Gas Flow Rate Ratio Research Articles

Heat transfer and shear stress in a gas-liquid flow in an inclined flat channel

Results of experimental investigation of a bubbly gas-liquid flow in an inclined flat channel are presented. Themeasurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and with different values of the volumetric gas flow rate ratio. The hydrodynamic structure wasmeasured bymeans of an electrochemical method using miniature shear stress probes. Values of average shear stress and heat transfer coefficient for different orientation of the channel were found. It is shown that in a bubbly gas-liquid flow the shear stress and heat transfer depend substantially on the channel inclination angle.

Scaling of the bubble formation in a flow-focusing device: Role of the liquid viscosity

The present work studies the bubble formation in viscous liquids with the viscosity ranging from 5 to 400mPas by using a high-speed digital camera. The experiment was carried out in a flow-focusing device with square cross-section of 600×600μm. Results show that the viscous shear stress strongly influences the dynamics of bubble formation, including the shape and size of bubbles. The bubble size follows power-law relations with the gas flow rate, the flow rate and viscosity of the liquid phase respectively, indicating that bubbles formed in viscous fluids are controlled by a combination of squeezing mechanism and shearing mechanism. Therefore, the bubble size can be predicted by a power-law function depending on the flow rate ratio of gas and liquid phases φ representing the squeezing mechanism and capillary number Ca representing the shearing mechanism. In addition, the dynamics of bubble formation in viscous liquids in a flow-focusing device is also analyzed.

The growth characteristics of microcrystalline Si thin film deposited by atmospheric pressure plasma-enhanced chemical vapor deposition

Microcrystalline silicon thin film was grown by atmospheric pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique with a cylindrical rotary electrode supplied with 150 MHz very-high-frequency power. The crystalline volume fraction could be controlled by changing the flow rate ratio of silane and hydrogen gas during AP-PECVD. We could also control it by regulating the substrate scanning speed. At low substrate scanning speed, the silicon film had a low crystalline volume faction and layer-by-layer structure with alternating layers of amorphous and microcrystalline Si. On the other hand, at high substrate scanning speed, silicon crystals of sizes 25 nm grew homogeneously throughout the whole film.

Analysis of Design Factors Influencing the Oxygen Transfer of a Pilot-Scale Speece Cone Hypolimnetic Aerator

The objective of this research was to characterize the performance of a pilot scale downflow bubble contact (DBCA) hypolimnetic aerator, the Speece Cone. The effect of two key design factors, inlet water velocity and the ratio of gas flow rate to water flow rate on four standard units of oxygen transfer, was examined: (a) the oxygen transfer coefficient, KLa, corrected to 20°C, KLa20 (h−1); (b) the standard oxygen transfer rate, SOTR (g O2·h−1); (c) the standard aeration efficiency, SAE (g O2·kW h−1); and (d) the standard oxygen transfer efficiency, SOTE (%). Two sources of oxygen, pressure swing adsorption (PSA) oxygen (87% purity oxygen) and air (∼21% oxyzen) were compared. KLa20, SOTR, and SAE increased with an increase in the ratio of gas flow rate to water flow rate for both air and oxygen, over a range of 0.5% to 5.0%; while SOTE deceased. An increase in inlet water velocity resulted in a decrease in KLa, corrected to 20°C, SOTR, and SAE, but an increase in the SOTE. Experimental treatments with air showed similar, but much less dramatic effect of the gas flow rate to water flow rate ratio and water inlet velocity on KLa20, SOTE, SAE, and SOTE, when compared to treatments with PSA oxygen. The best oxygen transfer performance was achieved with an inlet water velocity of 6.9–7.6 m·s−1 and oxygen flow rate to water flow rate ratio of about 2.5%. At this combination, the SOTE was about 66–72%. Further experimentation with inlet water velocity is required to increase oxygen transfer performance to the >95% range which has been reported in the literature for Speece Cone oxygenation systems.

Optimization of Aerosol Jet Printing for High-Resolution, High-Aspect Ratio Silver Lines

Aerosol jet printing requires control of a number of process parameters, including the flow rate of the carrier gas that transports the aerosol mist to the substrate, the flow rate of the sheath gas that collimates the aerosol into a narrow beam, and the speed of the stage that transports the substrate beneath the beam. In this paper, the influence of process parameters on the geometry of aerosol-jet-printed silver lines is studied with the aim of creating high-resolution conductive lines of high current carrying capacity. A systematic study of process conditions revealed a key parameter: the ratio of the sheath gas flow rate to the carrier gas flow rate, defined here as the focusing ratio. Line width decreases with increasing the focusing ratio and stage speed. Simultaneously, the thickness increases with increasing the focusing ratio but decreases with increasing stage speed. Geometry control also influences the resistance per unit length and single pass printing of low-resistance silver lines is demonstrated. The results are used to develop an operability window and locate the regime for printing tall and narrow silver lines in a single pass. Under optimum conditions, lines as narrow as 20 μm with aspect ratios (thickness/width) greater than 0.1 are obtained.

