Ultra low permittivity and near zero temperature coefficient in eucryptite ceramics with magnesium and nickel

Currently, most of the microwave ceramics have dielectric constant ( ε r ) above 5. Novel materials with ultra low ε r are extremely needed to realize high-speed signal transmission. In this work, LiAl 1- x Ni x SiO 4- x /2 and LiAl 1- y Mg y SiO 4- y /2 ceramics were synthesized via the solid-state method. X-ray diffraction (XRD) analysis revealed that all compositions formed single-phase solid solutions with hexagonal structure. Ni 2+ and Mg 2+ , as heterovalent dopants, induced oxygen vacancies and enhanced the relative density of the samples. The grain size also gave a positive correlation with density. According to the Phillips-Van Vechten-Levine (P-V-L) theory, variations in ε r were influenced by bond ionicity. Notably, the quality factor ( Q×f ) significantly increased from 30,520 GHz to 41,100 GHz (40,300 GHz) with the help of Ni 2+ (Mg 2+ ) doping, primarily attributed to the variations of lattice energy and oxygen vacancies content. Furthermore, the temperature coefficient of resonant frequency ( τ f ) was predominantly governed by the bond energy and linear expansion coefficient. After sintering at 1150 °C, LiAl 0.985 Ni 0.015 SiO 3.9925 and LiAl 0.985 Mg 0.015 SiO 3.9925 ceramics exhibited excellent microwave dielectric properties: ε r =3.94 (3.92), Q×f =41,100 (40,300) GHz, and τ f =-9.45 (-19.44) ppm/°C, respectively. Ultra-low ε ᵣ and near zero τ f were obtained. These results demonstrated promising performance for microwave applications.