Fabrication and properties of porous tungsten by freeze-drying process

Porous W with controlled pore characteristics was fabricated by a freeze-drying process. WO ₃ powder and camphene were used as the source materials of W and sublimable vehicles, respectively. Camphene slurries with WO ₃ contents of 10 and 15 vol% were prepared by milling at 50°C with a small amount of oligomeric polyester dispersant. Freezing of a slurry was done in a Teflon cylinder attached to a copper bottom plate cooled at -25°C while the growth direction of the camphene was unidirectionally controlled. Pores were generated subsequently by sublimation of the camphene during drying in air for 48 h. The green body was hydrogen-reduced at 800°C for 30 min and sintered in a furnace at 900°C for 1 h under a hydrogen atmosphere. Microstructural observation revealed that all of the sintered samples were composed of only W phase and showed large pores which were aligned parallel to the camphene growth direction. The porosity and pore size increased with increasing camphene content. The difference in the pore characteristics depending on the slurry concentration may be explained by the degree of powder rearrangement in the slurry. The results strongly suggest that a porous metal with the required pore characteristics can be successfully fabricated by a freeze-drying process using metal oxide powders.