Фазовая диаграмма системы H-Ni
К оглавлению: Другие диаграммы (Others phase diargams)
H-Ni (Hydrogen-Nickel)
M.L. Wayman and G.C. Weatherly
The behavior of the Ni-H system depends strongly on the activity (pressure) of
hydrogen. In equilibrium with hydrogen pressures of 1 atm or less, Ni exhibits
the features of a classical "endothermic occluder" of hydrogen, in that it
reacts endothermically with hydrogen. The result is a very low solubility (
interstitial) of hydrogen in Ni at room temperature (H/Ni › 3 x 10-5) and,
most characteristically, a positive temperature dependence of solubility. The
solubility of hydrogen is an order of magnitude higher at 1000 atm than at 1
atm. The assessed evaluation of the Ni-H system is based primarily on review
of the experimental data of [67Jan, 67Maj, 73Rob, 75Mcl, 78Pon, 80Ser, 81Bar,
and 84Mcl].
At hydrogen pressures in the range of 10 to 100 MPa, hydrogen appreciably
reduces the melting point of Ni, but this effect diminishes above 40 MPa
hydrogen pressure. The system exhibits a eutectic point at 1679 K (1406 C)
and H/Ni › 0.036 [81Sha]. The behavior at a hydrogen pressure of 100 MPa is
comparable [80Ser]. The microstructure of Ni that has been melted and then
quenched from the liquid phase in a hydrogen atmosphere (melt composition up
to H/Ni = 0.02) consists of a eutectic mixture of solid metal with hydrogen
gas bubbles [79Sha]. Nickel hydride is not observed following such treatments.
When Ni is exposed to environments in which the hydrogen activity is very high,
corresponding to hydrogen pressures well in excess of 100 MPa, the behavior
of the Ni-H system changes markedly. Such high hydrogen activity can be
obtained by electrochemically (cathodically) charging the Ni in an electrolyte,
for example 1N H2SO4 containing a hydrogen recombination poison such as
thiourea or arsenic trioxide. Alternatively, Ni can be subjected to high
pressures of hydrogen in a pressure cell; cells capable of hydrogenation at
pressures as high as 7000 MPa of hydrogen have been reported [78Pon].
The high-pressure cell techniques yield a controlled atmosphere of well-
defined hydrogen activity, which can be maintained long enough to ensure that
the system has reached equilibrium [81Bar]. The attainment of high hydrogen
activity is facilitated by the high fugacity coefficient of hydrogen, which
can often exceed a value of 100, thus permitting higher hydrogen activity at
lower hydrogen pressure than would otherwise be the case [84Bar].
With the application of pressures of hydrogen in excess of 1000 MPa, the Ni-H
system exhibits an equilibrium surface in pressure-temperature-composition
space. This surface is markedly similar to the pressure-temperature-vapor
surface for a free gas. The critical point, below which the Ni-H solid
solution decomposes to form a hydrogen-poor a solid solution and a hydrogen-
rich nonstoichiometric bNi hydride, lies in the vicinity of 623 to 703 K and
1600 to 1900 MPa of hydrogen [79Bar]. The strong pressure dependence of the Ni-
H system corresponds to a transition from an endothermic reaction with
hydrogen at low pressures to an exothermic reaction at high hydrogen pressures.
The hydrogen pressure necessary for the formation of bNi hydride-about 600 MPa-
is significantly higher than its decomposition pressure of 340 MPa. The
decomposition pressure is believed to represent the equilibrium value of
pressure for a-b coexistence [63Sch].
As hydrogen is absorbed at room temperature under high pressure conditions,
the terminal solid solubility amax is soon reached at H/Ni < 0.1. Further
absorption of hydrogen occurs by the formation of the b hydride in equilibrium
with the a solid solution. The b hydride phase has the same fcc crystal
structure as Ni and the a solid solution, but with a high concentration of
interstitial H atoms and, consequently, a larger lattice parameter [67Jan].
