- Email: [email protected]
Influence of Au ions irradiation on the deuterium permeation behavior in the oxidized Fe–Cr–Al ferritic steel
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 5 8 3 4 e2 5 8 3 9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Influence of Au ions irradiation on the deuterium permeation behavior in the oxidized FeeCreAl ferritic steel Yi-Ming Lyu a,b, Yu-Ping Xu a,*, Xin-Dong Pan a,b, Hao-Dong Liu a,b, Xiao-Chun Li a, Hai-Shan Zhou a, Qian Xu a, Zhong-Shi Yang a, Guang-Nan Luo a,b a b
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China Science Island Branch of Graduate, University of Science & Technology of China, Hefei, 230031, China
highlights Oxidized FeeCreAl ferritic steel (OFFS) has been irradiated by Au ions. The defects in OFFS have been characterized by DBS-PA experiments. D2 GDP experiments have been performed for OFFS. Vacancy-type defects have influence on D2 permeation behavior of OFFS.
article info
abstract
Article history:
Our previous study has shown that an effective tritium permeation barrier (TPB) with Al2O3
Received 30 April 2019
layer found could be obtained by oxidization of a FeeCreAl ferritic steel. In this study,
Received in revised form
irradiation effects on deuterium permeation behavior through oxidized FeeCreAl ferritic
11 July 2019
steel (OFFS) have been investigated by Au ions irradiation followed by deuterium gas driven
Accepted 1 August 2019
permeation (GDP) experiments. The deuterium permeability of the irradiated and original
Available online 6 September 2019
OFFS samples has been obtained and compared. Oxide layer has been characterized by Xray photoelectron spectroscopy (XPS) experiment and scanning electron microscopy (SEM)
Keywords:
experiment. The defects in the oxide layer with and without Au ions irradiation have been
Tritium permeation barrier
characterized by Doppler broadening spectrometry of positron annihilation (DBS-PA) ex-
Irradiation
periments. The deuterium permeation behavior of OFFS changed owing to the Au ions
Permeation
irradiation, which could be attributed to the increased density of vacancy-type defects.
Hydrogen isotope
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Oxidation
* Corresponding author. E-mail address: [email protected] (Y.-P. Xu). https://doi.org/10.1016/j.ijhydene.2019.08.006 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 5 8 3 4 e2 5 8 3 9
Introduction Tritium issues are of significant importance to the safety of the fusion reactor [1]. Alumina has been investigated as the most promising candidate tritium permeation barrier (TPB) coating and been widely studied [2e8]. In our previous study, an effective TPB with Al2O3 layer found has been obtained by oxidization of a FeeCreAl ferritic steel [9]. The permeability of oxidized FeeCreAl ferritic steel (OFFS) could be 3 orders of magnitude less than that of reduced activation ferritic/ martensitic (RAFM) steels like the F82H or the CLF-1 steels. The deuterium permeability of the original FeeCreAl ferritic steel is about 30 times lower than that of the F82H steel. After gas-driven permeation (GDP) experiments, the color of the original FeeCreAl ferritic steel changes from silver white to light gold, which means that the FeeCreAl ferritic steel is oxidized during the high temperature GDP experiments. Thus, the deuterium permeability of the original FeeCreAl ferritic steel should higher than the measured results and close to the deuterium permeability of the F82H steel. As the binding technology of the FeeCreAl ferritic steel with RAFM steels is relatively easy because the thermal coefficient of expansion of these two steels is similar, the FeeCreAl ferritic steel may be applied in the blanket as the coating materials, which after proper binding process and oxidization process will has high enough permeation reduction factor (PRF). During the binding process, the interface between coating material and RAFM steel could have large impact on the permeation behavior of the bonded material, which might make the hydrogen permeation behavior of bonded material different from bare coating material. Proper and optimized bonding process is needed to form a high performance bonded material. Different from the TPBs serving in the tritium factory or tritium storage vessels outside the reactor, the TPBs in the blanket will suffer from the neutron irradiation, which will introduce huge density of defects into the coatings and may influence the PRFs of the TPBs. When considered into the long-lived activation of aluminum in the neutron irradiation environment, the aluminum content in the coating material should be reduced as low as possible. The efficient TPBs are materials that usually have long-lived activation. A balance between low activation and high PRF of the TPB materials should be found. Efforts have been done to investigate the influence of irradiation on the hydrogen isotope permeation behavior in TPBs [10e12]. However, the conclusions drawn from these studies are in contrary, i.e. three sets of experiments performed in the high flux reactor (HFR) Petten reactor show that the PRFs of tritium in TiC and Al2O3 reach 3.2 and 3.4 respectively [13], which are much lower than those obtained in laboratory experiments. On the other hand, another set of experiments in HFR Petten reactor and one set of experiments in a research reactor, IGV, indicate the value of PRF for tritium in the in-pile experiments was close to that for deuterium in the out-of-pile experiments [12]. With the forwarding of the International Thermonuclear Experimental Reactor (ITER) project and the Chinese Fusion Engineering Testing Reactor (CFETR), the evaluation of irradiation effects on PRFs of TPBs becomes urgent. Owing to the lack of neutron sources and the troublesome activation of the
