Critical Behavior of Two Interacting Linear Polymer Chains in a Good Solvent

A model of two interacting (chemically different) linear polymer chains is solved exactly using the real-space renormalization group transformation on a family of Sierpinski gasket type fractals and on a truncated 4-simplex lattice. The members of the family of the Sierpinski gasket-type fractals are characterized by an integer scale factorb which runs from 2 to ∞. The Hausdorff dimensiond _F of these fractals tends to 2 from below asb → ∞. We calculate the contact exponenty for the transition from the State of segregation to a State in which the two chains are entangled forb = 2-5. Using arguments based on the finite-size scaling theory, we show that forb→∞, y = 2 - v(b) d _F, wherev is the end-toend distance exponent of a chain. For a truncated 4-simplex lattice it is shown that the system of two chains either remains in a State in which these chains are intermingled in such a way that they cannot be told apart, in the sense that the chemical difference between the polymer chains completely drop out of the thermodynamics of the system, or in a State in which they are either zipped or entangled. We show the region of existence of these different phases separated by tricritical lines. The value of the contact exponenty is calculated at the tricritical points.     

A cellular automaton technique for modelling of a binary dendritic growth with convection

A two-dimensional model for the simulation of a binary dendritic growth with convection has been developed in order to investigate the effects of convection on dendritic morphologies. The model is based on a cellular automaton (CA) technique for the calculation of the evolution of solid/liquid (s/l) interface. The dynamics of the interface controlled by temperature, solute diffusion and Gibbs–Thomson effects, is coupled with the continuum model for energy, solute and momentum transfer with liquid convection. The solid fraction is calculated by a governing equation, instead of some approximate methods such as lever rule method [A. Jacot, M. Rappaz, Acta Mater. 50 (2002) 1909–1926.] or interface velocity method [L. Nastac, Acta Mater. 47 (1999) 4253; L. Beltran-Sanchez, D.M. Stefanescu, Mat. and Mat. Trans. A 26 (2003) 367.]. For the dendritic growth without convection, mesh independency of simulation results is achieved. The simulated steady-state tip velocity are compared with the predicted values of LGK theory [Lipton, M.E. Glicksmanm, W. Kurz, Metall. Trans. 18(A) (1987) 341.] as a function of melt undercooling, which shows good agreement. The growth of dendrite arms in a forced convection has been investigated. It was found that the dendritic growth in the upstream direction was amplified, due to larger solute gradient in the liquid ahead of the s/l interface caused by melt convection. In the isothermal environment, the calculated results under very fine mesh are in good agreement with the Oseen–Ivanstov solution for the concentration-driven growth in a forced flow.     

Shortcut method for evaluation and design of a hybrid process for enantioseparations

Hybrid processes for enantioseparations have a considerable potential for reducing investment and operational costs. An example is the combination of simulated moving bed (SMB) chromatography and selective crystallisation. However, the design of integrated processes is a difficult task. A shortcut method is presented that can serve as a tool for design and estimation of the potential of such processes. The approach requires only limited experimental data and thus allows for systematic parameter studies. The method is based on the determination of the purity-performance characteristic of the SMB process and rigorous application of mass balances. The use of relative mass fluxes allows derivation of simple algebraic expressions for essential process parameters. The significant potential of combining SMB and crystallisation is demonstrated for the example of the separation of mandelic acid enantiomers.     

Structures of CoAl(1 1 1) surface: A first principles study

The structures of the CoAl(1 1 1) surface are studied by first principles calculations. Our calculations show that the surface layer is always occupied by pure Al for all concentrations studied here, which indicates the dominant role of the Al segregation tendency. This is different from the CoAl(0 0 1) surface, where a number of Co anti-sites are found on the top most layer. The calculated surface phase diagram of ground states shows that there are three stable structures. The diffusion barriers of the metastable structure evolving to the stable structure are also calculated. The high diffusion barrier can explain the appearance of metastable structures at low temperature in experiment.     

Two types of diffusions at the cathode/electrolyte interface in IT-SOFCs

Analytical transmission electron microscopy, in particular with the combination of energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS), has been performed to investigate the microstructure and microchemistry of the interfacial region between the cathode (La_0.6Sr_0.4Co_0.8Fe_0.2O₃, LSCF) and the electrolyte (Gd-doped ceria, GDC). Two types of diffusions, mutual diffusion between cathode and electrolyte as well as the diffusion along grain boundaries, have been clarified. These diffusions suggest that the chemical stability of LSCF and GDC are not as good as previously reported. The results are more noteworthy if we take into consideration the fact that such interdiffusions occur even during the sintering process of cell preparation.     

