Characterization of Nano‐Particles Using Laser‐Induced Incandescence

Laser‐induced incandescence (LII) is introduced as a valuable tool for the characterization of nanoparticles. This optical measurement technique is based on the heating of the particles by a short laser pulse and the subsequent detection of the thermal radiation. It has been applied successfully for the investigation of soot in different fields of application, which is described here in the form of an overview with a focus on work done at the LTT‐Erlangen during the last 10 years. In laboratory flames the soot primary particle size, volume concentration, and relative aggregate size have been determined in combination with the number density of primary particles. Furthermore, the primary particle sizes of carbon blacks have been measured in situ and online under laboratory conditions and also in production reactors. Measurements with different types of commercially available carbon black powders, which were dispersed in a measurement chamber yielded a good correlation between LII results and the specified product properties. Particle diameters determined by LII in a furnace black reactor correlate very well with the CTAB‐absorption number, which is a measure for the specific surface area. It turned out that the LII method is not affected by variations of the aggregate structure of the investigated carbon blacks. The LII signal also contains information on the primary particle size distribution, which can be reconstructed by the evaluation of the signal decay time at, at least, two different time intervals. Additionally, soot mass concentrations have been determined inside diesel engines and online measurements were performed in the exhaust gas of such engines for various engine conditions simultaneously providing information about primary particle size, soot volume, and number concentration. The LII results exhibit good correlation with traditional measurement techniques, e.g., filter smoke number measurements. In addition to the soot measurements, primarily tests with other nanoparticles like TiO₂ or metal particles are encouraging regarding the applicability of the technique for the characterization of such different types of nanoparticles.     

Laser-induced incandescence of flame-generated soot on a picosecond time scale

This paper presents measurements of time-resolved laser-induced incandescence (LII) from soot recorded on a picosecond time scale. The 532-nm output from a picosecond Nd:YAG laser was used to heat the soot, and a streak camera was used to record the LII signal. The results are compared with data collected on a nanosecond time scale and with a time-dependent model that solves the energy- and mass-balance rate equations. Relative to the laser timing, the picosecond and nanosecond results are very similar. Signals increase during the laser pulse as soot temperatures increase and decrease after the laser pulse. The signal decay rates increase significantly with increasing laser fluence. The LII model gives good agreement with the nanosecond data at fluences ≤0.2 J/cm² and underpredicts the signal decay rates at higher fluences. The picosecond temporal profiles increase significantly faster and earlier in the laser pulse than predicted by the model. This disagreement between the model and picosecond LII data may be attributable to perturbations to the signal by laser-induced fluorescence from polycyclic aromatic hydrocarbons or other large organic species. The excited state or states responsible for this fluorescence appear to be accessed via a two-photon transition and have an effective lifetime of 55 ps. PACS 44.40.+a; 78.67.Bf; 78.47.+p     

Environmental application of pulsed laser-induced incandescence

A portable instrument based on two-color laser-induced incandescence (LII) technique has been designed and developed for the detection of carbonaceous particles for environmental applications. The instrument has been calibrated by performing LII measurements at the exhaust of a home-made soot generator. The incandescence signal from particles sampled into the instrument has been compared with in situ, calibrated, LII measurements to correlate the incandescence signal by the instrument with particles concentration. Measurements of particulate with the LII instrument were then conducted in different environmental conditions, covering a wide range of concentration (from ambient air to cars’ exhaust). The detection limit of the LII instrument has been estimated to be in the range of 200 ng/m³. These measurements have been also compared with results obtained with a commercial aethalometer. The results show a linear relationship between the two sets of measurements, also in the case where significant variation of the carbon particles concentration has been observed over time. These observations allow us to infer that the two instruments are responding in the same way to different carbon particles load, size and nature.     

Modeling laser-induced incandescence of soot: enthalpy changes during sublimation, conduction, and oxidation

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.     

