PAH structure analysis of soot in a non-premixed flame using high-resolution transmission electron microscopy and optical band gap analysis
PAH structure analysis of soot in a non-premixed flame using high-resolution transmission electron microscopy and optical band gap analysis
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DOI：
10.1016/j.combustflame.2015.11.022
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发表时间：
2016-02
影响因子：
4.4
通讯作者：
María L. Botero;E. Adkins;Silvia González-Calera;H. Miller;M. Kraft
中科院分区：
文献类型：
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作者：
María L. Botero;E. Adkins;Silvia González-Calera;H. Miller;M. Kraft
通过原位可见光消光和热感取样高分辨率透射电子显微镜测量（HRTEM）表征了在支撑于芯送式烟点测试燃烧器（ASTM D1322-08）上的非预混液体燃料火焰系统中形成的碳烟颗粒。研究的燃料是庚烷、甲苯及其等体积混合物（H50 T50），因为它们作为替代燃料具有相关性。消光测量用于计算烟灰体积分数，Fv，并确定作为火焰位置的函数的光学带隙（OBG）。利用Tauc/Davis-Mott分析，从近边吸收特征推导出OBG。一个直接带隙（r = 0.5）被选择为这个分析假设的电子性质的烟尘是由多环芳烃的分子结构为主。对于HRTEM分析，烟灰样品收集在火焰中的不同位置，使用热泳取样和快速插入技术。然后使用“网格条纹”算法分析图像，以确定重要参数，如条纹长度。多环芳烃（PAH）的大小估计从共轭长度从OBG测量和条纹长度从HRTEM测量。在所有研究的火焰中，峰值Fv范围从庚烷火焰中的3.4 ppm到甲苯火焰中的17.6 ppm。尽管有这个宽范围，但不同火焰的平均OBG仅从H50 T50中的2.05 eV变化到甲苯火焰中的2.10 eV，这与0.96 nm和0.99 nm之间的分子长度一致。晶格条纹分析产生了略低的平均条纹长度在0.91和0.99纳米之间的整个不同的火焰。原位和非原位表征的烟灰的结果表明，不同的燃料性质和烟灰阶段的火焰经历了从化学到物理生长在一个类似的大小，约环芘的大小的过渡。
Soot particles formed in a system of non-premixed liquid fuel flames supported on a wick-fed, smoke point test burner (ASTM D1322-08) were characterized by in-situ visible light extinction and thermophoretically-sampled high-resolution transmission electron microscopy measurements, HRTEM. The fuels studied were heptane, toluene and their iso-volumetric mixture (H50T50), given their relevance as surrogate fuels. Extinction measurements were used to calculate the soot volume fraction,Fv, and determine the optical band gap (OBG) as a function of flame position. The OBG was derived from the near-edge absorption feature using Tauc/Davis-Mott analysis. A direct band gap (r = 0.5) was selected for this analysis assuming that the electronic properties of soot are dominated by the molecular structure of the PAHs. For the HRTEM analysis, soot samples were collected at different locations in the flame using thermophoretic sampling and a fast-insertion technique. The images were then analyzed using a ‘lattice-fringe’ algorithm, to determine important parameters such as the fringe length. Polycyclic aromatic hydrocarbon (PAH) sizes were estimated from conjugation lengths obtained from OBG measurements and fringe lengths from HRTEM measurements. Across all studied flames, the peakFvranged from 3.4 ppm in the heptane flame to 17.6 ppm in the toluene flame. Despite this wide range, the average OBG across the different flames only varied from 2.05 eV in the H50T50 to 2.10 eV in the toluene flames, which is consistent with molecule lengths of between 0.96 nm and 0.99 nm. Lattice fringe analysis yielded slightly lower average fringe lengths between 0.91 and 0.99 nm throughout the different flames. Results of in-situ and ex-situ characterization of soot suggests that flames of different fuel nature and sooting stage undergo the transition from chemical to physical growth at a similar size, about the size of circumpyrene.