UMD Theses and Dissertations
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Item Functionalized Thin-Film Shape Memory Alloys for Novel MEMS Applications(2023) Curtis , Sabrina M.; Takeuchi, Ichiro; Quandt, Eckhard; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nickel-titanium (NiTi) shape memory alloy (SMA) films are already implemented into microelectromechanical system (MEMS) devices such as sensors, actuators, and implantable medical devices. In this thesis, I used DC magnetron sputter deposition to study the influence of film composition, microstructure, and annealing conditions on the stability of the phase transformation for the NiTi-based SMA thin films TiNiCu, TiNiCuCo, and TiNiHf. SMAs are a type of smart material that can undergo stress or temperature-induced solid-to-solid phase transformation between two different crystalline phases. In NiTi-based SMAs, the two phases are known as martensite with a monoclinic crystalline structure and austenite with a cubic crystal structure. The temperature-induced phase transformations can be used to switch between the martensite and austenite phases, and thus switch between two sets of material properties in the SMA. For example, in NiTi-based SMAs the Young’s modulus, electrical resistivity, and coefficient of thermal expansion of the austenite phase are typically 2X larger than that of the martensite phase. The transformation temperatures, recovery strains, enthalpy of transformation, and fatigue properties of NiTi SMAs can be tuned by alloying NiTi with other elements like copper (Cu), cobalt (Co), and hafnium (Hf). For example, certain compositions of sputtered TiNiCu and TiNiCuCo are known to be ultra-low fatigue SMAs, able to reversibly undergo the phase transformation for 10+ million cycles without degradation in the mechanical or thermal properties. The primary focus of this thesis was the integration of these sputtered NiTi-based SMA thin-films into the following four novel MEMS devices: 1) TiNiCu for magneoelectric sensors, 2) TiNiHf for bistable actuators, 3) TiNiCuCo for stretchable electronics and 4) thin-film SMA stretchable auxetic structures for wearable and implantable medical devices. The shape memory effect was observed in TiNiCu and TiNiHf films when the film thickness and lateral dimensions are downscaled to micro and nano dimensions. In the research publication “Integration of AlN piezoelectric thin films on ultralow fatigue TiNiCu shape memory alloys.”, I showed the reproducibility of the thermal-induced phase transformation of Ti50Ni35Cu15 is attractive for integration into MEMS devices that require a high cycle lifetime. I showed how the SMA’s phase transformation can be used to tune the resonant of bending cantilever-type sensors like magnetoelectric sensors. I also demonstrated excellent thin-film piezoelectric and shape memory alloy properties for 2 μm AlN/ 5 μm TiNiCu films composites deposited onto silicon substrates. The large work densities and high strength-to-weight ratio offered by SMAs are attractive for the development of micro and nano actuators. The thermal induced phase transformation between martensite and austenite is also used to develop bi-directional micro-actuators with TiNiHf/Si and TiNiHf/SiO2/Si composites. In another research publication, “TiNiHf/SiO2/Si shape memory film composites for bi-directional micro actuation”, I demonstrated the influence of film thickness and substrate on the phase transformation properties of TiNiHf thin-films. Ti40.4Ni48Hf11.6 films with thicknesses as low as 220 nm on SiO2/Si substrates can undergo the phase transformation with high transformation temperatures (As > 100 °C) and a wide thermal hysteresis (ΔT > 50 °C). In this publication, we explain how the wide hysteresis and high transformation temperature obtained in TiNiHf films can be used to develop micro and nano-scale bistable actuators based on PMMA/TiNiHf/Si composites. Even though thin-film NiTi-based SMAs are known to reversibly recover superelastic strains of up to 8%, surprisingly, they have not yet been exploited in the growing field of stretchable electronics. In the technical article “Thin-Film Superelastic Alloys for Stretchable Electronics” I demonstrate the first experimental and numerical studies of freestanding thin-film superelastic TiNiCuCo structured into a serpentine geometry for use as stretchable electrical interconnects. Fabricated electropolished serpentine structures were demonstrated to have low fatigue after cycling external strains between 30% - 50% for 100,000 cycles. The electrical resistivity of the austenite phase of a Ti53.3Ni30.9Cu12.9Co2.9 thin-film at room temperature was measured to be 5.43 × 10-7 Ω m, which is larger than reported measurements for copper thin-films (1.87 × 10-8 Ω m). Expanding upon this work, in the conference