ISRN Materials ScienceVolume 2013 (2013), Article ID 126805, 9 pageshttp://dx.doi.org/10.1155/2013/126805

Research Article

Role of ZnO in Dc Electrical Conductivity of Lithium Bismuthate Glasses

Shashidhar Bale and Syed Rahman

Department of Physics, Osmania University, Hyderabad 500 007, India

Received 18 June 2013; Accepted 7 August 2013

Academic Editors: T. Pasinszki, M. Sliwa, and A. Vergara

Copyright © 2013 Shashidhar Bale and Syed Rahman. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Glasses of various compositions belonging to the Bi₂O₃-B₂O₃-ZnO-Li₂O quaternary system were prepared using melt quench technique. Dc electric measurements were done on the samples, and activation energies are determined. Arrhenius plots showed straight line behaviour. It is observed that the conductivity of the samples increased with temperature and also with Li₂O content, whereas the activation energy decreased with Li₂O content. The isothermal plots for constant ZnO and constant Bi₂O₃ glasses revealed that the conduction in these glasses is due to lithium ions only. The isothermal plots for constant lithium containing glasses varied nonlinearly with two maxima, which is attributed to mixed former effect. The variation is explained based on Anderson-Stuart model.

1. Introduction

Glasses and glass-ceramics are technologically important materials when compared with their crystalline counterparts. These materials show superior thermomechanical, electrical and other physicochemical properties, which make them suitable for use in vacuum, high-voltage, and biomedical applications [1].

Conventional glass formers such as P₂O₅ and TeO₂ containing transitional metal ions have been studied earlier [2–5]. In recent years, bismuth-based glasses have attracted the attention of researchers due to technological applications, useful physical properties and among them bismuth borates are of interest [6–9]. The introduction of alkali ions into these glasses exhibits high electrical conductivity and can be used as solid electrolytes in high energy density batteries, sensors, and so forth [10]. Further, transition metal ion glasses based on unconventional glass network formers such as Bi₂O₃ and PbO have been reported [11–14]. Especially, zinc-oxide based glasses/ceramics have special applications in the area of varistor designing, dielectric layers, barrier ribs in plasma display panels, and so forth [15, 16]. In the literature, it is reported that Bi₂O₃ occupies both network forming and network modifying positions. Therefore, the physical properties of such glasses exhibit discontinuous changes when the structural role of the cation changes [17, 18]. Especially, efforts are made to enhance the conductivity in lithium ion conducting glasses in this way [19, 20]. There have been two main approaches to improve the conductivity of the glass. The first approach is to dissolve alkali compounds such as Li₂O, LiCl, and Na₂O into an oxide glass. The second strategy is to combine the network forming oxides, which is known as mixed former effect, although the reason for this is not yet well understood.

The purpose of the present paper is to study the variation of dc electrical conductivity, activation energy in Bi₂O₃-B₂O₃-ZnO-Li₂O glasses. Bi₂O₃ and ZnO play the roles of network modifier and network former depending on the composition and their content. The role played by the ZnO in the dc electrical conductivity, in the entire composition range, is of interest. The compositional dependence of the dc electrical conductivity and activation energy has been compared with that of other traditional glasses.

2. Experimental

2.1. Chemicals

In the present investigation, the glasses are prepared by using high purity chemicals Li₂CO₃, ZnO (both GR grade Merck), H₃BO₃ (GR grade), and Bi₂O₃ (99.8% purity, Fluka) as starting materials.

2.2. Glass Preparation

The quaternary glasses with composition Bi₂O₃-B₂O₃-ZnO-Li₂O are prepared by conventional melt quench technique. The appropriate ratios of the mixture of the chemicals taken are designated and are given in Table 1. These mixtures are thoroughly grounded in an agate mortar for uniformity. Calcination is done in porcelain crucibles at 450°C for 1 hr, and the mixture is then melted at 1100–1200°C depending on the glass composition. The liquids were swirled frequently to ensure homogeneous mixture. The clear liquid (free of bubbles) was quickly casted in a stainless steel mould kept at 200°C and pressed with another steel disc maintained at same temperature. Glass discs of  mm and 1 mm thickness were obtained. The colour of the glasses varied from light brown to dark brown with increase in Bi₂O₃ content. The two surfaces of the sample were grounded parallel and polished with cerium oxide on a leather surface until a fine glassy finish is obtained.

Table 1: Glass composition and glass name.

