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The effect of surface oxides and grain sizes on the deuterium permeation behavior of niobium membranes
Fusion Engineering and Design 149 (2019) 111340
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
The effect of surface oxides and grain sizes on the deuterium permeation behavior of niobium membranes
T
Yakun Guoa, Xin Zhoua, Bangjun Maa, Dongli Zoub, Qifa Pana, Chuanhui Liangc, Xiaoqiu Yea, ⁎ Li Dengc, Chang’an Chena, a
Science and Technology on Surface Physics and Chemistry Laboratory, Jiangyou 621908, PR China Institute of Materials, China Academy of Engineering Physics, Mianyang 621700, PR China c Chengdu Science and Technology Development Center, Chengdu, 610200, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Niobium Deuterium Permeation Grain size Surface oxides Gas-driven permeation
Group 5 transition metal niobium is considered as the promising hydrogen separation membrane material because of its theoretical highest atomic hydrogen permeability, good mechanical strength, low price and easy shaping. However, the permeability of hydrogen in the dense metal membrane is affected by surface oxides, gain sizes, defects and so on. In this paper, the effect of surface oxides and grain sizes on the permeation behavior of deuterium through niobium membranes is studied. The niobium membranes with micro- and nanocrystalline were first treated by mechanically or electro-polished to remove the surface oxides, and then the deuterium permeability tests were performed in the temperature range of 600–800 ℃ at the driving pressures of 50 kPa by gas-driven permeation method. Results showed that the niobium membrane with nanocrystalline and “clean” surface had the highest deuterium permeation than that with microcrystalline or unclean surface. More importantly, grain size and grain boundary had a greater influence on the deuterium permeation behavior of niobium membranes than the thin surface oxides. These results shed new light on the enhancement of hydrogen and its isotopes permeation in the niobium-based hydrogen separation membranes. Further work is to study the plasma-driven permeation of deuterium through nanocrystalline niobium membrane in order to obtain the super-permeation rate.
1. Introduction Hydrogen separation and purified technology has been widely applied in the production of high purity hydrogen in the hydrogen fuel cell industry or the deuterium and tritium treatment process of the International Thermonuclear Experimental Reactor (ITER) [1,2]. Dense palladium membrane and its alloys are one of the available separation options, due to their extremely high levels of selectivity for hydrogen and its isotopes (ideally 100%) [3–6]; however, these metals are quite expensive and rare [7]. In the last few years, group 5 metals (e.g. Nb, V, Ta) or their alloys have been proposed as alternative hydrogen permeation materials [8–12]. The theoretical hydrogen permeability values of Nb, V and Ta found in the literatures are one or two orders of magnitude higher than that of Pd [13], but their surfaces are easily to be oxidized due to the high reactivity of group 5 metal atoms with oxygen, especially in the humid atmosphere at elevated temperatures, which leads to inactivation for the dissociation of hydrogen molecules into atoms [6,9,14]. This drastically limits their utilization as hydrogen
⁎
separation membranes unless the surface oxides is somehow removed or modified. Therefore, it is necessary to remove the oxides and obtain a relative “clean” surface of niobium membrane. Grain size and grain boundary is another interesting problem, which affects the diffusivity and permeation of hydrogen atom in the metal [15˜18] . For decades, scientists and engineers have paid great attention to studying the effect of grain size and grain boundary on diffusion and trapping of hydrogen in the dense metal, such as nickel [15,17–19], tungsten [20,21], silver [16] and so on. For substitutional diffusion (e.g., self-diffusion), hydrogen migrates faster around grain boundaries than in the bulk, because grain boundaries are with lower activation energy barrier and larger free volumes which can provide open spaces for hydrogen atoms to jump from one site to the other [18,19,22]. Another suggestion from molecular dynamics studies is that the diffusivity of hydrogen at the grain boundaries is lower than the bulk because there is strong attraction between hydrogen atom and grain boundaries, which believes that the role of grain boundaries is to act as trap sites rather than to provide fast diffusion paths of hydrogen atoms
Corresponding author. E-mail address: [email protected] (C. Chen).
https://doi.org/10.1016/j.fusengdes.2019.111340 Received 28 April 2019; Received in revised form 26 August 2019; Accepted 23 September 2019 Available online 22 October 2019 0920-3796/ © 2019 Published by Elsevier B.V.
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Fig. 1. (a) Schematic of deuterium permeation apparatus and (b) VCR fittings. P1: pressure gauge, P2: pressure sensor, SP1: scroll pump, SP2: scroll pump, TP1: turbo molecular pump, P3: pressure gauge, DPF1: pressure control meter, DPF2: pressure control meter, HMSLD: helium mass spectrometer leak detector.
deuterium permeation behavior was discussed.
[23]. There are some opposite opinions presented in the experiment and theoretical calculation. The crystal structure also has some influence on the substitutional diffusion of hydrogen [6]. For example, the polycrystalline material with body center cubic (BCC) crystal structure is more permeable for hydrogen than that with face center cubic (FCC) crystal structure due to more octahedral (O) and tetrahedral (T) interstitial sites available [6]. In addition, the lattice defects (e.g., vacancies, contaminant atoms, or dislocations) also have some impacts on the diffusion and permeation of hydrogen [19]. Despite extensive studies in the past for FCC metals have been reported, many fundamental studies are also required to better understand grain boundary diffusion for BCC metal of niobium. In this paper, the micro- and nanocrystalline niobium membranes were first treated by mechanically and electro-polished in order to remove the surface oxides. Then the deuterium permeation tests were determined in the temperature range of 600–800 ℃ at the driving pressures of 50 kPa by gas-driven permeation method and the apparent permeation activation energies of different niobium membranes were calculated according to Arrhenius equation. The surface morphology and chemical composition of nanocrystalline niobium membranes before and after deuterium permeation was evaluated by SEM and XPS in detail. Finally, the effect of surface oxides and grain sizes on the
2. Experimental 2.1. Materials The microcrystalline niobium membrane with the purity of 99.8 wt % and grain size of 50˜200 μm was supplied by Alfa Aesar (China) Chemical Co. Ltd. The nanocrystalline niobium membrane with purity of 99.95 wt% and grain size of 20˜50 nm was supplied by Baoji Boda Metal materials Co. Ltd. They were cut into disks with the diameter of 11.9 mm and thickness of 500 μm by line cutting machine. Hydrofluoric acid (HF), sulphuric acid (H2SO4), phosphoric acid (H3PO4), nitric acid (HNO3), lactic acid, acetone and absolute ethanol were bought from Sinopharm Chemical Reagant Co., Ltd. 2.2. Surface treatment Both sides of the Nb membranes were abraded with grain silicon carbide abrasive papers (500#, 800#, 1000#, 2000#), polished with diamond paste (2.5 μm and 1 μm) and then ultrasonically degreased with acetone and ethanol in sequence (noted as MP-Nb). After that, the 2
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temperature of the sample was dropped to 600 ℃ and the D2 was continuously delivered to the feed side of the membrane. The leakage rate of deuterium permeation through the sample was recorded by quadrupole mass spectrometer collected at the permeate side until the leakage rate readings became stable. After that the deuterium in the sample was removed again in vacuum at 800 ℃ until the background was below to 5 × 10-12 A. Then the temperature was increased to 650 ℃, and the deuterium permeation was conducted again. According to the above method, the deuterium permeation behavior of Nb membrane at 700 ℃, 750 ℃ and 800 ℃ were also obtained. Laser-induced breakdown spectroscopy (LIBS) was built by our laborary.
Nb membrane was electro-polished in an acid mixture solution of HF: H2SO4: lactic acid (volume ratio of 1:2:4) at 25 ℃ for 15 min under a direct voltage of 17 V to remove the residual oxide layer and washed with deionized water (noted as EP-Nb). 2.3. Membrane characterization The crystal structures of Nb membranes were characterized by X-ray diffraction (XRD, X′Pert PRO) with CuKa radiation (λ = 1.5418 Å). The microstructure of Nb membranes was observed by optical microscope (OM, TH4-200) and transmission electron microscope (TEM, FEI Titan G2 60–300). The diffraction pattern of niobium was determined by selected area electron diffraction (SAED, Titan) affiliated on TEM. The crystal orientation and misorientation grain boundaries of Nb were analyzed by SEM (FEI Helios Nanolab 600i) in electron back-scattered diffraction (EBSD) mode. Before EBSD analysis, the surface of pure niobium was sputtered by Ar+ for 30 min. The surface morphology of Nb membranes with different surface treatments or before and after deuterium permeation tests were also observed by SEM. The element composition analysis was determined by energy dispersive X-ray spectroscopy spectrometer (EDS, Oxford) equipped in the SEM. The surface chemical composition of niobium membranes with different surface treatment and different exposure time, or before and after deuterium permeation were analyzed by X-ray photoelectric spectroscopy (XPS, ESCALAB 250). The depth profile of niobium membranes after mechanically or electro-polished was studied by Auger electron spectroscopy (AES) instrument using the depth analysis function with a base pressure below 1 × 10−6 Pa. The native oxide layer was further removed by Ar+ plasma etching for 30 min at an argon pressure of 9.0 Pa and a negative bias of 470 V. The surface chemical composition of niobium membrane after electro-polished at different temperatures in vacuum (better than 1.0 × 10−7 Pa) was also measured by Time of flight secondary ion mass spectrometry (ToF-SIMS) and the NbO+/Nb value was calculated by the integrated area ratio.
