Ni-H System

H-Ni

The H-Ni (Hydrogen-Nickel) System By M.L. Wayman

University of Alberta and

G.C. Weatherly University of Toronto

Equilibrium Diagram

At Low Pressures---0.1 MPa (< 1 atm)-- of H2

The behavior of the Ni-H system depends strongly on the activity (pressure) of H. In equilibrium with H2 pressures on the order of I atm or less, Ni exhibits the features of a classical "endothermic occluder" of H [48Smi]--it reacts endothermically with H. The result is a very low solubility (interstitial) of H in (Ni) at room temperature (H/Ni - 3 • 10-5)* and most charac- teristically a positive temperature dependence of solubility. It has been possible to gather solubility data over a wide range of temperature, because the rapid rate of diffusion of H in (Ni) allows equilibrium conditions to be attained in reasonable periods of time c.g. 5 to 20 minutes at 250 ~ (523 K) for some experiments of [78Pon]. Solubility data for Ni in equilibrium with 1 atm of H2 gas have been compiled in several published reports [73Rob, 76Fro, 80Ser, 83Kiu, 84Mcl] repre- senting the results of more than 20 different solubility investigations carried out since 1907. A recent compilation by [84Mcl] excluded all data that might be in- fluenced by spurious surface-related experimental effects; these "clean" data for polycrystalline samples are shown, replotted, in Fig. 1.

At Moderate Pressures--10 to 100 MPa (100 to 1000 atm)--of H2

At H2 pressures in the range of 10 to 100 MPa, the H solubility and the effect of H2 pressure on melting behavior of Ni have been studied [79Shal, 79Sha2, 80Ser, 81Sha]. H appreciably reduces the melting point of Ni (Fig. 2), but this effect diminishes above 40 MPa H2 pressure. The Ni-H phase diagram in the vicinity of the melting temperature of 50 MPa of H2 is shown in Fig. 3. The system exhibits a eutectic point at 1406 ~ (679 K) and H/Ni = 0.036 [81Sha]. The behavior at H2 pressure of 100 MPa is comparable [80Ser]. The microstructure of Ni that was melted and then quenched from the liquid phase in H atmosphere (melt composition up to H/Ni = 0.02) consists of a eutectic mixture of solid metal with H2 gas bubbles, i.e. porosity [79Sha2]. Nickel hydride is not observed following such treatments.

*Throughout this assessment, H concentrations are reported as the atomic ratio of H to Ni;

At High Pressures-->100 MPa of H2

When Ni is exposed to environments in which the H activity is very high, corresponding to H2 pressures well in excess of 100 MPa, the behavior of the Ni-H system changes markedl:~ Such high H activity can be obtained by electrochemically (cathodically) charging the Ni in an electrolyte, for example 1N H2SO4 containing an H recombination poison such as thiourea or arsenic trioxide.

Fig. 1 Arrhenlue Plot of the Solubility of H In NI at Atmospheric Pressure

Atomic percent H

10 .3 i0-2

7

15

19

23

27

/ 35

�9 2 9 Sie �9 3 4 Smi [3 43 Arm x 68 81a t~ 70 Cot. o Z3 Mcl er ?4 Sta

84 Mc/

~ 0 10-5 i0-4 10-3

Atomic ratio x = HINt

10 -I

1500

iiooo

700

50O

30o

I00

5 0

From [84Mcl]. M. Wayman and G.C. Weatherly, 1989.

Bulletin of Alloy Phase Diagrams Vol. 10 No. 5 1989 569

H-Ni

Fig. 2 Effect of H2 Pressure on the Melting Temperature of Ni

1460 t

~ 144C

142C

,4o% I I I I I

I0 20 30 40 50 Hydrogen pressure (MPa)

From [79Sha2]. M. Wayman and G.C. Weatherly, 1989.

Alternatively, the Ni can be subjected to high pressures of H2 in a pressure cell; cells capable of hydrogenation at pressures as high as 7000 MPa of H2 were reported by [78Pon]. The electrochemical techniques are simple and flexible with regard to experimental conditions, being especially useful at low equilibrium H2 pressures [76Boe]. The high-pressure cell techniques give a controlled atmosphere of well-defined H activity, which can be maintained long enough to ensure that the system has reached equilibrium [72Bar, 78Bar, 81Bar]. The attainment of high H activity is facilitated by the high fugacity coefficient of H, which can often exceed a value of 100, thus permitting higher activity at lower H2 pressure than would otherwise be possible [72Bar, 84Bar]. The two techniques are sometimes combined, for example when samples charged with H electrochemically are placed in a high-pressure cell for the determination of the equilibrium pressure-temperature-composition relationships.

The high-pressure technique often is considered to give more reproducible results; however, sample preparation normally plays a major role in determining reproducibility It is frequently advantageous to carry out "sample training," whereby a sample is pretreated by a series of charge-discharge cycles to activate the surface before carrying out the measurements of interest [67Barl, 78Bar].

With the application of pressure of H2 in excess of 1000 MPa, the Ni-H system exhibits an equilibrium surface in pressure-temperature-composition (P-T-X) space. This surface is markedly similar to the P-T-V surface for a free gas. The critical point, below which the Ni-H solid solution decomposes to form the H-poor (Ni) solid

Fig. 3 Upper Corner of the NI-H Phase Diagram at a H2 Pressure of 50 MPa

1460

1440

1420

1400

Atomic percent H

I 2 3 4 I I ] I

H2

L~ O 011 ~ O. 036

I I I I Ni 0.01 0.02 0.03 0.04

Atomic ratio x = H / N i

From [79Sha2]. M. Wayman and G.C. Weatherly, 1989.

solution and the H-rich nonstoichiometric nickel hydride/3, lies in the vicinity of 350 to 430 ~ (623 to 703 K) and 1600 to 1900 MPa of H2 [76Pon, 79Bar]. The strong pressure dependence of the Ni-H systenl corresponds to a transition from an endothermic reaction with H2 at low pressures to an exothemnic reaction at high H2 pressures. Such a transition is predicted by the model of [80Yur], which allows calculation of the T-P conditions at which a sign reversal occurs in the temperature dependence of H solubility. This transition can also be seen in the calculated solubilities of [83Fuk].

