Role of ZnO in Dc Electrical Conductivity of Lithium Bismuthate Glasses
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.
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 .
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 . 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.
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.
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.
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 . 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%.
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.
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 , Li2O-SeO2-B2O3 glasses , and Li2O-B2O3-Al2O3 glasses . 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.
Chowdari and Rong  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 . Until now this model has been applied to the traditional ionic glasses such as sodium silicates  and sodium borates  and sodium thioborates  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.
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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