Femtosecond laser; Laser-induced breakdown spectroscopy; Nanoparticle; Double pulse;FIELD ENHANCEMENT; METALLIC SAMPLES; PLASMA; LIBS; CONFINEMENT; IMPROVEMENT; DISCHARGE; ABLATION; SURFACE; ARRAYS

Metal nanoparticle deposition on sample surface is a promising approach to enhance the emission signal during laser-induced breakdown spectroscopy(LIBS). In this article, strong optical emission enhancement was achieved by combining nanoparticle enhanced LIBS (NELIBS) and double pulse LIBS (DP-LIBS) on a crystal SiO2 sample by using femtosecond laser. Thermal dewetting was used to deposit gold (Au) nanoparticles (NPs) on sample surface and NPs with different size were obtained by altering the thickness of the Au film. It was found that both the size and distribution of Au NPs significantly affected the enhancement effect of NELIBS and the Au NPs made from 7.5 nm gold film had the best enhancement effect. The fundamental features of NELIBS enhancement for dielectric target were investigated by studying the photon absorption process. At a low laser fluence of 4.4 J/cm(2), an enhancement factor of about 13 for spectrum intensity was obtained in NELIBS and reached to 30 after being combined with DP-LIBS. (C) 2017 Elsevier Ltd. All rights reserved.

Nanoparticle; Counterflow flame; Laser diagnostics; Transport;INDUCED BREAKDOWN SPECTROSCOPY; LIGHT-SCATTERING MEASUREMENTS; SOOT-VOLUME FRACTION; INDUCEDINCANDESCENCE; TITANIA NANOPARTICLES; PARTICLES; SIZE; DEPOSITION

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 10 pm. 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.

Flame-assisted spray pyrolysis; Phase-selective LIBS; Transition from droplets to nanoparticles; Functional TiO2-based nanoparticle;INDUCED BREAKDOWN SPECTROSCOPY; PULVERIZED COAL COMBUSTION; NANOSTRUCTURED MATERIALS; AEROSOL SYNTHESIS; PARTICLES; OXIDATION; TIO2; COAGULATION; GROWTH; MODEL

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. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.