Plastic islands being used to restore African lake

One of the BioHaven Floating Islands, from a previous project

As the ever-growing giant flotilla of floating refuse known as the Great Pacific Garbage Patch will show us, we shouldn't be putting plastic waste in our waterways. A new project, however, is aimed at helping the environment by doing so ... in a roundabout way of speaking. The participants plan on taking several artificial floating islands made from post-consumer plastic, planting papyrus on them, and then using them to help rebuild the ecosystem of Africa’s Lake Naivasha.
Located in Kenya’s Rift Valley, the lake was crystal clear 30 years ago. Since then, a 20-fold increase in the local human population, along with foraging activity by water buffalo native to the region, has resulted in massive clearing of the lakeshore papyrus plants.
Like other aquatic plants, papyrus serve a vital role. Acting as biofilters, they trap suspended sediments, plus they remove toxic substances and excess nutrients from the water. Unfortunately, the destruction of much of the lake’s papyrus plants has led to a marked decline in its water quality.

One of the last stands of papyrus along Lake Naivasha (Photo: University of Leicester)

The restoration project is being funded by the German REWE Group. It involves the participation of UK tea producer and flower grower Finlays (which grows flowers in the region), and is being led by Dr. David Harper, a senior lecturer at the University of Leicester.
The team plan on introducing a new population of papyrus to the lake, at a spot where silty water from the Malewa River enters into it. Those papyrus will be planted on a series of BioHaven Floating Islands, made from recycled polyester drink bottles by North Carolina-based Floating Island Southeast.
Each BioHaven island is essentially a floating mat, made up of multiple layers of a plastic matrix bonded together with marine-grade foam. This provides a highly buoyant surface for plants to grow on, while allowing their roots (which dangle beneath the island) to both act as a sediment filter, and as a home for aquatic microbes. Those microbes feed on nutrients that might otherwise lead to excess algae growth – a service also provided by the plants themselves.
Additionally, the roots should serve as feeding grounds and nurseries for fish, while the five meter (16 foot)-tall stalks of the plants should make a good habitat for birds.
The islands have been ordered, and will be anchored in place once the papyrus is planted on them. If they prove successful at their first location, additional islands will be installed at other locations along the lake shore.
Sources: University of Leicester, Floating Island Southeast

World’s most efficient thermoelectric material developed

Thermoelectrics can be used to convert energy currently lost as heat wasted from industry and vehicle tailpipes into electricity

Approximately 90 percent of the world’s electricity is generated by heat energy. Unfortunately, electricity generation systems operate at around 30 to 40 percent efficiency, meaning around two thirds of the energy input is lost as waste heat. Despite this, the inefficiency of current thermoelectric materials that can convert waste heat to electricity has meant their commercial use has been limited. Now researchers have developed a thermoelectric material they claim is the best in the world at converting waste heat into electricity, potentially providing a practical way to capture some of the energy that is currently lost.
The new material, which is based on the common semiconductor telluride, is environmentally stable and is expected to convert from 15 to 20 percent of waste heat to electricity. The research team, made up of chemists, material scientists and mechanical engineers from Northwestern University and Michigan State University, say the material exhibits a thermoelectric figure of merit (or “ZT”) of 2.2, which they claim is the highest reported to date.
The higher a material’s ZT, the more efficient it is at converting heat to electricity. While there’s no theoretical upper limit to ZT, no known materials exhibit a ZT higher than 3. The researchers believe with a ZT of 2.2, the new material is efficient enough to be used in practical applications and could usher in more widespread adoption of thermoelectrics by industry.
"Our system is the top-performing thermoelectric system at any temperature," said Mercouri G. Kanatzidis, who led the research. "The material can convert heat to electricity at the highest possible efficiency. At this level, there are realistic prospects for recovering high-temperature waste heat and turning it into useful energy."
With the huge potential for thermoelectrics to recover some of the heat energy that is currently lost, they have been the focus of much research that has seen them improve significantly in recent years. So much so that the Mars rover Curiosity features lead telluride thermoelectrics, although its system only has a ZT of 1. BMW is also testing systems to harvest the heat from the exhaust systems and combustion engines of its cars.
Aside from capturing some of the wasted heat energy emitted through a vehicle’s tailpipe, the new material could be used in heavy manufacturing industries, including glass and brick making, refineries, and coal- and gas-fired power plants, and on large ships and tankers, where large combustion engines operate continuously. Such applications are seen as ideal as the waste heat temperatures in these areas can range from 400 to 600 degrees Celsius (750 to 1,100 degrees Fahrenheit),which is the sweet spot for thermoelectrics use.
The team’s paper describing the development of the new material is published in the journal Nature.
Source: Northwestern University

MIT researchers devise technique to clean up oil spills using magnets

The oil and water separation technique uses permanent magnets immersed in a reservoir containing oil and water

Possibly the only good thing to come out of the Deepwater Horizon disaster is the subsequent increase in research into finding more effective ways to clean up oil spills, including such initiatives as the X PRIZE Foundation's Wendy Schmidt Oil Cleanup X CHALLENGE. Joining the list is a new method devised by researchers at MIT who propose separating oil and water using magnets. The new technique would allow the oil to be recovered to help offset the costs of the cleanup operation.
Oil isn’t magnetic, but suspending magnetic nanoparticles within the oil turns it into a magnetic liquid known as a ferrofluid. Previous research efforts using ferrofluids typically involved pumping a water-and-ferrofluid mixture through a channel with magnets on the outside directing the flow of the water one way and the flow of the ferrofluid another. However, this technique will work only if the concentration of the ferrofluid is known beforehand and remains constant – neither of which is possible in water contaminated by an oil spill.
For their approach, the MIT researchers made two modifications to the existing method. Instead of placing the magnets on the outside of the stream, they were immersed within it, and instead of being oriented parallel to the flow of the stream, they run perpendicular to it.
Because the magnetic field of the cylindrical permanent magnets used by the MIT team is strongest at its edges, the oil is attracted to the tips of the magnets much more strongly than the sides. And as the bottoms of the cylindrical magnets were embedded under the waterline in the base of a reservoir and the tops of the magnets were positioned above water level, the oil didn’t collect around them. Rather, it shot up the sides of the magnets to form beaded spheres at the top.
Shahriar Khushrushahi, a postdoc in MIT’s Department of Electrical Engineering and Computer Science and lead author on a paper describing the approach says the technique provides excellent separation between oil and water. Additionally, its simplicity makes it feasible for large scale manufacture and deployment at sea for days or weeks a a time, where electrical power and maintenance facilities are limited.
While the team is yet to determine the most practical way to remove the oil from the magnets in an actual oil-recovery system, in their experiments they used a Halbach array. This is a special arrangement of permanent magnets where the magnetic field on one side is augmented, while the magnetic field on the other side is canceled out to near zero. This allowed the oil in the reservoir to remain unattracted to the bottom of the array, while the oil attached to the magnets was pulled off by the top of the array.
The team says that adding magnetic nanoparticles to oil mixed with water to produce a ferrofluid aboard a ship is not a challenge. Additionally, removing the nanoparticles can be achieved using a technique known as high-gradient magnetic separation. This has been done on a small scale and would allow the recovery of both the nanoparticles and the oil.
One remaining challenge is to determine how much water gets dissolved in the oil and the best way to remove it. “To our eye, you don’t see much moisture in there, but I’m sure that there is some moisture that adheres to it,” says team member Markus Zahn. “We might have to run it through multiple cycles.”
On a commercial scale, the magnetic separation method could be used in conjunction with existing oil recovery techniques such as skimming, which would perform an initial separation. The magnetic separation technique could then be used to finish the job.
The research team will present a paper detailing their work at the 13th International Conference on magnetic Fields (ICMF13) being held in New Delhi, India, in January 2013. The team has also filed two patents on its work.
MIT researchers Zahn and Khushrushahi explain the magnetic separation technique in the video below.
Source: MIT

