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.     

Si-induced superconductivity and structural transformations in DyRh4B4

DyRh₄B₄ has been known to crystallize in the primitive tetragonal (pt)-structure and to exhibit a ferromagnetic transition at 12 K, the highest magnetic transition temperature in the entire series of the RRh₄B₄ materials [For an extensive review on the magnetic and superconducting properties on the ternary superconductors, including RRh₄B₄, see, for example, Ø. Fischer, in: K.H.J. Buschow, E.P. Wohlfarth (Eds.), Ferromagnetic Materials, vol. 5, Elsevier Science Publishers B.V., 1990 (Chapter 6)]. We show here that our silicon-added samples of the nominal composition DyRh₄B₄Si_0.2 exhibit superconductivity below T_c  4.5 K and an antiferromagnetic transition below T_N  2.7 K. The 12 K transition observed in the pt-DyRh₄B₄ is completely suppressed. Our annealed samples mainly consist of domains of the chemical composition DyRh_3.9B_4.2Si_0.08. These domains contain two crystallographic phases belonging to the body-centred tetragonal (bct)-structure and the orthorhombic (o)-structure. We have reasons to suggest that superconductivity and antiferromagnetic ordering arise from bct-DyRh₄B₄ phase and, therefore, coexist below T_N  2.7 K.     

Synthesis, characterization and low temperature studies of iron chalcogenide superconductors

We have synthesized tetragonal iron selenide and telluride superconductors through solid state reaction at 450 °C and 550 °C, respectively. These synthesis temperatures have been established by optimization. Electrical resistivity and magnetic susceptibility measurements (4.2–300 K) confirm superconductivity with T_C of around 8.5 K in all selenide samples of nominal composition Fe_1+δSe (δ = 0.02–0.22). However, Scanning Electron Microscopy/Energy Dispersive Spectroscopy studies clearly indicate that the actual stoichiometric ratio of Fe:Se for all the samples synthesized is around 1:1. Also all the samples exhibit identical phase transition temperatures from tetragonal to orthorhombic structure below 100 K implying identical phase composition. The partial substitution of Se by Te leads to an enhancement of T_C to 13 K. The non-superconducting telluride Fe_1.09Te exhibits a metal–insulator phase transition at 82 K. Substitution studies of this telluride system by S and Si have in addition been carried out to investigate if chemical pressure induces superconductivity.     

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.     

Characteristic and synthesis mechanism of MgB2 nanoparticles in solid-state reactive sintering

MgB₂ nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994 K, and then directly cooled down at a rate of 40 K/min. The results of X-ray diffraction indicate that MgB₂ superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20 nm are considered to be at the onset for the formation of MgB₂ phase. Transition temperature (T_c) and critical current density (J_c) of the compound structures were determined to be 38.5 K and 1.8 × 10⁵ A cm⁻² by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB₂ nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994 K.     

dc-Magnetic susceptibility and EPR studies of vapour phase grown Cd1−xCoxTe crystals

Cd_1−xCo_xTe crystals (x = 0.001, 0.003, 0.005, 0.007 and 0.009) were grown by vapour phase technique. The grown diluted magnetic semiconducting (DMS) crystals were subjected to magnetization and dc-magnetic susceptibilities at room temperature. EPR spectra were recorded at 20 K for samples of all compositions. EPR spectra exhibited a broad resonance band around g  5.43. All the studies indicated the paramagnetic nature of the samples.     

Temperature dependent conductivity and thermoelectric power of the iodine doped poly(vinyl alcohol)–Cu chelate

We have investigated the temperature dependence of electrical conductivity and thermoelectric power (TEP) at 1.7 K < T < 300 K in an organo metallic complex, the iodine doped poly(vinyl alcohol)–Cu²⁺ chelate. We observed intrinsic metallic temperature dependence of resistivity from room temperature to 68 K with a broad minimum [ρ(68 K)/ρ(300 K) 0.75], which has not been observed previously in similar organo metallic complexes. There occurs an unusual metal-insulator transition at T 68 K and the resisitivity increases upon cooling below 68 K. However, the low temperature resistivity becomes finite (instead of going to infinity), [ρ(1.7 K)/ρ(300 K) 0.98] indicating that a quantum mechanical tunneling conduction is dominant at this low temperature. It is remarkable that the resistivity at 1.7 K is as small as that of room temperature. Such unusual temperature dependence of conductivity could be understood as thermally assisted hopping conduction between metallic islands. However, the observed intrinsic metallic temperature dependence of resistivity implies that such hopping conduction barrier is not important at high temperature (T > 68 K). The intrinsic metallic characteristics are confirmed by the quasi-linear temperature dependence of TEP for the whole measured temperature range (1.7 K < T < 300 K) with a small slope change at low temperature, T < 68 K, which is understood as an effect of variable range hopping (VRH) conduction at low temperature. The results of magneto resistance (MR) and magneto thermoelectric power (MTEP) are consistent with the above interpretation.     

