A method of controlling the crystal-growth habit in directional freezing has been devised. A substance can be purified by directional-freeze crystallization, wherein (1) the liquid phase of the substance is partially frozen, causing impurities to become preferentially concentrated in the remaining unfrozen liquid (they are more soluble in the liquid than in the solid), (2) the unfrozen liquid containing a large part of the impurities is extracted, and (3) the remaining crystalline phase is melted. The present method is directed toward reducing the entrapment of liquid within the crystalline structure, thus enhancing separation efficiency. The method is based on the observation that uniformity in heat-transfer conditions is of the utmost importance in forming uniform crystals in which little liquid is entrapped.

This work was done by William M. Conlon of Polar Spring Corp. for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Polar Spring Corp.
3501 Edison Way
Menlo Park, CA 94025

Refer to MSC-22518, volume and number of this NASA Tech Briefs issue, and the page number.