Spectral theory of the turbulent mean-velocity profile

It has long been surmised that the mean-velocity profile (MVP) of pipe flows is closely related to the spectrum of turbulent energy. Here we perform a spectral analysis to identify the eddies that dominate the production of shear stress via momentum transfer. This analysis allows us to express the MVP as a functional of the spectrum. Each part of the MVP relates to a specific spectral range: the buffer layer to the dissipative range, the log layer to the inertial range, and the wake to the energetic range. The parameters of the spectrum set the thickness of the viscous layer, the amplitude of the buffer layer, and the amplitude of the wake.     

Aggregation and chimney formation during the solidification of ammonium chloride

Experiments study large-scale pattern formation during the growth of ammonium chloride (NH4Cl) from solution in a thin (Hele-Shaw) geometry. In particular a solid-liquid mixture ("mushy layer") forms in which growing solid NH4Cl crystals form a solid network interspersed with liquid. There are different ways that the mushy layer can be formed, however. If the cell is heated from below and cooled from above, thermal convection generates large-scale recirculating flows that carry seed crystals from the upper (cold) boundary to the (warmer) side and bottom boundaries. Ballistic deposition of these seed crystals leads to aggregation patterns with significant voids (filled with liquid) with a wide range of length scales. If the cell is cooled from below with a warm environment, the solid NH4Cl grows dendritically without deposition, resulting in a compact mushy layer. Plume convection within this mushy layer produces one or two well-defined "chimneys." If the environment is cool (comparable to the liquidus temperature of the solution), the mushy layer forms by a combination of dendritic growth and ballistic deposition, resulting in a more permeable mushy layer and enhanced chimney formation. The effects of ballistic deposition are enhanced if the cell is tipped, in which case the voids reappear. Plume convection and chimney formation are dramatically enhanced in this case. Additional experiments are done in which fluid flows in the system are enhanced artificially to verify that enhancements in chimney formation are due primarily to the aggregation process, and not to the increases in fluid flows due to thermal and compositional convection.     

Superdiffusion in nearly stratified flows

In classical work, Mathéron and the Marsilly showed that superdiffusive scaling of mean-square displacements occurs in transport diffusion for stratified flows with steady simple shear layers and long-range spatial correlations. More recently the authors have calculated a formula for the non-Gaussian large-scale long-time renormalized Green function for these problems. Here the scaling laws and renormalized Green functions for diffusion in nearly stratified flows are studied; in such flows the simple shear layer with long-range correlations is perturbed by incompressible flows with short-range correlations. Here it is established that these flows belong to the same universality class as the simple shear layers, with a renormalized Green function with a similar structure but reflecting homogenization by the transverse displacements. The tools in the analysis involve a modification of homogenization theory and also rigorous diagrammatic perturbation theory.     

Ti,N共掺杂4H-SiC复合增强缓冲层生长及其对PiN二极管正向性能稳定性的改善

"双极型退化"现象严重阻碍了4H-SiC双极型器件如PiN二极管等的产品化,其微观机理是电子-空穴复合条件下层错由基面位错处的扩展.为遏制"双极型退化"现象,不仅要消除漂移层中的基面位错,还需要通过生长复合增强缓冲层的方法阻止少子空穴到达含高密度基面位错片段的外延层/衬底界面.本文采用钛、氮共掺杂的方式进行缓冲层的生长...     

The effects of sapphire nitridation on GaN growth by metalorganic chemical vapour deposition

In situ optical reflectivity measurements are employed to monitor the GaN epilayer growth process above a low-temperature GaN buffer layer on a c-plane sapphire substrate by metalorganic chemical vapour deposition. It is found that the lateral growth of the GaN islands and their coalescence are promoted in the initial growth stage if optimized nitridation time and temperature are selected when the substrate is pre-exposed to ammonia. As confirmed by atomic force microscopy observations, the quality of the GaN epilayers is closely dependent on the surface morphology of the nitridated buffer layer, especially grain size and nucleation density.     

Equilibrium Shape of Copper Crystals Grown on Sapphire

The equilibrium crystal shape (ECS) of copper has been studied by scanning electron microscopy on m-sized copper crystallites supported on single-crystals of -alumina. In addition, the orientation relationships between copper crystals and the sapphire substrate were investigated by X-ray techniques. A detailed discussion of the kinetic factors that can inhibit equilibration is provided, and it is shown that only crystals ranging in radius from 3 to 4.5 m can achieve equilibrium shapes under the conditions of the experiment. The maximum anisotropy of surface energy was found to be about 1.02, which is significantly lower than that of the other two fcc metals (lead and gold) for which reliable data are available. Another distinction between copper and those other fcc metals is that its ECS displays {110} facets, and possibly {311} facets, in addition to the commonly observed {111} and {100} facets, at temperatures where equilibration is possible. The observed facets connect tangentially to the curved parts of the ECS, so that all possible surface orientations are present on the copper ECS.     

