Fabrication of tungsten and carbon clad plates by sinter bonding methods for divertor system characterization

Fusion Engineering and Design 136 (2018) 116–119

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Fabrication of tungsten and carbon clad plates by sinter bonding methods for divertor system characterization

T

Hirotatsu Kishimotoa, , Yuuki Asakuraa, Minoru Matanoa, Naofumi Nakazatoa, Joon-Soo Parka, Tamaki Shibayamab, Masakatsu Fukumotoc ⁎

a

Muroran Institute of Technology, Muroran, Hokkaido, Japan Hokkaido University, Sapporo, Hokkaido, Japan c National Institutes for Quantum and Radiological Science and Technology, Naka, Ibaraki, Japan b

ARTICLE INFO

ABSTRACT

Keywords: CFC Tungsten Joining Divertor JT-60SA

Full tungsten divertor is employed for ITER, and the tungsten will be the most potential candidate for the surface material of plasma facing components in fusion reactor after DEMO. Under a Broader Approach (BA) activity, a satellite tokamak device JT-60SA is expected to contribute the investigation of many factors and issues related to the fusion plasma and components. JT-60SA has a future plan to transfer the surface of plasma facing components from carbon fiber reinforced carbon composite (CFC) to tungsten, and investigate the interaction of plasma and tungsten. For this plan, joining and coating technologies of tungsten on CFC is an important issue. Present research developed a fabrication method of the W/CFC clad plates and investigated the mechanical, microstructural and thermal properties of them.

1. Introduction

2. Experimental

The ITER council recommended in 2011 to start the research to possibly use a full tungsten armored divertor from the start of ITER operation. The physical and design issues have been investigated [1–6]. ITER will employ a full tungsten divertor, and DEMO in future is likely to employ tungsten as a plasma facing material [2]. Many researches such as mechanical, microstructural and thermal properties of tungsten, effects of high heat flux of plasma, charged particles and neutron irradiation effects on tungsten, are necessary for the use of divertors having tungsten surface [2]. Under the Broader Approach (BA) activities, a satellite tokamak program, JT-60SA, is on-going. JT-60SA is currently under construction [7]. The original surface of the first wall and the divertor of JT-60SA will be graphite tiles and carbon fiber reinforced carbon composite (CFC) tiles, respectively. Because the DEMO reactor is predicted to have metallic-wall, JT-60SA is also expected to contribute the engineering aspects for developing plasma-facing components having metallic-wall. In the JT-60SA program, it is planned to replace the surface of first wall and divertor from carbon to tungsten. Joining and coating technologies of tungsten and carbon materials will be issues for the replacement. Present research aims to develop joining technologies for tungsten with CFC plates, and investigates the mechanical, microstructural and thermal properties of them.

Tungsten used was 99.9% cold rolled plates. The CFC used was CX2002U provided by TOYO TANSO CO. LTD. The CX-2002U is an anisotropic carbon fiber reinforced composite having oriented microstructure and mechanical properties due to the fiber orientations. The strength and coefficient of thermal expansion (CTE) of each direction of materials are listed in Table 1. Two joining methods of “Diffusion bonding” and “Sinter bonding” were tested. For the search of conditions, graphite plates and CFC plated were used. The graphite and CFC plates were cut out to brocks having the dimension of 20 mmL × 20 mmW × 10 mmT. The tungsten plates were also cut out to 20 mmL × 20 mmW plates. The thicknesses of the tungsten plate prepared were 2 mm and 9 mm. The “Diffusion bonding” is a simple technique in which a tungsten plate and a CFC plate were stacked and hot-pressed without any insert materials. The “Sinter bonding” uses insert material. SiC green sheet is 30 μm thick and made by SiC nano-powders with polymer based binders [8]. The SiC green sheet was inserted between the tungsten and CFC plates, and all of materials were hot-pressed. Both “Diffusion bonding” and “Sinter bonding” were performed on the Y-Z plane of CFC. Schematic images of the bonding process are shown in Fig. 1. The contact surfaces of the plates were polished with diamond powders. The bonding was performed by a hot-pressing method under the pressure of 20 MPa in argon

⁎

Corresponding author. E-mail address: [email protected] (H. Kishimoto).

https://doi.org/10.1016/j.fusengdes.2018.01.004 Received 25 September 2017; Received in revised form 30 December 2017; Accepted 1 January 2018

Available online 19 January 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.

