Electrical Conduction in Bismuth Glasses

6.1.3.1. Electrical Conduction in Bismuth Glasses

The plot of log ρ vs. 1/T is shown in Figure 6.1 for the glasses 1 to 4. It is seen that for glasses 1 and 3 containing nano partcles of bismuth, there are two linear curves, whereas for the glasses 2 and 4 containing no bismuth, there are simple linear plots over the entire temperature range. These results indicate that for the glasses 1 and 3, there are two conduction mechanisms operative in two different temperature ranges. By assuming an Arrhenius type of variation of resistivity as a function of tempera- ture, which is written as :

(6.1) the activation energies of these glasses are estimated in the two different temperation ranges, i.e. be-

ρ =ρ 0 exp(E/kT)

tween 20° - 200°C and 200° - 500°C, which are shows in Table - 6.3. It is evident from the

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3 10 /T[k ] –1

Figure 6.1 : Log ρ against 1/T for glasses 1-4 at f = 1 kHz.

table that the resistivity of the glasses 1 and 3 in the higher temperature range arises due to the move- ment of Na + ions through the glass matrix, which obviously requires a higher activation energy due to the 'diffusional' jump of Na + ions in the glass matrix [6].

Table 6.3 : Activation energies of conduction in glasses 1 to 4.

Glass No.

Activation Energy (eV) :

Activation Energy (eV) :

20°C - 200°C

200°C - 500°C

1.10 ± 0.01 In oxide glasses, it is known that if the ratio of the network former (i.e. SiO 2 ,B 2 O 3 , etc.) to

network modifier (i.e. Na 2 O, CaO, Bi 2 O 3 , etc.) is reduced, the concentration of non-bonding oxygen ions is increased. This loosens the structure that makes it congenial for Na+ ion jump, which involves lower activation energy. The slightly lower activation energies of the glasses 1 and 3 compared to those

233 of glasses 2 and 4 can be attributed to the fact that the ‘coherence of silica network’ in the former is

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reduced owing to the presence of Bi 2 O 3 in these glasses, which act as network modifier [7]. There are various cases possible for the ‘electrical conduction’ due to Na + ions in the higher temperature range, which can be discussed as follows :

1. Between Glasses 1 and 2

Both the contents of silica and soda are constant, but borax is increasing from 18 to 26 mole%, which stiffens the network structure (total network former = 64 + 18 = 82% in glass 1 and 90% in glass

2) making the Na + ion jump difficult and thereby the activation energy increases from 1.10 to 1.42 ev (see Table 6.3)

2. Between Glasses 1 and 3

The total network former is reduced from 82% to 55%, and thereby the activation energy de- creases from 1.10 ev to 0.90 ev due to less structural cohesion. Moreover, the bisbuth content is higher in glass 3 than that of glass 1, which explains the effect of bismuth in loosening the structure from the point of view of energy.

3. Between Glasses 1 and 4

They have the same activation energy, even if the ratio of network former/modifier is higher in the former, where the presence of bismuth weakens the structure more. So, the effect of bismuth is more prominently seen in this case.

4. Between Glasses 2 and 3

In this case, the difference of activation energy is the highest (from 1.42 ev to 0.90 ev), since the network former is substantially reduced as well as there is bismuth present in glass 3, which makes the ‘weakening’ of the structure maximum and hence the lowest activation energy.

5. Between Glasses 2 and 4

Here, the reduction in activation energy is from 1.42 ev to 1.10 ev due to the substantial reduction of network former from glass 2 to 4, but the absence of bismuth in glass 4 does not allow a reduction of activation energy further. So, the effect is clear.

Therefore, it is seen that the above glasses 1 to 4 containing bismuth are quite interesting for a detailed analysis for activation energy. In the lower temperature region, the conduction in glasses 1 and

3 is via ‘electron hopping’ between isolated islands of nano-metallic bismuth particles, and hence the activation energy is extremely low at 0.03 ev.

Both the DC and AC resistivity data obtained at different frequencies for glass 1 are shown in

Figure 6.2.

A change in slope for the DC resistivity curve is also observed at a temperature of 120°C.

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Figure 6.2 : Log ρ against 1/T for glass 1 at different frequencies : Higher Curve = DC, Other Curves = AC.