Effect of cold rolling on the superconducting and electronic properties of two amorphous alloys; Nb50Zr35Si15 and Nb70Zr15Si15

The effect of cold rolling on the superconducting properties was examined for amorphous Nb₅₀Zr₃₅Si₁₅ and Nb₇₀Zr₁₅Si₁₅ superconductors. Cold rolling to 10 to 20% reduction in thickness results in a rise of superconducting transition temperature (T _c) and a decrease in transition width (T _c), upper critical field gradient near , critical current density [J _c(H)] and normal electrical resistivity (_n). Changes of about 7% forT _c 33% for T _c, 12% for and 70% forJ _c(H) are found. The rise ofT _c upon cold rolling was considered to originate from the increase in the electron-phonon coupling constant () due to an increase in the electronic density of states at the Fermi level [N(E _f)] and a decrease in the phonon frequency (), while the decreases in T_c,J _c(H) and _n were attributed to the decrease in fluxoid pinning force due to an increase in homogeneity in the amorphous structure. From the results described above, the following two conclusions were derived: (a) cold rolling causes changes in electronic and phonon-states in the quenched amorphous phase, and (b) deformation upon cold rolling occurs not only in the coarse deformation bands observable by optical microscopy, but also on a much finer scale comparable to the coherence length (7.7 nm).     

Superconductivity of Zr-Nb-Si amorphous alloys

Superconducting amorphous alloys with high strength and good ductility have been found in rapidly quenched alloys of the Zr-Nb-Si system. These alloys were produced in a continuous ribbon form of 1 to 2 mm width and 0.02 to 0.03 mm thickness using a modified single roller quenching apparatus. The amorphous alloys were formed over the whole composition range between zirconium and niobium, but the silicon content was limited to the relatively narrow range between about 12 and 24 at%. All the amorphous alloys showed a superconducting transition whose temperature, T _c, increased from 2.31 to 4.20 K with increasing niobium content or with decreasing silicon content. The upper critical magnetic field, H _c2, and the critical current density, J _c, for Zr₁₅Nb₇₀Si₁₅ alloy were of the order of 4.5 Tesla(T) and 5.5×10⁶ A m^–2 at 1.5 K in the absence of applied field. The upper critical field gradient at T _c, , and the electrical resistivity at 4.2 K, _n, decreased from 2.89 to 2.10 T K^–1 and from 2.70 to 1.80m, respectively, with the amount of niobium. The Debye temperature, _D, the electron-phonon coupling constant, , and the bare density of electronic states at the Fermi level, N(E _f ) were calculated from the experimentally measured values of _n, , Young's modulus and density by using the strong-coupling theories. From the comparison of T _c with their calculated parameters, it was found that is the most dominant parameter for T _c. The GL parameter, , and the GL coherence length, _GL(0), were estimated to be 70 to 100 and about 7.6 nm, respectively, from the experimental values of and _n by using the GLAG theory and hence it is concluded that the present amorphous alloys are an extremely dirty type-II superconductor having a very weak flux pinning force.     

Superconducting properties of Ce-substituted TaSr2(Gd,Ce)2 {\rm Cu}_2{\rm O}_y

A layered cuprate TaSr₂ Cu₂ for x = 0.4–0.8 has been successfully synthesized by solid state reaction. X-ray powder diffraction analysis indicates that nearly all the peaks from the samples can be indexed to a single phase of Ta-1222. The sample with nominal composition TaSr₂Gd_1.6Ce_0.4Cu₂ showed a tetragonal structure with lattice parameters a = 3.858 Å, c = 28.81 Å  with space group most likely I4/mmm. The TaSr₂ Cu₂ compound exhibits a narrow superconducting region near x = 0.6 with T_c-onset = 30 and T_c-zero=10 K as determined by d.c. electrical resistance versus temperature measurements.     

Comparative critical field study of superconducting ternary borides

The upper critical field (H _c2) and electrical transport properties have been determined for ternary rare-earth transition-metal borides with a variety of crystal structures; namely, ScRu₄B₄ (I4₁/acd), LuRh₄B₄ (I4₁/acd), ThRh₄B₄ (P4₂/nmc), LuOs₃B₂ (P6/mmm), LuRuB₂ (Pnma), and YRuB₂ (Pnma). For the majority of the compounds the critical field measurements are analyzed in terms of the dirty limit [l_tr/_BCS(0)1] theory of Werthamer, Helfand, and Hohenberg (WHH). For the superconductor YRuB₂, which is in the clean limit, critical field data are analyzed in terms of the Ginzburg-Landau theory. From the experimental data for ,T _c, and residual resistivity _res, we also have determined the heat capacity coefficient and BCS coherence length _BCS(0). The values indicate that these compounds are all strong type II superconductors.     

