Role of Bi₂O₃ in optimizing the dielectric properties of Ba_4.5Nd₉Ti₁₈O₅₄ based microwave ceramics

Small additions of Bi₂O₃ or bismuth titanate improve the dielectric properties of Ba_4.5Nd₉Ti₁₈O₅₄ at microwave frequencies. It was found that Bi³⁺ can replace Nd³⁺ up to 15 mol%, the limiting composition of solid solution being Ba_4.5(Nd_0.85Bi_0.15)₉Ti₁₈O₅₄. At the solid solution limit the temperature coefficient of resonant frequency attains its minimum value. The modified ceramic is distinguished by high permittivity of 99 and a Q-value of 5500. The temperature coefficient of resonant frequency is low, 15 ppm/K. After exceeding the solid solubility limit, additional Bi₂O₃ concentrates as a Bi-rich phase at the grain boundaries, causing considerable reduction of the Q-value and an increase of τ_f.