Understanding of Rare Earth Metals from Theory

The rare earth metals are becoming increasingly applicable in our everyday life. The enormous importance of rare earths in the technology, environment, and economy is attracting scientists all over the world to investigate them starting from the extraction to the physical and chemical properties measurements.  Although a lot of works have been done on the experimentation of rare earths, the true understanding from theory and modeling on these materials is lagging behind.

Solar to Chemical Energy Conversion with Photocatalytic Heterostructures made of Earth Abundant Materials

Cu2ZnSnS4 (CZTS) is one of the most promising materials for solar energy harvesting. Made of highly abundant, widely distributed and relatively biocompatible elements, and with a direct band gap of 1.5 eV, CZTS is an affordable, greener and more sustainable alternative to other semiconductors such as GaAs, CdTe, CuInS2 (CIS), or CuInxGa1-xSe2 (CIGS).

Software Interoperability for New Science

Within the Applied Mathematics and Computational Science (AMCS) program we advance the use of scalable computing in scientific and engineering computation, and develop new programming paradigms for novel hardware. Multiscale simulation methods is an indispensable tool in understanding chemical processes and designing new materials. When simulation spans multiple temporal or spatial scales, existing capabilities of a single software package are often insufficient, and a coupling of multiple programming packages developed by different research groups is strongly desirable.

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Graphene for metamaterials

Contacts:                                                                        For release: Aug. 9 2013

Costas Soukoulis, Physicist, 515-294-2816

Breehan Gerleman Lucchesi, Public Affairs 515-294-9750

Could graphene – a one-layer thick sheet of carbon atoms – be the ingredient needed for super-efficient solar harvesting with metamaterials?  Or for “light on wire” plasmonic data transmission? In the Aug. 9 issue of Science, U.S. Department of Energy’s Ames Laboratory physicists discuss the potential and challenges of using graphene in metamaterials and plasmonics in terahertz applications, which operate at frequencies between microwave and infrared waves.  

Metamaterials are man-made structures that exhibit properties not possible in natural materials, such as refracting light “backward” or absorbing all the light that hits them.  Soukoulis and fellow Ames Laboratory physicists Philippe Tassin and Thomas Koschny found that graphene may be a good candidate to replace the metals currently used to build metamaterials.

 “Graphene is a fascinating and promising material because it’s so thin, is very electronically responsive, and has electronic properties that are easily changed,” said Costas Soukoulis, Ames Laboratory physicist and Iowa State University Distinguished Professor of physics and astronomy. “Our review of the findings shows hurdles to cross before graphene could replace the thin metal films currently used in metamaterials and plasmonics.”

 “Graphene offers an advantage over metals because graphene’s properties can be more readily tuned to obtain the electrical response desired for a given application,” said Soukoulis.

Graphene’s tunability may also make it a good candidate for use in plasmonics, where a tiny structure uses light to carry information.

However, Soukoulis, Tassin and Koschny still see challenges. They surveyed both experimental measurements and theoretical simulations about graphene’s properties relevant for terahertz applications.  In general, Soukoulis’ team noted that the data indicate a “discrepancy between the experimentally realizable and the theoretically predicted performance.” For instance, experimental data have shown significantly higher electrical losses than has been estimated by theoretical work.

“Overcoming those dissipative losses will be the major obstacle for using graphene in terahertz applications of metamaterials and plasmonics,” said Soukoulis.

Soukoulis, Tassin and Koschny’s work at Ames Laboratory was supported by the U.S. Department of Energy’s Office of Science (funding for the search for better optical materials, properties of metals at optical frequencies, and understanding electrical losses in metamaterials) and the U.S. Office of Naval Research (graphene as conductors for resonant metamaterials). Iowa State University College of Liberal Arts Frances M. Craig Professorship also supports Soukoulis’ efforts.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at science.energy.gov/.

Ames Laboratory is a U.S. Department of Energy Office of Science national laboratory operated by Iowa State University. Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaborations to solve global problems.

Hybrid Nanostructured Organic-Inorganic Catalysts for Water Electrolysis

Hydrogen is a promising energy carrier, but also a valuable chemical used in a variety of industrial processes. Hydrogen is typically produced from natural gas by steam reforming, a process which requires high temperatures and generates CO2. More sustainable routes for H2 production exist. Water electrolysis, for example, is carried out at room temperature, H2 and O2 are the sole products of the reaction, and photovoltaics or wind turbines can supply the current. Unfortunately, several challenges remain.

The Long and the Short of It: Nanostructure of Thermoelectric Materials

Highlight Date: 
08/07/2013
Display Section: 
Broad Audience Highlights
Article Title: 
Analysis of Phase Separation in High Performance PbTe–PbS Thermoelectric Materials
Author(s): 
S. N. Girard, K. Schmidt-Rohr, T. C. Chasapis, E. Hatzikraniotis, B. Njegic, E. M. Levin, A. Rawal, K. M. Paraskevopoulos, and M. G. Kanatzidis
Article Link: 
Journal Name: 
Advanced Functional Materials
Volume: 
23
Year: 
2013
Page Number(s): 
747-757
Project Affiliation: 
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A study of thermoelectrics, materials that convert heat to electricity, demonstrates the importance of characterizing materials using several different methods. According to Vegard’s Law of Alloys, the size of a crystalline lattice (lattice parameter) changes linearly with composition. It is actually not a law, but an empirical observation that has been found to hold true for a majority of alloys. Researchers studied thermoelectrics made from lead, tellurium, and sulfur and varied the tellurium and sulfur content to answer the question “If a thermoelectric follows Vegard’s Law, does that make it an alloy?” The answer is: not necessarily.  High-resolution synchrotron X-ray diffraction data taken at the Department of Energy’s Advanced Photon Source, probing long-range order, shows that this material adheres to Vegard’s Law. Solid-state tellurium (125Te) nuclear magnetic resonance (similar to magnetic resonance imaging) was used to determine the short-range structure and composition of the samples with high accuracy; sulfur shows strong effects on the resulting NMR spectra, with the Te peaks shifted depending on the number and arrangement of sulfur atoms near Te.  Combined NMR and infrared spectroscopy results show that above 16% sulfur the samples are not alloys. Additionally, inhomogeneities improve the material’s ability to convert heat to electricity.  These findings suggest that there may be many thermoelectric systems that are not alloys as reported, but instead are nanostructured.  The work also demonstrates the importance of using probes of both long range and local chemical structure to differentiate between alloys and composites.

Predicting Unusual Deformation Behavior in Materials

Highlight Date: 
08/06/2013
Display Section: 
Broad Audience Highlights
Article Title: 
Stability Maps to Predict Anomalous Ductility in B2 Materials
Author(s): 
R. Sun and D. D. Johnson
Article Link: 
Journal Name: 
Physical Review B
Volume: 
87
Year: 
2013
Page Number(s): 
104107
Project Affiliation: 
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For the first time, researchers can now both explain and predict the behavior of different materials while they are being pulled apart.  Some materials are ductile, meaning they will deform without losing their toughness, and others are brittle.  The results explain even the unexpected ductility of a material within a class of rare-earth-containing materials that are otherwise known to be brittle.  To predict the behavior requires two maps.  The first map reveals whether a system has the ability to slip in a particular direction and form stable defects (a necessary condition for ductility), while the second map reveals if the required defects have multiple, active slip planes (a sufficient condition).  With both conditions satisfied, the material will be exhibit unusual ductility. The maps are accurate, with observed ductile to brittle transitions reproduced, too.  Similarly criteria can be formulated for a variety of systems — a direct form of computational materials discovery.