Continuous synthesis of iron oxide (Fe3O4) nanoparticles via thermal decomposition

Iron oxide nanoparticles have become of great interest in the medical field for their potential uses in areas such as biomagnetic imaging and hypothermia cancer treatment. Traditionally, particles for these applications are produced through batch-based methodologies. Herein, we demonstrate an alternative continuous flow production method for the synthesis of Fe₃O₄ iron oxide nanoparticles. Advantages of continuous flow over the batch method include consistent formation of uniformly spherical particles, thorough mixing of reactants, and capacity for high-volume particle production. In this study, a continuous flow reaction mechanism was proposed in which stoichiometric control of reactants had the potential to control final particle size. The project was conducted under the supposition that the iron oleate/ligand ratio in the precursor was the greatest size control factor, with a higher ratio resulting in smaller particles. The resulting particles produced by this continuous method were characterized by high-resolution transmission electron microscopy, X-ray diffraction, and magnetometry.     

Evaluation of particle growth systems for sampling and analysis of atmospheric fine particles

Three types of water-based condensational growth systems, which can enable particles to grow in size to facilitate sampling and subsequent chemical analysis, were evaluated. The first one is a mixing type growth system where aerosols are mixed with saturated water vapor, the second one is a thermal diffusive growth system where warm flow enters cold-walled tube, and the third one is a laminar flow type where cold flow enters a warm wet-wall tube. Hygroscopic sodium chloride (NaCl), ammonium sulfate ((NH₄)₂SO₄) and ammonium nitrate (NH₄NO₃), and non-hygroscopic polystyrene latex (PSL) particles, in the size range of 50–400 nm, were used to determine their growth factors through the growth systems. Our data showed that the third-type growth system could enable particles to grow most efficiently regardless of their hygroscopic property. Collection efficiency of particles in the size range of 0.05–2.5 μm, in a continuous aerosol sampler after they passed through the third-type growth system was about 100%, suggesting that the third-type growth system would be the most useful among the tested growth systems for sampling and subsequent chemical analysis of fine and ultrafine particles.     

A single-camera technique for simultaneous measurement of large solid particles transported in rapid shallow channel flows

This paper describes a measurement technique that was successfully applied in a study of bed load transport of large spherical solid particles in a shallow and supercritical flow (Fr?=?2.59–3.17) down a steep slope. The experimental condition was characterized by the relatively large solid particle size compared to the flow depth (d _p /h?=?0.23–0.35), and compared to the tracer diameter (d _p /d _t ?≈?130). The technique incorporated particle image velocimetry and particle tracking velocimetry (PTV) to simultaneously measure the characteristics of the two phases. In order to detect true solid particles and to distinguish them from each other and the unwanted objects, a particle characterization (PCR) algorithm based on Hough transform was employed. The output from the PCR process was utilized for PTV, as well as to generate the corresponding tracer images for special needs. Validation tests have confirmed the pixel accuracy and high reliability of the combined technique. Experimental results obtained with the developed technique include flow velocities, particle velocities, and concentration. The analysis has shown that the particle concentration profile followed an exponential relationship of the form similar to that of Rouse’s profiles, despite the large d _p /h ratio. It also revealed the effect of phase interaction, as a low loading rate of light particles on the order of O(10^?3) could yield a noticeable slowdown in the streamwise fluid velocity.     

Measurements of aerosol product in an axisymmetric co-flow jet

An experimental investigation explored the effects of varying reactant concentration and Reynolds number on the formation of product in a jet of air/N₂/HCl flowing into a co-issuing stream of air/NH₃. Turbulent mixing resulted in the production of NH₄Cl particles by a chemical reaction with negligible heat release. Laser light was elastically scattered in the transition regime between Rayleigh and Mie scattering from the particles. Scattered light intensity served as an indicator of particle mass concentration. Radial profiles of mean and root mean square concentrations were obtained in the self-similar far field region of the jet. The stoichiometric mixture fraction was varied by varying the concentration of NH₃ in the co-flowing stream. It was found that the “flame” length decreased with increasing stoichiometric mixture fraction, and was independent of Reynolds number. The overall amount of product decreased as the stoichiometric mixture fraction was increased from 0.06 to 0.27, while the amount of limiting reactant was the same in both cases. Received: 28 April 1998/Accepted: 16 November 1999     

An experimental investigation of single-phase natural circulation behavior in a rectangular loop with Al2O3 nanofluids

In this paper, the natural circulation behavior in a rectangular loop was investigated experimentally with water and different concentration of Al₂O₃ nanofluids (0.3–2% by wt. and particle size 40–80 nm). It was demonstrated that, not only the flow instabilities are suppressed but also the natural circulation flow rates are enhanced with nanofluids. The enhancement in natural circulation flow rate and suppression of instabilities were found to be dependent on the concentration of nanoparticles in water.     

