Bo-Hsiung Wu

Mixing behavior of the rhombic micromixers over a wide Reynolds number range using Taguchi method and 3D numerical simulations.

A planar micromixer with rhombic microchannels and a converging–diverging element has been systematically investigated by the Taguchi method, CFD–ACE simulations and experiments. To reduce the footprint and extend the operation range of Reynolds number, Taguchi method was used to numerically study the performance of the micromixer in a L9 orthogonal array. Mixing efficiency is prominently influenced by geometrical parameters and Reynolds number (Re). The four factors in a L9 orthogonal array are number of rhombi, turning angle, width of the rhombic channel and width of the throat. The degree of sensitivity by Taguchi method can be ranked as: Number of rhombi > Width of the rhombic channel > Width of the throat > Turning angle of the rhombic channel. Increasing the number of rhombi, reducing the width of the rhombic channel and throat and lowering the turning angle resulted in better fluid mixing efficiency. The optimal design of the micromixer in simulations indicates over 90% mixing efficiency at both Re ≥ 80 and Re ≤ 0.1. Experimental results in the optimal simulations are consistent with the simulated one. This planar rhombic micromixer has simplified the complex fabrication process of the multi-layer or three-dimensional micromixers and improved the performance of a previous rhombic micromixer at a reduced footprint and lower Re.

Thermally induced formation of SiC nanoparticles from Si/C/Si multilayers deposited by ultra-high-vacuum ion beam sputtering

A novel approach for the formation of SiC nanoparticles (np-SiC) is reported. Deposition of Si/C/Si multilayers on Si(100) wafers by ultra-high-vacuum ion beam sputtering was followed by thermal annealing in vacuum for conversion into SiC nanoparticles. The annealing temperature significantly affected the size, density, and distribution of np-SiC. No nanoparticles were formed for multilayers annealed at 500 °C, while a few particles started to appear when the annealing temperature was increased to 700 °C. At an annealing temperature of 900 °C, many small SiC nanoparticles, of several tens of nanometres, surrounding larger submicron ones appeared with a particle density approximately 16 times higher than that observed at 700 °C. The higher the annealing temperature was, the larger the nanoparticle size, and the higher the density. The higher superheating at 900 °C increased the amount of stable nuclei, and resulted in a higher particle density compared to that at 700 °C. These particles grew larger at 900 °C to reduce the total surface energy of smaller particles due to the higher atomic mobility and growth rate. The increased free energy of stacking defects during particle growth will limit the size of large particles, leaving many smaller particles surrounding the large ones. A mechanism for the np-SiC formation is proposed in this paper.

Determination of the strain energy release rate for C/a-Si composite film produced in nanoindentation tests

A general mechanical model that describes the contact behavior and deformations arising at all layers (including the substrate) is developed in the present study for multilayer specimens to evaluate the theoretical contact parameters. The governing differential equations for the depth solutions of the indenter tip formed at all layers of the specimen under their contact force and depth are developed individually. These two contact parameters allow the evaluation of the internal stress and strain using the membrane theory. The strain energy release rate can thus be determined if the internal stress is available. The mean value of these pop-in depths is almost constant when operating at various loading rates. The present model is precisely if it has good agreement with experiments. The pop-in internal stress was found to be strongly dependent on the C-film thickness (thus the material properties) but independent of the applied indentation system (thus indentation conditions). The pop-in internal stress and ...

Theoretical modeling developed to evaluate the hardness and reduced modulus for the C/a-Si composite film using nanoindentation tests.

A general mechanical model, which is composed of the mechanical models employed to describe the contact behaviors and deformations arising in all layers (including the substrate), is successfully developed in the present study for multilayer specimens in order to evaluate the contact projected area by a theoretical model, and thus the hardness and reduced modulus, using nanoindentation tests. The governing differential equations for the depth solutions of the indenter tip formed at all layers of the specimen under their contact load are developed individually. The influence of the material properties of the substrate on a multilayer specimens hardness and reduced modulus at various indentation depths can thus be evaluated. Transition and pop-in occurred at depths near, but still before, the C (top layer)/a-Si (buffer layer) interface and the a-Si/Si (substrate) interface, respectively. Using the present analysis, the depths corresponding to the transition and pop-in behaviors can be predicted effectively.

The nanoindentation applied to predict the interface delamination for the C/amorphous Si composite film

In the present study, the indentation depth corresponding to the pop-in arising in the loading process is found to be quite close to the C/amorphous Si composite film thickness, regardless of the C-film thickness. This load-depth behavior gives a clue that the occurrence of pop-in is perhaps related to the buckling of the composite film, which had already delaminated from the silicon substrate. This indentation depth of buckling predicted by the present model is quite close to the pop-in depth obtained from experimental results, regardless of the change in the C-film thickness. This characteristic reveals that the present model is developed successfully to predict the pop-in depth of a specimen, and the pop-in is indeed created due to the buckling of the composite film under a compression stress.

