Ionic conductivity of synthetic analcime,sodalite and offretite

The ionic conductivity of pressed pellets of dehydrated synthetic analcime, sodalite and offretite was determined by a.c. measurements within the range 10 Hz to 10 MHz. The ionic conductivity values of those zeolites exchanged with various monovalent cations were determined by the complex impedance plane method. The conduction activation energies range between 63 and 101 kJ mol^–1. Sodium analcime shows the best ionic conductivity, namely 1.8×10^–4–1 cm^–1 at 400° C. A comparison with the conductivity of other ion-conducting solids was made.     

A model of the cutting mechanism in grinding

In order to achieve optimum working results during grinding, information regarding both the kinematic relations and the structure of the multiple cutting-edge tool is necessary. In addition, the physical and metallurgical properties of the workpiece material and the specific influence of the interfacial frictional effects must be taken into account.This paper presents a contribution to the understanding of the cutting mechanisms in grinding. An analysis of the grinding mechanism is made on the basis of the cutting-edge geometry and the kinetics involved. One physical model has been developed to explain all the phenomena from friction to ploughing and cutting under plane strain conditions.Starting from the velocity relation at an averaged penetrating cutting edge and characterizing frictional conditions at the interface between the cutting edge and the workpiece material, it is possible to calculate a slip-line field which satisfies all the existing boundary conditions. The flow pattern of the material can be drawn taking the corresponding hodograph into account. This results in a distortion of the square grid characterizing the material on passing through the region of plastic deformation. Agreement with cross sections of actual chip formation zones during grinding is observed. The significance of this analysis lies in the fact that it establishes a relation between chip formation and the resultant surface integrity.     

‘Fragile Superconductivity’: A Kinetic Glass Transition in the Vortex Matter of the High-temperature Superconductor YBa2 Cu3O7-δ

Using high-resolution thermal expansion and magnetization measurements, we provide experimental evidence for a kinetic glass transition in the vortex matter of YBa₂Cu₃O_7-δ with some disorder. This transition, which represents the true superconducting transition in a magnetic field, exhibits many of the features of the usual glass transition found in supercooled structural liquids such as window glass. We demonstrate, using both kinetic and thermodynamic criteria, that this vortex matter is the most fragile system known to date, which we argue makes it possible to investigate the behavior very close to the Kauzmann temperature. Vortex matter, we suggest, may be a model system to study glassy behavior in general, which is expected to lead to a better understanding of the strong-fragile behavior in structural glasses.     

MDARTS: a multiprocessor database architecture for hard real-timesystems

Complex real time systems need databases to support concurrent data access and provide well defined interfaces between software modules. However, conventional database systems and prior real time database systems do not provide the performance or predictability needed by high speed, hard real time applications. The authors designed, implemented, and evaluated an object oriented database system called MDARTS (Multiprocessor Database Architecture for Real Time Systems). MDARTS avoids the client server overhead of most prior real time database systems and object oriented, real time systems by moving transaction execution into application tasks. By eliminating these sources of overhead and focusing on basic data management services for control systems (data sharing, serializable transactions, and multiprocessor support), the MDARTS prototype provides hard real time transaction times approximately three orders of magnitude faster than prior real time database systems. MDARTS ensures bounded locking delay by disabling preemption when a transaction is waiting for a lock, and hence, allows for the estimation of worst case transaction execution times. Another contribution of MDARTS is that it supports explicit declarations of real time requirements and semantic constraints within application code. The MDARTS library examines these declarations at application initialization time and attempts to construct objects that are compatible with the requirements. Besides local shared memory transactions with hard real time response time guarantees, MDARTS also supports remote transactions that use remote procedure calls for data access with less stringent timing constraints. The MDARTS prototype is implemented in C++ and it runs on VME based multiprocessors and Sun workstations     

