Pellets prepared with mechanically activated iron ore

This work analyses pellets prepared with iron ore that has been mechanically activated by high energy ball milling. Pellet feed iron ore was submitted to high-energy ball milling for 60 minutes, and the resulting material was analysed through measurements of particle size and specific surface area, as well as X-ray diffraction. Pellets were prepared from this material. The pellets were heated at temperatures ranging from 1000 to 1250 degrees C in a muffle furnace, and submitted to the maximum temperature during 10 - 12 minutes. The samples were then tested regarding crushing strength, densification and porosity, and were examined in a scanning electronic microscope. The results were compared to those obtained with similar samples made from non-milled pellet feed. It has been shown that through high-energy ball milling of iron ore it is possible to achieve pellets presenting high densification and compressive strength at firing temperatures lower than the usual ones.

Laboratory evaluation of recycled construction and demolition waste for pavements

The wide production of construction and demolition waste and its illegal deposition are serious current problems in Brazil. This research proposes to evaluate the feasibility of using aggregate from recycled construction and demolition waste (RCDW) in pavement applications. A laboratory program was conducted by geotechnical characterization, bearing capacity and repeated load triaxial tests. The results show that the composition and the compactive effort influence on the physical characteristics of the RCDW aggregate. The compaction process has promoted a partial crushing and breakage of RCDW particles, changing the grain-size distribution and increasing the percentage of cubic grains. This physical change contributes to a better densification of the RCDW aggregate and consequently an improvement in bearing capacity, resilient modulus and resistance to permanent deformation. The results have shown that the RCDW aggregate may be utilized as coarse base and sub-base layer for low-volume roads. (C) 2010 Elsevier Ltd. All rights reserved.

Democracy on the Edge: Limits and Possibilities in the Implementation of an Urban Reform Agenda in Brazil

The 1990s in Brazil were a time of institutional advances in the areas of housing and urban rights following the signing of the new constitution in 1988 that incorporated the principles of the social function of cities and property, recognition of the right to ownership of informal urban squatters and the direct participation of citizens in urban policy decision processes. These propositions are the pillars of the urban reform agenda which, since the creation of the Ministry of Cities by the Lula government, has come under the federal executive branch. This article evaluates the limitations and opportunities involved in implementing this agenda on the basis of two policies proposed by the ministry - the National Cities Council and the campaign for Participatory Master Plans - focusing the analysis on government organization in the area of urban development in its relationship with the political system and the characteristics of Brazilian democracy. Resume Au Bresil, les annees 1990 ont ete marquees par des progres institutionnels en matiere de logement et de droits urbains, dans le sillage de la Constitution de 1988 qui integre les principes d`une fonction sociale de la ville et de la propriete urbaine, ainsi que la reconnaissance du droit a la propriete pour les squatters urbains et la participation directe des citoyens aux processus d`elaboration des politiques urbaines. Ce sont egalement les piliers du programme de reforme urbaine qui releve de l`executif federal depuis la creation d`un ministere des Villes par le gouvernement Lula. Pour evaluer les limites et potentiels lies a la mise en place de ce programme, cet article s`appuie sur deux politiques publiques proposees par le ministere, le Conseil national des villes et la campagne en faveur des Plans directeurs participatifs, en analysant plus particulierement l`organisation gouvernementale en matiere d`urbanisme par rapport au systeme politique et aux caracteristiques de la democratie bresilienne.

Reactivity of chars and carbons: New insights through molecular modeling

A new model proposed for the gasification of chars and carbons incorporates features of the turbostratic nanoscale structure that exists in such materials. The model also considers the effect of initial surface chemistry and different reactivities perpendicular to the edges and to the faces of the underlying crystallite planes comprising the turbostratic structure. It may be more realistic than earlier models based on pore or grain structure idealizations when the carbon contains large amounts of crystallite matter. Shrinkage of the carbon particles in the chemically controlled regime is also possible due to the random complete gasification of crystallitic planes. This mechanism can explain observations in the literature of particle size reduction. Based on the model predictions, both initial surface chemistry and the number of stacked planes in the crystallites strongly influence the reactivity and particle shrinkage. Its test results agree well with literature data on the air-oxidation of Spherocarb and show that it accurately predicts the variation of particle size with conversion. Model parameters are determined entirely from rate measurements.

