Dissertations

Zelenska K. S.

27.06.2016

Thermal emission of light-absorbing surface layers under pulsed laser excitation
Doctor of Philosophy degree (Candidate of science in Physics and Mathematics)
01.04.05 – optics, laser physics

In this work laser-induced incandescence (LII) of light-absorbing surface layers was studied under the Q-switched YAG:Nd laser excitation. For the surfaces irradiated by a sequence of laser pulses, the non-monotonic behavior of LII intensity with the increase of number of irradiating laser pulses was observed. Computer simulation of pulsed laser heating of rough carbon surfaces revealed essential non-uniformity of the temperature field on the irradiated surface hence the surface relief is affected by the laser irradiation due to the processes of evaporation of the peaks on the irradiated surface. The intensity of LII was calculated as a function of height of the surface roughness. The results of calculations explain the observed features of LII of carbon surfaces.
Significant nonlinearity in the dependence of LII intensity on the laser pulse power density was observed. For LII signals integrated over the surface the non-dimensional rate of the surface emission non-linearity γ was also calculated. The obtained values of γ are within the margins of 7…10 that indicate significant nonlinearity of LII.
LII of polished carbon surface at different values of ambient air pressure was studied. A decrease in ambient air pressure led to an increase of the LII intensity in about 1.5 times and extension of LII pulse fall time by about 70 percent due to changes in the thermal conditions inside the sample material. The model of the porous carbon bulk is proposed for explanation of the observed effect.
The effect of laser-induced pore expansion on thermal emission with an increase in laser irradiation dose has been studied. The experimental results and calculation data demonstrate a significant impact of undersurface pore expansion on the LII behavior. Besides, LII strongly depends on the thickness of carbon layer covering a cavity which decreases under laser irradiation because of carbon evaporation.
The experiments show that the intensity of LII of carbon surface depends on the initial temperature of the investigated sample. A method is proposed for estimation of temperature of laser-heated surfaces. The method requires measurement of LII at a fixed wavelength with a moderate variation of initial sample temperature.
The observed phenomena are explained by changes in the morphology (structure) of the surface layer as a result of
(i) Non-uniform heating of the surface layer and
(ii) Evaporation of the material under the action of powerful laser radiation.
In particular, the following factors are important:
(i) Reduction of the height of irregularities on the rough surface;
(ii) Decrease in substance thickness above the pores, which border on the surface;
(iii) Expansion of subsurface pore.

Keywords: laser-induced incandescence, rough carbon surface, light-absorbing microparticles, surface temperature, temperature estimation, laser heating.

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