Based on an oxide material that expands and contracts dramatically in response to a small temperature variation, the actuators are smaller than the width of a human hair and are promising for microfluidics, drug delivery, and artificial muscles.
"We believe our microactuator is more efficient and powerful than any current microscale actuation technology, including human muscle cells," says Berkeley Lab and UC Berkeley scientist Junqiao Wu. "What's more, it uses this very interesting material—vanadium dioxide—and tells us more about the fundamental materials science of phase transitions."
Wu is corresponding author of a paper appearing in Nano Letters this month that reports these findings, titled "Giant-Amplitude, High-Work Density Microactuators with Phase Transition Activated Nanolayer Bimorphs." As often happens in science, Wu and his colleagues arrived at the microactuator idea by accident, while studying a different problem.
Vanadium dioxide is a textbook example of a strongly correlated material, meaning the behavior of each electron is inextricably tied to its neighboring electrons. The resulting exotic electronic behaviors have made vanadium dioxide an object of scientific scrutiny for decades, much of it focused on an unusual pair of phase transitions.
When heated past 67 degrees Celsius, vanadium dioxide transforms from an insulator to a metal, accompanied by a structural phase transition that shrinks the material in one dimension while expanding in the other two. For decades, researchers have debated whether one of these phase transitions drives the other or if they are separate phenomena that coincidentally occur at the same temperature.
Wu shed light on this question in earlier work published in Physical Review Letters, in which he and his colleagues isolated the two phase transitions in single-crystal nanowires of vanadium dioxide and demonstrated that they are separable and can be driven independently. The team ran into difficulty with the experiments, however, when the nanowires broke away from their electrode contacts during the structural phase transition.
"At the transition, a 100-micron long wire shrinks by about 1 micron, which can easily break the contact," says Wu, who has a dual appointment as a professor in UC Berkeley's department of Materials Sciences and Engineering. "So we started to ask the question: this is bad, but can we make a good thing out of it? And actuation is the natural application."
To take advantage of the shrinkage, the researchers fabricated a free-standing strip of vanadium dioxide with a chromium metal layer on top. When the strip is heated via a small electrical current or a flash of laser light, the vanadium dioxide contracts and the whole strip bends like a finger.
"The displacement of our microactuator is huge," says Wu, "tens of microns for an actuator length on the same order of magnitude—much bigger than you can get with a piezoelectric device—and simultaneously with very large force. I am very optimistic that this technology will become competitive to piezoelectric technology, and may even replace it."
Piezoelectric actuators are the industry-standard for mechanical actuation on micro scales, but they're complicated to grow, need large voltages for small displacements, and typically involve toxic materials such as lead. "But our device is very simple, the material is non-toxic, and the displacement is much bigger at a much lower driving voltage," says Wu. "You can see it move with an optical microscope! And it works equally well in water, making it suitable for biological and microfluidic applications."
The researchers envision using the microactuators as tiny pumps for drug delivery or as mechanical muscles in micro-scale robots. In those applications, the actuator's exceptionally high work density—the power it can deliver per unit volume—offers a great advantage. Ounce for ounce, the vanadium-dioxide actuators deliver a force three orders of magnitude greater than human muscle. Wu and his colleagues are already partnering with the Berkeley Sensing and Actuation Center to integrate their actuators into devices for applications such as radiation-detection robots for hazardous environments.
The team's next goal is to create a torsion actuator, which is a much more challenging prospect. Wu explains: "Torsion actuators typically involve a complicated design of gears, shafts and/or belts, and so miniaturization is a challenge. But here we see that with just a layer of thin-film we could also make a very simple torsional actuator."
The Nano Letters paper was coauthored by Kai Liu, Chun Cheng, Zhenting Cheng, Kevin Wang, and Ramamoorthy Ramesh. Wu's earlier work on separating phase transitions in vanadium dioxide appears in Physical Review Letters, titled "Decoupling of Structural and Electronic Phase Transitions in VO2," and is coauthored by Zhensheng Tao, Tzong-Ru T. Han, Subhendra D. Mahanti, Phillip M. Duxbury, Fei Yuan, and Chong-Yu Ruan, and Kevin Wang.
For a video of this work: http://www.youtube.com/watch?v=aXyTN_lyVF8"Giant-Amplitude, High-Work Density Microactuators with Phase Transition Activated Nanolayer Bimorphs"
Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov.
DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at http://science.energy.gov .
More information about Junqiao Wu's research can be found at http://www.mse.berkeley.edu/~jwu
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
28.04.2017 | Event News
20.04.2017 | Event News
18.04.2017 | Event News
28.04.2017 | Medical Engineering
28.04.2017 | Earth Sciences
28.04.2017 | Life Sciences