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Hydrogen absorption in beryllium substituted Mg2Ni
Int. J. Hydrogen Energy, Vol. 7, No. 10, pp. 783-785, 1982.
0360-3199/82/1007834)3 $03.00/0 Pergamon Press Ltd. (~) 1982 International Association for Hydrogen Energy.
Printed in Great Britain.
HYDROGEN ABSORPTION IN BERYLLIUM SUBSTITUTED Mg2Ni D. LuPu, A. BIRIS and E. INDREA Institute of Isotopic and Molecular Technology, R-3400 Cluj-Napoca, P.O. Box 243, Romania
(Receivedfor publication 22 January 1982) Abstract--The alloys Mg2Nix-xBex (x = 0.15 and 0.25) retain the Mg2Ni structure showing a lattice dilation proportional to the beryllium content. The pressure-composition isotherms are reported for the dissociation of hydrided samples. The results suggest that there are two type of interstices able to absorb up to 4 H atoms per formula unit. The heats of formation obtained from the van't Hoff relationship show an increased stability for the hydrides of the beryllium substituted alloys compared to the pure Mg2Ni. The results suggest that the electronic factors are more important for hydride stability than variations in the unit cell volume.
INTRODUCTION
RESULTS AND DISCUSSION
Since the discovery of the hydrogen absorption in Mg2Cu and Mg2Ni by Reilly and Wiswall [1, 2], the search for alloys with even higher capacities of hydrogen storage focuses the attention of large groups of 'hydriders'. Systematic studies of the influence of the substitutions in magnesium intermetallics on the absorption capacity and the hydride stability are of great interest in order to establish correlations between the hydride stability and the substituent characteristics. In this paper the pressure-composition isotherms are reported for two beryllium substituted magnesiumnickel alloys of the general formula Mg2Nil-xBex.
The diffraction patterns for all the samples were those corresponding to the Mg2Ni hexagonal structure. No 'extra' lines were observed. The lattice constants and the volume of the unit cell are given in Table 1. Table 1. The lattice constants and volume of the unit cell V for the alloys Mg2Ni~-xBex(x = 0, 0.15, 0.25) Compound
a (A)
c (A)
V (A3)
Mg2Ni* Mg2Ni0.asBe0.t5 Mg2Ni075Be0.25
5.191 5.245 5.277
13.209 13.332 13.416
308.2 317.6 323.5
EXPERIMENTAL
* The data are in good agreement with the values given in the literature (a = 5.19 ,~, c = 13.21 ,~,) [3].
Metals of 99.9% purity were used for the alloy preparation under high purity argon atmosphere in a high purity graphite crucible. A stainless-steel stirrer was used to homogenize the sample, which, after cooling, was overturned and remelted several times. The final compositions of the samples corresponded to the formulae Mg2Ni0.85Be015 and Mg2Ni075Be0.25. The X-ray diffraction patterns were determined using a powder diffractometer TUR-M61 with filtered C u K a radiation (34 kV, 20 mA). The scattered X-ray intensity was determined by the use of step scanning at 0.02° 20 intervals, with 100 s count-time per step. The 200 reflexion (Bragg angle 19.94 °) and 006 reflexion (Bragg angle 20.51 °) were employed to measure the lattice spacings in the a and c directions, respectively, with an accuracy of +0.003/~,. The finely powdered samples were outgassed at 350°C in a stainless-steel reactor after which high purity hydrogen at 60 atm. was admitted. The samples were hydrided and after several days desorption was started. The desorption kinetics does not differ essentially from that of pure Mg2Ni and the dissociation pressure was considered as the equilibrium value when no pressure increase was observed for at least 6 h.
The Mg2Ni structure is retained in the Mg2Nil-~Bex system up to x = 0.25. The substitution of Be for Ni atoms leads to a linear increase of the lattice constants with the increasing Be concentration. Taking into account that Be has a smaller metallic radius than Ni, the increase in the cell dimension of the beryllium substituted compounds was not anticipated. However, a pronounced expansion of the cell volume is observed. The desorption isotherms are reported in Figs. 1 and 2. Two plateau pressures are observed for the dissociation of the hydrided Mg2Ni085Be0.15 while for the Mg2Ni0.75Be0.25 only the upper plateau appears so that the remaining absorbed hydrogen ( - 0 . 3 H per metal) seems to be more strongly bound and has not been desorbed at hydrogen pressures above 10 -2 atm. H2. The dissociation pressure is lower than that for pure Mg2Ni and decreases with increasing beryllium content. A semilogarithmic plot of these dissociation pressures vs the reciprocal of the absolute temperature is shown in Fig. 3. The thermodynamic functions thus calculated from the van't Hoff equation are given in Table 2. The absorption of 4 H atoms (Mg2Nil-xBex)- ~suggests that the H atoms are located in the same type of interstices as in pure Mg2Ni.