On the optical and morphological properties of microstructured Black Silicon obtained by cryogenic-enhanced plasma reactive ion etching

Motivated by the need for obtaining low reflectivity silicon surfaces, we report on (sub-) micro-texturing of silicon using a high throughput fabrication process involving SF6/O2 reactive ion etching at cryogenic temperatures, leading to Black Silicon (BS). The corresponding high aspect ratio conical spikes of the microstructured surface give rise to multiple reflections and hence, enhanced absorption under electromagnetic radiation. Aiming a better understanding of this mechanism, we performed a systematic study by varying several plasma process parameters: O2/SF6 gas flow rate ratio, silicon temperature, bias voltage, and etching time. We determined the process window which leads to BS formation and we studied the influence of the process parameters on the surface morphology of the obtained BS samples, through analysis of scanning electron microscopy images. The measured optical reflectance of BS is in the order of 1% in the visible and near infrared ranges (400–950 nm). We noticed that the lowest reflectance is obtained close to the threshold parameters of BS formation. Absorptance spectral response of BS is measured from 1.3 to 17 μm, and we observed a great enhancement of absorptance up to about 75% compared to flat silicon. We also obtained through these experiments, a clear evidence of a correlation between the excellent optical properties and the aspect ratio of the BS conical microstructures in the measured wavelength ranges.

Application of SixNy:Hz (SiN) as index matching layer in a-Si:H thin film solar cells

The difference in refractive indices of glass substrate and transparent conducting oxide (TCO) electrode causes optical reflection in thin film solar cells, which results in lower absorption of light for devices. An anti-reflection layer between glass and TCO is required to reduce the loss of light due to optical reflection. Silicon nitride (SixNy:Hz) films have shown antireflection property. The refractive index of SixNy:Hz films can be engineered by changing the silicon or nitrogen content in the film. Here, we report the optimization of refractive index of SixNy:Hz to achieve a value between refractive index of glass (1.5) and TCO film (2.0). SixNy:Hz films have been deposited in a RF-plasma enhanced chemical vapour deposition system operating at a frequency of 13.56 MHz. The substrate temperature was fixed at 300 °C. Fourier transform infrared analysis has been used to determine the nature of Si-N, N-H, and Si-H bonding in the films. Refractive index of films has been measured using spectroscopic ellipsometer. The optical reflectance and transmission of SixNy:Hz and SixNy:Hz/TCO layers have been measured using UV/VIS spectrometer. The gas flow rate ratio of N2/SiH4 has been varied from 235 to 470. Decrease in transmittance of SixNy:Hz/TCO layer is observed with increase in silicon concentration in the film. Refractive index of SixNy:Hz also increased with an increase of the silicon content in the films. The reflectance of TCO films has been decreased from 15% to 8% when SixNy:Hz film is incorporated between glass substrate and TCO film. An improvement of around 20% has been observed in current density of solar cells having SixNy:Hz film as refractive index matching layer with refractive index 1.83. Thus, SixNy:Hz film as refractive index matching layer can be used to improve the solar cells device efficiency.

Mutate Chlorella sp. by nuclear irradiation to fix high concentrations of CO2

To improve biomass productivity and CO2 fixation of microalgae under 15% (v/v) CO2 of flue gas, Chlorella species were mutated by nuclear irradiation and domesticated with high concentrations of CO2. The biomass yield of Chlorella pyrenoidosa mutated using 500Gy of 60Co γ irradiation increased by 53.1% (to 1.12gL−1) under air bubbling. The mutants were domesticated with gradually increased high concentrations of CO2 [from 0.038% (v/v) to 15% (v/v)], which increased the biomass yield to 2.41gL−1. When light transmission and culture mixing in photo-bioreactors were enhanced at 15% (v/v) CO2, the peak growth rate of the domesticated mutant (named Chlorella PY-ZU1) was increased to 0.68gL−1d−1. When the ratio of gas flow rate (Lmin−1) to 1L of microalgae culture was 0.011, the peak CO2 fixation rate and the efficiency of Chlorella PY-ZU1 were 1.54gL−1d−1 and 32.7%, respectively.

