THERMODYNAMIC AND PARTICLE-DYNAMIC STUDIES ON SYNTHESIS OF SILICA NANOPARTICLES USING MICROWAVE-INDUCED PLASMA CVD

1. Introduction1.1 Silica nanoparticles and synthesis methods Silica (SiO2) nanoparticles are widely used in industry asan active filler for polymer reinforcement, a rheologicaladditive in fluids, a free flow agent in powders, and anagent for chemical mechanical polishing during IC (inte-grated circuit) fabrication (Sniegowski & de Boer, 2000).Silica powder is also used for producing silicon carbide(Koc & Cattamanchi, 1998) or opaque silica aerosols (Leeet al., 1995). Many methods can …     

Synthesis of TiO2 nanoparticles by propane/air turbulent flame CVD process

Synthesis of TiO2 nanoparticles by the oxidation of titanium tetrachloride (TiCl4) in high-strength propane/air turbulent flame is investigated tentatively for mass production ofTiO2 nanoparticles. Effects of reactor heat flux varying from 247 to 627 kJ/m2 s, initial TiO2 number density from 2×1020> to 1 × 1021 m-3, and apparent residence time of TiO2 nanoparticles in reactor from 0.06 to 0.9 s, on particle morphology, phase composition, UV absorption and photoluminescence (PL) spectra are studied. The TiO2 nanoparti-cles synthesized, with mean size of 30-80 nm and rutile mass fraction from 0.155 up to 0.575, exhibited a strong PL signal at the wavelength of 370-450 nm, with a wide peak signal at 400-420 nm, reflecting significant oxygen vacancies on the surface of the TiO2 nanoparticles.     

INTEGRAL COLLISION KERNEL FOR THE GROWTH OF AEROSOL PARTICLES

Integral collision kernel is elucidated using experimental results for titania, silica and alumina nanoparticles synthesized by FCVD process, and titania submicron particles synthesized in a tube furnace reactor. The integral collision kernel was obtained from a particle number balance equation by the integration of collision rates from the kinetic theory of dilute gases for the free-molecule regime, from the Smoluchowski theory for the continuum regime, and by a semi-empirical interpolation for the transition regime between the two limiting regimes. Comparisons have been made on particle size and the integral collision kernel, showing that the predicted integral collision kernel agreed well with the experimental results in Knudsen number range from about 1.5 to 20.     

Large-scale synthesis of hollow titania spheres via flame combustion

A one-step method for continuous large-scale synthesis of well-defined hollow titania spheres was established by feeding titanium tetrachloride mixed with ethanol vapor to a facile diffusion flame. A mixture of TiCl₄ and C₂H₅OH vapor was transported at 100 m/s into a flame reactor and condensed into mesoscale droplets due to Joule–Thomson cooling and the entrainment of cool gases into the expanding high-speed jet. Hollow crystalline TiO₂ spheres with good thermal stability were formed after the hydrolysis of TiCl₄ in the H₂/air flame at about 1500 °C. Structural characterization indicates that the hollow spheres, with uniform diameter of 300 nm and shell thickness of 35 nm, consist of 20–30 nm TiO₂ nanocrystallites. A formation mechanism of the hollow spheres was proposed, involving the competition between chemical reaction and diffusion during the flame process. The present study provides a new pathway for continuous and large-scale engineering of hollow nanomaterials.     

NUMERICAL SIMULATION OF THE GROWTH OF NANOPARTICLES IN A FLAME CVD PROCESS

The growth of titania nanoparticles in a flame CVD process has been simulated by computational fluid dynamics, based on the change rate of particle number density due to their collisions calculated from an integral collision kernel. The assumptions made on constant particle volume density nv (nd^3), constant density of particle surface area ns (nd^2), and constant entity nd^2.5 in coagulation process have been examined. Comparisons have been made on particle size distribution between measurement results and predictions from present model of particle growth and Kruis model of particle dynamics for titania nanoparticles synthesized by the flame CVD process. Effects of operational parameters such as O2 mole fraction and particle number density on mean particle size and size distribution have been discussed.     