Effect of gas composition on Si2N2O formation in the Si-C-N-O system

Formation of silicon oxynitride was observed during silicon nitride whisker synthesis from silica, carbon and nitrogen. The silicon oxynitride formation was limited to a bottom area of the charged powders and found both in whisker and powder. The possible reason for this localized Si₂N₂O formation is analysed, based on the effect of gas phase composition on the phase stability among β-Si₃N₄, β-SiC and Si₂N₂O. The phase stability is closely related to the ratio in the gas phase. To suppress Si₂N₂O formation, the ratio must be lower than that of the phase boundary in the Si₂N₄/Si₂N₂O equilibrium, whereas it must be higher than that of the phase boundary in the SiC/Si₂N₂O equilibrium. The formation of silicon oxynitride during silicon nitride whisker formation was caused most likely by fluctuation in the ratio in the gas phase surrounding the lower area.     

Effects of oxygen partial pressure control on the microstructure and PTCR properties of Ho doped BaTiO<Subscript>3</Subscript>

Effects of oxygen partial pressure ( ) control on the electrical properties and microstructural development of (Ba_1-xHo_x)_0.997TiO₃ were studied. An oxidation condition ( ∼ 1.0 atm) was maintained during the heating process, and then the specimen was sintered in a reducing atmosphere ( < 10⁻⁹ atm) at 1350 °C, followed by the annealing process at 1000 °C and = 1 atm. The switching temperature (T_S) from the oxidation atmosphere to the reducing condition was changed from 1100 to 1350 °C. A significant decrease in the room-temperature resisitivity (ρ₂₅) was observed as T_S was increased. The temperature coefficient of resistance (TCR) was independent of the change in T_S, and closed pores decreased with increasing T_S.     

Effect of Nb addition on the glass forming ability and mechanical properties in the Ni-Zr-Al-Hf-Nb alloys

The effect of substituting Nb for Zr on the glass forming ability (GFA) and mechanical properties of Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys was investigated. The substitution of Zr with Nb of 0 -- 14 at% improved the GFA. When increasing the Nb content x from 0 -- 14, the glass transition temperature, T_g, and the crystallization temperature, , of melt-spun Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys increased from 833 and 863 K to 877 and 914 K, respectively. The Ni₆₀ Al₆Hf₇ (x = 8, 10, 12 and 14) alloys exhibited high (>0.615), (>0.4) values and a wide supercooled liquid region, (= – T_g) (>36 K) enabling the fabrication of bulk metallic glass (BMG) with a diameter above 1 mm. As such, Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ and Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloys with a maximum diameter of 2 mm could be fabricated by injection casting. These bulk amorphous alloys also exhibited good mechanical properties, where the true ultimate compressive strength, strain and Vickers hardness (H_v) were approximately 3.0 GPa, 2.08% and 713, respectively, for the amorphous Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ alloy, and 3.1 GPa, 2.22% and 687, respectively, for the amorphous Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloy.     

Diffusivity and solubility of oxygen in solid palladium

The solid solubility c _O of oxygen in palladium in equilibrium with gaseous oxygen has been determined from absorption-desorption experiments for temperatures T of 1123 and 1173 K and oxygen partial pressures between 2.7 × 10³ and 4.0 × 10⁴ Pa. The relationship between c _O, and T is given by , where R = 8.314 JK⁻¹ mol⁻¹ is the universal gas constant, ΔH _s = −13.55 kJ/mol denotes the heat of solution of oxygen in palladium and the constant a amounts to or . The diffusion coefficient D _O of oxygen in solid palladium has been determined by incomplete isothermal internal oxidation of Pd–Fe alloys using the data on the oxygen solubility in palladium. The temperature dependence of D _O obeys the Arrhenius equation D _O = D ⁰ exp(−E _d/RT) with pre-exponential factor D ⁰ = 2.33 × 10⁻⁷ m²/s and activation energy of diffusion of oxygen in palladium, E _d = 102.76 kJ/mol     