Control growth of silicon nanocolumns’ epitaxy on silicon nanowires

The epitaxial growth of Si nanocolumns on Si nanowires was studied using hot-wire chemical vapor deposition. A single-crystalline and surface oxide-free Si nanowire core (core radius*21 ± 5 nm) induced by indium crystal seed was used as a substance for the vapor phase epitaxial growth. The growth process is initiated by sidewall facets, which then nucleate upon certain thickness to form Si islands and further grow to form nanocolumns. The Si nanocolumns with diameter of 10-20 nm and aspect ratio up to 10 can be epitaxially grown on the surface of nanowires. The results showed that the radial growth rate of the Si nanocolumns remains constant with the increase of deposition time. Meanwhile, the radial growth rates are controllable by manipulating the hydrogen to silane gas flow rate ratio. The optical antireflection properties of the Si nanocol- umns' decorated SiNW arrays are discussed in the text.

Postannealing Effect at Various Gas Ambients on Ohmic Contacts of Pt/ZnO Nanobilayers toward Ultraviolet Photodetectors

This paper describes a fabrication and characterization of ultraviolet (UV) photodetectors based on Ohmic contacts using Pt electrode onto the epitaxial ZnO (0002) thin film. Plasma enhanced chemical vapor deposition (PECVD) system was employed to deposit ZnO (0002) thin films onto silicon substrates, and radio-frequency (RF) magnetron sputtering was used to deposit Pt top electrode onto the ZnO thin films. The as-deposited Pt/ZnO nanobilayer samples were then annealed at450∘Cin two different ambients (argon and nitrogen) to obtain optimal Ohmic contacts. The crystal structure, surface morphology, optical properties, and wettability of ZnO thin films were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), photoluminescence (PL), UV-Vis-NIR spectrophotometer, and contact angle meter, respectively. Moreover, the photoconductivity of the Pt/ZnO nanobilayers was also investigated for UV photodetector application. The above results showed that the optimum ZnO sample was synthesized with gas flow rate ratio of 1 : 3 diethylzinc [DEZn, Zn(C2H5)2] to carbon dioxide (CO2) and then combined with Pt electrode annealed at450∘Cin argon ambient, exhibiting good crystallinity as well as UV photo responsibility.

Low-temperature crystallization and hardness enhancement of alumina films using the resputtering technique

In this study, alumina thin films were deposited on Si (100) substrates by reactive magnetron sputtering in an Ar–O2 atmosphere. The resputtering technique without any separated ion source was applied, as well as various substrate biases and deposition temperatures. Suitable oxygen/argon gas flow rate ratio was obtained to deposit stoichiometric Al2O3 films. As investigated by grazing incidence X-ray diffraction (GIXRD) and nanohardness measurements, the amorphous characteristics and hardness values of alumina films aren't altered with the various substrate biases up to −120V at the temperatures up to 600°C. The microstructure of alumina films with thin crystalline Al2O3 layer between the amorphous layers was achieved with the resputtering technique at elevated temperatures (≤300°C). This suggests that the reactive magnetron sputtering with the resputtering technique is a simple and effective method of preparing crystalline alumina films at relatively low temperature. The existence of thin crystalline Al2O3 layers between the amorphous layers makes the film harder than the amorphous films. In general, increasing the deposition temperature with the resputtering technique increased the hardness of the films, thus the crystalline-volume fraction of the films. However, the hardness was not a monotonic function of the deposition temperature.

An Investigation on the Exergo-Economic Performance of an Evaporator in Orc Recovering Low-Grade Waste Heat

Based on the perspective of exergy recovery (profit), an exergo-economic performance evaluation model of evaporator in ORC recovering low-grade waste heat is established. Selecting dry fluid R600a as a working fluid, an exergo-economic criterion was defined to estimate the effects of operating parameters and tube length on the performance of evaporator when the boiling temperature of evaporator is fixed. The optimal flow rate of organic working fluid Gt (flow rate of exhaust flue gas Gs), the flow rate ratio of exhaust flue gas to organic working fluid y, and tube length l have been discovered to maximize the annual net profit value NPV. Furthermore, there exists the critical values of Gt (Gs), y, and l which make NPV = 0. The results also show that, with the increase of the flow rate ratio of exhaust flue gas to organic working fluid, the maximum annual net profit value first increases sharply, and then decreases slowly, while the optimal tube length of evaporator reduces monotonously. Furthermore, the evaluation method from purely thermodynamic viewpoint is imperfect, and the economic considerations change the results obtained with only the maximization of recovered exergy.

