The sintering of particles with a size of a few µm with a 1064 nm laser shows unpredictable results, there is either no reaction or evaporation and ablation of material. My suggestion is that I get poor laser absorption, due to the small particle size, as soon as a few particles melted their size grows due to surface tension and the absorption coefficient increases, resulting in an avalanche effect which leads to material evaporation, instead of sintering. Can anyone confirm or negate this theory or point out papers where this has been observed?
The amount of energy a particle absorbs from a beam of light depends on the size and shape of the particle, and also on the particle's composition. To a very rough approximation, the absorption cross-section is proportional to the particle's projected area. Larger particles thus absorb more. Once a particle starts to melt, moreover, it is possible that their absorption coefficient may change due to chemical changes. Oxidation of organic materials, for instance, tends to increase the absorption coefficient, leading to just the sort of run-away effect you describe. A classic text on the subject is Bohren and Huffman, "Absorption and Scattering of Light by Small Particles," (Wiley, New York, 1983).
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