Glass composition
Glass name

55Bi₂O₃-10ZnO-15B₂O₃-20Li₂O
10Z55Bi

60Bi₂O₃-10ZnO-15B₂O₃-15Li₂O
10Z60Bi

65Bi₂O₃-10ZnO-15B₂O₃-10Li₂O
10Z65Bi

70Bi₂O₃-10ZnO-15B₂O₃-5Li₂O
10Z70Bi

50Bi₂O₃-15ZnO-15B₂O₃-20Li₂O
15Z50Bi

55Bi₂O₃-15ZnO-15B₂O₃-15Li₂O
15Z55Bi

60Bi₂O₃-15ZnO-15B₂O₃-10Li₂O
15Z60Bi

65Bi₂O₃-15ZnO-15B₂O₃-5Li₂O
15Z65Bi

45Bi₂O₃-20ZnO-15B₂O₃-20Li₂O
20Z45Bi

50Bi₂O₃-20ZnO-15B₂O₃-15Li₂O
20Z50Bi

55Bi₂O₃-20ZnO-15B₂O₃-10Li₂O
20Z55Bi

60Bi₂O₃-20ZnO-15B₂O₃-5Li₂O
20Z60Bi

45Bi₂O₃-25ZnO-15B₂O₃-15Li₂O
25Z45Bi

50Bi₂O₃-25ZnO-15B₂O₃-10Li₂O
25Z50Bi

55Bi₂O₃-25ZnO-15B₂O₃-5Li₂O
25Z55Bi

45Bi₂O₃-30ZnO-15B₂O₃-10Li₂O
30Zn45Bi

50Bi₂O₃-30ZnO-15B₂O₃-5Li₂O
30Zn50Bi

40Bi₂O₃-35ZnO-15B₂O₃-10Li₂O
35Z40Bi

45Bi₂O₃-35ZnO-15B₂O₃-5Li₂O
35Z45Bi

40Bi₂O₃-40ZnO-15B₂O₃-5Li₂O
40Z40Bi

2.3. Glass Characterization

The glass samples were characterized by X-ray diffraction using a PANalytical X-pert PRO model with Cu-K Alpha radiation ( Å).

Dc electrical conductivity measurements were made on the samples by usual technique of two electrode method. Silver paste was painted on the polished circular disc surfaces of the samples, and good ohmic contacts were found. The glassy sample (in the shape of a disc pellet) sandwiched between blocking silver electrodes is loaded in a cylindrical furnace using a spring. The current through the sample is recorded as a function of temperature using a Keithley electrometer model 614. The dc electrical conductivity was measured from 200°C up to below glass transition temperature of the respective glass sample. The temperature of the sample was recorded with a chromel-alumel thermocouple kept in close thermal contact with the sample surface.

3. Results and Discussion

3.1. X-Ray Diffraction Studies

The X-ray diffraction patterns of the present glasses reveal the amorphous nature and the absence of crystalline characteristics.

3.2. Dc Electrical Conductivity Studies

In the present study, 20 glass samples are prepared as listed in Table 1. In all the samples, B₂O₃ is kept constant at 15 mol%. The conductivity analysis is done by classifying the samples as constant ZnO, constant Bi₂O₃, and constant Li₂O glasses.

The reciprocal temperature dependence of the dc conductivity of ()Bi₂O₃-10ZnO-15B₂O₃-xLi₂O, ()Bi₂O₃-15ZnO-15B₂O₃-xLi₂O, ()Bi₂O₃-20ZnO-15B₂O₃-xLi₂O, ()Bi₂O₃-25ZnO-15B₂O₃-xLi₂O, and ()Bi₂O₃-35ZnO-15B₂O₃-xLi₂O glasses (constant ZnO containing glasses) is shown in Figures 1(a), 1(b), 1(c), 1(d), and 1(e), respectively. Similarly, the conductivity plots of 45Bi₂O₃-()ZnO-15B₂O₃-xLi₂O, 50Bi₂O₃-()ZnO-15B₂O₃-xLi₂O, 55Bi₂O₃-()ZnO-15B₂O₃-xLi₂O, and 60Bi₂O₃-()ZnO-15B₂O₃-xLi₂O glasses (constant Bi₂O₃ containing glasses) are given in Figures 2(a), 2(b), 2(c), and 2(d). It can be observed from the figures that the conductivity plots are straight lines. The Dc electrical conductivity follows Arrhenius relation: where is the dc activation energy for electrical conduction, is the Boltzmann’s constant, is the absolute temperature, and is the preexponential factor. It is observed that the dc electrical conductivity for all the samples increases with temperature in the range studied. These glasses possess electrical conductivity () from 1 × 10⁻¹⁰ to 8 × 10⁻⁷ (Ω·cm)⁻¹ at temperatures from 275 to 450°C. The activation energy and the pre-exponential factor were obtained from the slope and the intercept of the least square straight line fit of the conductivity plot. The values of and are presented in Table 2. The activation energy for conduction of the present glasses varies from 1.04 to 1.55 eV. Figures 3(a), 3(b), and 3(c) show the variation of conductivity and activation energy as a function of lithium oxide content in ()Bi₂O₃-10ZnO-15B₂O₃-xLi₂O, ()Bi₂O₃-15ZnO-15B₂O₃-xLi₂O, and ()Bi₂O₃-20ZnO-15B₂O₃-xLi₂O glasses. It is observed from Figure 3 that with the increase in lithium oxide content the conductivity increases and activation energy decreases. Other glass series also showed similar behaviour.