Absorption and desorption isotherms for the Ni-H system at 25 and 65 ~ (298 and 338 K) are shown in Fig. 4. This figure also illustrates the marked hysteresis that occurs in the Ni-H system. The H2 pressure necessary for the formation of/J--about 600 MPa--is significantly higher than its decomposition pressu~ 340 MPa. The decomposition pressure is believed to represent the equilibriun~ value of pressure for (Ni)-/J coexistence [63Sch].

The absorption isotherms show that as H is absorbed at room temperature under high-pressure conditions, the terminal solid solubility--(Ni)max--is reached soon at H/Ni <0.1. Further absorption of H occurs by the formation of/3 in equilibrium with (Ni)./3 phase has the same fcc crystal structure as Ni and the (Ni) solid

570 Bulletin of Alloy Phase Diagrams Vol. 10 No. 5 1989

H-Ni

Fig. 4 Absorption and Desorption Isotherms at 25 and 65 ~ for the NI.H System

I00<:

80C

60C

200

Atomic percent H

I0 20 30 40 50 i i

T=65~

_x ~x

T=25~

0.2 04 0.6 0.8 1.0 ! 2 Atomic ratio x = H iN/

From 178Bar]. M. Wayman and G.C. Weatherly, 1989.

solution, but with a high concentration of interstitial H atoms and consequently, a larger lattice parameter.

As the temperature is increased, the maximum H content in (Ni) increases, while the minimum H content of (Ni) decreases; thus, the ndscibility gap narrows and finally disappears at the critical point. Above the critical point, a continuous range of H contents occurs, and the terms (Ni) and #6 then refer to the same phase. This is illustrated by the temperature-composition diagram, Fig. 5, which is a projection of the P-T-Xbehavior on an isobaric plane. Values of (Ni)n~ and #6rain measured in the vicinity of 25 ~ (298 K) by numerous workers [59Bar2, 61Bau, 63Szk, 63Wol, 64Bar2, 65Bin; 66StI; 67Barl, 67Maj, 77Ant, 78Bar, 78Pon] lie in the range 0.02 _< ( N i ) , ~ _< 0.1 and 0.6 < #6rain -< 1.0, respectively.

At H contents between (Ni)n~x a n d / J n ~ (below the critical point) in the region of immiscibility, the (Ni) -~ #6 transformation can be followed by monitoring the electrical resistance, which decreases as #6 forms [67Barl, 76Pon]. Magnetic measurements are also helpful--~ is paramagnetic, whereas (Ni) is ferromagnetic; thus, as (Ni) transforms to/t, the saturation nmg- netization falls to zero in direct proportion to the anmunt of/3 (such a direct proportionality does not apply to electrical resistivity [77Bau, 81Bau]). In a similar manner, the high value of thermoelectrical power of #6 relative to (Ni) has been used to follow the (Ni) - -~ and/~ -* (Ni) phase transformations [71Sko].

Fig. 5 NI-H T-X Phase Diagram

3501

300'-

~ 250

2 0 0

Atomic percent H

0 I0 20 30 40 50 l I I I I

I , I , I , I ~ I

0 0.2 0.4 0.6 0.8 1.0

Atomic ratio x= H/N/

Open and closed circles show the data points for the (Ni),nax and/3min, respectively. From [77Anti.

M. Waynmn and G.C. Weatherly, 1989.

These electrical and nmgnetic effects are associated with the holes in the d band of Ni being filled with electrons from the H atoms.

The P-T-X behavior of Ni-H is notably similar to that of the Pd-H system, but because the critical point of the latter system is at a much more easily obtainable pressure ( - 2 MPa, a factor of 1000 lower than that of Ni-H), Pd-H has been studied more widely. Other than the coordinates of the critical point, the major difference between the two systems is that the formation of #6 causes a much greater lattice expansion in Ni (5.8% increase in lattice parameter, compared with 3.5% in Pd); this clearly is related to the observation that #6rain, the minimum hydrogen content of#6 at room temperature, is higher in the Ni-H system than in Pd-H. Thereis also a much greater hysteresis in the Ni-H system. These are quantitative rather than qualitative differences; consequently, the Pd-H system has provided a valuable analog for Ni-H behavior.

Techniques other than electrochemical and high-pressure H charging have produced nickel hydrides. Dia- tomic NiH has been detected spectroscopically when nickel carbonyl was introduced into a burner flame [35Gay]. Hydrides have been produced as well by proton bombardment of Ni foils [64Pal] (see also [67Maj]). In all these, it is apparent that the hydride phases form only when the activity of H, especially atomic H, is high. Hence, the hydrides are unstable at room temperature and pressure, and they decay by transforming back across the miscibility gap to (Ni), with a low H content.


H-Ni

One consequence of the lattice expansion that occurs when/5 is formed from the (Ni) solid solution (5.8% linear expansion corresponds to 18.3% volume expansion) is that only thin pieces of Ni can be transformed to /5 during reasonable charging times. Thus, during electrochemical charging, a surface layer of hydride on the order of 30 pm maximum in thickness is formed on the cathode surface [59Bar1, 59Bar3]. The surface is seen by optical microscopy to be swollen and bulged outwards by its conversion to hydride [61Bon, 80Kit] with grain boundaries and slip lines readily visible. Under ambient conditions, the hydride decomposes over a period of a few tens of hours by evolution of H, and this process can be followed by using optical microscopy; scanning electron microscopy [80Kit], and a Bit- ter method of revealing ferromagnetic phases [78Jon] have also been employed in studies of/5 decomposition. These reveal that decomposition occurs by the formation and growth of plate-like (Ni) grains on the 15 surface, followed by the advance of the transformation front into the bulk/5. The volume shrinkage results in cracking from the surface inwards; both intergranular and transgranular cracks are normally observed. These changes in the surface must be related to the afore- mentioned "sample training" or surface activation, which when carried out prior to the measurement of absorption and desorption isotherms, improves the reproducibility of the results.

