Shape memory ceramics (SMCs) are burgeoning functional materials based on zirconia with a reversible, stress-inducible martensitic phase transformation. Compared to metallic shape memory alloys, SMCs have broader operating temperatures, higher critical stresses, and larger mechanical hysteresis loops. These advantages make SMCs attractive for high-output actuation and sensing in extreme environments or energy dissipation applications; however, the key phase transformation generates large stresses and strains that accumulate at grain boundaries and result in fracture of monolithic SMCs. This means that material forms with decreased mechanical constraint are necessary. Transformation without fracture has been previously demonstrated with SMC micropillars and individual microparticles, but these material forms lack useful applications. By utilizing easily scalable granular packings of discrete free particles, the transformation can be triggered in bulk without fracture in much the same way. The granular packing material form introduces significant complexity as the internal stress distributions responsible for the phase transformation are highly heterogeneous on the macro-, meso-, and micro-scales. Moreover, the unconstrained phase transformation behaves differently than the constrained transformation, which is more studied in zirconia. The interactions of these various factors are explored from a fundamental perspective in this work, notably including (1) a unique 'continuous mode' of both forward and reverse transformation in granular packings, (2) the dependence of transformation behavior on macro-, meso-, and microstructure, and (3) the evolution of the granular packings' structure and energy dissipation capacity over 10,000 loading cycles. Diverse experimental techniques are employed, ranging from mechanical testing and calorimetry to in situ neutron diffraction, to support theory based on the martensitic phase transformation in zirconia, the shape memory and superelastic effects, and granular material physics. / Doctor of Philosophy / Shape memory materials are capable of remembering their original shape even when they are deformed, and can return to that shape when they are heated. This unique property stems from a phenomenon called martensitic phase transformation which bridges the gap between microscopic structural changes and macroscopic shape changes as a response to specific environmental changes. Most of the common shape memory materials are metallic, like nitinol (NiTi), which has uses in orthodontic wires and cardiological stents, but there are also ceramic materials that can display the shape memory effect. These shape memory ceramics are based on zirconia (ZrO2), and are distinct from metallic shape memory materials because of their brittle behavior and high temperature stability owing to their chemical structure. The work presented in this thesis concerns the behavior of shape memory ceramics in granular form (i.e., loose powders) over a range of external conditions. Diverse experimental techniques are employed to investigate differences between granular and non-granular shape memory ceramics. This work shows how the unique structure of a granular material, which is dominated by highly uneven force distributions and microscopic effects, interacts with the martensitic phase transformation in shape memory ceramics to produce a 'continuous' mode of transformation that differs from non-granular shape memory materials. This continuous mode is itself dependent on the granular material's macro-, meso-, and micro-structure, and on the shape memory material's composition and history. In the future, shape memory ceramics might leverage the insights gained from this work for applications including energy dissipation or on-demand shape changes (i.e., actuation).

Yes / We have performed shrinkage restraint force measurements on three shape memory polymers of polyethylene terephthalate (PET), polymethyl methacrylate (PMMA) and a blend of the two at a range of temperatures. Observations are made of the change in stress during temperature rise, hold and cooling. All materials show an increase in stress during rise and hold, but on cooling the three materials behave differently; the PET shows a drop in stress, the PMMA a rise and the blend a much smaller rise. This behaviour correlates with the reversible thermal dimensional change at below the shrinkage threshold temperature; the expansion coefficients are negative for PET, positive for PMMA and positive at a lower order of magnitude for the blend. We model the behaviour by supposing that the shrinkage forces are created by prestressed strains effective at long range within a matrix of shorter chains effective at short range. The total stress is the sum of the shrinkage stress and the thermal stress in the matrix. The drops in stress on cooling are modelled using an elastic analysis based on measured elastic moduli and thermal expansion coefficients. For the blend, downward jumps in temperature produce small transient increases in the total stress, leaving it effectively unchanged. This phenomenon and the results of the elastic model for the stress drops imply that the shrinkage stress from the long-range chain network is largely unaffected by the temperature change, and so is not entropic.

