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segunda-feira, 6 de julho de 2015

Graphene coating boosts battery performance

Jul 6, 2015

Extra charge: graphene and silicon can increase battery capacity

The capacity of a lithium-ion battery can be nearly doubled by using an anode made from tiny nanoparticles of silicon wrapped in several layers of graphene. Researchers from South Korea – including electronics giant Samsung – have found that the graphene coating boosts the electrical conductivity of the particles and stops them from being damaged as their volume expands when the battery is charged. The scientists describe their work as "a meaningful step" towards the development of commercial batteries with silicon anodes.

Ubiquitous in portable electronics, rechargeable lithium-ion batteries consist of two electrodes – anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, lithium ions move from the cathode through the electrolyte to the anode, where they are absorbed into the bulk of the anode material.

Expansion and contraction

When the battery is discharged, lithium ions come out of the anode and return to the cathode. This makes the anode first expand and then contract, which can damage the anode over repeated charge/discharge cycles. Anodes made from graphite, though, are resistant to this damage, which is why this material has been used in commercial batteries for three decades.

As portable devices become more energy-hungry, however, researchers have sought to boost the amount of energy that can be stored in lithium-ion batteries by developing anodes made from silicon. As well as being cheap and easy to work with, silicon can absorb 10 times more lithium ions per unit mass than graphite. Unfortunately, the volume of silicon expands by a factor of four when it absorbs lithium, which makes the silicon anodes prone to fracture and failure.

Cracking and coating

One way round this problem is to make the anode from an agglomeration of tiny spheres of silicon – each about 100 nm diameter – that are more resistant to cracking. But this approach also has its own challenges. Silicon is a semiconductor and to be an effective anode it must be coated with an electrical conductor. This coating must also remain intact as the nanospheres expand and contract.

Now Mark Rümmeli and colleagues at the Institute for Basic Science in Korea, at Samsung and at the Korea Advanced Institute of Technology have devised a way to coat silicon nanoparticles with multiple layers of graphene. Graphene is a layer of carbon just one atom thick that is both a good electrical conductor and an extremely strong material. These two properties combine to make the coated nanoparticles very good conductors that are able to increase in size without damage to the coating or to the nanoparticles.

An important challenge for Rümmeli and colleagues was how to coat silicon with graphene without creating a thin layer of silicon carbide between the two materials. This is because silicon carbide is an electrical insulator and also inhibits the flow of lithium ions. The team achieved silicon-carbide-free growth by heating the nanoparticles in the presence of methane and carbon dioxide.

High conductivity

Thanks to the graphene coating, a powder sample of nanoparticles has a conductivity that is 100 million times greater than a powder sample of uncoated particles. The team then made anodes from the coated nanoparticles and tested them in otherwise standard lithium-ion batteries. During the first charge–discharge cycle they found that the batteries held 1.8 times more energy than a battery with a conventional graphite anode. After 200 cycles, the batteries were still able to store 1.5 times more energy than a conventional device.

When the team took a closer look at individual nanoparticles using an electron microscope, the researchers found that each layer of graphene did not completely encapsulate a nanoparticle. This allowed the graphene layers to slide across each other as the nanoparticle grew in size, thereby creating an expandable shell. Rümmeli told physicsworld.com that a similar sliding effect has been seen in multiwalled carbon nanotubes – rolled up sheets of graphene – which can extend telescopically.

The team also believes that the sliding is offset by an inward "clamping" force that maintains the integrity of the graphene coating and reduces cracking in the nanoparticles. The incomplete layers also provide paths for the lithium ions to travel through the graphene coating to reach the anode.

The research is described in Nature Communications.

quinta-feira, 20 de novembro de 2014

New type of silicon could find use in solar cells and LEDs

A view through the channels of the new zeolite-type allotrope of silicon (Image: Timothy Strobel)

You probably wouldn't be reading this if it weren't for silicon. It's the second most-abundant element in the Earth's crust as well as the key to modern technology – used in the integrated circuits that power such electronics as computers, mobile phones, and even some toasters and refrigerators. It's also used in compound form in building, ceramics, breast implants, and many other areas. And now the ubiquitous element may have a plethora of new applications, thanks to a team of Carnegie scientists who synthesized an allotrope (new/different physical form) with the chemical formula Si24.

