Collection Efficiency of the Aerosol Mass Spectrometer for Chamber-Generated Secondary Organic Aerosols

The collection efficiency (CE) of the aerosol mass spectrometer (AMS) for chamber-generated secondary organic aerosol (SOA) at elevated mass concentrations (range: 19–207 μg m^?3; average: 64 μg m^?3) and under dry conditions was investigated by comparing AMS measurements to scanning mobility particle sizer (SMPS), Sunset semi-continuous carbon monitor (Sunset), and gravimetric filter measurements. While SMPS and Sunset measurements are consistent with gravimetric filter measurements throughout a series of reactions with varying parent hydrocarbon/oxidant combinations, AMS CE values were highly variable ranging from unity to <15%. The majority of mass discrepancy reflected by low CE values does not appear to be due to particle losses either in the aerodynamic lens system or in the vacuum chamber as the contributions of these mechanisms to CE are low and negligible, respectively. As a result, the largest contribution to CE in the case of chamber-generated SOA appears to be due to particle bounce at the vaporizer surface before volatilization, which is consistent with earlier studies that have investigated the CE of ambient and select laboratory-generated particles. CE values obtained throughout the series of reactions conducted here are also well correlated with the f ₄₄/f ₅₇ ratio, thereby indicating both that the composition of the organic fraction has an important impact on the CE of chamber-generated SOA and that this effect may be linked to the extent to which the organic fraction is oxidized.

Copyright 2013 American Association for Aerosol Research     

Classification of particles in particle-laden stream through a stainless steel fibrous filter

This investigation experimentally explores the penetration curve of particles shot onto a stainless steel fibrous filter or a flat surface. The effect of the pore size of the stainless steel fibrous filter, with or without an oil coating, on the particle penetration was examined at various flow rates, nozzle diameters and dimensionless particle diameters, Sqrt(Stk). The penetration of the flat surface by particles was also determined for comparison. Experimental results demonstrate that oleic acid particles larger than Sqrt(Stk)₅₀ are collected on the stainless steel fibrous filter with a low penetration, while smaller particles stay in the particle-laden stream with high penetration. The penetration of potassium chloride particles exceeds that of oleic acid particles, because potassium chloride particles bounce off the stainless steel fibrous filter and the flat surface. Particles bounce off the metal filter less easily than the flat surface. Coating the stainless steel fibrous filter with oil effectively reduces problems of particle bounce. The potassium chloride particles sucked the coated oil forming a small mountain on the surface. When the loaded particle mass on the coated stainless steel fibrous filter ranges between 0.4 and 2.3 mg, Sqrt(Stk)₅₀ is a constant 0.35.     

Evaluation of the new capture vaporizer for aerosol mass spectrometers (AMS) through field studies of inorganic species

The aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) are widely used for quantifying aerosol composition. The quantification uncertainty of these instruments is dominated by the collection efficiency (CE) due to particle bounce. A new “capture vaporizer” (CV) has been recently developed to achieve unit CE. In this study, we examine the performance of the CV while sampling ambient aerosols. AMS/ACSMs using the original standard vaporizer (SV) and CV were operated in parallel during three field studies. Concentrations measured with the CV (assuming CE = 1) and SV (using the composition-dependent CE of Middlebrook et al.), as well as SMPS and PILS-IC are compared. Agreement is good in all cases, verifying that CE ～ 1 in the CV when sampling ambient particles. Specific findings include: (a) The fragmentation pattern of ambient nitrate and sulfate species observed with the CV was shifted to smaller m/z, suggesting additional thermal decomposition. (b) The differences in fragmentation patterns of organic vs. inorganic nitrate and sulfur species are still distinguishable in the CV, however, with much lower signal-to-noise compared to the SV. (c) Size distribution broadening is significant, but its impact is limited in field studies since ambient distributions are typically quite broad. Consistent size distributions were measured with the SV and CV. (d) In biogenic areas, UMR nitrate is overestimated based on the default fragmentation table (～factor of 2–3 in SOAS) for both vaporizers, due to underestimation of the organic interferences. We also report a new type of small interference: artifact chloride signal can be observed in the AMS when high nitrate mass concentration is sampled with both the SV (～0.5% chloride/nitrate) or CV (～0.2% chloride/nitrate). Our results support the improved quantification with the CV AMS and characterize its chemical detection properties.

