1. Issues and Prospects of Silicon Carbide Composites for ITER-TBM
Yutai Katoh, Takashi Nozawa and Lance Snead Oak Ridge National Laboratory
For presentation at US ITER-TBM Meeting August 10–12, 2005, Idaho Falls

2. Issues of SiC/SiC Components for ITER TBM
SiC/SiC FCI for ITER DCLL Blanket System
No serious feasibility issues identified
(Dynamic chemical compatibility)
Issues related to performance and/or require design consideration
Thermal conductivity
Electrical conductivity
Deformation / internal stress
(SiC/SiC for ITER HCCB Blanket System)
Mechanical integrity issues
Helium tightness
Fabrication issues
Compatibility issues
Others
Non-destructive or proof testing
For SiC/SiC FCI for ITER DCLL, there has been no serious feasibility issue identified.
For FCCB blankets, these four items are feasibility issues

3. SiC/SiC FCI for DCLL Blanket Thermal Conductivity
Assumption:
Trans-thickness thermal conductivity KT < 5 W/m-K at 500°C maximum acceptable, < 2 W/m-K desirable.
In-plane thermal conductivity KIP does not matter.
Typical 2D CVI SiC/SiC with crystalline SiC fibers:
KT ~ 10 W/m-K at 500°C, unirradiated.
Neutron irradiation further decreases KT.

4. SiC/SiC FCI for DCLL Blanket Thermal Conductivity of Irradiated CVI SiC/SiC
Katoh, ISFNT-7
Hi-Nicalon Type-S
CVI Composites
(Snead)
Unirr.
Irrad.
LEFT:
Kth in a trans-thickness direction is typically ~10 W/m-K at ~500C for nuclear grade 2D composites.
Irradiation decreases Kth drastically.
RIGHT:
Another good news is that we already have developed reliable science base that enables the first order prediction.
The plot is result of calculation based on architectural modeling and linear defect thermal defect resistance.
Discuss numbers.

5. SiC/SiC FCI for DCLL Blanket Thermal Conductivity: Implications
KT of Type-S or Tyranno-SA CVI SiC-matrix composites at 500°C will be ~10 W/m-K unirradiated and ~3 W/m-K irradiated (>~1 dpa).
KT < 2 W/m-K should be achievable by (1) incorporating closed pores at around mid-plane (engineered porous mid-plane approach), or (2) impurity doping. Porous mid-plane approach is more compatible with other requirements (electrical, mechanical).
Modeling capability:
Sufficient science base exists for the estimation of irradiated thermal conductivity of SiC/SiC composites in various architectures.
Initial R&D Requirement:
Identify appropriate method(s) for engineering porous mid-plane components.
Micro-porous mid-plane
(Partial PIP, SiC foam)
Meso-porous mid-plane
(Loose stack 2D)
Open core weaves
(Truss core, etc.)

6. SiC/SiC FCI for DCLL Blanket Electrical Conductivity
Assumption:
Trans-thickness electrical conductivity sT < 500 S/m acceptable (for low pressure drop), < 20 S/m desirable, < 1 S/m admirable (flat velocity profile).
In-plane electrical conductivity sIP does not matter unless >> 103 S/m.
Typical CVI SiC/SiC:
sIP ~ 103 S/m for PyC-interphase composites (Snead, J. Nucl. Mater , 2004, 524, Scholz et al, J. Nucl. Mater , 2002, 1098). sIP is determined primarily by conduction through PyC (Katoh, ISFNT-7).
sT >(>>) 20 S/m for many CVI SiC matrix composites, although determined primarily by quality (impurity) of SiC matrices.

7. Usual n-type CVI SiC matrix will provide high composite conductivity.
SiC/SiC FCI for DCLL Blanket Electrical Conductivity of CVD SiC and Near-stoichiometric SiC Fibers
Usual n-type CVI SiC matrix will provide high composite conductivity.
<~20 S/m at 500C may be possible for CVI SiC matrix compensated with Al, etc. (R&D required)
Irradiation (and transmutation) effect has to be confirmed for high resistivity matrix.
For <<20 S/m at >500C, alternate approaches will be necessary.
R&H STD (Vender)
~60 meV (N)
HNLS (Scholz, 2002)
~940 meV (?)
TySA (Scholz, 2002)
~220 meV (Al)
20 S/m
R&H HR (ORNL/PNNL)
~420 meV (?)
500°C
Intrinsic RT resistivity of SiC is very high (insulating). So is band gap.
Resistivity of SiC with finite impurity levels is determined by impurity, in proportion to impurity concentration. Major impurity elements are N or B, still in order of ppm or sometimes even less. N makes n-type SiC, B p-type.
Responsible mechanism for T dependence may change with T. That means it is necessary to perform measurement up to the highest application T.
R&H HR (Vender)
~470 meV (?)

