ELECTRICAL CONDUCTIVITY STUDY OF ZINC OXIDE DOPED LEAD-BORATE GLASSES

Transcription

1 ELECTRICAL CONDUCTIVITY STUDY OF ZINC OXIDE DOPED LEAD-BORATE GLASSES S.G.MOTKE Department Of Physics, Phulsing Naik Mahavidyalaya, Pusad Dist: Yavatmal (M.S.) INDIA (Received : ; Revised : ; Accepted : ) Abstract Lead borate (x PbO- (100-x) B 2 O 3 ), where x = 35.20, 48.28, 59.11, 65.19, series I and ZnO doped Lead borate (x PbO ZnO-(100-x-11.11)B 2 O 3 ) where x=33.10, 46.36, 57.20, 63.19, series II glasses were prepared from the melts in appropriate proportions of PbO 2, H 3 BO 3 and (11.11 mol%) ZnO mixture in the temperature range K. The present paper deals with the dc-electrical conductivity of some Zinc free and Zinc containing lead borate glasses system in the temperature range from 313K to 573K. The dc conductivity of the samples in both the cases was found temperature and composition dependents. All the samples indicate a negative temperature coefficient as well as mixed type conduction. Keywords Activation Energy, DC Conductivity, Small Polaron Hopping Conduction, Polaron Hopping Energy 1 Introduction The role of PbO in lithium ion transport in Li 2 O-PbO-B 2 O 3 glasses was studied [1]. The presence of lead leads to a decrease in dc conductivity. Dcconductivity of V 2 O 5 -CoO-TeO 2 was also studied [2] at temperature K.In these glasses the most probable transport for the entire range of temperature and composition is concluded to be due to multi phonon tunneling of large polarons between the microclusters. Addition of PbF 2 to the lead telurite network can increase the conductivity by increasing both carrier concentration and mobility [3]. The electrical conductivity of PbO- 2B 2 O 3 was measured and possible mechanisms of ionic transport for Pb in the glass was proposed [4].The addition of oxides viz PbO or ZnO causes increase in the oxygen boron (O/B) ratio. PbO and ZnO can enter the glass network both as a network former and also as a network modifier and due to this the structure of this glass is expected to be different from that of alkali borate glasses [5]. This dual structural role of ZnO may also affect the conductivity. 2. Experimental DC electrical conductivity of present glass samples was measured by finding out the resistance of the glass samples. The resistance of the sample was measured using voltage drop method [6,7]. 3. Result and Discussion DC-electrical conductivity of the glass samples of series I and II is measured in the temperature range 313 to 573 K by measuring the resistance of the samples. It is observed that the value of resistance (R) depends on temperature. Over a limited temperature range, resistivity is usually described by the Arrhenius equation. Fig.1 163

2 and Fig.2 shows the plot of log σ versus 1/T for various samples in series I and II This plot contains three regions of temperature low temperature region (LTR) SM1 SM2 SM log σ (Ω-cm) /T(K-1) Fig.1 Temperature dependence of the dc-conductivity for lead borate glasses (series I) SM1 (35.20PbO-64.8B 2 O 3 ), SM2 (48.28PbO B 2 O), SM3 (59.11 PbO B 2 O 3 ), SM4 (65.19 PbO B 2 O 3 ), SM5 (73.36 PbO B 2 O 3 ) from 313 to 423 K, high temperature region (HTR) from 443 to 573K and knee temperature region from 423 to 443 K The conductivity of different glasses studied in series I range from 10 8 to (ohm-cm) -1 and for glass samples of series II conductivity ranges from 10-7 to (ohm-cm) -1. In general it is region II (HTR) conductivity increases linearly with increasing temperature at a considerably faster rate. observed that, the conductivity of all glass samples studied, increases with increasing temperature. In region I (LTR) conductivity of samples of series I and II increases linearly with increasing temperature at very slow rate whereas in Similar type of conductivity behavior in different glasses is reported in the literature [ SN /T(K) log σ (Ω-cm) -1 Fig.2:Temperature dependence of the dc-conductivity for lead borate glasses (series II) SN1(33.10PbO-11.10ZnO B 2O 3),SN2 (46.36 PbO ZnO B 2O 3),SN3(57.20 PbO ZnO B 2O 3),SN4(63.19 PbO ZnO B 2O 3),SN5(71.24 PbO ZnO B 2O 3) 164