Chromism and molecular weight of polyaniline derivatives

The striking solvatochromic shift observed in polyaniline derivatives correlates with two solvent polarity scales, donor number (DN) and hydrogen bond acceptor scale (Taft's β-scale). The large shift is caused by conformational changes in solution that result in dramatic differences in relative molecular weight values obtained by gel permeation chromatography (MW_GPC) in different solvents. For example, for poly-o-toluidine base in the emeraldine oxidation state, a low transition energy solvent like NMP (excitonic transition, λ_max 608 nm) yields a very high molecular weight value (M_w  15,000 g mol⁻¹) whereas a high transition energy solvent like CHCl₃ (excitonic transition, λ_max 570 nm) yields an absurdly low value (M_w  650 g mol⁻¹). An absolute molecular weight value, M_w  4700 g mol⁻¹, was obtained for the first time using laser light scattering (MW_LS) suggesting that CHCl₃ promotes a highly coiled chain conformation whereas NMP promotes a more expanded, rod-like conformation. A similar trend is observed for poly-o-toluidine base in the fully oxidized pernigraniline oxidation state (Pierels transition). This suggests that even though the Pierels and excitonic transitions have different molecular origins their conformation driven solvatochromic shifts trend in a similar fashion which is different from thermochromic trends reported in previous studies.     

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。     

Synthesis of nanocrystalline magnesium diboride (MgB2) metallic superconductor by mechano-chemical reaction and post-annealing

MgB₂ is recently discovered superconducting compound with a record-breaking transition temperature (T_c = 39 K) for a conventional metallic superconductor. Nanocrystallinity can improve its electrical properties by strong pinning. In the present work we report the results of the synthesis of nanocrystalline MgB₂ superconducting compound by mechano-chemical reaction followed by post-annealing. The first stage of synthesis was carried out from elemental crystalline Mg and amorphous B powders by controlled mechanical alloying (CMA) in the magneto-mill Uni-Ball-Mill 5. X-ray diffraction studies reveal that the nucleation of small amount of MgB₂ is initiated after two-step milling for combined 100 h under protective helium gas. Further reaction to form MgB₂ is accelerated by subsequent annealing of the milled powder at various temperatures. X-ray diffraction shows formation of a well-developed nanocrystalline MgB₂ after annealing of the pre-nucleated powder at the 630–650 °C range for a few hours.     

Milling and subsequent thermal annealing effects on the microstructural and magnetic properties of nanostructured Fe90Co10 and Fe65Co35 powders

The influence of milling and subsequent annealing on the microstructural and magnetic properties of Fe₉₀Co₁₀ and Fe₆₅Co₃₅ alloys is investigated. After milling for 8 h a body-centred cubic nanostructured Fe–Co alloy forms with an average crystallite size of about 12 nm. The magnetization saturation (M_S) increases 16% for Fe₆₅Co₃₅ and 5% for Fe₉₀Co₁₀ alloys by milling for 8 h. Subsequent annealing of Fe₉₀Co₁₀ and Fe₆₅Co₃₅ powders for 105 min at 550 °C improves the M_S about 6 and 11%, respectively. Before annealing, the coercivity increases (up to 60 Oe) by milling for 3 h, followed by a reduction on milling for longer periods (45 h). At the initial stage of the heating, a sharp decrease in H_C to 8–10 Oe occurs due to the relief of internal strain. Further heating leads to an increase in the coercivity (intermediate times) followed by a slight diminution on heating for final stage.     

