Effects of CaTiO₃ and SrTiO₃ additions on the microstructure and microwave dielectric properties of ultra-low-fire TeO ₂ ceramics

In this study, microwave dielectric ceramics of TeO₂ with various amounts of CaTiO₃ and SrTiO₃ were prepared. The densification, microstructural evolution, and dielectric properties of the ultra-low-fire ceramic composites were characterized. Three phases including TeO₂, TiTe₃O₈, and CaTe₂O ₅ or SrTe₂O₅ were observed in the CaTiO ₃- or SrTiO₃-added TeO₂ system. Eutectic reactions of the CaTe₂O₅-TeO₂ and SrTe ₂O₅-TeO₂ systems occurred at 650° and 590°C, respectively, enhancing the densification through liquid-phase sintering. The X-ray diffraction observations were confirmed as the Raman spectra located at 450-520 and 780-820 cm^-1 corresponded, respectively, toTiTe₃O₈ and CaTe₂O₅ or SrTe₂O₅. The densification temperaturesof TeO ₂-CaTiO₃ ceramics ranged from 630° to 670°Cdepending on the CaTiO₃ content, while all TeO ₂-SrTiO₃ceramics were densified at 610°C. TeO ₂-10 wt% CaTiO₃and TeO₂-15 wt% SrTiO ₃ ceramics were found to sustainbest performance in CaTe ₂O₅-TeO₂ and SrTe₂O ₅-TeO₂ systems. They showed the following major microwave dielectricproperties: eεr-28.6, Q×f≈15 600 GHz, and τf≈-3.24 ppm/°C, and eε_r≈28.7, Q×f≈12 400 GHz, and τf≈-3.81 ppm/°C,respectively.