Copyright © 2017 American Association for Aerosol Research     

Laboratory characterization of an aerosol chemical speciation monitor with PM2.5 measurement capability

The Aerodyne Aerosol Chemical Speciation Monitor (ACSM) is well suited for measuring non-refractory particulate matter up to approximately 1.0 µm in aerodynamic diameter (NR-sub-PM₁). However, for larger particles the detection efficiency is limited by losses in the sampling inlet system and through the standard aerodynamic focusing lens. In addition, larger particles have reduced collection efficiency due to particle bounce at the vaporizer. These factors have limited the NR-sub-PM₁ ACSM from meeting PM_2.5 (particulate matter with aerodynamic diameter smaller than 2.5 µm) monitoring standards. To overcome these limitations, we have redesigned the sampling inlet, the aerodynamic lens, and particle vaporizer. Both the new lens and vaporizer are tested in the lab using a quadruple aerosol mass spectrometer (QAMS) system equipped with light scattering module. Our results show that the capture vaporizer introduces additional thermal decomposition of both inorganic and organic compounds, requiring modifications to the standard AMS fragmentation table, which is used to partition ion fragments to chemical classes. Experiments with mixed NH₄NO₃ and (NH₄)₂SO₄ particles demonstrated linearity in the NH₄⁺ ion balance, suggesting that there is no apparent matrix effect in the thermal vaporization-electron impact ionization detection scheme for mixed inorganic particles. Considering a typical ambient PM_2.5 size distribution, we found that 89% of the non-refractory mass is detected with the new system, while only 65% with the old system. The NR-PM_2.5 system described here can be adapted to existing Aerodyne Aerosol Mass Spectrometer (AMS) and ACSM systems.

Copyright © 2017 American Association for Aerosol Research     

Characterization of Sintered Copper Wicks Used in Heat Pipes

Rectangular porous wicks for use in flat plate heat pipes were fabricated using copper powder (63 µm) sintered at 800°C and 1000°C. These wicks were characterized in terms of their porosity and pore size distribution using the techniques of mercury intrusion porosimetry and scanning electron microscopy (SEM). A uni-modal pore size distribution was obtained with most pores having sizes in the 30–40 µm range. Comparison was also made with cylindrical wicks fabricated by injection molding technique with the same binder and sintered at the same temperatures. Calculated permeability values of the rectangular wicks are comparable with commercially produced cylindrical wicks. When compared with conventional heat pipe wicks such as those using wire mesh, the advantage of these sintered wicks appears to be the existence of smaller pores and the controllability of porosity and pore size to optimize heat pipe performance.     

Evaporation rate of particles in the vaporizer of the Aerodyne aerosol mass spectrometer

We present calculations for evaporation rates of particles collected on the vaporizer of the Aerodyne aerosol mass spectrometer (AMS). These calculations provide insight on certain observed phenomena associated with the size-resolved mass spectrum (MS), because the time width of the MS signal from a particle can be limited by its evaporation rate upon contact with the vaporizer. We show that the counterintuitive weak dependence of observed MS signal widths (evaporation rates) on particle volatility is due to suppression of evaporation rates induced by latent heat release, which is more prominent at high volatilities. The same physics is responsible for the observed diminishing returns associated with increasing the vaporizer temperature to achieve narrower single particle pulses. We also show that the vaporizer typical operating temperature of 600°C is sufficient to evaporate extremely low volatility organic compounds (ELVOCs) rapidly enough to obtain reliable measurements for particles smaller than approximately 600 nm. However, the sizing resolution is compromised for large (near-micron) sizes regardless of particle volatility. Finally, our calculations indicate that the observed delayed particle signals, which lead to an artificial tail in AMS mass distributions, are not due to slow evaporation of particles deposited on a surface with lower temperature than the vaporizer, but particles bouncing in the ionizer cage and finally depositing on the vaporizer.

