《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_SiC-SiC COMPOSITE

Journal of Nuclear Materials 386-388(2009)622-627 Contents lists available at Science Direct Journal of Nuclear materials ELSEVIER journalhomepagewww.elsevier.com/locate/jnucmat Recent advances and issues in development of silicon carbide composites for fusion applications T Nozawa,, T Hinoki, A Hasegawa, A Kohyama, Y Katoh, L.L. Snead, C H. Henager ] r J. B ] Hegeman Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Ibaraki 319-1195. d Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011 ity. 6-6-01-2 Aramaki-aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan d oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831.USA Pacific Northwest National Laboratory, P 0. Box 999, Richland, WA 99352, USA I NRG Petten, P.O. Box 25, 1755 ZG Petten, The Netherlands A BSTRACT Radiation-resistant advanced silicon carbide(Sic/SiC) composites have been developed as a promising andidate of the high-temperature operating advanced fusion reactor. with the completion of the roof-of-principle' phase in development ofnuclear-grade'SiC/SiC composites, the R&D on Sic/SiC com- tes is shifting toward the more pra hase, i. e. industrialization of component manufacture and data-basing. In this paper, recent advances and issues in(1)development of component fabrication echnology including joining and functional coating, e.g., a tungsten overcoat as a plasma facing barrier, (2)recent updates in charact compatibility with solid lithium-based ceramics and lead-lithium liquid metal breeders, and (3 structural evolution, thermal and electrical conductivities and mechanical properties including the fiber/ minor influence on the composite performance in the design temperature range 2008 Elsevier B V. All rights reserved. 1 Introduction Of many proposed DEMO concepts utilizing SiC/SiC composites. a dual-coolant lead-lithium(Pb-17Li) breeder blanket system is icularly attractive and has been developed as a primary option carbide(SiC/Sic)composites, has been developed for fusion and in US and EU 5.6]. In this system, SiC/SiC composites are used as other nuclear systems incorporating high performance/high radia- flow channel inserts(FCi), which act as a thermal and electrical tion-resistant SiC fibers and matrix, and specially tailored fiber/ insulator for steel structures from Pb-17Li. Understanding the irra- atrix( F/M)interphases [1-3. While the development efforts car- diation effect on electrical conductivity is specifically important for nuclear-grade SiC/SiC composite [1], development continues to SiC composites in the solid-breeder blanket is still viable and lim- mprove on engineering properties of this composite. An example ited number of researches has been conducted. of such continued development is the nano-infiltration transient This paper reviews recent advances in component production eutectic-phase sintering(NITE) SiC/SiC composites [2, 3]. with the technology including joining and coating, research activities for completion of the 'proof-of-principle' phase, the R&d on SiC/Sic the development of the design basis and mechanical property data- composites is shifting to the more pragmatic phase of development base, high-temperature irradiation effects on SiC and Sic/ Sic com- of industrial foundation and material data-basing, which is recog- posites, and chemical compatibility with either solid or liquid nized as an essential tool for engineering design of Sic/Sic compo- breeder. nents. For that purpose, standardization of the material and test odology is of particularly importance and will be discussed 2. Advances in component fabrication technology framework of international collaborations such as broader ach(Ba)activities toward the demo design 4. 2. 1. Composite fabrication Corresponding author. Tel. +81 29 282 6146: fax: +81 29 284 3589. With many efforts on material development in the fundamental technologies for production of'nuclear-gradeSic/Sic 0022-3115/s- see front matter o 2008 Elsevier B V. All rights reserved. o:10.1016Jj 2008.1230

Recent advances and issues in development of silicon carbide composites for fusion applications T. Nozawa a,*, T. Hinoki b , A. Hasegawa c , A. Kohyama b , Y. Katoh d , L.L. Snead d , C.H. Henager Jr. e , J.B.J. Hegeman f a Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Ibaraki 319-1195, Japan b Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan cDepartment of Quantum Science and Energy Engineering, Tohoku University, 6-6-01-2 Aramaki-aza -Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan dOak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA e Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA f NRG Petten, P.O. Box 25, 1755 ZG Petten, The Netherlands article info abstract Radiation-resistant advanced silicon carbide (SiC/SiC) composites have been developed as a promising candidate of the high-temperature operating advanced fusion reactor. With the completion of the ‘proof-of-principle’ phase in development of ‘nuclear-grade’ SiC/SiC composites, the R&D on SiC/SiC com￾posites is shifting toward the more pragmatic phase, i.e., industrialization of component manufactures and