Abstract
Miniaturization of domains to the nanometer scale has been previously reported in many piezoelectrics with two-phase coexistence. Despite the observation of nanoscale domain configuration near the polymorphic phase transition (PPT) regionin virgin (K0.5Na0.5)NbO3 (KNN) based ceramics, it remains unclear how this domain state responds to external loads and influences the macroscopic electro-mechanical properties. To this end, the electric-field-induced and stress-induced strain curves of KNN-based ceramics over a wide compositional range across PPT were characterized. It was found that the coercive field of the virgin samples was highest in PPT region, which was related to the inhibited domain wall motion due to the presence of nanodomains. However, the coercive field was found to be the lowest in the PPT region after electrical poling. This was related to the irreversible transformation of the nanodomains into micron-sized domains during the poling process. With the similar micron-sized domain configuration for all poled ceramics, the domains in the PPT region move more easily due to the additional polarization vectors. The results demonstrate that the poling process can give rise to the irreversible domain configuration transformation and then account for the inverted macroscopic piezoelectricity in the PPT region of KNN-based ceramics.
Highlights
As compared to the micron-sized domains in the single-phase region, the nanodomains have different response mechanisms to the external loads
A two-phase coexistence zone composed of orthorhombic and tetragonal ferroelectric phases is formed at room temperature for the composition with x = 0.07
The x = 0.03 specimen is found to have an orthorhombic structure at room temperature, a two-phase coexistence region at about 50 °C, and a tetragonal structure above 50 °C up to 150 °C, as determined by in-situ X-ray diffraction (XRD) measurement (Figs S1 and S2)
Summary
As compared to the micron-sized domains in the single-phase region, the nanodomains have different response mechanisms to the external loads. Nanodomains in the polymorphic phase boundary can respond actively to external loads and lead to the enhancement of piezoelectric properties[25,26]. Orthorhombic and tetragonal phases do not converge near the line of Curie temperatures, as can be seen from the NaNbO3-KNbO3 phase diagram[27] Such a situation will result in a larger anisotropic energy contribution along the whole polymorphic boundary line compared with the phase convergence region in BZT-xBCT system[23]. A miniaturized nanoscale domain morphology was previously observed in the virgin state of KNN-based ceramics with two-phase coexistence and was believed to be responsible for the enhanced quasistatic piezoelectric coefficient of the poled ceramics[8,9]. The domain response to external loads in the virgin state can not account for the piezoelectric properties of the poled ceramics. The observed variations of the electro-mechanical properties before and after the poling process were related to the changes of domain configuration
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