New Way to Control Conductivity: Reversible Control of Electrical and Thermal Properties Could Find Uses in Storage Systems

"It's a new way of changing and controlling the properties" of materials -- in this case a class called percolated composite materials -- by controlling their temperature, says Gang Chen, MIT's Carl Richard Soderberg Professor of Power Engineering and director of the Pappalardo Micro and Nano Engineering Laboratories. Chen is the senior author of a paper describing the process that was published online on April 19 and will appear in a forthcoming issue ofNature Communications. The paper's lead authors are former MIT visiting scholars Ruiting Zheng of Beijing Normal University and Jinwei Gao of South China Normal University, along with current MIT graduate student Jianjian Wang. The research was partly supported by grants from the National Science Foundation.

The system Chen and his colleagues developed could be applied to many different materials for either thermal or electrical applications. The finding is so novel, Chen says, that the researchers hope some of their peers will respond with an immediate,"I have a use for that!"

One potential use of the new system, Chen explains, is for a fuse to protect electronic circuitry. In that application, the material would conduct electricity with little resistance under normal, room-temperature conditions. But if the circuit begins to heat up, that heat would increase the material's resistance, until at some threshold temperature it essentially blocks the flow, acting like a blown fuse. But then, instead of needing to be reset, as the circuit cools down the resistance decreases and the circuit automatically resumes its function.

Another possible application is for storing heat, such as from a solar thermal collector system, later using it to heat water or homes or to generate electricity. The system's much-improved thermal conductivity in the solid state helps it transfer heat.

Essentially, what the researchers did was suspend tiny flakes of one material in a liquid that, like water, forms crystals as it solidifies. For their initial experiments, they used flakes of graphite suspended in liquid hexadecane, but they showed the generality of their process by demonstrating the control of conductivity in other combinations of materials as well. The liquid used in this research has a melting point close to room temperature -- advantageous for operations near ambient conditions -- but the principle should be applicable for high-temperature use as well.

The process works because when the liquid freezes, the pressure of its forming crystal structure pushes the floating particles into closer contact, increasing their electrical and thermal conductance. When it melts, that pressure is relieved and the conductivity goes down. In their experiments, the researchers used a suspension that contained just 0.2 percent graphite flakes by volume. Such suspensions are remarkably stable: Particles remain suspended indefinitely in the liquid, as was shown by examining a container of the mixture three months after mixing.

By selecting different fluids and different materials suspended within that liquid, the critical temperature at which the change takes place can be adjusted at will, Chen says.

"Using phase change to control the conductivity of nanocomposites is a very clever idea," says Li Shi, a professor of mechanical engineering at the University of Texas at Austin. Shi adds that as far as he knows"this is the first report of this novel approach" to producing such a reversible system.

"I think this is a very crucial result," says Joseph Heremans, professor of physics and of mechanical and aerospace engineering at Ohio State University."Heat switches exist," but involve separate parts made of different materials, whereas"here we have a system with no macroscopic moving parts," he says."This is excellent work."

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Antibody-Based Biosensor Can Guide Environmental Clean-Ups, Provide Early Warning System for Spills

Testing of the biosensor in the Elizabeth River and Yorktown Creek, which both drain into lower Chesapeake Bay, shows that the instrument can process samples in less than 10 minutes, detect pollutants at levels as low as just a few parts per billion, and do so at a cost of just pennies per sample. Current technology requires hours of lab work, with a per-sample cost of up to$1,000.

"Our biosensor combines the power of the immune system with the sensitivity of cutting-edge electronics," says Dr. Mike Unger of VIMS."It holds great promise for real-time detection and monitoring of oil spills and other releases of contaminants into the marine environment."

The biosensor was developed and tested by Unger, fellow VIMS professor Steve Kaattari, and their doctoral student Candace Spier, with assistance from marine scientist George Vadas. The team's report of field tests with the sensor appears in this month's issue ofEnvironmental Toxicology and Chemistry.

