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Surface nano structures on W surface exposed to low-energy high flux D plasma
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Surface nano structures on W surface exposed to low-energy high flux D plasma
T
Y.Z. Jiaa,b, , W. Liub, , B. Xub, S.L. Qub, T.W. Morganc ⁎
⁎
a
Science and Technology on Reactor Fuel and Materials Laboratory, Nuclear Power Institute of China, Chengdu, Sichuan 610213, China Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c FOM Institute DIFFER-Dutch Institute for Fundamental Energy Research, 5612AJ Eindhoven, The Netherlands b
ARTICLE INFO
ABSTRACT
Keywords: Nanostructure Tungsten Deuterium TEM
The surface nano structures on W surface exposed to low-energy D plasma was studied by transmission electron microscopy. It was found that the surface nano structures on W surface induced by D plasma were mainly due to two reasons. First, the subsurface nano-sized bubbles will induce nano scale blistering of the surface, resulting in surface nano morphology. The nano scale blistering behaviour is most obvious on [1 1 1] surface at low temperature (500 K). Secondly, during the plasma exposure, oxide layer on W surface will be sputtered by low energy D particles, and the sputtering can also induce nano structures on some surfaces. At low temperature (500 K), the surface nano structures were formed due to the nano scale blistering behaviour and the sputtering of surface oxide layer. At high temperature (1000 K), the surface nano structures were mainly caused by the sputtering of surface oxide layer.
1. Introduction Tungsten (W) materials, due to its favourable properties, such as high thermal conductivity, high melting temperature, and low sputtering yield, etc. [1,2], will be used as the plasma-facing materials (PFMs) in divertor of ITER. During the operation, W material will be subjected to high flux and high fluence of D ions [3]. The main surface damage caused by D plasma is blister structures on the surface as reported before [4–6]. Besides blisters, surface nano structures on W surface induced by high flux D plasma were first reported by Xu et al. [7]. After that, many researches [8–11] reported similar surface nano structures under similar experimental conditions, and the morphology of nano structures is closely dependent on surface orientation. This kind of nano structures may affect the performance of W PFMs in ITER. Although the surface nano structures were reported and studied intensively, the mechanism for its formation is still not clear. Xu [12] speculated that the surface nano structures were surface reconstruction phenomenon due to D particles bombardment, but this theory lacks direct experimental evidence. The main present problem is that the subsurface structures beneath the surface nano structures were rarely studied before. In this study, the subsurface structures beneath the surface nano structures were studied by focused ion beam (FIB) and transmission
electron microscopy (TEM). The advantage of using FIB to fabricate TEM samples is that the surface morphology and cross-section morphology of the same surface area can be observed by scanning electron microscope (SEM) and TEM, so the subsurface structures beneath the nano structures can be well investigated. 2. Experimental W disc samples (Φ30 mm, 3 mm thick) were cut from a rolled sheet supplied by Advanced Technology & Materials CO., Ltd. (China). The textures of the sample are mainly {1 0 0} 〈1 1 0〉 and 〈1 1 1〉∥ND. The samples were polished by mechanical polishing and electrochemical polishing. After polishing, all the samples were stress relieved at 1273 K at a background pressure of 5 × 10−4 Pa for 1 h before plasma exposure. The samples were exposed to a high flux D plasma beam in the Pilot-PSI linear plasma generator [13], and the main species of the ions were D+ ions. The plasma beam was observed by optical emission spectroscopy, no obvious signal of impurity ions from the source was observed. Electron density and temperature of the plasma beam were determined using Thomson scattering (TS). The peak ion flux was about 1.5 × 1024 m−2 s−1. Because of the heating of the copper coils at high magnetic field, Pilot-PSI operates in a pulsed mode. For all samples we used identical shot durations of 70 s. During such a shot, the peak
⁎ Corresponding authors at: Science and Technology on Reactor Fuel and Materials Laboratory, Nuclear Power Institute of China, Chengdu, Sichuan 610213, China (Y.Z. Jia). E-mail addresses: [email protected] (Y.Z. Jia), [email protected] (W. Liu).
https://doi.org/10.1016/j.nimb.2018.10.022 Received 22 September 2017; Received in revised form 6 August 2018; Accepted 18 October 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
Y.Z. Jia et al.
