Crystalline δ–Bi2O3 is the best O2– ion conductor, but it is stable in a relatively narrow temperature range 729–825 °C only. Its very high ionic conductivity (1 S/cm at 750 °C) has motivated many researchers to look for a method to stabilize this fluorite-type structure to lower temperature. So far the successful strategies to achieve the stabilization of the delta phase have included doping (e.g. by rare-earth elements ) or synthesis in form of thin films . Our approach to reach the same goal is significantly different from those two strategies. It consists of two steps: i) preparation of the Bi2O3 glass (pure or only slightly doped) and ii) thermal nanocrystallization of the glass prepared in stage i). The advantages of such an approach are as follows: a) the final material is pure or nearly pure Bi2O3 and b) it can be prepared in bulk and is not limited to thin layers only. Our earlier experience with nanocrystallization of glassy analogs of some cathode materials for Li-ion batteries has shown that by the appropriate heat-treatment one can achieve a dramatic (even by a factor of 109) and irreversible enhancement of their electrical conductivity [3,4]. This effect is closely related to changes in the microstructure – namely to formation of nanoscale crystalline grains, with dimensions going down even to a few nanometers, confined in the glassy matrix.
In this research, we firstly obtained a pure Bi2O3 glass and then subjected it to an appropriate thermal treatment. Bismuth (III) oxide was melted at 1100 °C in a furnace and rapidly quenched. As a result, transparent orange glass with ca. 1 mm in thickness was obtained. Thermal events were observed by DTA measurements and the amorphousness of as-quenched material was confirmed by XRD. Crystallization and phase transitions taking place during heating and cooling were observed in-situ by temperature dependent XRD measurements. The microstructure of heat-treated samples was investigated by electron microscopy (both SEM and HRTEM). The total electrical conductivity of nanocrystallized samples as a function of temperature has been determined in preliminary measurements by impedance spectroscopy.
RESULTS AND DISCUSSION
It was observed by SEM that nanograins (20–40 nm) of δ–Bi2O3 phase were formed in glassy matrix (Fig. 1) upon heating to temperature within the 530–630 °C range, as confirmed by XRD. The phase remained stable after cooling down to room temperature even after ca 1 year of ageing. Heating to higher temperature led to formation of β–Bi2O3 phase, which remained stable down to room temperature. Preliminary electrical measurements showed an increase in the total conductivity at room temperature from 10–19 S/cm (for the initial glass) to 10–12 S/cm (for nanocrystalline samples). Analyses of Nyquist plots revealed the presence of a small arc at high frequencies that may be attributed to the conductivity of the interiors of nanocrystallites with δ–Bi2O3 phase.
Synthesis of δ–Bi2O3 phase stable at room temperature in form of nanocrystallites embedded in glassy matrix in bulk samples is an important result, from viewpoints of basic knowledge as well as applications. Further studies will be focused on syntheses of nanocrystallized samples with the highest possible ionic conductivity.
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