The instrument was developed in conjunction with Sapidyne Instruments, Inc., with funding from the state of Virginia, the Office of Naval Research, and the Cooperative Institute for Coastal and Estuarine Environmental Technology, a partnership between NOAA and the University of New Hampshire.

The tests in the Elizabeth River took place during clean up of a site contaminated by polycyclic aromatic hydrocarbons (PAHs), byproducts of decades of industrial use of creosote to treat marine pilings. The U.S. Environmental Protection Agency considers PAHs highly toxic and lists 17 as suspected carcinogens.

The biosensor allowed the researchers to quantify PAH concentrations while the Elizabeth River remediation was taking place, gaining on-site knowledge about water quality surrounding the remediation site. Spier says the test was"the first use of an antibody-based biosensor to guide sampling efforts through near real-time evaluation of environmental contamination."

In the Yorktown Creek study, the researchers used the biosensor to track the runoff of PAHs from roadways and soils during a rainstorm.

Biosensor development

Kaattari says"Our basic idea was to fuse two different kinds of technologies -- monoclonal antibodies and electronic sensors -- in order to detect contaminants."

Antibodies are proteins produced by the immune system of humans and other mammals. They are particularly well suited for detecting contaminants because they have, as Kaattari puts it, an"almost an infinite power to recognize the 3-dimensional shape of any molecule."

Mammals produce antibodies that recognize and bind with large organic molecules such as proteins or with viruses. The VIMS team took this process one step further, linking proteins to PAHs and other contaminants, then exposing mice to these paired compounds in a manner very similar to a regular vaccination.

"Just like you get vaccinated against the flu, we in essence are vaccinating our mice against contaminants," says Kaattari."The mouse's lymphatic system then produces antibodies to PAHs, TNT, tributyl tin {TBT, the active ingredient in anti-fouling paints for boats}, or other compounds."

Once a mouse has produced an antibody to a particular contaminant, the VIMS team applies standard clinical techniques to produce"monoclonal antibodies" in sufficiently large quantities for use in a biosensor.

"This technology allows you to immortalize a lymphocyte that produces only a very specific antibody," says Kaattari."You grow the lymphocytes in culture and can produce large quantities of antibodies within a couple of weeks. You can preserve the antibody-producing lymphocyte forever, which means you don't have to go to a new animal every time you need to produce new antibodies."

From antibody to electrical signal

The team's next step was to develop a sensor that can recognize when an antibody binds with a contaminant and translate that recognition into an electrical signal. The Sapidyne^®sensor used by the VIMS team works via what Kaattari calls a"fluorescence-inhibitory, spectroscopic kind of assay."

In the sensor used on the Elizabeth River and Yorktown Creek, antibodies designed to recognize a specific class of PAHs were joined with a dye that glows when exposed to fluorescent light. The intensity of that light is in turn recorded as a voltage. The sensor also houses tiny plastic beads that are coated with what Spier calls a"PAH surrogate" -- a PAH derivative that retains the shape that the antibody recognizes as a PAH molecule.

When water samples with low PAH levels are added to the sensor chamber (which is already flooded with a solution of anti-PAH antibodies), the antibodies have little to bind with and are thus free to attach to the surrogate-coated beads, providing a strong fluorescent glow and electric signal. In water samples with high PAH concentrations, on the other hand, a large fraction of the antibodies bind with the environmental contaminants. That leaves fewer to attach to the surrogate-coated beads, which consequently provides a fainter glow and a weaker electric signal.

During the Elizabeth River study, the biosensor measured PAH concentrations that ranged from 0.3 to 3.2 parts per billion, with higher PAH levels closer to the dredge site. In Yorktown Creek, the biosensor showed that PAH levels in runoff peaked 1 to 2 hours after the rain started, with peak concentration of 4.4 parts per billion.

Comparison of the biosensor's field readings with later readings from a mass spectrometer at VIMS showed that the biosensor is just as accurate as the more expensive, slower, and laboratory-bound machine.

