Materials in Focus
Oak Ridge National Laboratory (ORNL). See also the press release by Ron Walli of ORNL.
Image credit: ORNL. Click image to enlarge.
Image caption: Schematic of the piezoelectric effect in a non-polar block copolymer system.
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The discovery of piezoelectric behavior in non-polar block copolymers that is an order of magnitude higher than that found in more traditional inorganic ceramic piezoelectric materials caught researchers from Oak Ridge National Laboratory (ORNL) in Tennessee and Aachen University in Germany off guard. “People did not expect that non-polar block copolymers would show any strong response to electric fields, so that was quite a surprise,” says Volker Urban of ORNL. “When we observed how large the effect was at first we were not sure whether we should even call this a piezoelectric effect.” However, common characteristics with conventional piezoelectric ceramics, such as linear dependence on the electric field strength, convinced them that they were observing a new form of piezoelectric material having different physics at the molecular level.
Most piezoelectric materials found to date have been perovskite ceramics, like lead zirconate titanate (PZT), whose crystalline structure is responsible for piezoelectricity. As reported recently in Advanced Materials, Urban and his colleagues experimented with the non-polar block copolymer poly(styrene-b-isoprene) in solution with toluene. The solvent makes the block copolymers more mobile, but the system retains its underlying phase morphology, such as nanoscale lamellar structures. “So you retain this phase morphology but you make the system more flexible and then you can align the lamellae much more easily in an electric field,” Urban says. “That’s really unique to our research.”
Crystalline piezoelectric materials lose their anisotropy when heated above the Curie temperature, resulting in a loss of their piezoelectric properties. The block copolymer system, however, shows an increase in piezoelectricity when crossing the order-disorder transition temperature. The researchers explain this contrasting behavior by discussing the thermodynamics of the block copolymer system, specifically the entropic gain and the enthalpic penalty as the polymer chain reaches a more Gaussian conformation.
Urban sees potential long term applications for this discovery in the areas of batteries, capacitors, and fuel cells. Polymers have already been used as membranes in fuel cells. “But what has been overlooked until now is what effect electric fields, which of course are present in these electric storage devices, may have on the structure of the polymers that are involved in the system,” Urban says. “This has been neglected completely until now.” He acknowledges that fuel cell membranes have been made from polymers that are simpler than block copolymers, so the effects will be different. But he believes that this research provides a unique perspective that may open up new avenues for the improvement of such devices in the future. [Advanced Materials]
Nano Focus
Universitat Autònoma de Barcelona, Spain.
Image credit: Alvaro Sanchez, Universitat Autònoma de Barcelona, Spain. Click image to enlarge.
Image caption: (a) a magnet and its field; ((b) two adjacent magnets whose fields interfere; (c) antimagnetic cylinder (yellow) enclosing one of the magnets, preventing the internal magnetic field from leaking out, and preventing interference with the magnetic field of the other magnet.
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Building on recent revelations about metamaterials that can cloak a region in space from electromagnetic waves, researchers at the Universitat Autònoma de Barcelona have determined through simulations that it is possible to design an antimagnet using two materials that are within the realm of possibility: metamaterials and superconductors. According to a paper published recently in the New Journal of Physics, Alvaro Sanchez and his colleagues have outlined a method of designing an antimagnet that would “switch off the magnetic interaction of a magnetic material with existing magnetic fields without modifying them.” Such a device could have applications in medical MRI or in reducing the magnetic signatures of planes or ships.
To be precise about what the researchers are proposing, they define an antimagnet as “a material forming a shell that encloses a given region in space while fulfilling the following two conditions: (i) the magnetic field created by any magnetic element inside the inner region—e.g., a permanent magnet—should not leak outside the region enclosed by the shell; and (ii) the system formed by the enclosed region plus the shell should be magnetically undetectable from outside (no interaction—e.g., no magnetic force—with any external magnetic sources).”
Sanchez and his co-authors propose a cylindrical shell for their antimagnet, although they contend that other shapes are possible. The use of a superconductor with magnetic permeability μ = 0 on the inside of the cylinder satisfies condition (i). To satisfy condition (ii), the outer shell of the cylinder would have to be made of a magnetic material with homogeneous radial and angular magnetic permeabilities. Since no known material has these properties, they propose using alternating layers of available materials. A superparamagnet made by embedding ferromagnetic nanoparticles in a non-magnetic material could be used for the first type of layer, and arrays of superconducting plates could be used for the second type, according to the researchers. A ten-layer cylinder with carefully defined permeabilities could produce the desired antimagnetic properties, they contend. The researchers recognize the difficulty of producing a practical antimagnetic device at this time, but offer the results of their simulations as a significant step in the development process. [New Journal of Physics]
Bio Focus
University of Washington. See also the press release by Hannah Hickey of the University of Washington Office of News and Information.
