- Email: [email protected]
Effect of sulphur addition on the properties of FeTi alloy for hydrogen storage
Journal
of the Less-Common
Metals,
104
(1984)
199
EFFECT OF SULPHUR ADDITION ON THE PROPERTIES ALLOY FOR HYDROGEN STORAGE* RYOICHI
SUZUKI,
JIRO
Research and Development Nippon Steel Corporation, (Received
March
OHNO and HISASHI
199
- 206
OF Fe-Ti
GONDOH
Laboratories-I, Central Research and Development 1618 Ida, Nakahara-ku, Kawasaki 211 (Japan)
Bureau,
26, 1984)
Summary Addition of sulphur to the Fe-Ti alloy was found to produce a marked improvement in the properties of the alloy for hydrogen storage. The most which had a low activation temperaeffective composition was FeTir _OsSo~OZ, ture (80 - 100 “C), a short incubation time (10 - 30 h) and a good equilibrium plateau pressure over a wide concentration range of absorbed hydrogen. The degree of pulverization for this alloy after the absorptiondesorption cycle test was comparatively moderate. The sulphur added to the alloy was converted into TizS which was precipitated in a network structure with strips about 5 pm wide separated by spaces of dimensions about 20 50 pm.
1. Introduction Many types of metal hydride have been developed as interesting new materials for hydrogen storage and energy conversion [ 1, 21. Of the proposed metal hydrides, the FeTi alloy is claimed to be promising for the storage and transportation of hydrogen [3, 41. This alloy, however, has a disadvantage in that it requires activation for an extended period at temperatures above 400 “C under vacuum and at room temperature under pressures of 5 - 6 MPa. Many studies aimed at improving this performance have been conducted [5 - 81. These workers investigated the addition of a minor element to the Fe-Ti alloy to improve the activation performance while retaining its inherent advantages. The addition of sulphur to the alloy resulted in a marked improvement. The results of a study of the activation characteristics, hydriding and dehydriding behaviour, durability and metallurgical structure of the Fe-Ti-S ternary alloy are presented in this paper. *Paper presented at the International Symposium of Metal Hydrides IV, Eilat, Israel, April 9 - 13, 1984. 0022-5088/84/$3.00
0 Elsevier
on the Properties
Sequoia/Printed
and Applications
in The Netherlands
200
2. Experimental procedure Sample alloys were melted in an arc furnace under argon gas and were cast into button-shaped samples of mass 50 - 100 g. The samples were then pulverized in air with a vibrating mill to a size of 80 - 150 mesh. A computercontrolled apparatus was used to activate powder samples of mass 10 g without any heat treatment in a high temperature range by the repetition of a 30 min cycle of hydrogenation at elevated temperature under vacuum and at room temperature under a pressure of 3 MPa. The completion of the activation was determined from the degree of hydrogenation of the sample up to an [H] /[M] atomic ratio of 0.5 which was obtained by measuring the decrease in the hydrogen pressure in the test chamber. The activation performance of the alloy was evaluated from the temperature and the time required for activation. The durability of the sample was measured by allowing the sample first to absorb fresh hydrogen and then to desorb it into the atmosphere with a cycle time of 15 - 20 min.
3. Results and discussion 3.1. ~yd~~di~g characteristics of the Fe-Ti binary alloy The single-phase FeTi hydride exists in the ~Ti]/~Fe] atomic ratio range from 0.99 to 1.04. The hydriding characteristics of this alloy vary in this narrow range. The effects of the composition on the hydriding characteristics were investigated in detail by changing the [Ti] /[Fe] atomic ratio from 0.85 to 1.15. Typical results are shown in Fig. 1. When the [Ti] /[ Fe] atomic ratio is unity, the plateau in the pressurecomposition-temperature (P-C-T) diagram extends over a wide range and the alloy has a maximum relative hydrogen content [H]/[M] of 0.5 which is determined by the amount of dissociated hydrogen under pressure at the end point of the fl phase plateau. However, the activation of the alloy with this composition is difficult at temperatures exceeding 300 “C. When the [Ti]/[ Fe] ratio is below unity, the value of [~]/~M] decreases to 0.3 with decreasing value of [Ti]/fFe] but a large plateau still remains. The activation temperature gradually decreases to 150 - 200 “C with decreasing titanium content. When the [Ti]/[Fe] ratio exceeds 1.0, [HI/EM] decreases to below 0.45 and the plateau deteriorates; however, the activation is improved, with the temperature steeply dropping to 100 “C. 3.2. Effects of the addition of sulphur Fe-Ti-S alloys with various compositions were prepared by changing the [Ti]/[Fe] ratio from 0.85 to 1.15 and adding sulphur at [S]/[Ti] atomic ratios in the range 0.004 - 0.08. The results are summarized in Fig. 1. The maximum relative hydrogen content [H]/[M] of the Fe-Ti-S alloy reaches 0.54 at ~Ti]/~Fe] = 1.05 with [S]/[Ti] = 0.02. The activation temperature drops sharply to 100 “C for [Ti]/[Fe] = 1.05. If [Ti]/[Fe] is 1.05 with
201
J-----S/Ti
Ilium (b)
o. 085
OOli 090
095
100
105
I IO
115
0
0.1
0.2
0.3 Atom
T,/Fe
Fig. 1. Effects of alloy composition [S]/[Ti] = O;@, [S]/[Ti] = 0.02.
