Scientists develop heat-resistant materials that could vastly improve solar cell efficiency

Scientists have created a heat-resistant thermal emitter that could significantly improve the efficiency of solar cells.

The novel component is designed to convert heat from the sun into infrared light, which can than be absorbed by solar cells to make electricity – a technology known as thermophotovoltaics. Unlike earlier prototypes that fell apart at temperatures below 2200 degrees Fahrenheit (1200 degrees Celsius), the new thermal emitter remains stable at temperatures as high as 2500 F (1400 C).

“This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics,” said Shanhui Fan, a professor of electrical engineering at Stanford University. Fan and his colleagues at the University of Illinois-Urbana Champaign (Illinois) and North Carolina State University collaborated on the project. Their results are published in the October 16 edition of the journal Nature Communications.

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy. But silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass through the solar panel.

“In theory, conventional single-junction solar cells can only achieve an efficiency level of about 34 percent, but in practice they don’t achieve that,” said study co-author Paul Braun, a professor of materials science at Illinois. “That’s because they throw away the majority of the sun’s energy.”

Thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

“Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell,” Fan said. “That raises the theoretical efficiency of the cell to 80 percent, which is quite remarkable.”

So far, thermophotovoltaic systems have only achieved an efficiency level of about 8 percent, Braun noted. The poor performance is largely due to problems with the intermediate component, which is typically made of tungsten – an abundant material also used in conventional light bulbs.

“Our thermal emitters have a complex, three-dimensional nanostructure that has to withstand temperatures above 1800 F (1000 C) to be practical,” Braun explained. “In fact, the hotter the better.”

In previous experiments, however, the 3D structure of the emitter was destroyed at temperatures of around 1800 F (1000 C). To address the problem, Braun and his Illinois colleagues coated tungsten emitters in a nanolayer of a ceramic material called hafnium dioxide.

The results were dramatic. When subjected to temperatures of 1800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2500 F (1400 C), the samples remained thermally stable for at least an hour.

The ceramic-coated emitters were sent to Fan and his colleagues at Stanford, who confirmed that devices were still capable of producing infrared light waves that are ideal for running solar cells.

“These results are unprecedented,” said former Illinois graduate student Kevin Arpin, lead author of the study. “We demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage.”

Braun and Fan plan to test other ceramic-type materials and determine if the experimental thermal emitters can deliver infrared light to a working solar cell.

“We’ve demonstrated that the tailoring of optical properties at high temperatures is possible,” Braun said. “Hafnium and tungsten are abundant, low-cost materials, and the process used to make these heat-resistant emitters is well established. Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven’t been considered.”

Problem-Solving Parrots Understand Cause and Effect

Scientists speculate two factors may influence why some animal species are smarter than others: the foraging behavior of a species (for instance, how cognitively demanding it is for the animals to obtain food) and the social complexity of the animals’ society.

A new study looked at problem-solving skills, which reflect animals’ ability to understand and solve a novel situation, and whether they’re related to a species’ social complexity or foraging ecology. Anastasia Krasheninnikova, Stefan Bräger, and Ralf Wanker of the University of Hamburg, Germany, tested four parrot species with different social systems and diets: spectacled parrotlets, green-winged macaws, sulphur-crested cockatoos, and rainbow lorikeets.

“One of the characteristics of complex cognition in animals is the ability to understand causal relationships spontaneously, and one way of testing this is asking the animal to obtain a reward that is out of reach,” says Krasheninnikova. She and her colleagues gave the birds five variations on a string-pulling task, involving strings that varied in their relationship to each other or to a food reward, to see whether the birds really understood the means-end relationship between the string and the food.

