3 Giugno 2014 - Jeffrey J. Derby - A perspective on the history and future of Bridgman crystal growth

Relatore: Prof. Jeffrey J. Derby - Department of Chemical Engineering and Materials Science University of Minnesota

Luogo:  Aula Newton - Plesso Fisico

E-mail organizzatore: raffaella.burioni@unipr.it

Abstract:

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 find crystal growth conditions for the synthesis of new

materials and optimize crystal growth processes for existing materials.

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.