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Zhong-Lin Wang takes nanoscale piezo energy scavenging to heart
ACerS member Zhong-Lin Wang continues to make interesting progress on developing nanowire power generators and other energy-scavenging devices, and recently has demonstrated a nanogenerator that can be powered by the motion of a beating heart or the flexing of diaphragms and lungs.
When I last wrote about Wang in early 2009, he was demonstrating a “flex charge pump” generator constructed of zinc-oxide piezoelectric fine wires that measure three to five microns in diameter and 200 to 300 microns in length. Back then, he was thinking these tiny generators could be used in self-powered wireless sensing systems that gather, store and transmit data. He imagined then that his method could be scaled down to a nano size.
Since that time, however, it appears that Wang, a professor at Georgia Tech, has also become more interested in applications involving biomedical sensors. In fact, in a paper published in Advanced Materials, he and his fellow researchers report on what may be the first in vivo testing of nanoscale power generators activated by the breathing and heart beat of a rat. This could be a significant step forward in the creation of self-powered implanted nanodevices that could, for example, monitor blood pressure or blood glucose levels. (It should be noted that a group of Cleveland-area researchers reported in July 2009 on a larger-scale in vivo generator activated by a rabbit’s quadriceps).
Wang and his team sealed zinc-oxide nanowires in a polymer. The polymer served as a shield to the rat’s body fluids and to be a barrier to outside electrical sources. They then glued the 2 mm x 5 mm rectangular unit to the rat’s diaphragm muscle. The breathing motion generated 4 picoamps of current at a potential of 2 millivolts. Even more power was generated when the unit was glued to the rat’s heart: 30 picoamps at 3 millivolts.
Wang acknowledges that, while significant, this new work is more of a interim step than a final achievement, and that much more power is going to be needed for actual sensors. But Wang notes that his group has also figured out how to integrate a large number of nanowire energy harvesters into a single 4 mm2 power source (a vertically integrated nanogenerator, or VING) and has demonstrated the feasibility with a self-powered nanowire pH sensor and a nanowire UV sensor.
Interestingly, Wang has also demonstrated a hybrid generation system that could be used in vivo. This system, used to power a UV sensor combines a piezo nanogenerator with a biofuel cell that scavenges biochemical energy (glucose/O2).
Apparently the next step if to do in vivo testing of a VING–sensor system.
He is another video featuring an interview with Wang from about a year ago
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Georgia Tech group creates 3D photovoltaic system
Dye-sensitized nanowires cover the outer surface of a optical fiber to optimize photon collection. (Credit: Angewandte Chemie International.)
What if there was a way to create a material covered with tiny 3D solar collectors instead of the typical 2D flat photovoltaic systems (and in this context flexible PV sheets still count as two-dimensional)? And, what if you could “feed” these collectors with sunlight via optical fibers? Then you might be able to tuck these systems (architecturally speaking) into out-of-the-way locations or sites less obvious than rooftops.
That was some of the thinking motivating a group of researchers at Georgia Tech whose work is reported on in a new paper in Angewandte Chemie International.
The GT group figured out a way to improve upon existing dye-sensitized solar cell technology by growing nanostructures (on the optical fibers) that effectively increase the surface area of a collector. Compared to other approaches, DSSCs, generally speaking, are at a disadvantage because they relatively inefficient. On the other hand, the manufacturing costs of dye-sensitized cells are low. They also tend to be able to take more mechanical abuse.
The group grows the nanostructures by replacing in one section the outer layer of quartz optical fiber with a conductive coating. They then seed the surface with zinc oxide followed by solution-based techniques that grow aligned zinc oxide nanowires that radiate outward around the fiber. Finally, the nanowire–optical fiber is given a dye-sensitized materials coating. Groups of these nanowire-coated fibers are immersed in an electrolyte to harvest electrons. Length improves efficiency and the group has been able to make nanowire sections as long as 20 cm.
Closeup of single nanowire-coated fiber. (Credit: Georgia Tech and Gary Meek.)
According the the GT group, this internal axial illumination in this hybrid system multiplies six-fold the energy conversion efficiency of the DSSC nanowire array. “In each reflection within the fiber, the light has the opportunity to interact with the nanostructures that are coated with the dye molecules,” explains Z.L. Wang, who led the group. “You have multiple light reflections within the fiber, and multiple reflections within the nanostructures. These interactions increase the likelihood that the light will interact with the dye molecules, and that increases the efficiency.”
The team says it has reached an efficiency of 3.3 percent and think efficiencies of 7 to 8 percent are in reach if they make further modifications, such as using a better method for collecting the charges and a titanium oxide surface coating.
These efficiencies are still a long way off of current 2D PV units. But Wang says there would be several advantages to the group’s hybrid DSSC system. The already low production cost could be driven lower by using polymer fibers. The optical fibers used to feed the nanowire fibers could be placed fairly freely, providing a larger area for gathering light, and lenses could also be employed to focus the incoming light.
Another advantage is that it gives building designers new options. “This will really provide some new options for photovoltaic systems,” Wang said. “We could eliminate the aesthetic issues of PV arrays on building. We can also envision PV systems for providing energy to parked vehicles, and for charging mobile military equipment where traditional arrays aren’t practical or you wouldn’t want to use them.”
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Creating alternating current with piezoelectrics
Flexible charge pumps that produce alternating current by stretching and relaxing zinc oxide wires. (Credit: GTI, Gary Meek)
Researchers at the Georgia Institute of Technology have created a micro-scale “flexible charge pump” that produces alternating current by utilizing the piezoelectric properties of cyclically stretched and released zinc oxide wires.
