Influence of the surface morphology on the melting of polymer crystals. I. Loops of random length and adjacent reentry

A polymer crystal with a noncrystalline surface layer formed by chain loops of different lengths is considered. It is assumed that the length of each loop can be changed by longitudinal diffusion of the molecule through the crystal lattice. From the condition that the free energy of the system is minimum, the loop length distribution and the average loop length as function of temperature are calculated. In contrast to the results for loops of equal length, for the present model, a substantial thickness of the noncrystalline surface layer and a broad melting range is obtained also for the case of adjacent reentry. In order to get this result one has to take into account that even an “ideal fold” consists of at least four rigidly arranged CH2 groups in energetically unfavored conformation.     

Parametric study on a compound-drop spray flame

The introduction of compound-drop spray in a combustion system is a new concept. These droplets bear two gasification stages to cause an integral positive or negative effect on a premixed flame to raise or lower the local temperature of the gasification region. In this paper, we adopt a compound drop which contains a water core encased by a layer of shell fuel. A one-dimensional homogeneous lean or rich premixed flame with the dilute compound-drop spray was investigated by using large activation energy asymptotic analysis. The compound-drop spray burning mode was defined and divided into completely pre-vaporised burning (CPB), shell pre-vaporised burning (SPB) and shell partially pre-vaporised (SPP) burning modes by way of the gasification zones of the shell fuel and the core water relative to the flame position. The influences of the initial droplet radius, the shell-fuel mass fraction and the liquid loading of the compound-drop spray on the lean and rich flames were analysed. By means of the normalisation parameter of flame propagation mass flux (), enhancement, suppression or extinction of the compound-drop spray flame can be represented clearly. Furthermore, from the observation of extinction, the necessary conditions of extinction of a lean spray flame by the internal heat transfer are that the spray is a negative effect and causes a sufficient heat loss rate at flame sheet downstream side. For a rich spray flame, three extinction patterns were observed; they occur in SPP, SPB or at the critical SPB mode, but do not in CPB. The extinction maps of the compound-drop spray demarcate the patterns and also indicate the limitations and corresponding conditions of the flame extinction.     

Basal slip localization in zinc single crystals. The Considère analyses

The tensile and compression tests were performed on zinc single crystals oriented for slip in the basal slip system. During the first stage of the stress–strain curve, the localized necking was typical of strain localization in the tensile specimens. Single or multiple necks were formed along the specimen length. The range of temperatures and the strain rates for single necking of the sample was determined. The formation of such necking depends on strain hardening characteristics and can be predicted by the Considère criterion. On the other hand, propagation of the necked area along the sample length was not predictable by this criterion. Localized sliding and specimen kinking was indicative of the strain localization observed for different specimens compressed under the same conditions, i.e. temperature and strain rate. A decrease in the compression force and in the cross-sectional area with anvil displacements produced localized sliding. On the other hand, a continuous increase in the compression force was representative of tests leading to specimen kinking.     

Elastic mechanical grain interactions in polycrystalline materials; analysis by diffraction-line broadening

Abstract

Experimental investigations have revealed that the Neerfeld–Hill and Eshelby–Kröner models, for grain interactions in massive, bulk (in particular, macroscopically isotropic) polycrystals, and a recently proposed effective grain-interaction model for macroscopically anisotropic polycrystals, as thin films, provide good estimates for the macroscopic (mechanical and) X-ray elastic constants and stress factors of such polycrystalline aggregates. These models can also be used to calculate the strain variation among the diffracting crystallites, i.e. the diffraction-line broadening induced by elastic grain interactions can thus be predicted. This work provides an assessment of diffraction-line broadening induced by elastic loading of polycrystalline specimens according to the various grain-interaction models. It is shown that the variety of environment, and thus the heterogeneity of the stress–strain states experienced by each of the individual grains exhibiting the same crystallographic orientation in a real polycrystal, cannot be accounted for by traditional grain-interaction models, where all grains of the same crystallographic orientation in the specimen frame of reference are considered to experience the same stress–strain state. A significant degree of broadening which is induced by the heterogeneity of the environments of the individual crystallites is calculated on the basis of a finite element algorithm. The obtained results have vast implication for diffraction-line broadening analysis and modelling of the elastic behaviour of massive polycrystals.     

