Superconductivity at room temperature, with graphene

March 24, 2008

Graphene Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals. University of Maryland physicists have shown that in graphene the intrinsic limit to the mobility, a measure of how well a material conducts electricity, is higher than any other known material at room temperature – and 100 times faster than in silicon. A team of researchers led by physics professor Michael S. Fuhrer of the university’s Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.In any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place. As electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. This electrical resistance is “intrinsic” to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.

In graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area). This is about 35 percent less than the resistivity of copper, the lowest resistivity material known at room temperature. “Other extrinsic sources in today’s fairly dirty graphene samples add some extra resistivity to graphene,” explained Fuhrer, “so the overall resistivity isn’t quite as low as copper’s at room temperature yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper.”

In semiconductors, a different measure, mobility, is used to quantify how fast electrons move. The limit to mobility of electrons in graphene is set by thermal vibration of the atoms and is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm2/Vs in silicon, and 77,000 cm2/Vs in indium antimonide, the highest mobility conventional semiconductor known.

“Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we’ve shown that the mobility at room temperature is over 100,000 cm2/Vs” said Fuhrer (T. Dürkop, S. A. Getty, Enrique Cobas, and M. S. Fuhrer, Nano Letters 4, 35 (2004)).

Mobility determines the speed at which an electronic device (for instance, a field-effect transistor, which forms the basis of modern computer chips) can turn on and off. The very high mobility makes graphene promising for applications in which transistors much switch extremely fast, such as in processing extremely high frequency signals.

Mobility can also be expressed as the conductivity of a material per electronic charge carrier, and so high mobility is also advantageous for chemical or bio-chemical sensing applications in which a charge signal from, for instance, a molecule adsorbed on the device, is translated into an electrical signal by changing the conductivity of the device.

Graphene is therefore a very promising material for chemical and bio-chemical sensing applications. The low resitivity and extremely thin nature of graphene also promises applications in thin, mechanically tough, electrically conducting, transparent films. Such films are sorely needed in a variety of electronics applications from touch screens to photovoltaic cells.

Fuhrer and co-workers showed that although the room temperature limit of mobility in graphene is as high as 200,000 cm2/Vs, in present-day samples the actual mobility is lower, around 10,000 cm2/Vs, leaving significant room for improvement. Because graphene is only one atom thick, current samples must sit on a substrate, in this case silicon dioxide.

Trapped electrical charges in the silicon dioxide (a sort of atomic-scale dirt) can affect the electrons in graphene and reduce the mobility. Also, vibrations of the silicon dioxide atoms themselves can also have an effect on the graphene which is stronger than the effect of graphene’s own atomic vibrations. This so-called “remote interfacial phonon scattering” effect is only a small correction to the mobility in a silicon transistor, but because the phonons in graphene itself are so ineffective at scattering electrons, this effect becomes very important in graphene.

“We believe that this work points out the importance of these extrinsic effects, and creates a roadmap for finding better substrates for future graphene devices in order to reduce the effects of charged impurity scattering and remote interfacial phonon scattering.” Fuhrer said.

Article: J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer, ‘Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2, Nature Nanotechnology published online: 23 March 2008 | doi:10.1038/nnano.2008.58

Graphene crystals, “Vinograd19,” from Wikimedia Commons


Scientists fabricate non-cryogenic superconducting material

March 21, 2008

The chemical structure of silaneThis could be huge. Imagine the ability to transmit power from remote solar and wind sites without huge powerlines. From Next Energy News: A new breakthrough superconducting material fabricated by a Canadian-German team has been made out of a silicon-hydrogen compound and does not require cooling. The implications of the discovery are enormous and could transform the way people live by cutting power usage from everything from refrigeration to cell phones.

Instead of super-cooling the material, as is necessary for conventional superconductors, the new material is instead super-compressed. The researchers claim that the new material could sidestep the cooling requirement, thereby enabling superconducting wires that work at room temperature.

“If you put hydrogen compounds under enough pressure, you can get superconductivity,” said professor John Tse of the University of Saskatchewan. “These new superconductors can be operated at higher temperatures, perhaps without a refrigerant.”

He performed the theoretical work with doctoral candidate Yansun Yao. The experimental confirmation was performed by researcher Mikhail Eremets at the Max Plank Institute in Germany.

