Oak Ridge, TN, United States
Oak Ridge, TN, United States

Time filter

Source Type

News Article | March 25, 2016
Site: www.nanotech-now.com

Abstract: -Your car's bumper is probably made of a moldable thermoplastic polymer called ABS, shorthand for its acrylonitrile, butadiene and styrene components. Light, strong and tough, it is also the stuff of ventilation pipes, protective headgear, kitchen appliances, Lego bricks and many other consumer products. Useful as it is, one of its drawbacks is that it is made using chemicals derived from petroleum. Now, researchers at the Department of Energy's Oak Ridge National Laboratory have made a better thermoplastic by replacing styrene with lignin, a brittle, rigid polymer that, with cellulose, forms the woody cell walls of plants. In doing so, they have invented a solvent-free production process that interconnects equal parts of nanoscale lignin dispersed in a synthetic rubber matrix to produce a meltable, moldable, ductile material that's at least ten times tougher than ABS. The resulting thermoplastic--called ABL for acrylonitrile, butadiene, lignin--is recyclable, as it can be melted three times and still perform well. The results, published in the journal Advanced Functional Materials, may bring cleaner, cheaper raw materials to diverse manufacturers. "The new ORNL thermoplastic has better performance than commodity plastics like ABS," said senior author Amit Naskar in ORNL's Materials Science and Technology Division, who along with co-inventor Chau Tran has filed a patent application for the process to make the new material. "We can call it a green product because 50 percent of its content is renewable, and technology to enable its commercial exploitation would reduce the need for petrochemicals." The technology could make use of the lignin-rich biomass byproduct stream from biorefineries and pulp and paper mills. With the prices of natural gas and oil dropping, renewable fuels can't compete with fossil fuels, so biorefineries are exploring options for developing other economically viable products. Among cellulose, hemicellulose and lignin, the major structural constituents of plants, lignin is the most commercially underutilized. The ORNL study aimed to use it to produce, with an eye toward commercialization, a renewable thermoplastic with properties rivaling those of current petroleum-derived alternatives. To produce an energy-efficient method of synthesizing and extruding high-performance thermoplastic elastomers based on lignin, the ORNL team needed to answer several questions: Can variations in lignin feedstocks be overcome to make a product with superior performance? Can lignin integrate into soft polymer matrices? Can the chemistry and physics of lignin-derived polymers be understood to enable better control of their properties? Can the process to produce lignin-derived polymers be engineered? "Lignin is a very brittle natural polymer, so it needs to be toughened," explained Naskar, leader of ORNL's Carbon and Composites group. A major goal of the group is producing industrial polymers that are strong and tough enough to be deformed without fracturing. "We need to chemically combine soft matter with lignin. That soft matrix would be ductile so that it can be malleable or stretchable. Very rigid lignin segments would offer resistance to deformation and thus provide stiffness." All lignins are not equal in terms of heat stability. To determine what type would make the best thermoplastic feedstock, the scientists evaluated lignin from wheat straw, softwoods like pine and hardwoods like oak. They found hardwood lignin is the most thermally stable, and some types of softwood lignins are also melt-stable. Next, the researchers needed to couple the lignin with soft matter. Chemists typically accomplish this by synthesizing polymers in the presence of solvents. Because lignin and a synthetic rubber containing acrylonitrile and butadiene, called nitrile rubber, both have chemical groups in which electrons are unequally distributed and therefore likely to interact, Naskar and Chau Tran (who performed melt-mixing and characterization experiments) instead tried to couple the two in a melted phase without solvents. In a heated chamber with two rotors, the researchers "kneaded" a molten mix of equal parts powdered lignin and nitrile rubber. During mixing, lignin agglomerates broke into interpenetrating layers or sheets of 10 to 200 nanometers that dispersed well in and interacted with the rubber. Without the proper selection of a soft matrix and mixing conditions, lignin agglomerates are at least 10 times larger than those obtained with the ORNL process. The product that formed had properties of neither lignin nor rubber, but something in between, with a combination of lignin's stiffness and nitrile rubber's elasticity. By altering the acrylonitrile amounts in the soft matrix, the researchers hoped to improve the material's mechanical properties further. They tried 33, 41 and 51 percent acrylonitrile and found 41 percent gave an optimal balance between toughness and stiffness. Next, the researchers wanted to find out if controlling the processing conditions could improve the performance of their polymer alloy. For example, 33 percent acrylonitrile content produced a material that was stretchy but not strong, behaving more like rubber than plastic. At higher proportions of acrylonitrile, the researchers saw the materials strengthen because of the efficient interaction between the components. They also wanted to know at what temperature the components should be mixed to optimize the material properties. They found heating components between 140 and 160 degrees Celsius formed the desired hybrid phase. Using resources at ORNL including the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility, the scientists analyzed the morphologies of the blends. Scanning electron microscopy, performed by Chau Tran, explored the surfaces of the materials. Jihua Chen and Tran characterized soft matter phases using transmission electron microscopy, placing a thin slice of material in the path of an electron beam to reveal structure through contrast differences in the lignin and rubber phases. Small-angle x-ray scattering by Jong Keum revealed repeated clusters of certain domain or layer sizes. Fourier transform infrared spectroscopy identified chemical functional groups and their interactions. Future studies will explore different feedstocks, particularly those from biorefineries, and correlations among processing conditions, material structure and performance. Investigations are also planned to study the performance of ORNL's new thermoplastic in carbon-fiber-reinforced composites. "More renewable materials will probably be used in the future," Naskar said. "I'm glad that we could continue work in renewable materials, not only for automotive applications but even for commodity usage." ### ORNL's Technology Innovation Program, which reinvests royalties from the lab's patents in innovative, commercially promising projects, sponsored the study. The researchers conducted polymer characterization experiments (microscopy and X-ray scattering) at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. About Oak Ridge National Laboratory UT-Battelle manages ORNL for DOE's Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time.--by Dawn Levy For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article | March 23, 2016
Site: www.rdmag.com

