal X-ray diffraction peaks were indexed with tetragonal cell dimensions of a = 8.7693(4) and c = 12.3885(6) Å, space group I4/mcm (No.140). Ba₃Si₄C₂ has the Ba₃Ge₄C₂ type structure which can be described as a cluster-replacement derivative of perovskite (CaTiO₃), and contains isolated anion groups of slightly compressed [Si₄]⁴⁻ tetrahedra and [C₂]²⁻ dumbbells. The electrical conductivity measured for a not well-sintered polycrystalline sample was 2.6 × 10⁻²–7 × 10⁻³ S cm⁻¹ in the temperature range of 370–600 K and slightly increased with increasing temperature. The Seebeck coefficient showed negative values of around −200 to −300 μV K⁻¹.     

Phase formation of MgB2 superconducting materials fabricated by spark plasma sintering

Much research on MgB₂ has been carried out because MgB₂ has a higher transition temperature (T_c) of 39 K than that of other metallic superconductors and because the bulk form of MgB₂ has exhibited high current density. In this study, Mg powder of less than 10 μm and B powder of less than 3 μm with equivalent MgB₂ composition were mixed simply under argon atmosphere. In order to consider the effect of a pinning element on the superconducting properties, activated carbon of 5 at.% was added to mixed powders. The MgB₂ bulk was fabricated with mixed powders in graphite molds at the various temperatures by spark plasma sintering. The formation of the MgB₂ phase was confirmed with Differential Thermal Analysis (DTA) at 550 °C. The relative density of sintered MgB₂ was 97 %, which increased as the sintering temperature increased. The sintering proceeded initially in the solid state and then by liquid phase sintering with increasing temperature without abnormal grain growth. In the Physical Property Measurement System (PPMS) result, the Tc was about 37 K in the carbon-added sintered sample. The 300 nm size MgB₂ grains of hexagonal shape were formed after spark plasma sintering, but the MgB₄ phase did not produce precise T_c.     

镁扩散制备Pr6O11掺杂的 MgB2块体临界电流密度和磁场性能研究

采用镁扩散方法制备了Pr₆O₁₁纳米颗粒添加的MgB₂超导块体,研究了Pr₆O₁₁掺杂对其临界电流密度(J_c),不可逆磁场(H_irr)和上临界磁场(H_c2)的影响。实验结果表明Pr₆O₁₁纳米颗粒掺杂明显提高了块体的J_c,H_irr和H_c2,但没有降低其超导转变温度T_c。在20 K自场条件下,质量比为1 wt.% Pr₆O₁₁掺杂的MgB₂块体的J_c较没掺杂样品提高了将近5倍, J_c=3.61×10₅A/cm₂。在10 K温度下,MgB₂块体H_c2 和H_irr较没掺杂样品分别提高了1.9 T and 2.6 T。同时讨论了Pr₆O₁₁纳米颗粒掺杂对MgB₂块体的电性能和磁通钉扎机制的影响。     

Enhanced superconducting properties of bulk MgB2 prepared by in situ Powder-In-Sealed-Tube method

A systematic study on the superconducting properties of polycrystalline MgB₂ synthesized by in situ Powder-In-Sealed-Tube technique is carried out at different temperatures (750–900 °C). Both XRD and SEM results show well-crystallized MgB₂ grains in all the samples and grain size is found to be increasing with the sintering temperature. Sharp superconducting transitions are observed for all samples, irrespective of sintering temperatures, which implies the high degree phase purity and homogeneity of MgB₂ formed, while J_C(H) plot gives sample dependent critical current density. The samples heat treated at relatively low temperatures show enhanced flux pinning and hence improved J_C(H) performance. The reduced grain size and hence increased density of grain boundary pinning centers of MgB₂ bulks synthesized at low temperature are mainly responsible for the enhanced flux pinning and J_C.     

