The role of microexplosions in flame spray synthesis for homogeneous nanopowders from low‐cost metal precursors

One of the most versatile and rapid manufacturing processes for a variety of nanopowders is flame spray pyrolysis (FSP). The production costs of this scalable process are largely controlled by the raw materials, pushing for the utilization of low‐cost metal precursors. These, however, typically yield inhomogeneous products containing large particles up to micrometer size along with fine nanoparticles. Here, the release mechanism of nitrate and carboxylate precursors from spray droplets has been investigated by single‐droplet combustion experiments and thermogravimetric analysis. The results show that neither precursor evaporation nor choice of solvents is prerequisite for homogeneous nanopowders but droplet microexplosions with continuing combustion. It is shown that even low‐cost metal nitrates yield homogeneous nanopowders if precursors are formulated such that droplet microexplosions occur by internal superheating. The proposed precursor release mechanisms are verified with lab‐ and pilot‐scale FSP, demonstrating that single‐droplet combustion experiments can be employed to predict the product quality. © 2015 American Institute of Chemical Engineers AIChE J, 62: 381–391, 2016     

Miniemulsions for the Production of Nanostructured Particles

Nanostructured particles made from polystyrene and zinc oxide are synthesized by precipitation in miniemulsions and miniemulsion polymerization. There are two main applications for miniemulsions: the formation of sub‐micron or nano‐sized reactors for the precipitation of inorganic nanoparticles and the use of sub‐micron or nano‐sized droplets as templates for nanostructured particles. The latter includes the formation of the desired structures within a monomer droplet, which then is polymerized without changing its size or structure during the process. In this research article two approaches to combine both processes are presented: The zinc oxide nanoparticles are precipitated in an inverse miniemulsion of water droplets in a continuous monomer phase. The resulting miniemulsion is either distilled and the nanoparticles are forced into the monomer phase or the miniemulsion is used directly without distillation. In both cases the particle loaded monomer droplets are afterwards polymerized to hybrid nanoparticles. The focus is on the technological challenges in producing nano‐sized, hybrid particles, especially in regard to continuous processing.     

Surface force arising from adsorbed graphene oxide in alumina suspensions with different shape and size

The effects of graphene oxide (GO) on the yield stress‐pH of α‐Al₂O₃ (alumina) suspensions were investigated. For micron‐sized platelet alumina suspensions, micron‐sized GO additive increased the maximum yield stress by as much as six‐folds. This was attributed to GO‐mediated bridging interactions between the platelet particles. This type of bridging interactions was much less effective with submicron‐sized, spherical, and irregular shape alumina. Adsorption of the anionic GO reflected by the shift of pH of zero zeta potential to a lower pH is particularly high for platelet alumina. The 1.0 dwb % GO concentration added is sufficient to reinforce each platelet particle–particle bond, assisted by a directed GO–platelet interaction configuration. This is, however, not true with submicron‐sized particles as the particle concentration increases sharply with the inverse of the particle diameter to power of 3. Moreover, a GO sheet can adsorb several submicron‐sized particles and this does not produce the right interaction configuration. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3633–3641, 2013     

Nanoparticle formation through solid‐fed flame synthesis: Experiment and modeling

The preparation of silica nanoparticles through solid‐fed flame synthesis was investigated experimentally and theoretically. Monodispersed submicrometer‐ and micrometer‐sized silica powders were selected as solid precursors for feeding into a flame reactor. The effects of flame temperature, residence time, and precursor particle size were investigated systematically. Silica nanoparticles were formed by the nucleation, coagulation, and surface growth of the generated silica vapors due to the solid precursor evaporation. Numerical modeling was conducted to describe the mechanism of nanoparticle formation. Evaporation of the initial silica particles was considered in the modeling, accounting for its size evolution. Simultaneous mass transfer modeling due to the silica evaporation was solved in combination with a general dynamics equation solution. The modeling and experimental results were in agreement. Both results showed that the methane flow rate, carrier gas flow rate, and initial particle size influenced the effectiveness of nanoparticle formation in solid‐fed flame synthesis. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Al2O3–Ag nanopowders: new method of synthesis,characterisation and biocidal activity

Abstract

Abstract

The present paper describes an innovative method of producing silver nanoparticles incorporated into an aluminium nano‐oxide substrate. The method utilises thermal decomposition and reduction, which yields an Al₂O₃–Ag nanopowder with the average size of particles ranging from 43 to 60?nm and the average size of agglomerates between 330 and 870?nm. The average size of the silver nanoparticles incorporated in the aluminium nano‐oxide carrier ranges from 22 to 60?nm. The Al₂O₃–Ag nanopowders thus produced have a largely developed surface area (above 200?m²?g^?1) with a great number of open pores (above 5×10^?4?m³?g^?1), which gives evidence that their tendency to agglomeration is only slight and that the possible agglomerates have a loose structure. Moreover, the nanopowders show good bactericidal and fungicidal properties. The results obtained in the present experiments show that the Al₂O₃–Ag nanopowders produced by the proposed method can be used successfully as the raw material in the production of biocidal biomaterials.     