2.5. Permeation theory and calculations Generally, the concentrations of hydrogen isotopes adsorbed and dissolved on the surface of material are represented on the basis of Sieverts' law: (1)
c=S P
where, c represents the surface concentration of hydrogen isotope atoms with 0 and t as the coordinate and time, respectively; P denotes the pressure of gaseous hydrogen isotopes. The permeation of hydrogen through a membrane in one dimension can be expressed in term of permeation flux using the hydrogen concentration gradient with Fick's first law:
J = −D
əC əx
(2)
For the case of steady-state permeation process, taking account of the rate process limited by diffusion in the bulk rather than surface reaction [25], the permeation flux J mentioned above, which is the amount of substance per unit area per unit time, can be expressed by Richardson's law [26], namely:
J=D
S⋅( Pf −
2.4. Permeation experiment
Pp ) (3)
d −2 −1
Where, J is hydrogen permeation flux (mol·m ·s ), d is the thickness of metal membrane (m), D is the diffusion coefficient of the material with the thickness d (m2·s−1), S is the Sieverts' constant (mol·m−3 Pa0.5 ), Pf and Pp are the hydrogen partial pressures (Pa) in the feed and permeate sides of the membrane, respectively. If the Pf > > Pp≈0, the Pp will be omitted as usual. Denoted the hydrogen permeability Φ= D×S (mol·m−2·s−1 Pa−0.5), the Eq. (3) can be written as:
The permeation of deuterium through Nb membranes at elevated temperatures were conducted on the gas evolution permeation equipment (Fig. 1a) [24]. The prepared Nb membranes were first sealed in Swagelok VCR fittings (Fig. 1b, 1/4 in., the inside and outside diameters of the shim is 5.6 mm and 11.9 mm) with an exposing area of 0.24 cm2. The base pressure of the high pressure region (feed side) was fixed to 50 kPa and that of the low pressure region was below 10−5 Pa in order to obtain the optimal signals by quadrupole mass spectrometry (PFEIFFER, QME220, 1–200 amu) in the experimental temperature range of 600˜800 ℃. Before permeation experiment, the quadrupole mass spectrometer was calibrated by standard leakage hole (TDL Z08-8B-12, leakage rate = 2.3 × 10−6 Pa·m3·s−1, He, 23 ℃) and helium mass spectrometer leak detector (INFICON UL1000Fab) in order to obtain the linear relationship between ion current intensity and leakage rate, which was related with the filament state of quadrupole mass spectrometry. After that, the sample was fixed onto the permeation system according to Fig. 1 and the gas leakage test was done by delivering pure He to the feed side of the sample at room temperature in order to detect whether the permeation system (pipelines) was sealed well. Next, the pipelines of the feed side were flushed with deuterium gas (D2) with an isotopic abundance of 99.7% within 2 or 3 s for 3 times. Then the permeation system was heated slowly to 100 ℃ and kept for 5 h under vacuum condition (better than 10−4 Pa) and then heated continuously to 800 ℃ and kept for another 2 h in order to remove the thin oxide layer and the residual hydrogen isotope gas that may remain on the surface of niobium membrane. Finally, the temperature was reduced to the experimental temperature and the pressure of the gas pressure controller (ALICAT, 0–1 bar, absolute pressure) was set at 50 kPa, and the deuterium permeation test was performed. Typically, the
Φ=
J∙d Pf
(4)
It is obvious that a decrease in the metal thickness provokes an increase of the permeation capability. According to the ideal gas law P·V = n·R·T, the leakage of a closed vessel can be expressed by L = V·dp/ dt = d(PV)/dt = RT·dn/dt. The diffusion flux in/out of the vessel wall can be expressed by J = 1/A·dn/dt. Thus, we can obtain the following expression: (5)
J = L/ RTA 3 −1
Where, L is the leak rate (Pa·m ·s ), calculated by the relationship of leakage and ion current, R is universal gas constant (R = 8.314 J·mol−1 K−1), and T is the absolute temperature of sample (K), A is the exposing area of sample (m2). The apparent diffusion coefficient (D) can be estimated by time-lag technique [27] which can be described as:
D=
d2 6τ
(6)
where d is the thickness of the samples and τ is the time corresponding to the point on the current-time curve when the permeation flux matches 63% of steady-state current. 3
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Fig. 2. (a, d) OM, (b, e) TEM images and (c, f) diffraction pattern of microcrystalline and nanocrystalline niobium membranes.
According to Φ= D×S (mol·m−2·s−1 Pa−0.5), the solubility (S) could be calculated by S=Φ/D
mechanically polished and then by electro-polished in order to remove the tightly held oxide. Fig. 4 and 5 give the surface morphology and chemical composition of niobium membrane (microcrystalline) after different surface treatments. The original surface is rough (Fig. 4a) and covered with a layer of oxides with the thickness of about 5˜15 μm (Fig. 4b). After mechanically polished, the surface becomes smooth (Fig. 4c), but still some O, C and Si elements are observed by EDS which may be induced by the residual oxide, ethanol or the silica polishing agent (Fig. S2). The residual oxides are mainly niobium pentoxide identified by the occurrence of two photoelectron peaks with the binding energies of about 207.4 eV and 210.2 eV corresponding to the Nb3d5/2 and Nb3d3/2 for Nb atoms in Nb5+ state, respectively (Fig. 5a) [30]. The thickness is about 20 nm estimated by XPS and AES in depth profile analysis and the content of oxide is dropped down quickly with the increase of Ar+ sputtering time (Fig. 6a and Fig.S3). Finally, pure Nb atoms are exposed identified by the shift of Nb3d5/2 and Nb3d3/2 peaks from 210.2 eV and 207.4 eV for Nb atoms in Nb5+ state to 205.2 eV and 202.4 eV for Nb atoms in Nb0 state. Ion sputtering is a good method to obtain a “clean” surface, but it is uneconomical and not suitable for large-scale industrial applications. In order to remove the oxide layer by a relative simple method, electro-polishing treatment was implemented. The mechanically polished niobium membrane was dipped into acid mixture solution of HF, H2SO4, and lactic acid at room temperature in a controlled voltage of 17 V for about 15 min and some of thin oxides and possible part of the bulk material were dissolved. After that, the pure niobium and Nb2O5 with a chemical composition of about 1:1 calculated by the integrated area ratio are coexisted on the rough surface of niobium membrane (Figs. 4d and 5b), showing that the thickness of oxide is very thin, about 2˜3 nm (several atomic layers, Fig. 6b). It has been reported that under steady-state conditions, clean niobium surfaces existed only at temperatures above 1700 K in vacuum better than 10−10 mbar, and oxidation was inevitable for Nb handled at 298 K in air [31], especially in the wet oxide environment. Usually, the O element would be adsorbed, hopped on the Nb surface and formed into NbOx (x˜1) interface layer firstly and then Nb2O5 amorphous dielectric oxide layer quickly due to the large binding energy of O and Nb [30]. So it is reasonable to deduce that most of Nb2O5 on the surface of electro-polished niobium membrane may be originated from the reoxidation of exposed fresh niobium atoms with oxygen at room temperature in the experimental process since a period of time (about
(7)
3. Results and discussion 3.1. Characterization of Nb membrane Hydrogen trapping and diffusivity in materials is very important for hydrogen permeability, which may be affected by crystal structure, grain boundaries, defects and so on [15–22,28,29]. The niobium membranes (1# and 2#) are pure niobium with BCC structure and no obvious preferred orientation [Im-3 m (229)] identified by X-ray diffraction (Fig. S1), which allows for greater hydrogen solubility and more rapid diffusion [6]. The microscopic structure of them were characterized by OM, TEM and SAED (Fig. 2). The grain size of 1# Nb membrane is about 50˜200 μm (Fig. 2a and b) and there is no obvious participates or defects included in it. The body parameter is about 3.307 Å calculated from the clear diffraction pattern (Fig. 2c). In contrast, only diffraction rings are observed for 2# Nb membrane due to the small-size effect, indicating it is a nanopolycrystalline material (Fig. 2f). The grain size of 2# Nb estimated from the magnified TEM image (Fig. 2e) is about 20˜50 nm, which is far smaller than that of 1# Nb. In order to distinguish them, the 1# and 2# niobium membranes are named as microcrystalline and nanocrystalline Nb, respectively. The metal with smaller grain size and more grain boundaries would contribute for hydrogen trapping and permeation [6]. Fig. 3 presents the reconstructed microstructures of the EBSD analysis performed on the surface of pure niobium after Ar+ sputtering for 30 min. The grain sizes and their misorientation of microcrystalline and nanocrystalline niobium have been distinctly given in Fig. 3a˜h. The misorientation profile along the black line are shown in Fig. 3i and j. The results indicate that some subgrains with the boundary angle below 5° are present in the big grain, which reveals the high dislocations density or stored energy of them. 3.2. Surface treatment of niobium membranes The surface of niobium membranes was treated first by 4
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Fig. 3. (a∼h) EBSD images, (i) crystal orientations and (j) misorientation angle distribution of microcrystalline and nanocrystalline niobium membranes.
been reported that Nb2O5 has two isomers, α-Nb2O5 and β-Nb2O5 [32]. α-Nb2O5 is thermodynamically unstable and can be reduced under vacuum heating while β-Nb2O5 is stable and cannot be easily reduced under same conditions [33]. Also, α-Nb2O5 could transfer rapidly into
30 min) is needed to transfer the fresh niobium membrane after electrolysis to the chamber of XPS. From the above analysis, the reformation of Nb2O5 compound on the surface of “clean” niobium at room temperature is inevitable. It has 5
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Fig. 4. The surface or interfacial morphology of (a, b) original, (c) mechanically polished and (d) electro-polished microcrystalline niobium membranes.
