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Distribution by Scientific Domains
Physics and Astronomy50%

Selected Abstracts

Model development for semicontinuous production of ethylene and norbornene copolymers having uniform composition

AICHE JOURNAL, Issue 3 2009
Shaojie Liu
Abstract Terminal and penultimate models for controlling copolymer composition distribution (CCD) in ethylene and norbornene (NB) copolymerization were developed by taking into account the variation of active site concentration with the initial comonomer ratio. The models were validated by batch polymerization experimental data. The terminal model gave better correlation with the composition data whereas the penultimate model had a better fit to the rate data. The terminal model was then used to design NB feeding policies in semicontinuous processes for targeted CCD profiles. Based on the model results, a series of ethylene-NB copolymers with various NB contents were prepared. With the same NB content, the semicontinuous process produced a uniform composition, whereas the batch process yielded broad CCD. The batch samples had lower Tg values and broader transition ranges, even yielded crystalline materials. In contrast, the semicontinuous samples overcame the disadvantages. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Influence of Nb substituted for Fe on the microstructure and magnetic properties of Fe-based nanocomposite alloy

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 12 2007
N. Q. Hoa
Abstract The influence of Nb substituted for Fe on the microstructure and magnetic properties including the magnetoimpedance effect of a Fe-based have been investigated. The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix was obtained by annealing the Fe-based amorphous alloy prepared by rapidly-quenched method. The measurements of thermomagnetic curves indicated that the Curie temperature of the amorphous phase of the samples decreases with increasing Nb content. The optimal heat treatment was performed at Ta = 480 °C for 30 min and showed that the ultrasoft magnetic properties of nanocomposite materials were obtained. The magnetoimpedance (MI) of these samples has been studied in range frequency from 1MHz to 5 MHz and varying a dc magnetic field within 300 Oe. The correlation between the MI effect and the soft magnetic properties is discussed. The incremental permeability ratio (PR) showed the drastic changes of soft magnetic properties as a function of annealing temperatures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Structure, stability and electronic properties of composite Mo1,xNbxS2 nanotubes

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2006
V. V. Ivanovskaya
Abstract The effect of Mo , Nb substitution on the electronic structure of MoS2 nanotubes has been investigated using the density functional tight binding method (DFTB). It has been found that composite Mo1,xNbxS2 nanotubes (with Nb contents of 5, 10 and 25 at%) are more stable than the corresponding pure tubes, especially for larger tube diameters. The defect-formation energy indicates that the most stable dopant arrangement is a NbS2 stripe along the tube direction. However, entropy effects may favor a random arrangement of Nb dopant atoms at high temperatures in the tubes with a larger diameter. All of the studied Mo1,xNbxS2 nanotubes have metallic properties, independent of their chirality, diameters and ordering type of substitutional atoms. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]