Search

The paper studies nanoparticle formation in a glowing wire generator (GWG), in which the gas carrier flows around heated metal wire, producing aerosols from a vapor released from the surface. The device has been customized, enabling the use of a double-wire in different orientations in regard to the gas flow. Such alterations provided different effective distances between wires enabling investigation of their mutual influence. Concentration of particles produced in the GWG at different parameters (applied voltage and a gas flow) was carefully measured and analysed. Different regimes of a nanoparticle nucleation were identified that resulted from the applied voltage variation and the gas flow direction. In particular, independent nucleation of nanoparticles on both parts of the wire occurred in the wire plane’s configuration perpendicular to the gas flow, whilst dependent nucleation of nanoparticles was observed at a certain specific set of parameters in the configuration, in which the wire plane was parallel to the gas flow. Two corresponding functions were introduced in order to quantify those nucleation regimes and they tend to zero when either independent or dependent nucleation occur. The peculiarities found ought to be considered when designing the multi-wire GWGs in order to further extend the device’s range for industrial applications.

In this paper we report the results of investigation of a gas carrier velocity influence on morphology and structure of molybdenum oxide nanoparticles, produced by a glowing wire generator. The device operational principle is based on electrical resistance related heating of a metal wire and placement it into relatively cool and not ionized gas carrier for rapid solidification of vapour released from the wire surface. The results show that the suggested system is capable of producing a variety of nanostructures by changing a single process parameter—velocity of the gas carrier.

[1] Iron-rich nanoparticles in aeolian mineral dust are of considerable importance to biogeochemical cycles. A major determinant of the chemical characteristics of nanoparticles is the parent sediment they are sourced from. The abrasion of dune sand has previously been shown to produce coarse dust (>1 � m) during the occurrence of aeolian saltation. In this study, Australian red dune sands were laboratory abraded and emission of particles 18–414 nm was observed throughout the experiment duration (� 1 h). The mean size of particles was 130 nm at the start of the test, but this gradually decreased to 110 nm at the end. The number concentration of particles approximately trebled over the course of the experiment with results suggesting that collisions between mobile sand grains led to the production of new nanosized particles over time. Chemical analysis revealed that these nanoparticles were highly abundant in iron, with some aluminium present. This chemical composition suggests that nanoparticles are produced from the clay coatings surrounding the parent sand grains. The study shows that abrasion from saltation occurring in Australian dune sands can release iron-rich nanoparticles, making them available for downwind transport during blowing dust events.

Thermophoretic and electrophoretic coatings are the main viable mechanisms for the coating of objects with nanoparticles. Unlike electrophoretic coating, thermophoretic coating has the advantage that electrically conductive substrates are not a requirement. This paper investigates the thermophoretic deposition and uniformity of molybdenum oxide nanoparticles, generated by a glowing wire generator, on various surfaces at three different flow rates (0.3, 1 and 1.5 L min–1). The quantitative evidence of the presence of particles collected by a suggested thermophoretic precipitator at different flow rates has shown that a uniform distribution of the particles could be achieved across the whole area of the precipitator. SEM and TEM micrographs of the film confirmed that a homogeneous densely packed network of molybdenum oxide nanoparticles was built across the precipitation area at the flow rate of 1.5 L min–1.

A new thermophoretic particle precipitator has been developed for representative and efficient collection of aerosol particles from the ambient air and technological pipelines. The device consists of hot and cold plates (5×5cm) capable of operation at temperature gradients ranging from 20000 to 100000K/m. A gas sample is made to pass through a 1-mm slot between the plates at a flow rate of up to 1.5L/min, which makes the device suitable for operation in conjunction with common aerosol instruments including DMA and diffusion batteries with similar operational flow rates. It was shown that the efficiency of the device was highest for the lowest gas flow rate used (0.3L/min) reaching a level of above 99%. The efficiency was decreased reaching its minimal values at the highest flow rate investigated (1.5L/min). However, even for highest flow rate, the average efficiency for removal of particle smaller than 60nm was around 50%.

