Study of the effects of composition and sintering parameters on dielectric properties of zirconia and niobium oxide doped barium titanate ceramics

Abstract

In this research, the effects of ZrO2and Nb2O5addition to BaTiO3 and sintering parameters on the dielectric properties were observed. When BaTiO3 ceramics are doped with ZrO2 or Nb2O5, the substitution of Ti4+ ions by Zr4+or Nb5+ions is expected, which isa temperature dependent process and can significantly alter the dielectric properties. However, it is also proved that a very small addition of ZrO2 or Nb2O5 e.g. 0.1 to 2wt% has the potential to yield very good dielectric characteristics. In this thesis the optimum sintering temperature yielding the smallest grain size along with 95-97% of theoretical density was found to be 1290°C for both 1 and 2 wt% ZrO2 added samples and 1275°C for 0.1 to 0.3 wt% Nb2O5 added samples. The experiments revealed that low soaking time e.g. 0 minute, can yield the best combination of sintering outputs. Double stage sintering was found to give better control over the grain size and thus more effective for the dielectric properties. The smallest grain size achieved was 500 nm for ZrO2 added samples and 600 nm for Nb2O5 added samples. Images from SEM suggested that the inhibition to grain growth in BaTiO3is a function of the amount of dopants and sintering parameters. Above 1300°C, significant grain growth was observed for ZrO2 added samples and the grains reduced in size with increasing ZrO2 content from 1 to 2 wt% BaTiO3.Microstructures showed the grain size to be finer in 2 wt% samples than in 1 wt% samples. These observations suggested ZrO2 diffusion inside BaTiO3 lattice above 1300°C and therefore the need for higher amount of ZrO2 to inhibit grain growth. For Nb2O5 modified samples 1320°C was the maximum sintering temperature and yet no significant grain growth occurred. Nb2O5 was found to yield the same microstructural effects brought by ZrO2 with addition of much lower amount than that of ZrO2. 1275°C sintering was found to yield the best dielectric characteristics for Nb2O5 added samples.XRD analysis showed reduction in tetragonality in both pure anddoped BaTiO3samples after sintering followed by decrease in cubic to tetragonal transition temperature and suppression of curie peak to some extent. The shift of tetragonal to orthorhombic transition point to higher temperatures was also understandable from the experiments.Such shifting of cubic-tetragonal peak and tetragonal-orthorhombic peak towards each other is a plausible cause of the rise in dielectric constant at intermediate temperatures.Nb2O5 added samples showed similar characteristics. At low temperatures where ZrO2 is known to linger at grain boundaries, long sintering time by means of double stage sinteringhas shown to improve permittivity on many occasions.Nb2O5 added samples were found to yield increasing dielectric constant with the percent increase in Nb2O5content even with lower percent theoretical density achievement.The permittivity (k) was found to yield consistent and flat response with temperature.ZrO2doped samples have shown permittivity (k) values as much as 3500 and that for Nb2O5 samples were found to be as high as 6500.