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On the structural characteristics and hydrogenation behaviour of TiMn1.5 hydrogen storage material
International Journal of Hydrogen Energy 26 (2001) 817–821
www.elsevier.com/locate/ijhydene
On the structural characteristics and hydrogenation behaviour of TiMn1:5 hydrogen storage material B.K. Singha;∗ , A.K. Singha , A.M. Imamb , O.N. Srivastavaa a Department
of Physics, Banaras Hindu University, Varanasi-221005, India b Naval Research Lab., Washington, USA
Abstract We report our investigations done on TiMn1:5 intermetallics in relation to hydrogen storage behaviour. TiMn1:5 is known to exhibit the clear plateau in the series TiMn2−x (x = 0–1:25). This communication describes the synthesis of TiMn1:5 by using r.f. induction furnace and characterization by employing, X-ray di7ractometry, transmission electron microscopy and Sievert’s apparatus for hydrogen desorption characteristics. The correlation between the microstructural characteristics and the hydrogenation behaviour for TiMn1:5 is discussed. The clarity of the plateau pressure is supported by the occurrence of ordered √ superstructure in the partially hydrogenated sample. The superstructure has the periodicity amod = (8= 3)aparent . Electron micrographs of the hydrogenated samples at successive stages showed dendritic rupturing of the as-synthesized TiMn1:5: A model has been proposed to explain the stability and hydrogenation behaviour of TiMn1:5 with a modi:ed structure in which Ti atoms substitute for Mn vacancy and at the same time softens the void to accommodate hydrogen. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction In the last decade hydrogen has been proven to be the best alternative for petroleum. Therefore its reversible storage in solids has attracted attention of scientist since it provides a means to carry hydrogen safely and easily. Out of many intermetallics [1–7] used for hydrogen absorption, Ti–Mn system occupies a special place due to its rapid and easy activation [8]. An interesting feature exhibited by the series TiMn (=0:75–2:00) is the fact that homogenized TiMn1:5 exhibits a clear plateau pressure. This in turn makes a material most desirable. The hydrogen storage capacity (wt%) of this composition is reported to be 1.5 –1:8 wt%. The explanation for occurrence of clear plateau corresponding to the speci:c composition TiMn1:5 is, however, not explained in a comprehensive way. The purpose of the present investigation is to explain the above mentioned clear plateau on the basis of experimental observations, namely hydrogen ∗ Corresponding author. Tel.: +91-542-368-468; +91-542-368-468. E-mail address: [email protected] (B.K. Singh).
fax:
absorption and desorption in the material and corresponding microstructural characteristics. A model has been proposed to explain the stability and hydrogenation behaviour of TiMn1:5 with a modi:ed structure in which Ti atoms substitute for Mn vacancy and at the same time softens the void to accommodate hydrogen.
2. Experimental details The synthesis of the material was done by melting a pellet of stoichiometric mixture of TiMn ( = 1:3–1:65) using r.f. induction furnace in Ar atmosphere.The ingot so obtained was further annealed in the resistance heated furnace for 20 ◦ h at 1000 C. All the samples of TiMn1:5 investigated corresponded to annealed version of the as-synthesized samples. The :nally obtained material was then characterized by X-ray di7ractometry employing Philips X-ray di7ractometer with PW 1710 controller. Fig. 1 shows X-ray di7raction pattern of TiMn1:5 , depicting C14 hexagonal phase. The hydrogen absorption and desorption behaviours were investigated using a high
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 1 2 - X
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B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
Fig. 1. X-ray di7raction patterns of (a) as-synthesized and (b) annealed samples of TiMn1:5 .
