Lokesh Dhakar

Development of a Broadband Triboelectric Energy Harvester With SU-8 Micropillars

This paper describes a broadband energy harvester working on the principle of contact electrification or triboelectric charging. Design and fabrication of the device have been discussed. The device uses contact and separation mechanism using a cantilever to generate triboelectric charges. This mechanism introduces nonlinearity in the cantilever, which results in broadband behavior of triboelectric energy harvester. The device uses SU-8 micropillar arrays to enhance the triboelectric charging. A study is conducted to study the effect of the micropillar sizes on the power output of devices. The devices were tested at different acceleration levels. The peak power output achieved is 0.91 μW at an acceleration of 1g. The amplitude limiter based design of the energy harvester enables broadening of operating bandwidth as the acceleration level increases. A maximum operating bandwidth of 22.05 Hz was observed at 1.4g increasing from an operating bandwidth of 9.43 Hz at 0.4g.

Large Scale Triboelectric Nanogenerator and Self-Powered Pressure Sensor Array Using Low Cost Roll-to-Roll UV Embossing

Triboelectric nanogenerators (TENGs) have emerged as a potential solution for mechanical energy harvesting over conventional mechanisms such as piezoelectric and electromagnetic, due to easy fabrication, high efficiency and wider choice of materials. Traditional fabrication techniques used to realize TENGs involve plasma etching, soft lithography and nanoparticle deposition for higher performance. But lack of truly scalable fabrication processes still remains a critical challenge and bottleneck in the path of bringing TENGs to commercial production. In this paper, we demonstrate fabrication of large scale triboelectric nanogenerator (LS-TENG) using roll-to-roll ultraviolet embossing to pattern polyethylene terephthalate sheets. These LS-TENGs can be used to harvest energy from human motion and vehicle motion from embedded devices in floors and roads, respectively. LS-TENG generated a power density of 62.5 mW m−2. Using roll-to-roll processing technique, we also demonstrate a large scale triboelectric pressure sensor array with pressure detection sensitivity of 1.33 V kPa−1. The large scale pressure sensor array has applications in self-powered motion tracking, posture monitoring and electronic skin applications. This work demonstrates scalable fabrication of TENGs and self-powered pressure sensor arrays, which will lead to extremely low cost and bring them closer to commercial production.

Broadband Energy Harvester Using Non-linear Polymer Spring and Electromagnetic/Triboelectric Hybrid Mechanism

Over the years, several approaches have been devised to widen the operating bandwidth, but most of them can only be triggered at high accelerations. In this work, we investigate a broadband energy harvester based on combination of non-linear stiffening effect and multimodal energy harvesting to obtain high bandwidth over wide range of accelerations (0.1 g–2.0 g). In order to achieve broadband behavior, a polymer based spring exhibiting multimodal energy harvesting is used. Besides, non-linear stiffening effect is introduced by using mechanical stoppers. At low accelerations (<0.5 g), the nearby mode frequencies of polymer spring contribute to broadening characteristics, while proof mass engages with mechanical stoppers to introduce broadening by non-linear stiffening at higher accelerations. The electromagnetic mechanism is employed in this design to enhance its output at low accelerations when triboelectric output is negligible. Our device displays bandwidth of 40 Hz even at low acceleration of 0.1 g and it is increased up to 68 Hz at 2 g. When non-linear stiffening is used along with multimodal energy-harvesting, the obtained bandwidth increases from 23 Hz to 68 Hz with percentage increment of 295% at 1.8 g. Further, we have demonstrated the triboelectric output measured as acceleration sensing signals in terms of voltage and current sensitivity of 4.7 Vg−1 and 19.7 nAg−1, respectively.

Microelectromechanically tunable multiband metamaterial with preserved isotropy

We experimentally demonstrate a micromachined reconfigurable metamaterial with polarization independent characteristics for multiple resonances in terahertz spectral region. The metamaterial unit cell consists of eight out-of-plane deformable microcantilevers placed at each corner of an octagon ring. The octagon shaped unit cell geometry provides the desired rotational symmetry, while the out-of-plane movable cantilevers preserves the symmetry at different configurations of the metamaterial. The metamaterial is shown to provide polarization independent response for both electrical inductive-capacitive (eLC) resonance and dipolar resonance at all states of actuation. The proposed metamaterial has a switching range of 0.16 THz and 0.37 THz and a transmission intensity change of more than 0.2 and 0.7 for the eLC and dipolar resonances, respectively for both TE and TM modes. Further optimization of the metal layer thickness, provides an improvement of up to 80% modulation at 0.57 THz. The simultaneously tunable dual band isotropic metamaterial will enable the realization of high performance electro-optic devices that would facilitate numerous terahertz applications such as compressive terahertz imaging, miniaturized terahertz spectroscopy and next generation high speed wireless communication possible in the near future.

