The properties of thin-walled dual superconducting cylinders

We deal with the properties of dual superconducting cylinders (DSC) in the limitd ₁,d _{2 L} (d ₁ andd ₂ are the thicknesses of the inner and outer cylinders, and _L is the penetration depth). The Gibbs free energy is given at this limit, and the variational equations are established. From these equations, we have found: 1) coupling between the two cylinders are determined by their screening factors, 2) the character of the phase transitions is also determined by the screening factors; and 3) the two cylinders cannot perform complete phase transition together. The critical field that suppresses the cylindrical Josephson current is also given, which is found to agree with the experiment. The field distribution over a DSC and the total magnetic moment are presented in terms of the effective areas of DSC cavities and the susceptibilities of the superconducting cylinders.     

Existence of extra vortex and twin vortex of anomalous mode in Taylor vortex flow with a small aspect ratio

Summary This experimental work on Taylor vortex flow in a gap with a small aspect ratio is concerned with two extra vortices and a twin vortex system, each of which depends on an anomalous cell of the anomalous mode. Extra vortices are smaller than other vortices such as defined cells. At any Reynolds number and aspect ratio extra vortices can be found at the corner of the end plate and inner rotating cylinder and at the corner of the end plate and outer stationary cylinder. For a one-cell flow (anomalous one-cell mode) in a symmetric system, an outer extra vortex develops and grows to the same size as the main cell, only in an aspect ratio of less than one. A twin vortex is observed to form when two vortices are aligned in the direction of the radius. There are three flow fields on the end plate; two are extra vortex flows and the other is the main cell flow. The flow direction of the anomalous cell is from the inner cylinder to the outer one, at the end plate opposite of the flow direction of the normal cell.Nomenclature R ₁ Radius of inner cylinder (2R ₁=40.19±0.006 mm) - R ₂ Radius of outer cylinder (2R ₂=60.11±0.024 mm) - R _r Radius ratio (R ₁/R ₂=0.669) - d Clearance between cylinders (R ₂–R ₁=9.96±0.025 mm) - L Height of working fluid - Aspect ratio=L/d - Rotational angular speed - Kinematic viscosity - Re Reynolds number=R ₁ d/ Other nomenclature is defined as it appears     

The effect of particles on the critical strain associated with the Portevin-LeChatelier effect in aluminium alloys

The effect of particles on the critical strain, _c, associated with the Portevin-LeChatelier (PL) effect of aluminium alloys is studied using Al-Mg-Ni and Al-Si alloys. Al-Mg-Ni and Al-Si alloy matrixes are composed of Al₃Ni and Si particles, respectively. Tensile tests were performed in the temperature range 223–273 K in which the critical strain decreases with increasing temperature, and strain rates between 10^–5 and 10^–2 s^–1 were chosen. According to the apparent activation energies, Q, Mg and Si solute atoms are responsible for the flow instability in Al-Mg-Ni and Al-Si alloys, respectively. The experimental results also show that the critical strain decreases with decreasing particle spacing, d _p. Since the particle spacing is small compared to the corresponding grain size, the decrease in critical strain should be ascribed to the effect of particles. Considering that the dislocation density is increased by the particles, a modified model showing the critical strain, _c, as a function of particle spacing, d _p, is proposed as T ^–1 exp (–Q/kT), in which , T and k are the strain rate, temperature and Boltzmann constant, respectively. Linear fit of the plots of In _c versus In d _p and In _c versus In d _p indicates that this equation is appropriate to rationalize the particle effect on the critical strain.     

Microstructural Cause of the Peak Effect in Superconducting Y-Ba-Cu-O Melt-Grown Bulks

Superconducting Y-Ba-Cu-O bulks with J _c as high as 30 kA /cm ² (at 77 K and 1 T) containing both Y-123 and Y-211 phases were utilized in the present work . With different post-annealings, we obtained samples with different degrees of peak effect in the J _c -vs- H characteristics. Using specimens with TEM observable areas as wide as 20 × 20 (m) ² , large-area high-resolution transmission-electron-microscopic (LA-HRTEM) observations were made for the samples with different degrees of peak effect. Statistical analyses were made for the size of Y-211 particles and the distance between adjacent twin boundaries. It is revealed that (1) Y-211 particles including those of very small diameters have nothing to do with-the peak effect, while (2) the inter-twin-boundary distance is undoubtully correlated with the degree of peak effect. Further investigation on the twin boundary structure indicates that lower T _c regions may be formed in the vicinity of twin boundaries to provide fluxons with mesoscopically distributed weak pinning centers.     

