Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4

An extensive theoretical study is performed for wide bandgap crystalline oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}: $\alpha$-quartz, $\alpha$- and $\beta$-cristobalite and stishovite, for GeO_{2}: $\alpha$-quartz, and rutile, for Al_{2}O_{3}: $\alpha$-phase, for Si_{3}N_{4} and Ge_{3}N_{4}: $\alpha$- and $\beta$-phases. This work constitutes a comprehensive account of both electronic structure and the elastic properties of these important insulating oxides and nitrides obtained with high accuracy based on density functional theory within the local density approximation. Two different norm-conserving \textit{ab initio} pseudopotentials have been tested which agree in all respects with the only exception arising for the elastic properties of rutile GeO_{2}. The agreement with experimental values, when available, are seen to be highly satisfactory. The uniformity and the well convergence of this approach enables an unbiased assessment of important physical parameters within each material and among different insulating oxide and nitrides. The computed static electric susceptibilities are observed to display a strong correlation with their mass densities. There is a marked discrepancy between the considered oxides and nitrides with the latter having sudden increase of density of states away from the respective band edges. This is expected to give rise to excessive carrier scattering which can practically preclude bulk impact ionization process in Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure

Wax Anatomical Models and Neuroscience: From Artistic Italian Creation to Therapeutic Approach

Wax modelling has been used since ancient times with its first application in art in the fifteenth century involving famous artists with knowledge of anatomy. It was only in the seventeenth century that coloured ceroplastic began to be used for teaching anatomy as valid alternative to dissected human bodies, including also neuropathological. The origin of this scientific approach was born in central Italy, in Florence and Bologna in the eighteenth century, and immediately spread to other Italian cities, and Europe, and throughout the rest of the world. Wax neuro-models were shown as artefacts and destined to train young doctors in anatomical knowledge. Nowadays, wax is often considered an old-fashioned art form but what is not well known is that wax has a useful therapeutic application in medicine, with a particular emphasis in neurosurgery