Researchers Closer to The Ultimate Green 'Fridge Magnet'

Scientists are a step closer to making environmentally friendly "magnetic" refrigerators and air conditioning systems a reality, thanks to new research published today in Advanced Materials.

Magnetic refrigeration technology could provide a 'green' alternative to traditional energy-guzzling gas-compression fridges and air conditioners. They would require 20 percent-30 percent less energy to run than the best systems currently available, and would not rely on ozone-depleting chemicals or greenhouse gases. Refrigeration and air conditioning units make a major contribution to the planet's energy consumption -- in the USA in the summer months they account for approximately half of the country's energy use.

A magnetic refrigeration system works by applying a magnetic field to a magnetic material - some of the most promising being metallic alloys -- causing it to heat up. This excess heat is removed from the system by water, cooling the material back down to its original temperature. When the magnetic field is removed the material cools down even further, and it is this cooling property that researchers hope to harness for a wide variety of cooling applications.

The technology, based on research funded in the UK by the Engineering and Physical Sciences Research Council (EPSRC), has proved possible in the lab but researchers are still looking for improved materials that provide highly efficient cooling at normal room temperatures, so that the technology can be rolled out from the lab to people's homes and businesses.

They need a material that exhibits dramatic heating and cooling when a magnetic field is applied and removed, which can operate in normal everyday conditions, and which does not lose efficiency when the cooling cycle is repeated time after time.

The new study published today shows that the pattern of crystals inside different alloys - otherwise known as their microstructure - has a direct effect on how well they could perform at the heart of a magnetic fridge. The Imperial College London team behind the new findings say this could, in the future, help them to custom-design the best material for the job.

Lesley Cohen, one of the authors of the paper, explains that by using unique probes designed at Imperial, her team, led by James Moore, was able to analyze what happens to different materials on a microscopic level when they are magnetised and de-magnetised. This enabled the team to pinpoint what makes some materials better candidates for a magnetic fridge system than others.

Cohen, from Imperial's Department of Phsyics, says: "We found that the structure of crystals in different metals directly affects how dramatically they heat up and cool down when a magnetic field is applied and removed. This is an exciting discovery because it means we may one day be able to tailor-make a material from the 'bottom up', starting with the microstructure, so it ticks all the boxes required to run a magnetic fridge. This is vitally important because finding a low-energy alternative to the fridges and air conditioning systems in our homes and work places is vital for cutting our carbon emissions and tackling climate change."

This new research follows on from another study published by the same Imperial group in Physical Review B last month, in which they used similar probing techniques to precisely measure the temperature changes that occur when different materials are removed from a magnetic field, and to analyse the different ways they occur.

The lead scientist Kelly Morrison found that at the molecular level two different temperature change processes, known as first- and second-order changes, happen simultaneously in each material. The team think that the extent to which each of these two processes feature in a material also affects its cooling capabilities.

Cohen says this means that whilst the majority of research to perfect magnetic refrigeration worldwide has tended to involve analysing and testing large samples of materials, the key to finding a suitable material for everyday applications may lie in the smaller detail:

"Our research illustrates the importance of understanding the microstructure of these materials and how they respond to magnetic fields on a microscopic level," she says.

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Cheap Plastic Used in CDs Could Improve Aircraft, Computer Electronics

If one University of Houston professor has his way, the inexpensive plastic now used to manufacture CDs and DVDs will one day soon be put to use in improving the integrity of electronics in aircraft, computers and iPhones.

Thanks to a pair of grants from the U.S. Air Force, Shay Curran, associate professor of physics at Houston, and his research team have demonstrated ultra-high electrical conductive properties in plastics, called polycarbonates, by mixing them with just the right amount and type of carbon nanotubes.

The findings are chronicled in a paper titled "Electrical Transport Measurements of Highly Conductive Carbon Nanotube/Poly(bisphenol A carbonate) Composite," appearing in a recent issue of the Journal of Applied Physics, the archival publication of the American Institute of Physics.

Curran, who initially began this form of research a decade ago at Trinity College Dublin, started to look at high-conductive plastics in a slightly different manner. Curran's team has come up with a strategy to achieve higher conductivities using carbon nanotubes in plastic hosts than what has been currently achieved. By combining nanotubes with polycarbonates, Curran's group was able to reach a milestone of creating nanocomposites with ultra-high conductive properties.

"While its mechanical and optical properties are very good, polycarbonate is a non-conductive plastic. That means its ability to carry an electrical charge is as good as a tree, which is pretty awful," Curran says. "Imagine that this remarkable plastic can now not only have good optical and mechanical properties, but also good electrical characteristics. By being able to tailor the amount of nanotubes we can add to the composite, we also can change it from the conductivity of silicon to a few orders below that achieved by metals."

Making this very inexpensive plastic highly conductive could benefit electronics in everything from military aircraft to personal computers. Computer failure, for instance, results from the build up of thermal and electrical charges, so developing these polymer nanotube composites into an antistatic coating or to provide a shield against electromagnetic interference would increase the lifespan of computing devices, ranging from PCs to PDAs.

