Fusion Engineering and design ELSEVIER Fusion Engineering and Design 51-52(2000)11-22 www.elsevier.com/locate/fusengdes Status of the European r&d activities on Sicf Sic composites for fusion reactors B. Riccardi a,.P. Fenicib. A. Frias Rebelo b. L. giancarlicg. Le marois d E. Philippe e Associazione EURATOM-ENEA CR FRASCATI, C P.65-00044 Frascati, Rome, Italy b European Commission, JRC Ispra, Tp 202, 21020 Ispra(VA), Italy CEA Saclay, DRN/ DMT, F-91191 Gif-sur-Yuette Cedex, Franc CEA Grenoble, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, france SEP Division of SNECMA, Le Haillan-BP 37-33165 St Medard en Jalles Cedex, france Abstract Silicon carbide composites are a candidate for fusion reactors structural material because of their low activation and after heat properties and good mechanical properties at elevated temperatures. These materials, to be more suitable with their use for fusion energy production, need a strong r&D effort in order to solve some critical issues Ich as thermal conductivity and radiation stability, hermeticity, chemical compatibility with the fusion environment the capability to be formed in complex geometries, the joining process and long production time Constant progress in the fibre quality and matrix-fibre interfaces contribute to support the use of Sicr sic composites as structural material for fusion application. This paper presents an overview of the current status of the European r&d activities on SiCSic composites focussing on reactor design studies, composites manufacturing, material characterisation in particular after irradiation, chemical compatibility with different blanket environments and development of joining techniques. c 2000 Elsevier Science B V. All rights reserved Keywords: Status: European R&D activities; SIC / SIC composites; Fusion reactors 1. Introduction als(Lam) is fundamental as their use will reduce the risk related to accidents will facilitate mainte- The final goal of the Fusion Technology Pro- nance operations and will simplify decommission gramme is power generation under attractive eco- ing and waste management nomical and environmental conditions. In this Among LAMs, the SiCr Sic composites are the pursuit the use of structural low activation materi- ading candidates as structural material for fu- sion reactors due to their good mechanical prop- erties at high temperature, low chemical Corresponding author. Tel: +39-6-94005159: fax: +39. puttering, good resistance to oxidation at high 94005799 temperature(≤1000° and very low short and E-mail address: riccardia frascati. enea. it(B. Riccardi). medium term activation l] 0920-3796/00/S- see front matter c 2000 Elsevier Science B.v. All rights reserved. PI:s0920-3796(00)00311-2
Fusion Engineering and Design 51–52 (2000) 11–22 Status of the European R&D activities on SiCf /SiC composites for fusion reactors B. Riccardi a,*, P. Fenici b , A. Frias Rebelo b , L. Giancarli c , G. Le Marois d , E. Philippe e a Associazione EURATOM-ENEA CR FRASCATI, C.P.65-00044 Frascati, Rome, Italy b European Commission, JRC Ispra, Tp 202, 21020 Ispra (VA), Italy c CEA Saclay, DRN/DMT, F-91191 Gif-sur-Y6ette Cedex, France d CEA Grenoble, 17 rue des Martyrs,F-38054 Grenoble Cedex 9, France e SEP Di6ision of SNECMA, Le Haillan-BP 37-33165 St Medard en Jalles Cedex, France Abstract Silicon carbide composites are a candidate for fusion reactors structural material because of their low activation and after heat properties and good mechanical properties at elevated temperatures. These materials, to be more suitable with their use for fusion energy production, need a strong R&D effort in order to solve some critical issues such as thermal conductivity and radiation stability, hermeticity, chemical compatibility with the fusion environment, the capability to be formed in complex geometries, the joining process and long production time. Constant progress in the fibre quality and matrix–fibre interfaces contribute to support the use of SiCf /SiC composites as structural material for fusion application. This paper presents an overview of the current status of the European R&D activities on SiCf /SiC composites focussing on reactor design studies, composites manufacturing, material characterisation in particular after irradiation, chemical compatibility with different blanket environments and development of joining techniques. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Status; European R&D activities; SICf /SIC composites; Fusion reactors www.elsevier.com/locate/fusengdes 1. Introduction The final goal of the Fusion Technology Programme is power generation under attractive economical and environmental conditions. In this pursuit the use of structural low activation materials (LAM) is fundamental as their use will reduce the risk related to accidents, will facilitate maintenance operations and will simplify decommissioning and waste management. Among LAMs, the SiCf /SiC composites are the leading candidates as structural material for fusion reactors due to their good mechanical properties at high temperature, low chemical sputtering, good resistance to oxidation at high temperature (51000°C) and very low short and medium term activation [1]. * Corresponding author. Tel.: +39-6-94005159; fax: +39- 6-94005799. E-mail address: riccardi@frascati.enea.it (B. Riccardi). 0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0920-3796(00)00311-2
B. Riccardi et al./ Fusion Engineering and Design 51-52(2000)11-22 The SiC SiC composites have been conceived ous design studies which differ on the assumed and developed mainly for aerospace applications safety strategy [2-4]. within the eu the TAURO The optimisation of such material for fusion blanket concept [4] has been proposed by the needs a strong coordination of R&D efforts Commissariat a l' Energie Atomique (CEA).In among research institutes, including participation this concept the chosen safety strategy is based on of industry through all the development stages the minimisation of the energy available within The development of more advanced fibres and the reactor so that the amount of radioactive the enhancement of composites processing meth- material released. in case of severe accident. can ods as well as alternative solutions for fibre -ma e limited. This passive safety concept, which has trix interphase lead to improved thermo- to be applied to all in-vessel components, leads to mechanical characteristics at elevated tempera the use of low pressure coolants with reduced tures. Nevertheless critical issues related to chemical reactivity in air. Liquid Pb-17Li is the nuclear environment are still present; these issues most promising candidate. The main objectives of are mainly connected to the fibres and matrix the TAuRo Study were to find a credible alterna- stability under neutron irradiation, to the poor tive to existing high-pressure He-cooled blanket thermal properties and to the residual porosity In designs, to develop models and design criteria parallel, technology issues must be addressed, e.g. joining methodology and definition of design adapted MC) structural materials. to determine the main issues In parallel to the manufacturing development for a self cooled Pb-17Li using SiC/SiC materials and materials characterisation. fusion reactors de- and to evaluate the limit for fusion application of sign studies using SiCr Sic composites as struc the existing industrial SiCr/SiC composites tural material have been undertaken. These The tauro design is based on the reactor studies, relying on currently available materials specifications defined for the SEAFP Study [5]: 3 data. provide also guidelines for improvements of Gw of fusion power, neutron and heat wall load- ceramic composites, ranking the priorities of fu- Ing of, respectively, 2 and 0.5 MW m-2 and 5 ture developments years of full power continuous operation. The The present overview reports the main achieve- SEAFP conceptual reactor has 6 toroidal field ments of the European R&D activities on Sic/ coils and 48 outboard and 32 inboard segments SiC composites. a description of the design study (about 10 m high) of a SiCr/Sic composites blanket which uses liq The tauRo blanket consists essentially of a uid lithium lead as