samples after neutron irradiation, the data about the hydrogen isotopes transportation in the irradiation environment are limited and the mechanism of the irradiation effect is still unsettled. High-energy ions have been used to simulate the irradiation effects of neutron [14e17]. Chikada et al. have used the 6.4 MeV Fe ions to irradiate the Er2O3 coatings and the PRFs of the irradiated samples have been characterized [10]. They found that the deuterium permeability of the irradiated samples is lower than that of the original samples. It should be noted that the irradiation temperature in their study is higher than the service temperature of the RAFM steels which will lead to further recovery of the irradiation defects. In addition, the estimated irradiation depth is larger than the thickness of the coating which means that the results would covers the irradiation effect on the substrate and the gap between the coating and the substrate. While based on the results obtained from our previous studies [18,19], irradiation will influence the hydrogen isotope permeation behavior in the RAFM steel which serves as the substrate in Chikada’s study [10]. In this study, the OFFS is irradiated by Au ions within the oxide layer, and the evolution of the irradiation defects is characterized by Doppler broadening spectrometry of positron annihilation (DBS-PA). Then the deuterium GDP experiments of the irradiated and original OFFS samples are carried out to investigate the influence of irradiation on the hydrogen isotope permeation behavior.
Experimental Material preparation The material used in this work is a FeeCreAl ferritic steel bought from Goodfellow Corp. The detailed chemical compositions of this FeeCreAl ferritic steel are shown in Table 1. Steels with dimensions of F 20 mm 0.5 mm are cut and mechanically polished by SiC paper, then using polishing cloth to produce a mirror finish. Then the samples have been oxidized in air at 1073 K for 90 h to form an Al2O3 layer. The detailed characterization of the oxide can be found in [9]. However, it should be noted that during oxidation experiments, the humidity of air in this experiment might be slightly different with that in [9]. Thus, the oxidation products might be slightly different. To characterize whether there are difference in the oxidation products, X-ray photoelectron spectroscopy (XPS) experiment and scanning electron microscopy (SEM) experiment have been done again. Fig. 1 shows the surface morphology of the OFFS sample. Nanoscale granular oxide could be found on the surface of the OFFS. Fig. 2 shows the XPS depth profile of the oxide layer. The oxide layer thickness is about 400 nm. The measured chemical components and surface morphology of the OFFS are consistent with study [9], which means that the oxide layer properties are same with that in study [9]. Table 1 e Detailed chemical composition of the FeeCreAl ferritic steel. Element Content wt.%
Cr
Al
Y
Zr
C
Mn
Si
Fe
22.0
5.0
0.1
0.1
0.02
0.2
0.3
balance
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damage distribution calculated by SRIM 2008 and O and Al elements distribution in the oxide layer of the OFFS with irradiation fluence up to 2 1015 cm2 is shown in Fig. 4. As shown in Fig. 4, the irradiation damage is totally within the oxide layer, and the damage depth is about a third of the depth of the oxide layer. There is no irradiation damage in the FeeCreAl ferritic steel in our irradiation condition.
Positron annihilation
Fig. 1 e Surface morphology of the OFFS sample oxidized in air at 1073 K for 90 h.
After irradiation experiments, DBS-PA tests have been performed to characterize the damage in the irradiated and original OFFS samples. The DBS-PA method is an effective technique which is widely used to characterize the microstructure information in materials. The DBS-PA tests are performed in the Key Laboratory of Nuclear Analysis Techniques, the institute of high energy physics (IHEP), Chinese Academy of Sciences. The samples are implanted with positrons within the energy range of 0.18e20 keV, which is emitted from a 22Na source and moderated by tungsten. The detailed description of this setup and the DBS-PA theory could be found in [20,21]. The S parameter is correlated with the density of vacancytype defects, the higher S parameter represents higher density of vacancy-type defects. The different slopes of the lines in (S, W) plots mean different kinds of vacancy-type defects [21,22].
