0.93Bi0.5Na0.5TiO3-0.07BaTiO3 (BNTBT) and KNbO3 (KN) powders with average particle size of ∼50 nm and ∼300 nm were synthesized by sol-gel method and hydrothermal method, respectively. Then, (1 − x)(BNTBT)-xKN (BNTBT-KN, x = 0, 0.01, 0.03, 0.05, 0.07) ceramic samples were prepared using these two powder precursors. The structure, dielectric and energy-storage properties of BNTBT-KN ceramics were comprehensively investigated. All the ceramic samples were in single perovskite structure, indicating that KN can completely dissolve into BNTBT within the studied composition range. BNTBT-KN ceramics exhibited a high dielectric constant at room temperature, being in the order of 1430–1550. Ferroelectric hysteresis loops at room temperature became more slim with the increase of KN content, which largely improved energy-storage density and efficiency. For the composition of x = 0.05, the maximum recoverable energy-storage density reached 1.72 J/cm3 under 16.8 kV/mm, which is superior to linear dielectrics and even some Pb-based systems. All these results demonstrate that 0.95BNTBT-0.05KN fabricated by wet-chemical method is a promising lead-free dielectric material for energy-storage capacitors.

SrTiO3@SiO2 nanopowder was synthesized via a core-shell nano-scale technique that is known as the Stöber process. The effect of the SiO2 concentration on microstructure, dielectric response and energy storage properties of SrTiO3@SiO2 ceramics was investigated. Transmission electron microscopy (TEM) results confirmed the formation of core–shell nanostructures with controlled shell thicknesses between 2 nm and 13 nm. After increasing SiO2, a secondary phase with Sr2TiSi2O8 appeared due to inter-diffusion reactions between the SrTiO3 core and SiO2 shells during the sintering process. The results show that both breakdown strength and energy density improved apparently. The homogeneous coating of silica on ST cores is considered to dominate the contribution to improved breakdown strength. The composition for SrTiO3 coated with 2.5 wt% SiO2 shows the maximum energy storage density (1.2 J/cm3) and a breakdown strength of 310 kV/cm. The former is higher than for pure SrTiO3 (0.19 J/cm3). Measurements of the dielectric performance indicate that the SrTiO3@SiO2 ceramics possess good bias stabilities compared to pure ST ceramics.

Sr0.9775Sm0.015TiO3 ceramics with giant permittivity of about 40,000 and low dielectric loss of 0.0068 measured at 25 °C and 1 kHz were obtained by sintering the ceramics in N2 at 1500 °C. The surface structure was analyzed by XPS measurement, which implied that a large number of Ti3+ ions existed in the Sr0.9775Sm0.015TiO3 ceramics sintered in N2. The results indicated that making the oxygen vacancies fully ionized by N2 sintering could increase the permittivity and decrease the dielectric loss effectively.

The dielectric properties of SrTiO3 ceramics sintered in nitrogen (N2) exhibit a weak temperature- and frequency-dependent giant permittivity (>104) as well as a very low dielectric loss (mostly < 0.02) over a broad temperature range from −100 to 200 °C. Based on the results of ac conductivity and structural analysis, the giant permittivity and low dielectric loss were due to the fully ionized oxygen vacancies and giant defect-dipoles. When further sintering these ceramics in air, the materials exhibit a large temperature- and frequency-dependent high dielectric loss, which were due to the ionization and motion of oxygen vacancies.