1,720,973 research outputs found
Rare Earth-Doped SrTiO3 Perovskite Formation from Xerogels
Full text linkA synthesis process of rare earth doped SrTiO3 by modified sol-gel technique is described. Impervious strontium titanate doped
with rare earth was prepared by gelification and calcination of colloidal systems. Powders of thulium substituted strontium titanate
(SrTi1−xTmxO3-δ, where x = 0.005; 0.02; 0.05) were obtained through cohydrolysis of titanium, strontium, and thulium precursors
by sol-gel method. The xerogel obtained from the evaporation of solvents was milled and calcinated at 1100◦C to give a reactive
powder. Pure and doped SrTiO3 dense disks were formed by uniaxial pressing. Thermogravimetry (TGA), differential scanning
calorimetry (DSC) analysis, X-ray diffractometry (XRD), and scanning electron microscopy (SEM) have been used to study the
microstructural evolution of amorphous xerogel into crystalline reactive and sinterable powders. Hardness was measured for each
membrane by a Vickers microindenter. Dilatometric and TGA-DSC in pure CO2 flow tests have been performed to evaluate,
respectively, the thermal and chemical stability of the material. The optimized preparation route has allowed to synthesize highly
reactive easy sintering powders used for fully densified, impervious ceramics with high thermal and chemical stability at high
temperature
The origin of highly efficient selective emission in rare-earth oxides for thermophotovoltaic applications,
Full text linkRare-earth oxide materials emit thermal radiation in a narrow spectral region, and can be used for a variety of different high-temperature applications, such as the generation of electricity by thermophotovoltaic conversion of thermal radiation. However, because a detailed understanding of the mechanism of selective emission from rare-earth atoms has so far been missing, attempts to engineer selective emitters have relied mainly on empirical approaches. In this work, we present a new quantum thermodynamic model to describe the mechanisms of thermal pumping and radiative de-excitation in rare-earth oxide materials. By evaluating the effects of the local crystal-field symmetry around a rare-earth ion, this model clearly explains how and why only some of the room-temperature absorption peaks give rise to highly efficient emission bands at high temperature (1,000–1,500 °C). High-temperature emissivity measurements along with photoluminescence and cathodoluminescence results confirm the predictions of the theory
PROCESSO INNOVATIVO DI PRODUZIONE DI MATERIALE ANTIBATTERICO CONTENENTE ARGENTO SU SUPPORTO INORGANICO
No full textCeramic composites for automotive friction devices
No full textAdvanced braking devices can represent a promising application for ceramic matrix composites (CMC) with functional and structural properties. If the actual advanced braking materials could be at least partially replaced by CMCs, it might become the first consumer market for these materials. CMC containing three main phases, silicon carbide, graphite and carbon fibers were prepared. A systematic analysis of the processing-structure-properties relationship of the composite is carried out. In particular, silicon carbide provides the necessary hardness, whereas graphite is used for its lubricating properties, and carbon fibers are used as reinforcement. The samples, prepared using a reactive bonding technique, exhibited adequate mechanical properties, high resistance to thermal shocks and good stability after many thermal cycles. Morphological and structural investigations have been performed to optimize the content of each component. Preliminary tribological investigations are presented
Is nano-TiO2 alone an effective strategy for the maintenance of stones in Cultural Heritage?
