DC Conduction Data of Nano Particles

6.2.3. DC Conduction Data of Nano Particles

Although log σT against 1/T plots are normally shown to elucidate the 'small polaron' hopping conduction [20], log σ against 1/T plots are presented here, as has been done by various other workers [17]. Such plots are shown in Figure 6.12 in the temperature range 373 and 1000°K. There is some scatter in the experimental results, but they generally give straight lines. No appreciable time-dependent effects were observed during the conductivity measurements, thereby establishing the fat that the con-

duction is mainly electronic. The activation energy (E a ), calculated from the slopes of these straight lines, varies from 0.88 ev for the as-annealed glass to 0.60 ev for the basalt glass heat-treated at 900°C with a particle size of 7.0 nm. For a 2BaO-3B 2 O 3 glass contining about 10% Fe 2 O 3 , the activation energy was found to be 0.93 ev [18, 19]. Therefore, these data could be considered to be consistent with the ‘small polaron’ hopping between isolated Fe 2+ and Fe 3+ ions within the basalt glass for the as- annealed glass.

ELECTRICAL PROPERTIES

245 An abrupt change in both the conductivity and activation energy is observed as a function of

heat-treatment at higher temperature. The results are shown in Figure 6.13, in which log σ at 473°K (i.e. 200°C) and E a are plotted against heat-treatment temperature. It is noted that the abrupt changes take place around 700°C, which could be ascribed to the formation of nano particles of magnetite at this temperature. These data could be related to those described in the sections 5.4 and 5.5.

6.2.3.1. Correlation between Electronic Conduction and Magnetic Data

The value of saturation magnetization (M S ) at 270°K, and the quadrupole splitting and the iso- mer shift, showed similar changes around the heat-treatment around 700°C. These data have been inter- preted as the change of symmetry of the Fe ions. The remarkable ‘superparamagnetic’ behaviour of this 700°C sample due to smaller size of the nano particles (5.5 nm) of magneite is quite noteworthy. The increase of M S between 600 and 700°C could be associated with the increase of the ‘symmetry’ of the Fe ions. This has been revealed by the decrease of the ‘quadrupole splitting’ and the increase of ‘isomer shift’ in this range of heat-treatment temperature, i.e. within this nano range of magnetite particles, as also shown in the section 5.5 (see Figure 5.12).

600 700 800 900 T (°C)

Figure 6.13 : The DC conductivity at 473°K (filled circle) and activation energy (filled square) against heat-treatment temperature.

This effect is not so pronounced between 700 and 900°C, as explained by the cation redistribu- tion process of Neel [25], as the nano particles of magnetite grow in size from 5.5 nm to 7.0 nm. Moreover, the sharp decrease of activation energy after 700°C towards higher heat-treatment tempera- ture might indicate that it is easier for the small polaron hopping to take place between two sites within the ‘nano crystal’ than within a disordered system like glass.

A structural definition can be given to the DC conductivity data in correlation with the magnetic data as follows : Below 700°C, the Fe 2+ and Fe 3+ ions are mainly in ‘isolated octahedral sites’ in the glass matrix, and the conduction is via electron or small polaron hopping between these ion sites. The initial nucleation of magnetite in the region of 600 and 650°C removes some of the Fe 3+ ions from the glassy phase, and they are incorporated instead into the nano-crystalline particles of magnetite, where they are mainly in ‘tetrahedral coordination’. Thus, some of the Fe 3+ ions are in different structural coordination, and there are fewer number of “equivalent” Fe 2+ and Fe 3+ sites available for small polaron hopping conduction.

NANO MATERIALS

However, as the crystallization proceeds with increasing heat-treatment temperature, more and more of the Fe 2+ and Fe 3+ sites are created within the nano particles of magnetite, where the ratio of Fe 2+ and Fe 3+ ions in similar ‘octahedral’ coordination is optimized, and hence the conductivity will increase. The implication is that the ‘easy conduction path’ is via nano crystals of magnetite. Therefore, it should

be noted that the maximum amount of magnetite available for precipitation from the basalt glass is about

2 mole%. However, the magnetite is a good n-type semiconductor with a very low activation energy (≈ 0.0015 ev) and even a relatively small volume fraction of nano crystalline magnetite is just sufficient to cause a substantial change in the conductivity. Furthermore, the increase in conductivity is not simply due to a decrease in the activation energy. The ‘intercept’ of the straight lines of Figure 6.12 also changes, which is particularly noticeable in the sample heat-treated at 900°C. This is an additional evidence of a change of the ‘conduction path’.