Фазовая диаграмма системы H-Mg

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H-Mg (Hydrogen-Magnesium) A. San-Martin and F.D. Manchester Review of the current literature revealed no complete phase diagram for the Mg- H system. Only one (sketchy) temperature-composition isobar (T-X section) at 100 MPa [81Sha] and two contradicting sets [60Sta, 81Bel] of pressure- composition isotherms (P-X sections) were found. In a hydrogen-in-metal (metal hydride) system, the equilibrium pressure of the hydrogen surrounding the metal is always a significant thermodynamic variable, in contrast to most situations involving metallic alloys. Thus, sections of the P-X-T surface in a T-X plane and a P-X plane are always necessary. In the present assessment, P is the pressure (in pascals), T is the temperature ( plotted in both K and C), and X is the hydrogen concentration (expressed as X = H/Mg, the atomic ratio.) In the assessed T-X diagram, the principal features postulated by [81Sha] for an equilibrium pressure of 100 MPa were preserved. These were combined with the work of [60Sta], who provided more complete and reliable (P-X-T) equilibrium data (440 C < T < 560 C, P < 29 MPa). The assessed T-X diagram at 25 MPa consists of the cph a phase (the interstitial solid solution of H in Mg) and a b phase with tetragonal structure and nominal MgH2 stoichiometry. The a/(a + b) boundary constitutes the analytical approximation reported by [ 60Sta] for the terminal solid solubility. The temperatures for the reactions at 25 MPa, 643 C for L = a + H2 (eutectic type) and 566 C for a + H2 = b ( peritectic type) were determined from [81Sha] and [60Sta], respectively. Investigations of H dissolution in Mg encountered some difficulties due to the high vapor pressure and chemical reactivity of Mg. In addition, an ever- present Mg(HO)2 layer on the surface of the sample affected the H absorption or desorption. The final products of Mg(HO)2 decomposition are MgO and water below 420 C and oxide and H2 above 440 C. Under normal pressure, [81Fro] found that the MgO layer inhibits the H dissolution only between temperatures of 400 and 550 or 600 C. Nucleation of the b phase usually occurs at one or only a few points of a Mg particle surface [80Isl, 84Vig], where the oxide layer is unusually thin or is flawed in some respect. From TEM studies, [81Sch] reported that the a/b interface is rather planar and is oriented parallel to planes of the phase such as {011}, {001}, {110}, and {111}. [79Luz] showed that H diffused through the growing phase layer, although diffusion occurred slowly. Thus, when b phase nucleation takes place at many points or all over the surface of a Mg particle (depending on the particle prehistory), a continuous layer of the b phase is formed below the grain surface [84Ped]. In many situations, because of the low D values, a large amount of Mg is left in an unreacted state (unless an appropriately long time scale is involved). [78Sem] and [80Bas] reported that b, after being subjected to high compressive stress, partially transforms from the original tetragonal structure into a metastable orthorhombic phase (g) after release of the compressive stress. [ 85Lit] observed that the polymorphic transition b = g occurs at a pressure of 2.5 GPa and that the two phases coexist up to a pressure of 8 GPa. The metastable g phase totally reverts to the original b phase on heating at 350 C [78Sem]. [80Bas] observed that powdered samples of b phase, after being treated (1 to 3 h) at high pressures and temperatures (2.5 to 8 GPa and 650 to 800 C), sometimes produced a b phase modification, d phase. [80Bas] assumed that d was preserved by a favorable combination of the speed of cooling and/or pressure release, as well as by shear stresses developed in the samples. According to a recent investigation performed directly under high pressure [ 85Lit], the transformation from b to d phase occurs at a pressure of 8 GPa, and XRD patterns showing a "pure" d phase were obtained only at pressures >10 GPa. The d phase, with a density equal to 1.79 x 103 kg/m3 at room temperature [80Bas], is stable at atmospheric pressure up to temperatures of 400 C, where the b phase (density 1.417 x 103 kg/m3) begins to decompose around 380 C. [80Bas] also noted an endothermic effect between 350 and 400 C, and this was attributed to the reverse transition from the high-pressure phase (d) to the metastable (g) or to the original (b). 55Ell: F.H. Ellinger, C.E. Holley, Jr., B.B. McInteer, D. Pavone, R.M. Potter, E. Staritzky, and W.H. Zachariasen, J. Am. Chem. Soc., 77, 2647-2648 (1955). 60Sta: J.F. Stampfer, Jr., C.E. Holley, Jr., and J.F. Suttle, J. Am. Chem. Soc. , 82, 3504-3508 (1960). 63Zac: W.H. Zachariasen, C.E. Holley, Jr., and J.F. Stampfer, Jr., Acta Crystallogr., 16, 352-353 (1963). 78Sem: K.N. Semenenko, V.N. Verbestkii, Yu.A. Kalashnikov, N.V. Timofeeva, and M.I. Ioffe, Vest. Mosk. Univ. Ser. 2, Khim., 19, 718-722 (1978) in Russian. 79Luz: Z. Luz, J. Genossar, and P.S. Rudman, Scr. Metall., 14, 275-277 (1979). 80Bas: J.P. Bastide, B. Bonnetot, J.M. Letoffe, and P. Claudy, Mater. Res. Bull., 15, 1215-1224 (1980) in French. 80Isl: J.I. Isler, E. Joly, A. Barbet, and N. Gerard, C.R. Acad. Sci., Paris, Ser., C, 290, 317-320 (1980) in French. 81Bel: L. Belkbir, E. Joly, and N. Gerard, Int. J. Hydrogen Energy, 6, 285-294 (1981). 81Fro: R. Fromageau, J. Hillairet, E. Ligeon, C. Mairy, G. Revel, and P. Tzanetakis, J. Appl. Phys., 52, 7191-7195 (1981). 81Sch: T. Schober, Metall. Trans. A, 12, 951-957 (1981). 81Sha: V.I. Shapovalov, N.P. Serdyuk, and O.P. Semik, Dop. Akad. Nauk Ukr. RSR, A Fiz.-Mat. Tekh., (6) 99-101 (1981) in Ukrainian. 84Ped: A.S. Pedersen, J. Kjoller, B. Larsen, and B. Vigeholm, in Hydrogen Energy Progress V, Proc. 5th World Hydrogen Energy Conf., Toronto, Canada, 15- 20 Jul 1984, T.N. Veziroglu and J.B. Taylor, Ed., Pergamon Press, Vol. 3, 1269- 1277 (1984). 84Vig: B. Vigeholm, J. Koller, B. Larsen, and A.S. Pedersen, in Hydrogen Energy Progress V, Proc. 5th World Hydrogen Energy Conf., Toronto, Canada, 15- 20 Jul 1984, T.N. Veziroglu and J.B. Taylor, Ed., Pergamon Press, Vol. 3, 1455- 1463 (1984). 85Lit: L.M. Lityagina, T.I. Dyulleva, S.S. Kabalkina, T.N. Dimova, and V.G. Losev, Goekhim, (1), 118-120 (1985) in Russian. Published in Bull. Alloy Phase Diagrams, 8(5), Oct 1987. Complete evaluation contains 4 figures, 2 tables, and 62 references. 1