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Phase transformation and thermal expansion of MgNi alloys in a hydrogen atmosphere
Journal of the Less-Common
Metals, 88 (1982)
PHASE TR~SFORMATION ALLOYS IN A HYDROGEN
57
57 - 61
AND ~ERMAL ATMOSPMERE”
EXP~SION
OF Mg-Ni
S. ONO, Y. ISHIDO, K. IMANARI and T. TABATA Nationat Chemical 305 (~ap4~~
Laboratory
Y. K. CHO, R. YAMAIIOTO Tokyo
University,
for Industry,
Yatabe,
Tsuhuba
Research
Centre,
Ibaraki
and M. DOYAMA
Hongo, Bunkyo-ku,
Tokyo
113 (Japan)
(Received March 23,1982)
Summary The thermal expansion of the Mg-5.6wt.%Ni-H, and Mg-55wt.%Ni-H, systems was examined by in situ X-ray powder diffraction using a specially designed apparatus for measurements at 500 “C and a hydrogen pressure of 5 MPa. The temperature dependence of the unit cell parameters of each alloy and hydride phase and the volume change associated with each phase transformation were determined. The thermal expansion data are summarized. The volume expansions of magnesium to MgH,, Mg,Ni to high temperature Mg,NiH, and high temperature Mg,NiH, to low temperature Mg,NiII, were calculated from these data as 30.4% (0.2%) at 340 “C, 27.8% (0.2%) at 320 “C and 0.3% (0.1%) at 234 “C respectively (the standard deviations are given in parentheses).
1. Introduction The Mg-Hz system was first reported in 1960 [ 11. Reilly and Wiswall [2] have ~vestiga~d the Mg-Ni-H, system and have reported a ternary hydride MgZNiH,. We have previously investigated the kinetics [3 - 51 and the pressure-composition diagrams [6] of these systems. In order to use these materials in various applications [7, 81 accurate thermal expansion data are required. A high temperature and high pressure X-ray powder diffractome~r [ 93 was used in the present investigation, 2. Experimental
procedure
2.1. Starting materials Two samples (Mg-5%Ni and Mg-55%Ni) supplied by the Furukawa Ma~esium Co. were used in the present experiments. They were prepared by melting magnesium and nickel metal in carbon crucibles and pouring the *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides, Toba, Japan, May 30 - June 4,1982. 0022-5088/32/0000-0000/$02.75
@ Elsevier Sequoia/Printed
in The Netherlands
58 TABLE 1 Analytical results for the alloys Sample
Mg-5%Ni Mg-55%Ni
Ni
Zn
Fe
Pb
(wt.%)
(wt.%)
(wt.%)
(wt.%)
5.6 55.4
0.022 0.013
0.012 0.004
0.002 0.001
melt into hot iron moulds. The analytical results are given in Table 1. Singlephase Mg,Ni is difficult to be prepared and either excess MgNi, or magnesium is formed. The ingot was filed to a thickness of about 100 pm and hydrided in a microreactor at 350 “C and 5.9 MPa. After several hydridingdehydriding cycles a well-hydrided sample was removed in an argon dry-box in which it was stored. The hydrides obtained were all bright reddish brown, but soon discoloured to dark brown on exposure to air. Mg-5%Ni is a mixture of a major magnesium phase and a minor Mg,Ni phase. The addition of nickel is necessary to improve the kinetic properties of the magnesium. The pressure-composition isotherms of this sample were examined and the decomposition pressure was found to agree with that of the pure Mg-H, system [6]. The results for this sample will be described together with those for the Mg-H, system. The thermal analysis of this sample under a pressure of 0.5 MPa showed that the starting hydride had the following weight composition: 88.5% Mg, 5.3% Ni and 6.2% H. The average atomic ratio of hydrogen to magnesium was calculated to be 1.69. However, the X-ray diffraction data revealed that small amounts of unreacted magnesium and Mg,Ni were also present. Therefore the magnesium hydride phase in this sample must have a higher hydrogen content than MgH1,69 although its accurate composition cannot be obtained. The major component of Mg-55%Ni is Mg,Ni which coexists with small amounts of magnesium and MgNi,. The phase composition is reported elsewhere in this symposium [9]. The composition of the major phase was found to be Mg,NiH3.s4_ 2.2. X-ray powder diffraction For in situ experiments, a Rigaku Rotaflex RU-200 diffractometer was used in conjunction with a single-crystal graphite monochromator, a scintillation counter and a specially designed autoclave for measurements up to 500 “C and 5 MPa. The beryllium window for X-ray exposure was 3 mm thick. Cu Kcu radiation was used in this experiment. Some of the results are reported elsewhere in this symposium [9] . The computer program XRAY, which is based on the Crystal Analysis Universal Program System [lo] , was used for analysis of the experimental data. The least-squares program SALS developed at the computer centre of the University of Tokyo was used for the least-squares fitting of the thermal expansion data.