783
D. LUPU, A. BIRIS AND E. INDREA
784
+
I
10 - M+z. q'' NiO.'::Beo.15o..,
rr
++/'/z o
o
o
~
~
o
315"c
t
I
7
, 0.5
1.0 H/metal (cffomicratio) Fig. 1. Desorption isotherms for the Mg2Ni0.ssBeo]5-H2system.
The enthalpies of formation for the hydrides corresponding to the upper plateau pressure on the pressure-composition isotherms show that the stability of this hydride increases with increasing beryllium concentration in Mg2Nil-xBex. The hydride stabilities in the composition range corresponding to the lower plateau pressure are even higher, as reported in Table 2. Thus, it is reasonable to assume that the upper plateau pressure corresponds to the desorption of H atoms from interstices without Be atoms as nearest neighbours while the lower plateau pressure corresponds to the desorption of hydrogen atoms located in the interstices with Mg, Ni and Be atoms as nearest neighbours in the Mg2Nil-xBex lattice. The former could be associated with the influence of the Be atoms on the band structure of the entire lattice while the latter could be the result of a direct influence of Be atoms. Obviously neutron diffraction studies of these hydrides would be both helpful and interesting. 10
I
Mg2 Ni0.75Be0.25
I
The cell volume increases with increasing beryllium content as reported above, and this is in agreement with the equilibrium pressure-hole size correlation reported in the literature [4]. However, this correlation is considered fortuituous by Buschow and Miedema [5]. The importance of the electronic factors on the hydride stability was suggested by de Pous and Lutz [6] and Mendelsohn and Gruen [7]. Some limitations in the role of the geometrical factors on the stability of a metal hydride also result from the opinion expressed by Burch and Mason [8]. In fact, it has been recently shown by Bruzzone et al. [9] that the effect of beryllium in TiFel-xBe, is in contrast with the equilibrium pressure-hole size correlation. Thus, there are two cases where the presence of Be atoms increases the hydride stability: one with lattice contraction and the other with lattice expansion. This seems to be direct evidence of the prominent role played by the electronic factors on the hydride stability. In this connection very interesting quantitative correlations between hydride stability and band structure of the palladium alloys were recently reported [10]. The significant covalent character of the bonding in MgH2 is now largely accepted [11-13]. Taking into account that the A H values for all the magnesium intermetallic hydrides studied up to now are close to those of MgH2, which is not the case with the hydrides of La and LaNi5 or of Ti and TiFe (and related compounds), a significant covalent character (localized bond) is also to be expected for the magnesium intermetallic hydrides. A light hydride of relative low stability is needed for automotive applications and therefore more systematic studies on other substituted magnesium intermetallics
LnP
~. ~ "\\\
"~,~
-.\
t I
F~
-~
---o-- Mg2 Ni0.85Be0,15 l upper • Mg2 Nf0.75Be0.25~ ptateau ---B---Mg 2 Ni0,85Be0.15 ~)~atWe&u
Q\\
I rv-
_ ~
~
+++z+
. . . .
__0 . . . . .
4~---
;
---48---
/I
,'l
3s9"c o
o
,
g
337oC
,'l
i
t.,o
i i
i
ili
. . . . . . . . . . . i
I 0.5
i
i
h
,
i 10
'
H/metal (ot0rnic ralio) Fig. 2. Desorption isotherms for the MgzNi075Be0.zs--Hzsystem. The desorption isotherm for Mg2NiH4 at 337°C (. . . . ) is shown for comparison.
i
,
,
i
,
I
1.6
I
i
,
,
I
i
i
,
i
I
1.7
,
i
,
,
I
,
1/?'- '< 10 3
i
i
,
i
18
Fig. 3. Logarithmic plot of the dissociation pressure vs the reciprocal of the absolute temperature for the Mg2Nil-~BexH2 system.
HYDROGEN ABSORPTION IN BERYLLIUM SUBSTITUTED Mg2Ni
785
Table 2. The thermodynamic functions for the hydrides of the alloys MgNil-xBex (x = 0, 0.15, 0.25) Compound Mg2Ni* Mg2Nio.asBeoa5 Upper plateau Lower plateau Mg2Nio.vsBeo.25
AH(kcal (tool H2) -I)
AS(cal (mol H2) -1)
-15.4 -+ 1
-29.2 + 1.5
-16.9 -+ l -21.8 -- 1 -19.1 -+ 1
-31.3 -+ 1.5 -38.0 -+ 1.5 -33.6--+ 1.5
* Ref. [21. are necessary in order to establish general trends in hydride stability. Additional investigations on the effect of the substituents in Mg2Ni are now in progress.