data-basing. In this paper, recent advances and issues in (1) development of component fabrication technology including joining and functional coating, e.g., a tungsten overcoat as a plasma facing barrier, (2) recent updates in characterization of non-irradiated properties, e.g., strength anisotropy and chemical compatibility with solid lithium-based ceramics and lead-lithium liquid metal breeders, and (3) irradia￾tion effects are specifically reviewed. Importantly high-temperature neutron irradiation effects on micro￾structural evolution, thermal and electrical conductivities and mechanical properties including the fiber/ matrix interfacial strength are specified under various irradiation conditions, indicating seemingly very minor influence on the composite performance in the design temperature range. 2008 Elsevier B.V. All rights reserved. 1. Introduction A new class of high-temperature structural materials, silicon carbide (SiC/SiC) composites, has been developed for fusion and other nuclear systems incorporating high performance/high radia￾tion-resistant SiC fibers and matrix, and specially tailored fiber/ matrix (F/M) interphases [1–3]. While the development efforts car￾ried out over the past decade has resulted in a radiation-resistant ‘nuclear-grade’ SiC/SiC composite [1], development continues to improve on engineering properties of this composite. An example of such continued development is the nano-infiltration transient￾eutectic-phase sintering (NITE) SiC/SiC composites [2,3]. With the completion of the ‘proof-of-principle’ phase, the R&D on SiC/SiC composites is shifting to the more pragmatic phase of development of industrial foundation and material data-basing, which is recog￾nized as an essential tool for engineering design of SiC/SiC compo￾nents. For that purpose, standardization of the material and test methodology is of particularly importance and will be discussed in the framework of international collaborations such as Broader Approach (BA) activities toward the DEMO design [4]. Of many proposed DEMO concepts utilizing SiC/SiC composites, a dual-coolant lead–lithium (Pb–17Li) breeder blanket system is particularly attractive and has been developed as a primary option in US and EU [5,6]. In this system, SiC/SiC composites are used as flow channel inserts (FCI), which act as a thermal and electrical insulator for steel structures from Pb–17Li. Understanding the irra￾diation effect on electrical conductivity is specifically important for this application because of the paucity of data. Application of SiC/ SiC composites in the solid-breeder blanket is still viable and lim￾ited number of researches has been conducted. This paper reviews recent advances in component production technology including joining and coating, research activities for the development of the design basis and mechanical property data￾base, high-temperature irradiation effects on SiC and SiC/SiC com￾posites, and chemical compatibility with either solid or liquid breeder. 2. Advances in component fabrication technology 2.1. Composite fabrication With many efforts on material development in the past decade, fundamental technologies for production of ‘nuclear-grade’ SiC/SiC 0022-3115/$ - see front matter 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2008.12.305 * Corresponding author. Tel.: +81 29 282 6146; fax: +81 29 284 3589. E-mail address: nozawa.takashi67@jaea.go.jp (T. Nozawa). Journal of Nuclear Materials 386–388 (2009) 622–627 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat

T Nozawa et aL /Joumal of Nuclear Materials s8(2009)622-627 omposites have been established [1. Specifically, chemical vapor For instance, many attempts to detect internal flaws using a non- infiltration( Cvi)method is a well-developed composite fabrication distractive ultrasonic or an X-ray technique have been conducted technology and various components have been produced. For in- [121 stance, a four inch-diameter braided composite tube has been suc- ssfully fabricated by CVI [7. Presently, design-oriented tune of 2. 2. Joining the composites is specifically addressed. To assist densification of the sic matrix, nano whiskers were applied as seeds for the cvl Of many joining techniques [13-23. NITE-joining is very attrac Sic growth [8. In contrast, aiming to have thermal and electrical tive, since it allows a robust structure with the same composition insulation for the FCl application, EU parties have developed very of the matrix of NITE-SiCSiC composites [15]. A 200 mm-long porous CVI SiC/SiC composites by controlling the mid-plane struc- NITE-tube joint was successfully manufactured(Fig. 1(a)). Addi- ture of the composites replaced by porous layers[9). tionally, good applicability of mechanical joints was demonstrated A NITE SiC/SiC composite as another candidate is promising to using screw-threaded tubes(Fig. 1(b))[2, 3]. With advances in meet various industrial demands, i. e, mass production of various NITE-joining and good machinability of dense and robust NITE types of components with reasonably low production cost. Three SiC/SiC composites, the channel structure of a heat exchanger unit types of NITE-SiC/Sic