Copyright © 2017 American Association for Aerosol Research     

Image Analysis and Modeling of the Orientation of Pores in a Constrained Film on a Rigid Substrate

The nature of porosity in functional materials is often a critical parameter in determining their functionality, for example, in structure materials and fuel cell electrodes. Here, we study the development of the anisotropy of porous yttria‐stabilized zirconia (YSZ), focusing particularly upon the contribution of pore orientation to this anisotropy. Simulation from when the ink is deposited on the surface of a rigid substrate shows that platelike pores are found tend to align along the transverse direction of the substrate. Cross‐sectional image analysis of the pores from the attendant pores of YSZ particles or pore‐forming agent (PFA) matches with the simulated modeling when materials transport is insignificant in determining the shape of pores. The anisotropy created in the densification stage is separated from that in the green body by analyzing the image of porous structures formed with spherical glassy carbon which is unable to contribute to anisotropy during green‐state processing.     

A polydispersed particle system representation of the porosity for non-saturated cementitious materials

In this paper, the porosity of cementitious materials is described in terms of pore size distribution by means of a 3-dimensional overlapping sphere system with polydispersivity in size. On the basis of results established by Lu and Torquato [B. Lu, S. Torquato, Nearest-surface distribution functions for polydispersed particle systems, Phys. Rev. A 45(8) (1992) 5530-5544] and Torquato [S. Torquato, Random Heterogeneous Media: Microstructure and Macroscopic Properties. Springer-Verlag: New York, 2001] providing relations for nearest-neighbor distribution functions, the volume fraction of pores having a radius larger than a prescribed value is explicitly expressed. By adopting an appropriate size distribution function for the sphere system, it is shown that the pore size distribution of cementitious materials as detected for instance by mercury intrusion porosimetry (MIP), which generally points out several pore classes, can be well approached. On the basis of this porosity representation, the evaluation of the capillary pressure in function of the saturation degree is provided. The model is then applied to the simulation of the saturation degree versus relative humidity adsorption curves. The impact of the pore size distribution, the temperature and the thickness of the adsorbed water layer on these parameters are assessed and analyzed for three model materials having different pore characteristics.     

Collection Efficiencies in an Aerodyne Aerosol Mass Spectrometer as a Function of Particle Phase for Laboratory Generated Aerosols

The Aerodyne Aerosol Mass Spectrometer (AMS) is a useful tool to study ambient particles. To be quantitative, the mass or (number) of particles detected by the AMS relative to the mass (or number) of particles sampled by the AMS, or the AMS collection efficiency (CE), must be known. Here we investigated the effect of particulate phase on AMS CE for ammonium nitrate, ammonium sulfate, mixed ammonium nitrate/ammonium sulfate, and ammonium sulfate particles coated with an organic liquid. Dry, solid ammonium sulfate particles were sampled with a CE of 24 ± 3%. Liquid droplets and solid particles that were thickly coated with a liquid organic were collected with a CE of 100%. Mixed phase particles, solid particles thinly coated with liquid organic, and metastable aqueous ammonium sulfate droplets had intermediate CEs. The higher CEs for liquid particles compared with solid particles were attributed to wet or coated particles tending to stick upon impact with the AMS vaporizer, while a significant fraction of solid particles bounced prior to vaporization/detection. The consistency of single particle signals indicated that the phase (and hence CE) of mixed component particles did not affect the AMS sensitivity to a particular chemical species once volatilization occurred. Particle phase might explain a significant fraction of the variable AMS CEs reported in the literature. For example, ambient particles that were liquid (e.g., composition dominated by ammonium nitrate or acidic sulfate) have been reported to be sampled with 100% CE. In contrast, most ambient particle measurements report CEs of < 100% (typically~ 50%).     

