PhD Student, University of Waterloo
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.
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
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
Abstract: This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.
Pub.: 11 Nov '08, Pinned: 01 Nov '17
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
Abstract: Laser-induced incandescence measurements were conducted in the carbon arc discharge, used for synthesis of carbon nanostructures. The results reveal two spatial regions occupied by dominant populations of carbon particles with different sizes. Close to the axis of the arc, large micron size particles dominate the incandescence signal. In the arc periphery, the dominant population of nanoparticles has diameter of 20 nm. Using a heat transfer model between the gas, arc plasma and the particles, it is shown that such a drastic difference in the particle sizes can be explained by evaporation of the micron-scale particles which move across the arc plasma towards the arc periphery. It is also hypothesized that mass evaporated from the micro particles contributes to the carbon feedstock for the formation of nanostructures.
Pub.: 20 Feb '17, Pinned: 01 Nov '17
Abstract: Laser induced incandescence experiments were carried out in a flame reactor during titania nanoparticle synthesis. The structure of the reactor employed allowed for a rather smooth particle growth along the flame axis, with limited mixing of different size particles. Particle incandescence was excited by the 4th harmonic of a Nd:YAG laser. The radiation emitted from the particles was recorded in time and checked by spectral analysis. Results were compared with measurements from transmission electron microscopy of samples taken at the same locations probed by incandescence. This was done covering a portion of the flame length within which a particle size growth of a factor of about four was detected. The incandescence decay time was found to increase monotonically with particle size. The attainment of a process control tool in nanoparticle flame synthesis appears to be realistic.
Pub.: 09 May '09, Pinned: 01 Nov '17
Abstract: Abstract This work describes the application of temporally and spectrally resolved laser-induced incandescence to silicon nanoparticles synthesized in a microwave plasma reactor. Optical properties for bulk silicon presented in the literature were extended for nanostructured particles analyzed in this paper. Uncertainties of parameters in the evaporation submodel, as well as measurement noise, are incorporated into the inference process by Bayesian statistics. The inferred nanoparticle sizes agree with results from transmission electron microscopy, and the determined accommodation coefficient matches the values of the preceding study.AbstractThis work describes the application of temporally and spectrally resolved laser-induced incandescence to silicon nanoparticles synthesized in a microwave plasma reactor. Optical properties for bulk silicon presented in the literature were extended for nanostructured particles analyzed in this paper. Uncertainties of parameters in the evaporation submodel, as well as measurement noise, are incorporated into the inference process by Bayesian statistics. The inferred nanoparticle sizes agree with results from transmission electron microscopy, and the determined accommodation coefficient matches the values of the preceding study.
Pub.: 28 Oct '16, Pinned: 01 Nov '17
Abstract: Successful implementation of laser-induced incandescence (LII) relies upon judicious choice of excitation and detection conditions. Excitation conditions encompass choice of excitation wavelength and laser fluence. Detection conditions include choice of detection wavelength, spectral band pass about the central wavelength, detection delay and duration relative to the excitation laser pulse usually corresponding to the peak of the signal intensity. Examples of applying these parameters to LII are illustrated by way of examples: soot/polycyclic aromatic hydrocarbon and metal aerosol systems. Tradeoffs must be recognized. Laser-induced chemical and structural changes of the aerosol must be considered, particularly in light of heterogeneous aerosols. Diagnostics of such changes are outlined as they will affect interpretation of the LII signal. Finally, calibration (for LII) must be chosen to be appropriate for aerosols from practical sources as they may be mixed organic and inorganic composition.
Pub.: 05 May '09, Pinned: 01 Nov '17
Abstract: This paper presents an analysis of several equations used to model laser-induced incandescence (LII) of soot. The analysis focuses on sub-models of the change in particle enthalpy during sublimation, conduction, and oxidation. Assuming that pressure is constant, expressing the conductive cooling rate in terms of enthalpy instead of energy, thereby accounting for expansion work, increases the signal decay rate and has an effect comparable to increasing the thermal accommodation coefficient from 0.30 to 0.38. Accounting for oxidative heating decreases the signal decay rate and has an effect comparable to decreasing the accommodation coefficient from 0.30 to 0.25. As an estimate of magnitude of these effects, primary particle sizes inferred from signal decay rates measured at low fluences may be over-predicted by as much as 17% if oxidation is neglected in the model at O2 partial pressures of ∼0.2 bar, under-predicted by 24% if expansion work is neglected, and under-predicted by only 9% if both are neglected. This paper also provides updated parameterizations for average enthalpies of formation, molecular weights, and total pressures of sublimed carbon clusters for use in LII models.
