Microstructure-property relations for porous bismuth films

Metallorganic decomposition of metal carboxylates is used to prepare porous thin films of bismuth. The general approach is to spin coat a precursor solution, bismuth 2-ethylhexanoate in a solvent of 2-methyl-1-propanol on the substrate of interest and then pyrolyze the resulting film under conditions which evolve the organic ligands but do not melt the bismuth. The controlled release of ligands creates the nanoporous morphology which is then preserved in the final film. Subsequent plasma etching and heat treatments are used to produce films of variable porosity. The film microstructure consists of nanoporous channels separating grains of bismuth. For films in the range of 30 % porosity, thermal conductivity values are on the order of 0.35 W/mK. The interrelationships among processing conditions, microstructure and thermal conductivity are discussed.