SiC/graphite System for High-Heat-Flux Applications L. L. Snead 1, M. Balden 2, Rion Causey 3, H Atsumi 4 1 Oak Ridge National Laboratory, Oak Ridge, Tennessee. USA 2 Max-Planck-Institut für Plasmaphysik, Euratom Association, D Garching, Germany 3 Sandia National Laboratory, Livermore California. USA 4 Kinki 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 Tensile Strength (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 Defect Resistance Thermal conductivity is a function of interstitial migration energy at irradiation temp. Thermal defect resistance term can be used to calculate thermal conductivity of any pure ceramic (ie if grain boundary scattering can 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. Reference Conductivities ARIES 20 W/m-K DREAM W/m-K TAURO 50 W/m-K

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.)

T-3 Retention (appm) Irradiation / T-3 Loading Temperature (C) Non-irradiated, infinite charge time Non-Irradiated 1 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.

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. T irr =600°C Tload=1000°C T irr =200°C Tload=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%. FiberK-1100 P-55 Nicalon Type-S Kth ~ Diameter (micron) Tensile Strength (GPa) Tensile Modulus (GPa) Density (g/cc) P55 fiberK1100 fiber Architecture : Unbalanced weave. High TC

SEM Image of Polished SiC/g Surface Good inter-bundle infiltration (5-8% void) Large intra-bundle porosity (13% void) P55 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. Flexural Strength (MPa) Tensile Strength (MPa)

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).

Neutron Irradiation Data on Thermal Conductivity CVI SiC/P55 P55 fiber HFIR Irradiation Thermal flash diffusivity Thermal conductivity at measurement temperature

Comparison with High Quality Graphite Degradation Limited data set agrees with degradation expected from high quality graphite modeling.(thermal defect resistance.)

At fusion-relevant temp., SiC/g: --> irradiated TC exceeds max for SiC 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 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 < l c ( critical crack length ). -- bundle swelling causes significant matrix microcracking sample surface bundle shrinkage bundle swelling gap 500°C 800°C P55 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.