Thèse Frittage par Micro-Onde et Caractérisation de Matériaux Ferroélectriques Relaxeurs sans Plomb pour le Stockage de l'Énergie H/F

Doctorat.Gouv.Fr

Établissement : Université Polytechnique Hauts de France École doctorale : Ecole Doctorale Polytechnique Hauts-de-France Laboratoire de recherche : Laboratoire CERAMATHS - Département Matériaux et Procédés Direction de la thèse : Mohamed RGUITI ORCID 0000000168054898 Début de la thèse : 2027-01-01 Date limite de candidature : 2026-10-01T23:59:59 Les ferroélectriques relaxeurs sont une classe particulière de matériaux diélectriques caractérisés par une polarisation diffuse et dépendante de la fréquence, contrairement aux ferroélectriques classiques qui présentent une transition de phase nette. Ces relaxeurs présentent un désordre chimique au niveau du réseau cristallin ce qui conduit à la formation de nano-régions polaires (PNRs) qui fluctuent avec la température et la fréquence du champ électrique appliqué. Ils sont largement étudiés pour le stockage d'énergie électrique, notamment dans les condensateurs à haute densité d'énergie.
Les relaxeurs à base de plomb (PMN, PZN, PLZT) ont longtemps dominé en raison de leurs performances élevées. Cependant, les recherches actuelles se concentrent sur des alternatives sans plomb, telles que les systèmes à base de BaTiO modifié et les matériaux Bi.Na.TiO (BNT) et leurs solutions solides.
Le travail envisagé dans cette thèse concerne l'élaboration et l'optimisation de compositions à base de BNT dopé en mettant en évidence les relations structure-propriétés-performances énergétiques. Les matériaux développés seront mis en forme par pressage et par coulage en bande. Les couches épaisses (de quelques dizaines à quelques centaines de micromètres) peuvent en effet supporter des champs électriques relativement élevés donc une énergie récupérable plus forte. Des architectures multicouches, mono ou multi-matériaux, seront également envisagées ce qui permettra d'augmenter l'énergie stockée, d'améliorer la conduction locale, de réduire les pertes et d'optimiser les cycles d'hystérésis.
Le frittage micro-onde sera étudié dans ce travail afin de densifier ces matériaux et obtenir des microstructures fines, permettant l'augmentation du champ maximum de polarisation, tout en abaissant la température du frittage, ce qui limitera la volatilisation des éléments alcalins. En complément de l'approche chimique et de la modification de compositions, le frittage par micro-ondes pourrait favoriser la formation de PNRs plus petits et hautement dynamiques.
Relaxor ferroelectrics materials constitute a specific class of dielectrics characterized by the presence of dynamic nanopolar regions (PNRs), induced by chemical and structural disorder at the nanoscale [1]. Unlike conventional ferroelectrics, relaxors do not exhibit a sharp ferroelectric phase transition, but rather a diffuse, frequency-dependent transition, leading to a broad dielectric response over a wide temperature range [2].
From the perspective of electrostatic energy storage, these characteristics are particularly advantageous. The recoverable energy density depends directly on the shape of the polarization-electric field (P-E) hysteresis loop. Relaxing ferroelectrics generally exhibit high peak polarization combined with very low remanent polarization, and therefore narrow hysteresis, which allows for the simultaneous achievement of high energy density and high energy efficiency [3]. Efficiencies and energy densities exceeding 85-90% and 10 J·cm³ respectively have been reported in optimized relaxor ceramics and thin films [4]. Recent scientific advances based on several complementary strategies: chemical disorder engineering to stabilize typical relaxor behavior [2,3], high-entropy approaches aimed at delaying polarization saturation and increasing electric field strength, and the development of multiphase relaxor/antiferroelectric architectures to further reduce losses while maintaining high polarization [5]. In particular, relaxor thin films have achieved extremely high energy densities, around 100 J/cm³, thanks to their ability to withstand very strong electric fields [4].
From an economic and technological perspective, the development of high-performance materials for energy storage is closely linked to the global energy transition and the growth of intermittent renewable energies, smart grids, and power electronics. In this context, dielectric capacitors based on relaxor ferroelectrics occupy a strategic intermediate position between electrochemical batteries (high energy, low power) and supercapacitors (high power, low energy), offering ultrafast charge/discharge, excellent cyclic endurance, and high safety [3] by withstanding high electric fields, while exhibiting good thermal stability.
Although lead-based ferroelectric materials, such as Pb(Zr,Ti)O (PZT) or Pb(Mg/Nb/)O-PbTiO (PMN-PT), have long dominated dielectric applications due to their high performance (particularly in terms of maximum polarization, energy density, and energy efficiency), their toxicity and environmental impact currently represent a major obstacle to their industrial deployment. Strict environmental regulations (RoHS, REACH) mandate a drastic reduction, or even elimination, of lead use in electronic devices. In this context, lead-free ferroelectric relaxors appear as a strategic and sustainable alternative. Systems such as Bi.Na.TiO (BNT), (K,Na)NbO (KNN), and their modified solid solutions exhibit typical relaxor behavior, reduced hysteresis, and high induced polarization, perfectly meeting the requirements of capacitive energy storage [3,4]. Recent advances in doping and high-entropy design have made it possible to achieve, or even surpass, the energy performance of some lead-based systems [3].
From an industrial point of view, Relaxor ferroelectrics offer several key advantages: compatibility with conventional ceramic processes, potential for integration into large-scale production of multilayer ceramic capacitors (MLCCs), and the growing development of lead-free systems that comply with current environmental regulations. These advantages reinforce the economic interest of these materials for applications ranging from on-board electronics to high-power energy storage systems.
CERAMATHS has, in the past, collaborated with partners on the characterization of ferroelectric materials for energy storage [6-8].
The introduction of mixed phases (BaTiO, SrTiO), multicomponent solid solutions (BNT-BKT, BNT-KNN), or aliovalent dopants allows for fine-tuning of chemical disorder and the stability of highly dynamic polar nanoregions, thus enhancing the relaxor behavior [3, 4]. These strategies have led to substantial improvements in recoverable energy density, energy efficiency, and robustness under high electric fields [5]. In this context, the proposed work will focus on the main BNT-based systems, highlighting the structure-property-energy performance relationships, and adopting two main strategies: Microwave sintering and thick films elaboration.