on the hydrogen
Fig. 2. Effect of sulphur on the pressure-composition
0.4
0.5
0.6
Ratio
absorption
0.7
0.6
0.9
0
(H/M)
characteristics:
isotherm of FeTi,,$
0,
at 25 “C.
[S]/[Ti] = 0.02, the activation temperature decreases to between 80 and 100 “C and the incubation time is 10 - 30 h. As more sulphur was added, the activation temperature increased to 150 “C and the incubation time gradually increased. The effect of the sulphur content on a P-C-T diagram is shown in Fig. 2 for [Ti]f[Fe] = 1.05. The isothermal plateau improves with increasing sulphur content. The degree of plateau which is defined as degree of plateau
=
dissociation dissociation
pressure of hydrogen pressure of hydrogen
at [H]/[M]
= 0.25
at [H] / [ M] = 0.5
is used to characterize the plateau. Sulphur affects the degree of plateau as indicated in Fig. 3 in which the amount of sulphur is expressed as the [ S]/[ Ti] atomic ratio. As the sulphur content is increased, the degree of plateau improves and reaches nearly 1.0 for [S]/[Ti] = 0.03. The effects of the ~Ti]/~Fe] ratio on the degree of plateau are shown in Fig. 4 for the Fe-Ti-S alloy ([S]/[Ti] = 0.02) and the Fe-Ti binary alloy. The degree of plateau of Fe-Ti alloy deteriorates rapidly as [Ti]/[ Fe] exceeds 1.0. However, that of the Fe-Ti-S alloy with [S]/[Ti] = 0.02 stays almost constant up to a [Ti]/[Fe] ratio of 1.05 and does not exhibit a sharp decline. In view of the above results, it can be concluded that the optimum composition for the simultaneous improvement of both the hydriding and the activation characteristics is FeTi,+&&,,. 3.3. ChQr~c~eris~ics of the FeTi~,05So.02 alloy Hysteresis in the P-C-T diagrams in the Fe,.osSo_oz-H system is shown in Fig. 5. The enthalpy AH, of this alloy, which was obtained from the
202
I
L
0
0.01
I
I
002
0.03
I
004
005
085
090
095
S/Ti
too TI/
105
110
115
Fe
Fig. 3. Effect of sulphur on the degree of plateau given by the ratio of the dissociation pressure at [H]/[M J = 0.25 to that at [H]/[M] = 0.5 for FeTil.ssSx at 25 “C. Fig. 4. Effect of sulphur and titanium on the degree of plateau given by the ratio of dissociation pressure at [H]/[M] = 0.25 to that at [H]/[M] = 0.5 at 25 “C.
IO 5 B z f g
2 2oob------s----
I B z
05
a
FeogT~Mn,,
0.02 0.01
0
0
I
02
0.3 04 Atom
C!5 0.6 07 06 Ratio
Fig. 5. Hysteresis Fig. 6. Hydrogen dehydriding cycles.
