Monday, January 3, 2011


0 comentarios

A benchtop version of the world’s smallest battery — its anode a single nanowire one seven-thousandth the thickness of a human hair — has been created by a team led by Sandia National Laboratories researcher Jianyu Huang.

To better study the anode’s characteristics, the tiny rechargeable, lithium-based battery was formed inside a transmission electron microscope (TEM) at the Center for Integrated Nanotechnologies (CINT), a Department of Energy research facility jointly operated by Sandia and Los Alamos national laboratories.

Says Huang of the work, reported in the Dec. 10 issue of the journal Science, “This experiment enables us to study the charging and discharging of a battery in real time and at atomic scale resolution, thus enlarging our understanding of the fundamental mechanisms by which batteries work.”

Because nanowire-based materials in lithium ion batteries offer the potential for significant improvements in power and energy density over bulk electrodes, more stringent investigations of their operating properties should improve new generations of plug-in hybrid electric vehicles, laptops and cell phones.

“What motivated our work,” says Huang, “is that lithium ion batteries [LIB] have very important applications, but the low energy and power densities of current LIBs cannot meet the demand. To improve performance, we wanted to understand LIBs from the bottom up, and we thought in-situ TEM could bring new insights to the problem.”

Battery research groups do use nanomaterials as anodes, but in bulk rather than individually — a process, Huang says, that resembles “looking at a forest and trying to understand the behavior of an individual tree.”

The tiny battery created by Huang and co-workers consists of a single tin oxide nanowire anode 100 nanometers in diameter and 10 micrometers long, a bulk lithium cobalt oxide cathode three millimeters long, and an ionic liquid electrolyte. The device offers the ability to directly observe change in atomic structure during charging and discharging of the individual “trees.”

An unexpected find of the researchers was that the tin oxide nanowire rod nearly doubles in length during charging — far more than its diameter increases — a fact that could help avoid short circuits that may shorten battery life. “Manufacturers should take account of this elongation in their battery design,” Huang said. (The common belief of workers in the field has been that batteries swell across their diameter, not longitudinally.)

Huang’s group found this flaw by following the progression of the lithium ions as they travel along the nanowire and create what researchers christened the “Medusa front” — an area where high density of mobile dislocations cause the nanowire to bend and wiggle as the front progresses. The web of dislocations is caused by lithium penetration of the crystalline lattice. “These observations prove that nanowires can sustain large stress (>10 GPa) induced by lithiation without breaking, indicating that nanowires are very good candidates for battery electrodes,” said Huang.

“Our observations — which initially surprised us — tell battery researchers how these dislocations are generated, how they evolve during charging, and offer guidance in how to mitigate them,” Huang said. “This is the closest view to what’s happening during charging of a battery that researcher have achieved so far.”

Lithiation-induced volume expansion, plasticity and pulverization of electrode materials are the major mechanical defects that plague the performance and lifetime of high-capacity anodes in lithium-ion batteries, Huang said. “So our observations of structural kinetics and amorphization [the change from normal crystalline structure] have important implications for high-energy battery design and in mitigating battery failure.”

The electronic noise level generated from the researchers’ measurement system was too high to read electrical currents, but Sandia co-author John Sullivan estimated a current level of a picoampere flowing in the nanowire during charging and discharging. The nanowire was charged to a potential of about 3.5 volts, Huang said.

A picoampere is a millionth of a microampere. A microampere is a millionth of an ampere.

The reason that atomic-scale examination of the charging and discharging process of a single nanowire had not been possible was because the high vacuum in a TEM made it difficult to use a liquid electrolyte. Part of the Huang group’s achievement was to demonstrate that a low-vapor-pressure ionic liquid — essentially, molten salt —could function in the vacuum environment.

Although the work was carried out using tin oxide (SnO2) nanowires, the experiments can be extended to other materials systems, either for cathode or anode studies, Huang said.

