Phase-selective Laser-induced Breakdown Spectroscopy Research Articles

Molecular Emissions from Stretched Excitation Pulse in Nanosecond Phase-Selective Laser-Induced Breakdown Spectroscopy of TiO2 Nanoaerosols.

In phase-selective laser-induced breakdown spectroscopy (PS-LIBS), gas-borne nanoparticles are irradiated with laser pulses (∼2.4GW/cm2) resulting in breakdown of the nanoparticle phase but not the surrounding gas phase. In this work, the effect of excitation laser-pulse duration and energy on the intensity and duration of TiO2-nanoparticle PS-LIBS emission signal is investigated. Laser pulses from a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (532nm) are stretched from 8ns (full width at half maximum, FWHM) up to ∼30ns at fixed pulse energy using combinations of two optical cavities. The intensity of the titanium atomic emissions at around 500nm wavelength increases by ∼60%, with the stretched pulse and emissions at around 482nm, attributed to TiO, enhanced over 10 times. While the atomic emissions rise with the stretched laser pulse and decay around 20ns after the end of the laser pulse, the TiO emissions reach their peak intensity at about 20ns later and last longer. At low laser energy (i.e., 1mJ/pulse, or 80MW/cm2), the TiO emissions dominate, but their increase with laser energy is lower compared to the atomic emissions. The origin of the 482nm emission is explored by examining several different aerosol setups, including Ti-O, Ti-N, and Ti-O-N from a spark particle generator and Ti-O-N-C-H aerosol from flame synthesis. The 482nm emissions are attributed to electronically excited TiO, likely resulting from the reaction of excited titanium atoms with surrounding oxidizing (carbonaceous and/or radical) species. The effects of pulse length are attributed to the shift of absorption from the initial interaction with the particle to the prolonged interaction with the plasma through inverse bremsstrahlung.

Phase-selective laser–induced breakdown spectroscopy in flame spray pyrolysis for iron oxide nanoparticle synthesis

The flame spray pyrolysis of iron oxide nanoparticles using the new reference nozzle SpraySyn is a key step towards the understanding of the coupling of physicochemical steps such as precursor atomization, spray evaporation, combustion, particle formation and growth. Owing to the countless available solvents and precursors, systematic investigations are necessary to fully understand the impact of precursor formulation on the reaction route and, hence, the particle properties. In this work, the recently developed phase-selective laser-induced breakdown spectroscopy (PS-LIBS) technique is applied to an external mixing spray flame reactor in order to study iron oxide particle formation along the axial centerline of the spray under varying precursor solutions. The addition of 2-ethylhexanoic acid (EHA) to precursors is investigated and significant differences in the evolution of the atomic emission spectra are observed, enabling the differentiation between droplet-to-particle and gas-to-particle routes in situ. The observations from PS-LIBS are in good agreement with TEM images and XRD, where haematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) were observed. Raman spectroscopy (RS) in particle-free spray-flames revealed a significant gas-phase temperature difference of about ΔT∼500 K under addition of EHA to the spray and demonstrated accurate temperature measurements up to droplet rates of 103 Hz. The experimental results allow deep insights into the spray flame combustion and particle nucleation kinetics. Moreover, they can be coupled with population balance models and be used for the validation of numerical simulations.

Simultaneous Single-Shot Two-Dimensional Imaging of Nanoparticles and Radicals in Turbulent Reactive Flows

Recently, a phase-selective laser-induced breakdown spectroscopy (PS-LIBS) technique has been proposed and developed to diagnose nanoparticles. However, only a few kinds of nanoparticles can generate sufficient strong PS-LIBS signals for single-shot two-dimensional measurement. In this work, we report on the development and validation of a resonance-enhanced PS-LIBS for instantaneous two-dimensional imaging of nanoparticle formation and transport in a turbulent reactive flow. By matching the laser pulse with the absorption line of titanium atoms in the laser-induced nanoplasma, we find that the resonance-enhanced titanium atomic emissions are 1 order of magnitude stronger than the nonresonant spectra ranging from 498 to 502 nm. Based on the spectral characterization, a quantitative relation between the spectra and the particle volume fraction is theoretically established and validated by experiments. Combining the resonance-enhanced PS-LIBS and planar laser-induced fluorescence (PLIF) techniques, instantaneous two-dimensional distributions of the nanoparticle and the hydroxyl radical (OH radical) can be measured in situ in a turbulent reactive flow. The simultaneous measurement demonstrates a strong correlation between the nanoparticle formation and the reactive flow pattern in the turbulent region. The correlation further reveals that the precursor consumption is dominated by the turbulence effect, indicating the precursor chemistry-turbulence coupling.

