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In this study, we used the general dynamic equation (GDE) for aerosols, combined with particle charging by positive ions and electrons, to simulate particle generation in laser ablation plasmas. The population balance equations based on Brownian coagulation and diffusion charging theory were represented by using a discrete-sectional model. The temporal evolution of the particle size distribution was simulated at three different initial number concentrations of neutral monomers, positive ions and electrons, as well as at two different gas pressures. The simulated average particle size increased with time and with increasing initial concentration of neutral monomer and gas pressure. The particle growth was affected by the particle charging when both number ratios of positive ion to neutral monomer and of electron to neutral monomer are higher than 0.1. Simulated particle size distributions were compared with SEM images of particles collected at different sampling points and gas pressures in actual plasmas operating at conditions similar to those simulated. Changes of particle size with increasing distance between the target and the sampling point and with increasing the pressure were well reproduced by simulations. However, particle diameters calculated were much smaller than SEM images because physical values, such as, initial concentrations of monomer, positive ions and electrons and the recombination probability of electron and positive ions, used in calculations were invalid.

An experimental system employing a drift tube ion mobility spectrometer/mass spectrometer (DT-IMS/MS) and a cluster-differential mobility analyzer with Faraday cup electrometer (C-DMA/FCE) was developed for the study of ion-induced nucleation. Using this, both the time scales and size scales of ions to be measured can be extended significantly. The system offers the potential for tracing the entire process of ion-induced nucleation initiated by the radiative ionization of air. The main processes are the formation of stable cluster ions and their growth to nanometer-sized ultrafine particles. Negative ions and negatively charged nano-particles generated by α -ray radiolysis in SO 2 / H 2 O / air mixtures were examined. High-resolution mobility spectra in which ions are distributed in the relatively high mobility range of 2.1– 1.5 cm 2 V - 1 s - 1 were obtained by DT-IMS/MS. Alternatively, more aged ions with mobilities as low as 0.05 cm 2 V - 1 s - 1 were observed in the C-DMA spectra. The differences between both spectra reflect the nature by which cluster ions grow into nano-particles. In addition to mobility measurements, mass spectra of ions can be obtained by DT-IMS/MS, which allows the identification of nucleating ions and relevant ion chemistry to be investigated. In the mass spectra at low SO 2 and H 2 O concentrations, SO 2 - , SO 3 - , SO 4 - , and SO 5 - were observed as major ions, indicating that SO 2 is preferentially ionized by ion–molecule reactions with primary negative ions. When the concentrations of SO 2 and H 2 O were increased, HSO 4 - and HSO 4 - · H 2 SO 4 ions became prominent, suggesting that the oxidization of SO 2 into H 2 SO 4 is enhanced, probably due to the enhanced production of OH radicals by ion–molecule reactions involving H 2 O . This suggests that the role of SO 2 in particle formation differs depending on the concentrations of SO 2 and H 2 O . At low SO 2 and H 2 O concentrations, SO 2 tends to form core ions that lead to ion-induced particle formation, whereas more SO 2 molecules are transformed into gaseous H 2 SO 4 and homogenous nucleation involving H 2 SO 4 and H 2 O is enhanced at higher concentrations of SO 2 and H 2 O .

A modified version of the previously developed tubular particle-removal device based on the UV/photoelectron method (Shimada et al. (1997) J. Aerosol Sci. 28, 649) is manufactured to investigate removal efficiency of aerosol particles suspended in air at a reduced pressure down to 20 torr. The present measurements with monodisperse particles of 0.458 μm in diameter show the dependence of the removal efficiency on pressure, flow rate and electric field in the electrostatic precipitation device. To predict the removal efficiency, a theoretical analysis for the particle collection processes in the precipitator is made, in which particle charging by negative ions, and particle transport by air flow, diffusion and electrical migration are numerically calculated taking into account the effects of reduced pressure. The calculated results reproduce the measured removal efficiencies well. It is found in the calculation that the Fuchs' approach to ionic particle charging needs modification to predict charging of highly charged particles by ions of the same polarity. It is also demonstrated that the removal efficiency is increased to 100% when the precipitator is equipped with an additional set of electrodes that collects charged particles.

A new tubular particle-removal device based on the principles of charging of particles by ions and collecting by electrical force is developed and its performance is evaluated. In this device, negative ions of a high concentration are generated from continuously supplied photoelectrons emitted from the surface of an Au/ITO (Indium Tin Oxide) thin film irradiated by UV light. This removal method is called UV/photoelectron method. Airborne particles fed into the device are designed to encounter the ions, and to be attracted to a cylindrical positive electrode of which the outer wall of the device is composed. Measurements using submicron aerosol particles show that over 90% of the particles of 0.06-0.12 μm in diameter are collected by the device when the operating flow rate is below 1 1 min −1 However, removal efficiency depends on particle size, aerosol flow rate and electric field strength in the device. To investigate theoretically the removal efficiency, numerical calculation of particle behavior is made in which simultaneous charging of particles by negative ions and particle transport by air flow, Brownian diffusion and Coulomb force are taken into account. The results calculated successfully reproduce the dependence of the results measured on the operating conditions of the device.

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 TiO2 particles produced by low pressure chemical vapor deposition (LPCVD)

Uniformly-distributed ultrafine superconductive particles with the composition of Bi 2 CaSr 2 CU 2 O x have been prepared by the metalorganic chemical vapor deposition (MOCVD) method. A new type of electrostatic precipitator was developed for continuous collection of ultrafine particles. The particles produced were spherical and the crystalline phases of the particles were found to be greatly dependent on the decomposition temperature and the reaction time. The particles with the 80 K superconducting phase were only formed at around 800°C and a reaction time longer than about 14 s, and the average diameter of the particles was 0.05–0.06 μm.