Deuterium GDP experiments
Fig. 2 e XPS depth profile of the OFFS sample.
Au ions irradiation Au ions irradiation experiments have been performed at room temperature using a 2 1.7 MV tandem accelerator at the State Key Laboratory of Nuclear Physics and Technology, Peking University. In order to study the irradiation effects in the oxide layer and exclude the influence of steel substrate, the Auþ ions energy is set as 0.5 MeV based on the calculated result of SRIM 2008. In this case, irradiation damage within the depth of about 220 nm in the oxide layer of OFFS could be achieved. The samples are irradiated up to three irradiation fluence, 2 1014 cm2, 6 1014 cm2 and 2 1015 cm2 separately, and the corresponded peak value of displacement damage for these three irradiated samples is 0.88, 2.64 and 8.80 separately. The Au atom distribution and displacement damage distribution are calculated by SRIM 2008 with a threshold displacement energy of 30 eV. For the fixed implanted energy of Au ion, the calculated implanted Au atoms and damage distribution of the samples with different implanted fluence are similar. The only difference is the peak value of implanted Au atoms amount and dpa. Thus, the calculated result of irradiated sample with fuence up to 2 1015 cm2 is shown in Fig. 3 as demonstration. In order to show the damage distribution in the oxide layer clearly, the
GDP experiments have been performed for all irradiated and original OFFS samples in the device constructed in our lab. The GDP device has two different sections, which is separated by the disc samples. One of the two section is held at pressure lower than 105 Pa. The other section is held at the same pressure and then D2 gas is injected reaching a pressure up to 1 105 Pa. During the measurements, the experimental temperature is set between 623 K and 823 K, which is consistent with the service temperature of the structural materials in the
Fig. 3 e The implanted Au atoms and damage distribution of the irradiated sample with fluence up to 2 £ 1015 cm¡2 calculated by SRIM 2008 full-cascade simulation code in the “QUICK Kinchin-Pease calculation” mode with a threshold displacement energy of 30 eV. The peak dpa value is 8.80.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 5 8 3 4 e2 5 8 3 9
Fig. 4 e Damage distribution in the oxide layer of the irradiated OFFS with fluence up to 2 £ 1015 cm¡2. The damage distribution is calculated by SRIM 2008. The O and Al elements distribution is measured by XPS method.
blanket. In order to study the recovery effects of high temperature on the irradiation damage, the GDP experiments have been performed from low temperature to high temperature then back to low temperature again. The detailed description of the GDP device and experimental procedure could be found in our previous papers [23,24].
Results and discussion DBS-PA The profiles of the S parameters in terms of the depth for the irradiated and original OFFS samples are shown in Fig. 5. In the irradiated samples, the S parameter is higher than that of the original OFFS sample. With the increase of the irradiation fluence, the S parameter becomes higher, which indicated that the density of vacancy-type defects is increased after Au ions irradiation. As shown in Fig. 5, the DBS-PA results are in
Fig. 5 e Depth profile of S parameters for irradiated and original OFFS samples and distribution of displacement damage of irradiated sample with fluence up to 2 £ 1015 cm¡2. The distribution of displacement damage is calculated by SRIM 2008.
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relatively good agreement with SRIM 2008 prediction, suggesting that the irradiation damage are induced at the desired location as the calculated results of SRIM 2008. The SeW plots for all the irradiated and original OFFS samples are shown in Fig. 6. As mentioned above, the SeW plot can be used to identify the vacancy-type defects and different slopes in the (S, W) plots represent the different vacancy-type defects. In Fig. 6, there are slightly difference of slopes of the samples with and without Au ions irradiation. The irradiation fluence shows little influence on the slope, which means that in our experimental condition, even after irradiation and with different irradiation fluence, there are no new vacancy-type defects detected in the OFFS. H.-S. Zhou et al. [19] have studied the irradiation influence of 3.5 MeV He ions and 4.5 MeV Fe ions on the solid materials. In their study, even the irradiation condition is changed, there are same vacancy-type defects observed by TEM in the irradiated materials, which is consistent with our DBS-PA experimental results.