β-Nb2O5 in air as long as the thermodynamic conditions are satisfied such as the temperature is higher than 800 ℃, but the transition rate is very low [32]. Therefore, it is necessary to monitor the change of exposed fresh niobium at room temperature in order to study its keeping “clean” ability because it is closely related with the deuterium permeation rate. The microcrystalline niobium membrane after electropolished was exposed to air again at room temperature for different time, and the chemical composition of its surface was evaluated by XPS (Fig. 7). The characteristic peak intensity of Nb0 decreases greatly for the first 24 h while obvious increase of Nb5+ observed, showing that the Nb2O5 has been formed quickly in air at room temperature, which is similar with the results of Pasternak [34]. After that, the changes of Nb0 and Nb5+ slow down, indicating that the oxidation of niobium at room temperature in air is a kinetic-control process [35]. In the oxidation process of niobium, the adsorbed oxygen on the surface of Nb first becomes into oxygen ions and migrates from outside to inside. When meeting with Nb ions, they would combine with Nb ions quickly to form Nb2O5 in a short time due to the low diffusion restriction and the low Gibbs free energy (Table. S1) [36]. With the increase of exposure time, the oxides increases gradually and the diffusion of oxygen ions is
Fig. 5. XPS spectra of microcrystalline niobium membranes after (a) mechanically polished, (b) electro-polished and (c) Ar+ sputtering.
Fig. 6. Nb and O atomic concentration curves of (a) mechanically polished and (b) electro-polished niobium membranes with sputter time determined by AES. 6
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Fig. 7. (a) XPS spectra and (b) the integrated area ratio between Nb0 and Nb5+ of niobium membrane after electro-polished exposed in air for different exposed time.
Fig. 8. (a) SIMS curves and (b) NbO+/Nb ratio of electro-polished niobium membrane at different annealing temperatures.
polished niobium membrane. Therefore, the samples were heated to 800 ℃ and kept for 2 h under vacuum condition (better than 5 × 10-5 Pa) in deuterium permeation experiment in order to remove the reformed α-Nb2O5 covered on the surface of treated niobium. The nanocrystalline niobium presents the similar behavior with the microcrystalline niobium (e.g. similar XPS curves for mechanically polished and electro-polished niobium, shown in Figs. 5a, 7a and 11 in the back of the paper), so it is reasonable to assume that the surface treatment is also effective for the nanocrystalline niobium membrane.
restricted, which leads to the formation of some low valence oxides (e.g. NbOx, x = 0.02˜1). Compared the XPS curves of electro-polished niobium membrane exposed from 24 h to 912 h with that of the mechanically polished niobium (the top-end green dotted line in Fig. 7a), it is difficult to find that the characteristic peaks of Nb exist always and remain unchanged which has an obvious difference with that of the mechanically polished niobium, showing that the reformed Nb2O5 on the surface of fresh niobium membrane at room temperature is different from the bulk Nb2O5 covered on the original surface of niobium. They are lack of oxygen. Based on this, we deduce that the reformed Nb2O5 may be α-Nb2O5 and it cannot transfer into the β-Nb2O5 (bulk Nb2O5) easily at room temperature in air. In order to further identify it, high temperature treated experiment was carried out in vacuum. The electro-polished niobium membrane was first stored in air at room temperature for 30 days and then annealed from 25 ℃ to 800 ℃ in vacuum better than 1.0 × 10−7 Pa, the chemical composition was monitored by SIMS and the ratio of NbO+/Nb is given in Fig. 8. The NbO+/Nb ratio of the original exposed niobium membrane (the first point in Fig. 8b) is close to 1, indicating that there is a thin oxide layer existed on the surface of electro-polished niobium membrane, which is consistent with that of XPS (Fig. 7). As the temperature is risen, the NbO+/Nb ratio decreases gradually, and a minimum value of about 0.03 is obtained at 800 ℃, which shows that the reformed Nb2O5 on the surface of electro-polished niobium membrane is the thermodynamically unstable α-Nb2O5 and can be removed by high temperature annealing in vacuum. The higher the temperature, the lower the amount of niobium oxides present. This means that the reformed αNb2O5 has less impact on deuterium permeation behavior of electro-
3.3. Deuterium permeation behavior of niobium membranes The deuterium permeation behavior of niobium membranes with micro- and nanocrystalline structures after mechanically polished and electro-polished (sample A, B, C and D) were determined by gas permeation experiment and a steady increase of permeation flux of deuterium was detected in the measured temperature range (Fig.S4). The permeability (Φ), diffusion coefficient (D) and solubility (S) are calculated according to Eqs. ((4)–(7)) and the curves of Φ, D and S versus inverse temperature are given in Fig. 9. The permeability of all samples increases with the measured temperature, which are contrary to the theoretical prediction for pure Nb but in agreement with previous experimental findings [8,37,38]. These observations here indicate that the permeate rate is not just controlled by bulk diffusion, but also affected by surface process [39]. The maximum permeability of deuterium measured for microcrystalline and nanocrystalline Nb membranes after mechanically polished or electro-polished are ΦA,max = 7.91 × 10−11 mol·m−1·s−1 Pa−0.5, ΦB,max = 3.91 × 10−10 mol·m−1·s−1 Pa−0.5, 7
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Fig. 9. (a) permeability, (b) diffusion coefficients and (c) solubility constants of deuterium in niobium membranes with different grain sizes.
ΦC,max = 4.63 × 10−9 mol·m−1·s−1 Pa−0.5 and ΦD,max = 1.33 × 10−8 mol·m−1·s−1 Pa−0.5 at 800 ℃, respectively, which are lower than the theoretical hydrogen permeation value of niobium (500 ℃, 1.6 × 10−6 mol·m−1·s−1 Pa−0.5) or that of pure palladium membrane (500 ℃, 1.9 × 10−8 mol·m−1·s−1 Pa−0.5) [6], indicating that the readily formed oxide layers, even the very thin oxide layers, have severely hindered hydrogen dissociation and solubility, which causes the Nb metal to exhibit lower hydrogen permeability than Pd [13]. The niobium membranes after electro-polished have a relative higher deuterium permeation than that after mechanically polished (ΦB > ΦA, ΦD > ΦC), illustrating that the thickness and type of oxide layer also have a great influence on the deuterium permeation. The niobium membrane with a thinner α-Nb2O5 layer or no oxide layer is conducive to deuterium permeation [39]. Compared the niobium membranes with different grain sizes, it can be found that the Φ of nanocrystalline Nb is nearly two orders of magnitude higher than that of microcrystalline Nb (ΦD > ΦB, ΦC > ΦA). This is because in the same permeable area, the smaller the grain size, the more the grain boundary, that is, the more gas diffusion channels and the faster deuterium diffusion rate, so the nanocrystalline niobium has a higher deuterium permeation rate. Moreover, the grain size has a more influence on the deuterium permeation of niobium membranes than the oxide layer (ΦD > ΦC > ΦB > ΦA). The fitted Arrhenius relations between temperature T and deuterium permeability of microcrystalline and nanocrystalline niobium membranes after mechanically polished and electro-polished are ΦA (mol·m−1·s−1 Pa-0.5) = 1.06 × 10-7exp [-64.49(kJ·mol−1)/RT], ΦB (mol·m−1·s−1 Pa-0.5) = 3.90 × 10-6exp [-82.79 (kJ·mol−1)/RT], ΦC (mol·m−1·s−1 Pa-0.5) = 7.66 × 10-3exp [-127.75 (kJ·mol−1)/RT] and ΦD (mol·m−1·s−1 Pa-0.5) = 9.27 × 103 exp[-119.68 (kJ·mol−1)/RT]. In order to find out the essential reason for hindering the deuterium permeation, diffusion coefficient (D) and solubility (S) are given (Fig. 9b and c). The τ in the Eq. (6) is the time corresponding to the point on the current-time curve when the permeation flux matches 63% of steady-state current (Fig.S5). The diffusion coefficients of all the niobium membranes are in accordance with the Sieverts equation at 600 to 800 ℃ and increase with the
temperature (Fig. 9b). There are only some small differences between microcrystalline and nanocrystalline niobium membranes although the diffusion coefficient of nanocrystalline niobium is some higher than that of microcrystalline niobium, indicating that the diffusion of deuterium atom in the niobium matrix has less impact on the deuterium permeation. According to Φ=D×S, the solubility can be calculated. The solubility constants of them also increase with the temperature, but the S of nanocrystalline niobium are several times higher than that of microcrystalline niobium, which makes the permeability of nanocrystalline niobium about two orders of magnitude higher than that of microcrystalline niobium (Fig. 9c). These results are consistent with the previous findings that hydrogen migrates faster around grain boundaries than in the bulk due to low activation energy barrier and large free spaces for hydrogen atoms to jump from one site to the other [18,19,22]. The large number of grain boundaries presented in the nanocrystalline niobium and the high permeation temperature promote the rapid dissolution and migration of deuterium atoms along grain boundaries, which greatly increases the permeability rate of deuterium. This is also the reason reported for the super-permeation phenomenon occurred in the plasma-driven permeation [39]. From the above results, it can be concluded that the fresh niobium membranes with smaller grain and more grain boundaries can dissolve more deuterium atoms and is more helpful to transportation of deuterium. 3.4. The surface morphology and chemical composition of niobium membranes after deuterium permeation Fig. 10 presents the SEM images of permeate sides of mechanicallypolished and electro-polished nanocrystalline niobium membranes (named as MP-Nb and EP-Nb) before and after the deuterium permeation tests. The surface of MP-Nb membrane is smooth before deuterium permeation (Fig. 10a), while some nano-humps are observed after deuterium permeation experiment (Fig. 10b). The morphology differences of MP-Nb membrane before and after deuterium permeation may be originated from the surface oxide layer and the deformation of the Nb substrate in the deuterium permeation process [40]. As is known, 8
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Fig. 10. The surface morphology of (a, b) mechanically polished and (c, d) electro-polished nanocrystalline niobium membranes before and after deuterium permeation.
