Modeling for CVD of Solid Oxide Electrolyte

Because of its low thermal conductivity, high thermal expansion and high oxygen ion conductivity yttria-stabilized zirconia (YSZ) is the material of choice for high temperature electrolyte applications. Current coating fabrication methods have their drawbacks, however. Air plasma spray (APS) is a relatively low-cost process and is suitable for large and relatively complex shapes. it is difficult to produce uniform, relatively thin coatings with this process, however, and the coatings do not exhibit the columnar microstructure that is needed for reliable, long-term performance. The electron-beam physical vapor deposition (EB-PVD) process does produce the desirable microstructure, however, the capital cost of these systems is very high and the line-of-sight nature of the process limits coating uniformity and the ability to coat large and complex shapes. The chemical vapor deposition (CVD) process also produces the desirable columnar microstructure and--under proper conditions--can produce uniform coatings over complex shapes. CVD has been used for many materials but is relatively undeveloped for oxides, in general, and for zirconia, in particular. The overall goal of this project--a joint effort of the University of Louisville and Oak Ridge National Laboratory (ORNL)--is to develop the YSZ CVD process for high temperature electrolyte applications. This report describes the modeling effort at the University of Louisville, which supports the experimental work at ORNL. Early work on CVD of zirconia and yttria used metal chlorides, which react with water vapor to form solid oxide. Because of this rapid gas-phase reaction the water generally is formed in-situ using the reverse water-gas-shift reaction or a microwave plasma. Even with these arrangements gas-phase nucleation and powder formation are problems when using these precursors. Recent efforts on CVD of zirconia and YSZ have focused on use of metal-organic precursors (MOCVD). These are more stable in the gas-phase and can produce dense, crystalline films. With metal-organic CVD, consistent controlled delivery of the precursor vapor is sometimes a problem. Direct vaporization and vapor-phase metering is difficult due to marginal thermal stability of these compounds and changes in vaporization rate over time. A number of special precursor delivery systems have been designed to address these challenges. The direct liquid injection (DLI) method has several advantages for precursor delivery. Liquid metering provides accurate, stable control of precursor delivery rate. With a suitable solvent, a wide variety of precursor compounds can be used, including solids and other compounds not suitable for vapor delivery. Composite or multi-metal coatings require only one precursor source consisting of multiple precursors dissolved in the correct proportion in a single solution. Many uses of DLI-MOCVD involve deposition of thin films for electronic applications. In these applications, the substrate temperature is low and the deposition rate is relatively slow (< 1 {micro}m/hr). Under these conditions the deposition rate is kinetics-limited, i.e. controlled by the rate of reaction of adsorbed species on the substrate surface, and is strongly influenced by temperature. Thermal barrier applications require relatively thick films (50 to 100 {micro}m) and higher deposition rates. At the higher temperatures the deposition rate is ''transport-limited'', i.e. control by the transport rate of precursor to the surface. The purpose of this study is to investigate the deposition of zirconia under transport-limited conditions and to accurately model the deposition rate. Ultimately this model will be used to design a DLI-MOCVD reactor for coating of large, complex shapes