Temperature-programmed desorption and surface reaction of thiophene over co-mo/.gamma.-Al2O3 hydrodesulfurization catalysts

The temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) of thiophene over a series of Co-Mo/gamma-Al2O3, hydrodesulfurization (HDS) catalysts with varying Co to Mo ratios have been studied with the objective of understanding the promotional role of Co in the HDS reaction. As part of the study, the desorptions (TPD) and hydrogenations (TPSR) of butane, butene, and butadiene over these catalysts have also been investigated. The TPD of the hydrocarbons over catalysts containing no Co showed a single desorption profile while incorporation of Co created an additional site, with higher heats of desorption, without significantly affecting desorption from the original site. The TPSR measurements showed that the two sites had separate and independent activity for the hydrogenation of the C-4 hydrocarbons. The TPD of thiophene over catalysts with varying Co to Mo ratios showed a single desorption profile with identical heats of desorption, implying that Co does not affect or influence the adsorption sites for thiophene. The TPSR of the HDS of thiophene, however, showed that, although the products of the HDS reaction-butane, butene, and H2S-are the same irrespective of the Co content, the temperature profiles and the activation barriers for the formation of these species show considerable change with the Co/Co+Mo ratio. The results are discussed in light of the existing models for the promotional role of Co in the HDS reaction.

Effect of microstructure on the creep behaviour of Ti-24Al-11Nb

In this investigation, the influence of microstructure on the high temperature creep behaviour of Ti-24Al-11Nb alloy has been studied. Different microstructures are produced by devising suitable heat treatments from the beta phase field. Creep tests are conducted in the temperature range of 923-1113 K, over a wide stress range at each temperature, employing the impression creep technique. The creep behaviour is found tb be sensitive to the crystallographic texture as well as to the details of microstructure. Best creep resistance is shown when the microstructure contains smaller alpha(2) plates and a lower beta volume fraction. This can be understood in terms of the dislocation barriers offered by alpha(2) beta boundaries and the case of plastic flow in the beta phase at high temperatures.

Electron donor-acceptor complexes of I-2 with diethyl ether and diethyl sulphide. An ab initio MO study

The charge-transfer complexes of I-2 with the n-donors diethyl ether and diethyl sulfide were studied at the Hartree-Fock and MP2 levels. The structures were fully optimized using the 3-21G((*)) basis set as well as with effective core potentials. The calculations consistently yield a C-2v structure for the ether-I-2 complex, but an unsymmetrical form for the sulfide-I-2 complex. A natural bond orbital analysis and the BSSE-corrected complexation energies reveal stronger interactions in the sulfide complex. The computed orbital energies of the monomers and complexes reproduce the trends in experimentally observed vertical ionization potentials.

Coherent phonons in pyrochlore titanates A(2)Ti(2)O(7) (A = Dy, Gd, Tb): A phase transition in Dy2Ti2O7 at 110 K

We study the generation of coherent optical phonons in spin-frustrated pyrochlore single crystals Dy2Ti2O7, Gd2Ti2O7, and Tb2Ti2O7 using femtosecond laser pulses (65 fs, 1.57 eV) in degenerate time-resolved transmission experiments as a function of temperature from 4 to 296 K. At 4 K, two coherent phonons are observed at similar to 5.3 THz (5.0 THz) and similar to 9.3 THz (9.4 THz) for Dy2Ti2O7 (Gd2Ti2O7), whereas three coherent phonons are generated at similar to 5.0, 8.6, and 9.7 THz for Tb2Ti2O7. In the case of spin-ice Dy2Ti2O7, a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of lower-energy coherent phonon mode, indicating a subtle structural change at 110 K. Another important observation is a phase difference of pi between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes could be different in nature, unlike a purely impulsive or displacive mechanism.

Gibbs energies of formation of UMoC1.7 and UMoC2

The chemical potentials oi carbon associated with two three-phase fields in the system U-Mo-C were measured by using the methane-hydrogen gas equilibration technique in the temperature range 973 to 1173K. The technique was validated by measuring the standard Gibbs energy of formation of Mo2C. From the experimentally measured values of the chemical potential of carbon in the ternary phase fields UC+Mo+UMoC1.7 and UC+UMoC1.7+UMoC2 and data for UC from the literature, the Gibbs energies of formation of the two ternary carbides were derived:

Metastable phase formation during mechanical alloying of Al-Ge and Al-Si alloys

In this paper we report the mechanical alloying behaviour of elemental aluminium with diamond cubic elements Ge and Si. A metastable crystalline phase with rhombohedral crystal structure forms in Al-70 Ge-30 and Al-60 Ge-40 alloy compositions. The phase always coexists with elemental constituents and decomposes over a broad temperature range. No such metastable phase could be observed in the Al-Si system. We also report X-ray diffractometry and differential scanning calorimetry results suggestive of amorphization. Finally a comparison was made of the present result with that obtained in rapid solidification.

