Flame Aerosol Reactor Research Articles

Design of Turbulent Flame Aerosol Reactors by Mixing-Limited Fluid Dynamics

Nanoparticle synthesis in turbulent flame aerosol reactors is elucidated by computational fluid dynamics (CFD). Mixing-limited combustion is modeled, and total particle number, area, and volume concentration are described by transport equations including terms for particle dynamics. The spread of the particle size distribution at a given streamline is neglected as flame-made aerosols rapidly attain their self-preserving distribution. Results are in good agreement with primary particle data of turbulent diffusion flame synthesis of silica nanoparticles by oxidation of hexamethyldisiloxane vapor at different laboratories without adjustable parameters. Measured agglomerate mobility diameters best matched the predicted volume-equivalent soft-agglomerate diameters. The employed fractal-like dimensions (Df = 1.5−3) had no effect on the predicted primary particle and aggregate diameters and a rather small effect on volume-equivalent soft-agglomerate diameters.

Flame aerosol reactor synthesis of nanostructured SnO 2 thin films: High gas-sensing properties by control of morphology

Tin dioxide (SnO 2) thin films were deposited on commercial alumina chips using a flame aerosol reactor (FLAR) system. The morphology of the thin films was controlled by changing various parameters, such as precursor concentration, burner–substrate distance, and deposition time. Three typical morphologies of the thin films were achieved, i.e., columnar, column–granular, and granular structures. The as-prepared thin films with columnar structures demonstrated improved sensing performance to ethanol vapor, due to their preferred orientation, single crystal phase, and unique morphology.

Continuous Surface Functionalization of Flame-Made TiO2 Nanoparticles

Hydrophilic TiO(2) particles made in a flame aerosol reactor were converted in situ to hydrophobic ones by silylation of their surface hydroxyl groups. So the freshly formed titania aerosol was mixed with a fine spray of octyltriethoxysilane (OTES) in water/ethanol solution and functionalized continuously at high temperature. The extent of functionalization and structure of that surface layer were assessed by thermogravimetric analysis (TGA) coupled to mass spectroscopy (MS), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and Raman spectroscopy. Product particles were characterized also by transmission electron microscopy (TEM), X-ray diffraction, and nitrogen adsorption. The influence of titania specific surface area (SSA) and OTES solution concentration on the functional group surface density was investigated. The titanium dioxide surface was covered with functional groups (up to 2.9 wt %) that were thermally stable up to 300 degrees C in air at an average density of 2 OTES/nm(2). Such surface-functionalized particle suspensions in 2-ethylhexanoic acid and xylene were stable over several weeks. In contrast, as-prepared hydrophilic TiO(2) precipitated within days in these solvents.

Size distributions of aerosols in an indoor environment with engineered nanoparticle synthesis reactors operating under different scenarios

Size distributions of nanoparticles in the vicinity of synthesis reactors will provide guidelines for safe operation and protection of workers. Nanoparticle concentrations and size distributions were measured in a research academic laboratory environment with two different types of gas-phase synthesis reactors under a variety of operating conditions. The variation of total particle number concentration and size distribution at different distances from the reactor, off-design state of the fume hood, powder handling during recovery, and maintenance of reactors are established. Significant increases in number concentration were observed at all the locations during off-design conditions (i.e., failure of the exhaust system). Clearance of nanoparticles from the work environment was longer under off-design conditions (20 min) compared to that under normal hood operating conditions (4–6 min). While lower particle number concentrations are observed during operation of furnace aerosol reactors in comparison to flame aerosol reactors, the handling, processing, and maintenance operations result in elevated concentrations in the work area.

