Microwave dielectric properties of Sr3Ti2O7 ceramics with composite element doping of Nd & Al

Sr3Ti2O7 microwave dielectric ceramics with Ruddlesden-Popper (R–P) structure were successfully prepared via a solid-state reaction method, and its phase composition, microstructure and properties were systematically studied in this work. Sr3Ti2O7 with doping Nd & Al ceramics presented a single-phase Sr3Ti2O7 structure in I4/mmm space group. The refined results of XRD showed that the unit cell parameters gradually decreased with the increase of Nd & Al amount. The dielectric constant (εr) was not only affected by ion polarizability, but also related to the factors such as grain size and density. XPS indicated that Nd & Al co-doping effectively eliminated the effect of charge on performance. Raman spectrum showed that the quality factor (Qf) was mainly concerned with the FWHM of 633 cm−1 Raman mode and the grain size. In order to further evaluate the change of the temperature coefficient of the resonance frequency (τf), the thermal expansion coefficient (α) was further corrected and taken as 20 ppm/°C in this system. The optimal microwave dielectric properties were obtained for composition Sr2.94Nd0.06Ti1.94Al0.06O7 at 1500 °C: εr = 52.64， Qf = 29954 GHz， τf = 232.9 ppm/°C.