Search Results

You are looking at 11 - 19 of 19 items for

  • Author or Editor: M. Zaharescu x
  • Refine by Access: All Content x
Clear All Modify Search

Abstract  

In this paper the influence of alkaline-earth admixtures on the synthesis of lanthanum chromites of La1–xMxCrO3 (x=0; 0.3; M=Ca, Sr, Ca+Sr) type was studied. The formation mechanism as well as the phase composition evolution, under non-isothermal and isothermal conditions, were investigated by thermal analysis and X-ray diffraction. The structure of the compounds and of the solid solutions formed depends on the solute type by means of the structural distortion induced. The crystallinity of the chromites obtained is obviously influenced by both the temperature and the thermal treatment plateau.

Restricted access

The paper presents a new, non-traditional method for the synthesis of barium metaniobate, BaNb2O6, and of a mixed barium-strontium metaniobate, Ba0.29Sr0.71Nb2O6, through the thermal decomposition of coprecipitation products. The conditions of quantitative precipitation of the metals as niobic acid and barium or barium-strontium oxalate were established. The mechanism of thermal decomposition of the coprecipitate was deduced from differential thermal analysis and X-ray diffraction date. Barium metaniobate forms at 470°C, below the temperature required in the synthesis based upon the solid-state reaction between Nb2O5 and BaCO3 (1100°C). The mixed barium-strontium compound is formed at 700°C, below the 1100°C used in the reaction between Nb2O5, BaCO3 and SrCO3.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, M. Scurtu, S. Mihaiu, M. Caldararu, and M. Zaharescu

Abstract

Tin and cerium based oxide ceramics, due to their peculiar properties, are good candidates for using as sensors, solid electrolytes in fuel cells, and catalysts. In the present work, Sn–Ce–O powders with the composition of interest for catalysis applications were obtained by solid state reactions and by thermal decomposition of the different tin and cerium precursors. The structural characterization of the resulted samples was performed by X-ray diffraction. Morphological characteristics were evaluated from X-ray microstructural parameters and BET surface areas measurements. Surface evolution of the studied powders was investigated by electrical measurements in various atmospheres in the 25–400 °C temperature range. The samples studied behave as a n-type semiconductor. The catalytic activity for CO oxidation was measured in gas flow between 25 and 400 °C. The conversion degree depends on the preparation method of the samples. Magnetic susceptibility measurements at room temperature of the samples before and after catalytic test indicated a paramagnetic behavior. Higher values of magnetic susceptibility of the samples after catalytic test compared to initial ones could indicate the influence of CeO2 on the catalytic activity.

Restricted access

Abstract  

Composite SiO2—iron oxide materials were prepared by three experimental procedures. In the first case, the iron oxides were precipitated during a sol-gel process. In the second case, a SiO2 matrix was initially obtained, and the iron oxides were formed by thermal treatment after impregnation of a soluble Fe2+ salt in the previously processed matrix. In the third method, ferrite powders, prepared by wet chemical method, were embedded into a SiO2 based sol-gel matrix. Materials with convenient porosity and nano-sized iron oxide content could be prepared using the mentioned methods. The prepared composite has been tested for arsenic(V) removal.

Restricted access

Former studies concerning the formation of the compounds in the pseudobinary systems of Bi2O3-MO type (M =Ca, Sr, Ca+Sr) have shown that the reaction which occurs with the highest rate is that between Bi2O3 and CaO. In the present work CaCO3 was used as CaO source. We carried out an investigation of the thermal decomposition of CaCO3 in the presence of Bi2O3 in comparison with the decomposition of pure CaCO3.

Restricted access
Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: M. Zaharescu, A. Jitianu, A. Brãileanu, V. Bãdescu, G. Pokol, J. Madarász, and Cs. Novák

Abstract  

The hydrolysis-polycondensation of organically modified Si-alkoxides leads to the obtaining of inorganic-organic hybrid materials in which the organic moieties remain as permanent groups bonded to the inorganic network. The molecular species previously determinated by GC-MS during the gelation process have been significantly different according to the type of the alkoxide used. In the present work, thermal stability of SiO2-based inorganic-organic hybrid materials starting with TEOS (tetraethoxysilan), MTEOS (triethoxymethylsilan), VTEOS (triethoxyvinylsilan) and MTMOS (trimethoxymethylsilan) was studied. The molecular structure of the gels obtained determines differences in their thermal behaviour. Gels obtained starting with MTEOS show the highest thermal stability, while gels obtained using VTEOS the lowest, among the substituted alkoxides. A particular behaviour presents the gel obtained with MTMOS that decomposes in four steps. This could be explained by the presence in the gel of some prevalent types of molecular species with different thermal stability.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: A. Ianculescu, A. Brăileanu, M. Zaharescu, S. Guillemet, I. Pasuk, J. Madarász, and G. Pokol
Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: A. Ianculescu, A. Braileanu, M. Zaharescu, I. Pasuk, E. Chirtop, C. Popescu, and E. Segal

Abstract  

It is well known that the manganites-based solid solutions are interesting for their electric and magnetic properties. LaMnO3 exhibits a distorted perovskite structure due to Mn3+ ion, which determines the presence of the Jahn–Teller effect. Replacing La3+ host ions by cations of lower valence leads to the disappearance of this effect and changes the characteristics of these materials. Although the formation of manganites-based solid solutions has been intensively studied, there are some unelucidated aspects concerning their formation mechanism, depending both on the precursors used and on the thermal treatment applied in order to obtain suitable properties. In this work the formation mechanism of La0.7M0.3MnO3 (M=Ca, Sr, Ca+Sr in equimolecular mixture) solid solutions, in isothermal and non-isothermal conditions, was studied. For this purpose XRD, DTA/TG and spectral techniques were used. The solid solutions formation was found to be more dependent on the Mn-precursors type than the thermal treatment conditions.

Restricted access