Meilu He

A new high performance battery-operated electrometer

Design and analysis of a new low-noise, fast-response, high-sensitivity, compact electrometer are described. This electrometer are battery operated and capable of measuring currents down to the femtoampere level. The portable, high performance nature of the electrometer renders it applicable for deployment in compact instruments for applications such as aerosol particle counters. A parametric experimental study is conducted to determine the role of different components on the performance of the electrometer. Under an ideal configuration, the electrometer has a step-up response time of approximately 3 s. Experiments with the electrometer used for particle counting measurements suggest that the new electrometer has high accuracy and sensitivity in comparison to the Keithley 6514 electrometer. The response of the electrometer used in particle counting studies is consistent with that of an optical single particle counter used in comparison experiments. A damping component introduced to reduce possible spike currents in the electrometer is also seen to reduce noise and almost have no effect on response time. The electrometer response characteristics are presented in detail.

Aerosol Filtration with Mobility-Classified Particles: Role of Multiply Charged Particles in Skewing Penetration Measurements

Filter media exhibit strong particle-size dependent capture characteristics that have a complicated dependence to filter properties and operating conditions. As simulation-based determination of the filtration characteristics of real media is very challenging, accurate experimental approaches are critically necessary. Conventionally, filter penetration measurements were made using test particles extracted from a differential mobility analyzer (DMA) and measuring the ratio of particle counts downstream of the filter to that upstream. Size-dependent filter penetration can be directly determined from these measurements if the DMA was operated at high resolution and the contribution of multiply charged particles was negligible. The accuracy of this well-established approach has, however, never been fully assessed. In this study, the influence of multiply charged particles to the filter efficiency measurements is determined using a system-modeling approach. The model results suggest that the contribution of multiply charged particles could significantly influence penetration results when the mode size of the test particles entering the DMA or the most penetrating particle size (MPPS) of the test filter are larger than ∼100 nm. Under such conditions, even if multiply charged particles constitute a small fraction of the test aerosol, they can significantly skew filter penetration results. For accurate calculation of filter penetration, two approaches are proposed: an improved filter test protocol and a new multiple-charge correction approach for calculation of size-dependent filter penetration from mobility-based measurements. The validity of the proposed approaches is established with experimental data. Copyright 2015 American Association for Aerosol Research

Performance of bipolar diffusion chargers: Experiments with particles in the size range of 100 to 900 nm

ABSTRACT Accurate measurement of particle size distribution using electrical-mobility techniques requires knowledge of the charging state of the sampled particles. A consistent particle charge distribution is possible with bipolar diffusion chargers operated under steady-state condition. Theoretical steady-state charge distributions for bipolar charging are well established but recent studies have shown that the performance of particle chargers is a strong function of particle size, particle concentration, ion source, and charger operating conditions. Most of these studies have focused on particles smaller than 100 nm and the applicability of these results for particles larger than 100 nm must be investigated. In this study, experimentally obtained singly-charged and doubly-charged fractions are compared against theoretical predictions for particles in the size range of 100 to 900 nm. The experimental results show that the commercial soft X-ray charger performs as theoretically-predicted over the range of conditions studied while the performance of other commonly used radioactive chargers (85Kr and 210Po) are dependent on source strengths, flowrates, particle charge polarities, and particle sizes. From measurements of particle residence times and ion concentrations in different test bipolar chargers, prior observations of flowrate-dependent charging fractions can be explained. Additionally, the results from this study are used to determine an acceptable time period for usage of the commercial TSI 3077A 85Kr chargers for steady-state charging as a function of flowrate. Copyright

Characterization of filter performance under low-pressure operation

ABSTRACT Particulate gas filters are a critical element in the purification systems used to ensure defect-free manufacturing in semiconductor industry. In atomic layer deposition (ALD) processes, these filters are typically operated under sub-atmospheric pressure conditions, but their filtration characteristics are, often, only known at atmospheric pressure. In this study, performance of a metal filter that is typically used in low-pressure ALD precursor delivery systems is studied experimentally and theoretically down to 4.5 kPa. The experimental procedure was designed to minimize the presence of multiply charged particles in the test aerosol for different operating pressures and flowrates. The experimental results suggest that most penetrating particle size only slightly varies with pressure, but the shape of the penetration curve and the maximum value of the penetration changes significantly with pressure. The experimental data are used to test predictions of filter performance at low pressures made using classical theory. The comparison results suggest that the combination of classical theory and manufacturer-specified parameters results in large errors in calculated penetration values at low pressures. Accurate predictions are seen to be possible for particle Stokes numbers less than 0.1, when an inhomogenous filtration model is used in combination with effective filter parameters that are obtained from experimental measurements of filter efficiency and pressure drop at atmospheric pressures. Copyright