MICROSTRUCTURE OF POLARIZED ELECTROCHEMICAL VAPOR-DEPOSITION (PEVD) PRODUCTS

Many types of electrocrystallization microstructures have been studied for depositions from aqueous solutions and molten salt electrolyte sy stems. However, our knowledge is still limited for solid electrolyte s ystems. This paper presents initial microstructural results of the pro duct phases produced during polarized electrochemical vapor deposition (PEVD), which takes advantage of a solid state ionic material to tran sport the reacting species and utilizes electrochemical reaction of th e ionic species with the vapor phase for deposition. The microstructur e of PEVD products during various deposition conditions has mainly bee n characterized using scanning electron microscopy (SEM). In this stud y, a PEVD product (Na,CO,) was deposited on the cathode of a CO, poten tiometric sensor. Preferred growth and a faceted structure oriented al ong the porous Pt electrode surface were found. The unique electrocrys tallization microstructure of the PEVD products is due to the availabi lity of the reacting species for electrochemical reaction, electronic or ionic shorting at the substrate surface, and the preferred crystall ographic direction for ionic conduction in the product phase during bo th crystal nucleation and growth stages. The product microstructural s tudies indicate that PEVD is an attractive thin film deposition techni que to improve the solid electrolyte/electrode contact in a number of solid state ionics devices, such as sensors and fuel cells. Furthermor e, the studies in this paper also suggest that the microstructure of t he PEVD products is related to the thermodynamic considerations for th e electrochemical reaction in PEVD and electrical properties for the P EVD product phase. (C) 1998 Elsevier Science Ltd. All rights reserved.