Transcript Mechanical & Thermal Evaluation of SiC/SiC

Specialized SiC Components for Flow
Channel Insert Applications
R.J. Shinavski
Hyper-Therm HTC
Huntington Beach, CA
714-375-4085
[email protected]
FNST Meeting
UCLA, Los Angeles, CA
August 12, 2008
Hyper-Therm High-Temperature Composites, Inc.
Introduction
• SiC fiber-reinforced silicon carbide matrix (SiC/SiC) composites
combine the attributes of high temperature mechanical strength
and toughness with relative dimensional stability under high
neutron fluence that address the primary requirement of
survivability in application as a flow channel insert
• SiC/SiC composites composed of near-stoichiometric SiC fibers
(Hi-Nicalon Type S, Tyrannos SA3) and a CVI SiC matrix have
shown promise for use in high neutron flux environments and
have been termed nuclear grade SiC/SiC
• Mechanical, thermal, and electrical properties for nuclear grade
SiC/SiC composites will be discussed with regards to the flow
channel insert application
• Designs for a nuclear grade SiC/SiC flow channel insert will also
be discussed
Hyper-Therm High-Temperature Composites, Inc.
SiC/SiC Composites
MLSiC fiber coating
0º Fibers
90º Fibers
CVI SiC
Hi-Nicalon
Type S
• SiC matrix produced by • Hi-Nicalon Type S fiber
• Fiber coating is Hyper-
selected due to greater
existing database on
this fiber showing
radiation resistance
Therm HTC’s MLSiC
fiber coating (US
Patents 5,455,106 and
5,480,707)
•
isothermal/isobaric CVI
Composite bulk
densities 2.7 g/cm3
Hyper-Therm High-Temperature Composites, Inc.
Tensile Properties of Nuclear Grade SiC/SiC
• Tensile strength dependent on architecture
with consistent 0.5% failure strain
• SiC/SiC FCI needs to stay below the
proportional limit strength to maintain Pb-Li
impermeability
• High ultimate strength and failure strain
provide fail-safe behavior
• 5 HS construction considered primary
candidate with unidirectional used for joining
Fiber
Volume
σf
εf
Unidirectional
47%
720 MPa
0.47%
5 Harness Satin
36%
400 MPa
0.54%
Plain Weave
30%
350 MPa
0.45%
Triaxial Braid
23%
190 MPa
0.50%
Architecture
Hyper-Therm High-Temperature Composites, Inc.
Tensile Properties of Nuclear Grade SiC/SiC
• Tensile properties are insensitive to
temperature up to 1200ºC (data still
being accumulated)
• Room temperature tensile testing of 4
batches of nuclear grade SiC/SiC; 16
total samples were tested
• Statistical allowable calculated
• Acceptable stress levels should be less
than 157 MPa if no matrix cracking is to
occur
Mechanical Properties of Nuclear Grade SiC/SiC (5HS)
E
σf
εf
σPL
σILSS(RT)/σILSS(800C)
Mean
270
400 MPa
0.54%
180 MPa
42.3/37.9 MPa
B-basis Allowable*
---
344 MPa
0.39%
157 MPa
23.9/27.0 MPa
* 95% confidence that 90% of the material will be greater than the allowable
Hyper-Therm High-Temperature Composites, Inc.
Irradiation Dimensional Stability
• Dimensional change under neutron
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irradiation is fairly small
Depends on irradiation temperature
and is independent of irradiation
fluence after reaching saturation
Greater swelling at lower
temperatures
500ºC temperature difference will
induce ~0.21% strain
Strain of ~0.13% for 500ºC-300ºC
Irradiation induced strains opposite
thermal expansion strains
Newsome et al., J. Nuclear Mat’ls, 371, 2007, pp 76-89
Hyper-Therm High-Temperature Composites, Inc.
Thermal Expansion
• Thermal expansion of nuclear grade
SiC/SiC is essentially isotropic
• Mean CTE (RT-300ºC)=3.51 ppm/ºC
(RT-500ºC)=3.92 ppm/ºC
(RT-800ºC)=4.38 ppm/ºC
• Approximate strain for 500ºC-300ºC
thermal gradient is 0.09%
• Greater temperature difference
through wall of the FCI will result in
an increasing effect of thermal
expansion as compared to irradiation
dimensional change, but becomes
more unstable due to balance of
larger dimensional changes
• Balance of dimensional changes
(thermal and irradiation) within
nuclear grade SiC/SiC is within the
allowable strain even if restrained
Hyper-Therm High-Temperature Composites, Inc.
