A Hemi‐metallocene Chromium Catalyst with Trimethylaluminum‐Free Methylaluminoxane for the Synthesis of Disentangled Ultra‐High Molecular Weight Polyethylene

Recently, it has been shown that by using a single‐site catalytic system having titanium as a metallic center, it is possible to tailor the entanglement density in the amorphous region of a semi‐crystalline ultra‐high molecular weight polyethylene (UHMWPE). This route provides the possibility to make high‐modulus, high‐strength uniaxially and biaxially drawn tapes and films, without using any solvent during processing. In this publication, it is shown that a single‐site catalyst having chromium as metallic center, proposed by Enders and co‐workers, can also be tuned to provide control on the entanglement density during synthesis of the UHMWPE. However, to achieve the goal some modifications during the synthesis are required. The synthesized polymers can be processed in the solid state below the equilibrium melting temperature, resulting in uniaxially drawn tapes having tensile strength and modulus greater than 3.5 N/tex and 200 N/tex, respectively. Rheological studies have been performed to follow the increase in entanglement density in melt state with time.

     

Coplanar Waveguide Characterization with Thick Metal Coating

Coplanar waveguides (CPW) used in microwave integrated circuits are significantly dispersive. An analytical approximation to the frequency dependence of both _eff and _c of CPW with finite metallisation thickness and conductivity is discussed. The effect of increasing loss tangent on dielectric constant is also presented.     

Holographic moiré in real time

The body of knowledge necessary to observe holographic-moiré patterns in real time is introduced. The basic factors influencing fringe visibility in holographic moiré are analyzed and expressions to evaluate fringe visibility for any given displacement and deformation are given. The application of the introduced theory in the case of real-time observation is discussed. It is shown that the maximum benefits of this technique are achieved by combining it with closed-circuit TV. Several examples of application are given.     

High-resolution frequency estimation technique for recovering phase distribution in interferometers

An integral approach to phase measurement is presented. First, the use of a high-resolution technique for the pixelwise detection of phase steps is proposed. Next, the robustness of the algorithm that is developed is improved by incorporation of a denoising procedure during spectral estimation. The pixelwise knowledge of phase steps is then applied to the Vandermonde system of equations for retrieval of phase values at each pixel point. Conceptually, our proposal involves the design of an annihilating filter that has zeros at the frequencies associated with the polynomial that describes the fringe intensity. The parametric estimation of this annihilating filter yields the desired spectral information embedded in the signal, which in our case represents the phase steps. The proposed method offers the advantage of extracting the interference phase of nonsinusoidal waveforms in the presence of miscalibration error of the piezoelectric transducer. In addition, in contrast to previously reported methods, this method does not require the application of selective phase steps between data frames for nonsinusoidal waveforms.     

Maximum-likelihood estimator for dual phase extraction in holographic moire

A maximum-likelihood (ML) method based on spectral estimation theory for the extraction of dual phase distributions in holographic moire in the presence of nonsinusoidal waveforms, noise, and the miscalibration of piezoelectric (PZT) devices is proposed. The extraction of these phases requires incorporating two PZTs into the moire setup. ML estimators are asymptotically efficient for sufficient data samples. The approach presented uses a direct stochastic algorithm called probabilistic global search Lausanne for minimizing the ML function.     

Current instabilities in dynamic random access memory storage capacitor formed with electron beam deposited Y2O3 dielectric

