二维声子晶体色散关系的大尺度晶格动力学计算

提出了一种计算大尺度原胞的晶格动力学方法,对光滑硅薄膜的声子色散关系进行了检验.在不考虑声子区域折叠效应的条件下,通过比较超原胞和单原胞模型计算的声子色散关系,证实了所提方法的正确性.运用该方法研究了二维孔状硅声子晶体的色散关系.进一步研究了硅声子晶体色散关系与气孔率和膜厚的关系,结果发现,与相同厚度的光滑硅薄膜相比,孔的存在降低了三支声学的声子群速度;随着气孔率的增加,声子群速度快速降低;而增加薄膜厚度主要增加了法向声子群速度.

Nanofluids Transport Model Based on Fokker-Planck Equation and the Convection Heat Transfer Calculation

In current research about nanofluid convection heat transfer, random motion of nanoparticles in the liquid distribution problem mostly was not considered. In order to study on the distribution of nanoparticles in liquid, nanofluid transport model in pipe is established by using the continuity equation, momentum equation and Fokker-Planck equation. The velocity distribution and the nanoparticles distribution in liquid are obtained by numerical calculation, and the effect of particle size and particle volume fraction on convection heat transfer coefficient of nanofluids is analyzed. The result shows that in high volume fraction （ 0 _-- 0.8% ）, the velocity distribution of nanofluids characterizes as a ＂cork-shaped＂ structure, which is significantly different from viscous fluid with a parabolic distribution. The convection heat transfer coefficient increases while the particle size of nanoparticle in nanofluids decreases. And the convection heat transfer coefficient of nanofluids is in good agreement with the experimental result both in low （0 ~〈 0.1% ） and high （ q = 0.6% ） volume fractions. In presented model, Brown motion, the effect of interactions between nanoparticles and fluid coupling, is also considered, but any phenomenological parameter is not introduced. Nanoparticles in liquid transport distribution can be quantitatively calculated by this model.

Thermal conductivity measurement of InGaAs/InGaAsP superlattice thin films

The thermal conductivities of InGaAs/ InGaAsP superlattices with different period lengths were measured from 100 to 320 K using 3ω method. In this temperature range, the thermal conductivities were found to decrease with an increase in temperature. For the period length-dependant thermal conductivity, the minimum value does exist at a certain period length, which demonstrates that at a short period length, superlattice thermal conductivity increases with a decrease in the period length. When the period is longer than a certain period length, the interface thermal resistance dominates in phonon transport. The experimental and theoretical results confirmed the previous predictions from the lattice dynamics analysis, i.e. with the increase in period length, the dominant mechanisms of phonon transport in superlattices will shift from wave mode to particle mode. This is crucial for the cutoff of the phonons and lays a sound foundation for the design of superlattice structures.

Molecular dynamics simulation of ion transport in a nanochannel

A molecular dynamics (MD) model of the fluidic electrokinetic transport in a nano-scale channel with two bulk sinks was presented, and the process of ion transport in the nanochannel was simulated in this paper. The model consists of two water sinks at the two ends and a pump in the middle, which is different from a single pump model in previous MD simulations. Simulation results show that the charged surfaces of the nanochannel result in the depletion of co-ions and the enrichment of counterions in the nanochannel. A stable current is induced because of the motion of ions when an external electric field is applied across the nanochannel, and the current in the pump region is mainly induced by the motion of counterions. In addition, the ion number in the pump region rapidly decreases as the external electric field is applied. In the equilibrated system, the electrically neutral character in the pump region is destroyed and this region displays a certain electrical character, which depends on the surface charge. The ion distribution is greatly different from the results predicted by the continuum theory, e.g. a smaller peak value of Na+ concentration appears near the wall. The transport efficiency of counterions (co-ions) can be effectively increased (decreased) by increasing the surface charge density. The simulation results demonstrate that the ion distribution in the electric double layer (EDL) of a nanochannel cannot be exactly described by the classical Gouy-Chapman-Stern (GCS) theory model. The mechanism of some special experimental phenomena in a nanochannel and the effect of the surface charge density on the ion-transport efficiency were also explored to provide some theoretical insights for the design and application of nano-scale fluidic pumps.

Poisson-Fokker-Planck model for biomolecules translocation through nanopore driven by electroosmotic flow

A non-continuous electroosmotic flow model(PFP model)is built based on Poisson equation,Fokker-Planck equation and Navier-Stokse equation,and used to predict the DNA molecule translocation through nanopore.PFP model discards the continuum assumption of ion translocation and considers ions as discrete particles.In addition,this model includes the contributions of Coulomb electrostatic potential between ions,Brownian motion of ions and viscous friction to ion transportation.No ionic diffusion coefficient and other phenomenological parameters are needed in the PFP model.It is worth noting that the PFP model can describe non-equilibrium electroosmotic transportation of ions in a channel of a size comparable with the mean free path of ion.A modified clustering method is proposed for the numerical solution of PFP model,and ion current translocation through nanopore with a radius of 1 nm is simulated using the modified clustering method.The external electric field,wall charge density of nanopore,surface charge density of DNA,as well as ion average number density,influence the electroosmotic velocity profile of electrolyte solution,the velocity of DNA translocation through nanopore and ion current blockade.Results show that the ion average number density of electrolyte and surface charge density of nanopore have a significant effect on the translocation velocity of DNA and the ion current blockade.The translocation velocity of DNA is proportional to the surface charge density of nanopore,and is inversely proportional to ion average number density of electrolyte solution.Thus,the translocation velocity of DNAs can be controlled to improve the accuracy of sequencing by adjusting the external electric field,ion average number density of electrolyte and surface charge density of nanopore.Ion current decreases when the ion average number density is larger than the critical value and increases when the ion average number density is lower than the critical value.Our numerical simulation shows that the translocation velocity of DNA given by the PFP model agrees with the experimental,results better than that given by PNP model or PB model.