Novel CaLn4Mo3O16 (Ln = La, Nd, and Sm) ceramics: Sintering behaviour, phase structure and microwave dielectric performance

Abstract

In this study, novel CaLn4Mo3O16 (Ln = La, Nd, and Sm) ceramics were synthesised via a solid-state reaction method. Their microwave dielectric properties, crystal structure, and microscopic appearance were investigated. The ceramics exhibited a cubic fluorite-like structure similar to that of Nd5Mo3O16, with a Pn-3n (222) space group. In addition, second-phase CaMoO4 was present. The sintering mechanism of these ceramics was investigated via DSC-TG, which revealed that the sintering process occurred in two steps. Firstly, the raw materials generated CaMoO4 and Ln2MoO6 (Ln = La, Nd, and Sm). As the sintering temperature increased, the two phases underwent a chemical reaction to form the CaLn4Mo3O16 (Ln = La, Nd, and Sm) ceramics. The grains grew as the sintering temperature increased, and improvements in the microwave dielectric properties were also observed. An excessively high sintering temperature leads to overfiring and abnormal grain growth, resulting in degradation of the microwave dielectric properties. The microwave dielectric properties of the ceramics were closely related to their compositions, bulk densities, and grain sizes. The CaLn4Mo3O16 (Ln = La, Nd, and Sm) ceramics exhibited excellent microwave dielectric properties with εr, Q × ƒ, and τƒ of 13.6, 54182 GHz, and −26.0 ppm/°C for CaLa4Mo3O16 at 1375 °C, respectively; 13.84, 44560 GHz, −42.8 ppm/°C for CaNd4Mo3O16 at 1400 °C, respectively, and 14.57, 61920 GHz, −40.2 ppm/°C for CaSm4Mo3O16 at 1400 °C, respectively. These findings not only expand the system of microwave dielectric ceramics, but also indicate new materials for use in 5G construction.