Aluminum substitution for titanium in Ba_3.75Nd_9.5Ti₁₈O₅₄ microwave dielectric ceramics

Effects of aluminum substitution for titanium on microwave dielectric properties of Ba_3.75Nd_9.5Ti_18−zAl_zO₅₄ (z = 0–3.0) ceramics were investigated in this study. X-ray diffraction patterns of sintered samples revealed single-phase formation with an orthorhombic structure of all compositions. Scanning electron microscopy (SEM) and energy dispersive spectrometer results confirmed the substitution at b-site. A small amount of aluminum substitution into b-site lead to the decrease of relative dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f), but significantly improved the quality factor (Qf). Further aluminum substitution content of z ≥ 1.5 resulted in the drop of Qf value and restraint of Ti reduction. Excellent microwave dielectric properties with z = 1.5 sintered at 1,350 °C for 2 h in air would be obtained: ε_r = 72.7, Qf = 13,112 GHz and τ_f = +5 ppm/°C. Tolerance factor, electronegativity difference, ionic polarizability and b-site bond valence were calculated to clarify the relationship between microstructure and microwave dielectric properties.