Gerald E. Youngblood

Defect structure and evolution in silicon carbide irradiated to 1 dpa-SiC at 1100 °C

Transmission electron microscopy (TEM), swelling measurements, isochronal annealing, and thermal diffusivity testing were used to characterize the effects of radiation damage in SiC. Together, these techniques provided a comprehensive set of tools for observing and characterizing the structure and evolution of radiation-induced defects in SiC as a function of irradiation temperature and dose. In this study, two types of dense, crystalline, monolithic SiC were subjected to irradiation doses up to 1 dpa-SiC at a temperature of 1100 °C, as well as post-irradiation annealing up to 1500 °C. The microscopic defect structures observed by TEM were correlated to changes in the macroscopic dimensions, thermal diffusivity and thermal conductivity. The results demonstrated the value of using ultrapure β-SiC as an effective reference material to characterize the nature of expected radiation damage in other, more complex, SiC-based materials such as SiC/SiC composites.

Radiation response of SiC-based fibers

Abstract Loss of strength in irradiated fiber-reinforced SiC/SiC composite generally is related to degradation in the reinforcing fiber. To assess fiber degradation, the density and length changes were determined for four types of SiC-based fibers (Tyranno, Nicalon CG, Hi Nicalon and Dow X) after high temperature (up to 1000°C) and high dose (up to 80 dpa-SiC) irradiations. For the fibers with nonstoichiometric compositions (the first three types in the list), the fiber densities increased from 6% to 12%. In contrast, a slight decrease in density (

High thermal conductivity SiC/SiC composites for fusion applications

SiC/SiC composites offer excellent potential for fusion energy as well as numerous commercial applications. High manufacturing cost and very low through-the-thickness (transverse) thermal conductivity (below 15 W/m K) hinders wider applications of SiC/SiC composites. To overcome these obstacles, a low cost chemical vapor reaction (CVR) process was developed to fabricate crystalline, high-purity SiC fibers. Several variations of the CVR process were used to fabricate SiC/SiC composites. An unirradiated, 4-mm thick SiC/SiC composite was completely CVR-converted from a converted SiC fiber preform with a pitch graphite matrix, and exhibited a bulk density of 2.65 g/cc, 10% open porosity, room temperature (RT) and 1000°C thermal conductivity values of 70 and 35 W/m K, respectively, and an RT bend strength of 200 MPa.

Irradiation creep of advanced silicon carbide fibers

Abstract The bend stress relaxation (BSR) method was applied to study irradiation enhanced creep (IEC) of small diameter silicon carbide (SiC) fibers after 10 MeV proton irradiation. A first series of tests was conducted on Sylramic™ fibers irradiated at 600°C with average bending stresses of 400 and 667 MPa and for irradiation doses smaller than 0.04 dpa. The BSR results are compared to previously obtained torsional creep test results for the Textron SCS-6™ type SiC fibers by calculating the tensile equivalents for both testing methods. For the Sylramic fibers, the creep constant κ=4.7×10−6 Mpa−1 dpa−1, was a factor of 6 smaller than the κ-value determined for SCS-6 fibers at 600°C. In contrast, for T

Dimensional stability and tensile strength of irradiated Nicalon-CG and Hi-Nicalon fibers

Abstract Nicalon-CG and Hi-Nicalon fibers were characterized by measuring their density and tensile strength in the unirradiated, thermal annealed, and irradiated conditions. The results indicate the fibers that perform best after irradiation to 43 dpa SiC at 1000°C are those that approach stoichiometric and crystalline SiC. Hi-Nicalon fiber exhibited less than 1% densification, accompanied by a slight increase in tensile strength after irradiation. Nicalon-CG, in contrast, was significantly weakened in the annealed and irradiated conditions. In addition, Nicalon-CG exhibited substantial irradiation-induced shrinkage. Loss of fiber tensile strength after irradiation is shown to reduce the flexural strength of irradiated composites while fiber shrinkage, and resultant debonding from the matrix, are linked to a reduced composite elastic modulus.

Effect of irradiation on the microstructure of Nicalon fibers

Abstract Microstructural analyses were performed by TEM on two types of Nicalon fibers (CG and Hi) after neutron irradiation at nominally 1040°C to a relatively high dose (43 dpa). For comparison, microstructural analyses also were performed on unirradiated fibers that were thermally annealed at 1010°C for a time equivalent to the irradiation exposure time. No grain growth was observed for either type of unirradiated, but thermally annealed Nicalon fibers. However, significant grain growth was observed to have taken place in the irradiated Nicalon-CG fiber, presumably irradiation induced. In contrast, no significant amount of grain growth was observed in the irradiated Hi-Nicalon fibers. Void and other irradiation defect structures were not observed in either fiber. For such extreme irradiation conditions (43 dpa at 1040°C), the Hi-Nicalon fiber exhibits a much higher degree of microstructural stability than Nicalon-CG fiber. Composite SiC/SiC made with Hi-Nicalon promises to also exhibit improved irradiation performance.

