water filtering reduce electricity

Filtering liquids with liquids saves electricity

November 7, 2018
Harvard University 

Credit: CC0 Public Domain

Filtering and treating water, both for human consumption and to clean industrial and municipal wastewater, accounts for about 13% of all electricity consumed in the US every year and releases about 290 million metric tons of CO₂ into the atmosphere annually—roughly equivalent to the combined weight of every human on Earth.

One of the most common methods of processing wateris passing it through a membrane with pores that are sized to filter out particles that are larger than water molecules. However, these membranes are susceptible to "fouling," or clogging by the very materials they are designed to filter out, necessitating more electricity to force the water through a partially clogged membrane and frequent membrane replacement, both of which increase water treatment costs.

New research from the Wyss Institute for Biologically Inspired Engineering at Harvard University and collaborators at Northeastern University and the University of Waterloo demonstrates that the Wyss' liquid-gated membranes (LGMs) filter nanoclay particles out of water with twofold higher efficiency, nearly threefold longer time-to-foul, and a reduction in the pressure required for filtration over conventional membranes, offering a solution that could reduce the cost and electricity consumption of high-impact industrial processes such as oil and gas drilling. The study is reported in APL Materials.

"This is the first study to demonstrate that LGMs can achieve sustained filtration in settings similar to those found in heavy industry, and it provides insight into how LGMs resist different types of fouling, which could lead to their use in a variety of water processing settings," said first author Jack Alvarenga, a Research Scientist at the Wyss Institute.

LGMs mimic nature's use of liquid-filled pores to control the movement of liquids, gases and particles through biological filters using the lowest possible amount of energy, much like the small stomata openings in plants' leaves allow gases to pass through. Each LGM is coated with a liquid that acts as a reversible gate, filling and sealing its pores in the "closed" state. When pressure is applied to the membrane, the liquid inside the pores is pulled to the sides, creating open, liquid-lined pores that can be tuned to allow the passage of specific liquids or gases, and resist fouling due to the liquid layer's slippery surface. The use of fluid-lined pores also enables the separation of a target compound from a mixture of different substances, which is common in industrial liquid processing

The research team decided to test their LGMs on a suspension of bentonite clay in water, as such "nanoclay" solutions mimic the wastewater produced by drilling activities in the oil and gas industry. They infused 25-mm discs of a standard filter membrane with perfluoropolyether, a type of liquid lubricant that has been used in the aerospace industry for over 30 years, to convert them into LGMs. They then placed the membranes under pressure to draw water through the pores but leave the nanoclay particles behind, and compared the performance of untreated membranes to LGMs.

The untreated membranes displayed signs of nanoclay fouling much more quickly than the LGMs, and the LGMs were able to filter water three times longer than the standard membranes before requiring a "backwash" procedure to remove particles that had accumulated on the membrane. Less frequent backwashing could translate to a reduction in the use of cleaning chemicals and energy required to pump backwash water, and improve the filtration rate in industrial water treatment settings.

While the LGMs did eventually experience fouling, they displayed a 60% reduction in the amount of nanoclay that accumulated within their structure during filtration, which is known as "irreversible fouling" because it is not removed by backwashing. This advantage gives LGMs a longer lifespan and makes more of the filtrate recoverable for alternate uses. Additionally, the LGMs required 16% less pressure to initiate the filtration process, reflecting further energy savings.

"LGMs have the potential for use in industries as diverse as food and beverage processing, biopharmaceutical manufacturing, textiles, paper, pulp, chemical, and petrochemical, and could offer improvements in energy use and efficiency across a wide swath of industrial applications," said corresponding author Joanna Aizenberg, Ph.D., who is a Founding Core Faculty member of the Wyss Institute and the Amy Smith Berylson Professor of Material Sciences at Harvard's John A. Paulson School of Engineering and Applied Sciences (SEAS).

The team's next steps for the research include larger-scale pilot studies with industry partners, longer-term operation of the LGMs, and filtering even more complex mixtures of substances. These studies will provide insight into the commercial viability of LGMs for different applications, and how long they would last in a number of use cases.

