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Characterization of lithium borate–bismuth tungstate glasses and glass-ceramics by impedance spectroscopy
Solid State Ionics 139 (2001) 105–112 www.elsevier.com / locate / ssi
Characterization of lithium borate–bismuth tungstate glasses and glass-ceramics by impedance spectroscopy G. Senthil Murugan, K.B.R. Varma* Materials Research Centre, Indian Institute of Science, Bangalore – 560012, India Received 20 December 1999; received in revised form 19 September 2000; accepted 14 November 2000
Abstract Transparent glasses in the system (1 2 x)Li 2 B 4 O 7 –xBi 2 WO 6 (0 # x # 0.35) were prepared via melt quenching technique. Differential thermal analysis was employed to characterize the as-quenched glasses. Glass-ceramics with high optical transparency were obtained by controlled heat-treatment of the glasses at 720 K for 6 h. The amorphous nature of the as-quenched glass and crystallinity of glass-ceramics were confirmed by X-ray powder diffraction studies. High resolution transmission electron microscopy (HRTEM) shows the presence of nearly spherical nanocrystallites of Bi 2 WO 6 in Li 2 B 4 O 7 glass matrix. Capacitance and dielectric loss measurements were carried out as a function of temperature (300–870 K) in the frequency range 100 Hz–40 MHz. Impedance spectroscopy employed to rationalize the electrical behavior of glasses and glass-ceramics suggest the coexistence of electronic and ionic conduction in these materials. The thermal activation energies for the electronic conduction and ionic conduction were also estimated based on the Arrhenius plots. 2001 Published by Elsevier Science B.V. Keywords: Lithium borate; Bismuth tungstate; Glass; Glass-ceramic; Impedance spectroscopy
1. Introduction Glass-ceramics comprising nano / micro crystallites of ferroelectric materials have potential for pyroelectric, ferroelectric and electro-optic based device applications [1,2]. We have been making attempts to fabricate borate-based host glassy matrices consisting of Aurivillius family of layered ferroelectric oxides with the general formula [Bi 2 O 2 ] 21 22 [A n B n21 O 3n 11 ] , n 5 1–5 [3], in a bid to design transparent glass-ceramics for these technologically important applications. The borate-based materials *Corresponding author. Fax: 191-80-360-0683. E-mail address: [email protected] (K.B.R. Varma).
have been chosen for dispersing these oxides because of the fact that these could be quenched into glasses with ease at relatively lower temperatures than those of silica-based glasses. Recently we have reported the suitability of strontium tetraborate (SrB 4 O 7 ) (SBO) glasses to disperse vanadium analogue of the n 5 1 member (Bi 2 VO 5.5 ) and other ferroelectric oxides [4,5]. The glass-ceramics of this diphasic combination were proved to be interesting materials from the dielectric and pyroelectric properties point of view [3]. On the contrary, we had not been successful in crystallizing monophasic bismuth tungstate (Bi 2 WO 6 ) (BW) which is a well known n 5 1 member of the same family in SBO glass matrix.
0167-2738 / 01 / $ – see front matter 2001 Published by Elsevier Science B.V. PII: S0167-2738( 00 )00825-0
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However, our efforts to fabricate transparent glassceramics of lithium borate (Li 2 B 4 O 7 ) (LBO) consisting of BW over a wide range of compositions have been successful [6]. Though, the focus of the present investigations has been on the fabrication of transparent glass-ceramics for electro-optic and nonlinear optical device applications, to begin with we have investigated the electrical transport properties (which have indirect influence on the above properties) of the present glasses and glass-ceramics as functions of both frequency and temperature that were normally of interest in the applications of these materials. Impedance spectroscopy is known to be a powerful tool to probe into the complexities of ceramic electrolytes, glasses and electroceramics [7–11]. The purpose of the present work has been to demonstrate the effectiveness of impedance spectroscopy for the characterization of glasses and glass-ceramics of Li 2 B 4 O 7 containing ferroelectric nanocrystallites of BW. We report in this paper the studies concerning the fabrication, thermal, structural and complex impedance properties of the glasses and glassceramics of the composition corresponding to 0.30 in the system (1 2 x)Li 2 B 4 O 7 –xBi 2 WO 6 , though the samples have been synthesized for x ranging continuously from 0 to 0.35.