59
3. Results and discussion 3.1. The Mg-H, system The results of the present experiments on Mg-5%Ni are shown graphically in Figs. 1 and 2. Magnesium has an h.c.p. lattice. The unit cell parameters determined at room temperature in air agree with the values in the literature within the standard deviation. Magnesium hydride has a tetragonal TiOz structure. The experimental values in Figs. 1 and 2 were measured during runs under hydrogen pressures of 0.2 MPa for the magnesium phase and 0.5 MPa for the magnesium hydride phase because the changes in the unit cell parameters between these two pressure levels are within experimental error. In the case of magnesium a linear relationship was observed including the points at room temperature in air. This indicates that the solubility of hydrogen in magnesium is negligibly small under these conditions, as has already been pointed out elsewhere [6] . In Fig. 2 the volume per metallic atom is given as the unit cell volume divided by the number of magnesium and nickel atoms in the cell. The volume expansion of the hydriding reaction was calculated from Fig. 2 as 30.4% (0.2%) at 340 “C.
a
c
$
32-
m -? =:
MgH2 30-
s g
28-
k =
26-
(in
0.5
MPa.H2)
P z z 3.24
c!A” A” d”
5.26 Temp.
. 9:
_
22-
-
46” 3:” ’
24-
1
20:
lin
0
100
0.2?Pa.H2)
200
300
400 Temp.
Fig. 1. Temperature dependence of the unit cell parameters of magnesium magnesium at room temperature inair a = 3.209 8(5) A and c = 5.2117(12) gen pressures used were 0.2 MPa for magnesium and 0.5 MPa for MgH,. Fig. 2. Thermal
expansion
of the cell volumes
of magnesium
5 0
‘C
and MgHz. For A. The hydro-
and MgH2.
3.2. The Mg,Ni-H, system The results for the Mg,Ni-Hz system are shown in Figs. 3 and 4. The crystal structure of low temperature (LT) Mg,N&, which is stable below 234 “C, is described elsewhere in this symposium [9] as a monoclinic unit cell. At temperatures above this value high temperature (HT) Mg,NiH, which has a cubic CaF*-type metal structure is the stable phase. The unit cell of the LT form can be described as a distortion of that of the HT form. In Fig. 3 the a axis of the LT form is plotted as a/2 to facilitate comparison with the u axis of the HT form [9]. The runs were carried out under a hydrogen pressure of 2 MPa.
60
23
22
(in
Iin
2 MPa.H2)
$,2Ni,,in
ai,,.)
0 Temp..
HT-Hg2NiH4 2 HPa.H21
LT-Mg2NiH4
100
200
,7%P;:H21 300
“c
400 Temp.
Fig. 3. Temperature dependence of the unit cell pressure of 2 MPa.
parameters
eiCIO ‘C
of Mg,NiH4 under a hydrogen
Fig. 4. Thermal expansion of the cell volumes of Mg,Ni and MgzNiH4.
The volume change of this phase transformation is very small, being approximately 0.3% (0.1%). The enthalpy change is 2 kcal mol-’ 191. The calculated volume expansion associated with hydriding was 27.81% (0.02%) at 320 “C. TABLE 2 Summary of the thermal expansion data for the Mg-Hz and Mg,Ni-Hz systems gx 10s
Phase (lattice system)
xa
x0 (A)
Mg (hexagonal)
a
3.55(0.13) 3.74(0.08) ll.O(O.3)
V
3.2075(0.0011) 5.2075(0.0013) 23.20(0.02)
MgHz (tetragonal)
a c V
4.5198(0.0007) 3.0250(0.0006) 30.90(0.02)
1.39(0.07) 1.69(0.10) 4.6(0.2)
MgzNi (hexagonal)
a c V
5.26(0.01) 13.59(0.08) l&11(0.08)
1.3(0.6) O.O(l.3) 2.8(1.1)
HT MgaNiH4 (cubic)
a V
6.489(0.002) 22.77(0.02)
2.59(0.09) 7.9(0.3)
LT Mg,NiH4 (monoclinic)
a b
13.211(0.005) 6.4156(0.0017) 6.4952(0.0018) 93.23(0.04) 22.90(0.01)
C
C
V
l.Ol(O.21) 2.46(0.19) 3.13(0.22) -1.51(0.24) 6.9(0.4)
‘X = X0( 1 + gt) where X is the parameter of interest, X0 is the value of X at 0 C, g is a constant and t is the temperature in degrees Celsius. The standard deviations are given in parentheses.