“The methodology that we developed should stimulate extensive real-time studies of the microscopic processes in batteries and lead to a more complete understanding of the mechanisms governing battery performance and reliability,” he said. “Our experiments also lay a foundation for in-situ studies of electrochemical reactions, and will have broad impact in energy storage, corrosion, electrodeposition and general chemical synthesis research field.”

(Photo: DOE Center for Integrated Nanotechnologies)

Sandia National Laboratories


0 comentarios
What can scientists learn from watching a group of people sitting around, chatting, playing movies, reading, and happily making new friends? Quite a lot, says University of Melbourne, Australia acoustician Adam Vogel, who carefully observed this sort of group in a fatigue management study he and his colleagues describe this month in The Journal of the Acoustical Society of America.

Their report shows the effects of sustained wakefulness on speech and describes a novel method to acoustically analyze the effects of fatigue on the central nervous system as revealed through speech.

The findings are significant to workers, employers, public safety officials, and military leaders who are concerned with managing fatigue over long shifts, notes Vogel.

"There is increasing interest in the development of objective non-invasive systems that can be used to assist the identification and management of fatigue in both health and workplace settings," he says.

Measuring fatigue by analyzing a person's speech and quantifying any changes from their normal, rested speech may enable doctors to make objective decisions about a person's ability to function in a work environment. It may also be a useful tool for monitoring fatigue in clinical trials where alertness is a key measured outcome.

The Australian study involved 18 young adults who provided speech samples (sustained vowels, reading counting and reading tasks) every two hours. Vogel and his colleagues looked at components of speech such as length of pauses and total time to complete a spoken task. Their results showed that as fatigue progresses, speech slows and variations in pitch increase and tone diminishes. Their conclusion is that we have less control over the muscles that produce speech as we become more and more tired."Although remaining awake for 24 hours is physically and mentally exhausting, it's actually a great way to make new friends," notes Vogel. "Most of them just entertained themselves between testing by watching movies, reading or talking amongst themselves."

University of Melbourne


0 comentarios

About 50 miles from Bethlehem, a drilling project is determining the climate and earthquake activity of the Holy Land. Scientists from eight nations are examining the ground below the Dead Sea, by placing a borehole in this deepest basin in the world. The International Continental Scientific Drilling Program ICDP brings together research teams from Israel, Japan, Norway, Switzerland, the USA and Germany. Particularly noteworthy: Researchers from Jordan and Palestine are also involved.

Scientists and technicians of the GFZ German Research Centre for Geosciences have now completed a geophysical measurement procedure in the hole and helped with the initial examination of the cores in a field laboratory. "We have drilled through about half a million years of sedimentary deposits," estimates Dr. Ulrich Harms from the ICDP's operational support group at the GFZ. "From this, we can deduce not only the climate history, but also the earthquake activity in this seismically very active region." The direction and inclination of the well were determined with high precision below this lake which is around 300 meters deep here, and the physical properties of the rocks were measured down to the bottom of the 460 meters deep borehole.

These unique measurements are used to record a continuous survey of the deposits in the Dead Sea and to compare it with the recovered cores. Although scientific drilling attempts to recover cores over the entire length of a hole, it is not always possible. These special borehole measurements are conducted to cover the gaps. In addition, a second series of cores is obtained from a second well in order to verify and secure the data.

"If everything goes perfectly, we may soon be able to provide information about past climate and environmental changes in the Bethlehem area," says Ulrich Harms. His colleague Professor Achim Brauer, a paleo-climatologist at the GFZ, is one of the initiators of the ICDP project. He and his team will analyze the drill cores. They are not just interested in the climate at the time of Jesus' birth but in the climate of the whole history of mankind. The region of the Holy Land is considered a land bridge across which early man migrated in several waves from Africa to the north. The climate history of the land of the Bible is therefore closely connected with the history of mankind.

(Photo: © OSG-GFZ, ICDP)

The International Continental Scientific Drilling Program




Selected Science News. Copyright 2008 All Rights Reserved Revolution Two Church theme by Brian Gardner Converted into Blogger Template by Bloganol dot com