Investigating particle emissions and aerosol dynamics from a consumer fused deposition modeling 3D printer with a lognormal moment aerosol model

ABSTRACT

Particle emissions from consumer-fused deposition modeling 3D printers have been reported previously; however, the complex processes leading to observed aerosols have not been investigated. We measured particle concentrations and size distributions between 7 nm and 25 μm emitted from a 3D printer under different conditions in an emission test chamber. The experimental data was combined with a moment lognormal aerosol dynamic model to better understand particle formation and subsequent evolution mechanisms. The model was based on particles being formed from nucleation of unknown semivolatile compounds emitted from the heated filament during printing, which evolve due to condensation of emitted vapors and coagulation, all within a small volume near the printer extruder nozzle. The model captured observed steady state particle number size distribution parameters (total number, geometric mean diameter and geometric standard deviation) with errors nominally within 20%. Model solutions provided a range of vapor generation rates, saturation vapor pressures and vapor condensation factors consistent with measured steady state particle concentrations and size distributions. Vapor generation rate was a crucial factor that was linked to printer extruder temperature and largely accounted for differences between filament material and brands. For the unknown condensing vapor species, saturation vapor pressures were in the range of 10^?3 to 10^?1 Pa. The model suggests particles could be removed by design of collection surfaces near the extruder tip.

Copyright © 2018 American Association for Aerosol Research     

Sensitivity analysis of aggregate morphology on mass-mobility relationship and improved parameterizations

Past studies have shown that the diffusion-limited cluster aggregation mechanism yields aggregates with a mass fractal dimension (D_f) of around 1.8 and power-law prefactor (k_f) ranging between 1.2 and 2.5. For a fixed D_f, an increasing k_f physically manifests as decreasing shape anisotropy or the degree of “stringiness” of an aggregate. In this work, we investigate the effects of changing k_f, monomer size (d), and number of monomers (N) of computer-simulated aggregates on their mass-mobility scaling exponent (D_fm) and prefactor (k_fm). Our simulation results for a statistically significant number of D_f = 1.78 ± 0.10 aggregates yielded D_fm values of 2.20 ± 0.05. These values are in excellent agreement with previous experimental observations. While variations in D_fm were predominantly influenced by D_f, k_fm showed sensitivity to fluctuations in k_f. The validity and accuracy of the empirical power-law exponent 1.08 used for estimating N in three dimensions from two dimensional projection images was also evaluated. It was found that the exponent was only valid for aggregates with k_f close to unity. A correction has been proposed to account for the enhanced apparent screening effects at large k_f.

© 2016 American Association for Aerosol Research     

Sampling and dilution of nanoparticles at high temperature

Sampling and dilution of flame-generated, fractal-like ZrO₂ aerosols is investigated by aerosol mass/mobility measurements and microscopy. Two broadly used sampler configurations, a straight-tube (ST) and a hole-in-a-tube (HiaT), at three different in-flow orientations and hole diameters are evaluated. The mobility size distributions, mass-mobility exponent, D_fm, prefactor, k_fm, and average primary particle diameter are obtained at 10–60 cm height above the burner (HAB) of fuel-rich (hot) and fuel-lean (cold) spray flames by differential mobility analyzer (DMA) and aerosol particle mass (APM) measurements using a recent power law for fractal-like particles. The primary particle diameter, agglomerate size distributions, and corresponding standard deviations from aerosol measurements are compared to those by counting images of particles collected by thermophoretic sampling along the flame centerline. Once new particle formation is completed in the flame, both sampler configurations result in nearly identical particle size distributions. Furthermore, all HiaT samplers result in similar mobility size distributions at all orientations, regardless of hole size. Sampling using a downstream in-flow hole orientation results in slightly larger Sauter mean diameters than those obtained by upstream or sidestream ones, especially for the cold flame. Additionally, a correlation is developed by Discrete Element Modeling (DEM) for the agglomerate D_fm evolution to its asymptotic value of 2.2 as function of the average number of primary particles per agglomerate, n_va, or the relative particle density with pre-exponential constant k_fm = 1.18, regardless of primary particle size. This is in good agreement with an experimentally obtained correlation in terms of relative particle density as well as with experimental data for ZrO₂, Ag, and Cu nanoparticles.