Fig. 1. Morphology of W sample exposed to D plasma at 500 K. (a), (c), (e) surface morphology of surfaces with different NDs; (b), (d), (f) cross-section morphology of surfaces with different NDs.
fluence was about 8 × 1025 m−2–1.1 × 1026 m−2. In order to reach higher accumulated fluence, the same shot was repeated several times with a time interval between two shots of about 10 min. During the interval between two shots, the plasma source was not shut down, but the plasma species was switched to Ar plasma, and since the magnetic field was switched off between two shots, so basically no Ar ions reached the W surface between two shots. Before the next shot, the plasma species was switched back to D plasma. The total fluence for each sample in this study was about 6 × 1026 m−2 (each sample was exposed to 6 plasma shots). The ion energy was controlled by negatively biasing the sample with voltage about −48 V, so the ion energy of the plasma was ∼40 eV. No obvious mass change was measured before and after the plasma exposure. The surface temperature was controlled by the water cooling system from the back side of the samples and was a balance between the incoming power flux from the plasma and the cooling efficiency. In this study, the exposure temperature was set ∼500 K or ∼1000 K. The peak surface temperature was measured by a spectral pyrometer (FMPI SpectroPyrometer, FAR Associates) during the experiments, while the 2D surface temperature profile was measured by a fast infrared camera (FLIR A645 sc). It took about 10 s for the surface temperature to rise from room temperature to the stable temperature and after the exposure the temperature of the samples was back to room temperature typically within 5 s. After exposure, surface morphology changes were observed using a TESCAN MIRA 3 LMH high-resolution SEM. The orientations of the surface grains have been determined by Oxford instrument
NordlysMax2 electron backscatter diffraction technique (EBSD). Focused ion beam (FIB) was performed to fabricate the cross-section TEM samples using Ga+ ions, so that the cross-section morphology corresponding to the surface morphology can be observed by TEM. TEM observation was carried out at JEOL JEM-2100 TEM to obtain the crosssection morphology of the subsurface region, and EDS element distribution was carried out on JEOL JEM-2010F STEM. 3. Results Fig. 1 shows the surface morphology and cross-section morphology of W surface exposed to D plasma at 500 K. Fig. 1(a), (c) and (e) are the SEM surface morphology of surface with different surface normal directions (NDs), and Fig. 1(b), (d) and (f) are the TEM cross-section morphology of the surface in Fig. 1(a), (c) and (e) (TEM area) respectively. In the TEM images, note that during the FIB milling, Pt was deposited on W surface to protect the surface structure from Ga+ ions bombardment, so the TEM samples are composed of the W (the dark part) and Pt deposition (the bright part). As can be seen from the SEM surface morphology (Fig. 1(a), (c) and (e)), the surface nano structures exhibited different morphologies on different surfaces, which is similar to the results reported in previous literatures [7,11]. On the surface with ND close to [1 1 1], triangular nano structures were formed after the exposure, as shown in Fig. 1(a). Beneath the surface, nano-sized bubbles were observed by TEM, as shown in Fig. 1(b). The bubble size was about 20 nm, and the depth was about 20–30 nm, which is consistent with the results reported before 27
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
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Fig. 2. Morphology and element distribution on [1 1 0] surface exposed to D plasma at 500 K. (a) surface morphology, (b) Pt distribution, (c) W distribution, (d) O distribution.
exposure, as shown in Fig. 1(e), and the lamellar structures were parallel to the [0 0 1] direction and perpendicular to [1 1 0] direction. In Fig. 1(f), the cross-section morphology shows that the surface layer (marked by the red lines) was more obvious than that in Fig. 1(d). The thickness of this layer was about 30 nm. What should be noted is that the cross-section TEM samples were obtained along [1 1 0] direction and perpendicular to the lamellar structures. As can be seen in Fig. 1(f), the fluctuation of the surface layer was corresponding to the surface lamellar structures, indicating that the surface nano structures on some surfaces were directly due to the surface layer of unknown composition. In order to clarify the composition of this surface layer, the element distribution of the surface region was studied by STEM. Fig. 2(a) shows the cross-section morphology of [1 1 0] surface, and (b)–(f) were the Pt, W, O, C and Ga element distribution of the same region in Fig. 2(a). As can be seen from this image, the O element was concentrated in the surface layer between Pt deposition and W. In addition, the W concentration of the surface layer was lower than that of W matrix. Therefore, the composition of this layer was quite likely a W oxide. To further study the composition of this layer, the line scanning of element distribution across this layer was carried out. The line scanning results in Fig. 3 show that the signal of O element increased in the surface layer, and W signal decreased in this layer. On the contrary, on the surface without the obvious surface layer, the signals of W and O did not change a lot. Therefore, the composition of this layer should be a W oxide. There are some kinds of W oxide: WO2, WO3 and WO2.9. Also, previous study showed that D atoms may bond to W oxide [15], but clarifying the exact composition of oxide is beyond the scope of this study, which needs XPS or Raman spectral analyses. Fig. 4 shows the surface morphology and cross-section morphology of W surface exposed to D plasma at 1000 K. The surface morphology was different on surfaces with different NDs, as reported before [11]. The bubbles formed at 1000 K did not induce surface blistering or
Fig. 3. Line distribution of W and O of [1 1 0] surface exposed to D plasma at 500 K.
