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dra Häberli and Ralph Spolenak Laboratory for Nanometallurgy, Department of Materials, ETH Zürich, CH-8093 Zürich, Switzerland

David C. Dunanda) Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA (Received 5 January 2016; accepted 1 February 2016)

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 °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).

I. INTRODUCTION

There are many applications in which ceramic and metallic foams exhibit superior properties compared to their bulk counterparts, due to their lower density, lower heat conduction, and higher surface to volume ratio, e.g., for catalysis or sensor applications.1 In particular, porous tungsten oxide is used for a variety of energy and photo-therapeutic usages.2 Foams are additionally widely used as scaffolds for metal–metal- and ceramic–metalcomposites produced by melt-infiltration,3–5 important examples of which include W–Cu and W–Ag composites for arcing contacts in switchgears,4–6 W–Cu for resistance welding electrodes,6 low coefficient of thermal expansion substrates for heat spreader in power electronics6 and electrodes for electrical discharge machining.6,7 Common foam synthesis methods8,9 including traditional powder metallurgy (PM) approaches3,10,11 lead to equiaxed pores with an isotropic distribution over the whole foam. For tungsten controlling the pore architecture is difficult or not possible and the resulting structures exhibit isotropic materials properties. Especially in heat sink applications and arcing contacts, a high thermal and Contributing Editor: Paolo Colombo a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.62 J. Mater. Res., Vol. 31, No. 6, Mar 28, 2016

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electrical conductivity from the heat source into the material is desired to protect the arc region from arc erosion.12 Therefore, introducing a directional, anisotropic microstructure can minimize the resistance to current flow and simultaneously increase the thermal conductivity while modifying internal therma