Image credit: Uniiversity of Washington. Click image to enlarge.
Image caption: On the left is a colored photo of the University of Washington device overlaid on a graphic of the other components. On the right is a magnified image of the chitosan fibers. The white scale bar is 200 nanometers.
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Researchers at the University of Washington in Seattle and collaborators at the University of Waterloo in Canada have developed a protonic field effect transistor (H+-FET) that controls the flow of protons instead of electrons, making it a good potential starting point for bio-interface devices. In biology, it’s generally the movement of ions (H+, Na+, K+, and Ca2+), instead of electrons, that controls processes such as ATP synthesis, neuronal signaling, and cell communication. Protons specifically play a key role in biological energy transduction mediated by ATP.
Lead author Marco Rolandi of the University of Washington coined the word “bionanoprotonics” in a recent paper in Nature Communications to describe this novel field, complementing research being done in bionanoelectronics. “We had to learn and, at times, make up all new terminology because all of a sudden there are no electrodes, there are ‘protodes’ for contacts, and there’s no electronic current, there’s protonic current,” he says.
The H+-FET consists of maleic chitosan nanofibers bridging the source and drain of the transistor, which are made of proton-conducting PdHx. The prototype is built on a traditional Si/SiO2substrate, which would have to be replaced with a biocompatible and flexible material if these devices are ever used in biological systems. Maleic chitosan is a biodegradable, non-toxic polysaccharide chitin derivative that forms many hydrogen bonds when hydrated. When an electrostatic potential is applied between source and drain, the protons dissociated from the maleic acid groups “hop” along the hydrogen bond network as described by the Grotthus mechanism. This hopping results in a protonic current from source to drain, which can be modulated by a voltage applied to the gate. The measured mobility of this current is 4.9 x 10-3 cm2V-1 s-1. “We think it is actually a molecular level process rather than protons [as hydronium ions] just diffusing between the water molecules, pushing them around,” Rolandi says. Because the mechanism is specific to protons, this device will not be suitable for controlling Na+, K+, or Ca2+. “We wish we could work with those ions, but we’re happy with protons for now,” he says.
Future work will attempt to make a truly nanoscale device; the prototype is a microscale device with nanoscale fibers. Rolandi would like to bridge the source and drain contacts with a single nanofiber of maleic chitosan to see whether that improves the on/off ratio of the H+-FET, which is currently low compared to traditional semiconductors. A further goal is to interface these transistors with cell cultures. Ultimately, in the distant future, the goal is to optimize the materials and performance of H+-FETs in physiological conditions so that in vivo sensing and stimulation of proton-selective ion channels could become possible. [Nature Communications]
Energy Focus
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Energy Quartely in MRS Bulletin, September 2011
By Dr. Russell Chianelli, The University of Texas at El Paso, M.R.T.I
Opening the grid across continents: Desert visions.
The September issue of Energy Quarterly in MRS Bulletin reports on the Desertec initiative that promises to distribute solar power from the Middle East and North Africa (MENA) for distribution to Europe. Corinna Wu describes Desertec as a centerpiece of Europe’s plans to dramatically increase renewable sources in its energy supply mix. There are many obstacles to developing the system, some political and some technical. However, one issue that arises in centralized solar with power transmission is the transmission grids that must be constructed and the materials used. In some areas, for example in the Southwestern United States, distributed energy systems involving individual people, businesses, and institutions installing solar power devices on local buildings eliminates the need for long transmission systems. However, in this case, energy storage is the issue. We look forward with interest to the competition between these two approaches.
Solid-state lighting: The future looks bright.
Also, in the September issue is an article by Prachi Patel in which she describes the progress in making LEDs (light-emitting diodes) for practical solid-state lighting devices. LEDs promise to reduce energy use by 75% and increase lifetime by a factor of ten. But technical challenges remain. LEDs are made using semiconductor materials (e.g., InGaN or GaN). However, cost and spectral properties need improvement. These issues are covered in detail in this article.
Batteries for energy: generation and storage
An interview with Yet-Ming Chiang of MIT covers a crucial issue for efficient use of energy in electrical vehicles and solar installations: batteries. Chiang is an entrepreneurial leader in next-generation nano-phosphate lithium batteries, having formed companies such as A123 Systems, which now supplies these batteries to industry, and 24M Systems, which produces flow batteries. Flow batteries are preferred for solar-produced-energy storage. The interview also discusses Chiang’s experience as an entrepreneur and the process that he followed from laboratory research to marketable products.
Image in Focus
Anode Feathers
SEM image of an anode used in lithium ion batteries. This particular sample consists of 95% pure semiconducting single-walled carbon nanotubes.
Credit: Laila Jaber Ansari, Northwestern University
(Click image to enlarge.)
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