G. Desgardin

Les oxydes ATiMO5 : Echangeurs cationiques

Abstract The ion-exchange properties of the ATiMO 5 oxides (M = Ta, Nb), with a layer structure, are shown for the first time. By action of an acid solution on the ATiMO 5 compounds, the HTiMO 5 oxides were synthesized. The ion-exchange properties of the HTiMO 5 compounds were used to prepare new oxides which could not be obtained by direct action of the oxides - Tl Ti TaO 5 , NH 4 Ti MO 5 - and hydrated compounds - ( Na 2 3 ( H 3 O ) 1 3 ) TiMO 5 .H 2 O, ( Li 2 3 ( H 3 O ) 1 3 ) TiMO 5 . 4 3 H 2 O . The reversibility of the exchange properties was observed in every case. The crystallographical properties of these oxides were determined and discussed.

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

Abstract New nonstoichiometric oxides A1−x(Ti1−xNb1+x)O5 and tantalates ATiTaO5 with a layer structure of the KTiNbO5 type have been isolated, with A = K, Rb, Tl, Cs. These oxides, which are orthorhombic, space group Pnma, are characterized by a preferential occupation of one type of site 4c by the titanium atoms. The structural evolution as a function of composition and the stability of these compounds are discussed.

Influence of calcination, sintering and composition upon microwave properties of the Ba6−xSm8+2x/3Ti18O54-type oxide

Dielectric ceramic compositions for microwave applications belonging to the (BaO) (Sm2O3) (TiO2) ternary phase diagram were studied. Calcination, sintering, microwave properties and influence of secondary phases were investigated. By varying composition and/or sintering process, a high dielectric constant with low dielectric losses and modulable negative or positive temperature coefficient of the resonant frequency can be reach.

Rapid synthesis of the Bi(Pb)-2223 110 K superconductor by the EDTA sol-gel method

EDTA sol-gel methods have previously been employed successfully to synthesize Bi(Pb)-2223 phase. But heat-treatment optimization to transform the precursor solution into the final 2223 powder seems to have been neglected, leading to long heat-treatment times as in the classical powder method. In this paper, a route is proposed to accelerate 2223 phase kinetics formation, by improving calcination, sintering and intermediate grindings. Thus a nearly pure 2223 powder is obtained consecutive to a relatively short total duration of heat treatment.

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

Abstract 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.

Microstructure and flux pinning properties of melt textured grown doped YBa2Cu3O7−δ

Abstract The physical properties and the microstructure of highly textured YBa 2 Cu 3 O 7 /Y 2 BaCuO 5 ceramics, doped with platinum and/or cerium have been investigated by high resolution electron microscopy (HREM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The Ce, Pt and Ce + Pt doped samples show different J c behaviours as function of the applied field and a systematic comparison of the microstructure at different scales has been performed. The combination of both dopants leads to a drastic improvement of the critical current density either under zero field or with an external magnetic field (fishtail effect). A critical current density up to 56 000 A/cm 2 has been observed, which remains 43 000 A/cm 2 under 1 T in a sample doped with 2 wt% cerium oxide and 0.5 wt% platinum oxide. We suggest that a slight introduction of Ce in the YBa 2 Cu 3 O 7−δ matrix locally changes the superconducting properties, and enhances the flux pinning properties. Micron size BaCeO 3 particles, which appear with a preferential orientation in the YBa 2 Cu 3 O 7−δ matrix, are also potential candidates for strong pinning. Other secondary phases, formed during the growth and the presence of a nanocrystalline phase in the plate like gap of the Ce + Pt doped samples are other admixtures. Finally a high density of twins is observed in all samples with some differences between the cerium and the platinum doped samples. They most probably contribute to pinning under intermediate field.

Microwave dielectric properties of doped Ba6−x(Sm1−y′Ndy)8+2x/3Ti18O54 oxides

Dielectric ceramic compositions for microwave applications belonging to the (BaO)-(Nd2O3)-(Sm2O3)-(TiO2) phase diagram were studied. Two compositions were selected for study, the doping effect of MnO2, WO3, CaO on dilatometry, microstructure and microwave properties. The effect of the nature and the amount of dopants on microstructure and microwave properties were clearly demonstrated. The effect of the addition of 1 or 2wt% WO3, MnO2 and CaO to Ba4 (Sm0.6, Nd0.4)9.33Ti18O54 and Ba3.75(Sm0.5, Nd0.5)9.5Ti18O54 was studied. It was found that these two compositions lead to dense ceramics exhibiting excellent microwave properties, for instance ɛ=74–81, Q × f up to 9000 Ghz at 3 Ghz and τf around +9 p.p.m.°C−1.

Dielectric ceramics with relaxors and a tetragonal tungsten bronze

Abstract Type II ferroelectric ceramics with high dielectric constants and flat curves e = f(T) have, until now, only been obtained with oxides which exhibit the perovskite structure. Another family of oxides, the tetragonal tungsten bronze (TTB)-type niobates, should be of great interest for such applications because of their structure, which is, indeed, very closely related to that of perovskite. The authors report here on the sintering and dielectric properties of new ceramics based on the niobate K 0·2 Sr 0·4 NbO 3 (KSN) which exhibits the TTB structure, and show how the addition of a perovskite such as a relaxor or BaTiO 3 or a mixture of these perovskites can lead to a dielectric material compatible with the Y7 R or X7R Electronic Industries Association specifications with a dielectric constant e as high as 6000. Moreover, the use of a lithium salt as sintering agent has been shown to be important for the elaboration of those ceramics, and particularly allows the synthesis of type I materials with a dielectric constant e as high as 3000.

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

Abstract 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.

Synthese et evolution structurale de nouveaux pyrochlores au bismuth

Abstract The systems Cd 2 M 2 O 7 ue5f8Bi 3 MO 7 ue5f8Zn 2 M 2 O 7 (M = Ta and Nb) have been investigated: new ternary bismuth oxides with pyrochlore structure have been synthetized; their structural evolution is studied and discussed.