3.2.3. EDX Analysis

Since the EDX data are collected from the fracture surface of the sample, the relevant data are only used as references for the distribution law of each element. EDX analysis from Al-Ga-In3Sn-Zn ingots was performed to identify the element distribution in Al grains (G) and GB phases. SEM images of GB phase particles are illustrated in Figure 6, and the EDX data are listed in Table 2. It can be seen that the samples include a lot of O, this is owing to the rapid fracture surface oxidation when the sample is exposed to air during preparation. The elements of Al grains mainly include Al, Ga and Zn (if doped) besides O. Since Ga and Zn have high solid solubility in Al and an approximate atomic radius, it is easy for them to ge<sup>t</sup> into the Al crystal lattice. A small amount of In and Sn, found in the matrix, were quenched into Al grain due to a quick cooling rate during solidification. In Zones 2, 4, 6 and 8, the atomic ratio of In:Sn in these GB particles approaches 3:1, which is very close to the mole ratio of the two in the raw material during the preparation process. Meanwhile, the solid solubility of In and Sn in Al grains is particularly low. When the content of Zn increases, as shown in Figure 6e, a small amount of Zn is found almost in its particulate form (91.7 at.% Zn), which is dispersed in the alloy ingots.

Zn exists in both the G and GB phases. The typical characteristic of the GB phase is the high content of In and Sn elements. This explains why the reduction of In3Sn content in Figure 5 led to a relative decrease in the total area and number of GB phases. However, with an equivalent reduction in the amount of In3Sn, the variation law of the total area and single area of the GB phase (the slope of the solid line and the dotted line in Figure 5) are different. This shows that when a small amount of Zn is doped, Zn mainly forms a solid solution with Al, causing the GB phase area to decrease rapidly. When more Zn is added, Zn starts to enter the GB phase, so the decline rate of the solid line which shows the total area of the GB phase slows down, and the area of a single GB phase is basically unchanged, staying around 11–12 μm.

**Figure 6.** SEM images from fracture surface of Al-rich alloys, (**a**) 0 wt.% Zn, (**b**) 1 wt.% Zn, (**c**) 3 wt.% Zn and (**e**) 5 wt.% Zn; (**d**) EDX mapping scan spectrum of (**b**); (**f**) typical EDX mapping of Zone 9.


**Table 2.** Composition of the Al-Ga-In3Sn-Zn alloys (In brackets are the data after oxygen removal).

## 3.2.4. DSC Curves

Figures 7 and 8 show the typical DSC curves of Al-rich alloys. One wide endothermal peak or exdothermal peak emerges in the DSC trace. The peak temperature is about 120 ◦C, which is supposed to be due to the eutectic reaction of Al and In3Sn according to the In-Sn phase diagram (75 at.% In-25 at.% Sn). Compared with Al-Ga-In3Sn alloy, the diffusive peak moves to the cold side when a small amount of Zn is doped, and then moves to the opposite side slightly accompanied by the increase of Zn doping.

**Figure 7.** DSC curves of the Al-Ga-In3Sn-Zn alloys tested by heating cycles.

**Figure 8.** DSC curves of the Al-Ga-In3Sn-Zn alloys tested by cooling cycles.

## *3.3. Kinetic Parameters*

The formulas used to calculate the kinetic parameters of the Al hydrolysis reaction have been introduced in previous studies [36,37]. The activation energy (Ea) has been calculated (Figure 9) by the Arrhenius diagram of the hydrogen generation yields derived from various temperatures (Figure 10). When the Zn doping amount is increased from 1 to 5 wt.%, the activation energy of these alloys increases accordingly from 59 to 139 kJ/mol.

**Figure 9.** The activation energy versus Zn content.

**Figure 10.** Arrhenius plots for the isothermal reaction of alloy hydrolysis.

From previous studies, the factors affecting the activation energy are as follows:

(1) W. Wang and coworkers [30] found that the Ea of Al-Ga-In-Sn alloys ranged from 53 to 77 kJ/mol. The researchers believe that this change is the result of different grain size (increases from 23 to 258 μm). Our results of the Zn-free alloy (Al grain size: 33 ± 5 μm; Ea: 63 KJ/mol) are very close to their values (37 ± 16 μm, 56 ± 5 KJ/mol).

(2) Jeffrey T. Ziebarth and coworkers [36] speculated that the compositions might affect the activation energy. The eutectic reaction between GB phase and Al is identified to be a key factor in alloy hydrolysis. The addition of Zn changes the GB phase from a ternary alloy (Ga-In-Sn) to a quaternary alloy (Ga-In-Sn-Zn). Adding a small amount of Zn means that the relative contents of In and Sn are reduced, and the Ga-In-Sn mass ratio is closer to the eutectic point (Ga-In-Sn Ternary phase diagram), but as more low-melting metals (In3Sn) are replaced by Zn (more than 1 wt.%), the energy required for the reaction increases dramatically. Hence, it makes sense that the activation energy varies with chemical composition.

(3) The segregation in alloys may also cause an increase in the activation energy [35]. A Zn-rich particle was found in the high Zn alloy, which is most likely due to segregation. Although the number of these particles is very small, the degree of influence of these segregations on the hydrolytic activation energy needs further study.