[14]. The cross-section morphology shows that the bubbles are closely related to the surface fluctuations. Therefore, the nano structures on [1 1 1] surfaces were formed due to the nano scale blistering behaviour of the surface caused by the bubbles beneath the surface, as discussed in our previous article [14]. With NDs away from [1 1 1], nano-sized blisters were also formed on the surface due to the bubbles beneath the surface, as shown in Fig. 1(c) and (d). However, the deformation (or the blistering behaviour) of the surface was slight in Fig. 1(d) compared to that in Fig. 1(b). In addition, on the surface of W sample, a surface layer with different contrast was observed between the deposited Pt layer and the W matrix, as marked by the red lines in Fig. 1(d). On [1 1 0] surface, lamellar nano structures were observed after the 28
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
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Fig. 4. Morphology of W sample exposed to D plasma at 1000 K. (a), (c), (e) surface morphology of surfaces with different NDs; (b), (d), (f) cross-section morphology of surfaces with different NDs.
related surface nano structures, even on [1 1 1] surface, as shown in Fig. 4(a) and(b). Fig. 4(c) and (e) are the surface nano structures on different surfaces, and Fig. 4(d) and (f) are the cross-section morphology perpendicular to the lamellar structures in Fig. 4(c) and (e) respectively. Similar to the results at 500 K, a surface layer (pointed by the red arrows) was also observed between Pt and W, and this layer is responsible for the surface nano structures in Fig. 4(d) and (f). However, on [1 1 0] surface this layer was not continuous on the surface, and the thickness was only 10 nm, which is thinner than that formed on [1 1 0] surface at 500 K in Fig. 1(f). Fig. 5(a) is the line scanning of W and O element distribution of the subsurface region on [1 1 0] surface along the yellow line. It shows that the substance of the surface layer was also W oxide, which is same as that at 500 K only with smaller thickness. For the surfaces without nano structures, Fig. 5(b) shows that no oxide layer was observed and O element was not concentrated on the surface with ND near [0 0 1]. Therefore, the nano structures formed on the surface at 1000 K were mainly due to the oxide layer on the surface.
The subsurface nano-sized D bubbles may induce nano scale blistering on W surface during the growth of bubbles, as discussed in our previous article [14]. The nano scale blistering mainly induces nano structures at low temperatures (500 K). In addition, the surface with ND close to [1 1 1] was more prone to surface blistering, and when the ND is away from [1 1 1], the nano scale blistering behaviour was less obvious. For the D bubbles formed at 1000 K, the bubbles were smaller and deeper as analysed in previous study [14], so the bubbles were not able to induce obvious blistering on the surface. Therefore, surface nano structures formed at high temperature (1000 K) were not due to nano scale blistering. A more important factor of the nano structure formation is the surface oxide layer. In this study it is found that the oxide layer was formed on the surface of some grains, but not observed on other surfaces (as shown in Fig. 4(b) and Fig. 5(b)). The nano structures were only observed on grain surfaces with oxide layer, but not observed on grain surfaces without oxide layer (except the nano structures induced by nano scale blistering). This means the nano structures must be formed due to the surface oxide layer. In our previous study, the TEM results of pristine W samples before D plasma exposure showed no oxide layer on the surface [14]. Therefore, the oxide layer observed in this study must form during the process in Pilot-PSI. However, the chemical property of D is the same as H, and it is a strong reducing agent, so oxidation can hardly occur on W surface with the presence of D plasma. In Pilot-PSI, the vacuum before plasma exposure was not quite high (about 2 × 10−3 Pa), so it is assumed that the oxide layer may form during the interval between two plasma shots. After the
4. Discussion In previous study [12], the surface nano structures were thought to be the surface reconstruction phenomenon due to D particles bombardment and stress field in surface region. However, this theory lacks direct experimental evidence. In this study, it is found that at different temperatures, the nano structures induced by D plasma were formed due to two reasons: nano scale blistering and surface oxide layer. 29
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
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orientation as reported in previous papers [21–23], so in the oxidation behaviour is different on W surface with different orientation in this study. The thickness of surface oxide layer was different on surfaces with different NDs, and on some surfaces even no oxide layer was formed, which may be because of the different oxidation, reduction and sputtering rates on different surfaces. Therefore, the surface nano structures induced by D plasma were quite likely caused by the sputtering of the surface oxide layer. Previous research [9] on Pilot-PSI reported that 5 eV D ions cannot induce surface nano structures on W surface, which may be because 5 eV D particles cannot cause sputtering of the surface oxide layer. Similar nano-structures were also observed on W surfaces exposed to He plasma, and the orientation dependence of the nano-structures was similar to that caused by D plasma [24,25], so it may be also caused by sputtering of the surface oxide layer. 5. Conclusion In conclusion, in this study, it was found that the surface nano structures on W surface induced by low energy D plasma were mainly due to two reasons. Firstly, the subsurface nano-sized bubbles will induce nano scale blistering behaviour of the surface, resulting in the ordered nano scale surface morphology. The nano scale blistering behaviour is most obvious on [1 1 1] surface at low temperature (500 K). Secondly, during the plasma exposure procedure, oxidation would occur on W surface, and oxide layer will be sputtered by low energy D particles, resulting in the ordered nano structures on the surface. At low temperature (500 K), the surface nano structures were formed due to the nano scale blistering behaviour and the sputtering of surface oxide layer. At high temperature (1000 K), the nano structures were thought mainly caused by the sputtering of surface oxide layer.