0.9
IO
of the P-C-T absorption
2000
0
(HIM)
4000
0000
6000
Hydriding-Dehydriding
curve in the FeTiI_asSe.ez-H volume
as
a
function
P-C-T diagrams at 25, 50 and 80 “C, was -5.2
10000
12000
Cycles
system. of
the
number
of
hydriding-
kcal (mol HJ’. The hydrogenation rate was almost the same as that of the FeTi and MnNi5 alloys. The durability of FeTi,.,sS,.,, was compared with that of other Fe-Ti alloys by the repetition of a 20 min cycle of hydriding with fresh hydrogen under 3 MPa of pressure and dehydriding by evacuating the container at room temperature. As shown in Fig. 6, the hydrogen content of the alloy decreased by only 25% after 12 000 cycles. In contrast, the durabilities of other alloys decreased by as much as 70% after 12 000 cycles. The particle
203
size distribution is shown in Fig. 7. The initial particle size of the alloy ranged from 50 to 180 I.trn and its average value was 120 pm. The average particle size of the FeTi,.o&-,.oz alloy decreased to 90 pm after 20 cycles and to 46 pm after 12 000 cycles. In comparison with this alloy, the Fe-Ti alloys were easily pulverized. The average particle size of the FeTi,_Os alloy was 19 pm after 12 000 cycles.
as aft MV 120 FeTllo5 SOOZ 20 cycles
Particle
Fig. 7. Particle cycles.
J I’
I
Size
size distribution
for FeTi,.oSSo,,,
and FeTil.os
after hydriding-dehydriding
3.4. Metallurgical structure of the FeTi,~,,S,~,, alloy The electron probe microanalysis spectra of the FeTil.osS,.,, alloy showed that all the sulphur added reacted with titanium to form a precipitated layer of a Ti-S compound consisting of strips about 5 pm wide separated by spaces of dimensions 10 - 50 pm. The microstructure of this alloy was investigated by analysis using selective potentiostatic etching for electrolytic dissolution [ 91. The sample was electrolytically etched in a nonaqueous solution at a constant potential to dissolve the matrix in order to reveal the precipitate layer and was then observed by scanning electron microscopy. A white precipitate layer is distributed in a network structure as shown in Fig. 8(a). The alloy was analysed using an energy-dispersive X-ray spectrometer. The precipitate layer is confirmed to be T&S and the matrix is an Fe-Ti alloy with [Ti]/[Fe] = 1.0 as shown in Figs. 8(c) and 8(b). After dehydriding in the durability test, samples of this alloy were taken out of the test chamber and were analysed in air using an X-ray diffractometer. As shown in Fig. 9, the FeTi spectrum alone is detected up to 20 cycles, but the pattern of the fi phase hydride of FeTiH, is detected after 12 000 cycles. These spectra remained almost unchanged even when the sample was left in air for 1 week. The fl phase pattern was also observed for the FeTi, .05 and Fe,,sTiMn,,i alloys. The amount of hydrogen remaining was analysed using a high frequency melting gas analyser. The results are given in Table 1. The amount of hydrogen remaining in the alloys increases in the following order: FeTi,.o&-,oz, FeO.,TiMn,.i and FeTi,.O,.
204
(b)
(cl
(a)
Fig. 8. (a) Scanning electron micrograph of the FeTil.osSc.ez alloy after selective potentiostatic etching; (b) energy-dispersive X-ray spectrum of the matrix; (c) energy-dispersive X-ray spectrum of the precipitate.
(a)
(b) 29 (deg
1
Fig. 9. X-ray diffraction patterns (copper target) of FeTil.asSa.az 12 000 hydriding-dehydriding cycles: *, FeTi; 0, FeTiHr.
after (a) 20 and (b)
3.5. Features of the Fe-Ti-S alloy The microanalysis of FeTi I~0sS0~02alloy made it clear that sulphur was fixed in the form of Ti,S and that the [Ti]/[ Fe] ratio in the matrix was 1 .O. The Fe-Ti alloy with this composition provides both the best hydrogen storage capacity and an excellent plateau in the hydriding process; however,
205 TABLE
1
Amount
of remaining
hydrogen
determined
by vacuum
Condition
Alloy
After
20 cycles
FeTil.osSo.o:! FeTil.05 Feo.9TiMno.l
After
12 000
FeTil.osSO.O? FeTil.05 Feo.gTiMno.