composites:(1)high-ductility, (2) high- was successfully produced. With the multiple-joining steps ap strength, and(3)high-thermal conductivity types are present d, the well-controlled microstructure of the joint was obtained available [10]. In accordance with commercialization of NITE-tech-[16. One drawback to apply NITE-joining is a strict requirement of nology various shapes and sizes of components (plates, thick- pressure under controlled environments. Alternatively, either bricks, thin-wall tubes, large-diameter cylinders, etc. ) can also be high-strength joints using solid-state displacement reactions produced by the near-net shaping method [ 11] [17-22] or preceramic polymer joints [23. which are easily appli- With the recent progress in component fabrication technology, cable and that could be used in field repair situations, have been development of in-service inspection technology becomes an developed. In parallel, to obtain reliable strength data of these important technical issue for quality assurance of the components. joints, the torsional shear testing method, as one of the perceived a Fig. 1. Various types of joint components: (a)a 200 mm-long NITE-joined tube and (b)a screw-threaded NiTE-SiC/ SiC composite joint [3

composites have been established [1]. Specifically, chemical vapor infiltration (CVI) method is a well-developed composite fabrication technology and various components have been produced. For in￾stance, a four inch-diameter braided composite tube has been suc￾cessfully fabricated by CVI [7]. Presently, design-oriented tune of the composites is specifically addressed. To assist densification of the SiC matrix, nano whiskers were applied as seeds for the CVI￾SiC growth [8]. In contrast, aiming to have thermal and electrical insulation for the FCI application, EU parties have developed very porous CVI SiC/SiC composites by controlling the mid-plane struc￾ture of the composites replaced by porous layers [9]. A NITE SiC/SiC composite as another candidate is promising to meet various industrial demands, i.e., mass production of various types of components with reasonably low production cost. Three types of NITE-SiC/SiC composites: (1) high-ductility, (2) high￾strength, and (3) high-thermal conductivity types are presently available [10]. In accordance with commercialization of NITE-tech￾nology, various shapes and sizes of components (plates, thick￾bricks, thin-wall tubes, large-diameter cylinders, etc.) can also be produced by the near-net shaping method [11]. With the recent progress in component fabrication technology, development of in-service inspection technology becomes an important technical issue for quality assurance of the components. For instance, many attempts to detect internal flaws using a non￾distractive ultrasonic or an X-ray technique have been conducted [12]. 2.2. Joining Of many joining techniques [13–23], NITE-joining is very attrac￾tive, since it allows a robust structure with the same composition of the matrix of NITE-SiC/SiC composites [15]. A 200 mm-long NITE-tube joint was successfully manufactured (Fig. 1(a)). Addi￾tionally, good applicability of mechanical joints was demonstrated using screw-threaded tubes (Fig. 1(b)) [2,3]. With advances in NITE-joining and good machinability of dense and robust NITE￾SiC/SiC composites, the channel structure of a heat exchanger unit was successfully produced. With the multiple-joining steps ap￾plied, the well-controlled microstructure of the joint was obtained [16]. One drawback to apply NITE-joining is a strict requirement of pressure under controlled environments. Alternatively, either high-strength joints using solid-state displacement reactions [17–22] or preceramic polymer joints [23], which are easily appli￾cable and that could be used in field repair situations, have been developed. In parallel, to obtain reliable strength data of these joints, the torsional shear testing method, as one of the perceived Fig. 1. Various types of joint components: (a) a 200 mm-long NITE-joined tube and (b) a screw-threaded NITE-SiC/SiC composite joint [3]. T. Nozawa et al. / Journal of Nuclear Materials 386–388 (2009) 622–627 623

T Nozawa et al. Joumal of Nuclear Materials 386-388 (2009)622-627 reliable test methods, has been developed under the international off-axial strength for two-dimensional composites and inter-lami- collaboration nar shear strength for unidirectional composites. It is recognized that these shear properties depend significantly on the F/M inter 2.3. Coating facial strength. The off-axial strength was also evaluated for the braided tube and seemingly a good correlation with the plate re- Metal tungsten coating on the surface of SiC/SiC composites has sults was identified [ 9]. In support of an improved dynamic crack been developed for the plasma facing application of fusion 24-26]. growth model of sic/Sic composites at elevated temperatures A developmental hot pressing technique using nano-powders en- 30,31], time-dependent deformation of off-axial strength was also ables to provide a good bonding at the w/sic joint interface [25). evaluated( Fig. 2)[32 In the figure, experimental data was ob- To assess quality of the