Microactuator Systems (MAS)
Smart materials are characterized by their multifunctional properties, in particular, sensing and actuation functions. Important examples are shape memory alloys,(SMAs), piezoelectrics, as well as multiferroic materials. These materials show large abrupt changes of their physical properties near phase transformations. Therefore, they are predestined for applications on the micro- and nano-meter scale.
Our research contributes to the research program 3: "Materials Systems Engineering" (MSE), Topic 1: "Functionality by Information-Guided Design: From Molecular Concepts to Materials", in the research field „Information“ as defined by the Helmholtz Association. Link to MSE website.
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Thermal shape memory effectShape Memory Alloys (SMA) are able to return to their original shape on heating even after large deformation of up to 10% (thermal shape memory effect). This effect is illustrated in the stress-strain-temperature diagram. By mechanical loading using a weight, the SMA spring is deformed to a large extent (1→2). This pseudo-plastic deformation is enabled by reorientation of crystallographic variants in the cold temperature phase (martensite). In martensitic state, no energetic preference exists among the differently oriented crystallographic variants and, therefore, this state is metastable. Consequently, the deformation persists after load removal (2→3). On heating, the material undergoes a phase transformation to austenite. Because of the cubic crystal structure only one crystallographic variant exists in austenite state and, thus, only one possible shape that can be taken on heating (3→4). During shape recovery, large strain changes and large forces occur that are of special advantage for the development of microactuators. |
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Magnetic shape memory effectMagnetic SMAs are able to return to their original shape even after large deformation of up to 12%, by applying an external magnetic field (magnetic shape memory effect). The underlying mechanism is illustrated in the schematic for the tetragonal structure of a Ni-Mn-Ga single crystal in martensite state. By applying a pressure, the crystallographic variants are oriented such that the short c-axes are aligned along the force direction (1). This state is metastable and persists after releasing the force (2). Because of magnetic anisotropy the magnetic moments are aligned along the c-axis. On applying an external magnetic field perpendicular to the c-axis, magnetic moments and corresponding c-axis being coupled with each other reorient in field direction. This field-induced reorientation of martensite variants occurs by a movement of twin boundaries through the material (3) until all c-axes are aligned in field direction (4). This process may happen within less than 1 ms in contrast to the thermal shape memory effect. |
A short introduction to the shape memory effect and demonstration of this effect for a spiral coil microactuator of NiTi lifting a weight hundred times larger than its own weight.
Download video: MP4 format, 13.2 MB
Deutsche Forschungsgemeinschaft (DFG) decided to establish the Priority Programme SPP 2206, which will be funded for 2x3 years.
Objectives of this interdisciplinary Priority Programme are to show up new methods and concepts to understand the complex coupling and synergy effects in coupled microactuator systems as well as to fabricate platforms for cooperative and/or multistage microactuator systems. The development of bi-/multistable mechanisms will be encouraged.
Guest editors: M. Kohl, S. Seelecke, U. Wallrabe
This Special Isssue collects the emerging research activities in the field of cooperative microactuator systems, which are expected to generate new synergies, e.g. through parallelization, cascading and multistability as well as through inherent sensing.
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Researchers at KIT and Tohoku University (Japan) succeeded in significantly enhancing the electrical power of thermomagnetic generators with respect to footprint. This research is the front topic of the December 2020 Issue of the Energy Journal ‘Joule’.
Original Publication: Joel Joseph, Makoto Ohtsuka, Hiroyuki Miki, and Manfred Kohl: Upscaling of Thermomagnetic Generators Based on Heusler Alloy Films. Joule, 2020.
DOI: 10.1016/j.joule.2020.10.019
Awarded for the Development an Realisation of an Ultra-compact Photodetector: Prof. C Koos, Prof. M. Kohl and Physicist S. Mühlbrandt (from left to right, Photo: T. Niedermüller, GSS).
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M. Gültig, H. Ossmer, C. Megnin and C. Wessendorf from the start-up company memetis became founders of the month July 2017 and won the Elevator Pitch BW 2017 (Photo: KIT).
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