Paper TF-ThP37
Chemical Vapor Deposition of Samarium Doped Ceria from Metal-Organic Solid Precursors

Thursday, November 12, 2009, 6:00 pm, Room Hall 3

Doped ceria (where the dopant is a trivalent cation such as Sm or Gd) is an attractive electrolyte material for solid oxide fuel cells (SOFCs) owing to its high ionic conductivity at intermediate temperatures (IT). It is also a good anode material due to its mixed ionic-electronic conduction under the reducing conditions that exist at a fuel cell anode. For high power density, the electrolyte in a fuel cell must be fabricated in thin film form in order to lower the area specific resistance. Accordingly, many studies have been directed towards thin-film preparation of the conventional SOFC electrolyte material, yttria stabilized zirconia. In the present work, samarium doped ceria (SDC) thin films are grown via metal-organic chemical vapor deposition (MOCVD) as a first step towards high power density SDC based fuel cells.

A vertical cold-wall MOCVD reactor was built in-house. This cold-wall reactor has a showerhead which gives impinging multi-jet flow of precursor vapor onto a substrate of choice. Metal organic precursors, Ce(tmhd)₄ for Ce and Sm(tmhd)₃ for Sm, were utilized as the cation source compounds. They are commercially available as fine powders. These were used in the form of fine solid powder coated on steel balls by mechanical stirring. This step is expected to provide uniform mixing of the two precursors with sufficient, stable surface area for evaporation. During a deposition run, minimum surface area change for the solid precursors is desired. The compound mixture was placed in a single evaporation vessel for the simplicity of the system, and the evaporation temperature was controlled.

Precise and reproducible control of composition is not trivial for solid solutions such as samarium doped ceria. An UV optical cell is located between the precursor evaporator and the deposition chamber. Optical absorption of the incoming precursor vapors was monitored in-situ. Even though it is impossible to separate peaks from the two metal-organics due to severe peak overlap, gas phase UV absorption still provide valuable information on evaporation behavior. In-line gas phase reaction forms oxide powders before precursors reach the substrate, and therefore should be suppressed. UV absorption can detect this unwanted reaction. The influence of deposition conditions such as substrate temperature, evaporation temperature, and precursor mixing ratio, on the the samarium content in the oxide thin films is explored. Deposition is carried out on single crystalline oxide wafers, and nickel-SDC cermet pellets. The resulting films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction and energy dispersive X-ray spectroscopy (EDS).