Effect of sintering additives on the densification and dielectric properties of Sr0.5Ba0.5Nb2O6 ceramics synthesized by a soft-chemistry method

Abstract

Nano-crystalline Sr0.5Ba0.5Nb2O6 powders with addition of LiNbO3 or LiF as sintering additives were prepared by a soft-chemistry synthesis using polyethylene glycol. Calcination at 600 ​°C results in nanocrystalline powders (dcryst. ​≈ ​30 ​nm) which were sintered between 1000 and 1300 ​°C to ceramic bodies. By addition of 10 and 20 ​mol% LiNbO3, the sintering temperature was reduced by about 200 ​K and the activation energy of the initial stage of sintering decreases from 386 to 271 ​kJ ​mol−1. The sintering aid improves the grain growth and dense ceramic bodies were obtained after sintering at 1125 ​°C for 1 ​h. A higher LiNbO3 content favors the formation of a pillar-like microstructure. XRD patterns of ceramics sintered above 1000 ​°C show only reflections of the Sr0.5Ba0.5Nb2O6 phase indicating an incorporation of LiNbO3 in the Sr0.5Ba0.5Nb2O6 structure. Dielectric measurements reveal a diffuse phase transition and a relaxor-like behavior. The phase transition temperature (Tm) depends on the sintering conditions and is between 117 and 127 ​°C for 10 ​mol% LiNbO3, which is very close to pure Sr0.5Ba0.5Nb2O6, while addition of 20 ​mol% shifts Tm to around 200 ​°C and the transition becomes slightly more diffuse. The optical band gap of the samples is ​≈ ​3.3 ​eV and depends slightly on the sintering conditions. We also tried LiF as sintering aid, but this leads to the formation of considerable amounts of secondary phases in the ceramics and relative densities of only 82%.