Mass fractal characteristics of silica sonogels as determined by small-angle x-ray scattering and nitrogen adsorption

A sample series of silica sonogels was prepared using different water-tetraethoxysilane molar ratio (r(w)) in the gelation step of the process in order to obtain aerogels with different bulk densities after the supercritical drying. The samples were analyzed by means of small-angle x-ray-scattering (SAXS) and nitrogen-adsorption techniques. Wet sonogels exhibit mass fractal structure with fractal dimension D increasing from similar to2.1 to similar to2.4 and mass-fractal correlation length xi diminishing from similar to13 nm to similar to2 nm, as r(w) is changed in the nominal range from 66 to 6. The process of obtaining aerogels from sonogels and heat treatment at 500degreesC, in general, increases the mass-fractal dimension D, diminishes the characteristic length xi of the fractal structure, and shortens the fractal range at the micropore side for the formation of a secondary structured particle, apparently evolved from the original wet structure at a high resolution level. The overall mass-fractal dimension D of aerogels was evaluated as similar to2.4 and similar to2.5, as determined from SAXS and from pore-size distribution by nitrogen adsorption, respectively. The fine structure of the secondary particle developed in the obtaining of aerogels could be described as a surface-mass fractal, with the correlated surface and mass-fractal dimensions decreasing from similar to2.4 to similar to2.0 and from similar to2.7 to similar to2.5, respectively, as the aerogel bulk density increases from 0.25 (r(w)=66) up to 0.91 g/cm(3) (r(w)=6).

Estudo da competitividade em empresas industriais: o caso do arranjo produtivo local (APL) de Votuporanga

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Estudo de caso de avaliação dos ativos intangíveis utilizando o modelo de excelência de gestão do prêmio nacional da qualidade

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Desafios e oportunidades das incubadoras de empresas para a incorporação de estratégias ambientais

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

From sol to aerogel: a study of the nanostructural characteristics of TEOS derived sonogels

The nanostructural characteristics of acid-catalyzed sonogels are studied along the aging process at 60 degreesC in saturated conditions and after the CO, supercritical extraction (aerogel). The structural evolution was studied by means of small-angle X-ray scattering (SAXS) and UV-Visible absorption techniques. The sonogel exhibits a mass fractal structure in a length scale between zeta - 1/q(0) similar to 5.3 and a(1) similar to 1/q(m) similar to 0.22 nm, as the length scale probed by SAXS. The apparent mass fractal dimension lightly increases from 2.0 for fresh gel until 2.2 for 14 days aging in wet conditions. The UV absorption also increases with the aging time in wet conditions. Both observations are consistent with the syneresis process accompanying the polycondensation progress during aging in saturated conditions. For long aging times, the wet sonogels show a light transition from a mass to a surface fractal. in a very small interval of the length scale, developing an extremely rough surface with fractal dimension D-S similar to 2.9, the fractal characteristics of the sonogels practically do not change with the alcohol exchange. With the CO2 supercritical extraction (aerogel). The interval in the length scale in which the surface fractal is defined increases, while the surface fractal dimension diminishes to D-S similar to 2.5. The mass fractal characteristics are less apparent in the aerogels. (C) 2001 Elsevier B.V. B.V. All rights reserved.

Effect of the temperature on the TMOS sono-hydrolysis and a study of the structure of the resulting aged sonogels

The effect of temperature on the oxalic acid catalyzed sono-hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method. The activation energy of the process was measured as (24.5 +/- 0.8) kJ/mol in the temperature range between 10 and 50 degreesC. The structural characteristics of the resulting sonogels, after long period of aging in saturated conditions, were studied by means of small angle X-ray scattering. The structure can be described as formed by similar to2.7 nm mean size mass fractal-like aggregates (clusters) of primary silica particles of similar to0.3 nm mean size, all imbibed in a liquid phase. The average mass fractal dimension of the clusters was found to be 2.58. The primary particle density was estimated as 2.23 g/cm(3), in good agreement with the value frequently quoted for fused silica. The volume fraction of the clusters, in the saturated sonogels was estimated as about 28%. The moment in which the meniscus of the liquid phase penetrates into the clusters under rapid evaporation process has been detected by an inflection in the first derivative of the curve of weight loss in a simple thermogravimetric test. (C) 2003 Elsevier B.V. All rights reserved.

Structural evolution of aerogels prepared from TEOS sono-hydrolysis upon heat treatment up to 1100 degrees C

The structural evolution of aerogels prepared from TEOS sono-hydrolysis was studied as a function of the temperature of heat treatment up to 1100 degreesC by means of small angle X-ray scattering (SAXS) and density measurements. The mass fractal structure of the original wet sonogel (with scattering exponent alpha similar to 2.2) apparently transforms to a surface fractal structure in a length scale lesser than similar to1.5 nm, upon the process resulting in aerogel. Such a structural transformation is interpreted by the formation of new particles with characteristic dimension of similar to1.5 nm, with rough boundaries or electronic density fluctuations (or ultra-micropores) in their interior. The structural arrangement of these particles seem to preserve part of mass fractal characteristics of the original wet sonogel, now in a length scale greater than similar to1.5 nm. The electronic density heterogeneities in the particles start to be eliminated at around 800 degreesC and, at 900 degreesC, the particles become perfectly homogeneous, so the structure can be described as a porous structure with a porosity of similar to68% with similar to9.0 nm mean size pores and similar to4.3 nm mean size solid particles. Above 900 degreesC, a vigorous viscous flux sintering process sets in, eliminating most of the porosity and increasing rapidly the bulk density in an aerogel-glass transformation. (C) 2003 Elsevier B.V. All rights reserved.