Wood pulp extract stronger than carbon fiber or Kevlar

 

Underlying structure of the wall of a wood cell, showing the substructure of load-bearing cellulose microfibrils

The Forest Products Laboratory of the US Forest Service has opened a US$1.7 million pilot plant for the production of cellulose nanocrystals (CNC) from wood by-products materials such as wood chips and sawdust. Prepared properly, CNCs are stronger and stiffer than Kevlar or carbon fibers, so that putting CNC into composite materials results in high strength, low weight products. In addition, the cost of CNCs is less than ten percent of the cost of Kevlar fiber or carbon fiber. These qualities have attracted the interest of the military for use in lightweight armor and ballistic glass (CNCs are transparent), as well as companies in the automotive, aerospace, electronics, consumer products, and medical industries.
Cellulose is the most abundant biological polymer on the planet and it is found in the cell walls of plant and bacterial cells. Composed of long chains of glucose molecules, cellulose fibers are arranged in an intricate web that provides both structure and support for plant cells. The primary commercial source for cellulose is wood, which is essentially a network of cellulose fibers held together by a matrix of lignin, another natural polymer which is easily degraded and removed.

Cellulose structures in trees from logs to molecules

Wood pulp is produced in a variety of processes, all of which break down and wash away the lignin, leaving behind a suspension of cellulose fibers in water. A typical cellulose wood fiber is only tens of microns wide and about a millimeter long.

Micrographs of cellulose fibers from wood pulp

The cellulose in wood pulp, when dry, has the consistency of fluff or lint - a layer of wood pulp cellulose has mechanical properties reminiscent of a wet paper towel. Not what you might expect to be the source of one of the strongest materials known to Man. After all, paper is made from the cellulose in wood pulp, and doesn't show extraordinary strength or stiffness.

Cellulose fibers and the smaller structures within them - a) fiber from wood pulp; b) microcrystalline cellulose; c) microfibrils of cellulose; d) nanofibrils of cellulose; e) cellulose nanocrystals from wood pulp; f) CNCs from sea squirts (the only animal source of cellulose microfibrils); and g,h) cellulose nanofibrils from other sources

Further processing breaks the cellulose fibers down into nanofibrils, which are about a thousand times smaller than the fibers. In the nanofibrils, cellulose takes the form of three-dimensional stacks of unbranched, long strands of glucose molecules, which are held together by hydrogen bonding. While not being "real" chemical bonds, hydrogen bonds between cellulose molecules are rather strong, adding to the strength and stiffness of cellulose nanocrystals.

The upper figure shows the structure of the cellulose polymer; the middle figure shows a nanofibril containing both crystalline and amorphous cellulose; the lower figure shows the cellulose nanocrystals after the amorphous cellulose is removed by acid hydrolysis

Within these nanofibrils are regions which are very well ordered, in which cellulose chains are closely packed in parallel with one another. Typically, several of these crystalline regions appear along a single nanofibril, and are separated by amorphous regions which do not exhibit a large degree of order. Individual cellulose nanocrystals are then produced by dissolving the amorphous regions using a strong acid.
At present the yield for separating CNCs from wood pulp is about 30 percent. There are prospects for minor improvements, but the limiting factor is the ratio of crystalline to amorphous cellulose in the source material. A near-term goal for the cost of CNCs is $10 per kilogram, but large-scale production should reduce that figure to one or two dollars a kilo.

Cross-sectional structure of various types of cellulose nanocrystals showing various crystalline arrangements of the individual cellulose polymer molecules (the rectangular boxes)

CNCs separated from wood pulp are typically a fraction of a micron long and have a square cross-section a few nanometers on a side. Their bulk density is low at 1.6 g/cc, but they exhibit incredible strength. An elastic modulus of nearly 150 GPa, and a tensile strength of nearly 10 GPa. Here's how its strength to compares to some better-known materials:

• Material...........................Elastic Modulus................Tensile Strength
• CNC......................................150 GPa.............................7.5 GPa
• Kevlar 49..............................125 GPa.............................3.5 GPa
• Carbon fiber.........................150 GPa.............................3.5 GPa
• Carbon nanotubes..............300 GPa............................20 GPa
• Stainless steel.....................200 GPa............................0.5 GPa
• Oak..........................................10 GPa.............................0.1 GPa

The only reinforcing material that is stronger than cellulose nanocrystals is a carbon nanotube, which costs about 100 times as much. Stainless steel is included solely as a comparison to conventional materials. The relatively very low strength and modulus of oak points out how much the structure of a composite material can degrade the mechanical properties of reinforcing materials.
As with most things, cellulose nanocrystals are not a perfect material. Their greatest nemesis is water. Cellulose is not soluble in water, nor does it depolymerize. The ether bonds between the glucose units of the cellulose molecule are not easily broken apart, requiring strong acids to enable cleavage reactions.
The hydrogen bonds between the cellulose molecules are also too strong in aggregate to be broken by encroaching water molecules. Indeed, crystalline cellulose requires treatment by water at 320° C and 250 atmospheres of pressure before enough water intercalates between the cellulose molecules to cause them to become amorphous in structure. The cellulose is still not soluble, just disordered from their near-perfect stacking in the crystalline structure.
But cellulose contains hydroxyl (OH) groups which protrude laterally along the cellulose molecule. These can form hydrogen bonds with water molecules, resulting in cellulose being hydrophilic (a drop of water will tend to spread across the cellulose surface). Given enough water, cellulose will become engorged with water, swelling to nearly double its dry volume.
Swelling introduces a large number of nano-defects in the cellulose structure. Although there is little swelling of a single CNC, water can penetrate into amorphous cellulose with ease, pushing apart the individual cellulose molecules in those regions. In addition, the bonds and interfaces between neighboring CNC will be disrupted, thereby significantly reducing the strength of any material reinforced with CNCs. To make matters worse, water can move easily over the surface/interfaces of the CNCs, thereby allowing water to penetrate far into a composite containing CNCs.
There are several approaches to make CNC composite materials viable choices for real world applications. The simplest, but most limited, is to choose applications in which the composite will not be exposed to water. Another is to alter the surface chemistry of the cellulose so that it becomes hydrophobic, or water-repelling. This is easy enough to do, but will likely substantially degrade the mechanical properties of the altered CNCs. A third approach is to choose a matrix material which is hydrophobic, and preferably that forms a hydrophobic interface with CNCs. While not particularly difficult from a purely chemical viewpoint, there is the practical difficulty that interfaces between hydrophobic and hydrophilic materials are usually severely lacking in strength.
Perhaps the most practical approach will simply be to paint or otherwise coat CNC composite materials in some material that keeps water away. For such a prize - inexpensive strong and rigid materials - we can be sure that innovations will follow to make the theoretical practical.
Source: US Forest Service