© 2016 American Association for Aerosol Research     

Using silver as a surrogate for plutonium: An experimental study of the explosive silver aerosol source term

An experimental method is developed for the purpose of simulating plutonium aerosol source terms with conventional metals in laboratory. In this method, metal samples are aerosolized by high explosive detonation in a containment vessel. Aerosols having aerodynamic diameter (AD) less than 10 µm are then collected by a cascade impactor and analyzed by atomic absorption spectroscopy. Two sets of experiments were conducted. In the first set, five candidate metal samples (Ag, W, Sn, Ce, and V) were tested. It is found that the cumulative mass distribution of silver under certain conditions was in good agreement with that of plutonium from the Operation Roller Coaster-Double Track experiment. Thus, silver is chosen as a surrogate to simulate the plutonium aerosol source term. In the second set, silver aerosol source term was studied in detail with different test configurations. The results demonstrate that the peak of the mass-size distribution of silver is in the AD range 1.1–3.3 µm. The amount and fraction of relatively small silver aerosols decrease significantly with time due to coagulation and deposition. Interestingly, the amount of silver in aerosols could be expressed as a quadratic function of the peak detonation pressure.

© 2016 American Association for Aerosol Research     

Cu-Sn binary metal particle generation by spray pyrolysis

Cu-Sn binary particles were generated via spray pyrolysis from metal salt precursors with ethylene glycol as the co-solvent and reducing agent. The morphology, crystallinity, and elemental distribution of particles were tunable by changing the reaction temperature, residence time, and quench gas flow rate. Hollow porous particles were fabricated with a higher Sn concentration on the particle surface when the furnace set point was 500°C, while solid particles with a lower surface Sn concentration were generated when the furnace set point was 1000°C. Particles with spherical morphologies were obtained at long residence time conditions (4.5 s). Cu-Sn binary particles with irregular structures (e.g., pores on the particle surface, fragmented spherical particles, and lamellar fragments) were formed at short residence time conditions (0.92 s). A possible spray pyrolysis mechanism was proposed that incorporates chemical reaction steps and structural progression. By this mechanism, the metal salts are believed to sequentially undergo hydrolysis to metal hydroxides, decomposition to metal oxides, reduction to metals, and finally diffusion of Sn into the Cu matrix to generate the Cu-Sn solid solution.

Copyright © 2017 American Association for Aerosol Research     

The effect of nanosilica (SiO2) and nanoalumina (Al2O3) reinforced polyester nanocomposites on aerosol nanoparticle emissions into the environment during automated drilling

The aim of this study is to investigate the effect nanosilica and nanoalumina has on nanoparticle release from industrial nanocomposites due to drilling for hazard reduction whilst simultaneously obtaining the necessary mechanical performance. This study is therefore specifically designed such that all background noise is eliminated in the measurements range of 0.01 particles/cm³ and ±10% at 10⁶ particles/cm³. The impact nano-sized SiO₂ and Al₂O₃ reinforced polyester has on nanoparticle aerosols generated due to drilling is investigated. Real-time measurement was conducted within a specially designed controlled test chamber using a condensation particle counter (CPC) and a scanning mobility particle sizer spectrometer (SMPS). The results show that the polyester nanocomposite samples displayed statistically significant differences and an increase in nanoparticle number concentration by up to 228% compared to virgin polyester. It is shown that the nanofillers adhered to the polyester matrix showing a higher concentration of larger particles released (between 20 – 100 nm). The increase in nanoparticle reinforcement weight concentration and resulting nanoparticle release vary considerably between the nanosilica and nanoalumina samples due to the nanofillers presence. This study indicates a future opportunity to safer by design strategy that reduces number of particles released concentration and sizes without compromising desired mechanical properties for engineered polymers and composites.

