Preparation and Characterization of Directionally Freeze-cast Copper Foams

Abstract

Because of their excellent thermal and electric conductivities, copper foams are ideally suited for applications such as heat exchangers, catalyst supports and EMI-shields. Here, we demonstrate the preparation of copper with ~80% aligned, elongated, interconnected pores via directional freeze casting, a well established processing technique for porous ceramics. First, an aqueous slurry of 40−80 nm cupric oxide powders was directionally solidified, resulting in a preform consisting of elongated, aligned dendrites of pure ice separated by interdendritic ice walls with high oxide powder content. Oxide rather than metallic nanometric particles are used, as the latter would oxidize rapidly and uncontrollably when suspended in the aqueous solution used during directional casting. The preforms were then freeze-dried to sublimate the ice and sintered in a hydrogen-bearing atmosphere to reduce the copper oxide to metallic copper particles and densify these copper particles. Microstructural analysis of the copper foams shows that three types of porosities are present: (i) aligned, elongated pores replicating the ice dendrites created during the freeze-casting process; (ii) micro-porosity in the partially sintered copper walls separating the elongated pores; and (iii) cracks in these copper walls, probably created because of shrinkage associated with the reduction of the oxide powders.

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

“…This is comparable to the typical structures presented in the literature for freeze-casting of aqueous suspensions. [14][15][16][17]33 The structure can be explained by the growth kinetics of the ice crystals which is replicated as pores or channels, as explained in detail in the literature. 13 Decreasing the casting temperature as well as decreasing the particle volume fraction leads to smaller wall thickness in the final foam [ Fig.…”

“…If a metal is desired, an additional step, often carried out prior to the fourth, consisting of chemical reduction of the ceramic to metal, usually via hydrogen, while maintaining the constructed large porosity [13][14][15][16][17] needs to be introduced. Most literature focuses on freeze casting of ceramics, but a few metallic foams have been created by freeze-casting such as copper from cupric oxide, 15 nickel from metallic nickel using a carboxymethylcellulose/gelatin mixture, 18 porous stainless steels tapes from ferritic metallic powder 19 and titanium foams from metallic titanium powder. 14,16,17 In the latter case, the oxygen content has a strong influence on the mechanical properties 16 since surface oxides of titanium are not readily reduced under H 2 .…”

Metallic Architectures from 3D‐Printed Powder‐Based Liquid Inks

“…Traditional powder metallurgy utilizes metal powders that are shaped and compacted into dense metallic objects through hot isostatic pressing or dynamic compaction . An alternative, indirect approach is to first fabricate objects of the desired architectures from metal precursors, such as metal oxides, and then thermochemically reduce and sinter them to form metallic counterparts, including porous, cellular architectures that cannot be formed via direct metal casting.…”

“…Among the many established approaches for using metallic powders to create metallic architectures directly, there have been several previous examples that utilize the general approach of creating metal oxide powder objects and transforming them into disordered metallic foams and linear cellular architectures . In these instances, powder‐based, metal‐oxide green bodies are created and subsequently thermochemically reduced and sintered at elevated temperatures in reducing atmosphere (e.g., H 2 gas) to produce metallic structures with water vapor as a byproduct.…”

High porosity micro- and macro-cellular copper foams with semi-open cell microstructure toward its physical and mechanical properties

“…Till now, numerous techniques such as casting, electrodeposition or vapor deposition and powder metallurgy (PM) for foaming process have been developed [10,11]. According to some limitations of casting and deposition methods in control of cellular morphology, microstructures, and economic considerations, the PM method has been introduced [12,13].…”