Nicolae Dragan

Al2TiO5 preparation starting with reactive powders obtained by sol-gel method

Abstract Individual Al 2 O 3 , TiO 2 and SiO 2 , as well as binary Al 2 O 3 -TiO 2 powders have been obtained by sol-gel method, polymeric route. The obtained powders have been characterized by X-ray diffraction (XRD), IR-spectrometry and transmission electron microscopy (TEM) methods. In the used experimental conditions amorphous, submicronic, monodisperse powders have been obtained. The preparation of Al 2 TiO 5 and of some compositions situated in the Al 2 TiO 5 -Al 6 Si 2 O 13 pseudobinary system has been realized by solid-state reaction starting with the mentioned above powders. The thermal treatment schedule of the pressed powder mixtures has been established according to the results of differential thermal and thermogravimetric analysis (DTA/TGA). The formation of the compound has been followed by XRD, dimensional variation of the samples and apparent density measurements. The results obtained starting with reactive powders have been compared with those obtained starting with commercial powders. The samples have shown a high reactivity and the formation of tialite was enhanced. However the sintering tendency was not essentially increased.

The structure properties correlation in the Ce-doped SnO2 materials obtained by different synthesis routes

Abstract Ce-doped SnO 2 materials with rutile type structure ( SnO 2 SS ) were obtained as porous ceramic, powders and films from different precursor mixtures and by using different preparation routes. The samples were characterized in terms of their structural, morphological and electrical properties. Depending on the specific precursor mixture, different microstructural parameters were obtained. The lattice parameters of the host oxide (SnO 2 ) decrease by CeO 2 doping and the mean crystallite size D varies in the range of 670–1278 A; different values of lattice strain S were also noticed. The morphological characteristics as shrinkage (Δ l / l ), porosity ( Pa ), density ( d ) and BET surface area were determined. The electrical properties were measured on the porous ceramics and on the powders. All investigated samples showed n-type semiconducting behavior.

Reactivity of the Bi, Sr, Ca, Cu oxalate powders used in BSCCO preparation

Abstract The solution chemistry of oxalate coprecipitation of Bi, Sr, Ca, Cu has frequently been used in the synthesis of Bi-based ceramic systems, although there are different opinions referring to some major aspects, such as coprecipitation pH and precipitating agent. In the present work we established the precipitation conditions of Bi, Sr, Ca, Cu individual oxalates and of the quaternary mixture. The initial and thermally treated oxalate powders were characterized by XRD, IR, BET methods and TEM observation. The decomposition conditions were established based on a DTA/TG study. The powders resulted from the decomposition of Bi and Cu oxalates contain the corresponding oxides and the Ca and Sr oxalates decomposition lead to the carbonates obtaining. In each case a high reactivity and a very small powder particle size were observed. A very good powder homogeneity has also been noticed in the final mixture of oxalates. Compared to super-conducting ceramic materials realized by a classic route, the reactive oxalate powders allow the decrease of both thermal treatment periods and temperature, emphasizing a high microstructure homogeneity of the ceramic body.

Functional Properties of the (1-x)BiFeO3—xBaTiO3 Solid Solutions

Solid solutions of (1-x)BiFeO 3 –xBaTiO 3 (0 ≤ x ≤ 0.30) were prepared by a two-step solid state reaction procedure. After pre-sintering and sintering at 800°C/1h and slow cooling, pure single phase compositions were obtained for all the analyzed mixtures in the limit of XRD accuracy, including for x = 0. A gradual attenuation of the rhombohedral distortion with the increase of BaTiO 3 content was found. Small dielectric losses (tan δ < 3 %) and permittivities in the range of (180, 240) were found for all the compositions at room temperatures. The magnetic investigations shown a composition-dependent ferromagnetism with a maximum magnetization and coercivity for the composition x = 0.05 and perfect antiferromagnetic behavior for the BiFeO 3 ceramics. The complex impedance is sensitive to the magnetic field at small temperatures below 300 K, with a stronger variation in the range of fields of (0, 2 T) and a tendency of saturation at higher fields, demonstrating a kind of magnetoelectric coupling, with both magnetocapacitance and magnetoresistence phenomena.

Influence of the powder characteristics on the properties of the Bi-based superconductors

Abstract Bi-based ceramic materials with superconducting properties were prepared using the solid state reaction technique, starting with reactive powders obtained via oxalate coprecipitation, as well as by the traditional powders route. The powder characteristics were evaluated using BET method and transmission electron microscopy (TEM). The stages of the high T c supeconducting phases formation process were investigated by TG, DTA, XRD and IR methods. The dependence of the electric resistance versus temperature was recorded. The densities of the obtained samples have been also determined. The results emphasised high reactivity and shorter reaction times of the ceramic bodies obtained by the coprecipitation method.

Study on the Lattice Defects in the Mixed Oxides of Neodymium and Alkaline Earth Elements and Their Role on the Oxidative Coupling of Methane

Along with realising the shortage of natural energy feedstock nowadays, their most effective and wise utilisation gains growing importance. For more then two decades now a great interest is manifested in the utilization of the huge natural gas resources. Different ways of methane activation are studied, as it is the main component of the natural gas. One of the most challenging tasks is the direct catalytic conversion of methane into higher hydrocarbons. It appears to be rather hard to realize with satisfactory yield mainly because of the high symmetry and very low reactivity of the methane molecule which is normally less reactive than the desired products. A huge number of articles dealing with the catalytic oxidative coupling of methane (OCM) report data concerning almost all of the elements of the Periodic Table. It seems that a significant progress can be made only by designing complex catalytic compositions to improve the methane conversion and the selectivity towards the desired products. According to Efstatiou et al. [1], the introduction of altervalent cations into the oxide lattice gives an opportunity to control the activity towards