V.Fokin, E.Fokina, and B.Tarasov
Institute of Problems of Chemical Physics of the Russian Academy of Sciences, Chernogolovka, Russia Email: fvn@icp.ac.ru
Earlier by us [1] it has been shown, that at interaction of titanium powder with the size of particles about 100 microns with ammonia under initial pressure of 0.7–0.8 MPa at the temperature 250–300°C in the presence of NH4Cl the high-dispersed dihydride of titanium is formed, containing 0.1–0.15 atoms of nitrogen on a molecule of dihydride, with parameters of a tetragonal lattice a = 0.4468 and c = 0.4391 nm. The powder has a specific surface area of 30–55 m2/g and is steady in an inert atmosphere up to 600°C. Such value of a specific surface area corresponds to the sizes of powder particles of 15–30 nm.
In the given work the processes occurring in systems TiAl–NH3, Ti3Al–NH3, Ti10.1Al–NH3
and Ti15.7Al–NH3 in an interval of the temperatures 100–500°C under ammonia pressure of
~1.5 MPa are investigated in the presence of activator NH4Cl. Initial powders of alloys TiAl and Ti3Al with the average size of particles about 100 microns represent single-phase intermetallic compounds, and powders of alloys Ti10.1Al and Ti15.7Al are the solid solutions of 9 and 6 at. % Al in α-Ti.
It is established, that at interaction of the such solid solutions, i.e. practically of the titanium with small additives of aluminium, with ammonia the formation of cubic and/or tetragonal modification of titanium dihydride with the size of particles of 35–45 nm are observed only at temperature of 300°C, that can testify to decrease of a rate of titanium hydrogenation at doping by aluminium.
The appreciable interaction of TiAl with NH3 begins at temperature of 100°C. At treatment at 150°C the hydride phases of composition TiAlH0.7–1.5 with minor alteration of parameters of a crystal lattice of initial intermetallide are formed. At the temperature 300°C the hydride-nitride of titanium aluminide of composition TiAlH0.6N0.2 was obtained as a powder with the size of particles about 0.3 microns.
Intermetallide Ti3Al in an ammonia atmosphere is steadier – the hydride-nitride of titanium aluminide with the size of particles of 0.25–0.15 microns is formed only at temperatures of 450–500°C.
Reference
1. V. Fokin, E. Fokina, B. Tarasov, S. Shilkin, Int. J. Hydrogen Energy, 24 (1999), 111- 114.
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Formation of Transition Metal Hydrides at High Pressures
O. Degtyareva, C.L. Guillaume, J.E. Proctor, E. Gregoryanz
Centre for Science at Extreme Conditions and School of Physics & Astronomy, The University of Edinburgh, Edinburgh, EH9 3JZ, United Kingdom
Email: o.degtyareva@ed.ac.uk
There is a considerable interest in producing metallic, and possibly superconducting, states of hydrogen at multimegabar pressures; the pressures that are thought to be needed for metallization (>400 GPa) are however currently not achievable with static compression techniques. It has been recently suggested that hydrogen-rich compounds such as CH4, SiH4, and GeH4 with hydrogen being “chemically pre-compressed” will require pressures far less than expected for pure hydrogen at equivalent densities to enter metallic states [1,2]. As is the case for pure hydrogen, these compounds are considered to be good candidates for high temperature superconductors in their dense metallic forms. For silane (SiH4), however, there is a disagreement between different theoretical studies with metallization pressure ranging from above 91 GPa [2] to as high as 220-250 GPa [3,4].
Recent experimental study claimed a discovery of metallization and superconductivity of silane at pressures above 50 GPa and reported a hexagonal close-packed (hcp) structure for the metallic phase of silane [5]. On further compression above 110 GPa this phase is found to partially transform to a molecular insulating phase with a positive volume change of 25% which co-existed with the metallic hcp phase up to 190 GPa [5] – an observation that contradicts thermodynamic rules (i.e. Le Chatelier's principle). Subsequent ab initio calculations [4,6] showed that the proposed metallic hcp structure is mechanically unstable suggesting a possible partial dissociation and a phase of a different composition. These discrepancies between various theoretical studies as well as experimental work prompted us to further investigate the crystal structure of silane at high pressures.
