"For a long time, high-temperature superconductors were considered to be difficult to handle, too brittle, and too expensive for general industrial applications," explains project manager Wilfried Goldacker from Karlsruhe Institute of Technology."The second generation of high-temperature superconductor wires based on YBCO ceramics is much more robust. Properties have been improved." Superconducting current limiters work reversibly. In case of current peaks after short circuits in the grid, no components are destroyed. The limiter automatically returns to the normal state of operation after a few seconds only. Consequently, the power failure is much shorter than in case of conventional current limiters, such as household fuses, whose components are destroyed and have to be replaced with a high time and cost expenditure.

"Superconducting current limiters have a number of advantages for the stability of medium- and high-voltage grids," explains Mathias Noe, Head of the Institute of Technical Physics of Karlsruhe Institute of Technology. Reliable, compact current limiters enhance the operation stability of power grids and allow for a simplification of the grid structure. As they are protected against current peaks, decentralized energy generators, such as wind and solar systems, can be integrated much better in grids. Expensive components in the existing grid are protected efficiently, components in future grids can be designed for smaller peak currents, and transformers will no longer be necessary. Investment costs of power plants and grids will be reduced. Moreover, superconducting current limiters on the basis of YBCO can also be applied in high-voltage grids of more than 100 kilovolts for better protection against power failures in the future.

YBCO stands for the constituents of the superconductor: Yttrium, barium, copper, and oxygen. An YBCO crystal layer of about 1 micrometer in thickness is grown directly on a stainless steel strip of a few millimeters in width that gives the ceramics the necessary stability. Below a temperature of 90° Kelvin or minus 183° Celsius, the material becomes superconductive. However, superconductivity collapses abruptly when the current in the conductor exceeds the design limits. This effect is used by the current limiter. In case of current peaks in the grid, the superconductor loses its conductivity within fractions of a second and the current will flow through the stainless steel strip only, which has a much higher resistance and, thus, limits the current. The heat arising is removed by the cooling system of the superconductor. A few seconds after the short circuit, it is returned to normal operation in the superconducting state. YBCO superconducting layers on stainless steel strips are more stable and operation-friendly than first-generation superconductors based on BSCCO ceramics. Moreover, their production does not require any noble metals, such as silver, and will presumably be cheaper.

The superconducting current limiter was developed in the past two years under the ENSYSTROB project. The project partners are Karlsruhe Institute of Technology, Nexans SuperConductors, TU Dortmund, and BTU Cottbus. The field test will be carried out at the user, the Vattenfall utility company. The project was funded with about EUR 1.3 million by the Federal Ministry of Economics. The results of the project are of high relevance, as the functionality of current limitation may be integrated in superconducting transformers and energy cables in the future.

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The 'backjet' phenomenon on which the method turns can be compared to a pebble being dropped into water. Tightly focused ultrafast laser pulses carry sufficient energy to locally melt the surface of a gold film. When a laser pulse of light hits the film, a nanoscale backjet -- a nanojet -- of molten gold surges upward.

As the name suggests, nanojets on the surface of a homogeneous gold film are incredibly small, their size being determined by the distribution of energy in the light pulse. This distribution of energy is in turn dependent on the wavelength of light. Initially, scientists anticipated that nanojets could not be significantly smaller than the wavelength of light. In this study however, Ventsislav Valev and his colleagues show that nanojets can in fact be made much smaller with the help of 'plasmonic hotspots'.

Plasmonic hotspots are regions on the surface of metal nanostructures where light causes very strong oscillation of the electrons. Because electron oscillations constitute an electric current and because electric currents heat up the material the same way an electric stove heats up in the kitchen, the plasmonic hotspots are extremely hot. So hot that they can melt the gold in a spot much smaller than the wavelength of light. Dr. Valev and his colleagues were successfully able to demonstrate that this tiny little pool of molten gold can give rise to the smallest nanojets ever observed.

The gold nanodroplets propelled upward by the nanojets solidify in flight, producing perfectly spherical nanoparticles. These gold nanodroplets can be collected and used for medical applications including cancer treatment. The nanoparticles can be attached to molecules and injected in the blood. Once the molecules attach to cancer cells, light can be used to heat up the gold nanodroplets and destroy the cancer cells. Currently, the gold nanoparticles used in medications are chemically synthesised. These chemically synthesised gold nanoparticles have an unavoidably granular aspect. Conversely, gold nanodroplets created by the plasmonic nanojet method detailed by Dr. Valev and his colleagues are perfectly spherical, ensuring a better efficiency.