Rapid thermal annealing enhanced crystalline SiC particles at lower formation temperature

Formation of nanoparticle SiC (np-SiC) from three-layer Si/C/Si multilayers on Si(100) substrates were investigated using ultra-high-vacuum ion beam sputtering and post annealing by conventional furnace annealing (FA) and rapid thermal annealing (RTA). Fixing the thickness of bottom Si-layer at 50 nm, different thicknessses of the top Si and C layers were designed to study the effect of annealing on the reaction of np-SiC formation, that is, three-layer Si/C/Si structures with thicknesses of 50/200/50 nm by FA and 10/100/50 nm by RTA. There are almost no particle appears at 700degC 1.0 h by FA due to low thermal energy. It was observed that np-SiC appeared at a density order about 108 cm-2 by FA at 900degC for 1.0 h, but many np-SiC can be realized at 750degC for annealing time as short as 1 min at a density order about 1010 cm-2 by RTA. The density is much higher than conventional nanoparticles synthesis using CVD or PVD. The reaction temperature of SiC is also lower than the conventional CVD or FA because of RTA enhanced SiC crystallization behavior at high heating rate. The annealing method influences the particle formation. The particle size, distribution and density are concerned with the top and middle layer thickness. Thermal energy is the diving force for the crystalline SiC formation through interdiffusion between C and Si.

Design and Simulation of a Novel Micro-mixer with Baffles and Side-wall Injection into the Main Channel

A novel passive micromixer with rapid mixing and low pressure drop has been successfully demonstrated by simulations and micro-fabrication. The structure of this micro-mixer contains one main channel, two sub-channels for injecting the other specie and some baffles in the main channel to form mixing chambers. Adding baffles to the main channel will produce the recirculation to increase the contact area of the fluids and form nozzles. So, this type of design can improve mixing efficiency. The better stream mixing is attributed to the higher baffle, the more baffle number and the higher flow rates. From the design aided by simulations, the design with 3 baffles of 300 mum height is adaptive for uniform mixing ( 90% mixing) and low supplied pressure of 4750 Pa at Re of 100 in the 3D simulation.

The Interfacial Transition of the C/Si Composite Film and Si Substrate Evaluated to Predict the Pop-In Behavior in Nanoindentation

In this study, an analytical model is proposed in order to determine the indentation depth of pop-in appearing in the loading process and investigate the effects of the C-film thickness of the C/a-Si composite film on the parameters of the indentation depth of pop-in, the inclined face angle, and the Si-substrate semiangle shown at the substrate after finishing indentation. This model is developed on the basis of the elastoplastic deformation model for the specimen in combination with the concept of applying a virtual indenter with a variable semiangle to the Si substrate. From good agreement between the predicted value and the experimental result, the present model is proved to be trustworthy in the predictions of the indentation depth of pop-in varying with the C-film thickness. Due to a higher gradient demonstrated in the stress of the composite film and stress of the Si-substrate verse indentation depth, a specimen with a thinner C-film is easier to reach the pop-in as compared to that shown in a thicker C-film. The present model is applicable for hard film/substrate specimen as occurring the pop-in behavior in nanoindentations.

Design of a novel microreactor for microfluidic synthesis of silica nanoparticles

Conventional microreactors for nanoparticle synthesis were operated at low flow velocity as well as more residence time because of low mixing efficiency of the meander micromixer. Here, we purpose a simple obstacle micromixer to be operated at a wide range of flow velocity for increasing single-channel synthesis production rate of silica nanoparticles. Results show our obstacle micromixer has over 85% mixing efficiency covering the flow regions in both convection and diffusion mixing. This microreactor was used to synthesize silica nanoparticles with an average diameter of 200–250 nm. Enhancing fluid mixing at high flow velocity can result in high production rate.

Reaction of carbon and silicon at high temperature deposition

In order to study the reaction mechanism of in- situ formation of silicon carbide (SiC), the carbon was deposited on the crystalline silicon (c-SiC) substrate at high temperature of 400 - 600 degC using ultra-high-vacuum ion beam sputtering. X-ray diffraction, Raman spectra, Auger electron spectroscopy and high resolution scanning electron microscopy (SEM) with the attached dispersive X-ray (EDX) detector were used to examine the effect of substrate temperature on the reaction mechanism. Amorphous carbon was formed at room-temperature deposition and increased its disorder state with increasing deposition temperature to 500 degC corresponding to higher ratio of disorder peak to graphite peak intensity in Raman spectrum. The crystalline silicon carbide (c-SiC) was formed at 600 degC from the diffracted SiC(111) peak, which is much lower than conventional CVD c-SiC formed at more than 1000 degC. Also, a nanoweb-like morphology of c-SiC was observed on the surface from the SEM image. The atomic composition ratio of Si to carbon was about 54/46 from EDX analysis. Thermal energy is the diving force for the crystalline SiC formation through the interdiffusion between carbon and c-Si. The nanoweb-like morphology may be attributed the high surface energy of SiC with strong Si-C bonding.