The new family of superconductors are based on a hydrogen compound called “silane,” which is the silicon analog of methane–combining a single silicon atom with four hydrogen atoms to form a molecular hydride. (Methane is a single carbon atom with four hydrogens).

Researchers have speculated for years that hydrogen under enough pressure would superconduct at room temperature, but have been unable to achieve the necessary conditions (hydrogen is the most difficult element to compress). The Canadian and German researchers attributed their success to adding hydrogen to a compound with silicon that reduced the amount of compression needed to achieve superconductivity.

Tse’s team is currently using the Canadian Light Source synchrotron to characterize the high pressure structures of silane and other hydrides as potential superconducting materials for industrial applications as well as a storage mechanism for hydrogen fuel cells….

The chemical structure of silane,  SiH4, “Jrockley,” from Wikimedia Commons

A smart grid plan for the city of Boulder, Colorado

March 18, 2008

Boulder, ColoradoClean Edge: Xcel Energy announced recently it will put in motion its vision to make Boulder, Colo. the nation’s first fully integrated Smart Grid City.

The advanced, smart grid system – when fully implemented over the next few years – will provide customers with a portfolio of smart grid technologies designed to provide environmental, financial and operational benefits. Xcel Energy anticipates funding only a portion of the project, and plans to leverage other sources including government grants for the remainder of what could be up to a $100 million effort.

“Smart Grid City is the first step toward building the grid of the future,” said Dick Kelly, Xcel Energy chairman, president and CEO. “In Boulder, we’ll collaborate with others to integrate all aspects of our smart grid vision and evaluate the benefits. The work we’re doing will benefit not only Boulder, but also customers throughout our eight-state service territory. We’re pleased to partner with the city and our Boulder customers as we begin this journey.”

…In addition to its geographic concentration, ideal size and access to all grid components, Boulder was selected as the Smart Grid City because it is home to the University of Colorado and several federal institutions, including the National Institute of Standards and Technology, which already is involved in smart grid efforts for the federal government.

Smart Grid City could feature a number of infrastructure upgrades and customer offerings – for the first time fully integrated through the partnership’s efforts in Boulder – including:

  • Transformation of existing metering infrastructure to a robust, dynamic electric system communications network, providing real-time, high-speed, two-way communication throughout the distribution grid;
  • Conversion of substations to “smart” substations capable of remote monitoring, near real-time data and optimized performance;
  • At the customer’s invitation, installation of programmable in- home control devices and the necessary systems to fully automate home energy use; and
  • Integration of infrastructure to support easily dispatched distributed generation technologies (such as plug-in hybrid electric vehicles with vehicle-to-grid technology; battery systems; wind turbines; and solar panels).

The potential benefits of the Smart Grid City include operational savings, customer-choice energy management, better grid reliability, greater energy efficiency and conservation options, increased use of renewable energy sources, and support for plug-in hybrid electric vehicles and intelligent-home appliances.

Xcel Energy launches wind-to-battery project

February 29, 2008

Energy Daily: Xcel Energy soon will begin testing a cutting-edge technology to store wind energy in batteries. It will be the first use of the technology in the United States for direct wind energy storage.

Integrating variable wind and solar power production with the needs of the power grid is an ongoing issue for the utility industry. Xcel Energy will begin testing a one-megawatt battery-storage technology to demonstrate its ability to store wind energy and move it to the electricity grid when needed. Fully charged, the battery could power 500 homes for over 7 hours.

“Energy storage is key to expanding the use of renewable energy,” said Dick Kelly, Xcel Energy Chairman, President and CEO. “This technology has the potential to reduce the impact caused by the variability and limited predictability of wind energy generation. As the nation’s leader in distributing wind energy, this will be very important to both us and our customers.”

Xcel Energy has signed a contract to purchase a battery from NGK Insulators that will be an integral part of a project. The sodium-sulfur battery is commercially available and versions of this technology are already being used in Japan and in a few US applications, but this is the first U.S. application of the battery as a direct wind energy storage device.

The 20 50-kilowatt battery modules will be roughly the size of two semi trailers and weigh approximately 80 tons. They will be able to store about 7.2 megawatt-hours of electricity, with a charge/discharge capacity of one megawatt. When the wind blows, the batteries are charged. When the wind calms down, the batteries supplement the power flow….