Fig. 5. Line distribution of W and O of surface exposed to D plasma at 1000 K. (a) surface with nano structures, (b) surface without nano structures.
Acknowledgement
plasma exposure, the surface temperature returns to room temperature slowly, and during this cooling down period oxidation likely occur on W surface. In addition, the reduction effect of D plasma is stronger at higher temperature. Therefore, as can be seen from the cross-section morphology of [1 1 0] surface of 500 K (Fig. 1(f)) and 1000 K (Fig. 4(f)), the surface oxide layer formed at 1000 K is not continuous, and also thinner than that formed at 500 K. We exposed the W material to the air at 500 K for 10 min, after which the surface was covered with W oxide. However, no surface nano structures were observed on the surface. Also, no similar surface nano structures were reported in previous oxidation research. Therefore, the oxidation alone cannot induce the surface nano structures observed on W exposed to D plasma. Previous literatures [16,17] show that when oxidation occurs on W surface, the sputtering threshold energy of D ions will decrease. Roth et al. [16] shows that the sputtering threshold by D ions will decrease from 200 eV to ∼18 eV at an oxygen background pressure of 8 × 10−5 Torr, mainly due to oxidation during the sputtering measurement. Therefore, the particle energy used in this study (∼40 eV) can hardly cause sputtering of W surface, but may induce the sputtering of the surface oxide layer. Many previous researches reported that ordered surface nano morphology will form on sputtering surface, and it is also closely dependent on the surface orientation [18–20], which is similar to the orientation dependence of surface nano structures observed on W surface exposed to D plasma [7,11]. The ordered surface structure induced by ion sputtering was attributed to the distribution of energy deposited and the self-diffusion of atoms on the surface [18]. Because the diffusion barrier is different along different directions on different surfaces, the surface structures induced by sputtering are related to the grain orientation [20]. However, in this study, the lattice structure of the surface oxide layer was not studied in detail, so the detailed relationship between the grain orientation and sputtering effect may be studied in future research. In addition, the oxidation or nitridation behaviour on surface is dependent to surface
This work was supported by National Magnetic Confinement Fusion Science Program of China under Grant 2013GB109004, 2014 GB117000, and the National Nature Science Foundation of China under Contract No. 51471092. The authors gratefully thank the help of TEM observation from Beijing National Centre for Electron Microscopy. References [1] T. Hirai, F. Escourbiac, S. Carpentier-Chouchana, et al., Phys Scripta T159 (2014) 014006. [2] R.A. Pitts, S. Carpentier, F. Escourbiac, et al., J Nucl Mater 438S (2013) S48–S56. [3] G. De Temmerman, T. Hirai, R.A. Pitts, Plasma Phys Contr F 60 (2018) 44018. [4] W.M. Shu, Appl Phys Lett 92 (2008) 21190421. [5] Y.Z. Jia, G. De Temmerman, G.N. Luo, et al., J Nucl Mater 457 (2015) 213–219. [6] Y.Z. Jia, W. Liu, B. Xu, et al., J Nucl Mater 477 (2016) 165–171. [7] H.Y. Xu, Y.B. Zhang, Y. Yuan, et al., J Nucl Mater 443 (2013) 452–457. [8] M. Balden, A. Manhard, S. Elgeti, J Nucl Mater 452 (2014) 248–256. [9] H.Y. Xu, G.N. Luo, H. Schut, et al., J Nucl Mater 447 (2014) 22–27. [10] M.H.J. 't Hoen, M. Balden, A. Manhard, et al., Nucl Fusion 54 (2014) 083014. [11] Y.Z. Jia, W. Liu, B. Xu, et al., J Nucl Mater 463 (2015) 312–315. [12] H.Y. Xu, G. De Temmerman, G.N. Luo, et al., J Nucl Mater 463 (2015) 308–311. [13] G. De Temmerman, J.J. Zielinski, S. van Diepen, et al., Nucl Fusion 51 (2011) 073008. [14] Y.Z. Jia, W. Liu, B. Xu, et al., Nucl Fusion 57 (2017) 034003. [15] V.K. Alimov, B. Tyburska, M. Balden, et al., J Nucl Mater 409 (2011) 27–32. [16] J. Roth, J. Bohdansky, W. Ottenberger, “Data On Low Energy Light Ion Sputtering,” (Max-Planck-Institut fur Plasmaphysik (1979). [17] A.L. Suvorov, Phys Atom Nucl+ 65 (2002) 2021–2028. [18] R.M. Bradley, J. Harper, Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 6 (1988) 2390–2395. [19] S. Rusponi, C. Boragno, U. Valbusa, Phys Rev Lett 78 (1997) 2795–2798. [20] S. Rusponi, G. Costantini, C. Boragno, et al., Phys Rev Lett 81 (1998) 4184–4187. [21] B. Jeon, S.K.R.S. Sankaranarayanan, A.C.T. van Duin, et al., Philos Mag 91 (2011) 4073–4088. [22] H. He, T. Czerwiec, C. Dong, et al., 163 (2003) 331 - 338. [23] T. Sasada, Y. Nakakita, M. Takenaka, et al., J Appl Phys 106 (2009) 073716. [24] M. Miyamoto, S. Mikami, H. Nagashima, et al., J Nucl Mater 463 (2015) 333–336. [25] Y. Jia, W. Liu, T.W. Morgan, Atomic Energy Science and Technology 50 (2016) 2027–2033.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Surface nano structures on W surface exposed to low-energy high flux D plasma
