Domain structures in KNbO3 crystals and their piezoelectric properties

Abstract

The electromechanical coupling factor for the thickness-extensional mode, kt, in KNbO3 crystals has been predicted to be as high as 69% for the 49.5° rotated X-cut about the Y-axis, which is the highest among known piezoelectrics. This paper presents the experimental confirmation of the high coupling factor and elucidates the relationship between the domain structure and the piezoelectric properties in the pseudocubic (001)pc cut, which is close to the maximum kt cut and has a piezoelectric strain constant about 2.9 times that of the Z-cut. Etching of crystals is found to have an outstanding effect on the piezoelectric characteristics. That is, in the case of “as-cut” crystals, the strain versus electric field curves have a large hysteresis and instability due to the occurrence of 60° domains in a high electric field region, whereas etched crystals remain the single domain state and exhibit linear piezoelectric properties even in the high electric field region. It is also demonstrated that the (001)pc cut crystal poled along the 〈001〉pc direction has a polar multidomain structure composed of two kinds of 90° domains, not four kinds of equivalent domains, and that it exhibits almost the same piezoelectric properties as those of single-domain crystals.

Domain engineering of the transverse piezoelectric coefficient in perovskite ferroelectrics

“…This accounts for the largest part of the equivalent, experimental value ͑92 pC/ N͒ measured for the 4O structure by Nakamura et al 10 The transverse piezoelectric coefficient d 31 * ͑ =90°, =−45°, ͒ is a strong function of , varying between X = −13. pending on our choice of x 1 Ј, there will be a set of three different contributions to d 31 * from the three variants.…”

“…Following the rediscovery of their very high piezoelectric coefficients for rhombohedral or orthorhombic compositions oriented along the nonpolar ͓001͔ C ͑C: pseudocubic͒ direction, 1 much work has concentrated on the concept of domain engineering 2 not only in these materials [3][4][5][6][7] but in simpler perovskite crystals as well. [8][9][10] A good definition of "domain engineering" is that given by Bell: 2 A domain-engineered crystal is one which has been poled by the application of a sufficiently high field along one of the possible polar axes of the crystal other than the zerofield polar axis, creating a set of domains in which the polarizations are oriented such that their angles to the poling direction are minimized. In a perovskite material there are therefore three possible sets of poling directions ͗111͘ C , ͗101͘ C , and ͗001͘ C ͑if monoclinic phases are ignored͒.…”

What Can Be Expected From Lead-Free Piezoelectric Materials?

“…For example, the thickness coupling coefficient k t of single crystal KNbO 3 and Limodified (K,Na)NbO 3 is as large as 70%. 76,77 Together with a low permittivity and density, this crystal and its derivatives, make an excellent choice for high frequency single element transducers. 78 Unfortunately, other piezoelectric properties of KNbO 3 are not high enough to make it a viable alternative to PZT for actuators and sensors.…”

High Resolution Transmission Electron Microscopy Observations on Sintering Processes in KNbO3 Ceramics

“…The lowtemperature sintering of NKN-based ceramics has been also studied to prevent the evaporation of Na 2 O (or K 2 O) during sintering. The KNbO 3 (KN) ceramic could also be a candidate for leadfree piezoelectric ceramics since KN single crystal exhibited good piezoelectric properties and high T c (Nakamura et al, 2002). However, densification of the KN ceramics using the conventional solid-state method was very difficult because of the evaporation of K 2 O and formation of unwanted secondary phases (Birol et al, 2005).…”