Wind turbine from below, Dirk Ingo Franke, Wikimedia Commons

Storing solar energy in graphite

February 10, 2008

Canberra Times: Once a supply depot and airstrip for the mighty Snowy Mountains Hydro-electricity Scheme, Polo Flat, east of Cooma, is now headquarters for a new energy enterprise for rural Australia. When the scheme was completed, some of its engineers formed the Snowy Mountains Engineering Corporation, now known as SMEC, a worldwide engineering and development consultancy.

About seven years ago, a group within SMEC began developing Australian scientist Bob Lloyd’s invention for storing thermal energy in purified graphite. It was decided the best way to test and commercialise the technology was to form a separate company with an agreement that SMEC would provide all the engineering services. A group of investors formed Lloyd Energy Systems, which now has a laboratory, workshop, heat-storage blocks and a graphite-cleaning pilot plant at Polo Flat. Now an accredited research agency, the site also has demonstration storage plants, a wind-to-heat demonstration and a small-scale commercial plant incorporating water treatment, storage and steam turbine generation.

Lloyd Energy’s chief executive, Steve Hollis, said the past six years had been spent testing the storage of solar energy at very high temperatures and preparing the technology for market. The research has generated strong interest, culminating last year in a $5million Federal Government grant for an advanced energy storage program in the western NSW town of Lake Cargelligo. Country Energy will take the power for its grid network in a project worth $10million.

Lloyd Energy also has an agreement with Ergon Energy in Queensland to build a $30million plant, three times larger than Lake Cargelligo’s, and has a $7million contribution from the Queensland Government. Mr Hollis said large amounts of coal-fired energy were lost during long transmission to remote areas.

As power loads built up over time, mainly because of demand for air-conditioners, the grid could no longer cope in peak periods. Towns at the end of the line suffered the most from power shortages. “We’re putting environmentally friendly generation out at the end of the branches of the tree if you like, so it can pump energy back in when the branches are in trouble,” he said.

“It actually serves three purposes. Firstly, it is a renewable energy replacement for coal. Secondly, it avoids the country energy authorities having to upgrade their transmission lines so they can get more power out in the peak.” The third benefit was having an energy source at the end of the line that could return power into the grid.

“This is a modular system,” Mr Hollis said. “We have made it modular so it can be redeployed in remote rural locations in rural Australia and overseas, without involving monstrous towers….

Supergrid could supply nearly a third of Europe’s electricity, says company

January 30, 2008

The holy grail of load management would be to bring an entire region into a grid, so that alternative energy sources from the entire area could be directed as needed. Such a plan exists: A high voltage electricity grid connecting countries from the North Sea to the Bay of Biscay could provide almost a third of Europe’s power by 2030, according to the company behind the idea. The system would improve energy security, cut emissions, and even reduce the price of power at times of peak demand.

The supergrid is the brainchild of Irish wind generator Airtricity – recently acquired by Scottish and Southern Energy for €1.1 billion – and would connect countries as far apart as Norway and Spain to each other’s offshore wind farms. When the wind blows in one country but not in others, power would be directed through the high voltage direct current (HVDC) network to wherever it is needed most.

According to Mark Ennis, Airtricity’s Executive Director for Strategy and Public Policy, the system will solve the Achilles heel of wind generation. In an interview with lastoilshock.comand Global Public Media, Ennis said “By having a very large grid over several thousand kilometers you take the variability out, you almost come out with base load energy”

Speaking on the sidelines of the World Future Energy Summit in Abu Dhabi last week, Ennis went on to say that the HVDC technology is proven, that existing financial incentives are already sufficient to make the idea viable, and that a regulatory agreement between countries is close: “I think we are nearly there”. If work starts soon, Ennis claimed, the supergrid could supply 30% of Europe’s power by 2030….

Image by Duesentrieb, Wikimedia Commons.

Load management gets some attention

January 12, 2008

Over at CleanTechCollective, Joseph Romm has a post about load management: Energy efficiency isn’t just a term that should refer to appliances. At just about every point between its generation and end use, electricity is lost, comprising an entirely inefficient process. But never fear, there are a growing number of advocates for what’s been termed the “Smart Grid,” a more efficient, flexible, and modern electricity system:

(From Greenwire, subs. req’d)
Smartgrid group touts enlistment of industry backers

The GridWise Alliance, a collection of public and private-sector groups that advocate modernizing the nation’s electric grid, has enlisted major new backers over the past several months as it lobbied for concessions in the recently passed energy bill…