Domain boundaries in silicene:Density functional theory calculations on electronic properties

By using density functional theory(DFT)-based first-principles calculations, the structural stability and electronic properties for two kinds of silicene domain boundaries, forming along armchair edge and zigzag edge, have been investigated. The results indicate that a linkage of tetragonal and octagonal rings(4|8) appears along the armchair edge, while a linkage of paired pentagonal and octagonal rings(5|5|8) appears along the zigzag edge. Different from graphene, the buckling properties of silicene lead to two mirror symmetrical edges of silicene line-defect. The formation energies indicate that the 5|5|8 domain boundary is more stable than the 4|8 domain boundary. Similar to graphene, the calculated electronic properties show that the 5|5|8 domain boundaries exhibit metallic properties and the 4|8 domain boundaries are half-metal.Both domain boundaries create the perfect one-dimensional(1D) metallic wires. Due to the metallic properties, these two kinds of nanowires can be used to build the silicene-based devices.

FREE BOUNDARY VALUE PROBLEM FOR THE CYLINDRICALLY SYMMETRIC COMPRESSIBLE NAVIER-STOKES EQUATIONS WITH DENSITY-DEPENDENT VISCOSITY

In this paper, we investigate the free boundary value problem （FBVP） for the cylindrically symmetric isentropic compressible Navier-Stokes equations （CNS） with density- dependent viscosity coefficients in the case that across the free surface stress tensor is balanced by a constant exterior pressure. Under certain assumptions imposed on the initial data, we prove that there exists a unique global strong solution which tends pointwise to a non-vacuum equilibrium state at an exponential time-rate as the time tends to infinity.

Numerical and theoretical investigations of the effect of the gangue-coal density ratio on the drawing mechanism in longwall top-coal caving

Discrete element calculations of the top-coal drawing process for diferent gangue-coal density ratios were conducted to investigate the efect of the gangue-coal density ratio on the drawing mechanism in longwall top-coal caving.The efects were analyzed for the drawing body,the top-coal boundary,and the recovery of top coal.The results show that for increasing density ratio,the initial drawing body on the goaf side is farther away from the drawing support and its width and volume gradually increase.The upper part of the sickle-shaped drawing body extends near the initial drawing body with increasing density ratio in the normal cycling stage,and the distance from the drawing body to the initial drawing body is its maximum width.The larger the density ratio,the smaller the height of the top coal above the goaf at the end of the initial drawing process.The height of the top-coal boundary decreases with increasing density ratio,until it reaches a limit.In a normal cycle,due to hysteretic development,the top-coal boundary moves toward the goaf until the density ratio is approximately 2.0,which is consistent with the physical experiment results.Finally,increasing the advance length of the working face is benefcial for increasing the overall recovery of top coal.

Effect of dislocation structure evolution on low-angle grain boundary formation in 7050 aluminum alloy during aging

The effect of dislocation structure evolution on low-angle grain boundary formation in 7050 aluminum alloy during aging was studied by using optical microscopy, transmission electron microscopy, and electron backscatter diffraction analysis of misorientation angle distribution, cumulative misorientation and geometrically necessary dislocation （GND） density. Experimental results indicate that coarse spindle-shaped grains with the dimension of 200 μm- 80 μm separate into fine equiaxed grains of 20μm in size as a result of newborn low-angle grain boundaries formed during the aging process. More specifically, the dislocation arrays, which are rearranged and formed due to scattered dislocations during earlier quenching, transform into low-angle grain boundaries with aging time. The relative frequency of 3°-5° low-angle grain boundaries increases to over 30%. The GND density, which describes low-angle grain boundaries with the misorientation angle under 3°, tends to decrease during initial aging. The inhomogeneous distribution of GNDs is affected by grain orientation. A decrease in GND density mainly occurs from 1.83 × 10^13 to 4.40 × 10^11 m^-2 in grains with 〈111〉 fiber texture. This is consistent with a decrease of unit cumulative misorientation. Precipitation on grain boundaries and the formation of a precipitation free zone （PFZ） are facilitated due to the eroding activity of the Graft etchant. Consequently, low-angle grain boundaries could be readily viewed by optical microscopy due to an increase in their electric potential difference.

