Microscopic Damage of Tungsten and Molybdenum Exposed to Low-Energy Helium Ions

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1 Plasma Science and Technology, Vol.17, No.4, Apr Microscopic Damage of Tungsten and Molybdenum Exposed to Low-Energy Helium Ions FAN Hongyu ( ) 1,3, YANG Qi ( ) 1,3,LIXin( ) 1,3, NI Weiyuan ( ) 1,3,NIUJinhai( ) 1,3, LIU Dongping ( ) 1,2,3 1 School of Physics and Materials Engineering, Dalian Nationalities University, Dalian , China 2 Fujian Key Laboratory for Plasma and Magnetic Resonance, Department of Electronic Science, Aeronautics, School of Physics and Mechanical & Electrical Engineering, Xiamen University, Xiamen , China 3 Liaoning Key Laboratory of Optoelectronic Film and Materials, Dalian , China Abstract Polycrystalline tungsten (W) and molybdenum (Mo) materials both non-annealed and annealed at temperatures of o C have been irradiated with low-energy (220 ev), high-flux ( ions/m 2 s) He + at an irradiation temperature of 600 o C and at a dose of ions/m 2. This non-destructive conductive atomic force microscopy technique provides direct observation and comparison of surface swellings with growth of nanoscale defects in the irradiated materials. A coral-like surface structure and nanostructured defects were formed in W when irradiated at a He + dose of ions/m 2. Increasing the annealing temperature resulted in an increase in the size of nanostructured defects and serious surface damage of W. Compared to W, Mo suffered much less surface damage after being irradiated at various He + doses. irradiation damage, tungsten, molybdenum, conductive atomic force mi- Keywords: croscopy PACS: Dq DOI: / /17/4/13 (Some figures may appear in colour only in the online journal) 1 Introduction High-Z materials such as tungsten (W) and molybdenum (Mo) have been regarded as potential candidates for plasma-facing materials (PFMs) in fusion reactors because of their good material properties, such as high melting temperature, low sputtering yield and low tritium inventory. PFMs suffer surface damage such as blistering, erosion and sputtering caused by low-energy (tens of ev to several kev), high-flux ( ions/m 2 s ions/m 2 s) helium and hydrogen ions [1,2]. During normal operations of ITER, the expected surface temperature of vacuum vessels can be up to > 600 o C [3]. The understanding of the behavior of W and Mo materials irradiated with low-energy ions at an elevated temperature will overcome a major part of the material-related problems encountered with fusion reactors. Here, a comparative study is presented on the microscopic damage of W and Mo materials irradiated with low-energy He +. Conductive atomic force microscopy (CAFM) measurements were used to observe surface features and electron emission defects of W and Mo samples irradiated at various He + doses. Compared with the traditional transmission electron microscopy (TEM) measurement, the specimen preparation for CAFM analysis is rather simple; it can be performed at room temperature in an atmospheric environment. Even though the theory of CAFM is rather preliminary, its measurement can provide a direct observation of the sizes and distribution of nanostructured defects, and conductivity and surface features of irradiated materials, which is necessary for understanding the damage formation, defect accumulation and growth of gas bubbles in irradiated fusion materials. In this study, we report on the evolution of microstructures of W and Mo materials subjected to low-energy He + irradiation. 2 Experimental details A He + source was used to generate low-energy (220 ev), high-flux ( ions/m 2 s) He + to irradiate the surface of prepared samples. A detailed description of the irradiation experiment system has been reported elsewhere [4].Inthissystem,He + was extracted supported by the National Magnetic Confinement Fusion Program of China (No. 2011GB108011), National Natural Science Foundation of China (No ) and the Scientific Research Fund of Liaoning Provincial Education Department (No. L ) and the Fundamental Research Funds for the Central Universities of China (No. DC ) 331

2 Plasma Science and Technology, Vol.17, No.4, Apr from a MHz RF plasma source with a density of /m 3 and accelerated when one W or Mo sample on the sample holder was negatively biased. He + ions bombarded W or Mo samples at normal incidence (90 o ). The irradiation area was a circle with a diameter of 10 mm. A bias voltage of 100 V was applied to the W sample, resulting in an incident energy of 220 ev/he + when taking into account the plasma potential of 120 V as measured by a Langmuir probe. A variable-power semiconductor laser was used to heat the back side of W samples to maintain a constant sample temperature of 600 o C during irradiation. He + irradiation of W and Mo samples was performed at a He + dose of ions/m 2 to ions/m 2. The samples used in this study were polycrystalline W and Mo with a purity of > 99.9 at.