Engineering 3D architected metamaterials for enhanced mechanical properties and functionalities.

Compared with conventional materials, architected metamaterials have shown unprecedented mechanical properties and functionalities applications. Featured with controlled introduction of porosity and different composition, architected metamaterials have demonstrated unprecedent properties not found in natural materials. Such design strategies enable researchers to tailor materials and structures with multifunctionalies and satisfy conflicting design requirements, such as high stiffness and toughness; high strength with vibration mitigation properties, etc. Furthermore, with the booming advancement of 3D printing technologies, architected materials with precisely defined complex topologies can be fabricated effortlessly, which in turn promotes the research significantly. The research objectives of this dissertation are to achieve the enhanced mechanical properties and multifunctionalities of architected metamaterials by integrative design, computational modeling, 3D printing, and mechanical testing. Phononic crystal materials are capable of prohibiting the propagation of mechanical waves in certain frequency ranges. This certain frequency ranges are represented by phononic band gaps. Formally, band gaps are formed through two main mechanisms, Bragg scattering and local resonance. Band gaps induced by Bragg scattering are dependent on periodicity and the symmetry of the lattice. However, phononic crystals with Bragg-type band gaps are limited in their application because they do not attenuate vibration at lower frequencies without requiring large geometries. It is not practical to build huge models to achieve low frequency vibration mitigation. Alternatively, band gaps formed by local resonance are due to the excitation of resonant frequencies, and these band gaps are independent of periodicity. Therefore, lower frequency band gaps have been explored mostly through the production of phononic metamaterials that exploit locally resonant masses to absorb vibrational energy. However, despite research advances, the application of phononic metamaterials is sill largely hindered by their limited operation frequency ranges. Designing lightweight phononic metamaterials with low-frequency vibration mitigation capability is still a challenging topic. On the other hand, conventional phononic crystals usually exhibit very poor mechanical properties, such as low stiffness, strength, and energy absorption. This could largely limit their practical applications. Ideally, multifunctional materials and structures with both vibration mitigation property and high mechanical performance are demanded. In this work, we propose architected polymer foam material to overcome the challenges. Beside altering the topological architecture of metamaterials, tailoring the composition of materials is another approach to enhance the mechanical properties and realize multifunctionalities. Natural materials have adopted this strategy for long period of time. Biological structural materials such as nacre, glass sea sponges feature unusual mechanical properties due to the synergistic interplay between hard and soft material phases. These exceptional mechanical performance are highly demanded in engineering applications. As such, intensive efforts have been devoted to developing lightweight structural composites to meet the requirements. Despite the significant advances in research, the design and fabrication of low-cost structural materials with lightweight and superior mechanical performance still represent a challenge. Taking inspiration from cork material, we propose a new type of multilayered cellular composite (MCC) structure composed of hard brittle and soft flexible phases to tackle this challenge. On the other hand, piezoelectric materials with high sensitivity but low energy absorption have largely limited their applications, especially during harsh environment where external load could significantly damage the materials. Enlightened by the multiphase composite concept, we apply this design motif to develop a new interpenetrating-phased piezoelectric materials by combining PZT material as skeleton and PDMS material as matrix. By using a facial camphene-templated freeze-casting method, the co-continuous composites are fabricated with good quality. Through experiment and simulation studies, the proposed composite demonstrates multifunction with exceptional energy absorption and high sensitivity. Based on the above experimental studies, we further propose to use topology optimization framework to obtain the composites with the best performance of multifunctionalities. Specifically, we will use the solid isotropic material with penalization (SIMP) approach to optimize the piezoelectric materials with multi-objectives of 1) energy absorption and 2) electric-mechanical conversion property. The materials for the optimization design will be elastic PZT as skeleton and elatic material PDMS as matrix. To enable the gradient search of objective function efficiently, we will use adjoint method to derive the shape sensitivity analysis

Optimal Material Selection for Transducers

When selecting an active material for an application, it is tempting to rely upon prior knowledge or commercial products that fit the design criteria. While this method is time effective, it does not provide an optimal selection. The optimal material selection requires an understanding of the limitations of the active material, understanding of the function, constraints and objectives of the device, and rigorous decision making method to ensure rational and clear material selection can be performed. Therefore, this work looks into the three most researched active materials (piezoelectrics, magnetostrictives and shape memory alloys) and looks at how they work and their difficulties. The field of piezoelectrics is vast and contains ceramics, plastics and cellular structures that couple the mechanical and electrical domain. The difficulty with piezoelectric ceramics is their small strains and the dependence of their coefficients on the ferroelectric domains. Giant magnetostrictives materials couple the mechanical and magnetic domains. They are generally better suited for low-frequency operations since they properties deteriorate rapidly with heat. Shape memory alloys are materials that couple thermal and mechanical domains. They have large strain but are limited in their force output, fatigue life and cycle frequency. Optimal material selection requires a formalized material selection method. In mechanical material selection, the formal material selection method uses function, constraints and objectives of the designer to limit the parameter space and allow better decisions. Unfortunately, active materials figures of merit are domain dependent and therefore the mechanical material selection method needs to be adapted. A review of device selection of actuators, sensors and energy harvesters indicates a list of functions, constrains and objectives that the designer can use. Through the analysis of these devices figures of merit, it is realized that the issue is that the simplification that the figures of merit perform does not assist in decision making process. It is better to use decision making methods that have been developed in the field of operational research which assists complex comparative decision making. Finally, the decision making methods are applied to two applications: a resonant cantilever energy harvester and an ultrasound transducer. In both these cases, a review of selection methods of other designers provides guidance of important figures of merit. With these selection methods in consideration, figures of merit are selected and used to find the optimal material based upon the designer preference

