Molecular Piezoelectrics for Self-Healing and Flexible Energy Harvesters: Crystal Engineering

Bhunia, Surojit (2022) Molecular Piezoelectrics for Self-Healing and Flexible Energy Harvesters: Crystal Engineering. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Piezoelectricity is the interconversion between electrical potential and mechanical pressure, which arises in non-centrosymmetric crystalline materials. Piezoelectrics are essential for high-voltage sources, sensors, actuators, frequency standard, motors, vibration reducer, and other applications. All these applications involve mechanical agitation which often leads to formation of cracks and other types of defects at macro-to-microscopic level which can affect the structural integrity of the active materials, therefore, their performances. The durability of piezoelectrics can be boosted either by making them mechanically flexible or self-healing. Crystal engineering approaches have been utilized in many ways to tune and design various properties in crystals, and mechanical property design of molecular crystals is one of them. However, achieving self-healing property in crystals is difficult because they are typically stiff and rigid, and diffusion is difficult. Moreover, a lack of systematic studies and characterization tools adds to the complexity. In my second Chapter, I have demonstrated that our piezoelectric organic crystals, when mechanically fractured, reunite on their own by electrostatic attractive force and heal instantly with perfection. The healed crystals appear practically indistinguishable from their pristine state and show no sign of cracks. Using state of the art techniques, such as SCXRD, PFM, KPFM, Mueller matrix microscopy, we provide an in-depth molecular level mechanistic understanding of our observations. Our study overcomes a key obstacle in achieving the long-sought self-repairing behaviour in mechanically hard and crystalline matter. In my 3rd Chapter, I have explored another new self-healing and provided a full mechanism with design criteria by the help of state-of-the-art experiments and theoretical calculations. The identified design principles of non-centrosymmetry and crystal packing for favouring some degree of plastic deformation was evident from our study. One can use CCDC database to pick suitable systems with non-centrosymmetry and possible plasticity. In last two Chapters, I have demonstrated the fabrication of piezoelectric energy harvesting devices using self-healing and flexible crystals, respectively. Thereafter, electrical energy harvesting from mechanical force and realistic device application were demonstrated with good durability. In summery along with fundamental development of self-healing in piezoelectric organic crystals, I have also explored their proof-of-concept device applications for future technology.

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