%, obtained from Honglu Corporation, China. The samples were cut into pieces with a dimension of 10 mm 10 mm 2 mm. W and Mo surfaces were mechanically mirror-polished to a surface roughness of < 0.1 μm. Then, W and Mo samples were annealed at o C for 2 h in a vacuum with a background pressure of 10 5 Pa to relieve internal stresses and reduce the large concentration of nanoscale defects. Characterization of the surface modification on the irradiated W and Mo samples was performed using scanning electron microscopy (SEM) (Hitachi S4800). CAFM measurements were used to observe surface features and electron emission defects of W and Mo samples irradiated with low-energy (220 ev) He +. A detailed description of the CAFM method has been provided previously [5,6]. Briefly, one laser system is used to keep the constant deflection of the PtIr-coated tip in contact with the measured sample. A constant voltage (V sample ) is applied between the PtIr-coated AFM tip and the sample, and the electron emission image is obtained simultaneously with surface topography at a typical scan rate of 0.30 Hz. He + irradiation into W materials usually induces structural damage, and thus can affect their electron emission properties. When the conductive tip is scanned across the surface of irradiated samples, the difference in the electron emission intensity across the surface of the irradiated samples is obtained from the current image. This method can be used to efficiently compare the surface microstructures and observe the distribution of defects or He bubbles in the destroyed layer. ing temperature. Fig. 1(a) shows that for W samples (both non-annealed and annealed at a temperature of o C), most of the surface was covered by a onedirectional nanometer-sized structure. The surfaces exhibited coral-like structures, which is consistent with the previous report, where W materials were irradiated with 30 kev He + at a sample temperature of 900 o C [7]. The coral-like structure appears to be thin strips of material orientated in a certain direction on each grain. W samples were notably damaged due to He + irradiation at 600 o C. Ohno et al. reported the influence of crystal orientation on the damage of W materials irradiated with ev He + at a surface temperature of K [8]. Regular wavy nanostructures were observed on the crystal grains of W surfaces. The formation of He + -induced coral-like or wavy structures could be attributed to the larger angle between a slip face and the face of each grain, such as {101} [8]. SEM measurements show that no obvious damage is formed for Mo samples, both non-annealed and annealed at a temperature of o C. This indicates that compared to W, Mo materials can be more stable when irradiated with low-energy He + at a sample temperature of 600 o C. 3 Results 3.1 SEM analysis Fig. 1 shows the SEM micrograph of W (a) and Mo (b) samples irradiated at a He + dose of ions/m 2 and an irradiation temperature of 600 o C. Before irradiation, the annealing temperature of W and Mo samples varied from 800 o C to 1750 o C. As shown in Fig. 1, crystal grains of polycrystalline W and Mo materials grew rapidly with increasing anneal- Fig.1 SEM micrograph of W (a) and Mo (b) samples irradiated at a He + dose of ions/m 2 at a sample temperature of 600 o C. Before irradiation, the annealing temperature of W and Mo samples varied from 800 o Cto 1750 o C 332

3 FAN Hongyu et al.: Microscopic Damage of Tungsten and Molybdenum Exposed to Low-Energy Helium Ions 3.2 Effect of voltage polarity on CAFM measurement Fig. 2(a) shows typical I-V curves of Mo samples annealed at 1100 o C and irradiated at a He + dose of ions/m 2. The I-V curves were obtained at asamplebiasof 180 mv to 180 mv. The negative sample bias means that the electrons flow through the W sample into the PrIt-coated tip while a positive one means that electron emission takes place at the AFM tip. Measurements show that the voltage polarity of the measured samples or the PtIr-coated tip does not make a significant difference for the measured I-V curves. The emission current is rapidly improved with increasing sample bias. The current limit of the CAFM current sensor is ±10 na. The voltage polarity does not affect the shape of the I-V curves and the current of electron emission through the sample. Fig.2 (a) Typical I-V curves of Mo samples annealed at 1100 o C and irradiated at a He + dose of ions/m 2. The I-V curves were obtained at a sample bias of 180 mv to 180 mv, (b), (c) The surface topography (left) and the simultaneously measured current (right) images of Mo samples annealed at 1100 o C and irradiated at a He+ dose of ions/m 2. CAFM measurements were performed at a V sample of (b) 10 mv and (c) 10 mv Fig. 2(b) and (c) show the surface topography (left) and the simultaneously measured current (right) images of Mo samples annealed at 1100 o C and irradiated at a He + dose of ions/m 2. CAFM