| Ba(Ti0.8Zr0.2)O3-(Ba0.7Ca0.3)TiO3 (abbreviated as BZT-BCT) piezoelectric solid solution, when the composition is close to morphotropic phase boundary (MPB), 0.5BZT-0.5BCT piezoelectrics can exhibit an excellent piezoelectric constant d33 about 560-620pC/N, which is comparable with lead zirconate titanate, Pb(Zn1-xTix)O3) (PZT). This is a great progress in the lead-free piezoelectric field over the past fifty years. For practical applications, piezoelectric thin film devices should not only meet the electrical properties including dielectric constant ?, remnant polarization Pr, piezoelectric coefficient d33, temperature stability T', but also the mechanical factors such as wear, stiffness, and hardness should achieve the appropriate security index. In order to explore the potential application of BZT-BCT thin films, the electrical properties and their corresponding temperature stability, and mechanical properties are well studied.The 0.5BZT-0.5BCT thin films were fabricated on Pt(111)/Ti/SiO2/Si(100) substrate via sol-gel technique, and the effect of solvent, additive, annealing temperature on the microstructure, morphology and dielectric, ferroelectric, and piezoelectric properties were discussed. The results showed that the 2-methoxyethanol solvent with higher boiling point and saturated vapor pressure is more helpful for improving the microstructure and morphology of 0.5BZT-0.5BCT thin films when compared with absolute ethanol solvent. And the film becomes denser, smoother, and more homogeneous after the ethylene glycol, polyethylene glycol with 400 molecular amount were further added into the precursor fabrication, respectively. The electric measurements indicated that the prepared film with ethylene glycol monomethyl ether as solvent and polyethylene glycol as additive shows a high ? up to 1756, Pr around 17.2?C/cm2, Ec about 39.0kV/cm. In order to explore the annealing temperature effect on the piezoelectric property, four different temperatures,650?, 700?,750?,800? were set according to thermal gravity-differential thermal analysis (TG-DSC) curve. The results showed that as the annealing temperature increases, the film is more crystallized, and the grain size increases while the root mean square roughness decreases. As a result, the piezoelectricity improves with the elevated annealing temperature and the maximum d33 can reach 85pm/V for 800?-annealed film.The compositional gradient (1-x)BZT-xBCT (x=0.3,0.4,0.5,0.6,0.7) thin films were deposited on Pt(111)/Ti/SiO2/Si(100) substrate to improve the electrical temperature stability. The gradient film is called up-graded when the x increases from film surface to substrate, on the contrary, being called down-graded. X-ray diffraction results showed that the gradient film has deviated from MPB area, and tetragonality c/a is less than 1. Dielectric constant-temperature curve indicated that two gradient films display favorable temperature stability over the whole testing temperature, from -25? to 140?, and the dielectric constant decreased only 8%,12% for up-graded and down-graded, respectively. The dielectric loss also exhibits good temperature stability with the tan8 controlled between 0.03 and 0.08. Dielectric Constant-Voltage measurements discovered that the tunability of up-graded and down-graded films are about 37% and 35%, and keep stable in the temperature range from -25? to 90?. Meanwhile, the piezoelectric property displays excellent temperature stability, and when the testing temperature was set 180?, the displacement-voltage butterfly loop is well saturated and the d33 reaches 21pm/V. Studies indicated that the electric properties of up-graded film is better than down-graded one, which can be attributed to the good lattice matching between first layer and Pt electrode. Transmission electron microscope (TEM), and energy disperse spectroscopy (EDS) measurements were adopted to observe the morphology and analyze the atom diffusion law in the up-graded film, and initially identify that good temperature stability for compositional gradient film is closely related to the gradient strain in films.The nanomechanical properties and tribological behavior of BZT-BCT thin films were studied by nanoindentation and atomic force microscope (AFM), respectively. The grain size influence on elastic modulus (Er) and hardness (H) are well discussed and the wear properties of BZT-BCT thin films are closely related to the ratio of hardness and elastic modulus (H/Er), and the residual tensile stress in BZT-BCT film can effectively reduce the adhesive force during friction. Investigation on the variation of nanomechanical properties during phase transition showed that the MPB area has special mechanical characteristics. |
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