Fine-grained BaTiO 3 -based ceramics with an average grain size of 120−140 nm were prepared by a chemical coating method. The effect of Ho-Dy concentration on the microstructure, dielectric properties, and reliability of BaTiO 3 -based nanoceramics was investigated. Results showed that appropriate doping contents of Ho and Dy provide the highest dielectric constant of 2323 at room temperature and the temperature stability satisfied EIA X7R specification.Higher doping concentration specimens exhibit lower dielectric constant but gentler temperature stability, indicating thicker grain shells. The failure time under the highly accelerated lifetime test increased along with the increase of Ho-Dy and was consistent with the impedance analysis results. Doping elements are mainly distributed in grain shells, leading to an increase of resistance and activation energy. The compositions for the highest dielectric constant and best reliability were not the same, providing support for the composition design of dielectric layer material for different BME-MLCC applications.

Lead‐free 0.955K1‐xNaxNbO3‐0.045Bi0.5Na0.5ZrO3+0.4% mol MnO ceramics (Abbreviated as K1‐xNxN‐0.045BNZ+0.4Mn) were prepared by a conventional solid‐state sintering method in a reducing atmosphere (oxygen partial pressure pO2: 1 × 10−11 MPa). All K1‐xNxN‐0.045BNZ+0.4Mn samples show a pure perovskite structure with a polymorphic phase boundary (PPB) composed of rhombohedral (R) and tetragonal (T) phases. A high Na/K ratio and a low Na/K ratio can both induce an increase in the rhombohedral phase. The reverse piezoelectric coefficient d33∗ and its temperature stability in K1‐xNxN‐0.045BNZ+0.4Mn ceramics can be improved by controlling the Na/K ratio. The increase in the Na/K ratio from x = 0.46 to x = 0.56 can decrease the A‐site cation vacancies. The activation energy of the grain is higher than that of the grain boundary due to the accumulation of oxygen vacancies at the grain boundary. K1‐xNxN‐0.045BNZ+0.4Mn ceramics with excellent piezoelectric properties (quasi‐static piezoelectric coefficient d33 = 326 pC/N, and d33∗ = 472 pm/V at Emax = 25 kV/cm) were obtained at x = 0.52.

The defect mechanisms of rare earth (RE) doped BaTiO 3 have a strong impact on the electrical performance of the multilayer ceramics capacitors (MLCCs). Oxygen vacancy is the main reason for the device degradation over longtime use, while the effect of the doping strategy on controlling the oxygen vacancies is not yet quantitatively understood. In this work, the grand canonical thermodynamic defect model based on first-principle calculations is applied to evaluate the defect mechanism of RE-doped BaTiO 3 under practical experimental condition. The charge compensation and prior site occupancy of RE are found not only associated with ionic size but also exhibit transitions with oxygen partial pressure and doping concentration. Furthermore, the oxygen vacancy trapping ability of RE ions is evaluated from the perspectives of thermodynamics and kinetics. The migration barrier among first nearest oxygen sites dramatically changed depending on the RE site occupancy. The large trapping ability is contributed by the relatively large negative binding energy of the defect complex and comparable RE concentrations substituted on Ba and Ti sites. The two conditions can be achieved in amphoteric ions doped systems, while in pure donor doped BT only one of these conditions can be satisfied. Although the self-compensated defect complexes exhibit the highest binding energy, the trapping ability contributed by different defect complexes (RE ′ Ti , RE ⋅ Ba , RE Ba − RE Ti ) is generally comparable in these systems. This feature of amphoteric RE ions accounts for the improvement of the lifetime and reliability of MLCCs.

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