Search

The aim of this work is to highlight the correlation between treatment conditions, composition, and structure of reduced zinc oxide (ZnO:Zn) with optical and radioluminescence properties for the production of high yield scintillating powders. Purchased zinc oxide (ZnO) powders (≈0.35 ± 0.15 μm) were thermally reduced in Ar/H2 (3%) atmosphere at temperatures ranging between 600 (≈0.40 ± 0.17 μm) and 1000 °C (≈6.4 ± 1.6 μm), obtaining ZnO:Zn powders. The reduction degree, evaluated through energy dispersive spectroscopy, showed a linear increase from 0% up to 20% (800 °C sample), reaching a plateau condition at about 25% for the highest treatment temperatures (1000 °C sample). Structural analysis highlighted the existence of a threshold temperature, previously reported in the literature, around 800 °C, beyond which (i) the grain size remarkably increases, (ii) the shape changes from hexagonal to round-shaped, and (iii) the intensity of electron spin resonance (ESR)-active defects at g = 1.9596 decreases for a new resonance at g = 2.0105. Additionally, a substantial variation, from 5.63 ± 0.02 down to 5.2 ± 0.1 g/cm3, of the skeletal density and a slight contraction of the optical bandgap are observed. Unlike the structural properties, PL and x-ray radioluminescence showed a continuous increase of the green emission intensity, associated with ESR-active ionized oxygen vacancies and ESR non-active recombination centers, and of the PL defective lifetimes (33 ns < τfast < 88 ns, 395 ns < τslow < 930 ns). [ABSTRACT FROM AUTHOR]

This work is aimed at a deeper understanding of the polarity-sensitive electro-optical response observed in a liquid crystal (LC) cell with asymmetric insertion of thin films of TiO[SUB2]-V[SUB2]O[SUB5] having a Ti/V atomic ratio of 1/1, prepared by sol-gel synthesis on a transparent indium tin oxide substrate. After preliminary structural characterization of the films, the electro-optical response of the liquid crystal cells containing a TiO[SUB2]-V[SUB2]O[SUB5] layer has been analyzed. The voltage thresholds of the Freedericksz transition are increased or decreased for anodic or the cathodic polarization, respectively, of the TiO[SUB2]-V[SUB2]O[SUB5] film. In such a way a polarity-sensitive electro-optical response is generated that has the same frequency as the field applied. Impedance and cyclic voltammetry measurements were performed on liquid crystal cells having TiO[SUB2]-V[SUB2]O[SUB5] films inserted as the electrode, for as-deposited films as well as for films annealed at 400 °C. The LC cell containing thermally annealed Ti/V 1/1 film showed a rectified square wave response instead of the usual impulsive one, quadratic versus electric field. On the contrary, the LC cell containing as-deposited Ti/V 1/1 film exhibits more complex electro-optical behavior with a weak asymmetric response. All the measurements suggest that charge redistribution of the ions, always present in the films, occurs during the intercalation-deintercalation processes induced by the voltage applied, and this is responsible for changes of the effective electric field that act on the liquid crystal layer. [ABSTRACT FROM AUTHOR]

Dissolution and gelation procedures have a great influence on gelation time, microstructure and mechanical properties of reconstituted collagen products. We have investigated the dissolution of atelocollagen in CO2/water solutions at low temperature (4 °C) at different CO2 pressures (0.3-0.9 MPa), as well as gelation kinetics and physico-chemical properties of the hydrogel obtained after CO2 removal. Compared to conventional methods, the CO2-assisted technique resulted in faster soluble collagen dissolution and faster gelation into transparent gels characterized by thin 10 nm fibrils. Electrophoresis and CD spectroscopy demonstrated that the process did not denature the soluble collagen. The possibility to obtain collagen dissolution and gelation without the use of chemical agent other than water and CO2 makes this process particularly appealing for biomedical applications. [ABSTRACT FROM AUTHOR]

Kenaf fibres of different aspect ratios (L/D = 30 and 160) were melt compounded in an internal mixer with two types of starch-grafted-polypropylene matrices (G906PF and G906PJ) at various loadings, i.e. 10, 20 and 30 wt%. The compound was then compression-moulded into plaques of 1-mm thickness. Thermal, rheological and dynamic mechanical properties of the composite samples were investigated by several techniques involving differential scanning calorimetry, thermogravimetric analysis, melt flow index, Vicat softening point and dynamic mechanical thermal analysis. The results indicated that both thermal and mechanical properties of the composites were remarkably improved by kenaf fibres. Loss modulus and loss factor showed a shift of peak transitions to higher temperatures. Finally, the properties of the investigated composite materials were not affected by the fibre aspect ratio. [ABSTRACT FROM AUTHOR]

Aim of the present study is to investigate how synthetic boehmite alumina (BA) nanoparticles modify the viscoleastic and fracture behaviour of linear low-density polyethylene. Nanocomposites containing up to 8 wt% of untreated and octyl silane-functionalized BA nanoparticles, were prepared by melt compounding and hot pressing. The BA nanoparticles were finely and unformly dispersed within the matrix according to scanning electron microscopy inspection. The results of quasi-static tensile tests indicated that nanoparticles can provide a remarkable stiffening effect at a rather low filler content. Short term creep tests showed that creep stability was significatively improved by nanofiller incorporation. Concurrently, both storage and loss moduli were enhanced in all nanocomposites, showing better result for surface treated nanoparticles. The plane-stress fracture toughness, evaluated by the essential work of fracture approach, manifested a dramatic increase (up to 64%) with the BA content, with no significant differences among the various types of BA nanoparticles. [ABSTRACT FROM AUTHOR]

