ZnAl₂O₄-ZnO heterostructured nanocomposite with giant dielectric permittivity and epsilon-negative response:...

The development of multifunctional oxide heterostructures exhibiting simultaneously enhanced dielectric, electrical, and electromagnetic properties is highly desirable for next-generation electronic and radio-frequency devices. In this work, a ZnAl₂O₄@ZnO heterostructured nanocomposite was successfully synthesized via a combination of sol-gel auto-combustion and chemical precipitation techniques. X-ray diffraction combined with Rietveld refinement confirmed the coexistence of highly crystalline cubic ZnAl₂O₄ (15.1 wt%) and hexagonal ZnO (84.9 wt%) phases without detectable impurities, while elemental, morphological, and internal microstructure analyses verified the successful formation of a homogeneous ZnO phase surrounding the ZnAl₂O₄ cores. The nanocomposite exhibits a giant low-frequency dielectric permittivity reaching approximately 4 × 10⁴ at elevated temperatures, which is attributed primarily to strong Maxwell-Wagner interfacial polarization associated with the ZnAl₂O₄/ZnO heterointerfaces. Increasing the excitation frequency suppresses the interfacial polarization and produces a Lorentz-type dielectric resonance centered at approximately 43-44 MHz, accompanied by a transition of the real permittivity from positive to negative values, reaching approximately -64, together with an ε' = 0 crossover. The temperature-dependent dielectric response also exhibits a relatively stable permittivity variation within approximately ±10% over the -55 to +125 °C range at 10 MHz, corresponding to X7P-like temperature stability based on conventional EIA temperature-stability criteria. These results demonstrate that the ZnAl₂O₄@ZnO heterostructured nanocomposite architecture provides strongly tunable dielectric behavior, spanning giant positive permittivity at low frequencies and elevated temperatures to negative-permittivity response in the MHz regime. While these dielectric characteristics may be of interest for future investigations of temperature-stable capacitive components and electromagnetic structures, device-level performance, including RF resonators, electromagnetic shielding, metamaterials, and ENZ devices, was not directly evaluated in the present work.