(Fig. 10b). For the EP-Nb membrane, most of the bulk β-Nb2O5 compounds have been dissolved in the electrolysis process and the detected Nb2O5 compounds would be reformed α-Nb2O5 by fresh Nb and oxygen at room temperature in air. These niobium oxides are very thin and can be been decomposed in the deuterium permeation process at high temperatures identified by high temperature treated experiment (Fig. 8). Also, Scientific researches have shown that the reformed thin α-Nb2O5 layer can be regarded as two-dimensional oxide film, which has higher specific surface energies, more surface structural defects (e.g. disorder) and chemical defects (e.g. O voids) than that of bulk oxide
most of the surface oxides on the MP-Nb membrane is bulk Nb2O5 (βNb2O5), which are thermodynamically stable (ΔHp=-1898 kJ/mol) and cannot be reduced or decomposed easily by heating in the vacuum condition [31–33]. Also, deuterium absorption in the Nb membrane would result in Nb-lattice expansion but not induce the size change of βNb2O5, and thus, in an increase of the misfit between the D/Nb film and the β-Nb2O5 oxides [40]. Therefore, some of the β-Nb2O5 covered on the surface of Nb substrate would loose and take off under the loading of deuterium, especially in the position of lattice grain boundaries, which leads to the formation of nano-humps on the surface of MP-Nb
Fig. 11. Nb3d characteristic peaks of (a) mechanically polished and (b) electro-polished nanocrystalline niobium membranes before and after deuterium permeation. 9
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Fig. 12. The permeability mechanism of deuterium in micro- and nanocrystalline niobium membranes after mechanically polished (MP-Nb) or electro-polished (EPNb).
within the detecting range of XPS is observed on the surface of niobium substrate before deuterium permeation (Fig. 11b), it could easily be removed by Ar+ sputtering or reduced in the high temperature deuterium permeation process, which is identified by the occurrence of strong Nb2+ characteristic peak. The NbO occurred for MP-Nb and EPNb after deuterium permeation may be formed by reaction of Nb in the subsurface with the adjacent Nb2O5 or the decomposed oxygen permeated into the subsurface under the pressure-driven of deuterium [41]. The above results demonstrate that the type of oxides and its thickness has a significant influence on the deuterium permeation property of niobium membrane again. From the above discussion, the effect of surface oxides and grain boundaries on the permeability behavior of deuterium in micro- and nanocrystalline niobium membranes can be described as follows (Fig. 12). The permeation of deuterium in niobium membranes is controlled not only by the surface catalytic activity of niobium atom but also by the diffusion rate of deuterium atom. When some oxides were covered on the surface of niobium, the tightly held oxide layers would make some obstruction for the dissociation of deuterium molecules into deuterium atoms and the subsequent dissolution and absorption of deuterium atoms within the niobium substrate, which makes the permeability of deuterium in the mechanically polished niobium membrane slower than that after electro-polished. Second, when the deuterium atoms entered into the niobium substrate, substitutional diffusion and grain boundaries diffusion are the main modes for diffusion of deuterium atom. Since the grain boundaries are the fast channels for deuterium atom, the niobium substrate with smaller grain sizes and more grain boundaries would have a higher permeation rate of deuterium. So the deuterium permeation rate of nanocrystalline niobium is higher.
materials, which would weaken the bond force between the oxygen atom and the metal and reduce the thermodynamic stability [31,33]. In addition, the Nb substrate or its suboxides is favorable for the α-Nb2O5 decomposition or reduction [31]. So there are no nano-humps observed on the surface of EP-Nb membrane (Fig. 10d). The microstructure rearrangement was also observed (Fig. 10d), which may be ascribed to deuterium-induced or oxide-induced topography changes of the Nb membrane or the locally grown of Nb deuteride grains [40]. The occurrence of nano-humps for MP-Nb membrane while not for EP-Nb membrane further identifies that the type of oxides and its thickness has a significant influence on the deuterium permeation property of niobium membrane. The surface chemical composition of mechanically-polished and electro-polished nanocrystalline niobium membranes before and after deuterium permeation test were further characterized by XPS (Fig. 11). Nb2O5 compounds are observed both on the surface of MP-Nb membranes before and after deuterium permeation due to the occurrence of Nb3d photoelectron peaks in Nb5+ state (Fig. 11a), but there are some differences in the binding energies after Ar+ sputtering. The peaks of Nb3d5/2 and Nb3d3/2 are shifted from 210.2 eV and 207.4 eV (Nb atoms in Nb5+ state) to 205.2 eV and 202.4 eV (Nb atoms in Nb0 state) for the MP-Nb membrane before deuterium permeation, while three photoelectron peaks with the binding energies of 210.6 eV, 207.8 eV and 204.6 eV are occurred after deuterium permeation. Generally, the binding energy of Nb3d increases about 1 eV while increasing one valance from Nb0 to Nb5+, and the oxides of niobium can be identified by different valence states according to chemical shifts [31]. So the peak with binding energies of 204.6 eV is the characteristic peak of Nb3d3/2 for Nb atom in Nb2+ state while the other (Nb3d5/2) is coincided with that of Nb3d3/2 for Nb2O5, as can be seen an intensive and wide characteristic peak for Nb3d in Fig. 11a. The surface of niobium membrane after deuterium permeation was also evaluated by Laserinduced breakdown spectroscopy (LIBS), and there was no deuterium signal detected, indicating that little deuterium were resided in the niobium membranes. Therefore, it can be deduced that the subsurface of the mechanically-polished niobium substrate after deuterium permeation is mainly composed of NbO (because for NbH2, ΔH=-60 kJ/ mol which can be decomposed at 1023 K [6]) and β-Nb2O5 compounds. For the EP-Nb membrane, although a thin layer α-Nb2O5 compound
4. Conclusions Two kinds of polycrystalline niobium membranes with micro- and nanocrystalline structures were treated by mechanically polished, electro-polished, ultra-high vacuum high temperature annealing or/and argon ion sputtering respectively, and the results showed that a relatively "clean" niobium surface could be obtained by mechanical polishing combined with electrolytic polishing and ultra-high vacuum high 10
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temperature annealing. The niobium surface has the ability to keep in a relatively "clean" state at room temperature in air. The surface oxides and grain sizes had a great impact on the permeation behavior of deuterium through niobium membranes, especially the grain sizes and grain boundaries. A niobium membrane with small grain size and “clean” surface had the highest deuterium permeation and solubility than that with big grain size or unclean surface.
[12]
[13]
[14]
Declaration of Competing Interest
[15]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[16]
Acknowledgments
[18]
[17]
[19]
This work is financially supported by the National Magnetic Confined Fusion Energy Development Project (Grant No. 2015GB109004), National Natural Science Foundation of China (Grant No.11775194, 21805252, 21908209), Discipline Development Foundation of Science and Technology on Surface Physics and Chemistry Laboratory (Grant No. XKFZ201710) and the Authorized Exploration Program (TCSQ2018109). The authors would like to thank Mr. Rongguang Zeng for TEM observation, Mr. Dongli Zou and Mrs. Chunli Jiang for SEM and EBSD characterization, Mr. Qifa Pan and Chuanhui Liang for XPS measurement, and Mr. Li Deng for deuterium permeation experiment.
