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SiC/graphite System for High-Heat-Flux Applications
L. L. Snead1, M. Balden2, Rion Causey3, H Atsumi4
1 Oak Ridge National Laboratory, Oak Ridge, Tennessee. USA 2 Max-Planck-Institut für Plasmaphysik, Euratom Association, D-85748 Garching, Germany
3Sandia National Laboratory, Livermore California. USA4Kinki University, Osaka, Japan
Goal
Production of low activation composite with mechanical performance similar to SiC/SiC but with intrinsically higher thermal conductivity.
SiC/Graphite System
Advantages, #1: Literature indicates similar or enhanced mechanical properties (strength/toughness) #2: Significant thermal conductivity enhancement. #3: Reduced tritium retention over best carbon fiber composites
Disadvantage : Unknown radiation performance and limited manufacturing experience
Introduction
Advantage #1: Literature indicates similar or enhanced mechanical properties
TensileStrength (MPa)
SiC/SiC Composite (2-D lay-up) SiC/graphite Composite (2-D lay-up)
* Strength (and toughness) as good or superior to SiC/SiC
Advantage #2: Significant thermal conductivity enhancement
K (T)[ ]−1
=1
Ku(T)+
1Kgb(T)
+1Kd0
+1Krd
⎡
⎣ ⎢
⎤
⎦ ⎥
DefectResistance
0.001
0.01
0.1
1
0.001 0.01 0.1 1 10dpa
Graphite CompositeIrradiated at 300°C
Graphite CompositeIrradiated at 60°C
CVD SiCIrradiated at 60 and 300°C
1/Krd Comp SiC-g
Thermal conductivity is afunction of interstitial migrationenergy at irradiation temp.
Thermal defect resistance termcan be used to calculate thermalconductivity of any pure ceramic(ie if grain boundary scatteringcan be ignored.)
• Maximum irradiated thermal conductivity for SiC is estimated to be ~ 10 W/m-K for T < 500°C, ~37 W/m-K at 700°C.
0
50
100
150
200
250
300
350
0 200 400 600 800 1000 1200
K (W/m-K)
Temperature (C)
Saturation Conductivity for Morton CVD SiC
unirradiatedhigh conductivity
Kirr(T) : W/m-K1/Krd sat.(m-K/W)13.07240-270°C10.09490-510°C37.018690-720°C
1/Krd model
Reference Conductivities
ARIES 20 W/m-KDREAM 15-60 W/m-KTAURO 50 W/m-K
8
10
12
14
16
18
20
22
24
0 200 400 600 800 1000
Sylramic compositeNicalon Type-S Composite
Hi-Nicalon Composite
Temperature (°C)
3-Composite TC
Non-Irradiated
Irradiated
SiC/SiC Composite Thermal Conductivity
• Thermal conductivity of SiC/SiC composites is limited by low conductivity of fiber,low conductivity of matrix, and presence of interfaces (voids, f/m interface, etc.)
1
10
100
1000
200 400 600 800 1000 1200 1400
T-3
Ret
enti
on (
app
m)
Irradiation / T-3 Loading Temperature (C)
Non-irradiated, infinite charge time
Non-Irradiated1 hr Charge Time
High Quality Irradiated CFC (Causey, Snead)
Intermediate Quality Irradiated Graphite (Causey, Snead)
Advantage #3: Reduced tritium retention over best carbon fiber composites
NRL IFE 2/2001
• T-3 attaches to basal plane edges and highly defected structure. More perfect material and/or high temperature allows less retention.
10
100
1000
104
0 20 40 60 80 100
UnirradiatedNeutron Irradiated
Hydrogen Solubility (appm)
Graphitic Perfection (%)
1
10
100
1000
104
0.001 0.01 0.1 1 10
N3M graphiteFMI-222 CFCMKC-1PH CFC
Tritium Retention (appm)
Radiation Damage (dpa)
• Tritium retention, non-irradiated and irradiated, is highly dependent on graphite perfection. K-1100 type fibers are nearly perfect. SiC has very low retention.
• By replacing the lower perfection matrix of CFC’s with SiC, SiC/graphite will have lower retention.
Tirr=600°CTload=1000°C
Tirr=200°CTload=1000°C
Advantage #3: Reduced tritium retention over best carbon fiber composites
Reduced Basal
Plane Edge
Engineered High Thermal Conductivity SiC/G Composite
• Matrix : CVI SiC , no interphase• Fibers : Z-direction either Amoco P55 or Thornel K-1100 fiber X-Y direction Amoco P-55 fiber. Total Volume Fraction 44%.
Fiber K-1100 P-55 Nicalon Type-S
Kth (W/m-K@RT) ~950 120 15Diameter (micron) 10 10 13Tensile Strength (GPa) 3.1 1.9 2.6Tensile Modulus (GPa) 965 379 420Density (g/cc) 2.2 2.0 3.2
P55 fiber K1100 fiber
• Architecture : Unbalanced 1-1-6 weave.