Elongated aerosol particle removal on fibrous filters has been investigated. It was shown that particle agglomerates are removed much more efficiently compared to the regularly shaped single particles with identical electrical mobility diameter at two filtration velocities tested. The experimental results were compared with the classical filtration theory and it was shown that the theoretical predictions, which are based on the assumption that the particles are spherical, are significantly different compared to an agglomerate filtration efficiency value. In order to account for a particle shape non-regularity, dominating nanoparticle removal mechanisms were firstly evaluated for a regular particle of certain size and then adjusted by fitting coefficients k1 (for diffusion component) and k2 (for interception). These coefficients were determined by fitting the theoretical values that gives the best coincidence with the measured data points. As was further demonstrated theoretically, the coefficient k1 is identical to the ratio of the actual particle surface area to the surface area of the spherical particle of the equivalent diameter. On the other hand, the coefficient k2 was found to be equal to the ratio of the projection of a given particle on a plane perpendicular to a streamline, to that of the spherical particle of the equivalent diameter. The reported findings would allow undertaking more accurate evaluation of the removal efficiency of non-regular aerosol particle, which is especially important for industrial applications where non-regular aerosols are frequently met.

The filtration efficiency of a conventional fibrous filter was investigated with particular emphasis on the removal of particles with different shapes. A previous study has shown that particles of spherical shape are removed from the gas carrier with efficiencies which are higher when compared to cubic particles of the same aerodynamic size. In this project, to challenge our previously made explanation, spherical PSL and cubic MgO particles were tested along with particles of sodium chloride (NaCl) of intermediate shape (cubic particles with rounded edges) at a range of filtration velocities from 5 to 20 cm/s. It was found that particles of NaCl are removed with efficiencies lower then those for PSL particles but higher than the efficiencies for cubic particles of MgO, at the lowest filtration velocity when inertial effects are negligible. The rounded NaCl particles, depending on the geometry of the contact, could either land on the rounded corner and hence roll, land on a sharp edge and hence tumble, or slide. This range of options alters the probability of detachment of the particle. The difference between the filter efficiencies for cubic MgO particles and intermediate shaped NaCl particles is decreasing with the increase in velocity. With increasing velocity, the filtration efficiency of the cubic MgO particles, exceeds the filtration efficiency for the intermediate shaped NaCl particles, due to the dominating inertial effects of the denser, and hence heavier, MgO particles. This paper shows the results of these experiments and, we hope, will ignite the interest of the aerosol community towards further theoretical analysis of the phenomenon.

In our previous work it has been shown that perfectly spherical polystyrene latex (PSL) particles have higher filtration efficiency compared to cubic magnesium oxide (MgO) particles of the same electrical mobility as PSL particles. This disparity was ascribed to the different nature of motion of the spherical and cubic particles along the fibre surface, following the initial collision. After touching the fibre surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could slide or tumble. During tumbling, the area of contact between the particle and the fibre changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of particle trajectory. In this project, the polypropylene filter was coated with a thin layer of mineral oil that was used to absorb the energy and, respectively, to minimize particle motion along the fibre after collision. The filtration efficiency of spherical PSL, and cubic MgO particles was measured in the size range of 50–300 nm, for filtration velocity of 10 and 20 cm/s. It was found that, regardless of shape, both particle types have very similar filtration efficiency. The theoretical predictions are in good agreement with our experimental results. Therefore, the conclusion can be drawn that the oil coating minimizes the amount of particle motion along the fibre after initial collision, making results for all particle shapes similar.

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50-300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.

Publisher Summary Fibrous filters have long been recognized as an efficient means of removing aerosol particles from the carrier gas or air stream. There are various mechanisms by which the aerosol particles are captured on a filter material, and these include interception, inertia, diffusion, electrostatic attraction, and gravity. The aerosol particle size and filter properties determine the removal mechanisms that dominate for a particular filtration application. This chapter gives a review of removing fine particles on fibrous filters. Many theories have been developed to describe the particle interaction with the surface of the filter and to estimate the probability of the particle adhesion onto a surface. The collection efficiency of a fibrous filter depends on the structure of the filter (porosity, fiber diameter, and filter thickness), the operational conditions (filtration velocity, temperature, and humidity), and, in particular, the filtering aerosol characteristics (particle density, size, and shape). The possibility of the particle bouncing depends on its composition, its shape, its velocity, and the type of impaction surface. When a solid particle contacts a surface at low velocity, the particle loses its kinetic energy by deforming itself and the surface. At higher velocities, part of the kinetic energy is dissipated in the deformation process (plastic deformation) and part is converted elastically to kinetic energy of rebound. If the rebound energy exceeds the adhesion energy—the energy required to overcome the adhesive forces—a particle will bounce away from the surface. When the materials comprising the particle and surface become harder, the particle becomes larger, or its velocity becomes greater, then the likelihood of the particle bouncing from the surface also becomes greater.