Porous Nanowire Monoliths for Thermal Conductivity Engineering of TiC: Implications for Thermal Protection Systems

High Resolution Image Download MS PowerPoint Slide A bottom–up method of welding nanowires for the formation of porous nanowire monoliths (or foams) is presented. The method presented, which allows for controlling the monoliths’ porosities, also allows for independently engineering the mechanical and thermal properties of the monoliths. For example, control over thermal transport through these assemblies can be achieved by varying both the diameters of the nanowires and the sizes of the bridges formed between the nanowires and the overall porosities of the monoliths. Simultaneously, nanowire welding allows for imparting robustness to the nanowire foams. More specifically, these monoliths were prepared in this work by welding presynthesized TiC nanowires exhibiting an average diameter of 11 nm. Here, Ni nanoparticles were employed as the catalysts during the carbothermal reduction of TiO 2 to guide the one-dimensional growth of TiC into nanowires. The same Ni nanoparticles were also used as a liquid phase epitaxy (LPE) medium during the assembly of TiC nanowires into monoliths for the formation of TiC bridges between the TiC nanowires. Porosity control can be achieved during nanowire assembly in this method by using a sacrificial material, such as (NH 4 ) 2 CO 3, as a placeholder. To experimentally realize porous nanowire assemblies, nanowire welding and pore structure formation were simultaneously achieved in this work by heating mixtures of Ni-decorated (NH 4 ) 2 CO 3 microsalt crystals and presynthesized TiC nanowires. The method presented for nanowire foam formation allowed for reducing the thermal conductivities of TiC to 0.153–0.285 Wm – 1 K –1 in the temperature range of 298–1473 K, which is much lower than the 5–45 Wm – 1 K –1 observed in bulk samples of TiC. The TiC nanowire foams retained their structure upon heating to high temperatures of 1473 K for the thermal conductivity measurements, which served as a preliminary indicator of their mechanical robustness. These results demonstrate that the methodology presented is highly desired for next-generation thermal protection system fabrication.