© 2017 American Association for Aerosol Research     

Cluster formation mechanisms of titanium dioxide during combustion synthesis: Observation with an APi-TOF

Few studies reported the formation of Ti-containing clusters in the initial stages of TiO₂ flame synthesis. The conversion from synthesis precursor to TiO₂ monomers was commonly assumed to take place through global reaction such as thermal decomposition and/or hydrolysis at high temperatures. More recent studies have been able to identify stable intermediates of Ti-containing monomers, most commonly Ti(OH)₄, as the final step before the formation of TiO₂. However, no larger Ti-containing cluster formation mechanisms or interactions between these monomers have been tracked. To investigate cluster formation pathways of TiO₂ during flame synthesis, Charged clusters were measured in an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer. TiO₂ nanoparticles were synthesized by adding titanium tetraisopropoxide (TTIP) precursor to a premixed CH₄/O₂/N₂ flat flame aerosol reactor. Pure TiO₂ clusters were not detected by the APi-TOF. Results from measured mass spectra and mass defect plots show that for positively charged clusters, the abstraction of CH₂ groups occurs simultaneously with the clustering of larger intermediate organometallic species. For negatively charged clusters, NO_x formation pathways in the flame may play a role during the initial stages of TiO₂ formation, since a lot of Ti-containing clusters were attached with nitrate-related species. These research findings provide insights on quantum dot synthesis and molecular doping where rapid dilution of the flame synthesized nanoparticles is needed to better control the particle size and chemical composition. The possible influences of and potential artifacts brought by the dilution system on observing the incipient particle formation in flames were also discussed.

© 2017 American Association for Aerosol Research     

Controlled mesoporous film formation from the deposition of electrosprayed nanoparticles

Thin films of controlled morphology were fabricated by electrospray drying a colloidal nanoparticle suspension using a conductive and volatile solvent and impacting the nanoparticles on a substrate. Three parameters were used for control: impact velocity, size of the nanoparticles or nanoparticle agglomerates, and solvent volatility. The impact velocity was controlled by charging nanoparticles through electrospray dispersion and varying the electric field driving the particle impaction. It was found that the structure is governed by the relative importance of charged particle drift imposed by the external electric field and the thermal velocity due to Brownian motion. Peclet number correlates with the morphology of the deposit where columnar structures result from high Pe, corresponding to ballistic deposition and porous, fractal-like structures result from small Pe. These patterns match predictions based on Monte Carlo simulations in the literature. For dispersions with higher nanoparticle concentrations, droplet evaporation causes densification of the particle ensemble to form a spherical aggregate that deposits in a predominantly ballistic manner, with smaller aggregates forming denser films. If the droplet evaporation lifetime is altered for the aggregates to be partially wet upon impacting the substrate, the subsequent rapid evaporation of the remaining solvent on the substrate leads to formation of films with high interconnectivity. Films formed by the electrospray technique have large-scale uniformity and their structure is independent of thickness. The interpretation of the observed morphologies in terms of Peclet number and Damkhöler number provides a conceptual framework for a rational design of film structures as required by many applications.

Copyright © 2017 American Association for Aerosol Research     

Real-time measurement of size-resolved elemental composition ratio for flame synthesized composite nanoparticle aggregates using a tandem SMPS-ICP-OES

Composite nanoparticles find application in catalysis, drug delivery, and energy storage and require increasingly fine control of their physical properties and composition. While composite nanoparticles have been widely synthesized and characterized, little work has systematically correlated the initial concentration of precursors and the final composition of flame synthesized composite nanoparticles. This relationship is explored in a diffusion flame aerosol reactor by coupling a scanning mobility particle sizer (SMPS) with an inductively coupled plasma optical emission spectrometer (ICP-OES). A framework for studying the relationship between the initial precursor concentrations of different elements and the final nanoparticle composition is explored. The size-resolved elemental composition was measured by directly injecting size-selected fractions of aggregated magnetite and silicon dioxide composite nanoparticles into the ICP-OES plasma. This work showed a correlation between precursor molar ratio and the measured elemental ratio in the mobility size range of 50 to 140 nm. Building on previous work studying size resolved elemental composition of engineered nanoparticles, the analysis is extended to flame synthesized composite nanoparticle aggregates in this work.

