Synthesis, structure and mechanical properties of ice-templated tungsten foams

Tungsten foams with directional, controlled porosity were created by directional freeze-casting of aqueous WO3 powder slurries, subsequent freeze-drying by ice sublimation, followed by reduction and sintering under flowing hydrogen gas to form metallic tungsten. Addition of 0.51 wt% NiO to the WO3 slurry improved the densification of tungsten cell walls significantly at sintering temperatures above 1250 degrees C, yielding densely sintered W-0.5 wt% Ni walls with a small fraction of closed porosity (<5%). Slurries with powder volume fractions of 15-35 vol% were solidified and upon reduction and sintering the open porosity ranges from 27-66% following a linear relation with slurry solid volume fraction. By varying casting temperature and powder volume fraction, the wall thickness of the tungsten foams was controlled in the range of 10-50 mu m. Uniaxial compressive testing at 25 and 400 degrees C, below and above the brittle-to-ductile-transition temperature of W, yields compressive strength values of 70-96 MPa (25 degrees C) and 92-130 MPa (400 degrees C).