7.
REFERENCES 1. J. J. Reilly and R. H. Wiswall, Inorg. Chem. 6, 2220 (1967). 2. J. J. Reilly and R. H. Wiswall, lnorg. Chem. 7, 2254 (1968). 3. Powder Diffraction File 1-1268, Joint Committee on Powder Diffraction Standards (1960). 4. C. P. Lundin, F. E. Lynch and C. B. Magee, J. lesscommon Metals 56, 19 (1977). 5. K. H. Buschow and A. R. Miedema, in Hydrides for Energy Storage, Proc. Int. Syrup. Geilo, 14--19 August 1977 (A. F. Andresen and A. J. Maeland, eds.), Pergamon Press, Oxford (1977). 6. O. de Pous and H. M. Lutz, in Hydrogen Energy Systems,
8. 9. 10. 11. 12. 13.
Proc. 2nd World Hydrogen Energy Conf., Ziirich, 21-24 August 1978 (T. N. Veziro~lu and W. Seifritz, eds.),Vol. 3, p. 1597, Pergamon Press, New York (1978). G. H. Mendelsohn and D. M. Gruen, in Proc. Int. Syrup. Metal-Hydrogen Systems 13-15 April, 1981, Miami Beach, Florida (1981). R. Burch and N. B. Mason, J. less-common Metals 63, 57 (1979). G. Bruzzone, G. Costa, M. Perretti and G. L. Olcese, Int. J. Hydrogen Energy 5, 317 (1980). R. V. Bucur and D. Lupu, J. Phys. Chem. Solids (in press). B. Siegel and G. G. Libowitz, in Metal Hydrides (W. M. Mueller, J. P. Blackledge and G. C. Libowitz, eds.), pp. 545-674. Academic Press, New York (1968). C. M. Stander and R. A. Pacey, J. Phys. Chem. Solids 39, 829 (1978). G. Krasko, in Proc. Int. Syrup. Metal-Hydrogen Systems, 13-15 April 1981, Miami Beach, Florida (1981).
0360-3199/82/1007834)3 $03.00/0 Pergamon Press Ltd. (~) 1982 International Association for Hydrogen Energy.
Printed in Great Britain.
HYDROGEN ABSORPTION IN BERYLLIUM SUBSTITUTED Mg2Ni D. LuPu, A. BIRIS and E. INDREA Institute of Isotopic and Molecular Technology, R-3400 Cluj-Napoca, P.O. Box 243, Romania
(Receivedfor publication 22 January 1982) Abstract--The alloys Mg2Nix-xBex (x = 0.15 and 0.25) retain the Mg2Ni structure showing a lattice dilation proportional to the beryllium content. The pressure-composition isotherms are reported for the dissociation of hydrided samples. The results suggest that there are two type of interstices able to absorb up to 4 H atoms per formula unit. The heats of formation obtained from the van't Hoff relationship show an increased stability for the hydrides of the beryllium substituted alloys compared to the pure Mg2Ni. The results suggest that the electronic factors are more important for hydride stability than variations in the unit cell volume.
INTRODUCTION
RESULTS AND DISCUSSION
Since the discovery of the hydrogen absorption in Mg2Cu and Mg2Ni by Reilly and Wiswall [1, 2], the search for alloys with even higher capacities of hydrogen storage focuses the attention of large groups of 'hydriders'. Systematic studies of the influence of the substitutions in magnesium intermetallics on the absorption capacity and the hydride stability are of great interest in order to establish correlations between the hydride stability and the substituent characteristics. In this paper the pressure-composition isotherms are reported for two beryllium substituted magnesiumnickel alloys of the general formula Mg2Nil-xBex.
The diffraction patterns for all the samples were those corresponding to the Mg2Ni hexagonal structure. No 'extra' lines were observed. The lattice constants and the volume of the unit cell are given in Table 1. Table 1. The lattice constants and volume of the unit cell V for the alloys Mg2Ni~-xBex(x = 0, 0.15, 0.25) Compound
a (A)
c (A)
V (A3)
Mg2Ni* Mg2Ni0.asBe0.t5 Mg2Ni075Be0.25
5.191 5.245 5.277
13.209 13.332 13.416
308.2 317.6 323.5
EXPERIMENTAL
* The data are in good agreement with the values given in the literature (a = 5.19 ,~, c = 13.21 ,~,) [3].