On the use of trace additions of Sn to enhance sintered 2xxx series Al powder alloys

The mechanical properties of a typical sintered aluminium alloy (Al-4.4Cu-0.8Si-0.5Mg) have been improved by the simultaneous use of trace additions of Sn, high sintering temperatures and modified heat treatments. Tin increases densification, but the Sn concentration is limited to less than or equal to 0.1wt% because incipient melting occurs during solution treatment at higher Sn levels. A sintering temperature of 620 degrees C increases the liquid volume over that formed at the conventional 590 degrees C sintering temperature. However, the higher sintering temperature results in the formation of an embrittling phase which can be eliminated if solution treatment is incorporated into the sintering cycle (a modified TS heat treatment). These conditions produce a tensile strength of 375 MPa, an increase of nearly 20% over the unmodified alloy. (C) 1999 Elsevier Science S.A. All rights reserved.

α′ phase stability in Nd-Li-sialon systems

The formability and stability of the alpha-sialon (alpha') phase was investigated in multi-cation Nd-Li-sialon systems. Four samples were prepared, ranging from a pure Nd-sialon to a pure Li-sialon, with two intermediate samples being prepared with either lithium or neodymium replacing the other alpha'-stabilising additive by 20 eq.%, as to maintain an equivalent design composition in all samples. After sintering, all samples were subsequently heat treated up to 192 h at 1450 and 1300 degreesC. While significant quantities of the beta'-sialon (beta' phase were found in most samples, the high-lithium Li-Nd-sialon sample was found to be almost pure a' phase after sintering. Furthermore, the long-term stability of the a' phase on heat treatment was also found to be superior in both multi-cation samples than in either of the single-alpha'-stabilising-cation samples. This is thought to be related to improved retention of the lithium in the multi-cation systems, as much of the lithium was found to volatilise during sintering in the neodymium-free sample. (C) 2002 Elsevier Science Ltd. All rights reserved.

Sol-gel preparation of near-infrared broadband emitting Er(3+)-doped SiO(2)-Ta(2)O(5) nanocomposite films

This article reports a study on the preparation, densification process, and structural and optical properties of SiO(2)-Ta(2)O(5) nanocomposite films obtained by the sol-gel process. The films were doped with Er(3+) and the Si:Ta molar ratio was 90:10. Values of refractive index, thickness and vibrational modes in terms of the number of layers and thermal annealing time are described for the films. The densification process is accompanied by OH group elimination, increase in the refractive index, and changes in film thickness. Full densification of the film is acquired after 90 min of annealing at 900 degrees C. The onset of crystallization and devitrification, with the growth of Ta(2)O(5) nanocrystals occurs with film densification, evidenced by high-resolution transmission electron microscopy. The Er(3+)-doped nanocomposite annealed at 900 degrees C consists of Ta(2)O(5) nanoparticles, with sizes around 2 nm, dispersed in the SiO(2) amorphous phase. The main emission peak of the film is detected at around 1532 nm, which can be assigned to the (4)I(13/2)->(4)I(15/2) transition of the Er(3+) ions present in the nanocomposites. This band has a full width at half medium of 64 nm, and the lifetime measured for the (4)I(13/2) levels is 5.4 ms, which is broader compared to those of other silicate systems. In conclusion, the films obtained in this work are excellent candidates for use as active planar waveguide. (C) 2010 Elsevier B.V. All rights reserved.

Effect of temperature and heating rate on the sintering of leucite-based dental porcelains

The aim of this work was to determine the effect of temperature and heating rate on the densification of four leucite-based dental porcelains: two low-fusion (Dentsply Ceramco and Ivoclar) and two high-fusion commercial porcelains (Dentsply Ceramco). Porcelain powders were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), particle size distribution, helium picnometry, and by scanning electron microscopy. Test specimens were sintered from 600 to 1050 degrees C, with heating rates of 55 degrees C/min and 10 degrees C/min. The bulk density of the specimens was measured by the Archimedes method in water, and microstructures of fracture surfaces were analyzed by scanning electron microscopy (SEM). The results showed that densification of specimens increased with the increase in temperature. The increase in the heating rate had no effect on the densification of the porcelains studied. Both high-fusion materials and one of the low-fusing porcelains reached the maximum densification at a temperature that was 50 degrees C lower than that recommended by the manufactures. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Hydrophobicity of Templated Silica Xerogels for Molecular Sieving Applications

Silica xerogels were prepared by a sol-gel process catalyzed by acid with tetraethylorthosilicate, and using an organic covalent ligand template (methyltriethoxysilane) or a noncovalent template C6 surfactant (triethylhexylammonium bromide). The influence of hydrotreatment on the structure of templated xerogels is examined in terms of surface area, micropore volume, average pore size, and pore size distribution, and compared against a blank xerogel (nontemplated). The role of surface functional groups was evaluated using Si-29 nuclear magnetic resonance. The structural integrity of the xerogel was maintained to a large extent in samples that had a high contribution of Q(4) species (siloxane groups). Xerogel matrix densification occurred when there was a large concentration of Q(3) and Q(2) species (silanol groups), which also were responsible for increased hydrophilicity. The templated xerogels resulted in up to a 25% concentration of methyl functional groups (T-3 and T-2 species), leading to hydrophobic xerogels. The best results in terms of structural integrity and hydrophobicity were obtained with templated xerogels prepared with the C6 surfactant. The results in this study suggest that surfactant-enhanced condensation reactions lead to structures with a high contribution of Q(4) groups, which are not susceptible to water attack, but are strong enough to oppose matrix densification during rehydration.