GDP Fig. 7 shows that the deuterium permeability of the samples irradiated with fluence up to 2 1014 cm2 and 6 1014 cm2 is increased compared with that of the original OFFS sample. After high temperature experiments, the deuterium permeability of irradiated samples is decreased and close to the original OFFS sample. Same decreasing temperature GDP experiments have been performed for the original OFFS. The experiments show that there are no change of the deuterium permeability compared with that of the increasing temperature GDP experiments. Ions irradiation can produce defects in the irradiated materials [25,26], and vacancy-type defects can contribute to the H-transport in the alumina [27], which might be the reason why the irradiated OFFS samples have higher deuterium permeability compared with that of original OFFS sample in the lower temperature. The DAS-PA results shown in Fig. 5 confirm that the density of vacancy-type defects is increased after Au ions irradiation. After high temperature permeation experiments, the deuterium permeability of samples irradiated with fluence up to 2 1014 cm2 and 6 1014 cm2 are reduced and close to that of the original OFFS sample, which could be
Fig. 6 e (S, W) plots of irradiated and original OFFS samples.
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irradiated sample with highest fluence up to 2 1015 cm2, more carbon contamination might present on this sample and lead to the change of surface condition. Even though the defects are formed in the oxide layer, the change of the surface condition might decrease the deuterium permeability [32] and lead to the similar permeability with that of the original sample. After high temperature experiments, the deuterium permeability of the irradiated sample with fluence up to 2 1015 cm2 is increased. This phenomenon might be attributed to that carbon impurities could react with the water vapor [33] and release from the surface of the irradiated sample, which lead to the reduction of the carbon content on the surface of the irradiated sample and result in the increase of the deuterium permeability of the irradiated sample with fluence up to 2 1015 cm2 after high temperature GDP experiments. Fig. 7 e Temperature dependence of deuterium permeability of the OFFS samples irradiated by 0.5 MeV Auþ ions with fluence up to 2 £ 1014 cm¡2 and 6 £ 1014 cm¡2 compared with that of the original OFFS sample. The experimental sequence of measured points has been marked using arrows.
attributed to the recovery of the vacancy-type defects during the high temperature annealing [28e30]. As shown in Fig. 8, for the irradiated sample with fluence up to 2 1015 cm2, the deuterium permeability in the lower temperature is close to the original sample and after high temperature experiments the permeability is increased. This phenomena might be attributed to the influence of carbon contamination introduced on the oxide layer during Au ions irradiation. G.S. Was et al. [31] found that during ions irradiation, high energy ions may break the chemical bond of the hydrocarbons remained in the irradiation device and introduce carbon impurities into the irradiated materials. For the
Conclusion Au ions irradiation experiments have been performed for the OFFS. The irradiation damage is within the oxide layer. Defects analysis and deuterium GDP experiments have been performed for the irradiated and original OFFS. The permeability of the samples with the peak displacement damage of 0.88 dpa and 2.64 dpa is increased compared with that of original sample in the low temperature GDP experiments. After high temperature GDP experiments, the permeability is decreased, but is still higher than that of the original sample. The permeability of the sample with the peak displacement damage of 8.8 dpa is similar with that of the original sample. After high temperature GDP experiments, the deuterium permeability is increased. The presented results suggest that the Au ions irradiation can produce more vacancy-type defects in the oxide layer, which may be the reason why the deuterium permeability of the irradiated samples is increased after Au ions irradiation. But with the increase of the irradiation fluence, changes of surface condition may also have influence on the deuterium permeation behavior and lower down the deuterium permeability of OFFS. In this article, the investigation of the influence mechanism is preliminary. More characterization of the irradiation defects should be done. Simulation work will be done in the future. Meantime, it should be noted that the displacement damage is not high enough and the damage depth is limited compared with the thickness of the oxide layer, more work will be done in the future.
Acknowledgments
Fig. 8 e Temperature dependence of deuterium permeability in the OFFS samples irradiated by 0.5 MeV Auþ ions with fluence up to 2 £ 1014 cm¡2, 6 £ 1014 cm¡2 and 2 £ 1015 cm¡2 compared with that of the original OFFS sample. The experimental sequence of measured points has been marked using arrows.
