Scandium (Sc) is a metal characterized by low specific gravity and a relatively high melting point, making it a valuable addition to metals, alloys, and ceramics for enhancing material functionality. Al-Sc alloys, including intermetallic compounds of scandium (Sc) and aluminum (Al) (Shubin et al., 2008), are expected to be low-density materials with high strength at high temperatures. Al alloys with small amounts of Sc are uniquely advantageous as they combine light weight with exceptional strength (Shubin et al., 2008). For example, Sc-doped Al alloys exhibit significant hardening after quenching (Røyset and Ryum, 2005). This hardening was attributed to the presence of very fine equilibrium A1₃Sc precipitates in the ordered L₁₂ structure (Jo and Fujikawa, 1993), which is a type of regular atomic arrangement found in intermetallic compounds. As this case, intermetallic compounds with specific composition ratios can sometimes exhibit properties that differ from those of alloys. For example, Al-Zr-Sc and Al-Mg-Sc, alloys demonstrate anti-recrystallization effect (Røyset and Ryum, 2005) and superplastic behavior (Nieh et al., 1998), leading to excellent mechanical properties and high ductility. Additionally, nitrides of Al-Sc alloys exhibit high thermal conductivity and are promising as electronic component materials due to their wide-gap semiconductor characteristics (Talley et al., 2018; Liauh et al., 2016; Berkok et al., 2013). These Sc-doped intermetallic compounds and ceramics are anticipated to serve as functional materials in diverse applications, including aerospace, automotive, dielectric, semiconductor, and MEMS devices.

Although scandium (Sc) is classified as a rare earth element, it is relatively abundant in the Earth’s crust and is found in various ores. However, its dispersion in the Earth’s crust and the challenges associated with smelting Sc₂O₃, one of the most thermodynamically stable oxides, make its reduction a complex and energy-intensive process. A common method of Sc smelting involves converting Sc₂O₃ to ScF₃ and reducing it with Ca (Daane and Hampel, 1971). This process is inherently complicated, costly, and environmentally unfriendly due to the involvement of fluorides. In recent years, alternative methods for directly preparing alloys from Sc sources, bypassing the need to produce Sc metal before forming Al-Sc alloys, have been actively explored. For example, the electrochemical production of Al-Sc alloys has been investigated by several groups (Harata et al., 2009; Liu et al., 2019; Liao et al., 2024). Additionally, thermal reduction methods utilizing collector metals have been reported. For example, Fisher et al. described a method for extracting Sc-Zn alloys using ScF₃ as a starting material, with Zn serving as the collector metal (Spedding et al., 1960; Fischer et al., 1937). Similarly, Harata et al. obtained Sc-Al alloys from Sc₂O₃ at 1,000°C by employing Al as a collector metal during Ca thermal reduction (Harata et al., 2008). Furthermore, Fujii et al. successfully obtained Al-Sc alloys via microwave irradiation without the use of molten salt (Fujii and Fukushima, 2023; Fukushima et al., 2024; Fujii et al., 2020). The direct production of Al-Sc alloys from Sc₂O₃ without involving fluorides, offers significant advantages in terms of cost-effectiveness and environmental impact, making it a focus of intensive research.

The synthesis of Al₃ Sc alloys using microwave irradiation is anticipated to save energy by reducing processing time and lowering required temperatures. However, the underlying smelting mechanism remains unclear. Based on the XRD results from previous basic experiments, the crystalline phases in the samples obtained using the Mg plasma generator with a single-mode cavity were identified as Al₃Sc, MgO, and an unreacted raw material phase; notably, Al₂O₃ was not detected (Fujii and Fukushima, 2023). Consequently, the overall reaction equation, excluding unreacted raw materials, is proposed as follows: Sc 2 O 3   s + 6 Al  s  or  l + 3 Mg  g  or plasma = 2   Al 3 Sc + 3 MgO (1)

The Mg consumption during the reaction, determined from the pre-and post-experimental weights, was nearly identical to the Mg requirement calculated using Equation 1 for the synthesis of Al₃Sc. This finding confirms the plausibility of Equation 1. According to the Sc-Al phase diagram, equilibrium intermetallic compounds such as Al₃Sc, Al₂Sc, AlSc, and AlSc₂ were formed, with the thermodynamic equilibrium phase being Al₃Sc (Røyset and Ryum, 2005). However, while Al₃Sc was identified as the primary product, the driving force for the reduction of Sc₂O₃ remained unclear. Although the experiment was conducted under vacuum conditions, the base pressure was approximately 2 Pa. Based on the Ellingham diagram, Al and Mg cannot act as Sc₂O₃ reductants at the experimental temperature range (Chase, 1998). The sample temperature during the microwave process did not exceed 1,000°C (Fujii et al., 2020), although this measurement was uncertain as it was based on crucible temperatures from a previous study. These observations suggest that the reduction mechanism of Sc₂O₃ does not involve a simple reduction to Sc. The liquidus line of the Al₃Sc intermetallic compound is 1,050°C, yet the experimental temperature remained below 700°C. This indicates that the formation of Al₃Sc intermetallic compounds occurred through a mechanism distinct from the precipitation of Al₃Sc from molten Al-Sc alloy during cooling. On the other hand, Mg in its plasma state likely contributed to the reduction of Sc₂O_3, possibly due to the high-energy characteristics of the plasma, though the specific mechanism remains unclear. Moreover, the simultaneous heating of Sc₂O₃–Al pellets and the introduction of Mg plasma were performed in a single microwave furnace; consequently, the complexity of the system precluded a clear elucidation of the driving forces of the chemical reaction (Fujii et al., 2020).