Table 2: Conductivity at 400°C, activation energy, pre-exponential factor , and Li ion concentration () of present glasses.

Glass name
400°C (Ω·cm)⁻¹
(eV)

 ×10²¹/cc

10Z55Bi

1.29
2.51
4.84

10Z60Bi

1.31
2.12
3.46

10Z65Bi

1.32
2.08
2.25

10Z70Bi

1.35
1.20
1.07

15Z50Bi

1.26
3.54
5.02

15Z55Bi

1.33
3.04
3.63

15Z60Bi

1.44
2.57
2.33

15Z65Bi

1.55
3.37
1.13

20Z45Bi

1.13
3.97
5.20

20Z50Bi

1.15
3.18
3.80

20Z55Bi

1.32
1.92
2.45

20Z60Bi

1.45
0.42
1.17

25Z45Bi

1.22
1.95
3.96

25Z50Bi

1.24
1.53
2.55

25Z55Bi

1.28
1.76
1.24

30Zn45Bi

1.23
3.03
2.64

30Zn50Bi

1.25
2.15
1.30

35Z40Bi

1.15
1.71
2.80

35Z45Bi

1.04
0.95
1.34

40Z40Bi

1.11
0.36
1.49

(a)

(b)

(c)

(d)

(e)

Figure 1: The reciprocal temperature dependence of the dc conductivity of constant ZnO glasses.

(a)

(b)

(c)

(d)

Figure 2: The reciprocal temperature dependence of the dc conductivity of constant Bi₂O₃ glasses.

(a)

(b)

(c)

Figure 3: Variation of dc electrical conductivity and activation energy.

According to the mechanism of ion transport, the conduction in lithium oxide glasses is due to successive jumping of Li⁺ ions from one nonbridging oxygen to another [27]. Therefore, Li⁺ concentration and non-bridging oxygen number can both affect the glass conductivity. The calculated data of Li⁺ concentration in the present glass system (Table 2) increases with the increase in Li₂O content. However, it has been found that within the composition range of present study, as the Li₂O content is increased, the solubility of Li⁺ ion acting as a charge compensator is exceeded, and thus some of the relatively stronger Bi–O and Zn–O bonds in the glass network are replaced by weak ionic Li⁺–O⁻ bonds which manifest in the decrease of the activation energy and the increase in conductivity.

Similarly, the reciprocal temperature dependence of the dc conductivity of the ()Bi₂O₃-xZnO15B₂O₃-5Li₂O, ()Bi₂O₃-xZnO-15B₂O₃-10Li₂O, ()Bi₂O₃-xZnO-15B₂O₃-15Li₂O, and ()Bi₂O₃-xZnO-15B₂O₃-20Li₂O glasses containing constant Li₂O content is shown in Figures 4(a), 4(b), 4(c), and 4(d). The conductivity of these glasses also follows Arrhenius relation given in (1). It is found that the straight lines are almost overlapping, and there is not much change in conductivity with change in ZnO content. The activation energy and for the above glasses were obtained from the least square straight line fits of the conductivity plots. Table 2 lists the conductivity at 400°C, activation energy, and pre-exponential factor of all the glass samples under study. From Figure 4, it is observed that the conductivity of these glasses increases with increase in the temperature. At a given temperature, the conductivity of these glasses varies slightly as the content of ZnO is increased from 10 to 40 mol%.

(a)

(b)

(c)

(d)

Figure 4: The reciprocal temperature dependence of the dc conductivity of constant Bi₂O₃ glasses.

The isothermal conductivity plots of ()Bi₂O₃-10ZnO-15B₂O₃-xLi₂O and ()Bi₂O₃-15ZnO-15B₂O₃-xLi₂O, and 50Bi₂O₃-()ZnO-15B₂O₃-xLi₂O glasses as a function of Li₂O at 350°C, 375°C, 400°C, and 425°C are shown in Figures 5(a), 5(b), and 6, respectively. It is clear from the isothermal plots that the conductivity of the above glasses increases linearly with increase in Li₂O content. Therefore, it is understood that the conductivity in these glasses is due to lithium ions.