"The concept of using a liquid to help filter other liquids, while perhaps not obvious to us, is prevalent in nature. It's wonderful to see how leveraging nature's innovation in this manner can potentially lead to huge energy savings," said Wyss Founding Director Donald Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children's Hospital, as well as Professor of Bioengineering at SEAS.

Sun-free photovoltaics

Sun-free photovoltaics: Materials engineered to give off precisely tuned wavelengths of light when heated are key to new high-efficiency generating system.

A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. While the principle involved is not new, a novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths of light — selected to match the wavelengths that photovoltaic cells can best convert to electricity — makes the new system much more efficient than previous versions.

The key to this fine-tuned light emission, described in the journal Physical Review A, lies in a material with billions of nanoscale pits etched on its surface. When the material absorbs heat — whether from the sun, a hydrocarbon fuel, a decaying radioisotope or any other source — the pitted surface radiates energy primarily at these carefully chosen wavelengths.

Based on that technology, MIT researchers have made a button-sized power generator fueled by butane that can run three times longer than a lithium-ion battery of the same weight; the device can then be recharged instantly, just by snapping in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope that steadily produces heat from radioactive decay, could generate electricity for 30 years without refueling or servicing — an ideal source of electricity for spacecraft headed on long missions away from the sun.

According to the U.S. Energy Information Administration, 92 percent of all the energy we use involves converting heat into mechanical energy, and then often into electricity — such as using fuel to boil water to turn a turbine, which is attached to a generator. But today's mechanical systems have relatively low efficiency, and can't be scaled down to the small sizes needed for devices such as sensors, smartphones or medical monitors.

"Being able to convert heat from various sources into electricity without moving parts would bring huge benefits," says Ivan Celanovic ScD '06, research engineer in MIT's Institute for Soldier Nanotechnologies (ISN), "especially if we could do it efficiently, relatively inexpensively and on a small scale."

It has long been known that photovoltaic (PV) cells needn't always run on sunlight. Half a century ago, researchers developed thermophotovoltaics (TPV), which couple a PV cell with any source of heat: A burning hydrocarbon, for example, heats up a material called the thermal emitter, which radiates heat and light onto the PV diode, generating electricity. The thermal emitter's radiation includes far more infrared wavelengths than occur in the solar spectrum, and "low band-gap" PV materials invented less than a decade ago can absorb more of that infrared radiation than standard silicon PVs can. But much of the heat is still wasted, so efficiencies remain relatively low.

An ideal match

The solution, Celanovic says, is to design a thermal emitter that radiates only the wavelengths that the PV diode can absorb and convert into electricity, while suppressing other wavelengths. "But how do we find a material that has this magical property of emitting only at the wavelengths that we want?" asks Marin Soljačić, professor of physics and ISN researcher. The answer: Make a photonic crystal by taking a sample of material and create some nanoscale features on its surface — say, a regularly repeating pattern of holes or ridges — so light propagates through the sample in a dramatically different way.

"By choosing how we design the nanostructure, we can create materials that have novel optical properties," Soljačić says. "This gives us the ability to control and manipulate the behavior of light."

The team — which also includes Peter Bermel, research scientist in the Research Laboratory for Electronics (RLE); Peter Fisher, professor of physics; and Michael Ghebrebrhan, a postdoc in RLE — used a slab of tungsten, engineering billions of tiny pits on its surface. When the slab heats up, it generates bright light with an altered emission spectrum because each pit acts as a resonator, capable of giving off radiation at only certain wavelengths.

This powerful approach — co-developed by John D. Joannopoulos, the Francis Wright Davis Professor of Physics and ISN director, and others — has been widely used to improve lasers, light-emitting diodes and even optical fibers. The MIT team, supported in part by a seed grant from the MIT Energy Initiative, is now working with collaborators at MIT and elsewhere to use it to create several novel electricity-generating devices.

Mike Waits, an electronics engineer at the Army Research Laboratory in Adelphi, Md., who was not involved in this work, says this approach to producing miniature power supplies could lead to lighter portable electronics, which is "critical for the soldier to lighten his load. It not only reduces his burden, but also reduces the logistics chain" to deliver those devices to the field. "There are a lot of lives at stake," he says, "so if you can make the power sources more efficient, it could be a great benefit."