Using raman and x-ray diffraction techniques we found [7] that silane partially decomposes at high pressures and room temperature into pure Si and hydrogen, where released hydrogen readily reacts with the surrounding metals in the diamond anvil cell chamber forming metal hydrides. We find a formation of Re hydride after decomposition of silane and reaction of hydrogen with the Re gasket. We also identify the recently reported metallic hcp phase of silane [5] as PtH [8], that forms upon the decomposition of silane and reaction of released hydrogen with platinum metal that is present in the sample chamber. Thus, silane is shown to be acting as an internal hydrogen source in the high-pressure synthesis of metal hydrides.
References
1. N.W. Ashcroft, Phys. Rev. Lett. 92 (2004), 187002.
2. J. Feng et al., Phys. Rev. Lett. 96 (2006), 017006.
3. C. Pickard and R. Needs, Phys. Rev. Lett. 97 (2006), 045504.
4. M. Martinez-Canales et al., Phys. Rev. Lett. 102 (2009), 087005.
5. M. Eremets et al., Science 319 (2008), 1506.
6. D. Kim et al., PNAS 105 (2008), 16454
7. O. Degtyareva et al., Solid State Commun. 149 (2009) 1583-1586
8. N. Hirao, H. Fujihisa, Y. Ohishi, K. Takemura, T. Kikegawa, International Symposium on Metal-Hydrogen Systems, Reykjavik, Iceland, 2008; See also Acta Cryst. A 64 (2008) C609-C610.
183
Hydrogenation of TiNi Shape Memory Alloy Produced by Mechanical Alloying
T. Saito, T. Yokoyama and A. Takasaki
Department of Engineering Science and Mechanics, Shibaura Institute of Technology, Tokyo, Japan Email: m408030@shibaura-it.ac.jp
TiNi shape memory alloy has shape memory effect due to thermo-elastic martensitic transformation between B2 parent and monoclinic martensitic phases. Effect of hydrogen on the martensitic transforamation has been studied by several researchers. However, the conclusion is still contraversial. Mutiple-stage transformation was shown after hydrogenation [1], on the other hand, there was report that martensitic transformation temperature was lowered by cathodic hydrogen charging [2]. Mechanical alloying (MA) and direct current sintering are one of ways to produce powder and bulk alloy by solid state reaction, for which we have previously reported [3, 4]. In this study, influence of hydrogen on martensitic transformation behavior of TiNi shape memory alloy produced by a combination of MA and direct current sintering was investigated. Commercially pure Ti and Ni elemental powders with chemical composition of Ti50Ni50 (at%) were mechanically alloyed in stainless steel vials (45ml) with several stainless steel balls in an argon atmosphere. The powder after MA was compacted and sintered at 973K for 10min under vacuum condition (2Pa) at a pressure of 20MPa. The sintered bulk sample was then annealed in a furnace at 773K for 1 hour under a high vacuum condition (10-3Pa). Gaseous hydrogen charging was conducted at several hydrogen pressures, temperatures and times to control hydrogen concentration in the sample in order to evaluate the relation between hydrogen concentration and their martensitic transformation behaviors. The phases of the samples were determined by X-ray diffraction (XRD) measurement. The martensitic transformation behavior was determined by differential scanning calorimetry (DSC), and the hydrogen desorption property was measured by thermal desorption spectroscopy (TDS).
The bulk samples after MA and direct current sintering consisted of TiNi (B2) phase with a small amount of Ti2Ni phase, and showed martensitic transformation (exthothermic) and reverse transformation (endothermic). The TiNi hydride, with TiNi (B2) phase and unknown phase, was observed after hydrogenation. The martensitic and reverse transformations were observed after hydrogenation, however, enthalpy of the martensitic transformaiton was smaller than that of the reverse transforamtion. Eventually, enthalpy of the martensitic transformation turned to be similar to that of the reverse transformation after 1 cycle of the transformations. It is implied that hydrogen-induced martenstic transfroamtion occured during hydrogenation.
References
1. T. Ohba, F. Yanagita, M. Mitsuka, T. Hara and K. Kato, Mater. Trans., 43 5, (2002), 798.
2. W. Jihong, Z. Xiaotao, W. Zhiguo, L. Yanzhang, Rare Metals, 24 2, (2005), 190.
3. A. Takasaki, Phys. Stat. Sol. (a), (1998), 183.
4. T. Saito, A. Takasaki, Trans. Mater. Res. Soc. of Japan, 34[3], (2009), 403.
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