Nitrogen absorption behavior and mechanism of TiZrMnFe getter alloy

Highlights

• The as-produced TiZrMnFe getter alloys have an excellent nitrogen absorption capacity than ZrMnFe getter.

• The volume-weight conversion method is adopted for long-time nitrogen absorption experiments.

• The XPS with ion etching methods was used to characterize the surface and sub-surface after nitriding reaction.

• Nitrogen absorption process has been illustrated through analyzing the absorption performance and alloy structure.

Abstract

The getter alloy of Ti_xZr_1-xMnFe(x = 0, 0.1, 0.2, 0.3, 0.4) was prepared by arc melting and its nitriding behavior was investigated. The as-produced TiZrMnFe alloys exhibit a single ZrMn₂ C14 laves structure, and Ti is enriched on grain boundary. The nitrogen absorption kineticsshows that initial absorption rate slows down but final absorption amount increases with the substitution of Ti. The maximum absorption capacity improved to 0.00367 mol/g, a 31% rise compared to ZrMnFe. A new phase of (Zr,Ti)N formed during nitriding absorption, while the chemical states of the nitride alloys on surface was confirmed by XPS. A small quantity of oxygen was observed and the content of nitrogen decreases with depth increase. Nitrogen absorption process may be include absorbing and dissociating of N₂ on surface, diffusing of nitrogen atoms, forming of (Zr,Ti)N phases, and finally (Zr,Ti)N particles growing up into dense barrier, resulting in absorption gradually saturated.

Introduction

Nuclear fusion energy has great prospects in application for it has the advantages of high efficiency of reaction discharge, huge reserves of raw materials, and low radioactivity of nuclear waste, etc. [1,2]. However, there are still many issues to be solved in the fields of nuclear fusion, and hydrogen isotope handing is one of them. In the nuclear industry, many options could be used for handling tritium. Among the various options under consideration, tritium extracted by metallic getters seems to be a simple and safe solution [3,4].

Uranium beds have been applied in the tritium interim storage and are widely studied in the early years, whereas due to their high chemical reactivity and restrictive use of nuclear material, alternative materials have been investigated [5]. Researchers have evaluated that Zr-based AB₂ laves alloy getter can play a crucial role in hydrogen isotope separation and gas purification [6,7], in viewing of the main impurities in various systems are CQ₄, NQ₃, Q₂O, CO₂, N₂(where Q is any hydrogen isotope) [8].

ST909 is the trade name of a ZrMnFe AB₂ type getter produced by Italy SAES Getters, which is composed of equimolar quantities of zirconium (40.5 wt%), manganese (24.5 wt%), iron (25 wt%), and aluminum binder (10 wt%). It has potential application in such areas as hydrogen isotope chemical compression, tritium handling, purification, and storage due to its special property [8,9]. For example, it has been demonstrated to crack methane, tritiated water and ammonia at elevated temperatures, and can absorb carbon monoxide and carbon dioxide [10,11]. Given the reaction property of ZrMnFe to multiple gases, the investigation of nitrogen absorption is significant because nitrogen exposure will affect its performance on other gases [8]. Meanwhile, ammonia in the exhaust of the nuclear industry also needs to be absorbed, thus the interaction of nitrogen atom with getter metal matrix has a guiding role in the reaction of ammonia cracking by ZrMnFe [12].

According to Ref. [8], the reaction temperature of nitrogen with ST909 getter generally requires more than 670 °C, and the reaction rate is also relatively slow (needing more than 48 h to reach saturation). In the previous work, many efforts have been devoted to improve their absorption rate and capacity, in which the multi-element alloying is the most efficient one [13]. In the Zr(Al_xV_1-x)₂, Zr(Al_xFe_1-x)₂ and Zr(Al_xCo_1-x)₂ getter alloys, large substitution quantities of Al inhibit the hydrogen absorption which probably caused by a decreased elastically-mediated attraction between the hydrogen atoms [14]. The substitution of Ni at Fe side in ZrMnFe alloy reveals that the unit cell volume change will affect absorption properties of the alloys [15]. In AB₂ laves alloy, the partial substitution of Zr by Ti has been proved to be an efficient way to improve the hydrogenation properties of Zr-based Laves alloys [16]. Zhang et al. [17] prepared Zr_0.9-xTi_0.4+xV_1.7 non-stoichiometric hydrogen storage alloys, found that Ti substitution changed the phase constituent and microstructure of ZrV₂, and their hydrogen absorption capacity have been improved.

In this study, TiZrMnFe alloys with different stoichiometry of Ti have been prepared, and their nitrogen absorption behavior has been investigated. Furthermore, the possible absorption mechanism was discussed by the results of nitrogen absorption performance and structure characteristic. This work is helpful to understand the high-temperature absorption performance of Zr-base alloys and provides a new idea for the further study of getter alloys for nuclear fusion application.