Critical conditions for laser initiation of a chemical reaction in thin layer placed on substrate

Abstract

In this work, we propose a model for the laser initiation of a chemical reaction in a film supported by an inert substrate. Materials of the film and the substrate have different thermal and physical properties. The model includes the equations of heat conductivity and chemical kinetics of the overall reaction. The reaction rate depends on the reactant concentration by power law and on the temperature by Arrhenius law. Consideration is given to possible changes in optical properties of the film due to accumulation of reaction products as well as to features of laser absorption and reflection from the film-substrate interface. For a detailed numerical study, the model is presented in dimensionless variables selected by fitting spatial and temporal scales that characterize the model. This reduces the number of variable parameters and gives the most general results. Critical conditions are found between different modes of the process. At the subcritical pulse duration, either no chemical reaction takes place or a reaction stops immediately after it started. At the supercritical pulse duration, complete conversion occurs in the laser hot spot. It is found that the dependence of optical properties of the film on the degree of conversion strongly affects the temperature, conversion dynamics, size of the reaction product zone, and critical conditions. In this work, for the first time, the dependence of the critical conditions separating different modes of process development (sharp temperature rise with complete transformation in the center of the heating spot and extinction, which may be accompanied by partial transformation) on model parameters was investigated. For the first time, the dynamics of the reaction front in the film located on the substrate was analyzed for conditions of changing optical properties. The results of calculations can be useful in discussing the mechanisms of destruction of polymer coatings, in describing methods of protection against external influences, as well as in developing methods for modifying surface structures, creating three-dimensional objects with specified properties, synthesis of intermetallic products in 3D - technologies, etc.