Electrical properties of ZnO-modified Li(Nb,Ti)O3 composite ceramics and their application in microwave dielectric resonators

The present study shows the combustion-assisted solid-state synthesis of ZnO-modified Li(Nb,Ti)O₃ ceramics for potential use in microwave dielectric resonators. The changes in microstructure and dielectric properties caused by the addition of ZnO were analyzed in detail. Using scanning electron microscopy, the characteristic microstructure consisting of anisotropic grains in the form of rods and plates was confirmed. At the same time, X-ray diffraction measurements confirmed the formation of polycrystalline ZnO-modified Li(Nb,Ti)O₃ composites with the main structure of M−phase type and the appearance of a new Li_1.33Zn₂Ti_2.67O₈ phase. Increasing wt% ZnO slightly increases the bulk density and intensifies the peaks of the secondary phase in the X-ray diffraction patterns. The impedance responses of the ZnO-modified Li(Nb,Ti)O₃ composites were recorded at different temperatures and the obtained experimental data in all cases describe only one semicircle of different diameters, the size of which depends on the temperature and wt% ZnO. By fitting the impedance spectra, it was determined that the changes in grain boundary resistance of all samples with increasing temperature are characterized by a negative temperature coefficient of resistance behavior, which agrees with the very high measured values of direct resistance at room temperature. Finally, optimal microwave performance with good temperature stability was obtained for a cylindrical resonator based on a Li(Nb,Ti)O₃ composite ceramic modified with 5 wt% ZnO which has a dielectric constant of ${\epsilon }_r=58.1$ and $Q\times f=43856\mathrm{GHz}$ at a resonant frequency of $9.85GHz$, and a temperature coefficient of the resonant frequency of around $-14.1ppm{/}^{°}C$.