Characteristics of Radiation Absorption in Metallic Particles

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Thermal radiation absorption in metallic particles is an important phenomenon in many contemporary laser-processing techniques, including laser cladding of coating materials and laser cleaning of particulate contaminations. In this work, the Drude free-electron theory and electromagnetic wave theory are utilized to characterize the internal absorption of CO2 laser radiation in aluminum, chromium, and nickel particles. The results show that metallic particles have unique radiation properties. Radiation absorption in large particles occurs only in a very narrow region of the front particle surface, which results in inefficient radiation absorption. On the other hand, micron and submicron particles can absorb radiation very efficiently, due to the strong diffraction effect at the particle surface. For extremely small particles (e.g., nanometer particles), radiation absorption becomes less efficient. The particle absorption efficiency is found to increase with temperature, and this temperature dependence can be determined from those of flat metal surfaces at the normal incidence.