Synthesis, microstructure and microwave dielectric properties of Ca4-xMgxLa2Ti5O17 ceramics

Abstract

Ca4-xMgxLa2Ti5O17ceramics were prepared by a solid state ceramic route for x = 0, 0.5, 1, 2, 3 and 4. The structure and microstructure of the ceramics were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction results show that the Ca4-xMgxLa2Ti5O17 adopts an orthorhombic crystal structure with no secondary phase observed for x from 0 to 0.5. Secondary phase, MgTiO3 occurs with further increasing doping level (1 ≤ x ≤ 3). When x = 4, mixture phases La0.66TiO2.993, MgTiO3 and a trace of unknown phase coexist. Ca4La2Ti5O17 ceramic exhibits a relative permittivity (er) ~ 65, quality factor (Q × f) ~13,338 GHz (at ~4.75 GHz), and temperature coefficient of resonant frequency (τf) ~ 165 ppm/°C. The sintering temperature was distinctly reduced from 1,580 °C for x = 0 to 1,350 °C for x = 4. With increasing Mg content, er and τf obviously decrease, while Q × f value initially decreases and then increases. The ceramic for x = 2 shows er ~ 50, Q × f ~ 9,451 and τf ~ 62.5 ppm/°C. By the complete replacement of Ca with Mg, Mg4La2Ti5O17 ceramic sintered at 1,350 °C for 4 h combines a high dielectric permittivity (er = 31), high quality factor (Q × f ~ 15,021) and near-zero temperature coefficient of resonant frequency (τf ~ 4.0 ppm/°C). The materials are suitable for microwave applications.