Mixed oxides, with ionic conductivity

or Sm in a content below 5 mol% led to materials containing cubic and tetragonal mbced phases. 

The formation of the cubic F-phase solid solutions was also evidenced for other binary Zr02-Me203 oxide systems (Me = La, Nd-Lu) [32], The phase diagrams of these materials also show the presence of tetragonal (T) and mono-chnic (M) phases coexisting with zirconia-rich phase mixtures. 

Differently, for the Zr02-M0 systems with alkaline earth elements (M = Be, 

no solid solutions with fluorite structure were produced at temperatures below 1500 °C. The conductivity of such multiphase ceramic systems containing monoclinic or tetragonal zirconia-based phases was considerably low. 

The method of sintering is also important for the phase content, densification, and ionic conductivity. This was well demonstrated for zirconia stabilized with 10 mol% scandia and 1 mol% ceria. Changing the temperature and time [33], a rhombohedric phase was detected in the 1150-1300 °C range, while conventionally sintered pellets, after 2h at 1200 °C, showed a cubic single phase. 

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Mixed oxides with ionic conductivity are electrolyte materials. A large variety of mixed oxides, especially nano-oxides with single or complex phases, were... 

The electrolytes are required to be solid in applications where the electrochemical device must operate at temperatures too high for liquid electrolytes or thin films of the electrolytes. Solid electrolytes are materials exhibiting high ionic conductivity with negligible electronic conductivity and are thermodynamically stable under conditions of the application. Many of the mixed oxide systems satisfy this requirement. Mixed oxides with high conductivities of both ions and electrons are named mixed conductors and are used together with electrolytes in many applications. 

Many types of oxide layers have a certain, not very high electrical conductivity of up to 10 to 10 S/cm. Conduction may be cationic (by ions) or anionic (by or OH ions), or of the mixed ionic and electronic type. Often, charge transport occurs by a semiconductor hole-type mechanism, hence, oxides with ionic and ionic-hole conduction are distinguished (in the same sense as p-type and n-type conduction in the case of semiconductors, but here with anions or cations instead of the electrons, and the corresponding ionic vacancies instead of the electron holes). Electronic conduction is found for the oxide layers on iron group metals and on chromium. 

The R-based mixed oxides with p-type electronic conductivity as well as the superconducting oxides were found to be inactive for the OCM reaction, and only total combustion of methane was observed. The only exception was Lao Sro MnOj 5, which is probably an n-type conductor under the experimental conditions used. This solid is likely to be n-type under reducing atmosphere due to the stability of the Mn2+ ion. LaftsSro.2Mn03and LaFeo.sNdo.2O3 5, both considered n-type conductors, had low selectivity to C2, whereas La-Sr-Y-O systems, either ionic conductors or insulators, presented a good selectivity to those compounds (table 8). 

Polypyrrole, polythiophene, polyaniline, polyfuran, polyacetylene, and poly-methylthiophene may exhibit a mixed electronic and ionic conductivity, similar to inorganic intercalation, or insertion compounds [43]. In the conductive form these polymers are partly oxidized and these positive charges are equilibrated by inorganic anions, which can diffuse through the polymer net. Alternatively, the conductive polymers can be partly reduced, with cations as counter ions. The eleetronic conductivity originates Irom partial oxidation of conjugated jt-bonds. The positive charge... 

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

AU these features—low values of a, a strong temperature dependence, and the effect of impurities—are reminiscent of the behavior of p- and n-type semiconductors. By analogy, we can consider these compounds as ionic semiconductors with intrinsic or impurity-type conduction. As a rule (although not always), ionic semiconductors have unipolar conduction, due to ions of one sign. Thus, in compounds AgBr, PbCl2, and others, the cation transport number is close to unity. In the mixed oxide ZrOj-nYjOj, pure 0 anion conduction t = 1) is observed. 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

Hitherto we have dealt with model FICs that are mostly useful as solid electrolytes. The other class of compounds of importance as electrode materials in solid state batteries is mixed electronic-ionic conductors (with high ionic conductivity). The conduction arises from reversible electrochemical insertion of the conducting species. In order for such a material to be useful in high-energy batteries, the extent of insertion must be large and the material must sustain repeated insertion-extraction cycles. A number of transition-metal oxide and sulphide systems have been investigated as solid electrodes (Murphy Christian, 1979). 

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