A. U-Zr and U-Ti alloys

Hydrogen absorption-desorption properties and the reactivity with air of U-Zr and U-Ti alloys were investigated for use in depleted uranium based energy storage systems as well as for tritium storage systems [3-8]. Some of the U-Zr and U-Ti alloys have hydrogen absorption-desorption properties as good as those of uranium. Further, the pyrophilicity of Zr- or Ti-rich uranium alloys is more moderate than that of U-rich alloys due to the higher resistance against powdering upon hydrogenation [3, 7]. Table 1 summarizes the hydrogen storage capacities of U-Zr and U-Ti alloys [3]. On hydrogenation, U-Zr and U-Ti alloys decompose into UH₃ and ZrH_x (x<2) and UH₃ and UTi₂H_x (x<5) phases, respectively, so that P-c-T diagrams consist of the plateau regions of corresponding phases. Because of low equilibrium H pressure of ZrH_x phases, the rechargeable hydrogen capacities of U-Zr alloys depended on the fraction of UH₃ phase (or U content in the alloy), except UZr₂ where the UH₃ region becomes smaller after cycles. In U-Ti alloys the ternary phase UTi₂H₅ formation consumes some uranium to narrow the UH₃ plateau region, while utilizing the lower pressure UTi₂H₅ plateau can extend the rechargeable capacity. Alloying U with Zr or Ti basically keeps the equilibrium pressure of UH₃, while powdering is greatly reduced, especially with high content of Zr or Ti. Table 1 also shows the degree of powdering after up to 20 hydrogen absorption-desorption cycles [3]. UZr, UZr₂, and UTi₂ alloys showed significant resistance against severe disintegration into fine powders even after more than 10 cycles. UZr especially possess a good balance of large capacity and high resistance against powdering.

B. U-Mn, U-Fe and U-Ni alloys

Uranium-rich intermetallic compounds, namely U₆M (M = Fe, Mn, Ni), commonly forms in these binary systems. Cubic MgCu₂-type UM₂ phase is the only other intermetallic phase in the U-Mn and U-Fe systems, while four additional phases form in the U-Ni system, including UNi₅ and UNi₂ of known AuBe₅- and Cu₂Mg-type crystal structure, respectively, as well as U₅Ni₇ and U₇Ni₉. The hydrogen absorption properties of these compounds have been studied [6, 10], where none of the alloys except the U₆M-type compounds were found to be reactive to hydrogen. All U₆M type compounds absorbed hydrogen to form UH₃ and the corresponding UM₂ phases. The P-c-T isotherms of these alloys basically consist of a wide single plateau of UH₃. As shown in Fig. 1, the plateau pressures in these U₆M alloys are all higher than pure U. Among the alloys, U₆Fe has the highest desorption pressure.

Fig. 1.Full-screen View

(Color online) Hydrogen desorption pressure of U₆M type alloys in comparison with uranium.

C. UCo

UCo, one of the six known intermetallic compounds in the U-Co system, is known to form UCoH_2.7 [9, 11]. Our study on the hydrogen absorption—desorption behaviors of the compound has shown that there are two stages of desorption reactions at higher temperatures than 473 K, while only a single stage was observed below 423 K [9]. UCo readily absorbs hydrogen to form UCoH_2.7 at temperature below 423 K under the hydrogen pressure of 10⁵ Pa. It simply dehydrogenates reversibly at 423 K, while two-phases of UH₃ and UCo₂, appear during the desorption of UCoH_2.7 above 473 K. This is due to the following two competing reactions:

The standard Gibbs energy changes, $\Delta G_1^0$ and $\Delta G_2^0$, for reactions (1) and (2), respectively, as functions of T and hydrogen pressure, p, were estimated as follows

here, po is the standard pressure (101325 Pa), R is the gas constant (8.3 J/(mol K)), and each Gibbs energy is calculated per mole of UCoH_2.7. Fig. 2 plots the $\Delta G_1^0$ and $\Delta G_2^0$ vs. T in lines with unfilled and filled symbols, respectively, for two cases of p. Solid lines with circles are for p=10⁴ Pa, while broken lines with squares are for p=10⁵ Pa. For lower p (=10⁴ Pa), $\Delta G_2^0$ is lower than $\Delta G_1^0$ over a wide temperature range below 650 K, while at p=10⁵ Pa, $\Delta G_2^0$ is higher at T > 400 K. This results in the observation of a single stage reaction (2) at lower T and lower p, while two stages, including the decomposition into UCo₂ and UH₃, result at higher T and higher p.