In this work we report our preliminary results on the synthesis and characterization of the optical properties of 3-hydroxyflavone (3-HF) doped sol-gel derived glass, to be applied as a wavelength shifting medium in GaAs based near UV-VIS detectors in order to extend the device response down to 350 nm. As already reported by several authors, the intramolecular excited-state proton transfer process (ESIPT) in the 3-HF molecule is strongly affected by the chemical environment. For this reason the evolution of the emission features of the synthesized glasses was investigated by changing the chemical nature of precursor alkoxides used in the sol-gel process. The influence of chemical surroundings on the ESIPT process was studied by analysing the samples with fluorescence spectroscopy. FT-IR spectroscopy was performed in order to study the chemical interactions between the host matrix and the encapsulated dye molecule. The main result of this work is that the Stokes-shifted emission from the 3-HF tautomeric form is enhanced at increasing trifunctional alkoxide amount and at decreasing polarity of the non-hydrolizable groups. [ABSTRACT FROM AUTHOR]

Mixed systems TiO2-V2O5 are promising candidates for the improvement of counter-electrodes in electrochromic cells. In principle, they must give a good compromise between the performances of the separate components, in terms of transparency, chemical stability and charge capacity. The sol-gel method assures a better homogeneity of the starting precursors, thus a greater stability of the deposited films. The chemical and structural homogeneity of these films has been analysed by using a micro-Raman mapping, on as grown samples, as well as on samples undergoing annealing at increasing temperatures. We have indications that most of the samples are in amorphous state, with the remarkable occurrence of polyvanadate chains, even after annealing at 400 °C, but a separate crystallization can be observed for some point, and an inhomogeneous distribution, consisting of stripes of amorphous material is observed, for samples grown under specific pH condition during the sol-gel route. [ABSTRACT FROM AUTHOR]

Titania and titania/vanadia thin films were prepared via solgel process, starting from alcoholic solution of Ti (OBu) 4 for pure titania films and from Ti (OBu) 4 /VO(OPr i ) 3 for mixed oxide films. The main requirements of these materials for applications in electrochromic devices were investigated, i.e. high charge capacity and electrochemical stability. Microstructural and electrochemical characterizations were carried out both on gels and films leading to the following results: titania films showed a high electro-chemical stability associated with a good charge capacity, whereas mixed oxide systems displayed a high stability again but a lower intercalation/deintercalation charges. The first electro-optic measurements were also performed for two films with different Ti:V atomic ratio, to check the interaction with liquid crystals in modified nematic liquid crystal cells: the cell containing films poorer in vanadia give some rectification effect, but they have a very complex dependence on the applied potential and very slow relaxation times; on the contrary films richer in vanadia exhibits very good response in the liquid crystal cells, compared with another mixed conductivity oxides. [ABSTRACT FROM AUTHOR]

The variety of SiO2-gel structures allows the use of sol-gel derived materials as host matrices for prolonged release of chemicals. In this work, the release of perfumed essences is studied in the case of hybrid SiO2 materials obtained from alkyl-modified silicon alkoxides. Thermogravimetric analysis in isothermal conditions at various temperatures provide data on the kinetic release process. The effects of physical parameters (porosity reduction, vapour pressure) and chemical interactions (organic molecules with matrix) are discussed for the interpretation of the results. [ABSTRACT FROM AUTHOR]

The variety of SiO2-gel structures allows the use of sol-gel derived materials as host matrices for prolonged release of chemicals. In this work, the release of perfumed essences is studied in the case of hybrid SiO2 materials obtained from alkyl-modified silicon alkoxides. Thermogravimetric analysis in isothermal conditions at various temperatures provide data on the kinetic release process. The effects of physical parameters (porosity reduction, vapour pressure) and chemical interactions (organic molecules with matrix) are discussed for the interpretation of the results. [ABSTRACT FROM AUTHOR]

A multitechnique approach, consisting of x-ray diffraction, differential thermal analysis, low frequency Raman scattering from the acoustic vibrations of nanoclusters, and transmission electron microscopy associated with selected area diffraction, has been used to study the nucleation and crystallization processes in SiO[sub 2]–Ga[sub 2]O[sub 3]–K[sub 2]O glasses. The specific aim was to determine the structure and the size distribution of nanoparticles embedded in the glass matrix. It has been found that nearly spherical nanocrystals of β-Ga[sub 2]O[sub 3], with a size of ∼2–3 nm, nucleate during thermal treatments at 900 °C. Crystallization was observed after annealing at higher temperature. The amount of the crystalline phase and the mean size of the nanocrystals increased with heat treatment, time and temperature. β-Ga[sub 2]O[sub 3] was the only crystalline phase to appear in all glass samples. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural evolution with temperature of some V2O5 gels and thin films is presented, and the role of the hydrolysis conditions is investigated. Several techniques, i.e., x-ray diffraction, differential thermal analysis, infrared, and temperature-dependent Raman spectroscopy, have been used to follow the thermal behavior of the samples. When the bulk xerogels begin to change from a nanocrystalline phase to the orthorhombic a-V2O5, in the temperature range 280 to 300 °C, a growth of vanadium oxide nanotubes also occurs, while at higher temperatures the crystallization into the a phase prevails. A slightly different evolution is observed for heat treated thin films, which show a structure containing polyvanadate chains near room temperature. They also present a growth of nanotubes for intermediate temperatures and a complete crystallization into the a phase when the temperature is further increased. [ABSTRACT FROM AUTHOR]