Laser Densification Modeling
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LASER DENSIFICATION MODELING
TAIPAU CHIA*, L. L. HENCH*, CHAOBIN QIN** AND C. K. HSIEH** *Advanced Materials Research Center, University of Florida, One Progress Blvd., #14, Alachua, FL 32615. **Department of Mechanical Engineering, University of Florida, Gainesville, FL 32611.
ABSTRACT A three-dimensional transient model for heat conduction in silica glass is developed. The model simulates a three-dimensional temperature distribution in a silica glass irradiated by a moving CO2 laser. Both the reflectivity of the glass surface and the strong attenuation of the laser energy in the glass medium are accounted for by a detailed radiation analysis. The energy absorbed by the glass is determined to be confined in a 10 pm thickness; the laser irradiation is thus treated as a boundary condition. The heat diffusion equation is solved by an alternatingdirection-implicit method.
INTRODUCTION Many models have been developed for predicting the temperature distribution inside materials treated with laser heating [1-3]. In this paper the temperature rise is analyzed. The results are useful to determine the effect of applying a CO2 laser on a porous gel silica glass monolith. Previous papers report the processing and properties of the alkoxide derived gel silica investigated in this paper [4-6]. The laser energy is assumed to be absorbed and totally transformed into heat. Based on high absorption coefficient [7] of the glass, the laser energy is determined to be absorbed primarily within a very thin layer (-10 pm ) into glass surface. The Gaussian distribution of this energy is integrated within the absorption layer, and the absorbed energy is treated as a boundary condition in the analysis. The model developed in this paper simulates the three dimensional heat transfer in the silica glass irradiated by a moving CO2 laser. Relative temperature distributions at various laser power settings and travelling velocities are evaluated numerically. Parametric studies for the effects of the heat transfer coefficient on the glass surface and the thermophysical properties of the glass medium are also investigated. It is well known that the properties of gel silica glasses are strongly related to their processing temperature. The results presented in this paper can thus be used to evaluate the property changes due to the temperature rise in the gel silica glass.
FORMULATION OF THE PHYSICAL MODEL The physical system under investigation is shown in Figure 1. A focused CO 2 laser beam is travelling in the x direction. The thickness of the glass substrate is in the z direction, and because of the motion of the beam, the temperature distribution in glass is symmetrical with respect to the x-z plane; only one half of the medium is thus needed for analysis. Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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