Correlation of resistivity and microstructure in dilute titaniulll–nitrogen alloy

The precipitation kinetics of the quenched dilute Ti-1.6 at.-%N alloy has been followed by resistivity measurements at 77 K using the four probe method. Resistivity behaviour has been studied for various durations for the alloys aged in the temperature range 273-373 K. The resistivity behaviour has been explained on the basis of the growth and decay of interfacial strain fields associated with the precipitation process. In addition, the resistivity changes have been correlated with transmission electron microscopy observations. (C) 1995 The Institute of Materials.

Quasicrystals, crystals and multiple twins in rapidly solidified Al-Cr-Si, Al-Mn-Si and Al-Mn-Cr-Si alloys

The coexistence of quasicrystals and rational approximant structures (RAS) has been observed in melt-spun Al80Cr14Si6, Al80Mn14Si6 and Al75Mn10Cr5Si10 alloys. The presence of a b.c.c. alpha-AlMnSi phase in Al-Mn-Si and alpha-AlMnSi(Cr) phase in Al-Mn-Cr-Si has been seen. A multiple twinning around an irrational axis of the RAS has been reported in an aggregate of fine size cubic crystallites in all three alloys. Selected area diffraction patterns show that the crystalline aggregate symmetry is linked to the icosahedral point group symmetry (m35). Various ways of expressing the twin relationship in the cubic crystalline aggregates have been discussed. The thermal stability of the icosahedral phase at high temperatures reveals that the icosahedral phase in Al-Mn-Si and Al-Mn-Cr-Si alloys transforms to alpha-AlMnSi at temperatures of 690 and 670 K, respectively. In Al-Cr-Si alloy, heating to a high temperature (615 K) leads to the transformation of the icosahedral phase into a new metastable phase having an ordered cubic structure equivalent to alpha-AlMnSi. The occurrence of multiple twinning leading to icosahedral symmetry in the as-spun Al-Cr-Si alloy is presumably due to this metastable phase. Copyright (C) 1996 Acta Metallurgica Inc.

Electron microscopy and diffraction of ordering in Ni---W alloys

Electron microscopy and diffraction studies of ordering in stoichiometric Ni-20%W and off-stoichiometric Ni-15%W alloys have been carried out. The specimens of Ni-20%W were first disordered at 1398 K for 4 h and then quenched rapidly into water. Short range order (SRO) spots were observed at {1 1/2 0}* positions. Two hitherto unknown metastable phases: D-2h(25)-Ni2W and DO22-Ni3W were observed in the diffraction patterns. Long range order (LRO) transformations were studied at 1103 and 1213 K. Kinetics and mechanism of transformations have been identified. Ni-15%W specimens were solution treated at 1523 K for 1 h followed by quenching in water. SRO spots similar to those found in Ni-20%W were observed in this alloy as well. The transition to LRO was studied at 1093 K. Distinct Ni4W precipitates could be observed after 5 h of annealing at this temperature. After 100 h of annealing precipitates were found to grow into faceted shape coherent with the disordered matrix. After prolonged annealing for over 150 h the Ni4W precipitates began to lose coherency by the generation of misfit dislocations. The microstructural observations have been compared for the stoichiometric and off-stoichiometric alloys.

The influence of pH on the corrosion of medium strength aerospace alloys 8090, 2091 and 2014

The influence of pH on the corrosion behaviour of two aluminium-lithium-copper-magnesium-zirconium (8090 and 2091) alloys was studied and compared with a standard aircraft alloy, 2014 (Al-4.4% Cu) and 99.9% pure Al. In constant exposure and potentiodynamic polarization studies conducted in 3.5% NaCl solution having different pH values, all the alloys exhibited high corrosion rates in acidic and alkaline environments, with a minimum in less hostile environments close to neutral pH. The pitting potentials for aluminium-lithium alloys were slightly lower than those for 2014 and pure Al. The effect of pH on the passive current density was also less for aluminium-lithium alloys.