Electronic Structure of SiC (310) Twin Boundary Doped With B, N, Al and Ti

Doping of boron, nitrogen, aluminum and titanium in the SiC （310） twin boundary was investigated, and the first-principle calculation was used to analyze the underlying mechanism of excellent creep resistance and strength of Sylramic and Tyranno SA SiC fibers. The electronic structures were also analyzed and compared. The results of Mulliken overlap populations, electron density differences and density of states reveal that doping of B or N atom reinforces SiC GBs bonding, however, doping of Al or Ti atom weakens SiC GBs bonding. The reinforced SiC GBs will largely prevent atoms from sliding near GBs. The experimental results would be one of the reasons which lead to the reinforcement of either creep resistance or the strength of SiC fibers.

DISTRIBUTION OF RANDOM ELEMENTS SUBJECTED TO A FLEXIBLE BOUNDARY CONDITION

The probability distribution function of n random elements subjected to the flexible boundary condition is derived. The probability density is a descending curve and converges to a delta function as n tends to infinity. The distribution of the minimum value is discussed in context of ordered statistics.

Density Functional Theory Investigation of Structures and Electronic Spectra of N-protonated Corroles

The geometries and electronic spectra of a series of N-protonated corroles, including unsub- stituted H4Cor＋ and meso-triaryl substituted H4TPC＋, H4TpFPC＋, and H4TdCPC＋, were theoretically studied with density functional theory （DFT）. The results indicate that all these compounds have two conformers, one with C2 symmetry （denoted as Sl） is more stable than the other （denoted as $2, C1 symmetry） by 15.8-18.5 kJ/mol. The corrole macrocycles of these compounds show significant out-of-plane deformation. The enantiomerizations of the chiral S1 conformers were found to be a multi-step process with the $2 conformers as the intermediates. Electronic absorption spectra and electronic circular dichroism （ECD） of these compounds were calculated with time-dependent DFT. In comparison with H4Cor＋, the UV- Vis absorptions of meso-triaryl species are significantly red-shifted and their Q bands are enhanced due to the π-π conjugation between the aryl and corrole rings. Several neighboring electronic transitions were calculated with opposite signs in rotatory strengths, suggesting that ECD spectroscopy may be a useful tool in studying the electronic transitions of these compounds.

Spatially Resolved Analytical Electron Microscopy at Grain Boundaries of α-Al_2O_3

In this work the electronic structure and the impurity excess of the basal and rhombohedral twin grain boundaries are investigated, using electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS). The measurability of electronic structures of the twin grain boundaries are discussed by comparing theoretical density of states (DOS) from bulk material with interfacial DOS, obtained from local density functional theory (LDFT) calculations.

Numerical Simulation on Partially Liquid-Filled Sloshing with Baffle Under Different Density Ratios by the CLSVOF/IB Method

The density and viscosity ratios on partially liquid-filled sloshing with baffle have been investigated numerically in this study.As the key to success in the present simulation,the Coupled Level Set and the Volume of Fluid(CLSVOF)method and the Immersed Boundary(IB)method are used to capture gas/liquid and fluid/structure interfaces,respectively.Within the CLSVOF method,surface normal in weighting factors is calculated by the level set function,resulting in a more accurate solution.Furthermore,the Tangent of Hyperbola for INterface Capturing(THINC)coupled with the Weighted Linear Interface Calculation(WLIC)scheme is used for capturing moving interface.As a standard practice,we first validate the code by comparing it with experimental results of liquid sloshing,which involves large deformation of interface.In addition to the validation study of the present method,the problems of liquid sloshing with baffle are investigated to understand kinematics and dynamics behaviors under different density and viscosity ratios.

New expressions for the surface roughness length and displacement height in the atmospheric boundary layer

An alternative model for the prediction of surface roughness length is developed. In the model a new factor is introduced to compensate for the effects of wake diffusion and interactions between the wake and roughness obstacles. The experiments are carried out by the use of the hot wire anemometry in the simulated atmospheric boundary layer in a wind tunnel. Based on the experimental data, a new expression for the zero-plane displacement height is proposed for the square arrays of roughness elements, which highlights the influence of free-stream speed on the roughness length. It appears that the displacement height increases with the wind speed while the surface roughness length decreases with Reynolds number increasing. It is shown that the calculation results based on the new expressions are in reasonable agreement with the experimental data.