碳酸钙填充聚丙烯体系的流变行为研究

利用高压毛细管流变仪研究了纳米CaCO3和微米CaCO3分别单独填充和复配填充聚丙烯(PP)体系的流变行为。对纳米CaCO3填充PP体系,CaCO3的加入使体系的表观黏度(ηa)下降,但随着CaCO3的质量分数增加,ηa呈上升趋势。当CaCO3的质量分数为10%时,两种微米CaCO3(1.8μm和25μm)填充体系的ηa接近,但低于纳米CaCO3填充体系的;随着CaCO3的质量分数增加(如30%),三种体系的ηa接近。对纳米CaCO3和微米CaCO3复配填充PP体系,CaCO3的总质量分数为10%时,当微米CaCO3的质量低于CaCO3总质量的50%时,体系的ηa与纳米CaCO3单独填充体系相比呈显著下降;但随着CaCO3的总质量分数增加,ηa下降的幅度变小。     

The influence of operating conditions on the dense gas precipitation of model proteins

Dense gas techniques provide a suite of clean technology options for the processing of pharmaceuticals. Monodisperse, micron‐sized particles can be produced at mild operating temperatures and with negligible solvent residue. In this study, protein was precipitated from organic solutions using dense carbon dioxide as antisolvent. The gas antisolvent precipitation process (GAS) was used to produce biologically active lysozyme, insulin, and myoglobin powders. The effects of operating temperature, solute concentration and the rate of antisolvent addition on the morphology, size, activity and residual solvent concentration of lysozyme and insulin precipitates have been examined. The powders produced consisted of uniformly sized non‐aggregated spherical particles. Precipitate size was controlled between 0.05 µm and 2.0 µm by changes to the solvent and antisolvent compositions. In general the concentration of residual organic solvent was found to be dependent on the mass of antisolvent used during the washing cycle. Residual concentrations as low as 300 ppm were easily achievable in a single step. © 2000 Society of Chemical Industry     

Preparation and application of silver nanoparticles on silk for imparting antimicrobial properties

Silver nanoparticles were produced by a chemical reduction method that reduced silver nitrate with reducing agents such as hydrazine and glucose. The silver nanoparticles were characterized with transmission electron microscope, scanning electron microscope, and optical microscope. The effects of process parameters such as the stirring speed, temperature, type of reducing agent, and dispersing agent on the particle size were studied. The particle size decreased with an increase in the stirring speed and a decrease in the process temperature. Smaller particles were formed when the silver nitrate was reduced by glucose versus those that were formed by reduction with hydrazine. Silver nanoparticles with average sizes of 10 and 35 nm, produced by reduction with hydrazine at 5 and 40°C, were applied to silk by an exhaust method. Silk fabrics treated with 40 ppm silver hydrosol produced at 5°C and 60 ppm silver hydrosol produced at 40°C showed 100% antimicrobial activity against the gram‐positive bacterium Staphylococcus aureus. The durability of the antimicrobial property of the treated silk fabric to washing was also examined and is presented. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of an organic dicarboxylic acid salt on fractionated crystallization of polypropylene droplets

The effect of a particulate nucleating agent on fractionated crystallization of polypropylene (PP) was studied. A novel method utilizing breakup of PP nanolayers was used to obtain a dispersion of PP droplets in a polystyrene (PS) matrix. An assembly with hundreds of PP nanolayers alternating with thicker PS layers was fabricated by layer‐multiplying coextusion. The concentration of an organic dicarboxylic acid salt (HPN) nucleating agent in the coextruded PP nanolayers was varied up to 2 wt %. When the assembly was heated into the melt, interfacial driven breakup of the thin PP layers produced a dispersion of PP particles in a PS matrix. Analysis of optical microscope images and atomic force microscope images indicated that layer breakup produced a bimodal particle size distribution of submicron particles and large, micron‐sized particles. Almost entirely submicron particles were obtained from breakup of 12 nm PP layers. The fraction of PP as submicron particles dropped dramatically as the PP nanolayer thickness increased to 40 nm. Only large, micron‐sized particles were obtained from 200 nm PP nanolayers. The crystallization behavior of the particle dispersions was characterized by thermal analysis and wide angle X‐ray diffraction. Only part of the PP was nucleated by HPN. It was found that HPN was not effective in nucleating the population of submicron particles. The particulate HPN was too large to be accommodated in the submicron PP particles. On the other hand, the amount of nucleated crystallization qualitatively paralleled the fraction of PP in the form of large, micron‐sized particles. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Flame‐based synthesis and in situ functionalization of palladium alloy nanoparticles