coolant is discussed. advances SiCSic box containing the Pb-17Li which in manufacturing routes dealing with chemical works as coolant. tritium breeder. neutron multi vapour infiltration(CVI) and polymeric infiltra plier and tritium carrier. The maximum velocity tion and pyrolysis(PIP)are also presented includ- of Pb-17Li is about I m s-l in the channel ing recent results on joining and coating located just behind the Fw. The design is based techniques. Studies on radiation effects on ther- on the assumption, to be experimentally verified, mal conductivity and mechanical properties in that the Sic Sic has enough low electrical con- cluding irradiation creep and compatibility with uctivity to avoid large magneto-hydro-dynamic some solid breeders for long exposure time (MHD) induced pressure drops. Recent studies (10 000 h)are also reported carried out at JRC Ispra [6] indicate, for commer- cially available Sicr/Sic composites, a measured electrical conductivity ranging from 350(@2m) 2. Design at 200oC to 550(Q2m)at 1000C. Therefore, the item need further investigations taking into ac The use of SiC/SiC composite as structural count that the neutron irradiation tends to in- material for fusion power reactor has been pro- crease the electrical conductivity, being the design posed by different institutions by means of vari- reference value 500(Q2m)
12 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 The SiCf /SiC composites have been conceived and developed mainly for aerospace applications. The optimisation of such material for fusion needs a strong coordination of R&D efforts among research institutes, including participation of industry through all the development stages. The development of more advanced fibres and the enhancement of composites processing methods as well as alternative solutions for fibre–matrix interphase lead to improved thermomechanical characteristics at elevated temperatures. Nevertheless critical issues related to the nuclear environment are still present; these issues are mainly connected to the fibres and matrix stability under neutron irradiation, to the poor thermal properties and to the residual porosity. In parallel, technology issues must be addressed, e.g. joining methodology and definition of design criteria. In parallel to the manufacturing development and materials characterisation, fusion reactors design studies using SiCf /SiC composites as structural material have been undertaken. These studies, relying on currently available materials data, provide also guidelines for improvements of ceramic composites, ranking the priorities of future developments. The present overview reports the main achievements of the European R&D activities on SiCf / SiC composites. A description of the design study of a SiCf /SiC composites blanket which uses liquid lithium lead as coolant is discussed. Advances in manufacturing routes dealing with chemical vapour infiltration (CVI) and polymeric infiltration and pyrolysis (PIP) are also presented including recent results on joining and coating techniques. Studies on radiation effects on thermal conductivity and mechanical properties including irradiation creep and compatibility with some solid breeders for long exposure time (10 000 h) are also reported. 2. Design The use of SiCf /SiC composite as structural material for fusion power reactor has been proposed by different institutions by means of various design studies which differ on the assumed safety strategy [2–4]. Within the EU the TAURO blanket concept [4] has been proposed by the Commissariat a l’Energie Atomique (CEA). In this concept the chosen safety strategy is based on the minimisation of the energy available within the reactor so that the amount of radioactive material released, in case of severe accident, can be limited. This passive safety concept, which has to be applied to all in-vessel components, leads to the use of low pressure coolants with reduced chemical reactivity in air. Liquid Pb–17Li is the most promising candidate. The main objectives of the TAURO Study were to find a credible alternative to existing high-pressure He-cooled blanket designs, to develop models and design criteria adapted to ceramic matrix composites (CMC) structural materials, to determine the main issues for a self cooled Pb–17Li using SiCf /SiC materials and to evaluate the limit for fusion application of the existing industrial SiCf /SiC composites. The TAURO design is based on the reactor specifications defined for the SEAFP Study [5]: 3 GW of fusion power, neutron and heat wall loading of, respectively, 2 and 0.5 MW m−2 and 5 years of full power continuous operation. The SEAFP conceptual reactor has 16 toroidal field coils and 48 outboard and 32 inboard segments (about 10 m high). The TAURO blanket consists essentially of a SiCf /SiC box containing the Pb–17Li which works as coolant, tritium breeder, neutron multiplier and tritium carrier. The maximum velocity of Pb–17Li is about1ms−1 in the channel located just behind the FW. The design is based on the assumption, to be experimentally verified, that the SiCf /SiC has enough low electrical conductivity to avoid large magneto-hydro-dynamic (MHD) induced pressure drops. Recent studies carried out at JRC Ispra [6] indicate, for commercially available SiCf /SiC composites, a measured electrical conductivity ranging from 350 (Vm)−1 at 200°C to 550 (Vm)−1 at 1000°C. Therefore, the item need further investigations taking into account that the neutron irradiation tends to increase the electrical conductivity, being the design reference value 500 (Vm)−1
B. Riccardi et al./ Fusion Engineering and Design 51-52(2000)11-22 The design activities have been focussed on the about 1.5 MPa). Stiffeners have also the functions blanket outboard segments. In the most recent of Pb-17Li flow separators design version [7 each outboard segment is di- Because of the low pressure of the coolant fle vided in the poloidal direction in four straight 2.5 FW thickness of 3 mm could be acceptable m high modules, attached on a common thick However, plasma erosion has to be taken into back plate but cooled independently(Fig. 1). Pre- account and the minimum thickness has still to be liminary estimation of the back plate thickness present at the submodule end of life. At this stage leads to the choice of 80 mm. The feasibility of of the design uncertainties are present concerning such a structure has not been analysed in detail the erosion rate of SiC/SiC and the acceptability but a possible solution could be the use of a of direct exposure of Sic/sic to the plasma multilayer plate of SiC/Sic or, because of the regarding the last point the use of protective lower neutron flow at the back plate location, the coatings or monolithic Sic armour could be use of composites with lower resistance to neutron envisaged dose such as the carbon-SiC composites. Each A preliminary fabrication sequence was deter modulus is divided in the toroidal direction in five mined for the TAURO submodule. The different submodules and each of them is supported by the basic components to be manufactured and then back plate and cooled in parallel through a com assembled are shown in Fig. 2. The joining tech mon top horizontal collector formed by two let nique require relatively large contact surfaces els. one for the inlet and one for the outlet flow possible technologies are textile assembling by stitching and co-infiltration during manufacturing module. Within each submodule. the Pb_ILi or brazing. The use of half finished product T flows, at first, poloidally downwards (U=lm )in a thin channel (1.25 cm thickness) located Joints and to improve the stiffness of the module 2-D neutronic analysis was performed by means just behind the FW (6 mm thickness), at the of Montecarlo code TRIPOLI 4, by using the bottom turns in a second channel and flows up, ENDBF-VI transport cross section library [8] then down and up again(at gradually reduced The analysis was aimed at evaluating the overall elocity down to 0.06 m s-) for entering in the performance of the blanket in terms of tritium outlet collector. Toroidal stiffeners plates are re breeding ratio TBR), power density deposition quired for reinforcing the submodule box in order and coolant temperature at the outlet. The results to enable it to withstand the Pb-17Li hydrostatic obtained have shown that the tauro blanket pressure (whose maximum estimated value with 90% enrichment Li can widely fulfil TBR and that lower enrichment can be envisaged. The power density deposition distribution has been used for successive thermal-mechanical analysis For the initial TaURo blanket version [4, the design criteria used were those defined in the ARIES reactor studies [2]. More more severe criteria has recently be defined based mental results on 2D-CERASEPs N2-1 composite [9]. These new criteria take into ac count the orthotropic characteristics of the com- osite and do not distinguish between primary (mechanical) and secondary (thermal)stresses. The assumed limits, which depends on the specific composite and, therefore, are likely to change when more advanced composites will be evalu- Fig. I. The TAURO blanket concept. ated. are 145 MPa for the von mises stresses in
