硅酸盐学报

研究了二氧化锰(MnO₂)掺杂对铌酸钾钠(K_0.5Na_0.5NbO₃)基陶瓷介电性能的影响。采用固相法制备了不同掺杂量的KNN陶瓷，并对其介电性能进行了全面表征和分析。结果表明，所有掺杂MnO₂的KNN陶瓷样品均呈现出典型的四方钙钛矿结构，适量的MnO₂掺杂显著提高了KNN陶瓷的介电性能。然而，过量掺杂会导致性能下降，但仍优于纯KNN陶瓷。在最佳的MnO₂掺杂条件下，介电常数逐渐升高，表明极化强度和介电性能得到增强。此外，MnO₂掺杂降低了KNN陶瓷的介电损耗。在低频下，由于掺杂引起的结构变化，所有KNN陶瓷样品的介电常数均随温度升高而增大；然而，超过某一温度阈值后，其介电常数开始下降，这一发现对于在特定温度范围内工作的电子设备至关重要。

Introdution 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. Methods 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., Na_2CO₃, K_2CO₃, Nb_2O₅, 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 k V/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. Results and discussion The XRD patterns confirm that all the compositions retain a pure tetragonal perovskite structures(space group P4 mm). 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. Conclusions 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.