w-coating, in situ observation of crack tained at ambient temperature using single-edge-notched beams high voltage electron microscopy a piezoelectric driven set to a ratio of 200 compared to the interfacial shear friction. indentation system 26 The model was also used to simulate a case where matrix spalla tion due to fiber bending would increase the fiber bending moment 3. Status of characterization of mechanical properties and reduce the toughness. The data point at 35 falls below all rea- There are two design concepts for engineering applications of tures a tougines3o tions An inclined fiber bridging model cap- sonable model assu ease with increasing fiber inclination angle. omposites: (1)'damage tolerant' composites and (2)'cracking resistant composites. The former composites are traditional ones 4. Irradiation effects with comparably high-ductility due to weak interfacial strength, while the latter is composites with high proportional limit stress 4.1. Microstructural evolution and swelling (PLS)due to the strong F/M interface. For the primary safety issue, considering the failure behavior associated with matrix cracking is more important rather than composites ultimate fracture since the ited(CVD)B-sic has been investigated by neutron and ion irradia- matrix cracking is closely linked to the durability of the component tion [33-35]. The recent updates 36-38 specified the irradiation functionality, e.g., structural stability, permeability and thermal/ temperature dependence of void formation as(1) sparse formation electrical insulation. In short, the high PLS, i.e., the high cracking- of voids at 1050C,(2)relatively dense and small voids preferen- resistance, is essentially important for the structural/functional de- tially formed at stacking faults above 1300C, and 3)significant ign of the composites. The damage tolerance is also important for growth of voids at 1460C. It is worth noting that the voids appear the secondary safety purpose, i. e, functionality durability, propos- to be faceted with(111) planes. In contrast, the dominant disloca ng a leak before break concept for instance. The damage tolerance tion structure at high-temperatures is concluded as(1)detectable and crack tolerance issues are then being discussed because of Frank loops and (2)black spot defects which were generally dom- their significance [27]. Also, establishment of such design basis inant at lower temperatures. When Tirr=1460.C, corresponding for composite materials is targeted as a primary goal in the ba with the temperature at where steep increase of void size com- activity [4]. For that purpose, standardization of test methodology mences, the dominant defects turn into dislocation networks to and development of a reliable and reproducible database deliver- the fluence of 8.5 x 102 n/m due to the progressive growth able to the dEMo design are ongoing. and development of defects. Of particular importance is that the ef- Strength anisotropy is an important composites specific issue fect of cavity swelling <1300C is negligibly small. Fig 3 shows a for the component design. The strength anisotropy of advanced resultant map of irradiation-induced micro re of Sic. There Sic/SiC composites was determined by the off-axial tensile test seems no significant impact on the application of advanced Sicl [28 or analytically estimated from the combination of in-plane SiC composites in the assumed operating temperat shear and transverse-thickness test results [ 29. Preliminary test results imply the importance of in-plane shear strength to improve Cox Model predictions Larger Loops Dislocation Nebwork Void 1400 e a Frank图… Small Loops e Frank Loops K BSD ar and/or Network MPavmh △ Voids 800F Small Loops 5. a6。5 Fiber Angleφ (egrees Fluence(dpa) Fig. 2. Model calculations of composite toughness, reported as Ko values, for static loading at ambient temperature as a function of fiber inclination angle [32]- Fig 3. Summary of irradiation-induced microstructure of CVD-SiC [361

reliable test methods, has been developed under the international collaboration. 2.3. Coating Metal tungsten coating on the surface of SiC/SiC composites has been developed for the plasma facing application of fusion [24–26]. A developmental hot pressing technique using nano-powders en￾ables to provide a good bonding at the W/SiC joint interface [25]. To assess quality of the W-coating, in situ observation of crack propagation at the W/SiC joining interface was conducted using a high voltage electron microscopy with a piezoelectric driven indentation system [26]. 