Your car's bumper is probably made of a moldable thermoplastic polymer called ABS, shorthand for its acrylonitrile, butadiene and styrene components. Light, strong and tough, it is also the stuff of ventilation pipes, protective headgear, kitchen appliances, Lego bricks and many other consumer products. Useful as it is, one of its drawbacks is that it is made using chemicals derived from petroleum. Now, researchers at the Department of Energy's Oak Ridge National Laboratory have made a better thermoplastic by replacing styrene with lignin, a brittle, rigid polymer that, with cellulose, forms the woody cell walls of plants. In doing so, they have invented a solvent-free production process that interconnects equal parts of nanoscale lignin dispersed in a synthetic rubber matrix to produce a meltable, moldable, ductile material that's at least ten times tougher than ABS. The resulting thermoplastic—called ABL for acrylonitrile, butadiene, lignin—is recyclable, as it can be melted three times and still perform well. The results, published in the journal Advanced Functional Materials, may bring cleaner, cheaper raw materials to diverse manufacturers. "The new ORNL thermoplastic has better performance than commodity plastics like ABS," said senior author Amit Naskar in ORNL's Materials Science and Technology Division, who along with co-inventor Chau Tran has filed a patent application for the process to make the new material. "We can call it a green product because 50 percent of its content is renewable, and technology to enable its commercial exploitation would reduce the need for petrochemicals." The technology could make use of the lignin-rich biomass byproduct stream from biorefineries and pulp and paper mills. With the prices of natural gas and oil dropping, renewable fuels can't compete with fossil fuels, so biorefineries are exploring options for developing other economically viable products. Among cellulose, hemicellulose and lignin, the major structural constituents of plants, lignin is the most commercially underutilized. The ORNL study aimed to use it to produce, with an eye toward commercialization, a renewable thermoplastic with properties rivaling those of current petroleum-derived alternatives. To produce an energy-efficient method of synthesizing and extruding high-performance thermoplastic elastomers based on lignin, the ORNL team needed to answer several questions: Can variations in lignin feedstocks be overcome to make a product with superior performance? Can lignin integrate into soft polymer matrices? Can the chemistry and physics of lignin-derived polymers be understood to enable better control of their properties? Can the process to produce lignin-derived polymers be engineered? "Lignin is a very brittle natural polymer, so it needs to be toughened," explained Naskar, leader of ORNL's Carbon and Composites group. A major goal of the group is producing industrial polymers that are strong and tough enough to be deformed without fracturing. "We need to chemically combine soft matter with lignin. That soft matrix would be ductile so that it can be malleable or stretchable. Very rigid lignin segments would offer resistance to deformation and thus provide stiffness." All lignins are not equal in terms of heat stability. To determine what type would make the best thermoplastic feedstock, the scientists evaluated lignin from wheat straw, softwoods like pine and hardwoods like oak. They found hardwood lignin is the most thermally stable, and some types of softwood lignins are also melt-stable. Next, the researchers needed to couple the lignin with soft matter. Chemists typically accomplish this by synthesizing polymers in the presence of solvents. Because lignin and a synthetic rubber containing acrylonitrile and butadiene, called nitrile rubber, both have chemical groups in which electrons are unequally distributed and therefore likely to interact, Naskar and Chau Tran (who performed melt-mixing and characterization experiments) instead tried to couple the two in a melted phase without solvents. In a heated chamber with two rotors, the researchers "kneaded" a molten mix of equal parts powdered lignin and nitrile rubber. During mixing, lignin agglomerates broke into interpenetrating layers or sheets of 10 to 200 nanometers that dispersed well in and interacted with the rubber. Without the proper selection of a soft matrix and mixing conditions, lignin agglomerates are at least 10 times larger than those obtained with the ORNL process. The product that formed had properties of neither lignin nor rubber, but something in between, with a combination of lignin's stiffness and nitrile rubber's elasticity. By altering the acrylonitrile amounts in the soft matrix, the researchers hoped to improve the material's mechanical properties further. They tried 33, 41 and 51 percent acrylonitrile and found 41 percent gave an optimal balance between toughness and stiffness. Next, the researchers wanted to find out if controlling the processing conditions could improve the performance of their polymer alloy. For example, 33 percent acrylonitrile content produced a material that was stretchy but not strong, behaving more like rubber than plastic. At higher proportions of acrylonitrile, the researchers saw the materials strengthen because of the efficient interaction between the components. They also wanted to know at what temperature the components should be mixed to optimize the material properties. They found heating components between 140 and 160 degrees Celsius formed the desired hybrid phase. Using resources at ORNL including the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility, the scientists analyzed the morphologies of the blends. Scanning electron microscopy, performed by Chau Tran, explored the surfaces of the materials. Jihua Chen and Tran characterized soft matter phases using transmission electron microscopy, placing a thin slice of material in the path of an electron beam to reveal structure through contrast differences in the lignin and rubber phases. Small-angle x-ray scattering by Jong Keum revealed repeated clusters of certain domain or layer sizes. Fourier transform infrared spectroscopy identified chemical functional groups and their interactions. Future studies will explore different feedstocks, particularly those from biorefineries, and correlations among processing conditions, material structure and performance. Investigations are also planned to study the performance of ORNL's new thermoplastic in carbon-fiber-reinforced composites. "More renewable materials will probably be used in the future," Naskar said. "I'm glad that we could continue work in renewable materials, not only for automotive applications but even for commodity usage." The title of the paper is "A New Class of Renewable Thermoplastics with Extraordinary Performance from Nanostructured Lignin-Elastomers."


