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A pinboard by
Timothy Sipkens

PhD Student, University of Waterloo

PINBOARD SUMMARY

We use advanced statistics to determine fundamental properties of soot and engineered nanoparticles.

Engineered nanoparticles are enabling emerging technologies that promise to change the world of tomorrow, with applications in medicine, electronics, environmental remediation, and combustion. Meanwhile, discussion of nanoparticle production raises concerns about their impact on the environment. Soot, for example, is a carbonaceous nanoparticle already produced in mass by combustion processes. Its release into the atmosphere is also one of the leading causes of respiratory illness. Determining the impact and use of nanoparticles requires that we first have ways to characterize them, demanding new, unique diagnostics that allow us to describe the nanoscale. Time-resolved laser-induced incandescence (TiRe-LII) is a diagnostic in which aerosolized nanoparticles are heated using a short laser pulse until they glow or emit incandescence. The magnitude of the signal can be used to estimate the particle loading in engine cylinders or during gas-phase nanoparticle synthesis. Further, as smaller nanoparticles cool more quickly, the rate of decay of the incandescence following the laser pulse can be related to more fundamental properties of the nanoparticles, most prominently their size and thermal characteristics. The literature contains a large set of competing models that are used to interpret TiRe-LII data from soot and engineered nanoparticles without a consensus on which model is best. Furthermore, practitioners often ignore uncertainties in their estimates, increasing the degrees-of-freedom in their models until they can fit the data. This violates Occam’s razor and causes practitioners to pick models that are unnecessarily complex and tuned to specific data sets. To address this, the current work applies an advanced statistical technique known as Bayesian model selection to TiRe-LII data. The technique acts to determine the model most likely to have produced a given set of data by taking into account a model’s degrees of freedom and uncertainty in model inputs. The resulting preferred model is the one that is most robust to different data sets, but still supports the underlying physics. We demonstrate this technique by determining the temperature and nanoparticle size dependence of the latent heat of vaporization and vapor pressure. We hope that the framework will enable practitioners to further the frontier of nanoparticle characterization so that we can sustainably realize the full potential of nanoparticles.

15 ITEMS PINNED

Nano-particle sizing by laser-induced-incandescence (LII) in a shock wave reactor

Abstract: Abstract. Laser-Induced Incandescence (LII) is a relatively new optical diagnostic for particle sizing which is currently used in combustion science. Its advantage against light extinction and light scattering methods is the possibility of getting size information with high time and space resolution even for nano-particles. LII is mostly applied to particle formation or particle removal in reactive stationary flows, but it can also be used in shock-induced reactive flows. This is demonstrated in three examples: soot particle formation during high temperature pyrolysis of benzene, iron particle formation from iron pentacarbonyl, and formation of carbon-coated iron particles. From the principles of LII, it is not possible to obtain a complete particle growth curve from one individual shock tube experiment. Therefore, the kinetics of particle growth evolution must be determined from several “identical” shock tube experiments with a delayed triggering of the heat-up laser. The principles of LII, the in-situ measurement of particle size, and the comparison to beam-collected particles, which were visualized by a high resolution transmission electron microscope (HRTEM), are demonstrated. It was found that the energy accommodation coefficient during the particle cooling is \(\alpha = 1\) for a soot surface but is significantly lower e.g. for an iron surface.

Pub.: 01 Mar '03, Pinned: 01 Nov '17

Molecular dynamics simulation of thermal accommodation coefficients for laser-induced incandescence sizing of nickel particles

Abstract: Extending time-resolved laser-induced incandescence (TiRe-LII), a diagnostic traditionally used to characterize soot and other carbonaceous particles, into a tool for measuring metal nanoparticles requires knowledge of the thermal accommodation coefficient for those systems. This parameter can be calculated using molecular dynamics (MD) simulations provided the interatomic potential is known between the gas molecule and surface atoms, but this is not often the case for many gas/surface combinations. In this instance, researchers often resort to the Lorentz–Berthelot combination rules to estimate the gas/surface potential using parameters derived for homogeneous systems. This paper compares this methodology with a more accurate approach based on ab initio derived potentials to estimate the thermal accommodation coefficient for laser-energized nickel nanoparticles in argon. Results show that the Lorentz–Berthelot combining rules overestimate the true potential well depth by an order of magnitude, resulting in perfect thermal accommodation, whereas the more accurate ab initio derived potential predicts an accommodation coefficient in excellent agreement with experimentally-determined values for other metal nanoparticle aerosols. This result highlights the importance of accurately characterizing the gas/surface potential when using MD to estimate thermal accommodation coefficients for TiRe-LII.

