Development of mechanically and chemically stable electrical contacts for thermoelectric oxide material

Oxide thermoelectric materials like Na_xCoO₂ are favourable candidates for thermoelectric energy conversion at high temperatures. One of the main technological problems of oxide thermoelectric devices is to provide mechanically and chemically stable contacts at the hot side. In order to identify suitable metals to be employed as contacts and diffusion barriers the reaction between the metals Ni, W, and Au, respectively, and Na_xCoO₂ has been investigated at different temperatures. A thin layer (5 μm) of Ni, W, and Au, respectively, was deposited on Na_xCoO₂ by sputtering or vapor deposition. In order to study the diffusion between the metal and the thermoelectric material, metallized samples were annealed in air, where Na_xCoO₂ remains stable. To avoid oxidation of the metals by the atmosphere, the coatings were covered by a thin Al₂O₃ protective layer of about 1 μm thick. After annealing the cross section of the samples has been investigated by means of scanning electron microscopy and energy-dispersive X-ray analysis. The condition of the metallic coating after exposure in air was studied and distinguished as: separated from substrate / adhering to substrate / continuous / cracked. The results of the reaction of Na_xCoO₂ with thin metal films of Ni, W and Au presented here show that Au exhibited only a narrow diffusion length after prolonged annealing while the Ni and W film were oxidized by the substrate Na_xCoO₂.