measurements were performed at a V sample of Fig. 2(b) 10 mv and Fig. 2(c) 10 mv. The characteristic shape and distribution of nanometer-sized protuberances in the surface topography images show good consistency with those of current defects in the current images. This indicates that nanostructured defects were formed due to the irradiation of low-energy He + at the dose of ions/m 2. Compared with the surface topography images, the current images are more accurate for 333 observing the nanometer-sized defects in the irradiated sample. The voltage polarity did not make a difference for the current images obtained at a V sample of 10 mv and 10 mv. 3.3 Effect of He + doses and annealing temperature on nanoscale electron emission through W and Mo materials Fig. 3(a) and (b) show the surface topography (left) and the simultaneously measured current (right) images of W and Mo materials annealed at a temperature of 1100 o C and irradiated at various He + doses. The CAFM measurements were performed at a V sample of 10 mv. Fig. 3(a) shows that no obvious change in the surface microstructure was formed for the W samples irradiated at He + doses less than ions/m 2. However, plenty of conductive nanostructured defects could be observed from their current images. The density of nanostructured defects rapidly increased when the He + dose increased from ions/m 2 to ions/m 2. When the He + dose was up to ions/m 2, high-density nanostructured protuberances were formed. The surfaces exhibited corallike structures, which is consistent with the SEM measurement. The coral-like structures appears to be thin strips of material orientated in a certain direction on each grain. The characteristic shape of nanometersized protuberances and their distribution show good consistency with the simultaneously measured conductive defects in the current images. The density of conductive defects was greatly improved at the higher He + dose of ions/m 2. The size of conductive defects increased when the He + dose varied from ions/m 2 to ions/m 2, as shown in Fig. 3(a). Increasing the He + dose from ions/m 2 to ions/m 2 did not significantly change the surface microstructure of Mo materials. Fig. 3(b) shows that nanometer-sized protuberances were formed for Mo materials irradiated at high He + doses. Both the density and size of conductive defects in current images increased with the increase in He + dose. Although high-density defects were formed at a high He + dose of ions/m 2, no coral-like structure was observed for the Mo material. The high-density nanostructured defects were almost uniformly distributed over the Mo sample irradiated at a He + dose of ions/m 2, which is inconsistent with the results of the W sample irradiated at the same He + dose of ions/m 2. Fig. 4 shows the effect of annealing temperature on the microstructure of (a) W and (b) Mo materials irradiated at a He + dose of ions/m 2. The CAFM measurements were performed at a V sample of 10 mv. High-density nanostructured protuberances were formed for W materials both non-annealed and annealed at temperatures of o C, as shown in Fig. 4(a). W materials were notably damaged after

4 Plasma Science and Technology, Vol.17, No.4, Apr Fig.3 Surface topography (left) and simultaneously measured current (right) images of (a) W and (b) Mo materials annealed at a temperature of 1100 o C and irradiated at various He + doses. The CAFM measurements were performed at a V sample of 10 mv Fig.4 The effect of annealing temperature on the microstructure of (a) W and (b) Mo materials irradiated at a He + dose of ions/m 2. The CAFM measurements were performed at a V sample of 10 mv being irradiated at a He + dose of ions/m 2. The size of the nanostructured protuberances increased with an increase in the annealing temperature. Corallike nanostructures were formed on crystal grains of the W surface after being annealed. Compared with the surface topography images of W materials, their 334

5 FAN Hongyu et al.: Microscopic Damage of Tungsten and Molybdenum Exposed to Low-Energy Helium Ions current images are more accurate for observing the nanometer-sized defects in the irradiated materials, as shown in Fig. 4(a). Current images show that the size of nanometer-sized conductive defects increased rapidly with the annealing temperature, indicating that the W samples were seriously damaged at a relatively high annealing temperature. The surrounding region of each nanostructured defect was relatively conductive. Fig. 4(b) shows that no obvious change in surface microstructure was formed for Mo materials both nonannealed and annealed at temperatures of o C. Compared with W, Mo materials suffered much less surface damage and exhibited higher erosion resistance when irradiated with low-energy He + at a sample temperature of 600 o C. However, the current images show that nanostructured conductive defects were formed for all Mo samples irradiated at a He + dose of ions/m 2. The density and size of nanostructured defects did not significantly depend on the annealing temperature. Nanostructured defects were almost uniformly distributed over the Mo samples, and coral-like surface structures were not observed from the surface topography and current images. 