SYNTHESIS OF HIGH PURITY TiO2 NANOPARTICLES FROM Ti（SO4）2 IN PRESENCE OF EDTA AS COMPLEXING AGENT

TiO2 nanoparticles were synthesized by a homogeneous controlled precipitation method using industrial titanium sulfate （Ti（SO4）2）. The obtained powders were characterized by X-ray diffraction （XRD）, transmission electron microscope （TEM）, Fourier transform infrared spectroscopy （FTIR） and ICP plasma spectrometer. EDTA was used as complexing agent to improve the purity and the formation of TiO2 nanoparticles. Experimental results indicated that the high-purity TiO2 nanoparticles were 20 nm in mean size and nearly monodispersed.     

A flow visualization technique for low-speed wind-tunnel studies

The paper describes a flow visualization technique (direct injection method) used in low-speed wind-tunnel studies. Mixture of titanium tetrachloride and carbon tetrachloride is used to produce closely spaced parallel streaklines of white smoke of long duration. The technique is described in detail and representative photographs of various flow patterns are presented.     

Predicting thermal conductivity of liquid suspensions of nanoparticles (nanofluids) based on rheology

A methodology is proposed for predicting the effective thermal conductivity of dilute suspensions of nanoparticles (nanofluids) based on rheology.The methodology uses the rheological data to infer microstructures of nanoparticles quantitatively,which is then incorporated into the conventional Hamilton-Crosser equation to predict the effective thermal conductivity of nanofluids.The methodology is experimentally validated using four types of nanofluids made of titania nanoparticles and titanate nanotubes dispersed in water and ethylene glycol.And the modified Hamilton-Crosser equation successfully predicted the effective thermal conductivity of the nanofluids.     

Predicting thermal conductivity of liquid suspensions of nanoparticles （nanofluids） based on rheology

A methodology is proposed for predicting the effective thermal conductivity of dilute suspensions of nanoparticles （nanofluids） based on rheology. The methodology uses the rheological data to infer microstructures of nanoparticles quantitatively, which is then incorporated into the conventional Hamilton-Crosser equation to predict the effective thermal conductivity of nanofluids. The methodology is experimentally validated using four types of nanofluids made of titania nanoparticles and titanate nanotubes dispersed in water and ethylene glycol. And the modified Hamilton-Zrosser equation successfully predicted the effective thermal conductivity of the nanofluids.     

THERMODYNAMIC ANALYSIS AND EXPERIMENTAL VERIFICATION FOR SYNTHESIZING SILICON NITRIDE NANOPARTICLES USING RF PLASMA CVD

Silicon nitride nanoparticles were synthesized by radio-frequency (RF) plasma chemical vapor deposition (PCVD) using silicon tetrachloride and ammonia as precursors, and argon as carrier gas. By assuming chemical thermodynamic equilibrium in the system, a computer program based on chemical thermodynamics was used to calculate the compositions of the system at different initial concentrations and final temperatures. At first, five elements and thirty-four species were considered. The effects of temperatures, and concentrations of ammonia, hydrogen and nitrogen on the equilibrium compositions were analyzed. It was found that the optimal reaction temperature range should be 1200 to 1500 K to obtain the highest conversion and yield of Si3N4. The inlet position of ammonia should be lower than that of silicon tetrachloride, and both should be located at the tail of the plasma torch. The best moleratio of ammonia to silicon tetrachloride was found to be about 6. Later, the influences of water (.and oxygen) were considered, and 17 additional species were included in the computations. It was found that oxygen or water content in the raw materials should be as low as possible in order to have high nitride content in the produced Si3N4. Nitrogen or hydrogen might be used to replace some or even all the argon to improve the yield of silicon nitride and reduce the cost. The ratio of ammonia to silicon tetrachloride should be high enough to obtain high conversion, but not excessively high to reduce the oxygen content due to the existence of water in ammonia. The simulated results were verified bv experiments.     

