Thermodynamic Performance Characteristics of a Brownian Microscopic Heat Engine Driven by Discrete and Periodic Temperature Field

A Brownian microscopic heat engine with a particle hopping on a one-dimensional lattice driven by adiscrete and periodic temperature field in a periodic sawtooth potential is investigated.In order to clarify the underlyingphysical pictures of the heat engine, the heat flow via the potential energy and the kinetic energy of the particles areconsidered simultaneously.Based on describing the jumps among the three states, the expressions of the efficiency andpower output of the heat engine are derived analytically.The general performance characteristic curves are plotted bynumerical calculation.It is found that the power output-efficiency curve is a loop-shaped one, which is similar to onefor a real irreversible heat engine.The influence of the ratio of the temperature of the hot and cold reservoirs and thesawtooth potential on the maximum efficiency and power output is analyzed for some given parameters.When the heatflows via the kinetic energy is neglected, the power output-efficiency curve is an open-shaped one, which is similar to onefor an endroeversible heat engine.

Parameterizations of different hydrometeor spectral relative dispersion in the convective clouds

Spectral relative dispersion of different hydrometeors is vital to accurately describe sedimentation.Here,the Weather Research and Forecasting model with spectral bin microphysics is used to simulate convective clouds in Shouxian of Anhui province in China to study the spectral relative dispersion of different hydrometeors.Firstly,regardless of clean or polluted conditions,the relative dispersion of ice crystal spectra and its volume-mean diameter are negatively correlated,while the relative dispersion of other hydrometeor spectra is positively related to their respective volume-mean diameter.The correlations for cloud droplets and raindrops are affected by the process of collision-coalescence;the correlations for ice crystals,graupel particles,and snow particles could be affected by the deposition,riming,and aggregation processes,respectively.Secondly,relative dispersion parameterizations are developed based on a comprehensive consideration of the relationships between the relative dispersion and volume-mean diameter under both polluted and clean conditions.Finally,the relative dispersion parameterizations are applied to terminal velocity parameterizations.The results show that for cloud droplets,ice crystals,graupel particles,and snow particles,assuming the shape parameter in the Gamma distribution is equal to 0 underestimates the shape parameter and overestimates the relative dispersion;and for raindrops,assuming the shape parameter is equal to 0 is close to the relative dispersion parameterizations.The most appropriate constant shape parameters are recommended for different hydrometeors.The relative dispersion parameterizations developed here shed new light for further optimizing the terminal velocity parameterizations in models.

Microstructure and properties of Al-doped ODS steels prepared by wet-milling and SPS methods

In this paper,15Cr-ODS steels containing 0,1 wt%,2 wt%and 3 wt%Al element were fabricated by combining wet-milling and spark plasma sintering(SPS)methods.The microstructure and mechanical properties of ODS steel were investigated by XRD,SEM,TEM,EBSD and tensile tests.The results demonstrate that the Al addition significantly refines the particle precipitates in the Fe-Cr matrix,leading to the obvious refinement in grain size of matrix and the improvement of mechanical properties.The dispersion particles in ODS steels with Al addition are identified as Al2O3 and Y_(2)Ti_(2)O_(7)nanoparticles,which has a heterogeneous size distribution in the range of 5 nm to 300 nm.Increasing Al addition causes an obvious increase in tensile strength and a decline in elongation.The tensile strength and elongation of 15Cr-ODS steel containing 3 wt%Al are 775.3 MPa and 15.1%,respectively.The existence of Al element improves the corrosion resistance of materials.The ODS steel containing 2 wt%Al shows corrosion potential of 0.39 V and passivation current density of 2.61×10^(−3)A/cm^(2)(1.37 V).This work shows that Al-doped ODS steels prepared by wet-milling and SPS methods have a potential application in structural parts for nuclear system.

基于模拟退火机制的自适应粘性粒子群算法

为了进一步提升粒子群算法在离散优化问题中的性能,针对粘性二进制粒子群算法缺乏全局搜索能力、容易陷入局部最优和收敛速度慢的缺点,提出一种新的自适应参数策略和粒子散度指标,并结合模拟退火机制改善该算法的寻优能力.为了检验算法性能,通过选取不同维数的背包问题算例库以及不同规模的UCI特征选择问题算例库进行仿真实验,并对实验数据进行统计分析.实验以及分析结果表明,所提算法在寻优精度、算法稳定性和收敛速度上均优于对比算法.

