Marija Kosec

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) Material for Actuator Applications

Due to its large piezoelectric and electrostrictive responses to an applied electric field the ()Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) solid solution has been widely investigated as a promising material for different actuator applications. This paper discusses some of the recent achievements in the field of PMN-PT piezoelectric and electrostrictive actuators manufactured from PMN-PT single crystals, bulk ceramics, or thick films. The functional properties of PMN-PT materials and some representative examples of the investigated PMN-PT actuator structures and their applications are reported.

Processing of Ferroelectric Ceramic Thick Films

The rapid development of the electronics industry has created the need for highperformance, high-reliability, miniaturised electronic components integrated into various electronic devices. Additional requirements, such as the desired size and weight, low cost, low power consumption, and portability, should be considered to make the devices user friendly and widely accessible. Attempts to miniaturise discrete elements have generally failed due to the difficulty in handling and assembly. A lot of waste material and high costs are also involved. In this approach, the ceramic parts are manufactured as a bulk ceramic, followed by a reduction in size by cutting, polishing, etc., to specified dimensions. The final step is the assembling of a thin layer of ceramic with the other components. This topdown approach imposes limits on the minimum dimensions of the manufactured parts. It constrains the geometry of the parts to simple shapes, like discs, plates, rings, cylinders, etc.

Growth and Characterization of Single Crystals of Potassium Sodium Niobate by Solid State Crystal Growth

Due to the toxic nature of PbO in Pb(Zr,Ti)O3 (PZT), the most common type of piezoceramic, many studies on lead-free materials are being conducted worldwide (Roedel et al., 2009). One of the most studied groups of lead-free ferroelectric materials is based on a solid solution of potassium sodium niobate, K0.5Na0.5NbO3 (KNN) (Jaffe et al., 1971; Kosec et al., 2008). However, activity in attempts to find a lead-free replacement for PZT really accelerated with the discovery of textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics, with properties comparable to those of PZT (d33> 300 pC/N, relative permittivity > 1500, planar coupling coefficient kp> 60%)(Saito et al., 2004). One way to enhance the piezoelectric properties of KNN is to grow KNN-based single crystals along certain crystallographic directions, as has been demonstrated for the case of KNbO3 (Wada et al., 2004) and relaxor-based ferroelectric single crystals (Park & Shrout, 1997). The most frequently used methods for growing alkali niobate based single crystals are top seeded solution growth and the flux methods (Chen et al. , 2007; Davis et al., 2007; Kizaki et al., 2007; Lin et al., 2009). However, these methods are not yet fully commercialized due to high costs and poor reproducibility related to compositional inhomogeneity within the crystals. A possible way to improve the homogeneity of crystals with complex composition is to grow them by the low cost Solid State Single Crystal Growth (SSCG) method. The SSCG method is essentially a form of induced abnormal grain growth, a phenomenon which is very well known in the solid state sintering community. A significant breakthrough in the solid state synthesis of lead-free materials has been achieved in recent years (Kosec at al., 2010; Malic et al., 2008a). However, due to the strongly hygroscopic nature of alkaline carbonates usually used in solid state synthesis, different diffusion rates of involved ionic species during annealing, and the high sublimation rates of the alkaline species at high temperatures (Bomlai et al., 2007; Jenko et al., 2005; Kosec & Kolar, 1975; Malic et al., 2008b), it may be rather difficult to obtain compositionally homogeneous alkali niobate-based single crystals by SSCG.

Ferroelectric Thin Films for Energy Conversion Applications

The chapter attempts to describe the potential of sol-gel derived ferroelectric thin films in energy conversion applications in two fields, namely in cooling by exploiting the electrocaloric effect and in energy harvesting. First, the basics of the sol-gel processing of ferroelectric thin films with the Pb(Zr,Ti)O3 solid solution as the representative material are described. The electrocaloric effect is described as a reversible temperature change in a material under an applied electric field at adiabatic conditions. Sol-gel derived relaxor ferroelectric thin films demonstrate a giant electrocaloric effect and they could be applied for solid-state cooling devices in microelectronics. In energy harvesting, the energy is derived from external sources, captured, stored and used, for example in small autonomous devices. In ferroelectric thin films, the piezoelectric effect, i.e. the generation of an electric charge upon applied stress, is exploited. Due to the miniaturization of electronic components, thin films with lateral dimensions in nanometer to micrometer range are appropriate candidates for future miniature applications related to energy.