Terminal Solid Solution At low H2 pressure, below 1 MPa, Ni is a classical endothermic occluder, with a room-temperature H solubility of about H/Ni - 3 • 10-5, increasing with temperature to about H/Ni - 10- 3 at the melting point of 1455 ~C (1728 K) (Fig. I). Compilations of solubility data [73Rob, 76Fro, 80SeI; 83Kiu, 84Mcl] have been fitted to a solubility equation of the form:

log solubility = a - B/T (Eq 1)

For example the [73Rob] compilation yielded "best fit" values:

Solubility (H/Ni) = a ' r exp - B ' / R T (E<t 2)

where a' = 0.18 _ 0.007, P is pressure in MPa, B' = 12.5 _+ 0.3 kJ/nml, and Tis in K.

Howevel; the envelope of results plotted by [73Rob] exhibited deviations from a linear relationship between log solubility and 1/T. This nonlinearity---an enhancement of H solubility at low temperatures, in contrast with values extrapolated from high temperatures--is visible in the data of other workers [71Vyk] and was both observed and considered by [84Mcl], who have compiled the data shown in Fig. 1. The nonlinearity is consistent with the trapping of H atoms in surface layers at sites provided by oxygen atoms [83Kiu]. Howevel; when data are screened to eliminate this spurious surface effect, the low-temperature enhancement remains [84Mcl]. [84Mcl] explained the nonlinearity as being a consequence of the usually

Fig. 6 Effect of H2 Pressure on the Solubility of H In (Ni)

8

Atomic percent H

0.5 1.0 I I

. 1350~ o IO00~ i t , A 800oc / �9 600~

O. 0.01

Atomic ratio x = HiNt

From |80Ser]. M. Wayman m~d G.C. Weatherly, 1989.

wide range of temperatures over which solubility data are available; because the chemical potentials of H both in the gas phase and in the solid are somewhat dependent on temperature, and because these parameters contribute to the term B' in Eq 2, their temperature dependence destroys what would otherwise be a linear relationship. The effect of the temperature dependency in the gas p h a s e can be removed by plotting In(solubility x T7/4) vs 1/T (see [64Mcl]), but the plot remains nonlinear because of the sol id effects.

The effect on H solubility of the transition from the ferromagnetic to the paramagnetic state at the Curie temperature is unclear. Some workers detected a drop in solubility in the vicinity of the Curie temperature [83Sha], as well as change in the heat of solution [83Vya, 83Sha]. On the other hand, in the data compiled by [84Mcl] (Fig. 1) and others, there is no discernable discontinuity at the Curie temperature.

The effects of H2 pressure on solubility in the pressure range around 0.1 MPa are in accord with Sieverte' law, as given by the v ~ term in Eq 2 above [67Ebi, 71Jon, 83Vya].

The effects of H traps on solubility have also been considered. In addition to the contamination surfaces referred to above [83Kiu], dislocations and grain boundaries are potential trapping sites, and studies


H-Ni

have been carried out on the effects of cold work and grain size on H solubility. The results of these studies do not give a clear picture. By comparing single-crystal behavior with that of polycrystals or by studying polycrystals of varying grain size, some workers [73Rob, 83Vya] found no evidence for the trapping of H at grain boundaries and arguments have been made [83Kiu, 64Mcl] as to why H-grain boundary or H dislocation interactions should not cause solubility anomalies. On the other hand, other studies [74Sta, 86Lee] showed an apparent trapping of H at grain boundaries. Similarly, there are reports of enhanced solubility in cold worked Ni as a result of H trapping by dislocations [86Lee], as well as reports of the absence of such an effect [59Gri]. Thus, at present, the effects of H traps on solubility remain uncertain.

In the intermediate range of H2 pressure (10 to 100 MPa), the H solubility has been measured over the temperature range 327 to 1077 ~ (600 to 1350 K) [79Sha2, 80Ser]; the data are shown in Fig. 6. The solubility of H can be seen to be an order of magnitude higher at 100 MPa than at 0.1 MPa. Sieverts' law is obeyed reasonably well over the full pressure range up to about 1027 ~ (1300 K), but above that temperature, negative deviations are observed (solubility lower than the Sieverts' law prediction). The deviations form ideality of the gas phase and any trapping of H would lead to positive deviations from Sieverts' law, possibly explaining the nonlinearity of the lower-temperature curves in Fig. 7. The negative deviations at higher temperature have been explained as due to hydrostatic compression, as well as to "the special features of the molecular- kinetic equilibrium at the gas-metal interface" [80Ser].

At H2 pressures high enough that/3 forms, terminal solubility is synonymous with (Ni)max, the maximum hydrogen content in the (Ni) solid solution in equilibrium with /3. The shapes of the absorption isotherms below the plateaus in Fig. 4 indicate that as the H2 pressure increases, the H solubility increases. Howeveg both the H2 gas and (Ni) increasingly deviate from ideality, causing strong positive deviations from Sieverts' law as (Ni)max is approached. Values of (Ni)max do not appear to be strongly dependent on temperature below 327 ~ (600 K), as seen in Fig. 5, but may increase as the critical temperature is approached. Data from above the critical point, where the terminal solid solubility is simply the H content in equilibrium with gaseous H2 (because (Ni) and/3 are no longer distinct entities), are not yet available.

Increasing the H2 pressure has also been found to lower the Curie temperature of (Ni) from about 359 ~ (632 K) in pure Ni to about 277 ~ (550 K) at a H2 pressure of about 1200 MPa [76Pon, 77Ant, 78Pon], where (Ni)max is reached at H/Ni - 0.02 [77Ant]. This is shown in Fig. 7.

The exothennic nature of the H-Ni system at these high H2 pressures, in contrast to the behavior at lower

Fig. 7 T-P Phase Diagram for the NI-H System

40C

50C

I00

n Curie temperature �9 ( N # ~ # o ~ - - ~ (Ni)

O 0 / I" I I I

500 1000 1500 2000

o

Hydrogen pressure (MPa)

From 177Ant]. The horizontal displacement between the forward and reverse transformations at any temperature represents the hysteresis (see also Fig. 4).

M. Wayman and G.C. Weatherly, 1989.

pressures, has been attributed [83Fuk] to the fact that AV, the volume increase due to H in solution, is smaller than Vf, the atomic volume in fluid H. As a result, the term P(AV- Vf) tends to offset the positive heat of solution at high values of P. The elastic H-H interaction in the solid solution also plays a role here, as well as being largely responsible for the immiscibility between (Ni) and/3 in the system [72Ale].

