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Nanostructured Ceramic Coatings Enable the Potential of Thermophotovoltaics

The concept of the thermophotovoltaic (TPV) device has been around for more than 50 years.  In that time, its promise of a theoretical conversion efficiency of over 80 percent has been a tantalizing improvement over the still meek conversion efficiencies found in the average commercially available single-junction, silicon-based solar cells that reach just 15 percent.

Despite their theoretical promise, TPV devices haven’t been able to achieve much higher than 8 percent conversion efficiency. The problem has been that the thermal emitter (one of the two main components that make up a TPV device, the other being the photovoltaic diode) has yet to be made of a material that can withstand the temperatures required to make it effective.

Now, in joint research at Stanford University, the University of Illinois at Urbana Champaign and North Carolina State University, researchers have discovered that if they coat the tungsten typically used in thermal emitters with a nanostructured layer of a ceramic material called hafnium dioxide that they developed, the emitter will withstand extreme temperatures.

The researchers, whose findings were published in the journal Nature Communications (“Three-dimensional self-assembled photonic crystals with high temperature stability for thermal emission modification”), determined that the nanostructured coating enabled thermal emitters to remain stable at temperatures as high as 1400 ºC (2500 ºF). Previous prototypes would fall apart when temperatures reached 1200 ºC. (You can see this destruction in the image [right].)

"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, in a press release.

The essential difference between a TPV and your typical photovoltaic device is the thermal emitter. In  standard photovoltaics, the sun itself serves as the thermal emitter.

Unfortunately, with the sun as the thermal emitter, traditional photovoltaics can't make the most of the available energy because they are designed to take in only infrared light. All of the other, higher-energy light waves emitted from the sun are wasted as heat. The thermal emitter in a TPV device acts as an intermediary between the sun and the photovoltaic diode. It converts the higher-energy light waves into infrared light that the photovoltaic diode can convert into electricity.

"Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell," Fan said in the release.

While the new material the researchers developed can manage to stay stable at high temperatures, they admit that it can’t yet remain stable for the length of time that would make it viable as a component in a commercial device.

The researchers observed that when the ceramic-coated emitters were subjected to temperatures of 1000 ºC, they retained their structural integrity for more than 12 hours. At 1400 ºC, the samples began to break down after about an hour.

While 12 hours and one hour do not exactly conjure up the idea of these devices changing the photovoltaic landscape any time soon, it does open up an avenue that many had not expected was available.

"These results are unprecedented," said former Illinois graduate student Kevin Arpin, lead author of the study, in the press release. "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."

Images: Kevin Arpin

Europe Invests €1 Billion to Become "Graphene Valley"

The European Commission (EC) last week announced a €1 billion ($1.3 billion) investment in graphene research and development that will be spread over 10 years. The aim of the huge funding initiative will be to smooth the path for pushing graphene from the research lab to the marketplace.

The initiative has been dubbed "The Graphene Flagship," and apparently it is the first in a number of €1 billion, 10-year plans the EC is planning to launch. The graphene version will bring together 76 academic institutions and industrial groups from 17 European countries, with an initial 30-month-budget of €54M ($73 million).

Graphene research is still struggling to find any kind of applications that will really take hold, and many don’t expect it will have a commercial impact until 2020. What's more, manufacturing methods are still undeveloped. So it would appear that a 10-year plan is aimed at the academic institutions that form the backbone of this initiative rather than commercial enterprises.

Just from a political standpoint the choice of Chalmers University in Sweden as the base of operations for the Graphene Flagship is an intriguing choice. One has to wonder whether it was a decision arrived at after pragmatic analysis or whether it was just a demonstration of fairness in the awarding of business meted out by the EU.

With Konstantin Novoselov—who shared the Nobel Prize for the discovery of graphene—on the Graphene Flagship board, you can’t help but ask yourself why this wasn’t all set up at the University of Manchester in the UK. (You can see Novoselov speak during the press conference for the project’s launch in the video below.) The UK government already made a $71 million investment last year to create a “Graphene Hub”.  Instead of building on an investment, it appears they just decided to make another “Graphene Hub” somewhere else in Europe.

Of course, I am somewhat dubious that these regionally focused nanotechnology investments will have the desired impact of making the next “Silicon Valley”, or “Graphene Valley” as the case may be.  We have seen what happens when a country provides financial support to the basic research, but turns away as soon as the start-ups generated from the research start to struggle: Those struggling startups get bought up by companies in other countries that don’t support the basic research. The fruits of all that national investment are enjoyed by other countries.