B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 13 The design activities have been focussed on the blanket outboard segments. In the most recent design version [7] each outboard segment is divided in the poloidal direction in four straight 2.5 m high modules, attached on a common thick back plate but cooled independently (Fig. 1). Preliminary estimation of the back plate thickness leads to the choice of 80 mm. The feasibility of such a structure has not been analysed in detail but a possible solution could be the use of a multilayer plate of SiC/SiC or, because of the lower neutron flow at the back plate location, the use of composites with lower resistance to neutron dose such as the carbon–SiC composites. Each modulus is divided in the toroidal direction in five submodules and each of them is supported by the back plate and cooled in parallel through a common top horizontal collector formed by two levels, one for the inlet and one for the outlet flow (Fig. 1). The feeding pipes are located behind the module. Within each submodule, the Pb–17Li flows, at first, poloidally downwards (6=1 m s−1 ) in a thin channel (1.25 cm thickness) located just behind the FW (6 mm thickness), at the bottom turns in a second channel and flows up, then down and up again (at gradually reduced velocity down to 0.06 m s−1 ) for entering in the outlet collector. Toroidal stiffeners plates are required for reinforcing the submodule box in order to enable it to withstand the Pb–17Li hydrostatic pressure (whose maximum estimated value is about 1.5 MPa). Stiffeners have also the functions of Pb–17Li flow separators. Because of the low pressure of the coolant flow, a FW thickness of 3 mm could be acceptable. However, plasma erosion has to be taken into account and the minimum thickness has still to be present at the submodule end of life. At this stage of the design uncertainties are present concerning the erosion rate of SiC/SiC and the acceptability of direct exposure of SiC/SiC to the plasma: regarding the last point the use of protective coatings or monolithic SiC armour could be envisaged. A preliminary fabrication sequence was determined for the TAURO submodule. The different basic components to be manufactured and then assembled are shown in Fig. 2. The joining technique require relatively large contact surfaces: possible technologies are textile assembling by stitching and co-infiltration during manufacturing or brazing. The use of half finished product (T and L shape) is useful to reduce the number of joints and to improve the stiffness of the module. 2-D neutronic analysis was performed by means of Montecarlo code TRIPOLI 4, by using the ENDBF-VI transport cross section library [8]. The analysis was aimed at evaluating the overall performance of the blanket in terms of tritium breeding ratio (TBR), power density deposition and coolant temperature at the outlet. The results obtained have shown that the TAURO blanket, with 90% enrichment 6 Li can widely fulfil TBR and that lower enrichment can be envisaged. The power density deposition distribution has been used for successive thermal-mechanical analysis. For the initial TAURO blanket version [4], the design criteria used were those defined in the ARIES reactor studies [2]. More realistic and more severe criteria has recently be defined based on experimental results on 2D-CERASEP® N2-1 composite [9]. These new criteria take into account the orthotropic characteristics of the composite and do not distinguish between primary (mechanical) and secondary (thermal) stresses. The assumed limits, which depends on the specific composite and, therefore, are likely to change when more advanced composites will be evaluFig. 1. The TAURO blanket concept. ated, are 145 MPa for the von Mises stresses in
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 DETAIL C二 Fig. 2. TAURO blanket exploded view the plane and 110 MPa for the stresses through cific component with fibre architecture. The most the thickness. The thermo-mechanical analysis has widely studied and used fibre are the been performed by using the above new criteria NICALONTM and an elastic behavioural model for the com Since the beginning of 1990s the SEP Division posite. A behavioural model capable of simulating of SNECMA has been involved with the manu- the non linear stress-strain relationship and of facturing of Sic/Sic composites for fusion power predicting the damage status of the composite is reactors [ll]. A standard 2-D composite named under development [9]. Moreover the data used CERASEP N2-1 was used to carry out the initial for the design calculation are related to the sep evaluation work at the start of the European CERASEP N3-I with the further assumption of programme and demonstrate the an improved thermal conductivity in the thickness such material. One of the characteristics of this direction(15W mK ) The calculated maxi- particular SiC/SiC composite was its two dimen- mum shear in the joints between the sub module sional strengthening feature, achieved using a fab- side wall and bottom cup is 60 MPa: this value is ric of NiCaloN CG fibre (with about 12% the limiting design parameter for the join xygen) and by increasing the density of the pre strength. A recently performed parametric study form by a SiC CVI matrix. Due to the geometric has shown that with a FW-thickness of 3 mm and complexity of the parts making up the TauRo a module height of about 80 cm a FW surface blanket and in order to improve the material's heat flux between 0. 7 and I MW m could be shear related properties, CERASEP N3-1 was tolerated by the tauRo design [10] subsequently developed. This material, also pro- duced with NICaloNTM CG fibres offered an innovative 3-D strengthening feature: the 3. Manufacturing GUIPEX texture. CERASEP N2-1 and N3-1 materials offered fairly similar mechanical and Today's industry has a large installed capa hermal properties (Table 1) but the main advan- for full scale production of Sic SiC comp tages of the 3-D material are as follows: improved parts. Moreover the industry is capable of thermal conductivity in the Z direction, increased neerability' of such materials by optimising a spe and more consistent interlaminar shear failure
