B. Bonnetot

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.

Porous boron nitride supports obtained from molecular precursors.: Influence of the precursor formulation and of the thermal treatment on the properties of the BN ceramic

Abstract Boron nitride (BN) porous samples have been prepared in order to be used as noble metal catalyst support from various molecular precursors, using classical thermal methods to expand and preceramise the precursors. Three types of precursors have been tested: ammonia borane, polyborazylene and several aminoborazines and derived polymers. Using trimethylamino borazine (MAB) and MAB polymers, the surface of the foams was shown to be decreasing when the polymerisation advancement of the precursor increased meanwhile the crystallisation of the samples decreased. All the samples issued from MAB presented the same SEM morphology: large pores with a glassy skin surface covering a disorganised inner part, hidden under the thin homogeneous BN skin, which appeared through the sample breaks. This was related to the melting property of MAB polymers which imbedded the internal part of the sample and avoided its expansion. Attempts were made, using the bulkier amino group as anilino- and benzyl-aminoborazine to enhance the porosity of the samples using a precursor with a lower ceramic yield. The change in the specific area was not important. However, a surface of 30 m 2 g −1 was currently obtained. The ability of the precursor to lead to high specific area seemed to be correlated to the expansion possibilities of the precursor in the solid state prior to fusion. Foams exhibiting a surface of more than 50 m 2 g −1 have been obtained when polyborazylene based precursor was pyrolysed.

Determination des constantes thermodynamiques des hydrures simples et complexes de l'aluminium. IV. Enthalpie de formation de LiAIH2 et Li3AIH6

Abstract Heats of formation of α and γ aluminium hydrides have been measured by means of a scanning Calvet microcalorimeter: Thermal dissociation of AIH 3 γ shows an exothermic effect associated with hydrogen evolution followed by the endothermic decomposition of AIH 3 α. On the basis of the quantities of gas evolved, a solvent-free aluminium alanate structure, AI (AIH 2 ) 3 , is proposed for AIH 3 γ.

Synthesis of [B12H12]2− based extractants and their application for the treatment of nuclear wastes

Abstract Several closo-hydroborates bearing phosphite oxide, phosphine oxide or CMPO (carbamoylmethylphosphine oxide) groups, Cs[(Ph2P(O))2N(H)B12H11] (1), K[Ph2P(O)CH2C(O)N(H)2B12H11] (2), Na[Bz2N(H)B12H10OCH2CH2OCH2CH2P(O)(OBu)2] (5), Na[Bz2N(H)B12H10O(CH2)4N(R)C(O)CH2P(O)(Ph)2] (9, R=Pri; 10, R= Octyl), Na[Bz2N(H)B12H10OCH2CH2OCH2CH2N(Bui)C(O)CH2P(O)(Ph)2]− (11) were synthesised employing novel neutral bipolar intermediates [Bz2N(H)B12H10O(CH2)4] (3) and [Bz2N(H)B12H10O(CH2CH2)2O] (4) as useful synthons. All new compounds were characterised by NMR spectroscopy and mass spectrometry techniques. Their abilities to extract selectively the radionuclides 241Am and 152Eu from nuclear waste solutions were investigated using a liquid–liquid extraction technique. Promising results were obtained with compound 10, which exhibits enhanced hydrophobicity and solubility in organic extraction solvent.

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 .

Etude par RMN de la structure dynamique de l'alanate Na3AlH6

Abstract A low temperature wide-line NMR study has allowed a determination of the AI-H bond lengths in cryolite-type Na 3 AlH 6 . It was found from the thermal behavior of the proton lines that the |AIH 6 | 3− octahedra reorient around a C 4 axis. Starting below 170 K, this rotation can be hindered by lattice defects. Above room temperature it becomes isotropic, and a quick protonic exchange appears. The thermal narrowing of the linewidth and the T 1 and T 1 ρ relaxation times lead to activation energies of about 0.38 eV for axial rotation and 0.51 eV for protonic exchange. The minimum in T 1 is in good agreement with the exchange model. Absorption phenomena, as well as partial decomposition of Na 3 AlH 6 during the heat treatments, explain the presence of small amounts of mobil hydrogen.

Études calorimétriques en milieu solvant organique: II Enthalpies de dissolution et de dilution du boranate de lithium (LiBH4) et de l'alanate de lithium (LiAlH4) dans le tétrahydrofuranne

Abstract The enthalpies of dissolution and dilution of LiBH4 and LiAlH4 in THF have been determined. Dissolutions are exothermic. The endothermic dilution of LiAlH4 suggests for this compound, either a dissociation or an association equilibrium.

Études calorimétriques en milieu solvant organique I. enthalpie de dissolution du bromure de lithium dans le tétrahydrofuranne

Abstract A calorimeter has been devised to determine heats of solution in the absence of air. The enthalpy of solution of lithium bromide in THF has been measured at different concentrations. The results do not show a large difference with the heat of solution in water.

Études calorimétriques en milieu solvant organique: III. Enthalpie de dilution de l'alanate de lithium LiAlH4 dans le tétrahydrofuranneen présence d'halogénure de lithium

Abstract Heats of dilution of lithium aluminum hydride in THF, in THFLiBr and in THFLiCl have been established. At a constant concentration of LiBr in THF, dilution of LiAlH 4 is exothermal, and consequently it is shown that lithium aluminum hydride is dissociated in THF.

Determination des constantes thermodynamiques des hydrures simples et complexes de l'aluminium. II. Mesures des chaleurs spécifiques molaires á 298 K

Abstract The molar heat capacity of the hydrides MgH 2 , Mg(AIH 4 ) 2 , LiAIH 4 , NaAIH 4 . Li 3 AIH 6 , Na 3 AIH 6 , AIH 3 α and AIH 3 γ have been measured at 298 K by means of a Calvet microcalorimeter.