Better fighter planes, space shuttles on the way, thanks to new research


FULL STORY

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A team of scientists led by Changhong Ke, associate professor of mechanical engineering at Binghamton University’s Thomas J. Watson School of Engineering and Applied Science, and researcher Xiaoming Chen were the first to determine the interface strength between boron nitride nanotubes (BNNTs) and epoxy and other polymers.

“We think that this could be the first step in a process that changes the way we design and make materials that affect the future of travel on this planet and exploration of other worlds beyond our own,” said Ke. “Those materials may be way off still, but someone needed to take the first step, and we have.”

 

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Ke’s group found that BNNTs in polymethyl metacrylate (PMMA) form much stronger interfaces than comparable carbon tubes with the same polymer. Furthermore, BNNT-epoxy interfaces are even stronger. A stronger interface means that a larger load can be transferred from the polymer to nanotubes, a critical characteristic for superior mechanical performance of composite materials. Future airplane wings and spacecraft hulls built of those BNNT composite materials could be lighter and more fuel efficient, while maintaining the strength needed to withstand the rigors of flight.

Since nanotube wall thickness and diameters are measured in billionths of a meter, Ke and Chen extracted single BNNTs from a piece of epoxy and then repeated the process with PMMA inside an electron microscope. Their conclusions were based on the amount of force needed to do the extractions. This was the first time that BNNTs –more chemically and thermally stable than the more common carbon nanotubes (CNTs) –were in this kind of experiment. BNNTs can shield space radiation better than CNTs, which would make them an ideal building material for spacecraft.

“They are both light and strong,” Ke said of the two kinds of tubes. “They have similar mechanical properties, but different electrical properties. Those differences help to add strength to the BNNT interfaces with the polymers.”

Metaphorically, think of the epoxy or other polymer materials with the BNNT nanotubes inside like a piece of reinforced concrete. That concrete gets much of its strength from the makeup of the steel rebar and cement; the dispersion of rebar within the cement; the alignment of rebar within the cement; and “stickiness” of the connection between the rebar and the surrounding cement. The scientists essentially measured the “stickiness” of a single nanotube ‘rebar’ — helped by molecular and electrostatic interactions — by removing it from polymer “cement.”

The work was funded by the US Air Force Office of Scientific Research — Low Density Materials program, with materials provided by NASA. Co-authors Xianqiao Wang and graduate student Liuyang Zhang from the University of Georgia provided verification and explanation data through computational simulations after the experiments were conducted in Binghamton.

Catharine Fay from the NASA Langley Research Center and Cheol Park of the Center and the University of Virginia are co-authors on the paper.

In September, Ke and his collaborators received three years of additional funding totaling $815,000 from the Air Force to continue research.

The paper, “Mechanical Strength of Boron Nitride Nanotube-Polymer Interfaces,” was published in the latest issue of Applied Physics Letters.


Story Source:

The above post is reprinted from materials provided by Binghamton University. Note: Materials may be edited for content and length.


Journal Reference:

  1. Xiaoming Chen, Liuyang Zhang, Cheol Park, Catharine C. Fay, Xianqiao Wang, Changhong Ke. Mechanical strength of boron nitride nanotube-polymer interfaces. Applied Physics Letters, 2015; 107 (25): 253105 DOI: 10.1063/1.4936755

Magnetic nanoparticles enhance performance of solar cells: Technical University of Munich


1-magneticnanoMagnetic nanoparticles can increase the performance of solar cells made from polymers – provided the mix is right. This is the result of an X-ray study at DESY’s synchrotron radiation source PETRA III. Adding about one per cent of such nanoparticles by weight makes the solar cells more efficient, according to the findings of a team of scientists headed by Prof. Peter Müller-Buschbaum from the Technical University of Munich. They are presenting their study in one of the upcoming issues of the journal Advanced Energy Materials (published online in advance).

Polymer, or organic, solar cells offer tremendous potential: They are inexpensive, flexible and extremely versatile. Their drawback compared with established is their lower efficiency. Typically, they only convert a few per cent of the incident light into electrical power. Nevertheless, organic solar cells are already economically viable in many situations, and scientists are looking for new ways to increase their efficiency.