3. Status of characterization of mechanical properties There are two design concepts for engineering applications of composites: (1) ‘damage tolerant’ composites and (2) ‘cracking￾resistant’ composites. The former composites are traditional ones with comparably high-ductility due to weak interfacial strength, while the latter is composites with high proportional limit stress (PLS) due to the strong F/M interface. For the primary safety issue, considering the failure behavior associated with matrix cracking is more important rather than composites’ ultimate fracture since the matrix cracking is closely linked to the durability of the component functionality, e.g., structural stability, permeability and thermal/ electrical insulation. In short, the high PLS, i.e., the high cracking￾resistance, is essentially important for the structural/functional de￾sign of the composites. The damage tolerance is also important for the secondary safety purpose, i.e., functionality durability, propos￾ing a leak before break concept for instance. The damage tolerance and crack tolerance issues are then being discussed because of their significance [27]. Also, establishment of such design basis for composite materials is targeted as a primary goal in the BA activity [4]. For that purpose, standardization of test methodology and development of a reliable and reproducible database deliver￾able to the DEMO design are ongoing. Strength anisotropy is an important composites’ specific issue for the component design. The strength anisotropy of advanced SiC/SiC composites was determined by the off-axial tensile test [28] or analytically estimated from the combination of in-plane shear and transverse-thickness test results [29]. Preliminary test results imply the importance of in-plane shear strength to improve off-axial strength for two-dimensional composites and inter-lami￾nar shear strength for unidirectional composites. It is recognized that these shear properties depend significantly on the F/M inter￾facial strength. The off-axial strength was also evaluated for the braided tube and seemingly a good correlation with the plate re￾sults was identified [9]. In support of an improved dynamic crack growth model of SiC/SiC composites at elevated temperatures [30,31], time-dependent deformation of off-axial strength was also evaluated (Fig. 2) [32]. In the figure, experimental data was ob￾tained at ambient temperature using single-edge-notched beams of each material. Note that snubbing friction was ignored or was set to a ratio of 200 compared to the interfacial shear friction. The model was also used to simulate a case where matrix spalla￾tion due to fiber bending would increase the fiber bending moment and reduce the toughness. The data point at 35 falls below all rea￾sonable model assumptions. An inclined fiber bridging model cap￾tures a toughness decrease with increasing fiber inclination angle. 4. Irradiation effects 4.1. Microstructural evolution and swelling Irradiation-induced microstructure of chemical-vapor-depos￾ited (CVD) b-SiC has been investigated by neutron and ion irradia￾tion [33–35]. The recent updates [36–38] specified the irradiation temperature dependence of void formation as (1) sparse formation of voids at 1050 C, (2) relatively dense and small voids preferen￾tially formed at stacking faults above 1300 C, and (3) significant growth of voids at 1460 C. It is worth noting that the voids appear to be faceted with {111} planes. In contrast, the dominant disloca￾tion structure at high-temperatures is concluded as (1) detectable Frank loops and (2) black spot defects which were generally dom￾inant at lower temperatures. When Tirr = 1460 C, corresponding with the temperature at where steep increase of void size com￾mences, the dominant defects turn into dislocation networks to the fluence of 8.5 1025 n/m2 due to the progressive growth and development of defects. Of particular importance is that the ef￾fect of cavity swelling <1300 C is negligibly small. Fig. 3 shows a resultant map of irradiation-induced microstructure of SiC. There seems no significant impact on the application of advanced SiC/ SiC composites in the assumed operating temperature range. Fig. 2. Model calculations of composite toughness, reported as KQ values, for static loading at ambient temperature as a function of fiber inclination angle [32]. Fig. 3. Summary of irradiation-induced microstructure of CVD-SiC [36]. 624 T. Nozawa et al. / Journal of Nuclear Materials 386–388 (2009) 622–627

T Nozawa et aL /Joumal of Nuclear Materials s8(2009)622-627 625 4. 2. Thermal conductivity 4.3. Electrical resistivity Thermal conductivity of neutron-irradiated Sic has been re- To enable adequate MHD/thermo-mechanical design of dual ported by many authors [36, 39-41 The rapid decrease and satu- coolant lead-lithium breeder blankets and to develop appropriate ration of the thermal conductivity of CVD-SiC in the point-defect FCI materials, it is essential to understand the irradiation effect regime was analyzed in terms of the thermal resistance induced on the electrical resistivity of Sic ceramics and composites. Post by neutron irradiation [41]. From the recent updates at higher- irradiation electrical conductivity for CVD-Sic and CVI-SiC/Sic temperature irradiation(Fig. 4), it is clearly shown that as the composites identified up to 1. 4-8. 