Phase transformation in hydrogenation and dehydrogenation of LaCo5-xAlx-H2 (x = 0, 0.25) systems

Kohta Asano, Takahiro Nomiyama, Yumiko Nakamura, Etsuo Akiba, Yoshiaki Iijima

Research output: Contribution to journalArticle

Abstract

The dehydrogenation behavior of the LaCo5-H2 system was studied by in situ X-ray diffraction (XRD) under a hydrogen pressure of 5 MPa in the temperature range of 433-505 K. A new hydride phase (LaCo5H2.2) which is formed between the α phase (solid solution) and the β phase (LaCo5H3.4 hydride) has a hexagonal structure and the crystal lattice expands mainly along the a-axis during the transformation from the α phase to the LaCo5H2.2 phase. To elucidate the effect of the substitution of aluminum for a part of cobalt in LaCo5 on the phase transformation behavior in the LaCo5-H2 system, the hydrogenation and dehydrogenation behavior of the LaCo4.75Al0.25-H2 system was studied by the pressure differential scanning calorimetry (PDSC) under a hydrogen pressure of 0.1-5 MPa in the temperature range of 323-593 K. In the heating and cooling runs of the LaCo4.75Al0.25-H2 system under a hydrogen pressure of 0.1 MPa, one endothermic and one exothermic peaks were observed, respectively. These PDSC peaks were due to the transformation between the α phase and the β phase. Under a hydrogen pressure of 0.2 MPa both peaks divided into two peaks. Combining the PDSC with the P-C isotherms, the division of the PDSC peaks is attributed to the formation of the LaCo4.75Al0.25H2.7 phase between the α phase and the β phase above 383 K in the LaCo4.75Al0.25-H2 system, whereas the LaCo5H2.2 phase is formed above 465 K in the LaCo5-H2 system. By the substitution of aluminum for a part of cobalt in LaCo5, the formation temperature of a new hydride phase lowers. The substitution of aluminum makes the β phase stable and the γ phase (LaCo5H4.5 hydride) unstable.

Original languageEnglish
Pages (from-to)424-428
Number of pages5
JournalJournal of Alloys and Compounds
Volume425
Issue number1-2
DOIs
Publication statusPublished - Nov 30 2006

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Dehydrogenation
Hydrogenation
Phase transitions
Hydrides
Hydrogen
Differential scanning calorimetry
Aluminum
Substitution reactions
Cobalt
Crystal lattices
Temperature
Isotherms
Solid solutions
Cooling
Heating
X ray diffraction

All Science Journal Classification (ASJC) codes

  • Mechanics of Materials
  • Mechanical Engineering
  • Metals and Alloys
  • Materials Chemistry

Cite this

Phase transformation in hydrogenation and dehydrogenation of LaCo5-xAlx-H2 (x = 0, 0.25) systems. / Asano, Kohta; Nomiyama, Takahiro; Nakamura, Yumiko; Akiba, Etsuo; Iijima, Yoshiaki.

In: Journal of Alloys and Compounds, Vol. 425, No. 1-2, 30.11.2006, p. 424-428.

Research output: Contribution to journalArticle

Asano, Kohta ; Nomiyama, Takahiro ; Nakamura, Yumiko ; Akiba, Etsuo ; Iijima, Yoshiaki. / Phase transformation in hydrogenation and dehydrogenation of LaCo5-xAlx-H2 (x = 0, 0.25) systems. In: Journal of Alloys and Compounds. 2006 ; Vol. 425, No. 1-2. pp. 424-428.
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T1 - Phase transformation in hydrogenation and dehydrogenation of LaCo5-xAlx-H2 (x = 0, 0.25) systems

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AB - The dehydrogenation behavior of the LaCo5-H2 system was studied by in situ X-ray diffraction (XRD) under a hydrogen pressure of 5 MPa in the temperature range of 433-505 K. A new hydride phase (LaCo5H2.2) which is formed between the α phase (solid solution) and the β phase (LaCo5H3.4 hydride) has a hexagonal structure and the crystal lattice expands mainly along the a-axis during the transformation from the α phase to the LaCo5H2.2 phase. To elucidate the effect of the substitution of aluminum for a part of cobalt in LaCo5 on the phase transformation behavior in the LaCo5-H2 system, the hydrogenation and dehydrogenation behavior of the LaCo4.75Al0.25-H2 system was studied by the pressure differential scanning calorimetry (PDSC) under a hydrogen pressure of 0.1-5 MPa in the temperature range of 323-593 K. In the heating and cooling runs of the LaCo4.75Al0.25-H2 system under a hydrogen pressure of 0.1 MPa, one endothermic and one exothermic peaks were observed, respectively. These PDSC peaks were due to the transformation between the α phase and the β phase. Under a hydrogen pressure of 0.2 MPa both peaks divided into two peaks. Combining the PDSC with the P-C isotherms, the division of the PDSC peaks is attributed to the formation of the LaCo4.75Al0.25H2.7 phase between the α phase and the β phase above 383 K in the LaCo4.75Al0.25-H2 system, whereas the LaCo5H2.2 phase is formed above 465 K in the LaCo5-H2 system. By the substitution of aluminum for a part of cobalt in LaCo5, the formation temperature of a new hydride phase lowers. The substitution of aluminum makes the β phase stable and the γ phase (LaCo5H4.5 hydride) unstable.

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