Electronic structure and magnetism in two-dimensional hexagonal 5d transition metal carbides, Tan+1Cn (n=1,2,3)

Density functional calculations are used to investigate the electronic structure of two-dimensional 5d tantalum carbides with honeycomb-like lattice structures. We focus on changes in the low-energy bands near the Fermi level with dimensionality. We find that the Ta 5d states dominate, but the extended nature of the wavefunctions makes them weakly correlated. The carbide sheets are prone to long range magnetic order. We evaluate the stability of these states to enhanced electron--electron interactions through a Hubbard U correction. Lastly, we find spin orbit interactions strongly renormalize the band structure for n=2, but play a minor role in n=1 and 3.Comment: 4 pages, 4 figure

Mechanical properties of the MAX phases

From Buschow, K. H. J. (2001). Encyclopedia of materials : science and technology. Amsterdam: New York: pp.1-16. Retrieved September 19, 2006 from http://www.ece.drexel.edu/mml/pubs/jps_paper.pdf

Isothermal Oxidation of Ti\u3csub\u3e2\u3c/sub\u3eSC in Air

The oxidation behavior of fully dense Ti2SC was studied thermogravimetrically in air in the 500–800°C temperature range. The oxidation product was a single-layer of rutile in all cases. At 800°C, the oxide layer was not protective and the oxidation kinetics were rapid. At 600 and 700°C, and up to ~50 h, the kinetics were parabolic before they became linear. It was only at 500°C that the weight gain reached a plateau after a 50 h initial parabolic regime. Mass spectrometry of the gases evolved during oxidation confirmed that both CO2 and SO2 are oxidation products. The overall oxidation reaction is thus Ti2SC + 4O2 → 2TiO2 + SO2 + CO2. On the basis of this and previous work, we conclude that oxidation occurs by the outward diffusion of titanium, sulfur, and carbon, the latter two either as atoms or in the form of CO2 and SO2 and, most probably, the inward diffusion of oxygen. Mesopores and microcracks were found in all rutile layers formed except those formed at 500°C. The presence of these defects is believed to have led to significantly higher oxidation rates as compared to other rutile-forming ternary carbides, such as Ti3SiC2

Vibrational Behavior of the M\u3csub\u3en+1\u3c/sub\u3e\u3cem\u3eAX\u3csub\u3en\u3c/sub\u3e\u3c/em\u3e Phases from First-Order Raman Scattering (\u3cem\u3eM\u3c/em\u3e=Ti,V,Cr, \u3cem\u3eA\u3c/em\u3e=Si, \u3cem\u3eX\u3c/em\u3e=C,N)

We report on the Raman spectra of Ti3SiC2 (312), M2AlC(211) (M=Ti, V, Cr, and Nb) and Ti4AlN3 (413), as representative compounds from the family of Mn+1AXn phases. Intense and narrow first-order Raman peaks are observed, and we present an analysis of the spectra based on symmetry considerations and from results of first-principles calculations of phonon frequencies. The agreement between experimental and calculated mode energies is excellent. The identification of the modes enables application of Raman scattering as a diagnostic tool for the detailed study of the structural and physical properties of this family of compounds and their engineered solid solutions

Isothermal oxidation of Ta2ALC in air

Journal of the American Ceramic Society, 89(9): pp. 2974-2976.The oxidation behavior, in air, of bulk polycrystalline Ta2AlC samples was studied in the 6001–9001C temperature range. A protective, essentially X-ray amorphous, oxide layer—which does not appear to be resistant to thermal cycling—forms at 6001C. This layer is comprised of Ta, Al, and O. At 7001C and above, it is compositionally quite uniform, but porous and highly cracked. No phase separation was observed at the micrometer scale. The oxide layers consisted of the crystalline phases, Ta2O5 and TaAlO4, and an X-ray amorphous phase. In the 7001–9001C temperature range, the oxidation kinetics were found to be linear

Understanding the Effect of Sodium Polyphosphate on Improving the Chemical Stability of Ti3c2tz Mxene in Water

Degradation of MXenes in aqueous environments severely limits the application and industrialization of this large family of two-dimensional (2D) materials. Hydrolysis and oxidation are now considered as two main degradation mechanisms and while significant efforts have been directed to prolonging the shelf-life of MXenes, separating and studying their degradation mechanisms have lagged behind. Herein, gas analysis via gas chromatography and Raman spectroscopy were used to investigate the effect of sodium polyphosphate, PP, on the degradation of Ti3C2Tz MXene. Transmission and scanning electron microscopies, as well as X-ray photoelectron spectroscopywere also used as complimentary techniques to support conclusions derived from gas analysis and to confirm the extent of degradation via characterization of solid reaction products. Based on these studies we have determined that the addition of PP to an equal mass of Ti3C2Tz solution can effectively suppress hydrolysis and protect Ti3C2Tz from degradation