Flame‐driven synthesis and functionalization of palladium‐containing nanoparticles is demonstrated using a high temperature reducing jet (HTRJ) process that decouples flame chemistry from particle formation chemistry and provides a reducing environment that enables synthesis of metal nanoparticles from low‐cost aqueous precursors. Nanoparticles with controlled palladium, copper, and silver content were synthesized and functionalized with amine‐containing ligands using both in situ and ex situ approaches. For in situ functionalization, octylamine was sprayed into the quench section of the HTRJ reactor to cap the nanoparticles in the gas phase. For the ex situ approach, the “bare” nanopowders were heated in various amines (hexylamine, octylamine, and oleylamine) to form stable dispersions. Use of oleylamine at high temperature allowed modification of the nanoparticle size and shape while maintaining the alloy composition. These in situ and ex situ functionalization methods provide flexibility to tailor particles for specific applications such as electrocatalysis or hydrogen purification. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3826–3834, 2018     

Deagglomeration of nanoparticle aggregates via rapid expansion of supercritical or high‐pressure suspensions

Deagglomeration of suspensions of alumina and titania nanopowders (i.e., nanoparticle aggregates) via rapid expansion of supercritical suspensions (RESS) or high‐pressure suspensions (REHPS) was studied. The size distribution of fragmented nanopowders was characterized by online Scanning Mobility Particle Spectrometer (SMPS) and Aerodynamic Particle Sizer (APS), and by offline Scanning Electron Microscopy (SEM). SMPS and SEM measurements indicate that the average agglomerate sizes were well below 1 μm, consistent with the length scales observed in our complementary RESS/REHPS mixing experiments using alumina and silica nanopowders. The APS measurements, on the other hand, were affected by reagglomeration during sampling and yielded an agglomerate size range of 1 to 3 μm. Analysis of the RESS/REHPS process through compressible flow models revealed that both the shear stress in the nozzle and the subsequent impact of the agglomerates with the Mach disc in the free expansion region can lead to micron or sub‐micron level deagglomeration. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Densification of rare-earth (Lu, Gd, Nd)-doped alumina nanopowders obtained by a sol-gel route under seeding

Rare-earth (RE: Lu, Gd, Nd, 0.10 mol%)-doped alumina nanopowders were prepared by a new sol-gel route using polyhydroxoaluminum (PHA) and RECl₃ solutions under α-alumina (∼ 75 nm) seeding. Among the rare-earth dopants studied, Lu yields the most suitable nanopowders for low-temperature densification. The 0.10 mol% Lu-doped nanopowders, which were obtained at a calcination temperature of 900 °C under 5 mass% α-alumina seeding, consisted of ∼ 80-nm α-alumina particles and γ-alumina nanoparticles. Using these Lu-doped alumina nanopowders, fully densified alumina ceramics with a uniform microstructure composed of fine grains with an average size of 0.61 μm could be obtained at 1400 °C by pressureless sintering. Clearly, the Lu-doped nanopowders obtained here represent a viable option for fabricating dense, finer-grained alumina ceramics because an undoped sample with 5 mass% seeds gave a microstructure with an average grain size of 1.78 μm at 1400 °C.     

