Defect engineering of high‐performance potassium sodium niobate piezoelectric ceramics sintered in reducing atmosphere

Abstract

Reducing atmosphere fired (K0.5Na0.5)NbO3‐based ceramics open up an important opportunity for manufacturing lead‐free multilayer piezoelectric devices cofiring with the base metal inner electrode. In this study, the effects of sintering atmosphere on the piezoelectric, dielectric, and insulating properties in Sn‐doped (Na0.52K0.44Li0.04)NbO3 (KNN) were investigated. To establish the relationship between the defect structure and electrical properties, the thermally stimulated depolarization current (TSDC) technique was implemented. The electrical properties degrade severely when the pure KNN ceramics are sintered in reducing atmosphere, compared with the ceramics sintered in air. The reason is that the oxygen vacancy concentration in reduced fired ceramics is much higher than that in air fired ceramics. However, the 0.6 mol% Sn‐doped KNN ceramics sintered in reducing atmosphere exhibit comparable electrical properties to the air fired ceramics. From the TSDC analysis, the reducing atmosphere fired ceramics have approximately the same oxygen vacancy concentration as the air fired ceramics because B‐site substituted Sn works as an acceptor.

Design of p‐type NKN‐based piezoelectric ceramics sintered in low oxygen partial pressure by defect engineering

“…Observed from the SEM images, Mn0‐A ceramics exhibit cubic or rectangular grains with a clear and well‐defined grain boundary marked with the red squares. Whereas the grain morphology becomes round‐shaped (marked with the red circles) for Mn0‐N, indicating the high defect concentration based on the two‐dimensional nucleation‐controlled theory . Besides, it is found that the grain growth of the NKN‐based ceramics is slightly supressed in low oxygen partial pressure, which is similar with some previous reports .…”

“…5,6 Unfortunately, it is a common fact that their piezoelectric properties would degrade seriously when NKN-based ceramics are sintered in low oxygen partial pressure. 7 The relevant mechanism was especially in this work ascribed to rapidly increased electron concentration in low oxygen partial pressure, because conventional NKN-based ceramics the concentrations of oxygen vacancies and electrons is responsible for severe degradation of resistivity and piezoelectric properties as sintered in N 2 . By comparison, p-type NKN-based ceramics by Mn doping exhibit excellent electrical properties (d 33 = 368 pC/N, d * 33 = 643 pm/V, tanδ = 0.019, and IR = 39.9 GΩ · cm) in N 2 sintering atmosphere, as well interpreted by a series of proposed defect chemistry equations.…”

“…In the meantime, the alkali metal can volatilize during sintering process, accompanied by the loss of their associated suboxides. The volatilization process of alkali metal can be expressed as following equations…”

“…To prevent inner electrodes from oxidation, sintering process should be finished in low oxygen partial pressure to enable NKN‐based piezoelectric multilayer structures based on Cu/Ni inner electrodes to work appropriately . Unfortunately, it is a common fact that their piezoelectric properties would degrade seriously when NKN‐based ceramics are sintered in low oxygen partial pressure . The relevant mechanism was especially in this work ascribed to rapidly increased electron concentration in low oxygen partial pressure, because conventional NKN‐based ceramics exhibit an n‐type conduction mechanism.…”

Phase transition, microstructure and electrical properties of K1‐xNaxNbO3‐based ceramic sintered in reducing atmosphere

“…Multilayer piezoelectric actuators have been widely used in precision equipment. In the current market, the internal electrode of the multilayer actuator is a silver palladium electrode . As these actuators are an engineering solution, we must pay attention to the manufacturing cost.…”