A study of a bluff-body combustor using laser sheet lighting

Laser sheet lighting is used to study reacting flows with and without heat release in an axisymmetric, unducted and vertically mounted bluff-body combustor. The fuel, which is seeded with titanium tetrachloride vapor, is ejected from a jet located in the center of the bluff-body. The TiCl₄ in the dry fuel reacts spontaneously with the water in the annulus air to form titanium dioxide particles. High speed movies and visual observations of vertically and horizontally located sheets of laser light provided remarkably detailed visualization (via Mie scattering) of the vortex dynamics in the near-wake region of the bluff-body.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 and printed in AMD, Vol. 66     

Growth characteristics of spherical titanium oxide nanoparticles during the rapid gaseous detonation reaction

The nanosize grain growth characteristics of spherical single-crystal titanium oxide (TiO₂) during the rapid gaseous detonation reaction are discussed. Based on the experimental conditions and the Chapman–Jouguet theory, the Kruis model was introduced to simulate the growth characteristics of spherical TiO₂ nanoparticles obtained under high pressure, high temperature and by rapid reaction. The results show that the numerical analysis can satisfactorily predict the growth characteristics of spherical TiO₂ nanoparticles with diameters of 15–300 nm at different affecting factors, such as concentration of particles, reaction temperature and time, which are in agreement with the obtained experimental results. We found that the increase of the gas-phase reaction temperature, time, and particle concentration affects the growth tendency of spherical nanocrystal TiO₂, which provides effective theoretical support for the controllable synthesis of multi-scale nanoparticles.     

Micro/nano-structured montmorillonite/titania particles with high electrorheological activity

A low-cost electrorheological (ER) material made of micro/nano-structured montmorillonite/titania particles was prepared by a one-pot solvothermal method. The micro/nano-structured particles were characterized by X-ray diffraction, Fourier transform infrared spectra, and scanning electron microscopy. It was found that the nanorod-like titania assembled on the surface of montmorillonite, the diameters of the nanorods were about 30 nm, and the lengths were about 300 nm. The electrorheological property of the micro/nano-structured particles in silicone oil was measured under dc electric fields. It was found that the micro/nano-structured montmorillonite/titania ER fluid exhibited much stronger electrorheological effect compared to pure montmorillonite and pure titania nanorod ER fluids, while its leaking current density was significantly lower than that of montmorillonite ER fluid. The stronger electrorheological effect might be attributed to the larger interfacial polarization and interparticle friction, which originated from the unique structure and morphology of micro/nano-structured particles, compared to pure montmorillonite and pure titania nanorods.     

Steady laminar mixed convection stagnation-point flow of a nanofluid over a vertical permeable surface in the presence of a magnetic field

A similarity solution for a steady laminar mixed convection boundary layer flow of a nanofluid near the stagnation point on a vertical permeable plate with a magnetic field and a buoyancy force is obtained by solving a system of nonlinear ordinary differential equations. These equations are solved analytically by using a new kind of a powerful analytic technique for nonlinear problems, namely, the homotopy analysis method (HAM). Three different types of nanoparticles, namely, copper (Cu), alumina (Al₂O₃), and titanium oxide (TiO₂), with water as the base fluid are considered. The influence of the volume fraction of nanoparticles, permeability parameter, magnetic parameter, and mixed convection parameter on the surface shear stress and surface heat transfer, as well as on the velocity and temperature profiles, is considered. It is observed that the skin friction coefficient and the local Nusselt number increase with the nanoparticle volume fraction for all types of nanoparticles considered in this study. The greatest values of the skin friction coefficient and the local Nusselt number are obtained for Cu nanoparticles.     

TC4钛合金在模拟海水中腐蚀?磨损交互行为研究

采用自制摩擦腐蚀装置研究了TC4钛合金在模拟海水中电化学腐蚀与机械磨损间的交互作用,探究了不同电化学状态对TC4钛合金腐蚀磨损行为的影响. 在摩擦腐蚀过程中,TC4钛合金的腐蚀电位发生负偏移,腐蚀电流随着外加电位升高而增大,在零电流电势(OCP)附近TC4钛合金获得最低摩擦系数. TC4钛合金总体积损失随着外加电位的增加而增大,证实了腐蚀磨损交的交互作用随着外加电位的增加而增强;当电位从–0.5 V增大至0.8 V时,腐蚀磨损交互作用导致的材料损失占总材料损失的比例由12%增加至66%,其中腐蚀诱导磨损导致的损失量占比由7%增加至44%. OCP及其以下外加电位条件下,TC4钛合金的磨损机制为磨粒磨损;0 V电位下TC4钛合金磨损机制为磨粒磨损和疲劳磨损;0.8 V电位下TC4钛合金的磨损机制为磨粒磨损和摩擦诱导的腐蚀磨损.      

