NUMERICAL STUDY OF MELTING OF LARGE-DIAMETER CRYSTALS USING AN ORBITAL SOLAR CONCENTRATOR

The melting of large-diameter crystals using an orbital solar concentr ator is studied numerically. In the proposed configuration, a paraboli c dish imaging concentrator is used to focus the sun's radiation onto an ampoule which holds the solid charge material to be processed. The charge will start melting in the vicinity of the focal height, after w hich it is tl translated in order for the melt to resolidify as a sing le crystal. A ray-trace method has been developed to determine the inc ident concentrated solar hear flux on the ampoule's surface for both p erfectly aligned and misaligned configurations. For the perfectly alig ned charge, a transient two-dimensional Conduction problem with phase change is formulated, whereas once the perfect alignment of the charge 's symmetry axis with the sun's incoming ray is perturbed, the problem becomes three-dimensional due to the complex surface heat flux bounda ry condition. The commercial code FIDAP is used to solve the governing transport equation. By ignoring the participation of the ampoule in t he heat transfer process, preliminary results highlighting the feasibi lity of growing GaAs, Ge, and Si crystals with diameters of the order of 20 cm using the orbital solar concentrator concept are presented. T he transient temperature fields within various charge materials during the heat-up process are quantified. The resulting melting pattern wit hin the charge due to the uncolumnated beam is observed to be uniform along the charge when compared to the idealized limiting case of colum nated beams. Finally, the effect of the misalignment angle on the melt ing process is quantified.