14 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 Fig. 2. TAURO blanket exploded view. the plane and 110 MPa for the stresses through the thickness. The thermo-mechanical analysis has been performed by using the above new criteria and an elastic behavioural model for the composite. A behavioural model capable of simulating the non linear stress–strain relationship and of predicting the damage status of the composite is under development [9]. Moreover the data used for the design calculation are related to the SEP CERASEP® N3-1 with the further assumption of an improved thermal conductivity in the thickness direction (15 W m−1 K−1 ). The calculated maximum shear in the joints between the sub module side wall and bottom cup is 60 MPa: this value is the limiting design parameter for the joint strength. A recently performed parametric study has shown that with a FW-thickness of 3 mm and a module height of about 80 cm a FW surface heat flux between 0.7 and 1 MW m−2 could be tolerated by the TAURO design [10]. 3. Manufacturing Today’s industry has a large installed capacity for full scale production of SiCf /SiC composites parts. Moreover the industry is capable of ‘engineerability’ of such materials by optimising a specific component with fibre architecture. The most widely studied and used fibre are the NICALON™. Since the beginning of 1990s the SEP Division of SNECMA has been involved with the manufacturing of SiC/SiC composites for fusion power reactors [11]. A standard 2-D composite named CERASEP® N2-1 was used to carry out the initial evaluation work at the start of the European programme and demonstrate the capability of such material. One of the characteristics of this particular SiC/SiC composite was its two dimensional strengthening feature, achieved using a fabric of NICALON™ CG fibre (with about 12% oxygen) and by increasing the density of the preform by a SiC CVI matrix. Due to the geometric complexity of the parts making up the TAURO blanket and in order to improve the material’s shear related properties, CERASEP® N3-1 was subsequently developed. This material, also produced with NICALON™ CG fibres, offered an innovative 3-D strengthening feature: the GUIPEX® texture. CERASEP® N2-1 and N3-1 materials offered fairly similar mechanical and thermal properties (Table 1) but the main advantages of the 3-D material are as follows: improved thermal conductivity in the Z direction, increased and more consistent interlaminar shear failure
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 stress with the absence of interlaminar delamina en and the increase of the matrix crystallinity tion during manufacturing and use, and lower and properties. Recently, the use of an hybrid dispersion of the shear strength process combining CVI and PIp processes al CERASEP N3-1 features can be improved by lowed the fabrication of composites with relevant high purity SiC fibres virtually stoichiomet thicknesses(6 and 10 mm) with an intermediate ric and oxygen freeand with thermal conductiv- matrix structure [13]. Flat panels were produced ity intrinsically higher than that of the with different duration of CVI process and num- NICALONTM CG. Benefits with respect neutron ber of PIP cycles. The maximum density for 10 irradiation induced change in the fibre properties mm thick panels was reached at 60 h of CvI and are expected. Other positive aspects related to the seven PIP cycles( 2.05 g cm-2) use of such fibres are: an increasing in the maxi mum operating temperature from 1100 to 1300oC and improvements in mechanical properties 4. radiation In parallel with Sic, SiC composite manufac turing by the CVI technique, alternative matrix Neutron and particle irradiation induces dis- processing routes are going to be investigated, in placement damage and helium production in SiC particular polymer infiltration and pyrolisis(Pip) based materials, In order to assess the effect of [12]. This technology is less expensive than CVI, irradiation damage and He production on the can be carried out at lower temperature and al- mechanical properties of Sic /SiC structural com- lows the manufacturing of more complex shapes posites a testing campaign was carried out at the ENEA has extensively used preceramic polymers Institute of Advanced Materials/Joint Research with SiC nanopowders. This composites exhibited Centre, Ispra-Italy up to 1998. In particular the interesting mechanical properties in particular material tested was produced by the SEP Division high strain to failure and toughness. As all com- of SNECMA using a 2-D woven laminate of posites produced by PIP, the material exhibits NICALONTM fibres and a CVI infiltration of lower thermal conductivity, poor crystallinity and B-SiC matrix. The neutron irradiation of SiC/ SiC high residual oxygen content. The availability specimens was carried out at the high flux reactor more advanced fibres allowing higher pyrolysis (HFR), Petten-NL up to accumulated damage temperature will permit the reduction of free oxy- doses of 1. 29, 2.69 and 5.23 dpa at 750C irradia- Table I SEP SiCSiC composites main properties roperty Temperature CERASEP N2-1 CERASEP N3. (2-D) 2.5 2.4 10±2 Fibre content (%) Tensile strength(in plane)(MPa) ile strain (in plane)(%) ung module Trans-laminar shear strength Inter-laminar shear strength(MPa) 00-0000000 0.8±0.25 200 200±20 200 MPa 200+20GPa Thermal conductivity (in plane)(Wm-K-) 15 Thermal conductivity(trough the thickness)(W m-K 3±2 Thermal conductivity(trough the thickness)(W m-K Thermal conductivity(trough the thickness)(W m-K Thermal expansion coefficient (in plane)(K-) 4.0×10=6 4.0×10 Thermal expansion coefficient(trough the thickness)(K-) 20 2.5×10
B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 15 stress with the absence of interlaminar delamination during manufacturing and use, and lower dispersion of the shear strength. CERASEP® N3-1 features can be improved by using high purity SiC fibres virtually stoichiometric and ‘oxygen free’ and with thermal conductivity intrinsically higher than that of the NICALON™ CG. Benefits with respect neutron irradiation induced change in the fibre properties are expected. Other positive aspects related to the use of such fibres are: an increasing in the maximum operating temperature from 1100 to 1300°C and improvements in mechanical properties. In parallel with SiCf /SiC composite manufacturing by the CVI technique, alternative matrix processing routes are going to be investigated, in particular polymer infiltration and pyrolisis (PIP) [12]. This technology is less expensive than CVI, can be carried out at lower temperature and allows the manufacturing of more complex shapes. ENEA has extensively used preceramic polymers with SiC nanopowders. This composites exhibited interesting mechanical properties in particular high strain to failure and toughness. As all composites produced by PIP, the material exhibits lower thermal conductivity, poor crystallinity and high residual oxygen content. The availability of more advanced fibres allowing higher pyrolysis temperature will permit the reduction of free oxygen and the increase of the matrix crystallinity and properties. Recently, the use of an hybrid process combining CVI and PIP processes allowed the fabrication of composites with relevant thicknesses (6 and 10 mm) with an intermediate matrix structure [13]. Flat panels were produced with different duration of CVI process and number of PIP cycles. The maximum density for 10 mm thick panels was reached at 60 h of CVI and seven PIP cycles ( 2.05 g cm−2 ). 4. Irradiation Neutron and particle irradiation induces displacement damage and helium production in SiCbased materials. In order to assess the effect of irradiation damage and He production on the mechanical properties of SiCf /SiC structural composites a testing campaign was carried out at the Institute of Advanced Materials/Joint Research Centre, Ispra-Italy up to 1998. In particular the material tested was produced by the SEP Division of SNECMA using a 2-D woven laminate of NICALON™ fibres and a CVI infiltration of b-SiC matrix. The neutron irradiation of SiCf /SiC specimens was carried out at the high flux reactor (HFR), Petten-NL up to accumulated damage doses of 1.29, 2.69 and 5.23 dpa at 750°C irradiaTable 1 SEP SiCf /SiC composites main properties Property Temperature CERASEP CERASEP ® N3-1 ® N2-1 (°C) (3-D) (2-D) Density (g/cm 20 2.5 3 ) \2.4 Porosity (%) 20 10 1092 Fibre content (%) – 40 40 Tensile strength (in plane) (MPa) 300 20 285 920 Tensile strain (in plane) (%) 0.75 0.8 20 90.25 Young modulus (in plane) (GPa) 200 20 200920 Trans-laminar shear strength 20 200 MPa 200920 GPa Inter-laminar shear strength (MPa) 44 20 – Thermal conductivity (in plane) (W m 1000 15 15 −1 K−1 ) Thermal conductivity (trough the thickness) (W m 20 9 1392 −1 K−1 ) Thermal conductivity (trough the thickness) (W m 5.8 7.6 −1 K−1 ) 800 Thermal conductivity (trough the thickness) (W m 1000 5.7 7.5 −1 K−1 ) 20 4.0×10−6 Thermal expansion coefficient (in plane) (K−1 ) 4.0×10−6 2.5×10 – −6 Thermal expansion coefficient (trough the thickness) (K−1 ) 20