[20] [21]
[22] [23]
[24]
[25]
Appendix A. Supplementary data
[26]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.fusengdes.2019. 111340.
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The effect of surface oxides and grain sizes on the deuterium permeation behavior of niobium membranes
T
Yakun Guoa, Xin Zhoua, Bangjun Maa, Dongli Zoub, Qifa Pana, Chuanhui Liangc, Xiaoqiu Yea, ⁎ Li Dengc, Chang’an Chena, a
Science and Technology on Surface Physics and Chemistry Laboratory, Jiangyou 621908, PR China Institute of Materials, China Academy of Engineering Physics, Mianyang 621700, PR China c Chengdu Science and Technology Development Center, Chengdu, 610200, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Niobium Deuterium Permeation Grain size Surface oxides Gas-driven permeation
Group 5 transition metal niobium is considered as the promising hydrogen separation membrane material because of its theoretical highest atomic hydrogen permeability, good mechanical strength, low price and easy shaping. However, the permeability of hydrogen in the dense metal membrane is affected by surface oxides, gain sizes, defects and so on. In this paper, the effect of surface oxides and grain sizes on the permeation behavior of deuterium through niobium membranes is studied. The niobium membranes with micro- and nanocrystalline were first treated by mechanically or electro-polished to remove the surface oxides, and then the deuterium permeability tests were performed in the temperature range of 600–800 ℃ at the driving pressures of 50 kPa by gas-driven permeation method. Results showed that the niobium membrane with nanocrystalline and “clean” surface had the highest deuterium permeation than that with microcrystalline or unclean surface. More importantly, grain size and grain boundary had a greater influence on the deuterium permeation behavior of niobium membranes than the thin surface oxides. These results shed new light on the enhancement of hydrogen and its isotopes permeation in the niobium-based hydrogen separation membranes. Further work is to study the plasma-driven permeation of deuterium through nanocrystalline niobium membrane in order to obtain the super-permeation rate.
1. Introduction Hydrogen separation and purified technology has been widely applied in the production of high purity hydrogen in the hydrogen fuel cell industry or the deuterium and tritium treatment process of the International Thermonuclear Experimental Reactor (ITER) [1,2]. Dense palladium membrane and its alloys are one of the available separation options, due to their extremely high levels of selectivity for hydrogen and its isotopes (ideally 100%) [3–6]; however, these metals are quite expensive and rare [7]. In the last few years, group 5 metals (e.g. Nb, V, Ta) or their alloys have been proposed as alternative hydrogen permeation materials [8–12]. The theoretical hydrogen permeability values of Nb, V and Ta found in the literatures are one or two orders of magnitude higher than that of Pd [13], but their surfaces are easily to be oxidized due to the high reactivity of group 5 metal atoms with oxygen, especially in the humid atmosphere at elevated temperatures, which leads to inactivation for the dissociation of hydrogen molecules into atoms [6,9,14]. This drastically limits their utilization as hydrogen
⁎
separation membranes unless the surface oxides is somehow removed or modified. Therefore, it is necessary to remove the oxides and obtain a relative “clean” surface of niobium membrane. Grain size and grain boundary is another interesting problem, which affects the diffusivity and permeation of hydrogen atom in the metal [15˜18] . For decades, scientists and engineers have paid great attention to studying the effect of grain size and grain boundary on diffusion and trapping of hydrogen in the dense metal, such as nickel [15,17–19], tungsten [20,21], silver [16] and so on. For substitutional diffusion (e.g., self-diffusion), hydrogen migrates faster around grain boundaries than in the bulk, because grain boundaries are with lower activation energy barrier and larger free volumes which can provide open spaces for hydrogen atoms to jump from one site to the other [18,19,22]. Another suggestion from molecular dynamics studies is that the diffusivity of hydrogen at the grain boundaries is lower than the bulk because there is strong attraction between hydrogen atom and grain boundaries, which believes that the role of grain boundaries is to act as trap sites rather than to provide fast diffusion paths of hydrogen atoms
Corresponding author. E-mail address: [email protected] (C. Chen).
https://doi.org/10.1016/j.fusengdes.2019.111340 Received 28 April 2019; Received in revised form 26 August 2019; Accepted 23 September 2019 Available online 22 October 2019 0920-3796/ © 2019 Published by Elsevier B.V.
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Fig. 1. (a) Schematic of deuterium permeation apparatus and (b) VCR fittings. P1: pressure gauge, P2: pressure sensor, SP1: scroll pump, SP2: scroll pump, TP1: turbo molecular pump, P3: pressure gauge, DPF1: pressure control meter, DPF2: pressure control meter, HMSLD: helium mass spectrometer leak detector.
deuterium permeation behavior was discussed.
[23]. There are some opposite opinions presented in the experiment and theoretical calculation. The crystal structure also has some influence on the substitutional diffusion of hydrogen [6]. For example, the polycrystalline material with body center cubic (BCC) crystal structure is more permeable for hydrogen than that with face center cubic (FCC) crystal structure due to more octahedral (O) and tetrahedral (T) interstitial sites available [6]. In addition, the lattice defects (e.g., vacancies, contaminant atoms, or dislocations) also have some impacts on the diffusion and permeation of hydrogen [19]. Despite extensive studies in the past for FCC metals have been reported, many fundamental studies are also required to better understand grain boundary diffusion for BCC metal of niobium. In this paper, the micro- and nanocrystalline niobium membranes were first treated by mechanically and electro-polished in order to remove the surface oxides. Then the deuterium permeation tests were determined in the temperature range of 600–800 ℃ at the driving pressures of 50 kPa by gas-driven permeation method and the apparent permeation activation energies of different niobium membranes were calculated according to Arrhenius equation. The surface morphology and chemical composition of nanocrystalline niobium membranes before and after deuterium permeation was evaluated by SEM and XPS in detail. Finally, the effect of surface oxides and grain sizes on the
2. Experimental 2.1. Materials The microcrystalline niobium membrane with the purity of 99.8 wt % and grain size of 50˜200 μm was supplied by Alfa Aesar (China) Chemical Co. Ltd. The nanocrystalline niobium membrane with purity of 99.95 wt% and grain size of 20˜50 nm was supplied by Baoji Boda Metal materials Co. Ltd. They were cut into disks with the diameter of 11.9 mm and thickness of 500 μm by line cutting machine. Hydrofluoric acid (HF), sulphuric acid (H2SO4), phosphoric acid (H3PO4), nitric acid (HNO3), lactic acid, acetone and absolute ethanol were bought from Sinopharm Chemical Reagant Co., Ltd. 2.2. Surface treatment Both sides of the Nb membranes were abraded with grain silicon carbide abrasive papers (500#, 800#, 1000#, 2000#), polished with diamond paste (2.5 μm and 1 μm) and then ultrasonically degreased with acetone and ethanol in sequence (noted as MP-Nb). After that, the 2
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temperature of the sample was dropped to 600 ℃ and the D2 was continuously delivered to the feed side of the membrane. The leakage rate of deuterium permeation through the sample was recorded by quadrupole mass spectrometer collected at the permeate side until the leakage rate readings became stable. After that the deuterium in the sample was removed again in vacuum at 800 ℃ until the background was below to 5 × 10-12 A. Then the temperature was increased to 650 ℃, and the deuterium permeation was conducted again. According to the above method, the deuterium permeation behavior of Nb membrane at 700 ℃, 750 ℃ and 800 ℃ were also obtained. Laser-induced breakdown spectroscopy (LIBS) was built by our laborary.
Nb membrane was electro-polished in an acid mixture solution of HF: H2SO4: lactic acid (volume ratio of 1:2:4) at 25 ℃ for 15 min under a direct voltage of 17 V to remove the residual oxide layer and washed with deionized water (noted as EP-Nb). 2.3. Membrane characterization The crystal structures of Nb membranes were characterized by X-ray diffraction (XRD, X′Pert PRO) with CuKa radiation (λ = 1.5418 Å). The microstructure of Nb membranes was observed by optical microscope (OM, TH4-200) and transmission electron microscope (TEM, FEI Titan G2 60–300). The diffraction pattern of niobium was determined by selected area electron diffraction (SAED, Titan) affiliated on TEM. The crystal orientation and misorientation grain boundaries of Nb were analyzed by SEM (FEI Helios Nanolab 600i) in electron back-scattered diffraction (EBSD) mode. Before EBSD analysis, the surface of pure niobium was sputtered by Ar+ for 30 min. The surface morphology of Nb membranes with different surface treatments or before and after deuterium permeation tests were also observed by SEM. The element composition analysis was determined by energy dispersive X-ray spectroscopy spectrometer (EDS, Oxford) equipped in the SEM. The surface chemical composition of niobium membranes with different surface treatment and different exposure time, or before and after deuterium permeation were analyzed by X-ray photoelectric spectroscopy (XPS, ESCALAB 250). The depth profile of niobium membranes after mechanically or electro-polished was studied by Auger electron spectroscopy (AES) instrument using the depth analysis function with a base pressure below 1 × 10−6 Pa. The native oxide layer was further removed by Ar+ plasma etching for 30 min at an argon pressure of 9.0 Pa and a negative bias of 470 V. The surface chemical composition of niobium membrane after electro-polished at different temperatures in vacuum (better than 1.0 × 10−7 Pa) was also measured by Time of flight secondary ion mass spectrometry (ToF-SIMS) and the NbO+/Nb value was calculated by the integrated area ratio.
