The toxicity of lead and the danger of its entry into the environment make the search for lead-free, less hazardous piezoelectric ceramics a significant research topic. The European Union's Restriction of Hazardous Substances (RoHS) Directive (2011/65/EU) reinforces the urgency of finding lead-free alternatives to lead zirconate titanate (PZT) for electronic devices. This work deals with the fabrication of extremely thin, lead-free barium titanate ceramics for application in actuators. The background is a project with the aim of developing multistable, programmable micro actuators using combined piezo and thermal actuation and lead-free piezoceramics. Compared to PZT, Barium titanate (BaTiO₃) has a lower Curie temperature (123 °C) and smaller piezoelectric charge coefficients [1, 2, 3]. These limitations are to be overcome by a novel combination of bending and buckling actuation as well as piezoelectric and thermal actuation, which at the same time allows a continuous adjustment of the actuator.

The special challenge of this work is the production of wafer-thin, planar BaTiO₃ ceramics with a layer thickness in the range of 50-150 µm without an underlying substrate. The thin ceramic layers are prepared by electrophoretic deposition (EPD): The dispersed BaTiO₃ particles are deposited on an electrode by applying an electric field. This technology is simple and cost-effective. Furthermore, the high deposition rate and good control over the achieved layer thickness are advantageous. The latter can be set via the deposition time t and the applied voltage U. The required suspension contains BaTiO₃ powder (1 vol%) dispersed in a mixture of 2-propanol and acetone. 3,6,9-trioxadecanoic acid and PMMA serve as dispersant and binder, respectively. In order to obtain 100 µm thick layers, the electrodes are dipped into the suspension for 240 s at 100 V. The use of metal electrodes for the deposition of BaTiO₃ is not possible, since a subsequent detachment and sintering of the BaTiO₃ ceramic does not work non-destructively. The solution to this problem is the use of graphite electrodes: After the EPD process, the coated graphite electrode is ashed in a chamber furnace in air at 700 °C. The graphite electrode disappears. The BaTiO₃ ceramic that remains is sintered at 900-1200 °C. This fabrication process yields 25x25 mm² ceramics with a layer thickness of approximately 100 µm.

The current experiments focus on the influence of the particle size of the used BaTiO₃ powder (200?700 µm) on the surface roughness as well as the sintering behaviour of the BaTiO₃ ceramic. Research is also being conducted into optimising the sintering temperature and time. The analyses carried out include light scattering for determination of the particle size distribution, white light interferometry, thermogravimetric analysis and electron microscopy.

[1]  Sakayori K et al. Curie temperature of BaTiO₃. Jpn J Appl Phys 1995; 34(95): 5443.
[2]  Gao J et al. Recent progress on BaTiO₃-based piezoelectric ceramics for actuator applications. Actuators 2017; 6, 24 (20 pages).
[3]  Bruno BP et al. Properties of piezoceramic materials in high electric field actuator applications. Smart Mater Struct 2019; 28: 015029 (14 pages).