Graphene paper anodes pave way for faster charging Li-ion batteries

A scanning electron microscope image of the treated graphene oxide paper

While the lithium-ion batteries commonly used in electric cars are capable of storing a fairly large amount of energy, they’re not able to accept or discharge that energy very quickly. That’s why electric vehicles require supercapacitors, to speedily deliver energy when accelerating, or to store it when braking. Recently, however, researchers from New York’s Rensselaer Polytechnic Institute created a new anode material, that allows Li-ion batteries to charge and discharge ten times faster than those using regular graphite anodes. It could make EV supercapacitors unnecessary, and vastly shorten the charging time required by electronic devices.
The team, led by Prof. Nikhil Koratkar, started by creating a large sheet of graphene oxide paper. About the thickness of a piece of printer paper, it was made up of layered sheets of graphene (each graphene sheet being composed of a one one-atom thick layer of linked carbon atoms).
That paper was cut into smaller pieces, which were then subjected either to a laser, or the flash from a compact camera. In either case, the resulting flash of heat caused “mini explosions” to take place throughout the thickness of the paper, as oxygen atoms were expelled from its structure. The result of this carnage was graphene sheets that were full of flaws such as cracks, pores and voids. Additionally, the pressure exerted by the escaping oxygen forced the stacked layers of graphene apart from one another, resulting in a five-fold increase in the thickness of the paper.
When samples of this paper were tested for use as anodes, however, the marked decrease in charge and discharge times was noted. This was due to the fact that the lithium ions could enter (or exit) the anode at almost any point, using its imperfections as points of entry – by contrast, on graphite anodes, the ions can only enter at the sides and then slowly work their way into the middle.
The graphene oxide paper anodes were found to still work perfectly after more than 1,000 charge/discharge cycles. According to Koratkar, the sheets can be easily and inexpensively made in just about any shape or size, and the production process should be easy to scale up. His team’s next order of business is to match the anodes up with a high-power cathode, within a full Li-ion battery.
A paper on the research was recently published in the journal ACS Nano.
Source: Rensselaer Polytechnic Institute

Cryogenic treatment could cut coal-fired power plant emissions by 90%

   

Cooling the emissions from coal-fired power plants would significantly reduce the levels of dangerous chemicals entering the atmosphere

A team of physicists from the University of Oregon (UO) has calculated that cooling the emissions from coal-fired power plants would result in a reduction of the levels of dangerous chemicals entering the atmosphere, including CO2, by 90 percent. While cryogenic treatment would also see a 25 percent drop in efficiency, and therefore result in electricity costs increasing around a quarter, the researchers believe these would be offset by benefits to society, such as reductions in health-care and climate-change costs.
Previous studies, including one conducted in the 1970s by the Bechtel Corp. of San Francisco, have shown that cryogenic treatment of flue gases from coal-fired power plants can work. While the Bechtel study was looking at its effectiveness in capturing sulfur dioxide emissions, it also noted that large quantities of CO2 would also be condensed – something that didn’t warrant much attention back in the 70s but is of tremendous interest now.
Building on previous research he carried out in the 1960s into using cryogenic treatment technology as a way to remove odor-causing gases being emitted from a paper mill in Springfield, Oregon, UO physicist Russell J. Donnelly and his team have now composed a math-driven formula on an electronic spreadsheet that could be used by industry to weigh up the potential benefits of the technology.

UO physicist Russell J. Donnelly

The team’s paper says that CO2 condensed and captured as a solid would then be warmed and compressed into a gas that could be delivered via pipeline at near ambient temperatures to dedicated storage facilities that could be located hundreds of miles away. Additionally, other chemicals including sulfur dioxide, some nitrogen oxides and mercury would also be condensed so they could be safely removed from the gases emitted by the power plants.
The team’s calculations show that a cryogenic system would capture at least 90 percent on CO2, 98 percent of sulfur dioxide, and virtually 100 percent of mercury emissions. This is more than the capturing of 41 percent of sulfur dioxide and 90 percent of mercury emissions called for by the new mercury and air toxic standards (MATS) issued by the EPA in December 2011.
The team’s formula doesn’t take into account the cost of construction or retrofitting of the cooling machinery to existing power plants, which would be much larger than existing systems that use scrubbers – potentially as large as a football stadium. Nor does it take into account the cost of disposing of the captured pollutants. However, Donnelly thinks such systems are affordable, “especially with respect to the total societal costs of burning coal.”
“In the U.S., we have about 1,400 electric-generating unit(s) powered by coal, operated at about 600 power plants," Donnelly said. “That energy, he added, is sold at about 5.6 cents per kilowatt-hour, according to a 2006 Congressional Budget Office estimate. "The estimated health costs of burning coal in the U.S. are in the range of $150 billion to $380 billion, including 18,000-46,000 premature deaths, 540,000 asthma attacks, 13,000 emergency room visits and two million missed work or school days each year."
A separate, unpublished and preliminary economic analysis carried out by the team estimates that implementing large-scale cryogenic systems into coal-fired plants would see an overall reduction in costs to society of 38 percent through a sharp cut in associated health-care and climate-change costs.
The team’s paper appears in the journal Physical Review E.
Source: University of Oregon

   

A chemical engineer has created a 'food biorefinery' that converts used coffee grounds and other food waste into succinic acid