Fig. 2. Pressure-composition desorption isotherm of TiMn1:5 alloy.
pressure volumetric system developed in our laboratory [9]. A known quantity of this alloy powder was placed in a reactor which was then evacuated up to 10−5 Torr. During evacuation the reactor was continuously heated to a ◦ temperature 400 ± 10 C. After evacuation, the sample was exposed to hydrogen at pressure of about 50 kg=cm2 . The stored hydrogen was desorbed at room temperature after the equilibrium was established. This process was repeated 15 times. After that the pressure composition isotherms were plotted. Such a representative plot is shown in Fig. 2. As depicted by the P–C isotherm, the maximum hydrogen storage capacity corresponded to 1:5 wt% with a lower plateau region as shown in Fig. 2. 3. Results and discussions With the aim to explain the clear plateau in the composition TiMn1:5 , the present study is focussed on the detailed
microstructural (TEM) investigations and their correlation with hydrogenation behaviour of TiMn1:5 . Such studies do not seem to have been carried out as yet. The occurrence of modulated phase and dendrite like structure, which turns porous on further hydrogenation are the curious features of the present study. The plateau pressure of fully hydrogenated sample corresponds to 4 – 6 atmosphere (Fig. 2) and the samples were taken out from the reactor at atmospheric pressure. The sample will, therefore, naturally get dehydrogenated. However, the residual hydrogen atom will still be left in the partially hydrogenated TiMn1:5 sample at one atmosphere pressure (ambient) and presumably also in the samples subjected to TEM explorations. Therefore, the samples on which the microstructural investigation have been carried out correspond to the samples with some amount of residual hydrogen. The amount of residual hydrogen is dif:cult to evaluate. However, in the discussion of partially hydrogenated sample, we will imply the material still containing certain left over hydrogen concentration. The microstructural evaluations of the as-synthesized, annealed and dehydrogenated versions of material were done by transmission electron microscopic technique employing a Philips EM-CM 12 version in imaging and di7raction modes. Fig. 3 shows a representative di7raction pattern taken from partially hydrogenated sample exhibiting periodicity (for example along XY ) which is not directly explicable in terms of the parent phase corresponding to C14 lattice structure. The long periodicity corresponding to sharp and a closely spaced di7raction spots along PQ in Fig. 4, cannot get explained based on parent C14 phase. The di7raction pattern is suggestive of the creation of modulated version of the parent phase. Careful analysis of the di7raction patterns revealed that closely spaced subsidiary di7raction spots in the patterns shown in Figs. 3 and 4 represented the periL The bright spots arranged on hexagonal odicity of 13:44 A. L The pararray in Fig. 3 corresponds to a periodicity 3:36 A. ent and the modulated phase are related to each other and periodicity corresponding to subsidiary spots (along XY in Fig. 3) is exactly four times that of bright spots arranged on hexagonal array. Based on the interrelationship between
B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
819
Fig. 3. Electron di7raction pattern of partially hydrogenated the occurrence of modulated phase with TiMn1:5 , showing √ amod = (8= 3)aparent .
Fig. 5. Transmission electron micrographs of partially hydrogenated version of TiMn1:5 in (a) bright :eld and (b) dark :eld imaging modes. Dendrite like features are discernible from these micrographs. Fig. 4. Yet another electron di7raction pattern taken from the fully hydrogenated version of TiMn1:5 . Analysis of this pattern revealed the same d-spacing of :ne spots along row PQ as in Fig. 3.
above described spots, di7raction pattern√is found to have (d1 0 0 )mod = 8(d1 1 0 )parent or amod = (8= 3)aparent , for the modulated phase. In fact, this was a typical structural feature invariably observed in the dehydrogenated version of the TiMn1:5 phase. Even though detailed lattice structure of the modulated phase representing the dehydrogenated version could not be worked out, it is clear that hydrogenation instead of a disordered solid solution bearing phase, leads to an ordered hydride phase. Presumably this phase arises due to ordering of H atoms. This H ordered phase is a modulated variant (d1 0 0 )mod = 8(d1 1 0 )parent of the parent C14 phase. Such modulations were not observed in other compositions e.g. TiMn1:3 and TiMn1:6 etc., where the occurrence of clear plateau region is also missing. We, therefore,
suggest that the above mentioned ordered hydride phase may be responsible for the clear plateau region. The TEM micrographs in Fig. 5(a) and (b) exhibit the microstructures in bright and dark :eld imaging modes of the samples subjected to few hydrogenation cycles. Fig. 6(a) and (b) similarly correspond to multicycled samples (10 runs). An interesting feature brought out by these micrographs relates to the creation of dendrite like features on hydrogenation. Observations spread on several samples revealed that these dendrite like features are invariably present in the samples subjected to two or three hydrogenation cycles. As regards the creation of these dendrite like features, it can be said that these are most likely formed due to rapid hydrogenation along directions emanating from grain boundaries. It is expected that the absorbed hydrogen concentrations will :rst be accommodated within the grain boundaries. After these get saturated, hydrogen atom will proceed along preferred direction having high concentration
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B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
Fig. 6. Transmission electron micrographs of fully hydrogenated version of TiMn1:5 in (a) bright :eld and (b) dark :eld imaging modes. The porous dendritic growth can be seen in these micrographs.