Fusion Engineering and Design 136 (2018) 116–119

H. Kishimoto et al. Table 1 Properties of CFC and graphite. CFC (CX-2002U)

Graphite

Direction

X

Y

Z

(IG-430U)

Flexural Strength (MPa) Compression Strength (MPa) Tensile Strength (MPa) CTE (10−6/K)

47 48 35 1.7

43 45 30 2.3

17 52 11 5.3

54 90 37 4.8

Fig. 2. Back scattered electron images of interphasse of a sinter bonded clad plate of tungsten with CFC composite. Fig. 1. Schematic images of the bonding process and bending test specimens.

affect the bonding. Fig. 2 shows back scattered electron images of tungsten, CFC and the interphase of a “Sinter bonded” specimen. Tungsten consists of isotropic crystal grains. Fig. 2 was taken along Y-axis of CFC. There were many pores in CFC, but carbon fibers and carbon matrix were difficult to distinguish. Microstructure at the interphase was distinguished into layers from (a) to (e). The reaction layer (e) seemed to be buried in CFC microstructure. The contrast of the back scattered image of the reaction layer (e) in Fig. 2 seems to be slightly brighter than the CFC, it suggests that the reaction layer (e) consists of slightly heavier atoms than the CFC. The EPMA characterization in Fig. 3 was performed for two regions, the boundary between the interphase and tungsten, and the boundary between the interphase and CFC. The EPMA characterization of the former indicated that tungsten (a) contains carbon. The result for the border between the interphase suggests that the reaction layer (b) consists of tungsten, silicon and oxygen. The reaction layer (c) consists of mainly tungsten, but the detail of the contents is not certain. A character of the reaction layer (d) is that this layer includes oxides. The reaction layer (e) in Fig. 3Figure 3 seems to consist of mainly carbon and silicon.

atmosphere. The holding temperature was 2173 K, and the holding times were ranged between 1 h and 10 h. The bonding strength was measured by 3-point bending tests at room temperature. The dimension of the bending test specimens were 19 mmL × 3.5 mmW × 2 mmT. Because the CFC has oriented mechanical property, the bending specimens were cut out along with X-axis of CFC, and the load applied along Y-axis. The schematic images of bending test specimens are also shown in Fig. 1. Thermal conductivities of the CFC, tungsten and W/CFC clad plates were measured at room temperature by a laser flash thermal constant measuring device (ULVAC TC7000). In case of 2 mm thick W/CFC clad plate, heat flux by laser loaded onto W surface and temperature measured on CFC surface. The thermal conductivities of tungsten and CFC plates were also measured. Microstructures were examined using a field-emission scanning electron microscope (FE-SEM, JEOL JSM6700F), an electron probe micro analyzer (EPMA, JEOL JXA-8900R) and a field-emission gun transmission electron microscope (FE-TEM, JEOL JEM2100F) equipped for EDX and a laser microscope. 3. Results

3.2. Mechanical and thermal property of tungsten and CFC clad plates

3.1. Fabrication and microstructure of tungsten and CFC clad plates

Fig. 4 shows the stress-displacement curve of the 3-point bending test. Three specimens were tested, and the fracture strengths of each specimen were 17 MPa, 30 MPa and 35 MPa, respectively. The average strength was 27 MPa. The fracture occurred at the interphase and each specimen was divided to tungsten and CFC pieces during the test. Table 3 lists the results of measured densities and thermal conductivities of CFC, tungsten and W/CFC clad plates. The measured thermal conductivity of W/CFC clad plate was 151 W/(m K), which is slightly lower than the tungsten plate.

An overview on the bonding experiments of graphite or CFC with tungsten is given in Table 2. In the case of diffusion bonding, even for holding time of 10 h, the bonding was not successful. On the other hand, the sinter bonding specimens were successfully joined by 1 h holding time for both CFC and graphite plates. Because both 2 mm and 9 mm thick tungsten were successfully joined by sinter bonding, the thickness of tungsten did not Table 2 Overview of results of diffusion and sinter bonding experiments. Diffusion bonding

1h 10 h

4. Discussion

Sinter Bonding

W-Graphite

W-CFC

W-Graphite

W-CFC

Failure –

– Failure

Success –

Success –

A mismatch of the coefficients of thermal expansion (CTE) of materials is one of the most important factors of dissimilar material bonding. The CFC had oriented CTE, and 2.3 × 10−6/K along Y-axis and 5.3 × 10−6/K along Z-axis, related to the bonding. The CTE of tungsten is about 4.5 × 10−6/K which is between the CTEs of Y axis 117