纳米尺度下Si/Ge界面应力释放机制的分子动力学研究

采用分子动力学方法研究了纳米尺度下硅(Si)基锗(Ge)结构的Si/Ge界面应力分布特征,以及点缺陷层在应力释放过程中的作用机制.结果表明:在纳米尺度下, Si/Ge界面应力分布曲线与Ge尺寸密切相关,界面应力下降速度与Ge尺寸存在近似的线性递减关系;同时,在Si/Ge界面处增加一个富含空位缺陷的缓冲层,可显著改变Si/Ge界面应力分布,在此基础上对比分析了点缺陷在纯Ge结构内部引起应力变化与缺陷密度的关系,缺陷层的引入和缺陷密度的增加可加速界面应力的释放.参考对Si/Ge界面结构的研究结果,可在Si基纯Ge薄膜生长过程中引入缺陷层,并对其结构进行设计,降低界面应力水平,进而降低界面处产生位错缺陷的概率,提高Si基Ge薄膜质量,这一思想在研究报道的Si基Ge膜低温缓冲层生长方法中初步得到了证实.     

Neutrino trapping in braneworld extremely compact stars

Extremely Compact Stars (ECS) contain trapped null geodesics. When such objects enter the evolution period admitting geodetical motion of neutrinos, certain part of neutrinos produced in their interior will be trapped influencing their neutrino luminosity and thermal evolution. We study neutrino trapping in the braneworld ECS, assuming uniform distribution of neutrino emissivity and massless neutrinos. We give the efficiency of the neutrino trapping effects in the framework of the simple model of the internal spacetime with uniform distribution of energy density, and external spacetime described by the Reissner-Nordström geometry characterized by the braneworld “tidal” parameter b. For b < 0 the external spacetime is of the black-hole type, while for b > 0 the external spacetime can be of both black-hole and naked-singularity type. Then the ECS surface radius R can be located also above the unstable (outer) photon circular orbit. Such basically new types of the spacetimes strongly alter the trapping phenomena as compared to the standard case of b = 0. It is shown that the neutrino trapping effects are slightly lowered by the presence of physically more plausible case of b < 0, as compared to the standard internal Schwarzschild spacetime, while they can be magnified by positive tidal charges if b < 1 and lowered for b > 1. However, potential astrophysical relevance of the trapping phenomena is strongly enhanced for negative tidal charges enabling a significant enlargement of the ECS surface radius to values coherent with recent observations.     

Control growth of catalyst-free high-quality ZnO nanowire arrays on transparent quartz glass substrate by chemical vapor deposition

ZnO nanowire arrays have been successfully synthesized on transparent quartz glass substrate by chemical vapor deposition technique. Our work demonstrates the critical role of the growth temperature and the buffer layer on the effective control of the morphology of ZnO nanowires. A proper growth temperature and the thicker buffer layer could promise the good alignment and high density of the nanowires. The room-temperature photoluminescence spectrum shows that the buffer layer has also great effects on optical properties of ZnO nanowire arrays. The integrated intensity ratio [I_UV/I_{Visible band}] of the ZnO UV emission peak to visible band emission decreases with the increase of the thickness of the buffer layers. The obtained nanowire arrays have transmittance of above 50% in the visible region.     

利用Ag_2O/PEDOT:PSS复合缓冲层提高P3HT:PCBM聚合物太阳能电池器件性能的研究

利用Ag2O/PEDOT:PSS(聚(3,4-乙撑二氧噻吩):聚苯乙烯磺酸盐)作为复合阳极缓冲层,制备了P3HT:PCBM(聚(3-已基噻吩):富勒烯衍生物)聚合物太阳能电池器件,并通过改变氧化银插入层的厚度来分析复合缓冲层对器件性能的影响.实验发现,具有阳极缓冲层修饰的器件在退火处理后,光伏性能得到了改善.相比于单一PEDOT:PSS缓冲层的器件,Ag2O/PEDOT:PSS复合缓冲层可以增大器件的短路电流密度和外量子效率,使器件效率得到提高.分析表明,退火处理可以有效改善活性层的薄膜形貌,增加光的吸收和激子的解离,而较薄氧化银的引入,可以有效降低阳极处空穴的输运势垒,提高器件空穴收集效率,并能充当化学间隔层,提高器件光伏性能和稳定性.     

