Dielectric properties of barium strontium titanate (BST)/yttrium aluminate (YAlO3) thick films under DC bias field

The DC bias dependence of the dielectric properties of BST thick films on YAlO3 substrates has been investigated for possible tunable microwave device applications. Rare earth aluminate (e.g. YAlO3) substrates were used to overcome the interfacial interactions between BST thick films and alumina substrates during high temperature sintering. The results show that the BST films exhibit good chemical compatibility with YAlO3 substrates at sintering temperatures up to 1,500 C. Improved density and enhanced grain growth in the films have been obtained compared to BST films on alumina substrates sintered at lower temperatures (\1,250 C). Consequently, the permittivity and tunability are increased significantly. The low frequency losses in the ferroelectric region are also increased due to the contribution of domain wall motion. However, compared to their bulk ceramic counterparts, the films still exhibit a relaxor-like behaviour and DC bias hysteresis in both ferroelectric and paraelectric regions, which indicates some extent of microstructural heterogeneity and existence of micro-/nano- polar phases within the films. Possible reasons are discussed in terms of the substrate constraint induced stress effect in the films during high temperature sintering due to the thermal expansion coefficient mismatch between the BST films and YAlO3 substrates.