of interstitial. Hydrogen atoms, through rapid hydrogenation along [1 1 0] direction which contains highest density of interstitials. As has been well established new hydrogenation at interstitial sites leads to pulverization. Hydrogen when absorbed can increase the volume at the site, which can be high. As for example in the case of Ni for H=Ni ≈ 1, the L 3 [10]. This di7erential increase volume expansion is ≈2 A in the volume causes pulverization. The pulverization reduces the particle size and thus provides larger surface area and at the same time shorter distances for hydrogen to di7use into the absorber. These aspects taken together improves the kinetics as reported in several cases [11,12]. The hydrogen saturated regions undergo pulverization leading to fragmentation into smaller regions=particles. On the other hand such regions which will not get saturated through hydrogen will be left behind as remaining portion of the grains. The saturation hydrogenation will remove portions
of grains through pulverization along high interstitial density direction [1 1 0]. The remaining portion of the grain which are only partially hydrided will posses multibranched segments resembling dendrite con:gurations. The portion of samples which would not have broken into the particles, would correspond to the specimens which are picked up for TEM explorations. On further cycling when saturation hydrogenation and consequent pulverization will take place, the sample will assume porous structure. Finally, the dendrite like portions of the samples will turn into :ne powder. The observed characteristics tally with the above envisaged features. In the few cycle hydrogenated samples, the large particles which are left behind after pulverization, exhibit dendrite like features as shown in Fig. 5(a) and (b). These dendrites give way to become porous (Fig. 6(a) and (b)) and :nally break into small particles and the dendrites disappear. The whole sample then consists of small particles L of sizes of 100 –1000 A. Thus, as described above we found that TiMn1:5 exhibits clear plateau in conformity with others [8]. In the following we proceed to describe a model to explain these observations: Fruchart et al. [13] have suggested that TiMn1:5 []0:5 is too improbable structure. Therefore, if we take the composition TiMn1:5 , some of the Ti will substitute for Mn. We therefore, envisage TiMn1:5 , as Ti0:833 (Mn1:5 Ti0:166 ) which correspond to composition TiMn2 , of course with Mn substituted by Ti, to the extent of ∼10%. Thus we propose that Mn atoms are substituted by Ti. Fig. 7a depicts the parent TiMn2 with voids and Fig. 7b depicts the distorted voids when Mn is substituted by Ti. This will naturally distort the void because the Ti is bigger than Mn. Softening of the void so caused can enhance the hydrogen absorption. Also an ordered substitution of Mn by Ti can lead to a clear plateau and superstructures. Indeed superstructures have been actually observed.
4. Conclusion In summary it can be said that in the present work Ti bearing AB2 -type hydrogen storage alloy typi:ed by TiMn1:5 has been synthesized and detailed investigations have been carried out to explore the correlation between microstructural features and hydrogenation characteristics particularly to the occurrence of clear plateau speci:cally in TiMn1:5 . It has been found that in the partially hydrogenated samples √ modulated structures with amod = (8= 3)aparent get formed. The modulated phases are thought to result due to hydrogen ordering. Also the partially hydrogenated samples exhibit multibranched microstructure resembling dendrite. This is thought to result due to preferred saturation hydrogenation and pulverization along [1 1 0] direction. We have proposed a structural model in which Ti atoms substitute Mn vacancies in the unit cell, and softens the void which makes the void more favourable sites for hydrogen storage.