8. SiC/SiC FCI for DCLL Blanket Measurement of Trans-thickness Electrical Conductivity
For 2-probe measurement, good Ohmic SiC-metal contacts are required. They are usually obtained using Ni, Co, etc. (contact resistivity << 10-3 ohm-cm2)
4-probe technique is not ideal because of the flat specimen geometry and structural non-uniformity. However, there will be a way to analytically derive trans-thickness conductivity. V A V
Metals with better catalytic graphitization activities form better Ohmic contacts on metal/SiC structures.

9. High Purity (Ellison et al)
SiC/SiC FCI for DCLL Blanket Methods for (Semi-) Insulating SiC-based Matrices
Fibers
R&H STD
20 S/m
1 S/m
R&H HR
V-doped,
Sublimation
(Mitchel et al)
Insulating interlayer
High Purity (Ellison et al)
SiCN Layer
V-doped,
Implantation
(Kimoto et al)
SiC
Matrix
1000°C
500°C
Fiber
Graded insulating interlayer
Impurity doping / purification

10. SiC/SiC FCI for DCLL Blanket Electrical Conductivity: Implications
Trans-thickness electrical conductivity (sT) of standard CVI SiC-matrix composites at 500°C will be in order of 100 – 1000 S/m. Combined with porous mid-plane approach, maximum acceptable <500 S/m will be achieved.
Compensated or purified ‘high resistivity’ matrix combined with porous mid-plane may achieve sT ~ 20 S/m. R&D required. Transmutation effect needs attention.
For sT < 20 S/m, more effective measure will have to be taken. Options include (1) incorporation of insulating interlayers [eg. oxide], (2) incorporation of (semi-) insulating matrix second phase [eg. SiCN], and (3) doping for semi-insulating SiC matrix [V].
Initial R&D Requirements:
Establish a reliable technique to measure trans-thickness electrical conductivity at elevated temperatures.
Identify appropriate method(s) for engineering porous mid-plane components.
Survey applicability of high resistivity matrix to CVI composites.
Identify a few most promising methods for (semi-)insulating SiC-based matrices, and develop R&D plans for them.
Develop strategy to identify irradiation and transmutation effect. (High resistivity SiC in 18J)

11. SiC/SiC FCI for DCLL Blanket Deformation and Internal Stress Issues
Trans-thickness temperature gradient (DT ) in FCI walls causes:
Deformation in the non-constrained direction
Internal stress in the constrained direction
Responsible mechanisms:
Differential thermal expansion (Deth)
Differential irradiation-induced swelling (Des)
Insufficient pressure equalization gives external stress
Need assessment
Analytical capability:
Deformation and/or internal stress can be analyzed based on currently available material data and assumed / modeled properties for wall structure specific to FCI, ignoring the effect of irradiation creep.
Irradiation creep may play a key role in moderating the internal stress.

12. SiC/SiC FCI for DCLL Blanket Differential Thermal Expansion
Deformation due to differential thermal expansion:
Mid-point deflection (D):
Thermal stress when flexurally constrained:
Thermal stress may cause interlaminar shear failure for poorly-infiltrated 2D composites or matrix cracking in dense composites.
t = 5 mm
L = 200 mm
a = 4.5E-6 K-1
DT = 200 K
Deth= 0.09%
R ~ 5.6E+3 mm
q ~ 2.1°
D ~ 0.9 mm
E = 200 GPa
smax ~ 90MPa DT t L R q

13. SiC/SiC FCI for DCLL Blanket Lattice Swelling of SiC
Swelling of beta-SiC
(single-beam)
1400°C
800°C ?
Pramono, et al.
~8x10-6 K-1
~6.8x10-6 K-1

14. SiC/SiC FCI for DCLL Blanket Differential Swelling
Bend radius (R) due to differential swelling:
Mid-point deflection (D):
Thermal stress when flexurally constrained:
This will cause interlaminar shear failure in poorly-infiltrated 2D composites and may induce matrix micro-cracking in properly infiltrated 2D composites.
t = 5 mm
L = 200 mm
b ~ 8E-6 K-1
DT=200 K
Des= 0.16%
R ~ 3.1E+3 mm
q ~ 3.7°
D ~ 1.6 mm
E = 200 GPa
smax ~ 160 MPa DT t L R q

15. SiC/SiC FCI for DCLL Blanket Effect of Irradiation Creep on Time-evolution of Internal Stress due to Differential Swelling: Preliminary Model Analysis
E = 200 GPa
Substantial irradiation-induced internal stress will develop at <1 dpa due to differential swelling.
No credible data are available for irradiation creep compliance of SiC.

