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Bang J.,Materials and Structures Laboratory | Matsuishi S.,Materials Research Center for Element Strategy | Hiraka H.,High Energy Accelerator Research Organization | Fujisaki F.,Graduate University for Advanced Studies | And 11 more authors.
Journal of the American Chemical Society | Year: 2014

Compositionally tunable vanadium oxyhydrides Sr2VO 4-xHx (0 ≤ x ≤ 1.01) without considerable anion vacancy were synthesized by high-pressure solid-state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands. © 2014 American Chemical Society.

Murao R.,Tohoku University | Kikuchi M.,Tohoku Fukushi University | Atou T.,Materials and Structures Laboratory | Kusaba K.,Tohoku University | And 4 more authors.
Journal of Physics: Conference Series | Year: 2010

High-pressure behaviour of Ta3Si intermetallic compound was investigated by shock compression and static compression methods. Superconducting phase with TC = 9.3 K was found in the sample shocked to 50-61 GPa, however most of the shock recovered sample indicated the starting stable phase with the Ti3P-type structure. The new superconducting phase was not obtained from static compression up to 15 GPa and 800 °C. Bulk modulus of Ta3Si with the Ti3P-type structure was determined to be K0 = 246(4) GPa. The present results suggest that a rapid phase transformation occurred during shock compression, but most of the high-pressure phase was reverted to the stable phase in the decompression process. © 2010 IOP Publishing Ltd.

Morin C.,Materials and Structures Laboratory | Moumni Z.,Materials and Structures Laboratory | Zaki W.,Khalifa University
International Journal of Plasticity | Year: 2011

In this paper, we examine the influence of thermomechanical coupling on the behavior of superelastic shape memory alloys subjected to cyclic loading at different loading rates. Special focus is given to the determination of the area of the stress-strain hysteresis loop once the material has achieved a stabilized state. It is found that this area does not evolve monotonically with the loading rate for either transient or asymptotic states. In order to reproduce this observation analytically, a new model is developed based on the ZM model for shape memory alloys which was modified to account for thermomechanical coupling. The model is shown to predict the non-monotonic variation in hysteresis area to good accord. Experimentally observed variations in the temperature of SMA test samples are also correctly reproduced for lower strain rates. © 2011 Elsevier Ltd. All rights reserved.

Zaki W.,Khalifa University | Moumni Z.,Materials and Structures Laboratory | Morin C.,Materials and Structures Laboratory
Mechanics of Advanced Materials and Structures | Year: 2011

In this article, the Zaki-Moumni (ZM) model for shape memory alloys is extended to account for tensile-compressive asymmetry over a wide temperature range. To this avail, a mathematical framework recently developed by Raniecki and Mroz is utilized to define new yield functions that are sign-sensitive. With respect to the original ZM model, the modifications are essentially made to the expressions of the Helmholtz free energy and of the internal constraints. The model is shown to properly simulate the asymmetric behavior of shape memory alloys both for martensite orientation and pseudoelasticity. Copyright © Taylor & Francis Group, LLC.

Morin C.,Materials and Structures Laboratory | Moumni Z.,Materials and Structures Laboratory | Zaki W.,Modeling and Simulation Unit
International Journal of Plasticity | Year: 2011

This paper presents a generalized Zaki-Moumni (ZM) model for shape memory alloys (SMAs) [cf. Zaki, W., Moumni, Z., 2007a. A three-dimensional model of the thermomechanical behavior of shape memory alloys. J. Mech. Phys. Solids 55, 2455-2490 accounting for thermomechanical coupling. To this end, the expression of the Helmholtz free energy is modified in order to derive the heat equation in accordance with the principles of thermodynamics. An algorithm is proposed to implement the coupled ZM model into a finite element code, which is then used to solve a thermomechanical boundary value problem involving a superelastic SMA structure. The model is validated against experimental data available in the literature. Strain rate dependence of the mechanical pseudoelastic response is taken into account with good qualitative as well as quantitative accuracy in the case of moderate strain rates and for mechanical results in the case of high strain rates. However, only qualitative agreement is achieved for thermal results at high strain rates. It is shown that this discrepancy is mainly due to localization effects which are note taken into account in our model. Analyzing the influence of the heat sources on the material response shows that the mechanical hysteresis is mainly due to intrinsic dissipation, whereas the thermal response is governed by latent heat. In addition, the variation of the area of the hysteresis loop with respect to the strain rate is discussed. It is found that this variation is not monotonic and reaches a maximum value for a certain value of strain rate. © 2010 Elsevier Ltd. All rights reserved.

Zaki W.,Modeling and Simulation Unit | Morin C.,Materials and Structures Laboratory | Moumni Z.,Materials and Structures Laboratory
IOP Conference Series: Materials Science and Engineering | Year: 2014

This paper presents an outline for a new uniaxial model for shape memory alloys that accounts for thermomechanical coupling. The coupling provides an explanation of the dependence of SMA behavior on the loading rate. 1D simulations are carried in Matlab using simple finite-difference discretization of the mechanical and thermal equations. © 2010 IOP Publishing Ltd.

Komanoya T.,Materials and Structures Laboratory | Komanoya T.,Frontier Research Center | Nakajima K.,Materials and Structures Laboratory | Nakajima K.,Precursory Research for Embryonic Science and Technology PRESTO | And 4 more authors.
Journal of Physical Chemistry C | Year: 2015

Early transition metal oxides, TiO2, ZrO2, and Nb2O5, were studied as heterogeneous catalysts for the Meerwein-Ponndorf-Verley (MPV) reduction of cyclohexanone in 2-propanol. Despite a small amount of Lewis acid sites and weak Lewis acid strength, ZrO2 was clearly superior to TiO2 and Nb2O5 with respect to reaction rate. Fourier transform infrared spectroscopy (FT-IR) and temperature-programmed-desorption (TPD) measurements revealed that ZrO2 has large amounts of base sites that activate the methylene groups in 2-propanol bonded to Lewis acid sites. Various analyses, including experiments using isotopic 2-propanol, suggest that efficient MPV reduction over ZrO2 is due not only to Lewis acid strength and density, but also to a synergistic effect of base and Lewis acid sites. © 2015 American Chemical Society.

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