A valuable field tool

Spier says"Using the biosensor allowed us to quickly survey an area of almost 900 acres around the Elizabeth River dredge, and to provide information about the size and intensity of the contaminant plume to engineers monitoring the dredging from shore. If our results had shown elevated concentrations, they could have halted dredging and put remedial actions in place."

Unger adds"measuring data in real-time also allowed us to guide the collection of large-volume water samples right from the boat. We used these samples for later analysis of specific PAH compounds in the lab. This saved time, effort, and money by keeping us from having to analyze samples that might contain PAHs at levels below our detection limit."

"Biosensors have their constraints and optimal operating conditions," says Kaattari,"but their promise far outweighs any limitations. The primary advantages of our biosensor are its sensitivity, speed, and portability. These instruments are sure to have a myriad of uses in future environmental monitoring and management."

One promising use of the biosensor is for early detection and tracking of oil spills."If biosensors were placed near an oil facility and there was a spill, we would know immediately," says Kaattari."And because we could see concentrations increasing or decreasing in a certain pattern, we could also monitor the dispersal over real time."

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High Temperature Milestone Achieved in Silicon Spintronics

The electron possesses an internal angular momentum called the spin. The International Technology Roadmap for Semiconductors has identified the electron's spin as a new state variable that should be explored as an alternative to the electron's charge for use beyond Moore's Law, a projection named after Intel co-founder Gordon E. Moore. Moore predicted in 1965 that the number of transistors per unit area in an integrated circuit would double approximately every two years as advances in fabrication technology enabled the devices to be made smaller. Although this approach has been remarkably successful, critical device dimensions now approach atomic length scales, so that further size scaling becomes untenable."Researchers have been forced to look beyond the simple reduction of size to develop future generations of electronic devices," states NRL senior scientist Dr. Berry Jonker."Electrical generation, manipulation and detection of significant spin polarization in silicon at temperatures that meet commercial and military requirements are essential to validate spin as an alternative to charge for a device technology beyond Moore's Law."

Using ferromagnetic metal / silicon dioxide contacts on silicon, NRL scientists Connie Li, Olaf van 't Erve and Jonker electrically generate and detect spin accumulation and precession in the silicon transport channel at temperatures up to 225°C, and conclude that the spin information can be transported in the silicon over distances readily compatible with existing fabrication technology. They thus overcome a major obstacle in achieving control of the spin variable at temperatures required for practical applications in the most widely utilized semiconductor.

To make a semiconductor spintronic device, one needs contacts that can both generate a current of spin-polarized electrons (called a spin injector), and detect the spin polarization of the electrons (spin detector) in the semiconductor. Because the magnetic contact interface is likely to introduce additional scattering and spin relaxation mechanisms not present in the silicon bulk, the region of the semiconductor directly beneath the contact is expected to be a critical factor in the development of any future spin technology. The NRL scientists probe the spin environment directly under the magnetic metal / silicon dioxide contact using the three terminal geometry illustrated in the accompanying

figure

. Demonstration of spin precession and dephasing in a magnetic field transverse to the injected spin orientation, known as the Hanle effect, is conclusive evidence of spin accumulation, and enables a direct measure of the spin lifetime, a critical parameter for device operation. The NRL researchers observed Hanle precession of the electron spin accumulation in the silicon channel under the contact for biases corresponding to both spin injection and extraction, and determine the corresponding spin lifetimes.

Electronic states can form at the contact interface and introduce deleterious effects for both charge and spin transport. These undesirable states can serve as traps which prevent propagation of either charge or spin in the silicon channel. In bulk silicon, the spin lifetime is known to depend upon the carrier density, and generally decreases as the electron density increases.."In this study we show that the spin lifetime determined from our measurements changes systematically as one changes carrier concentration of the particular silicon sample used," adds Jonker."Our results were obtained for a number of different carrier densities and show this trend, thus making it very clear that we obtain spin injection and accumulation in the silicon itself rather than in interface defect states." The result of this research rules out spin accumulation in interface states and demonstrates spin injection, accumulation and precession in the silicon channel.

