Tungsten foams with directional, controlled porosity were created by directional freeze-casting of aqueous WO 3 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 WO 3 slurry improved the densification of tungsten cell walls significantly at sintering temperatures above 1250°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 lm. Uniaxial compressive testing at 25 and 400°C, below and above the brittle-to-ductile-transition temperature of W, yields compressive strength values of 70-96 MPa (25°C) and 92-130 MPa (400°C).
Controlling anisotropy in self-assembled structures enables engineering of materials with highly directional response. Here, we harness the anisotropic growth of ice walls in a thermal gradient to assemble an anisotropic refractory metal structure, which is then infiltrated with Cu to make a composite. Using experiments and simulations, we demonstrate on the specific example of tungsten-copper composites the effect of anisotropy on the electrical and mechanical properties. The measured strength and resistivity are compared to isotropic tungsten-copper composites fabricated by standard powder metallurgical methods. Our results have the potential to fuel the development of more efficient materials, used in electrical power grids and solar-thermal energy conversion systems. The method presented here can be used with a variety of refractory metals and ceramics, which fosters the opportunity to design and functionalize a vast class of new anisotropic load-bearing hybrid metal composites with highly directional properties.
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