First-principles analysis of phonon thermal transport properties of two-dimensional WS_2/WSe_2 heterostructures

The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.     

颗粒滚动摩擦系数对堆积特性的影响

为探讨颗粒摩擦系数对堆积特性的影响,利用离散单元法,模拟静摩擦系数固定时,变化滚动摩擦系数对椭球形颗粒堆积角及堆积体的影响.利用自制斜面仪测定了颗粒静摩擦系数,并对滚动摩擦系数与堆积角建立线性方程.结果表明,滚动摩擦系数对颗粒堆积特性有显著影响,颗粒堆积角随滚动摩擦系数增大而增大,边界圆与连续圆间的间隙随滚动摩擦系数增大而减小.依据颗粒堆积过程中旋转动能的变化可以阐述上述结果,建立的滚动摩擦系数与堆积角的线性方程可为具体颗粒物料滚动摩擦系数的获取提供数值测量思路.模拟堆积的过程可为散体物料一些不易测量的参数进行虚拟实验标定.     

A family of single‐isomer,dicationic cyclodextrin chiral selectors for capillary electrophoresis: Mono‐6A‐ammonium‐6C‐butylimidazolium‐β‐cyclodextrin chlorides

The first member of the single‐isomer, dicationic cyclodextrin (CD) family, 6^A‐ammonium‐6^C‐butylimidazolium‐β‐cyclodextrin chlorides (AMBIMCD), has been synthesized, analytically characterized, and used to separate a variety of acidic enantiomers and amino acids by CE. Starting from mono‐6^A‐azido‐β‐cyclodextrin, the cationic imidazolium and ammonium moieties were subsequently introduced onto primary ring of β‐cyclodextrin via nucleophilic addition and Staudinger reaction. The analytically pure AC regio‐isomer CD was further obtained via column chromatography. This dicationic CD exhibited excellent enantioselectivities for selected analytes at concentration as low as 0.5 mM, which were even better than those of its mono‐imidazolium or ammonium‐substitued counterpart CDs at 10 equivalent concentrations. The effective mobilities of all studied analytes were found to decrease with the concentration of AMBIMCD. Inclusion complexation in combination with eletrostatic interactions seemed to account for the enhanced chiral discrimination process.     

Analysis of polished polycrystalline diamond using dual beam focused ion beam microscopy

This paper presents a study on polycrystalline diamond (PCD) polished by dynamic friction polishing (DFP) with the aid of advanced dual beam FIB (focused ion beam) microscopy. After disclosing a variety of wear tracks by DFP using electron imaging in combination with the ion channelling effect, a dual beam FIB was successfully employed at wear track sites to specifically create both the large cross-sectional specimen for microanalysis and thin foil for nanoanalysis. The study concluded that the polished PCD subsurface was free from microscale cracking. However, the attached debris layer on the top surface contained metal oxides and non-diamond carbon phase with inhomogeneous distributions of C, Fe, Cr, Ni, Si and O across the layer. An attached layer directly above a diamond grain was composed of essentially amorphous carbon, suggesting that a direct phase transformation from diamond crystalline to amorphous occurred during DFP.     

Direct Henry reactions with modified calcium oxide as solid catalyst

Commercial CaO was modified simply with benzyl bromide. The modified CaO had good water resistance, and characterization by FTIR and TG revealed the modifier was chemically bonded to the CaO surface. Commercial CaO and CaO modified with benzyl bromide were investigated as catalysts for the Henry reaction between benzaldehyde and nitromethane. It was found that the catalytic activity of the modified CaO was greatly improved, with high conversion of benzaldehyde to the (E)-phenyl nitroolefin and 1-phenyl-2-nitroethanol, and with different selectivity from commercial CaO. The effect of modification and reaction conditions on yield, selectivity, and mechanism were studied thoroughly.     

Dynamics of shock induced ignition in Fickett’s model: Influence of χ

Using Fickett’s model for reactive compressible flows, i.e., the reactive form of Burgers’ equation, we address the problem of shock induced ignition by a piston in a reactive medium characterized by a 2 step induction-reaction kinetics. Owing to the model’s simplicity, the ignition and acceleration mechanism is explained using the two families of characteristics admitted by the model. The energy release along the particle paths provides the amplification of forward-traveling pressure waves. These waves pre-compress the medium in the induction layer ahead of the reaction zone, therefore changing the induction delays of successive particles. The variation of the induction delay provides the modulation of the amplification of the forward traveling pressure waves by controlling the residence time of the pressure waves in the reaction zone. A closed form analytical solution is obtained by the method of characteristics and high activation energy asymptotics. The acceleration of the reaction zone was found to be proportional to the product of the activation energy, the ratio of the induction to reaction time and the heat release. This finding provides a theoretical justification for the previous use of this non-dimensional number to characterize the ignition regimes observed experimentally in detonations and shock induced ignition phenomena. Numerical simulations are presented and analyzed. Both subsonic and supersonic internal flame propagation are observed, consistent with experiment and previous reactive Euler models.     

On-line writing identical and weak fiber Bragg grating arrays

We investigate the on-line writing identical fiber Bragg grating (FBG) arrays using the phase mask technique. Given the limitation of laser power, the energy density uniformity and the horizontal width of the writing spot cannot be further optimized. The results show that the FBG arrays obtained in the optimal process (drawing speed of 12 ± 0.15 m/min and average tension of 38.2 g) have a central wavelength bandwidth of less than 0.1 nm and an average reflectivity of 0.26%. Thus, the phase mask method is a promising alternative for on-line writing identical FBG arrays.On-line writing identical and weak fiber Bragg grating arrays.     

Erdös Rényi随机网络上爆炸渗流模型相变性质的数值模拟研究

基于改进的Newman和Ziff算法以及有限尺寸标度理论, 通过对表征渗流相变特征物理量的序参量、平均集团尺寸、二阶矩、标准偏差及尺寸不均匀性的数值模拟, 分析研究了Erdös Rényi随机网络上Achlioptas爆炸渗流模型的相变性质.研究表明: 尽管序参量表现出了不连续相变的特征, 但序参量以及其他特征物理量仍具有连续相变的幂律标度行为.因此严格地说, Erdös Rényi随机网络中的爆炸渗流相变是一种奇异相变, 它既不是标准的不连续相变, 又与常规随机渗流表现出的连续相变处于不同的普适类. 关键词： Erdös Rényi随机网络 爆炸渗流模型 相变 幂律标度行为     

一级相变磁制冷材料的基础问题探究

由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.