3 The temperature dependence of the dc conductivity σ for the different glass compositions for series I and II are shown in Fig.1 and Fig.2. Here it is observed that these plots are not linear, indicating temperature dependent activation energy, characteristic of small-polaron-hopping conduction [8-13]. Using the value of average site spacing R, the polaron radius r p has been evaluated [12]. These values are ranges from 1.36A 0 to 1.44A 0 and are less than 1.5A 0 for the glass samples of series I and II, as expected for small polaron. This suggests that the polaron is strongly polarised and therefore, the conduction takes place by small polaron hopping [14]. The activation energies (W) have been evaluated from the slope of the plots of logσ versus l/t. Activation energies at different temperature are also calculated. It is seen that the activation energy is temperature dependent [15] and also depends on composition of glass sample. The corresponding activation energy in the HTR (443K, 503K and 543K) has been found to be greater than that in LTR (330K). In low temperature region the activation energy at 330K is very small and ranges from to ev (series I) and ev to ev (series II). In HTR the activation energy is high and ranges from to ev (series I) and to ev (series II). The variation of activation energy with composition in both LTR and HTR for series I and II is shown in Fig.3(a), Fig.3(b),Fig. 4(a) and Fig.4(b). For series I and II, the activation energy changes in zigzag trend with increasing PbO content. W(eV) LTR 330K W(eV) k 503k 543k Fig. 3(b) Fig: 3(a) and 3(b) Dependence of activation energy (W) on composition PbO mol % at LTR and HTR for series I W(eV) W(eV) LTR 330K Fig. 4(a) 500k 540k Fig. 4(b) Fig. 3(a) 165

4 -logσ(ω-cm) -1 Fig: 4(a) and 4(b) Dependence of activation energy (W) on composition PbO mol % at LTR and HTR for series II The observed values of W are found to be of the same order as that of other glasses like sol-gel silicate glasses[16,17], CuO- Bi 2 O 3 glass pellets[18], Chalcogenide Se- Te-Ge glass [19], Zinc-bismuth oxide glasses[20], CdO-PbO-B 2 O 3 glasses [7,21]. In LTR and HTR the samples under investigation indicates increase in conductivity and activation energy both with increase in temperature. The increase in concentration of modifier forms (both ZnO and PbO) may increase the mobile K 458K Fig ion density. This may lead to an increase of conductivity as observed in present study. Fig: 5 Dependence of conductivity on PbO composition at different temperature in series I increase of the charge carrier concentration and, or, its mobility. The Kink temperature (Ө c ) is the temperature at which the Arrhenius plot is divided into two linear regions of different slopes. The Ө c is determined from the plot of logσ versus 1/T. logσ (Ω-cm) K 458K 503K PbO(mol Fig. 6 %) Fig: 6 Dependence of conductivity on PbO composition at different temperature in series II. W (ev) θ c (K) Series1 The variation of conductivity (-logσ) with composition at a given temperature in LTR (313K) and HTR (458K, 503K and 543K) is shown in Fig.5 and Fig.6 for different glasses in series I and II respectively. From the graphs it seems that the conductivity of all the glasses is composition dependent and also the variation is linear [22]. For series I, the conductivity is minimum at mol % of PbO. For series II, the conductivity is minimum at mol % of PbO. The conductivity increases with different rates when PbO increases. The increase of conductivity (Fig.5 and Fig.6) reflects an Fig: 7 Change of activation energy with kink temperature for glasses in series I The activation energy at kink temperature is also calculated for series I and II. The plot of logσ versus 1/T shows two regions viz. LTR (313 K to 423K) and HTR (443 to 573K) from a kink temperature value (Ө c ). The variation of activation energy (W) with kink temperature (Ө c ) is shown in Fig.7 and Fig.8 for series I and II respectively. 166

