Title: Fabrication and Characterization of Nanoscale Shape Memory Alloy Mems Actuators

Author: Cory Ray Knick

Date & Time: July 1, 2020 – 2:30-3:30pm

Committee: Dr. Hugh Bruck, Associate Dean for Faculty Affairs, Chair

Dr. Christopher Morris, Branch Chief, US Army Research Laboratory

Dr. Patrick McCluskey, Professor, Dept. of Mechanical Engineering

Dr. Don DeVoe, Professor and Associate Chair, Dept. of Mechanical Engineering

Dr. Miao Yu, Professor, Dept. of Mechanical Engineering

Dr. Ichiro Takeuchi, Deans Representative, Professor of Materials Science and Engineering

Abstract: 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 bilayer structure, homogenous SMA films can experience reversible shape memory effects 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. 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. It was also possible to crystalize these SMA films at lower processing temperatures 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 actuators. By developing a process to create nanoscale thickness NiTi SMA films, 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).

Title: Development and In-Silico Evaluation of a Closed-Loop Fluid Resuscitation Control Algorithm with Mean Arterial Pressure Feedback.

Author: Mohammad Alsalti

Date & Time: June 26, 2020 – 2:00pm

Committee: 

• Dr. Jin-Oh Hahn (Chair) 
• Dr. Nikhil Chopra
• Dr. Axel Krieger 

Abstract: A model-based closed-loop fluid resuscitation controller using mean arterial pressure (MAP) feedback is designed and later evaluated on an in-silico test-bed. The controller is based on a subject specific model of blood volume and MAP response to fluid infusion. This simple hemodynamic model is described using five parameters only. The model was able to reproduce blood volume and blood pressure response to fluid infusion using an experimental data set collected from 23 sheep and is therefore suitable to use for control design purposes. A model-reference adaptive control scheme was chosen to account for inter-subject variability captured in the parametric uncertainties of the underlying physiological model. Three versions of the control algorithm were studied under different measurement availability scenarios. In-silico evaluation of the three controllers was done based on a comprehensive cardiovascular physiology model on a cohort of 100 virtually generated patients.In-silico results showed consistent tracking performance across all patients unlike estimation performance, which varied depending on measurement availability. 

Title: Personal cooling system with phase change material

Author: Yiyuan Qiao

Committee members:

Dr. Reinhard Radermacher(Chair)

Dr. Ichiro Takeuchi

Dr. Jelena Srebric

Dr. Bao Yang 

Dr. Amir Riaz

 Dr. Yunho Hwang

Date: June 26, 2020, 2:00 pm

Abstract: Personal cooling systems are attracting more attention recently since they can set back building thermostat setpoint to achieve energy savings and provide high-level human comfort by focusing on micro-environment conditions around occupants rather than the entire building space. Thus, a vapor compression cycle (VCC)-based personal cooling system with a condenser integrated with the phase change material (PCM) is proposed. The PCM heat exchanger (PCMHX) works as a condenser to store waste heat from the refrigerant in the cooling cycle, in which the PCM melting process can affect the system performance significantly. Different from most of the previous study, various refrigerant heat transfer characteristics along the condenser flow path can result in the uneven PCM melting, leading to the degradation of the system performance. Therefore, enhancing heat transfer in the PCM, investigating the proposed personal cooling system performance, improving PCMHX latent heat utilization in terms of the distribution of PCM melting, and developing a general-purpose PCM model are the objectives of this dissertation. 

Five PCMHX designs with different heat transfer enhancement including increasing heat-transfer area, embedding conductive structures, and using uniform refrigerant distribution among condenser branches are introduced first. Compared with non-enhanced PCM, the graphite-matrix-enhanced PCMHX performs the best with 5.5 times higher heat transfer coefficient and 49% increased coefficient of performance (COP). To investigate the proposed system performance, a system-level experimental parametric study regarding the thermostat setting, PCM recharge rates, and cooling time was conducted. Results show that the PCS can work properly with a stable cooling capacity of 160 W for 4.5 hours. A transient PCM-coupled system model was also developed for detailed system performance, PCM melting process and heat transfer analysis. From both experiment and simulation work, the uneven PCM melting was presented, which could result in an increase of condenser temperature and a degradation of system COP with time. One significant reason for the uneven PCM melting is the variation of the refrigerant temperature and heat transfer coefficient. Therefore, through experimental analysis, several solutions were proposed to minimize the negative effect of the uneven PCM melting. Results show that the less subcooling is recommended, and the new design with the PCM mass maldistribution can increase the cooling time by 28%. In addition, to extend the PCMHX application, a multi-tube PCMHX model was developed for general-purpose design. A new multi-tube heat transfer algorithm was proposed, and variable tube shape, connection, and topology for tubes and PCM blocks were considered. The comparison with other PCMHX models in the literature shows that the proposed model exhibits much higher flexibility and feasibility for comprehensive multi-tube configurations. The personal cooling system coupled with PCMHX could achieve energy savings for a range of 8-36% depending on the climate and building types in the U.S..

Title: High Wave vector acoustic metamaterials: Fundamental studies and applications

Date & Time: Wednesday, May 27th at 10:30 AM

Abstract: Acoustic metamaterials are artificially engineered structures with subwavelength unit cells that hold extraordinary acoustic properties. Their ability to manipulate acoustic waves in ways that are not readily possible in naturally occurring materials have garnered much attention from researchers in recent years. In this dissertation work, acoustic metamaterials that enable wave propagation with high wave vector values are studied. These materials render several key properties including energy confinement and transport, wave control enhancement, and enhancement of acoustic radiation, which are exploited for enhancing acoustic wave emission and reception.

The dissertation work is summarized as follows: First, to enable experimental studies of the deep subwavelength cavities in these metamaterials, a low dimensional fiber-optic probe was developed, which allows direct characterization of the intrinsic properties of the metamaterials without seriously disrupting the acoustic fields. Second, low dimensional acoustic metamaterials for enhancing acoustic reception were realized and studied. These metamaterials were demonstrated to achieve both passive and active functionalities including passive signal amplification and frequency filtering, as well as active tuning for switching and pulse retardation control. Third, a metamaterial emitter was realized and studied, which is capable of enhancing the radiative properties of an embedded emitter. Parametric studies enhanced the understanding of the effects of different geometric parameters on the radiation performance of the structure. Finally, the metamaterial emitter and receiver were combined to form a metamaterial-based sonar system. For the first time, the superior performance of the metamaterial enhanced sonar system over conventional sonar systems was analytically and experimentally demonstrated. As a proof of concept, a sonar robotic platform equipped with the metamaterial system was shown to
possess remarkably better tracking performance compared to the conventional system.

Through this dissertation work, an enhanced understanding of high-k acoustic metamaterials has been achieved and their applications in acoustic sensing, emission enhancement, and sonar systems have been demonstrated.