Origin of high electromechanical properties in $(\mathrm{K},\mathrm{Na}){\mathrm{NbO}}_3$-based lead-free piezoelectrics modified with $\mathrm{BaZr}{\mathrm{O}}_3$

After two decades of extensive research, selected lead-free piezoelectric materials based on the $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$ (KNN) system exhibit excellent electromechanical properties that are comparable or even superior to those of $\mathrm{Pb}\left(\mathrm{Zr},\mathrm{Ti}\right){\mathrm{O}}_3$. The origin of the superior properties, however, remains insufficiently understood. Here, we establish a correlation between electromechanical properties and domain-wall dynamics for a prominent piezoceramic $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$-based composition, modified with $\left(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{L}{\mathrm{i}}_{0.5}\right)\mathrm{Ti}{\mathrm{O}}_3$ and different amounts of $\mathrm{BaZr}{\mathrm{O}}_3$. This composition combines excellent piezoelectric properties, high fatigue resistance, and low leakage currents. Domain-wall dynamics were studied over a broad range of electric fields, combining subcoercive field and dynamic pulse measurements. We find that the enhanced electromechanical properties in these materials are caused by easier domain-wall movement. Besides providing insights into fundamental mechanisms of domain-wall dynamics in KNN-based materials, our results can serve as a future guidance for the design of high-performance KNN compositions.

Figure 1

Structural study of the polycrystalline $\mathrm{BZ}x$ ceramics. (a) XRD patterns, labeled based on pseudocubic symmetry. (b) Evolution of the pseudocubic 200 diffraction profiles with different $\mathrm{BaZr}{\mathrm{O}}_3$ contents. The arrows visualize the decrease of the splitting of the 200 peaks with increasing $\mathrm{BaZr}{\mathrm{O}}_3$ contents. (c) Relative permittivity, ${\varepsilon }_{33}/{\varepsilon }_0$ (normalized by the vacuum permittivity ${\varepsilon }_0$) and dielectric loss, $tan\delta$ (dashed lines) of unpoled $\mathrm{BZ}x$ materials, measured with a frequency of 10 kHz as a function of the temperature, $T$.

Figure 2

Electric-field-dependent small signal piezoelectric, $d_{33}$, coefficient, relative dielectric permittivity, ${\varepsilon }_{33}/{\varepsilon }_0$ (solid lines), and dielectric loss, $tan\delta$ (dashed lines) for (a) BZ2, (b) BZ4, and (c) BZ6 samples.

Figure 3

Large signal (a)–(c) polarization, $P$, and (d)–(f) strain, $S$, loops of the materials with different contents of $\mathrm{BaZr}{\mathrm{O}}_3$, measured under 3–5 kV/mm and 1 Hz. The definition of negative strain, $S_{\mathrm{neg},}$, and maximum strain, $S_{\max }$, in bipolar $S$($E$) loops is given in (d).

Figure 4

Subcoercive measurements of the relative dielectric permittivity, ${\varepsilon }_{33}/{\varepsilon }_0$, for the polycrystalline $\mathrm{BZ}x$ materials. Real components of the relative permittivity (normalized by the vacuum permittivity ${\varepsilon }_0$) measured as a function of applied field. The dielectric Rayleigh coefficients ${\alpha }_{\varepsilon }$ (in mm/V) and field-independent initial permittivity ${\varepsilon }_{\mathrm{init}}$ were extracted from linear regression analysis for the linear portions of the response (“Rayleigh” region) as indicated by the black lines.

Figure 5

Simultaneous measurements of the time dependence of the switched polarization, $\mathrm{\Delta }P$, and strain, S, of the polycrystalline $\mathrm{BZ}x$ materials. (a)–(c) Switched polarization and (d)–(f) strain for materials with different contents of $\mathrm{BaZr}{\mathrm{O}}_3$. Measurements were conducted under constant electric fields with pulse height, $E_{\mathrm{Sw}}$, indicated by the inset values in kV/mm. The horizontal solid line in the strain curves represents zero relative strain obtained by normalization with the poled state.

Figure 6

Influence of $\mathrm{BaZr}{\mathrm{O}}_3$ on the different regimes of polarization reversal [34], as described in the text. The measurements were performed at constant electric-field pulses of height $E_{\mathrm{Sw}}$, as indicated. The influence of $\mathrm{BaZr}{\mathrm{O}}_3$ on initial non-180° domain-wall movement (regime 1), main switching phase (regime 2), and creeplike domain-wall movement (regime 3) is highlighted in (a), (b), and (c) with arrows, respectively. The horizontal solid line in strain curves represents zero relative strain obtained by normalization with the poled state.

Figure 7

Field-dependent characteristic switching times, τ, for the main switching phase [regime 2, determined from $\mathrm{\Delta }P\left(t\right)$ curves in Figs. 5–5, where 50% of the polarization is reversed]. The dashed lines represent fits according to Eq. (3) (Merz law [58]), from which the activation fields, $E_a$, in kV/mm were derived (highlighted in the figure).

Figure 8

Dependence of (a) coercive field, $E_c$, (b) activation field, $E_a$, and (c) Rayleigh coefficient, ${\alpha }_{\varepsilon }$, on $\mathrm{BaZr}{\mathrm{O}}_3$ contents. The arrow on the top indicates the increase in lattice distortion with decreasing $\mathrm{BaZr}{\mathrm{O}}_3$ contents [compare Fig. 1].