B. Riccardi et al./ Fusion Engineering and Design 51-52(2000)11-22 (M P a) Fig 3. Stress displacement curve at RT and 800C for neutron irradiated Sic/SiC composites tion temperature [14]. The average strength of the Sic matrix was observed with the formation of SiC SiC composites in the 'as received condi- enlarged tube-like channels surrounding the tions, measured by a three-point flexural test, was fibres. It has been shown that the linear expansion 305 MPa without significant temperatur of the matrix is poorly prevented by the fibre: in dence. The mechanical response of irradiated ma ruIc ular for 1. 29 dpa the expansion dose deper terials showed a progressive strength degradation dence is below the saturation values and for 2.69 with increasing damage dose. The strength values 5.23 dpa is similar to those of monolithic SiC with partially recovered for tests carried out at a high fibre-matrix mismatches being observed since low temperature(800oC)for all the neutron doses. fluences. The fibre frictional work against the The composites submitted to higher neutron ex- matrix seems to be effective at low doses (1. 29 posures exhibited a brittle behaviour (sharp load dpa); but at higher dose(2.69, 5.23 dpa), the large drop following the point of maximum strength) fibre/matrix detachment makes the fibre frictional with a slight recovery of the elastic modulus(Fig. work rather ineffective and, consequently, deter- 3), which represents the main limitation for the mines a dramatic reduction of the toughness of irradiation response for these materials. At low these composites damage doses (1.29 dpa)an improvement of In parallel with neutron irradiation helium im- toughness was observed but at higher damage plantation campaigns were also carried out at the doses (i.e. 2.69 and 5.23 dpa) both the strength IAM-Ispra cyclotron [15]. The implantation was and the toughness are significantly reduced. The performed at 950oC and helium concentrations up reasons for this decrease became apparent from to 2500 appm where obtained. The mechanical composites morphology and dimensional changes response of the helium implanted composites was induced by the irradiation. The NICALONTM CG evaluated with a post irradiation flexural test car fibres shrank due to the presence of an oxygen ried out at Rt. The results showed a mean 38% rich glassy phase whilst a linear expansion of the decrease of strength and the stress displacement
16 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 Fig. 3. Stress displacement curve at RT and 800°C for neutron irradiated SiC/SiC composites. tion temperature [14]. The average strength of the SiCf /SiC composites in the ‘as received’ conditions, measured by a three-point flexural test, was 305 MPa without significant temperature dependence. The mechanical response of irradiated materials showed a progressive strength degradation with increasing damage dose. The strength values partially recovered for tests carried out at a hightemperature (800°C) for all the neutron doses. The composites submitted to higher neutron exposures exhibited a brittle behaviour (sharp load drop following the point of maximum strength) with a slight recovery of the elastic modulus (Fig. 3), which represents the main limitation for the irradiation response for these materials. At low damage doses (1.29 dpa) an improvement of toughness was observed but at higher damage doses (i.e. 2.69 and 5.23 dpa) both the strength and the toughness are significantly reduced. The reasons for this decrease became apparent from composites morphology and dimensional changes induced by the irradiation. The NICALON™ CG fibres shrank due to the presence of an oxygen rich glassy phase whilst a linear expansion of the SiC matrix was observed with the formation of enlarged tube-like channels surrounding the fibres. It has been shown that the linear expansion of the matrix is poorly prevented by the fibre: in particular for 1.29 dpa the expansion dose dependence is below the saturation values and for 2.69/ 5.23 dpa is similar to those of monolithic SiC with fibre-matrix mismatches being observed since low fluences. The fibre frictional work against the matrix seems to be effective at low doses (1.29 dpa); but at higher dose (2.69, 5.23 dpa), the large fibre/matrix detachment makes the fibre frictional work rather ineffective and, consequently, determines a dramatic reduction of the toughness of these composites. In parallel with neutron irradiation helium implantation campaigns were also carried out at the IAM-Ispra cyclotron [15]. The implantation was performed at 950°C and helium concentrations up to 2500 appm where obtained. The mechanical response of the helium implanted composites was evaluated with a post irradiation flexural test carried out at RT. The results showed a mean 38% decrease of strength and the stress displacement
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 curve showed similar features as those observed W/mk for nanocrystalli to 490 W/mK for 1.29 dpa neutron irradiation, but also a sig- for high purity single cI C according to nificant decrease in the values of the maximum impurity content, lattice density, average deflection. Such behaviour indicates that the typi- grain size, porosity and the presence of amor- cal toughening mechanisms, arising after first ma- phous and or interfacial phases. For current in- trix cracking in unirradiated materials, are dustrial NICALONTM CG/Sic CVi matrix severely degraded following He implantation. a composites the thermal properties are relatively possible explanation for the observed degradation modest, e. g. 12 W m-lK- for 3D SEP is the significant swelling(around 0.4%)[16] in- CERASEP N3-1. This strong reduction, in com- luced in the CVi B-Sic matrix which leads to a posites respect bulk Sic, is primarily due to the wider fibre matrix gap and induces void formation low thermal conductivity of SiC NICALONTM at the interfaces CG fibre. Further contributions to this reduction Irradiation creep tests were conducted at JRC- are related to the matrix porosity and grain siz spra on TEXTRon type SCS-6TM SiC fibres A theoretical thermal conductivity of CVI SiC which are representative of the chemical vapour was calculated from experimental values of com- infiltrated matrix of the Sic/Sic composite be- posite thermal conductivity, corrected to take into are emical v account the measured porosity and the fibre con- position [17]. Sic is known to undergo radiation tent by using a two phase model, which gave tion dose for temperatures below about 1000oC. different batches. The analysis of the granularity of the matrix in the two batches has shown that creep strain in a tensile experiment but plays a the second batch has a better uniform grain size minor role in torsional creep tests. The tests were (100-200 nm)with respect to the first batch, carried out keeping the fibres in torsion under whose grain size showed a relevant variation from irradiation with 14 mev deuterons at different the fibre interface to the outer surface. The sic temperatures(450, 600 and 800C). The curve of matrix thermal conductivity calculated values are torsional creep strain induced by irradiation con- significantly lower than the measured values of a sists of two parts: the former is related to a long CVd bulk Sic (5-10 um in grain size) which lasting transient (with decreasing creep rate) and ranges from 300 W m-K- at room tempera the latter is related to a steady state behaviour ture to 70 W mK at 1000oC Reduced grain whose creep rate decreases with increasing tem- size, lattice defects and growth interlayer are re- perature. a possible explanation of such be- sponsible for this difference Calculation of ther haviour is that the microstructural creep process mal conductivity as a function of grain size by a is based on grain boundary sliding: at low temper- general formulation show a strong influence of the ature the presence of mobile carbon interstitials grain size particularly up to 500 nm. enhances the sliding which is, conversely, blocked cantly reduces the at high temperature, because of the interstitial thermal conductivity. Room temperature thermal concentration reduction and the accumulation of conductivity measurements for 2D NICALONTM immobile vacancies CG/SiC CVI matrix irradiated at 750C shows a The enhancement of thermal conductivity is a sharp reduction to 20% of the initial value for reason to use Sic based CMCs as fusion struc- only I dpa damage level but without significant tural material for FPR blankets. In particular the further degradation at higher doses; for the SiC thermal conductivity values should comply with CVI matrix the estimated thermal conductivity design requirements for heat removal and thermal passes from 51 W m- K-I unirradiated to 9.2 stress reduction. It is well assessed that thermal W m-K- at 1. 29 dpa and 6.5-6.8 Wm- properties of silicon carbide are highly dependent K for 2.69-5.23 dpa(Fig 4). Since post irradi- on the microstructure [18]. At room temperature ation values are well below the design requi the thermal conductivity of Sic varies from 1 ments, the improvement of thermal conductivity