1 dpa at 800-1120C microstructure makes a transition from the point-defect swelling (Fig. 5)142, 43]. Through-thickness electrical conductivity of neu- regime to that associated with void swelling that the thermal tron-irradiated 2D SiC/Sic composites with thin pyrolytic carbon ductivity, as governed by the phonon scattering from these irradi-(Pyc) interphase appeared to be in the order of 10 S/m in the typ- ation-induced defects, undergoes a fundamental shift when ical operating temperature range for FCl. Applying SiC over-coating Tirr 1000C For irradiation above this temperature, the thermal appeared beneficial in reducing electrical conductivity at relatively conductivities are not expected to saturate due to the formation low temperatures in expense of more significant irradiation effect of voids and other complex defects in the high-temperature and steeper temperature dependence The experimental results were successfully analyzed by consti- tutive modeling of the anisotropic electrical transport in two- dimensional woven fabric composites. It was revealed that the interphase network provides imperfect bypass through fabric lay- ers for through-thickness conduction, while axial interphase con- Rpm— duction dominates in most conditions for in-plane ce Specifically, over-coating or internal layer of semi-cond dd a serial resister to the circuit. Electrical conductivity tailoring by engineered interphase structure and configuration in composite materials appears feasible with the aid of modeling Under neutron irradiation, Pyc interphase conductivity may increases, while Sic semi-conducting properties become dictated by radiation defects. Transmutation plays a minor role in fission neutron irradiation, however, that will unlikely be the case for fusion neutrons. 8.5da 4. 4. Mechanical properties The radiation tolerance of strength of high-crystallinity and near-stoichiometric Sic and Sic/Sic composites has been proven at intermediate temperatures by many authors 33, 36, 44-49. Sp ifically, high-temperature irradiation stability of CvI-SiC/SiC com- 200400600 000120014001600 posites has been proven up to Tirr= 1100C. Furthermore, good Irradiation Temperature(C) radiation stability of liquid-phase-sintered (LpS)Sic as a corre- Fig. 4. The thermal conductivity change of Sic by neutron irradiation at high- (a) JMTR: 04M-1 200100 1E+03 豆1E+02 君1E+01 1E+00 yranno-SAPyC 1E01 82 1E-02 (m=6.2) 2.025 Ultimate Flexural Stress, o [MPa (1 dpa-SIC=1.0×1025n/m2) Fig. 5. Neutron irradiation effects on electrical resistivity of sic and CVI-SiC/SiC Fig. 6. The effect of neutron irradiation on the weibull statistical bend strength of

4.2. Thermal conductivity Thermal conductivity of neutron-irradiated SiC has been re￾ported by many authors [36,39–41]. The rapid decrease and satu￾ration of the thermal conductivity of CVD-SiC in the point-defect regime was analyzed in terms of the thermal resistance induced by neutron irradiation [41]. From the recent updates at higher￾temperature irradiation (Fig. 4), it is clearly shown that as the microstructure makes a transition from the point-defect swelling regime to that associated with void swelling that the thermal con￾ductivity, as governed by the phonon scattering from these irradi￾ation-induced defects, undergoes a fundamental shift when Tirr = 1000 C. For irradiation above this temperature, the thermal conductivities are not expected to saturate due to the formation of voids and other complex defects in the high-temperature regime. 4.3. Electrical resistivity To enable adequate MHD/thermo-mechanical design of dual￾coolant lead-lithium breeder blankets and to develop appropriate FCI materials, it is essential to understand the irradiation effect on the electrical resistivity of SiC ceramics and composites. Post￾irradiation electrical conductivity for CVD-SiC and CVI-SiC/SiC composites were identified up to 1.4–8.1 dpa at 800–1120 C (Fig. 5) [42,43]. Through-thickness electrical conductivity of neu￾tron-irradiated 2D SiC/SiC composites with thin pyrolytic carbon (PyC) interphase appeared to be in the order of 10 S/m in the typ￾ical operating temperature range for FCI. Applying SiC over-coating appeared beneficial in reducing electrical conductivity at relatively low temperatures in expense of more significant irradiation effect and steeper temperature dependence. The experimental results were successfully analyzed by consti￾tutive modeling of the anisotropic electrical transport in two￾dimensional woven fabric composites. It was revealed that the interphase network provides imperfect bypass through fabric lay￾ers for through-thickness conduction, while axial interphase con￾duction dominates in most conditions for in-plane conductance. Specifically, over-coating or internal layer of semi-conducting SiC add a serial resister to the circuit. Electrical conductivity tailoring by engineered interphase structure and configuration in composite materials appears feasible with the aid of modeling. Under neutron irradiation, PyC interphase conductivity may increases, while SiC semi-conducting properties become dictated by radiation defects. Transmutation plays a minor role in fission neutron irradiation, however, that will unlikely be the case for fusion neutrons. 