Effects of methane flow rate on the optical properties and chemical bonding of germanium carbon films deposited by reactive sputtering

Amorphous hydrogenated germanium carbon (a-Ge1−xCx:H) films were prepared by radio frequency (RF) reactive magnetron sputtering of a pure Ge (111) target in a CH4 + H2+Ar mixture and their composition, optical properties, chemical bonding were investigated as a function of gas flow rate ratio of CH4/(Ar + H2). The results showed that the deposition rate first increased and then decreased as gas flow rate ratio of CH4/(Ar + H2) was increased from 0.125 to 0.625. And the optical gap of the a-Ge1−xCx:H films increased from 1.1 to 1.58 eV accompanied with the increase in the carbon content and the decrease in the relative content of Ge–C bonds of the films as the CH4 flow rate ratio was increased, while refractive index of the films decreased and the absorption edge shifted to high energy. Through the analysis of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, it was found that the formation of Ge–C bonds in the films was promoted by low CH4 flow rate which is connected with relatively high H2 concentration and Ge content. Especially in low CH4 concentration, the formation of sp2-hybridised C–C bonds was suppressed considerably both due to the etching effect on weak bonds of H and the fact that chemical bonding for germanium can be only sp3 hybridization.

Annealing effect on the hydrophobic property of Cr2N coatings

Cr–N coatings were deposited in Ar/N2 plasma at various nitrogen flow rate ratios by using the reactive magnetron sputtering process. The solid solution Cr(N), chromium nitride Cr2N and CrN were found to form at nitrogen gas flow rate ratios of 8%, 32% and 48%, respectively. The hydrophobic property of these three phases was studied by water contact angle (WCA) measurement. The Cr2N phase with higher WCA was further selected to study the high temperature oxidation and the hydrophobic properties. The Cr2N coatings were annealed in air for 2h between 300 and 900°C at 100°C interval, respectively. The experimental results show that WCA of the as-deposited Cr2N is about 106° and there is no obvious change when annealed below and at 500°C. As the annealing temperature increased to 700°C, WCA increased to 115° with rougher surface and retained Cr2N. After annealing at 900°C, the rough crystalline Cr2O3 covers the surface and roughness sharply increases to 57nm. WCA also rapidly drops down to 28°.

$\hbox{BCl}_{3}/\hbox{Cl}_{2}$-Based Inductively Coupled Plasma Etching of GaN/AlGaN Using Photoresist Mask

Gallium nitride/aluminium gallium nitride (GaN/AlGaN) etching in BCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based inductively coupled plasma (ICP) is investigated for high electron mobility transistor (HEMT) mesa etching using rarely preferred mask-photoresist. The critical issues related to photresist burning/deforming, resist removal, selectivity, mesa edge roughening, and nonuniform etching of GaN and AlGaN layers are discussed in detail using plasma of BCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> / Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gases. The effect of ICP process parameters like ICP power, RF power, pressure, and BCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow rate ratio on etch rate of GaN/AlGaN layers and mask is studied systematically for the optimization of a HEMT mesa etching process that results in smooth etched surface with sharp and highly anisotropic mesa edges. The photoresist mask selectivity is found to depend strongly on pressure and RF power, whereas the etched surface morphology changes significantly with the gas flow rate ratio and chamber pressure. The AlGaN etch rate and selectivity with respect to GaN is also characterized for different Al concentrations varying up to 33%. The etch process is finally applied to GaN/AlGaN HEMT mesa etching, where the mesa features with depth of ~ 1500 A° are etched successfully. The resultant process etch uniformity is found to be better than 5% over 2-in wafer.

Enhanced adhesion of epoxy-bonded steel surfaces using O2/Ar microwave plasma treatment

Steel surfaces have been modified using low pressure microwave plasma to enhance its adhesion with an epoxy adhesive. Optimization of the wettability of the surface was done using contact angle measurements for varying plasma parameters. Maximum wettability (19.9°) was obtained at 1000W microwave power with 20min of treatment time, −50V sample bias and 1.67% O2/Ar gas flow rate ratio. Enhanced wettability of the steel surface was attributed to increased surface roughness and oxide deposition. Using atomic force microscopy, surface roughness was observed to increase from 64.4nm for the untreated surface to 76.7nm for the O2/Ar plasma treated surface. Deposition of oxides on the steel surface was also confirmed by the energy dispersive x-ray spectroscopy. Moreover, the increase in the total surface energy to 53.2mN/m for the O2 plasma treated steel surface supported the enhancement of its wettability, and hence, the adhesion with epoxy. Based on tensile test results, the adhesion strength of epoxy-bonded O2/Ar plasma treated surfaces at optimum settings was increased to 3816.0N, which is significantly higher compared to 3038.3N for the epoxy-bonded untreated surfaces.

