SiC/SiC Composite Properties and Flow Channel Insert Design R.J. Shinavski Hyper-Therm HTC Huntington Beach, CA 714-375-4085 robert.shinavski@htcomposites.com FNST Meeting UCLA, Los Angeles, CA August 18, 2009
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 as a flow channel insert to isolate the molten Pb-Li from the steel structure Properties of Nuclear Grade SiC/SiC are being measured to provide preliminary guidance in assessing viability of design approaches for a SiC/SiC FCI Finite element analysis of several FCI design approaches provides insight into the magnitudes of the critical parameters Advantages and disadvantages of possible designs for a Nuclear Grade SiC/SiC flow channel insert will be discussed with respect to mechanical, thermal, and electrical properties Support technologies being examined such as potential joining/bonding methods will also be discussed
SiC/SiC Composites 0º Fibers MLSiC fiber coating 90º Fibers CVI SiC Hi-Nicalon Type S SiC matrix produced by isothermal/isobaric CVI Composite bulk densities 2.7 g/cm 3 Hi-Nicalon Type S fiber selected due to greater existing database on this fiber showing radiation resistance Fiber coating is Hyper- Therm HTC s MLSiC fiber coating (US Patents 5,455,106 and 5,480,707)
Mechanical Prop of Nuclear Grade SiC/SiC Mechanical properties database is being generated for both ambient and elevated temperature properties that consists of tensile stress-strain and interlaminar shear stress Tentative design stresses are being established to evaluate the viability of some approaches and to determine the dependency on the design criteria Matrix cracking stress as represented by the proportional limit stress is the design limiting mechanical property for in-plane stresses to prevent the possibility of Pb-Li ingress through the SiC/SiC composite Mean B-basis Allowable* Weibull Analysis** Mechanical Properties of Nuclear Grade SiC/SiC (5HS) E 270 --- --- 400 MPa 344 MPa --- ε f 0.54% 0.39% --- σ PL 180 MPa 157 MPa 96 MPa * 95% confidence that 90% of the material will be greater than the allowable ** Weibull analysis for 1x10-6 failure with calculated Weibull Modulus of 21.1 σ f σ ILSS(RT) /σ ILSS(800C) 42/38 MPa 24/27 MPa insufficient data
Pb-Li Compatibility Nuclear grade SiC/SiC has been exposed to Pb-Li at up to 360ºC with no signs of Pb-Li penetration Recent experiments conducted with Pb-Li at 475ºC and 100 psi (690 kpa) Ar overpressure were inconclusive as cut specimen edges were insufficiently sealed and allowed LM ingress from the edges Such behavior is of concern as FCI segments will likely have cut edges Experiment is being repeated with greater attention to re-sealing the edges after machining test specimens Overpressure has significant impact on wetting of Pb-Li on SiC Mechanically materials were unchanged
Electrical Properties In-plane electrical conductivity dominated by small amount of carbon in fiber coating and is not highly sensitive to orientation within the plane Data and material show good reproducibility between measurements, samples, and lots of material The more critical through thickness electrical conductivity shows a high dependence on the presence/absence of the CVI SiC seal-coat particularly at lower temperatures Need to establish a better understanding of t-t conductivity behavior and measurement due to influence on magnetohydrodynamic effect Testing performed by G. Youngblood at PNNL
Thermal Conductivity Through-thickness thermal conductivity of Nuclear Grade SiC/SiC is too high to be a sufficient thermal insulator In-plane measurements performed at +/-45 orientation showed at most a 5% difference and thus NG SiC/SiC can likely be considered inplane isotropic with respect to thermal conductivity Architectural design is required to meet the thermal conductivity target of 1-2 W/m/K Must measure the effective thermal conductivity on the architecturally designed material
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 to perform preliminary design 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
Low Thermal Conductivity Construction Equivalent thermal conductivity of 1.4 W/m/K is predicted from series-parallel resistance approach FEA being performed to refine expected thermal transport 1.0 mm face sheets with 0.5 mm thick struts 45 degree flute angle maximizes shear strength of truss Must determine preferred strut frequency (balance mechanical versus thermal performance) Possibility of engineered high compliance in core to mitigate deformation in the composite 5 mm 18 mm
Evaluation of Truss Structure Approach Demo articles fabricated with carbon fiber reinforced SiC were produced to establish viability of planned manufacturing approach Two truss repeat distances were examined with a constant truss angle of 45 degrees Plan to produce using NG SiC/SiC in near term Experimentally measure equivalent thermal conductivity using guarded hot plate technique on plates of material Test shear strength of structure to determine if core strength or bonding of core to face governs failure
Anticipated Loading of FCI Irradiation induced swelling is greater than thermal expansion difference, but thermal expansion changes more rapidly with temperature Slot allows free expansion and minimal stresses if unrestrained Edges create localized restraints, which result in bending stresses Deformation will be asymmetric due to slot Use FEA to examine the stress state in the FCI First level of complexity is to not consider architectural structure