The button-like device that uses hydrocarbon fuels such as butane or propane as its heat source — known as a micro-TPV power generator — has at its heart a "micro-reactor" designed by Klavs Jensen, the Warren K. Lewis Professor of Chemical Engineering, and fabricated in the Microsystems Technology Laboratories. While the device achieves a fuel-to-electricity conversion efficiency three times greater than that of a lithium-ion battery of the same size and weight, Celanovic is confident that with further work his team can triple the current energy density. "At that point, our TPV generator could power your smartphone for a whole week without being recharged," he says.

Celanovic and Soljačić stress that building practical systems requires integrating many technologies and fields of expertise. "It's a really multidisciplinary effort," Celanovic says. "And it's a neat example of how fundamental research in materials can result in new performance that enables a whole spectrum of applications for efficient energy conversion."

Science news source:

Massachusetts Institute of Technology

Down to the wire: Inexpensive technique for making high quality nanowire solar cells developed

Solar or photovoltaic cells represent one of the best possible technologies for providing an absolutely clean and virtually inexhaustible source of energy to power our civilization. However, for this dream to be realized, solar cells need to be made from inexpensive elements using low-cost, less energy-intensive processing chemistry, and they need to efficiently and cost-competitively convert sunlight into electricity. A team of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory has now demonstrated two out of three of these requirements with a promising start on the third.

Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division, led the development of a solution-based technique for fabricating core/shell nanowire solar cells using the semiconductors cadmium sulfide for the core and copper sulfide for the shell. These inexpensive and easy-to-make nanowire solar cells boasted open-circuit voltage and fill factor values superior to conventional planar solar cells. Together, the open-circuit voltage and fill factor determine the maximum energy that a solar cell can produce. In addition, the new nanowires also demonstrated an energy conversion efficiency of 5.4-percent, which is comparable to planar solar cells.

"This is the first time a solution based cation-exchange chemistry technique has been used for the production of high quality single-crystalline cadmium sulfide/copper sulfide core/shell nanowires," Yang says. "Our achievement, together with the increased light absorption we have previously demonstrated in nanowire arrays through light trapping, indicates that core/shell nanowires are truly promising for future solar cell technology."
Yang, who holds a joint appointment with the University of California (UC) Berkeley, is the corresponding author of a paper reporting this research that appears in the journal Nature Nanotechnology. The paper is titled "Solution-processed core–shell nanowires for efficient photovoltaic cells." Co-authoring this paper with Yang were Jinyao Tang, Ziyang Huo, Sarah Brittman and Hanwei Gao.

Typical solar cells today are made from ultra-pure single crystal silicon wafers that require about 100 micrometers in thickness of this very expensive material to absorb enough solar light. Furthermore, the high-level of crystal purification required makes the fabrication of even the simplest silicon-based planar solar cell a complex, energy-intensive and costly process.

A highly promising alternative would be semiconductor nanowires – one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch up to the millimeter scale. Solar cells made from nanowires offer a number of advantages over conventional planar solar cells, including better charge separation and collection capabilities, plus they can be made from Earth abundant materials rather than highly processed silicon. To date, however, the lower efficiencies of nanowire-based solar cells have outweighed their benefits.

"Nanowire solar cells in the past have demonstrated fill factors and open-circuit voltages far inferior to those of their planar counterparts," Yang says. "Possible reasons for this poor performance include surface recombination and poor control over the quality of the p–n junctions when high-temperature doping processes are used."

At the heart of all solar cells are two separate layers of material, one with an abundance of electrons that function as a negative pole, and one with an abundance of electron holes (positively-charged energy spaces) that function as a positive pole. When photons from the sun are absorbed, their energy is used to create electron-hole pairs, which are then separated at the p-n junction – the interface between the two layers - and collected as electricity.

