''Smart'' Metals Take Shape
03/20/2009
Departments: Mechanical Engineering and Materials Science
For decades, shape memory alloys (SMAs) have garnered widespread attention from medicine to industry for their unique ability to “remember” their shape and return to that shape after being deformed. In their bulk form, they are commonly used as fasteners, auto shut-off valves and medical stents. In recent years, however, attention has shifted to their behavior as thin films (approximately 1/60th the thickness of human hair) for smaller scale applications, including full integration into micromachines, or microelectromechanical systems (MEMS). Since her first publication in this field on the crystallization and phase transformation of nickel titanium (NiTi) thin films in 2004, associate professor of mechanical engineering, Ainissa Ramirez, has made continuous breakthroughs in this area, culminating in her most recent publication which she describes as a recipe for MEMS engineers. (Published March 2009 in Scripta Materialia).
NiTi thin films have unique shape memory properties and ability to move and generate large forces by shifting their atomic arrangement from one crystalline structure to another – rather like a marching band changing formation. When first deposited on a silicon substrate, there is no organized or crystalline structure; they are amorphous. Crystallization occurs only when the films are heated, and the resulting microstructure varies depending on heating temperature and time. Contrary to common thought, and of great significance in material processing, Ramirez found that larger grain sizes formed at lower temperatures and significantly below the minimum temperature believed necessary.
Just as grain size formation varies with temperature, the actuation force, or force generated when these materials transform, varies with grain size (i.e., larger grain size results in larger forces). After determining the optimal temperature range for crystallization, Ramirez was able to link the three – mapping temperature to grain size formation and grain size to actuation properties – essentially creating a cookbook for material scientists. In a field described by Ramirez as largely unpredictable, recipes like this are sure to find wide-scale application.
NiTi thin films have unique shape memory properties and ability to move and generate large forces by shifting their atomic arrangement from one crystalline structure to another – rather like a marching band changing formation. When first deposited on a silicon substrate, there is no organized or crystalline structure; they are amorphous. Crystallization occurs only when the films are heated, and the resulting microstructure varies depending on heating temperature and time. Contrary to common thought, and of great significance in material processing, Ramirez found that larger grain sizes formed at lower temperatures and significantly below the minimum temperature believed necessary.
Just as grain size formation varies with temperature, the actuation force, or force generated when these materials transform, varies with grain size (i.e., larger grain size results in larger forces). After determining the optimal temperature range for crystallization, Ramirez was able to link the three – mapping temperature to grain size formation and grain size to actuation properties – essentially creating a cookbook for material scientists. In a field described by Ramirez as largely unpredictable, recipes like this are sure to find wide-scale application.
Work supported by: NSF Career; Alfred P. Sloan Foundation
Published as: Effects of Crystallization Temperature on the Stress of NiTi Thin Films, H. -J. Lee, X. Huang, K.P. Mohanchandra, G. Carman, A. G. Ramirez, Scripta Materialia (2009)