Improved microwave dielectric characteristics in CaHf1-xTixO3 ceramics

In the present work, effects of Ti-substitution for Hf upon microwave dielectric characteristics for CaHfO3 ceramics were investigated. CaHf1-xTixO3 solid solution in space group Pnma was determined in the present ceramics with x = 0-0.075, and only minor amount of secondary phases of CaHfO3 (R3 over bar c$\overline{3}c$, 1.3 wt%) and Ca2Hf7O16 (R3 over bar $\overline{3}$, 0.8 wt%) was detected. The significantly improved microwave dielectric characteristics were achieved with Ti-substitution in CaHfO3 ceramics. With increasing x, the temperature coefficient of resonant frequency tau(f) was substantially tuned from negative to positive through zero, which was caused by the decrease of oxygen octahedral tilting, and the Qf value was apparently improved simultaneously and reached the maximum of 43670 GHz at x = 0.025, while the dielectric constant epsilon(r) increased monotonously. The remarkable improvement of Qf was attributed to the enhancement of octahedral framework rigidity and the decreased nonuniform distribution of inner chemical bonding in crystal based on the change of chemical bond parameters calculated by the Phillips-Van Vechten-Levine bond theory, which was further confirmed by the higher frequency and narrower width of stretch mode of octahedron (i.e., A(1g)(O) near 730 cm(-1)) in Raman spectra. An interesting phenomenon was found in titanium-containing perovskite ceramics that they had consistent trend of tau(f) changing with tolerance factor, which was helpful to design temperature-stable perovskite microwave dielectric ceramics. The best combination of microwave dielectric characteristics was achieved at x = 0.035: epsilon(r) = 27.2, Qf = 42 870, and tau(f) = +3 ppm/degrees C. For practical applications of the present ceramics, further improvement of Qf is required, and it is expected to be achieved through microstructure optimization.

Automated summary

In this study, the effects of Ti-substitution for Hf on the microwave dielectric characteristics of CaHfO3 ceramics were investigated. It was found that Ti-substitution significantly improved the microwave dielectric characteristics due to changes in chemical bonds and enhanced rigidity of the octahedral structure. Additionally, it was observed that titanium-containing perovskite ceramics exhibited a consistent trend of tau(f) changing with tolerance factor, which is helpful for designing temperature-stable perovskite microwave dielectric ceramics.