Jean-Marie Létoffé

Etude du comportement thermique du tetrahydroaluminate de sodium NaAlH4 et de l'hexahydroaluminate de sodium Na3AlH6 DE 298 A 600 K

The thermal behaviour of NaAlH4 and Na3AlH6 has been studied by means of thermogravimetry and differential scanning calorimetry in the range 298–600 K. For NaAlH4, the melting point, the enthalpy of melting (ΔHm) and the entropy of melting (ΔSm) have been determined. At T = 453.8 K, ΔHm = 23225 ± 860 J mole−1 and ΔSm = 51.15 J mole−1 K−1. The thermal decomposition into Na3AlH6 3 NaAlH4 → Na3AlH6α + 3 H2 ↑ 2 Al starts at about 480 K. The enthalpy of this reaction is very low. Na3AlH6 has a phase transition at 525 K with ΔHtrans = 3600 J mole−1. The thermal decomposition of Na3AlH6 Na3AlH6 → 3 NaH + Al + 32 H2 ↑ starts at 495 K. The enthalpy of decomposition has been determined as ΔH0530 = 41450 ± 1250 J mole−1.

Using thermal analysis methods to better understand asphalt rheology

Abstract The Strategic Highway Research Program allowed to define some parameters that characterize the rheological properties of asphalt cement(s) (ACs). Those parameters implied that ACs follow the time and temperature superposition principle. However, highly asphaltic or waxy ACs deviate from this rule since their structure changes as a function of time and temperature. Some of these transitions clearly appear on differential scanning calorimetry curves. This paper shows some analogies between thermo-analytical and rheological properties of ACs. In particular, it emphasizes the roles of crystallized fraction(s) (CFs) and of the glass-transition temperature ( T g ) on the whole temperature range of end use. Thus, at low temperatures, CF seems greatly responsible for the physical hardening (PH) occurring with time. PH magnitude also depends on the position of T g relative to the given conditioning temperature. At higher temperatures, CF also plays a role on the rheological behavior since their dissolution–precipitation mainly occurs at temperatures between 0° and 80°C.

Polymorphisme de l'hexahydroaluminate trisodique Na3AlH6

Abstract Structural investigations on the cryolite-like compound Na 3 AlH 6 have been carried out using high-temperature diffractometry. α-Na 3 AlH 6 which crystallises in a monoclinic pseudo-cubic system under room-temperature conditions exhibits at 525 K a polymorphic transition into a f.c.c. form β-Na 3 AlH 6 . This result has been established by high-pressure-high-temperature studies. From structural point of view Na 3 AlH 6 -β is to be related to the high temperature modification of cryolite Na 3 AlF 6

Etude comparative des tetrahydro- et -deuteroaluminates de lithium LiAlH4, LiAlD4. I — Preparation, etude radiocristallographique et comportement thermique — Mise en evidence d'une variete metastable de LiAlD4

Abstract Toluene can precipitate lithium tetrahydroaluminate LiA1H 4 and trahydrodeuterate LiA1D 4 from concentrated solutions in diethylether at 20°C. This method gives small, solvent-free crystals with a purity better than 99.9%. X-ray and D.S.C. characterization show that LiA1H 4 is obtained in the usual “α” crystalline form. A new phase, LiA1D 4−α 1 can also be isolated, which transforms irreversibly upon heating to LiA1D 4−α . The latter phase has the same structure as LiA1H 4−α .

β-Cyclodextrin hydration: a calorimetric and gravimetric study

Abstract The standard heat of solvation of β-cyclodextrin (βCD) with one mole of water is determined as −10.50 kJ per mol βCD, after measurement of the heat of dissolution of βCD samples containing various amounts of water. This value is constant in the composition range βCD · n H 2 O, with 0 n n is the number of water molecules attached). The thermogravimetric study of the kinetics of water absorption by βCD does not show a definite hydrate. The water content of the hydrate is controlled by the vapour pressure and temperature alone. Under ambient conditions, the number of water molecules bound to one βCD molecule lies between 10 and 12.

