Esther Hontañón

Single Charging of Nanoparticles by UV Photoionization at High Flow Rates

The feasibility of UV photoionization for single unipolar charging of nanoparticles at flow rates up to 100 l· min −1 is demonstrated. The charging level of the aerosol particles can be varied by adjusting the intensity of the UV radiation. The suitability of a UV photocharger followed by a DMA to deliver monodisperse nanoparticles at high aerosol flow rates has been assessed experimentally in comparison to a radioactive bipolar charger ( 85 Kr, 10 mCi). Monodisperse aerosols with particle sizes in the range of 5 to 25 nm and number concentrations between 10 4 and 10 5 cm −3 have been obtained at flow rates up to 100 l· min −1 with the two aerosol chargers. In terms of output particle concentration, the UV photoionizer performs better than the radioactive ionizer with increasing aerosol flow rate. Aerosol charging in the UV photoionizer is described by means of a photoelectric charging model that relies on an empirical parameter and of a diffusion charging model based on the Fuchs theory. The UV photocharger behaved as a quasi-unipolar charger for polydisperse aerosols with particles sizes less than 30 nm and number concentrations ∼10 7 cm −3 . Much reduced diffusion charging was observed in the experiments, with respect to the calculations, likely due to ion losses onto the walls caused by unsteady electric fields in the irradiation region.

A Differential Mobility Analyzer (DMA) for Size Selection of Nanoparticles at High Flow Rates

This article demonstrates the feasibility of scaling-up the technique for particle size selection in the gas phase based on differential mobility analysis. Nano-DMAs used to select the particle size in processes for the synthesis of nanomaterials in the laboratory operate at aerosol flow rates of a few liters per minute. A new DMA capable of classifying nanoparticles of up to 30 nm in size at aerosol flow rates as high as 100 l· min −1 will be presented (HF-DMA). A major advantage of the HF-DMA over current nano-DMAs covering the same particle size is that its resolution is almost unaffected by Brownian diffusion for particles as small as 3 nm. Monodisperse nanoparticles in the 5 to 25 nm size range have been produced at flow rates of up to 90 l· min −1 . The spread in particle size and the particle number concentration were studied with respect to their dependency on the flow rates in the HF-DMA. The measurements reflect the behavior predicted by the theory. The HF-DMA makes it possible to deliver nanoparticles of a well-defined size at yields two orders of magnitude higher than with current nano-DMAs.

Reducing Multiple Charging of Submicron Aerosols in a Corona Diffusion Charger

An existing corona diffusion charger (Büscher et al. 1994) is optimized in order to reduce multiple charging of submicron particles. A positive-zero rectangular wave voltage is applied to the inner electrode and the ion concentration in the charging region is controlled by varying the duty cycle of the pulse voltage, i.e., the fraction of time during which the voltage applied to the electrode is positive. The performance of the corona charger is assessed experimentally and theoretically for monodisperse aerosols with particle sizes in the range of 50 to 250 nm and number concentrations below 10 5 cm−3. Optimal corona voltage and pulse voltage values of +4.5 kV and +5 V are identified. A duty cycle about 10% resulted in multiple charging levels comparable to positively charged particles in bipolar chargers, with higher fractions of singly charged particles. Multiple charging can be lowered by reducing further the duty cycle. When using the corona ionizer for charging of polydisperse aerosols, operational corona voltage and pulse voltage values must be adjusted in dependency of the aerosol particle number concentration. The corona charger is suitable for size selection of submicron particles by differential mobility analysis (DMA).

Influence of the inter-electrode distance on the production of nanoparticles by means of atmospheric pressure inert gas dc glow discharge

This work is aimed at investigating the influence of the inter-electrode spacing on the production rate and size of nanoparticles generated by evaporating a cathode on an atmospheric pressure dc glow discharge. Experiments are conducted in the configuration of two vertically aligned cylindrical electrodes in upward coaxial flow with copper as a consumable cathode and nitrogen as a carrier gas. A constant current of 0.5 A is delivered to the electrodes and the inter-electrode distance spanned from 0.5 to 10 mm. Continuous stable nanoparticle production is attained by optimal coaxial flow convection cooling of the cathode. Both the particle production rate and the primary particle size increase with the inter-electrode spacing up to nearly 5 mm and strongly decrease with an increasing inter-electrode distance beyond 5 mm. Production rates in the range of 1 mg h −1 of very small nanoparticles (<10 nm) are attained by a micro glow discharge (<1 mm); while glow discharges of intermediate sizes (<5 mm) result in production rates of up to 10 mg h −1 and primary particles of sizes between 10 and 20 nm. No correlation is found between the measured spatially averaged plasma parameters and nanoparticle production. Since the latter is largely determined by the properties of the cathode surface, spatially resolved spectrometric measurements are needed to discern between the positive column and the cathode region of the glow discharge plasma.

Assessment of a Cylindrical and a Rectangular Plate Differential Mobility Analyzer for Size Fractionation of Nanoparticles at High-Aerosol Flow Rates

An existing differential mobility analyzer (DMA) of cylindrical electrodes and a novel DMA of rectangular plate electrodes are demonstrated for size fractionation of nanoparticles at high-aerosol flow rates in this work. The two DMAs are capable of delivering monodisperse size selected nanoparticles (SMPS σg < 1.1) at gas flow rates ranging from 200 slm to 500 slm. At an aerosol flow rate of 200 slm, the maximum attainable particle mean size is of about 20 nm for the cylindrical DMA and of nearly 50 nm for the rectangular plate DMA. The number concentration of the monodisperse nanoparticles delivered by the high-flow DMAs spans from 104 cm−3 to 106 cm−3 depending upon the particle mean size and particle size dispersion. Copyright 2014 American Association for Aerosol Research

A Sustainable Route for Antibacterial Nanofinishing of Textiles

In this paper, an aerosol-based process is shown for imparting antibacterial property to textiles. Metal nanoparticles (copper and silver) are produced by means of DC electrical discharges (glow and arc) between two electrodes in nitrogen at ambient pressure and passed through textile fabrics (cotton, polyester and lyocell) which act as filter media. The particle retention efficiency of the fabrics is measured in dependence of particle size and face velocity. The antibacterial performance of the fabrics treated with metal nanoparticles and its durability to wash is assessed according to industry standards. Loads of about 200 ppm (2x10-2 %wt.) of nanoparticles of copper or silver give strong antibacterial property but the colour and hand feeling of the fabrics are significantly affected. Nanosilver loads in the order of 50 ppm (5x10-3 %wt.) impart comparably high antibacterial property to the fabrics with no visible impact on colour and hand touching, and wash fastness is proven for 10 washes. Small silver nanoparticles (<5 nm) result in much less release of silver to wash water, with respect to larger nanoparticles (>20 nm).