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At the heart of these solid-state cooling systems are thermoelectric materials, which can convert electricity into a range of different temperatures -- from hot to cold. Thermoelectric refrigerators employing these principles have been available for more than 20 years, but they are still small and highly inefficient. This is largely because the materials used in current thermoelectric cooling devices are expensive and difficult to make in large quantities, and do not have the necessary combination of thermal and electrical properties. A new study, recently published in the journalNature Materials, overcomes these challenges and opens the door to a new generation of high-performance, cost-effective solid state refrigeration and air conditioning.

Rensselaer Professor Ganpati Ramanath led the study, in collaboration with colleagues Theodorian Borca-Tasciuc and Richard W. Siegel.

Driving this research breakthrough is the idea of intentionally contaminating, or doping, nanostructured thermoelectric materials with barely-there amounts of sulfur. The doped materials are obtained by cooking the material and the dopant together for few minutes in a store-bought$40 microwave oven. The resulting powder is formed into pea-sized pellets by applying heat and pressure in a way that preserves the properties endowed by the nanostructuring and the doping. These pellets exhibit properties better than the hard-to-make thermoelectric materials currently available in the marketplace. Additionally, this new method for creating the doped pellets is much faster, easier, and cheaper than conventional methods of making thermoelectric materials.

"This is not a one-off discovery. Rather, we have developed and demonstrated a new way to create a whole new class of doped thermoelectric materials with superior properties," said Ramanath, a faculty member in the Department of Materials Science and Engineering at Rensselaer."Our findings truly hold the potential to transform the technology landscape of refrigeration and make a real impact on our lives." 

Trying to engineer thermoelectric materials is somewhat like playing a game of"tug of war," Ramanath said. Researchers endeavor to control three separate properties of the material: electrical conductivity, thermal conductivity, and Seebeck coefficient. Manipulating one of these properties, however, necessarily affects the other two. This new study demonstrates a new way to minimize the interdependence of these three properties by combining doping and nanostructuring in well-known thermoelectric materials such as tellurides and selenides based on bismuth and antimony.

The goal of tweaking these three properties is to create a thermoelectric material with a high figure of merit, orZT, which is a measure of how efficient the material is at converting heat to electricity. The new pea-sized pellets of nanomaterials developed by the Rensselaer team demonstrated aZTof 1 to 1.1 at room temperature. Since such high values are obtained even without optimizing the process, the researchers are confident that higherZTcan be obtained with some smart engineering.

"It's really amazing as to how nanostructures seasoned with just a few atoms of sulfur can lead to such superior thermoelectric properties of the bulk material made from the nanostructures, and allows us to reap the benefits of nanostructuring on a macroscale," Ramanath said.

An important facet of the discovery is the ability to make both p-type (positive charge) and n-type (negative charge) thermoelectric nanomaterials with a highZT. Up until now, researchers around the world have only been able to make large quantities of p-type materials with highZT.

Additionally, the new study shows the Rensselaer research team can make batches of 10 to 15 grams (enough to make several pea-sized pellets) of the doped nanomaterial in two to three minutes with a microwave oven. Larger quantities can be produced using industrial-sized microwaves ovens.

"Our ability to scalably and inexpensively produce both p- and n-type materials with a highZTpaves the way to the fabrication of high-efficiency cooling devices, as well as solid-state thermoelectric devices for harvesting waste heat or solar heat into electricity," said Borca-Tasciuc, professor in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer.

"This is a very exciting discovery because it combines the realization of novel and useful thermoelectric properties with a demonstrated processing route forward to industrial applications," said Siegel, the Robert W. Hunt Professor of Materials Science and Engineering at Rensselaer.

Rensselaer graduate student Rutvik J. Mehta carried out this work for his doctoral thesis. Mehta, Ramanath, and Borca-Tasciuc have filed a patent and formed a new company, ThermoAura Inc., to further develop and market the new thermoelectric materials technology. Mehta has since graduated and is now a post-doctoral associate at Rensselaer. He also serves as president of ThermoAura.

Beyond refrigerators and air conditioning, the researchers envision this technology could one day be used to cool computer chips.

Along with Ramanath,Borca-Tasciuc, Siegel, and Mehta, co-authors of the paper are Rensselaer graduate students Yanliang Zhang, Chinnathambi Karthik, and Binay Singh.