Bacterial responses to Cu-doped TiO 2 nanoparticles

The toxicity of Cu-doped TiO 2 nanoparticles (NPs, 20 nm), synthesized by a flame aerosol reactor, to Mycobacterium smegmatis and Shewanella oneidensis MR-1, is the primary focus of this study. Both doped and non-doped TiO 2 NPs (20 nm) tended to agglomerate in the medium solution, and therefore did not penetrate into the cell and damage cellular structures. TiO 2 particles (< 100 mg/L) did not apparently interfere with the growth of the two species in aqueous cultures. Cu-doped TiO 2 NPs (20 mg/L) significantly reduced the M. smegmatis growth rate by three fold, but did not affect S. oneidensis MR-1 growth. The toxicity of Cu-doped TiO 2 NPs was driven by the release of Cu 2+ from the parent NPs. Compared to equivalent amounts of Cu 2+, Cu-doped TiO 2 NPs exhibited higher levels of toxicity to M. smegmatis ( P-value < 0.1). Addition of EDTA in the culture appeared to significantly decrease the anti-mycobacterium activity of Cu-doped TiO 2 NPs. S. oneidensis MR-1 produced a large amount of extracellular polymeric substances (EPS) under NP stress, especially extracellular protein. Therefore, S. oneidensis MR-1 was able to tolerate a much higher concentration of Cu 2+ or Cu-doped TiO 2 NPs. S. oneidensis MR-1 also adsorbed NPs on cell surface and enzymatically reduced ionic copper in culture medium with a remediating rate of 61 µg/(liter∙OD 600∙ hour) during its early exponential growth phase. Since the metal reducing Shewanella species can efficiently “clean” metal-oxide NPs, the activities of such environmentally relevant bacteria may be an important consideration for evaluating the ecological risk of metal-oxide NPs.

Quench-Ring Assisted Flame Synthesis of SiO2-TiO2 Nanostructured Composite

A flame aerosol reactor (FLAR) was used to synthesize SiO2-based nanocomposite materials of SiO2-TiO2 in different precursor molar ratios and quench ring positions. Processing conditions were determined that resulted in formation of different crystal phases at different precursor concentration molar ratios. The results showed that the addition of SiO2 inhibited TiO2 phase transformation from anatase to rutile. The different morphology of SiO2-TiO2 nanocomposite was primarily the result of changing the quench ring position. Quenching can stop the growth kinetics, and this was obtained by locating the quench ring at different positions in the particle formation process. At a lower position, a binary mixture of SiO2-TiO2 was obtained in a core–shell structure. The difference in residence time for sintering and coalescence yielded different morphologies of SiO2-TiO2 nanocomposites. The results from FTIR confirmed Si-O-Ti bonding for all samples. The samples of SiO2-TiO2 exhibited better suspension in liquid than pure TiO2 as confirmed by zeta potential measurements.

Synthesis of visible light-active nanostructured TiOx (x < 2) photocatalysts in a flame aerosol reactor

Titanium dioxide is a wide band gap (3.2eV) semiconductor which is photo-active when irradiated with UV light. For wider scale use of TiO2 as a photocatalyst, its activity needs to be extended to the visible light region (constituting 45% of total incident solar energy). A diffusion flame aerosol reactor (FLAR) with an oxygen lean environment in the particle formation zone has been used to synthesize oxygen deficient titanium suboxide (TiOx with x<2) nanoparticles. Using a standard-based electron energy loss spectroscopy (EELS) technique, the non-stoichiometry (x in TiOx) in the flame synthesized particles has been quantified with high accuracy (uncertainty less than 3%). Under an oxygen lean environment in the particle formation zone, the non-stoichiometry in the TiOx particles is a function of the flame temperature. The value of x in the flame synthesized TiOx nanoparticles is in the range of 1.88<x<1.94. Diffuse reflectance spectra confirmed that the oxygen deficient TiOx particles absorbed visible light. Visible light activity of the TiOx particles is demonstrated by photocatalytic degradation of methyl orange solution under visible light illumination.

One-step synthesis of noble metal–titanium dioxide nanocomposites in a flame aerosol reactor

