Effects of germanium oxide on properties of lead-free barium iron niobate ceramics

Abstract

In this work, effects of germanium oxide (GeO2) glass former on the sintering behavior, phase formation, physical and electrical properties of barium iron niobate (BaFe0.5Nb0.5O3) perovskite ceramics have been investigated in two main research aspects. In this case, two different preparation methods were employed. The first one is doping technique where GeO2 dopant was added into the calcined BFN powder from 1 to 5 wt.% and the second one is solid-solution technique of BaGex(Fe0.5Nb0.5)1-xO3 where x = 0.01 – 0.025. For the doping technique, the lead-free ferroelectric powder of single phase BaFe0.5Nb0.5O3 (BFN), having a cubic structure, was prepared by the conventional mixed-oxide method and calcined at 1200°C for 4 h in an ambient atmosphere. After that, the GeO2 was added to the prepared BFN powder with concentrations ranging between 1 and 5 wt.% and mixed by vibro-milling for 30 minutes in ethanol media. The mixed powders were pressed into pellets and subsequently sintered at various temperatures with a soaking time of 4 h to form dense ceramics. It was found that the GeO2 concentrations played an important role on the decreasing of sintering temperature as it could be lowered about 200-250C comparing with that of the pure BFN ceramic. The grain size of the GeO2 added BFN ceramics is approximately 5 times smaller than that of the pure BFN sample. It also was observed that the crystal structure of BFN changed from cubic to monoclinic with increasing GeO2concentration. The GeO2 doping could improve the dielectric properties of these ceramics at room temperature (25C). The 1 wt.% GeO2 doped sample exhibited higher dielectric constant of about 1800 and lower dielectric loss of 0.45. For conventional solid-solution technique, the fabrication of lead-free BaGex(Fe0.5Nb0.5)1-xO3: BGFN ferroelectric ceramics has been carried out. The ceramic powders were produced via mixed-oxide method with the desired compositions of BaGex(Fe0.5Nb0.5)1-xO3 where x = 0.01, 0.015, 0.02 and 0.025, and subsequently calcined at 1100 - 1200C for 4 h. To form the BGFN ceramic, the resulting powders were ground, pressed into pellets and then subsequently sintered at various temperatures from 1200 to 1350C with a soaking time of 4 h to obtain the ceramic with maximum density under each condition. It was observed that the Ge4+cations played an important role on the change of crystal structure and the phase formation. XRD patterns indicated that the structure of BFN was cubic and transformed to monoclinic where x  0.015. Dielectric and ferroelectric properties were greatly improved. Hysteresis loops of the BGFN ceramics exhibited large P-E loops and they became slimmer with increasing Ge4+ content. The optimum composition for BaGex(Fe0.5Nb0.5)1-xO3 system was found to be x = 0.01 and x = 0.015, where the maximum values of remnant polarization (~0.55 µC/cm2) and dielectric constant (~12282) with relatively lower dielectric loss at 1 kHz and at room temperature were achieved, respectively. Additionally, thermal annealing method was employed to the GeO2 doped BFN ceramics. After the ceramic samples sintered at 1125 – 1350C for 4 h, the resulting samples were investigated as a function of annealing temperature. It was found that the thermal annealing treatment caused the improvement in density and microstructure of these ceramics, leading to the change in their dielectric permittivity.Dielectric constant of these ceramics was greatly improved while dielectric loss slightly increased with increasing annealing temperature. In conclusion, the thermal annealing is considered as an effective and simple method to improve the dielectric constant of GeO2 doped BFN ceramics.