This work is supported by National Key R&D Program of China (No. 2017YFE0301502, National MCF Energy R&D Program of China under contract number of 2018YFE0303103, National Postdoctoral Program for Innovative Talents (BX201700248), China Postdoctoral Science Foundation (No. 2017M622035), the National Natural Science Foundation of China (No. 11875287).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 5 8 3 4 e2 5 8 3 9
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Influence of Au ions irradiation on the deuterium permeation behavior in the oxidized FeeCreAl ferritic steel Yi-Ming Lyu a,b, Yu-Ping Xu a,*, Xin-Dong Pan a,b, Hao-Dong Liu a,b, Xiao-Chun Li a, Hai-Shan Zhou a, Qian Xu a, Zhong-Shi Yang a, Guang-Nan Luo a,b a b
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China Science Island Branch of Graduate, University of Science & Technology of China, Hefei, 230031, China
highlights Oxidized FeeCreAl ferritic steel (OFFS) has been irradiated by Au ions. The defects in OFFS have been characterized by DBS-PA experiments. D2 GDP experiments have been performed for OFFS. Vacancy-type defects have influence on D2 permeation behavior of OFFS.
article info
abstract
Article history:
Our previous study has shown that an effective tritium permeation barrier (TPB) with Al2O3
Received 30 April 2019
layer found could be obtained by oxidization of a FeeCreAl ferritic steel. In this study,
Received in revised form
irradiation effects on deuterium permeation behavior through oxidized FeeCreAl ferritic
11 July 2019
steel (OFFS) have been investigated by Au ions irradiation followed by deuterium gas driven
Accepted 1 August 2019
permeation (GDP) experiments. The deuterium permeability of the irradiated and original
Available online 6 September 2019
OFFS samples has been obtained and compared. Oxide layer has been characterized by Xray photoelectron spectroscopy (XPS) experiment and scanning electron microscopy (SEM)
Keywords:
experiment. The defects in the oxide layer with and without Au ions irradiation have been
Tritium permeation barrier
characterized by Doppler broadening spectrometry of positron annihilation (DBS-PA) ex-
Irradiation
periments. The deuterium permeation behavior of OFFS changed owing to the Au ions
Permeation
irradiation, which could be attributed to the increased density of vacancy-type defects.
Hydrogen isotope
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Oxidation
* Corresponding author. E-mail address: [email protected] (Y.-P. Xu). https://doi.org/10.1016/j.ijhydene.2019.08.006 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Tritium issues are of significant importance to the safety of the fusion reactor [1]. Alumina has been investigated as the most promising candidate tritium permeation barrier (TPB) coating and been widely studied [2e8]. In our previous study, an effective TPB with Al2O3 layer found has been obtained by oxidization of a FeeCreAl ferritic steel [9]. The permeability of oxidized FeeCreAl ferritic steel (OFFS) could be 3 orders of magnitude less than that of reduced activation ferritic/ martensitic (RAFM) steels like the F82H or the CLF-1 steels. The deuterium permeability of the original FeeCreAl ferritic steel is about 30 times lower than that of the F82H steel. After gas-driven permeation (GDP) experiments, the color of the original FeeCreAl ferritic steel changes from silver white to light gold, which means that the FeeCreAl ferritic steel is oxidized during the high temperature GDP experiments. Thus, the deuterium permeability of the original FeeCreAl ferritic steel should higher than the measured results and close to the deuterium permeability of the F82H steel. As the binding technology of the FeeCreAl ferritic steel with RAFM steels is relatively easy because the thermal coefficient of expansion of these two steels is similar, the FeeCreAl ferritic steel may be applied in the blanket as the coating materials, which after proper binding process and oxidization process will has high enough permeation reduction factor (PRF). During the binding process, the interface between coating material and RAFM steel could have large impact on the permeation behavior of the bonded material, which might make the hydrogen permeation behavior of bonded material different from bare coating material. Proper and optimized bonding process is needed to form a high performance bonded material. Different from the TPBs serving in the tritium factory or tritium storage vessels outside the reactor, the TPBs in the blanket will suffer from the neutron irradiation, which will introduce huge density of defects into the coatings and may influence the PRFs of the TPBs. When considered into the long-lived activation of aluminum in the neutron irradiation environment, the aluminum content in the coating material should be reduced as low as possible. The efficient TPBs are materials that usually have long-lived activation. A balance between low activation and high PRF of the TPB materials should be found. Efforts have been done to investigate the influence of irradiation on the hydrogen isotope permeation behavior in TPBs [10e12]. However, the conclusions drawn from these studies are in contrary, i.e. three sets of experiments performed in the high flux reactor (HFR) Petten reactor show that the PRFs of tritium in TiC and Al2O3 reach 3.2 and 3.4 respectively [13], which are much lower than those obtained in laboratory experiments. On the other hand, another set of experiments in HFR Petten reactor and one set of experiments in a research reactor, IGV, indicate the value of PRF for tritium in the in-pile experiments was close to that for deuterium in the out-of-pile experiments [12]. With the forwarding of the International Thermonuclear Experimental Reactor (ITER) project and the Chinese Fusion Engineering Testing Reactor (CFETR), the evaluation of irradiation effects on PRFs of TPBs becomes urgent. Owing to the lack of neutron sources and the troublesome activation of the