The first test was a basic string-pulling task in which the bird must figure out how to pull up a piece of food suspended from a perch by a single piece of string. Almost all the birds of all species solved this test immediately.
In the second task, there were two hanging strings, but only one was attached to a piece of food. If the bird really understood the string as a means to obtain the reward, it should pull only the rewarded string. Most of the birds (more than 75%) were able to solve this test on their first try.
To make sure that the bird really understood the functional relationship between food and string and was not just pulling the string closest to the food reward, the third task used a pair of crossed strings. In this test, pulling the string directly above the food would not result in obtaining the food, while pulling the further string that is actually attached to the food would. The spectacled parrotlets and rainbow lorikeets outperformed the macaws and cockatoos on this test, and only the parrotlets were able to figure out the test when the strings were the same color. Krasheninnikova says this study is the first to document a parrot species solving the crossed-strings task spontaneously.
The fourth task probed the flexibility of the bird’s behavior. The string was longer, so the bird could obtain the food from the ground rather than pulling the string up. Several members of all species adapted their problem-solving strategies by stopping string-pulling behavior and obtaining the food from the ground, but only the parrotlets and lorikeets clearly preferred the alternative strategy.
In the fifth and final task, there were two rewarded strings, but one had a gap between its end and the reward. Solving this task required the bird to understand the mere presence of the reward does not guarantee the reward can be obtained; the food had to be connected to the string to work properly. Parrotlets were the only species to successfully solve task five.

When Krasheninnikova and her colleagues compared their results to the birds’ lifestyles, they found the pattern in performance was best explained by differences in the species’ social structures rather than their diets.
Spectacled parrotlets performed best of the four species tested and they live in what’s known as a fission-fusion society. These birds live in large groups where they form different social subunits that split and merge, providing the opportunity for many different kinds of social interactions. They are also the only one of the four species tested to form crèches where young birds pass through the socialization process.
Green-winged macaws and sulphur-crested cockatoos live in small, stable family groups centered around a breeding pair and their offspring. These species failed tests four and five.
The social organization of rainbow lorikeets falls somewhere between the parrotlets and the macaws and cockatoos — as does their performance on the string-pulling tasks. Lorikeets live in social groups of 10-40 individuals, but do not form subunits such as crèches. They performed better than macaws and cockatoos, but not as well as parrotlets.
While these results support the social complexity hypothesis, the correlation between social structure and cognitive performance is mostly indirect. The reasoning behind the hypothesis is that living in social groups is cognitively demanding. “Individuals have to recognize group members and infer relationships among them,” Krasheninnikova says. “These demands favor the evolution of understanding functional relationships, such as which actions cause which outcomes.” Socially living animals might be able to apply this cause and effect thinking to their physical world as well as their social lives.

Video games do not make vulnerable teens more violent

Do violent video games such as ‘Mortal Kombat,’ ‘Halo’ and ‘Grand Theft Auto’ trigger teenagers with symptoms of depression or attention deficit disorder to become aggressive bullies or delinquents?

No, according to Christopher Ferguson of Stetson University and independent researcher Cheryl Olson from the US in a study published in Springer’s Journal of Youth and Adolescence. On the contrary, the researchers found that the playing of such games actually had a very slight calming effect on youths with attention deficit symptoms and helped to reduce their aggressive and bullying behavior.

Ferguson and Olson studied 377 American children, on average 13 years of age, from various ethnic groups who had clinically elevated attention deficit or depressive symptoms. The children were part of an existing large federally funded project that examines the effect of video game violence on youths.

The study is important in light of ongoing public debate as to whether or not violent video games fuel behavioral aggression and societal violence among youths, especially among those with pre-existing mental health problems. Societal violence includes behavior such as bullying, physical fighting, criminal assaults and even homicide. And the news media often draws a link from the playing of violent video games to the perpetrators of school shootings in the United States.

Ferguson and Olson’s findings do not support the popular belief that violent video games increase aggression in youth who have a predisposition to mental health problems. The researchers found no association between the playing of violent video games and subsequent increased delinquent criminality or bullying in children with either clinically elevated depressive or attention deficit symptoms. Their findings are in line with those of a recent Secret Service report in which the occurrence of more general forms of youth violence were linked with aggressiveness and stress rather than with video game violence. Interestingly, the researchers of the current study found a few instances in which video game violence actually had a slight cathartic effect on children with elevated attention deficit symptoms and helped to reduce their aggressive tendencies and bullying behavior.

Although Ferguson and Olson warned that their results could not be generalized to extreme cases such as mass homicides, they strongly advocate for a change in general perceptions about the influence of violent video games, even within the context of children with elevated mental health symptoms.

“We found no evidence that violent video games increase bullying or delinquent behavior among vulnerable youth with clinically elevated mental health symptoms,” Ferguson stressed. Regarding concerns about some young mass homicide perpetrators having played violent video games, Ferguson stated, “Statistically speaking it would actually be more unusual if a youth delinquent or shooter did not play violent video games, given that the majority of youth and young men play such games at least occasionally.”