Metal-oxide-semiconductor (MOS) storage capacitors based on electron beam deposited Y₂O₃ extrinsic dielectric on Si show changes in capacitance density depending on the amorphous and crystalline phases. Bias stress cycle-dependent changes in capacitance density occur due to the non-equilibrium nature of defect states at the Y₂O₃/Si interface after O₂ annealing as a result of the emergence of a 4–8 nm thick SiO₂ film at the interface. Leakage currents show instability under repeated dc bias stress, the nature and extent of which depend upon the structure of the Y₂O₃ gate dielectric and the polarity of dc bias. With amorphous Y₂O₃, leakage currents drift to lower values under gate injection due to electron trapping, and to higher values under Si-injection due to the generation of holes. Though leakage current drift is minimal for crystalline Y₂O₃, its magnitude increases as the energy of injected electrons from mid-gap states is low and the local field due to asperity is high. The emergence of interfacial SiO₂ reduces the magnitude of Si-injection leakage current substantially, but causes transient changes resulting in switching to higher values at a threshold dc bias. Thermal detrapping of holes and reverse bias stress studies confirm that the instability of current is caused by an increase in the cathodic field from hole trapping at interface states. Leakage current instability limits the application of extrinsic high dielectric constant dielectrics in a high density DRAM storage capacitor, unless a new interface layer scheme other than SiO₂ and a method to form a defect-free dielectric layer can be implemented. Received: 29 October 2001 / Accepted: 22 April 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-413/545-4611, E-mail: rastogi@ecs.umass.edu     

Propagation characteristics of a segmented cladding fiber

We propose a novel optical fiber design that consists of a uniform core and a segmented cladding formed by alternate regions of high and low refractive indices in the azimuthal direction. The structure is analyzed by use of the radial effective-index method, and the propagation characteristics of the structure are studied. The fiber has a highly dispersive cladding and shows characteristics similar to those of photonic-crystal fibers and holey fibers. The novel fiber offers the possibility of single-mode operation over a wide range of wavelengths with a large core diameter.     

Shielded suspended dielectric guide

The dispersion characteristics of a shielded suspended dielectric are computed using mode matching technique. From the dispersion characteristics the dielectric and conductor loss, quality factor and wave impedances are derived. The dimensions of shielded suspended dielectric are same as that of a rectangular metallic wave guide at K_a band.     

Comparative holographic interferometry: A nondestructive inspection system for detection of flaws

Comparative holographic interferometry addresses a key problem of nondestructive evaluation: comparison of the resistance to strength in real time of two nominally identical specimens. After outlining the basis of comparative holography in flaw detection, the present paper reports on some new and complementary developments in comparative holographic interferometry. The proposed systems stand out by their ability to store the master displacement field in the interferometric setup. The integration of the storage capacity in the instrument considerably augments its potential in nondestructive routine inspection tasks. The visual display of the fringes contouring the difference in mechanical response is shown to improve considerably with the addition of auxiliary phase difference satisfying certain conditions. Methods for the generation of corresponding fringes are considered and their localization investigated in brief. Particular attention is devoted to the formation of the holographic moiré fringes. The influence of system misalignment on the moiré fringe interpretation is examined. A potential application of comparative holography to the quantitative evaluation of fatigue is described. Experimental evidence supporting the operational feasibility of the technique along with the results obtained in application to flaw detection are finally presented.     

Magnetic properties of stuffed copper chromium tellurides Cu1+xCr2+yTe4 (x=0-1, y<0.3)

We present a detail study of the effect of excess metal atoms on the magnetic properties of Cu_1+xCr_2+yTe₄ at 2-400 K. With the increase in x=0-1 and y<0.3, these compounds retain metallic behavior, while ferromagnetic ordering temperature reduces from 325 to 160 K. Our low field susceptibility χ_ac measurements reveal a second transition on cooling below the ferromagnetic ordering; the transition at around 160-180 K intensifies with the excess amount of copper and chromium atoms. The value of spontaneous magnetization at 2 K remains between 2.6 and 2.9μ_B across all the compositions and it reduces with temperature as M(T)∼A₀T^3/2+A₁T^5/2, as expected for the excitation of Bloch's spin waves in a model of the Heisenberg ferromagnet. Our terminal composition Cu_1.9Cr_2.25Te₄ showed only second transition at 160 K with short range magnetic order much above the transition temperature and in the absence of the specific heat jump at this temperature. The magnetic properties are explained as a result of random magnetic anisotropy in the excess-metal compositions induced by the interstitial atomic defects in their parent spinel structure. The large stuffing of cations has been made possible in the telluride compounds because of the large size of tellurium and also by the covalent bonding that stabilizes the defect structure.