2. Experimental Glasses of the composition (1 2 x)Li 2 B 4 O 7 – xBi 2 WO 6 (0 # x # 0.35, in molar ratio) were prepared by melting the mixture of prereacted Li 2 B 4 O 7 and Bi 2 WO 6 in a platinum crucible at 1373 K for 1 h. The melt was quenched by pouring on a preheated (400 K) stainless steel block and pressed by another to obtain flat plates (dimensions: 20–25 mm diameter, 1–2 mm thickness). Polycrystalline powders of the starting compounds that were used in the present study were synthesized via solid state reaction route which involved heating stoichiometric mixtures of high purity (Aldrich) Li 2 CO 3 , B 2 O 3 , Bi 2 O 3 and WO 3 at appropriate temperatures and durations to obtain monophasic compounds [12]. The formation of crystalline phases of Li 2 B 4 O 7 and Bi 2 WO 6 was confirmed by X-ray powder diffraction studies. The as-quenched samples were annealed at 473 K
for 6 h which is well below their glass transition temperature. The glassy state of the as-quenched samples was established by subjecting the powders (weighing ¯ 30 mg) to differential thermal analysis (DTA) (Polymer Laboratories STA 1500) in the 300 to 1073 K temperature range. A uniform heating rate of 15 K / min was employed for this purpose. The average values of the glass transition temperature (T g ) and the temperature of onset of crystallisation (T cr ) were evaluated based on the DTA data collected on four samples. The as-quenched samples and those heated at various temperatures were examined by X-ray powder diffraction (XRD) (Scintag, USA) using Cu Ka radiation. High resolution transmission electron microscopy (HRTEM) along with selected area electron diffraction (SAED) (Jeol, JEM 200CX) studies were carried out on the as-quenched as well as heat-treated samples to confirm their amorphous as well as crystalline nature. Rectangular plates (area ¯ 100 mm 2 and thickness ¯ 1 mm) of glasses and glass-ceramics were polished prior to the electrical property studies. The major faces of the polished samples were gold sputtered and silver epoxy was employed to bond the leads. The capacitance (Cp ) and the dielectric loss (tan d ) measurements were carried out using an impedance gain phase analyser (HP 4194 A) in the frequency range 100 Hz–40 MHz at different temperatures (300–873 K). The temperature of the sample was controlled to an accuracy of 60.5 K and it was monitored using a Chromel-Alumel thermocouple placed very close to the sample. A twoterminal capacitor configuration was employed for the present measurements. The real (´ r9 ), and the imaginary (´ r99 ) parts of the dielectric constant were evaluated and transformed into Z9 and Z0 (real and imaginary parts of the impedance) using the standard relations [13]. The results are presented in complex impedance plane in which Z0 versus Z9 is plotted in linear scale. The value of bulk resistance (R b ) is found by the low frequency intercept of the semicircle on the real axis (x-axis). The semicircle passes through a maximum at a frequency f0 (relaxation frequency) and satisfies the condition 2p f0 R b Cb 5 1, from which the value of bulk capacitance (Cb ) is evaluated. The DC conductivity of the sample is obtained by measuring the
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resistance of the sample in a separate experiment at different temperatures using an electrometer (600B Electrometer, Keithley Instruments).
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708 K (T cr1 ) and 753 K (T cr2 ) in addition to the intense exotherm that was obtained at 865 K indicating that there exists a possibility of obtaining a phase separated glass on subjecting it to heat-treatment around T cr1 .