4.4. Mechanical properties The radiation tolerance of strength of high-crystallinity and near-stoichiometric SiC and SiC/SiC composites has been proven at intermediate temperatures by many authors [33,36,44–49]. Spe￾cifically, high-temperature irradiation stability of CVI-SiC/SiC com￾posites has been proven up to Tirr = 1100 C. Furthermore, good radiation stability of liquid-phase-sintered (LPS) SiC as a corre- 0 20 40 60 80 100 120 0 200 400 600 800 1000 1200 1400 1600 Room Temperature Thermal Conductivity (W/m-K) Irradiation Temperature (°C) Saturatable Regime Amorphization Regime Non-Saturatable Regime Non-irradiated conductivity ~ 280 W/m-K Rohm Haas CVD SiC ~ 8.5 dpa ~ 5 dpa ~ 1.75 dpa Fig. 4. The thermal conductivity change of SiC by neutron irradiation at high￾temperatures [36]. 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 1000/T[K] Electrical Conductivity [S/m]. CVD SiC 1020ºC / 1.9 dpa CVD SiC 400ºC / 6.4 dpa CVD SiC 640ºC / 3.7 dpa Hi-Nicalon-S/PyC(ML ~100nm) 800ºC / 1.4 dpa Hi-Nicalon-S/PyC (ML ~100nm) 1120ºC / 8.1 dpa Unirr. CVD SiC, Unirr. Temperature [ºC] 1000 500 200 100 20 Tirr Fig. 5. Neutron irradiation effects on electrical resistivity of SiC and CVI-SiC/SiC composites [43]. Unirrad. (m=7.7) 200 500 1000 -5 -4 -3 -2 -1 0 1 2 3 5 5.5 6 6.5 7 7.5 Ultimate Flexural Stress, σi [MPa] ln ln(1/(1-Fi)) ln(σi ) JMTR: 04M-18U, 05M-19U NITE-SiC 400oC 0.50 dpa (m=3.9) (m=3.9) 600oC 0.52 dpa (m=6.2) 600oC 0.55 dpa (m=5.8) 750oC 0.64 dpa (m=5.6) (1 dpa-SiC = 1.0x1025 n/m2) Fig. 6. The effect of neutron irradiation on the Weibull statistical bend strength of LPS-SiC as the corresponding matrix phase of NITE-SiC/SiC composites [47]. T. Nozawa et al. / Journal of Nuclear Materials 386–388 (2009) 622–627 625

26 T Nozawa et aL. /Joumal of Nuclear Materials 386-388(2009)622-627 trilayer 740°℃ 540PC 1030°C 640 080c 380°c 置101ec 1080 380C Fig. 7. The effect on irradiation on(a)interfacial debond shear strength and (b)interfacial friction stress at the fiber matrix interface of CvI-SiC/SiC composites (511 sponding matrix phase of NITE-SiC/SiC composites has been dem- 6. Other critical issues toward DEMO onstrated up to 0.5 dpa at intermediate temperatures(Fig. 6)(47]. although some degradation had been concerned by the presence of Critical issues still remain for this class of materials, including sintering additives as inferred from the previous results for impure (1)determination of the strength limit, correlated with the failure Sic materials 36 behavior, including a strength anisotropy map and lifetime evalu Irradiation effect on the F/M interfacial strength was evaluated ation, i. e fatigue and creep, (2)high-neutron fluence and high by applying analytical models to the fiber push-out test results temperature irradiation effects with considerations about the He [50,51]. It was obvious that both an interfacial debond shear H synergistic effects and dynamic deformation behavior, e.g, irra- strength and an interfacial friction stress slightly decrease by irra- diation creep, (3)He/H permeability and retention, (4)influence of diation depending on neutron dose rather than irradiation temper- nuclear transmutation other than He/H with an origin of metal ature for both PyC monolayer and Pyc/Sic multilayer interphases impurities, and(5)component performance characterization when Tirr 1000C(Fig. 7). Contrarily, when Tirr>1000C. further including joining and coating under fusion relevant environments. deterioration of interfacial shear properties and bulk strength was identified under certain irradiation conditions. However, reason-7. Conclusions ably high interfacial shear strength even after irradiation results in very minor influence on the bulk strength [48, 49 Note that the high interfacial debond shear strength closely link to the high With advances in development of radiation-resistant 'nuclear PLS. As aforementioned, this results in improved cracking-resis- grade'sic/Sic composites, the r&D on SiC/SiC composites is shift tance rather than damage tolerance. In contrast, the damage toler ng toward the more pragmatic phase, i. e, industrialization of com- nent manufactures and data-basing. This paper reviewed recent ance depends on interfacial friction, crack density and others. For advances in(1) component production technology development further understanding at the interfacial stress interaction, the ef- including Joining and coating, (2)characterization of mechanical fect of stress-relief at the interface by irradiation creep needs to be discussed 52. properties. )irradiation effect studies and (4)chemical compati bility with breeding materials. It is concluded that considerable progress has been made in all 4 domains 5. Chemical compatibility Chemical compatibility of Sic/Sic composites with coolant and Reference breeding materials is of great importance to ensure the reliability [11 Y Katoh, LL Snead, C.H. Henager J segawa, A Kohyama, B Riccardi, H. 900006p山myru日0 of the fusion blanket system. Supposing a helium-cooled solid breeder blanket, compatibility between CVD-SiC and Li-based [21 A Kohyama, T Hinoki, T Mizuno, T. Kunugl, M Sato, Y Katoh, J.S. Park, Proc. specifies formation of (1)an adhering substance of Li2SiO3 on the Tsuchiya, A. Moslang. N. Baluc, A. Pizzuto, E.R. Hodgson, R. Laesser, M. Sic surface except for the SiC-LiAlOz pair at 900C and (2)a reac- asparotto, A Kohyama, R Kasada, ma, H. Takatsu, M. Araki, ]. NucL. tion phase only in the Sic-Li4SiO4 pair at 1000C Oxidation by a ntsev. B. Merrill. M limited amount of oxygen contained in a helium coolant is another Youssef, S. Reyes, D K. Sze, N.B. Morley, S. Sharafat, P. Calderoni, G. potential concern. Oxidation kinetics of SiC and Py c on a state of Sviatoslavsky. R. Kurtz, P. Fogarty, S. Zinkle, M. Abdou, Fus. Eng. Des. 81 the F/M interface is being identified [54]. In contrast, good chemi- (6P. Norajitra, L Buhler, U. Fischer, S Gordeev,S Malang, G Reimann, Fus. Eng. cal stability of SiC with Pb-17Li as a breeding material in the liquid blanket system was previously demonstrated <1100.C[55]. Pres- [ RJ. Shinavski, Tz. Engel, R. Battiste, R. Trejo, E. Lara-Curzio Proceedings of the ently erosio sion behavior of Sic Sic composites in a Pb- 181.08A Y, ee, D, choi. ong. CY Pane. w.1. Kim, Am. Ceram. Soc. 84 investigated [56. (2001)245