Finite Element Model of FCI Model simply applied a 500-300ºC temperature gradient on the FCI Deformed shape is magnified 30X (slot deforms more than 2.5 mm out of plane) Highest stress (Von Mises) occurs along center line of side opposite slit and is 86 MPa Highest principal direction stress 68 MPa Magnitude of maximum interlaminar shear stress 42 MPa Predicted interlaminar shear stress is much higher than acceptable
Mechanical Prop of Nuclear Grade SiC/SiC Mechanical properties database is being generated for both ambient and elevated temperature properties that consists of tensile stress-strain and interlaminar shear stress Tentative design stresses are being established to evaluate the viability of some approaches and to determine the dependency on the design criteria Matrix cracking stress as represented by the proportional limit stress is the design limiting mechanical property for in-plane stresses to prevent the possibility of Pb-Li ingress through the SiC/SiC composite Mean B-basis Allowable* Weibull Analysis** Mechanical Properties of Nuclear Grade SiC/SiC (5HS) E 270 --- --- 400 MPa 344 MPa --- ε f 0.54% 0.39% --- σ PL 180 MPa 157 MPa 96 MPa * 95% confidence that 90% of the material will be greater than the allowable ** Weibull analysis for 1x10-6 failure with calculated Weibull Modulus of 21.1 σ f σ ILSS(RT) /σ ILSS(800C) 42/38 MPa 24/27 MPa insufficient data
Alternate FCI Design Closed box section provides greater geometric stability and symmetric deformations Assumes only purpose of slot is pressure equalization Restraint in closed section reduces stresses Again analyze with FEA
Finite Element Modeling Results Deformations significantly reduced Localized near ends of FCI (deformation scale 300X, instead of 30 X) Maximum Von Mises stress reduced to 76 MPa Magnitude of maximum interlaminar shear stress 33 MPa FCI maintains shape better and is stressed less, but interlaminar shear stress is still likely too high for reliable performance
Nested FCI (nfci) Separates electrical and thermal isolation functionality into two separate sub-components conceived by Smolentsev and Malang (Fusion Science and Technology, July 2009) Allows each to be optimized for function Further reduces stresses as thermal (outer) FCI can be fabricated from loosely located plates without inducing magnetohydrodynamic losses and minimal temperature drop will be experienced by electrical (inner) FCI Outer Thermal FCI Inner Electrical FCI Currently planning FEA to examine stresses particularly shear stresses in electrical FCI at corners Will likely consume several mm more real estate within the duct as thermal insulation target of 1-2 W/m/K cannot be met with less than ~5 mm thickness for the thermal FCI
Joining of SiC/SiC Composites Anticipated need for strong, tough joints with low activation (standoffs, locators, close-outs, multi-piece construction) Pre-ceramic polymers & solid state displacement reactions (Ti 3 SiC 2 ) PNNL has demonstrated joints with strengths of 50 MPa when produced with an applied pressure of 30 MPa Pre-ceramic polymer joint strengths of <25 MPa; Pressureless joints typically <10 MPa Undertaking an examination of alternate pressureless processing routes to Ti 3 SiC 2 (MAX phase) formation Ti 3 SiC 2 selected due to exceptional thermomechanical properties and thermal shock resistance and low activation composition Preliminary ion irradiation of MAX phases with 1MeV Kr ions shows no appreciable damage at up to 12 dpa (Whittle et al, MRS Proceedings 1125 to be published)
MAX Phase Joining Significant additions of SiC filler to better match cte and to distribute residual porosity as very small defects Good bonding (can be atomically sharp) between SiC and Ti 3 SiC 2 Average strengths of in excess of 50 MPa have been obtained Strength a strong function of bond thickness for pressureless joining 60 50 ILSS Strength (MPa) 40 30 20 10 0 0 50 100 150 200 250 300 350 400 Bond Thickness ( m)
Summary Preliminary design data is useful in determining the potential viability of FCI design approaches Truss structure design of SiC/SiC thermal insulation allows the thermal properties to be engineered and is predicted to reduce the thermal conductivity by greater than an order of magnitude nfci is currently preferred design approach to minimize shear stresses Permeability with respect to Pb-Li not as simple as original data indicated when temperature is increased and overpressure is added Pressureless joining using MAX phase has been demonstrated Planned Work Continue database accumulation such that tentative design properties can be determined to evaluate potential FCI design approaches Additional testing in Pb-Li with overpressure FEA modeling using architecturally designed truss structure Directly measure effective through-thickness thermal conductivity of SiC/SiC engineered fluted core structure Produce sub-scale FCI and subject to thermal difference that simulates anticipated strain from combined irradiation and thermal loading
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 Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.