Counting and measuring particles sized from soot to pollen

For number concentration measurements of superfine particles a condensation nucleus counter （CNC） is frequently used. The combination of a new CNC module with a white light aerosol spectrometer and a passive collector makes possible accurate time-resolved determination of particle number within the overall size range of 10 nm to 40 μm and at concentrations up to 10^5 particles/cm^3. With the aerosol spectrometer a high time-resolved particle size determination is also possible in the size range of 0.3-40 μm up to the same high number concentrations of 10^5 particles/cm^3.     

Impact comminution of solids due to local kinetic energy of high shear strain rate: I. Continuum theory and turbulence analogy

The modeling of high velocity impact into brittle or quasibrittle solids is hampered by the unavailability of a constitutive model capturing the effects of material comminution into very fine particles. The present objective is to develop such a model, usable in finite element programs. The comminution at very high strain rates can dissipate a large portion of the kinetic energy of an impacting missile. The spatial derivative of the energy dissipated by comminution gives a force resisting the penetration, which is superposed on the nodal forces obtained from the static constitutive model in a finite element program. The present theory is inspired partly by Grady's model for expansive comminution due to explosion inside a hollow sphere, and partly by analogy with turbulence. In high velocity turbulent flow, the energy dissipation rate gets enhanced by the formation of micro-vortices (eddies) which dissipate energy by viscous shear stress. Similarly, here it is assumed that the energy dissipation at fast deformation of a confined solid gets enhanced by the release of kinetic energy of the motion associated with a high-rate shear strain of forming particles. For simplicity, the shape of these particles in the plane of maximum shear rate is considered to be regular hexagons. The particle sizes are assumed to be distributed according to the Schuhmann power law. The condition that the rate of release of the local kinetic energy must be equal to the interface fracture energy yields a relation between the particle size, the shear strain rate, the fracture energy and the mass density. As one experimental justification, the present theory agrees with Grady's empirical observation that, in impact events, the average particle size is proportional to the (−2/3) power of the shear strain rate. The main characteristic of the comminution process is a dimensionless number B_a (Eq. (37)) representing the ratio of the local kinetic energy of shear strain rate to the maximum possible strain energy that can be stored in the same volume of material. It is shown that the kinetic energy release is proportional to the (2/3)-power of the shear strain rate, and that the dynamic comminution creates an apparent material viscosity inversely proportional to the (1/3)-power of that rate. After comminution, the interface fracture energy takes the role of interface friction, and it is pointed out that if the friction depends on the slip rate the aforementioned exponents would change. The effect of dynamic comminution can simply be taken into account by introducing the apparent viscosity into the material constitutive model, which is what is implemented in the paper that follows.     

Aerosol composition,sources, and secondary processing during autumn at a regional site in the Beijing–Tianjin–Hebei region

Air pollution is serious during autumn in the Beijing–Tianjin–Hebei (BTH) region, but there are few studies that have utilized real-time observations and source apportionment of the autumn submicron aerosols in this region. In this study, a quadrupole aerosol chemical speciation monitor (Q-ACSM) was deployed for the real-time measurement of the non-refractory compositions of submicron aerosols (NR-PM₁) at a regional site (Xianghe) from October 3 to November 14, 2017. The results showed that nitrate was the largest inorganic aerosol, and the oxygenated organic aerosol (OOA) was the largest organic aerosol in Xianghe. Hydrocarbon-like OA (HOA) was the largest organic aerosol When the NR-PM₁ mass concentrations increased from the lowest to the highest bins, nitrate and biomass burning OA (BBOA) showed increasing trends in the suburban area. Enhanced nitrate formation during the pollution episodes resulted from both photochemical and aqueous processing. To reduce the particulate matter (PM_2.5) concentrations and eliminate heavy pollution episodes, control measures should focus on reducing NO_x, NH₃, and volatile organic compound (VOC_s) emissions.     