T
Y.Z. Jiaa,b, , W. Liub, , B. Xub, S.L. Qub, T.W. Morganc ⁎
⁎
a
Science and Technology on Reactor Fuel and Materials Laboratory, Nuclear Power Institute of China, Chengdu, Sichuan 610213, China Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c FOM Institute DIFFER-Dutch Institute for Fundamental Energy Research, 5612AJ Eindhoven, The Netherlands b
ARTICLE INFO
ABSTRACT
Keywords: Nanostructure Tungsten Deuterium TEM
The surface nano structures on W surface exposed to low-energy D plasma was studied by transmission electron microscopy. It was found that the surface nano structures on W surface induced by D plasma were mainly due to two reasons. First, the subsurface nano-sized bubbles will induce nano scale blistering of the surface, resulting in surface nano morphology. The nano scale blistering behaviour is most obvious on [1 1 1] surface at low temperature (500 K). Secondly, during the plasma exposure, oxide layer on W surface will be sputtered by low energy D particles, and the sputtering can also induce nano structures on some surfaces. At low temperature (500 K), the surface nano structures were formed due to the nano scale blistering behaviour and the sputtering of surface oxide layer. At high temperature (1000 K), the surface nano structures were mainly caused by the sputtering of surface oxide layer.
1. Introduction Tungsten (W) materials, due to its favourable properties, such as high thermal conductivity, high melting temperature, and low sputtering yield, etc. [1,2], will be used as the plasma-facing materials (PFMs) in divertor of ITER. During the operation, W material will be subjected to high flux and high fluence of D ions [3]. The main surface damage caused by D plasma is blister structures on the surface as reported before [4–6]. Besides blisters, surface nano structures on W surface induced by high flux D plasma were first reported by Xu et al. [7]. After that, many researches [8–11] reported similar surface nano structures under similar experimental conditions, and the morphology of nano structures is closely dependent on surface orientation. This kind of nano structures may affect the performance of W PFMs in ITER. Although the surface nano structures were reported and studied intensively, the mechanism for its formation is still not clear. Xu [12] speculated that the surface nano structures were surface reconstruction phenomenon due to D particles bombardment, but this theory lacks direct experimental evidence. The main present problem is that the subsurface structures beneath the surface nano structures were rarely studied before. In this study, the subsurface structures beneath the surface nano structures were studied by focused ion beam (FIB) and transmission
electron microscopy (TEM). The advantage of using FIB to fabricate TEM samples is that the surface morphology and cross-section morphology of the same surface area can be observed by scanning electron microscope (SEM) and TEM, so the subsurface structures beneath the nano structures can be well investigated. 2. Experimental W disc samples (Φ30 mm, 3 mm thick) were cut from a rolled sheet supplied by Advanced Technology & Materials CO., Ltd. (China). The textures of the sample are mainly {1 0 0} 〈1 1 0〉 and 〈1 1 1〉∥ND. The samples were polished by mechanical polishing and electrochemical polishing. After polishing, all the samples were stress relieved at 1273 K at a background pressure of 5 × 10−4 Pa for 1 h before plasma exposure. The samples were exposed to a high flux D plasma beam in the Pilot-PSI linear plasma generator [13], and the main species of the ions were D+ ions. The plasma beam was observed by optical emission spectroscopy, no obvious signal of impurity ions from the source was observed. Electron density and temperature of the plasma beam were determined using Thomson scattering (TS). The peak ion flux was about 1.5 × 1024 m−2 s−1. Because of the heating of the copper coils at high magnetic field, Pilot-PSI operates in a pulsed mode. For all samples we used identical shot durations of 70 s. During such a shot, the peak
⁎ Corresponding authors at: Science and Technology on Reactor Fuel and Materials Laboratory, Nuclear Power Institute of China, Chengdu, Sichuan 610213, China (Y.Z. Jia). E-mail addresses: [email protected] (Y.Z. Jia), [email protected] (W. Liu).
