相选择性激光诱导击穿光谱在线诊断火焰场纳米颗粒的实验和理论研究

Phase-selective laser-induced breakdown spectroscopy (Phase-selective LIBS) is a new concept for in-situ diagnostics of nanoparticles in flames. By controlling laser fluence, a phase-selective breakdown phenomenon can be achieved that only particle phase is broken down while no gas phase molecules involves. By this means the technique is able to visualize the formation of particle phase and has the potential to measure particle volume fraction, size and elemental composition. However the lack of understanding of the physical mechanisms constraints the development towards quantitative measurments.. The present project proposals both experimental and theoretical investigation on the physical mechanisms of phase-selective breakdown phenomenon, aiming at establishing a new diagnostic tool for nanoparticles in flames. A Mckenna flame surrounded by Hencken flame will create a quasi one-dimensional flow for nanoparticles to form and coagulate. By studying time-resolved atomic and ionic spectra while varying laser fluence, wavelength and particle size, important information can be revealed on the creation of initial free electrons, the melting and vaporization of nanoparticles and dynamics of nano-sized plasmas. Based on the framework of Fokker-Planck equation, a quantitative model will be built to solve the time-resolved energy distribution of free electrons. Further, a quantitative relation between the intensity of atomic/ionic spectra and particle properties will finally be established to realize the quantitative measurements of Phase-selective LIBS technique. . The project will not only contribute to the establishment of Phase-selective LIBS technique, but also uncover important mechanisms of laser-nanoparticle interaction, which could be useful to other particle diagnostics techniques e.g. Rayleigh scattering and LII.

相选择性激光诱导击穿光谱是一种针对火焰场纳米颗粒的新颖诊断原理。通过控制激光能量密度实现仅击穿颗粒相而不击穿气相，可观测颗粒相形成过程并具备在线测量颗粒体积分数、粒径和组分的潜力，但物理过程不明是限制其发展的瓶颈。本项目拟从实验和理论两方面深入研究相选择性激光诱导击穿现象的物理机制，以将其发展为成熟定量的颗粒相诊断技术。通过Hencken+Mckenna一维火焰场建立粒径稳定可控的纳米气溶胶环境。通过研究原子离子光谱信号与激光能量密度、波长，颗粒粒径等的关系，及时间演化特性，探索自由电子产生、颗粒熔化气化以及纳米等离子体演化等物理过程的机制。进而基于Fokker-Planck方程建立自由电子能量分布的演化模型，建立光谱信号与颗粒参数的定量关系。本项目不仅对相选择性激光诱导击穿光谱诊断技术的建立具有重要意义，也将显著地丰富和完善激光对纳米颗粒的作用机制，有益于整个颗粒相激光诊断领域。

本课题研究的是一种全新的火焰场颗粒物在线激光诊断方法，相选择性激光诱导击穿光谱。这种方法创新性地利用气体和颗粒击穿极限的不同，通过调节激光能量产生纳米级等离子体，从而实现仅击穿颗粒不击穿气体的目的。与传统激光诱导击穿光谱产生的可见等离子体不同，相选择性激光诱导击穿的纳米等离子体无可见击穿火花，等离子体寿命也很短。针对这样一种新奇的现象和由此衍生的新型的诊断方法，本课题首先从现象本身入手，建立了沿程温度、速度均匀可控的准一维火焰场环境和激光诊断系统，通过控制变量实验获研究了原子/离子谱线与激光能量、颗粒粒径等参数间的关系，建立了定量的测量模型。同时通过时间演化测量实验和基于Fokker-Planck方程的电子能量输运建模，厘清了相选择性激光诱导击穿现象中纳米等离子体的动力学行为，建立了吸收-烧蚀-激发的纳米等离子体物理模型。进而，将相选择性激光诱导击穿光谱方法推广到多组分颗粒相形成的复杂环境中。课题还成功地实现了平面二维测量，首次报道了边界层内高空间分辨率的颗粒浓度分布测量，以及基于单激光脉冲的二维颗粒浓度测量，对复杂火焰场中颗粒形成和输运的研究具有重要意义。