Metal nanoparticle-loaded porous carbon hollow spheres by twin polymerization

The formation of silver and gold nanoparticle-encapsulated hollow carbon spheres (HCS) by twin polymerization is reported. Therefore, silica spheres with different diameters (Aerosil^® AS90, d = 20 nm; Aerosil^® OX50, d = 40 nm; Stöber particles, d = 200 nm) were coated with the metal carboxylates [AgO₂C(CH₂OCH₂)₃H] (1) or [(PPh₃)AuO₂C(CH₂OCH₂)₃H] (2). Thermal treatment of the as-produced templates gave the respective metal nanoparticle-functionalized systems, which were characterized by powder X-ray diffraction (PXRD) and transmission electron microscopy. The plasmon resonance of the surface-bonded particles was determined by using UV–Vis spectroscopy showing absorptions at 412 nm for silver and 524 nm for gold. The metal nanoparticle-modified templates were then coated with a twin polymer layer as result of the acid-catalyzed twin polymerization of 2,2′-spiro-bi[4H-1,3,2-benzodioxasiline] (3). This coating consists of a phenolic resin and in situ formed silicon dioxide nanoclusters. After carbonization and removal of the SiO₂ phase by refluxing with a NaOH solution, the appropriate metal-loaded HCS were obtained. The thus prepared carbon materials were characterized by PXRD, electron microscopy and nitrogen adsorption–desorption isotherms. Mainly micro-porous materials (IUPAC type I) were obtained with a surface area between 910 and 1110 m²/g. These HCS materials were used for the catalytic reduction of methylene blue and 4-nitrophenol proving the accessibility of the incorporated M-NPs. A size-dependent influence of the used carbon hull was found.     

Transmission electron microscopy of SrAl2Si2O8: Feldspar and hexacelsian polymorphs

Sr-feldspar: The electron diffraction data obtained agree with the space group 12/c determined by X-ray methods. The antiphase domains present are considered to originate from a subsolidus phase transition due to ordering of Al and Si. Polysynthetic twins of the Carlsbad law have been found. Sr-hexacelsian: Three polymorphs with the probable space groups P6/mmm, P6₃/mcm and Immm have been observed, the latter two for the first time in this compound. Both P6/mmm and P6₃/mcm polymorphs were present at room temperature. When heated above 600° C, the P6₃/mcm polymorph transforms rapidly and reversibly to the pseudohexagonal orthorhombic phase with the space group Immm. P6₃/mcm-Sr-hexacelsian shows antiphase domains with the displacement vector 1/2c and Immm-Sr-hexacelsian contains twin domains with the three-fold twin axis parallel to c. The occurrence of antiphase and twin domains, respectively, agrees with the symmetry reductions involved with the phase transitions. This is shown by aid of the group—subgroup relationships among the space groups P6/mmm, P6₃/mcm and Immm.     

Ion-Conducting Defect Pyrochlore Phases in the K2O–Sb2O3–WO3 System

Potassium tungstate antimonates were prepared by calcining K₂CO₃ + Sb₂O₃ + 2(1 – – )WO₃ mixtures in air. Data on the phase composition of the obtained materials were used to locate the pyrochlore phase region in the Sb₂O _z –W₂O₆ – K₂O composition triangle at 1170 K. The distributions of the constituent ions and lattice defects over the crystallographic positions of space group Fd3m were inferred from structural and gravimetric data. The ac conductivity of the potassium tungstate antimonates was measured from 600 to 1000 K. The conductivity and its activation energy were shown to be correlated with the concentrations of cation (position 16d) and anion (position 8b) defects. The concentration and mobility of K⁺ ions involved in charge transport were determined.     

Dynamics of Driven Vortices in the Presence of a Regular Array of Columnar Defects

The effects of pinning by a square lattice of columnar defects (CD) on the low-temperature dynamics of vortex lines driven by a uniform force, F _d , oriented in the (1,0) CD-lattice direction, are investigated by Langevin dynamics simulations of a London model for straight vortex lines interacting with CD of radius comparable to the vortex-core radius. Starting the simulations with the vortex lines in the zero-drive ground-state, it is found that for F _d Fp (F _p = single CD pinning force) reordering into a new state occurs. The spatial order in this state is identified with that of the ground-state of stationary vortex lines interacting with the potential obtained by averaging the vortex-line CD-lattice interaction in the direction of motion. The dynamical phase diagram is investigated by starting the simulations from this ground-state and with F _d F _p , and progressively decreasing F _d . Evidence is reported for the existence of two dynamical phases: high-drive elastic flow and low-drive plastic flow. These are characterized by studying the behavior of the vortex lines density-density correlation function and velocity distribution as a function of F _d .     