This latest funding will likely result in some new research facilities being built.  To that extent, we know that this will generate some employment. However, I hope over the 10-year span of the initiative, it will become clear that actually supporting the development of graphene applications does not just involve the supporting of the local construction industry, but also includes funding the small businesses out there trying to bring this technology to market.

Photo: Chalmers

Computer Model Shows Carbyne is the Strongest Known Material

Researchers at Rice University have developed computer models that reveal that the long-theorized material carbyne is the strongest material in the world.

The research, which was published in the journal ACS Nano (“Carbyne from First Principles: Chain of C Atoms, a Nanorod or a Nanorope”), demonstrated that carbyne should have the greatest tensile strength of any other known material, double that of graphene which itself is 200 times stronger than steel.

Boris Yakobson, a theoretical physicist at Rice who led the research, has previously demonstrated through computer modeling important possibilities for the use of carbon materials, like graphene. In the case of this carbyne research, as with his research into graphene, it remains to be seen whether the computer models can be duplicated in the physical world.

As the press release covering the research describes it: “Carbyne is a chain of carbon atoms held together by either double or alternating single and triple atomic bonds.” It has been encountered in highly compressed graphite. While there have been some demonstrations of the material being synthesized at room temperature,  it’s not clear how it could be produced in bulk.

If such a method could be developed, the Rice researchers believe that the material could be useful in a range of applications.

“You could look at it as an ultimately thin graphene ribbon, reduced to just one atom, or an ultimately thin nanotube,” said Yakobson in the press release. “It could be useful for nanomechanical systems, in spintronic devices, as sensors, as strong and light materials for mechanical applications or for energy storage.”

While the mere fact that carbyne is the strongest possible assembly of atoms is exciting to the Rice researchers, it will take a bit more research to exploit that strength. It wasn’t until recently that the strength of graphene could be fully exploited in a composite material.

The next step for the researchers will be to investigate the possibility of one-dimensional chains of atoms for other elements.

Image: Vasilii Artyukhov/Rice University

There Is No Nanotechnology Equivalent to the Digital Divide

A recent article in the venerable Financial Times proposed a sort of nanotechnology equivalent to the oft-mentioned “digital divide (the idea that the benefits of digital technology appear to accrue to the wealthy while the poor are left out). At issue in the article is whether developed economies and poorer nations are separated in their respective access to nanotechnology.

So, is there a nanotech divide?

The answer probably lies somewhere between “yes” and “no,” depending on the metric you use. But I would argue that nanotechnology has been one of the most egalitarian fields in technology history.

First, you can look at where the money goes. On this count, nanotechnology is basically the same as any other emerging technology: the initial targeted applications are those that can sustain the price premium for using a new and expensive technology.

However, nanomaterials can already be found in inexpensive items ranging from odor-resistant socks to plastic beer bottles.  So as the manufacturing technologies have matured and ramped up, we have seen prices fall and the democratization of nano-enabled products spread.

But I've often argued that analyzing nanotech's impact purely on economic terms can be misleading. It is usually a stretch to try to draw parallels between an emerging technology like nanotech and a developed field like information technology. Nanotech is still in its commercial infancy.

Instead, let's focus just on nanotechnology research, which constitutes the bulk of the activity in the field today. With nanotech, we are probably witnessing the most democratic and open-access research ever in a new technology.

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Silicon and Graphene: Two Great Materials That Stay Great Together

The use of graphene as a transparent conducting film has been hotly pursued of late, in large part because it offers a potentially cheaper alternative to indium tin oxide (ITO) where a bottleneck of supply seems to be looming.

It has not been clear whether photovoltaic manufacturers have taken any interest in graphene as an alternative for transparent conducting films. This lack of interest may in part be the result of there being little research into whether graphene maintains its attractive characteristic of high carrier mobility when used in conjunction with silicon.

Now researchers at the Helmholtz Zentrum Berlin (HZB) Institute in Germany have shown that graphene does not lose its impressive conductivity characteristics even when mated with silicon.

"We examined how graphene's conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little," said Marc Gluba of the HZB Institute for Silicon Photovoltaics in a press release.