Etude comparative des hexahydrido-et des hexadeuteridoaluminates de lithium et de sodium. I - Spectres raman et infrarouge de Li3-et Na3AlH6, et Li3AlD6

Abstract Raman and Infrared spectra of Lithium and Sodium hexahydrio- and hexadeuteridoaluminates Li3AlH6, Na3AlH6 and Li3AlD6 have been investigated in order to study metal-hydrogen interactions. A vibrational analysis has been performed. Several force field models (Urey-Bradley, Orbital Valence force field, Shimanouchi, Murrell).have been used in order to compute the force constants of Al-H bonds. No significative difference has been observed between the results obtained from the various models, and the environnement of aluminum has been thus well determined : AlH63− ions form an almost perfect octahedron in Li3AlH6 and a distorted one in Na3AlH6.

Étude thermodynamique de la réaction d'hydratation de la β-cyclodextrine

Abstract The thermal behaviour of β-cyclodextrin has been studied in the temperature range −150 to + 300 °C by TG and DSC measurements. At room temperature, the stable hydrate contains 11 molecules of water. Dehydration occurs in two steps corresponding to the loss of 7 and 4 molecules of water. The enthalpy of hydration and solubility at 25 ° C have been determined by dissolution calorimetry.

Etude comparative des tetrahydro- et -deuteroaluminates de lithium LiAlH4, LiAlD4. III - Caracterisation de la variete γ haute pression

Abstract The γ-crystalline form of LiAlH 4 (or LiAlD 4 ) has been obtained at high pressure using a belt type apparatus. The γ-phase is quenched and studied at ambient pressure. The crystalline structure has been redeterminated. On the basis of Raman spectra an octahedral environment of Al 3+ is proposed. The α → γ transition is reflected in a change of the CN of Al 3+ and Li + from 4 to 6, and an important decrease of the volume ( ⋍30 % ). Upon warming, the γ-phase transforms into the α-form with a large endothermal effect, as expected for a true hig pressure phase. From these results, a tentative (P,T) phase diagram for LiAlH 4 (or LiAlD 4 ) is proposed.

Etudes calorimetriques en milieu solvant organique. V. Enthalpies de dissolution de l'alanate de sodium, NaAlH4, dans le tetrahydrofuranne

Abstract The enthalpies of dissolution of sodium tetrahydridoaluminate NaAlH 4 in THF have been determined for different concentrations. The enthalpies of dissolution and dilution are exothermic from 1 to 7 · 10 −3 M. The enthalpy of dissolution at infinite dilution has been calculated: Δ H ∝ diss = −6.38 kcal mole −1 .

Tude par ACD du domaine hors d'equilibre du systemexLiCl-(1−x)H2O (0<x<0.18)

The non-equilibrium region of the phase diagramxLiCl-(1−x)H2O (0<x< 0.18) has been studied by means of a Mettler TA 2000 B heat flow differential scanning calorimeter. The metastable lines of the diagram have been established and the different phases obtained explained. A region has been found where the glass formed cannot recrystallize, the eutectic line being below the temperature of the transition glass.ZusammenfassungDas Nicht-Gleichgewichtsgebiet des PhasendiagrammsxLiCl-(1−x)H2O (0⩽x⩽ 0.18) wurde mittels DSC unter Verwendung des Scanningkalorimeters Mettler TA 2000 B untersucht. Die metastabilen Zweige des Diagramms wurden ermittelt und die erhaltenen verschiedenen metastabilen Phasen erklärt. Es wurde ein Bereich festgestellt, in dem das gebildete Glas nicht rekristallisieren kann, da das Eutektikum unterhalb der Temperatur der Glasbildung liegt.РезюмеС помощью дифференци ального сканирующег о калориметра теплево го потока Меттлер ТА 2000 Б проведе но исследование нера вновесной области фазовой диаг раммы системыxLiCl-(1−х)Н2О, где 0 ⩽х ⩽ 0.18. Установлены метастабильные лини и диаграммы и объяснено образова ние различных фаз. Най дена область, где образующ ееся стекло не может рекристалли зоваться, поскольку л иния эвтектики лежит ниже температуры расстекловывания.