Abstract Noble metal–titanium dioxide nanocomposites (Pt/TiO 2 , Pd/TiO 2 and bimetallic Pt-Pd/TiO 2 ) were synthesized in one step using a flame aerosol reactor (FLAR). The specific surface area, crystal phase and morphology of the nanocomposites were controlled by adjusting the reactant concentration and the temperature–time history in the reactor. The synthesized nanomaterials were characterized using transmission electron microscopy (TEM), electron diffraction, X-ray diffraction (XRD) and nitrogen adsorption (BET). Nanocomposites with 0.5–3.0% (wt%) noble metal loading were synthesized. Nanosized noble metal particles (2–4 nm) were dispersed on the 30–40 nm TiO 2 surface with an overall specific surface area in the range of 40–60 m 2 /g. The specific surface area increased with increasing noble metal loading. For the chosen flame conditions, a mixture of anatase and rutile phase was obtained without noble metal addition. On incorporation of the noble metal, the formation of the rutile phase of titanium dioxide was suppressed. The synthesized nanocomposites were tested for the photocatalytic oxidation of methyl orange dye in an aqueous phase. Platinum particles dispersed on the TiO 2 surface enhanced the photocatalytic activity compared to pristine TiO 2 . The existence of an optimum platinum loading for the highest photocatalytic activity was confirmed, and was approximately 0.5–1.0% Pt. Palladium addition had a detrimental effect on the photocatalytic activity of titanium dioxide. Bimetallic noble metal catalysts (Pt-Pd/TiO 2 ) showed enhanced photocatalytic activity compared to pristine titanium dioxide, but lower than platinum (only)–titanium dioxide nanocomposites.

Hydrothermal Synthesis and Photoluminescence of Eu2−xSmxSn2O7 (x = 0–2.0) Nanophosphors

A flame aerosol reactor (FLAR) was used to synthesize SiO2-based nanocomposite materials of SiO2-TiO2 in different precursor molar ratios and quench ring positions. Processing conditions were determined that resulted in formation of different crystal phases at different precursor concentration molar ratios. The results showed that the addition of SiO2 inhibited TiO2 phase transformation from anatase to rutile. The different morphology of SiO2-TiO2 nanocomposite was primarily the result of changing the quench ring position. Quenching can stop the growth kinetics, and this was obtained by locating the quench ring at different positions in the particle formation process. At a lower position, a binary mixture of SiO2-TiO2 was obtained in a core-shell structure. The difference in residence time for sintering and coalescence yielded different morphologies of SiO2-TiO2 nanocomposites. The results from FTIR confirmed Si-O-Ti bonding for all samples. The samples of SiO2-TiO2 exhibited better suspension in liquid than pure TiO2 as confirmed by zeta potential measurements.

Nanostructured TiO2Films with Controlled Morphology Synthesized in a Single Step Process: Performance of Dye-Sensitized Solar Cells and Photo Watersplitting

Nanostructured titanium dioxide (TiO2) films were synthesized with controlled morphology and thickness in an ambient pressure single-step flame aerosol reactor (FLAR) for use in water splitting photocells and dye-sensitized solar cells. Two different morphologies were studied: a granular morphology and a highly crystalline columnar morphology. The granular morphology consisted of nanoparticles, approximately 10 nm in diameter, aggregated into fractal structures on the substrate. The granular morphology contained a large number of grain boundaries and other interfacial defects. The columnar morphology consisted of single-crystal structures, approximately 85 nm in width, oriented normal to the substrate. The well controlled deposition process was used to deposit films with thicknesses in the range from 98 nm to 12 μm to establish the relationship to water splitting and dye-sensitized solar cell performance. It was found that for watersplitting there was an optimum thickness (∼1.5 μm), which was a tradeoff between light absorption and electron transport losses, where the conversion efficiency was maximized. Due to differences in electron transport and lifetime in the TiO2 film, for both applications the columnar morphology outperformed the granular morphology, achieving a uv-light to hydrogen conversion efficiency of 11% for water splitting and a visible light to electricity conversion efficiency of 6.0% for the dye-sensitized solar cell.

Preparation of Homogeneous, Bulk Nanocrystalline Ni/Mo Alloys with Tripled Vickers Hardness Using Flame‐Made Metal Nanoparticles.

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Preparation of Homogeneous, Bulk Nanocrystalline Ni/Mo Alloys with Tripled Vickers Hardness Using Flame-Made Metal Nanoparticles

Metals and metal−alloys have been traditionally prepared by large-scale reduction and ore refining. The present work demonstrates how this reduction and alloying step can be executed within flame aerosol reactors on a sub-second time scale and gives access to metal−alloy nanoparticles of 10−50 nm size at high production rate. We demonstrate the preparation of Ni/Mo and Hastelloy like metal nanoparticles as examples for this process and showed that the product particles exhibited an exceptionally high air stability and narrow particle size distribution in comparison to presently accessible materials. Sintering and compaction gave access to centimeter-sized bulk nanocrystalline alloys with strongly enhanced Vickers hardness. The versatility of already broadly used aerosol processing of oxides suggests that the extension of flame synthesis to metals and alloys may offer an alternative for the preparation of multicomponent nanocrystalline metals.

