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Thin-Film Active Materials

GREG P. CARMAN

Department of Mechanical and Aerospace Engineering

University of California, Los Angeles

Los Angeles, California

This presentation describes recent progress and future trends associated with the development and potential uses of thin-film active materials. Active materials exhibit energy coupling, such as coupling between mechanical energy and electrical energy. Some widely recognized active materials are piezoelectric (electromechanical coupling), magnetostrictive (magneto-mechanical coupling), and shape-memory alloys (thermo-mechanical phase-transformation coupling). Many of the physical phenomena associated with bulk active materials are well known (cf., the work of the Curies in the 1800s). However, the same cannot be said about thin-film active materials, which have dimensions on the order of microns and represent a research area that is still in its scientific infancy.

Some physical-coupling phenomena that occur at small scales do not occur in bulk active materials. These unique phenomena include atomic coupling at the nanoscale regime and increased surface area-to-volume ratios, which could lead to the development of structures that were previously unimaginable. These advantages, as well as new active materials that are being discovered (e.g., ferromagnetic shape-memory alloys), suggest that active thin-film material systems will be pervasive throughout our society in the upcoming decades (Murray et al., 2000). Applications for these unique phenomena range from powerful solid-state actuators to miniaturized sensors to clean power-generation systems. This presentation suggests how thin-film active materials will change our society.

In the Active Materials Laboratory at the University of California, Los Angeles (UCLA), research is being conducted on a wide range of active materials,



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Tenth Annual Symposium on Frontiers of Engineering Thin-Film Active Materials GREG P. CARMAN Department of Mechanical and Aerospace Engineering University of California, Los Angeles Los Angeles, California This presentation describes recent progress and future trends associated with the development and potential uses of thin-film active materials. Active materials exhibit energy coupling, such as coupling between mechanical energy and electrical energy. Some widely recognized active materials are piezoelectric (electromechanical coupling), magnetostrictive (magneto-mechanical coupling), and shape-memory alloys (thermo-mechanical phase-transformation coupling). Many of the physical phenomena associated with bulk active materials are well known (cf., the work of the Curies in the 1800s). However, the same cannot be said about thin-film active materials, which have dimensions on the order of microns and represent a research area that is still in its scientific infancy. Some physical-coupling phenomena that occur at small scales do not occur in bulk active materials. These unique phenomena include atomic coupling at the nanoscale regime and increased surface area-to-volume ratios, which could lead to the development of structures that were previously unimaginable. These advantages, as well as new active materials that are being discovered (e.g., ferromagnetic shape-memory alloys), suggest that active thin-film material systems will be pervasive throughout our society in the upcoming decades (Murray et al., 2000). Applications for these unique phenomena range from powerful solid-state actuators to miniaturized sensors to clean power-generation systems. This presentation suggests how thin-film active materials will change our society. In the Active Materials Laboratory at the University of California, Los Angeles (UCLA), research is being conducted on a wide range of active materials,

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Tenth Annual Symposium on Frontiers of Engineering including bulk materials and thin-film materials. One example is a thin-film shape-memory alloy composed of nickel (Ni) and titanium (Ti) (thermo-mechanical coupled). This class of shape-memory materials, discovered in bulk form in the 1960s, represents a benchmark material for shape-memory alloys. Although there are many other binary and ternary shape-memory systems, NiTi is the most often studied and the best characterized. The first publications on thin-film sputter-deposited NiTi appeared in the early 1990s (Busch and Johnson, 1990; Ikuta et al., 1990; Walker and Gabriel, 1990). Thus, research on thin-film shape-memory alloys is relatively new. Shape-memory materials undergo a solid-phase transformation from a low-temperature martensite phase to a higher temperature austenite phase. The phase-transformation temperature can be tailored by altering the composition of the material. One unique property associated with NiTi is its ability to recover (actuate) a specific shape when heated through the austenite phase. For example, imagine a NiTi wire that is bent in the lower temperature martensite phase. When heated (e.g., using Joule heating), the wire quickly returns to its original straight configuration (i.e., shape memory). To illustrate this, Figure 1 shows a heating/ cooling cycle for a thin-film NiTi system produced at UCLA. In Figure 1a, the film is at room temperature. From Figure 1a to 1c, the film is heated, and from Figure 1d to 1f, the film is cooled. This somewhat magical behavior is simply a phase transformation to a specific crystallographic arrangement resulting in “shape memory.” This process, which converts thermal energy into mechanical energy, is used in a wide range of applications, including vascular stents (biocompatible), electrical connectors, satellite release bolts, and coffeepot thermostats. Although shape-memory materials have unique attributes, the bandwidth (1 Hz) of bulk materials limits their applicability in many situations. The bandwidth limitation is due to relatively slow cooling processes related to bulk materials. However, thin-films have very large surface-to-volume ratios so their cooling times can be orders of magnitude higher. Recently at UCLA, thin-film shape-memory alloy bandwidths higher than 100 Hz have been demonstrated (Shin et al., 2004); theoretical predictions approach the kHz regime. These large bandwidths, along with associated large stress and strain output, provide films with enormous values of power-per-unit mass (e.g., 40 kW/kg). The relatively large specific power (e.g., compared to <100 W/kg for other milli- or micro-motors) provides unique opportunities for small-scale applications (e.g., miniature motors and heart valves). One pump design with dimensions in centimeters is used to articulate the nose cone of a small missile system (Shin et al., in press). Another application is a pumping motor that can move small amounts of fluids for various laboratory (e.g., laboratory-on-a-chip) and biomedical applications. For example, the pump can be used as an embeddable drug-delivery system. Coupled with appropriate sensors, this miniature (submillimeter) pump could someday replace a human

