No abstract

Strong electromechanical coupling is observed in tetragonal Pb-free 0.7(Bi 0.5 Na 0.5 )-TiO 3 -0.3BaTiO 3 films, which is far from the morphotropic phase boundary, prepared by pulsed laser deposition on a Si substrate. The tensile strain induced during cooling causes in-plane polarization in an oriented film on a Si substrate, while an epitaxial film grown on a SrTiO 3 substrate exhibits out-ofplane polarization. S−E curve analysis reveals that the obtained piezoelectric coefficient for the film on the Si substrate (d 33,f ≈ 275 pm/V) is approximately eight times higher than that for the epitaxial film on the SrTiO 3 substrate (d 33,f ≈ 34 pm/V). In situ X-ray diffraction analysis confirms the occurrence of domain switching under an electric field from in-plane to out-of-plane polarization. An effective piezoelectric stress coefficient, e 31,eff , of ∼19 C/m 2 is obtained from a Si cantilever sample, which is the highest among the reported values for Pb-free piezoelectric films and is comparable to those for Pb-based films. The significant piezoelectric response produced by domain switching in the Pb-free materials with the composition far from the morphotropic phase boundary will expand future applications due to their both outstanding properties and environmental sustainability.

The composition dependence of ferroelectric properties was investigated for (111)-oriented epitaxial HfO2-CeO2 solid solution films. Twenty-nanometer-thick films with different compositions were prepared on (111)ITO//(111)YSZ substrates at room temperature by pulsed laser deposition and subsequent heat treatment at 1000 oC under atmospheric N2 and O2 flows. All of the films had fluorite structures, and their crystal symmetries changed from monoclinic to tetragonal/cubic through orthorhombic phases as the (Hf1-xCex)O2 (x =0.12-0.25) increased in the film. The orthorhombic phase was confirmed for films with x = 0.15 and 0.17. On the other hand, ferroelectric properties were observed in films with x = 0.15-0.20, suggesting that a field-induced phase transition for films with x = 0.20. The film composition showing ferroelectricity was the widest for the reported epitaxial HfO2-based films. Their remanent polarization (P r) and coercive field (E c) were almost the same, at 17-19 μC/cm2 and 2.0-3.0 MV/cm, respectively. This wide ferroelectric composition range with relatively similar ferroelectricity is due to the solid solution of the same fluorite structure of HfO2 and CeO2 with monoclinic and cubic symmetries, that is respectively lower and higher crystal symmetries of the ferroelectric orthorhombic phase.

Lead-free piezoelectric (1 − x)(Bi, Na)TiO3–xBaTiO3 (x = 0.06–1.0) thin films were deposited, and their crystal structures, ferroelectricity, and piezoelectric properties were studied. These films were epitaxially grown on SrRuO3 covered (100) SrTiO3 substrates by pulsed laser deposition using ceramic targets. For all the compositions, the tetragonal films were found to be grown with (001), polar-axis orientation, mainly due to the thermal strain caused by the different thermal expansion coefficients between the substrate and films. Tetragonal films with x = 0.3 and 0.5 showed higher tetragonality, c/a ratio, than the reported values for the bulk ceramics due to the clamp of the in-plane a-axis by underlying SrRuO3 lattice. Here, a and c are the lattice parameters along in-plane and out-of-plane directions, respectively. This larger c/a ratio results in higher Curie temperature. It was revealed that remanent polarization and coercive electric field correlate with the increase in the c/a ratio of the films. The film with x = 0.06 exhibits the maximum piezoelectric property.

(Bi,K)TiO3 films were grown on (001) c SrRuO3 // (100)SrTiO3 substrates by pulsed laser deposition (PLD) at various temperatures. The K/Ti and Bi/Ti ratios of the films were almost the same at 350 and 500 °C, but decreased markedly at 650 °C. The ferroelectric property was ascertained for epitaxial tetragonal (Bi,K)TiO3 films grown at 500 °C from polarization–electric field relationships, and their saturation polarization (P sat) and coercive field (E c) at a maximum electric field of 800 kV/cm were 18 µC/cm2 and 300 kV/cm, respectively. To improve the ferroelectric property, the films were heat-treated at 650 °C while keeping the films in the PLD chamber after film deposition. These films also maintained a (001)-oriented epitaxy and had a tetragonal symmetry. The P sat and E c obtained at an electric field of 800 kV/cm were changed to 22 µC/cm2 and 95 kV/cm, respectively. Moreover, these P sat and E c values increased to 31 µC/cm2 and 165 kV/cm, respectively, at a maximum electric field of 1500 kV/cm. These heat-treated (Bi,K)TiO3 films showed piezoelectricity with an apparent piezoelectric coefficient (d 33(AFM)) of 22 pm/V.