Initial data truncation for multivariate output of discrete-event simulations using the Kalman filter

Data truncation is a commonly accepted method of dealing with initialization bias in discrete-event simulation. An algorithm for determining the appropriate initial-data truncation point for multivariate output is proposed. The technique entails averaging across independent replications and estimating a steady-state output model in a state-space framework. A Bayesian technique called Multiple Model Adaptive Estimation (MMAE) is applied to compute a time varying estimate of the output's steady-state mean vector. This MMAE implementation features the use, in paralle, of a bank of Kalman filters. Each filter is constructed under a different assumption concerning the output's steady-state mean vector. One of the filters assumes that the steady-state mean vector is accurately reflected by an estimate, called the assumed steady-state mean vector, taken from the last half of the simulation data. As the filters process the output through the effective transient, this particular filter becomes more likely (in a Bayesian sense) to be the best filter to represent the data and the MMAE mean estimator is influenced increasingly towards the assumed steady-state mean vector. The estimated truncation point is selected when a norm of the MMAE mean vector estimate is within a small tolerance of the assumed steady-state mean vector. A Monte Carlo analysis using data from simulations of open and closed queueing models is used to evaluate the technique. The evaluation criteria include the ability to construct accurate and reliable confidence regions for the mean response vector based on the truncated sequences.     

Dynamic complexity of optimal paths and discount factors for strongly concave problems

We study the relationship between the dynamical complexity of optimal paths and the discount factor in general infinite-horizon discrete-time concave problems. Given a dynamic systemx _t+1=h(x _t ), defined on the state space, we find two discount factors 0 < ^{* **} < 1 having the following properties. For any fixed discount factor 0 < < ^*, the dynamic system is the solution to some concave problem. For any discount factor ^** < < 1, the dynamic system is not the solution to any strongly concave problem. We prove that the upper bound ^** is a decreasing function of the topological entropy of the dynamic system. Different upper bounds are also discussed.This research was partially supported by MURST, National Group on Nonlinear dynamics in Economics and Social Sciences. The author would like to thank two anonymous referees for helpful comments and suggestions.     

Mindlin板几何非线性分析的附加内部剪应变法

本文在几何非线性分析的Ｍｉｎｄｌｉｎ板元中引入单元附加内部剪应变，有效地解决了薄板情况下的剪切自锁问题．文中导出了相应的能量相容条件，给出了有限元非线性列式的全过程及有关簿板及中厚板大挠度问题的数值结果．     

理想弹塑性固体Riemann问题的解结构与初值条件

针对理想弹塑性固体材料的一维Riemann问题,在不考虑真空的情况下,讨论其所有可能存在的解结构,给出每一种解结构下对应的初值条件且证明该系列初值条件的完备性,即任意给定的物理量初值均有且只有一种解结构与之对应.基于该理论,在设计精确或近似理想弹塑性Riemann问题求解器时,可以依据初值条件对任意物理量初值直接判断其对应的解结构,从而提高求解器的精度和效率.数值试验验证了该系列初值条件的正确性和有效性.     

Efficient Quantum Dot Light-Emitting Diodes Based on Well-Type Thick-Shell CdxZn1−xS/CdSe/CdyZn1−yS Quantum Dots

Device grade quantum dots (QDs) require QDs ensembles to retain their original superior optical properties as in solution. QDs with thick shells are proven effective in suppressing the inter-dot interaction and preserving the emission properties for QDs solids. However, lattice strain–induced defects may form as the shell grows thicker, resulting in a notable photoluminescence quenching. Herein, a well-type Cd_xZn_1−xS/CdSe/Cd_yZn_1−yS QDs is proposed, where ternary alloys CdZnS are adopted to match the lattice parameter of intermediate CdSe by separately adjusting the x and y parameters. The resultant thick-shell Cd_0.5Zn_0.5S/CdSe/Cd_0.73Zn_0.27S QDs reveal nonblinking properties with a high PL QY of 99% in solution and 87% in film. The optimized quantum dot light-emitting diodes (QLEDs) exhibit a luminance of 31547.5 cd m⁻² at the external quantum efficiency maximum of 21.2% under a bias of 4.0 V. The shell thickness shows great impact on the degradation of the devices. The T₅₀ lifetime of the QLEDs with 11.2 nm QDs reaches 251 493 h, which is much higher than that of 6.5 and 8.4 nm QDs counterparts. The performances of the well-type thick-shell QLEDs are comparable to state-of-the-art devices, suggesting that this type of QDs is a promising candidate for efficient optoelectronic devices.