Influence of soot particle aggregation on time-resolved laser-induced incandescence signals

Laser-induced incandescence (LII) is a versatile technique for quantitative soot measurements in flames and exhausts. When used for particle sizing, the time-resolved signals are analysed as these will show a decay rate dependent on the soot particle size. Such an analysis has traditionally been based on the assumption of isolated primary particles. However, soot particles in flames and exhausts are usually aggregated, which implies loss of surface area, less heat conduction and hence errors in estimated particle sizes. In this work we present an experimental investigation aiming to quantify this effect. A soot generator, based on a propane diffusion flame, was used to produce a stable soot stream and the soot was characterised by transmission electron microscopy (TEM), a scanning mobility particle sizer (SMPS) and an aerosol particle mass analyzer coupled in series after a differential mobility analyzer (DMA-APM). Despite nearly identical primary particle size distributions for three selected operating conditions, LII measurements resulted in signal decays with significant differences in decay rate. However, the three cases were found to have quite different levels of aggregation as shown both in TEM images and mobility size distributions, and the results agree qualitatively with the expected effect of diminished heat conduction from aggregated particles resulting in longer LII signal decays. In an attempt to explain the differences quantitatively, the LII signal dependence on aggregation was modelled using a heat and mass transfer model for LII given the primary particle and aggregate size distribution data as input. Quantitative agreement was not reached and reasons for this discrepancy are discussed.     

LII–lidar: range-resolved backward picosecond laser-induced incandescence

A novel concept for remote in situ detection of soot emissions by a combination of laser-induced incandescence (LII) and light detection and ranging (lidar) is presented. A lidar setup based on a picosecond Nd:YAG laser and time-resolved signal detection in the backward direction was used for LII measurements in sooty premixed ethylene–air flames. Measurements of LII–lidar signal versus laser fluence and flame equivalence ratio showed good qualitative agreement with data reported in literature. The LII–lidar signal showed a decay consisting of two components, with lifetimes of typically 20 and 70 ns, attributed to soot sublimation and conductive cooling, respectively. Theoretical considerations and analysis of the LII–lidar signal showed that the derivative was proportional to the maximum value, which is an established measure of soot volume fraction. Utilizing this, differentiation of LII–lidar data gave profiles representing soot volume fraction with a range resolution of ~16 cm along the laser beam propagation axis. The accuracy of the evaluated LII-profiles was confirmed by comparison with LII-data measured simultaneously employing conventional right-angle detection. Thus, LII–lidar provides range-resolved single-ended detection, resourceful when optical access is restricted, extending the LII technique and opening up new possibilities for laser-based diagnostics of soot and other carbonaceous particles.     

High-vacuum time-resolved laser-induced incandescence of flame-generated soot

We have measured time-resolved laser-induced incandescence of flame-generated soot under high-vacuum conditions (4.1×10⁻⁶ mbar) at an excitation wavelength of 532 nm with laser fluences spanning 0.06–0.5 J/cm². We generated soot in an ethylene/air diffusion flame, introduced it into the vacuum system with an aerodynamic lens, heated it using a pulsed laser with a spatially homogeneous and temporally smooth laser profile, and recorded LII temporal profiles at 685 nm. At low laser fluences LII signal decay rates are slow, and LII signals persist beyond the residence time of the soot particles in the detection region. At these fluences, the temporal maximum of the LII signal increases nearly linearly with increasing laser fluence until reaching a plateau at ∼0.18 J/cm². At higher fluences, the LII signal maximum is independent of laser fluence within experimental uncertainty. At these fluences, the LII signal decays rapidly during the laser pulse. The fluence dependence of the vacuum LII signal is qualitatively similar to that observed under similar laser conditions in an atmospheric flame but requires higher fluences (by ∼0.03 J/cm²) for initiation. These data demonstrate the feasibility of recording vacuum LII temporal profiles of flame-generated soot under well-characterized conditions for model validation.     