Interfacial interaction of solid nickel with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

The dissolution process of nickel in liquid Pb-free 87.5% Sn–7.5% Bi–3% In–1% Zn–1% Sb and 80% Sn–15% Bi–3% In–1% Zn–1% Sb soldering alloys has been investigated by the rotating disc technique at 250–450 °C. The temperature dependence of the nickel solubility in soldering alloys obeys a relation of the Arrhenius type c_s = 4.94 × 10² exp(−39500/RT)% for the former alloy and c_s = 4.19 × 10² exp(−40200/RT)% for the latter, where R is in J mol⁻¹ K⁻¹ (8.314 J mol⁻¹ K⁻¹) and T is in K. Whereas the solubility values differ considerably, the dissolution rate constants are rather close for these alloys and fall in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. Appropriate diffusion coefficients vary from 0.16 × 10⁻⁹ to 2.02 × 10⁻⁹ m² s⁻¹. With both alloys, the Ni₃Sn₄ intermetallic layer is formed at the interface of nickel and the saturated or undersaturated melt at dipping times of 300–2400 s. The other Ni–Sn intermetallic compounds are found to be missing. A simple mathematical equation is proposed to evaluate the Ni₃Sn₄ layer thickness in the case of undersaturated melts. The tensile strength of the nickel-to-alloy joints is 94–102 MPa, with the relative elongation being 2.0–2.5%.     

Synthesis of Y2SiO5:Eu nanoparticles from a hydrothermally prepared silica sol

In this paper, a novel route for the synthesis of Y₂SiO₅ (YSO) nanoparticles doped with Eu³⁺ ions, based on the use of a hydrothermally prepared silica sol as a chemical precursor and nanostructuring template, is presented. X-ray powder diffraction revealed crystallization of nanoparticles in an X₁ monoclinic structural type (P2₁/c) with well pronounced diffraction peaks and without any sign of additional phases. Transmission electron microscopy showed that the particle diameter was around 40 nm and that the particles formed loose aggregates. The synthesized materials exhibited characteristic luminescence emission of the trivalent europium ion, with the strongest emission (⁵D₀ → ⁷F₂ transition) at 611.5 nm and a corresponding lifetime of 1.8 ms. Two site occupancy, as expected, were demonstrated.     

Graphene micro-substrate-induced π gap expansion in MgB2

The lattice effect induced by tensile strain on the superconductivity of graphene–MgB₂ composites was studied systematically to deduce the electron–phonon coupling (EPC) and the multiple superconducting gap behavior. Compared with nano-carbon doped MgB₂, graphene–MgB₂ composites show larger lattice parameters and higher critical superconducting transition temperatures (T_c). The EPC strength of MgB₂ with ∼2 wt.% graphene addition is even higher than that of the pure reference sample, as estimated from the Sommerfeld constant. The π gap was found to be expanded by graphene addition through the analysis of heat capacity data, and it is responsible for both the enhanced EPC strength and the weak dependence of T_c on the graphene content.     

Determination of the crystallization kinetic parameters of Ge22.5Te77.5 glass using model-free and model-fitting methods

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation for Ge_22.5Te_77.5 glass. The kinetic parameters of glassy Ge_22.5Te_77.5 under non-isothermal conditions are analyzed by the model-free and model-fitting approaches from a series of experiments at different constant heating rates (1–50 K/min). A strong heating rate dependence of the activation energy of crystallization was observed. The analysis of the present data shows that the activation energy of crystallization is not constant but varies with the degree of crystallization and hence with temperature. The crystallization mechanisms examined using the local Avrami exponents indicate that one mechanism (two-dimensional growth) is responsible for the crystallization process for heating rates 1–30 K/min and two mechanisms (one- and two-dimensional growth) are working simultaneously during the amorphous–crystalline transformation of the Ge_22.5Te_77.5 glass for a heating rate 50 K/min. The reaction model that may describe crystallization process of the Ge_22.5Te_77.5 glass is Avrami–Erofeev model (g(α) = [−ln(1 − α)]^1/n) with n = 2 for the heating range 1–30 K/min and n = 1.5 at a heating rate 50 K/min for the whole range of conversion crystallization fraction (α = 0.05–0.95). A good agreement between the experimental and the reconstructed (α − T) curves has been achieved. The transformation from amorphous to crystalline phase in glassy Ge_22.5Te_77.5 demonstrates complex multi-step involving several processes.     