News Article | April 20, 2016
Site: www.cemag.us

Scientists at the U.S. Naval Research Laboratory (NRL) have devised a clever combination of materials — when used during the thin-film growth process — to reveal that particle atomic layer deposition, or p-ALD, deposits a uniform nanometer-thick shell on core particles regardless of core size, a discovery having significant impacts for many applications since most large-scale powder production techniques form powder batches that are made up of a range of particles sizes. "Particle atomic layer deposition is highlighted as a technology that can create new and exciting designer core/shell particles to be used as building blocks for the next generation of complex multifunctional nanocomposites," says Dr. Boris Feygelson, research engineer, NRL Electronics Science and Technology Division. "Our work is important because shell-thickness is most often a crucial parameter in applications where core-shell materials can be used to enhance performance of future materials." Atomic layer deposition is a layer-by-layer chemical vapor deposition-based thin-film growth technique used extensively in the electronics industry to deposit nanometer-thick films of dielectric materials on devices. Combined with other deposition and shadowing masking techniques, ALD is an integral part of electronic chip and device manufacturing. The same gas-phase process can be applied in a rotary or fluidizing powder bed reactor to grow nanometer-thick films that are highly conformal and uniformly thick on individual particles. Previous research on p-ALD, patented by ALD NanoSolutions Inc., has shown that growth of each layer during the deposition process varies with particle size, with the underlying assumption that larger particles will always have less growth. To observe this growth phenomenon, the NRL team grew alumina on nano- and micron-sized particles of tungsten and measured the shell thickness in a transmission electron microscope. Because of the huge mass/density difference of the two materials, this pairing provides maximum contrast in the electron microscope and delineation was easily distinguishable between the particle core and shell. In their research, the scientists created core and shell powders consisting of a tungsten particle core and thin alumina shell that were then synthesized using atomic layer deposition in a rotary reactor. Standard atomic layer deposition of trimethylaluminum and water was performed on varying batches of powder with different average particle sizes. "Amazingly, we found that the growth per cycle of the alumina film on an individual particle in a batch was shown to be independent of the size of an individual particle, and therefore, a powder batch — which consists of particles sizes spanning orders of magnitude — has constant shell thicknesses on all particles. This result upsets the current understanding of ALD on particles," says Dr. Kedar Manandhar, ASEE postdoc, NRL Electronics Science and Technology Division and leading author of the research paper. The work, published recently in the Journal of Vacuum Science and Technology A, suggests that water, a reactant in the ALD process, is reason for the same rate of growth on different particles. This uniformity of thickness on different particle sizes in a particular batch is determined to be due to the difficulty of removing residual water molecules from the powder during the purging cycle of the atomic layer deposition (ALD) process. "Water is very sticky and it is very difficult to remove the last mono-layer from surfaces," Feygelson says. "And when you have a tumbling bed of powders, the water sticks around between the particles and results in consistent shell growth in the tumbling powder. Applications for this research demonstrate implications for use in materials like abrasion resistant paints, high surface area catalyst, electron tunneling barriers, ultra-violet adsorption or capture in sunscreens or solar cells and even beyond when core-shell nanoparticles are used as buildings blocks for making new artificial nanostructured solids with unprecedented properties. This research is a cross-disciplinary effort at NRL between the Materials Science and Technology Division and Electronics Science and Technology Division. The authors of the paper gratefully acknowledge Drs. Dev Palmer (Defense Advanced Research Projects Agency), Baruch Levush (NRL), and Fritz Kub (NRL). Source: U.S. Naval Research Laboratory