To elucidate the reaction mechanism of Al₃Sc alloy formation, it is essential to determine the role of Al, the contribution of Mg, and the effect of Mg plasma under conditions that exclude the influence of microwave electric and magnetic fields on oxide reduction (Fukushima et al., 2012; Mizuno et al., 2021). In this study, the mechanism of Al₃Sc formation was investigated by heating only mixed Sc₂O₃ + Al powder pellets, and the effect of Mg vapor on these pellets was evaluated using in situ gas analysis. Additionally, the influence of Mg plasma was assessed using in situ gas analysis within an experimental system where Mg plasma was applied to Sc₂O₃ -Al pellets heated in an electric furnace. This setup employed a technique that maintained Mg plasma for an extended duration and generated the plasma outside the microwave cavity.

A series of results related to Experiment (a) demonstrates that Al₃Sc can be obtained simply by heating a mixed powder of Sc₂O₃ and Al under a vacuum. During the reaction, oxygen was released, and no Al₂O₃ was detected in the sample after the experiment. These observations form the basis for discussing the mechanism by which Al₃Sc was formed in Experiment (a). First, based on the Ellingham diagram and the absence of Al₂O₃ formation, it is unlikely that aluminum in its solid or liquid phases directly reduced Sc₂O₃. The fact that oxygen was released from the pellet indicates that Sc₂O₃, the only oxide in the system, underwent reduction. However, according to the Ellingham diagram, Sc₂O₃ being highly stable is unlikely to be reduced to Sc metal under the experimental conditions. From the above, the following elementary reactions are proposed: S c 2 O 3 → Sc 2 O 3 − δ + δ / 2 O 2   g (2) Sc 2 O 3 − δ   + 3 δ Al → δ Al 3 Sc + 1 ‐ δ Sc 2 O 3   (3)

Oxygen is released according to Equation 2, and the formation of Al₃Sc proceeds via Equation 3. In this mechanism, aluminum acts as a collector metal, extracting Sc from the weakly reduced scandium oxide. Although the precise amount of oxygen loss(δ) has not been quantified, δ is expected to be small due to the high stability of Sc₂O₃. However, the conversion ratio of Al₃Sc exceeds 50%; suggesting that the driving force of Al as a collector metal is significant in this system.

Next, the effect of Mg vapor is discussed. The XRD results indicate the formation of MgO, but it is unlikely that Mg, which is considered to have a lower reducing power than Al, directly reduced Sc₂O₃. In-situ gas analysis revealed that the partial pressure of oxygen in the system decreased immediately after the generation of Mg vapor, This observation suggests that the oxygen released via Equation 2 reacts with Mg vapor to form oxides, as shown in Equation 4 δ / 2 O 2   g + δ Mg  g → δ Mg O   s (4)

The reaction in Equation 4 causes the equilibrium in the overall reaction in Equation 1 to shift to the right, increasing the amount of Al₃Sc produced. Due to this oxygen-gettering effect, the conversion ratio, which was 57% in the absence of Mg vapor, increased to 82%.

Finally, the effect of the Mg plasma is discussed. The XRD results revealed that the maximum conversion ratio reached 89%, indicating that the reaction is accelerated in the presence of Mg plasma compared to the Mg vapor. This demonstrates that Mg plasma enhances the reaction rate more effectively than Mg vapor. While Equation 4 suggests a possible effect of Mg plasma on lowering the oxygen partial pressure in the system, the in situ gas analysis results indicate that although the oxygen partial pressure was reduced, the oxygen partial pressure reduction per unit time was smaller than that in the Mg vapor experiment. These observations suggest that the effects of Mg vapor and Mg plasma on the reaction system differ significantly. From the formation of MgO on the right side of the overall reaction, it is strongly inferred that a portion of the Mg plasma contributes to the direct reduction of scandium oxide, thereby promoting the reaction. This can be represented by Equation 5: Sc 2 O 3 s + Mg plasma → Sc 2 O 3 − δ + δ MgO (5)

This finding is consistent with the results of a previous study (Fujii et al., 2020). From the above, Al₃Sc smelting is achieved through a mechanism where aluminum acts as a collector metal, extracting Sc from slightly reduced Sc₂O₃, Mg vapor further promotes the reaction by decreasing the system’s oxygen partial pressure, while the plasma conversion of Mg vapor partially contributes to the direct reduction of scandium oxide, thereby enhancing the reaction.

This study investigated the reaction mechanism involved in the synthesis of Al₃Sc alloys using Mg plasma applied to Sc₂O₃-Al mixtures via microwave irradiation, analyzed through in situ gas analysis. The findings revealed that Al acts as a collector metal, extracting Sc from slightly reduced Sc₂O₃, while Mg vapor promotes the reaction by functioning as an oxygen getter. Furthermore, the direct reduction of Sc₂O₃ by Mg in the plasma state was confirmed, leading to an increased yield of Al₃Sc compared to the Mg vapor case. These results emphasize the critical role of Al and Mg in the development of novel synthesis methods for Sc-containing alloys and demonstrate that Mg plasma enables smelting via a reaction pathway distinct from conventional methods. The outcomes of this study will contribute to the development of manufacturing processes for Sc-containing alloys, addressing the limitations of traditional methods that impose a large environmental burden. Moreover, these findings open the possibility of developing new alloy formation routes by leveraging the appropriate interactions of collector metals, metal vapors, and metal plasma, even with other difficult-to-reduce oxides as starting materials.