https://doi.org/10.1016/j.nimb.2018.10.022 Received 22 September 2017; Received in revised form 6 August 2018; Accepted 18 October 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
Y.Z. Jia et al.
Fig. 1. Morphology of W sample exposed to D plasma at 500 K. (a), (c), (e) surface morphology of surfaces with different NDs; (b), (d), (f) cross-section morphology of surfaces with different NDs.
fluence was about 8 × 1025 m−2–1.1 × 1026 m−2. In order to reach higher accumulated fluence, the same shot was repeated several times with a time interval between two shots of about 10 min. During the interval between two shots, the plasma source was not shut down, but the plasma species was switched to Ar plasma, and since the magnetic field was switched off between two shots, so basically no Ar ions reached the W surface between two shots. Before the next shot, the plasma species was switched back to D plasma. The total fluence for each sample in this study was about 6 × 1026 m−2 (each sample was exposed to 6 plasma shots). The ion energy was controlled by negatively biasing the sample with voltage about −48 V, so the ion energy of the plasma was ∼40 eV. No obvious mass change was measured before and after the plasma exposure. The surface temperature was controlled by the water cooling system from the back side of the samples and was a balance between the incoming power flux from the plasma and the cooling efficiency. In this study, the exposure temperature was set ∼500 K or ∼1000 K. The peak surface temperature was measured by a spectral pyrometer (FMPI SpectroPyrometer, FAR Associates) during the experiments, while the 2D surface temperature profile was measured by a fast infrared camera (FLIR A645 sc). It took about 10 s for the surface temperature to rise from room temperature to the stable temperature and after the exposure the temperature of the samples was back to room temperature typically within 5 s. After exposure, surface morphology changes were observed using a TESCAN MIRA 3 LMH high-resolution SEM. The orientations of the surface grains have been determined by Oxford instrument
NordlysMax2 electron backscatter diffraction technique (EBSD). Focused ion beam (FIB) was performed to fabricate the cross-section TEM samples using Ga+ ions, so that the cross-section morphology corresponding to the surface morphology can be observed by TEM. TEM observation was carried out at JEOL JEM-2100 TEM to obtain the crosssection morphology of the subsurface region, and EDS element distribution was carried out on JEOL JEM-2010F STEM. 3. Results Fig. 1 shows the surface morphology and cross-section morphology of W surface exposed to D plasma at 500 K. Fig. 1(a), (c) and (e) are the SEM surface morphology of surface with different surface normal directions (NDs), and Fig. 1(b), (d) and (f) are the TEM cross-section morphology of the surface in Fig. 1(a), (c) and (e) (TEM area) respectively. In the TEM images, note that during the FIB milling, Pt was deposited on W surface to protect the surface structure from Ga+ ions bombardment, so the TEM samples are composed of the W (the dark part) and Pt deposition (the bright part). As can be seen from the SEM surface morphology (Fig. 1(a), (c) and (e)), the surface nano structures exhibited different morphologies on different surfaces, which is similar to the results reported in previous literatures [7,11]. On the surface with ND close to [1 1 1], triangular nano structures were formed after the exposure, as shown in Fig. 1(a). Beneath the surface, nano-sized bubbles were observed by TEM, as shown in Fig. 1(b). The bubble size was about 20 nm, and the depth was about 20–30 nm, which is consistent with the results reported before 27
Nuclear Inst. and Methods in Physics Research B 438 (2019) 26–30
Y.Z. Jia et al.
