Weon Gyu Shin

Structural Property Effect of Nanoparticle Agglomerates on Particle Penetration through Fibrous Filter

Most filtration studies have been conducted with spherical particles; however, many aerosol particles are agglomerates of small primary spheres. Filtration efficiency tests were conducted with silver NP agglomerates, with the agglomerate structure controlled by altering the temperature of a sintering furnace. The mobility diameter and mass of the silver NP agglomerates were measured using a differential mobility analyzer together with an aerosol particle mass analyzer. From these measurements, it was found that the fractal-like dimension, D fm, varied from 2.07 to 2.95 as the sintering temperatures was increased from ambient to 600°C. The agglomerates were essentially fully coalesced at 600°C allowing direct comparison of the filtration behavior of the agglomerate to that of a sphere with the same mobility diameter. Other agglomerate properties measured include the primary diameter, the agglomerate length and aspect ratio, and the dynamic shape factor. Agglomerate filtration modeling with no adjustable parameters has been investigated in terms of diffusion, impaction, and interception. The model results agree qualitatively with the experimental results in the particle size range of 50 to 300 nm. The results indicated that the larger interception length of agglomerates is responsible for the smaller penetration through a fibrous filter in comparison to spherical particles with the same mobility diameters.

Measurement of Nanoparticle Agglomerates by Combined Measurement of Electrical Mobility and Unipolar Charging Properties

We have developed an instrument, Universal NanoParticle Analyzer (UNPA), for quasi-online measurement of gas-borne nanoparticle agglomerates. The UNPA utilizes a Differential Mobility Analyzer (DMA), a Condensation Particle Counter (CPC), and a Nanoparticle Surface Area Monitor (NSAM, including a unipolar charger combined with an electrometer), to determine the primary particle size and measure the number, surface area, and volume distributions of loose nanoparticle agglomerates. By loose agglomerates we refer to those that can be modeled as clusters of spherical primary particles with open structures and fractal dimensions less than two. The key parameter measured is the UNPA sensitivity, which is defined as the current measured by the NSAM divided by the number concentration measured by the CPC. Our experimental data and theoretical model have shown that the UNPA sensitivity S depends on the particle morphology. The sensitivity S is larger for loose agglomerates than for spheres at a fixed mobility diameter, and S for partly sintered agglomerates is between those for loose agglomerates and spheres. Thus the measured value of S gives a direct indication of the particle morphology. From UNPA sensitivities, the primary particle size is determined using a fitting procedure. Using the model of Lall and Friedlander (2006) for loose agglomerates, the number of primary particles in agglomerates can be computed. Then the surface area and volume of agglomerates can be obtained. Operated under the scanning mode, the UNPA can provide the number, surface area and volume distributions of loose agglomerates in the range of 50 to several hundred nanometers in several minutes.

Structural Properties and Filter Loading Characteristics of Soot Agglomerates

Nanoparticle agglomerates are pervasive in atmospheric sciences, air pollution, and manufacturing of powdered materials, yet studies for filtration of nanoparticle agglomerates are still scarce compared to those for spherical particles. We investigated loading of soot nanoparticle agglomerates on fibrous air filter media. The soot agglomerates were generated from a diffusion burner with propane gas as fuel and compressed air as oxidant/sheath. The mode of the number distribution was determined to be 120 nm. A Differential Mobility Analyzer (DMA)—Aerosol Particle Mass Analyzer (APM) system was used to measure the mass of agglomerates as a function of the mobility size, which gave a mass-mobility exponent of 1.9 ± 0.1. Using transmission electron microscopy (TEM), we found that the primary particles in the agglomerates had a mean diameter of 28 nm with a geometric standard deviation of 1.26. Loading experiments were carried out with the face velocity of 10 cm/s on a fiberglass filter media. The pressure drop increased approximately linearly with the loading mass. The porosity of the cakes was calculated using cake mass and cake thickness, and the average cake porosity was 0.95. We found that the model of Endo et al. (1998) for cake loading was applicable to soot agglomerate loading. The cake could be regarded as formed by primary particles in soot agglomerates, and agglomerates are indistinguishable once deposited in the cake. When the size distribution of the primary particles was used in the model of Endo et al. good agreement between the experimental and computed results was obtained.

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

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 real-time 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 under our experimental conditions between results given by UNPA, the effective density, and the gravimetric measurement. Copyright 2012 American Association for Aerosol Research

Ultrasonication assisted production of silver nanowires with low aspect ratio and their optical properties.

We investigated a facial method to produce silver nanowires with low aspect ratio by fragmentizing as produced long silver nanowires. The length of silver nanowires can be shortened in a controllable manner by increasing ultrasonication time or ultrasonication power. However, excessively large ultrasonication power caused a problem of agglomeration of nanowires. From UV absorption spectra, it was found that the UV absorption characteristic of silver nanowires is not affected by ultrasonication assisted fragmentation indicating that one dimensional structure of silver nanowires is maintained during the fragmentation process.