Nickel Hydride

/3 exists in equilibrium with high pressures of H2 at temperatures below the critical point. It consists of an fcc lattice of Ni atoms, with H atoms located in the octahedral interstices [63Wol]. Occupation of all the octahedral interstitial sites by H atoms would result in a hydride with H/Ni = 1.0; observed values for/3 lie in the range 0.6 to 1.0, with a most probable value of 0.8 to 0.9. Several studies have determined H/Ni in/3 to be higher than 1.0 [77Ant, 78Pon] (see also [79Kep]), but ques- tions have been raised with regard to this possibility [79Kep]. Values of H/Ni > 1.0 imply some occupation by H atoms of tetrahedral interstices, as discussed by several workers [67Bar2, 78Pon]. However, whether or not the H/Ni values do exceed 1.0, it is clear that high H2 pressure can raise the H/Ni ratio o f /3 above /3n~ [59Bar2, 67Barl, 68Bau, 71Way, 78Pon].

/3 is paramagnetic [61Bau, 77Bau, 81Bau]. It has a metallic character, with a positive temperature dependence of electrical resistivity.


H-Ni

Table I EqulUbrlum DecomposlUon and Forma. tlon Pressures for/~

Pressure, Temperature, R e a c t i o n MPa ~ R e f e r e n c e

Decomposition ......... 340 • 7 25 [65Bar] 335 25 [78Bar, 80Bar] 356 67 [78Bar, 80Bar]

Formation ................ 806 • 12 RT [67Bar1] 620 25 {78Bar, 80Bar] 645 67 [78Bar, 80Bari

The experimentally determined H2 pressures at which#6 forms and decomposes are given in Table I, and their dependencies on temperature in Fig. 7. The hysteresis is obvious, as is its diminishing as the critical temperature is approached. The spread of values may be related to the necessity for "sample training." The high decomposition pressure means that #6 is unstable at room temperature and decomposes readily. The kinetics of this decomposition and of the formation of #6 are controlled by thermally activated processes, with activation energies in the range 56 to 62 kJ/mol (decomposition) and 27 kJ/mol (fornmtion) [59Bar4, 78Bau, 80Bor]. The decomposition rate was found to be the same in air and in vacuum [60Jan1] and to be affected by both the density and distribution of dislocations [78Pie]. Magnetic studies indicated that the amount of fl increases in proportion to the square root of time during cathodic charging and decomposition [78Bau].

As noted above, other nickel hydride phases have been reported by several workers, but their existence requires confirmation under a range of conditions to assess how general their formation is. They are discussed further below.

Crystal Structures and Lattice Parameters

Crystallographic studies of the Ni-H system have been carriei out mainly by X-ray diffraction (XRD), although neutron diffraction has proved valuable for locating the positions of H atoms. XRD has been performed both during and after H charging by both the pressure and the electrochemical techniques. Most of the X-ray measurements have been made at room temperatures, although in a few instances, lower temperatures have been used.

As H is absorbed, little change in the XRD pattern is observed until the H content reaches (Ni)max, at which time theft pattern appears. With continued H charging, the (Ni) lines weaken, whereas the #6 lines strengthen, until at #6rain, the (Ni) lines are no longer present. Increases in the lattice parameter of a phase reflect its increasing H content, as shown for fcc metals in general by [71Bar], who found the partial molar volume of H to be constant up to H]Ni - 0.75 at a value of 1.65 cm3/mol of H (as calculated by [72Fla]).

X-ray studies in the Ni-H system are complicated by several factors. First, the limiting thickness of the/3 layer produced by electrochemical charging (-30pro) is approximately the same as the analysis depth for XRD using CuIZ~ radiation. Thus, in many instances, the sample being X-rayed has a heterogeneous H/Ni ratio (and often phase structure) within the analyzed volume. Secondly, the formation of #6 in (Ni) matrix across the miscibility gap gives rise to markedly broadened fl lines. This is believed to be due to the creation of a dislocation structure as a consequence of the volume expansion that accompanies the (Ni) -* fl transfornmtion. Hence, the XRD pattern of fl created in this way and of (Ni) produced by subsequent decomposition offl consists of lines that are broadened sufficiently to impair precise lattice parameter measurements.

The third factor that must be considered in interpreting XRD results from the Ni-H system is the time- dependent nature of the diffraction pattern as the H is desorbed at room temperature. #6 loses H as it approaches #6rain, so its lattice parameter changes during the first hour after H charging is stopped. A Debye-Scherrer pattern, with an exposure time of some tens of minutes, will therefore show extra broadening of the #3 lines caused by the shifting peak position. On the other hand, repeated scanning of a hydride peak with a diffractometer will detect the time dependence of the lattice parameter [68Bau, 71Way].

Despite these limitations, XRD has provided useful information on both the (Ni) and/3 phases of the Ni-H system.

(NI)

The (Ni) solid solution has the fcc crystal structure of Ni, with H dissolved interstitiallyBecause at 25 ~ (298 K), the maximum H content of the solid solution is low (H/Ni - 6 • 10-5 in equilibrium with 0.1 MPa of H2 and H/Ni - 0.03 in equilibrium with#6 at H2 pressures above 340 MPa), lattice parameter changes are not expected to be large. Many workers [61Bon, 67Barl, 6TMaj] were unable to detect a change in the lattice parameter of the (Ni) phase as pure Ni absorbs H. Howevel; [67Jan] reported a slight but significant difference between the lattice parameter (measured at 25 ~ of Ni (0.35213 • 0.00005 nm) and that of (Ni) in equilibrium with/3 (0.35218 • 0.00005 nm). Using the lattice expansion data of [71Bar], this corresponds to H/Ni ~ 0.02, in agreement with the estimated values of (Ni)nmx as determined from absorption isotherms and electrochemical potential measurements [64Bar2, 78Pon]. An expansion of the lattice of the (Ni) phase above that of Ni was also observed by [68Bau].

On the basis of results from the #6 phase, it is likely that the H atoms occupy octahedral interstices in the fcc lattice of the (Ni) solid solution, but there is only indirect evidence to confirm this.

