Oxide pyrochlores — A review

In recent years, many soft-chemical reactions of layered perovskites have been reported, and they can be classified into sets of similar reactions. Simple ion-exchange and intercalation reactions replace or modify the interlayer cations of layered perovskites, and more complex metathesis reactions replace interlayer cations with cationic structural units. Topochemical condensation reactions that involve dehydration and reduction provide access to a variety of metastable structural features in three-dimensional perovskites, and similar reactions can be used to convert among higher order layered perovskite homologues. Other techniques, such as high pressure and anion intercalation/deintercalation, also yield interesting metastable phases. When combined, the individual reactions complement each other, and a powerful toolbox of solid-state reactions emerges. By using layered perovskites as templates, it is possible to retrosynthetically design new product perovskites that retain the structural features of the...

Perovskites by Design: A Toolbox of Solid-State Reactions

TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17

Magnesium aluminate (MgAl2O4) spinel (MAS) is a synthetic material with cubic crystal structure and excellent chemical, thermal, dielectric, mechanical and optical properties. These properties made MAS an indispensable material for optically transparent windows, domes and armours, and for certain refractory applications. High processing cost of dense MAS ceramics is in the main responsible for its limited usage in certain important applications despite its excellent performance in them. The volume expansion (∼8%) associated with MAS phase formation from alumina and magnesia does not allow obtaining dense MAS bodies in a single-stage reaction sintering process. Therefore, dense MAS bodies are made by following a double stage firing process, which is expensive. The existing literature suggests that the processing cost of dense MAS ceramics could be reduced to a great extent by decisive selection of starting raw materials, powder processing and densification conditions, and by understanding the under...

A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications

The dielectric properties and chemical homogeneity of BaTiO3 ceramics sintered with additions of the pseudophase “CdBi2Nb2O9” were investigated using SEM, TEM, STEM, and EDX. In materials showing the “X7R” dielectric temperature characteristic, the microstructure exhibits the grain core-grain shell structure. The perovskite material in the shell shows a temperature characteristic determined by mixed crystals of BaTiO3 with the complex perovskites Ba(Bi1/2Nb1/2)O3 and Ba(Cd1/3Nb2/3)O3 having an approximate Curie point of -80°C. The chemical inhomogeneity emerges during a process of reactive liquid-phase sintering. Application of too-high sintering temperatures leads to uniform distributions of the additives via solid-state diffusion and to the loss of the X7R characteristic.

Temperature‐Stable Dielectrics Based on Chemically Inhomogeneous BaTiO3

By controlling the granulometric distribution of the powders, a density of >96% of the theoretical density has been attained for Ba(Zn1/3Ta2/3)O3 that has been sintered at a low temperature (1450°C) for a short soaking time (2 h). Attrition milling produces a very fine and monomodal powder that allows for good densification. The attrition conditions result from a compromise between granulometric distribution and the risk of pollution. We have found a great influence of the origin of the raw materials on the temperature coefficient of frequency, allowing a modulation in the range of 0-15 ppm/°C. Finally, we have studied the influence of each step of the process on the dielectric properties and identified the secondary phases that form with the evaporation of zinc: an eight-layered hexagonal perovskite and a tetragonal tungsten-bronze-type phase.

Low‐Temperature Sintering and Influence of the Process on the Dielectric Properties of Ba(Zn1/3Ta2/3)O3

Les oxydes ATiMO5 : Echangeurs cationiques

Nonstoichiometric oxides with a layer structure: The compounds A1−x(Ti1−xM1+x)O5

The synthesis and reaction mechanisms of the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconducting compound have been investigated. Based on density curves evolution, on simulation of different kinetic models, and on SEM observations confirming directly some proposed hypotheses, the formation mechanisms are explored and discussed in this paper. It was found that the conversion process of Bi-2212 to the Bi-2223 phase was competitive, consisting of a one-dimensional diffusion-controlled nucleation-growth reaction with edge dislocation at early sintering stages and a two-dimensional heterogeneous growth at grain interfaces with prolonged sintering. It was suggested that the origin of the liquid phase accounting for the fast transformation of 2212 to 2223 comes from the reaction between Ca 2 PbO 4 and PbO. A reproducible processing protocol to develop the synthesis of the 110 K high- T c phase in the solid-state regime in large quantitites is also presented, as is needed for applications.

Synthesis and reaction mechanism of the high-Tc 2223 phase in the (Bi,Pb)SrCaCaCuO system

A new oxygen-rich 123-phase, YBa 2 Cu 3 O 6.8 , with tetragonal symmetry has been synthesized with c / a =3. Its mean structure has been determined from powder neutron diffraction. The HREM study shows the existence of microdomains, oriented at 90°, resulting from the lack of mismatch between a and c ( c rigorously equal to 3 a ). Another striking characteristic concerns the very dense microtwinning, with 〈110〉 twin boundaries spaced from 22 A to 65 A. Structural models are proposed, taking into consideration copper disproportion and a reasonable coordination for copper. The lack of superconductivity in this oxide, in spite of its rather high Cu (III) content is discussed.

G. Desgardin

Papers

Low‐Temperature Sintering and Influence of the Process on the Dielectric Properties of Ba(Zn1/3Ta2/3)O3

Les oxydes ATiMO5 : Echangeurs cationiques

Nonstoichiometric oxides with a layer structure: The compounds A1−x(Ti1−xM1+x)O5

Synthesis and reaction mechanism of the high-Tc 2223 phase in the (Bi,Pb)SrCaCaCuO system

YBa2Cu3O6.8: an oxygen-rich “123”-type phase with tetragonal symmetry