61
3.3. Summary
of the thermal expansion
data
The thermal expansion data obtained in the present experiments are summarized in Table 2. It should be noted that the thermal expansion is more strongly anisotropic in the Mg,Ni-H, system than in the Mg-H, system. The volume expansion coefficients of LT Mg,NiH, and HT Mg,NiH, are approximately the same, whereas the former shows strong anisotropy in its linear thermal expansion coefficients.
References J. F. Stampfer,
Jr., C. E. Holley,
Jr., and J. F. Suttle,
J. Am.
Chem.
3505. J. J. Reilly and R. H. WiswaIl, Jr., Inorg. Chem., 7 (1968) 2254. K. Nomura, E. Akiba, S. Ono and S. Suda, Trans. Jpn. Inst. Met.,
4 5 6 7 8 9 10
Sot.,
82 (1960)
Suppl., 21 (1980) 353. E. Akiba, K. Nomura, S. Ono and S. Suda, Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 23 26, 1980, Pergamon, Oxford, 1980, p. 881. S. Ono, E. Akiba and K. Imanari, Proc. Int. Symp. on Metal-Hydrogen Systems, Miami, FL, April 1981, Pergamon, New York, 1982, p, 467. E. Akiba, K. Nomura, S. Ono and Y. Mizuno, J. Less-Common Met., 83 (1982) L43. S. Ono, M. Kawamura, Y. Ishido, E. Akiba and S. Higano, Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 23 - 26, 1980, Pergamon, Oxford, 1980, p. 937. N. Nishimiya, A. Suzuki and S. Ono, Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 23 - 26, 1980, Pergamon, Oxford, 1980, p. 917. S. Ono, H. Hayakawa, A. Suzuki, K. Nomura, N. Nishimiya and T. Tabata, J. LessCommon Met., 88 (1982) 63. T. Sakurai (ed.), Universal Crystallographic Computation Program System, Crystallographic Society of Japan, Tokyo, 1967.
Metals, 88 (1982)
PHASE TR~SFORMATION ALLOYS IN A HYDROGEN
57
57 - 61
AND ~ERMAL ATMOSPMERE”
EXP~SION
OF Mg-Ni
S. ONO, Y. ISHIDO, K. IMANARI and T. TABATA Nationat Chemical 305 (~ap4~~
Laboratory
Y. K. CHO, R. YAMAIIOTO Tokyo
University,
for Industry,
Yatabe,
Tsuhuba
Research
Centre,
Ibaraki
and M. DOYAMA
Hongo, Bunkyo-ku,
Tokyo
113 (Japan)
(Received March 23,1982)
Summary The thermal expansion of the Mg-5.6wt.%Ni-H, and Mg-55wt.%Ni-H, systems was examined by in situ X-ray powder diffraction using a specially designed apparatus for measurements at 500 “C and a hydrogen pressure of 5 MPa. The temperature dependence of the unit cell parameters of each alloy and hydride phase and the volume change associated with each phase transformation were determined. The thermal expansion data are summarized. The volume expansions of magnesium to MgH,, Mg,Ni to high temperature Mg,NiH, and high temperature Mg,NiH, to low temperature Mg,NiII, were calculated from these data as 30.4% (0.2%) at 340 “C, 27.8% (0.2%) at 320 “C and 0.3% (0.1%) at 234 “C respectively (the standard deviations are given in parentheses).