Investigating the role of solvent formulations in temperature-controlled liquid-fed aerosol flame synthesis of YAG-based nanoparticles

Abstract Key effects of solvent formulations on the structure and morphology of optical-quality yttrium-aluminum nanocomposites using liquid-fed aerosol flame synthesis are investigated. Employing a temperature-controlled flat flame burner with inexpensive nitrates as multi-component precursors, three different solvent formulations, i.e., ethanol, ethanol/2-ethylhexanoic acid (EHA), and butanol, are studied. Adding EHA into ethanol in a 1:1 volume ratio dramatically changes the flame-made yttrium-aluminum oxides from hollow inhomogeneous powders that contain non-uniform large particles to homogeneous nanopowders around 10 nm. As characterized by in-situ phase Doppler anemometry, droplet size with increasing burner height for the EHA/ethanol case remains constant at the beginning, whereas those for both ethanol and butanol cases reduce immediately. EHA likely causes a shift from the droplet-to-particle precipitation route to the gas-to-particle route because of the formed low-boiling-point 2-ethylhexanoates from nitrates via ligand exchange. By replacing ethanol with butanol, hollow particles are produced with better crystallinity because of its high calorific value that helps to heat precursors at the droplet surface. In-situ diagnostics using phase-selective laser-induced breakdown spectroscopy, which tracks only atomic emission from the nanoparticle phase. The result shows that the Al atomic emissions in the EHA/ethanol mixture case gradually increase along the burner height, while those for both ethanol and butanol cases fluctuate, further verifying the favoring of the gas-to-particle route for producing uniform, ultrafine solid multi-oxide particles by adding EHA in solvents.

Single-shot two-dimensional measurement of nanoparticles in turbulent jet-diffusion flames using phase-selective laser-induced breakdown spectroscopy

In this work, we demonstrate a single-shot two-dimensional measurement of vanadic oxide (V2O5) nanoparticles in a turbulent jet-diffusion flame based on phase-selective laser-induced breakdown spectroscopy (PS-LIBS). By collecting the atomic spectra of vanadium near 439 nm from nano-sized plasmas within the ∼30 ns lifetime, particle information can be revealed without interference from elastic scattering or Bremsstrahlung emissions. As the laser intensity increases, the signal intensity first increases and then saturates when the laser intensity reaches 0.5 GW/cm2. In the saturation regime, a proportional correlation is established between the signal intensity and the particle volume fraction. Based on the parametric study in the laminar condition, we image the instantaneous distribution of particle volume fraction in the turbulent condition using single-shot PS-LIBS measurements. The snapshots show that vanadic oxide nanoparticles concentrate near the diffusion flame surface, which may be caused by the rapid formation of particles on the oxidizer/precursor side and quick dilution across the reaction layer. The Proper Orthogonal Decomposition (POD) analysis further indicates that the fluctuations of the particle volume fraction originate from the unsteady flame surface at the upstream positions and large-scale mixing at the downstream positions. The single-shot PS-LIBS measurement shows promising potential for resolving complex processes of particle formation in turbulent flame synthesis.

In-situ laser diagnostic of nanoparticle formation and transport behavior in flame aerosol deposition

In this work, we investigate the flame aerosol deposition of TiO2 nanoparticles in a well-defined premixed stagnation flame based on two-dimensional phase-selective laser-induced breakdown spectroscopy (PS-LIBS). The deposited nanoparticles are in anatase phase with an average size of ∼10 nm. In PS-LIBS measurement, atomic emissions of titanium near the wavelength of 500 nm are extracted by spectral filter lenses and imaged for demonstrating the particle volume fraction distributions. The PS-LIBS signals can well depict the whole process of particle formation and transport, including the rapid gas-to-particle conversion near the flame front, dilution and concentrating of the nanoparticles induced by gas density variations, and dropping of particle volume fractions near the substrate. A quantitative model has been proposed to describe convection, thermophoresis, and diffusion of the nanoparticles. Combining the PS-LIBS measurement and the numerical simulation, it is found that, the nanoparticles concentrate outside the boundary layer at low substrate temperatures due to a joint effect of gas compression and thermophoresis. Further parametric analysis indicates that substrate temperatures and precursor loading rates can strongly affect the particle concentration boundary layer and the particle deposition rate.