samples after neutron irradiation, the data about the hydrogen isotopes transportation in the irradiation environment are limited and the mechanism of the irradiation effect is still unsettled. High-energy ions have been used to simulate the irradiation effects of neutron [14e17]. Chikada et al. have used the 6.4 MeV Fe ions to irradiate the Er2O3 coatings and the PRFs of the irradiated samples have been characterized [10]. They found that the deuterium permeability of the irradiated samples is lower than that of the original samples. It should be noted that the irradiation temperature in their study is higher than the service temperature of the RAFM steels which will lead to further recovery of the irradiation defects. In addition, the estimated irradiation depth is larger than the thickness of the coating which means that the results would covers the irradiation effect on the substrate and the gap between the coating and the substrate. While based on the results obtained from our previous studies [18,19], irradiation will influence the hydrogen isotope permeation behavior in the RAFM steel which serves as the substrate in Chikada’s study [10]. In this study, the OFFS is irradiated by Au ions within the oxide layer, and the evolution of the irradiation defects is characterized by Doppler broadening spectrometry of positron annihilation (DBS-PA). Then the deuterium GDP experiments of the irradiated and original OFFS samples are carried out to investigate the influence of irradiation on the hydrogen isotope permeation behavior.
Experimental Material preparation The material used in this work is a FeeCreAl ferritic steel bought from Goodfellow Corp. The detailed chemical compositions of this FeeCreAl ferritic steel are shown in Table 1. Steels with dimensions of F 20 mm 0.5 mm are cut and mechanically polished by SiC paper, then using polishing cloth to produce a mirror finish. Then the samples have been oxidized in air at 1073 K for 90 h to form an Al2O3 layer. The detailed characterization of the oxide can be found in [9]. However, it should be noted that during oxidation experiments, the humidity of air in this experiment might be slightly different with that in [9]. Thus, the oxidation products might be slightly different. To characterize whether there are difference in the oxidation products, X-ray photoelectron spectroscopy (XPS) experiment and scanning electron microscopy (SEM) experiment have been done again. Fig. 1 shows the surface morphology of the OFFS sample. Nanoscale granular oxide could be found on the surface of the OFFS. Fig. 2 shows the XPS depth profile of the oxide layer. The oxide layer thickness is about 400 nm. The measured chemical components and surface morphology of the OFFS are consistent with study [9], which means that the oxide layer properties are same with that in study [9]. Table 1 e Detailed chemical composition of the FeeCreAl ferritic steel. Element Content wt.%
Cr
Al
Y
Zr
C
Mn
Si
Fe
22.0
5.0
0.1
0.1
0.02
0.2
0.3
balance
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damage distribution calculated by SRIM 2008 and O and Al elements distribution in the oxide layer of the OFFS with irradiation fluence up to 2 1015 cm2 is shown in Fig. 4. As shown in Fig. 4, the irradiation damage is totally within the oxide layer, and the damage depth is about a third of the depth of the oxide layer. There is no irradiation damage in the FeeCreAl ferritic steel in our irradiation condition.
Positron annihilation
Fig. 1 e Surface morphology of the OFFS sample oxidized in air at 1073 K for 90 h.
After irradiation experiments, DBS-PA tests have been performed to characterize the damage in the irradiated and original OFFS samples. The DBS-PA method is an effective technique which is widely used to characterize the microstructure information in materials. The DBS-PA tests are performed in the Key Laboratory of Nuclear Analysis Techniques, the institute of high energy physics (IHEP), Chinese Academy of Sciences. The samples are implanted with positrons within the energy range of 0.18e20 keV, which is emitted from a 22Na source and moderated by tungsten. The detailed description of this setup and the DBS-PA theory could be found in [20,21]. The S parameter is correlated with the density of vacancytype defects, the higher S parameter represents higher density of vacancy-type defects. The different slopes of the lines in (S, W) plots mean different kinds of vacancy-type defects [21,22].