Leading the Charge in Bone Healing: Design of Compliant Layer Adaptive Composite Stacks for Electrical Stimulation in Orthopedic Implants

The overall aim of this research is to develop a robust, adaptable piezoelectric composite load-bearing biomaterial that when integrated with current implants, can harvest human motion and subsequently deliver electrical stimulation to trigger the natural bone healing and remodeling process. Building on the preclinical success of a stacked piezocomposite spinal fusion implant, compliant layer adaptive composite stacks (CLACS) were designed as a scalable biomaterial to increase efficiency of power generation while maintaining mechanical integrity under fatigue loading seen in orthopedic implants. Energy harvesting with piezoelectric material is challenging at low frequencies due to material properties that limit total power generation at these frequencies and brittle mechanical properties. Stacked generators increase power generation at lower voltage levels and resistances, but are not efficient at low frequencies seen in human motion. CLACS integrates compliant layers between the stiff piezoelectric elements within a stack, capitalizing on the benefits of stacked piezoelectric generators, while decreasing stiffness and increasing strain to amplify power generation. The first study evaluated CLACS under compressive loads, demonstrating the power amplification effect as the thickness of the compliant layer increases. The second study characterized the effect of poling direction of piezoelectric discs within a CLACS structure under multiaxial loads, demonstrating an additional increase in power generation when mixed poling directions are used to create mixed-mode CLACS. The final study compared the fatigue performance and power generation capability of three commercially fabricated piezoelectric stack generators with and without CLACS technology in modified implant assemblies. All configurations produced sufficient power to stimulate bone growth, and maintained mechanical strength throughout a high load, low cycle fatigue analysis, thus validating feasibility for use in orthopedic implants. The presented work in this dissertation provides a robust experimental understanding of CLACS and a characterization of how piezoelectric properties and composite structures can be tailored within the CLACS structure to efficiently generate power in low frequency, low impedance applications. The main motivation of this work was to develop a thorough understanding of CLACS behavior for implementation into medical implants to deliver therapeutic electrical stimulation and accelerate rate of bone growth, helping patients completely heal faster. However, the ability to tune composite stiffness by changing compliant material properties, type of piezoelectric material and poling direction, or volume fractions could benefit the energy harvesting potential in fields ranging from civil infrastructure to wind energy, to wearables and athletic equipment

Auxetic power amplification mechanisms for low frequency vibration energy harvesting

Energy harvesting from locally available small amplitude vibrations can struggle to generate sufficient power for wireless sensor nodes, which thereby constrains their use for structural health monitoring. This work discusses a selection of two-dimensional auxetic substrate designs used to increase a piezoelectric harvester’s power output by 2.18-14.5 times by concentrating the ambient strain energy into the piezoelectric material. The harvesters were modelled and their auxetic designs optimised in COMSOL before empirical testing under sinusoidal or dynamic strain oscillations. The investigated auxetic designs included re-entrant honeycombs, rotating squares, triangles and hexagrams, and -hole structures; the most effective of which was found to be the honeycomb design, with a gain of 5.66 and a raw output of 570 μW at 10 Hz, 100 με. This work also compared PZT (Lead Zirconate Titanate), LN (Lithium Niobate), and MFC (Macro-Fibre Composite) as materials for the active piezoelectric layer. The former was found to be detrimentally brittle but delivered the greatest output, while the LN was stronger but with a significantly lower output. The MFC was more flexible, with only a modest reduction in output compared to PZT, and was found to be the most viable of these materials for future research. A crucial issue during the design stages was appropriately modelling the mechanical losses associated with the bonding between substrate and piezoelectric material; this adhesion was modelled using thin elastic layers (TELs) to emulate each sample by comparing to its output. The value of the stiffness constant per unit area in these TELs was found to be consistent for each sample across a range of input excitations. These kinds of energy harvesters open up many new avenues for wireless self-powered structural health monitoring sensor nodes in infrastructure, buildings, and vehicles, where the ambient vibration energy would otherwise be too diffuse to harvest from.Engineering and Physical Sciences Research Council (EPSRC