Density Estimation Using Gumbel Kernel Estimator

In this article, our proposed kernel estimator, named as Gumbel kernel, which broadened the class of non-negative, asymmetric kernel density estimators. Such kernel estimator can be used in nonparametric estimation of the probability density function (pdf). When the density functions have limited bounded support on [0, ∞) and they are liberated of boundary bias, always non-negative and obtain the optimal rate of convergence for the mean integrated squared error (MISE). The bias, variance and the optimal bandwidth of the proposed estimators are investigated on theoretical grounds as well as on simulation basis. Further, the applicability of the proposed estimator is compared to Weibull kernel estimator, where performance of newly proposed kernel is outstanding.

Analysis of electron structure ofγ/α_2 phase boundaries in ternary TiAl intermetallics

The electron structure ofγ/α2 phase boundaries in lamellar colonies in Ti-47Al-2M(M=Nb, Cr, V) (mole fraction, %) alloys was theoretically investigated by Empirical Electron Theory of Solid and Molecules (EET) and the bond-length-difference (BLD) method. Average-Atom-Model was employed to calculate valence electron structure of TiAl intermetallics containing site substitution elements. On this basis, the boundary condition of electron movement was employed in the improved Thomas-Fermi-Dirac (TFD) theory to decide the continuity of the electron density of the lamellar colonies interface and it is found that ofγ/α2 interface is continuous(Δρ<10%). Furthermore, it is found that adding alloying elements (including Nb, Cr, and V) can improve the electron density (ρ) ofγ/α2 interfaces, and decrease the electron density difference(Δρ)ofγ/α2 interfaces. Adding V element decreasingΔρis more remarkable than other site substitution elements. According to electron structure study ofγ/α2 interfaces in Ti-47Al-2M alloys, the added elements improve mechanical properties of the alloy in the following order: V>Cr>Nb.

Wildebeest (<i>Connochaetes taurinus</i>) Population Densities and Distribution in Dry and Wet Season in the Kilimanjaro Landscape

The conservation of migratory wildlife species in the savannah habitat can be a challenge because of frequent and prolonged drought and their requirements for a large area. We investigated the performance of the wildebeest (Connochaetes taurinus) population in the 25,624 km2 Kilimanjaro landscape of Kenya and Tanzania, which comprises Amboseli-West Kilimanjaro-Magadi-Natron after 2009 drought. We used total aerial counts to determine the spatial distribution and numbers of wildebeests during wet and dry season in 2010 and 2013. Global Positioning System and digital voice recorders were used to count wildebeests along established transects within blocks. There was an increase in the wildebeest population by 103% during the wet season and 14% during the dry season between 2010 and 2013. The seasonal variation in density occurred between the four counting blocks with Natron and Magadi areas recording the highest densities. Generally, the increase in population could be attributed to the recovery of the population after the 2009 drought. The current cross border collaboration between Kenya and Tanzania in aerial surveys is an important step in the conservation of this migratory species in the landscape. This study demonstrates that detailed knowledge of density and spatial distribution of migratory species is required to plan effective conservation action.

Incompressible Limit of the Oldroyd-B Model with Density-Dependent Viscosity

This paper studies the existence and uniqueness of local strong solutions to an Oldroyd-B model with density-dependent viscosity in a bounded domain Ω ⊂ Rd, d = 2 or 3 via incompressible limit, in which the initial data is “well-prepared” and the velocity field enjoys the slip boundary conditions. The main idea is to derive the uniform energy estimates for nonlinear systems and corresponding incompressible limit.

结合聚类边界采样的主动学习

主动学习是一种机器学习方法,需要选择最有价值的样本进行标注。目前,主动学习在应用时面临着一些挑战,其依赖分类器的先验假设,这容易导致分类器性能意外下降,同时需要一定规模的样本作为启动条件。聚类可以降低问题规模,是主动学习的一种有效手段。为此,结合密度聚类边界采样,开展主动学习方法的研究。针对容易产生分类错误的聚类边界区域,通过计算样本密度,提出一种密度峰值聚类边界点采样方法;在此基础上,给出密度熵的定义,并利用密度熵对聚类边界区域进行启发式搜索,提出一种基于聚类边界采样的主动学习方法。试验结果表明,与文献中的5种主动学习算法相比,该算法能够以更少标记量获得同等甚至更高的分类性能,是一种有效的主动学习算法;在标记不足,无标签样本总量20%的情况下,算法在Accuracy、F-score等指标上取得较好的结果。

