Issues and Prospects of Silicon Carbide Composites for ITER-TBM Yutai Katoh, Takashi Nozawa and Lance Snead Oak Ridge National Laboratory For presentation

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

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

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.


Thermal Conductivity of Irradiated CVI SiC/SiC

0

2

4

6

8

10

12

14

16

18

20

0 200 400 600 800

Tmeas [C]

Kth

[W

/m-K

]

Non-irr.0.1 dpa at 200C0.1 dpa at 400C0.5 dpa at 400C0.1 dpa at 600C1-3 dpa at 800C1-4 dpa at 500C1-4 dpa at 300C

Unirr.

Irrad.

Hi-Nicalon Type-SCVI Composites

(Snead)

0

5

10

15

20

25

30

0 200 400 600 800 1000 1200

Irradiation / Test Temperature (°C)T

her

rmal

Co

nd

uct

ivit

y (W

/m-K

)

2D-PW TySA / PyC, Through-thickness, Model

5HSW HNLS / PyC, Through-thickness, Model

CVD SiC, Model

CVD SiC, Experiment (Snead, 2004)

Katoh, ISFNT-7


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.)


Electrical Conductivity

•Assumption: – Trans-thickness electrical conductivity T < 500 S/m acceptable

(for low pressure drop), < 20 S/m desirable, < 1 S/m admirable (flat velocity profile).

– In-plane electrical conductivity IP does not matter unless >> 103 S/m.

•Typical CVI SiC/SiC:– IP ~ 103 S/m for PyC-interphase composites (Snead, J. Nucl.

Mater. 329-333, 2004, 524, Scholz et al, J. Nucl. Mater. 307-311, 2002, 1098). IP is determined primarily by conduction through PyC (Katoh, ISFNT-7).

– T >(>>) 20 S/m for many CVI SiC matrix composites, although determined primarily by quality (impurity) of SiC matrices.


Electrical Conductivity of CVD SiC and Near-stoichiometric SiC Fibers

-6

-4

-2

0

2

4

6

8

10

0.000 0.001 0.002 0.003 0.004

1/T (1/K)

Ln[E

lect

rical

Con

duct

ivity

(S/

m)]

R&H HR (Vender)~470 meV (?)

R&H HR (ORNL/PNNL)~420 meV (?)

TySA (Scholz, 2002)~220 meV (Al)

R&H STD (Vender)~60 meV (N)

HNLS (Scholz, 2002)~940 meV (?)

•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.

50

0°C

20 S/m


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

AV

-30

-25

-20

-15

-10

-5

0

5

10

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

1/ T (1/ K)

Ele

ctri

cal C

onduct

ivit

y (

S/m

)

Fiber

SiCMatrix

SiCN Layer


Methods for (Semi-) Insulating SiC-based Matrices

Insulating interlayer

Graded insulating interlayerImpurity doping / purification

V-doped, Implantation (Kimoto et al)

R&H HR

R&H STDFibers

50

0°C

20 S/m

1 S/m

High Purity (Ellison et al)

10

00°C

V-doped, Sublimation

(Mitchel et al)


Electrical Conductivity: Implications•Trans-thickness electrical conductivity (T) 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 T ~ 20 S/m. R&D required. Transmutation effect needs attention.

•For T < 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)


Deformation and Internal Stress Issues

•Trans-thickness temperature gradient (T ) in FCI walls causes:– Deformation in the non-constrained direction– Internal stress in the constrained direction

•Responsible mechanisms:– Differential thermal expansion (th)– Differential irradiation-induced swelling (s)

•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.


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

T

ttR

th

t

L

R

R

LRD

2cos1

2max

TE

t = 5 mmL = 200 mm = 4.5E-6 K-1

T = 200 K

th= 0.09%R ~ 5.6E+3 mm ~ 2.1° D ~ 0.9 mm

E = 200 GPa

max ~ 90MPa


Lattice Swelling of SiC

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

200 600 1000 1400 1800

Irradiation temperature / K

Sw

elli

ng

/ %

Thorne (1967) Price (1969,1974) Blackstone (1971) Snead (1998) Snead (2001)Katoh, Si ion (2002)

~6.8x10-6 K-1~8x10-6 K-1

Pramono, et al.

0.1

1

10

0.01 0.1 1 10

Fluence / dpa

Sw

elli

ng

/ %

60°C

200°C

400°C600°C

1000°C1200°C

Swelling of beta-SiCSwelling of beta-SiC(single-beam)(single-beam)

1400°C

800°C

?


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

T

ttR

s

t

L

R

R

LRD

2cos1

2max

TE

t = 5 mmL = 200 mm ~ 8E-6 K-1

T=200 K

s= 0.16%R ~ 3.1E+3 mm ~ 3.7° D ~ 1.6 mm

E = 200 GPa

max ~ 160 MPa


Effect of Irradiation Creep on Time-evolution of Internal Stress due to Differential Swelling: Preliminary Model Analysis

0.01

0.1

1

10

0.001 0.01 0.1 1 10

DPA

Sw

elli

ng (

%)

400C, self ion data

600C, self ion data

400C, model

600C, model

0

50

100

150

200

250

300

350

400

450

500

0 1 2 3 4 5

DPAM

axim

um

Inte

rnal Str

ess

(M

Pa)

No Irradiation Creep

Kic = 1E-6 /MPa-DPA

Kic = 1E-5 /MPa-DPA

STbaS

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.


Deformation and Internal Stress Issues : Implications•Differential thermal expansion (th) and differential swelling (s) due to the trans-thickness temperature gradient (T) result in

bending and/or internal stress in FCI. •Bending and/or internal stress due to th and s occur in

opposite signs but would not cancel each other; th occurs immediately after starting operation, whereas s develops over a long period.

•Both th and s 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.


Summary / Perspective

1. 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.

2. 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.

3. 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.


SiC/SiC FCI Envisioned for Two R&D Cases

Minimum R&D

Poorly-Infiltrated 2DStandard SiC MatrixThin PyC Interphase

KT(irr) = ~3 W/m-K

T = ~100 S/m

Design has to account for very weak interlaminar shear strength

Extensive R&D

Architecturally Designed 3DInsulating SiC-based Matrix

PyC Interphase

KT(irr) = ~1 W/m-K

T << 1 S/m

Enhanced tolerance against internal shear stress


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.

Estimated SiC/SiC FCI Cost•Assumes a square tube of 1m x 20cm x 10cm x 5mm(t). •Matrix infiltration: 50-100k•Fiber (20% Vf): ~30k (Hi-Nicalon™ Type S) / ~10k (Tyranno™-

SA3)•Weaving: ~50k•Total cost per tube: 100-200k

CG-Nicalon™ Hi-Nicalon™ Type-S Tyranno™-SA3

Manufacturer Nippon Carbon Co. 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 (8dpa @300-

800°C)Proven (3dpa @600-

950°C)


R&D Plan (preliminary proposal)Year

1 2 3 4 5

Insulation

Establish measurement technique X

Materials development X X

Irradiation effect X X

Mechanical

Stress analysis X

Materials design X X

Test method development X X

Irradiation creep X X X X

Mock-up test X X X

Others

Database X X X X

NDT development X X

Documents

Issues and Prospects of Silicon Carbide Composites for ITER-TBM Yutai Katoh, Takashi Nozawa and Lance Snead Oak Ridge National Laboratory For presentation