Project position and novelty
Stress-induced transitions in ferroelectric materials
Aiming at stabilizing material characteristics, transitions are usually considered as undesirable. Works have thus been oriented towards limiting such effects. Temperature-induced transitions have been widely investigated, but a few works addressed effect of stress. Common composition consisted in Barium Titanate ([8, 9]) and Potassium Niobate ([10, 11]) as alternatives to PZT, showing ferroelectric-ferroelectric structural phase transitions in the proximity of ambient temperature. Required stress to ensure transition is several hundreds of MPa to Gpa however. Stress and temperature induced transitions are strongly thermodynamically linked through mechanisms at the material structural level. FIESTA thus targets materials with transitions near Room Temperature (RT). KTN (KTa1-xNbxO3) is of interest for both applications as its Trigonal (frequently called Rhombohedral, R) − Orthorhombic (O) - Tetragonal (T) transitions can be tuned close to RT and even below ([FEMTO 1], [12, 13]) while presenting good resistance to fatigue (). Relaxor ferroelectric crystals (PZN-PT) are a backup possibility, requiring stress as low as 100 MPa ().
Another aspect for energy harvesting consists in polymers (not for acoustic devices due to the need of high mechanical quality factor). PVDF and its derivative (e.g., P(VDF-TrFe-CFE), with low glass transition temperature and ferro-paraelectric transition near RT) are of interest to obtain low-stress transitions ([16, 17]). Their electromechanical activity is however limited compared to ceramics and crystals. Polymer composites ([LGEF 1]) enhance this coupling while allowing low stress transition. Stress-induced crystallization is besides expected to allow dramatic changes in the ferroelectric response.
Researches have been devoted to increase transition temperature and thus stress in the search of stable materials, and no work has gone towards their lowering for application into devices.
Tunable acoustic filter using electromechanical devices
Piezoelectric acoustic filters such as Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) devices are widely used in telecommunication due to their integration potentials and abilities to work at ultrahigh frequencies (), typically in GHz range, requiring microsystems preventing the use of magnetic devices, also prone to Electro-Magnetic Compatibility issues. Such filters consist for example in interdigitated piezoelectric devices converting back and forth an electrical signal into mechanical one, allowing high speed and selectivity thanks to the acoustic medium.
Meanwhile, with the multiplication of communication protocols (with soon 5G and IoT-dedicated protocols: SigFox, LoRa…) as well as spectrum spreading, means of making such filters tunable to reduce their number in a single terminal is mandatory. Recent works and projects aimed at enabling resonance frequency adaptation ([FEMTO 2], [FEMTO 3], ). Principles lie in modulating dielectric response through voltage, changing the mechanical properties thanks to piezoelectric coupling. Obtained tunability, related to the piezoelectric coupling coefficient, is still quite limited (even with high-performance dedicated materials - ) to less than 20% of the central frequency. Furthermore, such results consider low frequency devices. At high frequencies (6-9 GHz) required for 5G, the device electromechanical coupling is highly reduced and the tunability is extremely limited.
Novel aim of FIESTA is to provide frequency hopping characteristics thanks to the dramatic change of the dielectric constant enabled by the phase transition in targeted material (Figure 7)
Vibrational energy harvesting
Boosted by constant decrease of electronic device consumption (a sensing and communication cycle could be done with less than 1 mJ – ), modern energy harvesting has been developed for almost two decades. Among potential sources (), mechanical energy is of particular interest () due to its ubiquity, availability at small scale even in confined environments, and conversion potential of associated transducers. Piezoelectric effect using ferroelectrics presents good electromechanical coupling and integration potentials to this end (). Electrostatic devices, although requiring external polarization, provide very high integration and compatibility with MEMS process.
Enhancing the performance of Vibrational Electrical Harvesters (VEH) for additional functions or active parts reduction considered nonlinearities at several scales. One of the first lied in electrical interfaces (e.g., switching - [LGEF 2]). Recently, self-polarizing mechanisms using synchronous nonlinear converters have been unveiled (). Based on Bennet’s doubler (), these structures consist in a feedback loop triggering instability to obtain a virtuous cycle ([C2N 1], [C2N 2], [C2N 3], [C2N 4]). Two limitations however arise: (1) necessity of initial energy shoot and (2) minimal capacitance variation. Ferroelectrics will address these issues as (1) remnant polarization provides initial energy and (2) transitions ensure significant capacitance variation.
Ferroelectric transitions have been scarcely investigated for VEH. Closest evidence considered temperature-induced transitions ([LGEF/ELyTMaX 1], [LGEF/ELyTMaX 2], ) which, while staying at preliminary concept level with experiments, showed huge conversion potential improvement compared to linear materials. While FIESTA does not consider ferromagnetic devices (low integrability and electromagnetic noise sensitivity), recent works showed the benefit of stress-induced structural changes in magnetic moments for harvesting application ().
Following investigations of nonlinearities in electrical and mechanical domains (the latter not being considered here), it is expected that researchers will move deeper into nonlinearities, including material ones, with early works being very recently published considering other conversion effects. FIESTA proposes to be one of the first achievements of such a concept using KTN and polymers, providing cutting-edge founding publications. A by-product of the project encompasses both mechanical and thermal harvesting, providing multifunctional/multisource harvesters that are currently under high interest ().
FIESTA provides cutting edge innovations through intrinsic nonlinearities for energy harvesting, yielding magnification of power densities (Figure 10), while providing means of multifunctionality
Transdisciplinary approach for electroactive materials and devices
FIESTA’s ambition encompasses every step in the elaboration and fabrication of efficient electroactive devices for acoustic devices (SAW or BAW resonators) and energy harvesting, both using KTN (as well as ferroelectric polymers for energy harvesting), going from the material aspects to the system implementation, with scientific advances in all of these fields and at their interfaces. This transdisciplinary approach (e.g., multiscale nonlinearities and their interactions) corresponds to a recent trend in the field (FIESTA partners being very active in its promotion), leading for example to the following projects: BESTMEMS (self-powered sensors for transportation applications), LiLit (ferroelectric materials for acoustic filters), ENHANCE (self-powered sensors for automotive), HiPerTherMag (thermal microgenerator).
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