The demand for high-temperature dielectric materials arises from numerous emerging applications such as electric vehicles, wind generators, solar converters, aerospace power conditioning, and downhole oil and gas explorations, in which the power systems and electronic devices have to operate at elevated temperatures. This article presents an overview of recent progress in the field of nanostructured dielectric materials targeted for high-temperature capacitive energy storage applications. Polymers, polymer nanocomposites, and bulk ceramics and thin films are the focus of the materials reviewed. Both commercial products and the latest research results are covered. While general design considerations are briefly discussed, emphasis is placed on material specifications oriented toward the intended high-temperature applications, such as dielectric properties, temperature stability, energy density, and charge-discharge efficiency. The advantages and shortcomings of the existing dielectric materials are identif...

High-Temperature Dielectric Materials for Electrical Energy Storage

The development of lead-free bulk ceramics with high recoverable energy density (Wrec) is of decisive importance for meeting the requirements of advanced pulsed power capacitors toward miniaturization and integration. However, the Wrec (<2 J cm−3) of lead-free bulk ceramics has long been limited by their low dielectric breakdown strength (DBS < 200 kV cm−1) and small saturation polarization (Ps). In this work, a strategy (compositions control the grain size of lead-free ceramics to submicron scale to increase the DBS, and the hybridization between the Bi 6p and O 2p orbitals enhances the Ps) was proposed to improve the Wrec of lead-free ceramics. (K0.5Na0.5)NbO3–Bi(Me2/3Nb1/3)O3 solid solutions (where Me2+ = Mg and Zn) were designed for achieving large Ps, and high DBS and Wrec. As an example, (1 − x)(K0.5Na0.5)NbO3–xBi(Mg2/3Nb1/3)O3 (KNN–BMN) ceramics were prepared by using a conventional solid-state reaction process in this study. Large Ps (41 μC cm−2) and high DBS (300 kV cm−1) were obtained for 0.90KNN–0.10BMN ceramics, leading to large Wrec (4.08 J cm−3). The significantly enhanced Wrec is more than 2–3 times larger than that of other lead-free bulk ceramics. The findings in this study not only provide a design methodology for developing lead-free bulk ceramics with large Wrec but also could bring about the development of a series of KNN-based ceramics with significantly enhanced Wrec and DBS in the future. More importantly, this work opens a new research and application field (dielectric energy storage) for (K0.5Na0.5)NbO3-based ceramics.

Potassium–sodium niobate based lead-free ceramics: novel electrical energy storage materials

Perovskite solid solution ceramics of (1 − x)BaTiO3–xBi(Mg2/3Nb1/3)O3 (BT–BMN) (x = 0.05–0.2) were synthesized by solid-state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT-based ceramics. X-ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor-like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature-stable permittivity plateau (from −50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 1013 Ω cm at x = 0.15, and an energy density of about 1.13 J/cm3 is achieved in the sample of x = 0.1.

Relaxor Ferroelectric BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for Energy Storage Application

Ultrahigh discharge energy density (Wdis = 10.5 J cm−3) and efficiency (η = 87%) have been obtained in doped BiFeO3–BaTiO3 ceramic multilayers by achieving an electrically rather than chemically homogeneous microstructure. Back scattered scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy mapping of (0.7 − x)BiFeO3–0.3BaTiO3–xNd(Zn0.5Zr0.5)O3 (0.05 ≤ x ≤ 0.10) ceramics revealed a core–shell grain structure which switched from a bright to dark contrast as x increased. Compositions with x = 0.08 were at the point of cross over between these two manifestations of core–shell contrast. Dielectric measurements together with the absence of macrodomains in diffraction contrast TEM images suggested that compositions with x = 0.08 exhibited relaxor behaviour within both the core and shell regions. Impedance spectroscopy demonstrated that, despite being chemical dissimilar, the grains were electrically homogeneous and insulating with little evidence of conductive cores. Multilayers of x = 0.08 had enhanced breakdown strength, EBDS > 700 kV cm−1 and a slim hysteresis loop which resulted in large Wdis and high η which were temperature stable to <15% from 25 to 150 °C.