How well do discrete element granular flow models capture the essentials of mixing processes?

Flowing granular materials, undergoing both mixing and segregation, play important roles in industries ranging from minerals and food to pharmaceuticals and ceramics. Sometimes it is desirable to enhance the mixing and inhibit segregation and in other cases it is desirable to minimise the mixing and enhance the segregation. The fundamentals of these processes are poorly understood. Computational modelling of such granular flows offers a good opportunity to study these fundamentals. But how well do these discrete element based modelling techniques capture the essential features of mixing processes? The capabilities of our discrete element modelling package are described. Two different methods for measuring the rates of mixing are presented and three different configurations are studied. Qualitatively reasonable flows are obtained. A detailed study of the mixing demonstrates that the amount and nature of the mixing is quite sensitive to a range of physical parameters.     

硒蒸气浓度对制备CIGS薄膜的影响

采用“预制层硒化法”制备CuIn_1-xGa_xSe₂ (CIGS)薄膜. 基于自主设计的“双层管式硒化装置”, 通过控制硒蒸气浓度优化退火工艺, 研究硒蒸气浓度对薄膜光电性能的影响. 利用俄歇电子能谱(AES)和X射线衍射分析(XRD)等手段对不同硒浓度氛围下生成的CIGS薄膜的成分和物相进行表征, 并在AM1.5、1000 W·m^-2的标准光照条件下比较相应CIGS电池器件的输出性能. 实验结果表明: 饱和硒蒸气下退火得到的样品, 基底钼膜遭到严重腐蚀破坏, 失去背电极功能; 在低浓度硒气氛下退火不能有效消除CIGS薄膜的偏析和缺陷, 以致光电转换效率低; 而在无硒惰性氛围下退火的样品, 生成了物相均一化的CIGS薄膜, 由此制备的CIGS电池取得了8.5%的转换效率.     

Study of the selective retention of fluorinated compounds on perfluorinated stationary phases

Summary The basis of the selective retention of perfluorinated compounds on perfluorinated bonded phases is examined. It is shown that the selective retention increases dramatically by increasing chain length and strand multiplicity.     

A dynamical systems model of unorganized segregation

We consider Schelling’s bounded neighborhood model (BNM) of unorganized segregation, from the perspective of modern dynamical systems theory. We carry out a complete quantitative analysis of the system for linear tolerance schedules. We derive a fully predictive model and associate each term with a social meaning. We recover and generalize Schelling’s qualitative results.

For the case of unlimited population movement, we derive exact formulae for regions in parameter space where stable integrated population mixes can occur, and show how neighborhood tipping can be explained in terms of basins of attraction.

When population movement is limited, we derive exact criteria for the occurrence of new population mixes.

For nonlinear tolerance schedules, we illustrate our approach with numerical simulations.     

A dynamical systems approach to mixing and segregation of granular materials in tumblers

The physics of granular matter is one of the big questions in science. Granular matter serves as a prototype of collective systems far from equilibrium and fundamental questions remain. At the same time, an understanding of granular matter has tremendous practical importance. Among practical problems, granular mixing and its interplay with segregation is arguably at the top of the list in terms of impact. Granular mixing in three-dimensional systems is complicated, as flow induces segregation by particle size or density. Several approaches and points of view for analysis are possible in principle, ranging from continuum to discrete. Flow and segregation in three-dimensional systems is seemingly complicated; however, to a reasonable approximation, all of the dynamics takes place in a thin flowing surface layer. This observation, coupled with key experimental results, leads to a simple, compact and extensible continuum-based dynamical systems framework applicable to time-periodic flow in quasi-two-dimensional tumblers and three-dimensional systems (such as spheres and cubes) rotated about one or more axes of rotation. The case of time-periodic systems, in its simplest version, can be viewed as a mapping of a domain into itself. The placement of periodic points can be investigated using symmetry concepts; the character of the periodic points and associated manifolds provides a skeleton for the flow and a template for segregation processes occurring in the flow.