High quality, temperature stable MgAl2-x(Mg0.5Ge0.5)xO4 microwave dielectric ceramics: The dependence of microwave-millimeter-wave properties on the degree of inversion of A-B lattice sites

In this paper, a strategy for preparing MgAl 2- x (Mg 0.5 Ge 0.5 ) x O 4 (0 ≤ x ≤ 0.12) solid solution ceramics through the substitution of (Mg 0.5 Ge 0.5 ) 3+ for Al 3+ in the octahedral lattice of spinel crystal structure A(tetra)B 2 (octa)O 4 via traditional high-temperature solid-state reaction method is presented. The intrinsic and extrinsic factors influencing their microwave dielectric properties , including the inversion degree of A/B lattice sites (lattice occupancy), bond length and bond valence, as well as grain sizes, were investigated using Rietveld refinement of X-ray powder diffraction data and scanning electron microscopy. To verify the applicability for the next-generation 5 G-n257/n258 millimeter wave frequency band, the Q × f values were tested within the frequency range of 12–25 GHz. Notably, MgAl 1.91 (Mg 0.5 Ge 0.5 ) 0.09 O 4 ceramics exhibited a high Q × f value of 105, 384 GHz @11.83 GHz, low ε r of 8.7, and τ f of −66.4 ppm/℃. SrTiO 3 was added to tune the τ f to a near-zero value of +2.6 ppm/℃ with high Q × f value of 51, 152 GHz @11.53 GHz and ε r of 9.91. Finally, a microstrip filter with a center frequency of 3.6 GHz was designed using this composite ceramic, successfully demonstrating its practical application in the low-frequency band of 5 G networks.