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.