NaNbO3 crystals dispersed in a B2O3 glass matrix - Structural characteristics versus electrical and dielectrical properties

Sodium niobate (NaNbO3) is a crystal with a perovskite structure that exhibits, at room temperature, an anti ferroelectric behaviour. It is a very interesting material due to the several phase transitions that it presents as a function of the temperature (ferroelectric-antiferroelectric-paraelectric). Thus, the preparation of glass-ceramics containing NaNbO3 Crystals is scientifically and technologically important. Besides, there is actually few works available about the preparation of NaNbO3 crystals embedded in a glass matrix. The present work reports the preparation process and the study of glass and glass-ceramics in the B2O3-NaNbO3 system. The glass with the molar composition 60B(2)O(3)-30Na(2)O-10Nb(2)O(5) (mol%) was prepared by the melt-quenching method. Sodium niobate (NaNbO3) crystallites were precipitated through a controlled heat-treatment (HT) process. NaNbO3 crystallites were detected by X-ray diffraction (XRD) in the samples treated above 500 degrees C. The treatments above 600 degrees C favour also the formation of Na2B4O7 and Nb2O5 crystalline phases. Scanning electron microscopy (SEM) reveals that the crystallization occurs in volume and that the number of particles increases with the rise in HT temperature. The number of network modifier ions (Na+ and Nb5+) in the glass network is the main factor in the dc and ac conductivity behaviours. The dielectric constant (epsilon') value increases with the increase of the volume ratio between the particles and the glass matrix. The sample heat-treated at 550 degrees C shows two thermally stimulated depolarization current (TSDC) peaks. The high temperature peak can be related to the presence of NaNbO3 particles. (c) 2008 Elsevier Masson SAS. All rights reserved.