Capturing wasted heat to power devices
Imagine how much you could save on your electricity bill if you could use the excess heat your computer generates to actually power the machine. Towards this end, researchers at the UCLA Henry Samueli School of Engineering and Applied Science have taken an important step toward harnessing that heat and converting it for practical use, which could lead to more energy-efficient appliances and information processing devices.

The research team has demonstrated how to add power to a spintronics device, which uses the spin of electrons for energy rather than their charge. Excess heat, like that generated by extended use of a computer or other device, naturally creates what is known as a spin wave that can move a domain wall. A domain wall separates magnetic materials that point in different directions in certain magnetic devices.

According to the researchers, if housed within the central processing unit of a computer or other electrical device, a domain wall would serve as a sort of turbine, capturing the heat from the traveling spin wave and converting it into energy, just as a turbine harnesses the power of water and converts it into electrical energy that can be used to redirect the water or serve another purpose. The captured energy can then be used to help power the electrical device.

UCLA researchers have demonstrated the use of spin wave flow to move the domain wall in a magnetic device, allowing the harvest of wasted energy. (Source: UCLA)

The concept of using heat energy to move magnetic domain walls is not new, they said, but this paper is the first demonstration of moving a domain wall through propagation of a spin wave.

The capture of heat energy could serve to supplement the power provided by traditional CMOS circuits in devices from smartphones to computer servers and large electrical equipment. In the long run, the researchers expect the process to serve as an alternative to CMOS circuits in many devices.

‘Super-resolution’ microscope to view nanostructures
Purdue University researchers have found a way to see synthetic nanostructures and molecules using a new type of super-resolution optical microscopy that does not require fluorescent dyes, representing a practical tool for biomedical and nanotechnology research.

Conventional optical microscopes can resolve objects no smaller than about 300 nanometers, or billionths of a meter, a restriction known as the “diffraction limit,” which is defined as half the width of the wavelength of light being used to view the specimen. However, researchers want to view molecules such as proteins and lipids, as well as synthetic nanostructures like nanotubes, which are a few nanometers in diameter.

Such a capability could bring advances in a diverse range of disciplines, from medicine to nanoelectronics. The diffraction limit represents the fundamental limit of optical imaging resolution. Super-resolution imaging methods have been developed that require fluorescent labels but here the researchers demonstrate a new scheme for breaking the diffraction limit in optical imaging of non-fluorescent species. Because it is label-free, the signal is directly from the object so that we can learn more about the nanostructure.

A new type of super-resolution optical microscopy takes a high-resolution image (at right) of graphite "nanoplatelets" about 100 nanometers wide. The imaging system, called saturated transient absorption microscopy,or STAM, uses a trio of laser beams and represents a practical tool for biomedical and nanotechnology research. (Source: Purdue University)

The imaging system, called saturated transient absorption microscopy (STAM) uses a trio of laser beams, including a doughnut-shaped laser beam that selectively illuminates some molecules but not others. Electrons in the atoms of illuminated molecules are kicked temporarily into a higher energy level and are said to be excited, while the others remain in their “ground state.” Images are generated using a laser called a probe to compare the contrast between the excited and ground-state molecules.

The researchers demonstrated the technique, taking images of graphite “nanoplatelets” about 100 nanometers wide.

Low-cost, long-life battery
Researchers from the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford University have designed a low-cost, long-life battery that they believe could enable solar and wind energy to become major suppliers to the electrical grid.

For solar and wind power to be used in a significant way, a battery is needed that is made of economical materials that are easy to scale and still efficient. The researchers believe their new battery may be the best yet designed to regulate the natural fluctuations of these alternative energies.

Currently the electrical grid cannot tolerate large and sudden power fluctuations caused by wide swings in sunlight and wind. As solar and wind’s combined contributions to an electrical grid approach 20 percent, energy storage systems must be available to smooth out the peaks and valleys of this “intermittent” power – storing excess energy and discharging when input drops.

Among the most promising batteries for intermittent grid storage today are “flow” batteries, because it’s relatively simple to scale their tanks, pumps and pipes to the sizes needed to handle large capacities of energy. The new flow battery developed by the Stanford group has a simplified, less expensive design that presents a potentially viable solution for large-scale production.

~Ann Steffora Mutschler

Peel-and-stick solar panels
For all their promise, solar cells have frustrated scientists in one crucial regard – most are rigid. They must be deployed in stiff and often heavy fixed panels, limiting their applications. As such, researchers have been trying to get photovoltaics to loosen up. The ideal:  flexible, decal-like solar panels that can be peeled off like band-aids and stuck to virtually any surface, from papers to window panes.

Now the ideal is real. Stanford researchers have succeeded in developing the world’s first peel-and-stick thin-film solar cells.

Unlike standard thin-film solar cells, peel-and-stick thin-film solar cells do not require any direct fabrication on the final carrier substrate. This is a far more dramatic development than it may initially seem, the researchers stressed in that all the challenges associated with putting solar cells on unconventional materials are avoided with the new process, vastly expanding the potential applications of solar technology.

