**1. Introduction**

Alkali metal heat pipes (HPs) are initially designed for heat transfer in space nuclear power systems, of which the operating temperature is typically from 800 K to 1800 K. HPs using alkali metals are also promising in advanced energy and power systems such as highefficiency waste heat utilization [1], hypersonic vehicles [2], and molten salt reactors [3].

A heat pipe consists of a sealed shell, wick structure and a vapor chamber containing working fluid, which is normally filled after the shell is evacuated [4]. Heat transfer in a heat pipe is achieved passively by the phase change and the circulation of the working fluid [5]. Different types of working fluid and shell material are adopted in heat pipes used under different working conditions. The type of heat pipe can be divided into four main types according to their working temperature: low temperature heat pipe (−270~0 ◦C), normal temperature heat pipe (0~200 ◦C), medium temperature heat pipe (200~600 ◦C) and high temperature heat pipe (above 600 ◦C) [6].

Alkali metal heat pipes operating at temperature above 800 K have typically been constructed taking liquid alkali metal: potassium, sodium, or lithium as the working fluid due to their high-power capacity and great thermal stability [7]. For alkali metal heat pipes used in nuclear power systems, a key requirement is the compatibility of structure materials with both nuclear fuel and alkali metal [8]. Refractory metals and alloys, for owning both high creep strength at high temperatures and excellent compatibility with alkali liquid metals as well as nuclear fuel, are often applied as the shell material of alkali metal heat pipes [9]. Molybdenum (Mo) is a kind of refractory metal that is reported to be one of the greatest candidates for use of alkali metal heat pipe walls [10–13]. Recently, Mo

**Citation:** Zeng, Q.; Liu, Z.; Liang, W.; Ma, M.; Deng, H. A First-Principles Study on Na and O Adsorption Behaviors on Mo (110) Surface. *Metals* **2021**, *11*, 1322. https://doi.org/ 10.3390/met11081322

Academic Editor: Guy Makov

Received: 16 July 2021 Accepted: 16 August 2021 Published: 20 August 2021

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alloys are also considered as structural materials using in nuclear reactors [14,15]. However, pure Mo becomes brittle at about room temperature and below, which impacts heavily not only on the fabrication of heat pipes, but also the transportation process that easily leading to heat pipe breakage [16,17]. To tackle this problem, adding rhenium (Re) into pure molybdenum is found to be effective in enhancing low temperature ductility while also improving the high-temperature strength and creep resistance, known as the "rhenium effect" [18,19]. Mo-Re alloy has great advantage in high temperature heat pipes, where the operating environment is usually highly oxidizing and corrosive.

One issue for Mo-Re alloy in the heat pipe is the corrosion induced by the liquid metal working fluid and impurities. Studies have shown that dissolution, mass transfer and impurity reactions are the major corrosion mechanisms in refractory metal-alkali systems. Meanwhile, the most serious corrosion problems encountered are related to impurity reactions associated with oxygen [20]. Even though the addition of Re can improve the low temperature ductility, creep resistance and high temperature strength, it is not known that whether the Re atom could bring an improvement on the corrosion resistance of Mo. In addition, as the existence of oxygen (O) could lead to serious corrosion problems, it is important to learn the chemical interaction between O and Mo surface [21]. In this study, we used a first-principles approach to investigate basic properties such as adsorption, diffusion properties of Na atom and O atom on the pure Mo (110) surface and Mo-Re (110) surface. In addition, the formation of surface vacancy was calculated for evaluating if the adsorbates can prevent Mo from dissolution.