One promising method is the addition of nanoparticles. It has been shown, for example, that absorb additional sunlight, which in turn produces additional electrical charge carriers when the energy is released again by the gold particles.

Müller-Buschbaum’s team has been pursuing a different approach, however. “The light creates pairs of charge carriers in the solar cell, consisting of a negatively charged electron and a positively charged hole, which is a site where an electron is missing,” explains the main author of the current study, Daniel Moseguí González from Müller-Buschbaum’s group. “The art of making an organic solar cell is to separate this electron-hole pair before they can recombine. If they did, the charge produced would be lost. We were looking for ways of extending the life of the electron-hole pair, which would allow us to separate more of them and direct them to opposite electrodes.”

Crystalline structures within polymer solar cells cause characteristic diffraction patterns in experiments with synchrotron radiation. Credit: Credit: TU München

This strategy makes use of a quantum physical principle which states that electrons have a kind of internal rotation, known as spin. According to the laws of quantum physics, this spin has a value of 1/2. The positively charged hole also has a spin of 1/2. The two spins can either add up, if they are in the same direction, or cancel each other out if they are in opposite directions. The electron-hole pair can therefore have an overall spin of 0 or 1. Pairs with a spin of 1 exist for longer than those with an overall spin of 0.

The researchers set out to find a material that was able to convert the spin 0 state into a spin 1 state. This required nanoparticles of heavy elements, which flip the of the electron or the hole so that the spins of the two particles are aligned in the same direction. The iron oxide magnetite (Fe3O4) is in fact able to do just this. “In our experiment, adding magnetite nanoparticles to the substrate increased the efficiency of the solar cells by up to 11 per cent,” reports Moseguí González. The lifetime of the electron-hole pair is significantly prolonged.

Adding nanoparticles is a routine procedure which can easily be carried out in the course of the various methods for manufacturing organic solar cells. It is important, however, not to add too many nanoparticles to the solar cell, because the internal structure of organic solar cells is finely adjusted to optimise the distance between the light-collecting, active materials, so that the pairs of charge carriers can be separated as efficiently as possible. These structures lie in the range of 10 to 100 nanometres.

“The X-ray investigation shows that if you mix a large number of nanoparticles into the material used to make the solar cell, you change its structure”, explains co-author Dr. Stephan Roth, head of DESY’s beam line P03 at PETRA III, where the experiments were conducted. “The solar cell we looked at will tolerate magnetite nanoparticle doping levels of up to one per cent by mass without changing their structure.”

The scientists observed the largest effect when they doped the substrate with 0.6 per cent nanoparticles by weight. This caused the efficiency of the polymer solar cell examined to increase from 3.05 to 3.37 per cent. “An 11 percent increase in energy yield can be crucial in making a material economically viable for a particular application,” emphasises Müller-Buschbaum.

The researchers believe it will also be possible to increase the efficiency of other by doping them with nanoparticles. “The combination of high-performance polymers with holds the promise of further increases in the efficiency of organic in the future. However, without a detailed examination, such as that using the X-rays emitted by a synchrotron, it would be impossible to gain a fundamental understanding of the underlying processes involved,” concludes Müller-Buschbaum.

 Explore further: New technique helps probe performance of organic solar cell materials

More information: Advanced Energy Materials, 2015; DOI: 10.1002/aenm.201401770

New Catalyst Process Uses Light, Not Metal, for Rapid Polymerization


UC SB Polymer-Hawker-Fors-deAlaniz-Dec2014-webresUC Santa Barbara researchers develop a metal-free Atom Transfer Radical Polymerization process that uses an organic-based photocatalyst ~ January 13, 2015

A team of chemistry and materials science experts from University of California, Santa Barbara and The Dow Chemical Company has created a novel way to overcome one of the major hurdles preventing the widespread use of controlled radical polymerization.

In a global polymer industry valued in the hundreds of billions, a technique called Atom Transfer Radical Polymerization is emerging as a key process for creating well-defined polymers for a vast range of materials, from adhesives to electronics. However, current ATRP methods by design use metal catalysts, a major roadblock to applications for which metal contamination is an issue, such as materials used for biomedical purposes.