Thin-film photovoltaic cells are traditionally fixed on rigid silicon and glass substrates, greatly limiting their uses,and while the development of thin-film solar cells promised to inject some flexibility into the technology, scientists found that use of alternative substrates was problematic in the extreme.

Peel-and-stick solar decal process and as applied to a cell phone, a business card and a window. (Source: Stanford)

Nonconventional or ‘universal’ substrates are difficult to use for photovoltaics because they typically have irregular surfaces and they don’t do well with the thermal and chemical processing necessary to produce today’s solar cells, the researchers said. They got around these problems by developing this peel-and-stick process, which gives thin-film solar cells flexibility and attachment potential never seen before, and also reduces their general cost and weight.

Utilizing the process, researchers attached thin-film solar cells to paper, plastic and window glass, among other materials – without losing any of the original cell efficiency.

The new process involves a unique silicon, silicon dioxide and metal “sandwich.” First, a 300-nanometer film of nickel (Ni) is deposited on a silicon/silicon dioxide (Si/SiO2) wafer. Thin-film solar cells are then deposited on the nickel layer utilizing standard fabrication techniques, and covered with a layer of protective polymer. A thermal release tape is then attached to the top of the thin-film solar cells to augment their transfer off of the production wafer and onto a new substrate.

The solar cell is now ready to peel from the wafer. To remove it, the wafer is submerged in water at room temperature and the edge of the thermal release tape is peeled back slightly, allowing water to seep into and penetrate between the nickel and silicon dioxide interface.

The solar cell is thus freed from the hard substrate but still attached to the thermal release tape.  The researcher team heats the tape and solar cell to 90°C for several seconds, and the cell can then be applied to virtually any surface using double-sided tape or other adhesive. Finally, the thermal release tape is removed, leaving just the solar cell attached to the chosen substrate.

Tests have demonstrated that the peel-and-stick process reliably leaves the thin-film solar cells wholly intact and functional.

Using nanotechnology for more efficient power generation
Scalable nanopatterned surfaces designed by MIT researchers could make for more efficient power generation and desalination. Many industrial plants depend on water vapor condensing on metal plates: In power plants, the resulting water is then returned to a boiler to be vaporized again; in desalination plants, it yields a supply of clean water. The efficiency of such plants depends crucially on how easily droplets of water can form on these metal plates, or condensers, and how easily they fall away, leaving room for more droplets to form.

The key to improving the efficiency of such plants is to increase the condensers’ heat-transfer coefficient — a measure of how readily heat can be transferred away from those surfaces, according to MIT researchers that have designed, made and tested a coated surface with nanostructured patterns that greatly increase the heat-transfer coefficient.

On a typical, flat-plate condenser, water vapor condenses to form a liquid film on the surface, drastically reducing the condenser’s ability to collect more water until gravity drains the film, acting as a barrier to heat transfer. The researchers have focused on ways of encouraging water to bead up into droplets that then fall away from the surface, allowing more rapid water removal.

They found that the way to remove the thermal barrier is to remove the droplets as quickly as possible. Many researchers have studied ways of doing this by creating hydrophobic surfaces, either through chemical treatment or through surface patterning but the MIT researchers have made scalable surfaces with nanoscale features that barely touch the droplets.

The result is that the droplets don’t just fall from the surface, but actually jump away from it, increasing the efficiency of the process. The energy released as tiny droplets merge to form larger ones is enough to propel the droplets upward from the surface, meaning the removal of droplets doesn’t depend solely on gravity.

Other researchers have worked on nanopatterned surfaces to induce such jumping, but these have tended to be complex and expensive to manufacture, usually requiring a clean-room environment. Those approaches also require flat surfaces, not the tubing or other shapes often used in condensers. Finally, prior research has not tested the enhanced heat transfer predicted for these types of surfaces.

Previously the MIT researchers showed that droplet shape is important to enhanced heat transfer and have now gone a step further and developed a surface that favors these kinds of droplets, while being highly scalable and easy to manufacture. Furthermore, they’ve actually been able to experimentally measure the heat-transfer enhancement.

–Ann Steffora Mutschler

Recent advances in microtechnology and nanotechnology depend on microscopic spherical particles self-assembling into large-scale aggregates to form a relatively limited range of crystalline structures. Directed assembly is a new branch of this field, where scientists figure out how to make particles assemble to form a broad range of structures at given locations.

Current techniques for directed assembly typically use an applied field, such as an electric or magnetic field, to move particles and to assemble them into well-defined structures but researchers at the University of Pennsylvania have identified a simple new method to direct particle assembly based only on surface tension and particle shape.

Kathleen J. Stebe, professor in the Department of Chemical and Biomolecular Engineering and the school’s Deputy Dean for Research, led the research. Their results rely on the simple fact that a liquid surface will tend to minimize its surface area. “It’s the same reason that surface tension makes a drop of water want to be a sphere. But we can tune that phenomenon to do astonishing things.”