Effect of high-temperature buffer thickness on quality of AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition

The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm.     

缓冲层对量子阱二能级系统中电子子带间跃迁光吸收的影响

由于ZnO缓冲层对纤锌矿ZnO/Mg_xZn_(1-x)O有限深单量子阱结构左垒的限制作用,导致阱和右垒的尺寸、Mg组分值等因素将影响系统中形成二能级.本文考虑内建电场、导带弯曲及材料掺杂对实际异质结势的影响,利用有限差分法数值求解Schr?dinger方程,获得电子的本征能级和波函数,探讨ZnO缓冲层对此类量子阱形成二能级系统的尺寸效应及三元混晶效应的影响;利用费米黄金法则探讨缓冲层、左垒、阱及右垒宽度和三元混晶效应对此类量子阱电子子带间跃迁光吸收的影响.计算结果显示:对于加入ZnO缓冲层的ZnO/Mg_xZn_(1-x)O有限深单量子阱二能级系统,左垒宽度临界值会随着阱宽和Mg组分值的增大而逐渐减小,随着右垒宽度和缓冲层厚度的增大而逐渐增大;量子阱中电子子带间跃迁光吸收峰会随着左垒、右垒尺寸以及Mg组分的增大发生蓝移,随着阱宽增大而发生红移.本文所得结果可为改善异质结器件的光电性能提供理论指导.     

Kolmogorov scaling of turbulent flow in the vicinity of the wall

How to scale even the simplest of turbulent flows continues to be a cause for considerable controversy. In the present research, a data base compiling results from channel flow direct numerical simulations and turbulent boundary layer experiments is employed to investigate the properties of shear and normal Reynolds stresses very close to the wall. Two types of scaling based on Kolmogorov length and velocity scales are analyzed. It is shown that it is highly likely that large length scales of the order of the channel half-width or the boundary layer thickness play an important role even in the innermost regions of wall-bounded turbulent flows, which hints at the persistence of Reynolds number effects in even high Reynolds number flows.     

Nanoparticle aggregation due to dewetting of sandwiched buffer layers

The deposition of Au onto thin condensed volatile buffer layers produces small clusters. Sublimation of the buffer converts these clusters into compact or ramified structures, depending on the thickness of the buffer, in a process called buffer-layer-assisted growth. We have used bilayer structures of Xe on CO₂ or Xe on H₂O on amorphous carbon substrates to investigate effects of second layer dewetting and the impact of the initial particle size on aggregation. Compact particles formed by Xe desorption aggregate during removal of the CO₂ or H₂O layer but little aggregation occurs for ramified particles produced on Xe layers thicker than 100 ML. Instead, the large structures tend to break up on the CO₂ film, producing smaller, more compact particles. CO₂ and H₂O impurities in the Xe film significantly reduce particle coalescence and accelerate Xe dewetting.     

Deformation of AlGaN/GaN superlattice layers according to x-ray diffraction data

Three-crystal x-ray diffractometry is used for structural studies of nitride AlGaN/GaN superlattices (SLs) grown by metal-organic chemical vapor deposition on sapphire with GaN and AlGaN buffer layers with widely varied SL period (from 50 to 3500 Å), Al content in Al_x Ga_1?x N layers (0.1≤x≤0.5), and buffer layer composition. Satellite peaks characteristic of SLs are well pronounced up to the third order in θ-2θ scans of symmetric Bragg reflections and θ scans of the symmetric Laue geometry. The corresponding curves are well modeled by kinematic formulas. The average SL parameters, as well as the thickness, composition, and strain of individual layers, are determined using a combination of symmetric Bragg and Laue reflections. It is shown that all the samples under study are partially relaxed structures in which the elastic stresses between the entire SL and the buffer layer, as well as between individual layers, are relaxed. The AlGaN layers are stretched and the GaN layers are compressed. The GaN layer compression is larger in magnitude than the AlGaN layer tension because of thermoelastic stresses.     

