Search results for "Silice"

IS THE MACROSCOPIC CLASSIFICATION OF FLINT USEFUL? A PETROARCHAEOLOGICAL ANALYSIS AND CHARACTERIZATION OF FLINT RAW MATERIALS FROM THE IBERIAN NEOLIT…

2009

Casa Montero is a mining complex located outside Madrid (Spain), dated from the Early Neolithic (c. 5400-5000 cal bc). An area of some 4 ha has been investigated and some 4000 shafts recorded, of which 324 have been excavated. The characterization of its raw flint materials and the establishment of its diagnostic features are indispensable in the reconstruction of the distribution of the mine's products beyond the immediate site. This work reports the geological study of the mine's Miocene flint layers and their petrological characterization. Archaeological samples from the mine's shafts were classified according to macroscopic features and petrological characteristics.

New stratigraphical data on the Middle-Late Jurassic biosiliceous sediments from the Sicanian basin, Western Sicily (Italy)

2008

The reported data present the stratigraphy of several sections across a Middle–Late Jurassic Radiolaritic Unit, well exposed in different thrust sheets pertaining to the Maghrebian chain of Southwestern Sicily. The aim was to define the chronostratigraphical distribution of the Jurassic biosiliceous sedi- mentation in the Sicanian palaeogeographical zone, a deep water basin belonging to the Southern Tethys continental margin. The radiolarian biostratigraphy indicates that the switching from carbonate to siliceous sedimentation in the Sicanian Basin is referable to the Bajocian, as shown by the section of Campofiorito, near Corleone. The biostratigraphical dataset allows the correlation betw…

Codon usage in the siliceous sponge Geodia cydonium: highly expressed genes in the simplest multicellular animals prefer C- and G-ending codons

2001

Among a sample of 39 Geodia cydonium (Demospongiae, Porifera) genes, with an average Gh+hC content of 51.2%, extensive structural heterogeneity and considerable variations in synonymous codon usage were found. The G + C content of coding sequences and G + C content at silent codon positions (GC3S) varied from 42.4 to 59.2% and from 35.6 to 76.5%, respectively. Correspondence analysis of 39 genes revealed that putative highly expressed genes preferentially use a limited subset of codons, which were therefore defined as preferred codons in G. cydonium. A total of 22 preferred codons for 18 amino acids with synonyms in codons were identified and they all (with one exception) end with C or G. A…

Physico-chemical evolution of low-pH cements : influence of the temperature and the retention mechanisme of alkalins

2010

Because of their high alkalinity, Portland cement (OPC)-based materials may have deleterious effects in an underground waste repository. A solution would be to use low-alkalinity cements (also referred as low-pH cements) generating interstitial solutions with a reduced pH (11 instead of 13.5 for OPC), and thus showing an improved chemical compatibility with the repository environment. In this work, the investigated formulations were based on binary (OPC / silica fume) or ternary (OPC / silica fume / slag or fly ash) blends, with high substitution levels of CEM I (from 30% to 80%). This research project met two main objectives: (i) study the chemical evolution of low-pH cements at 50°C or 80…

Structural and luminescence properties of amorphous SiO2 nanoparticles

2011

We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO 2. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO2 nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the de…

Thermoelectric properties of atomic-thin silicene and germanene nano-structures

2014

The thermoelectric properties in one- and two-dimensional silicon and germanium structures have been investigated using first-principles density functional techniques and linear response for the thermal and electrical transport. We have considered here the two-dimensional silicene and germanene, together with nanoribbons of different widths. For the nano ribbons, we have also investigated the possibility of nano structuring these systems by mixing silicon and germanium. We found that the figure of merit at room temperature of these systems is remarkably high, up to 2.5.

Sintesi e caratterizzazione di compositi polimetilmetacrilato e nano-SiO2, -TiO2 o -ZrO2

2011

Formation of optically active oxygen deficient centers in Ge-doped SiO2 by γ- and β-ray irradiation

2010

Abstract We report an experimental study on the comparison between the γ- or β-ray induced Ge related point defects in Ge-doped silica. Silica samples doped with ∼2.2 1017 Ge atoms/cm3 produced with the sol–gel technique have been irradiated with γ-ray or with β-ray. The effects of the irradiation have been investigated by optical absorption, photoluminescence and electron paramagnetic resonance spectroscopy in order to evaluate the generation and the dependence on dose of the Ge(1), E’Ge, GLPC (Germanium lone pair center) and H(II) point defects. No relevant differences between the concentrations of γ- or β-ray induced Ge(1) and E’Ge point defects have been observed and, in addition, it ha…

Modification of Ce oxidation state by thermal treatments in controlled atmosphere of Ce-Ti-Eu doped SiO2 glasses

2010

Sponges (Porifera) as living metazoan witnesses from the Neoproterozoic: biomineralization and the concept of their evolutionary success

2010

Terra Nova, 22, 1–11, 2010 Abstract The emergence of the Metazoa can be dated back to the Neoproterozoic Era which comprises the Cryogenian Period during which two major glaciations occurred, the Sturtian and the Varanger-Marinoan. At that time, the phylum Porifera (sponges) evolved as the first animals and developed a hard skeleton. The two classes of siliceous sponges, the Hexactinellida and the Demospongiae, are already provided with the major genetic repertoire and gene regulatory networks that also exist in modern multicellular animals. Besides these metazoan innovations, the siliceous sponges display one autapomorphic character, silicatein, an enzyme which mediates bio-silica formatio…