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  • Author or Editor: M. Zaharescu x
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Abstract  

A thermoanalytical study in non-isothermal conditions was realized on Ga-substituted MBa2Cu3O7-δ superconducting compounds formation (M=Yb, Eu). The presence of different oxides in the reaction mixtures leads to different reactivity of the system. The type of lanthanoide ion present in the studied compositions determines the reactivity of the system. In the system containing Eu the solid state reactions start at lower temperature than in the system containing Yb. Ga-substitution does not change evidently the thermal behavior of the unsubstituted samples. Different reactivities of the studied reaction mixtures led, after similar thermal treatment in isothermal conditions, to samples with different structure and superconducting properties.

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Abstract  

This study reports experimental investigations by DTA/TG analysis of (1−x)SnO2xCuO compositions, up to 1773 K and at two oxygen partial pressures (i.e. air and argon). In air, DTA/TG results showed thermal effects due exclusively to CuO presence in the initial mixture. No binary compounds were formed. The reduction process of CuO to Cu2O over 1273 K as well as the formation over 1373 K of the liquid phase, have been evidenced. In argon atmosphere, CuO to Cu2O reduction reaction is shifted toward 1205 K, while the liquid phase appears in the studied mixtures over 1473 K. The formation of an eutectic composition between SnO2−Cu2O, melting at 1491 K, coordinates:0.932Cu2O+0.068SnO2, has been experimentally established in argon.

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Abstract  

The paper presents a new, nonconventional method, based upon coprecipitation, for the synthesis of niobium oxidic compounds. The coprecipitation product of niobic acid with calcium oxalate was used as precursor. Calcium metaniobate was obtained by appropriate thermal treatment of the coprecipitate. The coprecipitation mechanism was studied and the optimal conditions for quantitative precipitation of niobium and calcium were established. The mechanism of thermal decomposition of the coprecipitate was investigated by means of differential thermal analysis and X-ray powder diagrams. The final product of thermal decomposition, calcium metaniobate, is formed at 730°C.

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Journal of Thermal Analysis and Calorimetry
Authors: L. Marta, M. Zaharescu, and C. Macarovici
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Abstract  

In order to clarify the effect of PbO addition on the formation steps of the superconducting phases in the system Bi2O3−SrO−CaO−CuO, a study of solid-state reactions under non-isothermal conditions, in the PbO−MO (M=Ca, Sr, Ca+Sr) system has been carried out. Results suggest that the reactivity of the components in the system containing PbO and CaO is much higher than in the system containing SrO. The Ca2PbO4 compound is formed first even in the system whereM=Ca+Sr. It is confirmed that Ca2PbO4 systems containing PbO.

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Abstract  

The paper reports a new, nonconventional method for the preparation of oxygen-containing niobium compounds, based upon coprecipitation. The coprecipitation product of niobic acid with lead oxalate was used as precursor. Lead metaniobate was obtained by proper thermal treatment of the coprecipitate. The coprecipitate mechanism was studied and the optimal conditions for quantitative precipitation of niobium and lead were established. The mechanism of thermal decomposition of the coprecipitate was investigated by differential thermal analysis and X-ray powder diagrams. The final product of thermal decomposition, lead metaniobate, is formed at 850C.

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Abstract  

This work introduces results obtained during the preparation of a Bi-based material with superconducting properties by oxalate coprecipitation. The influence of Fe presence on the precursors thermal stability and on the superconducting phases formation mechanism are presented. The thermal decomposition and the stability in air of FeC2O42H2O and also of the components mixture were studied by DTA/TG. It was evidenced that iron oxalate decomposes at the lowest temperature compared to the decomposition temperatures of the individual oxalates. XRD, IR and TEM/ED studies were approached to investigate the individual oxalates and the mixture coprecipitates for the high-T c superconducting material synthesis.

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The paper presents a new, non-traditional method for the synthesis of barium metaniobate, BaNb2O6, and of a mixed barium-strontium metaniobate, Ba0.29Sr0.71Nb2O6, through the thermal decomposition of coprecipitation products. The conditions of quantitative precipitation of the metals as niobic acid and barium or barium-strontium oxalate were established. The mechanism of thermal decomposition of the coprecipitate was deduced from differential thermal analysis and X-ray diffraction date. Barium metaniobate forms at 470°C, below the temperature required in the synthesis based upon the solid-state reaction between Nb2O5 and BaCO3 (1100°C). The mixed barium-strontium compound is formed at 700°C, below the 1100°C used in the reaction between Nb2O5, BaCO3 and SrCO3.

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Abstract  

Following our previous research, this work is dedicated to the study of phase formation in the subsolidus domain of the Bi2O3-PbO-CaO system. Former investigations performed by DTA/TGA and XRD have pointed out that under non-isothermal conditions only the formation of binary compounds occurs. Under such conditions these compounds could be non-equilibrium phases. In order to establish the conditions of formation of equilibrium phases, a study of the Bi2O3-PbO-CaO system, in isothermal conditions, was carried out. The results obtained in isothermal conditions have confirmed the presence of Bi2O3-rich solid solutions and Ca2PbO4 as main equilibrium phases. An attempt to represent the phase relations of the mentioned system at 700C should be equally mentioned.

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