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  • Author or Editor: A. Sebaoun x
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Transformations de la phase supraconductrice a haute T c (Bi,Pb)2Sr2CaCu2O8+x

Etude des variations de stoechiométrie d'oxygène par optimisation de l'analyse thermogravimétrique

Journal of Thermal Analysis and Calorimetry
Authors: P. Satre and A. Sebaoun

Abstract  

DTA-DTG systematic study of highT c superconducting phase (Bi,Pb)2Sr2CaCu2O8+x, with and without lead, shows a quantitatively opposite variation of oxygen stoechiometry on heating and cooling. Multiplate sample container, designed for the study, allowed to dissociated PbO loss from oxygen variations. Phases transformations have been studied under severalP O2 by scanning DTA and DTG. Oxygen variations evolution has been evaluated by isothermal TG. The results proved that phase diagram representation must take into consideration theP O2 coordinate.

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The solid ⇌ solid equilibria in the binary system Ca3(PO4)2-CaNaPO4 have been studied. Different diagrams have been proposed for this system. The differences relate to the limits of the field of existence of a non-stoichiometric phase (phase A) stable at high temperature and undergoing eutectoid decomposition during cooling; they can be explained by the existence of two metastable equilibria diagrams which are superposed in the stable diagram. These metastabilities depend on both the cooling rates and the maximum temperatures reached before this cooling.

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The phase diagram of solid-solid equilibria in the system Ca3(PO4)-CaNaPO4 has been studied by thermal analysis. The results are confirmed by X-ray diffraction at different temperatures. This study was limited to that of mixtures in a temperature range determined by previous results. The present work confirms the existence of two metastable diagrams which are superimposed on the stable diagram.

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Phase diagram in the Bi-Pb-Sr-Ca-Cu-O system

Buffer effect of non-superconducting phases on oxygen exchange of the superconducting ones

Journal of Thermal Analysis and Calorimetry
Authors: A. Sebaoun, P. Satre, M. Mansori and A. L'Honore

TG-DTA and X-ray diffraction measurements at different temperatures and under different oxygen partial pressures were carried out on the species Bi1.6Pb0.4Sr2CuO6±x, Bi1.6Pb0.4Sr2CaCu2O8±y and Bi1.6Pb0.4Sr2Ca2Cu3O10±z to analyse the influence of the oxygen chemical potential on the phase transformations. A new phase was found at isobaric invariant equilibrium for 0.5≤ /PTot≤1 at 884°C. This new phase presents a buffer effect towards oxygen exchange between superconducting oxides and the gas phase.

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Abstract  

A systematic study of Al-Zn-Sn ternary alloys by using a Tian-Calvet calorimeter with slow heating and cooling rate was carried out and supplemented by scanning electron microscopic observations. The results have shown that crystallization coupled with dissolution of tin into the αss′ ternary solid solution on heating is an endothermic process, while melting coupled with tin expulsion on cooling is an exothermic one. It seems that the thermal effects of phase transition are outweighed by much stronger ones due to a large composition change of the αss′ ternary phase.

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Abstract  

The Al–Ga–Zn ternary phase diagram presents two isobaric invariant reactions: a eutectic at 231C and a metatectic at 1231C [1–3]. Calorimetric measurements on the two isobaric invariant reactions have been carried out. First the Tammann method has enabled us to determine the composition of their limits on five isopletic cross sections. Then, the compositions of the invariant phases have been determined.

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Abstract  

The phase diagram of the binary AgNO3–CsNO3 system was constructed using differential thermal analysis (DTA) technique in the range 300–700 K. The apparatus is described briefly. The results exhibit a congruently melting compound CsNO3·AgNO3 (m.p.=453 K) characterized by two allotropic varieties and , an incongruently melting compound AgNO3·CsNO3 (m.p.=450 K) with three forms , and , two eutectics (16 mol% CsNO3, 442 K and 32.5 mol% CsNO3, 445 K) and a peritectic (38mol% CsNO3, 450 K). The occurrence of the transitions of intermediates was confirmed by X-ray diffraction at variable temperatures. The phase diagram exhibits also two plateaus at 429 K and 435 K corresponding to the phase transitions of CsNO3 and AgNO3, respectively.

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Abstract  

The Al-Zn-Ga ternary phase diagram was earlier established by thermodynamic modellization [1], but no experimental study appears to have been carried out on this system, except for measurements of mixing enthalpies in the liquid [2]. The present experimental study was carried out by thermal analysis and X-ray diffraction at various temperatures, using the isopletic cuts method. Four isopletic cuts were established and two others were partly studied in the Al-rich corner of the diagram. On these cuts, two isobaric ternary invariant reactions were determined: a eutectic reaction at 231C, and a metatectic reaction at 1231C. Evidence was found for the existence of a retrograde miscibility of Ga in a solid solution α′SS which protrudes into the ternary system starting from the Al-Zn binary up to a Ga concentration of about 30%*.

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