The control of electrostatic charge on particles in gas-solids pipe flow has been studied experimentally and theoretically. Alumina particles of 3.3μm in mass median diameter were dispersed in airflow and pneumatically transported in dilute phase. Five different materials were used for the transport pipes, and the relationships between the particle charge and the pipe length were obtained. The polarity and the amount of particle charge were found to depend on the pipe material and the length. In order to control the particle charge, a system combining two different pipe materials was proposed depending on the particle charging characteristics. The particle charge controlled by this method was in good agreement with the theoretical calculation, in which the distribution of particle charge as well as the average was well estimated.

View full abstract

Charge accumulating process of a single particle due to successive impacts was numerically simulated based on a charge relaxation model. In the calculation, a patch-like charge generated by each impact was localized randomly on the particle surface, and the fixed patch-like charges worked as the initial charge at the next impact. Due to such non-uniform charge distribution of initial charge on the particle surface, impact charge upon each impact scattered around a ‘charging line, ’ which is a linear relationship between the initial charge and impact charge derived with an assumption of uniform initial charge diatribution. Despite the variation in impact charge, it was shown that the total charge accumulation follows a normal exponential decay and reached a saturated value which corresponds to the linear charging line.

View full abstract

A single particle is brought into contact with a metal target, and the force acting on the particle is measured by using an atomic force microscope (AFM). Focusing on the measurement of force curve rather than the maximum adhesive force, the electrostatic interaction was successfully observed by separating it from other interations such as liquid bridge and intermolecular forces. In order to evaluate the force curve and the method, a force curve was numerically calculated with an approximation of disk-to-disk interaction with image force and a good analytical approximation was developed. The successful agreement between the observed force curve and the theory revealed that the force curve observed can be explained by the electrostatic interaction, and that the amount of charge on the particle and the radius of the charged (contact) area can be estimated from the analysis. The order of magnitude of the measured charge density was 10^-2C/m², which is much greater than that obtained with the impact charging experiment of 10^-4C/m². From this it was concluded that the force curve measurement with AFM can measure the net amount of the charge generated before the charge relaxation due to gas discharge or other charge relaxation took place. In the experiment using 8 kinds of metal (Al, Au, Cr, Ni, Pt, Ti, Zn, and Zr) and polystyrene particles, the net charge generated was shown to be compared fairly well with the conventional simple condenser model based on metal-to-metal contact model with the term of contact potential difference in its order of magnitude. Although clear correlation was not obtained between the measured charge density and the work functions of metal targets because of a large scatter in the data, a strong relationship between the charge density and contact area was found. The underlying mechanisms for the relationship is not known at this moment, however the finding gives a good hint for the next attempt.

View full abstract

Nanoparticle generation system via electrospray pyrolysis with AC voltage (called as AC electrospray pyrolysis) was developed in order to mass-produce nanoparticles. The system was applied to ZnS nanoparticles generation from the solution of ethyl alcohol with zinc nitrate and thiourea. The measurements of size distributions of droplets sprayed with AC voltages revealed that the droplet size decreases with increasing AC voltage. Furthermore, the AC electrospray pyrolysis was found to reduce the particle deposition in the reactor. The collection efficiency of electrostatic precipitator used in the system was less than 5% and the precipitator should be improved to scale up the system. X-ray diffraction patters showed that the hexagonal ZnS nanoparticles were directly synthesized using AC electrospray pyrolysis.

View full abstract

Magnetic powder dispersed carrier (MC) is studied in order to extend the life of developer, while achieving the fast charging rate for both positive and negative toners. This paper reports the influence of MC composition on the charging characteristics by clarifying the charging mechanism. The charging property of MC varies depending on which of the resins or magnetic materials becomes a doner or an acceptor of electrons. The polarity and the charging level of MC are evaluated by measuring the electronegativity of metal oxide in magnetic powder and the chemical property of functional group in resin. The charging rate of MC developer is faster by increasing the fraction of magnetic material in MC. It is found that MC attains fast charging rate without a charge control agent, (CCA) although the charging rate of conventional coated carrier is very low without AAC. Furthermore, the charge of MC does not decrease linearly with the carrier spent volume. The charge gradually increases for 200-300K prints from the start and then slowly decreases to the initial charge volume for 300-800K prints. Therefore, the life of the developer is the same as the machine life and no maintenance of the developer is required. The peculiar behavior of MC is due to the surface which possesses both electron donor site (magnetic particles) and acceptor sites (resin). The charge of toners mixed with MC, which are pre-stirred before the mixing, is larger than that without the pre-stirring. This phenomenon of MC can be explained by the mechanism that the charge arises not only by the difference in work function between toner and carrier but also by donation of electron from toner to positive site generated by pre-stirring of carriers. This charging mechanism enables the MC developer to be in a good toner charge condition during the printing with a fast charging rate in MC developer.

View full abstract

Electrostatic precipitator (ESP) was applied to remove automobile-exhaust particulates in expressway tunnels to prevent drivers from inhaling and to avoid the diffusion into surrounding environment. Although the conventional ESP has relatively high collection efficiency, the collection efficiency is apt to decrease for large particles because of particle re-entrainment. The re-entrained particles may deposit on the wall downstream the ESP. In order to solve this problem, the authors developed an ESP with AC electric field (ACESP). In this paper, experiments were carried out to investigate the effectiveness of ACESP on tunnel-air cleaning. The fractional collection efficiency and the deposition efficiency onto the downstream wall were measured using ACESP and ESP with DC electric field (DCESP). As a result the ACESP attained a high collection efficiency for large particles for 801 hours without the particle re-entrainment, whereas the particle re-entrainment was significant after 127 hours with the DCESP. The particle deposition onto the walls downstream the ACESP was significantly reduced compared to the DCESP.

View full abstract

Powder electrification is applied to important systems such as electrophotography, and development of many new systems using this phenomenon is now progressing. In most of these systems, considerably high charging states of particles are needed, and the particle size tends to be in nanometer region.
In the present paper, first, the effect of plasma processing was described for the purpose of improving the contact charging characteristics of polymer powder after surveying the mechanisms of contact charging. Then, we introduced methods to obtain fine solid particles by drying the droplets containing solid ingredients. By these methods, solid particles of nanometer size were easily generated and fine particles with extremely high surface-charge density can be obtained. In order to obtain fine droplets, systems using supersonic atomization, electrostatic spraying and straight-through microchannel were very interesting.

View full abstract

In electrophotography consisting of charged toner transfer processes, the toner charge is of great importance. Toner charge of two component developer is determined by work function, toner content, charging sites density etc. These parameters are dependent on not only the toner materials and their composition but also the manufacturing condition. Charging property of each material is estimated by their chemical properties, i. e., whether they are the electron donor or the acceptor. Toner charge stability during printing is influenced by the properties of developer and mixing process as well as the stress exerting on toner in a developing device. By employing an easy simulation method, it is possible to estimate the best constitution of the device and developer in various printing speed ranges without conducting experiments.

View full abstract