Every year, the individual stores that make up Starbucks Hong Kong produce almost 5,000 tons (4,536 tonnes) of used coffee grounds and unconsumed bakery items. As it stands now, all of that waste is incinerated, dumped in a landfill, or composted. In the future, however, it may be used to produce a key ingredient in laundry detergents, plastics, and many other items. A recent experiment showed that it can indeed be done.
The project began when the non-profit organization The Climate Group approached Dr. Carol S. K. Lin, a chemical engineer at the City University of Hong Kong. Lin’s team was already developing biorefineries, which are used to convert plant-based materials such as corn and sugar cane into biofuel or other products. In this case, The Climate Group was hoping that she could adapt the technology to process food waste from one of its corporate members, Starbucks Hong Kong.
In the resulting “food biorefinery,” the coffee grounds and baked goods are blended with a mixture of fungi. These fungi excrete enzymes, which break the carbohydrates in the food down into simple sugars. The food/fungi mixture is then transferred into a fermenting vat, where Actinobacillus succinogenes bacteria convert the sugars into succinic acid. It is that acid which is used in products such as detergents, plastics and medicines.
Potentially, the technology could greatly reduce the amount of food waste going into landfills and incinerators, plus it might also provide a more eco-friendly, sustainable alternative to the petroleum products currently used for plastic production. It could also provide the suppliers of the food waste with a source of income, and reduce the need for crops that are currently grown specifically to be biorefined.
Lin is now working on scaling up the process, and plans to test it in a pilot plant in Germany.
Source: American Chemical Society

New nanocrystals let solar panels generate electricity and hydrogen gas

   

Scientists have developed new nanocrystals that allow solar panels to generate both electricity and hydrogen gas

At first glance, photovoltaic solar panels are brilliant. They’re self-contained, need no fuel and so long as the sun is shining, they make lots of lovely electricity. The trouble is, they’re expensive to make, batteries are poor storage systems for cloudy days, and the panels have a very short service life. Now, Dr. Mikhail Zamkov of Ohio's Bowling Green State University and his team have used synthetic nanocrystals to make solar panels more durable as well as capable of producing hydrogen gas.
Solar panels using inorganic molecules as part of their construction have a short service life. The effects of UV radiation and heat degrade them, and they end up with a life of only about 20 years. Given how expensive it is to make solar panels, it’s not surprising that the cost per kilowatt is so much higher than conventional energy sources. In a video paper published in the Journal of Visualized Experiments (JoVE), Zamkov outlines his team’s process that involves replacing the organic molecules with two inorganic nanocrystals made from zinc selenide and cadmium sulfide, with a platinum catalyst added.

Structure of the nanocrystal (Image: Bowling Green State University)

According to Zamkov, "The main advantage of this technique is that it allows for direct, all inorganic coupling of the light absorber and the catalyst." In other words, these are very durable crystals compared to their organic counterparts. Not only are they less susceptible to heat and UV radiation, they also don’t suffer from degradation problems that plague their organic counterparts – where those are often irreversibly “poisoned” while in service, the nanocrystals can be recharged with a methanol wash.
The other advantage is that the nanocrystals don’t just generate electricity, they produce hydrogen gas as well. When immersed in water and exposed to light, the rod-shaped cadmium sulfide nanocrystal breaks down the water into hydrogen and oxygen.
Meanwhile, the nanocrystal – that is composed of stacked layers of zinc selenide – is photovoltaic and generates electricity. With this dual capacity, a panel made with the nanocrystals would not only generate power during the day, but also hydrogen to run a fuel cell at night.
Source: Journal of Visualized Experiments

Bioengineered bacteria could produce fuel from CO2

In the near future, genetically-altered Ralstonia eutropha bacteria could be used to convert carbon dioxide gas into fuel

Scientists at the Massachusetts Institute of Technology (MIT) have succeeded in genetically altering Ralstonia eutropha soil bacteria in such a way that they are able to convert carbon into isobutanol, an alcohol that can be blended with or even substituted for gasoline. It is hoped that once developed further, this technology could help reduce our reliance on fossil fuels, and lessen the amount of carbon dioxide released into the atmosphere by smoke stacks.
When their regular carbon food sources become scarce, R. eutropha ordinarily respond by synthesizing a type of polymer, in which they store whatever carbon they’re able to find. By “knocking out a few genes, inserting a gene from another organism and tinkering with the expression of other genes,” the team of MIT biologists were able to get the bacteria to produce isobutanol instead of that polymer.
Unlike certain other biofuels, isobutanol can be used directly as is, requiring no refining. The bacteria produce the alcohol continuously, releasing it into their fluid environment, from which it can be filtered. This differs from experiments conducted at other institutions, in which various types of bacteria have had to be destroyed in order to harvest the desired biofuel byproducts from their bodies.
Currently, the genetically modified microbes are getting their carbon from fructose. It is expected that with further alterations, however, they should be able to draw it from industrial carbon dioxide gas emissions. In fact, the scientists believe that properly bioengineered R. eutropha should be able to feed on carbon from almost any source, such as agricultural or municipal waste.
The team is now looking into increasing the bacteria’s isobutanol production levels, and scaling the technology up for use in industrial-scale bioreactors. If successful, such facilities should lessen the need for biofuel-dedicated crops such as corn, that compete with food crops for land and water.
In fact, MIT isn’t the first place to experience success in this area of research. In 2009, scientists from UCLA announced that they had been able to harvest isobutanol from CO2-consuming Synechoccus elongatus bacteria.
A paper on the Massachusetts research was published this month in the journal Applied Microbiology and Biotechnology.
Source: MIT

Scientists develop material that's harder than diamonds

Simulated structure of buckyballs and new super-hard material (Image: Lin Wang, Carnegie Institution of Washington)

Diamonds may be forever, but they aren’t what they were. True, they shine just as brightly and they’re as hard as ever, but scientists from the Carnegie Institution of Washington are giving them some competition. An international team led by Carnegie’s Lin Wang has discovered a new substance that is not quite crystalline and not quite non-crystalline, yet is hard enough to dent diamonds.
The new substance, which has yet to be named, is described by Wendy Mao, a Stanford University professor, as a “hybridization of crystalline and amorphous structures at an atomic level.” It was also something of a surprise to the Carnegie team.
The super-hard material started out as clusters of carbon-60 – the soccer-ball shaped molecules of carbon commonly known as "“buckyballs." These were mixed with m-xylene solvent, which is used in the manufacture of soft drink bottles. The mixture was then placed in a diamond cell anvil at the Argonne National Laboratory's Advanced Photon Source in Argonne, Illinois.

