1. Field of the Invention
The present invention relates generally to an aerosol charger, and more specifically to a corona-wire unipolar aerosol charger.
2. Description of the Related Art
Nanoparticles possess many unique physical, chemical and biological properties which lead to their diverse applications. However, some of the same unique properties which make nanoparticles useful are also properties which may cause nanoparticles harmful to humans or the environment. Many parameters such as size, shape, density, surface characteristics and composition influence the behavior, fate, transport, and toxicity of nanoparticles. Among them, size is one of the most important parameters. Therefore, it is important to characterize the size of nanoparticles accurately.
The differential mobility analyzer (DMA) shows the greatest promise for accurate sizing and classification of nanoparticles. To operate the DMA, aerosol particles must first be charged electrically to a known charge distribution on which nanoparticles sizing and classifying is based. In electrical aerosol instruments, the most commonly used techniques for charging particles is diffusion charging. Diffusion charging of particles can be either unipolar or bipolar, depending on the polarity of the ions colliding with particles. In unipolar charging, ions of only one polarity are present, and particles increase their charge with time. In bipolar charging, both positive and negative ions are present, and particles will acquire charges with time by attachment of ions of the opposite polarity until they reach an equilibrium charge distribution. This process is also known as charge neutralization.
Bipolar diffusion chargers are commonly used with DMAs in scanning mobility particles sizers for the measurement of particle size distribution because of a well defined charge distribution. However, the charging efficiency for nanoparticles is low because both charging and neutralization mechanisms happen at the same time, which is only 0.7%-4% and 0.8%-5%, respectively, for positively and negatively charged particles of 2-10 nm in diameter. In other words, lots of nanoparticles would be wasted during the classification process. In addition, the extremely low nanoparticles charging efficiency of bipolar chargers could lead to low sensitivity in detecting nanoparticles with low concentration. Therefore, it is desirable to have high concentration of charged nanoparticles from the charger and before they are classified by the DMA.
Unipolar diffusion chargers provide higher charging efficiency than bipolar diffusion chargers because the recombination of charged particle with the ions of opposite polarity is avoided. To achieve high charging efficiency, various unipolar chargers were developed using a variety of techniques to generate ions for diffusion charging. Among these techniques, corona discharge can produce unipolar ions at a high enough concentration for efficient diffusion charging.
Numerous corona-based unipolar chargers were designed using either a wire or a needle as the discharge electrode. Some designs involved mixing ion jet flow with aerosol flow in the charging chamber without an external electric field to reduce charged particle loss. However, high efficiency charging is still difficult to obtain for nanoparticles smaller than 10 nm. Therefore, an aerosol charger with higher charging efficiency in this size range is needed to improve the sensitivity and accuracy of monitoring instruments for nanoparticles.
Some unipolar charger designs have an additional sheath air flow either near the wall of the charger to reduce charged particle loss or around the discharge wire to prevent accretion of particles in the charging chamber.
For example, U.S. Pat. No. 5,247,842, U.S. Pat. No. 6,093,557 and U.S. Pat. No. 7,972,661 disclose electrospray systems that are able to generate submicrometer droplets substantially uniform in size. A liquid sample is supplied at a controlled rate to a capillary needle of the system, and droplets are formed due to an electrical field in the region about the needle discharge. All of the three also disclose an introduction of sheath air flowing concentrically and axially about the capillary needle to guide the droplets downstream. However, such design does not help to reduce the charged particle loss of the system.
U.S. Pat. No. 5,973,904 discloses a unipolar charger utilizing a radioactive source of polonium. A confined electric field is applied within a charger housing and is parallel to the flow of a stream of aerosol particles. The design further uses a sheath air surrounding the aerosol flow to keep charged particles in the core region to minimize electrostatic loss. So far, this design has the highest extrinsic charging efficiency for particles smaller than 5 nm in diameter among all unipolar chargers. However, the issue of tight safety regulations on using radioactive sources remains, and the unipolar charger using radioactive sources required an electric field to separate the positive and negative ions is also referred to as a relatively complicated design. Moreover, the concentration of the charged particles is lowered at an outlet of the charger subject to the utilization of the sheath air.
U.S. Pat. No. 8,044,350 discloses a corona-needle unipolar charger consisting of two major components. The outer includes a radial inlet tube and axial outlet tube. The second is the corona discharge module, consisting of a pointed needle electrode placed coaxially in the outer tube capped with a perforate dome. The corona discharge module is installed in the case at the end opposite the axial exit tube. Although an axial sheath is introduced to avoid accumulation of charged particles on the corona needle, such design cannot mitigate the particle loss in the charger.
Chien et al. (2011) and U.S. Pat. No. 8,400,750 disclose a particle charger for enhancing the charging efficiency with a sheath air of high velocity introduced from an annular slit formed by a shroud and an outer casing to minimize charged particle loss. The numerical results of Chien et al. (2011) indicate that there are flow recirculation zones generated inside the charger due to the axial high velocity sheath air, and therefore the charged particles are restricted from rapidly leaving the charger.