Abstract
As the power consumption requirements for the electronics decreases, the notion of powering the electric circuits by harvesting the ambient energy deems more practical. Particularly, energy harvesters could be used for powering the next-generation “flexible” microelectronics capable of being stretched, bent, twisted, and deformed without affecting their normal functionality. So far, several energy harvesters have been developed by the researchers, however, less attention has been paid to the performance of the energy harvesters in harsh i.e., hot humid, environment. Thin film technologies, used to protect the MEMS energy harvesters, continue to play a key role in the development of stretchable electronics, flexible displays, and a host of other applications. Nevertheless, movable components can be exposed to cyclic loading in these applications, which can result in fatigue failure of the MEMS device, the thin film coating, or both. While the applied coating is expected to prevent the moisture and oxygen ingress into the energy harvester hence effectively improve its fatigue life in the harsh environments, it will also add such parameters to the design as the critical permeation thickness and will possibly move the crack initiation location to the bond line between the coating and the substrate. As such, it is important to characterize the fatigue behavior of the microstructures under different loading and environmental conditions. Hence, the investigation of fatigue degradation mechanisms of thin films is linked to the reliability concerns. Particularly, extreme stress gradients and stress ratio can occur for notched and unnotched components in bending mode. The geometry and the presence of extreme stress gradient under different stress amplitude necessitate characterizing the effect of extreme stress gradients and stress ratios on microstructurallysmall crack growth behavior to address the major reliability concerns for the thin film applications. The complex behavior of short cracks is due to less crack closure effects in short cracks as well as the interaction with the surrounding microstructural features in the materials such as grain boundaries, precipitates, inclusions, and phase boundaries. In addition to the experimental studies, finite element analysis has been employed to evaluate thermal stresses and temperature distribution in the coated energy harvesters during the fatigue test. This paper reviews the state-of-the-art in fabrication and characterization in flexible energy harvesters. The aim of this literature survey is to promote creation, development and application of novel experimental approaches to answer major and currently open questions on short fatigue mechanism of microstructural materials and also to provide a deeper understanding of mechanics of coated materials.
