FS&T Template - Nasr Ghoniem's Matrix Laboratory - UCLA

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Sharafat et al. SiC-FOAM BASED FLOW CHANNEL INSERT FOR DCLL TBM of the FCI. They can take the form of either pressure equalization holes or slots. However, these features have not yet been designed and are currently not included in the analysis. the proposed SiC materials are produced by a CVD process. A potential advantage of using the combination of SiC-foam with CVD face sheets is that CVD SiC materials have been established to have good resistance to neutron irradiation damage. However, the radiation performance of SiC foam materials has yet to be validated. Figure 5 shows a micrograph of the proposed FCI structure, which consists of SiC foam between two CVD SiC face sheets. The parts with 12% SiC foam exhibited flexure strength of 50.3 ± 1.4 MPa, while the parts with 22% SiC foam had a strength of 115.4 ± 7.4 MPa (Ref. 11). The SiC foam is integrally attached (mechanically and chemically bonded) to the CVD SiC faceplate. For a 100 pores per linear inch SiC-foam, there are 10,000 support points per square inch for the SiC face sheet. The foam is integrally fused to the CVD-SiC face sheet, because of the nominally 1 to 2 pore depth penetration of the face sheet into the foam. The reticulated ligamental structure of SiC foams provides favorable thermomechanical properties, such as high thermal shock resistance and the ability to withstand steep temperature gradients, by reacting stress through small movements of individual ligaments. Fully dense SiC structures are not capable of reducing stress through this micro-distortion behavior, and can result in microcracks. Fig. 3: Schematic of the segmented FCI structure; also a cross sectional view of a single sector and a close-up view of the bracket are shown (dimensions are in mm). III.A. SiC Foam-based FCI Material SiC/SiC composites have been suggested as structural materials of FCIs. However, these advanced SiC/SiC composites have thermal and electrical conductivities, which are above the desired limits. Figure 4 shows typical un-irradiated SiC/SiC composite materials properties. Depending on irradiation temperature, dose, and architecture, SiC/SiC can experience a wide range in thermal conductivity degradation of K irr /K o ratios between 0.1 and 0.6 between 400 o C and 1000 o C (Ref. 10). An effort is currently underway to develop new SiC/SiC composites with 2-D fiber architecture to satisfy the key requirements of electrical- and thermal conductivity limits outlined earlier. Here we report on the use off-the-shelf CVD-based SiC materials for the FCI, consisting of open-cell SiC foam sandwiched between CVD SiC face sheets. SiCfoams are routinely manufactured at low cost compared with SiC/SiC composites, which fulfill FCI conductivity requirements. No significant R&D development, especially neutron irradiation data is necessary, as all of Fig. 4: Transverse thermal (top) and electrical (bottom) conductivities of un-irradiated SiC/SiC composites. 9 Figure 6 shows the thermal and electrical conductivities of SiC foam as a function of temperature 886 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009
Sharafat et al. for 65 and 100 pores per linear inch (ppi) foam at 10% and 20% density. The average thermal conductivity of 10% dense, 100 ppi SiC foam is close to 1.0 W/m-K at 700 o C, which is less than the required 2 W/m-K for the FCI structure. The effective electrical conductivity of FCI development structure with 8 mm thick, 20% dense, 65 ppi foam and 0.08 mm thick face sheets was measured and found to be less than 0.2 S/m at a current of 1 A (Ref. 11). Prior to testing, the residual carbon at the core of the SiC foam ligaments was removed by oxidation. The low electrical conductivity of the FCI development piece indicates that design requirements of less than ~1 S/m are readily met using SiC foam sandwiched between two CVD SiC face sheets. SiC-FOAM BASED FLOW CHANNEL INSERT FOR DCLL TBM CVD SiC face sheets. For finite element analysis a three layered material model was used to represent the CVD- Foam-CVD structure of the FCI wall. Here, we assume that each layer consists of a homogeneous material. For the 1-mm thick face sheets, CVD-SiC material properties - measured SiC-foam properties were assigned. (Table II). This analysis is, thus by its nature not exact in that the local response of individual SiC-foam ligaments are not analyzed. Therefore, the uncertainty associated with this FEM analysis must be taken into account in ascertaining the performance of this structure. Fig. 5: (a) Cross-sectional photograph of 10-mm thick 12% dense CVD SiC foam with 1.8 mm thick CVD SiC face sheets; (b) 5-mm thick 22% dense CVD SiC foam with 1.8 mm thick CVD SiC face sheets; (both 5X); (c) FCI development specimen in 4-point bend fixture; (d) post-test photo of torch-tested SiC faceplate surface (1200 o C) (2X). 11 Limited room temperature mechanical property measurements of SiC foam have been made. Figure 7 shows the strength and modulus of SiC foam as functions of bulk density. 11 Under compression, 10% dense SiC foam (0.32 g/cc) had a modulus of about 6 GPa, which rises to about 15 GPa at a 20% density. In tension, the modulus of 10% and 20% dense SiC foams are about 3 GPa and 8 GPa, respectively. Tensile/compressive strengths of 10% and 20% SiC foams are about 3.5/10.5 MPa and 7.6/27.2 MPa, respectively. IV. THERMO-MECHANICAL ANALYSIS The FCI model used for thermo-mechanical analysis is shown in Fig. 3, which shows a cross sectional view of one of five FCI segment. The FCI panel consists of a 3- mm thick foam sandwiched between two 1-mm thick Fig. 6: Thermal conductivity versus temperature of 65 and 100 ppi SiC foam at 10% and 20% dense and electrical conductivity for 20% dense SiC foam with 1.8-mm CVD face-sheets (w/o carbon core). 11 Two thermo-mechanical analyses were conducted, one on a single segment with a constant heat flux from the inside of the FCI and the other on the entire structure using detailed temperature maps, generated in conjunction with magnetohydrodynamic models. 5 The thermal loading conditions for the single FCI segment were chosen such that a mean temperature difference of about 40 o C (Ref. 1) would exist between the inside and the outside of the FCI. A heat flux of 0.2 MW/m 2 was applied to all inner areas of the FCI and the outside surfaces were cooled assuming a PbLi temperature of 450 o C with a heat transfer coefficient of 10 000 W/m 2 -K. The brackets are not bonded to the FCI SiC-foam panels, rather they are simply slid into each other. Hence, there is a thermal resistance across the contact area between the bracket and the face sheet. The heat transfer coefficient of ceramic-toceramic joints ranges between 500 W/m 2 -K to 3000 FUSION SCIENCE AND TECHNOLOGY VOL. 56 AUG. 2009 887
Page 1 and 2: DEVELOPMENT STATUS OF A SiC-FOAM BA
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Page 7 and 8: Sharafat et al. 100% dense CVD SiC
Page 9: We presented here the design, the t

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