This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

The synthesis of porous W by freeze-casting and vacuum drying is investigated. Ball-milled WO3 powders and tert-butyl alcohol were used as the starting materials. The tert-butyl alcohol slurry is frozen at –25oC and dried under vacuum at –25 and –10oC. The dried bodies are hydrogen-reduced at 800oC and sintered at 1000oC. The XRD analysis shows that WO3 is completely reduced to W without any reaction phases. SEM observations reveal that the struts and pores aligned in the tert-butyl alcohol growth direction, and the change in the powder content and drying temperature affects the pore structure. Furthermore, the struts of the porous body fabricated under vacuum are thinner than those fabricated under atmospheric pressure. This behavior is explained by the growth mechanism of tert-butyl alcohol and rearrangement of the powders during solidification. These results suggest that the pore structure of a porous body can be controlled by the powder content, drying temperature, and pressure.

The effects of drying temperature on the microstructure of porous W fabricated by the freeze-casting process of tert-butyl alcohol slurry with WO3 powder was investigated. Green bodies were hydrogen-reduced at 800oC for 1 h and sintered at 1000oC for 6 h. X-ray diffraction analysis revealed that WO3 powders were completely converted to W without any reaction phases by hydrogen reduction. The sintered body showed pores aligned in the direction of tertbutyl alcohol growth, and the porosity and pore size decreased as the amount of WO3 increased from 5 to 10v ol%. As the drying temperature of the frozen body increased from -25oC to -10oC, the pore size and thickness of the struts increased. The change in microstructural characteristics based on the amount of powder added and the drying temperature was explained by the growth behavior of the freezing agent and the degree of rearrangement of the solid powder during the solidification of the slurry.

The effect of tert-butyl alcohol (TBA) as a freezing solvent on the pore structure of a porous tungsten body prepared by freeze-drying is analyzed. TBA slurries with a WO3 content of 10 vol% are prepared by mixing with a small amount of dispersant and binder at 30oC. The slurries are frozen at -25oC, and pores are formed in the frozen specimens by the sublimation of TBA during drying in air. After hydrogen reduction at 800oC and sintering at 1000oC, the green body of WO3 is completely converted to porous W with various pore structures. Directional pores from the center of the specimen to the outside are observed in the sintered bodies because of the columnar growth of TBA. A decrease in pore directionality and porosity is observed in the specimens prepared by long-duration drying and sintering. The change in pore structure is explained by the growth of the freezing solvent and densification.