About a year ago, working with silicon, Yang and members of his research group developed a relatively inexpensive way to replace the planar p-n junctions of conventional solar cells with a radial p-n junction, in which a layer of n-type silicon formed a shell around a p-type silicon nanowire core. This geometry effectively turned each individual nanowire into a photovoltaic cell and greatly improved the light-trapping capabilities of silicon-based photovoltaic thin films.

Now they have applied this strategy to the fabrication of core/shell nanowires using cadmium sulfide and copper sulfide, but this time using solution chemistry. These core/shell nanowires were prepared using a solution-based cation (negative ion) exchange reaction that was originally developed by chemist Paul Alivisatos and his research group to make quantum dots and nanorods. Alivisatos is now the director of Berkeley Lab, and UC Berkeley's Larry and Diane Bock Professor of Nanotechnology.

"The initial cadmium sulfide nanowires were synthesized by physical vapor transport using a vapor–liquid–solid (VLS) mechanism rather than wet chemistry, which gave us better quality material and greater physical length, but certainly they can also be made using solution process" Yang says. "The as-grown single-crystalline cadmium sulfide nanowires have diameters of between 100 and 400 nanometers and lengths up to 50 millimeters."
The cadmium sulfide nanowires were then dipped into a solution of copper chloride at a temperature of 50 degrees Celsius and kept there for 5 to 10 seconds. The cation exchange reaction converted the surface layer of the cadmium sulfide into a copper sulfide shell.

"The solution-based cation exchange reaction provides us with an easy, low-cost method to prepare high-quality hetero-epitaxial nanomaterials," Yang says. "Furthermore, it circumvents the difficulties of high-temperature doping and deposition for typical vapor phase production methods, which suggests much lower fabrication costs and better reproducibility. All we really need are beakers and flasks for this solution-based process. There's none of the high fabrication costs associated with gas-phase epitaxial chemical vapor deposition and molecular beam epitaxy, the techniques most used today to fabricate semiconductor nanowires."

Yang and his colleagues believe they can improve the energy conversion efficiency of their solar cell nanowires by increasing the amount of copper sulfide shell material. For their technology to be commercially viable, they need to reach an energy conversion efficiency of at least ten-percent.

Provided by Lawrence Berkeley National Laboratory [August 31, 2011]

Manufacturing method paves way for commercially viable quantum dot-based LEDs

University of Florida researchers may help resolve the public debate over future light source of choice: Edison's incandescent bulb or the more energy efficient compact fluorescent lamp. It could be neither.

Instead, future lighting needs may be supplied by a new breed of light emitting diode, or LED, that conjures light from the invisible world of quantum dots. According to an article in the current online issue of the journal Nature Photonics, moving a QD LED from the lab to market is a step closer to reality thanks to a new manufacturing process pioneered by two research teams in UF's department of materials science and engineering.

"Our work paves the way to manufacture efficient and stable quantum dot-based LEDs with really low cost, which is very important if we want to see wide-spread commercial use of these LEDs in large-area, full-color flat-panel displays or as solid-state lighting sources to replace the existing incandescent and fluorescent lights," said Jiangeng Xue, the research leader and an associate professor of material science and engineering "Manufacturing costs will be significantly reduced for these solution-processed devices, compared to the conventional way of making semiconductor LED devices."

A significant part of the research carried out by Xue's team focused on improving existing organic LEDs. These semiconductors are multilayered structures made up of paper thin organic materials, such as polymer plastics, used to light up display systems in computer monitors, television screens, as well as smaller devices such as MP3 players, mobile phones, watches, and other handheld electronic devices. OLEDs are also becoming more popular with manufacturers because they use less power and generate crisper, brighter images than those produced by conventional LCDs (liquid crystal displays). Ultra-thin OLED panels are also used as replacements for traditional light bulbs and may be the next big thing in 3-D imaging.

Complementing Xue's team is another headed by Paul Holloway, distinguished professor of materials science and engineering at UF, which delved into quantum dots, or QDs. These nano-particles are tiny crystals just a few nanometers (billionths of a meter) wide, comprised of a combination of sulfur, zinc, selenium and cadmium atoms. When excited by electricity, QDs emit an array of colored light. The individual colors vary depending on the size of the dots. Tuning, or "adjusting," the colors is achieved by controlling the size of the QDs during the synthetic process.

