A recent article in Nature Communications proposed a high-entropy tungsten bronze-type relaxor ferroelectric ceramic with an ultrahigh recoverable energy density of 11.0 J/cm3 and a high efficiency of 81.9%.
Study: High-entropy relaxor ferroelectric ceramics for ultrahigh energy storage. Image Credit: sommart sombutwanitkul/Shutterstock.com
Background
Modern electrical devices require dielectric ceramic capacitors with ultrahigh power densities. Due to their unique electrostatic energy storage mechanism, dielectric capacitors exhibit high power density, ultrafast charge/discharge rate, and long service life. Consequently, they find applications in high-tech industries, including medical, military, and electric vehicles.
However, the poor energy storage density resulting in the low breakdown strength is a major hurdle in developing dielectric ceramics for commercial use.
Thus, novel ceramic compositions are being explored for pulsed power system applications. High-entropy ceramics are anticipated to be effective relaxors with enhanced energy storage performance.
Tetragonal tungsten bronze (TTB) structures, with the general formula A12A24C4B10O30, are promising candidates due to their complex structure and abundant dielectricity and ferroelectricity despite low polarizability.
Thus, this study aimed to achieve high polarization and entropy by selecting the common Sr2+ and Ba2+ ions as TTB A-site from the classical (Sr0.5Ba0.5)Nb2O6 ceramic, Pb2+ with high polarizability and La3+ and Na+ as other heterovalent ions.
Methods
Firstly, the researchers performed phase-field simulations to design the high-entropy ceramics by investigating the evolution of domain structures and polarization loops with the increasing A-site element from 1-ary to 2-ary, 3-ary, and 5-ary.
Subsequently, SrCO3, BaCO3, Pb3O4, La2O3, Na2CO3, and Nb2O5 were used as starting materials to synthesize (Sr0.2Ba0.2Pb0.2La0.2Na0.2)Nb2O6 (SBPLNN) bulk ceramics via conventional solid-state method.
The crystal structures of the samples were characterized by an X-ray diffractometer (XRD), while their temperature-dependent Raman spectra were recorded on a Raman spectrometer.
Additionally, the microstructure of the ceramics was analyzed by a tabletop microscope, and piezoelectric force microscopy (PFM) was employed for ferroelectric domain characterization.
The ceramics were characterized at the atomic scale using transmission electron microscopy (TEM) incorporating energy dispersive spectroscopy (EDS). This data was used to determine the atomic structure precisely by implementing a specialized MATLAB (Matrix Laboratory) software algorithm.
Finally, the SBPLNN ceramics were characterized for electrical properties, including polarization, leakage current density, dielectric permittivity, loss tangent, complex impedance, and charge-discharge performances.
Results and Discussion
The phase field simulations predicted strong relaxors' energy storage capacity and temperature stability in 5-ary high-entropy ceramics. Accordingly, the researchers designed SBPLNN high-entropy ceramics by introducing five A-site elements with an equimolar ratio.
The macroscopic polarization of these ceramics was temperature-insensitive, including the polarization distribution state and domain size variation. X-ray Rietveld refinement indicated the presence of a pure tungsten bronze structure with a P4bm tetrago nal space group in SBPLNN.
Overall, the ceramic had a dense microstructure comprising refined and equiaxed grain sizes of ~1.57 μm. Additionally, EDS mapping demonstrated a uniform element distribution without any segregation. These results verified the successful fabrication of single-phase high-entropy TTB ferroelectrics.
The hysteresis loops maintained a slender feature with a high maximum polarization (~45.9 μC/cm2) and a low remanent value (~ 2.8 μC/cm2) under the maximum test field. Thus, the proposed equimolar ratio high-entropy design proved to be a practical and efficient method for developing advanced pulse power dielectric materials.
The fatigue resistance of SBPLNN ceramics remained robust up to 106 cycles under 470 kV/cm. Simultaneously, the ceramics exhibited excellent temperature stability of between −120 °C and 120 °C. In addition, the thermal structural evolution was revealed by the temperature-dependent XRD and Raman spectroscopy. The positions and numbers of diffraction peaks remain unaltered in a temperature range of −160 °C to 290 °C.
EDS mapping revealed that high-entropy effects broke the inherent site selectivity of TTB structures, confirming enhanced local compositional inhomogeneity of SBPLNN high-entropy ceramics. This resulted in local structure disorder and polar fluctuations, which modulated the relaxor features.
The statistical analysis of the lattice structure exhibited random distribution of the displacement directions from −180° to 180°, indicating a non-periodic lattice distortion in SBPLNN ceramics.
Such an atomic disorder and lattice distortion introduces grain refining in the microstructure, accompanied by higher resistivity, superior electrical homogeneity, and low leakage current density. Thus, SBPLNN ceramics were capable of ultrahigh-energy storage performance.
Conclusion
Overall, the researchers effectively fabricated high-entropy TTB-structured ceramics with (Sr0.2Ba0.2Pb0.2La0.2Na0.2)Nb2O6 composition by introducing equimolar-ratio elements with distinct valence and radii at A sites.
These ceramics demonstrated significantly enhanced energy storage performance, including energy density, energy efficiency, frequency/temperature/fatigue stability, and discharging performance.
The atomic-scale microstructural analysis helped attribute this excellent comprehensive energy storage performance to the increased atomic-scale compositional heterogeneity due to high-entropy configuration.
This modulated the relaxor features and induced lattice distortion, reducing polarization hysteresis and enhancing breakdown endurance.
Consequently, SBPLNN ceramics exhibit great potential for practical applications as high-power pulse capacitors.
The equimolar-ratio element high-entropy strategy proposed in this study can be a universal, practical, and efficient method for developing next-generation dielectrics with exceptionally high energy storage capabilities.
Journal Reference
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