Lead‐free (K,Na)NbO₃‐based ceramics with high optical transparency and large energy storage ability

Abstract

Highly transparent lead‐free (1‐x)K0.5Na0.5NbO3–xSr(Zn1/3Nb2/3)O3 (KNN–xSZN) ferroelectric ceramics have been synthesized via a conventional pressureless sintering method. All samples are optically clear, showing high transmittance in the visible and near‐infrared regions (~70% and ~80% at 0.5 mm of thickness, respectively). This exceptionally good transmittance is due to the pseudo‐cubic phase structure as well as the dense and fine‐grained microstructure. In addition, a high energy storage density of 3.0 J/cm3 has been achieved for the 0.94K0.5Na0.5NbO3–0.06Sr(Zn1/3Nb2/3)O3 ceramics with submicron‐sized grains (~136 nm). The main reason is likely to be the typical relaxor‐like behavior characterized by diffuse phase transition, in addition to the dense and fine‐grained microstructure. This study demonstrates that the 0.94K0.5Na0.5NbO3–0.06Sr(Zn1/3Nb2/3)O3 ceramic is a promising candidate of lead‐free transparent ferroelectric ceramics for new areas beyond transparent electronic device applications.

Simultaneous realization of high transparency and piezoelectricity in low symmetry KNN‐based ceramics

“…A similar phenomenon was also observed in other KNN‐based materials . To comprehensively evaluate the multifunctional properties of this work and other reported values, we made a systematic comparison of ε r P r and transmittance and showed the results in Figure D. It is seen that relatively large ε r P r value and high transmittance were simultaneously obtained in SNN‐modified KNN ceramics, exhibiting their great potentials in application of multifunctional optoelectronic device.…”

“…Furthermore, when x increases to 0.09, the transmittance goes so high over ~80% at 780 nm that a red flower can be seen vividly through the KNN–0.09SSN sample (see inset of Figure B). In this work, the transmittance is superior to those KNN‐based and even PLZT materials via hot‐pressed sintering . The improvement of optical transmittance with increasing x is shown in Figure B.…”

“…Translucency in KNN‐based ceramics was discovered in 2004, and drew few attentions until Du et al reported translucent (1− x )KNN– x (Bi 0.5 Na 0.5 )TiO 3 ceramics in 2008. Thereafter, growing interest was shown in the optical transmittance of KNN‐based ceramics . For example, Kwok et al reported the Li and Bi comodified K 0.5 Na 0.5 NbO 3 ceramics have high optical transmittance of 60% in the near‐IR region .…”

“…On this account, two possible routes can be used to improve the transparency of anisotropic KNN ceramics: one is to modify KNN to pseudocubic or cubic structure, and the other is to reduce the grain size to submicrometer scale while retaining its low‐symmetry anisotropic structure. For the first route, it has already been utilized to improve the optical transmittance (60%‐74% in the visible region) in KNN‐based ceramics . However, it is known that the high‐symmetry pseudocubic structure will severely deteriorate the piezoelectric performance of KNN, making the second route a seemingly only choice for us.…”

“…Liu et al achieved a uniform grain size of 250‐260 nm in KNN by SrZrO 3 alloying. Chai et al prepared KNN with 6%Sr(Zn 1/3 Nb 2/3 )O 3 and achieved submicrometer‐sized grains of about ~136 nm. However, it should be noted that overreduction in grain size can result in severe decrease in domain density and thus poor electrical properties, considering the strong coupling between grain boundaries and domain walls .…”