Processes have been developed to deposit metal onto disposable open-cell polymer foams. Once the polymer is fully coated with metal the structure can be sintered to remove the polymer.
Electrodeposition
An electrodeposition approach is based on an open-cell polyurethane foam, which is made to conduct, either by immersing it in a colloidal fluid dispersion of carbon black, or by vapourising a thin layer of metal onto it. It is then electroplated with metal, and sintered to remove the polymer, leaving an open-cell network of hollow struts. This process is used commercially to produce CELMET™ and METAPORE™, net-shape open-cell nickel foams with approximately 95% porosity and cell size between 400 and 5000 µm, principally for use as battery electrodes.
Deposition from the gas phase
A variation on this process, also used for nickel and with similar results, uses a gas-phase reaction to directly deposit metal. At high temperatures, Ni(CO)4 decomposes to form Ni.
The main steps in the process is shown in Figure 1. An open-cell polymer preform (1) is coated with a strongly infrared-absorbing material (2), usually carbon black or appropriate pigments. In order to avoid spontaneous decomposition of the gas away from the surface, the coating is chosen to strongly absorb infrared radiation of a frequency which is only weakly absorbed by the carbonyl gas. Infrared radiation is used to heat the coated polymer so that the Ni(CO)4 decomposes at the surface and deposits a layer of nickel the polymer (3 & 4). The cell structure is then sintered to remove the polymer (5 & 6). Hydrogen gas can be added to the nickel carbonyl to catalyse the decomposition.
Figure 1: The production of open-cell nickel foams by deposition from the gas phase onto a disposable preform.
It is claimed that foams produced using this method are mechanically stronger and show better conductivity than electroplated foams, mainly due to the smoother surfaces obtainable. This deposition technique is used commercially to produce INCOFOAM™, an open-cell nickel foam with cell sizes between 20µm and 400µm and porosities as high as 99%.