Soot Particle Size Distribution Functions in a Turbulent Non-Premixed Ethylene-Nitrogen Flame

A scanning mobility particle sizer with a nano differential mobility analyzer was used to measure nanoparticle size distribution functions in a turbulent non-premixed flame. The burner utilizes a premixed pilot flame which anchors a C₂H₄/N₂ (35/65) central jet with Re _D = 20,000. Nanoparticles in the flame were sampled through a N₂-filled tube with a 500- μm orifice. Previous studies have shown that insufficient dilution of the nanoparticles can lead to coagulation in the sampling line and skewed particle size distribution functions. A system of mass flow controllers and valves were used to vary the dilution ratio. Single-stage and two-stage dilution systems were investigated. A parametric study on the effect of the dilution ratio on the observed particle size distribution function indicates that particle coagulation in the sampling line can be eliminated using a two-stage dilution process. Carbonaceous nanoparticle (soot) concentration particle size distribution functions along the flame centerline at multiple heights in the flame are presented. The resulting distributions reveal a pattern of increasing mean particle diameters as the distance from the nozzle along the centerline increases.     

Vibration induced flow in hoppers: DEM 2D polygon model

A two-dimensional discrete element model （DEM） simulation of cohesive polygonal particles has been developed to assess the benefit of point source vibration to induce flow in wedge-shaped hoppers. The particle-particle interaction model used is based on a multi-contact principle. The first part of the study investigated particle discharge under gravity without vibration to determine the critical orifice size （Bc） to just sustain flow as a function of particle shape. It is shown that polygonal-shaped particles need a larger orifice than circular particles. It is also shown that Bc decreases as the number of particle vertices increases. Addition of circular particles promotes flow of polygons in a linear manner. The second part of the study showed that vibration could enhance flow, effectively reducing Bc. The model demonstrated the importance of vibrator location （height）, consistent with previous continuum model results, and vibration amplitude in enhancing flow.     

Effects of deformed volume, volume fraction and particle size on the deformation behaviour of W/Cu composites

Tungsten/copper (W/Cu) particle reinforced composites were used to investigate the scaling effects on the deformation and fracture behaviour. The effects of the volume fraction and the particle size of the reinforcement (tungsten particles) were studied. W/Cu-80/20, 70/30 and 60/40 wt.% each with tungsten particle size of 10 μm and 30 μm were tested under compression and shear loading. Cylindrical compression specimens with different volumes (D_S = H) were investigated with strain rates between 0.001 s⁻¹ and about 5750 s⁻¹ at temperatures from 20 °C to 800 °C. Axis-symmetric hat-shaped shear specimens with different shear zone widths were examined at different strain rates as well. A clear dependence of the flow stress on the deformed volume and the particle size was found under compression and shear loading. Metallographic investigation was carried out to show a relation between the deformation of the tungsten particles and the global deformation of the specimens. The size of the deformed zone under either compression or shear loading has shown a clear size effect on the fracture of the hat-shaped specimens.The quasi-static flow curves were described with the material law from Swift. The parameters of the material law were presented as a function of the temperature and the specimen size. The mechanical behaviour of the composite materials were numerically computed for an idealized axis-symmetric hat-shaped specimen to verify the determined material law.     

Experimental study of turbulence-induced coalescence in aerosols

In this paper, measurements of the rate of aerosol coalescence in a well characterized turbulent flow are presented. The time dependence of the aerosol droplets’ mean radius upon initiation of flow in an oscillating grid generated turbulence chamber is determined using a phase-Doppler method. Together with a measurement of the aerosol number density from a light attenuation probe, the observed rate of change of the aerosol droplets’ mean radius can be related to the rate constant for the coalescence of two droplets. The Kolmogorov shear rate, which is the primary parameter in theories predicting coalescence rate, is determined from measurements of the root-mean-square fluctuating velocity and the integral length scale. Our experimental results are compared with theoretical predictions, obtained by solving of the population balance equation. Various expressions are considered for the coalescence rate constant to be used in the population balance equation. First, we considered various combinations of ideal coalescence rate constants, i.e. obtained theoretically neglecting particle interactions. Our data are then found to be in good agreement with theoretical predictions that take into account the simultaneous effects of turbulent shear induced and Brownian motion induced coalescence. Second, our results are compared with a theory that considers the effects of turbulent shear and Brownian motion as well as the non-continuum hydrodynamic and van der Waals interparticle interactions. The measured experimental values are generally 50–100% higher than those predicted by this theory. This discrepancy could be explained by the small polydispersity of the aerosol which may result in coalescence induced by differential sedimentation and turbulent acceleration.     