Tiny Twist in Graphene May Solve a Mystery

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a unique new twist to the story of graphene, sheets of pure carbon just one atom thick, and in the process appear to have solved a mystery that has held back device development.

Electrons can race through graphene at nearly the speed of light – 100 times faster than they move through silicon. In addition to being superthin and superfast when it comes to conducting electrons, graphene is also superstrong and superflexible, making it a potential superstar material in the electronics and photonics fields, the basis for a host of devices, starting with ultrafast transistors. One big problem, however, has been that graphene’s electron  conduction can’t be completely stopped, an essential requirement for on/off devices.

The on/off problem stems from monolayers of graphene having no bandgaps – ranges of energy in which no electron states can exist. Without a bandgap, there is no way to control or modulate electron current and therefore no way to fully realize the enormous promise of graphene in electronic and photonic devices. Berkeley Lab researchers have been able to engineer precisely controlled bandgaps in bilayer graphene through the application of an external electric field. However, when devices were made with these engineered bandgaps, the devices behaved strangely, as if conduction in those bandgaps had not been stopped. Why such devices did not pan out has been a scientific mystery until now.

Working at Berkeley Lab’s Advanced Light Source (ALS), a DOE national user facility, a research team led by ALS scientist Aaron Bostwick has discovered that in the stacking of graphene monolayers subtle misalignments arise, creating an almost imperceptible twist in the final bilayer graphene. Tiny as it is – as small as 0.1 degree – this twist can lead to surprisingly strong changes in the bilayer graphene’s electronic properties.

“The introduction of the twist generates a completely new electronic structure in the bilayer graphene that produces massive and massless Dirac fermions,” says Bostwick. “The massless Dirac fermion branch produced by this new structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. This explains why bilayer graphene has not lived up to theoretical predictions in actual devices that were based on perfect or untwisted bilayer graphene.”

Bostwick is the corresponding author of a paper describing this research in the journal Nature Materials titled “Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene.” Keun Su Kim of the Fritz Haber Institute in Berlin is the lead author Other coauthors are Andrew Walter, Luca Moreschini, Thomas Seyller, Karsten Horn and Eli Rotenberg, who oversees the research at ALS Beamline 7.0.1.

Monolayers of graphene have no bandgaps – ranges of energy in which no electron states can exist. Without a bandgap, there is no way to control or modulate electron current and therefore no way to fully realize the enormous promise of graphene in electronic and photonic devices. Berkeley Lab researchers have been able to engineer precisely controlled bandgaps in bilayer graphene through the application of an external electric field. However, when devices were made with these engineered bandgaps, the devices behaved strangely, as if conduction in those bandgaps had not been stopped.

To get to the bottom of this mystery, Rotenberg, Bostwick, Kim and their co-authors performed a series of angle-resolved photoemission spectroscopy (ARPES) experiments at ALS beamline 7.0.1. ARPES is a technique for studying the electronic states of a solid material in which a beam of X-ray photons striking the material’s surface causes the photoemission of electrons. The kinetic energy of these photoelectrons and the angles at which they are ejected are then measured to obtain an electronic spectrum.

“The combination of ARPES and Beamline 7.0.1 enabled us to easily identify the electronic spectrum from the twist in the bilayer graphene,” says Rotenberg. “The spectrum we observed was very different from what has been assumed and contains extra branches consisting of massless Dirac fermions.  These new massless Dirac fermions move in a completely unexpected way governed by the symmetry twisted layers.”

Massless Dirac fermions, electrons that essentially behave as if they were photons, are not subject to the same bandgap constraints as conventional electrons. In their Nature Materials paper, the authors state that the twists that generate this massless Dirac fermion spectrum may be nearly inevitable in the making of bilayer graphene and can be introduced as a result of only ten atomic misfits in a square micron of bilayer graphene.

“Now that we understand the problem, we can search for solutions,” says lead author Kim. “For example, we can try to develop fabrication techniques that minimize the twist effects, or reduce the size of the bilayer graphene we make so that we have a better chance of producing locally pure material.”

Beyond solving a bilayer graphene mystery, Kim and his colleagues say the discovery of the twist establishes a new framework on which various fundamental properties of bilayer graphene can be more accurately predicted.