B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
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Acknowledgements The authors are grateful to Prof. A.R. Verma, Prof. B.B. Rath (ONR, U.S.A) and to Prof. A. Ramachandran and Prof. M.V.C. Sastri for encouragement and discussions. Financial support from ONR: Washington, DC (U.S.A) under the INDO-US collaboration programme (No. N00014-96-1-1233) and also from MNES (New Delhi) are gratefully acknowledged. References
Fig. 7. (a) parent TiMn2 structure, voids can be seen (b) substitution of Mn vacancy by Ti atoms distorts voids as shown .
•
[1] Wallace WE, Craig RS, Rao VUS. Adv Chem Ser 1980;186:207. [2] Van Vucht JHN, Kuijpers FA, Burning HCA. Philips Res Rep 1970;25:133. [3] Reilly JJ, Wiswall Jr. RH. Inorg Chem 1967;6:2220. [4] Singh BK, Singh AK, Srivastava ON. Int J Hydrogen Energy 1996;21:111–7. [5] Singh BK, Singh AK, Pandey CS, Srivastava ON. Int J Hydrogen Energy 1990;24:1077–82. [6] Reilly JJ, Wishwall RH. J Inorg Chem 1968;7:2254. [7] Ivey DG, Northwood DO. Z Phys Chem NF 1986;147:191. [8] Gamo T, Moriwaki Y, Yanagihara N, Yamashita T, Iwaki T. Int J Hydrogen Energy 1995;10(1):39–47. [9] Ramakrishana K, Singh SK, Singh AK, Srivastava ON, Dahiya RP, editors. Progress in hydrogen energy, 1987;7:81–110. [10] Fukai YZ. Phys Chem NF 1989;163:165. [11] Libowitz GG, Hayes HF, Gibb Jr. TRP. J Phys Chem 1958;62:76. [12] Simonovic DR, Mentus S, Dimitrijevic R, Susic M. Int J Hydrogen Energy 1999;24:449. [13] Fruchart D, Soubeyroux JL, Hemopelmann RJ. Less Common Metals 1984;99:307–19.
www.elsevier.com/locate/ijhydene
On the structural characteristics and hydrogenation behaviour of TiMn1:5 hydrogen storage material B.K. Singha;∗ , A.K. Singha , A.M. Imamb , O.N. Srivastavaa a Department
of Physics, Banaras Hindu University, Varanasi-221005, India b Naval Research Lab., Washington, USA
Abstract We report our investigations done on TiMn1:5 intermetallics in relation to hydrogen storage behaviour. TiMn1:5 is known to exhibit the clear plateau in the series TiMn2−x (x = 0–1:25). This communication describes the synthesis of TiMn1:5 by using r.f. induction furnace and characterization by employing, X-ray di7ractometry, transmission electron microscopy and Sievert’s apparatus for hydrogen desorption characteristics. The correlation between the microstructural characteristics and the hydrogenation behaviour for TiMn1:5 is discussed. The clarity of the plateau pressure is supported by the occurrence of ordered √ superstructure in the partially hydrogenated sample. The superstructure has the periodicity amod = (8= 3)aparent . Electron micrographs of the hydrogenated samples at successive stages showed dendritic rupturing of the as-synthesized TiMn1:5: A model has been proposed to explain the stability and hydrogenation behaviour of TiMn1:5 with a modi:ed structure in which Ti atoms substitute for Mn vacancy and at the same time softens the void to accommodate hydrogen. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction In the last decade hydrogen has been proven to be the best alternative for petroleum. Therefore its reversible storage in solids has attracted attention of scientist since it provides a means to carry hydrogen safely and easily. Out of many intermetallics [1–7] used for hydrogen absorption, Ti–Mn system occupies a special place due to its rapid and easy activation [8]. An interesting feature exhibited by the series TiMn (=0:75–2:00) is the fact that homogenized TiMn1:5 exhibits a clear plateau pressure. This in turn makes a material most desirable. The hydrogen storage capacity (wt%) of this composition is reported to be 1.5 –1:8 wt%. The explanation for occurrence of clear plateau corresponding to the speci:c composition TiMn1:5 is, however, not explained in a comprehensive way. The purpose of the present investigation is to explain the above mentioned clear plateau on the basis of experimental observations, namely hydrogen ∗ Corresponding author. Tel.: +91-542-368-468; +91-542-368-468. E-mail address: [email protected] (B.K. Singh).
fax:
absorption and desorption in the material and corresponding microstructural characteristics. A model has been proposed to explain the stability and hydrogenation behaviour of TiMn1:5 with a modi:ed structure in which Ti atoms substitute for Mn vacancy and at the same time softens the void to accommodate hydrogen.