The research, which was published in the journal Applied Physics Letters (“Embedded graphene for large-area silicon-based devices”), used the method of growing the graphene by chemical vapor deposition on a copper sheet and then transferring it to a glass substrate. This was then covered with a thin film of silicon.

The researchers experimented with two different forms of silicon commonly used in thin-film technologies: amorphous silicon and polycrystalline silicon. In both cases, despite completely different morphology of the silicon, the graphene was still detectable.

"That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," said Norbert Nickel, another researcher on the project, in a press release.

In their measurements, the researchers determined that the carrier mobility of the graphene layer was roughly 30 times greater than that of conventional zinc oxide-based contact layers.

Although the researchers concede that connecting the graphene-based contact layer to external contacts is difficult, it has garnered the interest of their thin-film technology colleagues. "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it,” Nickel adds.

Illustration: Marc A. Gluba/HZB

High Quality Graphene Could Soon Be Produced in Bulk

Photo: Graphene Frontiers
A Graphene CVD furnace in operation.

Back in September, the National Science Foundation awarded $744,600 to a University of Pennsylvania graphene start-up called Graphene Frontiers. The money is intended to help the young company ramp up a new manufacturing technique for producing graphene in a roll-to-roll process, which could enable large-scale production of high-quality graphene. The company is attempting to be the first to adapt a chemical vapor deposition (CVD) technique, often used to create polycrystalline single layers of graphene, to a roll-to-roll process.

But not all graphene is created equal. While polycrystalline graphene has found use in applications like advanced composites and RF transistors, it's monocrystalline graphene that has the properties worthy of a “wonder material.” Monocrystalline graphene can still only be produced through what are known as mechanical cleavage techniques, in which the graphene is pulled off in single layer flakes from graphite. (One of these became known as the “Scotch Tape” method). These techniques tend to be unscalable for manufacturing large quantities of the material.

Despite this limitation, there are a host of companies like Graphene Frontiers that are already producing graphene. There are several different manufacturing techniques that are suited for producing varying qualities of graphene.

One method is to start with mined graphite and separate out the carbon layers using plasma, mechanical exfoliation, and chemical techniques. This tends to produce lower quality graphene that could eventually be useful in anti-corrosion paints or battery electrodes.

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Researchers Discover Giant Refractive Index in Graphene Oxide

One of the downsides of optical discs is that they are not particularly durable, leading researchers to look at using quartz glass as a replacement for the typical plastic used. The fact that these discs are fragile has also demanded the rather expensive practice of keeping multiple duplicates at large data centers in order to safeguard against loss of data through breakage.

Now researchers at Swinburne University of Technology in Australia have developed a polymer using graphene oxide that makes it possible to retrieve all the information from the disc even in the case of it breaking.

It should be noted that graphene oxide is different from graphene in a couple of ways. First, the manner in which it is produced is fundamentally different. It is made via a chemical reduction process rather than the mechanical or thermal exfoliation processes used to create single layer graphene. Secondly, it differs significantly in its properties. It lacks graphene's unmatched electron mobility, so it's not the great conductor that graphene is. It does, however, have a property that makes it ideal for optical storage applications: a fundamental fluorescence that is useful for multimode optical recording.

The Swinburne researchers took advantage of that fluorescence, demonstrating that when a laser is focused onto a graphene oxide polymer, the beam increases the material's refractive index by 10 to 100 times. All materials have a refractive index, which is essentially a measurement of the speed of light and how much it bends when it passes through a medium.

“The unique feature of the giant refractive-index modulation together with the fluorescent property of the graphene oxide polymer offers a new mechanism for multimode optical recording," said Professor Min Gu, one of the researchers, in a press release.

In their demonstration, which was published in the journal Scientific Reports (“Giant refractive-index modulation by two-photon reduction of fluorescent graphene oxides for multimode optical recording”), the researchers encoded the image of a kangaroo in a computer-generated hologram. The researchers then rendered the hologram as a three-dimensional recording on the graphene oxide polymer.

One other attribute of the recorded hologram is that it lends itself to secure coding. You can’t actually see the encrypted images of the hologram with a microscope, but they can be seen when the images are diffracted.

"The giant refractive index of this material shows promise for merging data storage with holography for security coding," Professor Gu said in the release. "This exciting feature not only boosts the level of storage security, but also helps to reduce the operation costs of big data centers that rely on multiple physical duplicates to avoid data loss."

