Structural characteristics of P123-modified supercritical drying and hydrophobic ambient pressure drying aerogels

Supercritical drying (SCD) and hydrophobic ambient pressure drying (APD) aerogels were prepared from hydrolysis of tetraethoxysilane in solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (P123) in the range of composition below the threshold for the ordered mesoporous silica precipitation. APD was carried out after silylation of wet gels with trimethylchlorosilane (TMCS) or hexamethyldisilazane (HMDZ). The samples were analyzed by small-angle X-ray scattering and nitrogen adsorption. Wet gels are formed by mass-fractal domains, with fractal dimension close to 2, and larger pores superposing the pores belonging to the fractal structure in case of high P123 concentrations. Aerogels exhibit smaller-sized mass-fractal domains with larger mass-fractal dimension accounting for some porosity elimination on drying. The pore volume of the aerogels increases significantly with the P123 amount and it is even larger in the APD aerogels than in the SCD aerogels. © 2013 Elsevier B.V. All rights reserved.

Structure of Hydrophobic Ambient-Pressure-Dried Aerogels Prepared by Sonohydrolysis of Tetraethoxysilane with Additions of N,N-Dimethylformamide

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

Structural features of silicas prepared in n-heptane/water/ethanol/sodium dodecylsulfate microemulsions

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

Estudo da fotossensibilidade de materiais híbridos de sílica dopados com heteropoliânions de tungstênio

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Preparação e caracterização de sílicas mesoporosas

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

Propriedades nanoestruturais de géis de sílica preparados com adiçoes de surfactante anionico

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Preparação e caracterização estrutural de aerogéis de sílica com diferentes concentrações de F127

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

Propriedades estruturais de géis de sílica preparados com adições de dodecil sulfato de sódio

This work aims to study the structural characteristics of silica gels obtained from the acid hydrolysis of tetraethoxysilane (TEOS) in water solutions with different concentrations of sodium dodecyl sulfate (SDS). The structural characteristics were studied in stages ranging from the wet gel to the dry stages of the gels (aerogels and xerogels). Aerogels were obtained by ambient pressure drying (APD) after silylation process using trimethylchlorosilane (TMCS) as silylating agent. Xerogels were obtained by conventional evaporating the liquid phase from non silylated gels. The samples were characterized by nitrogen adsorption and small angle X-ray scattering (SAXS). The structure of the wet gels and of the aerogels prepared with the surfactant exhibited characteristics of mass-fractal structures with fractal dimension D in the range 2.1-2.2 for the wet gels and 2.3-2.4 for the aerogels. The characteristic size  of the fractal domain reduces while the size a0 of the primary silica particle composing the fractal structure increases with the drying of the gels, in a process in which share of the porosity is eliminated. Aerogels exhibited typical values for the specific surface of 900 m2g-1 and of 3.5 cm3.g-1 for the total pore volume. These values are correspondingly comparable to those of the aerogels prepared by supercritical drying, since the silylation process replaces hydrophilic –OH groups by hydrophobic –Si-R3 ones, inhibiting the porosity elimination on drying. The silica particle size also increases lightly with the silylation because the attachment of the –Si-R3 groups on the silica surface. The pore size distribution curves of the aerogels are similar for all samples exhibiting a maximum in around 40 nm, independent the concentration of surfactant. This suggests that the characteristic size of 40 nm is due to the association of surfactant micelles... (Complete abstract click electronic access below)

Características estruturais de aerogéis hidrofílicos e hidrofóbicos de sílica modificados com Dodecil Sulfato de Sódio

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Pressure-induced phase transitions in Zr-rich PbZr1-xTixO3 ceramics

A Raman study of structural changes in the Zr-rich PbZr1-x TixO3 (PZT) system under hydrostatic pressures up to 5.0 GPa is presented. We observe that externally applied pressure induces several phase transitions in PZT ceramics among phases with orthorhombic (Ao), rhombohedral low-temperature (RLT), and rhombohedral high-temperature (RHT) symmetries (all found in PZT at ambient pressure and room temperature). Each of the compositions investigated (0.02 ≤ x ≤ 0.14) exhibits a high-pressure phase with orthorhombic (OI′) symmetry. We further report a detailed study of the pressure dependence of Raman frequencies to elucidate the phase transitions and to provide a set of pressure coefficients for the high-pressure phases.

High-resolution thermal expansion measurements under helium-gas pressure

We report on the realization of a capacitive dilatometer, designed for high-resolution measurements of length changes of a material for temperatures 1.4K ≤ T ≤ 300K and hydrostatic pressure P ≤ 250MPa. Helium ( 4He) is used as a pressure-transmitting medium, ensuring hydrostatic-pressure conditions. Special emphasis has been given to guarantee, to a good approximation, constant-pressure conditions during temperature sweeps. The performance of the dilatometer is demonstrated by measurements of the coefficient of thermal expansion at pressures P ≃ 0.1MPa (ambient pressure) and 104MPa on a single crystal of azurite, Cu 3(CO 3) 2(OH) 2, a quasi-one-dimensional spin S = 1/2 Heisenberg antiferromagnet. The results indicate a strong effect of pressure on the magnetic interactions in this system. © 2012 American Institute of Physics.

Effect of pressure-assisted thermal annealing on the optical properties of ZnO thin films

ZnO thin films were prepared by the polymeric precursor method. The films were deposited on silicon substrates using the spin-coating technique, and were annealed at 330°C for 32h under pressure-assisted thermal annealing and under ambient pressure. Their structural and optical properties were characterized, and the phases formed were identified by X-ray diffraction. No secondary phase was detected. The ZnO thin films were also characterized by field-emission scanning electron microscopy, Fourier transform infrared spectroscopy, photoluminescence and ultraviolet emission intensity measurements. The effect of pressure on these thin films modifies the active defects that cause the recombination of deep level states located inside the band gap that emit yellow-green (575nm) and orange (645nm) photoluminescence. © 2012 John Wiley & Sons, Ltd.

Síntese de aerogéis e xerogéis de sílica com troca de solvente e secagem a pressão ambiente

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Treatment of polycarbonate by dielectric barrier discharge (DBD) at atmospheric pressure

Generally most plastic materials are intrinsically hydrophobic, low surface energy materials, and thus do not adhere well to other substances. Surface treatment of polymers by discharge plasmas is of great and increasing industrial application because it can uniformly modify the surface of sample without changing the material bulk properties and is environmentally friendly. The plasma processes that can be conducted under ambient pressure and temperature conditions have attracted special attention because of their easy implementation in industrial processing. Present work deals with surface modification of polycarbonate (PC) by a dielectric barrier discharge (DBD) at atmospheric pressure. The treatment was performed in a parallel plate reactor driven by a 60Hz power supply. The DBD plasmas at atmospheric pressure were generated in air and nitrogen. Material characterization was carried out by contact angle measurements, and X-ray photoelectron spectroscopy (XPS). The surface energy of the polymer surface was calculated from contact angle data by Owens-Wendt method using distilled water and diiodomethane as test liquids. The plasma-induced chemical modifications are associated with incorporation of polar oxygen and nitrogen containing groups on the polymer surface. Due to these surface modifications the DBD-treated polymers become more hydrophilic. Aging behavior of the treated samples revealed that the polymer surfaces were prone to hydrophobic recovery although they did not completely recover their original wetting properties.

Bioactive coating on titanium implants modified by Nd:YVO4 laser

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)