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Fig. 3. EPMA mappings and schematic image of chemical composition of interphase on a sinter bonded clad plate of tungsten with CFC composite.

and maybe oxygen but the detail of this reaction layer is still not certain. Microstructural investigation suggests that the bonding strength of the clad plates is caused by the silicon diffusion and its reacted phases with tungsten and CFC. The flexural strength of the CFC has anisotropy. Because load applied to specimens along Y-axis in the 3-point bending tests, the results need to be compared with Y-axis bending strength of 43 MPa of CFC in Table 1. The fracture strengths of 3-point bending test of the clad plates was between 19 and 35 MPa, and the average was 27 MPa. The flexural strengths correspond to be between 40% and 81% strength of the bending strength of CFC along Y-axis. Because the weakest bending strength of the CFC is 17 MPa of Z-axis which is worse than the bonding strength, we need to pay attention to the strength of the CFC Z-axis. Because tungsten had well crystallized grains, it seemed to have high thermal conductivity. But the interphase has complex structure including several kinds of oxides which tend to have lower thermal conductivity. In the rule of mixture on two layered W and CFC composite, thermal conductivity of the W/CFC clad plate is shown as following;

Fig. 4. Stress-displacement curve of 3-point bending test at room temperature of a sinter bonded clad plates. Table 3 Densities and thermal conductivities.

CFC plate Tungsten plate W/CFC clad plate

Density (g/cm3)

Thermal Conductivity (W/(m K))

1.56 19.0 9.84

228 168 151

(Thickness of W/CFC/Thermal conductivity of W/CFC) = (Thickness of W/Thermal conductivity of W) + (Thickness of CFC/Thermal conductivity of CFC). The thermal conductivity of W/CFC clad plate is predicted to be 193 W/(m K) by the rule of mixture, while the measured value of it was 151 W/(m K). The thermal conductivity of W/CFC clad plate is lower than those of tungsten and CFC. The precision of data is still uncertain, but it is possible that high thermal resistivity is caused by the interphase [10]. In the case of a 3 layered composite of W, interphase and CFC, because (Thermal Resistivity) = (Thickness)/(Thermal conductivity), the thermal resistivity of interphase is able to be calculated using data on Table 3. The total thermal resistivity of the clad plate will be following;

and Z axis of CFC. Such mismatch tends to cause the debonding or fracture of materials. But, in present research, the sinter bonding was succeeded to produce clad plates, and any significant cracks were not observed. The microstructure of CFC on Fig. 2 indicated that the CFC used had many pores, and the CFC consists of reinforcements and matrix, thus, the CFC had better toughness than monolithic ceramics materials. The other factor is that the absolute strains caused by the CTE mismatch were small because of the small CTEs of both CFC and tungsten. But it is considered that we need to pay attention that residual stress may be involved in the clad plates. Thermal shock and fatigue effects will be needed to be evaluated. SEM and EPMA analysis in Figs. 2 and 3 indicated that silicon diffused into both tungsten and CFC. The reaction layers (b) in Fig. 2 are relative silicon rich phases which seemed to consist of silicon, tungsten and maybe, oxygen. Carbon seemed into tungsten uniformly as shown in the reaction layer (a) in Figs. 2 and 3. Such reaction layers of W-C phase of and W-Si phase of reaction layers were observed in the interphase of the tungsten and SiC clad plates [9]. The reaction layer (e) was also silicon rich phase but the phase dose not included tungsten. EPMA data of Fig. 3 indicates the layer consists of silicon and carbon

(Thermal resistivity of W/CFC) = (Thermal resistivity of W) + (Thermal resistivity of interphase) + (Thermal resistivity of CFC). Thickness of CFC is shown as following; (Thickness of W/CFC) = (Thickness of W) + (Thickness of CFC). Weight of CFC has also the same relationship with W and CFC as following; (Density of W/CFC) × (Thickness of W/CFC) = (Density W) × (Thickness of W) + (Density of CFC) × (Thickness of CFC).

of

Then, thickness of tungsten and CFC in the W/CFC clad plate was estimated by the above two formulas. The calculated thicknesses of CFC and tungsten are 0.88 mm and 1.10 mm, respectively. The thermal resistivity of the W/CFC interphase was calculated to be 118

