An Experimental and Theoretical Insights into the Dielectric Properties of (Li, Nd) Co-doped ZnO Ceramics (1905.07226v2)

Abstract: In this work, we report the combined effect of donor (Nd) and acceptor (Li) co-doping at the Zn-site of ZnO ceramics on structural, microstructural and dielectric properties. Combining experimental observations with DFT based theoretical study, we have shown that before experimental fabrication DFT based first principles study can be used as a good indication to have prior qualitative assessment of a dielectric medium. For implementing this objective various Li and Nd co-doped ZnO ceramics have been synthesized through the conventional solid-state reaction route. Quantitative XRD analysis reveals the formation of wurtzite hexagonal structured ZnO having space group P63mc. Meanwhile, FESEM micrographs confirm the formation of randomly aligned non-uniform grains in size and shape. We show that the average grain size distribution and density of the studied compositions are two tuning factors to control the dielectric properties of these compounds. Though the value of dielectric constant is decreased with the increase in doping content, the optimum composition Nd0.005Li0.005Zn0.99O exhibits slightly lower dielectric constant (2066 at 1 KHz) than pristine ZnO but relatively very low dielectric loss (0.20 at 1 KHz) at room temperature than pure ZnO ceramics sintered at 1623 K. For understanding the dielectric relaxation mechanism in the studied ceramics, complex impedance spectra analysis have also been performed and discussed thoroughly. This study provides a new insight for further development of colossal permittivity (CP) ceramics and extends the current understanding of CP mechanism in ceramic materials.

• The paper demonstrates that (Li, Nd) co-doping in ZnO ceramics significantly lowers dielectric loss while slightly reducing the dielectric constant.
• Experimental solid-state synthesis paired with XRD, FESEM, and DFT simulations validates the impacts on structural, morphological, and electrical properties.
• Results show that the Nd0.005Li0.005Zn0.99O composition achieves optimal performance with a dielectric constant near 2066 and a loss tangent around 0.20.

An In-Depth Examination of "(Li, Nd) Co-Doped ZnO Ceramics" Dielectric Properties

This essay provides a comprehensive analysis of the dielectric properties in (Li, Nd) co-doped zinc oxide (ZnO) ceramics. The paper combines experimental techniques and Density Functional Theory (DFT) simulations to explore the impacts of co-doping on structural, morphological, and dielectric characteristics.

Experimental Methodology and DFT Simulation

The manufacturing process employs the solid-state reaction technique, targeting compositions of (Nd\textsubscript{0.5}Li\textsubscript{0.5})\textsubscript{x}Zn\textsubscript{1-x}O with varying x values. Rigorous XRD analysis certifies the wurtzite hexagonal structure, while FESEM micrographs confirm non-uniform, randomly aligned grains—a consequence of elevated sintering temperatures. Conversely, DFT simulations use the CASTEP code to pre-assess these properties computationally, serving as predictive models before actual synthesis.

Impact of Co-Doping on Structural Properties

Analysis reveals secondary phases, predominantly Nd\textsubscript{2}O\textsubscript{3}, when doping exceeds thresholds, attributed to Nd's significant ionic radius compared to Zn. Rietveld refinement with XRD substantiates partial incorporation at the Zn site, manifesting in peak shifts linked to Nd and Li substituents. While the realignment suggests increased lattice strain, the non-uniform peak adjustments indicate potential disruption by Nd's incomplete incorporation.

Dielectric Characterization and Morphology

The dielectric evaluation underscores a decrease in dielectric constant with increased co-doping, yet it highlights the Nd\textsubscript{0.005}Li\textsubscript{0.005}Zn\textsubscript{0.99}O composition's superior performance in reducing dielectric loss. This composition, achieving a dielectric constant of approximately 2066 at 1 kHz, demonstrates a lower loss tangent (approximately 0.20) than pristine samples—indicating improved energy storage capabilities. The morphological analysis elucidates substantial grain growth variability, largely attributed to the melting profile of the Li dopant and Nd-induced stresses, constraining larger crystal formations in higher dopings.

Conductivity and Impedance Analysis

AC conductivity, evaluated through frequency-dependent models, aligns with small polaron hopping theories. Complex impedance spectroscopy indicates non-Debye relaxation, bolstered by Cole-Cole plots highlighting the interplay between grains and grain boundaries. Variations in semicircular arc formations suggest grain-boundary limited dielectric contributions, with equivalent RC circuit modeling corroborating these interactions.

Theoretical Insights from DFT Studies

The DFT simulation informs about dielectric functions under high-frequency regimes, predicting minimal changes in real dielectric constants upon co-doping, yet observing a diminishment in the imaginary component, suggesting decreased ohmic losses. These theoretical projections concur with empirical observations, reinforcing DFT's utility in predictive analyses prior to synthesis.

Conclusion

While (Li, Nd) co-doped ZnO ceramics demonstrate a minor reduction in dielectric constants, the paper highlights significant advancements in reducing dielectric losses through strategic dopant selection and synthesis optimization. The integration of experimental findings with DFT simulations provides a robust framework for predicting dielectric material behavior, offering valuable insights into material engineering's next steps in tailoring dielectric properties for advanced applications in multilayer capacitors and energy storage systems. This research contributes to the nuanced understanding of tuning oxide ceramics for optimized electronic performance.