Self-assembling spherical particles have been used to make new materials with unique optical and mechanical properties, but non-spherical, or anisotropic, particles may hold even greater promise. By having a definable directionality, the properties of the materials the particles make up can be altered based on their orientations.

What is particularly powerful is that the research team discovered that it does not depend on the particle having a certain shape or being made from a certain material. Surfaces studded with strategically placed and shaped posts could direct and orient particles into almost any configuration. And because the mechanism behind the particles’ movement is simply the surface curvature, their movement could be “programmed” by changing the arrangement of the posts or the shape of the interface.  “I could go in with needle, for example, and dynamically pull the surface up at different locations, or over different times,” Stebe said.

“Very often when we think about using micro- or nanotechnology, we’re not thinking about properties on that tiny scale: It’s going to be the organized structure made from micro- or nanoparticles that’s going to be useful, perhaps as a lens or a smart surface,” she said. “This phenomenon could be used to make new structures by sending particles to certain locations. We could define paths and say ‘here’s a docking site: go there’ or ‘here’s a spot where we want nothing; don’t go there.’ “This is a clear demonstration of directed assembly. Like self-assembly, things come together from the bottom up, but here they come together exactly where we want them to.”

–Ann Steffora Mutschler

Solar Cells Like LEDs
Researchers from the University of California at Berkeley have suggested and demonstrated a solar cell concept more like that of LEDs to allow solar cells to be able to emit light as well as absorb it—a counterintuitive concept because currently to produce the maximum amount of energy, solar cells are designed to absorb as much light from the Sun as possible.

Principal researcher and UC Berkeley professor of electrical engineering Eli Yablonovitch said, “What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce.”

Yablonovitch and his colleagues have been trying to understand why there has been such a large gap between the theoretical limit of 33.5% and the limit that researchers have been able to achieve of 26%. As they worked, a “coherent picture emerged,” explained Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch’s group. They came across a relatively simple, if perhaps counterintuitive, solution based on a mathematical connection between absorption and emission of light. “Fundamentally, it’s because there’s a thermodynamic link between absorption and emission.”

Eli Yablonovitch and Owen Miller, who worked out the theory for the new solar cell efficiency. The monitor in the picture illustrates the new physics concept where increased light emission yields higher efficiency. Source: The Optical Society (OSA)

Designing solar cells to emit light—so that photons do not become “lost” within a cell—has the natural effect of increasing the voltage produced by the solar cell. If there is a solar cell that is a good emitter of light, it also makes it produce a higher voltage, which in turn increases the amount of electrical energy that can be harvested from the cell for each unit of sunlight.

The Berkeley research team will present its findings at the Conference on Lasers and Electro Optics (CLEO: 2012), to be held May 6-11 in San Jose, Calif.

Harvesting Energy with Nanotechnology
With 58% of the energy generated in the United States wasted as heat, researchers at Purdue University are working on a technique that uses nanotechnology to harvest energy from hot pipes or engine components to potentially recover energy wasted in factories, power plants and cars.  “If we could get just 10% back, that would allow us to reduce energy consumption and power plant emissions considerably,” said Yue Wu, a Purdue University assistant professor of chemical engineering.

The technique utilizes glass fibers coated with a thermoelectric material containing nanocrystals of lead telluride they developed, then exposed to heat in an annealing process to fuse the crystals together.  When the materials are heated on one side, electrons flow to the cooler side, generating an electrical current. These coated fibers could be used to create a solid-state cooling technology that does not require compressors and chemical refrigerants or even be woven into a fabric to make cooling garments. The fibers could be wrapped around industrial pipes in factories and power plants, as well as on car engines and automotive exhaust systems, to recapture much of the wasted energy. The ‘energy harvesting’ technology might dramatically reduce how much heat is lost, Wu said.

This image shows glass fibers coated with a thermoelectric material that generates electrical current when exposed to heat. The technology might be used to harvest energy from hot pipes or engine components, possibly representing a way to recover energy wasted in factories, power plants and cars. Source: Purdue University

These findings were detailed in a research paper appearing last month in the journal Nano Letters. The paper was written by Daxin Liang, a former Purdue exchange student from Jilin University in China; Purdue graduate students Scott Finefrock and Haoran Yang; and Wu.

Today’s high-performance thermoelectric materials are brittle. The devices are formed from large discs or blocks, which requires using a lot of material. But the new flexible devices would conform to the irregular shapes of engines and exhaust pipes while using a small fraction of the material required for conventional thermoelectric devices, researchers said.

“This approach yields the same level of performance as conventional thermoelectric materials but it requires the use of much less material, which leads to lower cost and is practical for mass production,” Wu said. Further, this approach promises a method that can be scaled up to industrial processes, making mass production feasible.

—Ann Steffora Mutschler