3. Results and discussion
3.2. Phase identification 3.1. Thermal analysis The as-quenched samples were subjected to DTA in order to determine annealing ranges, glass transition (T g ) and crystallization (T cr ) temperatures. Typical DTA plots obtained for the samples of the composition (12x)Li 2 B 4 O 7 –xBi 2 WO 6 (x50, 0.1 and 0.3) are depicted in Fig. 1a–c. Fig. 1a shows an endotherm (760 K) and an exotherm (833 K) corresponding to the glass transition and the crystallisation temperatures, respectively, for the parent glass (x50). As the composition of BW in LBO glass increases the crystallization peak (T cr ) shifts towards higher temperatures while T g tends to shift to lower temperatures. However, the sample for which x50.3 exhibits two less intense exotherms at
Fig. 2a and b shows the XRD patterns recorded for the as-quenched glass and the one heat-treated at 720 K (T cr1 ) for 6 h. The pattern that was obtained for the as-quenched sample (Fig. 2a) confirms its amorphous nature. Interestingly, XRD of the glasses heat-treated at a temperature around the T cr1 and T cr2 gave rise to intense Bragg peaks (Fig. 2b) which could be indexed to an orthorhombic BW phase ˚ associated with the lattice parameters a55.4596 A, ˚ ˚ b55.4511 A and c516.6462 A. These are indeed in good agreement with the values reported in the literature for the polycrystalline BW [14]. These findings establish the crystallization and growth of BW phase on subjecting the as-quenched glasses of LBO and BW to an isothermal heating at 720 K for 6 h. The transmission electron micrograph and the selected area electron diffraction (SAED) pattern of the as-quenched and heat-treated samples (x50.3) are shown in Fig. 3a and b, respectively. The SAED pattern (Fig. 3a) confirms the amorphous nature of the as-quenched glass and the micrograph reveals the presence of nearly spherical crystallites that are more or less uniformly dispersed in the LBO glass matrix. The average crystallite size that is estimated based on electron microscopic analysis is around 20 nm, which is very close to that obtained via X-ray full width at half maximum (FWHM) studies. The crystallite size is found to increase from 20 nm to 30 nm on subjecting the as-quenched sample to an isothermal heat-treatment at 720 K for 6 h. The d-spacings and the lattice parameters that are computed based on the SAED pattern are found to compare well with those obtained via XRD studies for the heat-treated glass composite.
3.3. Impedance analysis
Fig. 1. DTA curves for the as-quenched samples.
In Fig. 4a and b, we show the characteristic complex impedance plots at various temperatures
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Fig. 2. XRD patterns for (a) as-quenched sample and (b) one heat-treated at 720 K / 6 h for the composition corresponding to x50.3.
covering the glass transition and the crystallization temperatures of the as-quenched glass of the composition corresponding to x50.3. The presence of a single semicircle in the range of frequencies that are covered (100 Hz–40 MHz) in the present studies refers to the AC response of the crystallites embedded in the glass matrix. This may be interpreted in terms of the bulk response of the sample and a single parallel RC element [15]. There is no clear indication of a low frequency electrode spike. It is evident from the transmission electron microscopy that the bismuth tungstate crystallites are separated within the glassy lithium borate matrix. It is thought to be a situation analogous to that of grains and grain boundaries in a polycrystalline sample. The glassy region that separates the crystallites from each other is considered to be similar to that of grain boundaries in the above sample. Hence we ascribe the first semicircle in our impedance plot to the contribution from the bismuth tungstate crystallites which are dispersed in the glassy lithium borate matrix. The intercept of the semicircle with the real axis on the low frequency side is normally referred to as the bulk resistance of the sample. In the present case
it is considered to be the resistance of the crystallites embedded in the LBO glass matrix. The intercept of the semicircle shifts towards lower Z9 values on increasing the temperatures indicating the decrease in bulk resistance (R b ). The measurements made at higher temperatures especially in the vicinity of T cr1 , reveal the symptoms of poorly resolved semicircle at low frequency. The impedance plots that are obtained for the glass heat-treated at the crystallization temperature of BW are shown in Fig. 5a and b. The impedance characteristics of this sample are similar to that of the as-quenched glass, but for a change in values of the impedance. Even though there is a sign of second semicircle appearing, we could not really resolve it due to the frequency limitation that is associated with our experimental facility. The relaxation times that are noted for both glasses and glassceramics (720 K heat-treated) at 720 K are 8.1 ns and 7.6 ns. The bulk resistance and capacitance values noted from the complex impedance plots at various temperatures for both the samples are elucidated in Table 1. Arrhenius plots of the bulk conductivity (sb ), obtained from the complex impedance plot using
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Fig. 3. HRTEM and SAED patterns of (a) as-quenched sample and (b) heat-treated (720 K / 6 h) sample for the composition corresponding to x50.3.