Superconductivity in 4-Angstrom carbon nanotubes--a short review

We give an up-to-date review of the superconducting phenomena in 4-Angstrom carbon nanotubes embedded in aligned linear pores of the AlPO(4)-5 (AFI) zeolite, first discovered in 2001 as a fluctuation Meissner effect. With the introduction of a new approach to sample synthesis around 2007, new data confirming the superconductivity have been obtained. These comprise electrical, specific heat, and magnetic measurements which together yield a consistent yet complex physical picture of the superconducting state, largely owing to the one-dimensional (1D) nature of the 4-Angstrom carbon nanotubes. For the electrical transport characteristics, two types of superconducting resistive behaviors were reproducibly observed in different samples. The first type is the quasi 1D fluctuation superconductivity that exhibits a smooth resistance drop with decreasing temperature, initiating at 15 K. At low temperatures the differential resistance also shows a smooth increase with increasing bias current (voltage). Both are unaffected by an applied magnetic field up to 11 Tesla. These manifestations are shown to be consistent with those of a quasi 1D superconductor with thermally activated phase slips as predicted by the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. The second type is the quasi 1D to 3D superconducting crossover transition, which was observed to initiate at 15 K with a slow resistance decrease switching to a sharp order of magnitude drop at ～7.5 K. The latter exhibits anisotropic magnetic field dependence and is attributed to a Berezinskii-Kosterlitz-Thouless (BKT)-like transition that establishes quasi-long-range order in the plane transverse to the c-axis of the aligned nanotubes, thereby mediating a 1D to 3D crossover. The electrical data are complemented by magnetic and thermal specific heat bulk measurements. By using both the SQUID VSM and the magnetic torque technique, the onset of diamagnetism was observed to occur at ～15 K, with a rapid increase of the diamagnetic moment below ～7 K. The zero-field-cooled and field-cooled branches deviated from each other below 7 K, indicating the establishment of a 3D Meissner state with macroscopic phase coherence. The superconductivity is further supported by the specific heat measurements, which show an anomaly with onset at 15 K and a peak at 11-12 K. In the 3D superconducting state, the nanotube arrays constitute a type-II anisotropic superconductor with H(c1)≈ 60 to 150 Oe, coherence length ξ≈ 5 to 15 nm, London penetration length λ≈ 1.5 μm, and Ginzburg-Landau κ≈ 100. We give a physical interpretation to the observed phenomena and note the challenges and prospects ahead.     

Observation of Room Temperature Ferromagnetism in Conducting and Insulating Cu doped ZnO Thin Films

With pulsed laser deposition, the Cu_0.04Zn_0.96O thin films are grown at 600 °C under three different oxygen pressures, namely PO₂ = 0.00, 0.02, and 1.00 Pa. X-ray diffraction shows single-phase material for the samples grown under PO₂ = 0.00 and 1.00 Pa and CuO secondary phase for the PO₂ = 0.02 Pa grown sample. The observation of satellite structures in the Cu 2p core level X-ray photoelectron spectroscopy (XPS) spectra suggest the presence of Cu²⁺ and CuO secondary phases in the samples grown at PO₂ = 0.02 and 1.00 Pa. The sample grown under vacuum (PO₂ = 0.00 Pa) shows mixed Cu oxidation state of 1 + or 2 + . The sample grown without oxygen is n-type and those grown with oxygen are highly insulating. The insulating sample grown at PO₂ = 0.02 Pa shows highest magnetization due to possible collective behavior of Cu²⁺ – O _v – Cu²⁺ network in the form of bound magnetic polaron (BMP) and ferromagnetic superexchange interaction coming from uncompensated surface spins of the Cu ions in the CuO secondary phase. Both delocalized electrons (～3.32 × 10¹⁸) due to oxygen deficient defects and reduced amount of effective Cu²⁺ ions discredit the BMP model for this vacuum grown sample, and magnetism is suggested due to O _v and presence of possible CuO secondary phase.     

Muon-Spin Rotation Study of the Ternary Noncentrosymmetric Superconductors Li2Pd x Pt3−x B

We investigated the superconducting state of the noncentrosymmetric superconductors Li₂Pd _x Pt_3−x B with superconducting transition temperature T _c=5.16(8) K (x=2.25), 3.56(8) K (x=1.5) and 2.60 K (x=0) by means of muon-spin rotation (μSR) and specific heat experiments. The μSR relaxation rate σ _sc was found to be constant at low temperatures for all the compounds. Data taken at different magnetic fields show that the magnetic penetration depth λ is field-independent for Li₂Pd_2.25Pt_0.75B and Li₂Pt₃B. The electronic contribution to the specific heat measured in Li₂Pd_1.5Pt_1.5B and Li₂Pt₃B increases exponentially at the lowest temperatures. These features suggest that the whole family of Li₂Pd _x Pt_3−x B comprises single-gap s-wave superconductors across the entire doping regime.     

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La_0.65Ca_0.35MnO₃ and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at T _c, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.     

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La_0.65Ca_0.35MnO₃ and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at T _c, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.     

High-Resolution Thermal Expansion of MgB2

The thermal expansion of polycrystalline MgB₂ from 5–300 K is studied using high-resolution capacitance dilatometry. The thermal expansivity exhibits a small jump of –5.8×10^–8 K^–1 at T _c (in accord with expectations from the Ehrenfest relationship and published specific heat and pressure data) and a negative peak-like feature close to 5 K. No indications of any structural instabilities are observed.