Fig. 2. Morphology and element distribution on [1 1 0] surface exposed to D plasma at 500 K. (a) surface morphology, (b) Pt distribution, (c) W distribution, (d) O distribution.
exposure, as shown in Fig. 1(e), and the lamellar structures were parallel to the [0 0 1] direction and perpendicular to [1 1 0] direction. In Fig. 1(f), the cross-section morphology shows that the surface layer (marked by the red lines) was more obvious than that in Fig. 1(d). The thickness of this layer was about 30 nm. What should be noted is that the cross-section TEM samples were obtained along [1 1 0] direction and perpendicular to the lamellar structures. As can be seen in Fig. 1(f), the fluctuation of the surface layer was corresponding to the surface lamellar structures, indicating that the surface nano structures on some surfaces were directly due to the surface layer of unknown composition. In order to clarify the composition of this surface layer, the element distribution of the surface region was studied by STEM. Fig. 2(a) shows the cross-section morphology of [1 1 0] surface, and (b)–(f) were the Pt, W, O, C and Ga element distribution of the same region in Fig. 2(a). As can be seen from this image, the O element was concentrated in the surface layer between Pt deposition and W. In addition, the W concentration of the surface layer was lower than that of W matrix. Therefore, the composition of this layer was quite likely a W oxide. To further study the composition of this layer, the line scanning of element distribution across this layer was carried out. The line scanning results in Fig. 3 show that the signal of O element increased in the surface layer, and W signal decreased in this layer. On the contrary, on the surface without the obvious surface layer, the signals of W and O did not change a lot. Therefore, the composition of this layer should be a W oxide. There are some kinds of W oxide: WO2, WO3 and WO2.9. Also, previous study showed that D atoms may bond to W oxide [15], but clarifying the exact composition of oxide is beyond the scope of this study, which needs XPS or Raman spectral analyses. Fig. 4 shows the surface morphology and cross-section morphology of W surface exposed to D plasma at 1000 K. The surface morphology was different on surfaces with different NDs, as reported before [11]. The bubbles formed at 1000 K did not induce surface blistering or
Fig. 3. Line distribution of W and O of [1 1 0] surface exposed to D plasma at 500 K.
[14]. The cross-section morphology shows that the bubbles are closely related to the surface fluctuations. Therefore, the nano structures on [1 1 1] surfaces were formed due to the nano scale blistering behaviour of the surface caused by the bubbles beneath the surface, as discussed in our previous article [14]. With NDs away from [1 1 1], nano-sized blisters were also formed on the surface due to the bubbles beneath the surface, as shown in Fig. 1(c) and (d). However, the deformation (or the blistering behaviour) of the surface was slight in Fig. 1(d) compared to that in Fig. 1(b). In addition, on the surface of W sample, a surface layer with different contrast was observed between the deposited Pt layer and the W matrix, as marked by the red lines in Fig. 1(d). On [1 1 0] surface, lamellar nano structures were observed after the 28
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Fig. 4. Morphology of W sample exposed to D plasma at 1000 K. (a), (c), (e) surface morphology of surfaces with different NDs; (b), (d), (f) cross-section morphology of surfaces with different NDs.
related surface nano structures, even on [1 1 1] surface, as shown in Fig. 4(a) and(b). Fig. 4(c) and (e) are the surface nano structures on different surfaces, and Fig. 4(d) and (f) are the cross-section morphology perpendicular to the lamellar structures in Fig. 4(c) and (e) respectively. Similar to the results at 500 K, a surface layer (pointed by the red arrows) was also observed between Pt and W, and this layer is responsible for the surface nano structures in Fig. 4(d) and (f). However, on [1 1 0] surface this layer was not continuous on the surface, and the thickness was only 10 nm, which is thinner than that formed on [1 1 0] surface at 500 K in Fig. 1(f). Fig. 5(a) is the line scanning of W and O element distribution of the subsurface region on [1 1 0] surface along the yellow line. It shows that the substance of the surface layer was also W oxide, which is same as that at 500 K only with smaller thickness. For the surfaces without nano structures, Fig. 5(b) shows that no oxide layer was observed and O element was not concentrated on the surface with ND near [0 0 1]. Therefore, the nano structures formed on the surface at 1000 K were mainly due to the oxide layer on the surface.