“A lesson learned here is that even such a tiny structural distortion of atomic-scale materials should not be dismissed in describing the electronic properties of these materials fully and accurately,” Kim says.
This research was supported by the DOE Office of Science.

Better droplet condensation could boost power efficiency

Researchers at MIT have developed an innovative approach to improving heat transfer in power plants and cooling systems. The new system could provide a 100 percent improvement in the efficiency of heat transfer over conventional systems, the researchers say.

Heat transfer by condensation is key to the operation of today’s power plants, where fossil fuels are used to boil water and the resulting steam drives turbines to generate electricity. The steam must then condense back to water, which is collected and sent back to the boiler to start the cycle again.
The new system is an improvement of the condensers used to turn steam back into water. The same principle might also be used to improve condensers in desalination plants and in thermal-management systems.
Three key qualities contribute to the efficiency of heat transfer in such systems: Droplets must form easily and abundantly on a condenser surface; the area of contact between the droplet and the surface must be large enough to easily conduct heat; and the droplets must quickly fall away from that surface to allow new droplets to start condensing.
While most previous research on improving condensers has focused on the third part, the new work improves all three aspects at once, says associate professor of mechanical engineering Evelyn Wang, senior author of a paper just published in the journal Scientific Reports. The report was co-authored by Rong Xiao and Nenad Miljkovic, both of whom just completed their PhDs at MIT, and former postdoc Ryan Enright.
The innovation combines two properties: First, a nanopatterned surface, etched with tiny pillars, reduces contact between droplets and the surface. Second, a layer of oil coats the surface, helping droplets to form abundantly on the surface and also making it easy for them to slide off.
“We know it’s a combination of these qualities that is optimal,” Wang says. “We believe the big contribution of this work is to drastically enhance [droplet] densities. … We see [droplets] form on every single one of those pillar tops.”

Condensers’ contradictory need to enhance both droplet formation (requiring a hydrophilic surface) and droplet release (requiring a hydrophobic surface) is satisfied by the combination of nanostructures and an oily surface. This results in a surface that is locally hydrophilic — attracting droplets to the tops of the tiny pillars — while still being hydrophobic overall, causing droplets to fall away quickly as they grow in size. The new system produces much greater density of droplets than has been achieved on most other nanopatterned surfaces, Wang says.
Because the droplets condense right through the thin coating of oil, and end up being immersed in oil, the researchers coined the term “immersion condensation” to describe their new system. The new approach can be applied to ordinary copper plates or tubes, typically used in today’s condensers, so it should be relatively easy to incorporate into existing plants, Wang says.
The group initially did computer modeling of the system, then carried out experiments to verify the models’ predictions. The experiments confirmed a 100 percent enhancement of heat-transfer efficiency, compared to untreated copper surfaces.
Further research, using different kinds of oil and different texture patterns, could yield even greater improvements, Wang says. “There’s lots of opportunity for optimization of the structures to get better performance,” she says.
Anthony Jacobi, a professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign, says this research “is very exciting to me, because it not only demonstrates the innovative use of surface heterogeneity to promote immersion condensation, but it carries that idea to a potentially scalable and inexpensive deployment that may be useful in real power-generation or water-recovery systems. … The impact on energy efficiency could be tremendous.”
This work was supported by the Office of Naval Research and MIT’s Solid State Solar Thermal Energy Conversion Center, which is funded by the U.S. Department of Energy.
 

Researchers use video game tech to steer roaches on autopilot

North Carolina State University researchers are using video game technology to remotely control cockroaches on autopilot, with a computer steering the cockroach through a controlled environment. The researchers are using the technology to track how roaches respond to the remote control, with the goal of developing ways that roaches on autopilot can be used to map dynamic environments – such as collapsed buildings.

The researchers have incorporated Microsoft’s motion-sensing Kinect system into an electronic interface developed at NC State that can remotely control cockroaches. The researchers plug in a digitally plotted path for the roach, and use Kinect to identify and track the insect’s progress. The program then uses the Kinect tracking data to automatically steer the roach along the desired path.