Synthesis of Pd–alumina and Pd–lanthana Suspension for Catalytic Applications by One-step Liquid Flame Spray

Catalytic materials of alumina and lanthana supported nanosized palladium particles (7 wt%) in a water suspension were prepared by Liquid Flame Spray (LFS) method. The particle production rate was 90 g/h, using liquid precursors containing Al(NO₃)₃ · 9H₂O, La(NO₃)₃ · 6H₂O and Pd(NH₃)₄NO₃ in water solution. In the LFS method, a turbulent, high-temperature (T_max ∼ 2,700 °C) H₂–O₂ flame is used. The liquid precursor is atomized into micron sized droplets by high velocity H₂ flow and introduced into the flame where the droplets will evaporate. The evaporated compounds decompose and the reaction product re-condenses into particulate material. Here, the nanosized particles are formed by gas-to-particle conversion and the micron sized particles via liquid-to-solid route. In this study, the produced particulate material was collected by thermophoresis along with condensing water into a suspension (nanoparticles in water) in a one-step process. Thus, the whole suspension was produced from the end products of the flame. According to TEM-EDS analysis, the particulate material contained micron sized porous aluminum oxide or lanthanum oxide carrier particles, coated by nanosized palladium particles (∼2–10 nm). The surfactant (Rhodasurf-La 42) was injected into the suspension just after collection to reduce agglomeration. Catalytic performance of the produced Pd–lanthana containing suspension was tested in laboratory with synthetic gases, in order to use it as a possible raw material for three-way catalyst (TWC). The suspension was used as Pd raw material in TWC washcoat and dispersed onto a metallic honeycomb.     

Microemulsion route to fabrication of silver and platinum‐polymer nanocomposites

Polymethylmethacrylate (PMMA)‐platinum and PMMA‐silver nanocomposites have been produced using polymerization of W/O microemulsions. MMA monomer was used as the oil or continues phase of the microemulsion system and polymerized following formation of Pt and Ag nanoparticles in the fluid medium. The UV‐vis absorption spectra have been used to trace the growth process of the nanoparticles in the microemulsion system. Scanning electron microscopy and transmission electron microscopy (TEM) have been used to determine the morphology and particle size of the Pt and Ag particles in the synthesized nanocomposites. Image analyses of TEM micrographs confirm that the Pt and Ag particles in the synthesized nanocomposites have a narrow size distribution. Meanwhile, Fourier‐transform infrared spectroscopy was used to verify polymer‐nanoparticles interaction in nanocomposite bulk. POLYM. COMPOS., 35:2023–2028, 2014. © 2014 Society of Plastics Engineers     

Narrow size distribution nanoparticle production by electrospray processing of ferritin

Electrospraying and in-flight heating of ferritin, the iron-storage protein, was used to produce controlled size, monodisperse aerosol particles which can be used as size standards for instrument calibration. As aerosol particles can be collected in liquids or on a substrate, standard size aerosol nanoparticles can be used for the calibration and development of not only aerosol instrumentation, but also colloid instrumentation and electron microscopes. Differences in the sizes of apoferritin and ferritin were detectable using scanning mobility particle spectrometry. Apoferritin has a mobility diameter of 11.8 nm, while iron-rich ferritin had a mobility diameter of 13.1 nm and the size distribution function of both apoferritin and ferritin had geometric standard deviations of 1.05. In-flight heating in a furnace aerosol reactor was used to remove the ferritin protein coat and produce monodisperse iron oxide particles 7.9 nm in diameter and a size distribution function geometric standard deviation of 1.07. Ferritin dimers and higher order n-mers, produced from multiple ferritin complexes being present in a single electrospray droplet, remained bound to each other after in-flight heating. Monte Carlo simulations of the electrospray process showed that as long as the electrospray droplets are sufficiently monodisperse, monodisperse standard size nanoparticles can also be produced from ferritin n-mers.     

Nanometer Sized Silver Particles Embedded Silica Particles—Spray Method

Spherical shaped, nanometer to micro meter sized silica particles were prepared in a homogeneous nature by spray technique. Silver nanoparticles were produced over the surface of the silica grains in a harmonized manner. The size of silver and silica particles was effectively controlled by the precursors and catalysts. The electrostatic repulsion among the silica spheres and the electro static attraction between silica spheres and silver particles make the synchronized structure of the synthesized particles and the morphological images are revealed by transmission electron microscope. The silver ions are reduced by sodium borohydride. Infra red spectroscopy and X-ray photoelectron spectroscopy analysis confirm the formation of silver–silica composite particles. Thermal stability of the prepared particles obtained from thermal analysis ensures its higher temperature applications. The resultant silver embedded silica particles can be easily suspended in diverse solvents and would be useful for variety of applications.     

