One of the elements modern diesel engines require to become energy-efficient and clean are precisely controllable injection nozzles using piezo crystals. How exactly these crystals work has not been fully understood to date. In a project funded by the Austrian Science Fund FWF, a group of researchers from the Materials Center Leoben (MCL) in Austria has now gained insight into the mechanics of this technology—which can make the systems more reliable and efficient. Their results are also of interest for medical applications or energy harvesting.
Diesel engines are currently the object of strong criticism because of their emissions. While the share of nitrogen oxides can be reduced only by adding chemicals or by lower combustion temperatures, the generation of soot depends on the quality of the combustion process itself. To optimize combustion, injection schemes and timing has become complex. The multiple injections need to happen in a fraction of a second and require high precision injection nozzles. Magnetic valves are often too slow, which is why piezo crystals are used.
Piezo crystals expand when electric voltage is applied to them. Conversely, they generate voltage when subjected to mechanical stress. Hence, an electrically activated piezo crystal can open a valve. The most well-known material with this property is quartz, which was used typically as a clockwork generator.
The automotive industry uses ceramic materials that are known as ferroelectric and have slightly different properties, explains Marco Deluca, the project lead:
There’s a difference compared to quartz. If you exert pressure you create electrical voltage. In quartz, this property cannot be modified, but with ferroelectric materials it is possible to influence the direction of the material’s expansion.
Whereas the atoms in a quartz crystal are very ordered, ferroelectric ceramics consist of tiny domains that are smaller than one millionth part of a millimeter (one nanometer). If sufficiently high voltage is applied, these domains go through a cooperative reorientation. Because of the domains’ reorientation, the same voltage applied to ferroelectric ceramics results in a greater expansion than with quartz-like materials that are merely piezoelectric, Deluca explains. This greater elongation is of prime importance for injection nozzles.
In the automotive industry, common rail injection nozzles with piezo injectors have been used for several years, but there are some technical problems. Engineers struggle with cracks in the ceramic elements—which is why these elements are installed with a certain amount of pre-stress. Performance is improved if the actuators are installed in the engine under a load of about 50 mega pascal—but the manufacturers did not know why, said Deluca.
It was one of the objectives of the project to gain a better understanding of this effect. To do so, the team investigated commercially available piezo actuators in action using Raman laser spectroscopy and high-energy x-rays.
For his project, Deluca cooperated with the North Carolina State University, which has access to a particle accelerator (Advanced Photon Source) where the measurements were conducted.
Investigations showed that mechanical pre-stressing changes the orientation of the domains: the pre-stressing orders the domains in a certain direction perpendicular to the electric field axis. Once they are electrically actuated, more domains can change phase than is possible in the absence of mechanical pre-stressing, thereby creating greater changes in the length of the material. Based on this insight, the researchers were able to determine the optimum pre-stressing for technical applications.
Another objective of the project was to avoid the formation of cracks. Crack formation can be stopped if the original orientation of the ferroelectric domains is controlled, Deluca said. To do that one needs to first capture the direction in which the domains are oriented.
These insights are already being used by industry, reports Deluca. Not only the automotive industry is interested in this development, there are other technology sectors that also opt for piezoelectric methods.
In the end there’s always one problem: which orientation is best for this application? How can the orientation of domains be changed? And under how much strain can they be put without being destroyed? This will be primarily important for energy harvesting or energy self-sufficient sensors.—Marco Deluca
P. Kaufmann, S. Röhrig, P. Supancic, and M. Deluca (2017) “Influence of ferroelectric domain texture on the performance of multilayer piezoelectric actuators” Journal of the European Ceramic Society doi: 10.1016/j.jeurceramsoc.2016.12.029 (Open access)
G. Esteves, C. Fancher, S. Röhrig, G. Maier, J. Jones, M. Deluca (2017) “Electric-field-induced structural changes in multilayer piezoelectric actuators during electrical and mechanical loading” Acta Materialia 132 doi: 10.1016/j.actamat.2017.04.014 (Open access)
S. Röhrig, C. Krautgasser, R. Bermejo, J. L. Jones, P. Supancic, and M. Deluca (2015) “Quantification of crystalline texture in ferroelectric materials by polarized Raman spectroscopy using Reverse Monte Carlo modelling” Journal of the European Ceramic Society 35 doi: 10.1016/j.jeurceramsoc.2015.08.003 (Open access)