Mechanical Force at the Push of a Button
Magnetically activatable supraparticles convert heat into work and can lift 25,000 times their own weight
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magnetic nanoparticles can be heated without physical contact. Researchers at Friedrich-Alexander University Erlangen-Nuremberg (FAU), in collaboration with researchers at the University of Helsinki, are now using this effect to generate mechanical forces: Micrometer-sized superparticles convert the heat generated by an alternating magnetic field into gas pressure and volume expansion. As little as 20 milligrams of the material can lift a mass of 500 grams.
Heat Becomes Mechanical Work
Magnetic nanoparticles can efficiently convert electromagnetic energy into heat. When exposed to a high-frequency alternating field, their temperature rises sharply within a very short time. This principle of inductive heating is being investigated for the contactless heat treatment of materials.
The team led by Karl Mandel from the Chair of Particle-Based Materials Chemistry is pursuing a different approach: The magnetic nanoparticles are combined with a second component that reacts to the generated heat by expanding. Through spray drying, this results in micrometer-sized supraparticles in which the heat source and the functional component are directly linked.
When an alternating magnetic field is applied, the nanoparticles heat up. The heat generated locally then triggers rapid gas formation or vaporization within the supraparticles. The resulting pressure causes expansion—and can perform mechanical work.
Two Different Mechanisms
The researchers developed two systems for this purpose.
The first system uses azodicarbonamide (ADCA). The material acts as a chemical propellant and decomposes upon heating, rapidly releasing gas. Because the heat is generated directly by the neighboring magnetic nanoparticles, a large amount of gas can be produced in less than a second. The pressure causes the structure of the supraparticles to break apart.
The second system is based on a superabsorbent polymer made of poly(acrylamide-co-acrylic acid). This polymer initially absorbs water. After subsequent drying, a portion of the water remains firmly bound within the polymer structure. Upon inductive heating, it evaporates. Since the water vapor cannot initially escape from the polymer fast enough, the internal pressure rises and the material expands significantly.
The two systems thus exhibit different responses to the same external stimulus: While the ADCA system primarily releases large amounts of gas rapidly, the superabsorbent system results in significant macroscopic volume expansion.
20 milligrams lift 500 grams
The researchers demonstrated just how great the resulting forces can be with a simple lifting experiment. 20 milligrams of supramolecular particles were able to lift a mass of 500 grams—equivalent to 25,000 times the mass of the material used.
Remote Activation
A key advantage of the system is its contactless activation. The supraparticles require neither an electrical connection nor a direct heat source. An external alternating magnetic field is sufficient to trigger the expansion.
It is not only the temperature reached that matters. How quickly the magnetic nanoparticles generate heat is also crucial. Only if the heat is generated quickly enough can sufficient pressure build up inside the supraparticles before gas or heat is lost to the surroundings. Slow or uniform heating, such as in an oven, does not produce the same reaction.
The researchers were also able to show that the maximum temperature can be influenced by the choice of magnetic nanoparticles. For example, in addition to nanoparticles with the maximum heating rate, zinc-cobalt-ferrite nanoparticles can be used, whose lower Curie temperature limits the maximum achievable temperature. This allows for more controlled activation and helps prevent damage to the surrounding environment.
Epoxy resin can be selectively broken down
This concept is particularly interesting for materials that are normally difficult to separate. The researchers therefore integrated the supraparticles into an epoxy resin matrix.
When cured, epoxy resins form permanently cross-linked structures that cannot be easily broken down or reshaped. However, when the embedded supraparticles are exposed to an alternating magnetic field, they generate local gas pressure or volume expansion. The resulting mechanical stresses can cause the epoxy resin to break apart. In the experiments, the composite could be destroyed within a maximum of ten seconds.
This means that magnetically activatable supraparticles could become useful in the future, for example, for debonding on demand, the disassembly of composite materials, or the targeted release of individual components. Such locally and contactlessly triggered mechanical activation could open up new possibilities, particularly for materials with permanently cross-linked structures.
Microscopic Actuators for Adaptive Materials
The study thus demonstrates a new approach to using magnetic nanoparticles not only as heat sources but also as components of microscopic actuators. An external magnetic field triggers a chain of processes: the nanoparticles generate heat, which activates a chemical or physical expansion, and the resulting pressure is converted into mechanical work.
The combination of contactless activation, rapid response, and high force relative to the material’s mass could prove useful in the future for adaptive materials, switchable material composites, and microscopic actuators. The researchers see particular potential for systems in which mechanical functions need to be triggered without cables and without direct access to the material.
The study “Inductively Expandable Supraparticles as Microscopic Force Generators for Remote Mechanical Actuation” by Leoni Luthardt, Stephan Müssig, Robert Luxenhofer, and Karl Mandel was published in the journal Advanced Materials.
Note: This article has been translated using a computer system without human intervention. LUMITOS offers these automatic translations to present a wider range of current news. Since this article has been translated with automatic translation, it is possible that it contains errors in vocabulary, syntax or grammar. The original article in German can be found here.
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