By integrating the work of both teams, researchers created a high-performance hybrid LED, comprised of both organic and QD-based layers. Until recently, however, engineers at UF and elsewhere have been vexed by a manufacturing problem that hindered commercial development. An industrial process known as vacuum deposition is the common way to put the necessary organic molecules in place to carry electricity into the QDs. However, a different manufacturing process called spin-coating, is used to create a very thin layer of QDs. Having to use two separate processes slows down production and drives up manufacturing costs.

According to the Nature Photonics article, UF researchers overcame this obstacle with a patented device structure that allows for depositing all the particles and molecules needed onto the LED entirely with spin-coating. Such a device structure also yields significantly improved device efficiency and lifetime compared to previously reported QD-based LED devices.

Spin-coating may not be the final manufacturing solution, however.
"In terms of actual product manufacturing, there are many other high through-put, continuous "roll-to-roll" printing or coating processes that we could use to fabricate large area displays or lighting devices," Xue said. "That will remain as a future research and development topic for the university and a start-up company, NanoPhotonica, that has licensed the technology and is in the midst of a technology development program to capitalize on the manufacturing breakthrough."

Provided by University of Florida [August 31, 2011]

millimeter-scale energy harvester

Most powerful millimeter-scale energy harvester generates electricity from vibrations

April 26, 2011

A new energy harvester developed by University of Michigan researchers can harness energy from vibrations and convert it to electricity with five to ten times greater efficiency and power than other devices in its class.

"In a tiny amount of space, we've been able to make a device that generates more power for a given input than anything else out there on the market," said Khalil Najafi, one of the system's developers and chair of Electrical and Computer Engineering.

This new vibration energy harvester is specifically designed to turn the cyclic motions of factory machines into energy to power wireless sensor networks. These sensor networks monitor machines' performance and let operators know about any malfunctions.

The sensors that do this today get their power from a plug or a battery. They're considered "wireless" because they can transmit information without wires. Being tethered to a power source drastically increases their installation and maintenance costs, said Erkan Aktakka, one of the system's developers and a doctoral student in Electrical and Computer Engineering.

Long-lasting power is the greatest hurdle to large-scale use of pervasive information-gathering sensor networks, the researchers say.

"If one were to look at the ongoing life-cycle expenses of operating a wireless sensor, up to 80 percent of the total cost consists solely of installing and maintaining power wires and continuously monitoring, testing and replacing finite-life batteries," Aktakka said. "Scavenging the energy already present in the environment is an effective solution."

The researchers have built a complete system that integrates a high-quality energy-harvesting piezoelectric material with the circuitry that makes the power accessible. (Piezoelectric materials allow a charge to build up in them in response to mechanical strain, which in this case would be induced by the machines' vibrations.)

"There are lots of energy sources surrounding us. Lightning has a lot of electricity and power, but it's not useful," Najafi said. "To be able to use the energy you harvest you have to store it in a capacitor or battery. We've developed an integrated system with an ultracapacitor that does not need to start out charged."

The active part of the harvester that enables the energy conversion occupies just 27 cubic millimeters. The packaged system, which includes the power management circuitry, is in the size of a penny. The system has a large bandwidth of 14 Hertz and operates at a vibration frequency of 155 Hertz, similar to the vibration you'd feel if you put your hand on top of a running microwave oven.

"Most of the previous vibration harvesters operated either at very high frequencies or with very narrow bandwidths, and this limited their practical applicationsoutside of a laboratory environment," Aktakka said.

The new harvester can generate more than 200 microwatts of power when it is exposed to 1.5g vibration amplitude. (1g is the gravitational acceleration that all objects experience by Earth's gravity.) The harvested energy is processed by an integrated circuitry to charge an ultracapacitor to 1.85 volts.

In theory, these devices could be left in place for 10 or 20 years without regular maintenance. "They have a limitless shelf time, since they do not require a pre-charged battery or an external power source," Aktakka said.