聚氨酯型固-固相变储能材料对沥青调温效果的影响研究

聚氨酯型固-固相变储能材料可以有效提高沥青的相变储能能力,从而在调节路面温度的同时,减小温度波动。基于上述背景,针对聚氨酯型固-固相变储能材料对沥青调温效果的影响展开研究。确定相变材料的分类标准,并分别研究其储能原理及具体应用情况,完成对聚氨酯型固-固相变储能材料特性的分析。在此基础上,定义周期性储热边界条件,根据沥青调温过程中内聚能密度的变化形式,推导相变储能材料的能量变化规律,从而确定聚氨酯型固-固相变储能材料对沥青调温效果的影响能力。

熔体旋甩法快速制备热电材料及性能表征

熔体旋甩法结合放电等离子体烧结技术制备了P型化合物CuInTe_(2),采用XRD、LFA、DSC以及LSR-3等技术手段研究不同制备工艺对材料晶体结构与热电性能的影响规律。结果表明:熔体旋甩工艺可快速制备CuInTe_(2)化合物,大大缩短材料制备周期,降低了成本;可实现材料结构低维化增大晶界密度,从而增强对声子的散射作用,显著降低材料的晶格热导率,达到优化热电性能的目的;与传统工艺下的CuInTe_(2)材料的最大zT@815 K相比,熔体旋甩工艺下CuInTe_(2)材料的最大zT@787 K,提高了约20%。

快速热压烧结制备高性能B_(4)C-LaB_(6)复合陶瓷

为了解决碳化硼(B_(4)C)难以烧结致密化和韧性差的问题,以B_(4)C和氧化镧(La_(2)O_(3))为原料,在2000℃和30 MPa下快速热压烧结制备B_(4)C-LaB_(6)复合陶瓷,研究La_(2)O_(3)含量对B_(4)C-LaB_(6)复合陶瓷微观结构和力学性能的影响,分析LaB_(6)的增韧机制。结果表明:w(La_(2)O_(3))=3%的B_(4)C-LaB_(6)复合陶瓷综合力学性能较好,其相对密度、维氏硬度、抗弯强度和断裂韧性分别为99.51%、36.56 GPa、547.43 MPa和4.18 MPa·m^(1/2)。LaB_(6)可细化B_(4)C-LaB_(6)复合陶瓷晶粒,同时提高其相对密度和断裂韧性。LaB_(6)的增韧机制与裂纹偏转和晶粒裂解有关。

基于时均流场的湍流边界层内壁面压力脉动功率谱计算方法研究

针对湍流边界层内壁面压力脉动功率谱的计算问题,该文结合Grasso修订的TNO-Blake解析模型、RANS(Reynolds average Navier-Stokes)时均流场解,发展一种脉动压力计算方法,近壁面附近采用RANS数值解,对于壁面附近湍流各向异性特性,流向和横向异性模型参数采用Stalnov推荐数值,法向异性模型参数采用试验参数,对某平板上一点处的压力脉动功率谱进行了计算,分析了湍流能谱模型、迁移速度等影响,并与Goody模型结果进行对比。研究表明,该文计算方法合理可行,能快速获取壁面压力脉动功率谱,可为工程装备设计的振动噪声分析提供输入。

金刚石基GaN界面热阻控制研究进展

GaN高电子迁移率晶体管(HEMT)在雷达、5G通信、航空航天等领域发挥了重要作用,随着GaN HEMT功率密度的提升,器件热效应显著,散热成为了GaN HEMT性能提升的瓶颈。解决GaN HEMT器件散热问题的有效途径是采用高导热的衬底材料取代现役的SiC、Si衬底。金刚石是目前已知热导率(>2000 W·m^(-1)·K^(-1))最高的材料,高导热金刚石衬底能够全面解决GaN HEMT器件的热效应,成倍提升GaN器件的功率密度。本文阐述了金刚石基GaN的技术优势、主要实现途径,以及界面热阻对金刚石上GaN器件性能的影响。综述了国内外金刚石基GaN界面热阻控制的最新研究进展,分析了金刚石上GaN界面热阻研究过程中面临的主要问题和发展趋势。明确了在介质层材料选择有限的条件下,需要从GaN和金刚石的界面质量入手进一步降低界面热阻,提升GaN器件的性能。