Lead-free programmable multistable piezo-thermal actuators (LEAP)

• Applicants: Professor Dr.-Ing. Thomas Hanemann; Professor Dr. Matthias Wapler
• Subject Area: Microsystems
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 438866249

We aim to develop novel multistable and programmable actuators based on the combination of piezoelectric and thermal actuation. Whilst generally creating new performance and functionality compared to present piezo actuators, we address the key challenge of implementing lead-free alternatives to conventional lead-ziconate-titanate (PZT) based ceramics: As lead is highly toxic and can accumulate in the environment (e.g. due to improper recycling and disposal), it is generically banned from electronic products and PZT relies on a temporary ROHS exemption. Lead-free piezo materials, piezo actuation and thermal actuation alone pose the following set of problems and limitations that we aim to overcome:a) The displacement is typically enhanced with lever mechanisms - bending and buckling - but at the cost of reduced force, keeping their product approximately constant. b) Lead free piezo materials have much lower charge coefficients than PZT.c) Thermal actuators have a high power consumption, in particular also when holding a position.d) Buckling actuators typically have only one ground state, not true multistability where a position is held without an applied electric signal.e) In addition to wanted or unwanted deformations, the temperature affects the piezoelectric response and may depolarize the piezo material above the Curie temperature.f) Lead-free piezo materials also have a lower Curie temperature than PZT. One key concept are mechanically multistable designs that use thermal actuation in combination with piezo actuation to switch between the different stable states, allowing us to design higher energy barriers that would not be overcome by piezo actuation alone (d). This keeps the power consumption low (c) and it enables higher forces and larger displacements, (a) and (b). Piezo actuation is then used to provide fine tuning around a stable point, and we aim to develop systems where the fine tuning range around different stable states overlaps, providing continuous displacement. The second key concept is to re-pole the piezo material with different micro-polarization patterns with a combination of in- and out-of-plane polarization of the piezo sheets assisted by heating to the Curie temperature - turning (e) and (f) into virtues. The remnant displacement then defines additional stable states, and the actuators can be re-programmed to have different electromechanical responses. While this will provide additional functionality, it will also increase the overall range of strains of the piezo material. Mechanically, we will use combined bending and buckling and aim to develop pre-tensioned planar spring systems with non-trivial order buckling states and transitions between these states. To avoid separate heating layers we aim to heat with ultrasound using the piezo material. Following the idea of structural simplicity, we will explore self-sensing through the frequency-dependent impedance of the piezo actuators.

• DFG Programme: Research Grants