cycles
fusion analysis
Remaining
H (at.%)
2.9 3.8 3.5
I
4.8 20.0 14.2
the activation performance of this alloy is very poor. FeTi,.o&oz can be easily activated, probably because Ti2S serves as a catalyst for the dissociation of hydrogen molecules into hydrogen atoms, which facilitates the activation process. To confirm the effect of the matrix composition control, Fe-Ti-S alloys were prepared such that the [Ti] /[ Fe] ratio was greater than unity and various amounts of sulphur were added. The relationship between the amount of excess titanium and the sulphur addition at which the plateau became optimum was obtained. If the alloy composition is signified by FeTi i +XSy, the linear relation y = 0.5~ holds. It is clear that even if the [Ti]/[ Fe] ratio is larger than unity the Fe-Ti-S alloy has excellent hydriding performance because the composition of the matrix is controlled to [ Ti] / [ Fe] = 1 .O by the addition of sulphur. The network structure of Ti2S is assumed to prevent the pulverization of the alloy, which serves as the structure for stress relaxation, and to reduce the amount of remaining hydrogen. The cost of the raw materials for the Fe-Ti-S alloy is low and it is expected to perform satisfactorily in practice.
Acknowledgments The authors wish to thank Mr. R. Matsumoto of New Energy Development Organization and Dr. H. Okada and Dr. Y. Nakamura of the Central Research and Development Bureau, Nippon Steel Corporation, for their helpful suggestions, and Mr. T. Iwata for his support in the laboratory.
References 1 J. J. Reilly
and J. R. Johnson, Proc. 1st World Hydrogen Energy Conf., Miami Beach, 14 - 19, 1976, University of Miami, Coral Gables, FL, 1976, p. 1. 2 G. D. Sandrock, in A. E. Andresen and A. J. Maeland (eds.), Proc. Int. Symp. on Hydrides for Energy Storage, Geilo, August 14 - 19, 1977, Pergamon, Oxford, 1978, p. 353.
FL, August
206 3 4 5 6 7 8
R. H. Wiswall and J. J. Reilly, U.S. Patent 3,315,479, April 25, 1967. J. J. Reilly and R. H. W&wall, U.S. Patent 3,375,676, April 2, 1968. E. L. Huston and G. D. Sandrock, J. Less-Common Met., 74 (1980) 435. Y. Sasaki and K. Nakamura, Bull. Jpn. Inst. Met., 19 (7) (1980) 494. T. Mizuno and T. Morozumi, J. Less-Common Met., 84 (1982) 237. Y. Osumi, H. Suzuki, A. Kato, M. Nakane and Y. Miyake, J. Less-Common Met., (1980) 79. 9 F. Kurosawa, I. Taguchi and R. Matsumoto, J. Jpn. Inst. Met., 44 (1980) 539.
72
of the Less-Common
Metals,
104
(1984)
199
EFFECT OF SULPHUR ADDITION ON THE PROPERTIES ALLOY FOR HYDROGEN STORAGE* RYOICHI
SUZUKI,
JIRO
Research and Development Nippon Steel Corporation, (Received
March
OHNO and HISASHI
199
- 206
OF Fe-Ti
GONDOH
Laboratories-I, Central Research and Development 1618 Ida, Nakahara-ku, Kawasaki 211 (Japan)
Bureau,
26, 1984)
Summary Addition of sulphur to the Fe-Ti alloy was found to produce a marked improvement in the properties of the alloy for hydrogen storage. The most which had a low activation temperaeffective composition was FeTir _OsSo~OZ, ture (80 - 100 “C), a short incubation time (10 - 30 h) and a good equilibrium plateau pressure over a wide concentration range of absorbed hydrogen. The degree of pulverization for this alloy after the absorptiondesorption cycle test was comparatively moderate. The sulphur added to the alloy was converted into TizS which was precipitated in a network structure with strips about 5 pm wide separated by spaces of dimensions about 20 50 pm.
1. Introduction Many types of metal hydride have been developed as interesting new materials for hydrogen storage and energy conversion [ 1, 21. Of the proposed metal hydrides, the FeTi alloy is claimed to be promising for the storage and transportation of hydrogen [3, 41. This alloy, however, has a disadvantage in that it requires activation for an extended period at temperatures above 400 “C under vacuum and at room temperature under pressures of 5 - 6 MPa. Many studies aimed at improving this performance have been conducted [5 - 81. These workers investigated the addition of a minor element to the Fe-Ti alloy to improve the activation performance while retaining its inherent advantages. The addition of sulphur to the alloy resulted in a marked improvement. The results of a study of the activation characteristics, hydriding and dehydriding behaviour, durability and metallurgical structure of the Fe-Ti-S ternary alloy are presented in this paper. *Paper presented at the International Symposium of Metal Hydrides IV, Eilat, Israel, April 9 - 13, 1984. 0022-5088/84/$3.00
0 Elsevier
on the Properties
Sequoia/Printed
and Applications
in The Netherlands
200
2. Experimental procedure Sample alloys were melted in an arc furnace under argon gas and were cast into button-shaped samples of mass 50 - 100 g. The samples were then pulverized in air with a vibrating mill to a size of 80 - 150 mesh. A computercontrolled apparatus was used to activate powder samples of mass 10 g without any heat treatment in a high temperature range by the repetition of a 30 min cycle of hydrogenation at elevated temperature under vacuum and at room temperature under a pressure of 3 MPa. The completion of the activation was determined from the degree of hydrogenation of the sample up to an [H] /[M] atomic ratio of 0.5 which was obtained by measuring the decrease in the hydrogen pressure in the test chamber. The activation performance of the alloy was evaluated from the temperature and the time required for activation. The durability of the sample was measured by allowing the sample first to absorb fresh hydrogen and then to desorb it into the atmosphere with a cycle time of 15 - 20 min.
