In recent years, lead titanate (PZT) based ceramics are used in the electronic ceramics due to their excellent piezoelectric and dielectric properties. However, PZT contains a high proportion of toxic heavy metal lead, which poses a serious threat to the environment and human-being health during production, use, and disposal. With increasingly strict global environmental regulations, the development of high-performance lead-free piezoelectric-based materials becomes an urgent task and consensus in the field of materials science. Among these ceramic materials, K_0.5Na_0.5NbO₃ (KNN) ceramics have attracted much attention as a lead-free alternative to conventional piezoelectric materials due to their excellent electromechanical properties and environmental compatibility. This study was to address these limitations via implementing a manganese dioxide (MnO₂) doping strategy. The primary objectives were to establish structure-property relationships and optimize dielectric performance for high-temperature/high-frequency electronic applications, while elucidating the underlying defect-mediated mechanisms governing the performance enhancement.

The KNN ceramics with controlled MnO₂ doping concentrations (i.e., x = 0, 0.010%, 0.020%, 0.035%, 0.060%, and 0.095%) were synthesized via conventional solid-state reaction. High-purity precursors (i.e., Na₂CO₃, K₂CO₃, Nb₂O₅, MnO₂) were stoichiometrically weighed, ground in a planetary mill with ethanol, calcined (at 800-900 ℃), re-ground, and pressed into pellets with a polyvinyl alcohol binder. The sintering was conducted at 1050-1070 ℃ for 3 h. The poling was performed at 2 kV/mm for electrical characterization. The phase purity and crystal structure were analyzed by X-ray diffraction (XRD). The microstructure and elemental distribution were characterized by scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS). The dielectric permittivity and loss tangent were determined at 40 Hz-10 MHz and varied temperatures using an LCR meter. The electrical conductivity, nonlinear "J-E" characteristics, and impedance spectroscopy were systematically evaluated.

The XRD patterns confirm that all the compositions retain a pure tetragonal perovskite structures (space group P4mm). As 0.060% as an optimal doping concentration, Mn²⁺/Mn³⁺/Mn⁴⁺ substitution for Nb⁵⁺ induces lattice expansion and forms defect dipoles. The SEM images reveal a substantial grain growth from 1.2 μm (undoped) to 3.5 μm (doped at 0.060%), due to liquid-phase sintering and lattice strain. The doping concentration of 0.060% can achieve a synergistic enhancement of dielectrical property, i.e., ε_r increases significantly, while tanδ decreases dramatically at low frequencies, governed by grain homogenization suppressing electron-hopping conduction. Conversely, the doping concentration of 0.095% has anomalous high-frequency losses due to the intensified grain-boundary polarization and skin effects. The temperature-dependent studies demonstrate a reduced dielectric maximum temperature from 240 ℃ (undoped) to 200 ℃ (doped), with a superior stability in tan δ at10⁴-10⁶ Hz. The impedance spectra reveal an accelerated ion mobility at > 200 ℃. The optimal operational window is identified at 200-240 ℃ and 10⁴-10⁶ Hz. The excessive doping concentration (i.e., 0.095%) degrades the performance due to the secondary phase formation, reduces relative density, and increases porosity. The moderate improvements in DC conductivity (attributed to defect-assisted hopping) and nonlinear coefficient α are obtained.

In this study, a series of K_0.5Na_0.5NbO₃ (KNN) ceramics with different manganese oxide (MnO₂) concentrations were synthesized. The results showed that the optimal doping concentration of MnO₂ was 0.060%. At this optimal concentration, manganese ions in multiple oxidation states replaced Nb⁵⁺, causing the lattice expansion and the formation of beneficial defect dipoles. This doping strategy simultaneously improved the microstructure, promoted a significant grain growth from 1.2 µm to 3.5 µm, and enhanced dielectric properties, achieving a significant synergistic enhancement effect. This improvement could be mainly attributed to the uniformity of the microstructure, which inhibited a harmful electron hopping conduction. The study also clarified that the optimal window for stability performance could be 200 -240 ℃ and 10⁴-10⁶ Hz. Beyond the optimal doping level, the excessive doping concentration (i.e., 0.095%) led to a performance degradation due to the formation of secondary phases, increased porosity, and increased losses at high frequencies due to grain boundary polarization and surface effects. This study could provide a basic framework for developing manganese-doped potassium sodium nickel oxide ceramics for specific electronic applications.