Relaxor-ferroelectric PMN–PT Thick Films

Abstract

Electroceramic thick films are 2D (planar) structures that, in their simplest form, consist of a substrate, a bottom electrode, a ceramic film and a top electrode, with the thickness of an individual layer being typically between 1 and 100 μm. The processing of thick films is similar to the processing of bulk ceramics, i.e., it involves powder synthesis, shaping and sintering. The major difference, however, is in the clamping of the film onto the substrate. The consequences of this are a constrained sintering, a possible reaction with the electrode or the substrate during firing, and the thermal stresses that are generated during the cooling. To minimize the chemical interaction of the film and the substrate the sintering temperature must be kept relatively low in comparison to that for the bulk. This requires a fine particle size for the powder and additives that form a liquid phase at the sintering temperature. This liquid phase must also be present to enable the densification under the constrained conditions, as was predicted and demonstrated in Kosec et al. (1999). The effect of clamping the active film to the relatively stiff substrate results in smaller displacements in comparison to substrate-free structures. Due to the mismatch of the thermal expansion of the thick-film and substrate materials during the processing the properties of the thick films differ from those of the respective bulk ceramics. Furthermore, an additional drawback can be the deterioration of the thick film’s properties due to its chemical interaction with the substrate. To ensure a sufficient displacement required for a certain application, different solutions have been proposed, such as a reduction in the thickness of the substrate, the processing of thick-film multilayer structures or the preparation of “substrate-free” structures. In the world of technology there is a continuous, global increase of interest in the miniaturisation of devices, materials and system integration. Thick-film structures are a good example of the opportunities offered for the miniaturization of electromechanical systems by the successful implementation of new functional materials and technologies. Driven by the versatility of conventional thick-film technology, the processing of functional structures with thick films on different substrates is possible, along with many design possibilities. In this chapter the progress in relaxor-ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) thick films is reported. The preparation procedures, the processing of PMN–PT thick films on different substrates, the structural and electrical characterization as well as PMN–PT thick-film devices are discussed. The phase composition and functional properties of PMN– PT thick films are discussed in terms of the thermal stresses generated in the films during