Electrical poling below coercive field for large piezoelectricity

Appl. Phys. Lett. 102, 092902 (2013).

Hanzheng Guo, Cheng Ma, Xiaoming Liu and Xiaoli Tan.

Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA.

 

Abstract

Isotropic polycrystalline ferroelectric ceramics have to be electrically poled to develop a net macroscopic polarization and hence piezoelectricity. It is well accepted that a sufficient poling can only be realized under an electric field that is much higher than the coercive field. In this study, we observed in (Bi1/2 Na 1/2)TiO3-BaTiO3 ceramics that large piezoelectricity can develop at poling fields far below the measured coercive field. Using in situ transmission electron microscopy, such an unusual behavior, is interpreted with the polarization alignment of polar nanodomains in the non-ergodic relaxor phase.

© 2013 American Institute of Physics

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Additional Information

Piezoelectric materials, which enable the conversion between mechanical and electrical energies, have been in commercial use for over half century and formed a multibillion-dollar piezoelectric industry nowadays. They are crucial to numerous important technologies, including energy harvesting, noise cancellation, and minimally invasive surgery.

For an originally isotropic polycrystalline ferroelectric ceramic, the piezoelectricity can be achieved by a poling process, where a strong direct current electric field is applied to switch and align ferroelectric domains and finally form themacroscopic net remnant polarizations. Therefore,it is commonlybelieved that poling fields have to be much greater than the coercive field EC, which characterizes the difficulty of polarization reversal in ferroelectrics. Such an empirical rule is based on and supported by numerous experimental observations on all of the widely studied piezoelectrics so far, including Pb(ZrxTi1-x)O3-based, BaTiO3-based, K0.5Na0.5NbO3-based, and (Bi1/2Na1/2)TiO3-basedcompositions. These investigations indicate thatoptimal or well-developed piezoelectricity (represented as piezoelectric coefficient d33) can only be achieved when poled at or above 2EC or even as high as 16EC. Poling fields below or around EC only yield very small d33values that are close to zero.Therefore, the rule that poling field should excess the coercive field is commonly accepted in the piezoelectric research community and even written into the American National Standard.

The present study breaks such a long-standing empirical rule.It reports the first observation of large piezoelectricity realized at poling fields significantly below both coercive field ECand phase transition field EFin a limited composition range (6%≤x≤9%) of non-ergodicrelaxor (1-x)(Bi1/2Na1/2)TiO3xBaTiO3ceramics. For all of the studied compositions, the d33suddenly increases and then saturates at a critical poling field EP. It is surprisingly to notice that all of the EP values are found to be significantly lower than EC and EF. For x=9%, EP is even smaller than one third of its EC. The microstructural origin of this highly unusual phenomenon is rationalized using our uniqueelectric-field in-situ transmission electron microscopy (TEM). It clearly demonstrates that the poling of the non-ergodicrelaxorferrielectric phase takes place through the irreversible coalescence of individual nanodomains into thin lamellar domains without any phase transition from P4bm phase, which is characterized by the preservation of ½{ooe} supper diffraction spots. This breakthrough suggests that non-ergodicrelaxors should be included in the search of lead-free piezoelectrics for replacing the industrial standard, but environmentally hazardous, lead-containing ones.

 

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