Low-temperature fabrication and characterization of porous SiC ceramics using silicone resin as binder

Commercially available silicone resin and silicon carbide (SiC) powders were adopted as the starting materials for the fabrication of porous SiC ceramics. During the heat treatment process, silicone resin experienced an organic–inorganic transformation and acted as the bonding material between SiC particles at a low temperature of 1000 °C. The mean particle size of starting SiC powders and silicone resin content can control the pore size, open porosity and fracture strength. The flexural strength of porous SiC ceramics increases with increasing silicone resin content and decreasing mean particle size of SiC powders. Larger pores can be obtained with coarser starting SiC powders and higher silicone resin content. The fracture surface of porous SiC ceramics was observed.     

How the coarse fraction influences the microstructure and the effective thermal conductivity of alumina castables – An experimental and numerical study

This study investigates the particle size distribution's effect on the microstructure and effective thermal conductivity (ETC) of alumina castables. The ETC was measured by the transient plane source method and predicted numerically based on a two-scale model describing the structure on a fine and coarse scale. The prediction considered particle and pore size distributions, porosity (around 20%) and grain morphology. The microstructure was investigated by scanning electron microscopy. For a constant fines content, increasing the coarse grain fraction while decreasing the medium fraction enhanced sintering of the matrix. Small pores (≤250 nm) increased the sintering activity. The densest castable contained the most small pores. The particles’ and pores’ contributions to the sintering activity led to intensified microcracking and a decreased ETC. The numerical model did not consider constituents ≤500 nm like the small pores and microcracks and the calculated ETC values consequently deviated from the measured values.     

Evaluation of the new capture vaporizer for aerosol mass spectrometers: Characterization of organic aerosol mass spectra

The Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Speciation Monitor (ACSM) are widely used for quantifying submicron aerosol mass concentration and composition, in particular for organic aerosols (OA). Using the standard vaporizer (SV) installed in almost all commercial instruments, a collection efficiency (CE) correction, varying with aerosol phase and chemical composition, is needed to account for particle bounce losses. Recently, a new “capture vaporizer” (CV) has been shown to achieve CE～1 for ambient aerosols, but its chemical detection properties show some differences from the SV due to the increased residence time of particles and vaporized molecules inside the CV. This study reports on the properties and changes of mass spectra of OA in CV-AMS using both AMS and ACSM for the first time. Compared with SV spectra, larger molecular-weight fragments tend to shift toward smaller ions in the CV due to additional thermal decomposition arising from increased residence time and hot surface collisions. Artifact CO⁺ ions (and to a lesser extent, H₂O⁺), when sampling long chain alkane/alkene-like OA (e.g., squalene) in the CV during the laboratory studies, are observed, probably caused by chemical reactions between sampled OA and molybdenum oxides on the vaporizer surfaces (with the carbon derived from the incident OA). No evidence for such CO⁺ enhancement is observed for ambient OA. Tracer ion marker fractions (f_m/z =, i.e., the ratio of the organic signal at a given m/z to the total OA signal), which are used to characterize the impact of different sources are still present and usable in the CV. A public, web-based spectral database for mass spectra from CV-AMS has been established.

