(K, Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics are considered to be one of the important candidates to replace lead-based piezoelectric ceramics due to their high Curie temperature and excellent comprehensive performance. To further improve its performance, the strategy of element doping is employed. The doping behavior of the Mn element in pressure-sintered KNN ceramics has been extensively studied. However, there are significant differences in the density of Mn-doped KNN ceramics in different works, which poses a challenge to the analysis of the underlying mechanism of Mn doping and its modification effect. Therefore, this study takes hot-pressed KNN ceramics as the research object due to their high densification, and systematically explores the impact of Mn doping on the microstructure and electrical properties of KNN ceramics.

In this study, KNN ceramics with MnO₂ doping were prepared by hot-pressed sintering. The specific composition is (K_0.5Na_0.5)NbO₃-x% MnO₂ (abbreviated as KNN-x% MnO₂, x = 0, 0.5, 1.0, 1.5). The raw materials include K₂CO₃ (99.0%), Na₂CO₃ (99.8%), Nb₂O₅ (99.99%), and MnO₂ (98.8%), all of which were purchased from Sinopharm Chemical Reagent Co., Ltd. First, carbonate and Nb₂O₅ were placed in a nylon ball mill with zirconia balls according to the formula ratio, and ball milled for 24 h using anhydrous ethanol as the medium; the ball-milled slurry was dried and calcined at 730 ℃ for 4 h, and then ball-milled again and calcined again at 930 ℃ for 4 hours. Subsequently, different amounts of MnO₂ were added to the calcined powder, mixed and ball-milled for 24 h, and then dried and ground to obtain Mn-doped KNN ceramic powder. Finally, the ceramic block was prepared by hot pressing and sintering at 950 ℃ and 30 MPa for 2 h under argon atmosphere. After cutting, grinding, and annealing at different temperatures, the sample surfaces were subsequently coated with silver electrode for electrical measurements. The poling process was performed at 3.5 kV/mm in 120 ℃ silicone oil for 30 min.

The surface micromorphology of the ceramics was characterized by Merlin scanning electron microscope (SEM). The crystal structure was analyzed by D/Max 2500 X-ray diffractometer (XRD). The density of the samples was measured using the Archimedean drainage method. The dielectric properties were tested using a TH2827 impedance analyzer. The ferroelectric properties were obtained using a TF Analyzer 2000 ferroelectric analyzer. The piezoelectric constant d₃₃ was measured using a ZJ-3A quasi-static d₃₃ tester. The mechanical quality factor Q_m was measured using a TH2839 impedance analyzer. The local domain structure and domain switching behavior were tested and analyzed using an MFP-3D atomic force microscope with piezoelectric force microscopy (PFM).

High-density (relative density > 98%) KNN-x% MnO₂ ceramics were successfully prepared by hot pressing sintering process. XRD results show that MnO₂ doping does not change the perovskite phase structure of KNN. With the increase of Mn doping amount, the P_r of the ceramic shows a trend of first increasing and then slightly decreasing. At x = 1.0-1.5%, P_r reaches 14.92-18.32 μC/cm², d₃₃ can reach up to 106 pC/N, and both positive and negative strains are improved, reflecting enhanced ferroelectric and piezoelectric responses after Mn doping. PFM test shows that the switching voltage of the local ferroelectric domain gradually increases from 5 V to 20 V, indicating that Mn doping enhances the domain wall pinning effect. At the same time, the mechanical quality factor Q_m of the ceramic is significantly improved, with the highest value reaching 175, reflecting the typical "hard" doping behavior.

This study found that Mn-doped KNN ceramics exhibit an atypical "hard" doping effect for the high-density hot-pressed ceramics that is different from the traditional doping mechanism in lead-based ceramics, in which Q_m and P_r are simultaneously improved and d₃₃ is maintained at a high value. This atypical "hard" doping effect induced by Mn doping provides a new perspective for understanding the mechanism of element doping in the KNN system, and also provides significant guidance for the optimized design of high-performance lead-free piezoelectric ceramics.