1. Introduction The Mg-Hz system was first reported in 1960 [ 11. Reilly and Wiswall [2] have ~vestiga~d the Mg-Ni-H, system and have reported a ternary hydride MgZNiH,. We have previously investigated the kinetics [3 - 51 and the pressure-composition diagrams [6] of these systems. In order to use these materials in various applications [7, 81 accurate thermal expansion data are required. A high temperature and high pressure X-ray powder diffractome~r [ 93 was used in the present investigation, 2. Experimental
procedure
2.1. Starting materials Two samples (Mg-5%Ni and Mg-55%Ni) supplied by the Furukawa Ma~esium Co. were used in the present experiments. They were prepared by melting magnesium and nickel metal in carbon crucibles and pouring the *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides, Toba, Japan, May 30 - June 4,1982. 0022-5088/32/0000-0000/$02.75
@ Elsevier Sequoia/Printed
in The Netherlands
58 TABLE 1 Analytical results for the alloys Sample
Mg-5%Ni Mg-55%Ni
Ni
Zn
Fe
Pb
(wt.%)
(wt.%)
(wt.%)
(wt.%)
5.6 55.4
0.022 0.013
0.012 0.004
0.002 0.001
melt into hot iron moulds. The analytical results are given in Table 1. Singlephase Mg,Ni is difficult to be prepared and either excess MgNi, or magnesium is formed. The ingot was filed to a thickness of about 100 pm and hydrided in a microreactor at 350 “C and 5.9 MPa. After several hydridingdehydriding cycles a well-hydrided sample was removed in an argon dry-box in which it was stored. The hydrides obtained were all bright reddish brown, but soon discoloured to dark brown on exposure to air. Mg-5%Ni is a mixture of a major magnesium phase and a minor Mg,Ni phase. The addition of nickel is necessary to improve the kinetic properties of the magnesium. The pressure-composition isotherms of this sample were examined and the decomposition pressure was found to agree with that of the pure Mg-H, system [6]. The results for this sample will be described together with those for the Mg-H, system. The thermal analysis of this sample under a pressure of 0.5 MPa showed that the starting hydride had the following weight composition: 88.5% Mg, 5.3% Ni and 6.2% H. The average atomic ratio of hydrogen to magnesium was calculated to be 1.69. However, the X-ray diffraction data revealed that small amounts of unreacted magnesium and Mg,Ni were also present. Therefore the magnesium hydride phase in this sample must have a higher hydrogen content than MgH1,69 although its accurate composition cannot be obtained. The major component of Mg-55%Ni is Mg,Ni which coexists with small amounts of magnesium and MgNi,. The phase composition is reported elsewhere in this symposium [9]. The composition of the major phase was found to be Mg,NiH3.s4_ 2.2. X-ray powder diffraction For in situ experiments, a Rigaku Rotaflex RU-200 diffractometer was used in conjunction with a single-crystal graphite monochromator, a scintillation counter and a specially designed autoclave for measurements up to 500 “C and 5 MPa. The beryllium window for X-ray exposure was 3 mm thick. Cu Kcu radiation was used in this experiment. Some of the results are reported elsewhere in this symposium [9] . The computer program XRAY, which is based on the Crystal Analysis Universal Program System [lo] , was used for analysis of the experimental data. The least-squares program SALS developed at the computer centre of the University of Tokyo was used for the least-squares fitting of the thermal expansion data.
59
3. Results and discussion 3.1. The Mg-H, system The results of the present experiments on Mg-5%Ni are shown graphically in Figs. 1 and 2. Magnesium has an h.c.p. lattice. The unit cell parameters determined at room temperature in air agree with the values in the literature within the standard deviation. Magnesium hydride has a tetragonal TiOz structure. The experimental values in Figs. 1 and 2 were measured during runs under hydrogen pressures of 0.2 MPa for the magnesium phase and 0.5 MPa for the magnesium hydride phase because the changes in the unit cell parameters between these two pressure levels are within experimental error. In the case of magnesium a linear relationship was observed including the points at room temperature in air. This indicates that the solubility of hydrogen in magnesium is negligibly small under these conditions, as has already been pointed out elsewhere [6] . In Fig. 2 the volume per metallic atom is given as the unit cell volume divided by the number of magnesium and nickel atoms in the cell. The volume expansion of the hydriding reaction was calculated from Fig. 2 as 30.4% (0.2%) at 340 “C.
a
c
$
32-
m -? =:
MgH2 30-
s g
28-
k =
26-
(in
0.5
MPa.H2)
P z z 3.24
c!A” A” d”
5.26 Temp.
. 9:
_
22-
-
46” 3:” ’
24-
1
20:
lin
0
100
0.2?Pa.H2)
200
300
400 Temp.
Fig. 1. Temperature dependence of the unit cell parameters of magnesium magnesium at room temperature inair a = 3.209 8(5) A and c = 5.2117(12) gen pressures used were 0.2 MPa for magnesium and 0.5 MPa for MgH,. Fig. 2. Thermal
expansion
of the cell volumes
of magnesium
5 0
‘C
and MgHz. For A. The hydro-
and MgH2.