Copyright © 2018 American Association for Aerosol Research     

New insights on mass transfer kinetics in chromatography

The mass transfer kinetics of thiourea, phenol, ethylbenzene, propylbenzene, butylbenzene, and amylbenzene were studied on a Gemini‐C₁₈ (5 μm, 110 A?, 375 m²/g) column (150 mm × 4.6 mm) eluted with methanol/water solutions (100, 90, and 20% v/v). Each of the successive steps of the mass transfer of these solutes (axial diffusion, eddy dispersion, film mass transfer resistance, and transparticle mass transfer resistance) was unambiguously measured, using a combination of the peak parking method, the total pore blocking method, and moment analysis, in a wide range of reduced linear velocities. The results obtained offer new insights on the mass transfer kinetics in chromatographic columns. They show first that the eddy dispersion A‐term is strongly correlated with the particle porosity. The complex, anastomosed transcolumn flow pattern causes extra band broadening. This transcolumn effect was found to be markedly smaller with porous particles than with nonporous particles of the same size. Second, the film mass transfer coefficient of retained compounds is smaller for porous than for nonporous particles, a result consistent with concentration gradients being steeper at the wall of solid particles than across the entrance surface of pores. The external mass transfer coefficient decreases with increasing fraction of the surface area of the particles that is open to pores, e.g., with increasing particle porosity. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

ON THE PREDICTION OF POROSITY OF BEDS COMPOSED OF MIXTURES OF SPHERICAL PARTICLES†

A semi-theoretical study of porosity of particulate beds composed of mixtures of various size spherical particles has been performed. A model for mixtures of only two particle sizes is developed and is used to identify several dimensionless parameters which are correlated by comparing predictions with the data. The model is then extended to evaluate porosity of mixtures of a large number of particle sizes and distributions. The predictions from the model are found to compare quite well with the data taken in this work and the existing data in the literature when unavailability of a certain fraction of pores formed of largest size particles in the mixture is accounted for.     

包混材料

包混材料是在研制高温气冷堆用炭／石墨材料的过程发展出来的，它与常规炭／石墨材料的区别在于独特的粉料制备工艺，这种工艺使每一个骨料颗粒的表面都均匀地包覆上一层粘结剂，因此骨料之间的每一个接触点都是有效的连接，这种材料在相同的强度下，其密度可以比常规炭／石墨材料的低，孔隙均匀，通孔发达，具有广阔的应用前景，这种粉料制备工艺也可用于SiC及其他陶瓷制品。     

Biodegradable polycaprolactone/cuttlebone scaffold composite using salt leaching process

We prepared biodegradable polycaprolactone/cuttlebone scaffold composite by salt leaching process. In the first step, a co-continuous blend of biodegradable materials, polycaprolactone (PCL) and cuttlebone (CB), and an amount of sodium chloride salt particles were mixed using a stirrer. Next, the extraction of mineral salts using de-ionized distilled water was performed using a biodegradable PCL/CB scaffold with fully interconnected pores. Finally, the durable morphology of the scaffolds was fabricated by freeze-drying process at ?53 °C for 24 hrs in a vacuum. In addition, the quadrilateral pres ranged from about 250 to 300 ??m in diameter. Scanning electron microscopy (SEM) and mercury intrusion porosimeter techniques were carried out to characterize the pore morphology. By increasing the CB and sodium chloride salt particle content, the number of interconnected pores, material properties, and pore morphology were dramatically changed. The average compressive strengths (load at 50% strain) of the different porous PCL/CB scaffolds were found to decrease from 133 to about 79 (load at 50% strain, gf) with an increase in porosity. The values of the porosity increased as the sodium chloride salt volume fraction increased     

Physical modeling of the electrical double layer effects on multispecies ions transport in cement-based materials

The transport of ions through cement-based materials is described at a microscopic scale with a pore modeled by two infinitely large flat plates. The theory of the electrical double layer (EDL) shows that (i) the overlapping between the diffuse layers occurring in the pore is more important as the pore diameter will be small (low than the Debye length) and the pore walls will be strongly charged, (ii) the fluxes of coions and counterions will be respectively increased and attenuated in such pores. The gel pores of cement based materials have similar characteristics. As the capillary pores of the cement based materials with low porosity are connected between them by the pores gel, the transport of ions at a macroscopic scale could be greatly influenced by the overlapping effect of the diffuse layers.     