sb 5 t /R b A (where t is the thickness and A is the electroded area of the sample) and the measured DC conductivity (sdc ) at different temperatures for both the as-quenched glass and heat-treated samples are shown in Figs. 6 and 7, respectively. In the entire temperature region covered in the present studies the DC conductivity is lower than the bulk conductivity. The deviation of DC conductivity from the bulk conductivity implies a considerable contribution from ionic conductivity. It is reported [16] that LBO glass which happens to be the host matrix in the present investigation is a good example for mixed conductor in which electronic and ionic conduction coexists. In such a case the bulk conductivity is a
combined effect arising out of both electrons (and or holes) and ions. Therefore
sbulk 5 sion 1 sdc 5 sion 1 selec where sdc refers to the electronic conductivity selec . Since silver electrodes (ion blocking ones) were employed for making DC conductivity measurements, it is presumed that it is dominated only by the electronic contribution. Hence sbulk 2 sdc yields ionic conductivity. From Figs. 6 and 7 it is concluded that the present composite has both ionic and electronic conductivity in the entire range of temperatures under study.
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Fig. 4. Complex impedance plot at different temperatures for the as-quenched sample, with the corresponding equivalent circuit.
Fig. 5. Complex impedance plot at different temperatures for the sample heat-treated at 720 K / 6 h.
The glass and glass-ceramic follows the Arrhenius law which is suitable for thermally activated systems
associated with the bulk conductivity is higher than that of the DC conductivity. However, the difference (0.01 eV) between the activation energies associated with the bulk conductivity (0.78 eV) and DC conductivity (0.77 eV) is smaller in the case of glassceramic. The lower activation energies (than that of pure BW) associated with the present sample may be attributed to the mechanisms that have extrinsic origin. The possible error in the activation energy calculation is associated with the conductivity measurement which is found to be less than 1%.
S D
2 Ea s 5 s0 exp ]] kT
where s0 is the pre-exponential factor and Ea , k and T are the activation energy for conduction, Boltzmann constant and absolute temperature, respectively. For both the glass and glass-ceramic (the one heat-treated at 720 K / 6 h) the activation energy
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Table 1 Bulk resistance (R b ), relaxation frequency ( f0 ) and bulk capacitance (Cb ) of the as-quenched and heat-treated (720 K / 6 h) sample at various temperatures T (K)
R b (V)
f0 (Hz)
Cb (F)
As-quenched 470 520 570 610 650 690 720 750
7.67310 5 1.22310 5 2.57310 4 1.03310 4 4.55310 3 1.92310 3 6.80310 2 2.90310 2
3.53310 4 2.15310 5 9.49310 5 2.49310 6 7.01310 6 1.33310 7 1.96310 7 2.09310 7
5.85310 212 6.05310 212 6.52310 212 6.17310 212 4.98310 212 6.19310 212 1.19310 211 2.61310 211
4.21310 3 3.53310 4 1.56310 5 5.66310 5 1.22310 6 2.49310 6 5.07310 6 9.67310 6 1.52310 7 2.09310 7 3.09310 7
1.00310 211 9.69310 212 1.01310 211 1.01310 211 1.09310 211 1.11310 211 1.10310 211 1.06310 211 1.13310 211 1.34310 211 1.35310 211
Heat-treated at 720 K / 6 h 420 3.75310 6 470 4.64310 5 510 1.00310 5 550 2.77310 4 580 1.18310 4 610 5.74310 3 640 2.84310 3 670 1.55310 3 695 9.23310 2 720 5.63310 2 745 3.80310 2
Fig. 7. Arrhenius plot of bulk and DC conductivity of the sample heat-treated at 720 K / 6 h.