News Article | April 20, 2016
Site: www.nrl.navy.mil

Scientists at the U.S. Naval Research Laboratory (NRL) have devised a clever combination of materials - when used during the thin-film growth process - to reveal that particle atomic layer deposition, or p-ALD, deposits a uniform nanometer-thick shell on core particles regardless of core size, a discovery having significant impacts for many applications since most large scale powder production techniques form powder batches that are made up of a range of particles sizes. "Particle atomic layer deposition is highlighted as a technology that can create new and exciting designer core/shell particles to be used as building blocks for the next generation of complex multifunctional nanocomposites," said Dr. Boris Feygelson, research engineer, NRL Electronics Science and Technology Division. "Our work is important because shell-thickness is most often a crucial parameter in applications where core-shell materials can be used to enhance performance of future materials." Atomic layer deposition is a layer-by-layer chemical vapor deposition-based thin-film growth technique used extensively in the electronics industry to deposit nanometer-thick films of dielectric materials on devices. Combined with other deposition and shadowing masking techniques, ALD is an integral part of electronic chip and device manufacturing. The same gas-phase process can be applied in a rotary or fluidizing powder bed reactor to grow nanometer-thick films that are highly conformal and uniformly thick on individual particles. Previous research on p-ALD, patented by ALD NanoSolutions, Inc., has shown that growth of each layer during the deposition process varies with particle size, with the underlying assumption that larger particles will always have less growth. To observe this growth phenomenon, the NRL team grew alumina on nano- and micron-sized particles of tungsten and measured the shell thickness in a transmission electron microscope. Because of the huge mass/density difference of the two materials, this pairing provides maximum contrast in the electron microscope and delineation was easily distinguishable between the particle core and shell. In their research, the scientists created core and shell powders consisting of a tungsten particle core and thin alumina shell that were then synthesized using atomic layer deposition in a rotary reactor. Standard atomic layer deposition of trimethylaluminum and water was performed on varying batches of powder with different average particle sizes. "Amazingly, we found that the growth per cycle of the alumina film on an individual particle in a batch was shown to be independent of the size of an individual particle, and therefore, a powder batch - which consists of particles sizes spanning orders of magnitude - has constant shell thicknesses on all particles. This result upsets the current understanding of ALD on particles," said Dr. Kedar Manandhar, ASEE postdoc, NRL Electronics Science and Technology Division and leading author of the research paper. The work, published recently in the Journal of Vacuum Science and Technology A, suggests that water, a reactant in the ALD process, is reason for the same rate of growth on different particles. This uniformity of thickness on different particle sizes in a particular batch is determined to be due to the difficulty of removing residual water molecules from the powder during the purging cycle of the atomic layer deposition (ALD) process. "Water is very sticky and it is very difficult to remove the last mono-layer from surfaces," Feygelson says. "And when you have a tumbling bed of powders, the water sticks around between the particles and results in consistent shell growth in the tumbling powder. Applications for this research demonstrate implications for use in materials like abrasion resistant paints, high surface area catalyst, electron tunneling barriers, ultra-violet adsorption or capture in sunscreens or solar cells and even beyond when core-shell nanoparticles are used as buildings blocks for making new artificial nanostructured solids with unprecedented properties. This research is a cross-disciplinary effort at NRL between the Materials Science and Technology Division and Electronics Science and Technology Division. The authors of the paper gratefully acknowledge Drs. Dev Palmer (Defense Advanced Research Projects Agency), Baruch Levush (NRL), and Fritz Kub (NRL). About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.