Bimodal model of slurry viscosity with application to coal-slurries. Part 2. High shear limit behavior

It is shown that in a truly bimodal coal-water slurry the hydrodynamic interactions between the coarse particles impose on the fine fraction a shear rate higher than that applied externally by the viscometer walls. A semi-empirical function of the coarse volume fraction is obtained for this correction factor to the applied shear rate. The derivation of this shear correction factor is based on lubrication concepts and introduces the maximum packing fraction,ø _m, at which flow can take place.ø _m is obtainable from a simple dry packing experiment. It is shown that the contribution of the coarse particles to the viscosity rise can be successfully described by a viscosity model employing the same concepts used to derive the shear correction factor. The bimodal model is applied in the high shear limit to polymodal coal slurries with a continuous particle size distribution. In the model, the contribution of the coarse particles to the viscosity rise is taken from separate viscosity measurements for the coarse coal particles, while the contribution to the viscosity of the fine coal particles is taken to be that given by the measured viscosity of colloidal suspensions of monomodal rigid spheres. It is shown that there is a ratio of coarse to fine fraction volumes in the continuous size distribution, corresponding to a specific separating particle size, for which the measured viscosities of the polymodal slurries match almost perfectly over the whole solids volume fraction range with the viscosity values obtained using the bimodal approach. The match is found to be relatively insensitive to the precise value of the separating particle size.     

Development,verification and application of a versatile aerosol calibration system for online aerosol instruments

Traditional calibration methods mostly focus on the calibration of detection systems while the calibration from the sampling and pre-condition systems to the detection system is usually ignored. In this regard, a Primary Standard Aerosol Mass Concentration Calibration System (PAMAS) is developed for the whole-process calibration of time-resolved aerosol measurement instruments. PAMAS is composed of a particle generation chamber, an ultrasonic atomizer, a dilution system, and a syringe pump. It is designed to steadily generate standard aerosol particles of known concentrations (≤250 μg/m³), chemical compositions, and stable particle size distributions. Monodispersed aerosol can be generated in the size range of hundreds of nanometers to several micrometers with a narrow size distribution. The generated particles with different compositions generated by PAMAS have been well verified by the filter-based gravimetric method, yielding accuracy and R² of more than 95% and 0.999 in a wide concentration range. The response time by changing the target concentration of reference particles is 1–2 min. PAMAS has been applied to various types of time-resolved aerosol measurement instruments, including particle mass concentration monitors (Beta Attenuation and Tapered Element Oscillating Microbalance), online Ion Chromatograph, and semi-continuous OCEC carbon aerosol analyzer. Very consistent results between PAMAS and calibrated instruments can be obtained if the instruments are functioning well. As for instruments with certain technical issues, PAMAS can serve as a good tool for performance evaluation and quality assurance of the instruments and the accuracy of the measurement data can be adjusted based on the calibration results.     

Pore-structure-mediated hierarchical SAPO-34: Facile synthesis,tunable nanostructure,and catalysis applications for the conversion of dimethyl ether into olefins

Hierarchical cross-like SAPO-34 catalysts with different pore size distributions were obtained via hydrothermal synthesis with polyethylene glycol (PEG) as the mesopore-generating agent. The hierarchical SAPO-34 molecular sieves were characterized using X-ray diffraction, scanning electron microscopy, N₂ adsorption–desorption, thermogravimetric analysis, and temperature-programmed NH₃ desorption. The cross-like SAPO-34 catalysts exhibited enriched multi-porosity, and the sizes of their mesopores ranged from 10 to 50 nm. Both the mesoporous structures and morphologies of the hierarchical SAPO-34 could be further tuned through adjustments of the amount of PEG used. The as-obtained SAPO-34 showed dramatic catalytic performance in the conversion of dimethyl ether into olefins. A maximum selectivity of olefins of 96% was achieved, which was attributed to the rapid transport of the reactants and products in zeolitic micropores through mesopores.     

The structure of multidimensional strained flames under transcritical conditions

Strained flames are commonly used to study the structure of reactive layers and describe the local properties of turbulent combustion. This model is attractive because constant strain rate flames only depend on a transverse coordinate and can be treated as a one-dimensional problem. This configuration is considered in a multidimensional context in which the strained flow is obtained by two counterflowing streams of reactants. It is used to examine the structure of transcritical strained flames in which one or two reactants are injected at a high pressure exceeding the critical value while their temperature is below the critical value. Calculations are carried out in a two-dimensional domain to test numerical models developed for multidimensional simulations and test thermodynamic and transport models devised to deal with high pressure real gas effects. Multidimensional strained flame calculations carried out in this study serve to check the validity of a new version of a Navier–Stokes flow solver (AVBP) conceived to deal with transcritical combustion of interest to liquid propellant rocket applications. This article describes the basic elements of such simulations and discusses results of calculations. It is shown that the calculated multidimensional strained flames have the expected features in terms of structure and response to the imposed strain rate. To cite this article: L. Pons et al., C. R. Mecanique 337 (2009).     