Characterisation of templated xerogels for molecular sieve application

This paper presents the results of the characterisation of templated silica xerogels as precursor material for molecular sieve silica membranes for gas separation. The template agent integrated in the xerogel matrix is a methyl ligand covalently bended to the siloxane network in the form of methyltriethoxysilane (MTES). Several surface and microstructural characterisation techniques such as TGA, FTIR, NMR, and nitrogen adsorption have been employed to obtain information on the reaction mechanisms involved in the sol-gel processing of such molecular sieves. The characterisation results show the effects of processing parameters such as heat treatment temperature, and the concentration of the covalently bonded template on the development of the pore structure. It was found that calcination temperature significantly enhanced the condensation reactions thus resulted in more Si-O-Si groups being formed. This was also confirmed with the data of FTIR characterisation showing enhanced silicon bands at higher heat treatment temperatures. As a result of the promoted densification and shrinkable pore network the micropore volume also reduced with increasing methyl ligand molar ratio. However, the mean pore diameter does not change significantly with calcination temperature. While the contribution of the templates towards controlling pore size is less precise, increasing the methyl ligand molar ratio results in the broadening of the pore size distribution and lower pore volume. Higher template concentration induces the collapse of the xerogel matrix due to capillary stress promoting dense xerogels with low pore volume (C) 2001 Elsevier Science B.V. All rights reserved.

Freeform fabrication of aluminum metal-matrix composites

A series of metal-matrix composites were formed by extrusion freeform, fabrication of a sinterable aluminum alloy in combination with silicon carbide particles and whiskers, carbon fibers, alumina particles, and hollow flyash cenospheres. Silicon carbide particles were most successful in that the composites retained high density with up to 20 vol% of reinforcement and the strength approximately doubles over the strength of the metal matrix alone. Comparison with simple models suggests that this unexpectedly high degree of reinforcement can be attributed to the concentration of small silicon carbide particles around the larger metal powder. This fabrication method also allows composites to be formed with hollow spheres that cannot be formed by other powder or melt methods.

Finite, three-dimensional adsorbed phase growth in activated carbon mesopores

The role of PACs (primary adsorption centers) in the mesopore (i.e., transport) region of activated carbons during adsorption of polar species, such as water, is unclear. A classical model of three-dimensional adsorption on finite PACs is presented. The model is a preliminary, theoretical investigation into adsorption on mesopore PACs and is intended to give some insight into the energetic and physical processes at work. Work processes are developed to obtain isotherms and three-dimensional sorbate growth on PACs of varying size and energetic characteristics. The work processes allow two forms of adsorbed phase growth: densification at constant boundary and boundary growth at constant density. Relatively strong sorbate-sorbent interactions and strong surface tension favor adsorbed phase densification over boundary growth. Conversely, relatively weak sorbate-sorbent interactions and weak surface tension favor boundary growth over densification. If sorbate-sorbate interactions are strong compared to sorbate-sorbent interactions, condensation with hysteresis occurs. This can also give rise to delayed boundary growth, where all initial adsorption occurs in the monolayer only. The results indicate that adsorbed phase growth on PACs may be quite complex.

Variation of the crystalline structure of coal char during gasification

The variation of the crystallite structure of several coal chars during gasification in air and carbon dioxide was studied by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. The XRD analysis of the partially gasified coal chars, based on two approaches, Scherrer's equation and Alexander and Sommer's method, shows a contradictory trend of the variation of the crystallite height with carbon conversion, despite giving a similar trend for the crystallite width change. The HRTEM fringe images of the partially gasified coal chars indicate that large and highly ordered crystallites exist at conversion levels as high as 86%. It is also demonstrated that the crystalline structure of chars can be very different although their pore structures are similar, suggesting a combination of crystalline structure analysis with pore structure analysis in studies of carbon gasification.

Harmonização da fiscalidade directa na UE: evolução e perspectivas: a directiva da poupança

Mestrado em Fiscalidade

Télédetection en milieu périurbain. Détection et localisation du changement de l'occupation du sol par intégration des donnés-satellite SPOT HRV dans un système d'information géographique

Thèse pour obtenir le grade de DOCTEUR DE L' UNIVERSITÉ PARIS XII, Discipline: Urbanisme Aménagement