Preparation of yttria-stabilized zirconia (YSZ) films on La0.85Sr0.15MnO3 (LSM) and LSM–YSZ substrates using an electrophoretic deposition (EPD) process
Introduction
Solid oxide fuel cells (SOFCs) have emerged as a leading technology to provide clean and efficient power sources of the future. The advantages offered by the operation of SOFCs at low or intermediate (600–800°C) temperatures, such as high energy efficiency and reduced material and maintenance cost, have stimulated intense investigations into new materials with fast transport and high catalytic activities and new processes for fabrication of thin film SOFCs.1, 2
Yttria-stabilized zirconia (YSZ) is still the best electrolyte among all solid electrolytes studied for SOFCs because of its excellent long-term stability, mechanical strength and durability. Due to its limited ionic conductivity, however, YSZ must be fabricated in a thin-film form for intermediate or low temperature SOFCs. A number of techniques have been successfully developed for preparation of YSZ films on porous substrates, including electrochemical vapor deposition (EVD),3 chemical vapor deposition (CVD),4 physical vapor deposition (PVD),5 electrostatic spray deposition,6 sputter deposition,7 sol–gel processing,8 solution deposition9 and colloidal deposition.10 However, each of these processes has some limitations, including difficulty in obtaining good compositional homogeneity, not suitable for mass production, not cost-effective, or difficult to reproduce.
Another alternative thin-film forming approach is electrophoretic deposition (EPD),11 in which charged particles dispersed in a stable suspension are driven by a dc electric field to move towards an oppositely charged electrode, upon which they ultimately deposit and build up a closely-packed particulate layer. EPD is a combination of two processes: electrophoresis and deposition. Electrophoresis is the motion of charged particles in a suspension under the influence of an electric field. Deposition is the coagulation of charged particles to a dense mass. As the green ware contains no organics, no burn out procedures are required. EPD is inexpensive and has been used for many years to fabricate green ceramic bodies and coatings with different shapes for applications ranging from ceramic/ceramic and metal/ceramic composites to thin/thick film coatings for electronic devices.12, 13 EPD has the advantages of short formation times, little restriction in the shape of substrates, simple deposition apparatus, and suitable for mass production. It is a cost-effective, reproducible process for the preparation of electrolyte films for SOFCs. However, gas tight films are difficult to obtain by EPD from aqueous suspensions due to the evolution of hydrogen and oxygen by water electrolysis at the substrate, resulting in the formation of holes in the film. Fortunately, this can be avoided by using organic solvents such as ketones or ethanol to form the suspension.
The preparation of YSZ films for ceramic coatings using EPD was first investigated by Nicholson and co-workers.14, 15 YSZ films as thin as 2 μm were deposited on a graphite cathode from YSZ-ethanol suspensions using constant current EPD. Ishihara and co-workers16 applied EPD to deposit YSZ films onto a porous NiO–CaO-stabilized zirconia (NiO–CSZ) cermet using ketone as the solvent. Since the conductivity of the NiO–CSZ cermet was very low, a Pt electrode was plated on one face of the cermet using electroless-plating prior to the deposition of YSZ powder. The deposition was conducted at a constant voltage of 5 V for 3 min and the process was repeated five times to yield a 5 μm nonporous YSZ film after being sintered at 1375°C for 1 h.
In this article, we report our findings in fabricating YSZ films on porous La0.85Sr0.15MnO3 (LSM) substrates using an EPD process. LSM is superior to NiO–CSZ cermet as the substrate because LSM is conductive. Further, LSM–YSZ composite was also used as substrate for deposition of YSZ films because the addition of YSZ to the LSM electrode significantly enhances the catalytic activity of the LSM–YSZ composite electrode.17 Another anticipated benefit of using the composite substrate is the improved adhesion between the film and the substrate due to the intimate sintering of YSZ particles to the LSM–YSZ substrate. The processing parameters affecting the film microstructure are systematically investigated and subsequently optimized to improve the quality of films. The electrochemical properties of the films and the performance of the solid-state ionic devices based on these films will be discussed in future communications.
Section snippets
Preparation of LSM and LSM–YSZ substrates
LSM powders were prepared using the Pechini process, as summarized in Fig. 1. The precursors were La(NO3)3·4H2O (Alfa), Mn(CH3COO)2·4H2O (Aldrich), and Sr(NO3)2 (Aldrich). Stoichiometeric amount of the nitrates (yielding the composition of La0.85Sr0.15MnO3) was dissolved into de-ionized water. 90/10 mole ratio of citric acid/ethylene glycol was first dissolved in de-ionized water and then mixed with the nitrate solution. The solution was heated to 70°C under continuous stirring until it turned
Characteristics of the LSM powder obtained from the Pechini process
Shown in Fig. 3(a) is a XRD pattern obtained for the LSM powder after calcination at 600°C for 1 h, indicating that the powder has a pure perovskite phase. The average particle size is about 100 nm, as calculated from the line broadening according to Scherrer formula.18 Shown in Fig. 3(b) is a representative TEM image of the LSM powder, showing that the average particle size is about 100 nm. The BET surface area of the LSM powder is 37 m2/g, as determined from the surface area analyzer. Shown
Conclusions
The Pechini process has been used to prepare fine LSM powders with an average particle size of about 0.1 μm. Both porous LSM and porous LSM–YSZ pellets are used as substrates for preparation of YSZ films using an EPD process. Uniform, crack-free green YSZ films with high packing density of YSZ particles are obtained by optimizing the key processing variables. Dense, crack and pinhole-free sintered YSZ films with a thickness of about 10 μm have successfully been prepared using an one-step EPD on
Acknowledgements
The authors wish to gratefully acknowledge partial support of this project by the National Science Foundation under Award No. DMR-9357520, by Georgia Tech Research Cooperation, and by the Georgia Institute of Technology Molecular Design Institute, under prime contract N00014-95-1-1116 from the Office of Naval Research. Special thanks are given to Dr Z. L. Wang at Georgia Institute of Technology Electron Microscope Center for SEM assistance.
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