2.5. Permeation theory and calculations Generally, the concentrations of hydrogen isotopes adsorbed and dissolved on the surface of material are represented on the basis of Sieverts' law: (1)
c=S P
where, c represents the surface concentration of hydrogen isotope atoms with 0 and t as the coordinate and time, respectively; P denotes the pressure of gaseous hydrogen isotopes. The permeation of hydrogen through a membrane in one dimension can be expressed in term of permeation flux using the hydrogen concentration gradient with Fick's first law:
J = −D
əC əx
(2)
For the case of steady-state permeation process, taking account of the rate process limited by diffusion in the bulk rather than surface reaction [25], the permeation flux J mentioned above, which is the amount of substance per unit area per unit time, can be expressed by Richardson's law [26], namely:
J=D
S⋅( Pf −
2.4. Permeation experiment
Pp ) (3)
d −2 −1
Where, J is hydrogen permeation flux (mol·m ·s ), d is the thickness of metal membrane (m), D is the diffusion coefficient of the material with the thickness d (m2·s−1), S is the Sieverts' constant (mol·m−3 Pa0.5 ), Pf and Pp are the hydrogen partial pressures (Pa) in the feed and permeate sides of the membrane, respectively. If the Pf > > Pp≈0, the Pp will be omitted as usual. Denoted the hydrogen permeability Φ= D×S (mol·m−2·s−1 Pa−0.5), the Eq. (3) can be written as:
The permeation of deuterium through Nb membranes at elevated temperatures were conducted on the gas evolution permeation equipment (Fig. 1a) [24]. The prepared Nb membranes were first sealed in Swagelok VCR fittings (Fig. 1b, 1/4 in., the inside and outside diameters of the shim is 5.6 mm and 11.9 mm) with an exposing area of 0.24 cm2. The base pressure of the high pressure region (feed side) was fixed to 50 kPa and that of the low pressure region was below 10−5 Pa in order to obtain the optimal signals by quadrupole mass spectrometry (PFEIFFER, QME220, 1–200 amu) in the experimental temperature range of 600˜800 ℃. Before permeation experiment, the quadrupole mass spectrometer was calibrated by standard leakage hole (TDL Z08-8B-12, leakage rate = 2.3 × 10−6 Pa·m3·s−1, He, 23 ℃) and helium mass spectrometer leak detector (INFICON UL1000Fab) in order to obtain the linear relationship between ion current intensity and leakage rate, which was related with the filament state of quadrupole mass spectrometry. After that, the sample was fixed onto the permeation system according to Fig. 1 and the gas leakage test was done by delivering pure He to the feed side of the sample at room temperature in order to detect whether the permeation system (pipelines) was sealed well. Next, the pipelines of the feed side were flushed with deuterium gas (D2) with an isotopic abundance of 99.7% within 2 or 3 s for 3 times. Then the permeation system was heated slowly to 100 ℃ and kept for 5 h under vacuum condition (better than 10−4 Pa) and then heated continuously to 800 ℃ and kept for another 2 h in order to remove the thin oxide layer and the residual hydrogen isotope gas that may remain on the surface of niobium membrane. Finally, the temperature was reduced to the experimental temperature and the pressure of the gas pressure controller (ALICAT, 0–1 bar, absolute pressure) was set at 50 kPa, and the deuterium permeation test was performed. Typically, the
Φ=
J∙d Pf
(4)
It is obvious that a decrease in the metal thickness provokes an increase of the permeation capability. According to the ideal gas law P·V = n·R·T, the leakage of a closed vessel can be expressed by L = V·dp/ dt = d(PV)/dt = RT·dn/dt. The diffusion flux in/out of the vessel wall can be expressed by J = 1/A·dn/dt. Thus, we can obtain the following expression: (5)
J = L/ RTA 3 −1
Where, L is the leak rate (Pa·m ·s ), calculated by the relationship of leakage and ion current, R is universal gas constant (R = 8.314 J·mol−1 K−1), and T is the absolute temperature of sample (K), A is the exposing area of sample (m2). The apparent diffusion coefficient (D) can be estimated by time-lag technique [27] which can be described as:
D=
d2 6τ
(6)
where d is the thickness of the samples and τ is the time corresponding to the point on the current-time curve when the permeation flux matches 63% of steady-state current. 3
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Fig. 2. (a, d) OM, (b, e) TEM images and (c, f) diffraction pattern of microcrystalline and nanocrystalline niobium membranes.
According to Φ= D×S (mol·m−2·s−1 Pa−0.5), the solubility (S) could be calculated by S=Φ/D
mechanically polished and then by electro-polished in order to remove the tightly held oxide. Fig. 4 and 5 give the surface morphology and chemical composition of niobium membrane (microcrystalline) after different surface treatments. The original surface is rough (Fig. 4a) and covered with a layer of oxides with the thickness of about 5˜15 μm (Fig. 4b). After mechanically polished, the surface becomes smooth (Fig. 4c), but still some O, C and Si elements are observed by EDS which may be induced by the residual oxide, ethanol or the silica polishing agent (Fig. S2). The residual oxides are mainly niobium pentoxide identified by the occurrence of two photoelectron peaks with the binding energies of about 207.4 eV and 210.2 eV corresponding to the Nb3d5/2 and Nb3d3/2 for Nb atoms in Nb5+ state, respectively (Fig. 5a) [30]. The thickness is about 20 nm estimated by XPS and AES in depth profile analysis and the content of oxide is dropped down quickly with the increase of Ar+ sputtering time (Fig. 6a and Fig.S3). Finally, pure Nb atoms are exposed identified by the shift of Nb3d5/2 and Nb3d3/2 peaks from 210.2 eV and 207.4 eV for Nb atoms in Nb5+ state to 205.2 eV and 202.4 eV for Nb atoms in Nb0 state. Ion sputtering is a good method to obtain a “clean” surface, but it is uneconomical and not suitable for large-scale industrial applications. In order to remove the oxide layer by a relative simple method, electro-polishing treatment was implemented. The mechanically polished niobium membrane was dipped into acid mixture solution of HF, H2SO4, and lactic acid at room temperature in a controlled voltage of 17 V for about 15 min and some of thin oxides and possible part of the bulk material were dissolved. After that, the pure niobium and Nb2O5 with a chemical composition of about 1:1 calculated by the integrated area ratio are coexisted on the rough surface of niobium membrane (Figs. 4d and 5b), showing that the thickness of oxide is very thin, about 2˜3 nm (several atomic layers, Fig. 6b). It has been reported that under steady-state conditions, clean niobium surfaces existed only at temperatures above 1700 K in vacuum better than 10−10 mbar, and oxidation was inevitable for Nb handled at 298 K in air [31], especially in the wet oxide environment. Usually, the O element would be adsorbed, hopped on the Nb surface and formed into NbOx (x˜1) interface layer firstly and then Nb2O5 amorphous dielectric oxide layer quickly due to the large binding energy of O and Nb [30]. So it is reasonable to deduce that most of Nb2O5 on the surface of electro-polished niobium membrane may be originated from the reoxidation of exposed fresh niobium atoms with oxygen at room temperature in the experimental process since a period of time (about
(7)
3. Results and discussion 3.1. Characterization of Nb membrane Hydrogen trapping and diffusivity in materials is very important for hydrogen permeability, which may be affected by crystal structure, grain boundaries, defects and so on [15–22,28,29]. The niobium membranes (1# and 2#) are pure niobium with BCC structure and no obvious preferred orientation [Im-3 m (229)] identified by X-ray diffraction (Fig. S1), which allows for greater hydrogen solubility and more rapid diffusion [6]. The microscopic structure of them were characterized by OM, TEM and SAED (Fig. 2). The grain size of 1# Nb membrane is about 50˜200 μm (Fig. 2a and b) and there is no obvious participates or defects included in it. The body parameter is about 3.307 Å calculated from the clear diffraction pattern (Fig. 2c). In contrast, only diffraction rings are observed for 2# Nb membrane due to the small-size effect, indicating it is a nanopolycrystalline material (Fig. 2f). The grain size of 2# Nb estimated from the magnified TEM image (Fig. 2e) is about 20˜50 nm, which is far smaller than that of 1# Nb. In order to distinguish them, the 1# and 2# niobium membranes are named as microcrystalline and nanocrystalline Nb, respectively. The metal with smaller grain size and more grain boundaries would contribute for hydrogen trapping and permeation [6]. Fig. 3 presents the reconstructed microstructures of the EBSD analysis performed on the surface of pure niobium after Ar+ sputtering for 30 min. The grain sizes and their misorientation of microcrystalline and nanocrystalline niobium have been distinctly given in Fig. 3a˜h. The misorientation profile along the black line are shown in Fig. 3i and j. The results indicate that some subgrains with the boundary angle below 5° are present in the big grain, which reveals the high dislocations density or stored energy of them. 3.2. Surface treatment of niobium membranes The surface of niobium membranes was treated first by 4
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Fig. 3. (a∼h) EBSD images, (i) crystal orientations and (j) misorientation angle distribution of microcrystalline and nanocrystalline niobium membranes.
been reported that Nb2O5 has two isomers, α-Nb2O5 and β-Nb2O5 [32]. α-Nb2O5 is thermodynamically unstable and can be reduced under vacuum heating while β-Nb2O5 is stable and cannot be easily reduced under same conditions [33]. Also, α-Nb2O5 could transfer rapidly into
30 min) is needed to transfer the fresh niobium membrane after electrolysis to the chamber of XPS. From the above analysis, the reformation of Nb2O5 compound on the surface of “clean” niobium at room temperature is inevitable. It has 5
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Fig. 4. The surface or interfacial morphology of (a, b) original, (c) mechanically polished and (d) electro-polished microcrystalline niobium membranes.
