Nano and Grain-Orientated Ferroelectric Ceramics Produced by SPS

Abstract: Nano-powders of BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3, a mixture of the composition (BaTiO3)0.6(SrTiO3)0.4 with particle sizes in the range of 60 to 80 nm, and Bi4Ti3O12 with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO3)0.6(SrTiO3)0.4 mixture is 125 oC, ~75 oC for Bi4Ti3O12, ~25oC for BaTiO3 and SrTiO3, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba0.6Sr0.4TiO3. During the densification of the (BaTiO3)0.6(SrTiO3)0.4 mixture the reaction 0.6BaTiO3+0.4SrTiO3 → Ba0.6Sr0.4TiO3 takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures.The dielectric properties of selected samples of (Ba, Sr)TiO3 ceramics have been studied. The results are correlated with the microstructural features of these samples, e.g. to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region.The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi4Ti3O12 (BIT) and CaBi2Nb2O9 (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism.A new processing strategy for obtaining highly textured ceramics is described. It is based on a directional dynamic ripening mechanism induced by superplastic deformation. The new strategy makes it possible to produce a textured microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties.The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi4Ti3O12 ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi4Ti3O12 ceramics. Textured CaBi2Nb2O9 ceramics exhibited a very high Curie temperature, d33-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.