Surgical procedures conducted without proper guidance and dynamic feedback mechanism could lead to unintended consequences. In-vivo diagnostics and imaging (the Gastro-Intestinal tract) has shown to be inconvenient for the patients using traditional endoscopic instruments and often these conventional methods are limited in terms of their access to various organs (e.g. small intestines). Embedding sensors inside the living body is complex and further the communication with the implanted sensors is challenging using the current RF technology. Additionally, continuous replacement and/or batteries recharging for wireless sensors networks both in-vivo and ex-vivo adds towards the complexity. Advances in diagnostics and prognostics techniques require development at multiple levels through systems approach, guided by the futuristic intelligent decision making environment that reduces the human interference. The demands are not only at the component level, but also at the connectivity of the components such that secure, sustainable, self-reliant, and intelligent environment can be realized. This thesis provides important breakthroughs required to achieve the vision of intelligent healthcare environment. The research contributions of this thesis provide foundation for developing a new architecture for continuous medical diagnostic and monitoring. The chapters in this thesis cover four fundamental technologies covering the in-vivo imaging, ex-vivo imaging, energy for sensors, and acoustic communication. These technologies are: locomotion mechanism for wireless capsule endoscope (WCE), multifunctional image guided surgical (MIGS) platform, shape memory alloy (SMA) thermal energy harvester and thermo-acoustic sonar using carbon nanotube (CNT) sheets. First, two types of locomotion mechanisms were developed, the first one inspired by millipede legged type mechanism and the second one based on the traveling waves that were induced onto the walls of the WCEs through vibration. Both mechanisms utilize piezoelectric actuators and couple their dynamics and actuation capability in order to achieve propulsion. This controlled locomotion will provide WCE advantage in terms of conducting localized diagnostics. Next, in order to conduct ex-vivo surgical procedures using the OCT such as removing the unwanted tissue and tumors short distance beneath the skin, MIGS platform was developed. The MIGS platform is composed of three key elements: optical coherence tomography (OCT) probe, laser scalpel and high precision miniature scanning and positioning stage. The focus in this dissertation was on design and development of the programmable scanning and positioning stage. The combination of in-vivo tool such as WCE and ex-vivo tool such as MIGS will provide opportunity to conduct many non-invasive procedures which will save time and cost. In order to power the feedback sensors that assist in remote operation of surgical procedures and automation of the diagnostic algorithms, an energy harvester technology based on the SMA thermal engine was designed, fabricated, and characterized. A mechano-thermal model for the overall SMA engine was developed and experimentally validated. Finally, the thermo-acoustic sound generation mechanism using CNT sheets was investigated with the goal of developing techniques for acoustic localization of WCE and customized sound generation devices. CNT thermo-acoustic projectors were modeled and experimentally characterized to quantify the dynamics of the system under varying drive conditions. The overall vision of this thesis is to lay down the foundation for intelligent healthcare environment that provides the ability to conduct automated diagnostics, prognostics, and non-invasive surgical procedures. In accomplishing this vision, the thesis has addressed several key fundamental aspects of various technologies that will be required for implementing the automation algorithms. / Ph. D.

Aerospace structures have stringent pointing and shape control requirements during long-term exposure to a hostile environment with no scheduled maintenance. This makes them excellent candidates for a smart structures approach as current passive techniques prove insufficient. This study investigates the feasibility of providing autonomous dimensional control to aerospace structures by embedding shape memory alloy elements inside composite structures. Increasing volume fractions of nitinol wire were embedded in cross-ply graphite/ epoxy composite panels. The potential of this approach was evaluated by measuring the change in longitudinal strain with increasing temperature and volume fraction. Reduction of thermal expansion is demonstrated and related to embedded volume fraction. Classical lamination theory is used to formulate a two-dimensional model which included the adaptive properties of the embedded nitinol. The model was used to predict the increased modulus and reduction of thermal strain in the modified plates which was verified by the experimental data. / Master of Science

