No abstract

Bismuth ferrite (BiFeO 3 ) is difficult to pole because of the combination of its high coercive field and high electrical conductivity. This problem is particularly pronounced in thick films. The poling, however, must be performed to achieve a large macroscopic piezoelectric response. This study presents evidence of a prominent and reproducible self-poling effect in few-tens-of-µm-thick BiFeO 3 films. Direct and converse piezoelectric measurements confirmed that the as- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 self-poling effect only appears in cases when the films are heated and cooled through the ferroelectric-paraelectric phase transition (Curie temperature T C~8 20 o C). These self-poled films exhibit a microstructure with randomly oriented columnar grains. The presence of a compressive strain gradient across the film thickness cooled from above the T C was experimentally confirmed and is suggested to be responsible for the self-poling effect. Finally, the macroscopic d 33 response of the self-poled BiFeO 3 film was characterized as a function of the driving-field frequency and amplitude.

In this work, the effects of thermal annealing at 500 °C on aerosol-deposited 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 thick films on stainless-steel substrates are investigated using two complementary methods at high and low applied external electric fields. The first one is the positive up negative down method, which allows us to obtain information about the switching and non-switching contributions to the polarization. It shows that the as-deposited film is ferroelectric before annealing, since it has a switching contribution to the polarization. After annealing, both the switching and non-switching contributions to polarization increased by a factor of 1.6 and 2.33, respectively, indicating a stronger ferroelectric behavior. The second method is based on impedance spectroscopy coupled with Rayleigh analysis. The results show that post-deposition thermal annealing increases the reversible domain wall contribution to the dielectric permittivity by a factor of 11 while keeping the threshold field similar. This indicates that, after annealing, domain wall density is larger while domain wall mobility remains similar. These two complementary characterization methods show that annealing increases the ferroelectric behavior of the thick film by increasing the domain wall density, and its influence is visible both on polarization vs electric field loop and dielectric permittivity.