Abstract
The application of pyrometry to retrieve particle temperature in particulate-generating flames strictly requires the knowledge of the spectral behavior of emissivity of light-emitting particles. Normally, this spectral behavior is considered time-independent. The current paper challenges this assumption and explains why the emissivity of oxide nanoparticles formed in flame can change with time. The suggested phenomenon is related to transitions of electrons between the valence and conduction energy bands in oxides that are wide-gap dielectrics. The emissivity change is particularly crucial for the interpretation of fast processes occurring during laser-induced experiments. In the present work, we compare the response of titania particles produced by a flame spray to the laser irradiation at two different excitation wavelengths. The difference in the temporal behavior of the corresponding light emission intensities is attributed to the different mechanisms of electron excitation during the laser pulse. Interband transitions that are possible only in the case of the laser photon energy exceeding the titania energy gap led to the increase of the electron density in the conduction band. Relaxation of those electrons back to the valence band is the origin of the observed emissivity drop after the UV laser irradiation.
Highlights
Emission properties of flame-generated oxide nanoparticles are a subject of a longlasting discussion within the combustion community
We studied nanoparticle light emission intensities that were collected from the non-irradiated flame and the flame irradiated by the laser at different delay times (100–800 ns) after the laser pulse (IR and UV), and compared the temporal behavior of the gray body temperature and the relative volume fraction inferred according to the procedure described in the previous section
The comparison of the temporal behavior of the spectra emitted by titania particles generated by a flame spray after the laser irradiation of different wavelengths carried out in the current paper revealed a noticeable difference in the effect of the laser photon energy on the character of the evolution of the flame characteristics inferred from the light emission spectra
Summary
Emission properties of flame-generated oxide nanoparticles are a subject of a longlasting discussion within the combustion community. The essential condition for the condensation growth of an oxide nanoparticle is a formation of defects within the forbidden band of material and an appearance of non-equilibrium electrons in the conduction band This is required for dissipating the condensation energy, which is in the order of 5 eV per condensing molecule. Since the concentration of defects and non-equilibrium electrons depends on the condensation rate, the inferred nanoparticle emissivity may vary depending on the flame conditions that control this rate The justification of this explanation requires multiple steps, which should preferably be done in a system that allows for the most straightforward interpretation of experimental results. The resulting emissivity after the UV laser pulse is expected to be a weaker function of the wavelength compared to that after the IR laser pulse We apply this concept interpreting the temporal difference in the laser-induced emission from flame-generated titania nanoparticles under two different excitation wavelengths
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