Thermal behavior of gadolinium-silica gels prepared via sol–gel route combined to γ-irradiation process

Abstract

Gadolinium-silica (Gd-silica) gels were prepared via sol–gel route combined to γ-irradiation (0–30 kGy), starting from Tetra-Ethyl-Orthosilicate (Si(OCH2CH3)4, TEOS) and gadolinium nitrate Gd(NO3).6H2O used as raw materials. The phase transformation and morphology of the gels derived from both precursors were characterized by XRD and TEM analysis. Differential scanning calorimetry (DSC) under non-isothermal conditions was used to determine the crystallization activation energy (E c) at different γ-doses (0, 8, 15, 25 and 30 kGy), using the methods suggested by Kissinger, Moynihan and Ozawa. XRD analysis of the samples calcined up to 1100°C for 2 h reveals the formation of gadolinium silicate (Gd4.67 (SiO4)3O) apatite crystalline phase with the presence of small traces of gadolinium pyrosilicate (Gd2Si2O7), the average particle sizes about 50 nm. TEM observations show spherical Gd-silicate nanoparticles homogeneously dispersed in silica matrix, the mean diameter size is about 40 nm. From DSC analysis, it was observed that the onset crystallization temperature (T x) and peak crystallization temperature (T c) both increase with increasing γ-dose. According to Kissinger, Moynihan and Ozawa models, the crystallization activation energies for Gd-silicate phase formation were found to be 465.2, 474.5 and 465.7 kJ mol−1, respectively for unirradiated samples and 429.1, 447 and 438 kJ mol−1 when the γ-irradiation reaches 30 kGy respectively. Results revealed that the values of E c decreased when the γ-dose increases and attains the minimum value at the dose of 15 kGy and then increase at higher doses. The crystallization rate factor (K) was also seen to decrease with increasing of γ-dose. Gamma irradiation increases the crystallization peak temperature (T c). The crystallization activation energy (E c) estimated from the three models is in good agreement and much well with those reported for silicate systems. Gamma irradiation decreases the E c and crystallization rate factor (K). These preliminary results can provide useful technical information for a better understanding and control of the crystallization process of this type of nanomaterial.