No full textTiO2-based nanocoatings have been becoming more and more widespread during last years in Cultural Heritage: they seem to be able to keep stone surfaces self-cleaned and to prevent the formation of biofouling. However, the efficiency of these coatings is strongly dependent on the substrate (i.e.: porosity and roughness) and on the amount of TiO2. Thus, this study experimentally investigates on the self-cleaning and anti-biofouling efficiency of a nano-TiO2 dispersion (without any organic or inorganic additive) applied on six different types of natural stones (three limestones, two sandstones and one tuff) usually used in Cultural Heritage, where high porosity and roughness can be found and the TiO2 amount cannot be increased in order to avoid any chromatic variation of the substrate. Water was used as solvent so as to reduce the risk of exposition of hazardous materials and to eliminate any chemical action on stones. The self-cleaning power of the coating was evaluated by measuring its ability at discolouring organic dye Methylene Blue, while its anti-biofouling efficiency was assessed by an accelerated growth test under controlled climatic conditions of two algal microorganisms, namely Chlorella mirabilis and Chroococcidiopsis fissurarum. Results show that, even if the photocatalytic and biocide power of nano-TiO2 itself is well known in literature, its application for the maintenance of stones in Cultural Heritage does not seem to be an effective strategy, especially when stones are highly porous and rough. Roughness and porosity of stones, in fact, can limit the efficiency of TiO2, which is thus not able to powerfully keep the stone substrate cleaned or slow down algal proliferatio
The challenge of high performance selective emitters for thermophotovoltaic applications
Full text linkWe present a brief survey of the most significant contributions to the study and the development of selective emitters for high-temperature applications. After a brief introduction and some necessary notes on definitions and experimental methods, this review presents the many different solutions proposed so far from the point of view of both the optimization of the functional properties of selective emitters and the fulfillment of the severe thermostructural requirements imposed by most high-temperature applications such as thermophotovoltaics
Magnetic and x-ray diagnostics for plasma position measurements in the ignition experiment ignitor
No full textThe ignition experiment Ignitor will produce high performance plasma regimes with neutron fluxes at the first wall comparable to those expected in power generating reactors (1015n/cm2/s at the first wall). An appreciable degradation of the inorganic insulators surrounding the conductors is expected and the measurement of basic plasma parameters, such as plasma current and position, by electromagnetic diagnostics can become problematic. Thus an R&D program, aimed at the development of effective and affordable devices for electromagnetic diagnostics with higher damage threshold, is being carried out. Prototype diagnostic coils of different shapes have been manufactured, in collaboration with SALENTEC and Università di Lecce. At the same time, an alternative plasma position control method is being explored, based on the diffraction and detection of the soft X-ray radiation emitted, mostly by Molybdenum impurities, near the top or bottom of the plasma column, where the distance of the plasma last closed magnetic surface from the wall is only few millimeters. The underlying principles and general layout of the new system is essentially an adaptation of a 2-dimensional curved crystal spectrometer. © 2008 American Institute of Physics
Lanthanum glass infiltrated alumina/alumina composites for dental prosthetic applications
Full text linkLanthanum glass frits suitable for infiltration in pre-sintered alumina were developed in order to obtain innovative alumina–glass composites, which are promising materials for all-ceramic dental prosthetic restorations, such as single crowns and fixed partial dentures. Glass compositions belonging to the lanthanum-silicate-borate system were synthesized in order to reinforce and give an improved esthetic appearance to the pre-sintered Al2O3.
Fifteen formulations, defined by using a mixture design approach, were melted and quenched in water at room temperature in order to obtain the frits. Those exhibiting the lower melting temperatures were tested for infiltration into Al2O3 to obtain Al2O3/glass composites. The frits and the composites were characterized by X-Ray diffraction, differential thermal analyses, hot stage microscopy, colorimetry (Hunter color scale) and scanning electron microscopy. The mechanical properties of the composites, in terms of flexure strength, were evaluated using a three-point bending tests
Sol gel preparation of selective emitters for thermophotovoltaic conversion
Full text linkSelective emitters are materials characterized by a high temperature emissivity significantly changing in different spectral regions. One of the crucial steps for the development of Thermophotovoltaic (TPV) generators is given by an optimal matching of the spectral emissivity of an IR radiation source with the spectral region where is maximum the efficiency of photovoltaic cells. The emitters should retain good structural properties at the working temperature above 1300°C and they can be either an external coating for the a burner or, as a structural material, a burner and emitter at once.
In this work, oxide glass and ceramics containing rare earths have been prepared and characterized as selective emitters candidates. Different approaches and materials have been attempted all based on a colloidal route. Rare earths oxides (erbium and holmium) have been incorporated in transparent silica glass and in polycrystalline alumina and zirconia using their hydrated salts as oxide precursors. Rare earth modified silica glass were obtained by sintering silica xerogel containing fumed silica and hydrolysed ortholisicate. Rare earth modified alumina and mixed alumina-zirconia ceramics were obtained from slurries containing alumina colloidal particles and milled ceramic fibres. Functional properties i.e. the high temperature spectral emissivities have been measured by means of a specially designed apparatus where the working conditions of the selective emitters can be reproduced and monitored
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