Synthesis of nanoparticles in a flame aerosol reactor with independent and strict control oftheir size, crystal phase and morphology

A flame aerosol reactor (FLAR) was developed to synthesize nanoparticles with desired properties(crystal phase and size) that could be independently controlled. The methodology was demonstratedfor TiO2 nanoparticles, and this is the first time that large sets of samples with the same size but differentcrystal phases (six different ratios of anatase to rutile in this work) were synthesized. The degree ofTiO2 nanoparticle agglomeration was determined by comparing the primary particle sizedistribution measured by scanning electron microscopy (SEM) to the mobility-based particlesize distribution measured by online scanning mobility particle spectrometry (SMPS). Bycontrolling the flame aerosol reactor conditions, both spherical unagglomerated particlesand highly agglomerated particles were produced. To produce monodisperse nanoparticles,a high throughput multi-stage differential mobility analyser (MDMA) was used in serieswith the flame aerosol reactor. Nearly monodisperse nanoparticles (geometricstandard deviation less than 1.05) could be collected in sufficient mass quantities (ofthe order of 10 mg) in reasonable time (1 h) that could be used in other studiessuch as determination of functionality or biological effects as a function of size.

Nanostructured photoactive films synthesized by a flame aerosol reactor

AbstractA flame aerosol reactor (FLAR) system was used to deposit nanostructured photocatalytic films of titanium dioxide with well controlled morphologies. Nanoparticles were generated in the aerosol phase and then deposited onto a water‐cooled substrate via thermophoresis. Two important parameters that influenced film characteristics were the titanium precursor feed rate and substrate temperature, through their effect on particle sintering dynamics on the substrate. The size of the particles as they arrived at the substrate was controlled by varying the titanium precursor feed rate. When the size was below ∼8 nm, sintering was completed in the time available to obtain films with columnar nanostructures. Larger particle sizes resulted in granular, particulate films. The temperature of the substrate was also an important parameter as it controlled the sintering rate and the resultant crystal phase of the film. The thickness of the films was controlled by varying the precursor feed rate and deposition time. The performance of the as‐synthesized photocatalytic films was established by measuring the resultant photocurrents. Well sintered columnar morphologies and thicker films (in the range of 40–900 nm) resulted in the largest photocurrents. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Silica-based composite and mixed-oxide nanoparticles from atmospheric pressure flame synthesis

Binary TiO2/SiO2 and SnO2/SiO2 nanoparticles have been synthesized by feeding evaporated precursor mixtures into an atmospheric pressure diffusion flame. Particles with controlled Si:Ti and Si:Sn ratios were produced at various flow rates of oxygen and the resulting powders were characterized by BET (Brunauer–Emmett–Teller) surface area analysis, XRD, TEM and Raman spectroscopy. In the Si–O–Ti system, mixed oxide composite particles exhibiting anatase segregation formed when the Si:Ti ratio exceeded 9.8:1, while at lower concentrations only mixed oxide single phase particles were found. Arrangement of the species and phases within the particles correspond to an intermediate equilibrium state at elevated temperature. This can be explained by rapid quenching of the particles in the flame and is in accordance with liquid phase solubility data of Ti in SiO2. In contrast, only composite particles formed in the Sn–O–Si system, with SnO2 nanoparticles predominantly found adhering to the surface of SiO2 substrate nanoparticles. Differences in the arrangement of phases and constituents within the particles were observed at constant precursor mixture concentration and the size of the resultant segregated phase was influenced by varying the flow rate of the oxidant. The above effect is due to the variation of the residence time and quenching rate experienced by the binary oxide nanoparticles when varying the oxygen flow rate and shows the flexibility of diffusion flame aerosol reactors.