A novel silicon micromachining technique allows the engineers to fabricate the harvesters in bulk with a high-quality piezoelectric material, unlike other competing devices.

The market for power sources for wireless sensor networks in industrial settings is expected to reach $450 million by 2015, Aktakka said.

These new devices could have applications in medicine and the auto industry too. They could possibly be used to power medical implants in people or heat sensors on vehicle motors, Najafi said.

The researchers will present this work next at the 16th International Conference on Solid-State Sensors, Actuators, and Microsystems (TRANSDUCERS 2011) in Beijing in June.

Provided by University of Michigan

light to predict molecular crystal structures

Syracuse University chemist develops technique to use light to predict molecular crystal structures

March 23rd, 2011

A Syracuse University chemist has developed a way to use very low frequency light waves to study the weak forces (London dispersion forces) that hold molecules together in a crystal. This fundamental research could be applied to solve critical problems in drug research, manufacturing and quality control. The research by Timothy Korter, associate professor of chemistry in SU's College of Arts and Sciences, was the cover article of the March 14 issue of Physical Chemistry Chemical Physics. The journal, published by the Royal Society of Chemistry, is one of the most prestigious in the field. A National Science Foundation Early Career Development (CAREER) Award funds Korter's research. "When developing a drug, it is important that we uncover all of the possible ways the molecules can pack together to form a crystal," Korter says. "Changes in the crystal structure can change the way the drug is absorbed and accessed by the body." One industry example is that of a drug distributed in the form of a gel capsule that crystallized into a solid when left on the shelf for an extended period of time, Korter explains. The medication inside the capsule changed to a form that could not dissolve in the human body, rendering it useless. The drug was removed from shelves. This example shows that it is not always possible for drug companies to identify all the variations of a drug's crystal structure through traditional experimentation, which is time consuming and expensive. "The question is," Korter says, "can we leverage a better understanding of London and other weak intermolecular forces to predict these changes in crystal structure?" Korter's lab is one of only a handful of university-based research labs in the world exploring the potential of THz radiation for chemical and pharmaceutical applications. THz light waves exist in the region between infrared radiation and microwaves and offer the unique advantages of being non-harmful to people and able to safely pass through many kinds of materials. THz can also be used to identify the chemical signatures of a wide range of substances. Korter has used THz to identify the chemical of signatures of molecules ranging from improvised explosives and drug components to the building blocks of DNA. Korter's new research combines THz experiments with new computational models that accurately account for the effects of the London dispersion forces to predict crystal structures of various substances. London forces are one of several types of intermolecular forces that cause molecules to stick together and form solids. Environmental changes (temperature, humidity, light) impact the forces in ways that can cause the crystal structure to change. Korter's research team compares the computer models with the THz experiments and uses the results to refine and improve the theoretical models. "We have demonstrated how to use THz to directly visualize these chemical interactions," Korter says. "The ultimate goal is to use these THz signatures to develop theoretical models that take into account the role of these weak forces to predict the crystal structures of pharmaceuticals before they are identified through experimentation." Source: Syracuse University

INVISIBILITY

Invisibility cloaks may be just around the corner

March 4, 2011

In 1897, H.G. Wells created a fictional scientist who became invisible by changing his refractive index to that of air, so that his body could not absorb or reflect light. More recently, Harry Potter disappeared from sight after wrapping himself in a cloak spun from the pelts of magical herbivores.

Countless other fictional characters in books and films throughout history have discovered or devised ways to become invisible, a theme that long has been a staple of science fiction and a source of endless fascination for humans. Who among us has never imagined the possibilities? But, of course, it's not for real.

Or is it?

While no one yet has the power to put on a garment and disappear, Elena Semouchkina, an associate professor of electrical and computer engineering at Michigan Technological University, has found ways to use magnetic resonance to capture rays of visible light and route them around objects, rendering those objects invisible to the human eye. Her work is based on the transformation optics approaches, developed and applied to the solution of invisibility problems by British scientists John B. Pendry and Ulf Leonhardt in 2006.

"Imagine that you look at the object, which is placed in front of a light source," she explains.

