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151.
Solid bisphenol-A epoxy
resin of medium molecular mass was cured using a Lewis acid initiator (ytterbium(III)
trifluoromethanesulfonate) in three different proportions (0.5, 1 and 2 phr).
A kinetic study was performed in a differential scanning calorimeter. The
complete kinetic triplet was determined (activation energy, pre-exponential
factor, and integral function of the degree of conversion) for each system.
A kinetic analysis was performed with an integral isoconversional procedure
(free model), and the kinetic model was determined both with the Coats-Redfern
method (the obtained isoconversional value being accepted as the effective
activation energy) and through the compensation effect. All the systems followed
the same isothermal curing model simulated from non-isothermal ones. The growth-of-nuclei
Avrami kinetic model A3/2 has been proposed as the
polymerization kinetic model. The addition of initiator accelerated the reaction
especially when 2 phr was added. 0.5 and 1 phr showed very few kinetic differences
between them. 相似文献
152.
The phases Bi14MO24 (M=Cr, Mo, W) have been studied using differential scanning calorimetry, variable temperature X-ray powder diffraction and neutron powder diffraction. All three compounds were found to undergo a phase change, on cooling, from the previously reported tetragonal symmetry (I4/m) to monoclinic symmetry (C2/m). Transition temperatures were determined to be ∼306 K (M=W) and ∼295 K (M=Mo), whereas a gradual transition between 275 and 200 K was observed for M=Cr. The high and low temperature structures are very similar, as indicated by the relationship between the monoclinic and tetragonal unit cell parameters: am=√2at, bm=ct, cm=at, β∼135°. High-resolution neutron powder diffraction data, collected at 400 and 4 K, were used to establish the nature of the transition, which was found to involve a reduction in the statistical possibilities for orientation of the MO4 tetrahedra. However, in both tetragonal and monoclinic variants, a degree of orientational disorder of the tetrahedra occurs to give partially occupied sites in the average unit cell. 相似文献
153.
Frank J. Berry Alberto Bohorquez Colin Greaves Julia McManus Elaine A. Moore Michael Mortimer 《Journal of solid state chemistry》1998,140(2):619
The defect structure of divalent magnesium-dopedα-Fe2O3has been examined by Rietveld structure refinement of the X-ray powder diffraction data. The results show that the Mg2+ions occupy the vacant interstitial octahedral sites as well as substituting on the two adjacent octahedral Fe3+sites in the corundum-relatedα-Fe2O3structure. The structure therefore involves a linear cluster of three Mg2+ions replacing two Fe3+ions. Interatomic potential calculations indicate that this is the most energetically favorable defect cluster for the system. 相似文献
154.
155.
156.
D.C.O. Marney L.J. Russell T. Nguyen N. Rigopoulos M. Greaves 《Polymer Degradation and Stability》2008,93(10):1971-1978
This work presents the first study on the fire behaviour of halloysite nanotubes-nylon 6 composites. The nylon 6-halloysite composites were prepared at 5-30 wt% of halloysite loadings by a simple melt extrusion process. A range of standard fire tests and characterization techniques were used to assess the efficacy and mechanism by which the halloysite nanotubes inhibited the burning of nylon. We found that for such systems, relatively high concentrations of additive (≥15 wt%) were required to achieve the levels of fire retardant property normally associated with nanoclay (or layered silicate) additives. We proposed that the primary mechanism of flame inhibition for halloysite nanotubes was similar to that of conventional nanoclays; however, the ease of composite preparation is an attractive consideration for further development or study of such systems. 相似文献
157.
A numerical fluid–structure interaction model is developed for the analysis of viscous flow over elastic membrane structures. The Navier–Stokes equations are discretized on a moving body‐fitted unstructured triangular grid using the finite volume method, taking into account grid non‐orthogonality, and implementing the SIMPLE algorithm for pressure solution, power law implicit differencing and Rhie–Chow explicit mass flux interpolations. The membrane is discretized as a set of links that coincide with a subset of the fluid mesh edges. A new model is introduced to distribute local and global elastic effects to aid stability of the structure model and damping effects are also included. A pseudo‐structural approach using a balance of mesh edge spring tensions and cell internal pressures controls the motion of fluid mesh nodes based on the displacements of the membrane. Following initial validation, the model is applied to the case of a two‐dimensional membrane pinned at both ends at an angle of attack of 4° to the oncoming flow, at a Reynolds number based on the chord length of 4 × 103. A series of tests on membranes of different elastic stiffness investigates their unsteady movements over time. The membranes of higher elastic stiffness adopt a stable equilibrium shape, while the membrane of lowest elastic stiffness demonstrates unstable interactions between its inflated shape and the resulting unsteady wake. These unstable effects are shown to be significantly magnified by the flexible nature of the membrane compared with a rigid surface of the same average shape. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
158.
Rita Punn Frank Berry Colin Greaves 《Journal of Physics and Chemistry of Solids》2008,69(11):2687-2690
The local environments of cations in a recently reported δ-Bi2O3-related phase stabilised by erbium and rhenium, and which shows exceptionally high oxide ion conductivity at low temperatures, have been examined by Bi LIII-, Er LIII- and Re LIII- edge extended X-ray absorption fine structure (EXAFS) measurements. The Bi LIII- and Er LIII- edge data recorded from the compound Bi12.5Er1.5ReO24.5 revealed a high level of oxygen disorder and the adoption by bismuth and erbium of different local environments than those previously observed in erbium-doped δ-Bi2O3. The Re LIII edge EXAFS recorded from Bi12.5Er1.5ReO24.5 endorsed the highly disordered nature of the oxygen system and showed rhenium to adopt fourfold oxygen coordination and to be significantly different from that of the local environment of rhenium in Bi28Re2O49. 相似文献
159.
Y. Kanai M. Saiki K. Hirasawa T. Tsukamoto K. Yoshida S.J. Greaves H. Muraoka 《Journal of magnetism and magnetic materials》2008,320(14):e287-e290
A Landau–Lifshitz–Gilbert (LLG) micromagnetic analysis of the recording field of single-pole-type (SPT) heads was carried out. The whole volume comprising the SPT head and the double-layered medium was treated micromagnetically using the finite-difference method with cubic cells as small as 5 nm, giving a total number of cells of more than 10.8 million. A parallelized fast Fourier transform (FFT) method was used to solve this large-scale problem. Dynamic recording fields were calculated for various head structures and head materials. The timing (synchronization) between the dynamic head field and land location in bit-patterned media (BPM) is discussed and the design methodology is discussed for a fast-switching SPT head. 相似文献
160.
Y. Kanai K. Hirasawa T. Tsukamoto K. Yoshida S. Greaves H. Muraoka 《Journal of magnetism and magnetic materials》2008
Single-pole-type (SPT) heads for perpendicular magnetic recording were investigated using a Landau–Lifshitz–Gilbert (LLG) micromagnetic analysis program running on a PC cluster system. Dynamic recording fields were calculated for various driving currents, head structures and head materials. The dynamic head field response is discussed with regard to the write timing window for bit-patterned media, as described in previous papers. 相似文献