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Using Solar Power to Sterilize Medical Instruments

The student's used Capteur Soleil, a device created decades ago by French inventor Jean Boubour to capture the energy of the sun in places where electricity -- or fuel of any kind -- is hard to get. In attaching an insulated box containing the autoclave, the students transform the device into a potential lifesaver.

The Capteur Soleil, which sits outside Rice's Oshman Engineering Design Kitchen, looks something like an ultramodern lawn swing. Its spine is a steel A-frame, and a bed of curved mirrors beneath the frame produces steam by focusing sunlight along a steel tube at the frame's apex. Rather than pump steam directly into the autoclave, the Rice team's big idea was to use the steam to heat a custom-designed conductive hotplate.

"It basically becomes a stovetop, and you can heat anything you need to," said Sam Major, a member of the team with seniors Daniel Rist, David Luker and William Dunk, all mechanical engineering students."As long as the autoclave reaches 121 Celsius for 30 minutes (the standard set by the Centers for Disease Control and Prevention), everything should be sterile, and we've found we're able to do that pretty easily."

He said one person could easily adjust the Capteur Soleil by ratcheting up the back leg to align the mirrors with the sun. Within half an hour of receiving strong sunlight, the Capteur Soleil will begin to produce steam, which will in turn heat the patterned hotplate and then the standard-issue, FDA-approved autoclave. With good midday sun, Major said, it takes 40 minutes to an hour to begin significant heating of the autoclave.

The autoclave, which looks like a tricked-out pressure cooker, has a steamer basket inside."We put about an inch of water inside, followed by the basket with the tools and syringes," Major said."We've used some biological spores from a test kit, steamed them, and then incubated them for 24 hours and they came back negative for biological growth. That means we killed whatever was in there."

The autoclave, tucked inside a plywood frame, is wrapped in silicon-based Thermablok insulation, which has the highest R-value of any known material and is a spinoff from NASA research into thermal protection for the space shuttle."This thin layer does most of the work," Major said."We used standard pink insulation around the inside just to make the box stronger."

"This is really the latest iteration of a much larger project," said Doug Schuler, the team's faculty adviser and an associate professor of business and public policy at Rice's Jones Graduate School of Business."We already have a version of the Capteur Soleil being used in Haiti for cooking, but we felt it could do more."

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Portable Tech Might Provide Drinking Water, Power to Villages

Such a technology might be used to provide power and drinking water to villages and also for military operations, said Jerry Woodall, a Purdue University distinguished professor of electrical and computer engineering.

The alloy contains aluminum, gallium, indium and tin. Immersing the alloy in freshwater or saltwater causes a spontaneous reaction, splitting the water into hydrogen and oxygen molecules. The hydrogen could then be fed to a fuel cell to generate electricity, producing water in the form of steam as a byproduct, he said.

"The steam would kill any bacteria contained in the water, and then it would condense to purified water," Woodall said."So, you are converting undrinkable water to drinking water."

Because the technology works with saltwater, it might have marine applications, such as powering boats and robotic underwater vehicles. The technology also might be used to desalinate water, said Woodall, who is working with doctoral student Go Choi.

A patent on the design is pending.

Woodall envisions a new portable technology for regions that aren't connected to a power grid, such as villages in Africa and other remote areas.

"There is a big need for this sort of technology in places lacking connectivity to a power grid and where potable water is in short supply," he said."Because aluminum is a low-cost, non-hazardous metal that is the third-most abundant metal on Earth, this technology promises to enable a global-scale potable water and power technology, especially for off-grid and remote locations."

The potable water could be produced for about$1 per gallon, and electricity could be generated for about 35 cents per kilowatt hour of energy.

"There is no other technology to compare it against, economically, but it's obvious that 34 cents per kilowatt hour is cheap compared to building a power plant and installing power lines, especially in remote areas," Woodall said.

