No abstract

We have carried out dielectric and Raman spectroscopy studies at the 298 -623 K temperature range in polycrystalline Pb 0.70 Sr 0.30 TiO 3 thin films grown by a soft chemical method. The diffuse phase-transition behavior of the thin films was observed by means of the dielectric constant versus temperature curves, which show a broad peak. Such behavior was confirmed later by Raman spectroscopy measurements up to 823 K, indicating that a diffuselike phase transition takes place at around 548 -573 K. The damping factor of the E(1TO) soft mode was calculated using the damped simple harmonic oscillator model. On the other hand, Raman modes persist above the tetragonal to cubic phase transition temperature although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the microscopic local cubic symmetry by chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands at some temperature interval above the ferroelectric-paraelectric phase-transition temperature suggested a diffuse nature of the phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in this thin film.

No abstract

No abstract

Strontium-modified lead titanate thin films with composition Pb 1−x Sr x TiO 3 were grown on Pt/Ti/SiO 2 /Si substrates using the polymeric precursor method. The structural phase evolution as a function of the Sr contents was studied using micro-Raman scattering, specular reflectance infrared Fourier transform spectroscopy, and x-ray diffraction. The results showed a gradual change from tetragonal to cubic structure, the transition occurring at about x ‫ס‬ 0.58. The infrared reflectance spectra showed that the frequency of several peaks decreases as the strontium concentration increases. These features are correlated with a decrease in the tetragonal distortion of the TiO 6 octahedra as the strontium concentration increases.

Resumo:Nesse trabalho investigamos o Conhecimento Pedagógico do Conteúdo de uma professora de Química experiente, do nível Superior de ensino, com histórico de bom desempenho. Um estudo de caso qualitativo foi desenvolvido a partir: do registro audiovisual de suas aulas, aplicação do instrumento Representação do Conteúdo (CoRe), realização de entrevistas semiestruturadas, e reflexão estimulada. Observamos que a professora consegue mobilizar, em sua prática, a maioria das categorias do Conhecimento Pedagógico do Conteúdo (PCK), adotando a “Didática” como orientação para o ensino de ciências e demonstrando grande preocupação com a transmissão dos fatos científicos. Apesar de suas aulas serem predominantemente expositivas e centradas na figura do professor, foi possível identificar momentos de abordagem interativa, na qual considera os diferentes pontos de vista dos alunos sobre o conteúdo ministrado. Finalmente, constatamos que a professora apresenta uma prática pedagógica refletida, sobretudo no que se refere às suas estratégias de ensino.

No abstract

No abstract