Modeling the Transverse Thermal Conductivity of 2-D SiCf/SiC Composites Made with Woven Fabric

Abstract The hierarchical two-layer (H2L) model describes the effective transverse thermal conductivity (keff ) of a two-dimensional (2-D) SiC f /SiC composite plate made from stacked and infiltrated woven fabric layers in terms of constituent properties and microstructural and architectural variables. The H2L model includes the effects of fiber-matrix interfacial conductance, high-fiber packing fractions within individual tows, and the nonuniform nature of 2-D fabric/matrix layers that usually include a significant amount of interlayer porosity. Previously, H2L model keff predictions were compared to measured values for two versions of 2-D Hi-NicalonTM/pyrocarbon (PyC)/isothermal chemical vapor infiltration (ICVI)-SiC composite, one with a “thin” (0.11-μm) and the other with a “thick” (1.04-μm) PyC fiber coating, and for a 2-D TyrannoTM SA/thin PyC/forced flow chemical vapor infiltration SiC composite. In this study, H2L model keff predictions were compared to measured values for a 2-D SiC f /SiC composite made using the ICVI process with Hi-Nicalon type S fabric and a thin PyC fiber coating. The values of keff determined for the latter composite were significantly greater than the keff values determined for the composites made with either the Hi-Nicalon or the Tyranno SA fabrics. Differences in keff values were expected for the different fiber types, but major differences also were due to observed microstructural and architectural variations between the composite systems, and as predicted by the H2L model.

Failure mechanisms in continuous-fiber ceramic composites in fusion energy environments

Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These composites are relatively new materials with a limited database; however, there is sufficient understanding of their performance to identify key issues in their application. To date, dimensional changes of the constituents, microstructural evolution, radiation-enhanced creep, and slow crack growth have been identified as potential lifetime limiting mechanisms. Experimental evidence of these mechanisms, the factors that control them, and their implications on component lifetime will be discussed.

Time-dependent failure mechanisms in silicon carbide composites for fusion energy applications

Abstract Silicon carbide has many properties that are attractive for applications in fusion energy systems. The reliability of monolithic silicon carbide, however, is insufficient for its use in large components. Ceramic matrix composites offer greater flaw tolerance and reliability, but their failure mechanisms are less well understood. This work has focussed on studying potential failure mechanisms in silicon carbide fiber-reinforced, silicon carbide matrix (SiCf/SiCm) composites. In the event of pre-existing cracks, subcritical crack-growth may occur due to creep of fibers that bridge the crack faces. Irradiation-enhanced creep will enhance the subcritical crack-growth rate. The presence of oxygen leads to oxidation of the interphase material and subcritical crack-growth controlled by the rate of interphase recession. In addition, fiber shrinkage or weakening due to exposure to radiation can promote additional failure mechanisms, including embrittlement. These mechanisms, the conditions, under which they occur, and the current state of models of the crack-growth mechanisms will be discussed.

Electrical Conductivity of 2D-SiCf/CVI-SiC

Abstract Electrical conductivity (EC) data for several plate forms of two-dimensional, silicon carbide composite made with chemical vapor infiltration matrix and with Hi NicalonTM type S fibers (2D-SiCf/CVI-SiC) were acquired. The composite fibers were coated with pyrocarbon (PyC) of various thicknesses (50 to 310 nm) and an outer thin (˜60 μm) SiC “seal coat” was applied by CVD to the infiltrated plates. The EC was highly anisotropic in the transverse and in-plane directions. In-plane EC ranged from ˜150 to 1600 S/m, increased slowly with increasing temperature, and depended primarily on the total PyC thickness. High in-plane EC-values occur because it is dominated by conduction along the numerous, continuous PyC fiber coating pathways. Transverse EC ranged from ˜1 to 60 S/m, and increased strongly with increasing temperature up to 800°C. The transverse EC is controlled by conduction through the interconnections of the carbon-coating network within and between fiber bundles, especially at moderate temperatures (˜300 to 700°C). Below ˜300°C, the electrical resistance of the pure SiC seal coat becomes increasingly more important as temperatures are further lowered. Importantly, a “3-layer series” model predicts that transverse EC-values for a standard seal-coated 2D-SiCf/CVI-SiC with a monolayer PyC fiber coating of ˜50-nm thickness will be <20 S/m for all temperatures up to 800°C, as desired for a flow channel insert in a fusion reactor blanket component.