4. Conclusions In conclusion, glasses and glass-ceramics of good optical quality in the composition (12x)Li 2 B 4 O 7 – xBi 2 WO 6 (0#x#0.35) were fabricated. The DC and bulk conductivities of both glasses and glassceramics have been evaluated via impedance spectroscopy and demonstrated that in these materials both electronic and ionic conduction processes are active. The thermal activation energies for the electronic conduction for the as-quenched glass and glass-ceramic are 0.59 eV and 0.77 eV, respectively, whereas the activation energies for ionic conduction for the glass and glass-ceramic are, respectively, 0.17 eV and 0.01 eV. The present findings demonstrate the effectiveness of impedance spectroscopy in rationalizing the dielectric behavior of glass-ceramic diphasic composites.
Acknowledgements
Fig. 6. Arrhenius plot of bulk and DC conductivity for the as-quenched sample.
The authors thank Department of Science and Technology, Government of India for financial grant. We also acknowledge Dr. G.N. Subbanna for his help in carrying out electron microscopy.
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References [1] N.F. Borreli, J. Appl. Phys. 38 (1967) 4243. [2] K. Tanaka, K. Kashima, K. Hirao, N. Soga, A. Mito, H. Nasu, J. Non-Cryst. Solids 185 (1995) 123. [3] M.V. Shankar, K.B.R. Varma, J. Non-Cryst. Solids 226 (1998) 145. [4] K.B.R. Varma, M.V. Shankar, G.N. Subbanna, Mater. Res. Bull. 31 (1996) 475. [5] M.V. Shankar, K.B.R. Varma, Phys. Chem. Glasses 40 (1999) 44. [6] G. Senthil Murugan, G.N. Subbanna, K.B.R. Varma, J. Mater. Sci. Lett. 18 (1999) 1687. [7] A. Huanosta, A.R. West, J. Appl. Phys. 61 (1987) 5386. [8] D.C. Sinclair, A.R. West, Phys. Rev. B 39 (1989) 13486. [9] A. Huanosta, O. Alvarez-Fregoso, E. Amano, C. TabaresMunoz, M.E. Mendoza-Alvarez, J.G. Mendoza-Alvarez, J. Appl. Phys. 69 (1991) 404.
[10] N. Hirose, A.R. West, J. Am. Ceram. Soc. 79 (1996) 1633. [11] J.T.S. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 132. [12] G. Senthil Murugan, K.B.R. Varma, Mater. Res. Bull. 34 (1999) 2201. [13] J.R. Macdonald (Ed.), Impedance Spectroscopy, Wiley, New York, 1987. [14] R.W. Wolf, R.E. Newnham, M.I. Kay, Solid State Commun. 7 (1969) 1797. [15] S. Lanfredi, A.C.M. Rodrigues, J. Appl. Phys. 86 (1999) 2215. [16] T. Matsuo, T. Yagami, T. Katsumata, J. Appl. Phys. 74 (1993) 7264.
Characterization of lithium borate–bismuth tungstate glasses and glass-ceramics by impedance spectroscopy G. Senthil Murugan, K.B.R. Varma* Materials Research Centre, Indian Institute of Science, Bangalore – 560012, India Received 20 December 1999; received in revised form 19 September 2000; accepted 14 November 2000
Abstract Transparent glasses in the system (1 2 x)Li 2 B 4 O 7 –xBi 2 WO 6 (0 # x # 0.35) were prepared via melt quenching technique. Differential thermal analysis was employed to characterize the as-quenched glasses. Glass-ceramics with high optical transparency were obtained by controlled heat-treatment of the glasses at 720 K for 6 h. The amorphous nature of the as-quenched glass and crystallinity of glass-ceramics were confirmed by X-ray powder diffraction studies. High resolution transmission electron microscopy (HRTEM) shows the presence of nearly spherical nanocrystallites of Bi 2 WO 6 in Li 2 B 4 O 7 glass matrix. Capacitance and dielectric loss measurements were carried out as a function of temperature (300–870 K) in the frequency range 100 Hz–40 MHz. Impedance spectroscopy employed to rationalize the electrical behavior of glasses and glass-ceramics suggest the coexistence of electronic and ionic conduction in these materials. The thermal activation energies for the electronic conduction and ionic conduction were also estimated based on the Arrhenius plots. 2001 Published by Elsevier Science B.V. Keywords: Lithium borate; Bismuth tungstate; Glass; Glass-ceramic; Impedance spectroscopy
1. Introduction Glass-ceramics comprising nano / micro crystallites of ferroelectric materials have potential for pyroelectric, ferroelectric and electro-optic based device applications [1,2]. We have been making attempts to fabricate borate-based host glassy matrices consisting of Aurivillius family of layered ferroelectric oxides with the general formula [Bi 2 O 2 ] 21 22 [A n B n21 O 3n 11 ] , n 5 1–5 [3], in a bid to design transparent glass-ceramics for these technologically important applications. The borate-based materials *Corresponding author. Fax: 191-80-360-0683. E-mail address: [email protected] (K.B.R. Varma).
