Grain-size effects on the structural, electrical properties and ferroelectric behaviour of barium titanate-based glass–ceramic nano-composite

Grain-size effects on the structural and electrical properties as well as ferroelectric behaviour of 10BaTiO3–70V2O5–20Bi2O3 glass–ceramic nano-composite have been studied by scanning electron micrographs (SEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), dc conductivity (σ) and dielectric (ε) measurements over a wide temperature range. The present glass has been transformed into glass–ceramic nano-composite by annealing at temperatures close to crystallization temperature (Tcr). The XRD and SEM observations have shown that by heat treating at Tcr, the sample under study undergoes structural changes: from amorphous to partly crystalline for 1 and 8 h and to colossal crystallization for 24 h. After heat treated at Tcr for 1 and 8 h, the samples under load consist of small nano-crystallites (average size ca. 20–50 nm) embedded in glassy matrix. However, when the glass heat treated at Tcr for 24 h, the microstructure of the sample changes considerably. It is found that the glass–ceramic nano-composite obtained by heat treated at Tcr for 1 and 8 h exhibit giant improvement of electrical conductivity that is up to four order of magnitude. The electrical conductivity increases with increasing grain-size. The major role in the conductivity enhancement of this glass–ceramic nano-composite is played by the developed interfacial regions “conduction tissue” between crystalline and amorphous phases, in which the concentration of V4+–V5+ pairs responsible for electron hopping, is higher than inside the glassy matrix. The heat treated at Tcr for 24 h leads to decrease of the electronic conductivity. This phenomena lead to disappearance of most “conduction tissue” for electrons and substantial reduction of electronic conductivity. The experimental results were discussed in terms of a model proposed in this contribution which is based on a “core–shell” concept. The glass heat-treated at different times (1, 8 and 24 h) exhibited broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature. The Curie temperature (Tc), corresponding to ferroelectric phase transition increases with increasing grain-size. The observation of the glass–ceramic nano-composite being studied here can be used to control BaTiO3 grain-size and hence transition temperature by proper adjustment of annealing times.