16. Action / Initial R&D Requirements:
SiC/SiC FCI for DCLL Blanket Deformation and Internal Stress Issues : Implications
Differential thermal expansion (Deth) and differential swelling (Des) due to the trans-thickness temperature gradient (DT) result in bending and/or internal stress in FCI.
Bending and/or internal stress due to Deth and Des occur in opposite signs but would not cancel each other; Deth occurs immediately after starting operation, whereas Des develops over a long period.
Both Deth and Des result in slight distortion onto cross-sectional geometry (non-constrained) and may impose substantial internal stress in longitudinal bending (constrained).
The internal stress may be reduced by (1) architectural design for low flexural modulus, (2) geometrical design that allows longitudinal bending deformation, and (3) irradiation creep.
Action / Initial R&D Requirements:
Thoroughly assess external stresses.
Confirm if expected cross-sectional distortion is within design allowance.
Develop material design strategy for low flexural modulus and compatibility with other requirements (eg. porous mid-plane)
Develop FCI design strategy that allows longitudinal bending deformation of each face.
Develop method to determine stiffness matrix of relevant materials.
Develop research plan to be able to predict irradiation creep compliance of FCI material.

17. SiC/SiC FCI for DCLL Blanket Summary / Perspective
2D composites, poorly or more properly infiltrated, will achieve the minimum insulation requirements. Mechanical integrity under thermal stress will be the primary issue for such 2D composite FCI. Flexure and interlaminar shear are the most likely failure modes.
Electrical conductivity can be substantially reduced by incorporation of oxide or SiCN matrix interlayers. These two approaches are industrially available and therefore feasible with limited R&D effort.
Thermal insulation and the thermal stress are incompatible issues, unless employing suitable architectural fabric features. The architectural approach will be feasible, as several appropriate specialty weaves are commercially available.

18. SiC/SiC FCI for DCLL Blanket SiC/SiC FCI Envisioned for Two R&D Cases
Minimum R&D
Extensive R&D
Poorly-Infiltrated 2D
Standard SiC Matrix
Thin PyC Interphase
KT(irr) = ~3 W/m-K
sT = ~100 S/m
Design has to account for very weak interlaminar shear strength
Architecturally Designed 3D
Insulating SiC-based Matrix
PyC Interphase
KT(irr) = ~1 W/m-K
sT << 1 S/m
Enhanced tolerance against internal shear stress

19. SiC/SiC FCI for DCLL Blanket R&D Items (preliminary proposal)
For electrical (and thermal) insulation:
Establish a reliable technique to measure trans-thickness electrical conductivity of SiC/SiC plates at elevated temperatures.
Identify appropriate method(s) for engineering porous mid-plane components, compensated high resistivity SiC matrix, and insulating SiC-based matrices. Trial-fabricate flat plates of these composites and evaluate baseline properties.
Perform small scale irradiation experiment (rabbit type) on insulating SiC matrix composites.
For mechanical integrity:
Perform detailed evaluation of cross-sectional and longitudinal stress / strain due to thermal gradient. Interact with design community.
Design materials / components for low flexural / trans-thickness shear moduli and compatibility with other requirements (eg. porous midplane)
Determine stiffness matrix and other design properties for FCI material. Develop appropriate test methods.
Determine irradiation creep compliance of FCI material.
Perform mock-up (-like) testing to ensure mechanical integrity and sealing are maintained up to design maximum thermal gradient.

20. Estimated SiC/SiC FCI Cost
Assumes a square tube of 1m x 20cm x 10cm x 5mm(t).
Matrix infiltration: k
Fiber (20% Vf): ~30k (Hi-Nicalon™ Type S) / ~10k (Tyranno™-SA3)
Weaving: ~50k
Total cost per tube: k
CG-Nicalon™
Hi-Nicalon™ Type-S
Tyranno™-SA3
Manufacturer
Nippon Carbon Co.
Ube Industries, Ltd.
Price
~800$/kg
~14k $/kg
~5k $/kg
Production Capacity (2005)
>10,000 kg/yr
~30 kg/yr
~200,000 kg/yr
Max. Temp. (non-oxidizing)
~1,200°C
~1,400°C
~1,800°C
Thermal Conductivity (RT)
~2 W/m-K
~18 W/m-K
~65 W/m-K
Irradiation Stability
Poor (<<1 dpa)
Proven
Proven

21. SiC/SiC FCI for DCLL Blanket R&D Plan (preliminary proposal)
Year 1 2 3 4 5
Insulation
Establish measurement technique X
Materials development
Irradiation effect
Mechanical
Stress analysis
Materials design
Test method development
Irradiation creep
Mock-up test
Others
Database
NDT development