proceedings paper “Auxetic Superelastic TiNiCuCo Sputtered Thin-Films for Stretchable Electronics”, I present a new platform for functionalized wearable electronics and implantable medical devices based on superelastic thin-film SMA substrates structured into novel stretchable auxetic geometries. Since thin-film SMAs are conductive, the structured substrate itself could serve as the current collector for such stretchable and flexible devices, or a more conductive electrode can be deposited on top of the stretchable auxetic SMA substrate. Overall, the results discussed in this doctoral thesis look to the future of harnessing the functional properties of thin-film sputtered SMAs for novel uses in next-generation MEMS devices.Item Fabrication and Characterization of Nanoscale Shape Memory Alloy MEMS Actuators(2020) Knick, Cory R.; Bruck, Hugh; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The miniaturization of engineering devices has created interest in new actuation methods capable of large displacements and high frequency responses. Shape memory alloy (SMA) thin films have exhibited one of the highest power densities of any material used in these actuation schemes with thermally recovery strains of up to 10%. With the use of a biasing force, such as from a passive layer in a “bimorph” structure, homogenous SMA films can experience reversible shape memory effect provided they are thick enough that the crystal structure is capable of transforming. However, thick films exhibit lower actuation displacements and speeds because of the larger inertial resistance. Therefore, there is a need to find a way to process thinner SMA films with grain structures that are capable of transformation in order to realize larger actuation displacements at higher speeds. In this work, a near-equiatomic NiTi magnetron co-sputtering process was developed to create nanoscale thick SMA films as thin as 120 nm. By using a metallic seed layer, it was possible to induce the crystallization of epitaxial, columnar grains exhibiting the shape memory effects in nanoscale films ranging from 120 – 400 nm. It was also possible to crystalize these SMA films at lower processing temperatures (as low as 325 °C) compared to directly sputtering thicker films onto Si wafers. The transformation behavior associated with the SME in these films were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), and stress-temperature measurements at wafer level. After quantifying the shape memory effects at wafer-level, the SMA films were used to fabricate various microscale MEMS actuators. The SMA films were mated in several “bimorph” configurations to induce out of plane curvature in the low-temperature Martensite phase. The curvature radius vs. temperature was characterized on MEMS cantilever structures to elucidate a relationship between residual stress, recovery stress, radius of curvature, and degree of unfolding. SMA MEMS actuators were fabricated and tested using joule heating to demonstrate rapid electrical actuation of NiTi MEMS devices at some of the lowest powers (5-15 mW) and operating frequencies (1-3 kHz) ever reported for SMA or thermal actuators. By developing a process to create nanoscale thickness NiTi SMA film, we enabled the fabrication of MEMS devices with full, reversible, actuation as low as 0.5 V. This indicated the potential of these devices to be used for high frequency, low power, and large displacement applications in power constrained environments (i.e. on chip).Item AN INVESTIGATION OF CONCENTRATED AND DISTRIBUTED STRAIN INDUCING CONSTRAINTS FOR TRAINING SHAPE MEMORY ALLOYS(2012) Parent, Pauline M.; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research explores concentrated and distributed strain inducing constraints for Two Way Shape Memory (TWSM) training of cantilevered NiTiNOL shape memory alloy strips via the constrained thermal cycling of deformed Martensite training method. The goal is to evaluate the performance of a tip-moment trained sample actuator, which is characterized by constant strain along the sample length, and compare it with the performance of tip-force trained sample actuators, which have root concentrated strain. The shape and net tip displacement of trained specimens is expected to vary with training constraint type and training load magnitude, rendering it uncertain which sample will have greater work potential. A training structure and systems for introducing thermal and mechanical loads were developed to provide the explored training constraints and induce TWSM. The work performance was evaluated by measuring vertical displacement of tip weights. The experimental results indicate that samples subject to strain distributing constraints during training have higher work potential than samples subject to concentrated strain inducing constraints.