Structural study of composites of aerosil fumed silica and tetraethoxysilane-derived sonogels

Composites of aerosil fumed silica and tetraethoxysilane-derived sonogel were prepared by changing the aerosil content between 0 and 30wt% with respect to the silica content in the original tetraethoxysilane (TEOS). The structural characteristics were studied by density and Vickers microhardness measurements and analyzed by means of small-angle X-ray scattering (SAXS). The structure of the composite aerosil/TEOS-derived sonogel can be described as inclusions of the aerosil particles embedded in the matrix of the TEOS-derived sonogel, forming an aerosil/matrix interfacial surface inside the composite. The weakening of the bonding of aerosil/matrix interface, as suggested by the reported decrease in microhardness, increases the fracture toughness of the composite. The additive effect of the aerosil particles on the structure of the sonogel accounts for the increase of the bulk density and reduction of the specific surface of the composite. Some internal structure associated with the microclusters making up the sonogels is apparent from systematic deviations from Porod's law found in the system with small aerosil contents. (c) 2005 Elsevier B.V. All rights reserved.

Structural study of aged saturated silica gels obtained from tetramethoxysilane sonohydrolysis with different water/tetramethoxysilane molar ratio

The structural characteristics of saturated silica sonogels were studied by means of small-angle x-ray scattering (SAXS) and thermogravimetric analysis (TG), after a long time of aging in saturated conditions. The sonogels were obtained by a sol-gel routine from ultrasound stimulated tetramethoxysilane (TMOS) hydrolysis carried out with the initial water/TMOS molar ratio (r) ranging from 2 to 10. The saturated sonogel structure can be described as composed by mass fractal-like aggregates (clusters) of primary silica particles, all imbibed in a liquid phase. The values of the mass fractal dimension (D) of the clusters was found all around 2.5, while the characteristic size of the clusters (ξ) was found generally increasing with r, going from approximately 2.3 nm (r = 2) to 4.5 nm (r = 10). The volume fraction of the clusters was estimated from the SAXS data. The results were compared to the values of weight loss fraction at the inflection point that has been found in the derivative of the TG curve, which should accounts for the instant in which the meniscus of the liquid phase penetrates into the clusters under a rapid evaporation process as in a TG test.

Danos morais: compensabilidade e quantificação

Pós-graduação em Direito - FCHS

Fatores facilitadores e inibidores no uso das comunidades de prática: um estudo de caso em uma empresa alemã

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Prospecção e monitoramento informacional no contexto da inteligência competitiva em microempresas do ramo óptico da região de Marília - SP

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Gestão do Conhecimento aplicada à redes de Fast-Food

The knowledge currently stands as one of the intangible assets of fundamental importance in the competitive market in which it is organizations. Knowledge management has become mandatory in the competitive environment of today, as a tool to systematize knowledge within the organization. Underscoring the importance of this management process, this thesis aimed to identify possible problems and generate recommendations for improving the performance of Knowledge Management. The objective was achieved by conducting the analysis of a case study of a fast-food franchise through a case study in the ground plan of all the franchise restaurants. For the preparation of the case study was required to respect a theoretical review on the introduction of the concepts of knowledge management in fast-food restaurants, in addition to reviewing the theory on the concepts of organizational learning and on Standardization. When finalizing the theoretical review and analyze the case study was proposed recommendations and highlight difficulties and good practice found in the analyzed organization, and procedures for demonstrating the success of knowledge management in organizations

Bioimperialismo e direito de propriedade intelectual: disputas pelo acesso ao cupuaçu e ao conhecimento tradicional

Pós-graduação em Ciências Sociais - FFC

Análise dos benefícios do edi para a competitividade logística das organizações

The concept of EDI -Electronic Data Interchange is normally used to determine integration technologies between companies. Logistically, this implies in the integrationbetween enterprises in supply chain, involving the electronic transmission of data and thus reducing human intervention in the process, still favoring organizational performance. This study investigated the main benefits of EDI for organizational competitiveness and its possible impacton improving logistics performance of four companies comprising large national networks. Through multiple case studieswere able to identify common features that the use of EDI can favorer these companies. The results were significant and assume that the use of this tool can add value to logistics, primarily through streamlining processes, inventory optimization, cost reduction and performance improvement potential. Thus, the use of Electronic Data Interchange as a strategic tool for logistics proved, through this study, an efficient alternative for business improvement and good practice, able to leverage competitive advantages not only for individual companies, but also for the entire supply chain.