Design and Fabrication of a Complex LTCC-Based Reactor for the Production of Hydrogen for Portable PEM Fuel Cells

The development process for complex 3D ceramic structures in the integrated evaporator, mixer, reformer and combustor (EMRC) system for the production of hydrogen needed in portable polymer-electrolyte membrane (PEM) fuel cells is presented. Low-temperature co-fired ceramic (LTCC) technology was used to fabricate the ceramic structures of the system. The process started with the design and fabrication of each separate (stand-alone) component, which was developed and tested, and then optimized to fulfill the requirements for the integrated EMRC system. The final 3D ceramic structure consists of 45 LTCC tapes (Du Pont 951PX). The dimensions of the structure are 75 × 41 × 9 mm3 and the weight is about 73 g.

Relaxor-ferroelectric PMN–PT Thick Films

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

Ink-jet Printing of TiO2 Suspensions

A general approach to prepare aqueous suspensions of inorganic particles suitable for ink-jet printing was developed. Titanium dioxide was used as the model material. Stable colloidal suspensions of TiO2 particles in water were prepared by adjusting the pH. The milling conditions were optimized in order to effectively mill the starting micron-sized powder. After 280 min of milling, the particle size was decreased to dmean = 170 nm. The fluid properties of the suspensions, i.e. viscosity and surface tension, were optimized by increasing the solid load of TiO2 powder and by addition of the surfactant triton X-100. Additionally, glycerol was added to the suspensions to control the drying behavior. The patterns of TiO2 were successfully printed on glass substrates.

Low-Temperature Processing

Low-temperature processing incorporates a variety of strategies implemented in chemical solution deposition (CSD) of functional oxide thin films with the aim of decreasing the final heating temperature and thus overcoming the problems related to interaction with the substrate and/or integration into microelectronic elements/devices, or deterioration of thin-film materials. The crystallization temperatures of thin films with different material compositions have been decreased by modification of the precursor solution and the design of the heating profile to ensure a high level of chemical homogeneity already in the liquid state and throughout the processing. In Pb(Zr,Ti)O3 thin films nucleation layers have been successfully implemented to decrease the temperature of perovskite crystallization. Some combined approaches to achieve lower processing temperatures are presented, including hydrothermal reaction, photochemical activation to facilitate the organics thermal decomposition, and laser activation of the as-deposited films. Oxide semiconductors on the other hand can be used as amorphous inorganic films, therefore the focus is on the design of low-temperature decomposable precursors.

Inkjet Printing of Piezoelectric Lead Magnesium Niobate-Lead Titanate Thick Films

This study shows the fabrication of lead magnesium niobate-lead titanate (PMN–PT) thick-films by inkjet printing on ceramic substrates for the first time. PMN–PT powder, prepared by mechanochemical synthesis, is dispersed in an organic solvent using a three roll mill to form a high-viscosity paste. For printing, the paste is thinned to a low-viscosity ink. Small amounts of ethyl cellulose in the ink prevent sedimentation. Glass particles are added to the paste to improve adhesion on the substrate surface. A two step inkjet-printing process forming homogeneous layers is introduced. After sintering at 950 °C for 2 h, piezoresponse force microscopy (PFM) is used to investigate the piezoelectric response of the printed films.

Large Strain Actuation of 0.65PMN–0.35PT/Pt Thick-Film Bimorphs

The effect of clamping a piezoelectric layer to a relatively stiff substrate results in smaller displacements in comparison to substrate-free structures. To ensure a sufficient amount of the bending of actuators, it is important to reduce the thickness of the substrate. For this reason an approach to preparing “substrate-free”, large-displacement actuators using the screen-printing method was developed. The “substrate-free” 0.65PMN–0.35PT/Pt actuators were prepared by screen-printing the 0.65PMN–0.35PT and Pt pastes on alumina substrates. After the screen printing and the subsequent firing the 0.65PMN–0.35PT/Pt composites were peeled off from the substrates. The normalized displacement (the displacement per unit length) of 55 μm/cm at 18 V was achieved, which is approximately five times higher than the displacement of related actuators from the literature.