Porous metals with complex shape and designed microstructure

The synthesis of metal particles in the micrometer or nanometer range and their assembly into larger units has led to fascinating results in recent decades. However, producing macroscopic objects with complex geometrical shapes and, at the same time, a complex microstructure is still a challenging task. Modern production strategies to porous metals suffer from similar limitations, causing a lack of porous metal architectures with complex and tailored structural features on several length scales. Here, we propose routes to porous metals with complex macroscopic shapes and a tailored microstructure to fill the above-mentioned gap in available manufacturing strategies by developing fabrication approaches beyond state of the art. In addition, it should be possible to develop novel metal architectures with unique structures and special properties (e.g. thermal or catalytic) that are not achieved by materials from conventional manufacturing processes. During the abroad phase (at ETH Zürich) of the proposed research project, porous metals with a deliberately arranged pore structure shall be produced. For this purpose, metals will be deposited on preformed particles that serve as a template, and these particles will be arranged and assembled in the desired manner or further processed by means of 3D printing. First, anisotropic (and magnetic) particles will be prepared and coated with copper. These particles will be aligned by mechanical forces or magnetic fields and then assembled into larger objects with anisotropic microstructure. Removal of the template results in copper foams / sponges with a deliberately arranged pore structure. The next step is to combine these synthesis and assembly techniques with additive manufacturing (3D printing) to create macroscopic objects with complex geometries. 3D printing approaches based on two different inks (shaping and texturing ink) or only one comprehensive ink will be carried out and the necessary inks will be developed. In the final step, the above-mentioned synthesis approaches for porous copper shall be extended to other metals such as gold or nickel. During the return phase at the University of Salzburg, hierarchical porous metal structures will be produced by metal deposition on hierarchical templates. In addition, these templates will also be arranged in an anisotropic fashion or processed by 3D printing in order to explore further design potentials and to increase the complexity in both the macro- and the microscale.

Within this project, different routes for the fabrication of porous metals with complex macroscopic shapes and deliberately designed micro- or nanostructures were investigated. The first approach was based on the synthesis of anisotropic template particles in combination with wet-chemical deposition of metal. A synthesis route towards anisotropic calcium sulfate rods in aqueous solution was used for the creation of the template. The wet-chemical deposition of metallic copper on thus synthesized template particles was successfully implemented by using copper(II) acetylacetonate as copper source and benzyl alcohol as reducing agent. For the arrangement of the resulting microstructure, two different strategies based on the application of mechanical and magnetic forces, respectively, were explored. For both approaches, an alignment of single coated particles as well as a particle alignment on the global scale of a macroscopic object could be achieved. However, the final assembly of aligned particles into a monolithic material led either to a breakdown of the structure after template removal or to a disturbance of the alignment on the global scale. Hence, the resulting material was in both cases a porous metal monolith with random pore structure. The second approach focused on the wet-chemical deposition of metal on hierarchically structured porous materials used as templates. A synthesis route based on sol-gel chemistry was applied to create hierarchical carbon monoliths with pores on three different hierarchy levels. While the macropore structure of these materials could be coated by wet-chemical deposition of copper, smaller features like micro- and mesopores could not be accessed via this approach. Moreover, a gradient in the quality of metal coating due to diffusion limitations from the outside to the inside of the cylindrical monolith could be observed. In addition to the main objective, a few side aspects related to the highly versatile wet-chemical deposition process for metallic copper were investigated within the project. One of these projects focused on the deposition of copper on different threads and fabrics, which showed excellent electrical conductivity after coating and could thus be used as current collectors for flexible electronics. In another side project, 3D printed macroporous polyvinylidene fluoride (PVDF) structures were investigated as templates. However, as the printed objects were not stable under the metal-deposition conditions, another approach based on 3D printing of a special PVDF-ink containing copper nanowires (CuNWs) was applied. This procedure facilitated the creation of PVDF-CuNW composite objects, which could be interesting candidates for sensing and actuation applications due to their piezoelectric properties.