Nanowire and Fiber Composite Electromechanical Sensor

Fiber or nanowire composites offer many benefits for piezoelectric sensor and actuator applications. Piezoelectric composite is comprised of piezoelectric ceramics lain in polymer matrix. The composite with the piezoelectric ceramics connected in one direction and the polymer in three directions is named as 1-3 composite. 1-3 composites are most ordinary used and the anisotropic alignment of PZT in the composite may substantially lower lateral piezoelectric coupling and increases the sensitivity of the transducer mechanically. Piezoelectric fiber composites are suitable for sensor applications, medical diagnostics and nondestructive testing. Single crystal zinc-oxide nanowires were synthesized through a simple hydrothermal route and subsequently mixed with polyimide matrix to form ZnO nanocomposites. Superimposed a.c. and d.c. electric fields were applied to microscopically tailor the alignment of ZnO nanowires in polyimide matrix to form anisotropic nanocomposites. Piezoresistive property of ZnO nanocomposite was investigated for strain sensor application. A large gauge factor was obtained from the monotonic uniaxial stress-strain experiment for this nanocomposite and it is much higher than that of ordinary metal strain sensor. A low frequency fiber composite vibration sensor was fabricated and experimentally studied. The global parameters of the composite were substituted into lumped and distributed element constituent equations for piezoelectric unimorph to theoretically predict the sensitivity and effective frequency response range of the vibration sensor. An experiment was carried out to validate the result from the theoretical model. The output voltage per unit input displacement keeps stable in a wide frequency range with a suitable damping ratio. This PZT fiber composite sensor was also applied for soft material strain measurement and soft biomaterial surface morphology and elastic modulus characterization. From the theoretical evaluation and experiment result, this strain sensor is suitable for strain measurement with high sensitivity and high softness. A rectangular breathing sensor and an annular breathing sensor were fabricated for breathing rate and depth monitoring. Both sensors were tested under different physiological conditions and measurement results could be utilized for precaution and monitoring of breathing diseases. Both of them are excellent for monitoring breathing rate and depth and be nice choices for daily use and diagnose purpose

Ferroelectrics

Ferroelectric materials exhibit a wide spectrum of functional properties, including switchable polarization, piezoelectricity, high non-linear optical activity, pyroelectricity, and non-linear dielectric behaviour. These properties are crucial for application in electronic devices such as sensors, microactuators, infrared detectors, microwave phase filters and, non-volatile memories. This unique combination of properties of ferroelectric materials has attracted researchers and engineers for a long time. This book reviews a wide range of diverse topics related to the phenomenon of ferroelectricity (in the bulk as well as thin film form) and provides a forum for scientists, engineers, and students working in this field. The present book containing 24 chapters is a result of contributions of experts from international scientific community working in different aspects of ferroelectricity related to experimental and theoretical work aimed at the understanding of ferroelectricity and their utilization in devices. It provides an up-to-date insightful coverage to the recent advances in the synthesis, characterization, functional properties and potential device applications in specialized areas

A comprehensive review of nanocomposite PVDF as a piezoelectric material: evaluating manufacturing methods, energy efficiency and performance

Given the escalating concerns surrounding high energy consumption during manufacturing and the environmental impact of piezoelectric materials, the pursuit of sustainable alternatives has emerged as a critical challenge in shaping our technological future. In light of this imperative, this review paper investigates the domain of polymeric piezoelectric materials, with a specific focus on Polyvinylidene fluoride (PVDF) as a promising avenue for sustainable piezoelectric materials with a low-energy production process. The primary objective of this investigation is to conduct a comprehensive assessment of the existing research on the manufacturing processes of polymeric piezoelectric materials to enhance piezoelectric properties while minimizing energy-intensive production techniques. Through rigorous evaluation, the effectiveness of each manufacturing method is scrutinized, enabling the identification of the most energy-efficient approaches. This review paper paves the way for sustainable development and advancement of piezoelectric technologies

Plasma engineering of microstructured piezo – Triboelectric hybrid nanogenerators for wide bandwidth vibration energy harvesting

We introduce herein the advanced application of low-pressure plasma procedures for the development of piezo and triboelectric mode I hybrid nanogenerators. Thus, plasma assisted deposition and functionalization methods are presented as key enabling technologies for the nanoscale design of ZnO polycrystalline shells, the formation of conducting metallic cores in core@shell nanowires, and for the solventless surface modification of polymeric coatings and matrixes. We show how the perfluorinated chains grafting of polydimethylsiloxane (PDMS) provides a reliable approach to increase the hydrophobicity and surface charges at the same time that keeping the PDMS mechanical properties. In this way, we produce efficient Ag/ZnO convoluted piezoelectric nanogenerators supported on flexible substrates and embedded in PDMS compatible with a contact–separation triboelectric architecture. Factors like crystalline texture, ZnO thickness, nanowires aspect ratio, and surface chemical modification of the PDMS are explored to optimize the power output of the nanogenerators aimed for harvesting from low-frequency vibrations. Just by manual triggering, the hybrid device can charge a capacitor to switch on an array of color LEDs. Outstandingly, this simple three-layer architecture allows for harvesting vibration energy in a wide bandwidth, thus, we show the performance characteristics for frequencies between 1 Hz and 50 Hz and demonstrate the successful activation of the system up to ca. 800 Hz.EMERGIA Junta de Andalucía programUniversity of Seville the VI PPIT-USICMS and the CITIUS from the University of Sevill