Preparation of carbon coated ceramic foams by pyrolysis of polyurethane

Electronically conducting carbon coatings over alumina foams were prepared by the foams' impregnation in a polyurethane solution, followed by pyrolysis of the polymer layer. An optimal coating procedure was developed, using a commercial polyurethane lacquer. Pyrolysis was performed by heating the coated foams to 650-1,200 DGC in Argon for 2-8 h. Coating characterization included surface area, phase composition, morphological and electrical conductivity measurements. Auger electron spectroscopy (AES) showed the composition was mostly carbon, with trace levels of oxygen impurities. Thickness, microstructure and interface between the alumina foam surface and the carbon film were analyzed using scanning electron microscopy (SEM and HR-SEM).The carbon film's specific electrical resistivity was 1-10 (Omegamx10-2), depending on the pyrolysis time, temperature and number of coatings. The resistivity was found to decrease by a factor of six when the pyrolysis temperature was increased from 750 to 1,200 DGC. A second carbon layer reduced the resistivity further by about a factor of two. These effects are attributed to densification, improved connectivity between the carbon grains and an overall thickening of the carbon layer. Thermal analysis and Raman measurements on the carbon films point to a grain rearrangement that is consistent with the improved conductivity of the films.