Suspensions of monodisperse spheres in polymer melts: particle size effects in extensional flow

Suspensions in polymeric, viscoelastic liquids have been studied in uniaxial extensional flow. The fibre wind-up technique has been used for this purpose. The effects of particle size and particle volume fraction have been investigated, using monodisperse, spherical particles. The results have been compared with shear flow data on the same materials. The values of the relative extensional viscosities at low stretching rates are in agreement with the relative shear viscosities and relative moduli. This indicates that hydrodynamic forces are stronger than the particle interaction forces. At larger strain rates strain hardening occurs; it is suppressed when particles are added. Small aggregating particles reduce the strain hardening more strongly than larger particles; strain hardening can even be totally eliminated. When further increasing the stretching rate, hydrodynamic effects dominate again and the effect of particle size effect on strain hardening disappears.     

Size effects on stress concentration induced by a prolate ellipsoidal particle and void nucleation mechanism

There generally exist two void nucleation mechanisms in materials, i.e. the breakage of hard second-phase particle and the separation of particle–matrix interface. The role of particle shape in governing the void nucleation mechanism has already been investigated carefully in the literatures. In this study, the coupled effects of particle size and shape on the void nucleation mechanisms, which have not yet been carefully addressed, have been paid to special attention. To this end, a wide range of particle aspect ratios (but limited to the prolate spheroidal particle) is considered to reflect the shape effect; and the size effect is captured by the Fleck–Hutchinson phenomenological strain plasticity constitutive theory (Advance in Applied Mechanics, vol. 33, Academic Press, New York, 1997, p. 295). Detailed theoretical analyses and computations on an infinite block containing an isolated elastic prolate spheroidal particle are carried out to light the features of stress concentrations and their distributions at the matrix–particle interface and within the particle. Some results different from the scale-independent case are obtained as: (1) the maximum stress concentration factor (SCF) at the particle–matrix interface is dramatically increased by the size effect especially for the slender particle. This is likely to trigger the void nucleation at the matrix–particle interface by cleavage or atomic separation. (2) At a given overall effective strain, the particle size effect significantly elevates the stress level at the matrix–particle interface. This means that the size effect is likely to advance the interface separation at a smaller overall strain. (3) For scale-independent cases, the elongated particle fracture usually takes place before the interface debonding occurs. For scale-dependent cases, although the SCF within the particle is also accentuated by the particle size effect, the SCF at the interface rises at a much faster rate. It indicates that the probability of void nucleation by the interface separation would increase.     

Extensional-flow-induced crystallization of isotactic polypropylene

A filament stretching extensional rheometer with a custom-built oven was used to investigate the effect of uniaxial flow on the crystallization of polypropylene. Prior to stretching, samples were heated to a temperature well above the melt temperature to erase their thermal and mechanical histories and the Janeschitz-Kriegl protocol was applied. The samples were stretched at extension rates in the range of 0.01 s^-1 ￡ [(e)\dot] ￡ 0.75 s^-10.01\,\mbox{s}^{-1}\le \dot{{\varepsilon }}\le 0.75\,{\rm s}^{-1} to a final strain of ε = 3.0. After stretching, the samples were allowed to crystallize isothermally. Differential scanning calorimetry was applied to the crystallized samples to measure the degree of crystallinity. The results showed that a minimum extension rate is required for an increase in percent crystallization to occur and that there is an extension rate for which percent crystallization is maximized. No increase in crystallization was observed for extension rates below a critical extension rate corresponding to a Weissenberg number of approximately Wi = 1. Below this Weissenberg number, the flow is not strong enough to align the contour path of the polymer chains within the melt and as a result there is no change in the final percent crystallization from the quiescent state. Beyond this critical extension rate, the percent crystallization was observed to increase to a maximum, which was 18% greater than the quiescent case, before decaying again at higher extension rates. The increase in crystallinity is likely due to flow-induced orientation and alignment of contour path of the polymer chains in the flow direction. Polarized light microscopy verified an increase in number of spherulites and a decrease in spherulite size with increasing extension rate. In addition, small angle X-ray scattering showed a 7% decrease in inter-lamellar spacing at the transition to flow-induced crystallization. Although an increase in strain resulted in a slight increase in percent crystallization, no significant trends were observed. Crystallization kinetics were examined as a function of extension rate by observing the time required for molten samples to crystallize under uniaxial flow. The crystallization time was defined as the time at which a sudden increase in the transient force measurement was observed. The crystallization time was found to decrease as one over the extension rate, even for extension rates where no increase in percent crystallization was observed. As a result, the onset of extensional-flow-induced crystallization was found to occur at a constant value of strain equal to ε _c  = 5.8.