Copyright © 2017 American Association for Aerosol Research     

Comparison of nanoparticle generation by two plasma techniques: Dielectric barrier discharge and spark discharge

Dielectric barrier discharge (DBD) and spark discharge, two versatile atmospheric pressure plasma-based techniques, have been employed to generate nanoparticles. This study compares the characteristics of metal nanoparticles generated by a DBD reactor and a spark discharge generator with argon as the working gas. The gas temperature in the discharge region of the DBD reactor remained near room temperature, while that of the spark reactor varied from 470 to 1120 K and generally increased with increasing applied voltage amplitude in the range of 2–10 kV and driving frequency in the range of 1–10 kHz. Comparing to spark-generated nanoparticles under the same voltage, frequency, and flow rate, DBD-generated nanoparticles have smaller sizes, better monodispersity, and lower number concentrations. The number concentration of DBD-generated particles decreases significantly under high working voltage and frequency, while the number concentration of spark-generated particles increases with increasing working voltage. Under continuous operations over several hours, the DBD reactor has better temporal stability in generating nanoparticles than the spark generator.

© 2017 American Association for Aerosol Research     

Studies on detachment behavior of micron sized droplets: A comparison between pure fluid and nanofluid

Resuspension is considered as a source of indoor air pollutants. These airborne pollutants can be in the form of liquid or solid. It has been previously found that the detachment mechanism of liquid droplets is different from the solid particles on the poly(methyl methacrylate) (PMMA) surface. Liquid droplets detach by portion when they are under an increasing normal force field while droplets detach completely when under a tangential force field. In this research, droplet detachment experiments are extended to different substrate materials, which are PMMA, glass, and stainless steel by the means of centrifuge. Also, the differences in detachment between pure glycerol-water (pure fluid) and a glycerol solution with the addition of nanoparticles (nanofluid) are investigated under different substrate materials. It is found that liquid droplets, again, detach by portion under normal force for all the substrate materials. For tangential force, the droplets detach completely if the exerted force was sufficiently large and the threshold values are material dependent, which is further elaborated by retention theory. After the addition of nanoparticles, a higher removal force was required compared to the droplets of pure fluid within the same size range. Also, solid residues with a negligible amount of fluid were found on the substrate after each removal of droplets under both normal and tangential force. The involvement of nanoparticles could be the pioneer work for future studies on commonly found liquid pollutants, which are prone to be contaminated by solid particles, such as in salivary excretion.

Copyright © 2018 American Association for Aerosol Research     

The initial stages of multicomponent particle formation during the gas phase combustion synthesis of mixed SiO2/TiO2

The ability to properly scale the synthesis of advanced materials through combustion synthesis routes is limited by our lack of knowledge regarding the initial stages of particle formation. In flame aerosol reactors, the high temperatures, fast reaction rates, and flame chemistry can all play a critical role in determining the properties of the resulting nanomaterials. In particular, multicomponent systems pose a unique challenge as most studies rely on empirical approaches toward designing advanced composite materials. The lack of predictive capabilities can be attributed to a lack of data on particle inception and growth below 2 nm. Measurements for the initial stages of particle formation during the combustion synthesis of SiO₂ and composite SiO₂/TiO₂ using an atmospheric pressure inlet time-of-flight mass spectrometer are presented. Both positively and negatively charged clusters can be measured and results show the presence of silicic acid species which grow through dehydration, hydrogen abstraction, and interactions with hydroxyl radicals. In the case of composite SiO₂/TiO₂ particle formation, new molecular species containing Ti atoms emerge. Tandem differential mobility analysis-mass spectrometry (DMA-MS) provided further insight into the size-resolved chemistry of particle formation to reveal that at each cluster size, further hydroxyl-driven reactions take place. From this we can conclude that previous assumptions on collisional growth from simple monomer species of SiO₂ and TiO₂ do not sufficiently describe the collisional growth mechanisms for particle growth below 2 nm.

Copyright © 2018 American Association for Aerosol Research     

Deposition of ultrafine aerosol particles on wire screens by simultaneous diffusion and image force