A new system for experimental investigation of ion-induced nucleation, based on a continuous flow-mixing type device, is introduced. Measurements of the ion-induced nucleation of dibutylphthalate (DBP) vapor in the presence of unipolar ions are presented. When an ionized water vapor/N2 mixture comes into contact with DBP vapor, nucleation of DBP particles is significantly enhanced relative to that in the absence of ions for both positive and negative ions. The number concentration of DBP particles activated by ions increases with increasing ion number concentration and DBP supersaturation ratio. A definite ion polarity effect is observed; at equal saturation ratios, positive ions induce larger number concentrations than negative ions. In the absence of water vapor, an ionized N2 stream produces nucleation rates higher than those for pure homogeneous nucleation but well below those in the presence of water vapor. The present data set should be valuable in future assessments of ion-induced nucleation theories.

A new design of a unipolar aerosol charger has been developed. The charger consists of a radioactive source placed between two screen electrodes enclosed by a Plexiglass tube. The electric field in the charger is aligned with aerosol flow. The new charger is capable of charging ultrafine aerosols efficiently and with low particle losses. The charger was evaluated thoroughly both theoretically and experimentally. The basic equations for the charging process in this charger were numerically solved for monodisperse, ultrafine aerosols under various operating conditions. From the calculation results, the particle charge level and loss rate within the charger were found to depend on two dimensionless parameters defined in the paper. In the experiment, the particle charge level and penetration rate were measured for particle sizes of 10–30 nm, operating voltages of 3–9 kV, flow rates of carrier gas of 2–5.21 min−1, and pressures of 0.46 and 1 atm. The experimental results which were obtained under similar conditions estimated by the dimensionless parameters were found to agree with the theoretical predictions. The charger gave a unit charge to 50% of the 10 nm diameter particles with 20% particle losses. The charger was found to work also under low pressure as a high-efficiency charger.

A new system for experimental investigation of ion-induced nucleation, based on a continuous flow-mixing type device, is introduced. Measurements of the ion-induced nucleation of dibutylphthalate (DBP) vapor in the presence of unipolar ions are presented. When an ionized water vapor/N2 mixture is contacted with DBP vapor, nucleation of DBP particles is significantly enhanced relative to that in the absence of ions for both positive and negative ions. The number concentration of DBP particles activated by ions increased with increasing ion number concentration and DBP supersaturation ratio. A definite ion polarity effect is observed; at equal saturation ratios, positive ions induce larger number concentration than negative ions. In the absence of water vapor, an ionized N2 stream produced nucleation rates higher than those for pure homogeneous nucleation but well below those in the presence of water vapor.

The behavior of aerosols undergoing Brownian coagulation, Brownian diffusive deposition and gravitational sedimentation in a flow type chamber was studied by solving the population balance equation numerically for a polydisperse aerosol system and analytically for a monodisperse system, and then the results were examined by experiment. In solving the basic equation, three dimensionless parameters, which characterize the relative effects of Brownian coagulation and of diffusive and gravitational deposition to flowing conditions, were introduced in order to generalize complicated aerosol behavior. The calculated results of the time-dependent changes in particle number concentration, particle mass concentration and particle size distribution were presented graphically by using the above three parameters. The controlling domains of coagulation and of diffusive and gravitational deposition, under various flowing conditions, were mapped to give an easier understanding of the phenomena. Some of the calculated results were verified by experiment using the ultramicroscopic size analyzer and the piezoelectric crystal mass monitor.

Unipolar and bipolar diffusion charging of monodisperse aerosol particles of 4–100 nm in diameter has been studied experimentally and theoretically. Particles are charged by unipolar or bipolar ions produced by α rays from Am-241. They are enlarged by condensation of DBP vapor on them using the particle size magnifier and the enlarged particles are admitted into an observation cell having parallel d.c. electrodes to distinguish charged particles from uncharged particles by TV camera. In the case of unipolar charging, the ratios of charged particles to total particles at various values of the ion concentration and charging time are found in quite good agreement with the solution of equations based on the Fuchs' formula for the combination probability of ion with particle and introducing ion properties reported in previous papers. From these results, the necessary conditions to bring uncharged particles to a desired charge distribution are clarified for various particle sizes. In the case of bipolar charging, the number ratios of charged particles to total particles in a steady-state are again found in quite good agreement with the solution of basic equations similar to those of unipolar charging using the Fuchs' theory for the combination probability and the same ion properties as those used for unipolar charging. The ratio of positively to negatively charged particles is found to be approximately 0.35:0.65, which is explained by the difference in physical properties between positive and negative ions.

An evaporation-reaction type aerosol generator for ultrafine tin dioxide particles has been developed, and the effect of its evaporating temperature on particle size distributions and number concentrations of the generated ultrafine tin dioxide particles has been studied. The particles are drawn with high velocity into a deposition chamber with a pressure of about 400 Pa, and are deposited by inertia on a glass substrate, where they form a thin film. The electrical characteristics of this film are evaluated. The conductance of the film increases with the decrease of film thickness and with the increase of the substrate temperature. The current-voltage relation of the film is ohmic. It is shown that the ultrafine tin oxide particle film can selectively detect the gases N2 and O2 and is very sensitive to H2.