β-Nb2O5 in air as long as the thermodynamic conditions are satisfied such as the temperature is higher than 800 ℃, but the transition rate is very low [32]. Therefore, it is necessary to monitor the change of exposed fresh niobium at room temperature in order to study its keeping “clean” ability because it is closely related with the deuterium permeation rate. The microcrystalline niobium membrane after electropolished was exposed to air again at room temperature for different time, and the chemical composition of its surface was evaluated by XPS (Fig. 7). The characteristic peak intensity of Nb0 decreases greatly for the first 24 h while obvious increase of Nb5+ observed, showing that the Nb2O5 has been formed quickly in air at room temperature, which is similar with the results of Pasternak [34]. After that, the changes of Nb0 and Nb5+ slow down, indicating that the oxidation of niobium at room temperature in air is a kinetic-control process [35]. In the oxidation process of niobium, the adsorbed oxygen on the surface of Nb first becomes into oxygen ions and migrates from outside to inside. When meeting with Nb ions, they would combine with Nb ions quickly to form Nb2O5 in a short time due to the low diffusion restriction and the low Gibbs free energy (Table. S1) [36]. With the increase of exposure time, the oxides increases gradually and the diffusion of oxygen ions is
Fig. 5. XPS spectra of microcrystalline niobium membranes after (a) mechanically polished, (b) electro-polished and (c) Ar+ sputtering.
Fig. 6. Nb and O atomic concentration curves of (a) mechanically polished and (b) electro-polished niobium membranes with sputter time determined by AES. 6
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Fig. 7. (a) XPS spectra and (b) the integrated area ratio between Nb0 and Nb5+ of niobium membrane after electro-polished exposed in air for different exposed time.
Fig. 8. (a) SIMS curves and (b) NbO+/Nb ratio of electro-polished niobium membrane at different annealing temperatures.
polished niobium membrane. Therefore, the samples were heated to 800 ℃ and kept for 2 h under vacuum condition (better than 5 × 10-5 Pa) in deuterium permeation experiment in order to remove the reformed α-Nb2O5 covered on the surface of treated niobium. The nanocrystalline niobium presents the similar behavior with the microcrystalline niobium (e.g. similar XPS curves for mechanically polished and electro-polished niobium, shown in Figs. 5a, 7a and 11 in the back of the paper), so it is reasonable to assume that the surface treatment is also effective for the nanocrystalline niobium membrane.
restricted, which leads to the formation of some low valence oxides (e.g. NbOx, x = 0.02˜1). Compared the XPS curves of electro-polished niobium membrane exposed from 24 h to 912 h with that of the mechanically polished niobium (the top-end green dotted line in Fig. 7a), it is difficult to find that the characteristic peaks of Nb exist always and remain unchanged which has an obvious difference with that of the mechanically polished niobium, showing that the reformed Nb2O5 on the surface of fresh niobium membrane at room temperature is different from the bulk Nb2O5 covered on the original surface of niobium. They are lack of oxygen. Based on this, we deduce that the reformed Nb2O5 may be α-Nb2O5 and it cannot transfer into the β-Nb2O5 (bulk Nb2O5) easily at room temperature in air. In order to further identify it, high temperature treated experiment was carried out in vacuum. The electro-polished niobium membrane was first stored in air at room temperature for 30 days and then annealed from 25 ℃ to 800 ℃ in vacuum better than 1.0 × 10−7 Pa, the chemical composition was monitored by SIMS and the ratio of NbO+/Nb is given in Fig. 8. The NbO+/Nb ratio of the original exposed niobium membrane (the first point in Fig. 8b) is close to 1, indicating that there is a thin oxide layer existed on the surface of electro-polished niobium membrane, which is consistent with that of XPS (Fig. 7). As the temperature is risen, the NbO+/Nb ratio decreases gradually, and a minimum value of about 0.03 is obtained at 800 ℃, which shows that the reformed Nb2O5 on the surface of electro-polished niobium membrane is the thermodynamically unstable α-Nb2O5 and can be removed by high temperature annealing in vacuum. The higher the temperature, the lower the amount of niobium oxides present. This means that the reformed αNb2O5 has less impact on deuterium permeation behavior of electro-
3.3. Deuterium permeation behavior of niobium membranes The deuterium permeation behavior of niobium membranes with micro- and nanocrystalline structures after mechanically polished and electro-polished (sample A, B, C and D) were determined by gas permeation experiment and a steady increase of permeation flux of deuterium was detected in the measured temperature range (Fig.S4). The permeability (Φ), diffusion coefficient (D) and solubility (S) are calculated according to Eqs. ((4)–(7)) and the curves of Φ, D and S versus inverse temperature are given in Fig. 9. The permeability of all samples increases with the measured temperature, which are contrary to the theoretical prediction for pure Nb but in agreement with previous experimental findings [8,37,38]. These observations here indicate that the permeate rate is not just controlled by bulk diffusion, but also affected by surface process [39]. The maximum permeability of deuterium measured for microcrystalline and nanocrystalline Nb membranes after mechanically polished or electro-polished are ΦA,max = 7.91 × 10−11 mol·m−1·s−1 Pa−0.5, ΦB,max = 3.91 × 10−10 mol·m−1·s−1 Pa−0.5, 7
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Fig. 9. (a) permeability, (b) diffusion coefficients and (c) solubility constants of deuterium in niobium membranes with different grain sizes.
ΦC,max = 4.63 × 10−9 mol·m−1·s−1 Pa−0.5 and ΦD,max = 1.33 × 10−8 mol·m−1·s−1 Pa−0.5 at 800 ℃, respectively, which are lower than the theoretical hydrogen permeation value of niobium (500 ℃, 1.6 × 10−6 mol·m−1·s−1 Pa−0.5) or that of pure palladium membrane (500 ℃, 1.9 × 10−8 mol·m−1·s−1 Pa−0.5) [6], indicating that the readily formed oxide layers, even the very thin oxide layers, have severely hindered hydrogen dissociation and solubility, which causes the Nb metal to exhibit lower hydrogen permeability than Pd [13]. The niobium membranes after electro-polished have a relative higher deuterium permeation than that after mechanically polished (ΦB > ΦA, ΦD > ΦC), illustrating that the thickness and type of oxide layer also have a great influence on the deuterium permeation. The niobium membrane with a thinner α-Nb2O5 layer or no oxide layer is conducive to deuterium permeation [39]. Compared the niobium membranes with different grain sizes, it can be found that the Φ of nanocrystalline Nb is nearly two orders of magnitude higher than that of microcrystalline Nb (ΦD > ΦB, ΦC > ΦA). This is because in the same permeable area, the smaller the grain size, the more the grain boundary, that is, the more gas diffusion channels and the faster deuterium diffusion rate, so the nanocrystalline niobium has a higher deuterium permeation rate. Moreover, the grain size has a more influence on the deuterium permeation of niobium membranes than the oxide layer (ΦD > ΦC > ΦB > ΦA). The fitted Arrhenius relations between temperature T and deuterium permeability of microcrystalline and nanocrystalline niobium membranes after mechanically polished and electro-polished are ΦA (mol·m−1·s−1 Pa-0.5) = 1.06 × 10-7exp [-64.49(kJ·mol−1)/RT], ΦB (mol·m−1·s−1 Pa-0.5) = 3.90 × 10-6exp [-82.79 (kJ·mol−1)/RT], ΦC (mol·m−1·s−1 Pa-0.5) = 7.66 × 10-3exp [-127.75 (kJ·mol−1)/RT] and ΦD (mol·m−1·s−1 Pa-0.5) = 9.27 × 103 exp[-119.68 (kJ·mol−1)/RT]. In order to find out the essential reason for hindering the deuterium permeation, diffusion coefficient (D) and solubility (S) are given (Fig. 9b and c). The τ in the Eq. (6) is the time corresponding to the point on the current-time curve when the permeation flux matches 63% of steady-state current (Fig.S5). The diffusion coefficients of all the niobium membranes are in accordance with the Sieverts equation at 600 to 800 ℃ and increase with the
temperature (Fig. 9b). There are only some small differences between microcrystalline and nanocrystalline niobium membranes although the diffusion coefficient of nanocrystalline niobium is some higher than that of microcrystalline niobium, indicating that the diffusion of deuterium atom in the niobium matrix has less impact on the deuterium permeation. According to Φ=D×S, the solubility can be calculated. The solubility constants of them also increase with the temperature, but the S of nanocrystalline niobium are several times higher than that of microcrystalline niobium, which makes the permeability of nanocrystalline niobium about two orders of magnitude higher than that of microcrystalline niobium (Fig. 9c). These results are consistent with the previous findings that hydrogen migrates faster around grain boundaries than in the bulk due to low activation energy barrier and large free spaces for hydrogen atoms to jump from one site to the other [18,19,22]. The large number of grain boundaries presented in the nanocrystalline niobium and the high permeation temperature promote the rapid dissolution and migration of deuterium atoms along grain boundaries, which greatly increases the permeability rate of deuterium. This is also the reason reported for the super-permeation phenomenon occurred in the plasma-driven permeation [39]. From the above results, it can be concluded that the fresh niobium membranes with smaller grain and more grain boundaries can dissolve more deuterium atoms and is more helpful to transportation of deuterium. 3.4. The surface morphology and chemical composition of niobium membranes after deuterium permeation Fig. 10 presents the SEM images of permeate sides of mechanicallypolished and electro-polished nanocrystalline niobium membranes (named as MP-Nb and EP-Nb) before and after the deuterium permeation tests. The surface of MP-Nb membrane is smooth before deuterium permeation (Fig. 10a), while some nano-humps are observed after deuterium permeation experiment (Fig. 10b). The morphology differences of MP-Nb membrane before and after deuterium permeation may be originated from the surface oxide layer and the deformation of the Nb substrate in the deuterium permeation process [40]. As is known, 8
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Fig. 10. The surface morphology of (a, b) mechanically polished and (c, d) electro-polished nanocrystalline niobium membranes before and after deuterium permeation.