B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 17 curve showed similar features as those observed for 1.29 dpa neutron irradiation, but also a significant decrease in the values of the maximum deflection. Such behaviour indicates that the typical toughening mechanisms, arising after first matrix cracking in unirradiated materials, are severely degraded following He implantation. A possible explanation for the observed degradation is the significant swelling (around 0.4%) [16] induced in the CVI b-SiC matrix which leads to a wider fibre matrix gap and induces void formation at the interfaces. Irradiation creep tests were conducted at JRCIspra on TEXTRON type SCS-6™ SiC fibres which are representative of the chemical vapour infiltrated matrix of the SiC/SiC composite because they are produced by chemical vapour deposition [17]. SiC is known to undergo radiation induced swelling which occurs without an incubation dose for temperatures below about 1000°C. Such swelling in SiC may mask the irradiation creep strain in a tensile experiment but plays a minor role in torsional creep tests. The tests were carried out keeping the fibres in torsion under irradiation with 14 MeV deuterons at different temperatures (450, 600 and 800°C). The curve of torsional creep strain induced by irradiation consists of two parts: the former is related to a long lasting transient (with decreasing creep rate) and the latter is related to a steady state behaviour whose creep rate decreases with increasing temperature. A possible explanation of such behaviour is that the microstructural creep process is based on grain boundary sliding: at low temperature the presence of mobile carbon interstitials enhances the sliding which is, conversely, blocked at high temperature, because of the interstitial concentration reduction and the accumulation of immobile vacancies. The enhancement of thermal conductivity is a reason to use SiC based CMCs as fusion structural material for FPR blankets. In particular the thermal conductivity values should comply with design requirements for heat removal and thermal stress reduction. It is well assessed that thermal properties of silicon carbide are highly dependent on the microstructure [18]. At room temperature the thermal conductivity of SiC varies from 1 W/mK for nanocrystalline fibres to 490 W/mK for high purity single crystal SiC according to impurity content, lattice defect density, average grain size, porosity and the presence of amorphous and/or interfacial phases. For current industrial NICALON™ CG/SiC CVI matrix composites the thermal properties are relatively modest, e.g. 12 W m−1 K−1 for 3D SEP CERASEP® N3-1. This strong reduction, in composites respect bulk SiC, is primarily due to the low thermal conductivity of SiC NICALON™ CG fibre. Further contributions to this reduction are related to the matrix porosity and grain size. A theoretical thermal conductivity of CVI SiC was calculated from experimental values of composite thermal conductivity, corrected to take into account the measured porosity and the fibre content by using a two phase model, which gave, respectively, 51 and 63 W m−1 K−1 for two different batches. The analysis of the granularity of the matrix in the two batches has shown that the second batch has a better uniform grain size (100–200 nm) with respect to the first batch, whose grain size showed a relevant variation from the fibre interface to the outer surface. The SiC matrix thermal conductivity calculated values are significantly lower than the measured values of a CVD bulk SiC (5–10 mm in grain size) which ranges from 300 W m−1 K−1 at room temperature to 70 W m−1 K−1 at 1000°C. Reduced grain size, lattice defects and growth interlayer are responsible for this difference. Calculation of thermal conductivity as a function of grain size by a general formulation show a strong influence of the grain size particularly up to 500 nm. Neutron irradiation significantly reduces the thermal conductivity. Room temperature thermal conductivity measurements for 2D NICALON™ CG/SiC CVI matrix irradiated at 750°C shows a sharp reduction to 20% of the initial value for only 1 dpa damage level but without significant further degradation at higher doses; for the SiC CVI matrix the estimated thermal conductivity passes from 51 W m−1 K−1 unirradiated to 9.2 W m−1 K−1 at 1.29 dpa and 6.5–6.8 W m−1 K−1 for 2.69–5.23 dpa (Fig. 4). Since post irradiation values are well below the design requirements, the improvement of thermal conductivity
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 is mandatory. A tentative route for improving the ceramic breeder LiSiO4 industrially produced by composites thermal properties relies, first of all, Glaswerke Schott( Germany) using the melt spray upon the use of quasi stoichiometric advanced technique has a granular form with an average fibres(Hi-NICALONTM S Type, Dow Corning diameter of 0.55 mm and it contains about 2% Sylramic) with larger grain size. The Cvi process TeO2. Li,TiO, pebbles have a shape and dimen- has to be improved in order to obtain a Sic sions similar to the Li4SiO4 ones and have been matrix with larger grain size. The fibre matrix manufactured by ENEA-Casaccia, starting from interface volume, usually amorphous and lamella commercially available Li2T1O3 powder. The ce- shaped, should be reduced because it acts as a ramic composite investigated was the 3D SEP- thermal barrier between the crystalline phases. In CERASEP N3-1; the composite samples order to face the thermal conductivity decrease (50 x 10 x 3 mm) were subjected to a final uni- fter irradiation and to stay within the design form CVD SiC coating( about 100 um thick). requirements the SiC crystalline morphology of Three cells were operated with LigSiO4 and three fibres and matrix should likely assure, at room with Li,TiO, for 216, 1000 and 10000 h at 800C temperature, a thermal conductivity of 100 W containing 1000 ppm H2; do stream operating samples( so called,)and immersed samples were provided for each cell. The evolution of the mechanical properties was 5. Compatibility nvestigated on immersed and blank samples for each cell before and after test by means of a three As part of the long term European programme point bending test n low activation materials the compatibility of After 10000 h of test, on blank sample SiC /SiC commercial composites with a solid Li, SiO4, lithium meta-silicate Li2SiO3 and other breeders bed in fusion relevant conditions was complex silicates were found indicating lithium investigated [19]. Lithium orto-silicate(Li SiO4), migration out of the reaction chamber. On im foreseen in the European helium cooled pebble mersed samples the original Sic coating was still bed blanket concept and lithium titanate, consid oresent after exposure: its thickness ranged be ered the most promising alternative to it, have tween 80 and 100 um Over it a layer of about 30 been then used for the exposure experiments. The um of lithium meta-silicate (Li2SiO3), not adher ent and heavily cracked, was observed(Fig The transformation of the Sic coating is not protective since a linear law of weight increase was observed but no changes have been detected inside the specimens. Slight changes of the me- chanical properties were observed at different temperatures: in particular, no variations within the experimental error were detected for Young modulus. As far as the flexural strength is con- cerned only the values obtained at 800C showed a worsening of about 25% for samples after 10 000 h of exposure. Therefore, for Li4SiO4 ex posure, a long life time can be foreseen for such kind of composite depending on the SiC coating layer thickness On blank samples over LiTiO, only silica was DOSE LEVEL (dpa) found up to 10 000 h whilst on immersed samples Fig 4. Thermal conductivity of neutron irradiated 2D Nicalon the original Sic coating is not pr half the whole surface and a heavily cracked layer