5 W (ev) θ c (K) Fig. 8 series II Fig: 8 Change of activation energy with kink temperature for glasses in series II θc(k) series I samples in series I and II. is shown in Fig.9 and Fig.10. It is found that for series I and II the values of Ө c decreases linearly with increasing PbO (mol %) content. The samples studied in series I and II indicate a negative temperature coefficient as well as mixed type conduction, since the activation energy is up to 1 ev [21].The polaron binding energy (W p ) and the value of effective dielectric constant can be calculated under the approximation ε p = ε = n 2 [22]. Where n is the refractive index of the glass. This value is taken from the optical properties of the glasses. Using this value, the value of hopping energy (W H ) is calculated. The values of polaron binding energy (W p ), polaron hopping energy (W H ) for LTR and HTR for series I (SM1 to SM5) and series II (SN1 to SN5) are reported in Table 1. θc(k) Fig. 9 Varation of kink temperature with PbO content(series I) series II Fig. 10 Varition of kinc temperature with Pbo content (series II) The variation in W with Ө c is not linear for samples in all series. The variation of Ө c with composition for glass The values of different parameters reported here are in agreement with the values reported for Vandium -bismuth, Bismuth-borate and other types of glasses in the literature [23, 24]. The values of density of states at Fermi level N (E F ) are calculated from the hopping distance in low temperature (LTR 313K) and in high temperature (HTR 543K) region values are reported in Table 1. The values of N (E F ) calculated from hopping distance are of order of ev -1 cm -3 for series I and II at LTR. In HTR the values of N (E F ) calculated from hopping distance for series I and II are ev -1 cm -1 at HTR. 167

6 Table 1: Polaron binding energy (W p ), polaron hopping energy (W H ), density of states at fermi level [N(E F )] and wave function decay constant. Glass No. Polaron binding energy W p (ev) Polaron hopping energy W H (ev) Density of States at LTR(330K) [N(E F )] ev 1 cm -3 From hopping distance Density of States at HTR(540K) [N(E F )] ev 1 cm -3 From hopping distance Wave function decay constant at LTR α (A 0 ) Wave function decay constant at HTR α (A 0 ) SM SM SM SM SM SN SN SN SN SN The values reported in the literature are of the order of ev -1 cm -3 are reasonable for localized states. The values of wave function decay constant (α) for series I and II are reported in Tables 1 respectively. The α values are found to be of the order of 0.1 to 0.7 A 0 in LTR and 6 to 10 A 0 in HTR. α (A o ) LTR (330K) PbO (mol %) Fig. 11 Depedance of electron wave function decay constant with PbO constant at LTR for series I 168

7 α (A o ) α (A o ) LTR(330K) Fig. 13 Depedance of electron wave function decay constant with PbO constant at LTR for series II HTR(500K) PbO(mol %) Fig. 12 Depedance of electron wave function decay constant with PbO constant at HTR for series I α (A o ) HTR( PbO (mol %) Fig.14 Depedance of electron wave function decay constant with PbO constant at HTR for series II The variation of α with composition for all the glass samples in series I and II is not linear (Fig.11, Fig. 12, Fig. 13 and Fig. 14). In series I the values of α is minimum at mol % of PbO in LTR and HTR. In series II the value α is minimum at mol % and mol% of PbO in LTR and HTR respectively. 4. Conclusion The dc-electrical conductivity is measured in the temperature range from 313K to 573K. The dc conductivity of samples in series I and II is temperature and composition dependents. All the samples indicate a negative temperature coefficient as well as mixed type conduction. The nature of hopping conduction is examined and is found to be adiabatic for all the glass samples. Acknowledgement The author is very much thankful to the UGC WRO Pune for granting minor research project and also financial assistance to complete the project. Author is also thankful to Dr. S. P. Yawale, Head, department of Physics Government Vidharbh Institute of Science and Humanities Amravati & Dr. (Mrs.) S.S. Yawale Director, Pre Indian Administrative Services Training Institute Nagpur for providing necessary facilities 169

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