Takafumi Seto

Evaluation of Sintering of Nanometer-Sized Titania Using Aerosol Method

The sintering of titania agglomerates consisting of nanometer primary particles in a heated gas flow is investigated under gas temperatures from room temperature to 1673 K. The test titania agglomerates are produced by thermal decomposition or hydrolysis of TTIP (titanium tetraisopropoxide) vapor. The size changes of the agglomerates of 30–100 nm in diameter are measured using a TDMA (Tandem Differential Mobility Analyzer) system. At a temperature lower than about 1000 K, the agglomerates do not change in size with increasing heating temperature, but a sudden decrease in size is detected at 1000 to 1500 K. From TEM observation, densification of agglomerates accompanying primary particle growth is observed. These experimental results for the rate of reduction in surface area are explained quantitatively by solving the basic equation of sintering under the calculated temperature profile.

Size distribution measurement of nanometer-sized aerosol particles using dma under low-pressure conditions

Abstract A system for measuring nm-sized aerosol particles under low pressure conditions using a differential mobility analyzer (low pressure DMA, LPDMA) with a Faraday cup electrometer (FCE) as the detector is developed. The performance of the LPDMA is investigated experimentally using a tandem DMA technique. It is shown that LPDMA can be used to measure nm-sized particles (4–10 nm) under low-pressure conditions (60–760 Torr). The effect of Brownian diffusion on the performance of LPDMA is qualitatively discussed by comparing the results obtained using LPDMA with those of conventional and Vienna DMAs with dimensionless parameters. The equilibrium charge distribution of aerosol particles reached under low-pressure conditions is also evaluated by solving population balance equations for particle-ion collision. The LPDMA/FCE system is also applied to the measurement of nm-sized silver particles produced by physical vapor deposition (PVD) and TiO 2 particles produced by low pressure chemical vapor deposition (LPCVD)

Sintering of Polydisperse Nanometer-Sized Agglomerates

ABSTRACT In order to evaluate the sintering characteristics of polydisperse TiO2 and SiO2 fractal agglomerates consisting of nm-sized primary particles (nanoparticles), the change in size distribution of agglomerates in a heated pipe is measured using a differential mobility analyzer and a condensation nucleus counter. Change in the structure and the primary particle size of the agglomerates collected by a thermophoretic aerosol sampler is also measured by the transmission electron microscopy as a function of furnace temperature. Coalescence of the agglomerates and the growth of primary particles due to sintering are observed at temperatures corresponding to 50%–100% of the bulk melting points of the particle material. To model the changes in the size distribution of agglomerates and primary particles due to sintering, two-dimensional sectional representation of the size distribution is employed, and the population balance equation for sintering is solved numerically. The calculated results, considering a...

Condensation of supersaturated vapors on monovalent and divalent ions of varying size

Vapor nucleation induced by seed ions of controlled size and charge is studied for several monovalent positive and negative ions as well as divalent cations. Ions from dissolved salts are transferred into a gas via electrospray, purified by differential mobility selection, and introduced into a condensation nucleus counter of the mixing type using supersaturated dibutyl phthalate vapors. The number of drops nucleated on the ions is then measured as a function of the vapor supersaturation S. None of the anions tested is activated. Singly charged cations with mobilities Z from 0.48 up to 0.93u2009cm2/V/s are activated at values of lnu2009S 30% smaller than predicted by Thomson’s model. All doubly charged cations tested (Z from 0.46 to 1.08u2009cm2/V/s) fall very near the Thomson curve, independently of their size or of whether the double ionic charge is localized on one metal atom or on two separate monovalent sites.

Filtration of Multi-Walled Carbon Nanotube Aerosol by Fibrous Filters

Filtration efficiency of multi-walled carbon nanotube (MWCNT) aerosol by fibrous filter was evaluated experimentally. Mono-mobility test aerosols with electrical mobility diameter of 100, 200, and 300 nm were generated by the atomization of MWCNT aqueous suspension followed by mobility classification with a differential mobility analyzer (DMA). By analyzing the shape of classified aerosol particles under a scanning electron microscope, it was found that the DMA-classified 300 nm particles were fibrous in shape and had uniform diameter of about 60 nm and length of 2.1 micrometer. On the other hand, 100 nm and 200 nm particles contained a fairly large amount of multiply charged fibrous particles with a larger diameter. These test aerosols were challenged to a medium performance fibrous filter at various filtration velocities. As a result, fibrous particles were captured by fibrous filter at a higher collection efficiency than the spherical particles with the same mobility. By analyzing the single fiber capturing efficiency, interception incorporating the rotation of fibrous particles is found to be the dominant capturing mechanism for the fibrous particles in the studied size range.