18 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 is mandatory. A tentative route for improving the composites thermal properties relies, first of all, upon the use of quasi stoichiometric advanced fibres (Hi-NICALON™ S Type, Dow Corning Sylramic) with larger grain size. The CVI process has to be improved in order to obtain a SiC matrix with larger grain size. The fibre matrix interface volume, usually amorphous and lamella shaped, should be reduced because it acts as a thermal barrier between the crystalline phases. In order to face the thermal conductivity decrease after irradiation and to stay within the design requirements the SiC crystalline morphology of fibres and matrix should likely assure, at room temperature, a thermal conductivity of 100 W m−1 K−1 . 5. Compatibility As part of the long term European programme on low activation materials the compatibility of SiCf /SiC commercial composites with a solid breeders bed in fusion relevant conditions was investigated [19]. Lithium orto-silicate (Li4SiO4), foreseen in the European helium cooled pebble bed blanket concept and lithium titanate, considered the most promising alternative to it, have been then used for the exposure experiments. The ceramic breeder Li4SiO4 industrially produced by Glaswerke Schott (Germany) using the melt spray technique has a granular form with an average diameter of 0.55 mm and it contains about 2% TeO2. Li2TiO3 pebbles have a shape and dimensions similar to the Li4SiO4 ones and have been manufactured by ENEA-Casaccia, starting from commercially available Li2TiO3 powder. The ceramic composite investigated was the 3D SEPCERASEP® N3-1; the composite samples (50×10×3 mm) were subjected to a final uniform CVD SiC coating ( about 100 mm thick). Three cells were operated with Li4SiO4 and three with Li2TiO3 for 216, 1000 and 10000 h at 800°C in flowing He containing 1000 ppm H2; downstream operating samples ( so called ‘blank’) and immersed samples were provided for each cell. The evolution of the mechanical properties was investigated on immersed and blank samples for each cell before and after test by means of a three point bending test. After 10000 h of test, on blank samples over Li4SiO4, lithium meta-silicate Li2SiO3 and other complex silicates were found indicating lithium migration out of the reaction chamber. On immersed samples the original SiC coating was still present after exposure: its thickness ranged between 80 and 100 mm. Over it a layer of about 30 mm of lithium meta-silicate (Li2SiO3), not adherent and heavily cracked, was observed (Fig. 5). The transformation of the SiC coating is not protective since a linear law of weight increase was observed but no changes have been detected inside the specimens. Slight changes of the mechanical properties were observed at different temperatures: in particular, no variations within the experimental error were detected for Young’s modulus. As far as the flexural strength is concerned only the values obtained at 800°C showed a worsening of about 25% for samples after 10 000 h of exposure. Therefore, for Li4SiO4 exposure, a long life time can be foreseen for such kind of composite depending on the SiC coating layer thickness. On blank samples over Li2TiO3 only silica was found up to 10 000 h whilst on immersed samples the original SiC coating is not present over about half the whole surface and a heavily cracked layer Fig. 4. Thermal conductivity of neutron irradiated 2D Nicalon CG™f /SiC vs. dose level
Riccardi et al. /Fusion Engineering and Design 5 000)I-22 Scale Residual SiC coating Fibres Matrix Fig. 5. SEM cross section picture of SiC/SiC sample after 10 000 h of exposure to LigSiOa of 80-100 um, identified as various silicates and 6. Joining and coatings lica, was observed, The original Sic layer is full of voids and cracks across its entire thickness sic The SiC /SiC component of a fusion reactor fibres after 10 000 h are not more protected and cannot be realised directly in the finished form but are exposed to the reacting environment. In spite they have to be realised by assembling small of these observations mechanical properties do components or half finished products. In particu- not appear to be significantly varied, indicatin lar the tauRo blanket assembly require the the bulk is not damaged up to 10 000 h. At 800c joining of small and simple parts(flat and curved the composite's Youngs modulus value after panels, T and L sections, etc. ) Nevertheless limi- 10 000 h of exposure is worsened by 30.8% while tations occur in assembling complex geometry a worsening of about 26% of the flexural strength SiC/Sic elements because welding is not possible was observed. Therefore, since the SiC coating is before reaching the melting temperature of the consumed and the fibres are exposed to the gas SiC fibres, with a partial loss of their mechanical phase a rapid reduction of the mechanical proper- properties, and because diffusion bonding does ties are foreseen for longer exposure times not seem suitable since the inter diffusion of sic As far as the compatibility of Sicr/Sic with is very low even at high temperature. For these respect to Pb-17Li is concerned, only partial data reasons the availability of a reliable joining tech- are available and they concern tests performed at nique is fundamental to realise fusion reactors 800C in stagnant liquid lithium lead using 2-D structures. In general the joints shall utilise low SiC/SiC composites. Tests for high Pb-17Li ve- activation elements, should operate at high tem locity (up to 1 m/s), with relevant duration perature(800-1000oC), should have radiation sta (10 000 h)and proper H, content, aimed at evalu- bility even at high temperature and should be ating the impact of Pb-17Li penetration through chemically compatible with the coolant and the Sicrsic thickness the composite proper- breeder. Several joining techniques are under de- ties, are still to be performed velopment. They include: assembling by sewing at
B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 19 Fig. 5. SEM cross section picture of SiC/SiC sample after 10 000 h of exposure to Li4SiO4. of 80–100 mm, identified as various silicates and silica, was observed,. The original SiC layer is full of voids and cracks across its entire thickness. SiC fibres after 10 000 h are not more protected and are exposed to the reacting environment. In spite of these observations mechanical properties do not appear to be significantly varied, indicating the bulk is not damaged up to 10 000 h. At 800°C the composite’s Young’s modulus value after 10 000 h of exposure is worsened by 30.8% while a worsening of about 26% of the flexural strength was observed. Therefore, since the SiC coating is consumed and the fibres are exposed to the gas phase a rapid reduction of the mechanical properties are foreseen for longer exposure times. As far as the compatibility of SiCf /SiC with respect to Pb–17Li is concerned, only partial data are available and they concern tests performed at 800°C in stagnant liquid lithium lead using 2-D SiCf /SiC composites. Tests for high Pb–17Li velocity (up to 1 m/s), with relevant duration (10 000 h) and proper H2 content, aimed at evaluating the impact of Pb–17Li penetration through the SiCf /SiC thickness on the composite properties, are still to be performed. 6. Joining and coatings The SiCf /SiC component of a fusion reactor cannot be realised directly in the finished form but they have to be realised by assembling small components or half finished products. In particular the TAURO blanket assembly require the joining of small and simple parts (flat and curved panels, T and L sections, etc.). Nevertheless limitations occur in assembling complex geometry SiCf /SiC elements because welding is not possible before reaching the melting temperature of the SiC fibres, with a partial loss of their mechanical properties, and because diffusion bonding does not seem suitable since the inter diffusion of SiC is very low even at high temperature. For these reasons the availability of a reliable joining technique is fundamental to realise fusion reactors structures. In general the joints shall utilise low activation elements, should operate at high temperature (800–1000°C), should have radiation stability even at high temperature and should be chemically compatible with the coolant and breeder. Several joining techniques are under development. They include: assembling by sewing at