574 Bulletin of Alloy Phase Diagrams Vol. I0 No. 5 1989

p Nlckel Hydrlde

Numerous XRD investigations [60Jan2, 61Bon, 67Maj, 67Jan, 71Way] have reported that ~ has the fcc Ni lattice, with the lattice parameter between 0.371 and 0.3735 nm at room temperature. Many of these authors reported no increase in the lattice parameter at H contents above ~min. However, as discussed above, this may in some instances reflect the use of powder cameras with exposure times too long to detect the decay of p to #6rain after the completion of charging [68Bau, 71Way]. The lattice parameter at #6rain should be that measured when/3 is in equilibrium with (Ni); under these conditions, [67Jan] reported a lattice parameter of 0.37251 • 0.00008 nm. Using this value for~n~, which is assumed to be at H/Ni = 0.9, and the partial molar volume data of [71Bar], the lattice parameter at H/Ni = 1.0 can be estimated to be 0.37318 nm, in rough agreement with the values measured by [68Bau] and by [71Way] immediately atter charging at the onset of the decay toward #6rain.

The/~ lattice parameter of 0.37251 nm represents a linear expansion of 5.8%, or a volume expansion of 18.3%, compared with (Ni) at its solubility limit, (Ni)max [67Jan]. The hydride phase inherits the preferred orientation of the Ni from which it is formed [64Maj, 67Ms j].

Table 2 Heats of Solution of H In NI

Heat of solution, kJ/mol of [] Comment Reference

12.7 . . . . . . . . . . . . . . . . . . . . . . [57Lie] 15.1 • 0.3 . . . . . . . . . . . . [67Ebi] 12.5 _+ 0.3 ......... Compilation of 12 experiments [73Rob] 15.5 . . . . . . . . . . . . . . . . . . . . . . [73Rob] 15.1 . . . . . . . . . . . . . . . . . . . . . . [79Sha2] 17.9 ................... Average of seven experiments [80Ser]

23.0 ...................... [81Sha]

12.3 ...................... [83Fuk]

14.25 ................. Ferromagnetic state [83Sha]

8.03 ................... Paramagnetic state [83Sha]

16.93 • 0.41 ..... Ferromagnetic state [83Yva] 10.64 _+ 0.38 ..... Paramagnetic state 11.8 . . . . . . . . . . . . . . . . . . . . . . [86Lee]

H-Ni

Neutron diffraction studies [63Wol] showed that the H atoms are located in the octahedral interstices, giving an NaCl structure and implying a maximum H]Ni ratio of 1.0. As mentioned above, it has been suggested [67Bar2, 78Pon] that at about H/Ni ffi 0.8, some tetrahedral site occupation begins, thus explaining the change in the H concentration dependence of the partial molar volume of H [71Bar]. This would also account for the H/Ni ratios greater than 1.0 (if indeed reports of H/Ni > 1.0 are correct; see above).

Nickel hydrides other than the fi hydride discussed above have been reported. [48Snd] referred to a hexagonal nickel hydride, whereas the results of [59Bar4] suggested the possible existence of two hydrides. Finally, [60Janl] found by electron diffraction both /~ and another fcc hydride with t h e la t t i ce parameter 0.383 to 0.387 nm; the latter disappeared during annealing at 400 ~ (673 K).

Thermodynamics Heat of Solution The heats of solution of H in Ni have been determined by numerous workers from the temperature dependence of the H solubility. A "best fit" value as determined by [73Rob] was found to be 12.5 kJ/mol; values measured by various workers since the date of that compilation lie mainly in the range 8 to 18 kJ/mol (Table 2). Two reports [83Sha, 83Vya] of a nmrked difference between the heats of solution above and below the Curie temperature (14.25 and 16.93 kJ/mol in the ferromagnetic state and 8.03 and 10.64 kJ/mol in the paramagnetic state) are not in accord with the results of other workers. This magnetic effect clearly is not related to the nonlinearity of the log solubility vs 1/T curve (Fig. 1), because the magnetic effects acts to reduce the solubility in the ferromagnetic temperature range, whereas the observed nonlinearity is in the opposite sense.

The partial molar enthalpy and excess entropy of H atoms dissolved in Ni, with respectto H atoms at rest in a vacuum and also with respect to H2 gas at 1 atm pressure, are given in Table 3; these were calculated from solubility measurements [74Sta, 75Mcl, 86Suz]. Two experimentally determined values of the entropy of

Table 3 Enthslples and Entropies of NI-H Solutions

Temperature, HH, ~V~H, SetH, ~eXll, Reference K kJ/mol kJ/mol J/K.moi JfK. mol

[73Mcl, 75Mcl] ......................... 1000 to 1516 [74Sta] ...................................... 613 to 1573 [86Suz] ..................................... 673 to 1273 Entropies of solution obtained from solubility data

[71Jon] ......................................... [83Fuk] ........................................

-180.4 16.6 37.8 -48.72 -180.1 27.0 35.3 -49.21 -186 to -196 ... 21.1 to 34.4 ...

31.3 34.08


H-Ni

Table 4 Enthalples Calorlmetrlcally

of Formation and Decompoaltlon of Nlckel Hydrlde (NIHx) as Determined

P/T, Enthalpy, Reference Reaction X(H/Ni) atm/K kJ/mol of H2

[64Barl] ........................ Decomposition [66Cza] .......................... Format ion [85Tka] ......................... Formation

Decomposition Format ion Decomposition

0.3 to 0.5 1/298 10.0 _ 0.4 0.5 ... -8.8

... 6200/298 -10.0 • 0.21

... 34001298 6.27 • 0.42

... 1/298 -16.3 • 0.21

... 1/298 8.95 • 0.21

Note" [71Wol] reported values of"thermal contribution of the enthalpy" of NiH:~ wherex = 0.50, 0.58, and 0.68. These enthalpy values were calculated from specific heat measurements nmde between 0 and 200 K and by extrapolation to 300 K. This enthalpy at 300 Kis roughly 6 kJ/mol of NiI~.