The program also uses Kinect to collect data on how the roaches respond to the electrical impulses from the remote-control interface. This data will help the researchers fine-tune the steering parameters needed to control the roaches more precisely.
“Our goal is to be able to guide these roaches as efficiently as possible, and our work with Kinect is helping us do that,” says Dr. Alper Bozkurt, an assistant professor of electrical and computer engineering at NC State and co-author of a paper on the work.
“We want to build on this program, incorporating mapping and radio frequency techniques that will allow us to use a small group of cockroaches to explore and map disaster sites,” Bozkurt says. “The autopilot program would control the roaches, sending them on the most efficient routes to provide rescuers with a comprehensive view of the situation.”
The roaches would also be equipped with sensors, such as microphones, to detect survivors in collapsed buildings or other disaster areas. “We may even be able to attach small speakers, which would allow rescuers to communicate with anyone who is trapped,” Bozkurt says.
Bozkurt’s team had previously developed the technology that would allow users to steer technology remotely, but the use of Kinect to develop an autopilot program and track the precise response of roaches to electrical impulses is new.
The interface that controls the roach is wired to the roach’s antennae and cerci. The cerci are sensory organs on the roach’s abdomen, which are normally used to detect movement in the air that could indicate a predator is approaching – causing the roach to scurry away. But the researchers use the wires attached to the cerci to spur the roach into motion. The wires attached to the antennae send small charges that trick the roach into thinking the antennae are in contact with a barrier and steering them in the opposite direction.

 

Video Games an Effective Tool for Stroke Rehabilitation

In the months following a stroke, patients undergo hours of rehabilitation to restore movement, speech, and overall functionality. But many still return home without the ability to perform daily tasks, such as dressing, cooking or driving.

Now occupational therapist Dr. Debbie Rand of Tel Aviv University’s Stanley Steyer School of Health Professions at the Sackler Faculty of Medicine, has turned to common interactive video games as an affordable and effective alternative to traditional therapy. In a recent study, done in collaboration with a team from Sheba Medical Center and funded by the Marie Curie International Reintegration Grant, she found that people recovering from stroke who use video games as a therapeutic method are more physically active during rehabilitation sessions, making more movements overall than those who experience traditional motor therapy.
The results were presented at the 9th International Conference on Disability, Virtual Reality and Associated Technologies (ICDVRAT 2012).
Goal-directed movement
Interactive game consoles require players to move continuously to interact with the virtual games, Dr. Rand explains. In her study, not only did the players perform double the number of arm movements during each session compared to patients in traditional therapy, but all of their movements were purposeful or “goal-directed” and not just repetitive exercises.
When individuals plan their movements and move deliberately in order to accomplish a specific goal, it is likely to have a positive impact on brain plasticity — changes in the brain that are crucial for recovery from brain damage caused by stroke, Dr. Rand notes. Players’ movements require precision and balance, and there is a cognitive benefit in that video games require strategy and planning. And because the individuals are motivated and enjoy the activity, it’s more likely that they will continue the treatment regime long-term, she believes.
To test the effectiveness of interactive video games compared to traditional therapy, individuals who had experienced a stroke one to seven years before the study began were randomly assigned to one of two groups of 20 participants each — a traditional therapy group, who completed traditional rehabilitation exercises, and a video games group which played video games using Xbox Kinect, Sony PlayStation and Nintendo Wii gaming consoles. Each group received two sessions a week with occupational therapists for a period of three months.
Although both groups showed improvement in functions such as grip strength of their weaker and stronger hands and gait speed, participants in the video games group continued to improve their grip strength for three months following the intervention, while the traditional group did not.
Having fun with therapy
Beyond the physical advantages, Dr. Rand believes that video games could be an excellent alternative to traditional therapy simply because they’re more fun. In the video game group, 92 percent of participants reported enjoying the experience “extremely” or “very much,” opposed to 72 percent of the traditional group. If patients are enjoying the therapy experience, it’s more likely that they will adhere to the therapy regime long-term, noting that game consoles are now widely available and fairly inexpensive. Participants who were in the video game playing group reported: “It was lots of fun,” “it stimulated all of my senses,” and “I finished the sessions wet from sweat, which proves that I really worked hard.”
The group environment also contributed to the success of the therapy, Dr. Rand says. Often, individuals with stroke are isolated and don’t have a very active social life. This program allowed them to connect with people like themselves, and encourage and support one another’s efforts.
In future studies, she intends to investigate whether these interactive video games will be as effective if they are used independently by patients at home to keep up activity levels — a crucial element of rehabilitation following a stroke.