This research is funded by support from the National Science Foundation (NSF); IBM through the Rensselaer Nanotechnology Center; and the U.S. Department of Energy through the S³TEC Energy Frontiers Research Center at the Massachusetts Institute of Technology (MIT).

See a video of Ramanath explaining the study at:http://www.youtube.com/user/rpirensselaer?feature=mhee#p/u/12/hgmBwg3FeS4

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The device is the world's first fiber-coupled cryogenic radiometer that links optical fiber power measurements directly to fundamental electrical units and national standards. It uses a microscopic forest of carbon nanotubes -- the world's darkest material -- to measure values that are about one-thousandth of the levels typically attained with a cryogenic radiometer lacking direct fiber input capability. With improvements in temperature control and speed, the device could meet the needs for ultraprecise calibrations at ultralow power in telecommunications, medical devices and other industries.

Optical power and energy are traceable to fundamental electrical units. Radiometers absorb optical energy and convert it to heat. Then the electrical power needed to induce the same temperature increase is measured. Because optical and electrical heating are not exactly equivalent, measurement uncertainties can be relatively large from a metrology point of view.

The demonstration is also a step toward converting radiometry from a classical practice based on electrical units to a quantum practice based on single particles of light (photons).

"We have many customers who request optical power measurements in fiber, mainly for optical communications," project leader John Lehman says."Also, our single-photon measurements are done in fiber."

The new radiometer is about 70 millimeters (mm) long and incorporates a 1.45-mm-thick optical fiber capped by a light-trapping cavity at one end with the nanotube absorber and a heater. The ultra-dark nanotubes are grown on a tiny X-shaped piece of micromachined silicon. Light absorption was so high it was difficult to determine measurement uncertainties; Lehman travelled to a special facility at the National Physical Laboratory (the British equivalent of NIST) to make some measurements.

Experiments and calculations indicate the new radiometer can measure a power level of 10 nanowatts with an uncertainty of 0.1 percent. By comparison, typical measurements of optical power delivered through fiber have an uncertainty of 3 percent or more at similar power levels. More importantly, these commercial devices rely on a series of calibrations to establish traceability to national standards.

NIST aims to develop an absolute quantum standard for optical power and energy based on single photons. The effort includes development of sources and detectors spanning a wide range of optical power measurements, from single photon counts to trillions of photons. Single photons are already used in quantum communications systems, which offer novel capabilities such as detecting extremely weak optical signals and providing quantum guarantees on security.

Reference: D. Livigni, N. Tomlin, C.L. Cromer and J.H. Lehman. Fiber-coupled cryogenic radiometer with carbon nanotube absorber. Paper presented at 11th International Conference on New Developments and Applications in Optical Radiometry (NEWRAD 2011), Maui, Hawaii, Sept. 19-23, 2011.

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A team of researchers at the University of Notre Dame has made a major advance toward this vision by creating an inexpensive"solar paint" that uses semiconducting nanoparticles to produce energy.

"We want to do something transformative, to move beyond current silicon-based solar technology," says Prashant Kamat, John A. Zahm Professor of Science in Chemistry and Biochemistry and an investigator in Notre Dame's Center for Nano Science and Technology (NDnano), who leads the research.

"By incorporating power-producing nanoparticles, called quantum dots, into a spreadable compound, we've made a one-coat solar paint that can be applied to any conductive surface without special equipment."

The team's search for the new material, described in the journalACS Nano, centered on nano-sized particles of titanium dioxide, which were coated with either cadmium sulfide or cadmium selenide. The particles were then suspended in a water-alcohol mixture to create a paste.

When the paste was brushed onto a transparent conducting material and exposed to light, it created electricity.

"The best light-to-energy conversion efficiency we've reached so far is 1 percent, which is well behind the usual 10 to 15 percent efficiency of commercial silicon solar cells," explains Kamat.

"But this paint can be made cheaply and in large quantities. If we can improve the efficiency somewhat, we may be able to make a real difference in meeting energy needs in the future."

"That's why we've christened the new paint, Sun-Believable," he adds.

Kamat and his team also plan to study ways to improve the stability of the new material.

NDnano is one of the leading nanotechnology centers in the world. Its mission is to study and manipulate the properties of materials and devices, as well as their interfaces with living systems, at the nano-scale.

This research was funded by the Department of Energy's Office of Basic Energy Sciences.

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