This new method of radical polymerization doesn’t involve heavy metal catalysts like copper. Their innovative, metal-free ATRP process uses an organic-based photocatalyst–and light as the stimulus for the highly controlled chemical reaction.

“The grand challenge in ATRP has been: how can we do this without any metals?” said Craig Hawker, Director of the Dow Materials Institute at UC Santa Barbara. “We looked toward developing an organic catalyst that is highly reducing in the excited state, and we found it in an easily prepared catalyst, phenothiazine.”

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“It’s “drop-in” technology for industry,” said Javier Read de Alaniz, principal investigator and professor of chemistry and biochemistry at UC Santa Barbara. “People are already used to the same starting materials for ATRP, but now we have the ability to do it without copper.” Copper, even at trace levels, is a problem for microelectronics because it acts as a conductor, and for biological applications because of its toxicity to organisms and cells.

Read de Alaniz, Hawker, and postdoctoral research Brett Fors, now with Cornell University, led the study that was initially inspired by a photoreactive Iridium catalyst. Their study was recently detailed in a paper titled “Metal-Free Atom Transfer Radical Polymerization,” published in the Journal of the American Chemical Society. The research was made possible by support from Dow, a research partner of the UCSB College of Engineering.

ATRP is already used widely across dozens of major industries, but the new metal-free rapid polymerization process “pushes controlled radical polymerization into new areas and new applications,” according to Hawker. “Many processes in use today all start with ATRP. Now this method opens doors for a new class of organic-based photoredox catalysts.”

Controlling radical polymerization processes is critical for the synthesis of functional block polymers. As a catalyst, phenothiazine builds block copolymers in a sequential manner, achieving high chain-end fidelity. This translates into a high degree of versatility in polymer structure, as well as an efficient process.

“Our process doesn’t need heat. You can do this at room temperature with simple LED lights,” said Hawker. “We’ve had success with a range of vinyl monomers, so this polymerization strategy is useful on many levels.”

“The development of living radical processes, such as ATRP, is arguably one of the biggest things to happen in polymer chemistry in the past few decades,” he added. “This new discovery will significantly further the whole field.”

Two sensors in one: Nanoparticles that enable both MRI and fluorescent imaging could monitor cancer, other diseases


12-Sensors 141118125600-largeNovember 18, 2014 Source: Massachusetts Institute of Technology

MIT chemists have developed new nanoparticles that can simultaneously perform magnetic resonance imaging (MRI) and fluorescent imaging in living animals. Such particles could help scientists to track specific molecules produced in the body, monitor a tumor’s environment, or determine whether drugs have successfully reached their targets.

In a paper appearing in the Nov. 18 issue of Nature Communications, the researchers demonstrate the use of the particles, which carry distinct sensors for fluorescence and MRI, to track vitamin C in mice. Wherever there is a high concentration of vitamin C, the particles show a strong fluorescent signal but little MRI contrast. If there is not much vitamin C, a stronger MRI signal is visible but fluorescence is very weak.

Future versions of the particles could be designed to detect reactive oxygen species that often correlate with disease, says Jeremiah Johnson, an assistant professor of chemistry at MIT and senior author of the study. They could also be tailored to detect more than one molecule at a time.

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MIT chemists have developed new nanoparticles that can simultaneously perform magnetic resonance imaging (MRI) and fluorescent imaging in living animals.
Credit: Illustration by Christine Daniloff/MIT

“You may be able to learn more about how diseases progress if you have imaging probes that can sense specific biomolecules,” Johnson says.

Dual action

Johnson and his colleagues designed the particles so they can be assembled from building blocks made of polymer chains carrying either an organic MRI contrast agent called a nitroxide or a fluorescent molecule called Cy5.5.

When mixed together in a desired ratio, these building blocks join to form a specific nanosized structure the authors call a branched bottlebrush polymer. For this study, they created particles in which 99 percent of the chains carry nitroxides, and 1 percent carry Cy5.5.

Nitroxides are reactive molecules that contain a nitrogen atom bound to an oxygen atom with an unpaired electron. Nitroxides suppress Cy5.5’s fluorescence, but when the nitroxides encounter a molecule such as vitamin C from which they can grab electrons, they become inactive and Cy5.5 fluoresces.

