Aerogels—solid foams consisting mostly of air—are some of the lightest known materials. By infusing them with magnetic nanoparticles, researchers have now made ultralight magnets that can do aerial acrobatics: levitating, rotating, and moving up and down when controlled from a distance with another magnet (ACS Nano 2019, DOI: 10.1021/acsnano.9b04818).
The aerogel magnets could be used to produce lightweight, low-friction motors for robots and drones, says the material’s developer Ivan I. Smalyukh, a physicist at the University of Colorado Boulder. The magnets could also be used in aircraft and spacecraft components, miniature electronics, and drug-delivery devices.
Typical aerogels are made of silica or cellulose and are used as spacecraft heat shields and building insulation materials. To make them, researchers start with a suspension of silica precursor compounds or cellulose nanofibers, add a catalyst to trigger the formation of a gel, and then remove the liquid in the gel by freeze-drying, which leaves behind a web-like, porous material made of pure silica or cellulose.
To make magnetic aerogels, Smalyukh, along with physicist Yong Xie of Beihang University and their colleagues, suspended cobalt nanorods in a silica aerogel precursor and barium hexaferrite nanoflakes in a cellulose aerogel precursor. These nanomaterials are ferromagnetic, which means they become permanent magnets when exposed to a magnetic field.
The researchers placed the suspensions inside a magnetic field during the gelling process, which caused the embedded nanorods and nanoflakes to line up along that applied field. The nanomaterials maintained their positions as the aerogels solidified, resulting in rectangular, 1 cm sized pieces of aerogels with thousands of tiny rod- or flake-shaped permanent magnets trapped inside.
The researchers performed simple tricks with the aerogel magnets to showcase their potential. When they put one on top of a small ring magnet with like poles facing each other, the repulsive force made the aerogel magnet float.
They also made aerogels in which the two halves of the material have opposing magnetic poles. They used this material to make an electrical switch by inserting a copper wire through one edge and then, using another magnet’s repulsive or attractive force, made the aerogel pivot up and down so that the wire completed or broke an electrical circuit.
Xie says the group is now working on making magnets out of even lighter aerogels made of carbon. They are also exploring applying the same approach to flexible hydrogels that can be controlled to change shape, instead of to aerogels. These hydrogels could potentially be loaded with drugs and guided to specific sites in the human body using an external magnet, Xie says.
“This is an important advance because the technology makes it possible to move aerogels in a controlled manner,” says Lei Zhai, a chemist at the University of Central Florida. The high cost of making large quantities of aerogels might limit widespread commercial use for now. Nonetheless, adds Dirk J. Broer, a chemical engineer at Eindhoven University of Technology, these aerogel magnets “certainly open a pathway to new lightweight devices, miniaturized electromotors, and composites with other functional materials.”