“The flexible charge pump offers yet another option for converting mechanical energy into electrical energy,” says project leader Zhong-Lin Wang, director of GIT’s Center for Nanostructure Characterization. Wang reports details of the project in the November issue of Nature Nanotechnology. “This adds to our family of very small-scale generators able to power devices used in medical sensing, environmental monitoring, defense technology and personal electronics,” he says, defining the pump’s significance.
Wang reports that the generator can produce up to 45 millivolts of electricity by converting about seven percent of the mechanical energy applied to the zinc-oxide wires. He notes that arrays of such generators could be used to charge low-power devices like sensors.
On GIT’s website, Wang explains that earlier nanowire generators and microfiber nanogenerators developed by his team required “intermittent contact between vertically-grown zinc oxide nanowires and an electrode or the mechanical scrubbing of nanowire-covered fibers.” Such pumps were difficult to build and had a short lifetime because their need for mechanical contact created wear that eventually wore them out. Additionally, because zinc oxide is soluble in water, so they also had to be protected from moisture.
The design of the new flexible charge pump resolves all of the earlier pumps’ shortcomings, according to Wang. In the new pump, he says, when the zinc-oxide wires are mechanically stretched and released, their piezo properties cause the material to create a piezoelectric potential that grows and diminishes.
On GIT’s website, he explains that a “Schottky barrier” controls the electrons’ alternating flow, and that the driving force is an electric potential. “The electrons flow in and out, just like AC current,” Wang describes. “The alternating flow of electrons is the power output process.”
The newly developed generator is not comprised of nanometer-scale structures. For ease of fabrication, Wang has chosen to construct it with zinc-oxide piezoelectric fine wires that measure three to five microns in diameter and 200 to 300 microns in length. He notes, however, that the same piezoelectric principles “would apply at the nanoscale.”
The GIT research team used physical vapor deposition to grow the wires at about 600°C. Then they used an optical microscope to bond the wires onto a polyimide film and closed both ends (which serve as electrodes) with silver paste. Polyimide was then used to encapsulate both wires and electrodes to prevent them from becoming worn.
During the testing phase, a motor-driven mechanical arm was used to repeatedly bend the encased wires to measure the electric energy generated. The bending supplied the tensile strain that generated the piezoelectric potential field along the wires. “This, in turn, [drives] a flow of electrons into an external circuit, creating the alternating charge-and-discharge cycle and corresponding current flow,” Wang explains on GIT’s website. He notes that the team controls the amount of electricity produced in both voltage and current by increasing or decreasing the strain rate.
To confirm that they were measuring current produced by the generator, Wang’s team repeated the test under the same conditions with stretched carbon filters and Kevlar fibers coated with polycrystalline zinc oxide. They did not see current flow in these instances.
What does the future hold for such small-scale generators. Wang says he sees them being used in self-powered wireless sensing systems that gather, store and transmit data. “Self-powered nanotechnology could be the basis for a new industry. That’s really the only way to build independent systems,” he concludes.
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Learning from lizards how to improve dry adhesives
Change in vertically-aligned multi-walled carbon nanotubes during adhesion measurements.
What can a team of highly trained researchers from four different U.S. universities learn from the feet of a gecko lizard? According to the researchers, how to improve carbon nanotube-based material so that, for the first time, it demonstrates “directionally varied (anisotropic) adhesive force” and gripping power nearly three times the level of existing nanotube dry adhesives.
The team - comprised of researchers from the University of Dayton, the Georgia Institute of Technology, the Air Force Research Lab and the University of Akron - describes its achievement in a paper published in the Oct. 10 edition of Science magazine.
As Science reports, the team believes their advance could lead to solder-free connections between electronic devices, longer-lasting adhesives for use in outer space and a broad range of other important applications.
In the article, team members explain that a gecko’s ability to scale vertical walls is due to atomic-scale van der Waals interactions that occur naturally in the microscopic hairs on the lizard’s toes.
These hairs - actually minute setae - give the gecko resistance to perpendicular shear force, enabling it to grab vertical surfaces with surprising strength. The setae also allow the gecko to easily release its strong hold.
By manipulating carbon nanotubes to simulate and intensify the anisotropic adhesive forces at work in gecko hairs, the team has created a carbon nanotube dry adhesive that is “ten times better than a real gecko at resisting perpendicular shear forces.”
According to team member Zhong Lin Wang, a Regents Professor in Georgia Tech’s School of Material Science and Engineering, the newly developed adhesive’s performance depends on the use of “rationally designed multi-walled carbon nanotubes formed into arrays with curly entangled tops.”
Wang likens the tangled tops to a “jungle of vines” that replicates the structure of a gecko’s foot, down to its “branching hairs of different diameters.”
These tangled tops become aligned with a surface when pressed against it, significantly increasing the contact area between the tops and the surface, Wang says.
Wang says, “When lifted off the surface in a direction parallel to the main body of nanotubes, only the [nanotube] tips remain in contact [with the surface], minimizing the forces of attraction.” He claims this “allows us to truly mimic what the gecko does naturally.”
Wang explains that, “When you have line contact along [a surface], you have van der Waals forces acting along the entire length of the nanotubes but, when you have a point contact, the van der Waals forces act only at the tip of the nanotubes.”
As the Science article reports, the researchers have tested their new adhesive’s grip on a number of surfaces, including glass, polymer, Teflon and rough sand paper. Wang says they found it measured up to 100 newtons per square centimeter in the shear direction and only 10 newtons per square centimeter in the normal direction. The team’s conclusion, he says, is that resistance to shear increases with nanotube length, while resistance to normal force is independent of tube length.
Funded by NSF, the project is led by the University of Dayton’s Liming Dai, the Wright Brothers Institute Endowed Chair in the UD’s School of Engineering. The research team also includes the UD’s Liangti Qu, Morley Stone from the Air Force Research Lab and Zhenhai Xia from the University of Akron.
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