Effect of hydration on microstructure and microwave dielectric properties of CaSmAlO4-based ceramics

Ca1.15Sm0.85Al0.85Ti0.15O4 ceramics with different ball milling temperatures were prepared by the solid-phase method. The X-ray diffraction results showed that the high ball milling temperature (45 ± 5 °C) led to the hydration of the material phase, resulting in the formation of a large amount of (CaO)3Al2O3(H2O)6 phase. The results of the scanning electronic microscope and the energy dispersive spectroscopy confirmed that the hydrated powders resulted in heterogeneous microstructure characteristics with a wide grain size distribution of ceramics sintered, which degraded dielectric characteristics, especially reducing the quality factor (around 30,000). However, controlling the ball mill temperature in the range of 20 ± 5 °C, there was no hydration occurring in the slurry. Thus, the sintered ceramics at optimum temperature exhibited excellent microwave dielectric properties, a dielectric constant of around 19.6, a high-quality factor of 70,000 GHz, and a near zero temperature coefficient (< ± 4 ppm/°C) of resonant frequency in a wide temperature range from − 40 to 125 °C.