Structural, dielectric and ferroelectric studies of thermally stable and efficient energy storage ceramic material: (Na0.5-xKxBi0.5-xLax)TiO3

Abstract: The structural, dielectric and ferroelectric properties of lead-free (Na0.5-xKxBi0.5-xLax)TiO3 powders synthesized by sol-gel self-combustion method were investigated. Rietveld refinement of Synchrotron x-ray diffraction data confirms pure single phase rhombohedral crystal structure with R3c space group for all the compositions and anti-phase octahedral tilting angle decreased with increase in composition x. Homogeneity and elemental proportions were confirmed by Energy dispersive x-ray spectrometry. The temperature-dependent dielectric study has shown two diffuse type of dielectric anomaly for all the samples, due to A-site disorder in the lattice, which has been assigned to two-has transitions: ferroelectric to anti-ferroelectric and anti-ferroelectric to the paraelectric phase transition. The transition temperature of these phase transitions is found to decrease as a function of composition. Thermal stability range of dielectric constant increases from ~100C to 220C as a function of composition. Stable dielectric constant first increases, from 1557 10 % for parent compound, with the composition, highest for 6 % composition with emid ~ 2508 10 % for the temperature range ~180 C to 340C and after that decreases to 1608 10 % for 12 % but remain higher than the parent compound Na0.5Bi0.5TiO3. Ferroelectric measurements have shown monotonously decreasing coercive field as a function of the composition due to a decrease in grain size, confirmed by microstructural studies using Field Emission Scanning Electron Microscope. Exponential increases in the energy storage efficiency from ~ 17 % to 87 % as a function of composition have also observed. These types of materials, with stable high dielectric constant and low tan delta, have a vast scope in the field of the thermally stable dielectric constant materials and energy storage applications.

• The paper demonstrates that lead-free NKBLT-x ceramics exhibit improved energy storage efficiency, increasing from 17% to 87% with A-site substitutions.
• The study employs a sol-gel self-combustion method and synchrotron XRD to reveal structural transformations, including reduced octahedral tilting and lattice expansion.
• The results indicate stable dielectric performance over 80°C to 300°C and reduced hysteresis in ferroelectric behavior, promising for high-temperature electronic devices.

Structural, Dielectric, and Ferroelectric Studies of Energy Storage Ceramics

The paper "Structural, dielectric and ferroelectric studies of thermally stable and efficient energy storage ceramic material: (Na0.5-xKxBi0.5-xLax)TiO3" (1806.00587) explores lead-free ceramic materials' potential for energy storage, with a specific focus on (Na0.5-xKxBi0.5-xLax)TiO3 (NKBLT-x) compounds. These materials are evaluated for their structural, dielectric, and ferroelectric properties, aiming to address the environmental concerns of lead-based alternatives and contribute to advancements in high-temperature stable capacitor applications.

Synthesis and Structural Analysis

The NKBLT-x compounds were synthesized via a sol-gel self-combustion method. Synchrotron x-ray diffraction (SXRD) confirmed that all compositions maintain a pure single-phase rhombohedral structure with the R3c space group. As the substitution component $x$ increases, the structural analysis indicated reduced anti-phase ($a^-a^-a^-$) octahedral tilting from 8.47° to 5.89°. This change in tilting, along with the expansion of lattice parameters and increase in the unit cell volume, suggests successful substitution at the A-site in the lattice. The substitution results in increased static disorder and a reduced distortion factor, as reflected in increased thermal parameters and decreased Ti-O bond length variability.

Dielectric and Phase Transition Properties

Dielectric measurements revealed two key diffuse transitions: ferroelectric (FE) to anti-ferroelectric (AFE) and AFE to paraelectric (PE). With increased $x$, transition temperatures decrease significantly, reflective of suppressed distortion in the lattice structure. The dielectric constant peaks broaden and shift towards lower temperatures, allowing the materials to maintain stable dielectric constants over extended temperature ranges (80 ℃ to 300 ℃ for $x = 0.12$). These characteristics are advantageous for their application in high-temperature environments, such as aerospace or oil drilling.

The dielectric constant increased with K$^+$/La$^{3+}$ substitution, registering a highest value of approximately $2508 \pm 10\%$ for $x = 0.06$ within $180 ℃$ to $340 ℃$. A-site substitution enhances the polarizability, contributing to the elevated dielectric properties.

Ferroelectric and Energy Storage Efficiency

Ferroelectric P-E hysteresis loop analysis revealed a decrease in coercive field ($E_c$) and remnant polarization (P_r) with increasing substitution. The coercive field reduction is primarily attributed to reduced grain sizes resulting from inhibited grain growth, characteristic of rare-earth substitutions. While the remnant polarization initially increases with substitutions up to $x = 0.06$, a subsequent reduction is observed with further substitutions.

Energy storage efficiency witnesses exponential improvement, expanding from 17% for the parent compound to 87% for $x = 0.12$. This efficiency is facilitated by slimmer P-E loops indicative of reduced hysteresis loss. Consequently, NKBLT-x presents promising energy storage capabilities, especially for high-frequency electronics and devices where high energy efficiency is critical.

Conclusion

The paper delineates the synthesis and thorough characterization of NKBLT-x ceramics, detailing structural changes and enhancements in dielectric and ferroelectric properties across compositions. These materials demonstrate potential for integration into high-temperature dielectric applications, presenting a substantial advantage over traditional lead-based solutions. The remarkable increase in storage efficiency solidifies NKBLT-x as a formidable candidate for future high-efficiency energy storage systems, expanding the landscape for environmentally friendly and high-performance dielectric materials.