Lili Zheng

A Model of Pulsations in Communicating Hydrocephalus

The traditional theory of communicating hydrocephalus has implicated the bulk flow component of CSF motion; that is, hydrocephalus is generally understood as an imbalance between CSF formation and absorption. The theory that the cause of communicating hydrocephalus is malabsorption of CSF at the arachnoid villi is not substantiated by experimental evidence or by physical reasoning. Flow-sensitive MRI has shown that nearly all CSF motion is pulsatile, and there is substantial evidence that hyperdynamic choroid plexus pulsations are necessary and sufficient for ventricular dilation in communicating hydrocephalus. We have developed a model of intracranial pulsations based on the analogy between the pulsatile motion of electrons in an electrical circuit and the pulsatile motion of blood and CSF in the cranium. Increased impedance to the flow of CSF pulsations in the subarachnoid space redistributes the flow of pulsations into the ventricular CSF and into the capillary and venous circulation. The salient features of communicating hydrocephalus, such as ventricular dilation, intracranial pressure waves, narrowing of the CSF-venous pressure gradient, diminished cerebral blood flow, elevated resistive index and malabsorption of CSF, emerge naturally from the model. We propose that communicating hydrocephalus is the result of a redistribution of CSF pulsations in the cranium.

Simulations of Droplet Spreading and Solidification Using an Improved SPH Model

Smoothed particle hydrodynamics (SPH) method as one of the meshless Lagrangian methods has been widely used to simulate problems with free surface. The traditional SPH method suffers from so-called tensile instability, which may eventually result in numerical instability or complete blowup during the simulation of bubble/droplet dynamics. A new pressure-correction equation is proposed to efficiently transport the local pressure to the neighboring area during the impact of incompressible/compressible fluid and reduce the disorder of particle distribution. Consequently, the accuracy and the efficiency of the SPH method can be dramatically improved. New treatments to the surface tension and solidification are also proposed to manipulate SPH particles near the free surface and the solidification interface. The improved SPH method has been used to simulate droplet impact, spreading, and solidification. It is evident that the new method can handle the droplet contraction problem without causing numerical instability. The numerically predicted flattening ratio of the splat due to droplet impact is in good agreement with the analytical prediction. The results demonstrate that the improved SPH model is a powerful tool to study droplet spreading and solidification.

Application of Smoothed Particle Hydrodynamics Method to Free Surface and Solidification Problems

The smoothed particle hydrodynamics (SPH) method is developed to simulate free surface and solidification problems. A new treatment is included to handle particles near the free and solidification interfaces. The method is applied to a droplet impacting on substrates with different roughnesses. Spreading, solidification, oxide redistribution, and droplet pinch-off are presented in two- and three-dimensional geometry configurations. This work demonstrates the SPH model as a powerful tool to study transport phenomena in problems with free surface deformation and solidification.

Resonance and the Synchrony of Arterial and CSF Pulsations

Accessible online at: www.karger.com/pne Dear Sir, We thank Tenti et al. [1] for their thoughtful critique of our paper ‘A model of intracranial pulsations’ [2]. It was our hope that this paper would stimulate a closer look at the theories of intracranial dynamics, and it appears to have done just that. In their letter, they made a number of points which we would like to address. One general comment is that one should be cautious when applying numbers to the model described in our paper. This paper presents as a first step a simplified linear, lumped-parameter circuit with one or two degrees of freedom as a model of a complex three-dimensional, distributed parameter nonlinear system (the cranium). Secondly, the relevant parameters of the system are not known with any precision. This is precisely the reason that all of the analyses in our paper were purely qualitative. Of course, it is our goal to develop a fully quantitative model based on our ideas; however, we have much more work to do in improving and validating the resonance concept before such a model can be completed. The letter of Tenti et al. [1] raised three specific objections to our paper, and we would like to address each in turn. (1) Synchrony between the intracranial arterial pulse and the CSF pulse is the result of the nearly instantaneous transmission of sound in water. It is not the result of resonance. Normal intracranial pulsations are characterized by synchrony and similarity [2, p 286], but pulsations in disease states are often characterized by asynchrony [2, pp 291–293]. Transmission of the arterial pulse through the CSF at the speed of sound does explain synchrony, but fails to explain the other two important phenomena. Similarity means that the intracranial pressure (ICP) pulse has nearly the same amplitude, wherever you measure it within the intracranial CSF. The theory of instantaneous transmission cannot explain the similarity of the CSF pulse throughout a CSF space. A traveling wave of compression and rarefaction in a fluid is attenuated as it spreads, as follows: The pressure in a fluid that is produced by a spherical acoustic wave is given by [3]:

GaSe and GaTe anisotropic layered semiconductors for radiation detectors

High quality detector grade GaSe and GaTe single crystals have been grown by a modified vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). A state-of-the-art computer model, MASTRAPP, is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown crystals. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The crystals harvested from ingots of 8-10 cm length and 2.5 cm diameter, have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature photoluminescence (PL), atomic force microscopy (AFM), and optical absorption/transmission measurements. Single element devices up to 1 cm2 in area have been fabricated from the crystals and tested as radiation detectors by measuring current-voltage (I-V) characteristics and pulse height spectra using 241Am source. The crystals have shown high promise as nuclear detectors with their high dark resistivity (≥109 Ω.cm), good charge transport properties (μτe ~ 1.4x10-5 cm2/V and μτh ~ 1.5x10-5 cm2/V), and relatively good energy resolution (~4% energy resolution at 60 keV). Details of numerical modeling and simulation, detector fabrication, and testing using a 241Am energy source (60 keV) is presented in this paper.