The researchers believe that optical data storage is only one application that could be impacted by the technology. They believe it could revolutionize flat screen displays and solar cell technology.

Gu adds: "The giant refractive index we discovered shows the promise of graphene to merge electronics and photonics for the platform of the next generation information technologies."

Image: Swinburne University of Technology

Band-Gap Engineering of Nanowires Could Boost Batteries

The reason for replacing graphite in the electrodes of the ubiquitous lithium-ion (Li-ion) battery is clear to anyone who uses a smartphone: The batteries run out of charge in just a few hours under regular use.

One answer has been to replace the graphite with silicon. Unfortunately, the expanding and contracting that occurred as the lithium ions transported in and out of silicon electrodes quickly cracks it.

The next solution was to create “nanostructured silicon” electrodes, sometimes with the help of graphene or good old carbon nanotubes.

Now researchers at the University of California San Diego (UCSD) have brought a new perspective to the issue. They are  taking a page from band-gap engineering, in which heterostructures are used to create energy barriers between electrons and holes, and applied the concept to creating barriers to the ions as they enter into an electrode so they diffuse in a very specific way.

The research, which was published in the journal Nano Letters (“Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale”), describes a method by which the typical surface diffusion of lithium ions into a nanowire electrode is blocked and instead the ions are diffused layer-by-layer along the length of the nanowire.

As the Nano Letters article notes: “These results demonstrate for the first time that interface and band-gap engineering of electrochemical reactions can be utilized to control the nanoscale ionic transport / insertion paths and thus may be a new tool to define the electrochemical reactions in Li-ion batteries.” 

In the video below you can see the different way this impacts the nanowire. Instead of blowing up around its middle, it gradually grows along its axis as the lithium ions transport into the nanowire. The video also demonstrates some new breakthroughs in nanoscale imaging with transmission electron microscopy. It shows lithium-ion reactions in real time at nanoscale precision.

In the press release about his work, Shadi Dayeh, a professor at UCSD, explains that this control of how the ions diffuse could result in “an effective way to tailor volume expansion of lithium ion battery electrodes, which could potentially minimize their cracking, improve their durability, and perhaps influence how one could think about different electrode architectures.”

The new technology starts with germanium nanowires that are then coated with silicon.The research builds on previous work of Dayeh and his colleagues published in the journal Applied Physics Letters and again in Nano Letters in which they demonstrated control over the heterostructuring of germanium-silicon nanowires.

 According to Dayeh, the new electrodes would allow for battery designs in which the expansion of the electrodes would not cause any shorting between the cathode and the anode.

Images: UC San Diego Electrical and Computer Engineering

Previously Unheralded 2-D Material Exhibits High Charge-Storage Capacity

The landscape for two-dimensional materials has been getting a little crowded lately. We, of course, hear stories every day about graphene, but molybdenum disulfide has been getting a lot of interest and perhaps even edging ahead of graphene in transistor applications. Finally hexagonal boron nitride is gaining attention and rounding out this trilogy of 2-D materials that all promise to displace silicon in future electronics.

But another two-dimensional material has been in our midst for years and has largely gone unnoticed.

Researchers at Drexel University developed the 2-D material three years ago to little fanfare. But now they have discovered that the material has extraordinary energy storage capacity and could power flexible devices.

Michel W. Barsoum and Yury Gogotsi, both professors at Drexel’s college of engineering, dubbed the material an “MXene” because of its origin from the process of etching and exfoliating atomically thin layers of aluminum from layered carbides called “MAX phases.” [The M is for transition metal, the A for "A group" metal, and the X for carbon and/or nitrogen.]

Over the last three years, the engineers have been investigating applications for the materials. The research, which was published in the journal Science (“Cation Intercalation and High Volumetric Capacitance of Two-dimensional Titanium Carbide”), shows that MXenes store ions and molecules between layers of the material in a process known as intercalation.

Intercalation is the inclusion of a molecule between two other molecules. The process is sometimes required in order to give two-dimensional materials their unique properties. In the Drexel research, they were able to place lithium ions between MXene sheets, making the materials good candidates for use in lithium-ion batteries.

“Currently, nine MXenes have been reported by our team, but there are likely many more that will be discovered—the MXene-and-ion combinations that have been tested to date are by no means an exhaustive demonstration of the material’s energy storage capabilities,” said Gogotsi, who is also director of the A.J. Drexel Nanotechnology Institute, in a press release. “So even the impressive capacitances that we are seeing here are probably not the highest possible values to be achieved using MXenes. Intercalation of magnesium and aluminum ions that we observed may also pave the way to development of new kinds of metal ion batteries.”