Tuning excitation laser wavelength for secondary resonance in low-intensity phase-selective laser-induced breakdown spectroscopy for in-situ analytical measurement of nanoaerosols

In recent years, a novel low-intensity phase-selective laser-induced breakdown spectroscopy (PS-LIBS) technique has been developed for unique elemental-composition identification of aerosolized nanoparticles, where only the solid-phase nanoparticles break down, forming nanoplasmas, without any surrounding gas-phase breakdown. Additional work has demonstrated that PS-LIBS emissions can be greatly enhanced with secondary resonant excitation by matching the excitation laser wavelength with an atomic transition line in the formed nanoplasma, thereby achieving low limits of detection. In this work, a tunable dye laser is employed to investigate the effects of excitation wavelength and irradiance on in-situ PS-LIBS measurements of TiO2 nanoaerosols. The enhancement factor by resonant excitation can be 220 times greater than that for non-resonant cases under similar conditions. Moreover, the emitted spectra are unique for the selected resonant transition lines for a given element, suggesting the potential to make precise phase-selective and analyte-selective measurements of nanoparticles in a multicomponent multiphase system. The enhancement factor by resonant excitation is highly sensitive to excitation laser wavelength, with narrow excitation spectral windows, i.e., 0.012 to 0.023nm (FWHM, full width at half maximum) for Ti (I) neutral atomic lines, and 0.051 to 0.139nm (FWHM) for Ti (II) single-ionized atomic lines. Boltzmann analysis of the emission intensities, temporal response of emissions, and emission dependence on excitation irradiance are investigated to understand aspects of the generated nanoplasmas such as temperature, local thermodynamic equilibrium (LTE), and excitation mechanism.

Effect of CO2/H2O on the Incipient Ultrafine Particulate Matter Formation in Oxy-fuel Combustion of High-Sodium Lignite

In this work, the formation of incipient ultrafine particulate matter (PM) from high-sodium lignite combustion, under both dry and wet oxy-fuel conditions, is intensively investigated using an optically accessible flat-flame burner. The ambient temperature was set at 1500 K, and measurements were performed along the zones with the coal residence time of 0–40 ms. We simultaneously introduced thermophoresis sampling, dilution sampling, and phase-selective laser-induced breakdown spectroscopy (PS-LIBS) to characterize the morphologies, components, and particle size distributions (PSDs) of ultrafine PM as well as the behaviors of Na. The results indicate that incipient particles formed in CO2–O2 ambience are more abundant in Na and Si. The substitution of CO2 may accelerate the mass loss rate of coal in the early devolatilization stage, whereas the Na-based incipient particles form around a time of ∼10 ms that is similar to conventional N2–O2 ambience. In combination with the computational fluid dynamics (CFD...

In situ laser diagnostics of nanoparticle transport across stagnation plane in a counterflow flame

The transport of nanoparticles in the boundary layer is closely related to particle mixing or deposition. We present an in situ imaging of TiO2 particle volume fraction near gas stagnation plane in a counterflow flame by recently developed phase-selective laser-induced breakdown spectroscopy technique. The concentration boundary layer is well resolved with a spatial resolution of 10μm. Together with a numerical analysis of particle transport equation, the roles of convection, diffusion and thermophoresis are discussed. The calculated profile of particle volume fraction agrees well with experimental measurements, which indicates that current model of nanoparticle transport is capable to quantitatively predict the concentration profile in boundary layers. Further study shows that altering thermophoretic velocity shifts the concentration boundary layer but does not change the shape of concentration profile. The decaying slope is mainly controlled by diffusion process that is dependent on particle size.

Laser-based investigation of the transition from droplets to nanoparticles in flame-assisted spray synthesis of functional nanoparticles

Mechanisms involved in flame-assisted-spray-pyrolysis (FASP) synthesis of TiO2-based functional nanoparticles are investigated using in situ phase-selective laser-induced breakdown spectroscopy. Specifically, the transition from droplets to nanoparticles is examined, as well as the particle growth process in the post flame region. In contrast to vapor-fed flame synthesis, the existence of the precursor breakdown in sprayed droplets is observed by both Ti atomic signal and Bremsstrahlung emissions. The emission signal intensity variation reveals the transition from droplets to nanoparticles, indicating that the competition between precursor vaporization and nascent particle formation is the rate determining factor. For the ensuing particle growth regime, the prediction of the particle size evolution by a polydispersed population balance model is in good agreement with TEM and laser-diagnostics results, demonstrating that the particle growth is governed by the collision-coagulation mechanism. Besides, the transition from droplets to nanoparticles can be accelerated with sufficient OH radicles provided by combustible solvents. Furthermore, doping of TiO2 with V and Zr will lead to changes in band gap of the nanoparticle observed, i.e., a distinct strengthening by 23% with V doping and a weakening by 22% with Zr doping in the intensity of Ti signal, respectively.