Hot-wire synthesis of gold nanoparticles

Gold nanoparticles were synthesized by a hot-wire generator at atmospheric pressure using a gold-platinum composite wire. At low gas flow velocities the nanoparticles were found to be agglomerates of partially sintered primary particles. By reducing the tube size via the insertion of a nozzle with a throat diameter of 3 mm, the hot-wire generator was found to produce small (<10 nm diameter) crystalline gold particles. Elemental and x-ray photoelectron spectroscopy analysis of the particles showed that they were composed of gold with no platinum impurity. Charging analysis of the “as-produced” nanoparticles showed that fewer than 10% of the particles were charged, but the charge fraction increased as the applied power increased, as did the ratio of negatively-to-positively-charged particles.

Combustion of boron particles coated with an energetic polymer material

Elemental boron has attracted considerable attention as a potential high energetic material for explosives and propellants. However, its use has been hindered by its high vaporization temperature and surface oxide layer. In this study, boron particles were coated with glycidyl azide polymer (GAP) to improve their combustion characteristics. The coated particles were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy. XPS performed before and after Ar+ ion sputtering confirmed that the azide (−N3) group of GAP was positioned at the proximity of the boron surface. In addition, B@GAP particles could be decorated with metallic Ag (∼10 nm) nanoparticles. The combustion characteristics were examined using a newly designed pre-heated (1,800 K) drop tube furnace and a high speed camera. Two stages of combustion were observed for a dust cloud of GAP-coated boron particles. The burning time was estimated to be approximately 37.5 msec.

Silver Nanowire Penetration Through Screen Filter

Understanding the filtration characteristics of fibrous particles is important since those particles have caused health and environmental concerns. Due to the straight morphology of metal nanowires, unlike carbon nanotube (CNT) particles nanowires can be considered as appropriate test material to evaluate existing filtration theory for cylindrical particles. We measured the penetration of silver nanowires in the size range of dm = 200 to 400 nm through screen mesh filter. By using Li et al. (2012)s theory, we determined the orientation status of silver nanowires inside differential mobility analyzer (DMA) and calculated the dynamic shape factor of nanowires. Theoretical penetration was obtained by using single fiber theory with modified interception parameter including orientation angle between a filter wire and a particle. The orientation angle obtained by fitting experimental data into single fiber theory for the 1 layer of screen mesh filter is found to be close to 40° indicating random orientation of nanowires near filter. However, in the experiments with multi-layers of screen mesh, any tendency related to the orientation angle was not found. We performed numerical simulations for the filtration processes such as impaction, diffusion, interception, and interception of diffusing particles by introducing modified slip correction factor. Overall, when interception of diffusing particles is considered in addition to diffusion and interception, numerically simulation results and theoretical prediction agree better with experimental data regarding the penetration of silver nanowires through the 1 layer of screen mesh filter. Copyright 2014 American Association for Aerosol Research

Experimental investigation and numerical modeling of the orientation angle of silver nanowires passing through polyester filters

ABSTRACT In the present study, we measured the penetration of silver nanowires with the mobility diameter in the range from 200 to 400 nm through two different types of polyester filters: the screen filter with the solidity of 0.505 and fibrous filter with the solidity of 0.278. The orientation angles of silver nanowires passing through the single layer and multi-layers of polyester filter were experimentally estimated on the basis of the single fiber efficiency theory. In the case of the screen filter, the orientation angle obtained by fitting the experimental data for single layer was found to be close to 40˚, indicating a random orientation of nanowires near the filter. However, the fibrous filter has the orientation angle much larger than 40˚. The orientation angle can be affected by inhomogeneity of the filter. In particular, in the case of the fibrous filter, the solidity and fiber diameter may affect the orientation angle. For multi-layers of both screen and fibrous filters, it is difficult to determine the typical orientation angle and the fibrous filter tends to have a larger orientation angle than the screen filter. In addition, we carried out numerical simulations on the penetration of silver nanowires through the five layers of screen filter and the single layer of both screen and fibrous filters. Numerical prediction was carried out by using the three-dimensional numerical model determined by solidity and thickness of fibrous filter. Numerical predictions are highly congruent with experimental results and theoretical prediction.

Preparation of TiO2-Decorated Boron Particles by Wet Ball Milling and their Photoelectrochemical Hydrogen and Oxygen Evolution Reactions

TiO2-coated boron particles were prepared by a wet ball milling method, with the particle size distribution and average particle size being easily controlled by varying the milling operation time. Based on the results from X-ray photoelectron spectroscopy, transmission electron microscopy, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy, it was confirmed that the initial oxide layer on the boron particles surface was removed by the wet milling process, and that a new B–O–Ti bond was formed on the boron surface. The uniform TiO2 layer on the 150 nm boron particles was estimated to be 10 nm thick. Based on linear sweep voltammetry, cyclic voltammetry, current-time amperometry, and electrochemical impedance analyses, the potential for the application of TiO2-coated boron particles as a photoelectrochemical catalyst was demonstrated. A current of 250 μA was obtained at a potential of 0.5 V for hydrogen evolution, with an onset potential near to 0.0 V. Finally, a current of 220 μA was obtained at a potential of 1.0 V for oxygen evolution.