News Article | April 11, 2016
Site: www.nrl.navy.mil

Sixteen U.S. Naval Research Laboratory (NRL) scientists and engineers representing six research divisions were recognized with the prestigious Dr. Delores M. Etter Top Scientist and Engineer of the Year Award. The award ceremony was held on June 12th, with Assistant Secretary of the Navy (Research, Development & Acquisition) Sean Stackley, and Dr. Delores Etter presenting the awards. The Assistant Secretary of the Navy for Research, Development and Acquisition sponsors this annual award. Former Assistant Secretary of the Navy, Delores Etter established the awards in 2006 to recognize scientists and engineers who have made significant contributions to their fields and to the fleet. The NRL researchers honored as 2014 Top Scientists and Engineers are as follows: Dr. Dmitri Kaganovich, named an Emergent Investigator, is recognized for his enhancement of temporal contrast in ultra-short laser systems. He provided an elegant solution to the fundamental problem of ultrashort laser contrast. The source of low-intensity laser pedestals has been unknown and there has been no efficient technique for minimizing them. Kaganovich was not only able to pinpoint the source of the problem (natural narrowing of the laser spectrum in the amplification chain of the laser), but also provided a simple, cost effective (few thousand dollar modification to a multi-million dollar laser system) way to significantly increase the contrast in high-intensity laser beams. This contrast enhancement technique could also enable the development of compact laser-based X-ray sources. Dr. Geoffrey S. San Antonio, named an Emergent Investigator, is recognized for his HF Over-the-Horizon Radar (HFOTHR) technology advancement. San Antonio successfully devised and demonstrated numerous techniques, architectures and experiments that have significantly advanced the HFOTHR technology. This type of radar uses the ionosphere to effectively act like a mirror to bend the radar signal back toward the earth, allowing detection of targets well beyond the radar horizon. San Antonio's research has been instrumental in mitigating environmental factors that limit the performance of these systems, which has been essential in making these systems effective, persistent sensors, capable of target detection at any time of day, any day of the year. Dr. Daniel Gibson is recognized for his Infrared Gradient Index Optics. Gibson developed diffusion-based infrared gradient (IR-GRIN) optics technology that will reduce the size, weight and power consumption of multi-band infrared imaging systems for DoD systems. The optics Gibson developed will correct for chromatic aberrations across a wide range of infrared wavelengths, enabling compact multi-band infrared imagers for the first time. His IR-GRIN optics will provide warfighters in the field with new tactical and operational advantages in systems with reduced size, weight and power consumption. Dr. Michael H. Stewart is recognized for his advanced functional nanoparticles for chemical, biological and solid state optoelectronic applications. Stewart leads multifaceted research efforts developing and advancing colloidal semiconductor quantum dot (QD)-based technologies for chemical, biological and optoelectronic applications. His efforts are relevant to the Department of Navy/Department of Defense for developing nanotechnology to improve nanobiosystems for health and biomedical purposes and to address the future of solution-processed optoelectronics for remote power and detector technologies. In 2014, Stewart demonstrated groundbreaking advances in biosensing and imaging with biocompatible QDs and has demonstrated innovative techniques to design and fabricate QDs for optical interrogation of neuronal communication networks. Dr. Mark Sletten is recognized for his next-generation concepts for Synthetic Aperture Radar. Sletten is conducting research at the forefront of next-generation imaging radar systems that overcome the challenges of an environment in perpetual motion as occurs in the maritime domain. Motion associated with ocean waves and ships represent a challenge for radar systems to compensate for and then robustly characterize. This new multi-channel