Study on phase structures and compositions,microstructures, and dielectric characteristics of (1-x)NdGaO3-xBi0.5Na0.5TiO3 microwave ceramic systems

(1–x)NdGaO₃-xBi_0.5Na_0.5TiO₃ (1–x)NdGaO₃-xBNT, 0.05 ≤ x ≤ 0.7)] ceramic systems were fabricated using a conventional solid-state reaction, and their phase structures, microstructures, and microwave dielectric characteristics were systematically investigated. The XRD patterns results showed an orthorhombic perovskite structure as the main phase with a Pbnm space group at x = 0.05. In a range of x = 0.1–0.3, the main Nd₃Ga₅O₁₂ cubic structure phases (Ia-3d space group) had been formed due to the increase in the cation vacancies resulting from the Na and Bi ion volatilization at the higher densification sintering temperatures (1400–1450 °C). As (Na_0.5Bi_0.5)²⁺ ions increased and the sintering temperatures decreased, the orthorhombic perovskite-type Pnma space group solid solutions (the main phases) were detected at x = 0.4 and 0.5, accompanied with a certain amount of second phase Bi₂O₃ in the samples at x = 0.6 and 0.7. The results of Raman spectroscopy analysis, EDS data analysis, and surface morphologies observations were basically in keeping well with the XRD analysis results, wherein the Raman-active modes also implied that the crystal structure of the main phase actually had a tendency to change the perovskite structure when x = 0.3. The ε_r value increased gradually as the x value increased from 0.05 to 0.5 because of the increase in the ionic polarizability, and increased slowly at x = 0.6, then decreased slightly as the x value further increased to 0.7 due to the formation of the second phase. The Q × f values of the (1-x)NdGaO₃-xBNT (x = 0.05–0.7) ceramic systems were strongly influenced by the densification, lattice defects, and phase composition and content. The τ_f values could be well predicted by replacing Na, Bi and Ti ions with suitable substitutions for these ceramic systems. As a result, the new temperature-stable ceramics with optimal dielectric properties of ε_r ~43.1, Q × f ~6700 GHz (at 5.85 GHz), and τ_f ~ −24.5 ppm/°C and ε_r ~ 44.5, Q × f ~ 5600 GHz (6.12 GHz), and τ_f ~ +2.4 ppm/°C were obtained in the (1-x)NdGaO₃-xBNT composite series at x = 0.5 and x = 0.6 sintered at 1320 °C and 1250 °C for 4 h, respectively.