New thermodynamic data for CoTiO3, NiTiO3 and CoCO3 based on low-temperature calorimetric measurements

The low-temperature heat capacities of nickel titanate (NiTiO3), cobalt titanate (CoTiO3), and cobalt carbonate (CoCO3) were measured between 2 and 300 K, and thermochemical functions were derived from the results. Our new data show previously unknown low-temperature lambda-shaped heat capacity anomalies peaking at 37 K for CoTiO3 and 26 K for NiTiO3. From our data we calculate standard molar entropies (298.15 K) for NiTiO3 of 90.9 ± 0.7 J mol-1 K-1 and for CoTiO3 of 94.4 ± 0.8 J mol-1 K-1. For CoCO3, we find only a small broad heat capacity anomaly, peaking at about 31 K. From our data, we suggest a new standard entropy (298.15 K) for CoCO3 of 88.9 ± 0.7 J mol-1 K-1

FT-IR STUDY OF THE OH SURFACE GROUPS ON CORDIERITE AEROGEL

Nous avons étudié une cordiérite à haute surface spécifique par spectroscopie infrarouge en transmission. Après activation des échantillons, nous avons mis en évidence la présence des groupes hydroxyles. L'action du méthanol et l'échange isotopique avec le deutérium ont prouvé que tous ces groupes sont liés aux atomes de silicium de surface.A high specific surface area cordierite has been investigated by infrared transmission spectroscopy. After activation of the samples, we have evidenced the presence of hydroxyl groups. Treatments with methanol and isotopic exchange with deuterium have proved that all of them are bound to the surface silicium atoms

On the Vibrational-spectra and Structure of FeCrO3 and of the Ilmenite-type Compounds CoTiO3 and NiTiO3

The XRD patterns and the IR and Raman spectra of fine powders of the corundum-type sesquioxides alpha-Fe2O3 and alpha-Cr2O3, of the ilmenite-type mixed oxides NiTiO3 and CoTiO3 and of the mixed oxide alpha-FeCrO3 is obtained from the XRD pattern, the IR and Raman spectra provide evidence for the centrosymmetric ilmenite-type superstructure in the case of alpha-FeCrO3. The noncentrosymmetric alternative superstructure (LiNbO3-like) is excluded by the validity of the mutual exclusion rule. (C) 1994 Academic Press, Inc

AMORPHOUS SILICON NITRIDE THIN FILMS PERFORMED IN TWO PECVD EXPERIMENTAL DEVICES

Les films de nitrure de silicium sont réalisés par deux procédés de PECVD différents. Dans le premier, ces films sont formés dans une décharge à courant continu dans un plasma d'argon-silane-azote alors que dans le second dispositif, une espèce active d'azote est créée sélectivement dans une post-décharge d'azote et réagit avec le silane dans une zone de réaction où est positionné le substrat. Dans les deux cas, les films réalisés sont sous-stoechiométriques en azote. Leurs caractéristiques : morphologie, structure et composition sont étudiées et comparées.Silicon nitride thin films are obtained using two different PECVD devices. In the first one, these thin films are created in a DC discharge in an argon-silane-nitrogen gas mixture whereas in the second, nitrogen active species are selectively produced in a flowing nitrogen post-discharge and then react with silane in a reaction region where is positioned the substrate. In both cases, the obtained films are substoichiometric in nitrogen. Their characteristics : morphology, structure and composition are studied and compared