Research Highlights


 

Phonon softening and metallization of a narrow-gap semiconductor by thermal disorder

O. Delaire,1 K. Marty,1 M. B. Stone,1 P. R. C. Kent,1 M. S. Lucas,2 D. L. Abernathy,1 D. Mandrus,1 B. C. Sales1

1 Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, TN 37831
2 Air Force Research Laboratory, Wright-Patterson AFB, OH 45433

Achievement
We have shown how, in some materials, there can be a surprisingly strong coupling between certain features of the electronic structure and the way the atoms in a solid vibrate. This insight should help us understand better how heat is transported in a solid. Inelastic neutron scattering measurements of Fe1-xCoxSi alloys were combined with quantum mechanics based calculations to show why the alloys exhibit unusual softening as the temperature is increased. Our results show that for alloys with a rapidly changing concentration of electrons near the chemical potential, there are likely to be strong temperature-dependent interactions between the atom vibrations and electrons.

(Left) Phonon dispersions of FeSi measured via time-of-flight inelastic neutron scattering, compared with calculations (light blue lines) provides clear evidence of the unusual softening of atomic motion with increasing temperature. (Right) Phonon density of states for FeSi and CoSi from ab initio molecular dynamics simulations. A large phonon softening in FeSi between 300K and 1200K is predicted.

Significance
By combining extensive neutron scattering based analysis with the results of first principles molecular dynamics calculations, we have clearly demonstrated a strong coupling between the phonon and electron states when there are sharp electronic features around the Fermi level. These effects are likely to be common to many narrow gap materials including some superconductors, heavy-Fermion compounds, and many thermoelectric materials. Our results demonstrate the importance of including these effects in predicting or optimizing heat flow in these materials.

CreditThis work was published in Proceedings of the National Academy of Sciences (7 March 2011, 2010-14869RR, doi: 10.1073/pnas.1014869108). Research at ORNL’s Spallation Neutron Source, High Flux Isotope Reactor, and Center for Nanophase Materials Sciences (PRCK) was sponsored by the U. S. Department of Energy Scientific User Facilities Division. Computations by PRCK used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science, U.S. Department of Energy.



On-Demand Generation of Monodisperse Femtoliter Droplets by Shape-Induced Shear

Seung-Yong Jung,1 Scott T. Retterer,1-2 and C. Patrick Collier1

1Nanofabrication Research Laboratory, Center for Nanophase Materials Sciences
2 Biological and Nanoscale Systems Group, Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN

Achievement
This manuscript describes a method for creating individual femtoliter (10-15 L) scale aqueous droplets on demand at a microfabricated junction between an aqueous and oil channel in the absence of oil cross flow, based solely on a localized increase in the oil channel height relative to that of the aqueous channel. This increase in channel height permits a droplet to attain its lowest energy spherical shape, which creates an interfacial tension induced force on the droplet sufficient to detach it from the junction orifice.

Fig. 1 Series of bright field images spaced at 82 msec intervals of the formation and detachment of an individual 5.7 μm diameter droplet from an aqueous channel (1 x 1 μ;m) into the oil phase, at a constant backing pressure of 130.3 kPa.

Monodisperse droplets could be created at regular intervals under constant pressure conditions, allowing each droplet to be tracked and manipulated individually in real time, or pressure pulses could be applied to generate one, two or more droplets per pulse reproducibly, without the need for additional actuation or detection equipment. All that was needed was a pressure regulator.

Significance
Although the droplet formation mechanism described here involves shear, the magnitudes of the shear stresses appear to be significantly less than the shearing instabilities typically used to split off daughter droplets from aqueous mother plugs in rapid continuous water/oil segmented flows, based on bright field analyses of droplet deformation during the splitting off process. This implies that this method may be better suited for studying biochemical reactions and reaction kinetics in droplets of decreasing volume without loss of chemical reactivity due to redistribution of surfactant density at the oil/water interface.

Credit: This work was published in Lab on a Chip vol. 10 (2010) p. 2688, doi: 10.1039/c0lc00120a. This Research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Research sponsored in part by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.


 

Nanoscale mapping of ion diffusion in a lithium-ion battery cathode

N. Balke,1 S. Jesse,1 A. N. Morozovska,2 E. Eliseev,3 D. W. Chung,4 Y. Kim,5 L. Adamczyk,5 R. E. Garcia,4 N. Dudney,5 and S. V. Kalinin,1-5

1The Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
2Institute of Semiconductor Physics, National Academy of Science of Ukraine, Ukraine 41, pr. Nauki, 03028 Kiev, Ukraine
3Institute for Problems of Materials Science, National Academy of Science of Ukraine, Ukraine 3, Krjijanovskogo, 03142 Kiev, Ukraine
4School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA
5Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

Achievement
The movement of Li in and out of cathodes is a critical component of the design of new and better batteries, but is dominated by nanoscale processes which have been difficult to identify. We have developed a scanning probe

microscopy based method, electrochemical strain microscopy (ESM), to investigate the electrical-bias induced Li-ion transport in thin-film LiCoO2 electrode materials. ESM utilizes the intrinsic link between bias-controlled Li-ion concentration and molar volume of electrode materials, providing the capability for new types of studies with nanometer precision. Using ESM, local electrochemical processes can be studied on relevant length scales to unravel the complex interplay between structure, functionality, and performance in Li-ion batteries. This work demonstrates how ESM can be used to investigate the Li-ion transport in layered cathode materials, such as LiCoO2.

Figure 1 | ESM: Inducing local Li-ion transport in a layered cathode material through a biased atomic force microscopy tip.

Through its layered structure, the Li-ion transport and the corresponding volume change is strongly dependent on the crystallographic orientation of the LiCoO2 grains. With ESM it was possible to identify grains and grain boundaries with enhanced Li-ion kinetics.

Significance
The growing need of renewable energy sources is strongly tied to the need for advanced energy storage technologies which currently do not perform as demanded by many applications. The functionality of energy storage systems, such as Li-ion batteries, is based on and ultimately limited by the rate and localization of ion flows through the device on different length scales ranging from atoms over grains to interfaces. The fundamental gap in understanding ionic transport processes on these length scales strongly hinders the improvement of current and development of future battery technologies. The development of ESM has opened the pathway to understand Li-ion batteries on a level never achieved before. The unique information about the local Li-ion flow obtained with ESM will inevitably lead to breakthroughs in material development for battery applications. Knowledge of the interplay between the ionic flow, material properties, microstructure, and defects is the key to battery operation and can be used to optimize the device properties and understand what happens during battery fading.

Credit Research was sponsored as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number ERKCC61 (N.B., L.A., N.D., S.V.K.) and part of the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy in the projects CNMS2010-098 and CNMS2010-099 (N.B., S.J.). N.B. also acknowledges the Alexander von Humboldt foundation. R.E.G. and D.W.C. are grateful for the support provided by NSF grant CMMI 0856491.

“Nanoscale mapping of ion diffusion in a lithium-ion battery cathode,” N. Balke, S. Jesse, A. N. Morozovska, E. Eliseev, D. W. Chung, Y. Kim, L. Adamczyk, R. E. Garcia, N. J. Dudney, and S. V. Kalinin, Nat. Nanotechnol., DOI: 10.1038/NNANO.2010.174 (2010).