Deuterium GDP experiments
Fig. 2 e XPS depth profile of the OFFS sample.
Au ions irradiation Au ions irradiation experiments have been performed at room temperature using a 2 1.7 MV tandem accelerator at the State Key Laboratory of Nuclear Physics and Technology, Peking University. In order to study the irradiation effects in the oxide layer and exclude the influence of steel substrate, the Auþ ions energy is set as 0.5 MeV based on the calculated result of SRIM 2008. In this case, irradiation damage within the depth of about 220 nm in the oxide layer of OFFS could be achieved. The samples are irradiated up to three irradiation fluence, 2 1014 cm2, 6 1014 cm2 and 2 1015 cm2 separately, and the corresponded peak value of displacement damage for these three irradiated samples is 0.88, 2.64 and 8.80 separately. The Au atom distribution and displacement damage distribution are calculated by SRIM 2008 with a threshold displacement energy of 30 eV. For the fixed implanted energy of Au ion, the calculated implanted Au atoms and damage distribution of the samples with different implanted fluence are similar. The only difference is the peak value of implanted Au atoms amount and dpa. Thus, the calculated result of irradiated sample with fuence up to 2 1015 cm2 is shown in Fig. 3 as demonstration. In order to show the damage distribution in the oxide layer clearly, the
GDP experiments have been performed for all irradiated and original OFFS samples in the device constructed in our lab. The GDP device has two different sections, which is separated by the disc samples. One of the two section is held at pressure lower than 105 Pa. The other section is held at the same pressure and then D2 gas is injected reaching a pressure up to 1 105 Pa. During the measurements, the experimental temperature is set between 623 K and 823 K, which is consistent with the service temperature of the structural materials in the
Fig. 3 e The implanted Au atoms and damage distribution of the irradiated sample with fluence up to 2 £ 1015 cm¡2 calculated by SRIM 2008 full-cascade simulation code in the “QUICK Kinchin-Pease calculation” mode with a threshold displacement energy of 30 eV. The peak dpa value is 8.80.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 5 8 3 4 e2 5 8 3 9
Fig. 4 e Damage distribution in the oxide layer of the irradiated OFFS with fluence up to 2 £ 1015 cm¡2. The damage distribution is calculated by SRIM 2008. The O and Al elements distribution is measured by XPS method.
blanket. In order to study the recovery effects of high temperature on the irradiation damage, the GDP experiments have been performed from low temperature to high temperature then back to low temperature again. The detailed description of the GDP device and experimental procedure could be found in our previous papers [23,24].
Results and discussion DBS-PA The profiles of the S parameters in terms of the depth for the irradiated and original OFFS samples are shown in Fig. 5. In the irradiated samples, the S parameter is higher than that of the original OFFS sample. With the increase of the irradiation fluence, the S parameter becomes higher, which indicated that the density of vacancy-type defects is increased after Au ions irradiation. As shown in Fig. 5, the DBS-PA results are in
Fig. 5 e Depth profile of S parameters for irradiated and original OFFS samples and distribution of displacement damage of irradiated sample with fluence up to 2 £ 1015 cm¡2. The distribution of displacement damage is calculated by SRIM 2008.
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relatively good agreement with SRIM 2008 prediction, suggesting that the irradiation damage are induced at the desired location as the calculated results of SRIM 2008. The SeW plots for all the irradiated and original OFFS samples are shown in Fig. 6. As mentioned above, the SeW plot can be used to identify the vacancy-type defects and different slopes in the (S, W) plots represent the different vacancy-type defects. In Fig. 6, there are slightly difference of slopes of the samples with and without Au ions irradiation. The irradiation fluence shows little influence on the slope, which means that in our experimental condition, even after irradiation and with different irradiation fluence, there are no new vacancy-type defects detected in the OFFS. H.-S. Zhou et al. [19] have studied the irradiation influence of 3.5 MeV He ions and 4.5 MeV Fe ions on the solid materials. In their study, even the irradiation condition is changed, there are same vacancy-type defects observed by TEM in the irradiated materials, which is consistent with our DBS-PA experimental results.