2. Experimental details The synthesis of the material was done by melting a pellet of stoichiometric mixture of TiMn ( = 1:3–1:65) using r.f. induction furnace in Ar atmosphere.The ingot so obtained was further annealed in the resistance heated furnace for 20 ◦ h at 1000 C. All the samples of TiMn1:5 investigated corresponded to annealed version of the as-synthesized samples. The :nally obtained material was then characterized by X-ray di7ractometry employing Philips X-ray di7ractometer with PW 1710 controller. Fig. 1 shows X-ray di7raction pattern of TiMn1:5 , depicting C14 hexagonal phase. The hydrogen absorption and desorption behaviours were investigated using a high
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 1 2 - X
818
B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
Fig. 1. X-ray di7raction patterns of (a) as-synthesized and (b) annealed samples of TiMn1:5 .
Fig. 2. Pressure-composition desorption isotherm of TiMn1:5 alloy.
pressure volumetric system developed in our laboratory [9]. A known quantity of this alloy powder was placed in a reactor which was then evacuated up to 10−5 Torr. During evacuation the reactor was continuously heated to a ◦ temperature 400 ± 10 C. After evacuation, the sample was exposed to hydrogen at pressure of about 50 kg=cm2 . The stored hydrogen was desorbed at room temperature after the equilibrium was established. This process was repeated 15 times. After that the pressure composition isotherms were plotted. Such a representative plot is shown in Fig. 2. As depicted by the P–C isotherm, the maximum hydrogen storage capacity corresponded to 1:5 wt% with a lower plateau region as shown in Fig. 2. 3. Results and discussions With the aim to explain the clear plateau in the composition TiMn1:5 , the present study is focussed on the detailed
microstructural (TEM) investigations and their correlation with hydrogenation behaviour of TiMn1:5 . Such studies do not seem to have been carried out as yet. The occurrence of modulated phase and dendrite like structure, which turns porous on further hydrogenation are the curious features of the present study. The plateau pressure of fully hydrogenated sample corresponds to 4 – 6 atmosphere (Fig. 2) and the samples were taken out from the reactor at atmospheric pressure. The sample will, therefore, naturally get dehydrogenated. However, the residual hydrogen atom will still be left in the partially hydrogenated TiMn1:5 sample at one atmosphere pressure (ambient) and presumably also in the samples subjected to TEM explorations. Therefore, the samples on which the microstructural investigation have been carried out correspond to the samples with some amount of residual hydrogen. The amount of residual hydrogen is dif:cult to evaluate. However, in the discussion of partially hydrogenated sample, we will imply the material still containing certain left over hydrogen concentration. The microstructural evaluations of the as-synthesized, annealed and dehydrogenated versions of material were done by transmission electron microscopic technique employing a Philips EM-CM 12 version in imaging and di7raction modes. Fig. 3 shows a representative di7raction pattern taken from partially hydrogenated sample exhibiting periodicity (for example along XY ) which is not directly explicable in terms of the parent phase corresponding to C14 lattice structure. The long periodicity corresponding to sharp and a closely spaced di7raction spots along PQ in Fig. 4, cannot get explained based on parent C14 phase. The di7raction pattern is suggestive of the creation of modulated version of the parent phase. Careful analysis of the di7raction patterns revealed that closely spaced subsidiary di7raction spots in the patterns shown in Figs. 3 and 4 represented the periL The bright spots arranged on hexagonal odicity of 13:44 A. L The pararray in Fig. 3 corresponds to a periodicity 3:36 A. ent and the modulated phase are related to each other and periodicity corresponding to subsidiary spots (along XY in Fig. 3) is exactly four times that of bright spots arranged on hexagonal array. Based on the interrelationship between
B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
819
Fig. 3. Electron di7raction pattern of partially hydrogenated the occurrence of modulated phase with TiMn1:5 , showing √ amod = (8= 3)aparent .