synthetic aperture radar (MSAR), first developed as a ground-based system, has been transitioned to an airborne configuration where the challenging effects of in-scene motion are being overcome. MSAR has many Navy and Marine Corps maritime mission applications. The team of Drs. David Abe, Simon Cooke, Baruch Levush, and John Pasour is recognized for their Ka-band amplifier demonstration of 12 kW peak output power. This team developed and demonstrated a ground-breaking millimeter-wave power amplifier that dramatically advances the power of state-of-the-art amplifiers. The 12-kW amplifier is driven by a 20 kilovolt, 3.5-ampere sheet electron beam of 0.3 mm x 4 mm cross-section and produces 20 times the power of commercially available amplifiers of comparable frequency, bandwidth, and operating voltage. The team employed innovative vacuum electronic circuits and techniques to achieve these breakthroughs, which satisfy a critical Navy need for higher-power, broadband, millimeter-wave amplifiers to enable electronic warfare systems to counter new and emerging threats. The team of Drs. Boris Feygelson (Electronics Science and Technology Division) and James Wollmershauser (Materials Science and Technology Division) is recognized for their bulk (3D) fully dense nanocrystalline materials with unprecedented improved performance. Feygelson and Wollmershauser developed a nanomaterial fabrication technique capable of producing bulk nanocrystalline ceramics with unprecedentedly small grain sizes that exhibit dramatic increases in hardness; up to 50% greater than conventional ceramics. The research demonstrates for the first time that nanocrystalline ceramics obey the 60-year-old postulation that decreasing the grain size of a ceramic will increase the hardness. The work furthers the fundamental understanding of the mechanical response of nanostructured materials that is critical to the Department of Navy/Department of Defense and the greater scientific community since it can lead to the development of a new generation of structural materials with extraordinary properties. The team of Mr. Kenneth Sarkady, Dr. Gregory Lynn, Mr. Roger Mabe, Dr. Hugo Romero, and Mr. D. Merritt Cordray is recognized for developing and flight testing a light-weight integrated missile warning and directed infrared countermeasures system. The team developed an innovative infrared countermeasure system for DoD aircraft. It features a high-power, high-efficiency quantum cascade laser, new two-color infrared focal plane arrays for longer range threat detection, advanced algorithms for lower false alarm rates, novel switching technology for laser energy distribution around the aircraft, and low-weight miniaturized pointing devices. The team designed these new technologies into a system lowering weight, cost, power, and space requirements, while providing full spherical aircraft protection. The system demonstrated unprecedented effectiveness in field and live fire tests against all advanced threats. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.


Du M.H.,Materials Science and Technology Division
Journal of Materials Chemistry A | Year: 2014

Halide perovskites have recently been shown to exhibit excellent carrier transport properties. Density functional calculations are performed to study the electronic structure, dielectric properties, and defect properties of β-CH3NH3PbI3. The results show that Pb chemistry plays an important role in a wide range of material properties, i.e., small effective masses, enhanced Born effective charges and lattice polarization, and the suppression of the formation of deep defect levels, all of which contribute to the exceptionally good carrier transport properties observed in CH3NH3PbI3. Defect calculations show that, among native point defects (including vacancies, interstitials, and antisites), only iodine vacancy is a low-energy deep trap and non-radiative recombination centre. Alloying iodide with chloride reduces the lattice constant of the iodide and significantly increases the formation energy of interstitial defects, which explains the observed substantial increase in carrier diffusion length in mixed halide CH3NH3PbI2Cl compared to that in CH3NH3PbI3. © 2014 the Partner Organisations.