The subsurface nano-sized D bubbles may induce nano scale blistering on W surface during the growth of bubbles, as discussed in our previous article [14]. The nano scale blistering mainly induces nano structures at low temperatures (500 K). In addition, the surface with ND close to [1 1 1] was more prone to surface blistering, and when the ND is away from [1 1 1], the nano scale blistering behaviour was less obvious. For the D bubbles formed at 1000 K, the bubbles were smaller and deeper as analysed in previous study [14], so the bubbles were not able to induce obvious blistering on the surface. Therefore, surface nano structures formed at high temperature (1000 K) were not due to nano scale blistering. A more important factor of the nano structure formation is the surface oxide layer. In this study it is found that the oxide layer was formed on the surface of some grains, but not observed on other surfaces (as shown in Fig. 4(b) and Fig. 5(b)). The nano structures were only observed on grain surfaces with oxide layer, but not observed on grain surfaces without oxide layer (except the nano structures induced by nano scale blistering). This means the nano structures must be formed due to the surface oxide layer. In our previous study, the TEM results of pristine W samples before D plasma exposure showed no oxide layer on the surface [14]. Therefore, the oxide layer observed in this study must form during the process in Pilot-PSI. However, the chemical property of D is the same as H, and it is a strong reducing agent, so oxidation can hardly occur on W surface with the presence of D plasma. In Pilot-PSI, the vacuum before plasma exposure was not quite high (about 2 × 10−3 Pa), so it is assumed that the oxide layer may form during the interval between two plasma shots. After the
4. Discussion In previous study [12], the surface nano structures were thought to be the surface reconstruction phenomenon due to D particles bombardment and stress field in surface region. However, this theory lacks direct experimental evidence. In this study, it is found that at different temperatures, the nano structures induced by D plasma were formed due to two reasons: nano scale blistering and surface oxide layer. 29
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orientation as reported in previous papers [21–23], so in the oxidation behaviour is different on W surface with different orientation in this study. The thickness of surface oxide layer was different on surfaces with different NDs, and on some surfaces even no oxide layer was formed, which may be because of the different oxidation, reduction and sputtering rates on different surfaces. Therefore, the surface nano structures induced by D plasma were quite likely caused by the sputtering of the surface oxide layer. Previous research [9] on Pilot-PSI reported that 5 eV D ions cannot induce surface nano structures on W surface, which may be because 5 eV D particles cannot cause sputtering of the surface oxide layer. Similar nano-structures were also observed on W surfaces exposed to He plasma, and the orientation dependence of the nano-structures was similar to that caused by D plasma [24,25], so it may be also caused by sputtering of the surface oxide layer. 5. Conclusion In conclusion, in this study, it was found that the surface nano structures on W surface induced by low energy D plasma were mainly due to two reasons. Firstly, the subsurface nano-sized bubbles will induce nano scale blistering behaviour of the surface, resulting in the ordered nano scale surface morphology. The nano scale blistering behaviour is most obvious on [1 1 1] surface at low temperature (500 K). Secondly, during the plasma exposure procedure, oxidation would occur on W surface, and oxide layer will be sputtered by low energy D particles, resulting in the ordered nano structures on the surface. At low temperature (500 K), the surface nano structures were formed due to the nano scale blistering behaviour and the sputtering of surface oxide layer. At high temperature (1000 K), the nano structures were thought mainly caused by the sputtering of surface oxide layer.
Fig. 5. Line distribution of W and O of surface exposed to D plasma at 1000 K. (a) surface with nano structures, (b) surface without nano structures.
Acknowledgement
plasma exposure, the surface temperature returns to room temperature slowly, and during this cooling down period oxidation likely occur on W surface. In addition, the reduction effect of D plasma is stronger at higher temperature. Therefore, as can be seen from the cross-section morphology of [1 1 0] surface of 500 K (Fig. 1(f)) and 1000 K (Fig. 4(f)), the surface oxide layer formed at 1000 K is not continuous, and also thinner than that formed at 500 K. We exposed the W material to the air at 500 K for 10 min, after which the surface was covered with W oxide. However, no surface nano structures were observed on the surface. Also, no similar surface nano structures were reported in previous oxidation research. Therefore, the oxidation alone cannot induce the surface nano structures observed on W exposed to D plasma. Previous literatures [16,17] show that when oxidation occurs on W surface, the sputtering threshold energy of D ions will decrease. Roth et al. [16] shows that the sputtering threshold by D ions will decrease from 200 eV to ∼18 eV at an oxygen background pressure of 8 × 10−5 Torr, mainly due to oxidation during the sputtering measurement. Therefore, the particle energy used in this study (∼40 eV) can hardly cause sputtering of W surface, but may induce the sputtering of the surface oxide layer. Many previous researches reported that ordered surface nano morphology will form on sputtering surface, and it is also closely dependent on the surface orientation [18–20], which is similar to the orientation dependence of surface nano structures observed on W surface exposed to D plasma [7,11]. The ordered surface structure induced by ion sputtering was attributed to the distribution of energy deposited and the self-diffusion of atoms on the surface [18]. Because the diffusion barrier is different along different directions on different surfaces, the surface structures induced by sputtering are related to the grain orientation [20]. However, in this study, the lattice structure of the surface oxide layer was not studied in detail, so the detailed relationship between the grain orientation and sputtering effect may be studied in future research. In addition, the oxidation or nitridation behaviour on surface is dependent to surface
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