(a)

(b)

Figure 5: The isothermal conductivity plots of constant ZnO glasses.

Figure 6: The isothermal conductivity plots of constant Bi₂O₃ glasses.

Similarly, the conductivity isotherms of ()-Bi₂O₃-xZnO-15B₂O₃-5Li₂O and ()Bi₂O₃-xZnO-15B₂O₃-10Li₂O glasses (constant Li₂O glasses) as a function of ZnO content at 350°C, 375°C, 400°C, and 425°C are presented in Figures 7(a) and 7(b). The conductivity in these glasses varies nonlinearly as a function of ZnO content, which show the typical mixed former behaviour with two maxima. The maximum in conductivity is observed at 15 and 30 mole% of ZnO. Similar results were reported in NaI-Na₂O-V₂O₅-B₂O₃ glasses [28], Li₂O-SeO₂-B₂O₃ glasses [29], and Li₂O-B₂O₃-Al₂O₃ glasses [30]. It can be observed from Figure 7(a) that the maxima at 15 mol% of ZnO increases and maxima at 30 mol% remains almost constant when temperature increases from 350 to 425°C. It is assumed that at 15 mol%, ZnO is taking network forming positions (ZnO₄ units) which affect the conductivity.

(a)

(b)

Figure 7: The isothermal conductivity plots of constant Li₂O glasses.

Chowdari and Rong [21] reported the conductivity of 25Bi₂O₃-37.5B₂O₃-37.5Li₂O glass at 200°C as 9.07 × 10⁻⁸ (Ω·cm)⁻¹. In the present glass system when ZnO is replacing Bi₂O₃ of Bi₂O₃-B₂O₃-Li₂O glasses, it was observed that the conductivity decreases by an order of two. Since lithium oxide and B₂O₃ are kept constant, the lithium ion concentration is not expected to vary; the decrease in conductivity in the present glass system may be due to the decrease in the mobility of the lithium ions. The macroscopic explanation for the variation in the conductivity is given on the basis of Anderson and Stuart model [31]. Until now this model has been applied to the traditional ionic glasses such as sodium silicates [32] and sodium borates [33] and sodium thioborates [34] where the glass former cations are light metals. In present study, the glass former cations Bi₂O₃ are highly polarizable. Thus, the presence of a polarization energy is expected to be present in the activation energy term in addition to the binding energy and strain energy contribution of Anderson-Stuart model. Thus, the macroscopic explanation for the mixed former effect given on the basis of Anderson-Stuart model, as one of the glass former ions, is substituted by another network former ion; the average interionic bond distance becomes larger or smaller according to whether the substituting ion is larger or smaller. In the present study, zinc being slightly smaller in size than bismuth, the substitution of bismuth by zinc will decrease the inter-ionic bond distance, and ZnO plays the role of network former. Therefore, the glass structure becomes tight, and hence the conductivity decreases after 15 mole% of ZnO. The conductivity of different glasses is compared with that of the present glass system and is presented in Table 3.

Table 3: Comparison of conductivity data in various glasses.

Glass composition
Temp. (°C)
(Ω·cm)⁻¹
(eV)
Reference

25Bi₂O₃-37.5B₂O₃-37.5Li₂O
200

0.881
[21]

20Li₂O-80(PbO-Bi₂O₃)
227

1.31
[22]

30Li₂O-20BaO-50Bi₂O₃
227

1.09
[23]

20Li₂O-30CdO-50P₂O₅
RT

0.69
[24]

20Li₂O-10Bi₂O₃-70B₂O₃
200

1.05
[25]

20Li₂O-80Bi₂O₃
200

[26]

55Bi₂O₃-20ZnO-15B₂O₃-10Li₂O
400

1.32
Present work

60Bi₂O₃-20ZnO-15B₂O₃-5Li₂O
400

1.45
//

40Bi₂O₃-40ZnO-15B₂O₃-5Li₂O
400

1.11
//

65Bi₂O₃-10ZnO-15B₂O₃-10Li₂O
400

1.32
//

4. Conclusions

The conductivity of the present glasses increases with increase in temperature. The conductivity plots are straight lines and follows Arrhenius relation with temperature. The activation energy and pre-exponential factor were determined.

The conductivity of the present glasses increases and the activation energy decreases with Li₂O content for constant ZnO and constant Bi₂O₃ containing glasses.

The conductivity in constant Li₂O containing glasses varies non-linearly as a function of ZnO/Bi₂O₃ content, which shows the typical mixed former behaviour with two maxima.

When comparing the conductivity of Bi₂O₃-B₂O₃-Li₂O glasses, the conductivity in the present glasses (with by incorporation of ZnO) decreases by an order of two, which was attributed to the network forming character of ZnO.

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