Hot Pressing And Low Oxygen Partial Pressure Sintering (K 0.5 Na 0.5 ) NbO 3 Based Ceramics And Their Structural And Electrical Properties

(K_0.5Na_0.5)NbO₃?KNN?-based lead-free piezoelectric ceramics are perovskite-type ferroelectrics.The KNN-based ceramics have been attracted more and more attention because they are environmental friendly,as well as their good ferroelectric and piezoelectric properties.However,the volatilization of K+ and Na+ at the sintering process leads to poor densification and bad electrical properties of KNN ceramics prepared via a conventional sintering method.In this paper,the KNN-based ceramics were studied.Here,the KNN-based ceramics were prepared via conventional sintering method,hot-press sintering method,and low oxygen partial pressure sintering method,respectively.Effects of different preparation methods and dopants on structure,dielectric,ferroelectric and piezoelectric properties of the?KNN?-based ceramics were studied in detail.The results are as following:?1?(K_0.5Na_0.5)NbO₃ lead-free piezoelectric ceramics were prepared via a conventional sintering method and a hot-press sintering method.Effects of different sintering method on microstructure,dielectric,ferroelectric and piezoelectric properties of the KNN ceramics were studied in detail.X-ray diffraction?XRD?results show that both of the samples exhibited pure perovskite structure.Scanning electron microscope?SEM?results suggest that the hot-press sintered ceramic has compact structure and smaller grain size.Combined with the results of the ultraviolet-visible diffuse reflectance spectra and impedance analysis,it is expected that point defect concentration in the hot-press sintered KNN ceramic is lower.Dielectric constant-temperature curves demonstrate that the hot-press sintered KNN ceramic exhibits higher dielectric constant and lower dielectric loss at room temperature.The hot-press sintered KNN ceramic exhibits higher polarization?Pr=17.3 ?C·cm-2?and lower coercive field?Ec=16 kV·cm-1?compared to the ceramic prepared via the conventional solid-state reaction method.By means of scanning probe microscope,the behavior of electrical domains was explored.The corresponding mechanism was also discussed.?2?(K_0.5Na_0.5)NbO₃-0.5mol%Al2O₃?KNN-Al?lead-free piezoelectric ceramic was prepared via a hot-pressing sintering method.Structure and electrical properties of the ceramics were studied in detail.The XRD results indicate that the ceramic has the pure perovskite structure,implying that the Al3+ ions have entered the KNN lattice.The SEM results show that the addition of Al3+ caused an increase in grain size of the ceramics.The results of the impedance analysis show that the activation energy is 1.91eV.For each frequency,two dielectric peaks exist on the ?r-T curves.Combined with the results of the electric hysteresis loop and electrically induced strain,it is found that the present samples exhibit good ferroelectric and piezoelectric properties than the results reported by others.?3?(K_0.5Na_0.5)NbO₃-2mol%CuO?denoted as KNN-A,KNN-B and KNN-H,respectively?lead-free piezoelectric ceramics were sintered under different low oxygen partial pressure?air,10'12atm and 10'16atm?.Effects of oxygen partial pressure on structure and electric properties of the ceramics were studied in detail.The results show that all ceramics have the pure perovskite structure.With a decrease in oxygen partial pressure,the mean grain sizes of the KNN-A,KNN-B and KNN-C ceramics are 7.8,1.4 and 1.5 um,respectively.Dielectric constant-temperature curves demonstrate that the ceramics sintered under 10-12 atm have higher phase transition temperatures.The ferroelectric properties of ceramics decrease with the decrease of oxygen partial pressure.The results of the impedance ganalysis show that the activation energy are 0.91,1.97 and 1.62eV for KNN-A?KNN-B and KNN-C,respectively.