Ceramic capacitors with upper operating temperatures far beyond 200°C are essential for high-temperature electronics used in deep oil drilling, aviation, automotive industry and so on. Recent advances in existing lead-free dielectrics for potential high-temperature capacitor applications are reviewed and grouped into three categories according to the parent component of the solid solution. Their desirable temperature stabilities were summarised comprehensively. However, there are still some limitations in the current research, such as achieving low loss in a wide temperature range and maintaining stable dielectric properties with different frequencies or at different voltages. Furthermore, the successful implementation of multilayer ceramic capacitors is one of the biggest challenges, which will have far-reaching impacts on the realisation of high-temperature capacitor application in the future.
To build piezoceramics with high transduction coefficient (d 33 × g 33 ) is the key to improve the power generation capability of piezoelectric energy harvester. Here, a new targeted doping strategy has been proposed to significantly increase the energy density of piezoceramics. Taking the modification of 0.2Pb(Zn 1/3 Nb 2/ 3 )O 3 -0.8Pb(Zr 0.5 Ti 0.5 )O 3 (PZN-PZT) as an example, dual functions can be achieved by introducing appropriate amount of target-doped (Zn 0.1 Ni 0.9 )TiO 3 (ZTN9) based on its pyrolysis characteristics. On the one hand, Ni 2+ ions enter the perovskite matrix to replace Zn 2+ ions to form equivalent doping; on the other hand, it induces the formation of 0-3 ZnO/perovskite composite structure, and both of which promote the large increase in d 33 × g 33 due to the changes in the domain configuration are more conducive to the ferro-/piezoelectricity. In all compositions, 0.67 mol% ZTN9 added specimen has a maximum value (12 433 × 10 −15 m 2 /N) of the d 33 × g 33 . The cantilever piezoelectric energy harvester fabricated with this material generates up to 4.50 μW/mm 3 of power density at 1 g acceleration, which is capable of quickly charging a 47 μF electrolytic capacitor and then lit 135 parallel-connected commercial blue light-emitting diodes (LEDs), showing its important application in implementing self-powered microsensors.
An ultra-wide temperature stable ceramic system based on (1Àx) [0.94 (0.75Bi 0.5 Na 0.5 TiO 3 À0.25NaNbO 3 )À0.06BaTiO 3 ]ÀxCaZrO 3 (CZ100x) is developed for capacitor application in this study. All samples exhibit characteristics of pseudocubic structures in XRD patterns. With CaZrO 3 addition, the coupling effect of polar nanoregions (PNRs) is weakening, leading to greatly improved temperature stability of dielectric properties. Among all samples, the most attractive properties are obtained in the composition of CZ10 at <15% variation in dielectric permittivity spanning from À55°C to 400°C and lower than 0.02 of dielectric loss of between À60°C and 300°C, accompanied by high DC resistivity (10 7 Ω m at 300°C, calculated by fitting Jonscher's power law). Furthermore, tentative multilayer ceramic capacitors (MLCCs) composed of CZ10 dielectric and Ag:Pd (70:30) internal electrode layers were fabricated by tape casting and cofiring processes. Temperature-stable dielectric property in formation of MLCC was successfully realized, with small DC/C 25°C (<15%) and loss factor (≤ 0.02) between À55°C and 340°C. Meanwhile, CZ10-based MLCC showed temperatureinsensitive energy storage density of 0.31À0.35 J/cm 3 and high-energy efficiency of above 77% at 120 kV/cm in the range of À55 to 175°C. All of these exhibit wonderful temperature-stable dielectric properties and indicate the promising future of CZ10 dielectric as high-temperature ceramic capacitors.
K E Y W O R D Scapacitor, ultra-high temperature ceramics, dielectric materials/properties
The occupation behavior of Y 2 O 3 in nonreducible BaTiO 3 -based ceramics was investigated thoroughly. Based on XRD, SEM, TEM-EDS, and complex impedance analysis, it is ensured that there exists two turning points of Y 2 O 3 , 0.50 mol % and 1.00 mol%, and the latter is the solubility limit. Below 0.50 mol%, the introduced Y 3+ ions precede to enter the A sites of the perovskite lattice, causing an observed enhancement in dielectric constant and energy storage density accompanied by an increase in grain size. Once the addition exceeds 0.50 mol%, the Y 3+ ions will turn to occupy B sites to substitute for Ti 4+ ions, leading to the significant reduction in dielectric constant and energy storage density. Above the solubility limit of 1.00 mol%, the excess Y 3+ ions would segregate at grain boundary and even induce the formation of Y 2 Ti 2 O 7 phase, resulting in an abrupt enhancement of grain-boundary resistance. Doped with 0.75-1.50 mol% Y 2 O 3 , all nonreducible specimens meet X7R requirement.
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