The diamond-structured form of silicon normally used in technology applications has a semiconducting property called an indirect band gap, which differs from a direct band gap in that it requires an extra step to excite bound electrons into a free state so that they can participate in electrical conduction. Direct band gap semiconductors need only two entities to intersect; a photon imparts momentum on an electron. But indirect band gap semiconductors require a third entity – a lattice vibration called a phonon – because the minimum energy state of the conduction band and the maximum energy state of the valence band occur at different values of momentum.

This new form of silicon is a quasi-direct band gap material, which means not only that it can conduct electricity more efficiently than diamond-structured silicon but also that it can absorb and emit light – a property never before achieved. (I say quasi-direct because it is technically a very small and almost flat indirect band gap.) These properties make it ripe for use in next-generation solar cells, LEDs, and other semiconductor technologies.

To create Si24, the researchers first formed a polycrystalline compound of silicon and sodium (Na4Si24) with help from a tantalum capsule, very high temperature, and a 1,500 ton multi-anvil press that gradually reached a pressure of 10 gigapascals (1,450,377 pounds per square inch). This compound was then "degassed" in a vacuum at 400 Kelvin (260 F) for eight days, after which they had pure Si24 in an open framework called a zeolite-type structure.

The structure is comprised of five-, six-, and eight-membered silicon rings through which small atoms and molecules could spread, with potential applications in electrical energy storage and molecular-scale filtering, among other things.

Si24 could be just the tip of the iceberg for desirable new materials formed at high pressure, the researchers suggest. Lead researcher Timothy Strobel has gone so far as to call high-pressure precursor synthesis "an entirely new frontier in novel energy materials" that goes above and beyond silicon. And the stability of the new structures at atmospheric pressure means that low-pressure methods such as chemical vapor deposition could potentially allow large-scale production.

A paper describing the research was published in the journal Nature Materials.

Source: Carnegie Institution for Science

Space: The final frontier in silicon chemistry

Click to enlarge

Schematic figure of the present experiment. Reactive molecules produced in a supersonic jet come into the microwave cavity placed inside a vacuum chamber, where microwave radiation excites the molecules. Induced microwave radiation from the excited molecules is detected.

Silicon, which is one of the most common elements in Earth's crust, is also sprinkled abundantly throughout interstellar space. The only way to identify silicon-containing molecules in the far corners of the cosmos -- and to understand the chemistry that created them -- is to observe through telescopes the electromagnetic radiation the molecules emit.

Scientists from the University of Tokyo, in Japan, have now determined the unique electromagnetic emission spectrums of two new, highly-reactive silicon compounds. The research, which is published in The Journal of Chemical Physics from AIP Publishing, will help astronomers look for the molecules in the interstellar medium.

"Like human fingerprints and DNA sequences are the markers of human identity, we can identify molecules from the frequencies of the electromagnetic waves emitted by them," said Yasuki Endo, a researcher in the Department of Basic Science at the University of Tokyo.

Using spectroscopic techniques, scientists have already detected silicon-containing molecules in the gaseous clouds that envelop some stars and in the sparsely populated space between stars. In space, silicon is often found in dust grains containing stable compounds called silicates. However, highly reactive molecules, such as SiCN, have also been detected in the gas phase in the interstellar medium.

Searching for More Reactive Silicon Compounds

Endo and his colleagues wondered if compounds in the same family as SiCN, but with longer carbon chains, also existed in the interstellar medium. But there was big obstacle to answering the question: Researchers had not yet performed any laboratory experiments to determine the spectroscopic signatures of reactive, silicon and nitrogen-terminated carbon chain molecules.

To fill the knowledge gap, Endo and his team created molecules of SiC2N and SiC3N by mixing precursor gases in a supersonic jet and zapping the mixture with electric pulses. The researchers then measured the electromagnetic emissions of the molecules in a Fourier transform microwave spectrometer. To find the peaks in the emission spectrum, the researchers were guided by theoretical calculations.

"Our experiment now makes it possible to search for SiC2N and SiC3N in the interstellar medium," Endo said.