The diamond cell anvil was key to the experiment. This is a super high-pressure chamber made of two flat-faced diamonds. The buckyball/solvent mixture is placed in a cell between the diamonds and pressure is applied. As the diamonds squeeze together, the mixture is subjected to a pressure of, in this case, 600,000 atmospheres. Not surprisingly, the buckyballs were crushed. What was mildly surprising was that properties of the former buckyballs were altered until they became hard enough to dent the diamonds. That is not unprecedented, but what was very surprising was that the new substance retained its structure once the incredible pressure was removed. What was even more surprising was that it turned out to be a substance that no one had seen before.
All solid matter comes in one of two forms. Either it has an ordered, crystalline structure, like quartz or iron or diamonds, or it is non-crystalline or amorphous, like glass or gels. What this new substance has is both. If you apply massive pressure to buckyballs, you should get mashed buckyballs, but the m-xylene reacted with the carbon in some manner so that it retained a long-range, regular molecular structure. In other words, it retained the order of a crystal despite its crystalline structure being destroyed.
According to Wang, there is more here than a laboratory curiosity. “We created a new type of carbon material, one that is comparable to diamond in its inability to be compressed,” Wang said. “Once created under extreme pressures, this material can exist at normal conditions, meaning it could be used for a wide array of practical applications.”
Exactly what these applications are remain unknown, though it could be as a protective coating or find mechanical, electronic, and electrochemical uses.
Sources: Carnegie Institution for Science Stanford University

Mobile machine can make biofuel for military and humanitarian operations

A diagram of the process utilized by the Endurance Bioenergy Reactor 

 

Researchers at the U.S. Department of Energy's Argonne National Laboratory (ANL) have created a device called the Endurance Bioenergy Reactor (EBR) that can produce bioenergy on location, using waste from kitchens and latrines. The fuel can go directly into engines and generators without any need for refining, avoiding the complications of distribution and supply chains associated with fuel production. The researchers say the EBR can produce 25 to 50 gallons (94.6 to 189.2 liters) of biofuel a day from waste streams or processed cellulosic materials.
The EBR is based on an engineered photosynthetic bacterium, an organism that divides itself quickly. The technology combines plant enzymes with an efficient light-harvesting system that is found in abundance within these cells. The reactions from the combination of enzymes and bacteria result in fuel molecules that are foreign to the bacterium, which then expels them into a culture medium where they can be sequestered and separated from the fermentation broth. When it gets to that stage, the molecules can be used, without refining, as diesel surrogates in engines or generators.
Because of its inherent mobility, the system would be ideal for military settings, humanitarian activities in emergency zones, native peoples' villages, and in any other remote setting. The same version of the system can be used for military and civilian purposes. It is estimated that one EBR can fuel a generator that can charge up to 60 light- to medium-duty electric vehicles per day, with an estimated daily range of 50 miles (80.4 km).
The EBR is past its development phase, so all the team needs to do now is to deal with integration and scale-up issues. The researchers anticipate only a small investment will be necessary, somewhere between US$2 and $3 million.
The Argonne National Laboratory is part of the US Department of Energy. It’s a multidisplinary project that works on pressing national issues related to science and technology.
In the video below Argonne bioscientist Phil Laible talks about the EBR and its applications.
Source: ANL

Scientists develop catalyst that cleans diesel emissions without platinum

 

Nanostellar has developed a mineral catalyst that outperforms platinum at a fraction of the cost

Diesel engines are a classic example of good news and bad news. The good news is that diesel engines are much more fuel efficient than petrol engines. The bad news is that they belch out some pretty nasty emissions like nitric oxide and nitrogen dioxide. The good news is that catalytic converters can scrub those out. The bad news is that last Friday the platinum needed by the converters is selling for US$1,473.10 an ounce. Now the good news is that a team at Nanostellar in Redwood, California, has developed a mineral catalyst that outperforms platinum at a fraction of the cost.
Platinum is an excellent catalyst, though it does have a few problems. One of the biggest at the moment is that a violent labor dispute in South Africa sent the price skyrocketing. Also, with the World Health Organization classifying diesel exhaust as a carcinogen, the potential demand for platinum for catalytic converters for hundred of millions of vehicles far outstrips supply. The Nanostellar team, led by Dr. Kyeongjae “K.J.” Cho, professor of materials science and engineering and physics at UT Dallas and co-founder of Nanostellar, determined that a mineral catalyst would be a cheaper alternative.

Dr. Kyeongjiae "K.J." Cho, professor of materials science and engineering and physics at UT Dallas (Photo: University of Texas at Dallas)

Reporting their findings in the August 17 issue of Science, Cho relates that computer modelling showed that mullite was a cost-effective substitute. Mullite is a silicate mineral discovered on the Isle of Mull, Scotland in 1924. It’s rare in nature, but a synthetic version is produced commercially for use in various porcelains, such as crucibles and heating balls. It has a very high melting point of 1840 C (3344 F) and as a mixed-phase oxide mineral it makes a very attractive catalyst. In addition, laboratory tests indicate that converters using mullite would have 45 percent lower emissions than with platinum.

Ideal and stepped crystal surface of Noxicat (Image: Geoffrey McCool)

“Our goal to move completely away from precious metals and replace them with oxides that can be seen commonly in the environment has been achieved,” Dr. Cho said. “We’ve found new possibilities to create renewable, clean energy technology by designing new functional materials without being limited by the supply of precious metals.”
The new catalyst, called Noxicat, will be developed for commercial use and further work is planned to determine its application in fuel cells.
Source: University of Texas at Dallas via Phys.org

Agri-Cube grows mass quantities of vegetables in a one-car parking spot

Bountiful harvest fills the trays of a Daiwa Agri-Cube prefab garden factory

Daiwa House, Japan's largest homebuilder, has introduced a line of prefabricated hydroponic vegetable factories, aimed at housing complexes, hotels, and top-end restaurants. Called the Agri-Cube, these units are touted by Daiwa as the first step in the industrialization of agriculture, to be located in and amongst the places where people live, work, and play.
More and more people desire sustainable, organic produce for their own use, and are turning to urban farming in an effort to insure the highest degree of freshness. However, some municipalities, neighborhoods, and homeowners associations have rules that effectively block such endeavors in areas under their sway. Add drought and pest control to the picture, and suddenly urban farming may seem more trouble than it is worth. There is a growing need for local supplies of freshly grown produce that avoids the difficulties presented by conventional small farms and gardens.

External view of the 11.7 square meter Agri-Cube E garden factory

This is where the Agri-Cube comes in. Measuring less than five meters (about 16 feet) in length and 2.5 meters (about 8 feet) wide, Daiwa's Agri-Cubes are smaller than a twenty-foot equivalent shipping container. An Agri-Cube can be brought to an installation site on the bed of a light heavy-duty truck. A concrete foundation about 10 square meters (108 square feet) in size must be prepared before delivery, along with plumbing and electrical utility hookups. Daiwa claims each Agri-Unit can grow about ten thousand servings of fruits and vegetables each year at an operations cost of about US$4,500, which corresponds to only 45 US cents per head of lettuce.
An Agri-Cube is designed to require little maintenance or attention to the hydroponic and lighting systems. It is delivered ready to use, with all the hydroponic equipment, air conditioning to maintain ideal growing temperatures, a heat-exchanging ventilation system, and special growth lights to encourage faster plant growth installed and functioning. The basic structure is a steel frame building, with anti-rust treatment and floor, wall, and ceiling insulation. Solar panels and air curtains (to better maintain the controlled environment) are available as options.