have been chosen for dispersing these oxides because of the fact that these could be quenched into glasses with ease at relatively lower temperatures than those of silica-based glasses. Recently we have reported the suitability of strontium tetraborate (SrB 4 O 7 ) (SBO) glasses to disperse vanadium analogue of the n 5 1 member (Bi 2 VO 5.5 ) and other ferroelectric oxides [4,5]. The glass-ceramics of this diphasic combination were proved to be interesting materials from the dielectric and pyroelectric properties point of view [3]. On the contrary, we had not been successful in crystallizing monophasic bismuth tungstate (Bi 2 WO 6 ) (BW) which is a well known n 5 1 member of the same family in SBO glass matrix.
0167-2738 / 01 / $ – see front matter 2001 Published by Elsevier Science B.V. PII: S0167-2738( 00 )00825-0
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However, our efforts to fabricate transparent glassceramics of lithium borate (Li 2 B 4 O 7 ) (LBO) consisting of BW over a wide range of compositions have been successful [6]. Though, the focus of the present investigations has been on the fabrication of transparent glass-ceramics for electro-optic and nonlinear optical device applications, to begin with we have investigated the electrical transport properties (which have indirect influence on the above properties) of the present glasses and glass-ceramics as functions of both frequency and temperature that were normally of interest in the applications of these materials. Impedance spectroscopy is known to be a powerful tool to probe into the complexities of ceramic electrolytes, glasses and electroceramics [7–11]. The purpose of the present work has been to demonstrate the effectiveness of impedance spectroscopy for the characterization of glasses and glass-ceramics of Li 2 B 4 O 7 containing ferroelectric nanocrystallites of BW. We report in this paper the studies concerning the fabrication, thermal, structural and complex impedance properties of the glasses and glassceramics of the composition corresponding to 0.30 in the system (1 2 x)Li 2 B 4 O 7 –xBi 2 WO 6 , though the samples have been synthesized for x ranging continuously from 0 to 0.35.
2. Experimental Glasses of the composition (1 2 x)Li 2 B 4 O 7 – xBi 2 WO 6 (0 # x # 0.35, in molar ratio) were prepared by melting the mixture of prereacted Li 2 B 4 O 7 and Bi 2 WO 6 in a platinum crucible at 1373 K for 1 h. The melt was quenched by pouring on a preheated (400 K) stainless steel block and pressed by another to obtain flat plates (dimensions: 20–25 mm diameter, 1–2 mm thickness). Polycrystalline powders of the starting compounds that were used in the present study were synthesized via solid state reaction route which involved heating stoichiometric mixtures of high purity (Aldrich) Li 2 CO 3 , B 2 O 3 , Bi 2 O 3 and WO 3 at appropriate temperatures and durations to obtain monophasic compounds [12]. The formation of crystalline phases of Li 2 B 4 O 7 and Bi 2 WO 6 was confirmed by X-ray powder diffraction studies. The as-quenched samples were annealed at 473 K
for 6 h which is well below their glass transition temperature. The glassy state of the as-quenched samples was established by subjecting the powders (weighing ¯ 30 mg) to differential thermal analysis (DTA) (Polymer Laboratories STA 1500) in the 300 to 1073 K temperature range. A uniform heating rate of 15 K / min was employed for this purpose. The average values of the glass transition temperature (T g ) and the temperature of onset of crystallisation (T cr ) were evaluated based on the DTA data collected on four samples. The as-quenched samples and those heated at various temperatures were examined by X-ray powder diffraction (XRD) (Scintag, USA) using Cu Ka radiation. High resolution transmission electron microscopy (HRTEM) along with selected area electron diffraction (SAED) (Jeol, JEM 200CX) studies were carried out on the as-quenched as well as heat-treated samples to confirm their amorphous as well as crystalline nature. Rectangular plates (area ¯ 100 mm 2 and thickness ¯ 1 mm) of glasses and glass-ceramics were polished prior to the electrical