Nitroxides typically have a very short half-life in living systems, but University of Nebraska chemistry professor Andrzej Rajca, who is also an author of the new Nature Communications paper, recently discovered that their half-life can be extended by attaching two bulky structures to them. Furthermore, the authors of the Nature Communications paper show that incorporation of Rajca’s nitroxide in Johnson’s branched bottlebrush polymer architectures leads to even greater improvements in the nitroxide lifetime. With these modifications, nitroxides can circulate for several hours in a mouse’s bloodstream — long enough to obtain useful MRI images.

The researchers found that their imaging particles accumulated in the liver, as nanoparticles usually do. The mouse liver produces vitamin C, so once the particles reached the liver, they grabbed electrons from vitamin C, turning off the MRI signal and boosting fluorescence. They also found no MRI signal but a small amount of fluorescence in the brain, which is a destination for much of the vitamin C produced in the liver. In contrast, in the blood and kidneys, where the concentration of vitamin C is low, the MRI contrast was maximal.

Mixing and matching

The researchers are now working to enhance the signal differences that they get when the sensor encounters a target molecule such as vitamin C. They have also created nanoparticles carrying the fluorescent agent plus up to three different drugs. This allows them to track whether the nanoparticles are delivered to their targeted locations.

“That’s the advantage of our platform — we can mix and match and add almost anything we want,” Johnson says.

These particles could also be used to evaluate the level of oxygen radicals in a patient’s tumor, which can reveal valuable information about how aggressive the tumor is.

“We think we may be able to reveal information about the tumor environment with these kinds of probes, if we can get them there,” Johnson says. “Someday you might be able to inject this in a patient and obtain real-time biochemical information about disease sites and also healthy tissues, which is not always straightforward.”

Steven Bottle, a professor of nanotechnology and molecular science at Queensland University of Technology, says the most impressive element of the study is the combination of two powerful imaging techniques into one nanomaterial.

“I believe this should deliver a very powerful, metabolically linked, multi-combination imaging modality which should provide a highly useful diagnostic tool with real potential to follow disease progression in vivo,” says Bottle, who was not involved in the study.

The research was funded by the National Institutes of Health, the Department of Defense, the National Science Foundation, and the Koch Institute for Integrative Cancer Research.


Story Source:

The above story is based on materials provided by Massachusetts Institute of Technology. The original article was written by Anne Trafton. Note: Materials may be edited for content and length.


Journal Reference:

  1. Molly A. Sowers, Jessica R. McCombs, Ying Wang, Joseph T. Paletta, Stephen W. Morton, Erik C. Dreaden, Michael D. Boska, M. Francesca Ottaviani, Paula T. Hammond, Andrzej Rajca, Jeremiah A. Johnson. Redox-responsive branched-bottlebrush polymers for in vivo MRI and fluorescence imaging. Nature Communications, 2014; 5: 5460 DOI: 10.1038/ncomms6460

Fully 3D-printed quantum dot LEDs


1-3D LED Print id37985_1By Michael Berger – Nanowerk

To date, the 3D printing of electronic components has been limited to the printing of batteries, strain sensors, interdigitated-electrode capacitors and passive metallic structures such as interconnects and antennas on surfaces or within biological organs.

The ability to directly and seamlessly incorporate materials with a range of diverse functionalities with 3D printing is particularly attractive as it could allow the simultaneous, comprehensive, and direct printing of structural, biological, and electronic materials that capture the complete spectra of material properties. The free-form generation of active electronics in unique architectures which transcend the planarity inherent to conventional microfabrication techniques has been an area of increasing scientific interest. Yet, attaining seamless interweaving of electronics is challenging due to the inherent material incompatibilities and geometrical constraints of traditional micro-fabrication processing techniques.

