Ultrahigh Energy Storage Density in Superparaelectric-Like Hf0.2 Zr0.8 O2 Electrostatic Supercapacitors.

Electrostatic capacitors attract great interest in energy storage fields due to their advantages of high power-density, fast charge/discharge speed, and great reliability. Intensive efforts have been placed on the development of high-energy-density of capacitors. Herein, a novel supercapacitor with Hf0.2 Zr0.8 O2 /xAl2 O3 /Hf0.2 Zr0.8 O2 (HAHx) is designed to improve the breakdown strength (Eb ) through optimizing Al2 O3 (AO) film thickness. Low-temperature annealing is first proposed to enhance the polarization difference (Pm -Pr ) due to the formation of dispersed polar nanoregions, which is called "superparaelectric-like" similar to previous super-paraelectric behavior of perovskite structures. As results, both large Eb and Pm -Pr values are obtained, leading to an ultrahigh energy storage density of 87.66 J cm-3 with a high efficiency of 68.6%, as well as a reliable endurance of 107 cycles. This work provides a feasible pathway to improve both the polarization difference and breakdown strength of HfO2 -based films by the combination of insulation insertion layer and low-temperature annealing. The proposed strategy can contribute to the realization of high-performance electrostatic supercapacitors with excellent microsystem compatibility.

Achieving Ultrahigh Energy Storage Density of La and Ta Codoped AgNbO3 Ceramics by Optimizing the Field-Induced Phase Transitions.

Energy storage capacitors are extensively used in pulsed power devices because of fast charge/discharge rates and high power density. However, the low energy storage density and efficiency of dielectric capacitors limit their further commercialization in modern energy storage applications. Lead-free AgNbO3-based antiferroelectric (AFE) ceramics are considered to be one of the most promising environmentally friendly materials for dielectric capacitors because of their characteristic double polarization-electric field hysteresis loops with small remanent polarization and large maximum polarization. An enhancement of these characteristics allows achieving a synergistic improvement of both the energy storage density and efficiency of the antiferroelectric materials. This work reports on a feasible codoping strategy enabling the preparation of AgNbO3-based ceramics with high energy storage performance. An introduction of La3+ and Ta5+ ions into the AgNbO3 perovskite lattice was found to increase the structural stability of the antiferroelectric phase at the expense of a reduction of local polar regions, resulting in the shifting of the electric field-induced antiferroelectric-ferroelectric phase transition toward higher fields. An ultrahigh recoverable energy storage density of 6.73 J/cm3 and high energy storage efficiency of 74.1% are obtained for the Ag0.94La0.02Nb0.8Ta0.2O3 ceramic subjected to a unipolar electric field of 540 kV/cm. These values represent the best energy performance in reported lead-free ceramics so far. Hence, the La3+/Ta5+ codoping has been shown to be a good route to improve the energy storage properties of AgNbO3 ceramics.

Grain Size Effects in Mn-Modified 0.67BiFeO3-0.33BaTiO3 Ceramics.

Grain size can have significant effects on the properties of electroceramics for dielectric, piezoelectric, and ferroelectric applications. Here, we systematically investigate the effect of grain size on the structure and properties of Mn-modified 0.67BiFeO3-0.33BaTiO3 ceramics, an important lead-free piezoelectric ceramic that exhibits both a high piezoelectric coefficient and a high Curie point. Ceramics with average grain sizes ranging from 0.46 to 6.85 µm were prepared using conventional and spark plasma sintering. It was found that the morphotropic phase boundary compositions are composed of two polar structures, rhombohedral and tetragonal, with DC poling inducing an increase in the fraction of the rhombohedral phase. All ceramics show relaxor behavior and their freezing temperature moves to higher temperatures with increasing grain size, although their Burns temperature is independent of grain size. In fine-grained ceramics, which show pronounced relaxor behavior, significant grain size dependency is seen in dielectric, piezoelectric, and ferroelectric properties, which is attributed to the presence of single ferroelectric domains and high concentrations of polar nanoregions. In coarse-grained ceramics, a critical grain size of 2.83 µm yields the highest dielectric permittivity at room temperature, with the piezoelectric coefficient plateauing at this grain size, which can be attributed to the contribution of both polar nanoregions and high domain wall density.

Terahertz Reading of Ferroelectric Domain Wall Dielectric Switching.

Ferroelectric domain walls (DWs) are important nanoscale interfaces between two domains. It is widely accepted that ferroelectric domain walls work idly at terahertz (THz) frequencies, consequently discouraging efforts to engineer the domain walls to create new applications that utilize THz radiation. However, the present work clearly demonstrates the activity of domain walls at THz frequencies in a lead-free Aurivillius phase ferroelectric ceramic, Ca0.99Rb0.005Ce0.005Bi2Nb2O9, examined using THz-time-domain spectroscopy (THz-TDS). The dynamics of domain walls are different at kHz and THz frequencies. At low frequencies, domain walls work as a group to increase dielectric permittivity. At THz frequencies, the defective nature of domain walls serves to lower the overall dielectric permittivity. This is evidenced by higher dielectric permittivity in the THz band after poling, reflecting decreased domain wall density. An elastic vibrational model has also been used to verify that a single frustrated dipole in a domain wall represents a weaker contribution to the permittivity than its counterpart within a domain. The work represents a fundamental breakthrough in understanding the dielectric contributions of domain walls at THz frequencies. It also demonstrates that THz probing can be used to read domain wall dielectric switching.

Superior Thermal Stability of High Energy Density and Power Density in Domain-Engineered Bi0.5Na0.5TiO3-NaTaO3 Relaxor Ferroelectrics.

Thermal-stable dielectric capacitors with high energy density and power density have attracted increasing attention in recent years. In this work, (1 - x)Bi0.5Na0.5TiO3-xNaTaO3 [(1 - x)BNT-xNT, x = 0-0.30] lead-free relaxor ferroelectric ceramics are developed for capacitor applications. The x = 0.20 ceramic exhibits superior thermal stability of discharged energy density (WD) with a variation of less than 10% in an ultrawide temperature range of -50 to 300 °C, showing a significant advantage compared with the previously reported ceramic systems. The WD reaches 4.21 J/cm3 under 38 kV/mm at room temperature. Besides, a record high of power density (PD ≈ 89.5 MW/cm3) in BNT-based ceramics is also achieved in x = 0.20 ceramic with an excellent temperature insensitivity within 25-160 °C. The x = 0.20 ceramic is indicated to be an ergodic relaxor ferroelectric with coexisted R3c nanodomains and P4bm polar nanoregions at room temperature, greatly inducing large maximum polarization, maintaining low remnant polarization, and thus achieving high WD and PD. Furthermore, the diffuse phase transition from R3c to P4bm phase on heating is considered to be responsible for the superior thermal stability of the high WD and PD. These results imply the large potential of the 0.80BNT-0.20NT ceramic in temperature-stable dielectric capacitor applications.