Zirconia calcia-stabilized

The same principles apply when two solids react, as can be illustrated by the reaction of a crystal of zirconia, Zr02, with a small amount of calcia, CaO, to produce a crystal of calcia-stabilized zirconia. There are two principal ways that this reaction can be imagined to occur either the Ca atoms occupy Zr sites or they occupy interstitial sites. 

The final outcome for the reaction between the two ionic crystals is obtained by adding Eqs. (1.8) and (1.9) to give 

For chemical reasons it might be argued that the Ca2+ ions do not occupy Zr4+ sites but prefer interstitial positions, while the oxygen atoms occupy newly formed sites. The Ca interstitial atoms do not affect site numbers, but the oxygen atoms must maintain the site ratio of the ZrC 2 matrix, so that one Zr vacancy must be created for each pair of oxygen atoms added. Because neutral atoms are added, the vacancy and oxygen atoms will carry effective charges as above. The reaction is 

If the added species are ions (Ca2+ and O2-) the Ca2+ ions will have an effective charge of 2 , written Ca2. Each oxide ion has no effective charge compared to a normal oxide anion, written Oo- There is no change as far as the Zr vacancy is concerned. The transformation can formally be expressed as 

Experimental evidence is needed to decide which (if any) of these formal equations represents the true situation in the material (see Section 4.4.5). 

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M. J. Radler, J. B. Cohen, and J. B. Faber, J. Phys. Chem. Solids, 51, 217-228 (1990). References to calcia-stabilized zirconia ... 

Each added Cd2+ ion generates one cation vacancy. Similarly, each added Ca2+ ion into Zr02, as in calcia-stabilized zirconia (Section 1.11.6), generates a compensating anion vacancy ... 

The stabilized zirconia family of oxides, especially calcia-stabilized zirconia, are solids in which oxide ion conductivity has been increased to the extent that they are widely used solid electrolytes (Section 1.11.6, Section 4.4.5, and Section 6.8). 

CALCIA-STABILIZED ZIRCONIA AND RELATED FAST OXYGEN ION CONDUCTORS... 

A number of oxides with the fluorite structure are used in solid-state electrochemical systems. They have formulas A02 xCaO or A02 xM203, where A is typically Zr, Hf, and Th, and M is usually La, Sm, Y, Yb, or Sc. Calcia-stabilized zirconia, ZrC)2.xCaO, typifies the group. The technological importance of these materials lies in the fact that they are fast ion conductors for oxygen ions at moderate temperatures and are stable to high temperatures. This property is enhanced by the fact that there is negligible cation diffusion or electronic conductivity in these materials, which makes them ideal for use in a diverse variety of batteries and sensors. 

The parent phase is a stoichiometric oxide M02 with the fluorite structure. Substitution of a lower valence cation for Zr4+ is compensated by oxygen vacancies (Section 1.11.6 and Section 4.4.5). Taking calcia-stabilized zirconia as an example, addition of CaO drops the metal to oxygen ratio to below 2.0, and the formula of the oxide becomes Ca Zrj -x02-x. 

FIGURE 5.11 Phase diagram of the pseudobinaiy CaO-ZrOz system. The cubic calcia-stabilized zirconia phase occupies the central band in the diagram and is stable to about 2400°C. 

FICs are useful as electrochemical sensors, electrolytes and electrodes in batteries and in solid state displays (Farrington Briant, 1979 Ingram Vincent, 1984). If a FIC material containing mobile M ions separates two compositions with different activities of M, a potential is set up across the FIC that can be related to the difference in the chemical activities of M. By fixing the activity on one side, the unknown activity on the other can be determined. This principle forms the basis of a number of ion-selective electrodes LaFj doped with 5% SrF2 is used for monitoring fluoride ion concentration in drinking water. Similarly, calcia-stabilized-zirconia is used in cells of the type... 

Solid electrolytes, which show ionic conductivity in the solid state, are considered to be potential materials for practical use, some are already used as mentioned below. Solid electrolytes have characteristic functions, such as electromotive force, ion selective transmission, and ion omnipresence. Here we describe the practical use of calcia stabilized zirconia (CSZ), (Zr02)o,85(CaO)o 15, the structure and basic properties of which are discussed in detail in Sections 1.4.5 1.4.8. 

ICI Co. uses a sol-gel method to produce silica-stabilized alumina (Saffil) and calcia-stabilized zirconia fibre [15], The saffil fibre is a 8-alumina short staple fibre that has about 4% Si02 and a very fine diameter (3 pm). 

For nonstoichiometric phases the stabilization of extended defects depends on the ion polarizability. The formation of extended defects is associated with a drastic decrease in the ionic conductivity, as for instance in calcia-stabilized zirconia (Section 3.2.1). 

Molodetsky, 1, Navrotsky A, Lajavardi M, Brune A (1998) The energetics of cubic zirconia from solution calorimetry of yttria- and calcia-stabilized zirconia. Z Physik Chem 207 59-65 Molodetsky 1, Navrotsky A, Paskowitz MJ, Leppert VJ, Risbud SH (2000) Energetics of X-ray-amorphous zirconia and the role of surface energy in its formation. J Non-Crystalline Solids 262 106-113 Moloy EC, Davila LP, Shackelford JF, Navrotsky A (2001) High-silica zeolites a relationship between energetics and internal surface area. Microporous Mesoporous Materials (submitted)... 

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