Abstract
In this work we studied the influence of microwaves in the pore structure of silica gels obtained via sol-gel polymerization process. We analised the feasibility of replacing conventional heating by microwave heating during drying. Structural parameters such as specific volume of pores (Vp), specific surface area (Sp), average pore size (rp) and the bulk and true densities were measured. X-ray diffraction confirmed the obtention of a completely amorphous structure for the gels. Some characteristics of silica gels were evaluated by thermogravimetric analyses (TGA), differential thermal analyses (DTA) and Fourier transform infrared spectroscopy (FTIR). We observed that microwave drying is a feasible process to yield controlled pore nanostructures. The gels obtained via microwave heating presented average pore size of 1.2 nm, specific surface area of 112 m2/g and specific volume of pores of 0.06 cm3/g. The gels obtained via conventional heating presented average pore size of 1.3 nm, specific surface area of 748 m2/g and specific volume of pores of 0.49 cm3/g.
Highlights
The microwave processing makes possible to heat both small and large pieces very rapidly and uniformly, to efficiently remove volatile constituents from thick sections
We observed that the bulk density is bigger for porous silica made via microwave heating
According to de Boer definition[15], the isotherm obtained via conventional heating is type I isotherm, being characteristic of a microporous solid and represent cylindrical pores opened in both extremities, in agreement with
Summary
The microwave processing makes possible to heat both small and large pieces very rapidly and uniformly, to efficiently remove volatile constituents (binders, moisture, etc) from thick sections. The absorption of microwave energy varies with the composition and structure of different phases, being possible the selective heating[1,2,3]. As a result of this internal and volumetric heating, smaller thermal gradients are created in microwave-processed materials than in materials processed by conventional heating, reducing cracking during drying[4]. Another advantage of microwave heating is that the treatment time can be considerably reduced, which in many cases represents a reduction in the energy consumption as well[4]
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