Particle-size distribution

Summary

In granulometry, the particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. Significant energy is usually required to disintegrate soil, etc. particles into the PSD that is then called a grain size distribution. Significance The PSD of a material can be important in understanding its physical and chemical properties. It affects the strength and load-bearing properties of rocks and soils. It affects the reactivity of solids participating in chemical reactions, and needs to be tightly controlled in many industrial products such as the manufacture of printer toner, cosmetics, and pharmaceutical products. Significance in the collection of particulate matter Particle size distribution can greatly affect the efficiency of any collection device. Settling chambers will normally only co

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Physical and chemical characterization of gas-borne particles in the nanometer to sub-micrometer size range is important in many applications, such as nanoparticle related health studies, the monitoring of powder or dust distribution processes, assessing the release of engineered nanoparticles (ENP) from ENP reinforced materials due to abrasion or combustion, or exposure studies related to ENP containing spray products. Such investigations are also highly valuable in the development of technical processes, e.g. to determine distinct size fractions containing specific elements or isotopes in terms of dealing with nuclear waste, or to evaluate particulate matter in process gases and emissions from thermal waste treatment, gas turbines, or other combustion engines. Such particle speciation includes knowledge about particle size distribution and chemical composition, and if possible obtained with a high time resolution. However, most of the available techniques are offline methods and/or not able to provide such physical and chemical information simultaneously. The Scanning Mobility Particle Sizer (SMPS) is a well-established and widely used equipment for determining size distribution and number concentration of such particles, within scan durations of a few minutes. Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a highly sensitive multi-element technique, which allows determining the elemental composition of normally liquid samples, with excellent detection limits and a wide dynamic concentration range. In this work, SMPS and ICPMS are coupled to one hyphenated system. A Rotating Disk Diluter (RDD) allows directing a well-defined flow of a diluted aerosol into subsequent measuring equipment, which besides is mainly argon based, due to the dilution effect. Therefore an RDD is implemented as sample introduction interface, and the SMPS flow concept is re-designed to fulfil the requirements of both different analytical methods. This coupling strategy allows achieving simultaneous information on particle size and elemental composition, with SMPS-typical time resolutions of a few minutes, and offers high flexibility in terms of dealing with different aerosols, loaded with particles in the nanometer to sub-micrometer size range. Proper SMPS operation under argon atmosphere instead of air is validated. The developed setup is tested with a model aerosol containing air-borne silver nanoparticles. The capabilities are then demonstrated on uniformly composited metal aerosols, as well as an aerosol mixture containing smaller gold and larger silver nanoparticles. Sensitivity and limit of detection, related to particle number and mass concentration, are determined for several elements and particle diameters. After the successful characterization, the instrumentation is applied in two research applications. The first is a study on aerosol particles, released by commercial consumer spray products. In the second application, particulate metal emissions from the thermal treatment of differently impregnated wood samples are investigated, with the focus on alkali metals.

EPFL2016

CFD simulation of an aerosol mixing chamber

Andrea Giordano

In the last years, the correlation between air pollution and health issues related to respiratory, cardiovascular and digestive systems has become evident. Today, urban aerosols raise the interest of both scientific community and public opinion. METAS, the Swiss Federal Institute of Metrology, takes part in AeroTox, a European Union’s research project involving the development of a reference aerosol calibration infrastructure - a so-called mixing chamber. In this chamber, pure air and particles are injected on top and the resulting aerosol is sampled at the bottom. The quality of this aerosol is assessed according to its concentration homogeneity: the purpose of this master’s project is to improve it. In addition, two research questions were addressed. How much can the mixing chamber dimensions be reduced without affecting the concentration homogeneity? Dimensions are crucial because the mixing chamber must be transportable. Also, how much can the flow rates be reduced without affecting the concentration homogeneity? Computational Fluid Dynamics (CFD) simulations and experiments were employed. Numerical simulations were performed in COMSOL Multiphysics, implementing a particle tracing and a diluted species model. This allowed to investigate the structure of the flow and the involved mixing mechanisms: diffusion, convection and turbulent dispersion. However, only the diluted species model was successful. The simulated concentration at the outlet is perfectly homogeneous. Experiments were carried out using two particle size distributions: NaCl (size peak at 80 nm) and Polystyrene Latex (PSL, size peak at 900 nm). Empirical data validate simulations and show a concentration homogeneity within 5%. Furthermore, uncertainty on the measurements is of 4.24%: the simulated concentration homogeneity thus lies within the uncertainty of the experimental findings. Moreover, experiments show that salt particles reach a higher concentration homogeneity than PSL particles. Finally, in case of salt particles, experiments prove that the flow rates can be halved and even equalized and the length of the mixing chamber can be reduced to 50% without drastically affecting the concentration homogeneity.

2020

Manganese doped ZnS nanoparticles

Yvonne Axmann

Nanostructured materials can be defined as those materials whose structural elements - clusters, crystallites or molecules - have dimensions in the 1-100 nm range. The explosion in both academic and industrial interest over the past 20 years arises from the remarkable variations in fundamental electrical, optical and magnetic properties that occur as one progresses from an "infinitely extended" solid to a particle of material consisting of a countable number of atoms. In this work the attention was laid on the optical properties of Mn2+ doped ZnS nanoparticles with respect to a possible application as bio sensor in medical and biological analytics. A synthesis for this type of particles has been developed and the nanocrystals obtained have been investigated in terms of their composition, particle size and optical properties. The particles synthesized were stabilized with the amino acid L-cysteine and showed the typical ZnS:Mn emission spectrum with its orange emission due to the 4T1 –> 6A1 transition within the d-orbitals of the Mn2+ at 585 nm and its blue emission due to recombination over the ZnS band gap from shallow electron traps at 380 nm. The onset of the absorption was blue shifted from 340 nm (bulk) to 310 nm, indicating a quantum size effect. The particle sizes and particle size distributions were determined with methods such as transmission electron microscopy, photon correlation spectroscopy, analytical ultra centrifugation and field flow fractionation. The particle size has also been calculated from the shift in the band gap with the help of the effective mass approximation. The limits of the applied techniques are discussed and the particle size distribution determined to go from 3-20 nm with a first maximum at about 5 nm and a second maximum at about 17 nm. The particles' luminescence quantum yields have been determined between 1.5-2 %. They could be enhanced by a factor of three by coating the particles with a SiO2 shell. The quantum yields showed a strong dependence on the dispersion concentration and Mn2+ content. These effects have been investigated with quantum yield measurements at different temperatures and Mn2+ contents, and lifetime measurements. The lifetime measurements showed decay times in the ns region for the blue and in the μs-ms region for the orange emission, indicating a florescence mechanism for the ZnS and a phosphorescence mechanism for the Mn2+ radiation.

EPFL2004