Combinatorial investigation of nanolaminate ternary carbide thin films

MAX phases have shown a combination of metal- and ceramic-like properties making them candidates in aerospace and other high-performance applications currently dominated by superalloys. The MAX phases follow the general formula Mn+1AXn, where M is an early transition metal, A is an A-group element, X is C and/or N, and n = 1 to 3. The hexagonal structure consists of edge sharing M6X octahedra interleaved with Agroup element layers. This layered structure, referred to as nanolaminate, leads to the unique and interesting properties. With more than fifty MAX phases already identified, an almost unlimited number of solid solution possibilities exist. Combinatorial methods provide a technique which allows for a large number of thin film samples to be generated with minimal time and expense. In this research, investigations of thin film MAX phase ternary carbides synthesized by magnetron sputtering in the temperature range of RT-1000 °C are reported. The ultimate goal was to synthesize MAX phase thin film solid solutions by a combinatorial method in an attempt to identify enhanced properties.The M2AC MAX phases that formed in the following systems, Ti-Nb-Al-C, V-Cr-Al-C, V-Cr-Ge-C, were examined. In all solutions, only mixing of the M elements was investigated. All textured films grew epitaxially (c-axis) on c-sapphire substrates or deposited binary carbide buffer layers. The lowest synthesis temperature resulting in textured growth was for V2AlC at 600 °C, however; formation of nanocrystalline Cr2AlC was observed at 550 °C as indicated by Raman spectroscopy. High temperature X-ray diffraction of amorphous Cr-Al-C and Cr-Ge-C films showed textured growth of the MAX phase occurred around 650 °C, and 725 °C, respectively. All combinatorial studies were performed at 850 °C with (Ti1-xNbx)2AlC films grown on TiC buffer layers while (V1-xCrx)2AlC and (V1-xCrx)2GeC grown directly on sapphire. Complete solubility across the entire range of x was observed for all systems. Additionally, new thin film phases of V3AlC2, V4AlC3, Nb5Al3Cx, Cr5Ge3Cx, (Ti1-xNbx)3AlC2, (Ti1-xNbx)4AlC3, and (V1-xCrx)4AlC3 were discovered.The M-element impacts many different properties of MAX phase films. The surface of most films were rough, some containing large hexagonal crystals. Yet, this work has demonstrated that the surface roughness can be tuned using elemental substitutions on the M-sites. While friction testing found all films to have relatively low coefficients of friction (<0.12), this too was found to be influenced by the M-element. Raman spectroscopy of (Ti1-xNbx)2AlC films indicates possible stiffening around x = 0.75 explicitly demonstrating the role of the M-element in this solid solution. All films were good electrical conductors with metal-like conduction down to 2K with magnitude and temperature dependence of the resistance tunable through composition. The Hall coefficient and magnetoresistance were also controlled by M-element substitution. While dramatic changes have not been observed, it is clear that the application of combinatorial methods has improved the understanding of the role of the M-element in determining the properties of MAX phases improving the ability to use the materials for thin film applications and beyond.Ph.D., Materials Science and Engineering -- Drexel University, 200

Phase stability, elastic, electronic, thermal and optical properties of Ti3Al1-xSixC2 (0 LE x LE 1): First-principles study

The structural parameters with stability upon Si incorporation and elastic, electronic, thermodynamic and optical properties of Ti3Al1-xSixC2 (0 \leq x \leq 1) are investigated systematically by the plane wave psedudopotential method based on the density functional theory (DFT). The increase of some elastic parameters with increasing Si-content renders the alloys to possess higher compressive and tensile strength. The Vickers hardness value obtained with the help of Mulliken population analysis increases as x is increased from 0 to 1. The solid solutions considered are all metallic with valence and conduction bands, which have a mainly Ti 3d character, crossing the Fermi level. The temperature and pressure dependences of bulk modulus, normalized volume, specific heats, thermal expansion coefficient, and Debye temperature are all obtained through the quasi-harmonic Debye model with phononic effects for T = 0-1000K and P = 0-50GPa. The obtained results are compared with other results where available. Further an analysis of optical functions for two polarization vectors reveals that the reflectivity is high in the visible-ultraviolet region up to ~ 10.5 eV region showing promise as good coating material. Keywords: Ti3Al1-xSixC2; First-principles; Quasi-harmonic Debye model; Mechanical properties; Band structure; Optical propertiesComment: 12 pages, 8 figure

Enforcing Against Terrorism by State and Non-State Actors

Moderator: Horace B. Robertson, Professor Emeritus, Duke University School of Law Presenters: Malvina Halberstam, Professor of Law, Yeshiva University, Benjamin N. Cardozo School of Law Neil C. Livingstone, President, Institute on Terrorism and Subnational Conflict Discussants: W. Michael Reisman, Wesley N. Hohfeld Professor of Jurisprudence, Yale Law School Yonah Alexander, Research Professor of Law, George Washington Universit

Epitaxial growth and electrical-transport properties of Ti(7)Si(2)C(5) thin films synthesized by reactive sputter-deposition

Epitaxial predominantly phase-pure Ti(7)Si(2)C(5) thin films were grown onto Al(2)O(3)(0 0 0 1) by reactive magnetron sputtering. The c-axis lattice constant is similar to 60.2 angstrom; the Ti(7)Si(2)C(5) unit cell comprises alternating Ti(3)SiC(2)-like and Ti(4)SiC(3)-like half-unit-cell stacking repeated three times. Elastic recoil detection analysis showed a few percent of nitrogen in the films from the acetylene gas used. The nitrogen-induced stabilization mechanism for Ti(7)Si(2)C(5) relative to Ti(3)SiC(2) and Ti(4)SiC(3) is discussed. Electrical-transport measurements showed metallic temperature dependence and a room-temperature resistivity of similar to 45 mu Omega cm.Funding Agencies|European Research Council||Swedish Research Council||Swedish Foundation for Strategic Research||NSF|08214060960003