Conductivity of a new pyrophosphate Sn0.9Sc0.1(P2O7)1−δ prepared by an aqueous solution method

New pyrophosphate Sn_0.9Sc_0.1(P₂O₇)_1−δ was prepared by an aqueous solution method. The structure and conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ have been investigated. XRD analysis indicates that Sn_0.9Sc_0.1(P₂O₇)_1−δ exhibits a 3 × 3 × 3 super structure. It was found that Sn_0.9Sc_0.1(P₂O₇)_1−δ prepared by an aqueous method is not conductive. The total conductivity of Sn_0.9Sc_0.1(P₂O₇)_1−δ in open air is 2.35 × 10⁻⁶ and 2.82 × 10⁻⁹ S/cm at 900 and 400 °C respectively. In wet air, the total conductivity is about two orders of magnitude higher (8.1 × 10⁻⁷ S/cm at 400 °C) than in open air indicating some proton conduction. SnP₂O₇ and Sn_0.92In_0.08(P₂O₇)_1−δ prepared by an acidic method were reported fairly conductive but prepared by similar solution methods are not conductive. Therefore, the conductivity of SnP₂O₇-based materials might be related to the synthetic history. The possible conduction mechanism of SnP₂O₇-based materials has been discussed in detail.     

Structure,superconductivity and magnetic properties of mechanically alloyed Mg1 − xFexB2 powders with x=0–0.4

Mechanical milling of Mg_1 − xFe_xB₂ with x=0–0.4 led to the formation of amorphous phase. For x=0, the hexagonal MgB₂ phase was formed after a heat treatment at 450 °C or above with critical temperature T_c=38–40 K. Fe-substituted MgB₂ phase was formed after annealing at 450 °C or above when x=0.05, 0.1 and 0.2. Fe solubility in the MgB₂ phase decreased with increasing annealing temperature, while lattice parameters (a and c) decreased. It has been found that critical temperature T_c increased with increasing annealing temperature. For Mg_0.6Fe_0.4B₂ powder, MgB₂ phase could not be formed after crystallization. Single solid-solution MgB₂ phase could be formed in Mg_0.95Fe_0.05B₂ after annealing at 450 °C. This sample exhibited paramagnetic between 30 and 290 K with a high magnetic moment of μ_Fe=5.5–6.0μ_B. An anomaly (minimum in reciprocal magnetic susceptibility) was observed below 30 K, accompanied by magnetic splitting in Mössbauer spectroscopy.     

DSC, structural single crystal and X-ray powder diffraction study of the ammonium sulfate selenate tellurate mixed solid solution

Crystal structure of (NH₄)₂(SO₄)_0.73(SeO₄)_0.27Te(OH)₆ (NSSeTe) crystallizes in the monoclinic P2₁/c space group. It was analyzed at room temperature using X-ray diffractometer data. The unit cell parameters are a = 13.7340(2) Å, b = 6.6583(1) Å, c = 11.4582(2) Å, β = 106.8270(6)°, Z = 2, V = 1002.93(3) Å³, R = 0.014, R_w = 0.017 and D_x = 2.426 g cm⁻³. The main feature of this atomic arrangements is the coexistence of three and different anions (SO₄²⁻, TeO₆⁶⁻ and SeO₄²⁻ groups) in the unit cell, connected by hydrogen bonds which make the building of the crystal. The distribution of atoms can be described as isolated TeO₆ octahedra and SO₄ and/or SeO₄ tetrahedra occupying the same positions. The NH₄⁺ cations, are located between these polyhedra. The molecular species present in the lattice are S/SeO₄²⁻ tetrahedra and TeO₆⁶⁻ octahedra disposed in a number of sheets. The thermal analysis of the title compound show three distinct endothermal peaks at 398, 430 and 450 K. X-ray powder diffraction data at different temperatures confirm that the first anomaly at 398 K can be attributed to a structural phase transition.     