Community member tending the Agri-Cube's crops

Initially, Agri-Cubes will be marketed to the food service industry. Daiwa intends to extend that focused niche market to include apartment houses and other housing complexes, neighborhood co-ops of perhaps ten households, small-scale stores, and local organic food suppliers.
A video from DigInfo TV appears below that will give a clear overview of the makeup of an Agri-Cube.
Beginning at a price of $70,000, the Agri-Cube may soon be dispensing fresh fruit and vegetables in your neighborhood.
Source: Daiwa House (Google translation), DigInfo TV

Advance could turn wastewater treatment into viable electricity producer

Research at Oregon State University by engineer Hong Liu has discovered improved ways to produce electricity from sewage using microbial fuel cells

In the latest green energy – or perhaps that should be brown energy – news, a team of engineers from Oregon State University (OSU) has developed new technology they claim significantly improves the performance of microbial fuel cells (MFCs) that can be used to produce electricity directly from wastewater. With the promise of producing 10 to 50 times the electricity, per volume, than comparable approaches, the researchers say the technology could see waste treatment plants not only powering themselves, but also feeding excess electricity back to the grid.
The electricity-generating potential of microbes has been known for decades, however, it is only in recent years that efforts to increase the amount of electricity generated to commercially viable levels has started to bear fruit. In MFCs, bacteria are used to oxidize organic matter – be it in wastewater, grass straw, animal waste, and byproducts from such operations as the wine, beer or dairy industries – which produces electrons that run from the anode to the cathode within the fuel cell to create an electrical current.
By adopting a number of new concepts, including reducing the anode-cathode spacing, and using evolved microbes and new separator materials, the researchers say they have been able to produce MFCs that produce more than two kilowatts per cubic meter of liquid reactor volume. The researchers point out this power density is much higher than has been achieved previously and could see the new technology replacing the widely used “activated sludge” process that has been used for almost a century.
While the power density of the new technology is impressive, its potential would be hampered somewhat if it was lacking in the water treatment department. Thankfully, the researchers claim it treats wastewater more effectively than the alternative approach used to generate electricity from wastewater, which is based on anaerobic digestion and produces methane and hydrogen sulfide.
The team has proven the system “at a substantial scale in the laboratory” and is now seeking funding to scale things up with a pilot study. A contained system that produces a steady supply of certain types of wastewater that would provide significant amounts of electricity, such as a food processing plant, is seen as an ideal candidate for such a test.
A pilot study would also assist the team in efforts to reduce the high initial costs of the system by identifying potential reductions in material costs, further optimizing the use of the microbes, and improving the system’s function at commercial scales. If the high initial costs can be brought down through such advances, the OSU team estimates the construction costs of their new technology will be comparable that that of activated sludge systems, even without taking into account future sales of electricity generated at the facility.
“If this technology works on a commercial scale the way we believe it will, the treatment of wastewater could be a huge energy producer, not a huge energy cost,” said Hong Liu, an associate professor in the OSU Department of Biological and Ecological Engineering. “This could have an impact around the world, save a great deal of money, provide better water treatment and promote energy sustainability.”
While turning wastewater treatment from an electricity consumer to an electricity generator has obvious benefits for developed countries, the potential for the technology holds even greater promise in developing countries where both electricity access and sewage treatment is lacking.
The OSU team's findings appear in Energy and Environmental Science.
Source: Oregon State University

Flexible lithium-ion battery technology is on the march

 

A newly developed bendable thin-film lithium-ion battery could help bridge the gap to high-performance bendable electronics 

Researchers at the Korea Advanced Institute of Science and Technology (KAIST) have developed a promising solid state, thin-film lithium-ion battery that claims the highest energy density ever achieved for a flexible battery. The new design, which showed for the first time that high-performance thin films can be used for flexible batteries, may be commercialized as early as next year.
Lithium-ion batteries are a strong candidate for powering the flexible electronics of the future. A high-performance lithium-ion flexible battery would be a giant step toward fully-fledged flexible electronics systems and would open the door to flexible e-paper, wearable devices, and better piezoelectric systems that harvest energy from mechanical forces.
Research is progressing, but seems to have hit an invisible – though very real – performance wall. This is because most designs employ either low-performance flexible organic materials, or polymer binders that take up too much space and decrease the battery's power density. In addition, the cathodes have to be treated at high temperatures to improve performance, but this can't be done effectively on substrates made of flexible polymers.
The new approach developed at KAIST uses high energy density inorganic thin films that can be treated at high temperatures, resulting in the highest-performance flexible lithium-ion batteries yet. "There is no performance difference in energy density, capacity, and cycle life between our flexible battery and bulk batteries," Prof. Keon Jae Lee, who led the research efforts, told Gizmag. "On the contrary, performance is improved by about 10 percent because of the stress release effect."
The batteries are built by sequentially depositing several layers – a current collector, a cathode, an electrolyte, an anode, and a protective layer – on a brittle substrate made of mica. Then, the mica is manually delaminated using adhesive tape, and the battery is enclosed between two polymer sheets to improve mechanical resistance.
Bending the battery affects performance, but not to disastrous levels. With the battery constantly bent at a radius of sixteen millimeters (about the same curvature of a fifty-cent coin) the discharge capacity drops by about seven percent after 100 charge-discharge cycles, compared to a three percent drop when the battery is not bent. Voltage was shown to remain almost constant, dropping by a very modest 0.02 V after the battery was bent and released 20,000 times.
"The technology for commercializing this battery could come in a relatively short time, about a year," says Prof. Lee. But first, the researchers need to find a better, automated way to delaminate the mica substrate – the manual method, involving adhesive tape, is very unpractical and can take up to ten minutes per battery.
"We are investigating a laser lift-off [delamination] process to facilitate mass production of large area flexible lithium-ion batteriesr" says Lee. "Its feasibility is already proven and will be reported in a later paper."
The team is also interested in stacking the structures on top of each other to improve charge density.
A paper describing the battery was recently published on the journal Nano Letters. The video below illustrates the voltage performance of the batteries under mechanical stress.
Source: KAIST

Iron-air batteries may prove a cheap, eco-friendly solution for energy storage

Dr. Narayan is testing one of his new batteries by using it to power a small fan

   

