The effect of the hydrogenation process on the production of lattice defects in Pd

doi:10.1016/j.jallcom.2005.04.214

Journal of Alloys and Compounds

Volume 414, Issues 1–2, 13 April 2006, Pages 204-206

https://doi.org/10.1016/j.jallcom.2005.04.214 Get rights and content

Abstract

The effect of the hydrogen absorption and desorption process on the production of lattice defects in pure Pd was studied by means of positron lifetime spectroscopy. Pd hydrogenated and dehydrogenated at 623 K contains some dislocations but few metal vacancies. In contrast, a remarkable vacancy production was observed in Pd hydrogenated at 623 K and subsequently electrochemically dehydrogenated at ambient temperature. This result shows that vacancies are formed during the hydrogen desorption process at RT, and suggests that the vacancy formation is caused by the effect of stresses accompanied by the phase separation.

Introduction

It has been found that large numbers of metal vacancies are produced when hydrogen storage alloys such as LaNi₅, NdNi₅, ZrMn₂ and pure Pd absorb hydrogen at ambient temperature [1], [2], [3]. This vacancy production is difficult to explain by the currently accepted vacancy production mechanisms, namely, quenching, radiation and plastic deformation.

On the other hand, Shirai and co-workers have found [4], [5] that excess vacancies are produced by some phase transformations and they proposed a stress-induced vacancy formation mechanism. Excess vacancies are generated as a result of the nucleation and growth of a new phase having different lattice parameters and/or volume of the matrix.

In order to clarify the defect formation mechanism in more details, the effect of hydrogen absorption and desorption process on the production of lattice defects in Pd has been studied by positron lifetime spectroscopy. Above a critical temperature, Pd can absorb and desorb hydrogen without passing through the (α + β) two-phase region. The α phase is the hydrogen solution phase and the β phase is the hydride phase. Defect formation during dehydrogenation was also investigated.

Section snippets

Experimental procedure

Samples of pure Pd (99.99%) have a square surface of 10 mm × 10 mm and a thickness of 0.3 mm, and were fully annealed at 1173 K for over 4 h in a pure Ar atmosphere. Two sets of pure Pd samples were hydrogenated at 623 K without passing through the two-phase (α + β) region. For hydrogenation, hydrogen gas pressure was gradually increased up to 7 MPa, that was maintained for 2 h. One set of the Pd was dehydrogenated at 623 K by evacuation. The other set was cooled down to 296 K in the first place under

Results and discussion

Before the hydrogenation treatment, all samples showed a positron lifetime value of 106 ps, which is the same value as that calculated for Pd without defects [3]. This clearly shows that there were no detectable lattice defects in the starting samples.

Conclusion

The effect of the hydrogenation and the dehydrogenation process on the production of lattice defects was investigated by using positron lifetime spectroscopy. The following conclusions were drawn:

(1)
Dislocations were formed even in the hydrogen solution region above the miscibility gap.
(2)
During the β → α a phase transformation at 296 K, excess vacancies are formed.
(3)
The mechanism of vacancy formation at ambient temperature may be due to stress generated as the result of the phase transformation.

Acknowledgement

The authors wish to thank Dr. Miyamura and Dr. Senoh for kind help concerning the electrochemical desorption method. This work was partly carried out at “Handai Frontier Research Center” and partly supported by a 21st Century COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Sports, Culture, Science and Technology of Japan.

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