At the fundamental level, 3D printing should be entirely capable of creating spatially heterogeneous multi-material structures by dispensing a wide range of material classes with disparate viscosities and functionalities, including semiconducting colloidal nanomaterials, elastomeric matrices, organic polymers, and liquid and solid metals. “The big push in 3D printing these days is to try to print two or more polymers at once,” Michael McAlpine, an assistant professor of mechanical and aerospace engineering at Princeton University, tells Nanowerk. “In our latest research, we go way beyond that. We show that we can print interwoven structures of quantum dots, polymers, metal nanoparticles, etc, to create the first fully 3D printed LEDs, in which every component is 3D printed.” This demonstration represents a proof of concept in combining active nanoelectronic components with the versatility of 3D printing, which enables the three-dimensional free-form fabrication of active electronics.

3D printed quantum dot light-emitting diode (QLED)

3D printed quantum dot light-emitting diode (QLED) on a 3D scanned curvilinear substrate. This CAD model shows the QD-LED components and conformal integration onto the curvilinear substrate. (Reprinted with permission by American Chemical Society)

McAlpine and his team published their findings in Nano Letters (“3D Printed Quantum Dot Light-Emitting Diodes”). “Using this approach, we can create unique structures, such as 2x2x2 arrays of LEDs, in which the electrical wiring runs horizontal and vertical, to create a multi-color 3D stack of LEDs,” notes Yong Lin Kong, a graduate student in McAlpine’s group who led this project and first author of the paper. “We also use 3D scanning to carefully scan a contact lens and store the specific topology of that lens, and then alter our 3D printing to adjust to that topology, allowing us to conformally 3D print LEDs on a contact lens.

This may have use in electronic contact lens or bionic eye applications in the future.” “This work outlines an exciting breakthrough that enables the direct printing of functional, embedded, active 3D nanoelectronics using only a 3D printer,” he adds. “Indeed, this is the first time to our knowledge that semiconducting nanoparticles have been 3D printed, and the first time that such a broad array of diverse functional materials have been fully interwoven entirely using a 3D printer.” The team’s approach consists of three key steps. First, it identifies electrodes, semiconductors, and polymers that possess desired functionalities and exist in printable formats.

Next, care is taken to ensure that these materials are dissolved in orthogonal solvents so as not to compromise the integrity of underlying layers during the layer-by-layer printing process. Finally, the interwoven patterning of these materials is achieved via direct dispensing in a CAD-designed construct. As a proof of concept of this approach, the researchers demonstrate the 3D printing of quantum dot light-emitting diodes (QLEDs), which involves the design, integration and printing of five classes of materials with distinct material properties. “Specifically, we demonstrate the seamless interweaving of 1) emissive semiconducting inorganic nanoparticles; 2) an elastomeric matrix; 3) organic polymers as charge transport layers; 4) solid and liquid metal leads; and 5) a UV-adhesive transparent substrate layer,” explains Kong. “The printed QLEDs exhibit excellent performance characteristics. The combination of 3D scanning and 3D printing allows for the direct printing of active functional electronics onto the precise topology of a non-flat object.”

3D printed quantum dot light-emitting diode (QLED)

3D printed 2×2×2 multidimensional array of embedded QD-LEDs. (A) Layout of the multi-color 3D QD-LED array design. (Image: McAlpine Group)

He points out that, most excitingly, this approach allows for the free-form fabrication of multi-dimensional nanoelectronics within a complex, interwoven architecture such as a 3D array of embedded QLEDs. The QLEDs printed by McAlpine’s team capture the unique properties of quantum dots: tunable and pure color emission. Further, combining a complementary 3D light-scanning technique with this approach allows for the fabrication of electronics topographically tailored to curvilinear surfaces. “We anticipate that this general strategy can be expanded to 3D print other classes of active devices, such as MEMS devices, transistors, solar cells, and photodiodes,” says McAlpine. “Our results suggest a number of exciting applications, including the generation of geometrically tailored devices containing LEDs and multimodal sensors to provide a new tool for optogenetics for studying neural circuitry.”

Co-printing of active electronics with biological constructs could also lead to new bionic devices, such as prosthetic implants that optically stimulate nerve cells. According to the team, future work will address a number of key challenges. These include: 1) increasing the resolution of the 3D printer such that smaller devices can be printed; 2) improving the performance and yield of the printed devices; and 3) incorporating other classes of nanoscale functional building blocks and devices, including semiconductor, plasmonic, and ferroelectric.