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Microwave Dielectric Properties of Ba4.2(Nd0.5,Sm0.5)9.2Ti18O54 Ceramics Doped with Al2O3 and SiO2

The effects of Al2O3 and SiO2 additives on the microwave properties of Ba6-3x(Ndy,Sm1-y)8+2xTi18O5 (x=0.6 and y=0.5; BNST) materials have been investigated. The microstructure of BNST doped with Al2O3 or SiO2 was analyzed using X-ray diffraction (XRD) analysis and scanning electron microscopy. XRD analysis did not reveal any second phases or modification of the BNST crystal structure. The addition of SiO2 at more than 0.2 wt % increased grain growth and shortened the densification process. The addition of small amounts of Al2O3 (≤2 wt %) increased the Qf markably and improved the TCf, while it decreased the density and dielectric constant (εr) of BNST. Doping 0.5 wt % Al2O3, in addition to a small amount of SiO2 (=0.2 wt %), further increased the Qf of BNST. The dielectric constant, the Qf and the TCf of the materials degrade when doped with too much SiO2. The BNST doped with 0.5 wt % Al2O3 and 0.2 wt % SiO2 exhibited dielectric properties with εr=74.3, Qf=8700 GHz and TCf=8.8 ppm/°C at a sintering temperature of 1320°C for 2 h.