Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Numerical integration of 2‐D integrals based on local bivariate C 1 quasi‐interpolating splines

In this paper cubature formulas based on bivariate C ¹ local polynomial splines with a four directional mesh [4] are generated and studied. Some numerical results with comparison with other methods are given. Moreover the method proposed is applied to the numerical evaluation of 2‐D singular integrals defined in the Hadamard finite part sense. Computational features, convergence properties and error bounds are proved. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cu+(H2) and Na+(H2) adducts in exchanged ZSM-5 zeolites

Cu(I) ions in Cu-ZSM-5 form Cu+(H2) complexes, stable at room temperature and sub-atmospheric H2 pressure, which do not have any homogeneous analogue except for matrix-isolated [Cu(eta2-H2)Cl]. Comparison with the unstable Na+(H2) adducts formed in the parent Na-ZSM-5 zeolite allow the conclusion that the Cu(I)/H2 bond is governed by sigma-pi overlap forces.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

A new zirconium inorganic building brick forming metal organic frameworks with exceptional stability

Porous crystals are strategic materials with industrial applications within petrochemistry, catalysis, gas storage, and selective separation. Their unique properties are based on the molecular-scale porous character. However, a principal limitation of zeolites and similar oxide-based materials is the relatively small size of the pores, typically in the range of medium-sized molecules, limiting their use in pharmaceutical and fine chemical applications. Metal organic frameworks (MOFs) provided a breakthrough in this respect. New MOFs appear at a high and an increasing pace, but the appearances of new, stable inorganic building bricks are rare. Here we present a new zirconium-based inorganic building brick that allows the synthesis of very high surface area MOFs with unprecedented stability. The high stability is based on the combination of strong Zr-O bonds and the ability of the inner Zr6-cluster to rearrange reversibly upon removal or addition of mu3-OH groups, without any changes in the connecting carboxylates. The weak thermal, chemical, and mechanical stability of most MOFs is probably the most important property that limits their use in large scale industrial applications. The Zr-MOFs presented in this work have the toughness needed for industrial applications; decomposition temperature above 500 degrees C and resistance to most chemicals, and they remain crystalline even after exposure to 10 tons/cm2 of external pressure.     

Finite part integrals of local bivariate c1 quasi-interpolating splines

A local bivariate C¹ quasi-interpolating spline operator with a four directional mesh is considered and studied. Based on the above operator we present cubature formulas for 2-D singular integrals, defined in the Hadamard finite part sense. Convergence results are obtained for a wide class of functions. Moreover numerical tests are given.     

A matrix model of the axle vibration of a piezoelectric motor

In this work, a matrix model of the axle vibration of a piezoelectric motor is proposed. The stator of this motor is composed of a thin piezoelectric membrane and a steel axle fitted at the center of the membrane. The rotor consists of a cylinder-shaped permanent magnet, pressed in contact with the other end of the axle by means of the magnetic forces. A travelling wave is excited in the membrane by using four electrodes and four, properly delayed, driving signals. The rotating flexural displacement of the membrane produces a wide precessional motion of the axle. In this way, a continuous slipping takes place between the axle and the rotor, and therefore, a torque is transmitted to the rotor. In this paper, the precessional motion of the axle is modeled as the composition of two transverse vibrations belonging to two perpendicular planes passing through the axle. The axle, vibrating in its transverse mode, is modeled as a two-port system: the input is the bending moment supplied by the membrane, and the output is the transverse force at the terminal end of the axle. With this model, we have computed the trasmission transfer function as a function of frequency, and the transversal displacement along the axle at its resonance frequency. The computed results are in reasonable agreement with experimental interferometric measurements carried out on a prototype.