Numerical simulation of an energy deposition zone in quiescent air and in a supersonic flow under the conditions of interaction with a normal shock

The potential of using the Euler equations to numerically simulate the evolution of localized energy deposition zones interacting with a normal shock in quiescent air and in a supersonic channel flow is demonstrated. Simulation results are compared with available experimental data for an optical discharge in quiescent air and with results calculated for a supersonic flow using the Navier-Stokes equations with allowance for real gas effects. The possibility of predicting gasdynamic effects using the T- and q-models of energy deposition for perfect gas is justified. The variation of the gasdynamic structure and flow parameters near an energy deposition zone developing in a quiescent medium and interacting with a normal shock is analyzed in detail for different energy deposition powers.     

Yttrium oxide thin films prepared under different oxygen-content atmospheres: microstructure and optical properties

Yttrium oxide films were prepared on silicon wafers by reactive magnetron sputtering at different oxygen flow rates to investigate the microstructure and optical properties for desirable planar optical waveguiding applications. Under the different conditions of target surface, the deposition rate, structure, and optical properties have great changes. The deposition rate increases to the maximum, and then monotonically decreases. Oxygen content in films increases and the composition of films transforms from stoichiometry to an oxygen-rich state. Y₂O₃ films grow with the (222) preferred growth orientation at low oxygen flow rate, and then turn into the (622) orientation; the microstructure evolves from crystal structure to amorphous state as oxygen flow rate increases from 2 to 12 sccm. Sufficient oxygen makes films low-order structure and oxygen-rich films have poor crystallinity. Very smooth film has been obtained at oxygen flow rate of 12 sccm. The refractive index can be greatly modulated by the oxygen-content factor. It is convincing that the controllable structure and optical properties of Y₂O₃ films can be achieved by adjustment the oxygen flow rate for desired optical design and applications.     

Fluid simulation and X-ray CT images for soot deposition in a diesel filter

Recently, stricter diesel particulate emission standards have been set in many countries. As for the after-treatment of exhaust gas, a diesel filter has been developed to trap diesel particles inside small-scale porous structure. Since measurement of flow in the filter is impossible, the phenomena of particle deposition in the filter are not well understood. In this study, we conducted Lattice Boltzmann simulation on flow in the newly developed diesel filter. The soot deposition was included to consider the particle trap in the filter. The inner structure of the diesel filter as well as trapped soot region was scanned by an X-ray CT technique. Results show that the flow pattern is largely changed when the soot is attached to the filter surface. By comparing simulation results with CT images, soot accumulation region is well predicted. It is found that the amount of trapped soot is proportional to the filter back-pressure even when soot deposition probability is changed.     

Phase composition of nanocrystalline iron films deposited in a nitrogen atmosphere

The phase composition of iron films prepared by pulsed-plasma deposition in a controlled nitrogen atmosphere is investigated by Mössbauer spectroscopy. The observed changes in the phase composition are dictated by the nanocrystalline structure of the samples and the dynamics of the substrate temperature during film deposition.     

In situ oxygen plasma cleaning of a PECVD source for hard disk overcoats

Reducing the thickness of the diamond-like carbon protective overcoat to a thickness of about 2–3 nm is one major key to increase the recording density of magnetic disk drives. Plasma-enhanced chemical vapor deposition (PECVD) deposited carbon layers have been shown to be denser and harder than those produced by conventional sputter deposition. One key problem of PECVD deposited carbon is the contamination of the carbon film by particles produced inside the carbon source after long-time operation. This particle production limits the runtime of the source drastically. To avoid this particle generation the source was cleaned by an intermittent in situ oxygen plasma process. The cleaning efficiency was investigated by recording the pressure change inside the source during the cleaning process caused by the CO production. The ratio of the cleaning time and the deposition time shows no significant dependence on the deposition time. An almost linear increase of the ratio with the acetylene flow was observed. This results from a higher deposition rate at higher acetylene flow, leading to a higher contamination inside the source. A strong dependence of the cleaning rate on the oxygen flow in the cleaning process was measured. More oxygen leads to a strong decrease of the needed cleaning time. Adding Ar gas to the oxygen discharge shows no improvement of the needed cleaning time. The cleaning process seems to be dependent only on the amount of reactive oxygen species in the discharge . PACS 62.20.Qp; 52.50.Dg; 52.77.Bm; 52.80.Pi     

The relationship between the macroscopic properties of PECVD silicon nitride and oxynitride layers and the characteristics of their networks