Pub.: 10 Sep '08, Pinned: 01 Nov '17
Abstract: This paper provides an overview of a workshop focused on fundamental experimental and theoretical aspects of soot measurements by laser-induced incandescence (LII). This workshop was held in Duisburg, Germany in September 2005. The goal of the workshop was to review the current understanding of the technique and identify gaps in this understanding associated with experimental implementation, model descriptions, and signal interpretation. The results of this workshop suggest that uncertainties in the understanding of this technique are sufficient to lead to large variability among model predictions from different LII models, among measurements using different experimental approaches, and between modeled and measured signals, even under well-defined conditions. This article summarizes the content and conclusions of the workshop, discusses controversial topics and areas of disagreement identified during the workshop, and highlights recent important references related to these topics. It clearly demonstrates that despite the widespread application of LII for soot-concentration and particle-size measurements there is still a significant lack in fundamental understanding for many of the underlying physical processes.
Pub.: 09 May '06, Pinned: 01 Nov '17
Abstract: The capabilities of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used almost exclusively to measure soot primary particles, could potentially be extended to size aerosolized metal nanoparticles. In order to do this, however, it is necessary to characterize the thermal accommodation coefficient, α, which specifies the heat conduction rate between the laser-energized nanoparticles and the surrounding gas. This paper extends a molecular dynamics (MD) methodology to calculate α for Fe/He, Fe/Ar, Mo/He, and Mo/Ar systems. A comparative analysis of the results shows that α is most strongly influenced by the potential well between the gas molecule and nanoparticle surface. Finally, the MD-derived value for α is used to recover the nanoparticle size distribution for TiRe-LII measurements made on molybdenum nanoparticles in argon.
Pub.: 22 May '13, Pinned: 01 Nov '17
Abstract: This paper describes the application of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used mainly for measuring soot primary particles, to size silicon nanoparticles formed within a plasma reactor. Inferring nanoparticle sizes from TiRe-LII data requires knowledge of the heat transfer through which the laser-heated nanoparticles equilibrate with their surroundings. Models of the free molecular conduction and evaporation are derived, including a thermal accommodation coefficient found through molecular dynamics. The model is used to analyze TiRe-LII measurements made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon and hydrogen. Nanoparticle sizes inferred from the TiRe-LII data agree with the results of a Brunauer–Emmett–Teller analysis.
Pub.: 19 Dec '13, Pinned: 01 Nov '17
Abstract: While time-resolved laser-induced incandescence (TiRe-LII) shows promise as a diagnostic for sizing aerosolized iron nanoparticles, the spectroscopic and heat transfer models needed to interpret TiRe-LII measurements on iron nanoparticles remain uncertain. This paper focuses on three key aspects of the models: the thermal accommodation coefficient; the spectral absorption efficiency; and the evaporation sub-model. Based on a detailed literature review, spectroscopic and heat transfer models are defined and applied to analyze TiRe-LII measurements carried out on iron nanoparticles formed in water and then aerosolized into monatomic and polyatomic carrier gases. A comparative analysis of the results shows nanoparticle sizes that are consistent between carrier gases and thermal accommodation coefficients that follow the expected trends with bath gas molecular mass and structure.
Pub.: 10 Feb '15, Pinned: 01 Nov '17
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
Abstract: Iron, the most ubiquitous of the transition metals and the fourth most plentiful element in the Earth's crust, is the structural backbone of our modern infrastructure. It is therefore ironic that as a nanoparticle, iron has been somewhat neglected in favor of its own oxides, as well as other metals such as cobalt, nickel, gold, and platinum. This is unfortunate, but understandable. Iron's reactivity is important in macroscopic applications (particularly rusting), but is a dominant concern at the nanoscale. Finely divided iron has long been known to be pyrophoric, which is a major reason that iron nanoparticles have not been more fully studied to date. This extreme reactivity has traditionally made iron nanoparticles difficult to study and inconvenient for practical applications. Iron however has a great deal to offer at the nanoscale, including very potent magnetic and catalytic properties. Recent work has begun to take advantage of iron's potential, and work in this field appears to be blossoming.
Pub.: 29 Dec '06, Pinned: 01 Nov '17