Ultrahigh energy storage density lead-free multilayers by controlled electrical homogeneity

Ceramic-based dielectric materials are regarded as the best candidates for advanced pulsed power capacitors because of their excellent mechanical and thermal properties. Nevertheless, lead-free bulk ceramics show relatively low recoverable energy storage density (Wrec < 2 J cm−3) owing to their low dielectric breakdown strength (DBS < 200 kV cm−1). In order to significantly increase Wrec, we proposed a strategy (compositions drive the grain size to submicrometer) to improve the DBS of lead-free ceramics. In this work, (1 − x)(K0.5Na0.5)NbO3–xSrTiO3 (KNN–ST) ceramics were chosen as a representative to verify the validity of this strategy. The (1 − x)KNN–xST ceramics (x = 0.15 and 0.20) with submicrometer grains (about 0.3 μm) were prepared using pressureless solid state sintering. A large Wrec (4.03 J cm−3) and DBS (400 kV cm−1 with a thickness of 0.2 mm) were achieved for 0.85KNN–0.15ST ceramics. The value of 4.03 J cm−3 is superior to all other Wrec in lead-free bulk ceramics and 2–3 times larger than that of other lead-free bulk ceramics. A large Wrec (3.67 J cm−3) and energy storage efficiency (72.1%) were simultaneously achieved for 0.80KNN–0.20ST ceramics. The results confirm that the (1 − x)KNN–xST ceramics (x = 0.15 and 0.20) are desirable materials for advanced pulsed power capacitors. The findings in this study could push the development of a series of KNN-based ceramics with enhanced DBS and Wrec in the future. On the other hand, this work could broaden the applications of KNN materials in a new field.

Significantly enhanced recoverable energy storage density in potassium–sodium niobate-based lead free ceramics

Single layer air co-fired capacitors with Pt internal electrodes were prototyped for the compositions 0.8CaTiO3–0.2CaHfO3 (CHT) and 0.5 mol% Mn-doped 0.8CaTiO3–0.2CaHfO3 (CHT + Mn) to yield a material with a room-temperature relative permittivity of er ~170, thermal coefficient of capacitance (TCC) of ±15.8% to ±16.4% from −50°C to 150°C, and a band gap of ~4.0 eV. Impedance spectroscopy revealed that doping with Mn reduces both the ionic and electronic conductivity. Undoped CHT single layer capacitors exhibited ambient energy densities as large as 9.0 J/cm3, but showed a drastic decrease in energy density above 100°C. When doped with 0.5 mol% Mn, the temperature dependence of the breakdown strength was minimized, and energy densities similar to ambient values (9.5 J/cm3) were observed up to 200°C. At 300°C, energy densities as large as 6.5 J/cm3 were measured. The design rationale for these dielectrics centered on materials with large band gaps, linear or weakly nonlinear permittivities, and high breakdown strengths. These observations suggest that with further reductions in grain size and dielectric layer thickness, the CaTiO3–CaHfO3 system is a strong candidate for integration into future power electronics applications.

High Energy Density, High Temperature Capacitors Utilizing Mn-Doped 0.8CaTiO3–0.2CaHfO3 Ceramics

Microstructural control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the present day challenges for increasing capacitive volumetric efficiency and high voltage dielectric properties. The present paper continues a series of investigations aimed at engineering the stability of ultra-thin Ni layers in base-metal electrode MLCCs. A kinetic approach based on the control of sintering profiles is found to not only prevent Ni electrode discontinuities, but also to significantly improve the interfacial electrical properties. Increasing sintering heating rates from 200 to 3000°C/h leads to a decrease in its temperature dependence of capacitance. Faster heating rates also reduce the BaTiO3 grain size, which is beneficial to the reliability of multilayer capacitors. A direct correlation between heating rates, the thickness of an interfacial (Ni, Ba, and Ti) alloy reaction layer and the interfacial contact resistance has been observed. The decrease in the alloy layer thickness at high heating rates leads to an increased effective Schottky barrier height between the dielectric and electrode toward its theoretical value of 1.25 eV for pure Ni–BaTiO3 interfaces.

Effect of Heating Rates during Sintering on the Electrical Properties of Ultra-Thin Ni–BaTiO3 Multilayer Ceramic Capacitors

High Temperature, High Energy Density Dielectrics for Power Electronics Applications

Microstructural control in thin-layer multilayer ceramic capacitors (MLCC) is one of the present day challenges for maintaining an increase in capacitive volumetric efficiency. It is observed that the continuity of the Ni electrodes increases with increasing heating rate but behaves non-linearly on sintering temperature. It is proposed that an interfacial liquid alloy layer initiates when the Ni electrodes are under tension. This accelerates a stress-induced diffusion which is the key cause of the severe electrode discontinuities during heating. Kinetic and thermodynamic approaches based on the control of sintering profiles or the control of Ni-BaTiO3 interface chemistry are proposed to prevent the Ni electrode discontinuity.

Stability of Ni electrodes and Ni-BaTiO3 interface evolution in ultrathin BME MLCCs

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Papers

High Energy Density, High Temperature Capacitors Utilizing Mn-Doped 0.8CaTiO3–0.2CaHfO3 Ceramics

Effect of Heating Rates during Sintering on the Electrical Properties of Ultra-Thin Ni–BaTiO3 Multilayer Ceramic Capacitors

High Temperature, High Energy Density Dielectrics for Power Electronics Applications

Stability of Ni electrodes and Ni-BaTiO3 interface evolution in ultrathin BME MLCCs