Monitoring simultaneously the growth of nanoparticles and aggregates by in situ ultra-small-angle x-ray scattering

Ultra-small-angle x-ray scattering can provide information about primary particles and aggregates from a single scattering experiment. This technique is applied in situ to flame aerosol reactors for monitoring simultaneously the primary particle and aggregate growth dynamics of oxide nanoparticles in a flame. This was enabled through the use of a third generation synchrotron source (Advanced Photon Source, Argonne IL, USA) using specialized scattering instrumentation at the UNICAT facility which is capable of simultaneously measuring nanoscales to microscales (1nmto1μm). More specifically, the evolution of primary-particle diameter, mass-fractal dimension, geometric standard deviation, silica volume fraction, number concentration, radius of gyration of the aggregate, and number of primary particles per aggregate are measured along the flame axis for two different premixed flames. All these particle characteristics were derived from a single and nonintrusive measurement technique. Flame temperature profiles were measured in the presence of particles by in situ Fourier transform infrared spectroscopy and thermophoretic sampling was used to visualize particle growth with height above the burner as well as in the radial direction.

Gas-phase synthesis of nanoparticles: scale-up and design of flame reactors

The scale-up and design of flame aerosol reactors are investigated for synthesis of silica and titania nanoparticles. In specific, coflow burners of different dimensions are studied at various precursor, fuel, and oxygen flow rates. The influence of the flame enthalpy content on product primary particle size is investigated by changing the fuel from methane to propane or hydrogen. Operation lines relating product particle size with reactant outlet conditions, burner size, and flame enthalpy are developed, showing how the different reactors can produce silica or titania nanoparticles of the same size. A scale-up procedure developed for fumed silica is extended to the synthesis of titania nanoparticles covering production rates of 2–200 g/h. At high fuel-oxidant velocity difference at the burner outlet, the operation of diffusion flame reactors converges to that of premixed ones.

Carbon-coated titania nanostructured particles: Continuous, one-step flame-synthesis

Concurrent synthesis of titania-carbon nanoparticles (up to 52 wt.% in C) was studied in a diffusion flame aerosol reactor by combustion of titanium tetraisopropoxide and acetylene. These graphitically layered carbon-coated titania particles were characterized by high-resolution transmission electron microscopy (HRTEM), with elemental mapping of C and Ti, x-ray diffraction (XRD), and nitrogen adsorption [Brunauer-Emmett-Teller (BET)]. The specific surface area of the powder was controlled by the acetylene flow rate from 29 to 62 m2/g as the rutile content decreased from 68 to 17 wt.%. Light blue titania suboxides formed at low acetylene flow rates. The average XRD crystal size of TiO2 decreased steadily with increasing carbon content of the composite powders, while the average BET primary particle size calculated from nitrogen adsorption decreased first and then approached a constant value. The latter is attributed to the formation of individual carbon particles next to carbon-coated titania particles as observed by HRTEM and electron spectroscopic imaging.

Scale-up of nanoparticle synthesis in diffusion flame reactors

The scaling-up of diffusion flame aerosol reactors is investigated for synthesis of nanoparticles. Three co-flow burners of different dimensions are studied at various precursor, oxidant and fuel flow rates. An operation line relating product particle size with reactant outlet conditions, flow rate and burner size is developed showing how the three reactors can produce silica and titania nanoparticles of the same size and morphology. This operation line shows the limit of fast reactant mixing where the diffusion flame aerosol reactors perform as premixed ones. An operation diagram is obtained for different silica production rates and a scale-up procedure is developed.

The Aggregation of Thermally Stable Particles in a Premixed Flat Flame Aerosol Reactor

Using a premixed flat flame, the aggregation process of thermally stable particles was studied. Based on the theoretical descriptions of the reaction limited aggregation process (RLAP) and the diffusion limited aggregation process (DLAP), the entire aggregation process was explained by a combination of the RLAP and the DLAP. Experiments were done to test the possibility of this explanation for the dynamic aggregation processes in terms of the parameters that are used in a normal chemical conversion process. By defining the primary particle as the basic unit of the aggregates, the aggregation process was divided into the early stage--when the primary particles are formed--and the stage of aggregation thereafter. Changing the combustion equivalence ratio of the premixed flat flame from 0.7 to 0.98 controlled the sizes of the primary particles. The deficiency of the oxygen in the combustion process controlled the final size of the primary particles in the aggregate. The morphological evolution of particles w...