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 conductivityB. 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 sig￾nificant decrease in the values of the maximum deflection. Such behaviour indicates that the typi￾cal toughening mechanisms, arising after first ma￾trix 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] in￾duced 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 JRC￾Ispra on TEXTRON type SCS-6™ SiC fibres which are representative of the chemical vapour infiltrated matrix of the SiC/SiC composite be￾cause they are produced by chemical vapour de￾position [17]. SiC is known to undergo radiation induced swelling which occurs without an incuba￾tion 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 con￾sists 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 tem￾perature. A possible explanation of such be￾haviour is that the microstructural creep process is based on grain boundary sliding: at low temper￾ature 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 struc￾tural 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 amor￾phous and/or interfacial phases. For current in￾dustrial 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 com￾posites 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 com￾posite thermal conductivity, corrected to take into account the measured porosity and the fibre con￾tent 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 tempera￾ture to 70 W m−1 K−1 at 1000°C. Reduced grain size, lattice defects and growth interlayer are re￾sponsible for this difference. Calculation of ther￾mal 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 irradi￾ation values are well below the design require￾ments, the improvement of thermal conductivity