Twinning in ferroelectric and ferroelastic ceramics: stress relief

The regular twinning in ceramics and metals below the temperature of a ferroelastic or ferroelectric structural phase transition is a result of energy minimization. Here homogeneous elastic energy is reduced at the expense of twin wall energy. The twin density depends on the gram sizeg; under homogeneous stress the total elastic energy of a grain increases g ³. Any kind of twin wall, however, increases g ². Below the intersection of these two curves, stress reduction by twinning cannot lower the total energy. Thus there is a critical grain size below which twinning should not occur. Above this limit the width of the twin lamellae increases g ^1/2. The shape of the grain then adjusts to the surroundings in two dimensions only. Above another larger critical grain size more complex interfaces with higher surface energy are created, which allow stress relief in the third dimension. A semi-quantitative model is developed with the example of BaTiO₃ ceramic, of which the domain patterns are well known. It is representative for many ceramics. The highT _c superconductor YBa₂Cu₃O_7– also twins according to the same law. For three-dimensional adjustment here a proper interface is missing.     

Formation of metastable phases in flame- and plasma-prepared alumina

The formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of metastable to stable forms, and the thermal history of the particles during solidification. It is suggested that homogeneous nucleation of the solidification of liquid droplets at considerable undercooling results in the formation of-Al₂O₃ rather than-Al₂O₃ because of its lower critical free energy for nucleation. The phase finally observed depends upon the thermal history of the particles during evolution of the heat of fusion and upon the kinetics of the transformation of the nucleating phase to the stable phase. This means that the cooling rate of the particles is relatively unimportant and under the conditions existing in flames and plasmas, metastable alumina will be formed on solidification. The metastable form will be retained on cooling particles less than approximately 10 m diameter, but particles larger than this may transform to-Al₂O₃ during the solidification exotherm     

Deposition from the vapour phase during induction plasma treatment of alumina powders

A study was carried out of the induction plasma melting of alumina powders (particle mean diameter, ¯d _p=24.5 m), (Ar/H₂ or Ar/N₂ plasma, plate power, 40 kW) under reduced pressure conditions (400 torr). The results reveal that in the process, partial vaporization of the alumina powders takes place in the hot region of the discharge. As the molten particles cool down and solidify, the deposits from the vapour phase was formed with the spheroidized particles. In all treatments with the Ar/H₂ and Ar/N₂ plasmas, a condensate of ultrafine alumina fume (d _p<200 nm) was obtained. The fine particles consisted essentially of metastable -, - and -phases. Needle-like crystals(0.1–0.3 m diameter, by 5–15 m long) were observed when operating with an Ar/N₂ plasma at powder feed rates exceeding 10 g min^–1. Electron diffraction analysis revealed that the needles were whiskers, whose structure was very similar to - or -aluminas with an hexagonal close-packed oxygen lattice. The change of morphology is related to the degree of supersaturation in the vapour phase.     

On size effects in helium II below 1 K

The normal fluid density _n and the specific heatc _v are calculated for helium II flowing through plane parallel slits and capillaries with a square profile at temperatures below1 K. Both functions increase over their bulk values if the thermal phonon wavelength becomes comparable to or larger than the smallest dimensiond of the container. The asymptotic expansions which are obtained in this paper have an overlapping range of validity, and therefore describe _n andc _v rather accurately for all values of /d.     

Some factors affecting the reactive hot-pressing behaviour of alumina

The densification behaviour of metastable cubic aluminas during reactive hot-pressing exhibits a region of enhanced densification followed by a region of reduced rate of densification. It has been established that the transition to the region of reduced rate of densification occurs after the phase transformations to -Al₂O₃ are complete. Results are presented of further studies on the influence of applied pressure and minor additions of impurities on the nature of the region of reduced densification for three different cubic Al₂O₃ powders. It was observed that those parameters that assisted the phase transformations to -Al₂O₃ also reduced the transition temperature between the two regions by a differing amount for each powder. It is believed that the differences in hotpressing behaviour of the three cubic Al₂O₃ powders arise from their initial powder characteristics, and that these differences may lead to a better understanding of the region of reduced rate of densification.A further aspect of the present work has been a brief study of the influence of uniaxial pressure upon the high temperature phase transformation of cubic -Al₂O₃ to hexagonal -Al₂O₃. Over a limited pressure range up to 1 kbar the influence of pressure on the irreversible phase transformations was observed to be dT/dp – 0.1 K bar^–1.     

Effect of Ho addition on the properties of Y-system superconductors

(YHo)Ba₂Cu₃O_7–y is prepared by the powder melting process and its properties are investigated. It is found that Ho addition can change the melting temperature of a Y-system superconductor. TheJ _c values of YBCO can be improved by Ho substitution, which is due to the effect of Ho addition on the number and quality of grain boundaries and the size and distribution of 211 particles. The highestJ _c value was found to be 50,000 A/cm² (77 K, 1T,HI) in Y_0.8Ho_0.2Ba₂Cu₃O_7–y. Our results indicate that the number and quality of grain boundaries have a great influence on the superconducting properties. A discussion is given on the relationship betweenJ _c and the size and phase proportion of 211 particles. There are a lot of dense twins in the sample, and many crystal defects exist around the 211 phase.     

