颗粒分层过程三维离散元法模拟研究

采用软球干接触模型对球形及非球形颗粒的分层过程进行了三维离散元法模拟研究,从颗粒间作用力、转动力矩和能量变化的角度分析了颗粒分层机理,讨论了颗粒的粒度比对分层速度的影响规律.结果表明,分层过程中,大颗粒比小颗粒活跃,非球颗粒由于具有较高的动能而比球颗粒活跃,在一定程度上弥补了颗粒形状对分层过程的影响.大颗粒间的平均法向、切向作用力、平均力矩及平均动能均大于小颗粒.颗粒分层速度随着粒度比的增加而显著增大,当粒度比大于临界粒度比3时,分层速度的增幅减缓.     

The role of nitrogen in the preferential chromium segregation on the ferritic stainless steel (1 1 1) surface

The temperature dependence on the segregation behavior of the ferritic stainless steel single crystal (1 1 1) surface morphology has been examined by scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). AES clearly showed the surface segregations of chromium and nitrogen upon annealing. Nanoscale triangular chromium nitride clusters were formed around 650 °C and were regularly aligned in a hexagonal configuration. In contrast, for the ferritic stainless steel (1 1 1) surface with low-nitrogen content, chromium and carbon were found to segregate on the surface upon annealing and Auger spectra of carbon displayed the characteristic carbide peak. For the low-nitrogen surface, LEED identified a facetted surface with (2 × 2) superstructure at 650 °C. High-resolution STM identified a chromium carbide film with segregated carbon atoms randomly located on the surface. The facetted (2 × 2) superstructure changed into a (3 × 3) superstructure with no faceting upon annealing at 750 °C. Also, segregated sulfur seems to contribute to the reconstruction or interfacial relaxation between the ferritic stainless steel (1 1 1) substrate and chromium carbide film.     

Element segregation on the surfaces of pure aluminum foils

The surface segregation trend of trace elements in pure aluminum foils was investigated by density functional theory. The model of nine-layer Al(1 0 0) slab substituted partially by trace element atoms was proposed for calculating surface segregation energy. The calculating results show that (i) B, Mg, Si, Ga, Ge, Y, In, Sn, Sb, Pb and Bi exhibit negative segregation energy and possibly move to the surface, while Be, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zr exhibit positive segregation energies and migrated into the bulk; (ii) the segregation energy was found to be related with the covalent radius, the relaxed position at the surface of the substituting atom and the surface energy; (iii) the segregation behavior of trace element generates lots of defects and dislocation, which can increase the initial pitting nucleation sites in the surface of aluminum foils; (iv) the impurity atom concentration was tested with Pb-doped surfaces, the calculated negative segregation energies in all coverage increases rapidly with the Pb coverage. These conclusions are helpful for designing of the chemical composition and to advance the tunnel etching of aluminum foils.     

Direct observation of surface and internal phase-separated structure of the active layer buried in organic photovoltaic cells

A phase-separated structure of the active layer, of variable thickness, buried in organic thin film solar cells (OTSC) was directly observed by scanning force microscopy (SFM) with the aid of a surface and interface cutting analysis system (SAICAS). This deals with SFM observation to both the surface and the internal regions of the OTSCs, leading to discussion about the formation of the overlayer in the active layer.     

Highly nonlinear large-competition limits of elliptic systems

This article is concerned with the effect of different types of competitive interaction terms on the large-interaction limit of nonlinear elliptic systems modelling the steady states of populations that compete in some region. As the competition rate tends to infinity, we show that non-negative solutions of quite simple-looking systems converge to the positive and negative parts of a solution of a scalar limit problem which may be much more strongly nonlinear than the original system, possibly with quadratic growth in the gradient of the limit function.     

Simulation of high fluence fast neutron damage with heavy ion bombardment

316 stainless steel has been irradiated with 5 MeV Cu ions to a fluence of 2 × 10¹⁶ ions/cm² at 500°C. Transmission electron microscopy of this sample reveals that 6 × 10¹⁵ voids/cm² of average diameter equal to 180 Å were produced. A method for correlating the fluence of ions with equivalent neutron fluences is described. This method predicts that the Cu bombardment in this study should produce a microstructure similar to that found in steel irradiated with 2–5 × 11²² neutrons/cm². A comparison of the ion produced voids with those found after previous neutron irradiation experiments confirms this prediction.     

A deductive approach to friendship networks

Based on a simple actor model, I derive theory concerning satisfied networks of close, reciprocal friendships. By satisfied networks I mean networks of actors who would be unhappier with more or fewer close friends. Deductions concern characteristics of such networks, such as that a satisfied network component (connected subnetwork) containing an actor with only one friend must be a dyad. I show that the model of satisfied friendship networks fits eight empirical friendship networks significantly better than a random model. Finally, I show the theory's usefulness by making five additional predictions, including two involving intervention.     

A Markov Model of Referral-Based Hiring and Workplace Segregation

I construct a Markov model of referral hiring to look at two issues: inequality between groups and workplace segregation. The model differs from most models of referral hiring in that it explicitly considers a firm structure where employment opportunities arise. The model suggests that referral hiring does not directly produce inequality between groups at the population level, even though firms have a bias toward hiring workers from the same group they already employ. The result highlights a difference between outcomes of referral hiring at the population and individual levels in that individual firm biases balance out when aggregated. However, referral hiring does produce segregation of groups across firms in a given industry, and the degree of segregation monotonically increases in the amount of referral hiring.     

Ethnic Preferences,Social Distance Dynamics,and Residential Segregation: Theoretical Explorations Using Simulation Analysis∗

In this paper I consider theories of residential segregation that emphasize social distance and ethnic preference dynamics. I argue that these theories are more compelling than many critics have supposed, and I conclude that they deserve to be considered more carefully. I then use simulation methodology to assess the potential impact of social distance and ethnic preference dynamics on ethnic segregation under certain theoretically interesting conditions. Based on the results from the simulation analyses, I offer three conclusions: (1) status preferences and status dynamics have the capacity to produce high levels of status segregation but do not produce high levels of ethnic segregation under the specified simulation conditions; (2) ethnic preferences can, under certain theoretically interesting conditions specified in these simulations, produce high levels of ethnic segregation in the absence of housing discrimination; and (3) ethnic preferences and social distance dynamics can, when combined with status preferences, status dynamics, and demographic and urban-structural settings common in American cities, produce highly stable patterns of multi-group segregation and hyper-segregation (i.e., high levels of ethnic segregation on multiple dimensions) of minority populations. Based on these model-based theoretical explorations I speculate that the persistence of segregation in recent decades may have been overdetermined, that is, it may have been sustained by multiple sufficient causes including not only discrimination, but also social distance and preference dynamics. This raises the possibility that reductions in housing discrimination may not necessarily lead to large declines in ethnic segregation in the short run because social distance and preference dynamics may be able to sustain ethnic segregation at surprisingly high levels in the absence of housing discrimination.