Effects of ambient conditions on the quality of graphene synthesized by chemical vapor deposition

In this work, we investigated the effects of methane concentration and gas flow rate ratio between hydrogen and methane on the quality of graphene synthesized by chemical vapor deposition. It is found that a critical concentration of methane is needed to grow continuous graphene films, while discontinuous graphene flakes are formed at low methane concentrations. Under the condition without hydrogen, a graphene film in which monolayer areas are predominant is grown, whereas a great proportion of hydrogen causes thick graphene, which reduces the transmittance of the film. Our results present an instructive reference to the large-area synthesis of graphene for the potential applications in electronics.

Experimental studies of flame stability limits of biogas flame

Flame stability of premixed biogas flame for Reference Test Burner (RTB) was investigated. In this study, six kinds of test gases were used to simulate biogas in which CO2 volume fraction varied from 30% to 45%. A series of experiments were conducted on two RTBs with different port diameters and at different outlet unburned mixture temperatures. It was found that the lifting and yellow tipping limits show similar trends regardless of the biogas components, port diameters and mixture temperatures. A “float off” phenomenon could be observed at low gas flow rate and low primary air ratio. Low mixture temperature, small ports and high CO2 concentration in biogas can lead to the unstable condition of “float off”. The lifting limits are enhanced with an increase of port diameter or mixture temperature and with a decrease of CO2 concentration. The yellow tipping limits are extended with an increase of CO2 concentration and with a decrease of mixture temperature or port diameter. In addition, the lifting limit curve becomes a straight line when semi-logarithmic graph paper is applied. The intercept increases with a decrease of the CO2 concentration in biogas and with an increase of port diameter or gas temperature.

Resistance Switching in Ni/HfOx/Ni Nonvolatile Memory Device with CF4/O2 Plasma Post-treatment

The resistance switching characteristics of Ni/HfOx/Ni capacitor structures with CF4/O2 plasma post-treatment of different gas flow rate ratios were investigated. The HfOx film was deposited by an electron-gun evaporator, followed by the CF4/O2 plasma post-treatment with different gas flow rate ratios. According to the filament model, conducting filaments (CFs) are formed by the percolation of various types of defects such as oxygen ions and oxygen vacancies. Moreover, the incorporation of oxygen/fluorine may terminate the oxygen vacancies to form Hf–F bonds and eliminate both fixed and interface traps, which can help to form fixed CFs in the film owing to local stronger Hf–F bonds. In this work, the improvement in the stability of resistance switching and current in the high-resistance state (HRS) was achieved by suitable plasma post-treatment. This may be attributed to the formation of Hf–F bonds as observed through electron spectroscopy for chemical analysis.

Deposition of Cluster-Free B-doped Hydrogenated Amorphous Silicon Films Using SiH4+B10H14Multi-Hollow Discharge Plasma Chemical Vapor Deposition

We have deposited cluster-free B-doped hydrogenated amorphous silicon (a-Si:H) films using a SiH4+B10H14 multi-hollow discharge plasma chemical vapor deposition (CVD) method. We have studied the dependence of the deposition rate, band gap, and conductivity of the films on the gas flow rate ratio R=[B10H14]/[SiH4]. The deposition rate for SiH4+B10H14 plasmas is 2–3 times as high as that for pure SiH4 plasmas. Optical emission spectroscopy (OES) measurements indicate that SiH3 radical generation rate remains nearly constant regardless of R. These results suggest that BxHy radicals enhance the surface reaction probability and/or sticking probability of SiH3, being the predominant deposition precursor. Cluster-free B-doped a-Si:H films have a wide band-gap energy of 1.8–2.0 eV and a conductivity as high as 5.0×10-6 S/cm. These results demonstrate that cluster-free B-doped a-Si:H films deposited using SiH4+B10H14 multi-hollow discharge plasma CVD are promising as a p-layer of pin a-Si:H solar cells.