Fig. 5. Transmission electron micrographs of partially hydrogenated version of TiMn1:5 in (a) bright :eld and (b) dark :eld imaging modes. Dendrite like features are discernible from these micrographs. Fig. 4. Yet another electron di7raction pattern taken from the fully hydrogenated version of TiMn1:5 . Analysis of this pattern revealed the same d-spacing of :ne spots along row PQ as in Fig. 3.
above described spots, di7raction pattern√is found to have (d1 0 0 )mod = 8(d1 1 0 )parent or amod = (8= 3)aparent , for the modulated phase. In fact, this was a typical structural feature invariably observed in the dehydrogenated version of the TiMn1:5 phase. Even though detailed lattice structure of the modulated phase representing the dehydrogenated version could not be worked out, it is clear that hydrogenation instead of a disordered solid solution bearing phase, leads to an ordered hydride phase. Presumably this phase arises due to ordering of H atoms. This H ordered phase is a modulated variant (d1 0 0 )mod = 8(d1 1 0 )parent of the parent C14 phase. Such modulations were not observed in other compositions e.g. TiMn1:3 and TiMn1:6 etc., where the occurrence of clear plateau region is also missing. We, therefore,
suggest that the above mentioned ordered hydride phase may be responsible for the clear plateau region. The TEM micrographs in Fig. 5(a) and (b) exhibit the microstructures in bright and dark :eld imaging modes of the samples subjected to few hydrogenation cycles. Fig. 6(a) and (b) similarly correspond to multicycled samples (10 runs). An interesting feature brought out by these micrographs relates to the creation of dendrite like features on hydrogenation. Observations spread on several samples revealed that these dendrite like features are invariably present in the samples subjected to two or three hydrogenation cycles. As regards the creation of these dendrite like features, it can be said that these are most likely formed due to rapid hydrogenation along directions emanating from grain boundaries. It is expected that the absorbed hydrogen concentrations will :rst be accommodated within the grain boundaries. After these get saturated, hydrogen atom will proceed along preferred direction having high concentration
820
B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
Fig. 6. Transmission electron micrographs of fully hydrogenated version of TiMn1:5 in (a) bright :eld and (b) dark :eld imaging modes. The porous dendritic growth can be seen in these micrographs.
of interstitial. Hydrogen atoms, through rapid hydrogenation along [1 1 0] direction which contains highest density of interstitials. As has been well established new hydrogenation at interstitial sites leads to pulverization. Hydrogen when absorbed can increase the volume at the site, which can be high. As for example in the case of Ni for H=Ni ≈ 1, the L 3 [10]. This di7erential increase volume expansion is ≈2 A in the volume causes pulverization. The pulverization reduces the particle size and thus provides larger surface area and at the same time shorter distances for hydrogen to di7use into the absorber. These aspects taken together improves the kinetics as reported in several cases [11,12]. The hydrogen saturated regions undergo pulverization leading to fragmentation into smaller regions=particles. On the other hand such regions which will not get saturated through hydrogen will be left behind as remaining portion of the grains. The saturation hydrogenation will remove portions
of grains through pulverization along high interstitial density direction [1 1 0]. The remaining portion of the grain which are only partially hydrided will posses multibranched segments resembling dendrite con:gurations. The portion of samples which would not have broken into the particles, would correspond to the specimens which are picked up for TEM explorations. On further cycling when saturation hydrogenation and consequent pulverization will take place, the sample will assume porous structure. Finally, the dendrite like portions of the samples will turn into :ne powder. The observed characteristics tally with the above envisaged features. In the few cycle hydrogenated samples, the large particles which are left behind after pulverization, exhibit dendrite like features as shown in Fig. 5(a) and (b). These dendrites give way to become porous (Fig. 6(a) and (b)) and :nally break into small particles and the dendrites disappear. The whole sample then consists of small particles L of sizes of 100 –1000 A. Thus, as described above we found that TiMn1:5 exhibits clear plateau in conformity with others [8]. In the following we proceed to describe a model to explain these observations: Fruchart et al. [13] have suggested that TiMn1:5 []0:5 is too improbable structure. Therefore, if we take the composition TiMn1:5 , some of the Ti will substitute for Mn. We therefore, envisage TiMn1:5 , as Ti0:833 (Mn1:5 Ti0:166 ) which correspond to composition TiMn2 , of course with Mn substituted by Ti, to the extent of ∼10%. Thus we propose that Mn atoms are substituted by Ti. Fig. 7a depicts the parent TiMn2 with voids and Fig. 7b depicts the distorted voids when Mn is substituted by Ti. This will naturally distort the void because the Ti is bigger than Mn. Softening of the void so caused can enhance the hydrogen absorption. Also an ordered substitution of Mn by Ti can lead to a clear plateau and superstructures. Indeed superstructures have been actually observed.