property studies. The major faces of the polished samples were gold sputtered and silver epoxy was employed to bond the leads. The capacitance (Cp ) and the dielectric loss (tan d ) measurements were carried out using an impedance gain phase analyser (HP 4194 A) in the frequency range 100 Hz–40 MHz at different temperatures (300–873 K). The temperature of the sample was controlled to an accuracy of 60.5 K and it was monitored using a Chromel-Alumel thermocouple placed very close to the sample. A twoterminal capacitor configuration was employed for the present measurements. The real (´ r9 ), and the imaginary (´ r99 ) parts of the dielectric constant were evaluated and transformed into Z9 and Z0 (real and imaginary parts of the impedance) using the standard relations [13]. The results are presented in complex impedance plane in which Z0 versus Z9 is plotted in linear scale. The value of bulk resistance (R b ) is found by the low frequency intercept of the semicircle on the real axis (x-axis). The semicircle passes through a maximum at a frequency f0 (relaxation frequency) and satisfies the condition 2p f0 R b Cb 5 1, from which the value of bulk capacitance (Cb ) is evaluated. The DC conductivity of the sample is obtained by measuring the
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resistance of the sample in a separate experiment at different temperatures using an electrometer (600B Electrometer, Keithley Instruments).
107
708 K (T cr1 ) and 753 K (T cr2 ) in addition to the intense exotherm that was obtained at 865 K indicating that there exists a possibility of obtaining a phase separated glass on subjecting it to heat-treatment around T cr1 .
3. Results and discussion
3.2. Phase identification 3.1. Thermal analysis The as-quenched samples were subjected to DTA in order to determine annealing ranges, glass transition (T g ) and crystallization (T cr ) temperatures. Typical DTA plots obtained for the samples of the composition (12x)Li 2 B 4 O 7 –xBi 2 WO 6 (x50, 0.1 and 0.3) are depicted in Fig. 1a–c. Fig. 1a shows an endotherm (760 K) and an exotherm (833 K) corresponding to the glass transition and the crystallisation temperatures, respectively, for the parent glass (x50). As the composition of BW in LBO glass increases the crystallization peak (T cr ) shifts towards higher temperatures while T g tends to shift to lower temperatures. However, the sample for which x50.3 exhibits two less intense exotherms at
Fig. 2a and b shows the XRD patterns recorded for the as-quenched glass and the one heat-treated at 720 K (T cr1 ) for 6 h. The pattern that was obtained for the as-quenched sample (Fig. 2a) confirms its amorphous nature. Interestingly, XRD of the glasses heat-treated at a temperature around the T cr1 and T cr2 gave rise to intense Bragg peaks (Fig. 2b) which could be indexed to an orthorhombic BW phase ˚ associated with the lattice parameters a55.4596 A, ˚ ˚ b55.4511 A and c516.6462 A. These are indeed in good agreement with the values reported in the literature for the polycrystalline BW [14]. These findings establish the crystallization and growth of BW phase on subjecting the as-quenched glasses of LBO and BW to an isothermal heating at 720 K for 6 h. The transmission electron micrograph and the selected area electron diffraction (SAED) pattern of the as-quenched and heat-treated samples (x50.3) are shown in Fig. 3a and b, respectively. The SAED pattern (Fig. 3a) confirms the amorphous nature of the as-quenched glass and the micrograph reveals the presence of nearly spherical crystallites that are more or less uniformly dispersed in the LBO glass matrix. The average crystallite size that is estimated based on electron microscopic analysis is around 20 nm, which is very close to that obtained via X-ray full width at half maximum (FWHM) studies. The crystallite size is found to increase from 20 nm to 30 nm on subjecting the as-quenched sample to an isothermal heat-treatment at 720 K for 6 h. The d-spacings and the lattice parameters that are computed based on the SAED pattern are found to compare well with those obtained via XRD studies for the heat-treated glass composite.