Influences of Different Architectures on the Thermodynamic Performance and Network Structure of Aircraft Environmental Control System

The environmental control system (ECS) is one of the most important systems in the aircraft used to regulate the pressure, temperature and humidity of the air in the cabin. This study investigates the influences of different architectures on the thermal performance and network structure of ECS. The refrigeration and pressurization performances of ECS with four different architectures are analyzed and compared by the endoreversible thermodynamic analysis method, and their external and internal responses have also been discussed. The results show that the connection modes of the heat exchanger have minor effects on the performance of ECSs, but the influence of the air cycle machine is obvious. This study attempts to abstract the ECS as a network structure based on the graph theory, and use entropy in information theory for quantitative evaluation. The results provide a theoretical basis for the design of ECS and facilitate engineers to make reliable decisions.     

The role of a ZnO buffer layer in the growth of ZnO thin film on Al2O3 substrate

A ZnO buffer layer and ZnO thin film have been deposited by the pulsed laser deposition technique at the temperatures of 200 C and 400 C, respectively. Structural, electrical and optical properties of ZnO thin films grown on sapphire (Al₂O₃) substrate with 1, 5, and 9 nm thick ZnO buffer layers were investigated. A minute shift of the (101) peak was observed which indicates that the lattice parameter was changed by varying the thickness of the buffer layer. High resolution transmission electron microscopy (TEM) was used to investigate the thickness of the ZnO buffer layer and the interface involving a thin ZnO buffer between the film and substrate. Selected area electron diffraction (SAED) patterns show high quality hexagonal ZnO thin film with 30 in-plane rotation with respect to the sapphire substrate. The use of the buffer can reduce the lattice mismatch between the ZnO thin film and sapphire substrate; therefore, the lattice constant of ZnO thin film grown on sapphire substrate became similar to that of bulk ZnO with increasing thickness of the buffer layer.     

Scrape-off layer width set by non-linear streamer flows in drift wave turbulence

In this work, we discuss the physics behind the excitation of non-linear streamer flows in drift wave turbulence and how to selectively excite these flows. Streamer flows are modelled as a non-linear, radially elongated convective cell in drift wave turbulence. It is shown that density modulation is key for exciting streamer flows. We show that streamer flows have a finite frequency, albeit smaller than that of drift waves. Streamers propagate in the ion direction. These theoretical predictions are compared against experimental data, which shows reasonable agreement. Finally, the scrape-off layer width set by streamer flows is calculated, and a scaling law against macroscopic plasma parameters is obtained.     

Spatial resolution analysis of planar PIV measurements to characterise vortices in turbulent flows

The effects of spatial resolution of planar particle image velocimetry (PIV) on vortex size, swirling strength, circulation and population density characterisation are analysed using a series of experimental and numerical databases. The databases comprise a PIV database of an adverse-pressure-gradient turbulent boundary layer (APG TBL), a PIV database of a zero-pressure-gradient (ZPG) TBL in streamwise-wall-normal planes and streamwise-wall-normal slices of a direct numerical simulation (DNS) of a ZPG TBL. The effects of interrogation window and mesh sizes on the vortex parameters are analysed in the outer region of these flows using different qualitative and quantitative approaches. The quantitative analysis mainly capitalises on the possibility of mimicking the PIV data-sets with the DNS one. These approaches allow us to not only isolate the effects of mesh size and the interrogation window size but also to deduce the combined effects of other measurement errors in PIV. Typical values of mesh size and interrogation window size (0.01–0.03 of the boundary layer thickness) and typical levels of measurement uncertainties have significant effects on the vortex parameters. Moreover, each PIV error source affects the vortex parameters in different and frequently opposite manners. Hence, an optimal selection of measurement parameters such as the interrogation window size is indispensable in order to minimise the effects of spatial resolution and other measurement errors on the vortex parameters. Guidelines are presented in the Conclusions section of this paper. Finally, it is found that all the vortex parameters, when averaged across the outer region, are reasonably comparable in the ZPG and APG TBLs despite the fact that these are very different flows.     

F16CuPc作为阳极缓冲层对有机太阳能电池性能的显著改善

采用F₁₆CuPc作为有机太阳能电池的阳极缓冲层可使器件的性能得到显著提高。F₁₆CuPc的引入,一方面可以实现CuPc分子的定向生长,从而改善CuPc薄膜的结晶度,提高其空穴迁移率;另一方面在F₁₆CuPc/CuPc界面处可形成偶极层,改善空穴的输出效率。以上两个作用有效提高了器件的载流子收集效率,降低了器件的串联电阻和光生载流子复合几率,从而提高了器件的短路电流和填充因子。同时,F₁₆CuPc的引入使器件的内建电场增大,提高了器件的开路电压。