Cinética do processo de cristalização do La2(WO4)3 por técnicas in situ

Abstract

Understanding the kinetic phenomena involved in the crystallization process is essential in the acquisition of crystalline materials with controlled properties, sizes and shapes. This study comprises the kinetics of the crystallization process of lanthanum tungstate nanoparticles La2(WO4)3 induced by in situ heating in X-Ray Diffraction (XRD). The amorphous La2(WO4)3 nanoparticles were prepared by the basic method of direct exclusion. As a characteristic of the synthesis method, clusters of nanoparticles with an average size of 20 nm were observed. The thermal behavior of the amorphous nanoparticles indicated an exothermic peak referring to the phase transformation (crystallization) at 590 °C. The thermal agreements carried out at 550 and 600 °C for 2 hours showed that at 550 °C for 2 hours as non-ordering nanoparticles at long range, as expected by thermal analysis. The sample translated at 600 °C for 2 hours presented diffraction peaks referring to the crystalline phase of La2(WO4)3, and the refinement by the Rietveld method confirmed that the system is monophasic, with monoclinic structure, C2/c spatial group. Heat treatments carried out up to 900 °C to the monoclinic phase of La2(WO4)3 are stable. The micrographs of the nanoparticles treated at 600 °C for 2 hours clusters of nanoparticles with an average shape and size equal to that of the amorphous phase. The vibrational aspects by Raman spectroscopy active vibrational modes referring to WO4 tetrahedron geometry and external modes. The logarithmic curves of the crystallized fractions as a function of the time of the isothermal diffractograms obtain at 570, 575, 580 and 585 °C exhibited two linear regions, which through the equation of the JMAK theory students values of the exponent of Avrami equal to 1,72 and 0,52, showing two main mechanisms in the crystallization kinetics of La2(WO4)3 nanoparticles, nucleation and diffusion. The activation energy of linear regions 1 and 2 were 270,91 and 100,15 KJ/mol, respectively, demonstrating the need for high configurations to start the crystallization process of crystalline La2(WO4)3 nanoparticles. The size evolution average of the crystallites as a function of the experiment time of the lanthanum nanoparticles at 585 °C exhibited different kinetic behavior in the two linear regions.