Organometallic Chemistry of Diphosphazanes. 11. Reactions of fac-[Mo(CO)3(MeCN){.eta.2-Ph2PN(Pri)PPh(DMP)}] (DMP = 3,5-Dimethyl-1-Pyrazolyl) with Diphosphazanes and Diphosphinoalkanes and Oxidative Addition with Iodine

The use of fac-[Mo(CO)(3)(MeCN)(eta(2)-L(1))] (1a) {L(1) = Ph(2)PN(Pr-i)PPh(DMP)}(2) as a precursor to metalloligands and bimetallic, heterotrimetallic, and heptacoordinated complexes is reported. The reaction of 1a with diphosphazane, dppa, or a diphosphinoalkane such as dppm or dppe yields the fac-eta(1)-diphosphine substituted metalloligands, fac-[Mo(CO)(3)(eta(2)-L(1))(eta(1)-PXP)] {PXP = dppa (2), dppm (3), and dppe (4)}. These undergo isomerization to yield the corresponding mer-diphosphine complexes (5-7). Oxidation of the uncoordinated phosphorus atom of the mer-eta(1)-dppm-substituted complex eventually provides mer-[Mo(CO)(3)-(eta(2)-L(1)){eta(1)-Ph(2)PCH(2)P(O)Ph(2)}](8). The structure of the latter complex has been confirmed by single crystal X-ray diffraction {triclinic system, P ($) over bar 1; a = 11.994(3), b = 14.807(2), c = 15.855(3) Angstrom; alpha = 114.24(1), beta = 91.35(2), and gamma = 98.95(1)degrees; Z = 2, 4014 data (F-0 > 5 sigma(F-0)), R = 0.066, R(W) = 0.069}. Treatment of the dppe metalloligand 7 with [PtCl2(COD)] yields the heterotrimetallic complex cis-[PtCl2{mer-[Mo(CO)(3)(eta(2)-L(1))(eta(1)-dppe]}(2)] (9). Attempts to prepare a related trimetallic complex with the dppm-containing metalloligand were unsuccessful; only the tetracarbonyl complex cis-[Mo(CO)(4)(eta(2)-L(1))] (1b) and cis-[PtCl2(eta(2)-dppm)] were obtained. Reaction of la with dppe in the ratio 2:1 yields the mer-mer dinuclear complex [{mer-[Mo(CO)(3)(eta(2)-L(1))]}(2)(mu-dppe)] (10) bridged by dppe. Oxidation of 1a with iodine yields the Mo(II) heptacoordinated complex [MoI2(CO)(2)(eta(3)-L(1))] (11) with tridentate PPN coordination. The same Mo(II) complex 11 is also obtained by the direct oxidation of the tetracarbonyl complex cis-[Mo(CO)(4)(eta(2)-L(1))] (1b) with iodine. The structure of 11 has been confirmed by X-ray diffraction studies {monoclinic system, Cc; a = 10.471(2), b = 19.305(3), c = 17.325(3) Angstrom; beta = 95.47(2)degrees; Z = 4, 3153 data (F-0 > 5 sigma(F-0)), R = 0.049, R(W) = 0.051}. This complex exhibits an unusual capped-trigonal prismatic geometry around the metal. A similar heptacoordinated complex 12 with a chiral diphosphazane ligand {L(3) = (S,R)-P(h)2PN-(*CHMePh)*PPh(DMP)} has also been synthesized.

Oriented ferroelectric thin films of PbTiO3, (Pb,La)TiO3, and Pb(Zr,Ti)O3 by nebulized spray pyrolysis

Nebulized spray pyrolysis provides a good low?temperature chemical route for preparing thin films of PbTiO3, (Pb0.9,La0.1)TiO3 and Pb(Zr0.52,Ti0.48)O3. The films are a? or c? axis oriented, with spherical grains of ?30 nm and give satisfactory P?E hysteresis loops. © 1995 American Institute of Physics.

Low-temperature magnetoresistance of ?-phase FexNi80-xCr20 alloys near the critical composition for ferromagnetism

We present a comprehensive study of magnetoresistance (MR) of the crystalline pseudobinary ?-phase Fe alloy series FexNi80-xCr20 (50?x?66). This alloy series shows exotic magnetic phases as the composition (x) is varied. It has a critical composition for ferromagnetism at x=xc?59�60. MR was measured in the temperature range 1.7�110 K and up to a field of 7 T. The observed MR was small and the change was ?1%. The temperature dependence of MR was found to contain a positive and a negative contribution. The positive term was found to be ?H2 and it dominates at high field and high temperatures. We explain this as a manifestation of Kohler�s rule. The negative MR was found to have a quadratic dependence on magnetization M. The magnitude of the negative MR reaches a maximum as x?xc.

Mechanism of seizure of aluminium-silicon alloys dry sliding against steel

A steel ball was slid on aluminium-silicon alloys at different temperatures. After the coefficient of friction had been measured, the surface shear stress was deconvoluted using a two-term model of friction. The ratio of surface shear stress to bulk hardness was calculated as a function of temperature, silicon content and alloying additions. These results are qualitatively similar to those recorded for pre-seizure specimens slid against an En24 disc in a pin-on-disc machine. This similarity, when viewed in the context of the phenomenon of bulk shear, provides a model for seizure of these alloys.