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  • Author or Editor: Adelina Ianculescu x
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Abstract  

Bismuth ferrite (BiFeO3) was obtained by a combustion reaction staring from two precursors systems, namely Fe(NO3)3 · 9H2O–Bi5O(OH)9(NO3)4 · 9H2O–glycine/urea with different metal nitrate/fuel molar ratios. The precursors’ thermal behavior is dependent on the fuel nature but practically independent to the fuel content. In glycine containing systems not all Bi2O3 is included into mixed oxides during the decomposition. Its presence was identified through the existence of two endothermic phase transitions (TDTA max at 745 and 818 °C) assigned to Bi2O3 α→δ transition, and its melting. The thermal investigations performed on oxides samples reveal for all oxides, independent on the precursor system, a similar behavior. For all the oxides was identified both the Curie temperature (which decreases with the annealing cycles) and the incongruent melting point (which is with ~10 °C higher for glycine generated oxides comparative with urea ones). The structural analysis shows in the case of the oxides prepared using urea as fuel, a faster evolution toward a single phase composition with the temperature, the formation of the BiFeO3 perovskite phase being completed in the temperature range of 500–550 °C. Only some traces of Bi36Fe2O57 were identified at the detection limit. TEM analysis performed on the BiFeO3 thermally treated at 500 °C for 3 h revealed the presence of small particles with an average size of ~33 nm and polycrystalline agglomerates with an average size of ~100 nm for glycine/urea derived oxides.

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Abstract  

Two bismuth ferrite potential precursors systems, namely Fe(NO3)3·9H2O-Bi(NO3)3·9H2O-glycine/urea with different metal nitrate/organic compound molar ratios have been investigated in order to evaluate their suitability as BiFeO3 precursors. The presence into the precursor of both reducing (glycine and urea) and oxidizing (NO3 ) components, modifies dramatically their thermal behaviour comparative with the raw materials, both from the decomposition stoichiometries and temperature occurrence intervals points of view. Also, the thermal behaviour is dependent on the fuel nature but practically independent with the fuel content. The fuel nature influences also some characteristics of the resulted oxides (phase composition, morphologies). In the case of the oxides prepared using urea as fuel, a faster evolution toward a single phase composition with the temperature rise is evidenced, the formation of the BiFeO3 perovskite phase being completed in the temperature range of 500–550°C.

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Journal of Thermal Analysis and Calorimetry
Authors:
Adelina Ianculescu
,
Ana Brăileanu
,
M. Crişan
,
P. Budrugeac
,
N. Drăgan
,
G. Voicu
,
D. Crişan
, and
V. Marinescu

Abstract  

In order to obtain pure and fine BaTiO3 powders with controlled morphology, sol-precipitation methods involving the use of titanium iso-propoxide and of two different barium sources, i.e. barium nitrate and barium acetate, were proposed in this work. The thermal behaviour of the synthesized gels and the X-ray diffraction data obtained for the oxide powders pointed out that, by using Ba(NO3)2 as barium source, the decomposition process was completed at lower temperature (750C) and was accompanied by a more pronounced tendency to obtain a single phase BaTiO3 composition, by comparison with the synthesis where barium acetate was used as raw material (1100C). Scanning electron microscopy investigations emphasized the effect of the nature of barium source and synthesis conditions on the morphology of the oxide powders, as well as on the microstructure of the related ceramics.

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Journal of Thermal Analysis and Calorimetry
Authors:
Maria Crişan
,
Ana Brăileanu
,
D. Crişan
,
Mălina Răileanu
,
N. Drăgan
,
Diana Mardare
,
V. Teodorescu
,
Adelina Ianculescu
,
Ruxandra Bîrjega
, and
M. Dumitru

Abstract  

Among the great number of sol-gel materials prepared, TiO2 holds one of the most important places due to its photocatalytic properties, both in the case of powders and coatings. Impurity doping is one of the typical approaches to extend the spectral response of a wide band gap semiconductor to visible light. This work has studied some un-doped and Pd-doped sol-gel TiO2 nanopowders, presenting various surface morphologies and structures. The obtained powders have been embedded in vitreous TiO2 matrices and the corresponding coatings have been prepared by dipping procedure, on glass substrates. The relationship between the synthesis conditions and the properties of titania nanosized materials, such as thermal stability, phase composition, crystallinity, morphology and size of particles, and the influence of dopant was investigated. The influence of Pd on TiO2 crystallization both for supported and unsupported materials was studied (lattice parameters, crystallite sizes, internal strains). The hydrophilic properties of the films were also connected with their structure, composition and surface morphology. The methods used for the characterization of the materials have been: simultaneous thermogravimetry and differential thermal analysis, powder X-ray diffraction, electron microscopy (TEM, SAED) and AFM.

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