Magneli Phases Research Articles

Preparation of suboxides in the Ti-O system by reactive sputtering

The preparation of non-stoichiometric TiO x films by reactive sputtering of titanium in an argon-oxygen plasma was studied in an r.f. diode sputtering system at 2 keV bias voltage. The properties of titanium oxide films deposited from oxidized and non-oxidized Ti targets were explored to determine the composition of the deposited phases. The mole fraction x of oxygen in the deposited films was determined from a combined analysis of deposition rate, substrate density and UV and IR spectra. The results indicate that phases with x = 0, x = 1.0 and x>1.6 (the Magneli phases and TiO 2) can be readily produced during reactive sputtering in argon plasmas containing an oxygen mole fraction in the range 0-0.15. The deposition of suboxide phases other than α-TiO δ, TiO 1±δ required stringent control of the processing variables.

Improved solar energy conversion efficiencies for the photocatalytic production of hydrogen via TiO 2 semiconductor electrodes

Overall solar energy conversion efficiencies of at least 0.8%, somewhat greater than those previously reported, have been obtained in the photocatalytic production of hydrogen using TiO 2 semiconductor electrodes prepared by heat-treatment of titanium metal foils. A shift in the threshold frequency for hydrogen evolution toward energies slightly lower than the 3.02 eV bandgap of TiO 2 has been observed for some of these electrodes. This result supports the possibility of utilizing a greater portion of the solar spectrum, thereby increasing overall conversion efficiency even further. A tentative explanation of this shift involves the presence of mixed phases of the titanium-oxygen system (Magneli phases) in the semiconducting film. An outline of the work in progress is given.

Phase equilibria in the system MnO−TiO 2−Ti 2O 3 at 1473°K

The oxygen fugacity-composition isotherm at 1473°K has been established for the MnO−TiO2−Ti2O3 system for fo2 in the range 10−13.5–10−18 atm. The quenching method was used and control of oxygen fugacity was achieved with CO2/H2 mixtures. Phase equilibria results are presented for the reduced rutile Magneli phases, TinO2n−1−MnTin−1O2n−1, and for the partial solid solutions Ti3O5−“MnTi2O5” (M3O5), Ti2O3−MnTiO3 (α-oxide), and MnTi2O4−Mn2TiO4 (spinel). Loss of manganese by volatilization is a problem in this system at low oxygen fugacities.

Growth of titanium oxide crystals of controlled stoichiometry and order

Growth of single crystals of the intermediate oxides of titanium, important because of the unique electrical and magnetic properties of these materials, is difficult to achieve in a controlled manner. The difficulties arise for several reasons: the large number of phases in the TiO system, their defect structure and potential for both equilibrium and nonequilibrium disorder of the defects and the change of the oxygen fugacity of the co-existing equilibrium vapor phase with temperature. Precise phase equilibria including solid-vapor equilibria are required and these were first determined in detail.Once these are known several common techniques can be applied to the crystal growth. Reduced rutile has been grown by flame fusion methods for years. We have grown fully oxidized TiO2 at a much lower temperature in hydrothermal highly acidic environments. The higher titanium oxides - TiO2O3, Ti3O3, and the Magneli phases from Ti4O7 to Ti10O19 have been grown from borate fluxes by precise control of the oxygen fugacity in the furnace atmosphere. Extensive characterization of the oxides indicates an unexplored region of possible ordered Magneli phases between Ti10O19 and approximately Ti100O199. Growth of these phases from fluxes with ultra-precise temperature and atmosphere control does not seem impossible.The lower titanium oxides cannot be grown in open systems because of the extremely low oxygen fugacities. The arc-fusion technique of T. B. Reed has proved the best method for preparing high TiO and other phases stable at the melting point. Low TiO remains a crystal-growth problem, as do the oxygen-rich titanium metal phases.

Growth of the intermediate oxides of titanium from borate fluxes under controlled oxygen fugacities

The Magneli phases (TinO2n−1) and Ti3O5 can be grown from Na2B4O7-B2O3 fluxes under equilibrium conditions if the oxygen fugacity of the coexisting vapor phase is controlled within narrow limits. Different compounds can be obtained by adjusting po2 over a narrow range.