High TC
SEM Image of Polished SiC/g Surface
• Good inter-bundle infiltration (5-8% void) • Large intra-bundle porosity (13% void)
P
55
P55 tow
P55 tow
Bend Testing Results
• Total of 9 tests on CVI SiC/K1100 fiber
Ultimate Bend Strength 283 ± 30 MPa
Macroscopic Matrix Microcracking ~130 Mpa
• Published data on SiC/graphite composite report similar strength to SiC/SiC with some reporting up to 800 MPa for T-300 fiber.
• Published data suggests slightly higher fracture toughness for SiC/graphite.
0
50
100
150
200
250
300
350
0 1 2 3 4 5 6Deflection (mm)
3 x 4 x 50 mm
19 mm
38 mm
CVI SiC/K-1100,P-55 fiber unbalanced weave compositeMajor fiaber axis || to tensile axis
Fle
xura
l Str
engt
h (
MP
a)T
ensi
le S
tren
gth
(M
Pa)
Temperature Dependent Thermal Conductivity
• At fusion-relevant temp., SiC/g:
--> conductivity exceeds present SiC/SiC
--> conductivity exceeds SiC theoretical max.
--> Low TC direction on order of SiC/SiC thermal conductivity (for this composite).
0
50
100
150
200
250
300
350
400
0 200 400 600 800
Thermal Conductivity (W/m-K)
Temperature (C)
Type-S Composite (transverse)
P55 Graphite/CVI SiC (high TC)
Morton CVD SiC
K1100 Graphite/CVI SiC (high TC)
ICFRM10 SiC/G
Neutron Irradiation Data on Thermal Conductivity CVI SiC/P55
P55 fiber
• HFIR Irradiation
• Thermal flash diffusivity
• Thermal conductivity at measurement temperature0
20
40
60
80
100
120
0 200 400 600 800
Temperature (C)
P55 Graphite/CVI SiC (high TC direction)
ICFRM10 SiC/G
800°C , 0.1 dpa
400 and 600°C , 0.5 dpa
Comparison with High Quality Graphite Degradation
0
1
2
3
4
300 400 500 600 700 800 900 1000 1100 1200
Irradiation Temperature (°C)
Degradation in High Conductivity Graphite Composite as a Function of Radiation Damage and Temperature
non-irradiated
irradiated
0.001 dpa
0.005 dpa
0.01 dpa
0.05 dpa0.1 dpa
0.5 dpa
1 dpa
• Limited data set agrees withdegradation expected from high quality graphite modeling.(thermaldefect resistance.)
• At fusion-relevant temp., SiC/g:
--> irradiated TC exceeds max for SiC
0
50
100
150
200
250
300
350
400
0 200 400 600 800 1000 1200
Thermal Conductivity (W/m-K)
Temperature (C)
CVD SiC/K1100 Non-Irradiated
CVD SiC/K1100 Irradiated
CVD SiCNon-Irradiated
CVD SiC Irradiated
Application of graphite thermal conductivity degradation model to SiC/K1100
Summary
• The SiC/graphite systems offer the possibility of acceptable as-irradiated thermal conductivity.
• Composites are easily made by a number of routes. Materials shown in this study were first attempts using isothermal and forced flow CVI SiC, both of which yielded material of quality comparable to SiC/SiC
But…
• In addition to the issues regarding the use of SiC/SiC composites. here are significant issues regarding the use of this material, including.
-- tritium retention-- radiation stability of fiber and overall mechanical lifetime-- effect of fiber shrinkage on thermal conductivity-- erosion and codepositiom issues (if first wall)
Radiation stability of graphite fibers in composites
-4
-3
-2
-1
0
1
⊥
||
0 1 2 3 4 5
Fiber Axis
Fiber Axis
UNIDIRECTIONAL FIBER COMPOSITE
axis parallel to fiber axes
(%)Dimensional Change
dpa
• Graphite fiber composites first gain strength (< few dpa) then rapidly lose strength as c-axis expansion causes widespread microcracking
• Fiber can be expected to shrink axially and swell radially putting interface under tension.
• Loss in strength may occur due to following: -- micro-cracking length of fiber is < lc ( critical crack length ). -- bundle swelling causes significant matrix microcracking
samplesurface
bundleshrinkage
bundle swelling
gap500°C 800°CP55 fiber CFC (FMI-222)
Future Work
• Understand radiation effects in composites with dissimilar swelling and mechanical property changes.
• Confirm thermal conductivity degradation is following thermal defect resistance model.
• Understand tritium retention in very high quality graphite fibers.
• Composite processing optimization with combined SiC and graphite fibers. Eg. High Nicalon Type S SiC fiber combined with high thermal conductivity Pitch-based graphite fiber.