The collective excitations of the order parameter in HTSC and heavy fermion superconductors (HFSC) underD-pairing

Within the path integral model ofd-pairing for HTSC and HFSC developed recently by P. N. Brusov and N. P. Brusova¹, the whole collective mode spectrum has been calculated for the first time for five states of HTSC that arise in their symmetry classification as well as for three states of HFSC. For HTSC the calculations have been made for the state which is a candidate for the superconducting state of HTSC in light of numerous recent experiments as well as for , d_xy, d_xz, d_yz states. For HFSC we considered three states, among themdγ and Y₂₋₁ states treated by Hirashima and Namaizawa² who used the theory of response. The number of the collective modes in each phase of both superconducting systems is equal to 10, among which five are high frequency modes while five others are Goldstone or Goldstone-like ones. The spectrum can be used to identify the superconducting states through ultrasound and microwave absorption experiments as well as to interpret these experiments.     

Defects on TiBiTe2 single crystals

Single crystals of rhombohedral TiBiTe₂ examined by transmission electron microscopy consist of large areas separated by {100}- and {110}-type twins. By measuring the rhombohedral angle from the splitting of diffraction patterns in the twin areas, we calculated the residual strain which remained after the transformation O_h D _3d, ranging from 1.2 to 12.2 %. Dislocations with Burgers vector (a/2) 1¯10 were mobile and the main slip system was (111) 1¯10. Planar defects with complex - fringe contrast were also observed.     

Superconductivity in the high-pressure dhcp phase of Pb-Bi

The superconducting properties of a recently identified high-pressure dhcp phase of Pb-Bi have been studied. There is a small abrupt drop in the superconducting transition temperature from 8.43 to 7.96 K as the samples transform from the mixed - phase to the dhcp phase. Both the mixed - phase and the dhcp phase show a surprisingly small negative slope for the volume dependence of the superconducting transition temperature corresponding tod(lnT _c )/d(lnV)=0.39.     

Small angle X-ray scattering (SAXS) studies of sol to gel transition in K2O-Al2O3-SiO2 system

The structural evolution during a sol to gel transition reaction in the K₂O-Al₂O₃-SiO₂ system was investigated by using in situ small angle X-ray scattering (SAXS) technique. The results are interpreted as evidence of the presence of at least two different gel phases. The analysis shows that the primary phase is composed of small particles with characteristic size of 3.0 nm, and considerable size and shape variation. The primary phase is identified as a random phase in the Debye sense. The primary particles aggregate into larger formations with spherical symmetry. The size of larger aggregates (second phase) increases continuously in the course of gelation and levels off after reaching 15.0 nm. The volume fraction of the second phase keeps increasing until the end of the measurement which is far after the gelation (3-fold of the gelation time).     

Point-contact spectroscopy for itinerant-magnetic material of Fe x TiS2

The nonlinear current-voltage characteristics of a homojunction point contact of 1T-CdI₂-type layered crystal TiS₂ and its intercalation compound of itinerantmagnetic Fe _x TiS₂ have been measured at 1.4 K. For the magnetic samples, both a maximum and a minimum in thedI/dV-V curves are observed within a restricted range of the initial differential resistancesR _d(0). Electron-magnon scatterings are responsible for the appearance of the minimum at a bias voltageV _B(min), which depends onR _d(0). These results are discussed using the conventional spherical spreading out model, together with a heating model. The agreement between experiment and theory is apparently good if a large background is taken into account, but with unusually large andR _d(0)-dependent Lorenz number.     

Collective modes and a theory of response ofd-wave superconductors

We present a theory of the response ofd-wave superconductors to weak applied fields, by taking account of the Coulomb interaction and all the collective degrees of freedom as well as crystal symmetry. We choose two representative phases: the d phase, which has point nodes in the energy gap, and theY _2–1 phase, which has line as well as point nodes. The former is a self-consistent solution for cubic as well as spherical symmetries and the latter is one for spherical, cubic, and hexagonal symmetries. We obtain obviously gauge-invariant expressions for the order-parameter fluctuations and the currents, having forms common not only to thed-wave states, but also to thep-wave states studied earlier. We also investigate the collective excitations; in the long-wavelength limit for spherically symmetric systems, there are, on the frequency-temperature plane, seven branches for eachd-wave phase considered, in addition to the common plasma mode and orbital Goldstone modes resulting from the spontaneous breakdown of the rotational invariance. In theY _2–1-phase two eigenmodes are found to become gapless at a finite temperature, below which they are purely imaginary. This implies instability of the phase. The effect of crystal anisotropy on the collective spectra is also studied.A preliminary report on the present work was published inJpn. J. Appl. Phys. Suppl. 26-3, 167 (1987).