Metals of 99.9% purity were used for the alloy preparation under high purity argon atmosphere in a high purity graphite crucible. A stainless-steel stirrer was used to homogenize the sample, which, after cooling, was overturned and remelted several times. The final compositions of the samples corresponded to the formulae Mg2Ni0.85Be015 and Mg2Ni075Be0.25. The X-ray diffraction patterns were determined using a powder diffractometer TUR-M61 with filtered C u K a radiation (34 kV, 20 mA). The scattered X-ray intensity was determined by the use of step scanning at 0.02° 20 intervals, with 100 s count-time per step. The 200 reflexion (Bragg angle 19.94 °) and 006 reflexion (Bragg angle 20.51 °) were employed to measure the lattice spacings in the a and c directions, respectively, with an accuracy of +0.003/~,. The finely powdered samples were outgassed at 350°C in a stainless-steel reactor after which high purity hydrogen at 60 atm. was admitted. The samples were hydrided and after several days desorption was started. The desorption kinetics does not differ essentially from that of pure Mg2Ni and the dissociation pressure was considered as the equilibrium value when no pressure increase was observed for at least 6 h.
The Mg2Ni structure is retained in the Mg2Nil-~Bex system up to x = 0.25. The substitution of Be for Ni atoms leads to a linear increase of the lattice constants with the increasing Be concentration. Taking into account that Be has a smaller metallic radius than Ni, the increase in the cell dimension of the beryllium substituted compounds was not anticipated. However, a pronounced expansion of the cell volume is observed. The desorption isotherms are reported in Figs. 1 and 2. Two plateau pressures are observed for the dissociation of the hydrided Mg2Ni085Be0.15 while for the Mg2Ni0.75Be0.25 only the upper plateau appears so that the remaining absorbed hydrogen ( - 0 . 3 H per metal) seems to be more strongly bound and has not been desorbed at hydrogen pressures above 10 -2 atm. H2. The dissociation pressure is lower than that for pure Mg2Ni and decreases with increasing beryllium content. A semilogarithmic plot of these dissociation pressures vs the reciprocal of the absolute temperature is shown in Fig. 3. The thermodynamic functions thus calculated from the van't Hoff equation are given in Table 2. The absorption of 4 H atoms (Mg2Nil-xBex)- ~suggests that the H atoms are located in the same type of interstices as in pure Mg2Ni.
783
D. LUPU, A. BIRIS AND E. INDREA
784
+
I
10 - M+z. q'' NiO.'::Beo.15o..,
rr
++/'/z o
o
o
~
~
o
315"c
t
I
7
, 0.5
1.0 H/metal (cffomicratio) Fig. 1. Desorption isotherms for the Mg2Ni0.ssBeo]5-H2system.
The enthalpies of formation for the hydrides corresponding to the upper plateau pressure on the pressure-composition isotherms show that the stability of this hydride increases with increasing beryllium concentration in Mg2Nil-xBex. The hydride stabilities in the composition range corresponding to the lower plateau pressure are even higher, as reported in Table 2. Thus, it is reasonable to assume that the upper plateau pressure corresponds to the desorption of H atoms from interstices without Be atoms as nearest neighbours while the lower plateau pressure corresponds to the desorption of hydrogen atoms located in the interstices with Mg, Ni and Be atoms as nearest neighbours in the Mg2Nil-xBex lattice. The former could be associated with the influence of the Be atoms on the band structure of the entire lattice while the latter could be the result of a direct influence of Be atoms. Obviously neutron diffraction studies of these hydrides would be both helpful and interesting. 10
I
Mg2 Ni0.75Be0.25
I
The cell volume increases with increasing beryllium content as reported above, and this is in agreement with the equilibrium pressure-hole size correlation reported in the literature [4]. However, this correlation is considered fortuituous by Buschow and Miedema [5]. The importance of the electronic factors on the hydride stability was suggested by de Pous and Lutz [6] and Mendelsohn and Gruen [7]. Some limitations in the role of the geometrical factors on the stability of a metal hydride also result from the opinion expressed by Burch and Mason [8]. In fact, it has been recently shown by Bruzzone et al. [9] that the effect of beryllium in TiFel-xBe, is in contrast with the equilibrium pressure-hole size correlation. Thus, there are two cases where the presence of Be atoms increases the hydride stability: one with lattice contraction and the other with lattice expansion. This seems to be direct evidence of the prominent role played by the electronic factors on the hydride stability. In this connection very interesting quantitative correlations between hydride stability and band structure of the palladium alloys were recently reported [10]. The significant covalent character of the bonding in MgH2 is now largely accepted [11-13]. Taking into account that the A H values for all the magnesium intermetallic hydrides studied up to now are close to those of MgH2, which is not the case with the hydrides of La and LaNi5 or of Ti and TiFe (and related compounds), a significant covalent character (localized bond) is also to be expected for the magnesium intermetallic hydrides. A light hydride of relative low stability is needed for automotive applications and therefore more systematic studies on other substituted magnesium intermetallics
LnP
~. ~ "\\\
"~,~
-.\
t I
F~
-~
---o-- Mg2 Ni0.85Be0,15 l upper • Mg2 Nf0.75Be0.25~ ptateau ---B---Mg 2 Ni0,85Be0.15 ~)~atWe&u
Q\\
I rv-
_ ~
~
+++z+
. . . .