Influence of long pulse duration on time-resolved laser-induced incandescence

Time-resolved laser-induced incandescence (LII) signal of soot in an ethylene laminar diffusion flame was measured with varying laser pulse durations in the range 50–600 ns. This study presents original results since the majority of LII studies reported are based on 7–10-ns pulse duration. The LII signal from soot is a combination of heating and cooling processes of different time scales, and the influence of the pulse duration is therefore particularly relevant. The most striking finding is that when the pulse durations is longer than approximately 100 ns, the time-resolved LII signal reveals a rebound of the LII signal during its slow decaying part. This feature occurs preferably at high fluence and is unexpected as none of the physical and chemical processes known to control LII signal behaviour, and their models suggest such an effect. The phenomenon occurs with both top hat and near Gaussian temporal laser shapes. Inspection of the time-resolved emission spectra shows no indication of a laser-induced fluorescence effect, although gas-phase PAH generated during the laser heating of soot particles cannot be rejected. Other hypotheses are that the mechanism responsible for that behaviour is linked to a slow rate change of the soot morphological characteristics or to the generation of new particles during the long-duration laser excitation. Finally, experiments show that soot volume fraction measured by integrating the temporal LII signal is not affected by the pulse duration in any regions of the flame, implying that the LII method is applicable with long pulse duration lasers.     

Effects of volatile coatings on the laser-induced incandescence of soot

We have measured time-resolved laser-induced incandescence (LII) from combustion-generated mature soot extracted from a burner and (1) coated with oleic acid or (2) coated with oleic acid and then thermally denuded using a thermodenuder. The soot samples were size selected using a differential mobility analyzer and characterized with a scanning mobility particle sizer, centrifugal particle mass analyzer, and transmission electron microscope. The results demonstrate a strong influence of coatings on the magnitude and temporal evolution of the LII signal. For coated particles, higher laser fluences are required to reach signal levels comparable to those of uncoated particles. The peak LII curve is shifted to increasingly higher fluences with increasing coating thickness until this effect saturates at a coating thickness of ~75 % by mass. These effects are predominantly attributable to the additional energy needed to vaporize the coating while heating the particle. LII signals are higher and signal decay rates are significantly slower for thermally denuded particles relative to coated or uncoated particles, particularly at low and intermediate laser fluences. Our results suggest negligible coating enhancement in absorption cross-section for combustion-generated soot at the laser fluences used. Apparent enhancement in absorption with restructuring may be caused by less conductive cooling.     

Determining aerosol particle size distributions using time-resolved laser-induced incandescence

The particle size distribution within an aerosol containing refractory nanoparticles can be inferred using time-resolved laser-induced incandescence (TR-LII). In this procedure, a small volume of aerosol is heated to incandescent temperatures by a short laser pulse, and the incandescence of the aerosol particles is then measured as they return to the ambient gas temperature by conduction heat transfer. Although the cooling rate of an individual particle depends on its volume-to-area ratio, recovering the particle size distribution from the observed temporal decay of the LII signal is complicated by the fact that the LII signal is due to the incandescence of all particle size classes within the sample volume, and because of this, the particle size distribution is related to the time-resolved LII signal through a mathematically ill-posed equation. This paper reviews techniques proposed in the literature for recovering particle size distributions from TR-LII data. The characteristics of this problem are then discussed in detail, with a focus on the effect of ill-posedness on the stability and uniqueness of the recovered particle size distributions. Finally, the performance of each method is evaluated and compared based on the results of a perturbation analysis. PACS  44.05.+e; 47.70.Pq; 78.70.-g; 65.80.+n; 78.20.Ci     

Laser-induced incandescence for soot particle size and volume fraction measurements using on-line extinction calibration

A novel technique for two-dimensional measurements of soot volume fraction and particle size has been developed. It is based on a combined measurement of extinction and laser-induced incandescence using Nd:YAG laser wavelengths of 532 nm and 1064 nm. A low-energy laser pulse at 532 nm was used for extinction measurements and was followed by a more intense pulse at 1064 nm, delayed by 15 ns, for LII measurements. The 532-nm beam was split into a signal beam passing the flame and a reference beam, both of which were directed to a dye cell. The resulting fluorescence signals, from which the extinction was deduced, together with the LII signal, were registered on a single CCD detector. Thus the two-dimensional LII image could be converted to a soot volume fraction map through a calibration procedure during the same laser shot. The soot particle sizes were evaluated from the ratio of the temporal LII signals at two gate time positions. The uncertainty in the particle sizing arose mainly from the low signal for small particles at long gate times and the uncertainty in the flame temperature. The technique was applied to a well-characterized premixed flat flame, the soot properties of which had been previously thoroughly investigated. Received: 21 June 2000 / Revised version: 11 September 2000 / Published online: 7 February 2001     