3. Results and discussion 3.1. ~yd~~di~g characteristics of the Fe-Ti binary alloy The single-phase FeTi hydride exists in the ~Ti]/~Fe] atomic ratio range from 0.99 to 1.04. The hydriding characteristics of this alloy vary in this narrow range. The effects of the composition on the hydriding characteristics were investigated in detail by changing the [Ti] /[Fe] atomic ratio from 0.85 to 1.15. Typical results are shown in Fig. 1. When the [Ti] /[ Fe] atomic ratio is unity, the plateau in the pressurecomposition-temperature (P-C-T) diagram extends over a wide range and the alloy has a maximum relative hydrogen content [H]/[M] of 0.5 which is determined by the amount of dissociated hydrogen under pressure at the end point of the fl phase plateau. However, the activation of the alloy with this composition is difficult at temperatures exceeding 300 “C. When the [Ti]/[ Fe] ratio is below unity, the value of [~]/~M] decreases to 0.3 with decreasing value of [Ti]/fFe] but a large plateau still remains. The activation temperature gradually decreases to 150 - 200 “C with decreasing titanium content. When the [Ti]/[Fe] ratio exceeds 1.0, [HI/EM] decreases to below 0.45 and the plateau deteriorates; however, the activation is improved, with the temperature steeply dropping to 100 “C. 3.2. Effects of the addition of sulphur Fe-Ti-S alloys with various compositions were prepared by changing the [Ti]/[Fe] ratio from 0.85 to 1.15 and adding sulphur at [S]/[Ti] atomic ratios in the range 0.004 - 0.08. The results are summarized in Fig. 1. The maximum relative hydrogen content [H]/[M] of the Fe-Ti-S alloy reaches 0.54 at ~Ti]/~Fe] = 1.05 with [S]/[Ti] = 0.02. The activation temperature drops sharply to 100 “C for [Ti]/[Fe] = 1.05. If [Ti]/[Fe] is 1.05 with
201
J-----S/Ti
Ilium (b)
o. 085
OOli 090
095
100
105
I IO
115
0
0.1
0.2
0.3 Atom
T,/Fe
Fig. 1. Effects of alloy composition [S]/[Ti] = O;@, [S]/[Ti] = 0.02.
on the hydrogen
Fig. 2. Effect of sulphur on the pressure-composition
0.4
0.5
0.6
Ratio
absorption
0.7
0.6
0.9
0
(H/M)
characteristics:
isotherm of FeTi,,$
0,
at 25 “C.