Fig. 2.Full-screen View

(Color online) (a) Standard Gibbs energy changes of two competing reactions on dehydrogenation of UCoH_2.7 for the hydrogen pressure p of 10⁴ Pa (Solid lines) and 10⁵ Pa (dotted lines). The reaction (1) to decompose into two phases and (2) to reverse to UCo are shown in the lines with unfilled and filled symbols, respectively; (b) Equilibrium desorption pressures based on the enthalpy and entropy changes given in Ref. [9].

While desorption operations may be limited in the p range, UCo can reversibly absorb and desorb hydrogen at a relatively low temperature of 423 K with the desorption pressure significantly higher than that of UH₃.

D. UNiAl and other Fe₂P-type uranium compounds

UNiAl intermetallic compounds have a hexagonal Fe₂P-type (or also known as ZrNiAl-type) crystal structure. As illustrated in Fig. 3(a), the unit cell consists of a stack of two kinds of planes, one with three U and one Ni atom and the other with three Al and two Ni atoms. Table 2 shows more details of the atomic positions in terms of Wyckoff’s notations and representative partial coordinates. In the original crystal structure, there are two U atom triangles per unit cell, which with two Ni atoms on the adjacent planes make tetrahedral interstitial sites sharing the triangle, as illustrated also in Fig. 3(a). At hydrogen pressures below 10⁵ Pa, UNiAl absorbs up to two hydrogen atoms per unit cell, or around 0.7 H per UNiAl formula unit (F.U.) (H/F.U.) [11-13]. The neutron diffraction study of a deuteride, UNiAlD_0.7, showed that D atoms occupy the U₃Ni₁-type interstitial sites. At a higher hydrogen pressure it can absorb up to around 2 H/F.U. [14]. Yamamoto et al. showed that the crystal structure of UNiAlH_2.2 or UNiAlD_2.2 has significantly different atomic positions while retaining basic hexagonal structure. Table 2 also shows the detailed crystal structure of the deuterides, including the atomic positions. In UNiAlD_2.2 formation the uranium positions shift to increase x in the representative coordinate, (x, 0, 1/2), of 3g position. Aluminum atoms also move within the plane in the same direction as that of U atoms, while Ni atoms in the 1b position moves along the c-axis to z=0 from z=1/2. These changes are illustrated in Fig. 3(b), 3(c), and 3(d) show U and Ni atom positions on the z=1/2 plane for UNiAl, UNiAlD_0.7, and UNiAlD_2.2, respectively. U atoms shift to increase x from 0.580 to 0.592 and 0.659 makes the U triangle, forming an U₃Ni₁-type interstitial site larger by 4% and 17%, respectively, which presumably is required to secure the U-D distance. It is noticeable that these shifts also make the other U triangles similar equilateral ones. Deuterium occupying sites are also shown in Fig. 3(c) and 3(d) with three different triangle symbols for U₃Ni₁^I-type D(1), U₃Al₁-type D(2), and U₃Ni₁^II-type D(3) sites, respectively. The final D site structure suggests that it is the atomic position shifts that really enabled this compound to accept around 6 hydrogen atoms per unit cell.

Fig. 3.Full-screen View

(Color online) (a) Crystal structure of UNiAl showing four U₃Ni^I-type tetrahedral sites; (b) atomic arrangements on the U plane at z=1/2 in (a) UNiAl, (c) UNiAlD_0.7 and (d) UNiAlD_2.2.

Hydrogen desorption isotherms of UNiAl have been studied in detail by Yamanaka et al. [14]. Fig. 4 shows the hydrogen desorption pressures at two plateau regions, between α and β and β and γ phases, corresponding to the following nominal reactions, respectively:

Fig. 4.Full-screen View

(Color online) Hydrogen desorption pressures of UNiAl reproduced from P-x-T diagram in Ref. [14].

here, H compositions are all nominal values based on the number of H atoms occupying the 4h site and the 4h, 3g, and 2e sites for β and γ phases, respectively. The figure indicates that the desorption pressure for the reaction (6) reaches around 10⁵ Pa at 400 K, which is sufficiently high for hydrogen storage use of the material. The lower plateau reaches 10⁵ Pa at around 550 K, which may be acceptable, while using only higher plateau region allows recycling of around 2/3 of the total hydrogen capacity.