Bridging length scales in piezoceramics for commercial actuators

The performance and reliability of multilayer piezoelectric actuators (MPA) for fuel injection in modern automotive systems is crucial to produce high-efficiency engines with low fuel consumption. These devices are built as multilayer metal-ceramic composites in order to maximise the piezoelectric response by applying low voltage. This response is induced by the alignment of ferroelectric domains in the ceramic under the effect of electrical and/or mechanical fields (ferroelectric and ferroelastic domain switching, respectively). MPAs, however, suffer from reliability problems: Since cracks might be induced during production, the effect of cyclic electrical, mechanical and thermal loading not only degrades the performance, but also threatens the survival of the device by crack propagation. The risk of failure is enhanced due to the mechanical stress produced by strain mismatch between the active (i.e. electrically driven) and passive (i.e. not electrically driven) zones, and also due to the clamping between the ceramic and metal layers. Although external mechanical and thermal loads may disrupt the preferential domain orientation (worsening the in- service response), domain switching ahead of cracks can increase toughening (providing a way to design devices with longer lifetime), and ad-hoc domain engineering could enhance the overall performance. It is thus mandatory to investigate the domain structure in several areas of the layered structure and on several length scales. Macro- scale analyses are needed to correlate domains to overall material properties (which are a function of the average crystallite response). Micro-scale analyses are needed to assess the structures in regions crucial for the reliability of the device (i.e. the border between active and passive zones, the tip of electrodes, the areas around cracks, etc.). Within our project, we will establish an unprecedented combination of methods to investigate the crystallographic (domain) orientation distribution in piezoceramics for commercial actuators on several length scales (macro-, micro- and nanoscale). We will perform this on materials that have been conditioned with different levels of mechanical stress and temperature, concentrating especially on critical areas in MPAs. We will measure also the macroscopic piezoelectric response, and develop models of domain distribution to aid the interpretation of the experimental results. This combination of local structure investigation, measurement of macroscopic properties and modelling will allow bridging the link between microscopic crystallographic orientation and macroscopic properties of the device. This will help also explaining the influence of external loads on MPAs in terms of the local structural parameters. This novel understanding is a crucial step to design piezoelectric devices with tailored domain structures in order to maximise their performance under varied temperatures. It is therefore expected that the research carried out here will constitute a significant breakthrough in the field of piezoceramics.

Due to the strict regulations for exhaust gases in cars, the combustion processes inside car engines have to be optimized. In order to decrease the amount of emitted pollutants and improve fuel consumption, the fuel has to be injected into the combustion chamber at high pressure and many times per second. These requirements, especially concerning the high pressures involved, cannot be met by conventional electromagnetic valves. A sustainable alternative is represented by piezoceramic actuators. These components expand or contract under an applied electric field, and thus allow producing a very precise elongation on the microscale. These macroscopic properties (i.e. the field-induced elongation) are based on the modification of the so-called ferroelectric domains. These are micro- or nanosized areas of the material, which possess a coherent electric polarization in a definite direction. However, it is not yet clear how the orientation of domains influences the elongation of piezoactuators in dependence of temperature and mechanical stress. This know-how is highly needed by piezoceramic producers, in order to improve and tailor the properties of the components that they produce.The aim of the TRP 302-N20 project is to contribute to the further development and improvement of piezoactuators by targeted characterization experiments. By employing microanalytic experiments during the project the influence of the domain structure on the macroscopic properties is elucidated. In fact, using non-contact methods based on X-ray or laser radiation, it is possible to determine the domain orientation over several length scales (from micro to nano) close to application conditions (of temperature and pressure). The project started in 2013 and delivered groundbreaking results for the automotive industry: the piezoactuators show better performances if they are pre-stressed prior to installing them into the car engine. The diverse analytical methods used in the project on several length scales allowed to determine that this effect is the result of an increase in field-induced domain movement and phase transitions promoted by the pre-stress. The identification of these aspects, which allow tailoring the properties of the components by modifying their structure, is important not only for the automotive industry, but also for sensor applications (e.g. acceleration, pressure and particle sensors), aircraft/aerospace condition monitoring, and energy harvesting components.