Frequency and Temperature Dependent Mechanical Behavior of Three-Dimensional Cellular Structures of Graphene and Carbon Nanotubes: Role of Microscopic Interfacial Interaction
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Three dimensional (3D) cellular structures with high mechanical strength and stability are recently proven to provide a crucial platform for multifunctional application in various fields namely biomedical, soft robotics, actuation, sensing, supercapacitor, etc. For example, materials used for aerospace or automotive engineering require strong but lightweight structures with shock absorbing property to withstand any harsh mechanical vibrations. In this regard, graphene and carbon nanotubes (CNT) based 3D cellular architectures such as foam, hydrogels, aerogels, sponges and layered structures have attracted worldwide attention due to their promising mechanical properties. Structural interconnectivity along with additional advantages such as light weight, high porosity, large damping factor, high mechanical stability and compressibility have enabled these structures to serve as nanoscale building block for applications across multifunctional domains. However, there is always a trade-off between mechanical strength and energy absorption capability that leaves a vast scope to modulate the mechanical properties of 3D architectures, which are lightweight but mechanically strong, applicable for high performance and robust shock absorbers and actuators. The thesis focuses on the development of 3D cellular structures with two-dimensional graphene and one-dimensional CNT as building blocks and evaluation of the impact of microscopic interfacial interaction on compressive behavior under various physical conditions. Cellular structures were fabricated using graphene and CNT with an interface with polydimethylsiloxane (PDMS) polymer (a flexible supporting material). Detailed dynamic and quasi-static experiments were performed to gain insight into the macroscopic mechanical performance. Graphene-based 3D cellular structure (graphene foam) comprised of both graphene and polymer, demonstrated an enhancement in the mechanical strength, where both peak stress and storage modulus doubles as compared to pristine PDMS foam. Moreover, frequency dependent mechanical behavior further revealed an enhancement in the storage modulus and tan delta with the increase in driving frequency. Strain rate independent, highly reversible compressibility were measured up to several cycles demonstrating high mechanical stability. The study elucidated that the interaction between graphene and polymer plays a crucial role in enhancing the thermo-mechanical properties of the cellular structure. Furthermore, mechanical behavior of 3D graphene hydrogel revealed temperature and frequency dependent compressive behavior. The stiffness of the hydrogel was further tailored through encapsulation of iron-oxide nanoparticles to achieve an extraordinary enhancement in storage modulus (450%). The presence of encapsulated water within the hydrogel network also contributed largely to the enhanced mechanical strength. Moreover, CNT cellular structure with polymer interface revealed an interesting correlation between CNT-CNT interaction and its energy absorption capability. CNT network within the cellular structure primarily has two modes of interactions, namely short-range interaction (nodes) and long-range interaction (bundles). It was observed that the applied load on the structure was distributed through both the interactions, where nodes help in achieving high mechanical strength whereas bundles are responsible for energy absorption. The study reveals that unlike traditional foams, both the mechanical strength and the energy absorption enhanced simultaneously by 790% and 840% because of higher degree of CNT-CNT interactions. The finite element simulation showed that the individual CNT-CNT interaction plays a key role in modulating both the strength and the energy loss within the structure. A practical application of 3D structure is demonstrated by fabricating an outperforming actuator based on tri-layer structure of CNT, PDMS and cellulose polymer. The actuator utilizes the large mechanical flexibility of the structure showing an efficient conversion of electrical stimuli into mechanical motion. An actuation up to 14 mm was measured at a small input power density of 35 mW mm-3. Such actuation amplitude is nearly an order of magnitude higher than previously reported CNT-polymer based actuators. Moreover, the actuator showed a large load-handling capability, which was almost 25 times higher than the other thin-film based actuators reported earlier. The actuation performance was monitored under both dry as well as moist environment and demonstrated its suitability for applications in microrobotics, artificial muscle, microsensors, microtransducer, micromanipulator, microvalves, etc.