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Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350 deg C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters., Prepared in collaboration with the Dept. of Chemical and Materials Engineering, University of Dayton, Dayton, OH, the University of Dayton Research Institute, Dayton, OH, and the Dept. of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, PA. Published in Applied Physics Letters, v104 article ID 261604, 1 Jul 2014.

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a > 10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793203]

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a > 10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793203]

Vapor phase growth of gold, nickel and titanium metal nanoparticles on multiwall carbon nanotube (MWCNT) buckypaper by sputtering was investigated. The size and distribution of nanoparticles was dependent on the intrinsic binding energy of the metal elements, but could be altered to mimic that of metals with different binding energies by in situ modification of the MWCNT surfaces by energetic metal ions or annealing of the buckypaper. A range of average gold particle diameters from approximately 5 30 nm could be produced depending on the intrinsic sputter process parameters (especially metal ion flux and kinetic energy) and defect density of the MWCNT surfaces, which could also be controlled by annealing prior to sputtering. The diameter of the MWCNTs had a significant influence on the geometry of the nanoparticles. Particles were elongated along the nanotube axis for tube diameters 30 nm. Remarkably strong alignment of the particles along the nanotube axis was observed, especially for MWCNTs with higher defect densities., Published in Carbon, v57 p274-281, 1 Feb 2013. Prepared in cooperation with the Department of Chemical and Materials Engineering and the University of Dayton Research Institute, University of Dayton, Dayton, OH, and the George W. Woodruff School of Mechanical Engineering and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta.

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a > 10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793203]

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a > 10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793203]

We present a micro-electro-mechanical system-based experimental technique to measure thermal conductivity of freestanding ultra-thin films of amorphous silicon nitride (Si3N4) as a function of mechanical strain. Using a combination of infrared thermal micrography and multi-physics simulation, we measured thermal conductivity of 50 nm thick silicon nitride films to observe it decrease from 2.7 W (m K)(exp -1) at zero strain to 0.34 W (m K)(exp -1) at about 2.4% tensile strain. We propose that such strong strain-thermal conductivity coupling is due to strain effects on fraction-phonon interaction that decreases the dominant hopping mode conduction in the amorphous silicon nitride specimens., Published in Journal Of Micromechanics And Microengineering, v22 article 045001, 2 March 2012. Prepared in collaboration with Department of Mechanical and Nuclear Engineering, Penn State University, University Park, PA. The original document contains color images.

Measured friction coefficients of carbon nanotubes vary widely from microns less than 0.1-microns less than 1.0, while theoretical studies suggest intrinsically high friction coefficients, approaching unity. Here we report that measured friction coefficients of MWNT films are strong functions of surface chemistry and temperature, but are not dependent on the presence of water vapor. We hypothesize that the origin of the temperature dependence arises from the interaction of the surface chemical groups on the nanotubes and rubbing counterface. The friction coefficient of individual films can be easily tuned by changing the surface temperature and chemistry of either the countersurface or the nanotubes, we have demonstrated the ability to create and control high and low friction pairs through plasma treatments of the nanotube films with argon, hydrogen, nitrogen, and oxygen. This behavior is completely reversible, and when coupled with the superior strength, thermal, and electrical properties of nanotubes, provides a versatile tunable, multifunctional tribological system., Published in the Journal of Tribology Letters, p1-6, 2006.

High-performance polymer matrix composites (PMCs) are an appealing choice for materials in aerospace applications due to their high strength-to-weight ratio and stability over a broad temperature range for thousands of service hours. The use of PMCs in propulsion applications is currently limited only by insufficient resistance to erosion by abrasive media. Erosion-resistant coatings may provide the necessary protection, however their application is not straightforward, as surface preparation is a challenge. Specifically, it is because of the resin-rich outer plies, mold release agents, and fluorinated high-temperature polymer matrices that treatment prior to deposition is required. A low pressure oxygen plasma treatment process was developed to improve adhesion of CNx coatings to a polymer matrix composite. CNx was selected as a protective coating for its high hardness-to-elastic ratio, coupled with elastic resilience. In situ x-ray photoelectron spectroscopy was used to evaluate the effect of the plasma treatment on surface chemistry, and electron microscopy was used to identify changes in the surface morphology of the PMC substrate after plasma exposure. CNx coatings were then deposited on treated PMC substrates. The effect of the plasma predeposition treatment on coating adhesion and erosion resistance was evaluated. The combination of PMC pretreatment and coating with CNx reduced the erosion rate by an order of magnitude., Submitted to Journal of Vacuum Science and Technology A. The original document contains color images. All DTIC reproductions will be in black and white.