Grain growth behaviour of the Al-Cu eutectic alloy during superplastic deformation

Grain growth behaviour of the Al-Cu eutectic alloy was investigated as a function of strain (ε), strain rate and deformation temperature (T) over = 10⁻² s⁻¹ and T=400 to 540°C. The grain size increases with increase in strain and temperature. Upon deformation to a fixed strain, the grain growth is generally seen to be more at lower strain rates. The rates of overall grain growth and due to deformation alone , however, increase with increasing strain rate according to and , respectively. The increase in the grain growth rate with strain rate is attributed primarily to the shorter time involved at higher strain rate for reaching a fixed strain. The activation energy for grain growth under superplastic conditions is estimated to be 79 kJ mol⁻¹.     

Fracture toughness and fracture mechanisms of PBT/PC/IM blend

The static and impact fracture toughnesses of a polybutylene terephthalate/polycarbonate/impact modifier (PBT/PC/IM) blend were studied at different temperatures. The static fracture toughness of the blend was evaluated via the specific fracture work concept and the J-integral analysis. A comparison of these two analytical methods showed that the specific essential fracture work, W _e, was equivalent to the obtained by the ASTM E813-81 procedure, representing the crack initiation resistance of the material. The discrepancy between W _e and of ASTM E813-89 was caused by the extra energy component in consumed by a 0.2 mm crack growth. Impact fracture toughness was also analysed using the specific essential fracture work approach. When the fracture was elastic, W _e was equivalent to the critical potential energy release rate, G _IC, obtained via LEFM analysis. Temperature and strain-rate effects on the fracture toughness were also studied. The increase in impact toughness with temperature was attributed to two different toughening mechanisms, namely, the relaxation processes of the rubbery particles and the parent polymers in a relatively low-temperature range and thermal blunting of the crack tip at higher temperatures. The enhancement in static fracture toughness at temperatures below — 60 °C was thought to be caused by plastic crack-tip blunting, but the monotonic reduction in yield stress was largely responsible for the toughness decreasing with higher temperatures. The temperature-dependent fracture toughness data obtained in static tests could be horizontally shifted to match roughly the data for the impact tests, indicating the existence of a time-temperature equivalence relationship.     

Microstructure and microwave properties of {CaT{i}O}<Subscript>3</Subscript>–LaGaO<Subscript>3</Subscript> {solid solutions}

The phase assemblage, microstructure and microwave (MW) properties of xCaTiO₃-(1-x)LaGaO₃ (CT-LG) ceramics prepared by solid state synthesis from raw oxides and carbonates have been investigated. LG was predominantly single phase perovskite with space group, Pnma, but a small quantity of pyrochlore structured La₂Zr₂O₇ second phase was present due to the contamination from the ZrO₂ milling media. At MW frequencies, the temperature coefficient of resonant frequency (τ_{f}) of LG was --80~ppm/\({\circ}\)C, its permittivity, \(\varepsilon_{r} {=} \)27 and MW quality factor, \(Q^{*}f_{o} {=} \)97,000 (@5~GHz). As CT concentration increased, \(\tau_{f}\) and \(\varepsilon_{r}\) increased to The phase assemblage, microstructure and microwave (MW) properties of xCaTiO₃-(1-x)LaGaO₃ (CT-LG) ceramics prepared by solid state synthesis from raw oxides and carbonates have been investigated. LG was predominantly single phase perovskite with space group, Pnma, but a small quantity of pyrochlore structured La₂Zr₂O₇ second phase was present due to the contamination from the ZrO₂ milling media. At MW frequencies, the temperature coefficient of resonant frequency (τ_{f}) of LG was --80~ppm/ C, its permittivity, 27 and MW quality factor, 97,000 (@5~GHz). As CT concentration increased, and increased to $+$850~ppm/ C and 160, respectively but decreased to 20,000. Zero was achieved at 0.65 with 47, and 40,000, properties comparable with commercial compositions. However, for 0.5, a different second phase was observed which was rich in Ca and Ga. Electron diffraction patterns could be indexed according to a body centred cubic lattice, 12.5 ?. It is suggested that the presence of second phases in the CT-LG compounds may, in part, be responsible for the deterioration in .     

