The analysis of size effects in nanostructured ferroic materials implies the characterization of the domain configurations and stability within the grains and their interaction with the grain boundaries, together with the study of local physical properties. In this project, NANOTEC will lead a joint project to apply PFM capabilities for unveiling the origin of size effects in nanograined lead-free films and multilayers prepared at ICMM-CSIC. The ESR will then spend some time at Tyndall UCC for studying lead-free materials with complementary methods and assessing the possibility of their applications in microsystems. The effects of the grain size on domain assemblages and polarization dynamics will be investigated and analyzed. The spatial distribution of polarization relaxation and switching parameters will be studied by using Switching Spectroscopy PFM to assess inhomogeneous hysteretic behaviour. Part of the work will be devoted to the instrumentation development at NANOTEC with the implementation of advanced measurement modes for PFM in accordance with the project needs. In particular, special attention will be paid to increase the lateral resolution of PFM in order to address domains even in smallest crystallites.
The project aims at piezoresponse force microscopy (PFM) investigation of electromechanical response in novel classes of lead-free ceramics demonstrating so far the best properties among lead-free materials. European and international legislation has driven research to replace the ubiquitous lead zirconate titanate, Pb(Zr,Ti)O3 (PZT) , for imminent health concerns. Lead-free solid solutions of (1-x)(Bi0.5Na0.5)TiO3 –xBaTiO3 (BNT-BT) provide outstanding electromechanical characteristics, as a results of an electric field induced transition from an ergodic relaxor (ER) to a ferroelectric (FE) phase. In addition, moderate doping improves the electro-mechanical response, yielding a giant field-induced unipolar strain, e.g. of 0.45% for 2% (K0.5Na0.5)NbO3 doped BNT-BT system. Despite the abundance of phenomenological information on the macroscopic electrostrictive and piezoelectric characteristics, understanding of the corresponding microscopic mechanisms is lacking. At the present stage, the understanding of why such large strains are achieved in the composite ends at the understanding that the ferroelectric phase due to its large polarization induces large local fields in its vicinity and the relaxor phase responds to these by large induced strains. Effects of composition, microstructure, grain boundaries and defects on the intrinsic piezoresponse and domain structure will be addressed by PFM in conjunction with other measurements. To elucidate the extrinsic contribution to the electromechanical response the electric field induced domain wall movement and phase transitions will be investigated in-situ. The PFM results will be analyzed in the context of data obtained by using other techniques including macroscopic piezoelectric and broad-band dielectric spectroscopy and structural measurements. The research will determine the links between macroscopic and local properties and should lead to significant improvement in the electromechanical response.
Bismuth layer-structured ferroelectric materials in the Aurivillius phase family have received increasing interest as lead-free piezoelectric materials with high Curie temperatures and fatigue-free switching for memories. The films will be grown by Tyndall UCC, Cork, and then studied in the University of Aveiro (PFM spectroscopy and hysteresis) and NANOTEC, Madrid, using advanced PFM tools. The layered nature of these materials also allows for the incorporation of magnetic ions in the B sites of the perovskite units, allowing cations that drive both ferroelectricity and ferromagnetism to occupy adjacent unit cells. Investigations of these candidate multiferroic materials will be conducted by simultaneous PFM/magnetic force microscopy studies as a function of temperature using Tyndall-UCC capabilities. The potential of the materials for applications in adverse environments will be evaluated by Tyndall UCC.
The functioning of PFM/ESM is intrinsically related to the understanding of coupled strain/stress and the electric field produced by the SPM tip. In this project at University of Duisburg-Essen, continuum mechanical modelling of the linear or non-linear electromechanically coupled material behaviour will be performed based on Hilbert´s theorem. For the computation of remanent quantities a thermodynamically consistent framework using a “switching hyper-surface” will be used. This model will be applied to the materials and structures used by NANOMOTION. Nonlinearity of the materials response will be treated by a classical penalty method or by the method of Nitsche. The change of the free charge carrier density at the outer boundary of the piezoelectric domain correlates with the change of the polarisation during the contact process measured in the other projects. Nano-heterogeneous materials like nanorods, nanowires, and nanotubes will be calculated, too. The prospective ESR will continue in NUID-UCD with the calculations of the properties of specific nanostructures offered by these institutions and will receive training in the application of the calculation results to emergent materials systems.