Percy Williams Bridgman received the 1946 Nobel Prize in Physics “for the invention of an apparatus to produce extremely high pressures, and for the discoveries he made therewith in the field of high pressure physics.” He also invented an apparatus that has arguably proven to be even more important and pervasive, when in 1926 he developed a method to grow single crystals of non-cubic metals needed for his high-pressure studies. Of course, this technique is now commonly referred to as the Bridgman method.
The initial realization of the Bridgman method was quite crude, with a cylindrical tube being lowered into the air of the room or into a cooling bath of oil. Bridgman noted, “It is important that air drafts be kept from the emerging mold, as otherwise new centers of solidification may be started.” In a “radical change of technique,” Stockbarger (1935) pulled his samples of lithium fluoride from an upper furnace maintained at a temperature above the melting point into a lower furnace, whose temperature was set to achieve a suitable axial gradient.
Thus, Stockbarger was perhaps the first to advance the idea that careful control of temperatures and gradients would be needed to carry out the growth of high-quality single crystals. This idea will be examined and enlarged in this presentation, which endeavors to highlight many of the prior advances in understanding and technique that have led to the Bridgman-Stockbarger and gradient freeze processes of today. In particular, we will emphasize the role of heat transfer and furnace design in setting the macroscopic shape of the solidification interface.
It will be argued that modern ideas of model-based design and control can be used to influence this important characteristic of growth. Several examples from recent modeling of electrodynamic gradient freeze growth of cadmium zinc telluride will be presented. Notably, a strategy is presented to dynamically adapt the furnace profile so that uniform, convex interface shapes are maintained through an entire growth run. Realizing a convex solidification interface is postulated to result in better crystallinity and higher yields than obtained via conventional approaches.
______________________________________
Supported in part by DOE/NNSA, DE-FG52-08NA28768, the content of which does not necessarily reflect the position or policy of the United States Government, and no official endorsement should be inferred. |