The Formation of Porous Membranes by Filtration of Aerosol Nanoparticles

Flame-generated aerosol particles of Al₂O₃ were deposited by gas filtration on two types of porous and ceramic tubes of -Al₂O₃ with mean pore diameters of 450 and 2700nm, respectively. The particles were aggregates with average mobility diameters in the range of 30–100nm and primary particle diameters of 4–8nm. The particles are characterized by differential mobility analysis, transmission electron microscopy, and by their specific surface area. The deposited membranes are characterized by gas permeability measurements, scanning electron microscopy, and by their pore size distribution from nitrogen capillary condensation. The particles form a distinct, homogeneous membrane layer with a porosity of 90% on top of the substrate surface and only penetrate slightly into the substrate structure. The mean pore sizes of the deposited membranes determined by nitrogen condensation agree approximately with those determined by gas permeation and the specific surface area. The mean pore diameter varies in the range of 30–70nm. The gas permeability of the deposited membranes is related to the specific surface area but influenced by the high porosity. The mean pore size and the permeability of the membranes are almost independent of the substrate structure.The development of a membrane with uniform properties is preceded by a short initial period in which the deposited particles, with an equivalent membrane thickness of roughly 2m, have a significantly lower permeability than the ultimately developed uniform membrane layer. This effect is particularly significant for the aerosol particles with the lowest mean size, probably due to particles deposited in the pore mouths of the substrate.The particles and the deposited membranes are X-ray amorphous but retain their specific surface area on heating to even high temperatures. When the membranes are heated to 1473K for 10h, X-ray diffraction shows a mixture of - and -alumina, accompanied by a partial disintegration of the membrane and a considerable loss of surface area.     

Determination of the Recoilless Fraction in Iron Oxide Nanoparticles Using the Two-lattice Method

We propose a two-lattice method for direct determination of the recoilless fraction using a single room-temperature transmission Mössbauer measurement. The method is first demonstrated for the case of iron and metallic glass two-foil system and is next generalized for the case of physical mixtures of two powders. We further apply this method to determine the recoilless fraction of hematite and magnetite particles. Finally, we provide direct measurement of the recoilless fraction in nanohematite and nanomagnetite with an average particle size of 19nm. A list of values obtained for the recoilless fraction in various materials using the two-lattice method is given.     

A High Efficiency,High Throughput Unipolar Aerosol Charger for Nanoparticles

A novel aerosol charger has been developed, which has high efficiency and high throughput especially for nanometer particles in the size range of 3–50nm. Unipolar charging with high ion concentration and long charging time is used to obtain the high charging efficiency. High throughput is achieved by reducing particle loss within the charger. This is accomplished by directing ion flow and aerosol flow in the same direction and by the use of sheath air flow. The charger configuration is of a longitudinal design – the direction of aerosol stream and ion stream are flowing parallel along the longitudinal axis of the charger. The charger consists of four sections: the inlet zone, the ion production zone, the unipolar charging zone, and the exit zone. In the inlet and ion production zones, unipolar ions are generated using Po²¹⁰ radioactive sources with an electric field designed to separate the positive and negative ions, and to focus the selected unipolar ions into the core region of the charger. The ions with the selected polarity is then attracted to the charging zone by an uniform electric field created by a series of ring electrodes applied with a linear ramped voltage. Aerosol entering the charger is sheathed with clean gas flow in order to keep the aerosol in the core region. A novel exit design with a reversed electric field is incorporated in order to minimize the charged particles loss. The performance of the charger is first evaluated using computer simulation and then constructed for experimental validation. Experiment data have demonstrated that the charger achieves 90% and 95% charged-particles penetration efficiency and with 22% and 48% extrinsic charging efficiency at 3 and 5nm particle sizes, respectively. These performance data represent significant improvement, over a factor of 10, compared with the existing chargers.     

A New Probe for Mechanical Testing of Nanostructures in Soft Materials

We report a new application of the optical tweezers, where a harmonically driven oscillating tweezer is combined with the forward light scattering and lock-in amplification techniques, for probing the mechanics of nanostructures in soft materials in a broad frequency range. Model independent dynamic moduli G and G of the material at a localized, sub-micron area can be measured directly from the displacement and the phase shift of the particle in the oscillating trap. The probe particles can be as small as 200nm and the displacement of the particle was in the range of a few nanometers. To illustrate the new methodology, we show the microscopic viscoelastic properties of a transient polymer network in the vicinity of a silica bead.     

Engineering Aspects of Preparation of Nanocrystalline Particles in Microemulsions

Engineering aspects of the preparation of palladium nanoparticles in non-ionic w/o-microemulsions are examined. In order to achieve reproducible synthesis conditions a semi-batch reactor with a standardized design is used. Influences of the stirring rate and of different ways of concentration control on the product properties are observed. For reproducible synthesis it is important to establish appropriate and defined preparation conditions. Monodisperse palladium particles of around 5nm size are obtained by adding the microemulsion containing the palladium salt at a constant feed rate to the precharged microemulsion containing the reducing agent. A quantitative kinetic model is proposed to describe particle formation in microemulsions. Unknown parameters of the model have been estimated by independent examinations or can be achieved by fitting to the experimental data.     