Nanoparticle distribution in a rotating curved pipe considering coagulation and dispersion

We study the evolution of the particle number concentration, mass concentration, particle polydispersity, particle diameter and geometric standard deviation considering particle coagulation and dispersion in a rotating curved pipe at different Reynolds number, Schmidt number and F number. It is found that, when the Coriolis force and the centrifugal force point to the same direction, particles concentrate near the outside edge of the pipe, which becomes more obvious as time goes by. The particle number and mass concentration increase faster at the early stage than that at the later stage, and approach a stable value finally. As the coagulation proceeds, the particle diameter, polydispersity and geometric standard deviation increase and have high values in the region close to the outside edge of the pipe. When the Coriolis force and the centrifugal force point to the oppo- site direction and the Coriolis force is more dominant than the centrifugal force, particles concentrate near the inside edge of the pipe. The particles in the region with a high number concentration have high mass concentration, large diameter and high polydispersity as well as large geometric standard deviation. The particle distribution is dependent on the balance of the pipe curvature and rotating speed. The Reynolds number and the Schmidt number have effects on the particle distribution when other parameters remain unchanged. An increase in the Reynolds number leads to an increase in particle number concentration and mass concentration, and a decrease in particle polydispersity, particle diameter and geometric standard deviation. With the increase of Schmidt number the particle number concentration and mass concentration increase, and the particle polydispersity, particle diameter and geometric standard deviation decrease.

Convective Assembly of Linear Gold Nanoparticle Arrays at the Micron Scale for Surface Enhanced Raman Scattering

A convective assembly technique at the micron scale analogous to the writing action of a ＂pipette pen＂ has been developed for the linear assembly of gold nanoparticle strips with micron scale width and millimeter scale length for surface enhanced Raman scattering （SERS）. The arrays with interparticle gaps smaller than 3 nm are hexagonally stacked in the vicinity of the pipette tip. Variable numbers of stacked layers and clean surfaces of the assembled nanoparticles are obtained by optimizing the velocity of the pipette tip. The SERS properties of tile assembled nanoparticle arrays rely on their stacking number and surface cleanliness.

Asymmetrical free diffusion with orientation-dependence of molecules in finite timescales

Using molecular dynamics simulations, we show that free diffusion of a nanoscale particle (molecule) with asymmetric structure critically depends on the orientation in a finite timescale of picoseconds to nanoseconds. In a timescale of ~100 ps, there are ~10% more possibilities for the particle moving along the initial orientation than moving opposite to the orientation; and the diffusion distances of the particle reach ~1 nm. We find that the key to this observation is the orientation-dependence of the damping force to the moving of the nanoscale particle and a finite time is required to regulate the particle orientation. This finding extends the work of Einstein to nano-world beyond random Brownian motion, thus will have a critical role in the understanding of the nanoscale world.

Coarse-Grained Langevin Approximations and Spatiotemporal Acceleration for Kinetic Monte Carlo Simulations of Diffusion of Interacting Particles

Kinetic Monte Carlo methods provide a powerful computational tool for the simulation of microscopic processes such as the diffusion of interacting particles on a surface, at a detailed atomistic level. However such algorithms are typically computationatly expensive and are restricted to fairly small spatiotemporal scales. One approach towards overcoming this problem was the development of coarse-grained Monte Carlo algorithms. In recent literature, these methods were shown to be capable of efficiently describing much larger length scales while still incorporating information on microscopic interactions and fluctuations. In this paper, a coarse-grained Langevin system of stochastic differential equations as approximations of diffusion of interacting particles is derived, based on these earlier coarse-grained models. The authors demonstrate the asymptotic equivalence of transient and long time behavior of the Langevin approximation and the underlying microscopic process, using asymptotics methods such as large deviations for interacting particles systems, and furthermore, present corresponding numerical simulations, comparing statistical quantities like mean paths, auto correlations and power spectra of the microscopic and the approximating Langevin processes. Finally, it is shown that the Langevin approximations presented here are much more computationally efficient than conventional Kinetic Monte Carlo methods, since in addition to the reduction in the number of spatial degrees of freedom in coarse-grained Monte Carlo methods, the Langevin system of stochastic differential equations allows for multiple particle moves in a single timestep.