Enhancement of mechanical energy harvesting in flexible composites and thin films via versatile structures

Abstract

In the last half century, the formidable development trends of electronics has been portability and miniaturization. The near-future society concerns much smaller electronics than the size of a cell phone, like a smart phone, so that each person can already have many small electronic systems. Such small-sized electronics and their working modes largely reduces power consumption, which means that it is possible to use the energy harvested from our surroundings to operate them. It will become impractical if sensor networks must be powered entirely by batteries owing to a number of devices, large scale of distribution, and the difficulty of tracking and recycling to minimize environmental impacts, and possibly health hazards. Hence, power sources should be sustainable, independent, renewable, and maintenance-free for continuous operation of such small electronic units, which can be utilized widely for various chemical and biomolecular sensors, micro-electromechanical systems (MEMSs), nanorobotics, mobile environmental detectors, remote security, and even wearable electronics. Therefore, new technologies that can harvest energy from our living environment as self-sufficient micro/nano-power sources provide reasonable and possible solutions. This is a newly-emerging field ‘nanoenergy’, which is the applications of nanomaterials and nanotechnology for energy harvesting to power micro/nano-systems. It can be used to possibly replace batteries or at least extend the lifetime of them. In addition to targeting at the worldwide energy needs at large macro/plant-scale, many researchers have been developing the field of nanoenergy, to generate energy required for sustainable, independent, and maintenance-free operation of micro/nano-systems and mobile/portable electronics using diverse nanotechnologies. As first reported in 2006, various nanogenerators have been demonstrated using piezoelectric, triboelectric, and pyroelectric effects. The self-powering approaches developed here are a new paradigm of mechanical energy harvesting to improve performance and efficiency using nanotechnology for truly achieving renewable self-sufficient micro/nano-systems. In chapter 2, to overcome the size-related limitation the insufficient output performance of previous nanocomposite-based nanogenerator, the large-area nanocomposite generator (NCG) made of piezoelectric lead zirconate titanate (PZT) particles using a simple, low-cost, and scalable bar-coating method is demonstrated. The fabricated large-area PZT-based NCG device converted from human hand tapping motions to high-outputs of 100 V and 10 $\mu$ A which were used to directly power up the 12 commercial RGB LEDs without external energy or storage systems. In chapter 3, moreover, the high-output and lead-free NCG devices were fabricated by employing piezoelectric alkaline niobate-based particles (KNLN) and copper (Cu) nanorods filler. The lead-free flexible NCG made by a simple spin-casting method successfully converts mechanical energy to electricity up to 12 V and 1.2 $\mu$ A

these are higher than that of lead-free nanocomposite-based nanogenerators. Moreover, a large-area NCG device (30 cm × 30 cm) fabricated using the bar-coating method produced maximum output up to 140 V and 8 $\mu$ A (~0.5 mW). In chapter 4, a highly-stretchable and deformable nanogenerator is developed using the piezoelectric nanocomposite composed of PMN-PT particles, carbon nanotubes (CNTs), silicone rubber matrix and the very long silver (Ag) nanowire percolation network electrodes. This stretchable elastic-composite generator (SEG) can produce high-output electrical signals (voltage of ~4 V and current of ~500 nA) under stretching condition with stretching strain up to 200%. The noteworthy performance was achieved by using a rubbery-elastomeric piezoelectric elastic composite (PEC) and the very long nanowire percolation (VLNP) electrodes. The principles of good stretchability and well-distributed piezopotential generation were also studied utilizing finite element analysis (FEA). Our SEG could directly generate electrical energy by using various mechanical deformation (e.g., twisting, folding, crumpling, pressing), and drive commercial electronic units by stretching stimulation. This hyper-stretchable piezoelectric generator with efficient energy harvesting would open a route to self-powered stretchable electronics. In chapter 5, we present a high-performance, flexible nanogenerator using anisotropic $BaTiO_3$ (BTO) nanocrystals synthesized on an M13 viral template through the genetically programmed self-assembly of metal ion precursors. The filamentous viral template realizes the formation of a highly entangled, well-dispersed network of anisotropic BTO nanostructures with high crystallinity and piezoelectricity. Even without the use of additional structural stabilizers, our virus-enabled flexible nanogenerator exhibits a high electrical output up to ∼300 nA and ∼6 V, indicating the importance of nanoscale structures for device performances. This study shows the biotemplating approach as a facile method to design and fabricate nanoscale materials particularly suitable for flexible energy harvesting applications. In chapter 6, for realization of the high-efficient, mechanical flexible/stable, and lightweight energy harvester, the large-area PZT thin film was transferred onto flexible substrate by adopting the simple and practical laser lift-off (LLO) process

then, thin film-based nanogenerator was fabricated by employing the interdigitated electrodes (IDEs). During the regular bending/unbending motions, the fabricated nanogenerator harvested the high electric energies of ~140 V and ~10 ？A. In addition, to establish a fully-flexible light-emitting system, we fabricated the vertically structured light-emitting diodes (f-VLEDs) by anisotropic conductive film bonding and entire wafer etching. In this process, self-powered all-flexible electronic system with light emittance an be spontaneously achieved by the electricity produced from flexible thin-film generator by applying slight biomechanical energy without any externally applied energy storage. In chapter 7, the crystallographic characteristics such as crystal orientation, phase, and crystallinity are very crucial factors in piezoelectric materials for their electromechanical properties

however, it has been scarcely studied that the theoretical and ab initio approach for crystallographic modulations of piezoelectric thin films related to substrate-interfacial phenomena, which is hinder proper and practical implementation of high-performance piezoceramics for flexible energy harvesters, sensors, transducers, and miscellaneous. Here by applying a simple rock salt structured substrate, MgO wafer, chemically-unreactive to $PbZr_{1-x}Ti_xO_3$ (PZT), the polymorphic phase balance, crystallinity, as well as crystal orientation of PZT thin film at morphotropic phase boundary (MPB) can be firmly controlled to enhance the piezoelectricity. The newly-discovered crystallographic phenomena of PZT thin film are thoroughly interpreted by first principle physics modeling and energy formalism. MgO is rationally used for the laser lift-off (LLO) transfer of PZT thin film to fabricate a flexible energy harvester, similar to conventional sapphire $(Al_2O_3)$ wafers. The flexible textured PZT energy harvester shows higher performance than the flexional randomly-oriented PZT generator, and even the voltage, current and power densities are improved by 556 %, 503 %, and 822 %, respectively, in comparison with the previously-reported flexible single crystalline PMN-PZT device. Finally, our developed flexible generator is applied to vibrational energy harvesting in a traffic system, which clearly shows that atomic-scale designs can evoke big influences on gigantic-scale applications, such as transports and infrastructures. In chapter 8, we report a facile and robust route to nanoscale tunable triboelectric energy harvesters realized by the formation of highly functional and controllable nanostructures via block copolymer (BCP) self-assembly. Our strategy is based on the incorporation of various silica nanostructures derived from the self-assembly of BCPs to enhance the characteristics of triboelectric nanogenerators (TENGs) by modulating the contact-surface area and the frictional force. Our simulation data also confirm that the nanoarchitectured morphologies are effective for triboelectric generation.