This dissertation investigates the fundamental behavior of multifunctional materials for energy conversion. Multifunctional materials exhibit two or more functional properties, such as electrical, thermal, magnetic etc. In this dissertation, the emphasis is on understanding the principles for energy conversion from one domain to another (e.g. thermal to electrical; or mechanical to electrical) by utilizing nanomaterials and nanostructured materials such as carbon nanotubes, shape memory alloy (SMA), and flexible piezoelectric materials. Carbon nanotubes (CNTs) are known for their unique electrical and thermal properties. Development of solid-state suspended CNT sheets having extremely low heat capacity per unit area opens an opportunity for utilizing thermoacoustic phenomenon (electrical to thermal to acoustic energy conversion) that results in sound generation over a wide range of frequencies. Detailed theoretical modeling and experiments were conducted for understanding the acoustics generation from multi-wall carbon nanotubes (MWNTs) sheets. The sound pressure level (SPL) of CNT-based thermoacoustic projector (TAPs) is proportional to the frequency and hence the performance reduces in low frequency (LF) region which could be used for noise cancellation, SONAR and oceanography applications. Extensive analytical modeling in conjunction with experiments were conducted involving structure-fluid-acoustic interaction to determine the operational physical behavior of TAPs. Numerical model combines all the controlling steps from power input to acoustic wave generation to the propagation in outer fluid media. Power input to the computational domain is used to determine the frequency dependent thermal diffusive length which governs the generation of TA wave. MWNT yarns/fibers/threads were also designed to harvest ocean wave energy (mechanical to electrical energy conversion). These yarn-based harvesters electrochemically convert tensile or torsional mechanical energy into electrical energy without requiring an external bias voltage. Harvesters were developed by spinning sheets of forest-drawn MWNTs into high-strength yarns. SMA wires exhibit two unique properties: thermally induced martensite to austenite phase transformation and super-elasticity (stress-induced martensitic transformation). These properties were implemented for developing the low-grade thermal energy harvesters (thermal to electrical energy conversion). More than half of the energy generated worldwide is lost as unused thermal energy because of the lack of efficient methodology for harnessing the low-grade heat. A systematic study is presented here that takes into account all the key steps in thermal to electrical conversion such as material optimization, thermal analysis and electrical conditioning to deliver the efficient harvester. Next using thin sheets of piezoelectric materials, strain energy harvesting from automobile tires is studied (strain to electrical conversion. Flexible organic piezoelectric material was utilized for transduction in the harvester for continuous power generation and simultaneous sensing of the variable strain experienced by tire under different driving conditions. Using sensors mounted on a real tire of a mobile test rig, measurements were conducted on different terrains with varying normal loads and speeds to quantify the sensitivity and self-powered sensing operation. / Doctor of Philosophy / This dissertation studies the potential of carbon nanotubes yarns and sheets, piezoelectric sheets and shape memory alloy wires for energy conversion applications. Multiwalled carbon nanotubes (MWNTs) are known for their unique electrical and thermal properties. Large surface area, solid state self-suspended carbon nanotube sheets having extremely low heat capacity per unit area were utilized for design of thermoacoustic projectors operating over a wide range of frequencies. Detailed numerical modeling and experiments were conducted for understanding the acoustics generation from MWNT sheets. Another potential application for MWNT yarns is in ocean wave energy harvesting, where these yarn based harvesters convert tensile mechanical energy into electrical energy. Harvesters were developed by spinning sheets of MWNTs into high-strength yarns. SMA exhibits unique phase change behavior on mechanical and thermal loading, which were utilized for converting low-grade thermal energy into electrical energy. At low temperature gradients, where there is lack of methodologies for converting thermal energy into electrical energy, SMA wire-based energy harvesters are shown to provide ultra-high power density. Extensive experimentation in conjunction with multi-physics modeling is conducted to provide understanding of energy losses occurring during the thermal to electrical conversion. Lastly, this dissertation investigates the mechanical to electrical conversion using organic piezoelectric materials. Self-powered strain sensing mechanism for autonomous vehicle will provide new capabilities in monitoring the dynamics and allow developing additional automated controls to assist the driver performance.

