Capable of changing form with the help of electronic impulses, the solid state carbon fibre composite was demonstrated by researchers from KTH Royal Institute of Technology, in a proof-of-concept study published recently by Proceedings of the National Academy of Sciences of the United States of America (PNAS).
Co-author Daniel Zenkert says the material exhibits all of the advantageous properties of shape-morphing material – without the drawbacks that have prevented other development work from taking flight, such as weight and insufficient mechanical stiffness.
Daniel Zenkert says:
“State-of-the-art morphing technologies, which can be used in robotics and satellite booms, rely on systems of heavy mechanical motors, hydraulic and pneumatic pumps, or solenoids to create shape changes. These mechanically complicated systems add what is known as “parasitic weight” and are costly to maintain. One way to reduce the mechanical complexity is to use solid-state morphing materials. We have developed an entirely new concept. It’s lightweight, stiffer than aluminum and the material changes shape using electric current. The material is capable of producing large deformations and holding them with no additional power, albeit at low rates.”
Solid-state carbon fiber composite morphing material in a cantilever setup. (A) SEM image and schematic of carbon fiber and SBE. (B) The composite material consists of two unidirectional Li-activated carbon fiber layers, and one ceramic-based Li-ion battery separator layer. All three layers are embedded in an SBE. A representative cross-section of the material captured using a light microscope is also shown. (C) Li ions are discharged from one carbon fiber layer to the other by application of a current, causing the discharging layer to contract in the fiber direction, and the charging layer to expand. This creates an overall bending deformation.
The composite consists of three layers – two of which are commercial carbon fibre doped with lithium-ions on each side of a thin separator. When the carbon fibre layers each have an equal distribution of ions, the material is straight. As electric current is added, the lithium ions migrate from one side to the other causing the material to bend. Reversing the current enables the material to return to a state of equilibrium and regain its previous, unbent form.
Fabrication process for the carbon fiber composite morphing material. (A) Two unidirectional carbon fiber layers and one ceramic separator layer are laid up on a mold. Copper current collectors are attached to the carbon fiber layers. (B) The layup is sealed with a vacuum-sealing film and tape, and the SBE is infused into the three layers under vacuum, before being heat cured for 45 min at 90 °C. (C) The cured three-layer composite material, with a density of ∼1,600 kgm−3, is ready for the activation process. (D) Two layers of Li metal foil are used as a source of Li ions and are placed on either side of the composite material, separated by layers of glass fiber paper soaked with liquid electrolyte. This is then sealed under vacuum inside a pouch bag. A current is applied (18.6 mAg−1) between the Li foil and the carbon fibers and the carbon fiber layers are charged/discharged several times before being left charged at 50% state of charge. (E) The composite material is removed from the pouch bag and clamped to form a cantilever. A current is applied between the two carbon fiber layers, charging/discharging the layers between −1.5 and 1.5 V causing the cantilever to bend.
“We have for some time worked with structural batteries, such as carbon fiber composites that also store energy like a lithium-ion battery. Now we have further developed the work. We expect it lead to completely new concepts for materials that change shape only by electrical control, materials that are also light and rigid.”
The researchers are now moving forward with other lightweight and structural materials that use less energy during use, with the ultimate aim of resource efficiency and sustainability.
Experiment showing the morphing carbon fiber composite material in a cantilever setup. A current is applied between the carbon fiber layers with cutoff voltages of −1.5 and 1.5 V. This causes Li to transfer from one carbon fiber layer to the other. The charging carbon fiber expands in the fiber direction as it reaches a higher state of charge, while the carbon fiber layer that is discharging contracts, creating a bending deformation. Potential difference between the carbon fiber layers and cantilever tip displacement are shown as functions of time. Images showing displacements of the morphing material are also shown. For a cantilever length of 48 mm a maximum tip displacement of 35 mm is reached for a charge transfer of 79.6 mAhg−1. The specimen here has average layer thicknesses: carbon fiber 53.4 μm and separator 21.0 μm.
Comparison between experiment, analytical prediction, and FE model prediction of tip displacement and curvature vs. change in charge for the carbon fiber composite morphing material in a cantilever setup. The specimen here has average layer thicknesses: carbon fiber 40.4 μm and separator 21.3 μm, and a cantilever length of 59 mm.