Synthesis of In<Subscript>2</Subscript>O<Subscript>3</Subscript>nanoparticles by thermal decomposition of a citrate gel precursor

This paper describes the synthesis of indium oxide by a modified sol–gel method, and the study of thermal decomposition of the metal complex in air. The characterization of the intermediate as well as the final compounds was carried out by thermogravimetry, differential thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and small angle X-ray scattering. The results show that the indium complex decomposes to In₂O₃ with the formation of an intermediate compound. Nanoparticles of cubic In₂O₃ with crystallite sizes in the nanosize range were formed after calcination at temperatures up to 900°C. Calcined materials are characterized by a polydisperse distribution of spherical particles with sharp and smooth surfaces.This revised version was published online in August 2005 with a corrected issue number.     

Fabrication of copper oxide nanofluid using submerged arc nanoparticle synthesis system (SANSS)

The optimal parameters are found for preparing nanofluid in our submerged arc nanoparticle synthesis system (SANSS) using a copper electrode. A suspended copper oxide nanofluid is thus produced at the current of 8.5–10 A, voltage of 220 V, pulse duration of 12 μs, and dielectric liquid temperature of 2°C. The CuO nanoparticle are characterized by transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), electron diffraction pattern (SAD) and electron spectroscopy for chemical analysis (ESCA). The equality volume spherical diameter of the obtained copper oxide particle is 49.1 nm, regular shape and narrow size distribution.This revised version was published online in August 2005 with a corrected issue number.     

Formation of Si-nanoparticles in a microwave reactor: Comparison between experiments and modelling

The formation and growth of silicon-nanoparticles from silane in a microwave reactor was investigated. Experiments were performed for the following conditions: precursor concentration 380–2530 ppm, pressures of 20–30 mbar, microwave powers 120–300 W. The formed particles were examined in-situ with a particle mass spectrometer. Additionally, particles were collected on grids and analyzed by transmission electron microscopy, X-ray diffraction, and by determining the specific surface area by BET. The particle size was found to be in the range of 5–8 nm in diameter. A simple model was used to simulate the particle formation processes taking place inside the reactor. The microwave energy coupled into the reactor flow was treated as a spatially distributed energy source resulting in a local temperature increase. The particles were assumed to have a monodisperse size distribution. To allow an approximation of their shape they were characterized by their volume and surface area. The model takes nucleation, convection, coagulation, and coalescence into account. The fluid flow inside the microwave reactor was simulated with the commercial CFD-code Fluent.     

Synthesis of Silver Nanoparticles by the Ion Implantation Method and Investigation of their Optical Properties

We have investigated the process of metal nanoparticle (NP) synthesis in SiO₂ by implanting Ag₊ ions with an energy of 30 keV depending on the dose ((2–8)·10¹⁶ cm⁻²) and the ionic current density (4–15 µA/cm²). Analysis of the composite materials formed was performed with the use of optical spectroscopy and atomic-force microscopy (AFM). The NPs synthesized in the glass demonstrate a characteristic absorption line associated with the surface plasma resonance effect. A correlation of the spectral shift of the lines caused by a change in the NP size with the diameter of the hemispherical asperities on the SiO₂ surface registered by the AFM method has been revealed. It has been found that for the case of a fixed current density in the ion beam the silver NP sizes remain practically unaltered with increasing ion dose up to a certain value (6·10¹⁶ cm⁻²), and only an increase in the concentration of NPs is observed thereby. However, a further increase in the dose causes a decrease in both the NP density and size. On the other hand, at a fixed high dose an increase in the ionic current density leads to a gradual enlargement of the NPs. We have considered the mechanisms explaining the change in the NP sizes with increasing dose and ionic current density and evaluated the possibilities of carrying out controlled synthesis by varying the implantation conditions.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 218–223, March–April, 2005.     

The Extended Surface as a Factor Forming Distinctive Features of Nanodimensional Media

As a result of studying thin-film nano-sized media, it is shown that the extended surface exerts a substantial influence on their distinctive properties. It has been established that, by using special methods to obtain nano-sized media under certain conditions, it is possible to form crystal structures with a high concentration of defects of a prescribed type in the near-surface layers of nanocrystals. The correlations found between the conducting and optical properties of thin-film nano-sized structures are an additional factor that extends the capabilities of controlling their characteristics with a view toward developing miniature effective radiation sources, elements of modulation of laser radiation, dosimeters, and components of thin-film solar cells. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 287–291, May–June, 2005.