Electrical conduction of partially stabilized zirconia Zr0.94Ca0.06O1.94 as a function of temperature and oxygen partial pressure

Partially stabilized zirconia (PSZ), Zr_0.94Ca_0.06O_1.94was prepared by a hot kerosene drying method and a conventional oxide wet-mixing method. The total d.c. conductivities of these zirconia specimens were measured by the three-terminal technique as a function of temperature in the range 1088 to 1285 K and oxygen partial pressure in the range 1 to 10^–24 bar. The specimen prepared by the hot kerosene drying method showed near oxygen ion conduction with four times higher conductivity than the specimen prepared by the conventional mixing method at T=1088–1285 K and bar. The higher oxygen pressure conductivity tended approximately towards a to dependence, indicative of p-type conduction, whereas the lower oxygen pressure conductivity tended to be virtually independent of oxygen pressure, indicative of oxygenion conduction. The activation energy was found to be 130 kJ mol^–1 at T=1088–1285 K, bar (air) for pure electron-hole conduction and 153kJ mol^–1 at T=1088–1285 K for ionic conduction.     

Interface structure and strain development during compression tests of Al2O3/Nb/Al2O3 sandwiches

The interface structure of an Al₂O₃/Nb/Al₂O₃ sandwich produced by solid-state diffusion bonding was investigated in detail by various transmission electron microscopy (TEM) methods. The joint possessed at one interface a , , and on the other interface a and orientation relationship. At both interfaces, misfit dislocations formed to compensate the lattice mismatch as found by high-resolution transmission electron microscopy (HRTEM). Electron energy-loss near edge structure (ELNES) studies revealed that the interface is terminating with an Al layer resulting in Al–Nb bonds. Identical sandwiches were investigated on the meso- and macroscopic scale by performing compression tests and simultaneously monitoring the strain development at (001)_Nb and crystal faces. The full-field optical strain measurements (FFOM) revealed that the strain is localized at the interfaces when observed at the (001)_Nb face while it is along the maximum shear directions of 36–54° inclined to the interface when observed at the face. The strain localization along a specific maximum shear direction results in the cleavage of Al₂O₃, always initiating from the interface possessing the and orientation relationship.     

Rigid body translation in the (\bar 211) twin boundary in silicon

The rigid body translation accompanying a ( 11) twin boundary in silicon has been studied by transmission electron microscopy. From a detailed analysis of theα-type fringe systems in the 111, 311 and 2 0 common reflections, the following translation vector is deduced: [011], which is equivalent to [411] in the other crystal element. A slight deviation of this orientation is possible.     

Anisotropic flow and fracture in beryl [Be3Al2(SiO3)6]

Flow and fracture resulting from Vickers indentation testing on {0 0 0 1} and {10 0 } planar orientations have been examined. Flow characterized by indent shape differentiation was analysed to belong to the slip system with planes of the types { 10 0} and {11 0}. The ensuing fracture paths were resolved to propagate along {1 0 0} and {1 1 } cleavage planes whileK _c values obtained for them were 0.196 and 0.248 MPam^1/2, respectively.     

Mixed pronton–electron conducting properties of Yb doped strontium cerate

The electrical conductivity and hydrogen permeation properties of membranes were studied as a function of temperature and gradient. The bulk conductivity of was an order of magnitude higher than the grain boundary conductivity over the temperature range 100–250 °C in feed gas of 4% H₂/balance He (pH₂O = 0.03 atm). The significantly lower grain boundary conductivity indicates that larger-grained materials might be more suitable for proton transport. The hydrogen flux through the membranes is proportional to thickness down to 0.7 mm. The hydrogen permeation flux increases with an increase in gradient where the increase in hydrogen flux was explained by an increase in electron conduction as a function of temperature. The ambipolar conductivity calculated from hydrogen permeation fluxes shows the same and dependence as electron concentrations. The hydrogen and oxygen potential dependence of the ambipolar conductivity (, ) was understood from the defect structure. From this, it was confirmed that hydrogen permeation might be limited by electron transport at wet reducing atmosphere. From the temperature dependence of the electronic conductivity, the activation energy calculated at wet reducing conditions is 0.63 eV.     

