AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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In-situ Nanoparticle Characterization at Ambient Pressure by Small Angle X-Ray Scattering (SAXS)
PAULUS S. BAUER, Heinz Amenitsch, Paul M. Winkler, Universitaet Wien, Vienna, Austria
Abstract Number: 266 Working Group: Instrumentation and Methods
Abstract Direct characterization of aerosol nanoparticles is a challenging task in aerosol science. Common aerosol analyzers usually extract the particles from their original environment for analysis. Thereby, nanoparticles can be modified or get lost e.g. by wall collisions inside the instrument, which can affect the measured size distribution and concentration. Thus, there is an essential demand for an in-situ measurement technique. Small-angle X-ray scattering (SAXS), commonly used in material science or in biochemical process analysis, can fill this gap. It is capable of measuring in-situ particle size distribution in the nanometer range if the scattering contrast between nanoparticles and gas molecules is sufficiently large. SAXS has already been applied in nucleation studies with extremely high nanoparticle concentrations of ~1012/cc and carrier gas pressures ~2kPa. (Laksmono (2011), Phys.Chem.Chem.Phys.,13,5855) However, in order to compare the in-situ SAXS results to the DMPS and CPC measurements it is important to choose system settings as close to ambient conditions as possible.
Here we report experiments on nanoparticle characterization in helium by SAXS at concentrations of about 106/cc. The experiments were conducted at the Elettra synchrotron near Trieste, Italy, due to the available high beam intensity and the experience on aerosol studies in flow tubes (Jungnikl (2011), Aerosol.Sci.Technol.,45,805). To provide a representative environment for aerosols a flow tube was operated at ambient pressure. A critical issue for SAXS experiments with aerosols is the background scattering signal originating from the carrier gas. The air background usually is of the same order of magnitude as the signal from the tungsten nanoparticles. This issue was lately resolved by replacing air with helium as carrier gas. The usage of helium provides the opportunity of operating the flow tube under ambient conditions (temperature, pressure), and allows the parallel sampling by modified state-of-the-art aerosol instruments like CPC and DMPS system.
Due to the low background of helium and the high electron density of tungsten particles, we obtained defined signals from aerosol SAXS measurements. Thereby a comparison between in-situ results from SAXS and conventional measurement techniques can be achieved.