Periodic Array of Subwavelength MEMS Cantilevers for Dynamic Manipulation of Terahertz Waves

We experimentally demonstrate the active manipulation of terahertz (THz) waves using a periodic array of electrostatically actuated subwavelength microelectromechanical system cantilevers, which effectively behave like a metamaterial. The design methodology for achieving desired ON- and OFF-state resonance frequencies through electromechanical optimization is presented. The microcantilever metamaterial has a switching range of 0.29 THz and a modulation depth of 60% at 0.59 THz. Utilizing metal layer thickness to optimize the devices, an improvement of 40% is achieved in switching range. The microcantilever metamaterials are highly miniaturized, extremely scalable, and electrically controlled with attractive electro-optic performance. Multiple cantilevers can be placed in a desired fashion to form complex unit cell geometry to realize advanced THz manipulation, such as polarization switching, bandwidth tunable filters, multicolor imagers, and so on.

A Wideband Triboelectric Energy Harvester

Contact electrification or triboelectric charging has mostly been seen as a negative phenomenon in various engineering applications. In electronic applications, static charge buildup may even destroy some of the components. In this paper, we demonstrate a cantilever based design using triboelectric charging mechanism for energy harvesting applications. The voltage output of triboelectric energy harvester (TEH) continuously increases with increased mass applied force and proof mass loading on the cantilever. The voltage output increases from 0.4 V to 1.8V as the excitation acceleration increases from 0.4g to 1.8g. The design has been shown to demonstrate wide bandwidth characteristics. The FWHM increases from 3.76 to 12.64 Hz as the acceleration increases from 0.4g to 1.8g. The peak power output is found to be 0.18μW at 1.4g.

Skin based flexible triboelectric nanogenerators with motion sensing capability

This paper presents a novel triboelectric nanogenerator (TENG) using outermost layer of human skin i.e. epidermis as an active triboelectric layer for device operation. The device is demonstrated to generate an open circuit voltage of ~90V with mild finger touch. The device uses PDMS nanopillar structures to improve the performance of contact electrification process which causes the charging of two surfaces. The device is demonstrated as a wearable self-powered device which can be used as a motion and activity sensor for a variety of applications.

Ultra-broadband electromagnetic MEMS vibration energy harvesting

This paper presents the design, fabrication and characterization of an electromagnetic MEMS energy harvester with ultra-broad operating bandwidth. The beam stretching induced nonlinear spring stiffness enables the resonance to extend from 65 Hz to 340 Hz and 400 Hz at accelerations of 0.5g and 1.0g, respectively. The ultra-wide bandwidth could benefit the harvester device for more efficient vibration energy harvesting.

Study of Effect of Topography on Triboelectric Nanogenerator Performance Using Patterned Arrays

Triboelectric effect has been recently demonstrated as a potential method to harvest mechanical energy from various sources in the surroundings which will otherwise be go wasted. Triboelectric effect is a type of contact electrification that results in the generation of surface charge as two different materials are put in contact with each other. The charging of surfaces occurs due to difference in tendencies of the materials to attract and donate electrons. Different tendencies of materials to attract or donate electrons arises due to chemical property of material known as electronegativity.

Triboelectric Devices for Power Generation and Self-Powered Sensing Applications

Introduction -- Overview of Energy Harvesting Technologies -- Study of Effect of Topography on Triboelectric Nanogenerator Performance Using Patterned Arrays -- Skin Based Self-Powered Wearable Sensors and Nanogenerators -- Large Scale Fabrication of Triboelectric Energy Harvesting and Sensing Applications -- Triboelectric Mechanism for Bidirectional Tactile Sensing and Energy Harvesting -- Conclusion and Recommendations for Future Work.