[S]/[Ti] = 0.02, the activation temperature decreases to between 80 and 100 “C and the incubation time is 10 - 30 h. As more sulphur was added, the activation temperature increased to 150 “C and the incubation time gradually increased. The effect of the sulphur content on a P-C-T diagram is shown in Fig. 2 for [Ti]f[Fe] = 1.05. The isothermal plateau improves with increasing sulphur content. The degree of plateau which is defined as degree of plateau
=
dissociation dissociation
pressure of hydrogen pressure of hydrogen
at [H]/[M]
= 0.25
at [H] / [ M] = 0.5
is used to characterize the plateau. Sulphur affects the degree of plateau as indicated in Fig. 3 in which the amount of sulphur is expressed as the [ S]/[ Ti] atomic ratio. As the sulphur content is increased, the degree of plateau improves and reaches nearly 1.0 for [S]/[Ti] = 0.03. The effects of the ~Ti]/~Fe] ratio on the degree of plateau are shown in Fig. 4 for the Fe-Ti-S alloy ([S]/[Ti] = 0.02) and the Fe-Ti binary alloy. The degree of plateau of Fe-Ti alloy deteriorates rapidly as [Ti]/[ Fe] exceeds 1.0. However, that of the Fe-Ti-S alloy with [S]/[Ti] = 0.02 stays almost constant up to a [Ti]/[Fe] ratio of 1.05 and does not exhibit a sharp decline. In view of the above results, it can be concluded that the optimum composition for the simultaneous improvement of both the hydriding and the activation characteristics is FeTi,+&&,,. 3.3. ChQr~c~eris~ics of the FeTi~,05So.02 alloy Hysteresis in the P-C-T diagrams in the Fe,.osSo_oz-H system is shown in Fig. 5. The enthalpy AH, of this alloy, which was obtained from the
202
I
L
0
0.01
I
I
002
0.03
I
004
005
085
090
095
S/Ti
too TI/
105
110
115
Fe
Fig. 3. Effect of sulphur on the degree of plateau given by the ratio of the dissociation pressure at [H]/[M J = 0.25 to that at [H]/[M] = 0.5 for FeTil.ssSx at 25 “C. Fig. 4. Effect of sulphur and titanium on the degree of plateau given by the ratio of dissociation pressure at [H]/[M] = 0.25 to that at [H]/[M] = 0.5 at 25 “C.
IO 5 B z f g
2 2oob------s----
I B z
05
a
FeogT~Mn,,
0.02 0.01
0
0
I
02
0.3 04 Atom
C!5 0.6 07 06 Ratio
Fig. 5. Hysteresis Fig. 6. Hydrogen dehydriding cycles.
0.9
IO
of the P-C-T absorption
2000
0
(HIM)
4000
0000
6000
Hydriding-Dehydriding
curve in the FeTiI_asSe.ez-H volume
as
a
function
P-C-T diagrams at 25, 50 and 80 “C, was -5.2
10000
12000
Cycles
system. of
the
number
of
hydriding-
kcal (mol HJ’. The hydrogenation rate was almost the same as that of the FeTi and MnNi5 alloys. The durability of FeTi,.,sS,.,, was compared with that of other Fe-Ti alloys by the repetition of a 20 min cycle of hydriding with fresh hydrogen under 3 MPa of pressure and dehydriding by evacuating the container at room temperature. As shown in Fig. 6, the hydrogen content of the alloy decreased by only 25% after 12 000 cycles. In contrast, the durabilities of other alloys decreased by as much as 70% after 12 000 cycles. The particle
203
size distribution is shown in Fig. 7. The initial particle size of the alloy ranged from 50 to 180 I.trn and its average value was 120 pm. The average particle size of the FeTi,.o&-,.oz alloy decreased to 90 pm after 20 cycles and to 46 pm after 12 000 cycles. In comparison with this alloy, the Fe-Ti alloys were easily pulverized. The average particle size of the FeTi,_Os alloy was 19 pm after 12 000 cycles.
as aft MV 120 FeTllo5 SOOZ 20 cycles
Particle
Fig. 7. Particle cycles.
J I’
I
Size
size distribution
for FeTi,.oSSo,,,
and FeTil.os
after hydriding-dehydriding
3.4. Metallurgical structure of the FeTi,~,,S,~,, alloy The electron probe microanalysis spectra of the FeTil.osS,.,, alloy showed that all the sulphur added reacted with titanium to form a precipitated layer of a Ti-S compound consisting of strips about 5 pm wide separated by spaces of dimensions 10 - 50 pm. The microstructure of this alloy was investigated by analysis using selective potentiostatic etching for electrolytic dissolution [ 91. The sample was electrolytically etched in a nonaqueous solution at a constant potential to dissolve the matrix in order to reveal the precipitate layer and was then observed by scanning electron microscopy. A white precipitate layer is distributed in a network structure as shown in Fig. 8(a). The alloy was analysed using an energy-dispersive X-ray spectrometer. The precipitate layer is confirmed to be T&S and the matrix is an Fe-Ti alloy with [Ti]/[Fe] = 1.0 as shown in Figs. 8(c) and 8(b). After dehydriding in the durability test, samples of this alloy were taken out of the test chamber and were analysed in air using an X-ray diffractometer. As shown in Fig. 9, the FeTi spectrum alone is detected up to 20 cycles, but the pattern of the fi phase hydride of FeTiH, is detected after 12 000 cycles. These spectra remained almost unchanged even when the sample was left in air for 1 week. The fl phase pattern was also observed for the FeTi, .05 and Fe,,sTiMn,,i alloys. The amount of hydrogen remaining was analysed using a high frequency melting gas analyser. The results are given in Table 1. The amount of hydrogen remaining in the alloys increases in the following order: FeTi,.o&-,oz, FeO.,TiMn,.i and FeTi,.O,.