(Fig. 10b). For the EP-Nb membrane, most of the bulk β-Nb2O5 compounds have been dissolved in the electrolysis process and the detected Nb2O5 compounds would be reformed α-Nb2O5 by fresh Nb and oxygen at room temperature in air. These niobium oxides are very thin and can be been decomposed in the deuterium permeation process at high temperatures identified by high temperature treated experiment (Fig. 8). Also, Scientific researches have shown that the reformed thin α-Nb2O5 layer can be regarded as two-dimensional oxide film, which has higher specific surface energies, more surface structural defects (e.g. disorder) and chemical defects (e.g. O voids) than that of bulk oxide
most of the surface oxides on the MP-Nb membrane is bulk Nb2O5 (βNb2O5), which are thermodynamically stable (ΔHp=-1898 kJ/mol) and cannot be reduced or decomposed easily by heating in the vacuum condition [31–33]. Also, deuterium absorption in the Nb membrane would result in Nb-lattice expansion but not induce the size change of βNb2O5, and thus, in an increase of the misfit between the D/Nb film and the β-Nb2O5 oxides [40]. Therefore, some of the β-Nb2O5 covered on the surface of Nb substrate would loose and take off under the loading of deuterium, especially in the position of lattice grain boundaries, which leads to the formation of nano-humps on the surface of MP-Nb
Fig. 11. Nb3d characteristic peaks of (a) mechanically polished and (b) electro-polished nanocrystalline niobium membranes before and after deuterium permeation. 9
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Fig. 12. The permeability mechanism of deuterium in micro- and nanocrystalline niobium membranes after mechanically polished (MP-Nb) or electro-polished (EPNb).
within the detecting range of XPS is observed on the surface of niobium substrate before deuterium permeation (Fig. 11b), it could easily be removed by Ar+ sputtering or reduced in the high temperature deuterium permeation process, which is identified by the occurrence of strong Nb2+ characteristic peak. The NbO occurred for MP-Nb and EPNb after deuterium permeation may be formed by reaction of Nb in the subsurface with the adjacent Nb2O5 or the decomposed oxygen permeated into the subsurface under the pressure-driven of deuterium [41]. The above results demonstrate that the type of oxides and its thickness has a significant influence on the deuterium permeation property of niobium membrane again. From the above discussion, the effect of surface oxides and grain boundaries on the permeability behavior of deuterium in micro- and nanocrystalline niobium membranes can be described as follows (Fig. 12). The permeation of deuterium in niobium membranes is controlled not only by the surface catalytic activity of niobium atom but also by the diffusion rate of deuterium atom. When some oxides were covered on the surface of niobium, the tightly held oxide layers would make some obstruction for the dissociation of deuterium molecules into deuterium atoms and the subsequent dissolution and absorption of deuterium atoms within the niobium substrate, which makes the permeability of deuterium in the mechanically polished niobium membrane slower than that after electro-polished. Second, when the deuterium atoms entered into the niobium substrate, substitutional diffusion and grain boundaries diffusion are the main modes for diffusion of deuterium atom. Since the grain boundaries are the fast channels for deuterium atom, the niobium substrate with smaller grain sizes and more grain boundaries would have a higher permeation rate of deuterium. So the deuterium permeation rate of nanocrystalline niobium is higher.
materials, which would weaken the bond force between the oxygen atom and the metal and reduce the thermodynamic stability [31,33]. In addition, the Nb substrate or its suboxides is favorable for the α-Nb2O5 decomposition or reduction [31]. So there are no nano-humps observed on the surface of EP-Nb membrane (Fig. 10d). The microstructure rearrangement was also observed (Fig. 10d), which may be ascribed to deuterium-induced or oxide-induced topography changes of the Nb membrane or the locally grown of Nb deuteride grains [40]. The occurrence of nano-humps for MP-Nb membrane while not for EP-Nb membrane further identifies that the type of oxides and its thickness has a significant influence on the deuterium permeation property of niobium membrane. The surface chemical composition of mechanically-polished and electro-polished nanocrystalline niobium membranes before and after deuterium permeation test were further characterized by XPS (Fig. 11). Nb2O5 compounds are observed both on the surface of MP-Nb membranes before and after deuterium permeation due to the occurrence of Nb3d photoelectron peaks in Nb5+ state (Fig. 11a), but there are some differences in the binding energies after Ar+ sputtering. The peaks of Nb3d5/2 and Nb3d3/2 are shifted from 210.2 eV and 207.4 eV (Nb atoms in Nb5+ state) to 205.2 eV and 202.4 eV (Nb atoms in Nb0 state) for the MP-Nb membrane before deuterium permeation, while three photoelectron peaks with the binding energies of 210.6 eV, 207.8 eV and 204.6 eV are occurred after deuterium permeation. Generally, the binding energy of Nb3d increases about 1 eV while increasing one valance from Nb0 to Nb5+, and the oxides of niobium can be identified by different valence states according to chemical shifts [31]. So the peak with binding energies of 204.6 eV is the characteristic peak of Nb3d3/2 for Nb atom in Nb2+ state while the other (Nb3d5/2) is coincided with that of Nb3d3/2 for Nb2O5, as can be seen an intensive and wide characteristic peak for Nb3d in Fig. 11a. The surface of niobium membrane after deuterium permeation was also evaluated by Laserinduced breakdown spectroscopy (LIBS), and there was no deuterium signal detected, indicating that little deuterium were resided in the niobium membranes. Therefore, it can be deduced that the subsurface of the mechanically-polished niobium substrate after deuterium permeation is mainly composed of NbO (because for NbH2, ΔH=-60 kJ/ mol which can be decomposed at 1023 K [6]) and β-Nb2O5 compounds. For the EP-Nb membrane, although a thin layer α-Nb2O5 compound
4. Conclusions Two kinds of polycrystalline niobium membranes with micro- and nanocrystalline structures were treated by mechanically polished, electro-polished, ultra-high vacuum high temperature annealing or/and argon ion sputtering respectively, and the results showed that a relatively "clean" niobium surface could be obtained by mechanical polishing combined with electrolytic polishing and ultra-high vacuum high 10
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temperature annealing. The niobium surface has the ability to keep in a relatively "clean" state at room temperature in air. The surface oxides and grain sizes had a great impact on the permeation behavior of deuterium through niobium membranes, especially the grain sizes and grain boundaries. A niobium membrane with small grain size and “clean” surface had the highest deuterium permeation and solubility than that with big grain size or unclean surface.
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Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
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This work is financially supported by the National Magnetic Confined Fusion Energy Development Project (Grant No. 2015GB109004), National Natural Science Foundation of China (Grant No.11775194, 21805252, 21908209), Discipline Development Foundation of Science and Technology on Surface Physics and Chemistry Laboratory (Grant No. XKFZ201710) and the Authorized Exploration Program (TCSQ2018109). The authors would like to thank Mr. Rongguang Zeng for TEM observation, Mr. Dongli Zou and Mrs. Chunli Jiang for SEM and EBSD characterization, Mr. Qifa Pan and Chuanhui Liang for XPS measurement, and Mr. Li Deng for deuterium permeation experiment.
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Appendix A. Supplementary data
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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.fusengdes.2019. 111340.
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