As the temperature is increased, the maximum hydrogen content in the a phase
increases, whereas the minimum hydrogen content of the a phase decreases. Thus,
the miscibility gap narrows and finally disappears at the critical point.
Above the critical point, a continuous range of hydrogen contents occurs; the
terms a and b then refer to the same phase. Values of amax and bmin measured
in the vicinity of 298 K [67Maj, 78Pon] lie in the range 0.02 њ amax њ 0.1 and
0.6 њ bmin њ 1.0, respectively.
At hydrogen contents between amax and bmin (below the critical point) in the
region of immiscibility, the a <259> b transformation can be followed by
monitoring the electrical resistance, which decreases as b hydride forms. The
b hydride is paramagnetic, whereas a is ferromagnetic. Thus, as the a
transforms to b, the saturation magnetization falls to zero in direct
proportion to the amount of b phase. In a similar manner, the high value of
thermoelectric power of the b hydride relative to the a phase has been used to
follow the a <259> b and b <259> a phase transformations [71Sko]. These
electrical and magnetic effects are associated with the holes in the d band of
Ni being filled with electrons from the hydrogen atoms.
The Ni-H system is notably similar to the Pd-H system, but because the
critical point of the latter is at a much more easily obtainable pressure (
~2 MPa, a factor of 1000 lower than that of Ni-H), the Pd-H system has been
more widely studied. Other than the coordinates of the critical point, the
major difference between the two systems is that the formation of the b
hydride causes a much greater lattice expansion in Ni (5.8% increase in
lattice parameter as compared to 3.5% in Pd). This is clearly related to the
observation that bmin, the minimum hydrogen content of the b hydride at room
temperature, is higher in the Ni-H system than in the Pd-H system. There is
also a much greater hysteresis in the Ni-H system.
The effect on hydrogen solubility of the transition from the ferromagnetic to
the paramagnetic state at the Curie temperature is unclear. Some workers have
detected a drop in solubility in the vicinity of the Curie temperature [83Sha],
as well as a change in the heat of solution [83Sha, 83Vya]. On the other hand,
[84Mcl] and others indicated that there is no discernible discontinuity at
the Curie temperature.
Dislocations and grain boundaries are potential trapping sites, and studies
have been carried out on the effects of cold working and grain size on
hydrogen solubility. The results of these studies are inconclusive. By
comparing single-crystal behavior with that of polycrystals, or by studying
polycrystals of varying grain size, [83Vya] found no evidence for the
trapping of hydrogen at grain boundaries. [84Mcl] has stated that hydrogen
grain boundary or hydrogen dislocation interactions should not cause
solubility anomalies. [74Sta] and [86Lee], however, showed an apparent
trapping of hydrogen at grain boundaries. Similarly, there are reports of
enhanced solubility in cold worked Ni, as a result of hydrogen trapping by
dislocations [86Lee], as well as reports of the absence of such an effect [
59Gri]. Thus, at present, the effects of hydrogen traps on solubility remain
uncertain.
At hydrogen pressures high enough that the b hydride phase forms, terminal
solubility is synonymous with amax, the maximum hydrogen content in the a
solid solution in equilibrium with b hydride. The shapes of the absorption
isotherms indicate that as the hydrogen pressure increases the hydrogen
solubility increases. However, both the hydrogen gas and the a solid solution
increasingly deviate from ideality, causing strong positive deviations from
Sieverts' law as amax is approached. Values of amax do not appear to be
strongly dependent on temperature below 600 K, but may increase as the
critical temperature is approached. Data from above the critical point, where
the terminal solid solubility is simply the hydrogen content in equilibrium
with gaseous hydrogen (because a and b are no longer distinct entities), are
not yet available.
Increasing the hydrogen pressure has also been found to lower the Curie
temperature of the a phase from about 632 K in pure Ni to about 550 K at a
hydrogen pressure of about 1200 MPa [77Ant, 78Pon], where amax is reached at H/
Ni ~ 0.02 [77Ant].