Towards understanding of ultrasonic consolidation process with “process map”

Purpose – The purpose of this paper is to identify the key parameters that control the bonding formation of foils by the ultrasonic consolidation (UC) process and to build the correlations among process operating conditions and key control parameters through the concept of “process map”. Design/methodology/approach – The concept of “process map” is proposed based on the diffusion bonding mechanism for the UC process, and numerical simulations have been applied to the UC process to predict peak temperature and plastic strain at the contact interface by considering a wide range of process operating conditions. Findings – This map reveals that the formation of bonding among foils by the UC process requires a good match between temperature and plastic deformation at the contact interface. This limits the process operating window to a narrow region in the strain – temperature coordinate system. Originality/value – This work has identified the underlying mechanism for bonding formation and the key control param...

Thermal Environment Evolution and Its Impact on Vapor Deposition in Large Diameter AlN Bulk Growth

In an AlN sublimation growth system, to obtain a large and thick single crystal, it is very important to maintain the thermal environment suitable for growth inside the crucible during a long period of time (>100 Hours). In this paper, an integrated model capable of describing inductive, radiative and conductive heat transfer will be used to simulate the transient behavior of thermal environment inside the crucible during a 40-hour experiment growth. The effect of graphite insulation degradation on the temperature distribution inside the crucible will be investigated. Simulation results will be compared with the experiments data to study the effect of the insulation thermal conductivity and geometry change, the degradation induced particle deposition, and the crystal size enlargement on the temperature distribution achieved in the crucible and the growth rate. The relationship of graphite insulation degradation with power input change of the induction heated system will be established. The evolution of temperature difference between the surfaces of source material and crystal, which is the driving force of the growth, will be presented.

Vapor Growth of III Nitrides

Good understanding of transport phenomena in vapor deposition systems is critical to fast and effective crystal growth system design. Transport phenomena are complicated and are related to operating conditions, such as temperature, velocity, pressure, and species concentration, and geometrical conditions, such as reactor geometry and source–substrate distance. Due to the limited in situ experimental monitoring, design and optimization of growth is mainly performed through semi-empirical and trial-and-error methods. Such an approach is only able to achieve improvement in the deposition sequence and cannot fulfill the increasingly stringent specifications required in industry. Numerical simulation has become a powerful alternative, as it is fast and easy to obtain critical information for the design and optimization of the growth system. The key challenge in vapor deposition modeling lies in developing an accurate simulation model of gas-phase and surface reactions, since very limited kinetic information is available in the literature. In this chapter, GaN thin-film growth by iodine vapor-phase epitaxy (IVPE) is used as an example to present important steps for system design and optimization by the numerical modeling approach. The advanced deposition model will be presented for multicomponent fluid flow, homogeneous gas-phase reaction inside the reactor, heterogeneous surface reaction on the substrate surface, heat transfer, and species transport. Thermodynamic and kinetic analysis will be presented for gas-phase and surface reactions, together with a proposal for the reaction mechanism based on experiments. The prediction of deposition rates is presented. Finally, the surface evolution of film growth from vapor is analyzed for the case in which surface diffusion determines crystal grain size and morphology. Key control parameters for film instability are identified for quality improvement.

Controlling Te inclusion during direct mixed solution growth of large size CdZnTe crystal

CdZnTe (CZT) is proved to be a perfect material during the fabrication of detectors for X-ray and Gamma ray. However, Te inclusion is one of the main defects in CZT crystal which influences the electrical and spectroscopic properties of the detectors. This paper presents optimization of hot zone design and operating conditions by direct mixed solution growth (DMSG) method in order to reduce the formation of Te inclusion. The growth temperature is 890°C and the growth rate is 5mm/day. With the temperature gradient increased from 20K/cm to 40K/cm, the number of Te inclusion is reduced sharply while the size of Te inclusion is still large. When accelerated crucible rotation technique (ACRT) is introduced, the size of Te inclusion is reduced significantly. Large-size Te inclusion almost disappears under IR imaging. The density of Te inclusion which is larger than 1×104cm3. The resistivity of the as grown crystal is higher than 1010Ωxcm. At last, the influence of ACRT sequences on Te formation is discussed.

Thermo Mechanical Analysis of Ultrasonic Welding of Metal Joints Under Different Control Parameters

Ultrasonic welding is a complex process combining the processes of interface friction, heat transfer, plastic deformation heating, and atom diffusion and so on. Even though much work has been performed to understand ultrasonic welding process, the key characteristic process parameters of ultrasonic welding process and the key control parameters for the bond quality are still questions. Based on the interactions of bond factors and previous research of ultrasonic welding process, we believe that plastic deformation and temperature which represent the energy and strain condition at bonding interface are the key process parameters related to bond. A 3-D thermal-mechanical finite element model is built to analyze the thermal and mechanical files of ultrasonic welding process of two types of aluminum alloys under different control parameters. A possible mechanism between bond quality and control parameters based on max temperature and max plastic deformation of temperature-strain map of simulation is presented.Copyright