The unit, including the alloy, the reactor and fuel cell might weigh less than 100 pounds.

"You could drop the alloy, a small reaction vessel and a fuel cell into a remote area via parachute," Woodall said."Then the reactor could be assembled along with the fuel cell. The polluted water or the seawater would be added to the reactor and the reaction converts the aluminum and water into aluminum hydroxide, heat and hydrogen gas on demand."

The aluminum hydroxide waste is non-toxic and could be disposed of in a landfill.

The researchers have a design but haven't built a prototype.

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Chemist Designs New Polymer Structures for Use as 'Plastic Electronics'

Those tricks improve the properties of certain organic polymers that mimic the properties of traditional inorganic semiconductors and could make the polymers very useful in organic solar cells, light-emitting diodes and thin-film transistors.

Conductive polymers date back to the late 1970s when researchers Alan Heeger, Alan MacDiarmid and Hideki Shirakawa discovered that plastics, with certain arrangements of atoms, could conduct electricity. The three were awarded the 2000 Nobel Prize in Chemistry for the discovery.

Jeffries-EL, an Iowa State assistant professor of chemistry, is working with a post-doctoral researcher and nine doctoral students to move the field forward by studying the relationship between polymer structures and the electronic, physical and optical properties of the materials. They're also looking for ways to synthesize the polymers without the use of harsh acids and temperatures by making them soluble in organic solvents.

The building blocks of their work are a variety of benzobisazoles, molecules well suited for electrical applications because they efficiently transport electrons, are stable at high temperatures and can absorb photons.

And if the polymers are lacking in any of those properties, Jeffries-EL and her research group can do some chemical restructuring.

"With these polymers, if you don't have the properties you need, you can go back and change the wheel," Jeffries-EL said."You can change the chemical synthesis and produce what's missing."

That, she said, doesn't work with silicon and other inorganic materials for semiconductors:"Silicon is silicon. Elements are constant."

The National Science Foundation is supporting Jeffries-EL's polymer research with a five-year,$486,250 Faculty Early Career Development grant. She also has support from the Iowa Power Fund (a state program that supports energy innovation and independence) to apply organic semiconductor technology to solar cells.

The research group is seeing some results, including peer-reviewed papers over the past two years inPhysical Chemistry Chemical Physics, Macromolecules, the Journal of Polymer Science Part A: Polymer Chemistry,and theJournal of Organic Chemistry.

"This research is really about fundamental science," Jeffries-EL said."We're studying the relationships between structure and material properties. Once we have a polymer with a certain set of properties, what can we do?"

She and her research group are turning to the molecules for answers.

"In order to realize the full potential of these materials, they must be engineered at the molecular level, allowing for optimization of materials properties, leading to enhanced performance in a variety of applications," Jeffries-EL wrote in a research summary."As an organic chemist, my approach to materials begins with small molecules."

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Cells Send Signals Via Membrane Nanotubes

For nearly ten years, researchers have known that cells can"grow" ultra-thin tubes named tunnelling nanotubes (TNTs) between one another. These nanotubes -- the length of two to three cells and just 1/500th the thickness of a human hair -- are connections that develop between nearly all cell types to form a communication channel different from any previously known mechanisms.

In 2010, Dr. Xiang Wang and Professor Hans-Hermann Gerdes -- colleagues at the University of Bergen's Department of Biomedicine -- discovered that electrical signals were being passed through nanotubes from one cell to another at high speed (roughly 1-2 m/sec). Their research receives funding under the Research Council's large-scale research programme Nanotechnology and New Materials (NANOMAT).

The breakthrough

In their key experiment, Dr Wang used fluorescent dye that changes in intensity as the electric potential of the cell membrane changes. When two cells connected by forming a nanotube, he poked into one of them with a microinjection needle to depolarise that cell's membrane potential. This caused the fluorescent indicator on the cell membrane to light up like a firework, and it was soon followed by a similar light display in the cell on the other end of the nanotube.

