Measurement of Metal Nanoparticle Agglomerates Generated by Spark Discharge Using the Universal Nanoparticle Analyzer (UNPA)

Abstract

Nanoparticle agglomerates play an essential role in the manufacturing of many nanomaterials and are commonly found in combustion products. Conventional aerosol instruments based on equivalent spheres are not directly applicable to the measurement of nanoparticle agglomerates. The increasing interest in real-time assessment of the structure of engineered nanoparticle agglomerates and the mass concentration of potentially hazardous agglomerates (e.g., diesel soot, welding fume) makes an instrument devoted to online structure and mass measurements for nanoparticle agglomerates highly desirable. A recently developed instrument, universal nanoparticle analyzer (UNPA), utilizes the close relation between agglomerate structure and unipolar charging properties and infers agglomerate structure from measurement of the average charge per agglomerate. It was used in this study to characterize in situ the structure of metal nanoparticle agglomerates generated by spark discharge, to study the effects of sintering on the structure of these agglomerates, and to make realtime assessment of their airborne mass concentration. The primary particles sizes measured by UNPA for the gold (Au), silver (Ag), and nickel (Ni) agglomerates are in reasonable agreement with the TEM (transmission electron microscopy) sizing results, d p = 7.9 ± 1.5, 11.8 ± 3.2, and 6.6 ± 1.0 nm, respectively. In addition, findings from the study of agglomerate structural change during sintering using the UNPA sensitivity coincide with results from TEM and mobility analyses. With regard to the mass concentration of silver agglomerates at room temperature, good agreement was found un- Address correspondence to David Y. H. Pui, Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA. E-mail: dyhpui@umn.edu der our experimental conditions between results given by UNPA, the effective density, and the gravimetric measurement.

Rationale for Data Evaluation of the Size Distribution Measurements of Agglomerates and Aggregates in Gases with Extended SMPS-Technology

“…The developed model has been adjusted by making use of measured charge per particle data of spherical and loosely agglomerated silver particles. Several comparisons with TEM-measurements showed a rather good agreement of the mean primary particle sizes Liu et al, 2012), Wang and Pui (2013). The used model does not show any material dependency ), but other materials may not form the ideal loose agglomerates with equally sized primary particles.…”

“…For the transformation of mass into volume or vice versa the material density of the aggregates has to be known. Liu et al (2012) used the gravimetric method to measure the mass concentration of particles from a spark generator and to compare them with the total mass concentration derived from SMPS + ES-measurements determining the primary particle size and the number distribution of chain like loose agglomerates making use of the Lall et al (2006a, b) model, which showed rather good agreement. Sampling of the fractionated aggregates on a filter and counting and weighing them is however very time-consuming and laborsome, because of the lack of suitable online measurement techniques.…”

“…Loose agglomerates of metals, metal oxides or other kinds of particle material are produced in a furnace, a flame reactor or by an electric arc Liu et al, 2012), and go through a coagulation chamber. The agglomerates/ aggregates are fractionated with respect to electrical mobility with a DMA.…”

The Effect of Particle Morphology on Unipolar Diffusion Charging of Silver Nanowires

“…The charging properties of non-spherical particles can be used to obtain morphology information about nonspherical particles combined with electrical mobility analysis (Shin et al 2007Wang et al 2010;Liu et al 2012). Diffusion charging properties of non-spherical particles have been investigated for unipolar and bipolar charging environments.…”

“…In our study, we used a unipolar diffusion charger that has been implemented in three commercial instruments (Electrical Aerosol Detector, TSI 3070A; Nanoparticle Surface Area Monitor, TSI 3550, and AeroTrak 9000; UFP Monitor, TSI 3031). The unipolar charger was also used in UNPA Liu et al 2012). This charger proved that particle loss can be minimized by eliminating the electrical field in the charging zone (Medved et al 2000).…”