Despite the large amount of research, the mechanism of particle inception in flame is still undetermined. The complexity of the subject comes from dealing with intermediate objects between a large molecule, or a molecular cluster, and a bulk solid compound. The transition between the growth of molecular species and the inception of a solid particle can be hardly defined, it involves both chemical and physical growth mechanisms and, from an experimental point of view, different operative definitions of inception particles can be given depending on the employed technique and its detection limit. Several experimental evidences support the formation of primary nanoparticles with a typical size of less than three nanometres and the subsequent appearance of a bimodal size distribution following a further particle formation and growth. How the chemical-physical and optical properties differ for particles composing the two modes of the size distribution and how they change as a function of the flame residence time is not jet clear. To address these questions, particle characterization at the early stages of formation requires using methods able to detect polydisperse nanoparticles with size ranging between 2-100 nm, and able to distinguish and selectively monitor particles by size and chemical composition. Optical methods have a good space and time resolution and provide "in situ", not intrusive measurements. However, only moments of the size distribution function are measured. In the last ten years it is increasing the use of sampling probe - Scanning Mobility Particle Sizing (sp-SMPS) technique to measure particle size distribution in laminar flame because of the capability to accurately measure the size distribution for polydisperse samples with a size detection limit down to about 2 nm. This is a powerful complementary method, on the one hand has confirmed and strengthened the optical methods, but on the other, has also opened new perspective and challenges providing useful information to improve the interpretation of optical experiments. Elastic Light Scattering, light absorption, and laser induced incandescence in the visible and near i.r. are generally employed to study soot particles and in flame condition where visible absorbing soot particles is not detected, the light scattering in excess to that due to the gas phase compounds, has been correlated to the light absorption in the UV and UV excited laser induced fluorescence and all these three features were attributed to carbon nanoparticles. By light scattering/extinction, particles with size of about 2 nm with a significant number density were measured. Over the years, we have used a variety of methods to validate the existence and the size of such particles; to investigate their compositions and properties and their differences and relationship with the visible absorbing soot particles. In this talk, the laser diagnostics and complementary techniques we have used in situ and on sampled particles will be reviewed. In addition to Light scattering/extinction, DLS was used to determine particle size and estimate the particle refractive index when combined to light scattering/extinction; Time Resolved Fluorescence Anisotropy measurements by ultrafast lasers source, has furnished the average size of those particles which emit fluorescence following laser excitation instead of emitting incandescence; particle composition and properties have been further characterized by on line size-selected photoionization efficiency using the fifth harmonic of a Nd-Yag laser; Raman spectroscopy of sampled particles and the evaluation of the optical band gap from light absorption spectra. The comparative analysis of optical measurements and DMA size distributions has allowed adding new insight on the evolution from a mono-modal to a bimodal size distribution. Furthermore, it results that size cannot univocally identify inception particles since particles of the smallest size mode of the size distribution formed in different flames have different properties. Environmental related properties of inception particles will be also discussed as the water affinity and the OC/EC analysis by thermal decomposition.
Nucleation Particles in Premixed Flames: Optical Diagnostics and Environmental Issues
Patrizia Minutolo
2015
Abstract
Despite the large amount of research, the mechanism of particle inception in flame is still undetermined. The complexity of the subject comes from dealing with intermediate objects between a large molecule, or a molecular cluster, and a bulk solid compound. The transition between the growth of molecular species and the inception of a solid particle can be hardly defined, it involves both chemical and physical growth mechanisms and, from an experimental point of view, different operative definitions of inception particles can be given depending on the employed technique and its detection limit. Several experimental evidences support the formation of primary nanoparticles with a typical size of less than three nanometres and the subsequent appearance of a bimodal size distribution following a further particle formation and growth. How the chemical-physical and optical properties differ for particles composing the two modes of the size distribution and how they change as a function of the flame residence time is not jet clear. To address these questions, particle characterization at the early stages of formation requires using methods able to detect polydisperse nanoparticles with size ranging between 2-100 nm, and able to distinguish and selectively monitor particles by size and chemical composition. Optical methods have a good space and time resolution and provide "in situ", not intrusive measurements. However, only moments of the size distribution function are measured. In the last ten years it is increasing the use of sampling probe - Scanning Mobility Particle Sizing (sp-SMPS) technique to measure particle size distribution in laminar flame because of the capability to accurately measure the size distribution for polydisperse samples with a size detection limit down to about 2 nm. This is a powerful complementary method, on the one hand has confirmed and strengthened the optical methods, but on the other, has also opened new perspective and challenges providing useful information to improve the interpretation of optical experiments. Elastic Light Scattering, light absorption, and laser induced incandescence in the visible and near i.r. are generally employed to study soot particles and in flame condition where visible absorbing soot particles is not detected, the light scattering in excess to that due to the gas phase compounds, has been correlated to the light absorption in the UV and UV excited laser induced fluorescence and all these three features were attributed to carbon nanoparticles. By light scattering/extinction, particles with size of about 2 nm with a significant number density were measured. Over the years, we have used a variety of methods to validate the existence and the size of such particles; to investigate their compositions and properties and their differences and relationship with the visible absorbing soot particles. In this talk, the laser diagnostics and complementary techniques we have used in situ and on sampled particles will be reviewed. In addition to Light scattering/extinction, DLS was used to determine particle size and estimate the particle refractive index when combined to light scattering/extinction; Time Resolved Fluorescence Anisotropy measurements by ultrafast lasers source, has furnished the average size of those particles which emit fluorescence following laser excitation instead of emitting incandescence; particle composition and properties have been further characterized by on line size-selected photoionization efficiency using the fifth harmonic of a Nd-Yag laser; Raman spectroscopy of sampled particles and the evaluation of the optical band gap from light absorption spectra. The comparative analysis of optical measurements and DMA size distributions has allowed adding new insight on the evolution from a mono-modal to a bimodal size distribution. Furthermore, it results that size cannot univocally identify inception particles since particles of the smallest size mode of the size distribution formed in different flames have different properties. Environmental related properties of inception particles will be also discussed as the water affinity and the OC/EC analysis by thermal decomposition.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
