Nucleation And Growth of Yttria-Stabilized Zirconia Thin Films Using Combustion Chemical Vapor Deposition
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Nucleation And Growth of Yttria-Stabilized Zirconia Thin Films Using Combustion Chemical Vapor Deposition Zhigang Xu1, Jag Sankar1, Sergey Yarmolenko1, Qiuming Wei1,2 1 NSF Center for Advanced materials and Smart Structures, North Carolina A&T State University, Greensboro, NC 27411 2 Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218 ABSTRACT Liquid fuel combustion chemical vapor deposition technique was successfully used for YSZ thin film processing. The nucleation rates were obtained for the samples processed at different temperatures and total-metal-concentrations in the liquid fuel. An optimum substrate temperature was found for the highest nucleation rate. The nucleation rate was increased with the total-metal-concentration. Structural evolution of the thin film in the early processing stage was studied with regard to the formation of nuclei, crystallites and final crystals on the films. The films were found to be affected by high temperature annealing. The crystals and the thin films were characterized with scanning electron microscopy. INTRODUCTION Yttria stabilized cubic phase zirconia (YSZ) is an oxygen ion conductive material [1]. It is the most widely used electrolyte material available for high-temperature fuel cells. In order to obtain the best performance of the fuel cells, thin film of the YSZ electrolyte is favored to minimize the current path [2]. In the SOFCs, both the air and fuel electrodes are porous materials. The thin film of electrolyte is placed between these electrodes. It has to be gas-tight to avoid any crossover of the oxidant and fuel gases. A fundamental understanding of the nucleation and growth of thin YSZ films is very important to its successful application for SOFCs. Atmospheric combustion chemical vapor deposition (ACCVD) technique had been employed for YSZ thin film processing, because of its advantages as high growth rate, low setup cost and low run cost. Nucleation of particles on substrates is a fundamental topic in chemical vapor deposition. Many innovative methods were studied to enhance the nucleation rate and/or to control the quality of the nucleated crystals [3-7]. However, most of the reported studies were required film processing in vacuum reactors. The nucleation and particle growth for ACCVD, conducted in open air, have not been well understood. In this paper, nucleation and growth of YSZ crystals using the ACCVD technique are presented. The emphases of the research were to understand the effects various processing parameters on the nucleation and growth. The methods to enhance the nucleation density were studied. The evolutions of microstructures from nuclei to continuous films were also observed.
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EXPERIMENTAL DETAILS A liquid fuel ACCVD system similar to the one introduced in previous work [8] was used for YSZ thin film processing. The metal-organic reagents were dissolved in toluene that acts as both a solvent and a combustion fuel to supply sufficient thermal energy for chemical reaction in the gas phase a
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