In addition to lithium, the researchers have been looking at intercalating ions of sodium, magnesium, potassium, ammonium, and aluminum in between the MXene sheets. In each case, the resulting material demonstrated a high capacity for energy storage, they said.

“Two-dimensional, titanium carbide MXene electrodes show excellent volumetric super capacitance of up to 350 F/cm3 due to intercalation of cations between its layers,” Barsoum said. “This capacity is significantly higher than what is currently possible with porous carbon electrodes. In other words, we can now store more energy in smaller volumes, an important consideration as mobile devices get smaller and require more energy”

- See more at: http://www.drexel.edu/now/news-media/releases/archive/2013/September/MXenes-Science/#sthash.PS22z2pL.dpuf

“Two-dimensional, titanium carbide MXene electrodes show excellent volumetric super capacitance of up to 350 F/cm3 due to intercalation of cations between its layers,” Barsoum said. “This capacity is significantly higher than what is currently possible with porous carbon electrodes. In other words, we can now store more energy in smaller volumes, an important consideration as mobile devices get smaller and require more energy”

- See more at: http://www.drexel.edu/now/news-media/releases/archive/2013/September/MXenes-Science/#sthash.PS22z2pL.dpuf

“Two-dimensional, titanium carbide MXene electrodes show excellent volumetric super capacitance of up to 350 F/cm3 due to intercalation of cations between its layers,” Barsoum said. “This capacity is significantly higher than what is currently possible with porous carbon electrodes. In other words, we can now store more energy in smaller volumes, an important consideration as mobile devices get smaller and require more energy”

- See more at: http://www.drexel.edu/now/news-media/releases/archive/2013/September/MXenes-Science/#sthash.PS22z2pL.dpuf

“Two-dimensional, titanium carbide MXene electrodes show excellent volumetric super capacitance of up to 350 [farads per cubic centimeter] due to intercalation of cations between its layers,” Barsoum said in a press release. “This capacity is significantly higher than what is currently possible with porous carbon electrodes. In other words, we can now store more energy in smaller volumes, an important consideration as mobile devices get smaller and require more energy.”

Image: Drexel University

Spray-On Technique Could Bring Carbon Nanotubes to Retailers’ Shelves

Carbon nanotubes appear to be getting back some of their glory—after seemingly being eclipsed by graphene—with the news yesterday that an entire computer could be made from the material.  Now researchers at Technische Universität München (TUM) in Germany are continuing the carbon nanotube comeback with a new, inexpensive process that promises to enable their use in a wide range of applications including electronic skin and sensors integrated into food packaging.

The process, which involves simply spraying the carbon nanotubes onto a flexible, plastic substrate, is described in the journal Carbon (“Fabrication of carbon nanotube thin films on flexible substrates by spray deposition and transfer printing”)

"To us it was important to develop an easily scalable technology platform for manufacturing large-area printed and flexible electronics based on organic semiconductors and nanomaterials," said postdoctoral researcher Alaa Abdellah in a press release. "To that end, spray deposition forms the core of our processing technology."

In the food packaging application, the carbon nanotubes would serve as gas sensors. Nanotubes have long proven themselves to be good sensors. They can be made to bind to certain substances, which changes the electrical properties of the nanotubes in a way that can be measured with a high degree of sensitivity.

In practical applications, the carbon-nanotube-enabled plastic would cover a grocery store product like chicken. If the product contained the chemical indicators of, say, salmonella, the packaging would detect them and alert the consumer.

While this sounds great, the obstacle preventing this from becoming a reality has always been cost. Thin-film sensory packaging may make sense for a high-cost item, but for an inexpensive grocery store product, it’s hard to justify an additional cost that may be as much as the product itself. I made this point nearly a decade ago in report I authored titled, "The Future of Nanotechnology in Printing and Packaging".

This doesn’t even take into account the often biased opinion people have about nanotechnology in relation to food.

For these reasons, I recommend that the researchers focus their attention on high-ticket applications such as electronic skin for robotics and bionics. In those applications, they will likely find greater resistance to the fear mongering practiced by the NGOs. And it just might make more economic sense to adopt their technology for those applications.

Image: Uli Benz/TUM

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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