Yttria-stabilized zirconia (YSZ) nanocomposite coatings consisting of silver and molybdenum were produced by a hybrid process of filtered vacuum arc, magnetron sputtering and pulsed laser depositions for tribological investigations at different high temperatures. The microstructure of the coatings was determined by X-ray diffraction and transmission electron microscopy. The friction coefficients were measured using a high-temperature ball-on-disk tribometer at 25 to 700C. The coatings with 24 at.% Ag and 10 at.% Mo contents showed a friction coefficient of 0.4 or less for all temperatures from 25 to 700C. The wear scar surfaces and coating cross sections were studied using scanning electron, transmission electron, scanning transmission electron and focused ion beam microscopes, which provided the information on chemical composition distributions of silver and molybdenum along with microstructure features. The silver diffusion and coalescence on surfaces played an important part in the high-temperature lubrication mechanism of the YSZ-Ag-Mo coatings. Silver was found to be an effective lubricant at temperatures below 500C and its coalescence on the surface isolated molybdenum inside coatings from ambient oxygen. Lubricious oxides of molybdenum were formed and lubricated at temperatures above 500C when the silver was worn off the contact surface. For silver containment inside the coating at high temperatures, a multilayer architecture was build by inserting a TiN diffusion barrier layer in the composite coatings. Microscopic observations showed that this barrier layer prevented silver exit to the coating surface. At the same time, this enabled a subsequent lateral lubricant supply toward a wear scar location where the diffusion barrier layer was worn through and/or for a next thermal cycle. The multilayer coating maint

Nanocomposite YSZ-Ag-Mo-MoS2 with different MoS2 additions (0-100 atomic percent) coatings were deposited with a hybrid pulsed laser/magnetron sputtering/filtered cathodic arc process. Wear testing was performed from 25-700 degrees C for each of the coatings. Electron microscopy and other characterization techniques were used to examine the surfaces and wear tracks of the coatings to determine the mechanisms resulting in the measured tribological properties. Adaptive coatings containing 8 atomic percent MoS2 demonstrated a friction coefficient of 0.2 throughout the temperature range examined here, compared to 0.4 for YSZ-Ag-Mo with no MoS2. Characterization of the YSZ-Ag-Mo-8%MoS2 coating revealed that MoS2 and silver provided lubrication at temperatures less or equal 300 degrees C, while silver molybdate phases and MoO3 were lubricious at higher temperatures. Silver molybdate was not observed in the coatings containing 0% MoS2. The role of sulfur in the formation of silver molybdate is briefly discussed., Submitted to Elsevier for publication in the 2006 Proceedings of the International Conference on Metallurgical Coatings and Thin Films (ICMCTF), paper no. B8-2-7(Surface Coatings and Technology 2006).

LAPPS (Large Area Plasma Processing System) was invented in NRL's Plasma Physics Division and Materials Science and Technology Directorate. It uses a 2-5 keV electron beam to ionize and dissociate gas molecules, species that are then used to modify the surface properties of various materials. This novel approach to plasma generation provides a direct scalability of the system and higher control over reactive chemical species compared to conventional plasma processing systems used in the semiconductor and coatings industries. The scalability is simply dictated by the dimensions of the electron beam, allowing unprecedented surface areas to be treated uniformly. The process/chemical control is provided by the electron beam/molecule interactions, which ensures that all chemical reaction pathways are accessible, as opposed to conventional plasma systems. The ability to increase the hardness of a material while preserving its intrinsic properties provides tremendous advantages in choosing materials for demanding applications. Thin layers of well-known hard coatings are frequently used, but adhesion problems (lattice and temperature coefficient matching) limit their utility. For metals, incorporating nitrogen into the surface's lattice (surface nitriding) is highly desirable because the nitrided layer is physically part of, yet significantly harder than, the bulk material. Equally important in surface nitriding is maintaining sufficiently low temperatures to preserve the properties of the bulk material., Published in the 2005 NRL Review, p171-172.

Plastic microvascular casts have been used primarily to study the three-dimensional distribution of small blood vessels in soft tissues, but in the two studies reported in bone, one investigated the vascular distribution of rat vertebrae and the other the micro-vascular distribution to both teeth and bone in the dog mandible. To this point the technique has not been used widely under experimental conditions. In this study we used the distribution of microvessels within the periosteum covering the rat perietal bones to investigate the effects of high doses of dexamethasone and prostaglandin E1 and low doses of parathyroid hormone on the microvessels of bone. Dexamethasone has been reported to decrease bone formation while the other two hormones have been reported to do just the opposite. The results showed that either prostaglandin E1 or parathyroid hormones, when given at 100 ng/rat twice a day, stimulate an increase in the number of microvessels on the surface of bone. Neither parathyroid hormones injected at 100 ng/day nor dexamethasone had any significant affect on microvessel number or size although a slight decrease in the number of microvessels appeared to occur at the highest dose of dexamethasone given (results not shown). In conclusion, we have demonstrated that capillaries found within the periosteum are very sensitive to two known stimulators of bone formation and should receive greater consideration when planning strategies to increase bone formation during bone growth and repair. (KT)