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Yes / Here in, we describe a non-covalent (ionic interlocking and hydrogen bonding) strategy of self-healing in a covalently crosslinked organic-inorganic hybrid 15 nanocomposite hydrogel, with special emphasize on it's improved mechanical stability. The hydrogel was prepared via in-situ free radical polymerization of sodium acrylate (SA) and successive crosslinking in the presence of poly(2-(methacryloyloxy)ethyl trimethyl ammonium chloride) (PMTAC) grafted cationically armed starch and organically modified montmorillonite (OMMT). This hydrogel shows stimuli triggered self-healing following damage in both neutral and acidic solutions (pH=7.4 and pH=1.2). This was elucidated by tensile strength and rheological analyses of the hydrogel segments joined at their fractured points. Interestingly this hydrogel can show water based shape memory effects. It was observed that the ultimate tensile strength (UTS) of the self-healed hydrogel at pH = 7.4 was comparable to extensor digitorum longus (EDL) muscle of the New Zealand white rabbit. The as synthesized self-healable hydrogel was found to be non-cytotoxic against NIH 3T3 fibroblast cells. / Medical Research Council (MRC (MR/N501888/2))

Yes / The shrinkage force exerted by restrained shape memory polymers can potentially be used to close cracks in structural concrete. This paper describes the physical processing and experimental work undertaken to develop high shrinkage die-drawn Polyethylene Terephthalate (PET) shape memory polymer tendons for use within a crack closure system. The extrusion and die-drawing procedure used to manufacture a series of PET tendon samples is described. The results from a set of restrained shrinkage tests, undertaken at differing activation temperatures, are also presented along with the mechanical properties of the most promising samples. The stress developed within the tendons is found to be related to the activation temperature, the cross-sectional area and to the draw rate used during manufacture. Comparisons with commercially-available PET strip samples used in previous research are made, demonstrating an increase in restrained shrinkage stress by a factor of two for manufactured PET filament samples. / Thanks must go to the EPSRC for their funding of the Materials for Life (M4L) project (EP/K026631/1) and to Costain Group PLC. for their industrial sponsorship of the project and author.

The overarching goal of this research was to generate a menu of shape memory alloys (SMAs) actuator materials capable of meeting the demands of aerospace applications. Material requirements were recognized to meet the demand for high temperature SMAs with actuating temperatures above 85 °C and provide material options capable of performing over 100K actuation cycles. The first study is a preliminary characterization for the down selection of Ni-rich NiTiHf15 compositions chosen for a more in-depth examination of the nano-precipitation and evolution of the H-phase. To make this selection, the effect of Ni content in Ni-rich NiTiHf high temperature shape memory alloys (HTSMAs) on processability, microstructure, and hardness was analyzed for three compositions (Ni50.1TiHf15, Ni50.3TiHf15, Ni50.5TiHf15). Each composition was characterized under three conditions: homogenized, 25%, and 50% thickness reduction through hot-rolling. The second study emphasized the processing and aging response of an industrially produced, hot-extruded Ni50.3Ti29.7Hf20 (at%) HTSMA. The samples were sectioned into two halves with half remaining as-extruded and the other half hot-rolled to a 25% reduction in thickness. A portion of both conditions underwent conventional aging for 3 hours at various temperatures ranging from 450-750 °C, and the other portion was pre-aged for 12 hours at 300 °C followed by conventional aging treatments. After processing, the samples were characterized by differential scanning calorimetry (DSC), Vickers hardness (HV) testing, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron radiation X-ray diffraction (SR-XRD). The relationship between the introduction of texturing, pre-aging, and aging on Ni-rich and high Hf-content compositions was investigated.