B. Riccardi et al./Fusion Engineering and Design 51-52(2000)11-22 the textile stage, metallic brazing, homogeneous shear strength, measured by means of an almost joining by preceramic polymers, joining by glass pure shear test were obtained for a joining sin- and glass ceramic tered a-SiC (40 MPa). Concerning SiC Sic com a brazing alloy series for joining 2-and 3-d posites(SEP CERASEP N3-1)the best results SiC/SiC composites has been developed at CEa were obtained using SR 350 silicone resin with Grenoble and named Brasic" [20]. This brazing Al/Si powders used as additives(31.6 MPa maxi compound is composed of low activation elements mum) with pyrolysis carried out at 1200oC and was conceived to work at elevated tempera Ferraris and Salvo [22] from Politecnico of tures and to be compatible with SiC. In partices o the use of pure silicon giving a room temperature Turin developed a joining technology based on the brazing system contain a sufficient amount silicon to prevent reaction with the SiC substrate. shear strength of 22 MPa. Encouraging results promote good wetting and to induce some infiltra- were also obtained using a glass ceramic phase to tion in the composites. Silicon is associated with join SiC CMCs [22]: in this case a 33 MPa shear reactive elements to improve the joining strength. strength were reached at room temperature Glass By using different alloys and compositions(Brasic and glass ceramic compound were also used for H2 and V2 or V3)and brazing in vacuum or in an coating SIC/SiC CMCs. The formulation was re- inert atmosphere it was possible to control the cently optimised in order to give a coating with infiltration of the alloy. USing Brasic V3 and reduced neutron activation (M. Ferraris, Pol carrying out the joining at 1300C in a neutral technic of Turin, personal communication, 1999) atmosphere a sound joint was obtained with aa double layer coating was set up: the first con- perfect filling of the joint gap but no infiltration sists of a glass ceramic phase stable at 800C and of the composite(Fig. 6). For this joint a shea the second consists of a glassy phase which is able perature and about 100 MPa at 800 C. The main about 1100 r crack appearance when heated to strength of 174 MPa was obtained at room tem to self heal aft limitation of the method rely on the free Si con- tent and the open high porosity of the composite This low activation brazing alloy can in principle 7. Conclusions be used for coating SiC/SiC c An homogeneous joining technique has been A specific R&D effort on SiCr/SiC composites developed by ENEA and Padua University [21 is currently ongoing in order to support the use of This technique is based on the application of a the material for FPRs. Significant higher effort preceramic polymer which pyrolysed at high tem both on the theoretical aspects, such as modelica perature to provide an adhesive bonding lay tion and analyses, and on expe riment consisting of a silicon oxi-carbide phase. Relevar ufacturing aspects will be required overt the next Sicdsic improvement and to establish its vance for use as a structural material in in-vessel SiC/SiC On the theoretical side activities include the development of the TaURo blanket conceptual BraSic V3 design with the double objective of:(i) improving behavioural modelisation and results interpreta- tion al nd (ii) supplying useful guidelines for the Sic/SiC material development and characterisation in or der to relax some critical issues concerning the manufacturing of complex shapes. In this respect Fig. 6. Sicy Sic composite brazed joint with a BraSiC v3 alloy the production of small scale mock ups aimed at Q-no infiltration of the composite is observed reproducing the main features of the blanket de
20 B. Riccardi et al. / Fusion Engineering and Design 51–52 (2000) 11–22 the textile stage, metallic brazing, homogeneous joining by preceramic polymers, joining by glass and glass ceramic. A brazing alloy series for joining 2- and 3-D SiC/SiC composites has been developed at CEA Grenoble and named Brasic® [20]. This brazing compound is composed of low activation elements and was conceived to work at elevated temperatures and to be compatible with SiC. In particular the brazing system contain a sufficient amount of silicon to prevent reaction with the SiC substrate, promote good wetting and to induce some infiltration in the composites. Silicon is associated with reactive elements to improve the joining strength. By using different alloys and compositions (Brasic H2 and V2 or V3) and brazing in vacuum or in an inert atmosphere it was possible to control the infiltration of the alloy. Using Brasic V3® and carrying out the joining at 1300°C in a neutral atmosphere a sound joint was obtained with a perfect filling of the joint gap but no infiltration of the composite (Fig. 6). For this joint a shear strength of 174 MPa was obtained at room temperature and about 100 MPa at 800°C. The main limitation of the method rely on the free Si content and the open high porosity of the composite. This low activation brazing alloy can in principle be used for coating SiCf /SiC composites. An homogeneous joining technique has been developed by ENEA and Padua University [21]. This technique is based on the application of a preceramic polymer which pyrolysed at high temperature to provide an adhesive bonding layer consisting of a silicon oxi-carbide phase. Relevant shear strength, measured by means of an almost pure shear test were obtained for a joining sintered a-SiC (40 MPa). Concerning SiCf /SiC composites (SEP CERASEP® N3-1) the best results were obtained using SR 350 silicone resin with Al/Si powders used as additives (31.6 MPa maximum) with pyrolysis carried out at 1200°C. Ferraris and Salvo [22] from Politecnico of Turin developed a joining technology based on the use of pure silicon giving a room temperature shear strength of 22 MPa. Encouraging results were also obtained using a glass ceramic phase to join SiC CMCs [22]: in this case a 33 MPa shear strength were reached at room temperature. Glass and glass ceramic compound were also used for coating SiC/SiC CMCs. The formulation was recently optimised in order to give a coating with reduced neutron activation (M. Ferraris, Polytechnic of Turin, personal communication, 1999). A double layer coating was set up: the first consists of a glass ceramic phase stable at 800°C and the second consists of a glassy phase which is able to self heal after crack appearance when heated to about 1100°C. 7. Conclusions A specific R&D effort on SiCf /SiC composites is currently ongoing in order to support the use of the material for FPRs. Significant higher effort both on the theoretical aspects, such as modelisation and analyses, and on experimental and manufacturing aspects will be required overt the next several years in order to achieve the necessary SiCf /SiC improvement and to establish its relevance for use as a structural material in in-vessel FPR components. On the theoretical side activities include the development of the TAURO blanket conceptual design with the double objective of: (i) improving behavioural modelisation and results interpretation and (ii) supplying useful guidelines for the material development and characterisation in order to relax some critical issues concerning the manufacturing of complex shapes. In this respect the production of small scale mock ups aimed at reproducing the main features of the blanket deFig. 6. SiCf /SiC composite brazed joint with a BraSiC V3 alloy Q — no infiltration of the composite is observed