Microstructural Engineering of Shape Memory Thin Films and Nanowires

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This Award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:Materials like NiTi, which have the unique ability of ?remembering? their original shape when heated, can be used as actuators. These materials undergo a martensitic phase transformation from one crystallographic structure to another and provide large actuation forces. Despite these interesting properties the integration of shape memory alloys in microelectromechanical systems (MEMS) is limited because the details of the phase transformation that activates the shape changes is very sensitive to microstructural details. For example a small increase in grain size significantly changes the actuation force and the transformation temperature. In addition, the mechanical behavior of NiTi in thin-film form differs from the bulk and is a largely unexplored research topic. Objectives of this program are to evaluate the effects of grain structure, grain size and grain-size distributions on phase transformation temperatures, hysteresis behavior, actuation properties, and mechanical properties. Accordingly, we will explore the crystallization behavior of NiTi thin films and nanostructures; and broaden the understanding of thin film mechanical properties by examining materials that exhibit elastic nonlinearities. Structure-property relationships will be studied by observing microstructural development using in situ transmission electron microscopy. Evolution of the and grain structure will be evaluated using the Johnson?Mehl?Avrami?Kolmogorov theory. The resulting actuation properties of the engineered microstructures will be studied with wafer curvature methods and MEMS-based cantilevers; and the transformation temperature changes will be investigated with differential scanning calorimetry. The dependence of mechanical properties on microstructure will be examined with nanoindentation. This study will provide novel observations of the behavior of thin film shape memory materials, and provide guidance for their adoption into MEMS.NON-TECHNICAL SUMMARY:Knowledge of the link between phase transformations, microstructure, and mechanical properties will be studied in thin films by observing a new class of materials that undergo a martensitic (i.e. displacive) transformation. From this work, we will improve the fundamental understanding of thin-film shape memory alloys and learn how to control their properties in a predictable way, thereby illuminating the role of microstructure on the thermodynamics of martensitic transformations. This ability to control properties will benefit the MEMS community and enable future devices. Additionally, these materials provide a model to hone the ability to tailor microstructures and will benefit other research pursuits in amorphous silicon, amorphous carbon, and metallic glasses. This program?s broader impact consists of stimulating the interest in science for a range of individuals from the training of graduate students to the encouragement of school children. A revamped introductory materials science class that includes hands-on demonstrations and real-world examples will cultivate engineers with strong materials backgrounds. A compilation of classroom demonstrations disseminated on the web will serve a wider learning community. This educational program also provides informal opportunities to change the perception of science via a lecture series that showcases diverse scientists and presents enjoyable science events. The aim is to encourage all students, particularly students of color and girls, to consider science as a career. It furnishes richer connections to science for school children, their teachers, and their parents and is a simple model that can be extended within and between universities. By leveraging a partnership with the NISE network, this program will have large dissemination channels. Leaving no student behind, this program seeks to capture the attention of non-science majors in a liberal-arts environment by using them as demonstrators to teach science in a compelling way.