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.
Thermal conductivity and microhardness of MWCNTs/copper nanocomposites
The effects of dispersion states of carbon nanotubes on thermal conductivity and Micro-hardness of Multi-walled carbon nanotube (MWCNT) reinforced copper nanocomposites were investigated. The nanocomposites were fabricated in a novel method. It involves the synthesis of MWCNT-implanted copper composite spheres and the preparation of the MWCNT/copper bulk materials using vacuum hot pressing and hot rolling. The thermal conductivity of the composites with different concentration of MWCNTs were measured. Although the coefficient of thermal conductivity decreases with the increase of the MWCNT content, it is still high enough to be used as electronic packaging materials even the concentration of MWCNTS in the composite is up to 5 wt%. Furthermore, the microhardness of the nanocomposites are much higher than that of pure copper, which is ascribed to the good dispersion of the MWCNTs in matrix.
Flexural strength of proof-tested and neutron-irradiated silicon carbide
Proof testing before service is a valuable method for ensuring the reliability of ceramic structures. Silicon carbide has been proposed as a very low activation first-wall and blanket structural material for fusion devices, where it would experience a high flux of fast neutrons. Strips of three types of silicon carbide were loaded in four-point bending to a stress sufficient to break about a third of the specimens. Groups of 16 survivors were irradiated to 2 × 1026n/m2 (E>0.05 MeV) at 740°C and bend tested to failure. The strength distribution of chemically vapor-deposited silicon carbide (Texas Instruments) was virtually unchanged by irradiation. The mean strength of sintered silicon carbide (Carborundum Alpha) was reduced 34% by irradiation, while the Weibull modulus and the truncated strength distribution characteristic of proof-tested material were retained. Irradiation reduced the mean strength of reaction-bonded silicon carbide (Norton NC-430) by 58%, and the spread in strength values was increased. We conclude that for the chemically vapor-deposited and the sintered silicon carbide the benefits of proof testing to eliminate low strength material are retained after high neutron exposures.