Preparation of interconnected poly(ε-caprolactone) porous scaffolds by a combination of polymer and salt particulate leaching

This paper examines a new technique for the preparation of porous scaffolds by combining selective polymer leaching in a co-continuous blend and salt particulate leaching. In the first step of this technique, a co-continuous blend of two biodegradable polymers, poly(ε-caprolactone) (PCL) and polyethylene oxide (PEO), and a certain amount of sodium chloride salt particles are melt blended using a twin screw extruder. Subsequently, extraction of the continuous PEO and mineral salts using water as a selective solvent yields a highly porous PCL scaffold with fully interconnected pores. Since, the salt particles and the co-continuous polymer blend morphology lead to very different pore sizes, a particular feature of this technique is the creation of a bimodal pore size distribution. Scanning electron microscopy, mercury intrusion porosimetry and laser diffraction particle size analysis were carried out to characterize the pore morphology. The prepared scaffolds have relatively homogeneous pore structure throughout the matrix and the porosity can be controlled between 75% and about 88% by altering the initial volume fraction of salt particles and to a lesser extent by changing the PCL/PEO composition ratio. Compared to the conventional salt leaching technique and to its different variants, the proposed process allows a better interconnection between the large pores left by the salt leaching and a fully interconnected porous structure resulting from the selective polymer leaching. The average compressive modulus of the different porous scaffolds was found to decrease from 5.2 MPa to about 1 MPa with increasing porosity, according to a power-law relationship. Since, the blending and molding of the scaffold (prior to leaching) can be made using conventional polymer processing equipment, this process seems very promising for a large scale production of porous scaffold of many sizes and in an economic way.     

Pore evolution during high pressure atomic vapor deposition

The development of physical vapor deposition systems that employ inert gas jets to entrain and deposit atomic and molecular fluxes have created an interest in the atomic assembly of thin films under high pressure (10–500 Pa) deposition conditions. Thin films grown under elevated pressure and low surface mobility conditions can contain a higher volume fraction of porosity and a different pore morphology to coatings created by conventional, low pressure (<10⁻⁴ Pa) deposition processes. A recent direct simulation Monte Carlo simulation analysis of binary vapor–gas jet atom interactions has shown that the incident angle distribution (IAD) for vapor atom impacts with a substrate is strongly effected by the background pressure. Here, these results are combined with a kinetic Monte Carlo technique to simulate the high pressure growth of vapor deposited nickel films and identify the mechanisms of pore formation. We find that when the surface atom mobility is low, shadowing of oblique angle arrivals by features on the substrate result in the incorporation of porosity with a hierarchical size distribution that includes elongated, inter-columnar pores and finer scale intra-columnar pores. The nucleation of the inter-columnar pores is related not only to the IAD, but also to the height and spacing of the initial asperities on the substrate and to those that subsequently evolve during deposition. The volume fraction of the inter-columnar pores is found to increase as both the fraction of oblique atom arrivals and the height of the asperities increase. For each prescribed IAD and asperity height, an asperity spacing is found that maximizes the inter-columnar pore fraction. By varying the IAD for a given substrate surface topology, in conjunction with intermittent observations of the coating structure during the growth process, the flux shadowing mechanisms that govern the inter-columnar pore nucleation have been determined.     

Effects of Particle Packing Characteristics on Solid-State Sintering

Alumina compacts fabricated with different green densities and different pore size distributions were characterized and the changes of the pore characteristics during solid-state sintering were studied. A critical ratio of pore size to mean particle size for pore shrinkage was determined. Porosity in the compact could be classified into two classes: the first class contains pores smaller than the critical ratio, and the second class contains pores larger than the critical ratio. Pores belonging to a different class of porosity behaved differently during sintering. Pores larger than the critical ratio were not totally eliminated during sintering. The first class of porosity controlled the ultimate sintering shrinkage, and the second class of porosity controlled the final sintered density.