Phase analysis of multiple absorbed and desorbed Zr-Fe-V by hydrogen

P. Roupcová, O. Schneeweiss,

¹Institute of Physics of Material, Czech Academy of Science, v.v.i., Brno, Czech Republic

roupcova@ipm.cz

The commercial non-evaporable getter SAES St 707 with chemical composition (70 % Zr, 24.6 % V, and 5.4 % Fe) is using for protection vacuum systems sensitive to presence of hydrogen. We have investigated its phase stability during recharging by hydrogen with emphasis on the influence of impurity formed by residual gases in atmosphere (O₂, CO₂, H₂O).

The surface composition of the as-received getter exposed by surrounding atmosphere determined by XPS reported in [1-2] consists of the respective oxides of the getter compounds, i.e., ZrO₂, VO₂, and Fe₂O₃. The getter activated at 500 °C in vacuum contained of metallic Zr and V with the small amount of oxygen and carbon bound at Zr and V surfaces. Subsequently, the getter was exposed to the D₂O vapour at different temperature and caused decomposition of water and its absorption. Iron had not an important role on this process. At high temperatures, diffusion of oxygen from the surface into the bulk occured [2].

We have investigated structure and phase composition of the getter using X-ray powder diffraction (XRD) and Mössbauer spectroscopy (MS). XRD was performed using CoKα radiation with qualitative analysis carried out by HighScore software and the JCPDS PDF-2 database. For a quantitative analysis of the XRD patterns we took HighScore plus with Rietweld structural models based on the ICSD database. ⁵⁷Fe Mössbauer spectra were measured in a standard transmission geometry using ⁵⁷Co/Rh source. Isomer shifts δ were refereed relative to α-Fe at room temperature. The computer processing of the spectra done by CONFIT package [3] yielded intensities (atomic fraction of Fe atoms) I of the components, their hyperfine inductions B_hf, isomer shifts δ, quadrupole splittings Δ, and quadrupole shifts ε_S.

Figure 1. Getter – hydrogen uncharged state. (· cubic C15 and à hexagonal C14 Laves phases, ÿ monoclinic-ZrO2, Ñ cubic- ZrO2).	Figure 2. Getter - hydrogen charged state. (+ ZrH2, o ZrV2H3.6, ´ cubic-ZrO2, D monoclinic-ZrO2).

The getter was hydrided during annealing in H₂ (5N) at 550°C for 15 minutes and dehydrided in vacuum (10^-2 Pa) at 550°C for 15 minutes. This procedure was applied one times, five times and ten times in the same furnace without a contact with an oxygen containing (ambient) atmosphere. Finally the samples were removed form the furnace to an ambient atmosphere and XRD and MS experiments were performed. From XRD measurements cubic C15 and hexagonal C14 Laves phases were determined in the samples annealed in vacuum (figure 1) and ZrH₂ and ZrV₂H_3.6 in hydrogen charged state. In the samples after 5 and 10 cycles a small amount of cubic and monoclinic ZrO₂ was observed (figure 2). These phases were nucleated already during the first step of heat treatment in the vacuum and their amounts gradually increased by subsequent annealing. MS phase analysis of the sample after the annealing in vacuum revealed Zr(Fe, V)₂ phase (as mentioned in [4]) with the similar parameters as Zr₂Fe, and iron atoms in Zr-V matrix. In the charged state in the first step, ZrFeV hydrided phase [4] and residua of iron atoms in Zr-V matrix were observed. The content of the second phase was slightly increasing and its parameter was changed during hydriding and dehydriding cycles. After ten cycles, non-charged Zr(Fe,V)₂ phase was found. The Fe₂Zr which is insensitive to hydrogen charging [5] was not observed in the present material.

References

1. K. Ichimura, K. Ashida, K. Watanabe, J. Vac. Sci. Technol. A 3 (1985) 2, 346.

2. I. Vedel, L. Schlabbach, J. Vac. Sci. Technol. A., 11, (1993) 3, 539.

3. T. Žák, in Mössbauer Spectroscopy in Materials Science, edited by M. Miglierini and D. Petridis (Dordrecht: Kluwer), 1999, p. 385.

4. L. Rodrigo, J.A. Sawicki, J. Nucl. Mater. 265 (1999) 208.

5. M. Hara, R. Hayakawa, Y. Kaneko, K. Watanabe, J. Alloys. Comp. 352 (2003) 218.

Acknowledgements.

This work was supported by the Czech Ministry of Education, Youth and Sports (1M6198959201) and Academy of Sciences of the Czech Republic (AV0Z20410507).