Wall Deposition of Ultrafine Aerosol Particles by Thermophoresis in Nonisothermal Laminar Pipe Flow of Different Carrier Gas

Deposition of ultrafine aerosol particles by Brownian diffusion and thermophoresis in a nonisothermal laminar pipe flow is investigated theoretically and experimentally. The deposition rates of Ag and TiO2 particles of 8–30 nm in diameter suspended either in N2, Ar or He gas are measured using a circular pipe with a nonuniform wall temperature distribution. The numerical calculation results considering the gas flow, temperature and particle distributions in the pipe show good agreement with the measured values. The particle deposition is enhanced with the increase of the wall temperature variation, but does not depend on the particle material. The difference in the deposition rates among the carrier gases is mostly attributable to the values of the Brownian diffusion coefficient. The enhancement of deposition by thermophoresis is of a similar magnitude among the carrier gases, which is explained in terms of the temperature distribution of the gases in the pipe.

Influence of nitrogen excited species on the destruction of naphthalene in nitrogen and air using surface dielectric barrier discharge.

The destruction of naphthalene, as representative polycyclic aromatic hydrocarbons, by surface dielectric barrier discharge is investigated in air as well as dry and humidified nitrogen at ambient temperature. Naphthalene destruction efficiency is evaluated in terms of chemical change vis-a-vis energy utilization. The detected byproducts are qualitatively evaluated in order to understand the role of the active species in the destruction process. The results show that the destruction efficiency and the energy efficiency are higher in the dry nitrogen than in the humidified nitrogen, and these decrease with the increase of the humidity. Measured concentration of ozone as a byproduct qualitatively indicates the roles of oxygen and ozone in the destruction process in air. The analysis of the aerosol particles formed during the destruction process, both in the dry and humidified nitrogen, confirmed the adverse effects of the humidity on the byproducts formation and subsequently on the destruction process.

Decomposition of toluene with surface-discharge microplasma device

A surface-discharge microplasma device (SMD) was developed for the decomposition of volatile organic compounds (VOCs) in the gas phase. The device is composed of a microscale-patterned electrode, a dielectric substrate, and a ground electrode. As a result of localized dielectric barrier discharge (DBD), surface-discharge microplasma was generated when a piezoelectric-transformed high voltage (66.7 kHz, 3.5 kV) was applied to the microscale-patterned electrode. The discharge current and voltage characteristics of the DBD were analyzed under atmospheric conditions. Toluene decomposition rate was evaluated by gas chromatography–mass spectrometry and nondispersive IR-absorption CO2 analysis. A decomposition efficiency of more than 99% was achieved in batch experiments. When the SMD was operated in a flow reactor, 30–80% of toluene was reduced with the percentage depending on residence time. The carbon balance between the toluene starting material and the CO2 product indicates that toluene was almost completely decomposed into CO2 by atomic oxygen in the microplasma.

Performance of a surface dielectric barrier discharge based reactor for destruction of naphthalene in an air stream

A surface dielectric barrier discharge reactor is used to investigate the destruction of naphthalene, as an example of polycyclic aromatic hydrocarbon compound, in an air stream. The characteristics of the reactor including the current–voltage and the power consumed are examined to evaluate its effectiveness in destroying naphthalene. The effects of different operational parameters including applied voltage, frequency, flow rate and initial concentration of naphthalene on the destruction process are studied. The results indicated that the destruction efficiency decreases with increasing flow rate and increases with an increase in the applied voltage and frequency. Although the destruction efficiency is independent of the initial concentration, the energy efficiency is increased with increasing initial concentration. The study shows that naphthalene is totally destroyed in the reactor, but not completely transformed to CO and CO2. Ozone generated as a byproduct in the reactor did not play a role in the destruction process.

High-Efficiency Unipolar Charger for Sub-10 nm Aerosol Particles Using Surface-Discharge Microplasma with a Voltage of Sinc Function

Our group developed a high-performance unipolar charger for sub-10-nm particles using a surface-discharge microplasma by increasing the charging time and minimizing the electrostatic deposition loss of the charged particles. An investigation of the discharge voltages of various discharge voltage waveforms demonstrated that a sinc function of time, t, that is, (sin ωt)/ωt with a bias voltage, achieved a high extrinsic charging efficiency (a high yield of charged particles) by generating a high concentration of ions and suppressing the electrostatic deposition of the charged particles. In trial operation at the optimal discharge voltage and an aerosol flow rate of 2.5 L/min, the charger attained intrinsic and extrinsic charging efficiencies of 79.3 and 61.4% for 10-nm particles and intrinsic and extrinsic charging efficiencies of 48.0 and 34.6% for 5-nm particles, respectively. Copyright 2013 American Association for Aerosol Research