Table 5 Standard Entropies of Formation of Nickel Hydride (NIHx)

Entropy Reference X(H/Ni) of formation

[65Bar] ..................... 0.5 -107 • 2 J/K.mol of H2 [71WolJ ..................... 0.5(?} -106 J/K.mol of H2

0.5 -26.1 J/K.mol of NiHx 0.59 -31.0 J/K.mol of NiH.~ 0.68 -36.4 J/K.mol of NiI-[~

Note.' Values of the s tandard entropy of NiHx from 171Wol]: forX ffi 0.5, +36.3 J/K.mol of NiI-I~; forX = 0.59, +37.3 J/K.mol of NiHx; and for X = 0.68, +37.7J/K.mol of NiH,~.

Table 8 Glbba Energies of Formation of Nlckel Hydrlde

Gibbs energy, Reference Conditions kJ/mol of H2

[65Bar] ..................... S tandard 3400 a tm

[68Sie] ...................... From electrode potential

180Bar] ..................... 340 K

23.6 • 0.08 34.4 +_ 0.16 25

27.2

so lu t ion of H in Ni a re also given. The d a t a s h o w n in Table 3 come p r inc ipa l ly f rom the group [73Mcl, 74Sta, 75Mcl, 86Suz] who use moles of H a t o m s r a t h e r t h a n moles of H2 gas, so t h a t t he i r da ta m u s t be m u l t i p l i e d by two before c o m p a r i n g wi th those of o the r workers . HH a n d SeXH a re the pa r t i a l mo la r e n t h a l p y a n d excess en t ropy of H in the so lu t ion wi th respec t to H a t o m s a t res t in a v a c u u m . AHH and ASeXH a re the re la t ive pa r t i a l m o l a r e n t h a l p y a n d excess e n t r o p y of a t o m s in the so lu t ion wi th respect to H2 gas a t 1 a t m pressure . The hea t of so lu t ion g iven in Table 1 is the s lope of the log solubil i ty vs 1 / T l ine, hence c o r r e s p o n d i n g to the HH in Table 3.

Table 7 Heat Capacities (Cp) of Nickel Hydride (NiHx)

Temperature, Heat Capacity (Cp), J/mol.K K NiHo~o N i H 0 ~ NiHo.as

10 ............................ 0.125 20 ............................ 0.473 30 ............................ 1.141 40 ............................ 3.07 50 ............................ 5.31 60 ............................ 7.87 70 ............................ 10.2 80 ............................ 12.0 90 ............................ 14.0

100 ............................ 15.9 110 ............................ 17.7 120 ............................ 19.4 130 ............................ 20.8 140 ............................ 22.0 150 ............................ 23.0 160 ............................ 24.0 170 ............................ 25.1 180 ............................ 25.8 190 ............................ 26.4 200 ............................ 27.0 210 ............................ 27.7 220 ............................ 28.3 230 ............................ 28.9 240 ............................ 29.4 250 ............................ 30.0 260 ............................ 30.5 270 ............................ 31.0 280 ............................ 31.6 290 ............................ 32.1 300 ............................ 32.6

From [71Wol].

0.139 0.140 0.502 0.494 1.55 1.52 3.27 3.34 5.52 5.65 8.16 8.19

10.5 10.5 12.6 12.6 14.6 14.6 16.4 16.4 18.3 18.3 20.0 20.0 21.6 21.6 22.7 22.9 23.6 24.0 24.5 25.0 25.4 25.9 26.1 26.7 26.8 27.4 27.4 28.0 28.0 28.6 28.6 29.2 29.2 29.8 29.8 30.3 30.4 30.8 30.9 31.4 31.5 31.9 32.0 32.5 32.5 33.0 33.0 33.5

Calorimetry Very few c a l o r i m e t r i c i n v e s t i g a t i o n s have b e e n ca r r i ed ou t i n the N i - H sys tem, a t l eas t p a r t l y because of the diff icul ty of m a k i n g such m e a s u r e m e n t s a t h igh pressure . The e n t h a l p y of d e s o r p t i o n of n ickel hyd r ide (at 298 K a n d i a t m of H2 p re s su re ) has b e e n m e a s u r e d

576 Bullet in of Alloy Phase Diagrams Vol. 10 No. 5 1989

H-Ni

Table 8 Heat Capacities (Cp) of Nickel Hydride (NiHx) from 10 to 20 K

NIHo.es NIHos NiHo~o [Tempera- Tempera - Tempera - | [ T e m p e r a . T e m p e r a - [ [Tempera . T e m p e r a . I ture, Cp. ture, Cp, ture, Cp. ture . Cp, ture , Cp, ture , Cp, ture , Up, K J/tool . K K J /mol �9 K K J /mol - K K J /mol �9 K K J / m o l - K K J / m o l �9 K K J / m o i �9 K

10.18 ..... 0.148 10.93 ..... 0.163 11.19 ..... 0.167 10.31 .... 0.136 10.32 ..... 0.135 10.90 ..... 0.158 10.44 ..... 0.146 12.75 ..... 0.213 11.81 ..... 0.188 12.07 ..... 0.196 11.32 .... 0.163 11.40 ..... 0.160 11.81 ..... 0.185 12.11 ..... 0.193 13.45 ..... 0.227 12.62 ..... 0.210 12.80 ..... 0.210 12.91 .... 0.205 12.33 ..... 0.184 12.58 ..... 0.206 13.58 ..... 0.236 14.10 ..... 0.252 13.31 ..... 0.227 13.46 ..... 0.228 13.58 .... 0.222 13.86 ..... 0.223 13.27 ..... 0.226 14.21 ..... 0.255 14.74 ..... 0.266 13.93 ..... 0.243 14.09 ..... 0.254 15.97 .... 0.299 14.54 ..... 0.256 13.90 ..... 0.243 14.76 ..... 0.275 16.10 ..... 0.323 14.54 ..... 0.264 14.63 ..... 0.267 16.66 .... 0.331 15.16 ..... 0.274 14.47 ..... 0.256 15.29 ..... 0.290 16.70 ..... 0.341 15.17 ..... 0.282 15.15 ..... 0.284 17.30 .... 0.351 15.76 ..... 0.303 15.15 ..... 0.280 15.91 ..... 0.313 17.30 ..... 0.356 17.18 ..... 0.353 16.25 ..... 0.323 17.92 .... 0.380 16.42 ..... 0.316 15.65 ..... 0.309 16.59 ..... 0.347 17.85 ..... 0.389 17.75 ..... 0.378 16.88 ..... 0.351 18.48 .... 0.402 17.11 ..... 0.348 16.35 ..... 0.330 17.20 ..... 0.359 18.37 ..... 0.407 19.28 ..... 0.451 17.49 ..... 0.375 19.47 .... 0.449 17.77 ..... 0.370 16.99 ..... 0.362 17.79 ..... 0.386 18.83 ..... 0.427 18.07 ..... 0.393 18.92 ..... 0.426 18.20 ..... 0.406 18.84 ..... 0A~ A 19.27 ..... 0.452 18.58 ..... 0.412 19.94 ..... 0.483 18.69 ..... 0.428 19.94 ..... 0.515

19.07 ..... 0.439 19.16 ..... 0.462 19.53 ..... 0.465

From [71Woll.