"The object would be invisible for your eye if the light rays are sent around the object to avoid scattering, and are accelerated along these curved paths to reach your eye undistinguishable from direct straight beams exiting the source, when the object is absent."

At its simplest, the beams of light flow around the object and then meet again on the other side so that someone looking directly at the object would not be able to see it--but only what's on the other side.

"You would see the light source directly through the object," said Semouchkina. "This effect could be achieved if we surround the object by a shell with a specific distribution of such material parameters as permittivity and permeability."

She and her collaborators at the Pennsylvania State University, where she is also an adjunct professor, designed a nonmetallic "invisibility cloak" that uses concentric arrays of identical glass resonators made of chalcogenide glass, a type of dielectric material--that is, one that does not conduct electricity.

In computer simulations, the cloak made objects hit by infrared waves--approximately one micron, or one-millionth of a meter long--disappear from view.

The potential practical applications of the work could be dramatic, for example, in the military, such as "making objects invisible to radar," she said, as well as in intelligence operations "to conceal people or objects."

Furthermore, "shielding objects from electromagnetic irradiation is also very important," she said, adding, "for sure, the gaming industry could use it in new types of toys."

Multi-resonator structures comprising Semouchkina's invisibility cloak belong to "metamaterials"--artificial materials with properties that do not exist in nature--since they can refract light by unusual ways. In particular, the "spokes" of tiny glass resonators accelerate light waves around the object making it invisible.

Until recently, there were no materials available with the relative permeability values between zero and one, which are necessary for the invisibility cloak to bend and accelerate light beams, she said. However, metamaterials, which were predicted more than 40 years ago by the Russian scientist Victor Veselago, and first implemented in 2000 by Pendry from Imperial College, London, in collaboration with David R. Smith from Duke University, now make it possible, she said.

Metamaterials use lattices of resonators, instead of atoms or molecules of natural materials, and provide for a broad range of relative permittivity and permeability including zero and negative values in the vicinity of the resonance frequency, she said. Metamaterials were listed as one of the top three physics discoveries of the decade by the American Physical Society.

"Metamaterials were initially made of metallic split ring resonators and wire arrays that limited both their isotropy (uniformity in all directions) and frequency range," Semouchkina said. "Depending on the size of split ring resonators, they could operate basically at microwaves and millimeter (mm) waves."

In 2004, her research group proposed replacing metal resonators with dielectric resonators. "Although it seemed strange to control magnetic properties of a metamateral by using dielectrics, we have shown that arrays of dielectric resonators can provide for negative refraction and other unique properties of metamaterials," she said. "Low loss dielectric resonators promise to extend applications of metamaterials to the optical range, and we have demonstrated this opportunity by designing an infrared cloak."

Semouchkina and colleagues recently reported on their research in the journalApplied Physics Letters, published by the American Institute of Physics. Her co-authors were Douglas Werner and Carlo Pantano of Penn State and George Semouchkin, who teaches at Michigan Tech and has an adjunct position with Penn State.

The National Science Foundation (NSF) is funding her research on dielectric metamaterials and the team's applications with a $318,520 award, but she plans to apply for an additional grant to conduct specific studies into invisibility cloak structures.

Semouchkina, who received her master's degree in electrical engineering and her doctorate in physics and mathematics from Tomsk State University in her native Russia, has lived in the United States for 13 years, and has been a U.S. citizen since 2005. She also earned her second doctorate in materials in 2001 from Penn State.

She and her team now are testing an all-dielectric invisibility cloak rescaled to work at microwave frequencies, performing experiments in Michigan Tech's anechoic chamber, a cave-like compartment in an electrical energy resources center lab, lined with highly absorbent charcoal-gray foam cones.

There, "horn" antennas transmit and receive microwaves with wavelengths up to several centimeters, that is, more than 10,000 times longer than in the infrared range. They are cloaking metal cylinders two to three inches in diameter and three to four inches high with a shell comprised of mm-sized ceramic resonators, she said.

"We want to move experiments to higher frequencies and smaller wavelengths," she said, adding: "The most exciting applications will be at the frequencies of visible light."

Provided by National Science Foundation