Comments on the effect of liquid layering on the thermal conductivity of nanofluids

This article provides critical examinations of two mathematical models that have been developed in recent years to describe the impact of nano-layering on the enhancement of the effective thermal conductivity of nanofluids. Discrepancy between the two models is found to be an artefact of an incorrect derivation used in one of the models. With correct formulation, both models predict effective thermal conductivity enhancements that are not significantly greater than those predicted by classical Maxwell theory. This study indicates that nano-layering by itself is unable to account for the effective thermal conductivity enhancements observed in nanofluids.     

GPU-accelerated molecular dynamics simulation for study of liquid crystalline flows

We have developed a GPU-based molecular dynamics simulation for the study of flows of fluids with anisotropic molecules such as liquid crystals. An application of the simulation to the study of macroscopic flow (backflow) generation by molecular reorientation in a nematic liquid crystal under the application of an electric field is presented. The computations of intermolecular force and torque are parallelized on the GPU using the cell-list method, and an efficient algorithm to update the cell lists was proposed. Some important issues in the implementation of computations that involve a large number of arithmetic operations and data on the GPU that has limited high-speed memory resources are addressed extensively. Despite the relatively low GPU occupancy in the calculation of intermolecular force and torque, the computation on a recent GPU is about 50 times faster than that on a single core of a recent CPU, thus simulations involving a large number of molecules using a personal computer are possible. The GPU-based simulation should allow an extensive investigation of the molecular-level mechanisms underlying various macroscopic flow phenomena in fluids with anisotropic molecules.     

SOA-based loop mirror for tunable pulse-width generation

We propose and experimentally demonstrate a system for the generation of pulses of tunable pulse-width as those required in high spectral efficiency optically routed networks. Pulse narrowing of 500 ps pulses by 90% is accomplished through a SOA based non-linear loop mirror. Optical switching through the SOA loop mirror is used to shape and carve these large pulses (e.g., 500 ps) generated by non-expensive low-frequency optoelectronic components to narrow pulses (e.g., 50 ps). We also calculate the minimum loop size and optimum repetition rate of the original pulse train for the generation of the shorter pulse-width pulse train.     

Photoluminescence and optical absorption properties of silicon quantum dots embedded in Si-rich silicon nitride matrices

Silicon nitride (SiN_x) films were prepared with a gas mixture of SiH₄ and NH₃ on Si wafers using the plasma-enhanced chemical vapor deposition (PECVD) method. High-resolution transmission electron microscopy and infrared absorption have been used to reveal the existence of the Si quantum dots (Si QDs) and to determine the chemical composition of the silicon nitride layers. The optical properties of these structures were studied by photoluminescence (PL) spectroscopy and indicate that emission mechanisms are dominated by confined excitons within Si QDs. The peak position of PL could be controlled in the wavelength range from 1.5 to 2.2 eV by adjusting the flow rates of ammonia and silane gases. Absorbance spectra obtained in the transmission mode reveal optical absorption from Si QDs, which is in good correlation with PL properties. These results have implications for future nanomaterial deposition controlling and device applications.     

Study of structural stabilities and optical properties of HgTe under high pressure

A theoretical investigation on the structural stabilities and electronic properties of HgTe under high pressure was conducted using first principles based on density functional theory. Our results demonstrate that the sequence of the pressure-induced phase transitions of HgTe is from the zinc blende, to cinnabar, rocksalt, orthorhombic, and CsCl-type structures. The pressure effects on the optical properties were discussed and compared with previous calculations and experimental data whenever available.     

Green luminescence from triphenylphosphine functionalized single-wall carbon nanotubes

In a simple wet chemical process, purified single-wall carbon nanotubes (SWCNTs) are treated with triphenylphosphine (Ph₃P) at room temperature. The functionalized material is characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. HRTEM micrograph clearly reveals that triphenylphosphine nanocrystals of nearly uniform size are attached to the surfaces of SWCNTs. The hybrid structure shows remarkable green luminescence with peak emission at around 500 nm, under UV excitation. The photoluminescence may be attributed to charge transfer from the electron-donating phosphorous atoms to the carbon nanotubes.     

Development and construction of rotating polarizer analyzer ellipsometer

A detailed mathematical derivation and an experimental characterization of one to two ratio rotating polarizer analyzer ellipsometer (RPAE) are presented. The alignment, calibration, and testing of reference samples are also discussed. The optical properties of some known materials obtained by the proposed ellipsometer will be shown and compared to accepted values. Moreover, the constructed ellipsometer will be tested using two ellipsometry standards with different thicknesses.     

纳米粒子在方管中传输与沉降的研究

采用有限体积法离散并应用Simple方法对方截面弯曲管道内的纳米粒子传输和沉降进行了数值计算,结果表明Reynolds数和Schmidt数是影响纳米粒子传输和沉降的重要参数。粒子较小时,弯管中轴向速度较大的区域就是粒子的高浓度区域,沉降增强因子最大值出现在外弯侧的中心位置;粒子较大时,截面浓度的梯度值降低,沉降增强因子趋于平均,此时整个截面的粒子平均沉降。弯曲作用对于粒子较大且Dean数也较大时的影响更加明显。     

Pulling rate, thermal and capillary conditions setting for rod growth

The purpose of this paper is to elaborate a procedure, based on a mathematical model, for the setting of the pulling rate, capillary and thermal conditions, in order to grow a cylindrical rod with prescribed radius and length, by edge-defined film-fed growth (EFG) method. First, in the case of an axisymmetric meniscus, we use simultaneously the catching and the angle fixation conditions in order to find a formula, which describes the fluctuation of the angle between the horizontal axis Or and the tangent line to the free surface of the meniscus at the three phase point. This angle appears in the system of differential equations which describes the evolution of the radius of the rod. During the growth this angle can fluctuate due to the fluctuations of the crystal radius, or crystallization front level, or pressure, respectively. In the second part of the paper it is shown in which kind this formula together with the energy balance equation at the crystallization front level can be used for setting the pulling rate, the thermal and capillary conditions to grow a cylindrical rod with prescribed radius and length. Numerical illustration and simulation are presented for rods having thermo-physical properties similar to NdYAG and InSb. This type of results can be useful for the experiment planning, since personal computer simulation is less expensive than experiment. With this aim the present study was undertaken.     

Effect of sp-hybridized defects on the oscillatory behavior of carbon nanotube oscillators

Using molecular dynamics simulations, we investigate the oscillatory behaviors of carbon nanotube oscillators containing sp³-hybridized defects formed by hydrogen chemisorption. It is found that the presence of these defects significantly affects the kinetic and potential energies of the nanotube systems, which in turn affects their oscillation periods and frequencies. We have also studied the oscillatory characteristics of the oscillators containing sp³-hybridized Stone-Wales defects. Our results show that it is possible to control the motion of the inner nanotube by introducing sp³-hybridized defects on the outer nanotube, which provides a potential way to tune the oscillatory behavior of nanotube oscillators.