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2.73 × 10−6 Km2/W which is one order better than the mechanical joint using screws and bolts [11]. Though the thermal resistivity is considered to be changed with temperature, the temperature mismatch between tungsten and CFC under the thermal flux of 15 MW/m2 is estimated to be 41 K [7]. The CTE of CFC is from 2.3 × 10−6/K for Y-axis to 5.3 × 10−6/K for Z-axis while of tungsten is 4.5 × 10−6/K. The thermal stress caused by the thermal flux of 15 MW/m2 will range from 0.01 % to 0.02 % which is small enough to keep bonding. Present research shows that the bonding of CFC and thick tungsten is possible, and the clad plate has reasonable thermal conductivity. But for the use in divertor of JT-60SA, the evaluation and maximization of stability of the W/CFC clad plate against the thermal shock and fatigue will be necessary.

Agency under JAEA Contract Research program (25I118). References [1] S. Suzuki, K. Ezato, Y. Seki, K. Mohri, K. Yokoyama, M. Enoeda, Development of the plasma facing components in Japan for ITER, Fusion Eng. Des. 87 (2012) 845–852. [2] Y. Ueda, J.W. Coenen, G. De Temmerman, R.P. Doerner, J. Linke, V. Philipps, E. Tsitronee, Research status and issues of tungsten plasma facing materials for ITER and beyond, Fusion Eng. Des. 89 (2014) 901–906. [3] R.A. Pitts, S. Carpentier, F. Escourbiac, T. Hirai, V. Komarov, S. Lisgo, A.S. Kukushkin, A. Loarte, M. Merola, A. Sashala Naik, R. Mitteau, M. Sugihara, B. Bazylev, P.C. Stangeby, A full tungsten divertor for ITER: Physics issues and design status, J. Nucl. Mater. 438 (2013) S48–S56. [4] K. Ezato, S. Suzuki, Y. Seki, K. Mohri, K. Yokoyama, F. Escourbiac, T. Hirai, V. Kuznetcov, Progress of ITER full tungsten divertor technology qualification in Japan, Fusion Eng. Des. 98–99 (2015) 1281–1284. [5] K. Ezato, S. Suzuki, Y. Seki, H. Yamada, T. Hirayama, K. Yokoyama, F. Escourbiac, T. Hirai, Progress of ITER full tungsten divertor technology qualification in Japan: manufacturing full-scale plasma-facing unit prototypes, Fusion Eng. Des. 109–111 (2016) 1256–1260. [6] M. Merola, F. Escourbiac, R. Raffray, P. Chappuis, T. Hirai, A. Martin, Overview and status of ITER internal components, Fusion Eng. Des. 89 (2014) 890–895. [7] JT-60SA Research Unit, JT-60SA Research Plan, Version 3.2, (2015). [8] A. Kohyama, Y. Kohno, H. Kishimoto, J.-S. Park, H.C. Jung, Industrialization of advanced SiC/SiC composites and SiC based composites; intensive activities at muroran institute of technology under OASIS, IOP conf, Ser.: Mater. Sci. Eng. 18 (2011) 202002. [9] H. Kishimoto, T. Shibayama, K. Shimoda, T. Kobayashi, A. Kohyama, Microstructural and mechanical characterization of W/SiC bonding for structural material in fusion, J. Nucl. Mater. 417 (2011) 387–390. [10] Y. Asakura, H. Kishimoto, J.-S. Park, N. Nakazato, T. Shibayama, A. Kohyama, Thermal property and microstructural characterization of W/SiC clad plates, Fusion Eng. Des. 125 (2017) 484–489. [11] T. Fukuoka, M. Nomura, A. Yamada, Evaluation of thermal contact resistance at the interface composed of dissimilar materials, Jpn. Soc. Mech. Eng. Ser. A 76 (763) (2010) 344–350 (in Japanese).

5. Conclusion The sinter bonding method using SiC green sheets as insert materials successfully produced clad plates of CFC and thick, crystallized tungsten. Microstructural characterization revealed thet diffusion of silicon and formation of several reaction layers including oxides and SiC in the interphase. The flexural strengths of 3-point bending test of the clad plates was between 19 and 35 MPa, which correspond to 40% and 81% strength of the bending strength of CFC along Y-axis. Thermal conductivity evaluation suggested the interphase has relatively high thermal resistivity. But the temperature mismatch between tungsten and CFC was estimated to be roughly 41 K, it is suggested to be enough small to keep the stability of W/CFC clad plate under the thermal flux of 15 MW/m2. Acknowledgment This work was partially supported by the Japan Atomic Energy

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