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Programmatic/Technical Accomplishments
New High Performance X-Ray Diffractometer Commissioned In MSEL
The need for more sophisticated and, in some cases, faster x-ray diffraction characterization of materials has prompted the acquisition of a new state-of-the-art x-ray diffraction system in MSEL. The new equipment, maintained as a shared facility of the Ceramics and Metallurgy Divisions, is installed in building 223. The system consists of a Bruker D8 diffractometer equipped with an area detector capable of recording diffraction over a cone of orientation space with a semi-angle of at least 15° at the typical detector distance; a high intensity beam is produced using a focusing mirror (often called a Goebel mirror) that creates a beam with ≈ 0.05° divergence. The combination of high intensity and large capture area allows for rapid phase identification. The sample stage has a compact Eulerian cradle and also features significant x-y-z capabilities. Accurate sample positioning is achieved with a system that uses video imaging of a laser spot on the sample. Rapid phase identification allied with the ability to scan accurately over a sample and obtain diffraction data from areas as small as 0.3 mm in diameter is particularly useful in the context of combinatorial experiments. Texture measurements and measurements of stress are also facilitated by the area detector and Eulerian cradle.
The system is equipped with a furnace that can go to 900 °C to conduct high temperature experiments, as needed, for example, in studies of phase development, crystallization kinetics, and interdiffusion. The system also has a point detector, and the beam can be prepared with a 4-bounce Ge monochromator, allowing high resolution diffraction experiments to be carried out, and also reflectometry measurements for characterizing thin film and multi-layer samples.
Contact: Mark D. Vaudin (MSEL, Ceramics) x5799
MSEL Researchers Provide A New View of Organic Electronic Devices
In a recent issue of Advanced Materials, researchers from MSEL’s Polymers and Ceramics Divisions and the University of California at Berkeley reported success in using a nondestructive measurement method to detail three structural properties crucial to making reliable electronic devices with thin films of organic semiconductors. The new capability could help industry clear hurdles responsible for high manufacturing development costs that stand in the way of widespread commercial application of the materials.
Using near-edge x-ray absorption fine-structure spectroscopy (NEXAFS), the team tracked chemical reactions, molecular reordering, and defect formation over a range of processing temperatures. They then evaluated how process-induced changes in thin-film composition and structure affected the movement of charge carriers in organic field effect transistors, devices basic to electronic circuits. With NEXAFS measurements taken over the range from room temperature to 300 °C, the team monitored the conversion of a precursor chemical to an oligothiophene, an organic semiconductor. The molecular organization and composition achieved at 250 °C yielded the highest levels of charge carrier movement and, consequently, maximum electric-current flow. As chemical conversion progressed, the researchers calculated how the molecules arranged themselves on top of an electrical insulator. Top transistor performance corresponded to a vertical alignment of molecules. In addition, they used NEXAFS to determine the angles of chemical bonds and to assess the thickness and uniformity of film coverage, also critical to performance.
For further information see the article in Advanced Materials 17(19), 2340-2344 (2005), or visit the Polymers Division website at www.nist.gov/polymers
CONTACT: Dean DeLongchamp, ext. 5599
Simplifying Integrated Circuit Fabrication
Researchers in the Metallurgy Division of MSEL have demonstrated a simplified process for fabrication of submicron copper interconnects (“on-chip wiring”) for microelectronics. The work shows that iridium, as well as osmium, can be used as a layer upon which copper can be directly electroplated to obtain the bottom-up “superfill” required for defect-free filling of today’s high aspect ratio interconnects. Replacement of standard tantalum diffusion barriers by either of these materials would eliminate the need for the copper seed layer presently required to obtain wetting of the electrodeposition process on the barrier. This work extends earlier Division research that demonstrated the effectiveness of ruthenium barriers for this application. Additional studies are underway to assess the effectiveness of these materials as diffusion barriers to prevent “poisoning” of the underlying silicon-based structures.
Contact: Dr. Daniel Josell, ext. 5788
Water, supercooled below the freezing point such that it is still a liquid, exhibits certain properties that are not well understood. An unusual transition in supercooled bulk water was predicted several years ago in which fundamental changes take place in the diffusive behavior (dynamics) of individual molecules at a specific temperature. It is well known that supercooled water exhibits a glass-like, so-called "fragile," temperature dependence of its dynamic properties that reflect a disordered environment around each molecule. Such disorder presents a distribution of different energy barriers to diffusing molecules. At sufficiently low temperatures, however, a theoretical crossover was proposed to a state in which an individual water molecule must overcome a single, well-defined energy barrier in order to move to a new position. In this state the dynamics are governed by a thermally activated (Arrhenius) mechanism and the liquid is called “strong.” This so-called “fragile-to-strong” liquid transition was subsequently observed using neutron scattering techniques at the NIST Center for Neutron Research at a temperature of 225 K. However the underlying reasons responsible for this transition are unknown.
Recently, scientists at the NCNR probed the dynamics of water molecules confined to the surface of cerium oxide nano-particles in an effort to shed new light on this phenomenon. A neutron scattering experiment covering the temperature range from 200 K to 250 K was carried out on the High-Flux Backscattering Spectrometer at the NCNR, which provided detailed information about the dynamics of “surface” water on the time scale of hundreds of picoseconds. Down to about 220 K, the temperature dependence of the relaxation time, i.e. the time between diffusive jumps of the water molecules, exhibits “fragile” behavior. At 215 K the temperature dependence of the relaxation time abruptly changes to an Arrhenius form indicating “strong” behavior, and thus a fragile-to-strong liquid transition at about 10 K below what was observed in bulk water. To date there have been neither theoretical predictions nor experimental observations of this fragile-to-strong liquid transition in surface water. This behavior could prove to have significant consequences in biological systems, particularly with regard to protein function.
These findings will appear in a forthcoming issue of the Journal of Chemical Physics.
CONTACT: Eugene Mamontov (NCNR), ext. 6232.
Interactions
Advanced Coatings R & D for Pipelines and Related Facilities
The Materials Reliability Division has just completed publication and distribution of CD copies of the proceedings of a workshop entitled Advanced Coatings R & D for Pipelines and Related Facilities. The June 9 and 10, 2005 workshop in Gaithersburg, Maryland brought together 56 representatives of pipeline operators, coating manufacturers, pipeline fabricators, pipeline industry consortia, standards developing organizations, government agencies, and regulatory agencies. This group was able to thoroughly assess the opportunities for research and development in the areas of coatings test methods and materials, coating application technologies and quality control, coating identification and inspection, and in field technologies for repair and rehabilitation. This workshop was organized by Richard Ricker of the Metallurgy Division for the U.S. Department of Transportation (Office of Pipeline Safety). Additional support came from the American Gas Association, ASTM International, CANMET, Gas Technology Institute, NACE International, National Energy Board Canada, Pipeline Research Council International, U.S. Department of Commerce (National Institute of Standards and Technology), and U.S. Department of Interior (Mineral Management Service). The proceedings (NIST Special Publication 1044) are in electronic form and are available by request.
Contact: Tom Siewert x3523 (Boulder) for CDs
Richard Ricker x6023 (Gaithersburg) for information on the workshop
NIST is an agency of the U.S. Commerce Department's Technology Administration.
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Date created: 5/10/2005
Last updated: 10/21/2005