Silicon nitride and oxynitride films have been deposited on silicon wafers using plasma-enhanced chemical vapour deposition. Various amounts of ammonia, silane and nitrous oxide gases were applied at fixed total gas flow and at the same deposition temperature. The dependence of the macroscopic properties of the layers such as refractive index, internal stress and etch rate on the reaction atmosphere during deposition has been demonstrated. The chemical structure of amorphous layers was studied using infrared spectroscopy. The network was found to be characterised by SiN_xO_yH_z tetrahedra, joined to each other by common corners. The characteristic vibrational bands due to species that join tetrahedral units (N(-Si≡)₃, ≡Si-N-Si≡, ≡Si-O-Si≡) and species that stop this interconnection (Si-H, N-H) were determined and discussed with reference to the corresponding species available during deposition. The analysis resulted in the determination of the relationship between the chemical structure of the network and the layer’s refractive index, internal stress and etch rate. Received: 24 July 2000 / Accepted: 30 May 2001 / Published online: 30 August 2001     

叶顶间隙对冲压叶轮激波结构的影响

本文采用数值模拟方法研究了带有叶顶间隙的冲压叶轮内部流场结构,分析了叶顶间隙泄漏流对冲压叶轮内部激波波系的影响。研究表明,由于叶顶间隙泄漏流的影响,在叶片展向高度85%以上的区域,冲压叶轮通道内的激波波系受到较大的影响,甚至导致叶尖区域的激波波系完全被泄漏流的影响破坏,并在叶片吸力面形成一道斜激波,形成了新的激波波系,并且由于泄漏流的存在使得流动损失增加。     

Influence of total gas flow rate on microcrystalline silicon films prepared by VHF-PECVD

Hydrogenated microcrystalline silicon (\mu c-Si:H) films are fabricated by very high frequency plasma enhanced chemical vapour deposition (VHF-PECVD) at a silane concentration of 7% and a varying total gas flow rate (Hk₂+SiHk₄). Relations between the total gas flow rate and the electrical and structural properties as well as deposition rate of the films are studied. The results indicate that with the total gas flow rate increasing the photosensitivity and deposition rate increase, but the crystalline volume fraction (Xk_c) and dark conductivity decrease. And the intensity of (220) peak first increases then decreases with the increase of the total gas flow rate. The cause for the changes in the structure and deposition rate of the films with the total gas flow rate is investigated using optical emission spectroscopy (OES).     

高速微晶硅薄膜沉积过程中的等离子体稳态研究

通过光发射光谱监测高速沉积微晶硅薄膜过程中I(H_α^*)/I(SiH^*) 随沉积时间的变化趋势, 分析高速率微晶硅薄膜纵向晶化率逐渐增大的原因. 通过氢稀释梯度法, 即硅烷浓度梯度和氢气流量梯度法来改善材料的纵向均匀性.结果表明: 硅烷浓度梯度法获得的材料晶化率从沉积300 s时的53%增加到沉积600 s时的62%, 相比于传统方式下纵向晶化率从55%到75%的变化有了明显的改善. 在硅烷耗尽的情况下, 增加氢气流量一方面增加了气体总流量, 使得电子碰撞概率增加, 电子温度降低, 从而降低氢气的分解, 抑制SiH_x基团的放氢反应, 同时背扩散现象也得到了一定的缓解, 使得I(H_α^*)/I(SiH^*) 在沉积过程中逐渐增加的趋势有所抑制, 所制备的材料的纵向晶化率在240 s 后维持在53%-60%范围内, 同样改善了薄膜的纵向结构. 关键词： 光发射光谱 高速沉积 微晶硅 纵向结构均匀性     

Dissipation in the dynamics of a moving contact line: effect of the substrate disorder

We investigate the time-dependent flow of water around a solid triangular profile oscillating horizontally in a narrow rectangular container. The flow is quasi two-dimensional and using particle image velocimetry we measure 20 snapshots of the entire velocity field during a period of oscillation. From the velocity measurements we obtain the circulation of the vortices and study the vortex dynamics. The time-dependence of the flow gives rise to the formation of a jet-like flow structure which enhances the vorticity production compared to the time-independent case. We introduce a simple phenomenological model to describe the important dynamical parameters of the flow, i.e., the vortex circulation and the jet velocity. We solve the model analytically without viscous damping and find good agreement between the model predictions and our measurements. Our work adds to the recent effort to understand more complicated flows past sand-ripples and insect wings.Received: 6 January 2004, Published online: 20 April 2004PACS: 47.32.Cc Vortex dynamics - 47.32.Ff Separated flows     