__0 . . . . .
4~---
;
---48---
/I
,'l
3s9"c o
o
,
g
337oC
,'l
i
t.,o
i i
i
ili
. . . . . . . . . . . i
I 0.5
i
i
h
,
i 10
'
H/metal (ot0rnic ralio) Fig. 2. Desorption isotherms for the MgzNi075Be0.zs--Hzsystem. The desorption isotherm for Mg2NiH4 at 337°C (. . . . ) is shown for comparison.
i
,
,
i
,
I
1.6
I
i
,
,
I
i
i
,
i
I
1.7
,
i
,
,
I
,
1/?'- '< 10 3
i
i
,
i
18
Fig. 3. Logarithmic plot of the dissociation pressure vs the reciprocal of the absolute temperature for the Mg2Nil-~BexH2 system.
HYDROGEN ABSORPTION IN BERYLLIUM SUBSTITUTED Mg2Ni
785
Table 2. The thermodynamic functions for the hydrides of the alloys MgNil-xBex (x = 0, 0.15, 0.25) Compound Mg2Ni* Mg2Nio.asBeoa5 Upper plateau Lower plateau Mg2Nio.vsBeo.25
AH(kcal (tool H2) -I)
AS(cal (mol H2) -1)
-15.4 -+ 1
-29.2 + 1.5
-16.9 -+ l -21.8 -- 1 -19.1 -+ 1
-31.3 -+ 1.5 -38.0 -+ 1.5 -33.6--+ 1.5
* Ref. [21. are necessary in order to establish general trends in hydride stability. Additional investigations on the effect of the substituents in Mg2Ni are now in progress.
7.
REFERENCES 1. J. J. Reilly and R. H. Wiswall, Inorg. Chem. 6, 2220 (1967). 2. J. J. Reilly and R. H. Wiswall, lnorg. Chem. 7, 2254 (1968). 3. Powder Diffraction File 1-1268, Joint Committee on Powder Diffraction Standards (1960). 4. C. P. Lundin, F. E. Lynch and C. B. Magee, J. lesscommon Metals 56, 19 (1977). 5. K. H. Buschow and A. R. Miedema, in Hydrides for Energy Storage, Proc. Int. Syrup. Geilo, 14--19 August 1977 (A. F. Andresen and A. J. Maeland, eds.), Pergamon Press, Oxford (1977). 6. O. de Pous and H. M. Lutz, in Hydrogen Energy Systems,
8. 9. 10. 11. 12. 13.
Proc. 2nd World Hydrogen Energy Conf., Ziirich, 21-24 August 1978 (T. N. Veziro~lu and W. Seifritz, eds.),Vol. 3, p. 1597, Pergamon Press, New York (1978). G. H. Mendelsohn and D. M. Gruen, in Proc. Int. Syrup. Metal-Hydrogen Systems 13-15 April, 1981, Miami Beach, Florida (1981). R. Burch and N. B. Mason, J. less-common Metals 63, 57 (1979). G. Bruzzone, G. Costa, M. Perretti and G. L. Olcese, Int. J. Hydrogen Energy 5, 317 (1980). R. V. Bucur and D. Lupu, J. Phys. Chem. Solids (in press). B. Siegel and G. G. Libowitz, in Metal Hydrides (W. M. Mueller, J. P. Blackledge and G. C. Libowitz, eds.), pp. 545-674. Academic Press, New York (1968). C. M. Stander and R. A. Pacey, J. Phys. Chem. Solids 39, 829 (1978). G. Krasko, in Proc. Int. Syrup. Metal-Hydrogen Systems, 13-15 April 1981, Miami Beach, Florida (1981).