Structure-Property Relationships of Tantalum Carbide Foams and Synthesis of an Interpenetrating Phase Composite

Ceramic and refractory metal foams have a potential for use in extreme environments, such as in fuel elements within nuclear reactors both in space and terrestrial applications. In addition, infiltrating an open-cell ceramic foam with a continuous second phase can create an interpenetrating phase composite (IPC), consisting of a three-dimensional reinforcement structure. One aspect of investigation within this study was the influence of foam pore/strut size, foam composition, and foam density on neutronic and mechanical properties. Neutron transmission through open-cell tantalum carbide foams was measured using experimental techniques and modeled with Monte Carlo N-Particle (MCNP) transport code. Neutron transmission decreased linearly within tantalum carbide (TaC)/reticulated vitreous carbon (RVC) foams as areal TaC density increased. All MCNP modeling runs predicted slightly higher neutron transmission than what was experimentally measured, potentially indicating that the foam structure had a small influence on neutron transmission. Compressive strength and Young's moduli of tantalum carbide foams were measured for foam specimens that were exposed to thermal cycling and thermal shock, as well as for baseline specimens. Extensive micro-cracking was observed in the foams after 18 thermal cycles to 2100°C. However, thermal shock in liquid nitrogen did not produce observable micro-cracking in the TaC foams. The average strengths of baseline TaC/RVC foams ranged from 1.97 MPa - 3.82 MPa. The baseline TaC/PyC/RVC foams exhibited strengths ranging from 4.57 MPa - 12.60 MPa. The compressive strength of thermally cycled foams tended to be 1/3-1/2 that of baseline specimens.

Another aspect of this study investigated the infiltration of RVC foams with tungsten powder in an attempt to form a tungsten-ceramic foam interpenetrating phase composite (IPC). It was found that tungsten particle size influenced infiltrated densities more than foam pore size. Significantly lower infiltrated densities were obtained using sub-micron tungsten than with 5-10 micron tungsten as a result of particle agglomeration. Infiltrated 5-10 micron tungsten achieved densities ranging from 23-25% theoretical within RVC foams, whereas sub-micron tungsten densities ranged from 11-16% theoretical. Constrained densification was observed during sintering of tungsten-infiltrated foams.