This paper presents the results of an experimental investigation on the deposition of multiply charged particles on wire screens by the combined mechanisms of diffusion and image force. Experiments were performed with particles having diameters between 25 and 65 nm (transition regime), carrying 0, +1, +2 or +3 elementary charges, and using three different flow rates, two types of wire screen, and two types of test aerosol. The single fiber efficiencies for the mechanisms of image force, η_IM, and diffusion, η_D, are of the same order of magnitude and, furthermore, they are both much smaller than one. Under these conditions, the total capture efficiency can be approximated as the sum of the efficiencies by diffusion and image force deposition. Theoretically, η_IM is proportional to the square root of a dimensionless number, K_IM, which includes all the relevant parameters cited above (i.e., particle size and charge, aerosol flow rate and screen geometry). The available correlations for η_IM, obtained from experiments with particles carrying a large number of elementary charges (K_IM>10^-5), predict that image force should not have any effect in the case of the small particles with very few number of charges that we have tested in our experiments (in our experimentation, K_IM ranged between 10^-7 and 10^-5). Our results, the only ones available to date for this particle size range, show that there is indeed a clear, measurable effect. Although our experimental results are best fitted by the correlation , it is shown that the expression , which is in agreement with the theoretical 1/2 exponent for K_IM, also reproduces reasonably well the measured values.     

Photophoresis of aerosol particles

On the basis of the linearized gas-kinetic equation solution a force affecting a unilaterally-illuminated aerosol particle is calculated. Numerical calculations of the photophoretic force and the particle velocity are carried out for the whole range of Knudsen numbers taking into account the particle's thermophysical, optical and accommodation properties. The results obtained are compared to the known theoretical and experimental data.     

Coagulation of bipolarly charged ultrafine aerosol particles

Particle charging during coagulational growth is widely used in material synthesis processes as well as with industrial particle removal equipment. Coagulation behavior of charged particles is significantly different from that of neutral particles. To calculate the change in size/charge distribution of particles undergoing bipolar coagulation, a two-dimensional sectional model has been usually used. This method, however, needs considerable computation time although it gives very accurate prediction. In this study, the moment model, to solve the bipolar coagulation problem in the free-molecule regime, was developed to provide a time-efficient tool. Simultaneous particle charging by bipolar ions was also considered in this study. The developed model is based on the assumption that particles cannot have more than one unit charge and the particle size distribution remains log normal. The developed model was compared to the two-dimensional sectional model, with good agreement being shown. Some characteristics of bipolar coagulation were investigated using the developed model. The bipolar coagulation with simultaneous bipolar diffusion charging was shown to significantly increase the coagulation rate compared to the neutral Brownian coagulation. It was also shown from the simulation results that if one needs a higher coagulation rate in the initial stage, bipolar coagulation without ions is recommended, while bipolar coagulation with simultaneous charging by bipolar ions should be used if one wants a high coagulation rate for a long time.     

Proton microprobe PIXE-analyses of single aerosol particles

A method for elemental analyses of individual aerosol particles by means of a focused proton beam and the PIXE method is demonstrated. Preliminary tests of the method has been performed successfully and several aspects on the results are discussed.     

Free electron charging of ultrafine aerosol particles

Experiments of free electron charging of aerosol particles in the free molecule regime are reported. Monodisperse ultrafine silver particles of 5–30 nm were exposed to known concentrations of low energy electrons produced by ionization by -particles in a unipolar aerosol charger. The kinetic energy of the electrons was varied by changing the electric field intensity in the charger (i.e. between 93 and 279 V cm⁻¹). The range of Knudsen number for aerosol charging (i.e. the ratio of the electron mean free path to the particle radius) was from 30 to 261. The charged fraction was measured as a function of particle size in high-purity helium and nitrogen under different charging conditions. The experimental results for the combination coefficient between neutral particles and electrons suggested a free-molecule diffusion charging mechanism which was dependent on the electron mobility, transverse diffusion coefficient, mean free path, and mean kinetic energy (i.e. electron temperature). The functional dependence was similar to that given by the ionic charging theories of Natanson (1960, Sov. Phys. 5, 538–551), and Fuchs (1963, Geofis. Pura Appl. 56, 185–193) when the appropriate electron properties were used. The electron charging models of O'Hara et al. (1989, J. Aerosol Sci. 20, 313–330), and Zagnit'ko et al. (1989, Russ. J. Phys. Chem. 63, 883–888) did not fit the experimental results because they were not derived for the free-molecule regime. A modified Fuchs charging theory that uses an empirical accommodation coefficient for the electrons at the surface of the particle was used to fit the experimental results. Good agreement was found by using an accommodation coefficient of 0.4 for both helium and nitrogen.