Pb-Li Compatibility
• Nuclear grade SiC/SiC has been exposed to Pb-Li at up to 360ºC
• White residue remaining on surface identified to be LiOH
• No LM penetration or degradation observed within composite
• Planned testing includes higher temperatures; overpressure and as
a function of pre-stress level
Hyper-Therm High-Temperature Composites, Inc.
Electrical Properties
• Electrical conductivity measured in the through-thickness and in-plane
directions
• Through-thickness electrical conductivity is ~3 orders of magnitude
lower than in-plane conductivity
• In-plane conductivity dominated by small amount of carbon in fiber
coating
• Meets low through-thickness electrical conductivity requirement to
minimize magnetohydrodynamic pressure drop
Hyper-Therm High-Temperature Composites, Inc.
Thermal Conductivity
• Through-thickness thermal
conductivity of nuclear grade
SiC/SiC is too high to be a sufficient
thermal insulator
• Additions of N were examined
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SiCxNy composition reduced thermal
conductivity to 2.5 W/m/K
However composition not neutronically
favorable and this matrix material will
likely also have poor irradiation stability
Low N additions to CVI SiC had minimal
effect on t-c
• Through-thickness thermal
conductivity of nuclear grade
SiC/SiC does not meet the 1-2
W/m/K targeted requirement for FCI
Hyper-Therm High-Temperature Composites, Inc.
Architectural Construction of FCI
• Add thermal conductivities as thermal resistances in series with
flutes added in parallel to calculate equivalent “bulk” through
thickness thermal conductivity
• Examined strut angle and frequency
• For lower thermal and electrical conductivity, minimize strut crosssection and number of struts/unit length
• For lower thermal conductivity and a higher electrical conductivity,
maximize the core thickness and minimize the face sheet thickness
Hyper-Therm High-Temperature Composites, Inc.
Low Thermal Conductivity Construction
• Equivalent thermal
conductivity of 1.4
W/m/K is predicted
• 1.0 mm face sheets with
0.5 mm struts
• Possibility of engineered
high compliance in core
to mitigate deformation
in the composite
• Plan to fabricate and
measure equivalent t-c
5 mm
18 mm
Hyper-Therm High-Temperature Composites, Inc.
End Close-Out
• Address need to close-out
ends to prevent LM penetration
• Working with PNNL to adapt
Ti3SiC2 joining technology to
nuclear grade SiC/SiC and to Ti3SiC2
pressureless fabrication
SiC
• Current concept is to use
unidirectional nuclear grade
SiC/SiC pins to provide
mechanical restraint to bond
and Ti3SiC2 is for sealing only
• Plan to evaluate for LM
penetration and stress/strain
limit for joint region
Hyper-Therm High-Temperature Composites, Inc.
Anticipated Loading of FCI
• Dimensional change resulting from
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through-thickness thermal gradient
Irradiation Induced Swelling
dominates loading
Results in differential expansion of
inside and outside of FCI
Irradiation induced swelling is
greater than thermal expansion
difference
Slot would allow free expansion and
Thermal Expansion
minimal stresses if unrestrained
End close-out and edges create
localized restraints, which result in
interlaminar stresses
Deformation will be asymmetric
Can be modeled as combined effect
of irradiation and thermal expansion
Hyper-Therm High-Temperature Composites, Inc.
Alternate FCI Design
• Closed box section provides
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greater geometric stability and
symmetric deformations
Assumes only purpose of slot is
pressure equalization
Restraint in closed section
increases in-plane stresses, but
reduces interlaminar stresses
Very important for fluted core
construction
FEM planned to determine
which approach is minimal
stress
Hyper-Therm High-Temperature Composites, Inc.
Summary
• Developing database of mechanical, electrical and thermal properties for
Nuclear Grade SiC/SiC with respect to the flow channel insert
• Current data indicates that Nuclear Grade SiC/SiC meets all
requirements of FCI with the notable exception of through-thickness
thermal conductivity
• Design of composite as a structure itself allows thermal conductivity to be
engineered to meet FCI requirements within known manufacturing
capabilities
Planned Work
• Measure elevated temperature mechanical properties
• Directly measure effective through-thickness thermal conductivity of
SiC/SiC engineered fluted core structure
• Develop end close-out method to seal core of FCI
• Finite element modeling of SiC/SiC structure to address end effects and
demonstrate that SiC/SiC structure meets mechanical requirements
• Produce sub-scale FCI and subject to thermal difference that simulates
anticipated strain from combined irradiation and thermal loading
Hyper-Therm High-Temperature Composites, Inc.
Acknowledgment
• We would like to acknowledge Department of Energy – (National
Nuclear Security Administration) SBIR Funding under Award
Number DE-FG02-07ER84717
•
This report was prepared as an account of work sponsored by an agency of the United States
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Hyper-Therm High-Temperature Composites, Inc.