Table 9 Electronic Specific Heat Coefficients (7) and Oebye Temperatures (0D) for NI and NIHx

Composition, },, 0D, H/Ni J /mol .K K

0 .......................... 7.299 x 10 -3 380 0.50 ..................... (9.29 • 0.21) x 10 -~ 376 • 4 0.59 ..................... (9.92 • 0.06 • 10 -3 372 • 1 0.68 ..................... (11.09 • 0.17) x 10 -3 388 • 3

From [71Wol].

in a d i f fe rent ia l c a l o r i m e t e r [64BAr l ] u s i n g a " s t a t i c -dynamic" method , w h e r e a s l a t e r m e a s u r e m e n t s by a d y n a m i c m e t h o d [66Cza] yielded va lues for i ts s t a n d a r d e n t h a l p y of fo rma t ion . The d a t a o b t a i n e d a re shown in Table 4. More r ecen t ly [85Tka], ca lo r ime t r i c m e a s u r e m e n t s u n d e r high p r e s s u r e have yielded va lues for the fo rma t ion a n d decompos i t i on en tha lp i e s of ~6 nickel hydride; these were t h e n conver t ed to e n t h a l p i e s a t n o r m a l p ressure of H2. T h e d a t a ob ta ined (also g i ve n in Table 4), are i n r ea sonab l e a g r e e m e n t wi th the ea r l i e r work.

At lower H con ten t , the abso lu t e va lues of these e n t h a l - pies decrease un t i l a t H /Ni - 0.02, a c h a n g e of s ign occurs, in a g r e e m e n t wi th the e n d o t h e r m i c n a t u r e of so lu t ion of gaseous H a t 1 a t m p re s su re in Ni.

S t a n d a r d en t rop ies of f o r m a t i o n of/~ nickel hydr ide a re p r e sen t ed in Table 5. These h a v e been ca lcu la ted f rom the ca lor imet r ica l ly d e t e r m i n e d s t a n d a r d e n t r o p y of the /~ hydr ide phase a n d the k n o w n va iues of the s t a n d a r d en t rop ies of Ni a n d of H2, o r a l t e rna t i ve ly f rom the m e a s u r e d Gibbs ene rgy a n d e n t h a l p y of fo rmat ion .

Gibbs ene rg i e s of f o r m a t i o n h a v e b e e n d e t e r m i n e d by ca l cu la t ion f r o m ca lo r ime t r i ca l ly d e t e r m i n e d e n t r o p i e s a n d e n t h a l p i e s of fo rma t ion , f rom e lec t rode po t en t i a l m e a s u r e m e n t s , a n d also by m e a s u r e m e n t s of the decompos i t i on p r e s s u r e p la teaus . Ta b l e 6 gives mea - su red va lues of t he Gibbs e n e r g y of f o r m a t i o n offl n ickel hydride. I t h a s b e e n po in ted o u t [65Bar] t h a t only a m i n i m a l d i f f e rence c a n be expec ted b e t w e e n t he Gibbs energ ies of f o r m a t i o n a t e q u i l i b r i u m p r e s s u r e a n d a t 1 a t m of H2 p ressure .

Low-Temperature Heat Capacity The hea t capac i t i e s o f N i - H al loys of c o m p o s i t i o n s H/Ni ffi 0.50, 0.59, a n d 0.68 have b e e n m e a s u r e d over the t e m p e r a t u r e r a n g e 10 to 200 K a n d e x t r a p o l a t e d to 300 K [71Wol]. T h e s e da ta , s h o w n in Ta b l e 7 a n d in m o r e de ta i l b e t w e e n 10 a n d 20 K i n Tab le 8, f i t the equa t ion :

Cp = yT + f l 'T 3 (Eq 3)

The e lec t ron ic specific hea t coeff ic ients , 7, a n d Debye t e m p e r a t u r e s (OD, ca lcu la ted f r o m t h e / ~ ' coeff icients) a r e s h o w n in Tab le 9. These d a t a a r e for a l loys ly ing in the (Ni) + /~ mi xe d - pha se reg ion , a n d so do no t r e p r e s e n t e i t h e r phase alone.

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80Kit: S. Kitagawa, "Mechanism of Surface Crack Forma- tion by Hydrogen in Pure Nickel," Hydrogen in Metals, Proc. 2nd Jpn. Inst. Met. Int. Syrup., suppl, to Trans. dpn. Inst. Met., 21,497-501 (1980). (Equi Diagram; Experimen- tad

*80Set: N.P. Serdyuk and A.L. Chuprina, "High Pressure Anomalies in the Solubility of Hydrogen and Nickel and Cobalt," Russ. d. Phys. Chem., 54, 1615-1618 ~ 1980). (Equi Diagram, Thermo; Experimental; # I

80Yur: I.A. Yurichev and E.G. Ponyatovskii, "Isomorphic Transitions and Critical Phenomena in EC.C. Metals under Hydrogen Pressure," Phys. Status Solidi (a), 58, 57- 66 ( 1980 I. (Equi Diagram; Theory)

*81Bar: B. Baranowski, "Hydride Formation at High Hydrogen Pressure," Metal Hydrides, G. Bambakidis, Ed., Plenum Press, New York, 193-213 (1981). (Equi Diagram; Review}

81Bau: H.J. Bauer, "Phase Transitions in Hydrogenated Nickel and Nickel Alloys Investigated by Magnetic