Structure and stability of superconducting core of single-core MgB2/Cu,Nb tube composite with a high critical current

The core of a single-core MgB₂/Cu,Nb composite, which has been prepared by the ex-situ technique and exhibits a high critical current equal to 427 A (at 0 T and 4.2 K, j _c ≥ 10⁵ A/cm²), has been studied using various structural methods. Two kinds of MgB₂ crystals were observed; those of the first kind is large, highly dense crystals characterized by a low oxygen content (2–8 at %) and the others are fine, weakly coupled crystallites characterized by high oxygen content (4–21 at %). To perform a comparative analysis of the structures, we have also studied an MgB₂ bulk sample synthesized at 1000°C. It was found that two phases with the same lattice are formed; they differ in the magnesium and boron contents (within the homogeneity range), impurity oxygen content and microstructure as well but differ slightly in the lattice parameters. The two-phase state of MgB₂ bulk sample is due to the mechanism of its formation, which includes the melting of magnesium, the dissolution of solid boron in it, and the crystallization of MgB₂ from the melt with the formation of dendrite-like structure characterized by corresponding redistribution of components and impurities. To a certain degree, the two-phase structure of MgB₂ bulk sample is inherited by the MgB₂/Cu,Nd composite prepared by ex-situ technique (annealing of composite at 700°C). It was shown that oxygen in the MgB₂ compound is the destabilizing factor and leads to the transformation of the superconductor into MgO.     

Synthesis and crystal structure of a new calcium borate, CaB6O10

A new calcium borate, CaB₆O₁₀, has been prepared by solid-state reactions at temperature below 750 °C. The single-crystal X-ray structural analysis showed that CaB₆O₁₀ crystallizes in the monoclinic space group P2₁/c with a = 9.799(1) Å, b = 8.705(1) Å, c = 9.067(1) Å, β = 116.65(1)°, Z = 4. It represents a new structure type in which two [B₃O₇]⁵⁻ triborate groups are bridged by one oxygen atom to form a [B₆O₁₃]⁸⁻ group that is further condensed into a 3D network, with the shorthand notation 6: ∞³[2 × (3:2Δ + T)]. The 3D network affords intersecting open channels running parallel to three crystallographically axis directions, where Ca²⁺ cations reside. The IR spectrum further confirms the presence of both BO₃ and BO₄ groups.     

Composition-controlled synthesis, structure and magnetic properties of ternary FexCoyNi100−x−y alloys attached on carbon nanotubes

Fe_xCo_yNi_100−x−y alloy nanoparticles with controllable compositions attached on the surface of carbon nanotubes (CNTs) were synthesized using an easy two-step route including adsorption and reduction processes. The nanocomposites have been characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy-disperse X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM). The effect of the alloy composition on microstructure and magnetic properties of ternary FeCoNi alloys attached on carbon nanotubes have been studied. During the nominal composition range (x = 21, 24, 33, 37, 46 and y = 60, 46, 48, 48, 35), Fe_xCo_yNi_100−x−y alloy nanoparticles attached on CNTs are quasi-spherical, fcc–bcc dual phase, and the coercivity (H_c) and saturation magnetization (M_s) vary with the alloy composition. The H_c of Fe_xCo_yNi_100−x−y alloy nanoparticles attached on CNTs decreases and M_s increases with increasing Fe content. These demonstrate that the two-step route is promising for fabricating alloy nanoparticles attached on CNTs for magnetic storage and ultra high-density magnetic recording applications.     

Superplastic compressive flow in MgB2

Macroplasticity in the brittle, superconducting ceramic MgB₂ would allow for the mechanical drawing of thin, dense superconducting wires, as done for metallic superconductors. Here, we report very large uniaxial compressive deformation (engineering strain of 67% or true strain of −1.1) without fracture at 1000 °C for specimens densified from commercially available MgB₂ powders with MgO and MgB₄ second phases. Plastic flow occurs under a diffusion-controlled mechanism with activation energies of 255–447 kJ mol⁻¹ and stress exponents of 1.4–2.0, indicative of superplastic behavior.     

