The main achievements of the project during the mid-term of the project are as follows:
- We synthesized and characterized size- and shape-controlled nanoparticles of various SCO complexes. The particles were then dispersed in different commercial polymer matrices to obtain composite films, which serve us to construct actuator devices. Notably, we developed a novel family of complexes, Fe(NH2trz)3(BF4)2−x(SiF6)x/2, which appear quasi-ideal for actuation purposes with adjustable, near room temperature operation, large strains and needle shaped particle morphology. Beyond simple particle dispersion, we synthesized also chemically-coupled SCO-polymer composites as well as self-healing composites.
- We have synthesized a series of unprecedented compounds, which combine SCO-active and electroactive properties. These comprise a novel family of Hofmann clathrates of general formula {Fe(R-pbpy+)2[μ2-M(CN)4]2}, which couple the SCO with redox-active ligands, providing an innovative strategy for the development of SCO switches, which can be actuated via a reversible electrochemical reaction (instead of temperature change).
- We fabricated various spin crossover based composite materials using different electroactive polymer matrices, including piezopolymers and conducting polymers, and investigated the electromechanical couplings between the constituents. We also prepared multilayer structures comprising a piezoresistive organic semiconductor layer and an SCO-polymer composite layer. When inserted into a field effect transistor, the strain coupling between the two layers afforded for electrically sensing of the molecular spin state changes with high sensitivity.
- We analyzed structure - mechanical property relationships in bulk and nanoscale spin crossoover materials using variable temperature and pressure x-ray diffraction, DMA and nanoindentation techniques, combined with Molecular Dynamics simulations - providing us the necessary inputs to start working towards effective actuator designs. Remarkably, we discovered important anelastic effects at the spin transition, manifested by elastic softening and associated mechanical damping phenomena.
- We conducted a deep experimental (DMA) and theoretical analysis of the mechanical properties of the SCO-polymer composites. Thanks to these efforts, we are now able to predict material properties and design materials with enhanced performance.
- We fabricated a series of electrothermally actuated bending actuators, which were characterized using a custom-built bench, including open-loop identification and closed-loop PID control.
- We have embarked in the design (by means of finite element analysis) and fabrication (by means of 3D printing) of more complex actuator concepts. Notably, we developed shape-morphing actuators, which can realize various pre-programmed movements.