sponding matrix phase of NITE-SiC/SiC composites has been dem￾onstrated up to 0.5 dpa at intermediate temperatures (Fig. 6) [47], although some degradation had been concerned by the presence of sintering additives as inferred from the previous results for impure SiC materials [36]. Irradiation effect on the F/M interfacial strength was evaluated by applying analytical models to the fiber push-out test results [50,51]. It was obvious that both an interfacial debond shear strength and an interfacial friction stress slightly decrease by irra￾diation depending on neutron dose rather than irradiation temper￾ature for both PyC monolayer and PyC/SiC multilayer interphases when Tirr 1000 C, further deterioration of interfacial shear properties and bulk strength was identified under certain irradiation conditions. However, reason￾ably high interfacial shear strength even after irradiation results in very minor influence on the bulk strength [48,49]. Note that the high interfacial debond shear strength closely link to the high PLS. As aforementioned, this results in improved cracking-resis￾tance rather than damage tolerance. In contrast, the damage toler￾ance depends on interfacial friction, crack density and others. For further understanding at the interfacial stress interaction, the ef￾fect of stress-relief at the interface by irradiation creep needs to be discussed [52]. 5. Chemical compatibility Chemical compatibility of SiC/SiC composites with coolant and breeding materials is of great importance to ensure the reliability of the fusion blanket system. Supposing a helium-cooled solid￾breeder blanket, compatibility between CVD-SiC and Li-based ceramics (LiAlO2, Li4SiO4, Li2ZrO3, and Li2TiO3) was evaluated at 900 and 1000 C for 100 h in helium [53]. The preliminary result specifies formation of (1) an adhering substance of Li2SiO3 on the SiC surface except for the SiC–LiAlO2 pair at 900 C and (2) a reac￾tion phase only in the SiC–Li4SiO4 pair at 1000 C. Oxidation by a limited amount of oxygen contained in a helium coolant is another potential concern. Oxidation kinetics of SiC and PyC on a state of the F/M interface is being identified [54]. In contrast, good chemi￾cal stability of SiC with Pb–17Li as a breeding material in the liquid blanket system was previously demonstrated <1100 C [55]. Pres￾ently erosion/corrosion behavior of SiC/SiC composites in a Pb– 17Li fluid is being investigated [56]. 6. Other critical issues toward DEMO Critical issues still remain for this class of materials, including (1) determination of the strength limit, correlated with the failure behavior, including a strength anisotropy map and lifetime evalu￾ation, i.e., fatigue and creep, (2) high-neutron fluence and high￾temperature irradiation effects with considerations about the He/ H synergistic effects and dynamic deformation behavior, e.g., irra￾diation creep, (3) He/H permeability and retention, (4) influence of nuclear transmutation other than He/H with an origin of metal impurities, and (5) component performance characterization including joining and coating under fusion relevant environments. 7. Conclusions With advances in development of radiation-resistant ‘nuclear￾grade’ SiC/SiC composites, the R&D on SiC/SiC composites is shift￾ing toward the more pragmatic phase, i.e., industrialization of com￾ponent manufactures and data-basing. This paper reviewed recent advances in (1) component production technology development including joining and coating, (2) characterization of mechanical properties, (3) irradiation effect studies and (4) chemical compati￾bility with breeding materials. It is concluded that considerable progress has been made in all 4 domains. References [1] Y. Katoh, L.L. Snead, C.H. Henager Jr., A. Hasegawa, A. Kohyama, B. Riccardi, H. Hegeman, J. Nucl. Mater. 367–370 (2007) 659. [2] A. Kohyama, T. Hinoki, T. Mizuno, T. Kunugi, M. Sato, Y. Katoh, J.S. Park, Proc. ICAPP’05, 2005 (in CD, paper # 5282). [3] T. Hinoki, A. Kohyama, Ann. Chim. Sci. Mater. 30 (2005) 659. [4] T. Nishitani, H. Tanigawa, S. Jitsukawa, T. Nozawa, K. Hayashi, T. Yamanishi, K. Tsuchiya, A. Möslang, N. Baluc, A. Pizzuto, E.R. Hodgson, R. Laesser, M. Gasparotto, A. Kohyama, R. Kasada, T. Shikama, H. Takatsu, M. Araki, J. Nucl. Mater., in these proceedings. doi:10.1016/j.nucmat.2008.12.146. [5] C.P.C. Wong, S. Malang, M. Sawan, M. 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