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Tenth Annual Symposium on Frontiers of Engineering FIGURE 1 Heating/cooling cycle for a thin-film NiTi system.

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Tenth Annual Symposium on Frontiers of Engineering pancreas in individuals suffering from diabetes. Recently, a cardiologist working with UCLA researchers proposed using this material for a percutaneous heart valve (Stepan et al., 2004). This revolutionary idea would mean that a heart valve could be replaced without major surgery. These are only a few of the future applications for thin-film shape-memory materials. The potential for the general class of thin-film active materials is substantially larger than for thin-film shape-memory alloys alone. In the thin-film regime, atomic coupling can be used to improve the performance of active-material systems. For example, exchange coupling interaction, a phenomenon not present on the macroscale, can considerably decrease the magnetic field required to actuate magnetostrictive materials (Quandt and Ludwig, 1999). One advantage of magnetostrictive materials, noncontact actuation, can be generated because magnetic fields easily propagate through the air. For example, a permanent magnet easily rearranges iron particles on a plate without requiring direct physical contact with the particles. A number of other submicron coupling phenomena could further improve thin-film active materials. For example, newly developed multiferroic systems that transform energies between multiple states (compared to classical two-state systems) are being studied (Zheng et al., 2004). Multiple-state energy transfer (e.g., mechanical to electrical power or solar to electrical power) is useful in constructing high-fidelity sensors, such as magnetometers that were once unimaginable and even novel, clean, power-generation systems that are far superior to existing systems (e.g., solar cells). These advancements will certainly change the way our world operates and will lead to many new scientific discoveries in the next century. This presentation has described a few thin-film active materials and some of the concepts currently being pursued by academic and industrial researchers, such as the fabrication of, and applications for, new small-scale actuator and sensor systems. In the future, vast numbers of small-scale actuators may be massed together to achieve a common purpose, similar to the way ants interact during gathering operations. Another future application might be for small-scale systems to remove, or possibly prevent, blood clots, thereby preventing catastrophic strokes. In the more distant future, even-smaller-scale systems may interact and join together to form solid structures, similar to a fluid liquid that becomes a solid, morphing into different defined shapes. Material scientists are already designing materials at the atomic level, giving rise to the possibility that structures could be designed to interact and be built at the micron level. Therefore, the study of thin-film active materials, although still in its infancy, may revolutionize the future of our society.

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Tenth Annual Symposium on Frontiers of Engineering REFERENCES Busch, J.D., and A.D. Johnson. 1990. Prototype Micro-Valve Actuator. Pp. 40–41 in Proceedings of IEEE Micro Electro Mechanical Systems, An Investigation of Micro Structures, Sensors, Actu ators, Machines and Robots. New York: IEEE. Ikuta, K., H. Fujita, S. Arimoto, M. Ikeda, and S. Yamashita. 1990. Development of Micro Actuator Using Shape Memory Alloy Thin Film. Pp. 3–6 in Proceedings of the IEEE International Workshop on Intelligent Robots and Systems ’90: Towards a New Frontier of Applications. New York: IEEE. Murray S.J., M. Marioni, S. Allen, R.C. O’Handley, and T.A. Lograsso. 2000. 6% magnetic-field-induced strain by twin-boundary motion in ferromagnetic Ni-Mn-Ga. Applied Physics Letters 77(6): 886–888. Shin, D., K.P. Mohanchandra, and G.P. Carman. 2004. High frequency actuation of thin film NiTi. Sensors and Actuators A: Physical 111(2-3): 166–171. Shin, D., K.P. Mochanchandra, and G.P. Carman. In press. Development of hydraulic linear actuator using thin film SMA. Sensors and Actuators A: Physical. Stepan, L., D. Levi, M. Fishbein, and G.P. Carman. 2004. A Thin Film Nitinol Heart Valve. Proceedings of the 2004 ASME International Mechanical Engineering Congress, Aerospace Division, Adaptive Structures and Material Systems Committee. New York: ASME. Quandt, E., and A. Ludwig. 1999. Giant magnetostrictive multilayers. Journal of Applied Physics 85(8): 6232–6237. Walker, J., K. Gabriel, and M. Mehregany. 1990. Thin-film processing of TiNi shape memory alloy. Sensors and Actuators A: Physical 21(1-3): 243–246. Zheng, H., J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, and R. Ramesh. 2004. Multiferroic BaTiO3-CoFe2O4 nanostructures. Science 303(5658): 661–663.

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