26Metal deposition on cellular preforms

Open-cell polymer foams can serve as templates upon which metals are deposited by chemical vapor decomposition (CVD), by evaporation or by electrodeposition. In the INCO process, nickel is deposited by the decomposition of nickel carbonyl, Ni(CO)4. Figure 2.6 schematically illustrates one approach in which an open-cell polymer is placed in a CVD reactor and nickel carbonyl is introduced. This gas decomposes to nickel and carbon monoxide at a temperature of about 100° C and coats all the exposed heated surfaces within the reactor. Infrared or RF heating can be used to heat only the polymer foam. After several tens of micrometers of the metal have been deposited, the metal-coated polymer foam is removed from the CVD reactor and the polymer is burnt out by heating in air. This results in a cellular metal structure with hollow ligaments. A subsequent sintering step is used to densify the ligaments.

Nickel carbonyl gas is highly toxic and requires costly environmental controls before it can be used for manufacturing nickel foams. Some countries, such as the United States, have effectively banned its use and others make it prohibitively expensive to implement industrial processes that utilize nickel carbonyl gas. Electro- or electroless deposition methods have also been used to coat the preforms, but the nickel deposited by the CVD technique has a lower electrical resistance than that created by other methods. The pore size can be varied over a wide range. Foams with open pore sizes in the 100-300 |im diameter range are available. The method is restricted to pure elements such as nickel or titanium because of the difficulty of CVD or electrodeposition of alloys. It gives the lowest relative density (0.02-0.05) foams available today.

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