H-Ni In-La

Methods," Metal Hydrides, G. Bambakidls, Ed., Plenum Prem, New York, 313-327 (1981). (Equi Diagram; Review)

81Sha: V.I. Shapovalov, L.M. Poltoratskii, V~. Trofimenko, and N.P Serdyuk, "Phase Diagram of a Metal-Hydrogen System," Fazorye Ravnovesiya Met. Splavakh, M.E. Drits, Ed., Izd. Nauka, Moscow, 280-285 (1981) in Russian. (Equi Diagram, Thermo; Experimental)

83Fuk: Y. Fukai and H Sugimoto, "Enhanced Solubility of Hydrogen in Metals under High Pressure: Thermodynami- cal Calculation," Trans. Jpn. Inst. Met., 24, 733-740 (1983). (Equi Diagram, Thermo; Theory)

83Klu: I~ Kiuchi and R.B. McLellan, "The Effect of Surface Contamination on the Measured Hydrogen Solubilities of Metals," J. Less-Common Met., 95, 283-292 {1983L (Equi Diagram; Experimental)

83Sha: V.I. Shapovalov and L.B. Boyko, "Anomaly of Hydrogen Solubility in Ferromagnetic Metals near the Curie Point," Fiz. Met. Metalloved., 55, 1220-1221 (1983) in Russian. {Equi Diagram, Thermo; Experimental)

83Vya: A.F. Vyatkin, P.V. Zhorin, and E.M. Tseitlin, "In- fluence of Magnetic Ordering on the Solubility of Hydrogen in Pure Nickel," Zh. Fiz. Khim., 57, 419-422 (1983) in Rus-

sian; TR: Russ. J. Phys. Chem., 57, PA9-251 (1983). (Equi Diagram, Thermo; Experimental)

84Bar: B. Baranowski, "Investigation of Some Metal- Hydrogen Systenm in the High Pressure Region," J. Less- Common Met., 101, 115-129 (1984). (Equi Diagram; Review)

"84Mc1: R.B. McLellan and EL. Sutter, "Thermodynamics of the Hydrogen-Nickel System," Acta Metall., 32, 2233-2239 (1984). (Equi Diagram; Experimental, Compilation)

85Tka: M. Tkacz and B. Baranowski, "Heats of Formation and Decomposition of Nickel and Ni0.sCu0.2 Hydrides Measured in High Pressures of Gaseous Hydrogen," J. Less-Common Met., 113, 83-87 (1985). (Thermo; Ex- perimental)

86Lee: S.-M. Lee and J.-Y Lee, "The Trappingand Transport Phenomena of Hydrogen in Nickel," Metall. Trans. A, 17, 181-187 (1986). (Equi Diagram, Thermo; Experimental)

86Suz: Y. Suzuki and R.B. McLellan, "Thermodynamic Study of Ternary Nickel-Cobalt-Hydrogen So]id Solu- tions," Acta MetaU., 34, 125-130 (1986). (Thermo; Ex- perimental)

*Indicates key paper. #Indicates presence of a phase diagranl.

Ni-H evaluation contributed by M. Wayman, Department of Mining, Metallurgical and Petroleum Engineering, University of Alberta, Edmonton, Canada T6G 2G6, and G.C. Weatherly, Department of Metallurgy and Materials Science, University of Toronto, Toronto, Canada M5S 1A4. This work was supported by Ontario Hydro as a contribution from the Canadian Council of ASM INTERNATIONAL to the joint ASM/NIST Binary Alloy Pha~ Diagram Program. Literature searched through 1986. Part of the bibliographic search was prodded by ASM INTERNATIONAL and part by other sources. Professor Weatherly is the ASM/NIST Data Program Co-Category Editor for binary hydrogen alloys.

The In-La (Indium-Lanthanum) System By A. Palenzona and S. Ciraficl

University of Genova, Italy

Equilibrium Diagram investigations. The accuracy in the temperature measurements was _+ 5 ~

The assessed In-La phase diagram (Fig. i) is based on the work of [74Mcm], and the equilibrium phases include: (1) the liquid, L; (2) seven intermetallic compounds-In3La, In2La, In5La3, InxLa, InLa, InLa2, and InLa3; and (3) three terminal solid solutions, (yLa), (~La), and (In). No information is available for the solid solution of In in c~La. The a ,~/3 transformation for La is indicated by a dashed line at 310 ~

[74Mcm] investigated the In-La system by thermal analysis, ndcroscop~ and X-ray diffraction. The In used was 99.999% (weight percent?) pure. The La was prepared in their laboratory by Ca reduction of the fluoride, followed by a vacuum casting of the La. The major impurities (in atomic ppm) were: 1000 each oxygen and Ta, 370 each carbon and nitrogen, 690 hydrogen, and smaller quantities of Fe, Y, Ce, Gd, and fluorine. Eight master alloys, with compositions of 1.0, 48.4, 50.3, 55.9, 62.5, 85.1, 97.4, and 98.2 at.% La were prepared by arc melting and heat treated in sealed tantalum crucibles. Weighed amounts of either La or In were added to these master alloys, for a total of 60 samples for differential thermal analysis (DTA)

The phase diagram of [74Mcm] showed seven intermetallic compounds In3La, In5La3, and InLa (congruently melting at 1140, 1185, and 1125 ~ respectively), In2La, InLa2, and InLa3 (forming peritectically at 1153, 955, and 816 ~ respectively) and InxLa, which exists in the temperature range 905 to 1073 ~ between 38.5 and 50 at.% La. Four eutectic reactions were reported at 155 ~ and < 0.5 at.% La, at 1120 ~ and 28.8 at.% La, at 1086 ~ and 46.0 at.% La, and at 742 ~ and 86.5 at.% La. A eutectoid reaction was observed at 702 ~ and 91.9 at.% La, resulting from the lowering of the 865 ~ Y "~ ~ transformation temperature of La by In additions.

At the eutectoid temperature, the solubility of In in (~La) and in (yLa) was shown as 3.5 and 8.1 at.% In, respectively, and at the 742 ~ eutectic temperature, the solubility was reported as 10.2 at.% In in (TLa). No appreciable solid solubility of La in (In) was observed. Only InsLa3 seemed not to be a line compound; [74Mcm] found a variable composition, but did not give the range.


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Ni-H System