Multifunctional Materials for Energy Harvesting and Sensing

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Virginia Tech

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.

Shape Memory Alloy, Carbon Nanotubes, Piezoelectric Materials, Nanotube yarns