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Congruent melting of binary compounds with non-negligible vapour pressure

I. Theoretical aspects of the congruent melting of a hydrate

Journal of Thermal Analysis and Calorimetry
Authors: J. Rouland, A. Thorén, J. Fournival, and R. Céolin

Abstract  

A theoretical temperature-specific volume-molar fraction (T-v-x) phase diagram of the binary systemA-H2O presenting a hydrateH with congruent melting is studied. The phase diagram is divided into two subsystems by the one-component phase diagram of the hydrateH. The eutectic invariant planes which occur in both subsystems are described. The relationships between the invariants are given. Isochoric and isoplethal theoretical sections are given. The one-component phase diagram of the hydrateH is described. It is pointed out that the triple line solid-liquid-vapour which relates the congruent melting is not necessarily perpendicular to the molar fraction axis. The means that the three phases, solid, liquid and vapour, have not necessarily the same composition. TheT-v-x representation gives an explanation to the deviation between the maximum of the melting curve and the stoichiometric composition of the hydrate which is often observed in theT-x binary diagramA-H2O.

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Congruent melting of binary compounds with non-negligible vapour pressure

II. Application to the sulfaguanidine—water system

Journal of Thermal Analysis and Calorimetry
Authors: J. Rouland, S. Makki, J. Fournival, and R. Céolin

Abstract  

The sulfaguanidine—water (SG-H2O) system is a binary system with non-negligible vapour pressure which presents a monohydrate. The phase diagram of this system is drawn from DTA experimental results, using the temperature-specific volume-molar fraction (T-v-x) model which was described in part I of this work. The melting of the monohydrate (SG, H2O) is found to be congruent. Isochoric sections are drawn; they make it possible to determine the limits of the two eutectic invariant planes. The composition and specific volume of the vapour phase at the eutectic equilibrium of theSG-SG, H2O subsystem are given. The triple line solid-liquid-vapour of the one-component phase diagram of the monohydrate is drawn. The experimental results are consistent with the congruent melting of the monohydrate. These results also show that the solid, liquid and vapour phase at the triple line have not the same composition.

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Abstract  

The subject of the present study is the system SeO2-Bi2O3 that comprises two oxides with low melting points. All batches are thermal treatment in quartz ampoules, which are evacuated and sealed at a pressure P=0.1 Pa. On the basis of DTA (differential thermal analysis) and X-ray data, the most probable liquidus line of the system has been plotted. The eutectic composition lies about 90 mol% SeO2,with on eutectic temperature at 230C. Above 20 mol% Bi2O3 the liquidus temperature extremely increases. The formation of three compounds is proved:Bi2Se3O9 and Bi2Se4O11 are melting incongruently at 540 and 350C respectively and Bi2SeO5 congruently at 915C.

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Journal of Thermal Analysis and Calorimetry
Authors: Nadia Mayoufi, Didier Dalmazzone, Walter Fürst, Leila Elghoul, Adel Seguatni, Anthony Delahaye, and Laurence Fournaison

to each set of points. The phase boundaries evidence a congruent melting compound marked by the vertical line, having a TBMAC/H 2 O molar ratio very close to that of the incongruent melting TBMAC hydrate. Fig. 9

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Journal of Thermal Analysis and Calorimetry
Authors: Maria Kuhnert-Brandstätter and A. Burger

Abstract  

The stearic acid-urea binary system exhibits an unusual phase diagram, which, on the one hand, indicates an incongruently melting inclusion compound and on the other hand a miscibility gap in the liquid phases. The peritectic point lies near the melting point of urea and the unstable congruent melting point of the inclusion compound coincides with the melting point of urea. In addition to the processing of the phase diagram, the pure inclusion compound was prepared and its DSC curve, FTIR spectrum and X-ray diffractogram were recorded.

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The phase diagrams of binary organic systems of benzidine with pyrogallol andp-nitrophenol give a double simple eutectic type phase diagram showing formation of a 1∶1 molecular complex with congruent melting point and two eutectics. The growth data on the pure components, the eutectics and the molecular complexes, determined by measuring the rate of movement of growth front in a capillary, justify the square relationship between growth velocity and undercooling. While enthalpy of mixing values suggest intermolecular hydrogen bonding, the excess thermodynamic functions reveal strong interactions among the components forming eutectics and addition compounds.

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Abstract  

The pseudobinary systems NaCl—LnCl3 (Ln=Tm, Yb, Lu) were investigated by DTA and X-ray diffraction. Two types of ternary chlorides exist: congruently melting compounds Na3LnCl6 with the cryolite-structure, incongruently melting compounds NaLnCl4 with the NaErCl4-Ln (Ln=Tm) or the NaLnCl4-structure (Ln=Yb, Lu). All these structure types contain [LnCl6]-octahedra.By solution calorimetry and e.m.f. measurements in galvanic cells for solid electrolytes could be proved that all compounds are formed from NaCl and LnCl3 by gain in lattice enthalpy.

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Journal of Thermal Analysis and Calorimetry
Authors: M. Hichri, C. Favotto, H. Zamali, Y. Feutelais, B. Legendre, A. Sebaoun, and M. Jemal

Abstract  

Phase diagram of the binary system AgNO3—RbNO3 was studied using thermal analysis technique, differential scanning calorimetry and X-ray diffraction. This binary exhibits a congruently melting compound Ag0.5Rb0.5NO3 (m. p.=138°C), an incongruently melting one Ag0.33Rb0.66NO3 with two polymorphic varieties, two eutectics at (36 mol% RbNO3, 128°C) and at (60 mol% RbNO3, near 134°C) respectively and a peritectic at (60.5 mol% RbNO3, 141°C). This system contains also three invariant reactions at 164, 222 and 282°C due to the phase transitions of RbNO3 and another one at 164°C due to the phase transition of AgNO3.

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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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Na-Gd phosphate glasses

Preparation, thermal and scintillating properties

Journal of Thermal Analysis and Calorimetry
Authors: K. Nitsch, A. Cihlář, D. Klimm, M. Nikl, and M. Rodová

Summary The present paper deals with preparation, thermal properties and radioluminescence of Ce-doped Na-Gd phosphate glasses. Thanks to their high radioluminescence intensity, three times greater than that of BGO, these glasses are promising materials for the detection of neutrons, ?- and X-rays. The glasses with a Gd concentration up to 89 mol% were prepared by a rapid quenching technique in air. Their thermal properties were determined using DSC and temperatures of glass transition were measured in addition using TMA. Temperatures of glass transition, crystallization and melting depend on Gd concentration and they follow the liquidus line in a phase diagram of binary system in which two eutectics and a congruently melting compound exist. High glass-forming ability of this glass system was found evidenced. The glasses containing at least 30 mol% Gd were moisture resistant.

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