F. Chassagneux

Comparison between SBA-15 silica and CMK-3 carbon nanocasting for mesoporous boron nitride synthesis

The synthesis of mesoporous boron nitride, using an aminoborazine as the boron nitride source, with silica (SBA-15) or mesoporous carbon (CMK-3) as the template is compared. Tri(methylamino)borazine (MAB) is converted to BN inside the mesopores of silica or carbon molecular sieves through a mild thermal process. Boron nitride is obtained after the removal of the hard template using an ammonia thermal treatment (carbon template) or an HF treatment (silica template). X-Ray diffraction, TEM and pore size analysis show that the structure of the BN molecular sieves synthesized from the carbon template consisted of a 2D regular array of uniform mesopores 3.4 nm in diameter. The hydrophilic nature of the silica template prevents successful pore filling with the aminoborazine precursor. Ordered BN molecular sieves are obtained at 1000 °C with a specific surface area of 500 m2 g−1 using mesoporous carbon molecular sieves as a hard-template.

Zirconia formation by reaction of zirconium sulfate in molten alkali-metal nitrates or nitrites

Thermogravimetry up to 500 °C reveals several multi-stage reactions between Zr(SO4)2 and molten alkali-metal nitrates which form poorly crystallised mixtures of tetragonal and monoclinic zirconia. The use of NaNO3 gives incomplete transformations. The addition of NaCl as well as the use of LiNO3-KN03, leads the reaction to completion; the increase of basicity by the addition of Na2CO3 also has the same effect, if it is assumed that Na2ZrO3 is formed together with ZrO2. Pure and finely divided tetragonal zirconia is produced by a single-stage reaction at a temperature lower than 300 °C, when the basicity of the medium is increased by replacing LiN03–KNO3 by NaNO2–KNO3.

pH-metric and Raman investigations of zirconium aqueous solutions and gels submitted to a reflux

Abstract Molar solutions of zirconyl chloride corresponding to different steps of neutralization were submitted to reflux at 105°C and Raman spectroscopy and potentiometry measurements were made. The structure of the final products is found to be strongly dependent on the time of reflux as well as on the initial step of hydrolysis. When dried at 50°C, the samples obtained by reflux give amorphous powders whose Raman spectra, differential thermal analysis and powder X-ray diffractograms are described. The nature of the crystallized ZrO 2 (monoclinic or tetragonal) produced by annealing at 470°C is linked to the structure of the amorphous gel. A domain of pH of the initial solution inside which monoclinic zirconia crystallizes directly from the gel is identified.

Dehydration of ammonium trioxalatochromate(III) and trioxalatoferrate(III) in relation with the preparation of metallic mixed-oxides

Abstract The different steps appearing during the dehydration processes of ammonium trioxalatochromate (III) and ferrate(III) have been investigated by considering 1) - their crystallization diagrams, 2) - thermogravimetry (TG) under controlled water pressure and 3) - differential thermal analysis (DTA). In the case of ammonium trioxalatoferrate(III) trihydrate (NH 4 ) 3 [Fe (C 2 O 4 ) 3 ]. 3H 2 O the TG curves exhibit a plateau corresponding to the dihydrate intermediate. For the chromate(III) the dehydration equilibrium is divariant. These observations arc discussed in terms of the crystal structures of both investigated solids, especially on the basis of the specific role played by one of the three water molecules in the trihydrates.