Application of Mie theory and fractal models to determine the optical and surface roughness of Ag–Cu thin films

Thin films of Ag/Cu were deposited by reactive DC magnetron sputtering on (001)-oriented Si and glass substrates for various deposition times (4–24 min). These films were characterized by atomic force microscopy (AFM), and a power law scaling was performed on the obtained micrographs to investigate the self-affine nature of the sample morphology, which is indicative of a fractal structure. We applied the Higuchi’s algorithm to the AFM data to determine the fractal dimension of each sample, and the Hurst exponents were computed. The deposition time dependences of these parameters and the grain size distributions estimated from the UV–visible spectra using the Mie theory, allowed us to describe a particle formation mechanism during the deposition process, in which the length of continuous paths of conductive particles increases as the deposition time is increased. In agreement with this explanation, the electrical resistance decreased with the increment of the deposition time.     

Transport properties of a ferroelectric tunnel junction in bilayer ferroelectric/manganite structures

This paper presents the results of an experimental investigation of current flow through a ferroelectric nanolayer in a series of bilayer ferroelectric/manganite structures prepared by metalorganic aerosol deposition on a MgO substrate. It has been shown that a variation in the thickness of the ferroelectric layer in a bilayer ferroelectric/manganite structure leads to a change in the metal-insulator phase transition temperature of the manganite layer and also affects the transport properties of the ferroelectric/manganite structure. Based on the analysis of the experimental results, it is demonstrated that the ferroelectric layer up to 4 nm thick exhibits polarization properties. In the theoretical analysis, it is assumed that electrical conduction of bilayer structures is provided by a dual mechanism combining the electron tunneling through a ferroelectric barrier and the ohmic current flow in a manganite layer. The possibility of retaining the mechanism of charge carrier tunneling in a bilayer ferroelectric/manganite structure through the ferroelectric layer up to 6 nm thick is discussed.     

Effect of the settings of electrospray deposition method on the structure and performance of the fuel cell catalyst layer

Catalyst layers of proton exchange membrane fuel cells (PEMFC) are formed by electrospray deposition (ESD) method. The cathode catalyst layers are formed and characterized by varying the settings of the system, such as flow rate, applied voltage and the distance between the capillary and the substrate. The dryness of the aggregates during deposition is evaluated using the Damkhöler number (Da), and the structure of the catalyst layer is observed using SEM, which shows that the catalyst layer is porous when dry and non-porous when insufficiently dry. In the case of insufficient drying, the structure changed significantly depending on the position. Single cell tests show that the maximum power density varies from 105 to 253 mWcm⁻² depending on the settings, even with the same catalyst ink and the same amount of platinum. Electrochemical impedance spectroscopy shows that the charge transfer and mass transport resistances tend to decrease with increase in Da.     

POD analysis of near-wake structures of an elliptic cylinder adjacent to a free surface

The POD (Proper Orthogonal Decomposition) method was used to investigate the near-wake behind an elliptic cylinder located under a free surface. For two different depths of cylinder submergence, the first four eigenmodes of the flow field were calculated and their structures were analyzed. The first four eigenmodes reveal the details about the global mean flow structure, with the large-scale structure being mainly related to the most energetic flow motion. The convergence of accumulated eigenvalues shows a slight dependence on the number of instantaneous velocity fields used in POD analysis.     

Optimization of a thermal-CVD system for carbon nanotube growth

We illustrate the optimization of the operation of a thermal chemical vapor deposition (CVD) system for the growth of carbon nanotubes (CNT). We have studied the deposition parameters using the Taguchi matrix robust design approach. The CVD system, which employs solid precursors (camphor and ferrocene) carried by nitrogen gas flow through a hot deposition zone, where the deposition of carbon nanostructures takes place, involves a large number of tunable parameters that have to be optimized.With the aim of getting the best configuration for the development of massive and well-oriented CNT carpets, the Taguchi method allowed us to improve our system leading to the growth of extremely long CNTs (few millimeters) at a high deposition rate (500 nm/s) and yield (30% in weight of the carbon precursors feedstock), which were characterized by electron microscopy.We found that the growth temperature had the most important influence on the CNT diameter, whereas the substrate tilt wit respect to gas flow did not influence their growth (i.e. CNTs grow on every side of the silicon wafer substrates, always normal to the substrate surface). The carrier gas flow and catalyst concentration both showed a secondary impact on CNT growth, though they showed a consistent correlation to the growth temperature.     