Pub.: 08 Feb '12, Pinned: 01 Nov '17

Molecular dynamics simulations of laser-induced incandescence of soot using an extended ReaxFF reactive force field.

Abstract: Laser-induced incandescence (LII) of soot has developed into a popular method for making in situ measurements of soot volume fraction and primary particle sizes. However, there is still a lack of understanding regarding the generation and interpretation of the cooling signals. To model heat transfer from the heated soot particles to the surrounding gas, knowledge of the collision-based cooling as well as reactive events, including oxidation (exothermic) and evaporation (endothermic) is essential. We have simulated LII of soot using the ReaxFF reactive force field for hydrocarbon combustion. Soot was modeled as a stack of four graphene sheets linked together using sp(3) hybridized carbon atoms. To calculate the thermal accommodation coefficient of various gases with soot, graphene sheets of diameter 40 Å were used to create a soot particle containing 2691 atoms, and these simulations were carried out using the ReaxFF version incorporated into the Amsterdam Density Functional program. The reactive force field enables us to simulate the effects of conduction, evaporation, and oxidation of the soot particle on the cooling signal. Simulations were carried out for both reactive and nonreactive gas species at various pressures, and the subsequent cooling signals of soot were compared and analyzed. To correctly model N(2)-soot interactions, optimization of N-N and N-C-H force field parameters against DFT and experimental values was performed and is described in this paper. Subsequently, simulations were performed in order to find the thermal accommodation coefficients of soot with various monatomic and polyatomic gas molecules like He, Ne, Ar, N(2), CO(2), and CH(4). For all these species we find good agreement between our ReaxFF results and previously published accommodation coefficients. We thus believe that Molecular Dynamics using the ReaxFF reactive force field is a promising approach to simulate the physical and chemical aspects of soot LII.

Pub.: 12 Nov '10, Pinned: 01 Nov '17

Time-resolved laser-induced incandescence characterization of metal nanoparticles

Abstract: Abstract This paper presents a comparative analysis of time-resolved laser-induced incandescence measurements of iron, silver, and molybdenum aerosols. Both the variation of peak temperature with fluence and the temperature decay curves strongly depend on the melting point and latent heat of vaporization of the nanoparticles. Recovered nanoparticle sizes are consistent with ex situ analysis, while thermal accommodation coefficients follow expected trends with gas molecular mass and structure. Nevertheless, there remain several unanswered questions and unexplained behaviors: the radiative properties of laser-energized iron nanoparticles do not match those of bulk molten iron; the absorption cross sections of molten iron and silver at the excitation laser wavelength exceed theoretical predictions; and there is an unexplained feature in the temperature decay of laser-energized molybdenum nanoparticles immediately following the laser pulse.AbstractThis paper presents a comparative analysis of time-resolved laser-induced incandescence measurements of iron, silver, and molybdenum aerosols. Both the variation of peak temperature with fluence and the temperature decay curves strongly depend on the melting point and latent heat of vaporization of the nanoparticles. Recovered nanoparticle sizes are consistent with ex situ analysis, while thermal accommodation coefficients follow expected trends with gas molecular mass and structure. Nevertheless, there remain several unanswered questions and unexplained behaviors: the radiative properties of laser-energized iron nanoparticles do not match those of bulk molten iron; the absorption cross sections of molten iron and silver at the excitation laser wavelength exceed theoretical predictions; and there is an unexplained feature in the temperature decay of laser-energized molybdenum nanoparticles immediately following the laser pulse.

Pub.: 19 Dec '16, Pinned: 01 Nov '17