4 Discussion Previous studies have shown that H + /He + irradiation results in surface hardening, dislocation loops, surface etching and blistering of Mo and W crystallines [9 12]. The 250 nm-20 μm wavelength reflectivity of Mo and W mirrors was greatly decreased because of blisters generated on the mirror surfaces [10]. Retention of deuterium in W and Mo has been observed during 10 kev D + 2 implantation at room temperature [13]. Serious surface etching of Mo crystalline has been formed due to He + irradiation [14,15]. The net erosion of W is distinctly (more than seven times) lower than that of Mo [16]. After W materials were irradiated at higher doses of He +, the nanometer-sized surface swellings of W materials showed good consistency with the nanostructured defects in current images. This suggests that notable surface damage of W materials occurred due to the strong He + irradiation. After W materials are irradiated with low-energy He +,Heatoms can be trapped in the vacancy, or at interstitial and substitutional sites in the grain boundarys [17,18]. He trapped in W can affect the conductivity of irradiated W samples. The trapped He atoms as one impurity can impede the drift of electrons, and thus have an effect on the electron emission threshold or the conductivity of irradiated W materials. Indeed, we have found that the conductivity of irradiated W materials is greatly reduced due to He trapping into W materials. If there are vacancies as trapping sites in W, many He atoms can diffuse in W at an elevated sample temperature, and be trapped in the vacancy, particularly at an increasing He + dose, resulting in nanoscale bubbles [19]. An increase in the internal pressure of these trapping sites results in mutation of the crystal lattice, and thus an expansion of the bubbles [19]. The He bubbles can act as one captor and capture trapped He atoms, particularly at an elevated sample temperature [6]. This can lead to a decrease in the concentration of trapped He atoms in the vicinity of surface swellings. Thus, the surrounding region of each nanostructured surface swelling or defect is relatively conductive. The nanostructured conductive defects were also observed for Mo materials irradiated at relatively high He + doses. The density and sizes of the nanostructured defects in irradiated Mo materials were quite similar to those in irradiated W materials. This indicates that He atoms canalsodiffuseinmoatanelevatedtemperature,and be trapped in vacancy or interstitial sites, resulting in nanoscale defects. Crystal grains of polycrystalline W and Mo materials rapidly grew when the annealing temperature varied from 800 o C to 1750 o C. Crystal grains of W and Mo materials annealed at a higher temperature can contain less vacancies or interstitial sites, which would act as trapping sites. Thus, the density of nanoscale swellings or defects can be greatly reduced by increasing the annealing temperature. The diffusion and coalescence of trapped He atoms along the surface layer result in an increase in the size of He bubbles. This indicates that W materials can suffer serious surface damage at a relatively high annealing temperature. However, Mo materials exhibited excellent erosion resistance when irradiated with low-energy He + at an elevated temperature. Nanostructured surface swellings were not clearly observed on irradiated Mo materials. However, the mechanism of microscopic evolution of Mo materials remains unclear and is still under study. 5 Conclusions Both the annealing temperature and He + dose significantly affected the microscopic structure of W and Mo irradiated with 220 ev He + at the sample temperature of 600 o C. Nanometer-sized surface swellings of W materials irradiated to higher doses of He + showed good consistency with the nanostructured defects in current images. A coral-like surface structure and nanostructured defects were formed in W when irradiated at a He + dose of ions/m 2. All of these measurements indicate that the serious surface damage of W materials occurred due to the strong He + irradiation, particularly at a relatively high annealing temperature. Surface swelling of irradiated W materials resulted from the over-high internal pressure of nanometer-sized He bubbles formed via the diffusion of He atoms trapped in the sub-surface layer. Compared to W, Mo materials annealed at different temperatures suffered much less surface damage when irradiated with low-energy He + at various He + doses. The exact mechanism of microscopic evolution of irradiated Mo materials is still under study. 335

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