204
(b)
(cl
(a)
Fig. 8. (a) Scanning electron micrograph of the FeTil.osSc.ez alloy after selective potentiostatic etching; (b) energy-dispersive X-ray spectrum of the matrix; (c) energy-dispersive X-ray spectrum of the precipitate.
(a)
(b) 29 (deg
1
Fig. 9. X-ray diffraction patterns (copper target) of FeTil.asSa.az 12 000 hydriding-dehydriding cycles: *, FeTi; 0, FeTiHr.
after (a) 20 and (b)
3.5. Features of the Fe-Ti-S alloy The microanalysis of FeTi I~0sS0~02alloy made it clear that sulphur was fixed in the form of Ti,S and that the [Ti]/[ Fe] ratio in the matrix was 1 .O. The Fe-Ti alloy with this composition provides both the best hydrogen storage capacity and an excellent plateau in the hydriding process; however,
205 TABLE
1
Amount
of remaining
hydrogen
determined
by vacuum
Condition
Alloy
After
20 cycles
FeTil.osSo.o:! FeTil.05 Feo.9TiMno.l
After
12 000
FeTil.osSO.O? FeTil.05 Feo.gTiMno.
cycles
fusion analysis
Remaining
H (at.%)
2.9 3.8 3.5
I
4.8 20.0 14.2
the activation performance of this alloy is very poor. FeTi,.o&oz can be easily activated, probably because Ti2S serves as a catalyst for the dissociation of hydrogen molecules into hydrogen atoms, which facilitates the activation process. To confirm the effect of the matrix composition control, Fe-Ti-S alloys were prepared such that the [Ti] /[ Fe] ratio was greater than unity and various amounts of sulphur were added. The relationship between the amount of excess titanium and the sulphur addition at which the plateau became optimum was obtained. If the alloy composition is signified by FeTi i +XSy, the linear relation y = 0.5~ holds. It is clear that even if the [Ti]/[ Fe] ratio is larger than unity the Fe-Ti-S alloy has excellent hydriding performance because the composition of the matrix is controlled to [ Ti] / [ Fe] = 1 .O by the addition of sulphur. The network structure of Ti2S is assumed to prevent the pulverization of the alloy, which serves as the structure for stress relaxation, and to reduce the amount of remaining hydrogen. The cost of the raw materials for the Fe-Ti-S alloy is low and it is expected to perform satisfactorily in practice.
Acknowledgments The authors wish to thank Mr. R. Matsumoto of New Energy Development Organization and Dr. H. Okada and Dr. Y. Nakamura of the Central Research and Development Bureau, Nippon Steel Corporation, for their helpful suggestions, and Mr. T. Iwata for his support in the laboratory.
References 1 J. J. Reilly
and J. R. Johnson, Proc. 1st World Hydrogen Energy Conf., Miami Beach, 14 - 19, 1976, University of Miami, Coral Gables, FL, 1976, p. 1. 2 G. D. Sandrock, in A. E. Andresen and A. J. Maeland (eds.), Proc. Int. Symp. on Hydrides for Energy Storage, Geilo, August 14 - 19, 1977, Pergamon, Oxford, 1978, p. 353.
FL, August
206 3 4 5 6 7 8
R. H. Wiswall and J. J. Reilly, U.S. Patent 3,315,479, April 25, 1967. J. J. Reilly and R. H. W&wall, U.S. Patent 3,375,676, April 2, 1968. E. L. Huston and G. D. Sandrock, J. Less-Common Met., 74 (1980) 435. Y. Sasaki and K. Nakamura, Bull. Jpn. Inst. Met., 19 (7) (1980) 494. T. Mizuno and T. Morozumi, J. Less-Common Met., 84 (1982) 237. Y. Osumi, H. Suzuki, A. Kato, M. Nakane and Y. Miyake, J. Less-Common Met., (1980) 79. 9 F. Kurosawa, I. Taguchi and R. Matsumoto, J. Jpn. Inst. Met., 44 (1980) 539.
72