Influence of Mg particle size on the reactivity and superconducting properties of in situ MgB2

The effects of starting Mg particle size on the reactivity of Mg with B, and on remnant Mg in in situ MgB₂, and their influence on the superconducting system are studied. Samples were prepared by a powder-in-sealed tube (PIST) method, heat treated at 850 °C for 2 h in air and were characterized using XRD, SEM and magnetization measurements. As the particle size of Mg powder increases, residual Mg increases significantly. The MgB₂ prepared using smaller sized Mg powder does not have any residual Mg and show the best infield critical current density (J_C (H)).     

Preparation and characteristic of spherical Li3V2(PO4)3

Spherical Li₃V₂(PO₄)₃ was synthesized by using N₂H₄ as reducer. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that single-phase, spherical and well-dispersed Li₃V₂(PO₄)₃ has been successfully synthesized in our experimental process. Electrochemical behaviors have been characterized by charge/discharge measurements. The initial discharge capacities of Li₃V₂(PO₄)₃ were 123 mAh g⁻¹ in the voltage range of 3.0–4.3 V and 132 mAh g⁻¹ in the voltage range of 3.0–4.8 V.     

Mixed cerium valence and unusual Ce–Ru bonding in Ce23Ru7Cd4

The rare earth-rich compounds Ce₂₃Ru₇Cd₄ and Pr₂₃Ru₇Cd₄ were synthesized from the elements in sealed tantalum ampoules in an induction furnace. Both structures were refined on the basis of diffractometer data: P6₃mc, Z = 2, a = 988.7(3), c = 2241.6(5) pm, wR2 = 0.0439, 1976 F² values for Ce₂₃Ru₇Cd₄ and a = 992.7(2), c = 2236.4(7) pm, wR2 = 0.0466, 2528 F² values for Pr₂₃Ru₇Cd₄ with 74 variables per refinement. Striking structural motifs are ruthenium centered trigonal prisms RuCe₆ and RuPr₆ which are condensed via common edges and corners, building rigid three-dimensional networks. Larger voids within these networks are filled by slightly elongated Cd₄ tetrahedra. Five of the nine crystallographically independent cerium sites in Ce₂₃Ru₇Cd₄ show Ce–Ru distances which are shorter than the Pr–Ru distances in Pr₂₃Ru₇Cd₄. This strong hint for mixed cerium valence is supported by the magnetic behavior. Pr₂₃Ru₇Cd₄ shows Curie–Weiss behavior above 50 K with an experimental magnetic moment of 3.62 μ_B/Pr atom, indicating stable trivalent praseodymium. Complex magnetic ordering sets in at 13 K. Ce₂₃Ru₇Cd₄ shows a reduced magnetic moment of 2.05 μ_B/Ce atom. The trivalent cerium atoms show ferro- or ferrimagnetic ordering below T_C = 3.6 K.     

Crystal structure and thermoelectric properties of β-MoSi2

A powder sample of a metastable phase β-MoSi₂ having the C40-type crystal structure was prepared by heating Mo sheets with a Na–Si melt at 858 K for 12 h to determine details of the crystal structure. The lattice constants (a = 4.6016(3) Å and c = 6.5700(3) Å) and Si atom coordinate (y = 0.1658(2)) of β-MoSi₂ were determined by Rietveld analysis of the X-ray powder diffraction. The thermoelectric properties were refined for a bulk sample prepared by sintering the β-MoSi₂ powder at 773 K and 600 MPa. The electrical resistivity of the sintered β-MoSi₂ sample with a relative density of 65% of the theoretical one was 2.5 mΩ cm at 300 K, and slightly increased with increasing temperature from 300 to 725 K. The Seebeck coefficients changed from +60 to +89 μV/K in the temperature range from 330 to 725 K. The maximum thermoelectric power factor was 2.2 × 10^?6 W cm^?1 K^?2 at 725 K.