Du M.H.,Materials Science and Technology Division
Journal of Materials Chemistry C | Year: 2014

Heavy 6p and 5p ions in groups IIIB, IVB, and VB (Tl, Pb, Bi, In, Sn, Sb) are multivalent ions, which act as electron and hole traps and radiative recombination centers in many wide band gap materials. In this paper, Tl + as a prototypical ns2 ion (ns2 ions here refer to 6p and 5p ions with outer electronic configurations of ns2) is studied as a luminescent center in alkali halides. Density functional calculations reveal the chemical trend that determines the luminescence mechanism in ns2-ion activated alkali halides. The activator-halogen hybridization strength and the ionicity of the host material strongly affect the positions of the activator levels relative to the valence and conduction band edges. This determines whether the radiative recombination occurs within the activator ion or involves the hole polaron, or the Vk center. Strategies for exploring different combinations of host materials and activators for desired luminescence mechanisms are discussed. The insight obtained in this work will help the search and the design of more efficient scintillators and phosphors. This journal is © the Partner Organisations 2014.


Lin Z.,Materials Science and Technology Division | Liu Z.,Oak Ridge National Laboratory | Dudney N.J.,Materials Science and Technology Division | Liang C.,Oak Ridge National Laboratory
ACS Nano | Year: 2013

This work presents a facile synthesis approach for core-shell structured Li2S nanoparticles with Li2S as the core and Li 3PS4 as the shell. This material functions as lithium superionic sulfide (LSS) cathode for long-lasting, energy-efficient lithium-sulfur (Li-S) batteries. The LSS has an ionic conductivity of 10 -7 S cm-1 at 25 C, which is 6 orders of magnitude higher than that of bulk Li2S (∼10-13 S cm-1). The high lithium-ion conductivity of LSS imparts an excellent cycling performance to all-solid Li-S batteries, which also promises safe cycling of high-energy batteries with metallic lithium anodes. © 2013 American Chemical Society.


Cooper V.R.,Materials Science and Technology Division
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

In this Rapid Communication, an exchange functional which is compatible with the nonlocal Rutgers-Chalmers correlation functional [van der Waals density functional (vdW-DF)] is presented. This functional, when employed with vdW-DF, demonstrates remarkable improvements on intermolecular separation distances while further improving the accuracy of vdW-DF interaction energies. The key to the success of this three-parameter functional is its reduction in short-range exchange repulsion through matching to the gradient expansion approximation in the slowly varying/high-density limit while recovering the large reduced gradient, s, limit set in the revised Perdew-Burke-Ernzerhof (revPBE) exchange functional. This augmented exchange functional could be a solution to long-standing issues of vdW-DF lending to further applicability of density-functional theory to the study of relatively large, dispersion bound (van der Waals) complexes. © 2010 The American Physical Society.


Du M.H.,Materials Science and Technology Division
Journal of Materials Chemistry C | Year: 2014

Mn4+ is known to activate red emission in many materials. However, the existing Mn4+ activated red phosphors have relatively long emission wavelengths and are therefore inefficient for general lighting purposes. Density functional calculations are performed on a large number of Mn4+ doped materials with diverse crystal structures to understand how material properties of different hosts affect the emission energy of the Mn4+ dopant. The results show that weak Mn4+-ligand hybridization generally leads to higher Mn4+ emission energies. Host materials allowing long Mn-ligand distance and/or significant distortion of bond angles around the Mn octahedral site are shown to have higher emission energies. Several new oxide host materials are found for Mn4+. Their emission energies are found to be higher than those currently known for Mn 4+ doped oxides and should be closer to that of Y2O 3:Eu3+, which is the current commercial red phosphor for fluorescent lighting. © 2014 the Partner Organisations.

Loading Materials Science and Technology Division collaborators
Loading Materials Science and Technology Division collaborators