Generation of Submicron Mass/Volume-Monodisperse Aerosols with a Nebulizer-Impactor-Electrostatic Classifier System

A study was undertaken to investigate the mass size dispersion of particles classified according to their electrical mobilities. This is of primary concern in experiments that measure a concentration dependent property of the classified particles. Initially, the mass size distribution of particles produced by the 3-jet Collison, 6-jet Collison, and Misty-Ox nebulizers was measured. A 0–2 stage impactor was placed after the nebulizers and the size mass distribution was measured again. The polydisperse particle stream was then used to generate “size classified” aerosol and the mass size distribution of the equal mobility particles was calculated and measured. It was found that without an impactor in line, the aerosol stream contained a significant mass fraction of multiply charged particles. When an impactor was inserted directly after the nebulizers, the multiply charged particles were effectively removed and the particles were nearly monodisperse.     

In-Situ Characterization of Metal Nanopowders Manufactured by the Wire Electrical Explosion Process

In order to understand the size distributions of metal nanopowders inside manufacturing equipment operated at elevated pressures, a scanning mobility particle sizer is used to carry out in-situ measurements of metal nanopowders manufactured by the wire electrical explosion process. A pressure reducer and rotating disk diluter are used for conditioning metal nanopowder samples appropriate for real-time aerosol instruments operated at atmospheric pressure. Based on measurement data collected downstream of the evaporation chamber, the production of metal nanopowders shows good stability and uniformity for a total number concentration of approximately 5 × 10⁷ particles/cm³, and a unimodal size distribution with a mean diameter of approximately 170 nm. Using an aerosol electrometer and two sets of electrostatic classifiers, positively charged particles slightly outnumber negatively charged particles. The performance of the rotating disk diluter is confirmed by comparing the size distributions of metal nanopowders diluted with five different dilution factors, ranging from 235 to 2500. SEM and TEM image analysis indicates that most metal nanopowders manufactured by this process consist of aggregated particles, and their size distributions obtained from SEM images are similar to those measured by the SMPS. The changes in particle size distribution at each stage of the manufacturing process, including the evaporation chamber, trap buffer, cyclone, and mesh filter, are also monitored using the above in-situ monitoring system. The resulting in-situ measurement data can be used for design modifications of equipment, as well as for investigating the sources of nanopowder release to the workplace environment.     

Preparation of Scratch and Abrasion Resistant Polymeric Nanocomposites by Monomer Grafting onto Nanoparticles, 3. Effect of Filler Particles and Grafting Agents

After modification with different trialkoxysilanes, nano‐sized silica and alumina particles were used as fillers in transparent UV/EB curable acrylates for polymer reinforcement, particularly to attain scratch and abrasion resistant coatings. The acid catalyzed condensation of the organosilanes forms a polysiloxane shell which covers the nanoparticle like a nanocapsule. CP MAS NMR spectroscopy and MALDI‐TOF mass spectrometry proved to be useful for the characterization of the polysiloxane structures. Grafted oligomers with more than 20 monomeric units were observed. Nanoparticles modified by methacroyloxy(propyl)trimethoxysilane and vinyltrimethoxysilane can copolymerize with acrylates. Compared with the pure polymers, these crosslinked polyacrylate nanocomposites, containing up to 35 wt.‐% silica, exhibit markedly improved surface mechanical properties. Promising scratch and abrasion resistance of radiation‐cured nanocomposite materials were also obtained by propyltrimethoxysilane grafting which results in an organophilation of pyrogenic silica. Both colloidal and pyrogenic nano‐sized silica nanopowders were used as fillers in polyacrylate films. The concentration of colloidal SiO₂ in commercial acrylate formulations amounts up to 50 wt.‐%, whereas pyrogenic silica, notwithstanding their surface modification by silanes, results in a thickening effect which limits its content to about 35 wt.‐%. Nevertheless, a comparison showed a distinct improvement in the surface mechanical properties such as haze and diamond microscratch hardness for surface‐modified pyrogenic silica.     

Synthesis of greater than 10‐μm‐sized,monodispersed polymer particles by one‐step seeded polymerization for highly monomer‐swollen polymer particles prepared utilizing the dynamic swelling method

The thermodynamic simulation under the kinetic control state indicates that 1.77‐µm‐sized monodispersed polystyrene (PS) particles can absorb 500 times the amount of the styrene monomer with keeping the monodispersity by the “dynamic swelling method” (DSM) which the authors proposed in 1991. Actually, about 14.1‐µm‐sized monodispersed styrene‐swollen PS particles in which PS seed particles absorbed 500 times the amount of styrene monomer were successfully prepared utilizing DSM. By one‐step seeded polymerization for the dispersion of the swollen particles at 30°C for 48 h with the 2,2′‐azobis(4‐methoxy‐2,4‐dimethyl valeronitrile) initiator, 13.1‐µm‐sized monodispersed PS particles were produced. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 278–285, 1999