The breakthrough discovery began with an experiment demonstrating intercellular transmission of electrical signals via nanotubes in 2007. The researchers then carried out similar trials with a number of other cell types, observing similar occurrences.

"We confirmed that this is a common phenomenon between cells," explains Professor Gerdes."Still, this characteristic is not in every cell type."

The experiment was replicated a number of times to obtain statistically reliable data. The electrophysiology group at the University of Bergen took precise conductivity measurements of the cell systems to determine the strength of the electrical coupling. In autumn 2010 the results were published inProceedings of the National Academy of Sciences (PNAS).

Short lifespan

Intercellular nanotubes are far from permanent. Most of them last only a few minutes. This means the researchers cannot predict where and when the cells will form nanotube connections.

"It is truly painstaking work," says Professor Gerdes."You may sit there examining cells for hours through a microscope without seeing a single tube. If you are lucky, however, you catch sight of a nanotube being created and can film the event."

To raise the likelihood of finding nanotubes, the researchers developed a micro-matrix consisting of thousands of points and bridges on a plate surface. Smaller than a postage stamp, the plate is covered by a nano-structured material to which the cells adhere. The researchers place one cell onto each point and hope that nanotubes will form along the bridges between the points. The camera is focused on these bridges.

Once the nanotubes have been established, the researchers manipulate the cells at specified times; meanwhile the microscope is programmed to photograph, say, 50 preselected points every five minutes. The team can thus obtain data about many nanotube connections in a short time.

How do cells do this?

Dr. Wang quickly discovered that the mere presence of a nanotube was not sufficient to transmit an electrical signal. There had to be another mechanism involved as well.

Many cells form tiny membrane pores with each other called gap junctions, which are made up of ring-shaped proteins. Back in the 1960s it was discovered that directly adjacent cells could exchange electrical impulses through these gap junctions. What Dr Wang found was that one end of the nanotube was always connected to cells by a gap junctions before it transmitted its electrical impulses.

He also found that in some coupled cells voltage-gated calcium channels were involved in the forwarding of the incoming signals. When the electrical signal being sent through the nanotube reaches the membrane of the receiving cell, the membrane surface is depolarised, opening the calcium channel and allowing calcium -- a vital ion in cell signalling -- to enter.

"In other words," explains Professor Gerdes,"there are two components: a nanotube and a gap junction. The nanotube grows out from one cell and connects to the other cell through a gap junction. Only then can the two cells be coupled electrically."

Controls embryonic cells?

Now the scientists are seeking answers as to why the cells send signals to each other in this way.

"It's quite possible that the discovery of nanotubes will give us new insight into intercellular communication," asserts Professor Gerdes."The process could explain how cells are coordinated during embryo growth. In that phase cells travel long distances -- yet they demonstrate a kind of collective behaviour, and move together like a flock of birds can."

Nanotubes may also be a factor in explaining cell movement associated with wound healing, since cells move toward a wound in order to close it. We already know that electrical signals are somehow involved in this process; scientists can only speculate as to whether nanotubes are involved here as well, stresses Professor Gerdes.

Perhaps brain cells, too?

In terms of electronic signal processing, the human brain surpasses all other organs. If this same signalling mechanism proves to be present in human brain cells, it could add a new dimension to understanding how the brain functions. Communication channels involving synapses and dendrites that are already identified differ widely from nanotubes.

The Bergen-based neuroscientists see this research as an opportunity to formulate better explanations for phenomena related to consciousness and electrical connections in the brain. In the project"Cell-to-cell communication: Mechanism of tunnelling nanotube formation and function," they are now studying precisely how nanotube mechanisms function in brain cells.

Professor Gerdes is currently conducting research at the European Molecular Biology Laboratory in Heidelberg. By studying the electrical connections in vivo he hopes to figure out how the mechanisms work in live subjects. The results could enhance understanding of diseases that occur when cell mechanisms fail to function properly.

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