Revamping a concept that was first explored forty years ago, researchers at the University of Southern California (USC) are putting the final touches on a patent-pending design for cheap, rechargeable, high energy density iron-air batteries. Because of their unique features, the batteries look particularly well-suited to the kind of large-scale energy storage that could accelerate the adoption of renewable energy sources.
The quest for a cheap, environmentally friendly rechargeable battery stretches back for decades. For one, lithium-ion batteries were first proposed in the seventies, and only recent advances in materials technology have made this technology into one of the most common, high-performing solutions for today's portable electronics.
Now, a team of USC researchers may have found the key to resuscitating yet another design first proposed around the same time – the iron-air battery.
In the context of battery design, iron has more than a few perks: it is durable, it packs good amounts of energy per unit of mass, it is easily recycled and, last but not least, it is very cheap – in commercial quantities, it only costs around US$1/kg (2.2 lb).
Iron-air batteries were a prime candidate for electric vehicles and military applications after the "oil crisis" that started in 1973. However, research stopped abruptly only years later, when scientists realized that iron-air batteries presented a serious and seemingly insurmountable limitation: whenever the battery was being charged, a wasteful process of hydrolysis drained away about half of the battery's energy.
Back to the present, where researchers at USC have finally found a solution to this wasteful problem. They learned that adding a small amount of bismuth sulfide into the battery shut down the harmful reaction and reduced the waste of energy more than tenfold, from fifty down to just four percent. (Other possible choice materials such as lead or mercury were discarded because, even though they could have worked just as well, they wouldn't have been as safe.)
Another crucial strength of the system is the remarkably simple, cost-effective design of its iron electrode. The researchers combined iron powder with a polyethylene binder, heating the mix to obtain a "pressed-plate" electrode that is simple to make and has high specific energy. With this technology, a battery storing a kWh of energy – the equivalent of 24 new iPad batteries – would require only about $3 worth (3 kg/6.6 lb) of iron powder.
This cheaper iron electrode is driving costs down significantly, and the researchers are targeting an aggressive $100 per kWh for their batteries. For reference, research firm Lux Research puts the cost of lithium-ion batteries at roughly around $600/kWh and says their cost will decrease quite slowly, dropping below the $400/kWh mark no earlier than in 2020.
The iron-air battery is exhibiting very promising durability, with a target life of 5,000 charge-discharge cycles. Even more importantly, the batteries seem to retain good performance when they are being drained quickly: at a two-hour rate of discharge, the batteries are showing a twenty-fold increase in capacity compared to commercially available electrodes.
All in all, this battery design seems well-suited to meet the demands of fast-paced, large-scale energy storage applications, and could supply the ideal "energy grid buffer" for renewable but intermittent energy sources like solar and wind power.
A paper describing the battery was published on a recent issue of the Journal of the Electrochemical Society.
Sources: USC, Lux Research

Flamestick – the firestarter and cooking fuel made from recycled plastic

When used in a stove, a few Flamesticks put out quite a flame

Looking not unlike a plastic Popsicle stick, the Flamestick from Germany's AceCamp is a firestarter made from recycled thermoplastic that measures 3.5 inches (8.9 cm) long by 1/4 inch (6.4 mm) wide. While plastic may sound like a strange way to start a fire, the Flamestick offers several advantages over more traditional materials.

The Flamestick's recycled thermoplastic lights quickly and burns hot, providing a capable firestarter for lighting campfires, stoves, etc. According to AceCamp, a single Flamestick burns at up to 626° F (330° C), and a pair of sticks can reach temperatures over 800° F (426° C). With that much heat, and a claimed five minutes of burn time, Flamesticks can double as cooking fuel. AceCamp's managing director Darko Leo told us he's fried an egg in two minutes and 15 seconds using a solid-fuel backpacking stove powered by Flamesticks.
Flamesticks offer several benefits over other firestarters. Their plastic build means that they aren't susceptible to soaking like other tinder is. So you don't need to worry so much about sealing them up in a dry, waterproof container and can count on them even if they've been rained on or dragged through a river. The fact that Flamesticks can double as cooking fuel means that backpackers can use them for both cooking and firestarting purposes, saving space and weight. AceCamp is also quick to tout the eco-friendly aspects of using recycled materials; however, it's easy to find other recycled or re-purposed firestarters – dryer lint, for instance.

AceCamp recently introduced the Flamestick to its home market of Germany, where Leo told us the product has gone through testing and has proven safe and non-toxic. The company is also in the process of launching in the U.S. and says it has a U.S. patent pending.
I received a pair of sample Flamesticks and put one to the test. The stick lit quickly and began sizzling within seconds. Thanks to its long, thin design, I was able to comfortably hold it on the opposite end while lighting it and use it like a slow-burning match. Acecamp's claim of "no smell" is exaggerated – while it might not be as pungent as burning a non-treated piece of plastic or rubber, it definitely had a smell. The Flamestick burned for about seven minutes. When it went out, it left a small bit of melted residue. The residue did relight, but I ended up having to hold it over a steady lighter flame with a pair of tongs and relight it several times to get it to burn off completely.
While pricing isn't finalized, Leo estimated that a box of 20 Flamesticks will retail for around US$7.
Source: AceCamp GmbH

"Transfer engineering" eliminates toxins from edible part of rapeseed plant

The rapeseed plant is one of the most widely cultivated crops in the world and researchers have now found a way to stop toxins entering edible parts of the plant

As well as being the third largest source of vegetable oil in the world – after soybean and oil palm – rapeseed (also known as rape, oilseed rape, rapa, rappi and rapeseed) is cultivated in Europe primarily for animal feed. But due to high levels of glucosinolates that are harmful to most animals (including humans) when consumed in large amounts, its use must be limited. Now researchers at the University of Copenhagen have found a way to stop unwanted toxins entering the edible parts of the plant, thereby increasing the potential of the plant to be used as a commercial animal feed.
Unlike the healthy glucosinolates found in broccoli, the glucosinolate found in rapeseed has toxic effects in both humans and animals in high doses. It also results in feed meal that is very bitter and unappealing to animals. This has led to the development of breeds with reduced glucosinolate content, such as Canola, which produces edible oil suitable for consumption by humans. However, animal intake of the protein-rich rapeseed cake, which is produced using the byproduct of rapeseeds pressed for oil and used for pig and chicken feed, must be limited, meaning that Northern Europe still imports large amounts of soy cake for animal feed.
By uncovering two proteins responsible for transporting glucosinolates into the seeds of the thale cress plant, a close relative to the rapeseed plant, researchers from the University of Copenhagen were able to produce thale cress without these two proteins and found that their seeds were completely glucossinolate free and therefore suitable for animal feed.
Professor Barbara Ann Halkier, head of the Center of Excellence for Dynamic Molecular Interactions (DynaMo) at the University of Copenhagen’s Faculty of Science says the team calls their new technology for eliminating unwanted substances from the edible parts of plants and crops “transport engineering.”
The team’s research has attracted the attention of Bayer CropScience, one of the world’s biggest plant biotech companies, which is now negotiating with the University of Copenhagen’s Tech Transfer Unit to team up with the research group and apply their approach to producing rapeseed plants with glucosinolate-free seeds.
The team’s research is published in the journal Nature.
Source: University of Copenhagen

New lignin-solvent process harvests biofuel, paper and chemicals from plant material

Researchers have developed a new lignin-solvent process to separate cellulosic biomass into usable components (from left: lignin, hemicellulose, and cellulose)

In order to improve the sustainability credentials of biofuels, experts have been trying to figure out ways to produce them from non-food sources, such as cellulose – the material that makes up the cell walls of plants. Now, researchers from the Wisconsin Institute for Sustainable Technology (WIST) at the University of Wisconsin-Stevens Point have patented a process that they say paves the way for the creation of biofuels from cellulosic plant material.

