Friday, October 23, 2009
Etiquetas: Materials Science
New drive technologies combined with lighter and stronger materials will make the airplanes and automobiles of the future more fuel-efficient. But a number of technical details need to be resolved first. Magnetic ball valves are one example – for them to react extremely quickly, the balls must be as light as possible, and the same applies to rapidly moving bearings. Hollow spheres made of steel represent a solution.
Researchers at the Fraunhofer Institute for Manufacturing and Advanced Materials IFAM in Dresden working in cooperation with hollomet GmbH Dresden have created the technology for the manufacture of rapidly reacting ball valves and bearings. “In an injection valve the movement of a ball causes the valve to open and close. The lighter the ball, the quicker it moves,” explains Dr.-Ing. Hartmut Göhler, project manager at the IFAM. Until now it has only been possible to produce balls of this size as solid spheres, but a solid body is relatively heavy and therefore reacts slowly in a ball valve. “For the first time we’ve been able to produce metal hollow spheres in the required diameter of just two to ten millimeters. The hollow spheres are 40 to 70 percent lighter than solid ones.” The process starts with polystyrene balls which are lifted up and held by an air current over a fluidized bed while a suspension consisting of metal powder and binder is sprayed onto them. When the metal layer on the balls is thick enough, heat treatment begins, in which all the organic components, the polystyrene and the binder evaporate. The residual materials are gaseous and escape through the pores in the metal layer. A fragile ball of metal remains. This is now sintered at just below melting temperature, and the metal powder granules bind together, forming a hard and cohesive shell. The sphere is now stable enough to be ground in a machine, but the pressure must not be too high as otherwise the hollow body will deform. The wall thickness can be set to between a few tenths of a millimeter and one millimeter.
Göhler sees applications for the technique wherever a low mass inertia is required. “Hollow spheres will create applications which have not been possible up to now,” Göhler states. The scientists have already produced ground spheres made of steel, other metals such as titanium and various alloys are envisaged for the future.
(Photo: © Fraunhofer IFAM)
Recently, at Arizona State University's Biodesign Institute, N.J. Tao and collaborators have found a way to make a key electrical component on a phenomenally tiny scale. Their single-molecule diode is described in this week's online edition of Nature Chemistry.
In the electronics world, diodes are a versatile and ubiquitous component. Appearing in many shapes and sizes, they are used in an endless array of devices and are essential ingredients for the semiconductor industry. Making components including diodes smaller, cheaper, faster and more efficient has been the holy grail of an exploding electronics field, now probing the nanoscale realm.
Smaller size means cheaper cost and better performance for electronic devices. The first generation computer CPU used a few thousand transistors, Tao says noting the steep advance of silicon technology. "Now even simple, cheap computers use millions of transistors on a single chip."
But lately, the task of miniaturization has gotten much harder, and the famous dictum known as Moore's law—which states that the number of silicon-based transistors on a chip doubles every 18-24 months—will eventually reach its physical limits. "Transistor size is reaching a few tens of nanometers, only about 20 times larger than a molecule," Tao says. "That's one of the reasons people are excited about this idea of molecular electronics."
Diodes are critical components for a broad array of applications, from power conversion equipment, to radios, logic gates, photodetectors and light-emitting devices. In each case, diodes are components that allow current to flow in one direction around an electrical circuit but not the other. For a molecule to perform this feat, Tao explains, it must be physically asymmetric, with one end capable of forming a covalent bond with the negatively charged anode and the other with the positive cathode terminal.
The idea of surpassing silicon limits with a molecule-based electronic component has been around awhile. "Theoretical chemists Mark Ratner and Ari Aviram proposed the use of molecules for electronics like diodes back in 1974," Tao says, adding "people around world have been trying to accomplish this for over 30 years."
Most efforts to date have involved many molecules, Tao notes, referring to molecular thin films. Only very recently have serious attempts been made to surmount the obstacles to single-molecule designs. One of the challenges is to bridge a single molecule to at least two electrodes supplying current to it. Another challenge involves the proper orientation of the molecule in the device. "We are now able to do this—to build a single molecule device with a well defined orientation," Tao says.
The technique developed by Tao's group relies on a property known as AC modulation. "Basically, we apply a little periodically varying mechanical perturbation to the molecule. If there's a molecule bridged across two electrodes, it responds in one way. If there's no molecule, we can tell."
The interdisciplinary project involved Professor Luping Yu, at the University of Chicago, who supplied the molecules for study, as well as theoretical collaborator, Professor Ivan Oleynik from the University of South Florida. The team used conjugated molecules, in which atoms are stuck together with alternating single and multiple bonds. Such molecules display large electrical conductivity and have asymmetrical ends capable of spontaneously forming covalent bonds with metal electrodes to create a closed circuit.
The project's results raise the prospect of building single molecule diodes – the smallest devices one can ever build. "I think it's exciting because we are able to look at a single molecule and play with it," Tao says. "We can apply a voltage, a mechanical force, or optical field, measure current and see the response. As quantum physics controls the behaviors of single molecules, this capability allows us to study properties distinct from those of conventional devices."
Chemists, physicists, materials researchers, computational experts and engineers all play a central role in the emerging field of nanoelectronics, where a zoo of available molecules with different functions provide the raw material for innovation. Tao is also examining the mechanical properties of molecules, for example, their ability to oscillate. Binding properties between molecules make them attractive candidates for a new generation of chemical sensors. "Personally, I am interested in molecular electronics not because of their potential to duplicate today's silicon applications,” Tao says. Instead, molecular electronics will benefit from unique electronic, mechanical, optical and molecular binding properties that set them apart from conventional semiconductors. This may lead to applications complementing rather than replacing silicon devices.
Author Juliano Laran (University of Miami) tested subjects to determine how certain words and concepts affected consumers' decisions for self-control or indulgence. He found that consumer choices were affected by the actions most recently suggested to them by certain key words.
The tests involved a word-scramble containing words that suggested either indulgence ("weight") or self-control ("delicious"). "Participants who unscrambled sentences associated with self-control were more likely to choose a healthy snack (a granola bar) to be consumed right now, but an indulgent snack (a chocolate bar) to be consumed in the future," writes Laran. Participants who unscrambled sentences associated with indulgence were more likely to choose an indulgent snack to be consumed right now but a healthy snack to be consumed in the future."
A second study examined the same phenomenon, but it involved information associated with saving versus spending money. Again, when information about saving money was active (participants had been exposed to words associated with saving money), participants said that they imagined themselves trying to save money while shopping in the present, but spending a lot of money while shopping in the future. When words about spending money were suggested, the study showed the opposite result.
"The type of information (self-control or indulgence) that is currently active may influence a decision for the future," write Laran. "When information about self-control (indulgence) is currently active, decisions for the present will be virtuous (indulgent), while decisions for the future will be indulgent (virtuous). This result arises from people's need to balance behaviors performed in the present with behaviors that will be performed in the future."
Both marketers and consumers can benefit from being aware of these effects, Laran concludes.