Results
FIESTA's multi-scale, multidisciplinary and multiphysics approach has demonstrated the interest of stress-induced phase transitions for electromechanical conversion. The project thus uncovered new properties and potential for the application of materials, guided by specifically developed analysis and characterization tools. Combined with the broad spectrum of expertise of the project consortium, applications to energy harvesting and acoustic filters were demonstrated.
Materials
FIESTA targeted KTN because it has transitions close to room temperature. Different compositions of KTNs were studied in order to have an overview of the characteristics in the different crystallographic structures. The associated characterizations (XRD, Raman, EDS, SEM, Brillouin) confirmed the possibility, through the selection of a suitable composition, to obtain a phase transition around 20°C, facilitating the effect of a transition by applying mechanical stress.
Modeling and characterization
A modelling approach based on Landau-Devonshire theory was implemented, including the effect of temperature, electric field and stress. Its validation through a specific bench made it possible to estimate the energy densities using Ericsson cycles on a panel of materials. The use of the nonlinear regime and phase transitions showed very high energy densities (>100 mJ/cycle), as well as a convergence effect of performance under high solicitations.
Application to energy harvesting
A mechanical system for converting the input force exerted by a person walking on the prototype into a large mechanical stress showed an output energy of up to two decades above the typical energies in the literature. Specifically designed electrical interfaces have made it possible to maintain this performance in terms of harvested energy. Other approaches have also been considered, such as an impact system operating at high frequency.
Application to acoustic filters
A device based on interdigitated electrodes has been developed. Characterization indicates an extremely high TCF (Temperature Coefficient of Frequency) compared to other acoustic materials, and confirmed a significant frequency hopping effect induced by phase transitions. To obtain the latter by the application of stress, the resonator was assembled by gluing on the surface of a hardened steel specimen, whose straining allows the stress to be applied to the material.
Perspectives
FIESTA allowed the identification of new avenues of research in each of the involved disciplines, but also at the level of the interfaces between these fields, in a global scientific and application vision.
The study of the materials showed significant differences between low and high levels (electric field and stress), paving the way for future analyses and developments to precisely identify the phenomena in order to be able to select the best suited materials to given conditions. The interaction of three fields of physics (mechanical, electrical and thermal) on phase transitions is a great opportunity in the context of multi-source recovery, where it is important to ensure that the synergy between these sources occurs. When it comes to the development of KTN materials, the complexity and time required to complete such a study should not be underestimated, aiming at obtaining materials with a very high density of converted energy per cycle.
Beyond the composition of the materials, being able to use the material in different orientations would allow placing oneself in different configurations depending on the mechanical, thermal and electrical conditions of excitation.
The device for stressing acoustic filters will allow obtaining precise experimental results concerning the levels of strain necessary to obtain large frequency jumps thanks to mechanically induced phase transitions, research on which we have found no trace in the literature. This will lead to the definition of appropriate pathways to a fully integrated system.
The design of filters based on potassium niobate (KN) could be considered, as this material not only offers excellent thermal stability in terms of frequency in the vicinity of the phase transition, but also very high electromechanical coupling. Notably, the development of thin films of KN is a promising, albeit tricky, avenue for large-scale production.
Based on the energy harvesting demonstrator developed in FIESTA, the next step may be to envision smaller-scale devices. This may include the miniaturization of stress amplification (e.g. by 2D lever arms), the use of bending or via impacting systems, for which a preliminary test bench has been developed in the framework of the project.
The electrical interface has contributed to unprecedented converted energy levels, thanks to the use of self-synchronous converters. The extension of the architectures developed in FIESTA is an attractive path. New approaches combining triangular and rectangular cycles, studied in the framework of FIESTA, constitute very promising new possibilities, combining performance and simplicity.