Phase-selective laser-induced breakdown spectroscopy of metal-oxide nanoparticle aerosols with secondary resonant excitation during flame synthesis

Phase-selective LIBS with secondary resonant excitation demonstrates strongly-enhanced emission in the in situ study of flame synthesis of titania nanoparticles.

Ultrafine particulate matter formation in the early stage of pulverized coal combustion of high-sodium lignite

Abstract In this work, the pulverized coal combustion of Zhundong lignite was conducted in an optically-accessible, downward Hencken flat-flame burner to investigate the incipient formation of ultrafine particulate matter (PM). The ultrafine PM were collected in - flame from distinct positions under 1800 K, 1500 K and 1200 K using a self-designed thermophoretic sampler, together with extensive conventional transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) examinations. A novel in situ low-intensity phase-selective laser-induced breakdown spectroscopy (PS-LIBS) was further introduced to diagnose the dynamic behavior of particle-phase sodium during the pulverized coal combustion. The primary particles in the collected ultrafine PM have mean diameter of 8–15 nm, with Si and Na as main mineral components based on TEM/HRTEM–EDS results. The formation of ultrafine PM is regarded as a process of multi-component flame syntheses of mineral precursors, in which the competition between the devolatilization and the coalescence of particles are of most significance. Finally, based on an approach of time scale analysis, three characteristic times of minimum mean diameter of primary particles, the initial increment point of particle-phase sodium and the maximum devolatilization are found to be highly correlated.

Absorption-Ablation-Excitation Mechanism of Laser-Cluster Interactions in a Nanoaerosol System

The absorption-ablation-excitation mechanism in laser-cluster interactions is investigated by measuring Rayleigh scattering of aerosol clusters along with atomic emission from phase-selective laser-induced breakdown spectroscopy. For 532nm excitation, as the laser intensity increases beyond 0.16 GW/cm^{2}, the scattering cross section of TiO_{2} clusters begins to decrease, concurrent with the onset of atomic emission of Ti, indicating a scattering-to-ablation transition and the formation of nanoplasmas. With 1064nm laser excitation, the atomic emissions are more than one order of magnitude weaker than that at 532nm, indicating that the thermal effect is not the main mechanism. To better clarify the process, time-resolved measurements of scattering signals are examined for different excitation laser intensities. For increasing laser intensity, the cross section of clusters decreases during a single pulse, evincing the shorter ablation delay time and larger ratios of ablation clusters. Assessment of the electron energy distribution during the ablation process is conducted by nondimensionalizing the Fokker-Planck equation, with analogous Strouhal Sl_{E}, Peclet Pe_{E}, and Damköhler Da_{E} numbers defined to characterize the laser-induced aerothermochemical environment. For conditions where Sl_{E}≫1, Pe_{E}≫1, and Da_{E}≪1, the electrons are excited to the conduction band by two-photon absorption, then relax to the bottom of the conduction band by electron energy loss to the lattice, and finally serve as the energy transfer media between laser field and lattice. The relationship between delay time and excitation intensity is well correlated by this simplified model with quasisteady assumption.

Dynamic behavior of sodium release from pulverized coal combustion by phase-selective laser-induced breakdown spectroscopy

In this paper, we examine the dynamic behavior of sodium (Na) release during the pulverized coal combustion of Zhundong lignite using a laminar, Hencken flat-flame burner technique. By utilizing the gap between the excitation energies of the gas and particle phases, a new low-intensity laser-induced breakdown spectroscopy (LIBS) is developed to distinguish the existence of sodium in the particle or gas phase along the combustion process. Within the coal flame domain, Na atomic spectra in the particle phase are clearly detected that consistently agree with the NIST database. For the first time, the in situ verification of the gas phase Na release accompanying coal devolatilization is fulfilled when the ambient temperature is high enough. The residence time, indicating Na release from particle to gas phases, is determined from the signal. By using a theoretical analysis, the Na release time approximately coincides with the characteristic pyrolysis time of lignite, further confirming the observation above. The effects of ambient temperature, coal rank and oxygen concentration are further discussed. This work may provide a basis for exploring the formation mechanism of submicron fine particulates during coal combustion.