The next step of this research is to develop ink formulations to paint these polycarbonate nanocomposites onto various electrical components. Normally, metal plates are used to dissipate electrical charge, so it's not surprising that the availability of a paintable ink would be particularly appealing to the Air Force for its lightweight properties, resulting in lighter aircraft that guzzle less gas.

Another key component of this latest research is that pristine nanotubes disbursed in this polycarbonate were found to possess an even higher conductivity than acid-treated carbon nanotubes. Traditionally, the tubes are sonicated, or treated with acid, to clean them and remove soot to get a higher conductivity. This, however, damages the tubes and exposes them to defects. Instead, Curran and his group were able to centrifuge, or swirl, them. This takes a little longer, but increases the potential to have higher conductivities. He attributes this to the incredibly clean samples of carbon nanotubes obtained from fellow collaborator David Carroll in the physics department at Wake Forest University.

In addition to Curran and Carroll, the team behind these remarkable findings includes Donald Birx, professor of electrical engineering and vice president for research at UH, two of Curran's former post-doctoral students, Jamal Talla and Donghui Zhang, and a current Curran student, Sampath Dias.

Coincidentally, Curran's former thesis supervisor Werner Blau and his group in the department of physics at Trinity College Dublin have come out with similar findings recently in the journal ACS Nano. Both groups really have been pushing hard in the area of polymer nanotube composites during the course of the last decade. Curran said his group at Houston achieved the highest conductivity levels so far, but also is encouraged by Blau's success and said repeating these types of outcomes will open doors for even higher values.

"While these are phenomenal results, finding these unusual highly conductive properties has not even begun to scratch the surface," Curran says. "There is hard science behind it, so developing it further will require significant investment. And we are very thankful to the Air Force for giving us this auspicious start."

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An Impossible Alloy Now Possible

What has been impossible has now been shown to be possible –- an alloy between two incompatible elements. The findings are being published in this week's edition of Proceedings of the National Academy of Science, USA.

A research team led by H.K. Mao from Carnegie Institution of Washington and Rajeev Ahuja from Uppsala University have used high pressure experiments and theoretical calculations to study the behaviour of Ce3Al under high pressure.

"We were surprised to find that Cerium and Aluminium formed a so called substitutional alloy under high pressure. Forming these alloys has been limited to elements close in atomic radii and electronegativity up until now," says Ahuja.

The difference in radii and electronegativity of Cerium and Aluminium was diminished by applying pressure. Both synchrotron X-ray diffraction and ab initio calculations showed the same cause for bringing the two elements closer in radii and electronegativity, resulting in the new alloy phase. After the release of pressure the substitutional alloy still remained.

"This discovery opens up the possibility for finding new alloys with other ratios between Cerium and Aluminium, as well as alloys with Cerium and other incompatible elements. These new alloys may possess interesting and useful mechanical, electronic, and magnetic properties."

The new findings have been made possible by the recent development of high pressure diamond anvil cell and synchrotron radiation techniques. The extensive simulations were performed at Uppsala University's Multidisciplinary Center for Advanced Computational Science (UPPMAX).

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Single-Crystal Semiconductor Wire Built Into Optical Fiber

An international science team from Penn State University in the United States and the University of Southampton in the United Kingdom has developed a process for growing a single-crystal semiconductor inside the tunnel of a hollow optical fiber. The device adds new electronic capabilities to optical fibers, whose performance in electronic devices such as computers typically is degraded by the interface between the fiber and the device. The research is important because optical fibers -- which are used in a wide range of technologies that employ light, including telecommunications, medicine, computing, and remote-sensing devices -- are ideal media for transmitting many types of signals.

The development of the single-crystal device, which will be described in a paper to be published later this month in the journal Advanced Materials, builds on research reported in 2006, in which the team first combined optical fibers with polycrystalline and amorphous semiconductor materials in order to create an optical fiber that also has electronic characteristics. The group's latest finding -- that a single-crystal semiconductor also can be integrated into an optical fiber -- is expected to lead to even further improvements in the characteristics of optical fibers used in many areas of science and technology.

"For most applications, single-crystal semiconductor materials have better performance than polycrystalline and amorphous materials," says John Badding, associate professor of chemistry at Penn State. "We have now shown that our technique of encasing a single-crystal semiconductor within an optical fiber results in greater functionality of the optical fiber, as well."

The team used a high-pressure fluid-liquid-solid approach to build the crystal inside the fiber. First, the scientists deposited a tiny plug of gold inside the fiber by exposing a gold compound to laser light. Next, they introduced silane, a compound of silicon and hydrogen, in a stream of high-pressure helium. When the fiber was heated, the gold acted as a catalyst, decomposing the silane and thus allowing silicon to deposit as a single crystal behind the moving gold catalyst particle, forming a single-crystal wire inside the fiber.

"The key to joining two technologies lies not only in the materials, but also in how the functions are built in," says Pier Sazio, senior research fellow in the Optoelectronics Research Centre at the University of Southampton. "We were able to embed a nanostructured crystal into the hollow tube of an optical fiber to create a completely new type of composite device."

The research team sees potential to carry the application to the next level. "At present, we still have electrical switches at both ends of the optical fiber," says Badding. "If we can get to the point where the electrical signal never leaves the fiber, it will be faster and more efficient."

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