Piezoelectric composites based on structured ceramics and polymers
The development of new high-performance piezoelectric devices (sensors, acoustic transducers, etc.) requires the design of a new generation of materials which combine high electromechanical coupling, flexibility and modulable acoustic impedance. In the case of ultrasonic transducers, the commonly used piezoelectric materials are the macro-structured PbZrTiO3-polymer composites which combine the piezoelectric properties and the high acoustic impedance of ceramics with the flexibility properties and the low acoustic impedance of polymers. However, European directives aim to eliminate toxic substances such as lead (Pb) in electronic components. Thus, the ANR NanoPiC project aims to manufacture piezoelectric composite materials in thin films (< 1 µm) to be able to be integrated into devices, based on lead-free piezoelectric ceramics and electroactive polymers with a micro- or nano-metric structure and with a robust ceramic - polymer interface.
The fabrication of the composites was carried out by growth then etching of non-toxic inorganic thin films and organic grafting then polymer impregnation. (Bi0.5Na0.5)TiO3 (or BNT) appears to be a good candidate to replace PbZrTiO3 (or PZT, the reference material). Growth of BNT thin films was achieved by RF-magnetron sputtering. The fabrication of micrometric-sized pillars was then carried out by wet etching of these thin films. In order to improve adhesion at the ceramic / polymer interface, new adhesion promoter polymers based on piezoelectric polymer (PVDF or derivatives) or miscible with PVDF (based on PMMA) were synthesized by RAFT polymerization then functionalized by a catechol-based function having the ability to react with ceramic to create a robust interface. These polymers were then grafted to the surface of the ceramic pillars to form an interfacial layer. Finally, the grafted pillars were impregnated with a commercial fluorinated copolymer. The structure and properties of the materials have been characterized at different scales during the successive stages of the composite fabrication.
The main result of this project concerns the mastery of the different stages of structured piezoelectric composites manufacturing, carried out by impregnation of electroactive ceramic pillars with a piezoelectric polymer.
The fabrication of structured piezoelectric composites opens great prospects in the acoustic field with the modulation of the acoustic impedance according to the size of the ceramic domains relative to the polymer domains. One possible application of this new generation of composites relates to the medical environment with the integration of the latter in ultrasound probes with increased sensitivities.
The ANR NanoPiC project resulted in the publication of 7 scientific articles in journals with a high impact factor (with an average impact factor = 5.4) and 5 articles are currently being written or submitted.
NanoPiC project aims at creating innovative piezoelectric materials with enhanced properties, from ceramic-polymer structured composites. The structuration at different scales (from micron to nanometer) will be performed by thin film etching of BNT ceramic. The choice of these ceramics is based on the twofold aspect of non-toxicity (lead free) and the high piezoelectric properties (d33 = 80/120 pm/V). The passive or active fluorinated polymer (PVDF or PVDF-TrFe) will be incorporated by grafting from ceramic surfaces in order to increase the interfacial cohesion between the two materials. In polar crystalline phase, the fluoropolymers present the advantage to have a high piezoelectric coefficient compared to other polymer materials (|d33| = 20/30 pC/N). The combination of processes such as ceramic etching and polymer grafting for the fabrication of structured composites is an original and innovative approach.
In the case of PVDF, the structuration of domains from microns to nanometers present a high interest on the polymer crystalline structure and may imply a phase change (no polar/polar) induced by the confinement. The polymer initially passive can then become active.
The piezoelectric characterization of composites will be investigated at the macroscopic scale in order to probe the global piezoelectric answer of the material. In this case, the poling parameters (direction of the electrical field, time and temperature) will have a key role and will allow a parallel or anti-parallel poling of ceramic or polymer domains, i.e. a compensation or addition of piezoelectric properties. In addition, local piezoelectric properties will be performed by PFM (Piezoelectric Force Microscopy) to evaluate the impact of the size (micro- or nano-metric domains) and of the environment (for instance in the case of ceramic domain, the influence of passive or active polymer incorporation, influence of grafting...) of ceramic and polymer domains on the piezoelectric performances. The understanding of multi-scale piezoelectric behaviors of these structured composite materials is simultaneously a fundamental scientific challenge, and an innovative means to widen the scope of utilization of these materials for applications in pMUT (piezoelectric Micro machined Ultrasonic Transducers) area.
Madame Sophie Barrau (Unité Matériaux et Transformation)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
UCCS Université d'Artois - Unite de catalyse et Chimie du solide
UMET Unité Matériaux et Transformation
IEMN Institut d'Electronique de Microelectronique et de Nanotechnologie
ICGM Institut Charles Gerhardt Montpellier
Help of the ANR 500,579 euros
Beginning and duration of the scientific project: - 48 Months