3.2. The Mg,Ni-H, system The results for the Mg,Ni-Hz system are shown in Figs. 3 and 4. The crystal structure of low temperature (LT) Mg,N&, which is stable below 234 “C, is described elsewhere in this symposium [9] as a monoclinic unit cell. At temperatures above this value high temperature (HT) Mg,NiH, which has a cubic CaF*-type metal structure is the stable phase. The unit cell of the LT form can be described as a distortion of that of the HT form. In Fig. 3 the a axis of the LT form is plotted as a/2 to facilitate comparison with the u axis of the HT form [9]. The runs were carried out under a hydrogen pressure of 2 MPa.
60
23
22
(in
Iin
2 MPa.H2)
$,2Ni,,in
ai,,.)
0 Temp..
HT-Hg2NiH4 2 HPa.H21
LT-Mg2NiH4
100
200
,7%P;:H21 300
“c
400 Temp.
Fig. 3. Temperature dependence of the unit cell pressure of 2 MPa.
parameters
eiCIO ‘C
of Mg,NiH4 under a hydrogen
Fig. 4. Thermal expansion of the cell volumes of Mg,Ni and MgzNiH4.
The volume change of this phase transformation is very small, being approximately 0.3% (0.1%). The enthalpy change is 2 kcal mol-’ 191. The calculated volume expansion associated with hydriding was 27.81% (0.02%) at 320 “C. TABLE 2 Summary of the thermal expansion data for the Mg-Hz and Mg,Ni-Hz systems gx 10s
Phase (lattice system)
xa
x0 (A)
Mg (hexagonal)
a
3.55(0.13) 3.74(0.08) ll.O(O.3)
V
3.2075(0.0011) 5.2075(0.0013) 23.20(0.02)
MgHz (tetragonal)
a c V
4.5198(0.0007) 3.0250(0.0006) 30.90(0.02)
1.39(0.07) 1.69(0.10) 4.6(0.2)
MgzNi (hexagonal)
a c V
5.26(0.01) 13.59(0.08) l&11(0.08)
1.3(0.6) O.O(l.3) 2.8(1.1)
HT MgaNiH4 (cubic)
a V
6.489(0.002) 22.77(0.02)
2.59(0.09) 7.9(0.3)
LT Mg,NiH4 (monoclinic)
a b
13.211(0.005) 6.4156(0.0017) 6.4952(0.0018) 93.23(0.04) 22.90(0.01)
C
C
V
l.Ol(O.21) 2.46(0.19) 3.13(0.22) -1.51(0.24) 6.9(0.4)
‘X = X0( 1 + gt) where X is the parameter of interest, X0 is the value of X at 0 C, g is a constant and t is the temperature in degrees Celsius. The standard deviations are given in parentheses.
61
3.3. Summary
of the thermal expansion
data
The thermal expansion data obtained in the present experiments are summarized in Table 2. It should be noted that the thermal expansion is more strongly anisotropic in the Mg,Ni-H, system than in the Mg-H, system. The volume expansion coefficients of LT Mg,NiH, and HT Mg,NiH, are approximately the same, whereas the former shows strong anisotropy in its linear thermal expansion coefficients.
References J. F. Stampfer,
Jr., C. E. Holley,
Jr., and J. F. Suttle,
J. Am.
Chem.
3505. J. J. Reilly and R. H. WiswaIl, Jr., Inorg. Chem., 7 (1968) 2254. K. Nomura, E. Akiba, S. Ono and S. Suda, Trans. Jpn. Inst. Met.,
4 5 6 7 8 9 10
Sot.,
82 (1960)
Suppl., 21 (1980) 353. E. Akiba, K. Nomura, S. Ono and S. Suda, Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 23 26, 1980, Pergamon, Oxford, 1980, p. 881. S. Ono, E. Akiba and K. Imanari, Proc. Int. Symp. on Metal-Hydrogen Systems, Miami, FL, April 1981, Pergamon, New York, 1982, p, 467. E. Akiba, K. Nomura, S. Ono and Y. Mizuno, J. Less-Common Met., 83 (1982) L43. S. Ono, M. Kawamura, Y. Ishido, E. Akiba and S. Higano, Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 23 - 26, 1980, Pergamon, Oxford, 1980, p. 937. N. Nishimiya, A. Suzuki and S. Ono, Proc. 3rd World Hydrogen Energy Conf., Tokyo, June 23 - 26, 1980, Pergamon, Oxford, 1980, p. 917. S. Ono, H. Hayakawa, A. Suzuki, K. Nomura, N. Nishimiya and T. Tabata, J. LessCommon Met., 88 (1982) 63. T. Sakurai (ed.), Universal Crystallographic Computation Program System, Crystallographic Society of Japan, Tokyo, 1967.