Magnetic properties of (Fe)1−x–(Al2O3)x and (Fe50Ni50)1−x–(Al2O3)x nanocomposite magnetic media synthesized using gel like Al2O3 matrix

Composites with ferromagnetic nanoparticles, Fe and Fe₅₀Ni₅₀, dispersed in Al₂O₃ have been synthesized by a solution phase technique. The structure and magnetic properties of these composites with varying fractions of Al₂O₃ have been investigated. Both Fe and Fe₅₀Ni₅₀ nanoparticles are amorphous in the as-prepared state and become crystalline on heat treating with near equilibrium lattice parameters of 0.287 nm and 0.358 nm respectively. The interparticle distance increases with increasing Al₂O₃ from 0 wt.% to 20 wt.%. The size of Fe nanoparticles is 40 nm while the Fe₅₀Ni₅₀ nanoparticles are 20 nm in size. The Fe and Fe₅₀Ni₅₀ nanoparticles dispersed composites are found to be ferromagnetic at room temperature both in the as-prepared and heat treated conditions with clear coercive fields of 5.5–35 × 10³ A m⁻¹. The saturation magnetization increases by orders of magnitude on heat treatment, for e.g. from <1.0 emu g⁻¹ to 143.4 emu g⁻¹ for Fe–15 wt.% Al₂O₃ and 95.6 emu g⁻¹ for Fe₅₀Ni₅₀–15 wt.% Al₂O₃. The Fe-composites exhibit a Curie transition at 1000 K while the Fe₅₀Ni₅₀ composites exhibit a transition at 880 K, both temperatures close to bulk values.     

Influence of thickness on the material characteristics of reactively sputtered W2N layer and electrical properties of W/W2N/SiO2/Si capacitors

The material characteristics of W₂N layer and electrical properties of W/W₂N/SiO₂/Si metal–oxide–semiconductor (MOS) capacitors with different W₂N thickness upon annealing in N₂ + H₂ ambient at 500 °C for 20 min are investigated. The nitrogen concentration of W₂N for the W/W₂N stack with thin W₂N layer (≤10 nm) is lower than that for the W/W₂N stack with thick W₂N layer (≥15 nm). In addition, the crystallinity of W₂N in the W/W₂N (15 nm) stack is better than that in the W/W₂N (10 nm) stack. For all capacitors, the oxide charges decrease significantly after annealing and the amount of oxide charges is independent of the W₂N thickness. However, the work function (Φ_m) of the W/W₂N (≤10 nm) stack (4.6 eV) is smaller than that of W/W₂N (15 nm) stack (5.0 eV). The Φ_m of W/W₂N (15 nm) stack is close to that of W₂N single layer. After annealing, the Φ_m of W/W₂N (15 nm) stack and W₂N single layer decrease, especially for the W₂N single layer. But for the W/W₂N (≤10 nm) stack, the Φ_m increases after annealing.     

Effect of Pt on the superconducting and magnetic properties of ErNi2B2C

We show that the variation of T_c in Er(Ni_1−xPt_x)₂B₂C (T_c  10.6 K and T_N  5.7 K for x = 0) as a function of x proceeds in two steps: strong decrease of T_c for initial values of x (0 ≤ x < 0.10, T_c = 7.3 K at x = 0.1) and, thereafter, a relatively much weaker drop (almost a plateau) of T_c with further increase of x. T_N exhibits a slight, almost linear, decrease over the entire range of x studied here; T_N = 4.7 K for x = 0.2. Our results for x = 0.10 are in sharp disagreement with the results, namely, T_c < T_N, as reported by Felner et al. [I. Felner, D. Schmitt, B. Barbara, C. Godart, E. Alleno, J. Solid State Chem. 133 (1997), 5].     

Thermoelectric properties of novel skutterudites with didymium: DDy(Fe1−xCox)4Sb12 and DDy(Fe1−xNix)4Sb12

In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe_1−xCo_x)₄Sb₁₂ and (Fe_1−xNi_x)₄Sb₁₂. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD_0.4Fe₂Co₂Sb₁₂ was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material.     

