Structural studies of niquel oxide xerogels. / Estudos estruturais de xerogéis de óxido de níquel.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2000

RESUMO

Structural properties of several xerogels obtained by the sol-gel procedure and heat treated at different temperatures between 150 and 900ºC were studied. The initial systems were dry gels prepared from liquid solutions composed of nickel chloride, buthanol, water and acetic acid. During the different isothermal treatments, volatile species leave the inicial material and structural transformations occur. The resulting porous materials (xerogels) were studied by means of several structural techniques: small angle X ray scattering (SAXS), X ray absorption spectroscopy (XANES and EXAFS), X ray diffraction (XRD) and scanning electron microscopy. These studies were complemented with chemical analysis (infrared spectroscopy), differential thermal analisis, mass loss and density measurements. The experimental results demonstrated that the porous structure of the starting material, based on nickel chloride, transforms in a also porous biphasic structure composed of nickel chloride and nickel oxide, respectively, rich fase. The volume fraction occupied by the nickel oxide rich fase of the final material increases with temperature of heat treatment. For temperatures above 500ºC, practically all the sample volume transforms and is occupied by the NiO phase. It was demonstrated that the porous structure of all materials, studied by SAXS, is composed of nanopores (sizes ranging from 10 to 150Å ) and mesopores (larger than 150Å), their fraction depending on the heat treatment temperature. Particularly, nanopores are only present in samples treated below 500ºC. The average local order close to nickel atoms, determined by EXAFS, corresponds to the expected one for biphasic systems whose volume fractions depend on treatment temperature, in agreement with XRD results. The structural properties of the several materials obtained after different heat treatments were studied as functions of preparation conditions of precursor solutions. In a first series of experiments, acetic acid content in the solution was varied. It was demonstrated that higher acetic acid content promotes the formation of more compact structures. In a second series of experiments, water content was varied. It was concluded that water content only affects the structure of the materials heat treated at temperatures below 500ºC.

ASSUNTO(S)

phase system drx oxide scattering fase sistema xerogel angle exafs small lnls sol gel transition bifásico xrd sas x-ray transição xafs solution saxs sol-gel níquel byphasic óxido espalhamento

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