MHD mixed convection stagnation-point flow of a nanofluid over a vertical permeable surface: a comprehensive report of dual solutions

The steady laminar magnetohydrodynamic mixed convection boundary layer flow of a nanofluid near the stagnation-point on a vertical permeable plate with prescribed external flow and surface temperature is investigated in this study. Here, both assisting and opposing flows are considered and studied. Using appropriate similarity variables, the governing equations are transformed into nonlinear ordinary differential equations in the dimensionless stream function, which is solved numerically using the Runge–Kutta scheme coupled with a conventional shooting procedure. Three different types of nanoparticles, namely copper Cu, alumina Al₂O₃ and titania TiO₂ with water as the base fluid are considered. Numerical results are obtained for the skin-friction coefficient and Nusselt number as well as for the velocity and temperature profiles for some values of the governing parameters, namely, the volume fraction of nanoparticles ?, permeability parameter f _o , magnetic parameter M and mixed convection parameter λ. It is found that dual solutions exist for both assisting and opposing flows, and the range of the mixed convection parameter for which the solution exists, increases with suction, magnetic field and volume fraction of nanoparticles.     

Hollow spherical titanium dioxide nanoparticles for energy and environmental applications

Hollow spherical titanium dioxide(TiO_2) nanoparticles possess unique properties toward energy and environmental applications,because of the intrinsic properties of TiO_2 and benefits induced by their hollow structure.A detailed understanding of TiO_2 hollow spheres will promote their use in sustainable energy and environmental applications.This perspective details current methods for synthesizing hollow spherical TiO_2 nanoparticles,and their performance in dye-sensitized solar cells,photocatalysts,and batteries.This perspective will promote the design and innovative thinking on the application of hollow spherical metal oxide nanoparticles.     

Experimental and numerical studies on the burning of aluminum micro and nanoparticle clouds in air

An experimental study has been conducted to determine flame propagation velocities in clouds of micro- (4.8 μm) and nano- (187 nm) aluminum particles in air at various concentrations. The experimental results show faster flame propagation in nanoparticle cloud with respect to the case of microparticles. Maximum flame temperature has been measured using a high-resolution spectrometer operating in the visible range. Analysis of combustion residual shows that nanoparticles combustion is realized via the gas-phase mechanism. A three-stage particle combustion model has been proposed based on these observations. Model parameters have been fitted to match the experimental results on the flame velocity and maximum temperature. Particle burning time is estimated from the flame simulations.     

当量比对预混燃烧影响的大涡模拟研究

本文对在突扩燃烧室内甲烷和空气的预混燃烧进行了大涡模拟(LES)研究,考虑预混燃料的当量比对燃烧室提供的动力及产生的污染物的影响．利用LES计算了不同当量比条件下燃烧室内湍流预混燃烧反应流场的温度、浓度、涡量和压力分布,最后对当量比0.5时B点和C点的温度和速度进行EMD分解,得到了温度场和速度场的各阶模态的平均周期．结果表明：随着当量比从0.5增加至0.7,燃烧反应趋于剧烈,燃烧室的最高温度提高了350K,平均压力从32.876 Pa增大到34.833Pa,燃烧产生的瞬态径向最高浓度从0.5%增加到0.95%．     

The discrete heat source approach to dust cloud combustion

In the present paper, combustion of dust clouds from the discrete point heat source method has been addressed. Time-place temperature profile generated by single particle burning has been obtained to study the dust combustion. The summation of the temperature profiles of burned and burning particles predict the temperature in the preheating zone so that the ignition time of layer in flame front can be determined. Consequently the flame propagating speed was obtained based on the dust concentration corresponding to particles spacing and particle diameter. This method has been validated with aluminum dust cloud combustion. Decrease in the dust concentration leads to the lean limit of dust combustion. Increase in particles diameter or reduction in the dust concentration causes higher lean limit and also reduction in the flame propagating speed. Adding the ignition energy as igniter to this system, provides the path to study the effects of ignition energy in the dust combustion.