A new diagnostic for volume fraction measurement of metal-oxide nanoparticles in flames using phase-selective laser-induced breakdown spectroscopy

A new diagnostic has been developed for measuring nanoparticle volume fraction by using phase-selective laser-induced breakdown spectroscopy (LIBS) for TiO2 nanoparticles generated during flame synthesis. The volume fraction is determined from Ti atomic spectra by exciting all the nanoparticle-phase matter to the plasma state, while keeping the gas-phase unexcited, based on the selectivity of breakdown thresholds between gas phase and particle phase. The measurement is performed with no detection delay, as no Bremsstrahlung background is detected. The nonvisible nanoplasmas are likely confined around each nanoparticle, differing from the continuously expanding plasmas produced in conventional LIBS. The intensity of the atomic spectra increases proportionally with laser fluence and reaches a saturation regime above 35J/cm2, facilitating the measurement of volume fraction. It is found that smaller nanoparticles decrease in absorption efficiency due to the size-dependent band gap. Nevertheless, this effect diminishes for nanoparticle sizes above 8nm. In the saturation regime, a fairly-good linear relation exists between signal intensity and nanoparticle volume fraction, for volume fractions above 140ppb. Benefitting from locally-confined nanoplasmas without detectable sparks, two-dimensional planar measurements of TiO2 nanoparticle volume fraction is accomplished, resulting in the visualization of the rapid formation of nanoparticles across the flame sheet and conservation of their volume fraction in the post-flame region. When an impinged-upon substrate is added, the two-dimensional phase-selective LIBS imaging shows good resolution of nanoparticle volume fraction variation in the boundary layer, influenced by diffusion and thermophoresis.

Doping mechanism of Vanadia/Titania nanoparticles in flame synthesis by a novel optical spectroscopy technique

Flame synthesis of V-doping TiO2 is studied by in situ diagnostic of phase selective laser-induced breakdown spectroscopy (LIBS). We apply this novel optical spectroscopy to tracing the gas-to-particle phase transition of V and Ti elements, as low-intensity laser only excites V and Ti atoms present in the particle phase but not in the gas phase. Both V and Ti atomic signals appear early at the burner exit and plateau downstream after a distance about 14mm. Compared with signals in pure TiO2 synthesis, the signal of Ti in the doping synthesis is significantly strengthened due to the lower band gap of V-doped TiO2. The doping mechanism is then inferred from the observations. It is deduced that the substantial collision and mixing of the nucleated V and Ti oxides occur even at the burner rim and persist through the entire process. The signal intensities of both V and Ti atoms increase with laser power and tend to plateau at about 20mJ/pulse. In the flatten region, the ratio of V and Ti signal intensities is almost proportional to the doping ratio of V and Ti elements in the particle phase, showing feasibility of utilizing the optical method in the doping ratio measurement.

Novel low-intensity phase-selective laser-induced breakdown spectroscopy of TiO2 nanoparticle aerosols during flame synthesis

Novel low-intensity laser-induced breakdown spectroscopy (LIBS) is employed to conduct an in situ study on swirl flame synthesis of TiO2 nanoparticles. Collected emissions agree well with Ti atomic spectra from the NIST database. In contrast to traditional application of LIBS on particles, the power used here is much lower (∼35mJ/pulse or 28J/cm2 at 532nm); and no macroscopic spark is visually observed. Nevertheless, the low-intensity LIBS shows interesting selectivity—only exciting Ti atoms in particle phase, with no breakdown emission occurring for gas molecules (e.g. titanium tetraisopropoxide precursor, air). The emission intensity increases as the nanoparticles grow in the synthesis flow field, plateauing as the particles become larger than 6nm, indicating the absorption efficiency to be size-dependent for small particles. The signals saturate at a fluence of ∼16J/cm2. When the precursor concentration is larger than 150ppm (corresponding to particle sizes of 6–8nm), the emission intensity increases linearly with precursor concentration. The selectivity of the low-intensity LIBS for such application could be advantageous for tracking nanoparticle formation and for measuring particle volume fraction during gas-phase synthesis or other systems. The size-dependent absorption efficiency could be used to measure nanoparticle size in situ.

96/05301 Shock tube and modeling study of ketene oxidation

In this work, the pulverized coal combustion of Zhundong lignite was conducted in an optically-accessible, downward Hencken flat-flame burner to investigate the incipient formation of ultrafine particulate matter (PM). The ultrafine PM were collected in-flame from distinct positions under 1800 K, 1500 K and 1200 K using a self-designed thermophoretic sampler, together with extensive conventional transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) examinations. A novel in situ low-intensity phase-selective laser-induced breakdown spectroscopy (PS-LIBS) was further introduced to diagnose the dynamic behavior of particle-phase sodium during the pulverized coal combustion. The primary particles in the collected ultrafine PM have mean diameter of 8–15 nm, with Si and Na as main mineral components based on TEM/HRTEM–EDS results. The formation of ultrafine PM is regarded as a process of multi-component flame syntheses of mineral precursors, in which the competition between the devolatilization and the coalescence of particles are of most significance. Finally, based on an approach of time scale analysis, three characteristic times of minimum mean diameter of primary particles, the initial increment point of particle-phase sodium and the maximum devolatilization are found to be highly correlated.