Yttria ionic conductivity

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

SOE cells utilize solid ceramic electrolytes (e.g. yttria stabilized zirconia) that are good oxygen ion (0 ) conductors at very high temperatures in the range of 1000°C [8]. The operating temperature is decided by the ionic conductivity of the electrolyte. The feed gas, steam mixed with hydrogen, is passed through the cathode compartment. At the cathode side, the reaction is... 

As can be seen, the mechanical strength of the partially stabilized body is approximately twice that of the fully stabilized and the thermal expansion is approximately 30% lower. Because of this, the thermal shock resistance of the PSZ body is greatly improved. The ionic conductivity of the PSZ body is lower but is still adequate for automotive applications. Figure 10 compares the conductivity of a yttria partially stabilized zirconia body with several fully stabilized bodies. 

CaO has been used to some degree as a stabilizer and is attractive due to its low cost. Its ionic conductivity, however, is approximately an order of magnitude less than an equivalent yttria stabilized body. There has also been some question about the chemical stability of a CaO stabilized body, although this may be more of a factor with a partially stabilized body than a fully stabilized body. Calcia fully stabilized ZrO has been and may still be used in commercial production of oxygen sensors. 

Zirconia, stabilized with 8-9 % yttria (yttria stabilized zirconia - YSZ) is the most commonly used electrolyte for SOFCs because it exhibits predominant ionic conductivity (O2- transport number close to unity) over a wide range of oxygen partial pressures (1 to 10 20 atmospheres). YSZ provides sufficient conductivity at... 

For any particular solid, the relative activation barriers for the available mechanisms determine whether the anions or cations are responsible for the ionic conduction. For example, in a yttria-stabilized Zr02, with the formula Zri Y ,.02-(x/2). aliovalent substitution of Zr by Y generates a large number of oxygen vacancies, giving rise to a mechanism for oxide ion conduction. Indeed, it is found that the anions diffuse about six orders of magnitude faster than the cations. 

Goff, J.P., Hayes, W., Hull, S., Hutchings, M.T., and Clausen, K.N., Defect structure of yttria-stabilized zirconia and its influence on the ionic conductivity at elevated temperatures, Physical Review B, 1999, 59, 14202. 

A primary limitation of YSZ is its low ionic conductivity. To overcome this, thinner electrolyte layers have been developed and yttria has been replaced with other acceptors. Even with these developments, the electrolytes mnst operate at temperatures exceeding 600°C. Ce02 materials have a higher ionic condnctivity than YSZ and can operate in the temperatnre range of 500 to 700°C bnt snffer from strnc-tural instability in the reducing atmosphere of the cell. 

These dopants also stabilize the crystal structure in the cubic fluorite phase at the sintering ( 1450°C) and in-service operating temperatures (600-1100°). The mechanism of ionic conduction involves migration of ions in the anion sublattice by exchange with oxygen ion vacancies. The 0 ion conductivity of yttria-stabilized ziroconia at 1100°C is comparable that of Na ion conductivity in jS"-alumina at 300° C. 

In EVD, a modified form of chemical vapor deposition (CVD), an electrochemical potential gradient is used to grow a thin, dense layer of the ionic conducting oxide (e.g., yttria-stabilized zirconia) on a porous substrate. EVD is either a single-step or a two-step process depending on the nature of the substrate. For a porous substrate, the first step involves pore closure by CVD (i.e., deposition from the vapor of an oxide layer by reaction of a chloride gas precursor compound with water vapor or oxygen) ... 

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

Another type of electrical conductivity observed in ceramics is ionic conductivity, which often occurs appreciably at elevated temperature a widely used material exhibiting this behavior is zirconia doped with other oxides such as calcia (CaO) or yttria (Y2O3). For this material, atomic oxygen is the mobile ionic species. Doped zirconia finds widespread use as oxygen sensors, especially as part of automobile emission control systems, where the oxygen content of the exhaust gas is monitored to control the air-to-fuel ratio. Other applications of ionic conducting ceramics are as the electrolyte phases in solid-oxide fuel cells and in sodium-sulfur batteries. 

Figure 8.4. Experimental (full lines) and calculated (dashed lines) d.c. ionic conductivities at two different temperatures for yttria-doped ceria as a function of the composition (adapted from ref. 59)...

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