The research on piezoelectric materials have been in a rising curve during the last decades. Indeed, this type of materials is very interesting because of their ability to convert mechanical energy into electrical energy and vice versa. A large spectrum of applications can be targeted for piezoelectric materials, such as energy harvesters, pressure sensors, micro-actuators, etc. In accordance with the European directive on hazardous substances (RoHS), the future materials to be used in piezoelectric devices should be lead-free. Thus, an intense work is done in order to find potential candidates to replace high performance PZT (lead containing) at least for some applications.
Technically, the elaboration of piezoelectric energy harvesting devices requires the use of thick films with thicknesses in the range of 1 to 100 µm. An emerging deposition technique called AD (Aerosol Deposition method) allows the deposition of thick films with a wide thickness range from 1 to few hundred µm.
In this work, lead-free BaTiO₃ thick films were successfully deposited onto glass and kovar substrates at room temperature by AD. The as-deposited films showed a very high densification state with few stacking defects. The amount of observed defects is thickness-dependent, which confirms the hammering effect reported in the literature for AD films. The XRD analysis revealed that the perovskite crystallized phase was conserved in the as-deposited films. The broadening of XRD peaks is related to crystallite size reduction and high microstrain induced during the deposition process. In addition, the Raman spectroscopy analysis confirmed that the tetragonal symmetry of the raw powder was conserved in the deposited films.
The measured ferroelectric P-E loops showed linear dielectric-like behavior, with high resistivity values. The high resistivity values confirm the high densification of the deposited BaTiO₃ films. The absence of ferroelectricity is due to the size effect. Indeed, when the crystallite size is below a certain critical size, the long-range ferroelectric correlation is lost, which leads to the loss of macroscopic ferroelectricity.
In order to recover the piezoelectric and ferroelectric properties, different annealing strategies were followed. The annealing post-treatment has many aims such as to release residual stress due to the deposition mechanism, crystallize any amorphous phase formed during the particles impact, and promote the grain/crystallite growth to restore the long-range ferroelectric order.
The XRD results indicated that the thermal treatment of BaTiO₃ films has successfully promoted the grain growth. The AD films crystallinity is getting higher as long as the annealing temperature is increased. Raman spectroscopy analysis confirmed the residual stress release and the reduction of the size effect of grain structure. TEM images showed 30 to 100 nm faceted grains, with well-determined grain boundaries. The annealed films exhibited lossy ferroelectric behavior without fully restoring the hysteresis phenomena. Further experiments will be carried on in order to improve the ferroelectric characteristics of the AD deposited BaTiO₃ films.