GDP Fig. 7 shows that the deuterium permeability of the samples irradiated with fluence up to 2 1014 cm2 and 6 1014 cm2 is increased compared with that of the original OFFS sample. After high temperature experiments, the deuterium permeability of irradiated samples is decreased and close to the original OFFS sample. Same decreasing temperature GDP experiments have been performed for the original OFFS. The experiments show that there are no change of the deuterium permeability compared with that of the increasing temperature GDP experiments. Ions irradiation can produce defects in the irradiated materials [25,26], and vacancy-type defects can contribute to the H-transport in the alumina [27], which might be the reason why the irradiated OFFS samples have higher deuterium permeability compared with that of original OFFS sample in the lower temperature. The DAS-PA results shown in Fig. 5 confirm that the density of vacancy-type defects is increased after Au ions irradiation. After high temperature permeation experiments, the deuterium permeability of samples irradiated with fluence up to 2 1014 cm2 and 6 1014 cm2 are reduced and close to that of the original OFFS sample, which could be
Fig. 6 e (S, W) plots of irradiated and original OFFS samples.
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irradiated sample with highest fluence up to 2 1015 cm2, more carbon contamination might present on this sample and lead to the change of surface condition. Even though the defects are formed in the oxide layer, the change of the surface condition might decrease the deuterium permeability [32] and lead to the similar permeability with that of the original sample. After high temperature experiments, the deuterium permeability of the irradiated sample with fluence up to 2 1015 cm2 is increased. This phenomenon might be attributed to that carbon impurities could react with the water vapor [33] and release from the surface of the irradiated sample, which lead to the reduction of the carbon content on the surface of the irradiated sample and result in the increase of the deuterium permeability of the irradiated sample with fluence up to 2 1015 cm2 after high temperature GDP experiments. Fig. 7 e Temperature dependence of deuterium permeability of the OFFS samples irradiated by 0.5 MeV Auþ ions with fluence up to 2 £ 1014 cm¡2 and 6 £ 1014 cm¡2 compared with that of the original OFFS sample. The experimental sequence of measured points has been marked using arrows.
attributed to the recovery of the vacancy-type defects during the high temperature annealing [28e30]. As shown in Fig. 8, for the irradiated sample with fluence up to 2 1015 cm2, the deuterium permeability in the lower temperature is close to the original sample and after high temperature experiments the permeability is increased. This phenomena might be attributed to the influence of carbon contamination introduced on the oxide layer during Au ions irradiation. G.S. Was et al. [31] found that during ions irradiation, high energy ions may break the chemical bond of the hydrocarbons remained in the irradiation device and introduce carbon impurities into the irradiated materials. For the
Conclusion Au ions irradiation experiments have been performed for the OFFS. The irradiation damage is within the oxide layer. Defects analysis and deuterium GDP experiments have been performed for the irradiated and original OFFS. The permeability of the samples with the peak displacement damage of 0.88 dpa and 2.64 dpa is increased compared with that of original sample in the low temperature GDP experiments. After high temperature GDP experiments, the permeability is decreased, but is still higher than that of the original sample. The permeability of the sample with the peak displacement damage of 8.8 dpa is similar with that of the original sample. After high temperature GDP experiments, the deuterium permeability is increased. The presented results suggest that the Au ions irradiation can produce more vacancy-type defects in the oxide layer, which may be the reason why the deuterium permeability of the irradiated samples is increased after Au ions irradiation. But with the increase of the irradiation fluence, changes of surface condition may also have influence on the deuterium permeation behavior and lower down the deuterium permeability of OFFS. In this article, the investigation of the influence mechanism is preliminary. More characterization of the irradiation defects should be done. Simulation work will be done in the future. Meantime, it should be noted that the displacement damage is not high enough and the damage depth is limited compared with the thickness of the oxide layer, more work will be done in the future.
Acknowledgments
Fig. 8 e Temperature dependence of deuterium permeability in the OFFS samples irradiated by 0.5 MeV Auþ ions with fluence up to 2 £ 1014 cm¡2, 6 £ 1014 cm¡2 and 2 £ 1015 cm¡2 compared with that of the original OFFS sample. The experimental sequence of measured points has been marked using arrows.
This work is supported by National Key R&D Program of China (No. 2017YFE0301502, National MCF Energy R&D Program of China under contract number of 2018YFE0303103, National Postdoctoral Program for Innovative Talents (BX201700248), China Postdoctoral Science Foundation (No. 2017M622035), the National Natural Science Foundation of China (No. 11875287).
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