Ana Brăileanu

Composition and thermal stability of SiO2-based hybrid materials TEOS-MTEOS system

Hybrid materials with different amounts of organics permanently bound on the inorganic network obtained in the TEOS-MTEOS (tetraethoxysilan-methyltriethoxysilan) system are used for obtaining coatings with different optical and mechanical properties. To study the thermal stability of the mentioned materials, compositions with different molar ratios of the precursors were prepared. The influence of the solvent and water amounts on the gelation process was also investigated. The gels obtained were characterised by IR spectrometry and their decomposition temperatures were determined by DTA/TG. Thermal stability of the gels is rather influenced by their composition than the conditions of the gelation process.

Formation and properties of some Nb-doped SrTiO3-based solid solutions

It is well known that SrTiO3 is one of the widely used materials in the electronic ceramic industry. Donor-type dopants are often added in order to improve the electrical properties of SrTiO3-based ceramics. In the present study, the formation and characteristics of undoped and Nb-doped SrTiO3 were studied as a function of both the niobium dopant concentration and the Sr/Ti ratio (at the same Nb content). The formation mechanism of the mixed Sr(Ti,Nb)O3 crystals was investigated by XRD and thermal analysis. The niobium solubility in the SrTiO3 perovskite lattice was estimated by means of unit cell parameter data. The influence of Sr concentration on the microstructure of the ceramics obtained was investigated by SEM. Ceramic properties pointed out the influence of the dopant concentration and stoichiometry on the sintering behavior of these materials.

Properties of the Pb(Mg1/3Nb2/3)O3 Ceramics Prepared by Using Two Different Mg Precursors

Pb(Mg 1/3 Nb 2/3 )O 3 ceramics prepared via columbite method by using two types of Mg precursors, i.e. MgO and (MgCO 3 ) 4 ·Mg(OH) 2 ·4H 2 O exhibit high crystallinity and high densification after sintering at 1200°C. Higher amounts of pyrochlore phases were detected in the PMN sample obtained by using (MgCO 3 ) 4 · Mg(OH) 2 .4H 2 O as precursor. This compositional difference, determines, for all frequencies, a shift of ∼ 50 K of the temperature corresponding to the permittivity maximum T m for the sample derived from (MgCO 3 ) 4 ·Mg(OH) 2 ·4H 2 O. From this reason, better dielectric properties were obtained at room temperature for this PMN sample, although a higher dielectric permittivity at T m was obtained for the sample derived from MgO precursor.

Solid State Phase Formation in the Bi2O3-PbO-CaO System

Following our previous research, this work is dedicated to the study of phase formation in the subsolidus domain of the Bi2O3-PbO-CaO system.Former investigations performed by DTA/TGA and XRD have pointed out that under non-isothermal conditions only the formation of binary compounds occurs. Under such conditions these compounds could be non-equilibrium phases.In order to establish the conditions of formation of equilibrium phases, a study of the Bi2O3-PbO-CaO system, in isothermal conditions, was carried out. The results obtained in isothermal conditions have confirmed the presence of Bi2O3-rich solid solutions and Ca2PbO4 as main equilibrium phases. An attempt to represent the phase relations of the mentioned system at 700°C should be equally mentioned.