The effect of Fe atoms on the absorption of a W atom on W(100) surface

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Abstract

We report a first-principles calculation that models the effect of iron (Fe) atoms on the adsorption of a tungsten (W) atom on W(100) surfaces. The adsorption of a W atom on a clean W(100) surface is compared with that of a W atom on a W(100) surface covered with a monolayer of Fe atoms. The total energy of the system is computed as the function of the height of the W adatom. Our result shows that the W atom first adsorbs on top of the Fe monolayer. Then the W atom can replace one of the Fe atoms through a path with a moderate energy barrier and reduce its energy further. This intermediate site makes the adsorption (and desorption) of W atoms a two-step process in the presence of Fe atoms and lowers the overall adsorption energy by nearly 2.4 eV. The Fe atoms also provide a surface for W atoms to adsorb facilitating the diffusion of W atoms. The combination of these two effects result in a much more efficient desorption and diffusion of W atoms in the presence of Fe atoms. Our result provides a fundamental mechanism that can explain the activated sintering of tungsten by Fe atoms.


Full Article: http://jap.aip.org/resource/1/japiau/v103/i10/p106103_s1

Introduction

The sintering of tungsten can be enhanced by the addition of small amounts of alloying elements (0.5%–1.0%) from the iron group metals such as Fe, Co, and Ni. This phenomenon is called “activated sintering.”1,2 These additive species lower the activation energy for sintering, resulting in a much lower sintering temperature, shorter sintering time, or better properties.3 The classical theory explains this by the enhancement of grain-boundary diffusion in tungsten due to the presence of the respective element in the grain boundary. High densities (up to 99% of bulk) can be obtained even at 1100 °C (at this temperature, tungsten compacts are commonly presintered).3–5 Because of high melting point and large surface energy, tungsten has been considered to be one of the best substrates to grow thin magnetic films on and, consequently, its interaction with magnetic materials including Fe has been studied extensively.6–8 In this study, we investigated the activated sintering of tungsten by iron from a quantum mechanics point of view. In the presence of high diffusivity path (such as Fe layer), adsorption and desorption are two of the main mechanisms of sintering.

The effect of Fe on adsorption of W

Fig. 1. Adsorption of W on (a) clean W(100) and (B–D) Fe monolayer on W(100) surfaces. The side views (left column) and top views (right column) are given. The (3x3) surface unit cells are used for our calculations, as shown in top views. The black spheres represent W atoms in the substrate layers, while a red sphere represents the W adatom. The Fe atoms are represented by smaller green spheres.

In this study, we focus on the adsorption of W atoms on W(100) and Fe/W(100) surfaces. Figure 1 shows the optimized structures of adsorbed W atoms in different configuconfigurations. In Fig. 1(a), a W adatom is adsorbed on a clean W(100) surface, while Figs. 1(b)–1(d) show three different configurations of a W adatom adsorbed on W(100) surface covered by a Fe monolayer. For each configuration, all atoms are fully relaxed except for the atoms in the bottom layer. W(100) surface is known to exhibit a (sqrt(2) x sqrt(2))R45° reconstruction at low temperatures.19 It also has been observed that the surface reconstruction vanishes locally upon the adsorption of a Mn atom on W(100) surface.7 Our result is in good agreement with these observations. Two deltoids in Fig. 1(a) show the distortion of the nearest neighbor bond angles due to surface reconstruction. The upper-left deltoid containing the W adatom is indistinguishable from a square reflecting the fact that the reconstruction is suppressed due to the adsorption of the W atom. On the contrary, the lower-right deltoid has two equal obtuse angles of 96.1° indicating the distortion of bond angles due to surface reconstruction. However, because the deltoid is adjacent to the W adatom adsorption site, the angle is smaller than 106° observed for clean W(100) surfaces.


The adsorption energy of W

Fig. 2. The adsorption energy of a W adatom on a clean W(100) surface (black open circles) and on a Fe monolayer on a W(100) surface (red filled squares). The height of W adatom z is measured from the W atoms in the top layer of the W(100) surface.

Figure 2 also shows the adsorption energy of a W adatom on two different surfaces, a clean W(100) surface and an Fe monolayer on W(100) surface, as a function of the height. For the configurations (A–D), only the bottom layers are fixed and all other atom positions are fully relaxed. The other points in Fig. 2 are generated by additionally fixing the height of the W adatom.

Our result is consistent with the experimental observation that the addition of a small amount of Fe atoms enhances the sintering of tungsten. Our study shows that there are two main effects of Fe atoms on tungsten sintering. First, the existence of a stable adsorption site on an Fe monolayer (configuration B) leads to more efficient desorption of W atoms. Second, the existence of stable adsorption sites on an Fe monolayer makes the diffusion of W atoms on Fe atoms available for an efficient sintering process. The smaller adsorption energy of a W atom on an Fe monolayer strongly suggests that the Fe thin film layer will provide a much smoother platform for the adsorbed W atoms to diffuse. This enhanced diffusion will facilitate the movement of W atoms from one place to another.

Conclusion

we have presented a first-principles DFT investigation of the structure and energetics of a W adatom on W(100) and Fe/W(100) surfaces. We found that the W atom first adsorbs on top of the Fe monolayer. Then, the W atom can replace one of the Fe atoms through a path with an energy barrier of 0.99 eV and reduce its energy further. This intermediate site makes the adsorption (and desorption) of W atoms a two-step process in the presence of Fe atoms and lowers the overall adsorption energy by nearly 2.4 eV. This effect results in a more efficient adsorption and desorption of W atoms in the presence of Fe atoms. Our result provides a fundamental mechanism that can explain the activated sintering of tungsten by Fe atoms.

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