Space Chemistry Insights

Endo and his colleagues plan to use their new results to look for silicon and nitrogen-terminated carbon chain molecules in the gaseous cloud surrounding a giant infrared star called IRC+10216. Scientists had previously detected the single carbon SiCN surrounding this star.

"If [SiC2N and SiC3N] molecules are identified in astronomical objects and their abundances are determined, we will be able to obtain valuable information on the mechanisms for the formations of these molecules," Endo said. "In addition, the information may provide clues to understand formation pathways of other silicon-bearing molecules." The new information could give scientists clues about the chemical composition of the universe and the conditions that birth stars and planets.

Story Source:

The above story is based on materials provided by American Institute of Physics (AIP). Note: Materials may be edited for content and length.

Journal Reference:

Hiroya Umeki, Masakazu Nakajima and Yasuki Endo. Laboratory detections of SiC2N and SiC3N by Fourier transform microwave spectroscopy. The Journal of Chemical Physics, 2014 DOI: 10.1063/1.4900740

quarta-feira, 10 de setembro de 2014

Artificial membranes on silicon

September 9, 2014

American Institute of Physics

Artificial membranes mimicking those found in living organisms have many potential applications ranging from detecting bacterial contaminants in food to toxic pollution in the environment to dangerous diseases in people. Now a group of scientists has developed a way to create these delicate, ultra-thin constructs through a 'dry' process, by evaporating two commercial, off-the-shelf chemicals onto silicon surfaces.

Click to enlarge

Organic and inorganic materials grouped together to bridge the gap between biology and physics.

Artificial membranes mimicking those found in living organisms have many potential applications ranging from detecting bacterial contaminants in food to toxic pollution in the environment to dangerous diseases in people. Now a group of scientists in Chile has developed a way to create these delicate, ultra-thin constructs through a "dry" process, by evaporating two commercial, off-the-shelf chemicals onto silicon surfaces.

Described in The Journal of Chemical Physics, from AIP Publishing, this is the first time anyone has ever made an artificial membrane without mixing liquid solvents together. And because the new process creates membranes on silicon surfaces, it is a significant step toward creating bio-silicon interfaces, where biological "sensor" molecules can be printed onto cheap silicon chip holding integrated electronic circuits.

"Our idea is to create a biosensor that can transmit electrical signals through the membrane," said María José Retamal, a Ph.D. student at Pontificia Universidad Católica de Chile and first author of the paper.

Membranes for Technology, as for Life

The importance of lipid membranes to life is hard to overstate. They are a principal component of the cell, as fundamental as DNA or proteins, and all known organisms on Earth, from the bittiest bacteria to the biggest blue whales, use membranes in a multitude of ways.

They separate distinct spaces within cells and define walls between neighboring cells -- a functional compartmentalization that serves many physiological processes, protecting genetic material, regulating what comes in and out of cells, and maintaining the function of separate organs.

Synthetic membranes that mimic nature are of great interest to science because they offer the possibility of containing membrane proteins -- biological molecules that could be used for detecting toxins, diseases and many other biosensing applications.

Retamal and her colleagues created the first artificial membrane without using solvents on a silicon support base. They chose silicon because of its low cost, wide availability and because its "hydrophobicity" (how much it repels water) can be controlled chemically, allowing them to build membranes on top.

Next they evaporated a chemical known as chitosan onto the silicon. Chitosan is derived from chitin, a sugar found in the shells of certain crustaceans, like lobsters or shrimp. Whole bags of the powder can be bought from chemical companies worldwide. They chose this ingredient for its ability to form a moisturizing matrix. It is insoluble in water, but chitosan is porous, so it is capable of retaining water.

Finally they evaporated a phospholipid molecule known as dipalmitoylphosphatidylcholine (DPPC) onto the chitosan-covered silicon substrate and showed that it formed a stable "bilayer," the classic form of a membrane. Spectroscopy showed that these artificial membranes were stable over a wide range of temperatures.

More work is needed to standardize the process by which proteins are to be inserted in the membranes, to define the mechanism by which an electrical signal would be transmitted when a protein binds its target and to calibrate how that signal is detected by the underlying circuitry, Retamal said.

"This is a powerful tool," she added. "The idea is that it will be used by the rest of the scientific community in order to improve existing techniques."