The bNi hydride phase exists in equilibrium with high pressures of hydrogen at
temperatures below the critical point. It consists of a fcc lattice of Ni
atoms with hydrogen atoms located in the octahedral interstices [63Wol]. The
bNi hydride phase is paramagnetic [81Bau]. It has a metallic character, with a
positive temperature dependence of electrical resistivity.
29Sie: A. Sieverts, Z. Metallkd., 21, 37-46 (1929).
34Smi: J. Smittenberg, Rec. Trav. Chim. Pays-Bas, 53, 1065-1083 (1934).
43Arm: M.H. Armbruster, J. Am. Chem. Soc., 65, 1043-1054 (1943).
59Gri: H.H. Grimes, Acta Metall., 7, 782-786 (1959).
63Sch: N.A. Scholtus and W.K. Hall, J. Chem. Phys., 39, 868-870 (1963).
63Wol: E.O. Wollan, J.W. Cable, and W.C. Koehler, J. Phys. Chem. Solids, 24,
1141-1143 (1963).
67Jan: A. Janko and J. Pielaszek, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 15,
569-572 (1967).
67Maj: S. Majchrzak, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 15, 485-490 (1967)
.
68Bla: J.S. Blakemore, W.A. Oates, and E.O. Hall, Trans. AIME, 242, 332-333 (
1968).
70Com: P. Combette and P. Azou, Mem. Sci. Rev. Metall., 67, 17-31 (1970).
73Mcl: R.B. McLellan and W.A. Oates, Acta Metall., 21, 181-185 (1973).
73Rob: W.M. Robertson, Z. Metallkd., 64, 436-443 (1973).
74Sta: S.W. Stafford and R.B. McLellan, Acta Metall., 22, 1463-1468 (1974).
75Mcl: R.B. McLellan and C.G. Harkins, Mater. Sci. Eng., 18, 5-35 (1975).
77Ant: V.E. Antonov, I.T. Belash, and E.G. Ponyatovskii, Dokl. Akad. Nauk SSSR,
233, 1114-1117 (1977) in Russian.
78Bar: B. Baranowski, in Topics in Applied Physics, Vol. 29, Hydrogen in
Metals II, G. Alefeld and J. Volkl, Ed., Springer-Verlag, Berlin, 157-200 (
1978).
78Pon: E.G. Ponyatovskii, V.E. Antonov, and I.T. Belash, Neorg. Mater., 14,
1570-1580 (1978) in Russian.
79Bar: B. Baranowski, Z. Phys. Chem. (Neue Folge), 114, 71-93 (1979).
79Sha: V.I. Shapovatov and N.P. Serdyuk, Zh. Fiz. Khim., 53, 2187-2191 (1979)
in Russian; TR: Russ. J. Phys. Chem., 53, 1250-1252 (1979).
80Ser: N.P. Serdyuk and A.L. Chuprina, Russ. J. Phys. Chem., 54, 1615-1618 (
1980).
81Bar: B. Baranowski, in Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New
York, 193-213 (1981).
81Bau: H.J. Bauer, in Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New
York, 313-327 (1981).
83Sha: V.I. Shapovalov and L.B. Boyko, Fiz. Met. Metalloved., 55, 1220-1221 (
1983) in Russian.
83Vya: A.F. Vyatkin, P.V. Zhorin, and E.M. Tseitlin, Zh. Fiz. Khim., 57, 419-
422 (1983) in Russian; TR: Russ. J. Phys. Chem., 57, 249-251 (1983).
84Mcl: R.B. McLellan and P.L. Sutter, Acta Metall., 32, 2233-2239 (1984).
86Lee: S.-M. Lee and J.-Y. Lee, Metall. Trans. A, 17 181-187 (1986).
Published in Phase Diagrams of Binary Nickel Alloys, 1991, and Bull. Alloy
Phase Diagrams, 10(5), Oct 1989. Complete evaluation contains 7 figures, 9
tables, and 80 references.
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