Analyses for oxygen vacancies in (Na,K)NbO3-series piezoelectric ceramics by soft x-ray emission spectroscopy

Abstract

(Na0.55K0.45)NbO3 (NKN) and acceptor-doped (Na0.55K0.45)(Nb1-xZr5x/4)O3 (NKNZ; x = 0.005 and 0.01) piezoelectric ceramics were fabricated by the conventional solid-state reaction method. Both ceramics were applied to a fatigue test for 106 cycles under an electric field at 3 kV/mm with unipolar drive. After the fatigue test, the maximum polarization of NKN and NKNZ (x = 0.005) was decreased by approximately 15 and 5%, respectively. Therefore, we found that the fatigue resistance properties of NKNZ ceramics, which substituted the Nb5+ ion with the Zr4+ ion, were improved. To study fatigue phenomena in piezoelectric ceramics, the chemical bond strength of both ceramics before and after the fatigue test was analyzed by soft X-ray emission spectroscopy (SXES), which has high energy resolution and is suitable for the evaluation of light elements in a local region with the micrometer order. We evaluated the formation of oxygen vacancies in as-sintered, poled, and fatigued ceramics by analyzing a chemical shift of the O Ka(3) peak appeared at approximately 175 eV in the SXES spectra. In NKN ceramics, the O Ka(3) peak was shifted to the higher energy side by the poling treatment and fatigue test. The peak position of fatigued NKN ceramics was close to that of the standard SrTiO3 sample. It was considered that the oxygen vacancies of the fatigued NKN ceramics had accumulated at grain boundaries instead of the oxygen moving. The improvement of the fatigue resistance for the NKNZ ceramics was attributed to the formation of a defect dipole connected with the oxygen vacancy and acceptor Zr ion.

Stress dependent impedance spectroscopy of niobium based ceramics

“…Information about the microstructure can be found elsewhere. 17) Both compositions showed a monomodal grain size distribution with an average grain diameter of about 1 μm. In the same study, undoped NKN has shown a broad grain size distribution and average grain size of approximately 10 μm.…”

“…In a previous study, the average grain size of Zr-doped NKN was estimated to be 2 μm with homogenous distribution. 17) Since a homogenous microstructure is present in our material, the bricklayer is a reasonable approximation. 17) To describe the ceramics, an equivalent circuit comprising two serial R-CPE elements was used.…”

“…17) Since a homogenous microstructure is present in our material, the bricklayer is a reasonable approximation. 17) To describe the ceramics, an equivalent circuit comprising two serial R-CPE elements was used. In this context, R denotes a resistor element, while CPE represents a non-ideal capacitor element with a frequency-dependent response, often referred to as a constant phase element.…”

“…16) Tanaka et al observed similar materials and found indications of defect dipoles from the increase of coercive field and the peak shift in soft X-ray emission spectra. 17) The presence of defect dipoles in Zrdoped NKN ceramics effectively reduced the occurrence of independent oxygen vacancies, thereby improving fatigue resistance and resulting in ferroelectric hardening. 17) This improvement can be attributed to the ability of defect dipoles to deplete domain wall mobility, consequently impeding domain switching and promoting the stabilization of the domain structure.…”

“…17) The presence of defect dipoles in Zrdoped NKN ceramics effectively reduced the occurrence of independent oxygen vacancies, thereby improving fatigue resistance and resulting in ferroelectric hardening. 17) This improvement can be attributed to the ability of defect dipoles to deplete domain wall mobility, consequently impeding domain switching and promoting the stabilization of the domain structure. 4) Despite several investigations on Zr-doped NKN, there are no studies on the mechanical properties.…”