4. Conclusion In summary it can be said that in the present work Ti bearing AB2 -type hydrogen storage alloy typi:ed by TiMn1:5 has been synthesized and detailed investigations have been carried out to explore the correlation between microstructural features and hydrogenation characteristics particularly to the occurrence of clear plateau speci:cally in TiMn1:5 . It has been found that in the partially hydrogenated samples √ modulated structures with amod = (8= 3)aparent get formed. The modulated phases are thought to result due to hydrogen ordering. Also the partially hydrogenated samples exhibit multibranched microstructure resembling dendrite. This is thought to result due to preferred saturation hydrogenation and pulverization along [1 1 0] direction. We have proposed a structural model in which Ti atoms substitute Mn vacancies in the unit cell, and softens the void which makes the void more favourable sites for hydrogen storage.
B.K. Singh et al. / International Journal of Hydrogen Energy 26 (2001) 817–821
821
Acknowledgements The authors are grateful to Prof. A.R. Verma, Prof. B.B. Rath (ONR, U.S.A) and to Prof. A. Ramachandran and Prof. M.V.C. Sastri for encouragement and discussions. Financial support from ONR: Washington, DC (U.S.A) under the INDO-US collaboration programme (No. N00014-96-1-1233) and also from MNES (New Delhi) are gratefully acknowledged. References
Fig. 7. (a) parent TiMn2 structure, voids can be seen (b) substitution of Mn vacancy by Ti atoms distorts voids as shown .
•
[1] Wallace WE, Craig RS, Rao VUS. Adv Chem Ser 1980;186:207. [2] Van Vucht JHN, Kuijpers FA, Burning HCA. Philips Res Rep 1970;25:133. [3] Reilly JJ, Wiswall Jr. RH. Inorg Chem 1967;6:2220. [4] Singh BK, Singh AK, Srivastava ON. Int J Hydrogen Energy 1996;21:111–7. [5] Singh BK, Singh AK, Pandey CS, Srivastava ON. Int J Hydrogen Energy 1990;24:1077–82. [6] Reilly JJ, Wishwall RH. J Inorg Chem 1968;7:2254. [7] Ivey DG, Northwood DO. Z Phys Chem NF 1986;147:191. [8] Gamo T, Moriwaki Y, Yanagihara N, Yamashita T, Iwaki T. Int J Hydrogen Energy 1995;10(1):39–47. [9] Ramakrishana K, Singh SK, Singh AK, Srivastava ON, Dahiya RP, editors. Progress in hydrogen energy, 1987;7:81–110. [10] Fukai YZ. Phys Chem NF 1989;163:165. [11] Libowitz GG, Hayes HF, Gibb Jr. TRP. J Phys Chem 1958;62:76. [12] Simonovic DR, Mentus S, Dimitrijevic R, Susic M. Int J Hydrogen Energy 1999;24:449. [13] Fruchart D, Soubeyroux JL, Hemopelmann RJ. Less Common Metals 1984;99:307–19.