Freeze Casting of Porous Copper with Lamellar Morphology from Cupric Oxide Suspensions for Enhancing Through-Plane Thermal Conductivity

Abstract

Abstract This study reports porous copper with lamellar morphology produced with the freeze casting method, in which aqueous suspensions of cupric oxide particles (1 µm–2 µm) were frozen under controlled cooling rates, followed by ice sublimation, reduction to copper, and sintering. The effects of the cooling rate (0.008–0.08 °C·s⁻¹), the particle loading (6.0 vol %–13.0 vol %), and the concentration of polyvinyl alcohol (PVA) (1.2 wt %–3.6 wt %) on the through-plane efective thermal conductivity and structural characteristics of the as-produced material were investigated. Over a narrow range of cooling rates (0.016–0.026 °C·s⁻¹), continuous lamellae formed, and the porous copper structures with 6.0 vol %–13.0 vol % particle loadings demonstrated an average porosity of 66.7–89.5 %, an average through plane effective thermal conductivity of 9.5 W m⁻¹ ⋅ K⁻¹ –12.9 W m⁻¹ ⋅ K⁻¹, and average lamellar thickness and spacings less than 50 μm. The highest through-plane effective thermal conductivity of 16.7 Wm-1·K-1 was obtained at 65.7 % porosity with suspensions of 13.0 vol % particle loading. These results suggest that freeze cast porous copper has a higher through-plane effective thermal conductivity than commercial copper foams for a given porosity. The fastest cooling rate (0.08 °C·s⁻¹) resulted in engulfment of particle aggregates by the freezing front. The effective thermal conductivity along the freezing direction is not uniform, showing a less than 10.0 % difference in the samples produced with the cooling rate of 0.016 °C·s⁻¹. Increasing the PVA concentration from 1.2 wt % to 3.6 wt % showed an insignificant influence on the non-uniformity of this property, but decreased its value due to the enlarged tilt angles.