Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics

Abstract

In this letter, we use transmission electron microscopy to study the microstructure feature of recently reported Pb-free piezoceramic Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 across its piezoelectricity-optimal morphotropic phase boundary (MPB) by varying composition and temperature, respectively. The domain structure evolutions during such processes show that in MPB regime, the domains become miniaturized down to nanometer size with a domain hierarchy, which coincides with the d33-maximum region. Further convergent beam electron diffraction measurement shows that rhombohedral and tetragonal crystal symmetries coexist among the miniaturized domains. Strong piezoelectricity reported in such a system is due to easy polarization rotation between the coexisting nano-scale tetragonal and rhombohedral domains.

Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ferroelectrics

“…The absolute values of the activation barrier W B * of about $0.75 eV are close to those of soft piezoceramic PZT-5 H at the MPB which were reported to exhibit the activation barrier W B * ¼ 0.7 eV but one order of magnitude smaller nuclei volume V* of 3 Â 10 À25 m 3 . 28 Concerning the nuclei size Gao et al 17 reported by transmission electron microscopy that the nucleating domains can hardly be seen in the pure tetragonal and rhombohedral phases of BCZT100x, because in these phases, the critical nuclei rapidly develop to their equilibrium domain size which is in the micrometer range. In the phase transition region near the MPB, they report nanodomain features of 20 nm to 100 nm, which are in agreement with our reported V* values of about 2 to 3 Â 10 À24 m 3 at x $0.5 in Fig.…”

“…9 At room temperature, this boundary is at x $0.5 between a rhombohedral (R) and a tetragonal (T) phase, although the exact phase diagram in BCZT100x is still controversial. 9,[12][13][14][15][16][17][18] For example, a recent study 18 suggested that an intermediate orthorhombic (O) phase and a phase convergence region where four phases (R, O, T, and cubic (C)) may occur in a narrow temperature and compositional range. Thereby, the existence of such polymorphic regions seems to be a key to achieving the outstanding piezoelectric performance.…”

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…The high switching degree is ascribed to the high multiplicity of polarization states and considerably lower free energy anisotropy 156 , which in turn leads to a low barrier to reorient and hierarchical domain morphology. 319,347 A reduced domain size was previously shown to affect mostly irreversible switching, leading to a pronounced enhancement of this strain contribution. 227 Moreover, the soft character ( Figure 5.13 (a)) of the O phase and low thermal energy at room temperature are expected to favor irreversible switching due to the stress relief inherent to ferroelastic switching.…”

“…The hierarchical domain formation was ascribed to reduced free energy anisotropy. 319 In situ temperature-dependent TEM indicated a considerable change in the hierarchical domain structure of the system. 347 Upon heating wedge-shaped domains in the O phase of BZT-0.50BCT disappeared progressively.…”

“…347 In situ electric field TEM studies demonstrated that switching is also quite considerable leading to the formation of a monodomain state within grains. 321,339 The hierarchical domain morphology 319 and irreversible switching 347 were mentioned as contributors to the enhancement of electromechanical properties around the O to T PPB.…”