A contribution to the Al–Ni–Cr phase diagram

The Al–Ni–Cr phase diagram was specified at 1000 °C and partially at 900 °C. The results concerning the region below 60 at.% Al agreed qualitatively with the literature data. The binary Al–Cr phases μ and γ dissolve up to 1 and 3 at.% Ni, respectively, and Al₃Ni₂ up to 2.5 at.% Cr. Two ternary phases were revealed: hexagonal ζ (a ≈ 1.77, c ≈ 1.24 nm) in a wide range between Al₈₁Ni₃Cr₁₆, Al_76.5Ni₃Cr_20.5, Al_76.5Ni₉Cr_14.5 and Al_71.5Ni₉Cr_19.5, and high-temperature orthorhombic (a ≈ 1.26, b ≈ 3.48, c ≈ 2.02 nm) around Al_76.5Ni_2.0Cr_21.5.     

Properties of Mn-doped FINEMET

Structural, thermodynamical and magnetic properties of Fe_73.5−xSi_13.5B₉Cu₁Nb₃Mn_x amorphous alloys, with Mn content x=1,3–15, were studied by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC), Mössbauer spectroscopy (MS) and energy-dispersive X-rays (EDX), as-quenched and after annealing. The alloys with x≤7 suffer primary (above 550 °C) and secondary (below 680 °C) crystallisation, whereas the alloys with x≥9 only at 580 °C. Mn doping results in increase (for x≤7) and then (for x≥9) in decrease of grain size. The hyperfine field of the amorphous precursor and remainder substantially decrease with increase of Mn content x, whereas the fields in crystallites remain nearly independent of x. A paramagnetic component appears at x9 and grows with x.     

Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate

The present study contemplates the application of Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate (NaBH₄). Ru and Pt, RuCu, RuPd, RuAg and RuPt (atomic ratio 1:1), PtAg, and Ru_xPt_y (atomic ratios x:y of 2:1 or 1:2), all supported over titanium oxide, were prepared. Their activity decreased in the order RuRu₂Pt₁ > RuPtRu₁Pt₂ > RuPd > RuAgPt > RuCu > PtAg. Alloying Ru with an inactive metal like Cu, Pd or Ag did not improve the performances of Ru. The catalytic ability of Ru₂Pt₁-TiO₂ is in the range of the highest values reported so far in the literature with a hydrogen generation rate of 15.2 L(H₂) min⁻¹ g⁻¹(RuPt). After separation from the reaction medium and rinsing with deionised water, the used Ru₂Pt₁-TiO₂ catalyst was re-evaluated and almost the same catalytic activities as fresh catalyst were obtained during several cycles.     

Amorphous and nanocrystalline sputtered Mg–Cu thin films

Binary Mg–Cu amorphous alloys were first fabricated in 1980s via liquid quenching. In this study, the Mg_1−xCu_x (x varying from 38 at.% to 82 at.%) partially amorphous thin films are prepared via co-sputtering. Upon thermal annealing, the Mg₂Cu or MgCu₂ nanocrystalline phases are induced in the Mg-rich or Cu-rich thin films, respectively. Due to the presence of fine nanocrystalline Mg₂Cu or MgCu₂ particles in the Mg–Cu amorphous matrix, the as-sputtered thin films show satisfactory Young's modulus 100 GPa and hardness 4 GPa.     

Magnetism in PrRhSn studied on a single crystal

We have grown a single crystal of PrRhSn, analyzed the structure by X-ray diffraction methods and measured the specific heat, ac susceptibility, magnetization, and electrical resistivity as functions of temperature and magnetic field. Also we have calculated the electronic structure of this compound by ab initio methods. Ferromagnetism below T_C = 2.9 K characterized by strong uniaxial anisotropy (an anisotropy field 65 T) has been confirmed. Pronounced crystal field (CF) effects were observed on the temperature dependences of the magnetic susceptibility, electrical resistivity and specific heat. The calculations revealed that besides the stable Pr magnetic moment owing to the localized 4f-electrons a small magnetic moment of at most 0.2μ_B is induced at the Rh site (and 0.1μ_B at the Sn site) due to the polarized Rh 4d-electron states (Sn 5p-states) hybridizing with the Pr 5d-electron states, i.e. the Rh and Sn moments play some role in the total balance of the magnetic moments in this compound. This result is in agreement with the experimentally determined saturated magnetic moment of PrRhSn, which is about 0.3μ_B larger than the ordered moment value expected for the Pr³⁺ free ion.