An analytical model for number of fibre–fibre contacts in paper and expressions for relative bonded area (RBA)

This paper proposes an analytical model for calculating the number of fibre–fibre contacts per unit fibre length from the cross-sectional dimensions of the fibres in a sheet and sheet density. This model has been verified with data of the number of fibre–fibre contacts measured directly in handsheets. The measured fibre–fibre contacts in the handsheets were classified into full and partial contacts. The model best fits this data when 1.5 partial contacts are equated to one full contact. A plot of measured versus predicted equivalent full contacts produced a linear correlation with a slope of 0.99 and correlation coefficient of R ²  = 0.93. The model was used to derive expressions for the fraction of the available fibre surface, which is bonded to other fibres, a quantity called the Relative Bonded Area (RBA). The validity of the model was checked using experimental data for RBA. RBA was determined both by nitrogen adsorption ( ) and by scattering coefficient (RBA_sc). The extrapolation method of Ingmanson and Thode to determine S ₀, the scattering coefficient of an unbonded sheet, proved to be inaccurate. We estimated S ₀ for some samples from their linear relationship between scattering coefficient and nitrogen adsorption. The new model accurately predicted both and RBA_sc.     

Stability and direct conversion of mineral barite crystals in carbonated hydrothermal fluids

The pseudomorphic replacement of mineral barite (BaSO₄) crystals into barium carbonate was investigated in the present work by using carbonated alkaline hydrothermal fluids. Hydrothermal treatments were carried out over the temperature range from 150 up to 250 °C for intervals between 1 and 192 h, with different filling ratios (40–70%), and molar ratios of 1, 5, and 10. The reaction products were characterized by XRD and SEM techniques. The chemical reactivity of mineral barite crystals was markedly limited at temperatures below 200 °C, and only a tiny BaCO₃ layer on the surface of the original BaSO₄ crystal was formed on the crystal treated for 192 h. The rate of the pseudomorphic conversion of BaSO₄ into BaCO₃, was accelerated by increasing the reaction temperature and the molar ratio . Powder X-ray diffraction results showed that under hydrothermal conditions the replacement of ions by ions, in barite crystals was completed at 250 °C with a molar ratio = 10 for an interval of 192 h, resulting in the Witherite structure. The morphology of the completely converted BaCO₃ at 250 °C in a Na₂CO₃ solution for 192 h, showed that the conversion proceed without severe changes of the original shape and dimension of the original crystal, similar to that observed in mineral pseudomorphic replacement process.     

Three oxidation states and atomic-scale <Emphasis Type="Italic">p–n</Emphasis> junctions in manganese perovskite oxide from hydrothermal systems

Perovskite oxides have provided magical structural models for superconducting and colossal magnetoresistance, and the search for nano-scale and/or atomic-scale devices with particular property by specific preparations in the same systems has been extensively conducted. We present here the three oxidation states of manganese (Mn³⁺, Mn⁴⁺, Mn⁵⁺) in the perovskite oxide, La_0.66Ca_0.29K_0.05MnO₃, which most interestingly shows the rectifying effect as atomic-scale p–n junctions (namely FY-Junctions) of single crystals and films. The family of cubic perovskite oxides were synthesised by the so-called hydrothermal disproportionation reaction of MnO₂ under the condition of strong alkali media. The new concept of the atomic-scale p–n junctions, based on the ideal rectification characteristic of the p–n junctions in the single crystal, basically originates from the structural linkages of [Mn³⁺–O–Mn⁴⁺–O–Mn⁵⁺], where Mn³⁺ and Mn⁵⁺ in octahedral symmetry serve as a donor and an acceptor, respectively, corresponding to the localized Mn⁴⁺ .     

Critical currents and scaling laws in sputtered PbMo6±x S8−y thin films

Thin films with the Chevrel-phase structure were deposited using a dc getter sputtering method from targets with composition PbMo_5.1S₆ and PbMo_6.35S₈, respectively. Critical parameters asT _c , , andJ _c vs.B andT were measured. Based on theJ _c (B, T) relation it was possible to show that the pinning forcesP _v with respect to the reduced magnetic fieldb=B/B _c2 have the formb ^1/2(1–b)²for samples prepared from the target with the composition PbMo_5.1S₆ andb(1–b) ²for samples prepared from the target with the composition PbMo_6.35S₈. It is suggested that some of MoS₂ phase is responsible for pinning centers in the first set of samples This MoS₂ phase was indicted by x-ray diffraction. For the second set of samples it is supposed that point-type defects are the pinning centres.