Growth of sputter-deposited Ni-Ti thin films: Effect of a SiO2 buffer layer

In-situ X-ray diffraction (XRD) during the growth of Ni-Ti thin films was chosen in order to investigate their texture development using a deposition chamber installed at a synchrotron radiation beamline. Near-equiatomic films were co-sputtered from Ni-Ti and Ti targets. The texture evolution during deposition is clearly affected by the substrate type and the ion bombardment of the growing film. On naturally oxidized Si(100) substrates, the Ni-Ti B2 phase starts by stacking onto (h00) planes, and as the thickness increases evolves into a (110) fibre texture. For the deposition on thermally oxidized Si(100) substrates, this pronounced cross-over is only observed when a substrate bias voltage (-45 V) is applied. The oxide layer plays an important role on the development of the (100) orientation of the B2 phase during deposition on heated substrates (≈470 °C). If this layer is not thick enough (naturally oxidized Si substrate) or if a bias voltage is applied, a cross-over and further development of the (110) fibre texture is observed, which is considered as an orientation that minimizes surface energies. Electrical resistivity measurements showed different behaviour during phase transformation for the Ni-Ti film deposited on thermally oxidized Si without bias and those on thermally oxidized Si(100) with bias and on naturally oxidized Si(100) without bias. This is related to stresses resultant from the fact that the Ni-Ti films are attached to the substrates as well as with the existence of distinct textures. PACS 81.15.Cd; 61.10.Nz; 68.55.Jk     

Towards a gravity dual for the large scale structure of the universe

The dynamics of the large‐scale structure of the universe enjoys at all scales, even in the highly non‐linear regime, a Lifshitz symmetry during the matter‐dominated period. In this paper we propose a general class of six‐dimensional spacetimes which could be a gravity dual to the four‐dimensional large‐scale structure of the universe. In this set‐up, the Lifshitz symmetry manifests itself as an isometry in the bulk and our universe is a four‐dimensional brane moving in such six‐dimensional bulk. After finding the correspondence between the bulk and the brane dynamical Lifshitz exponents, we find the intriguing result that the preferred value of the dynamical Lifshitz exponent of our observed universe, at both linear and non‐linear scales, corresponds to a fixed point of the RGE flow of the dynamical Lifshitz exponent in the dual system where the symmetry is enhanced to the Schrödinger group containing a non‐relativistic conformal symmetry. We also investigate the RGE flow between fixed points of the Lifshitz dynamical exponent in the bulk and observe that this flow is reflected in a growth rate of the large‐scale structure, which seems to be in qualitative agreement with what is observed in current data. Our set‐up might provide an interesting new arena for testing the ideas of holography and gravitational duals.     

晶粒尺寸对1 MeV电子在金属Ni中能量沉积的影响

采用脉冲电沉积方法制备出高致密、高质量的纳米晶Ni, 并对其密度、组织成分和微观结构进行了表征. 利用高能粒子加速器产生的1 MeV高能电子为辐照源, 研究高能电子在纳米晶Ni和常规粗晶Ni中的能量损失. 通过辐照过程中放置的吸收剂量片来准确表征其电子的能量沉积. 结果表明, 晶粒尺寸对高能电子在材料中的能量沉积有明显的影响, 1 MeV电子在穿过一定厚度的金属Ni后, 在晶粒尺寸细小的纳米晶Ni中测得总的吸收剂量较大, 证明了高能电子在纳米材料中的总能量沉积较小, 从而表现出纳米材料抗辐照的优异性能. 关键词： 高能电子 纳米金属 辐射损伤     

ZrO2-based thin films synthesized by electrostatic spray deposition: Effect of post-deposition thermal treatments

Cubic and tetragonal Y₂O₃-doped ZrO₂ thin films were deposited with a dense surface morphology by electrostatic spray deposition. Four dependent process parameters – substrate temperature, precursor solution flow rate, nozzle to substrate distance and the deposition time – have been used to control the process. Temperature dependent Raman spectroscopy and X-ray diffraction were performed in order to investigate the crystallization behavior and structural properties.     

单体流量对苯乙烯等离子体聚合过程影响的光谱分析

单体流量是影响等离子体聚合沉积过程的重要宏观参数之一。本文应用发射光谱方法研究了苯乙烯单体流量变化时，ＣＨ，Ｃ４Ｈ＾＋２粒子发射强度的变化情况，用红外吸收光谱法分析了等离子体聚苯乙烯膜的本体结构，并测量了相应放电条件下膜的沉积速率。