Soot sublimation studies in a premixed flat flame using laser-induced incandescence (LII) and elastic light scattering (ELS)

Laser-induced incandescence (LII) as a diagnostic technique is based on rapid heating of soot particles to temperatures of several thousand Kelvin and subsequent detection of the thermal radiation from the laser-heated particles. At such high temperatures, soot sublimation effects must be considered when estimating uncertainties in LII measurements. In this work we have investigated the use of various laser fluences in LII using a Nd:YAG laser at 1,064 nm. Using another Nd:YAG laser at 532 nm, the elastic light scattering (ELS) signal from soot particles heated by the 1,064-nm laser was monitored. This approach makes it possible to determine at which fluence of the LII laser soot sublimation starts to become visible as a decrease in the ELS signal. By performing the measurements at various heights in a premixed sooting flat ethylene/air flame, the fluence threshold above which the ELS signal decreased was found to be higher at the lower flame heights corresponding to younger, smaller and less aggregated particles. The results from this work indicate that the different fluence thresholds for sublimation may be explained by a lower absorption function E(m) for the younger soot particles.     

Investigation on thermal accommodation coefficient and soot absorption function with two-color Tire-LII technique in rich premixed flames

Although the two-color laser-induced incandescence technique (2C-LII) has proved to be a significant tool for soot diagnostics, many efforts are still required to gain a whole understanding of the chemical and physical processes involved. Time-resolved two-color LII measurements are carried out in a rich ethylene/air premixed flame at different heights above the burner and by changing the laser fluence. The prompt LII at two wavelengths and the corresponding soot incandescence temperature are obtained at different stages of the soot growth and under different laser irradiations. The decay rate of the LII signals, as a method for soot sizing, is investigated at different laser fluence. The time-resolved LII curves, obtained in the low laser fluence regime, are analyzed by a numerical simulation, available on the web. By considering the gas/particle initial temperature obtained with thermocouple measurements and by knowing soot particle diameter with previous TEM and extinction/scattering measurements, information about soot parameters, such as absorption function and thermal accommodation coefficient are obtained. The presence of the so-called young or mature soot along the flame height is strictly related to different optical and heat-exchange properties necessary to fit all the experimental data available.     

Investigation on the influence of soot size on prompt LII signals in flames

Theoretical papers predict that prompt LII signals are weakly dependent on the soot size due to the fact that larger particles reach higher temperatures during the heating process by nanosecond laser pulses. This question is of crucial importance for establishing LII as a practical technique for soot volume fraction measurements. In this work two-color prompt LII measurements have been performed in several locations of diffusion and rich premixed ethylene-air flames. The experimental apparatus was carefully designed with a probe volume of uniform light distribution and sharp edges, a 4 ns integration time around the signal pulse peak and narrow spectral bandwidth. Measurements did not confirm the theoretical predictions concerning an increase of temperature for larger particles. On the contrary, larger particles in richer premixed flames exhibit a lower 400/700 signal ratio. This can probably be attributed to small differences in the refractive index of soot.     

Experimental and theoretical comparison of spatially resolved laser-induced incandescence (LII) signals of soot in backward and right-angle configuration

In-situ measurements of soot volume fraction in the exhausts of jet engines can be carried out using the laser-induced incandescence (LII) technique in backward configuration, in which the signal is detected in the opposite direction of the laser beam propagation. In order to improve backward LII for quantitative measurements, we have in this work made a detailed experimental and theoretical investigation in which backward LII has been compared with the more commonly used right-angle LII technique. Both configurations were used in simultaneous visualization experiments at various pulse energies and gate timings in a stabilized methane diffusion flame. The spatial near-Gaussian laser energy distribution was monitored on-line as well as the time-resolved LII signal. A heat and mass transfer model for soot particles exposed to laser radiation was used to theoretically predict both the temporal and spatial LII signals. Comparison between experimental and theoretical LII signals indicates similar general behaviour, for example the broadening of the spatial LII distribution and the hole-burning effect at centre of the beam due to sublimation for increasing laser pulse energies. However, our comparison also indicates that the current heat and mass transfer model overpredicts signal intensities at higher fluence, and possible reasons for this behaviour are discussed. PACS 42.62.Fi; 44.40.+a     

