Films of zirconia-based solid

STRUCTURE AND FORMATION OF FILMS OF ZIRCONIA-BASED SOLID ELECTROLYTE... 

Keywords structure of films, films of zirconia-based solid electrolytes, ion plasma sputtering, scanning electron microscope, transmission electron microscopy. 

The structure of films of zirconia-based solid electrolytes was analyzed in a scanning electron microscope. It was found that both one- and multilayer films (Fig. la, lb) of zirconia, which was stabilized to its cubic modification, up to 10 pm thick had a columnar structure, that is, consisted of mutually adjoining crystallites, which generally were oriented perpendicularly to the film surface. The observed deviation of the crystallites from the normal direction to the film plane was not over 15° and was explained by the mutual misorientation of the target and the substrate. 

Considering the obtained experimental data, it is possible to propose a model of the formation of a porous structure of the films of zirconia-based solid electrolytes. The model assumes the formation of pores and submicropores when vacancies, which are trapped during sputtering of the solid-electrolyte films (the sputtering temperature was Tf < 0.3Tmeit), pass to sinks and then condense [2,3,4,5], The sinks are boundaries between the crystallites forming the film structure. 

The transmission electron microscopy was used to study the initial stage of the formation of the films of zirconia-based solid electrolytes, which were prepared by ion plasma sputtering on a glass-ceramic substrate having a thin ( 10 nm) layer of amorphous carbon. 

The use of multilayered films of zirconia-based solid electrolytes in fuel cells is more promising than the use of one-layer films. 

The tape-casting method makes possible the fabrication of films in the region of several hundred micrometers thick. The mechanical strength allows the use of such a solid electrolyte as the structural element for devices such as the high-temperature solid oxide fuel cell in which zirconia-based solid electrolytes are employed both as electrolyte and as mechanical separator of the electrodes. 

The structure and the formation of films of a zirconia-based solid electrolyte, which were prepared by ion plasma sputtering, were studied using scanning and transmission electron microscopy methods. 

It was found that at the initial stage of its formation the film of the zirconia-based solid electrolyte consisted of nanocrystallites arranged randomly or in some order. The nanocrystallites had an irregular round shape and were 40 nm in size on the average. Ordered nanocrystallites generally formed squares, but sometimes they were shaped as parallelograms (Fig. 2). The nanocrystallites had a kind of self-organization. 

Figure 2a. Ordered nanocrystalline structure of a film of a zirconia-based solid electrolyte in the form of a parallelogram.

Since the nanocrystallites were separated from the substrate with a layer of amorphous carbon, one might think that the ordered arrangement of the nanocrystallites was due to an auto-orientation mechanism, which operated at all stages of the growth of the ordered nanocrystalline forms of the films of the zirconia-based solid electrolyte. 

The simultaneous analysis of the results of the study into the structure and the formation of the solid-electrolyte films led to the following conclusion the nanocrystalline structure of the solid-electrolyte films at the initial stage of their formation caused the appearance of a columnar structure of the films of the zirconia-based solid electrolyte during their sputtering. 

S. Wuttiphan, A. Pajares, B. Lawn, C. C. Berndt, Effect of substrate and bond coat on contact damage in zirconia based plasma sprayed coatings, Thin Solid Films, 293 251 260 (1997). 

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

Instead of the system silica/silicate also other systems such as titania/titanate, zirconia/zirconate can be used as a reference system [xiv]. The response time of freshly fabricated thick-film sensors based on thin-film /3-alumina is very short (about 15 ms at 650 °C). After several weeks of operating this time increases 10 times (150 ms) [xv]. Solid electrolyte C02 sensors using Ni/carbonate composite as measuring electrode are suited for measuring of C02 in equilibrated water gases [xiv]. Using semiconducting oxides and carbonates like ITO (indium tin oxide) Nasicon-based C02 sensors are able to measure at room temperature [xvi]. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

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