resumo
In this work, the crystallization process of a SiO2-3CaO center dot P2O5-MgO glass was studied by non-isothermal measurements using differential thermal analysis carried out at various heating rates. X-ray diffraction at room and high temperature was used to identify and follow the evolution of crystalline phases with temperature. The activation energy associated with glass transition, (E (g)), the activation energy for the crystallization of the primary crystalline phase (E (c)), and the Avrami exponent (n) were determined under non-isothermal conditions using different equations, namely from Kissinger, Matusita & Sakka, and Osawa. A complex crystallization process was observed with associated activation energies reflecting the change of behavior during in situ crystal precipitation. It was found that the crystallization process was affected by the fraction of crystallization, (x), giving rise to decreasing activation energy values, E (c)(x), with the increase of x. Values ranging from about 580 kJ mol(-1) for the lower crystallized volume fraction to about 480 kJ mol(-1) for volume fractions higher than 80 % were found. The Avrami exponents, calculated for the crystallization process at a constant heating rate of 10 A degrees C min(-1), increased with the crystallized fraction, from 1.6 to 2, indicating that the number of nucleant sites is temperature dependent and that crystals grow as near needle-like structures.
palavras-chave
DIFFERENTIAL THERMAL-ANALYSIS; SILICATE-PHOSPHATE GLASSES; CRYSTAL-GROWTH; ACTIVATION-ENERGY; TRANSITION TEMPERATURE; MECHANICAL-PROPERTIES; NACAPO4-SIO2 SYSTEM; CERAMICS; ALLOY; 45S5
categoria
Thermodynamics; Chemistry
autores
Davim, EJC; Senos, AMR; Fernandes, MHV
nossos autores
Ana Maria de Oliveira e Rocha Senos
Colaborador
Erika Judith Cardoso Rodrigues Davim
Bolseiro de investigação
Maria Helena Figueira Vaz Fernandes
Professor(a) Jubilado(a)Grupos
G2 - Materiais Fotónicos, Eletrónicos e Magnéticos
G5 - Materiais Biomiméticos, Biológicos e Vivos
agradecimentos
This work was financed by FEDER funds through the Operational Programme COMPETE and by FCT-Foundation for Science and Technology funds under the Grant SFRH/BD/48357/2008. We also acknowledge the program financing CICECO, Pest-C/CTM/LA0011/2011.