2D imaging of laser wing effects and of soot sublimation in laser-induced incandescence measurements

The distribution of Laser-Induced Incandescence (LII) signal in sooting flames along the laser beam is imaged using two directions of observation: one counter to the propagation direction of the incident laser (backward LII) and one at right angles. It is shown that the effective probe volume, in which the LII signal is observed, is highly dependent on the laser irradiance profile. At high fluence, the LII from the central part of the beam decreases because of soot sublimation. This decrease can be compensated by an increase in the LII from the wings of the laser beam. This interaction is particularly important in the extraction of quantitative information in the backward LII case, which is the configuration best suited to the practical application of LII for in-situ particle concentration measurements in the exhaust of aero-engines.     

Soot formation characteristics in a lab-scale turbulent pulverized coal flame with simultaneous planar measurements of laser induced incandescence of soot and Mie scattering of pulverized coal

Soot formation characteristics of a lab-scale pulverized coal flame were investigated by performing carefully controlled laser diagnostics. The spatial distributions of soot volume fraction and the pulverized coal particles were measured simultaneously by laser induced incandescence (LII) and Mie scattering imaging, respectively. In addition, the radial distributions of the soot volume fraction were compared with the OH radical fluorescence, gas temperature and oxygen concentration obtained in our previous studies [1], [2]. The results indicated that the laser pulse fluence used for LII measurement should be carefully controlled to measure the soot volume fraction in pulverized coal flames. To precisely measure the soot volume fraction in pulverized coal flames using LII, it is necessary to adjust the laser pulse fluence so that it is sufficiently high to heat up all the soot particles to the sublimation temperature but also sufficiently low to avoid including a too large of a change in the morphology of the soot particles and the superposition of the LII signal from the pulverized coal particles on that from the soot particles. It was also found that the radial position of the peak LII signal intensity was located between the positions of the peak Mie scattering signal intensity and peak OH radical signal intensity. The region, in which LII signal, OH radical fluorescence and Mie scattering coexisted, expanded with increasing height above the burner port. It was also found that the soot formation in pulverized coal flames was enhanced at locations where the conditions of high temperature, low oxygen concentration and the existence of pulverized coal particles were satisfied simultaneously.     

Numerical investigation of the effect of signal trapping on soot measurements using LII in laminar coflow diffusion flames

Laser-induced incandescence has been rapidly developed into a powerful diagnostic technique for measurements of soot in many applications. The incandescence intensity generated by laser-heated soot particles at the measurement location suffers the signal trapping effect caused by absorption and scattering by soot particles present between the measurement location and the detector. The signal trapping effect was numerically investigated in soot measurements using both a 2D LII setup and the corresponding point LII setup at detection wavelengths of 400 and 780 nm in a laminar coflow ethylene/air flame. The radiative properties of aggregated soot particles were calculated using the Rayleigh–Debye–Gans polydisperse fractal aggregate theory. The radiative transfer equation in emitting, absorbing, and scattering media was solved using the discrete-ordinates method. The radiation intensity along an arbitrary direction was obtained using the infinitely small weight technique. The contribution of scattering to signal trapping was found to be negligible in atmospheric laminar diffusion flames. When uncorrected LII intensities are used to determine soot particle temperature and the soot volume fraction, the errors are smaller in 2D LII setup where soot particles are excited by a laser sheet. The simple Beer–Lambert exponential attenuation relationship holds in LII applications to axisymmetric flames as long as the effective extinction coefficient is adequately defined.     

The concept of 2D gated imaging for particle sizing in a laminar diffusion flame

In this work, time-resolved laser-induced incandescence (TiRe LII) has been employed to measure primary particle diameters of soot in an atmospheric laminar ethylene diffusion flame. The generated data set complements existing data determined in one single location and takes advantage of the good spatial resolution of the ICCD detection. Time resolution is achieved by shifting the camera gate along the LII decay. One key input parameter for the analysis of time-resolved LII is the local flame temperature. This was determined on a grid throughout the flame by coherent anti-Stokes Raman scattering. The accurate temperature data, in combination with other published data from this flame, are well suited for soot model validation purposes while we showed feasibility of a shifted gate approach to deduce 2D particle sizes in the chosen standard flame.