WIST’s first patent is for a process that makes biofuels and other products from such material, including agricultural left-overs such as corn stover, or plants grown specifically for fuel production, such as hardwood and softwood trees. The method they’ve patented involves an aqueous solvent that separates cellulosic material into pure cellulose and lignin, the substance that gives woody biomass its rigidity. The lignin-solvent mixture can then be separated from the water and becomes a high-energy-density fuel that can be used independently or in combination with biodiesel.

But it’s not just cellulosic ethanol that can come out of this process. Pure cellulose can be used to make paper or can be converted into fermentable sugars. Besides biofuels, the sugars can also be used to make other renewable chemicals for industry including isoprene. It's a material used to make rubber, plastics and pharmaceuticals, but which comes mostly from petroleum.

The patent is an improvement on traditional processes for separating lignin from cellulose employed by the paper industry, which make it more difficult to convert the cellulose to sugars. Also, the lignin that's produced contains chemicals that cannot be easily or economically separated. The new lignin-solvent process results in a purified lignin and pure cellulose, which can be readily used to produce other renewable chemicals. This saves the lignin from being burned, which is the process conventional paper plants typically adopt to recover inorganic chemicals from the pulping process and to produce energy as well.

WIST is working with UW’s WiSys Technology Foundation to license the intellectual property to private industry for development. The researchers envisage several applications for the lignin, including carbon fiber.

Besides the lignin-solvent process, WIST hopes to develop a biorefinery that could be fitted to existing paper mills or to revive idle ones. They have performed the process in the lab and now are looking to develop it into a demonstration-scale plant.

Source: WIST

Breakthrough allows inexpensive solar cells to be fabricated from any semiconductor

A new technique allows photovoltaic solar cells to be produced using any semiconductor

Despite their ability to generate clean, green electricity, solar panels aren't as commonplace as the could be. The main sticking point, of course, is price. Due to their need for relatively expensive semiconductor materials, conventional solar cells don't yet have a price-efficiency combination that can compete with other sources of electricity. Now Profs. Alex Zettl and Feng Wang of Lawrence Berkeley National Laboratory and the University of California at Berkeley have developed seriously unconventional solar cell technology that allows virtually any semiconductor material to be used to create photovoltaic cells.

A solar cell works according to these steps. First, sunlight hits the solar cell and is absorbed by the semiconductor of which the solar cell is made. In the absorption process, electrons are freed from their atoms, allowing them to flow through the semiconductor. The presence of a p-n junction acts as a diode, only allowing the electrons to move in a single direction. (Electrons and holes move in opposite directions, but the electrical current only moves in one.) Metal electrodes then transfer the light-generated electron flow into an electric circuit for use. A p-n junction is the interface between a region of the semiconductor where the dominant charge carriers are holes and a region where the carriers are electrons.

How a solar cell works (Image: Brian Dodson)

A conventional solar cell is made of a thin wafer of a semiconductor with a metallic electrode deposited on its rear side. The side facing the light source is polished more finely than any optical lens, cleaned to the atomic level, and then dopant atoms are deposited onto the front side, whereupon the entire wafer is placed in a high-temperature diffusion furnace.

The purpose of a dopant is to change the dominant charge carrier in the semiconductor from hole-rich to electron-rich, or v.v. In this process the p-n junction that converts incident light into a flow of electrons is formed. Following diffusion, the wafer is again cleaned, and a metallic electrode is grown on the front surface, using arcane rituals to ensure an ohmic contact with the active semiconductor material. (An ohmic contact is an electrical contact that obeys Ohm's law, having no rectifying or diode-like properties.)

The efficiency of conventional solar cells is also limited by the semiconducting materials which are suitable for the manufacture of solar cells by some approximation of the above process. It must be possible for the dominant charge carrier of the semiconductor to be changed between p (hole) and n (electron)-dominated conduction by introduction of chemical dopants, so that a well-behaved p-n junction is formed. It must also be possible to make a satisfactory electrical contact between the electrodes and the semiconductor.

There are many semiconductor materials with optical properties and electronic band-gaps well suited to conversion of light to electricity for which one or another of the rather rigid criteria for manufacture of conventional solar cells fails. These include many metal oxides, sulfides and phosphides, which are plentiful and inexpensive, but have been considered unsuitable because it is so difficult to alter their electronic structure chemically (e.g., through doping). For example, zinc oxide is a semiconductor well suited for capturing violet and near ultraviolet light, which is wasted by most conventional solar cells.

We now have Zettl and Wang's unconventional solar cells. Broken down, the expensive parts of making regular solar cells are the semiconductor wafer, forming a high-quality p-n junction under the surface of the wafer, and making ohmic electrical contact with the front and back of the wafer. Aside from providing a semiconductor wafer (usually of a cheaper material), the new solar cells require none of this.

The new technology is called "screening-engineered field-effect photovoltaics" (SFPV). An electrode is deposited on the front of the semiconductor wafer, which partially screens the semiconductor from an electric field generated between the front and rear electrodes. Assume the semiconductor is naturally p-type, so that it has an excess of holes. The applied electric field then penetrates the semiconductor surface slightly, attracting electrons toward the surface and repelling holes. As a result, the semiconductor near the surface changes from p-type to n-type (electron-rich), and a buried p-n junction has been generated – not by chemistry, but by use of carefully tailored electric fields. An extra bonus is that the front electrode automatically forms an ohmic contact with the semiconductor wafer.

“Our technology requires only electrode and gate deposition, without the need for high-temperature chemical doping, ion implantation, or other expensive or damaging processes,” said lead author of a paper describing the new technology, William Regan. “The key to our success is the minimal screening of the gate field which is achieved through geometric structuring of the top electrode. This makes it possible for electrical contact to and carrier modulation of the semiconductor to be performed simultaneously.”

Two approaches to engineering the screening properties of SFPV electrodes - fingered electrodes on the left, graphene electrodes on the right

Two electrode configurations that exhibit the SFPV concept have been developed. In one, the electrode in contact with the semiconductor wafer is formed of a row of narrow fingers, while in the other the partial screening is accomplished by placing a layer of graphene atop the semiconductor wafer. In both cases, high-quality p-n junctions are formed in semiconductors for which this structure was previously impossible.

Low cost, high efficiency solar cells? Sounds like a winner here – let's see when and if it hits the market.

The team's research is published in the journal Nano Letters.

Source: Lawrence Berkeley National Laboratory