3.3. Impedance analysis
Fig. 1. DTA curves for the as-quenched samples.
In Fig. 4a and b, we show the characteristic complex impedance plots at various temperatures
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Fig. 2. XRD patterns for (a) as-quenched sample and (b) one heat-treated at 720 K / 6 h for the composition corresponding to x50.3.
covering the glass transition and the crystallization temperatures of the as-quenched glass of the composition corresponding to x50.3. The presence of a single semicircle in the range of frequencies that are covered (100 Hz–40 MHz) in the present studies refers to the AC response of the crystallites embedded in the glass matrix. This may be interpreted in terms of the bulk response of the sample and a single parallel RC element [15]. There is no clear indication of a low frequency electrode spike. It is evident from the transmission electron microscopy that the bismuth tungstate crystallites are separated within the glassy lithium borate matrix. It is thought to be a situation analogous to that of grains and grain boundaries in a polycrystalline sample. The glassy region that separates the crystallites from each other is considered to be similar to that of grain boundaries in the above sample. Hence we ascribe the first semicircle in our impedance plot to the contribution from the bismuth tungstate crystallites which are dispersed in the glassy lithium borate matrix. The intercept of the semicircle with the real axis on the low frequency side is normally referred to as the bulk resistance of the sample. In the present case
it is considered to be the resistance of the crystallites embedded in the LBO glass matrix. The intercept of the semicircle shifts towards lower Z9 values on increasing the temperatures indicating the decrease in bulk resistance (R b ). The measurements made at higher temperatures especially in the vicinity of T cr1 , reveal the symptoms of poorly resolved semicircle at low frequency. The impedance plots that are obtained for the glass heat-treated at the crystallization temperature of BW are shown in Fig. 5a and b. The impedance characteristics of this sample are similar to that of the as-quenched glass, but for a change in values of the impedance. Even though there is a sign of second semicircle appearing, we could not really resolve it due to the frequency limitation that is associated with our experimental facility. The relaxation times that are noted for both glasses and glassceramics (720 K heat-treated) at 720 K are 8.1 ns and 7.6 ns. The bulk resistance and capacitance values noted from the complex impedance plots at various temperatures for both the samples are elucidated in Table 1. Arrhenius plots of the bulk conductivity (sb ), obtained from the complex impedance plot using
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Fig. 3. HRTEM and SAED patterns of (a) as-quenched sample and (b) heat-treated (720 K / 6 h) sample for the composition corresponding to x50.3.
sb 5 t /R b A (where t is the thickness and A is the electroded area of the sample) and the measured DC conductivity (sdc ) at different temperatures for both the as-quenched glass and heat-treated samples are shown in Figs. 6 and 7, respectively. In the entire temperature region covered in the present studies the DC conductivity is lower than the bulk conductivity. The deviation of DC conductivity from the bulk conductivity implies a considerable contribution from ionic conductivity. It is reported [16] that LBO glass which happens to be the host matrix in the present investigation is a good example for mixed conductor in which electronic and ionic conduction coexists. In such a case the bulk conductivity is a
combined effect arising out of both electrons (and or holes) and ions. Therefore
sbulk 5 sion 1 sdc 5 sion 1 selec where sdc refers to the electronic conductivity selec . Since silver electrodes (ion blocking ones) were employed for making DC conductivity measurements, it is presumed that it is dominated only by the electronic contribution. Hence sbulk 2 sdc yields ionic conductivity. From Figs. 6 and 7 it is concluded that the present composite has both ionic and electronic conductivity in the entire range of temperatures under study.
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Fig. 4. Complex impedance plot at different temperatures for the as-quenched sample, with the corresponding equivalent circuit.
Fig. 5. Complex impedance plot at different temperatures for the sample heat-treated at 720 K / 6 h.
The glass and glass-ceramic follows the Arrhenius law which is suitable for thermally activated systems
associated with the bulk conductivity is higher than that of the DC conductivity. However, the difference (0.01 eV) between the activation energies associated with the bulk conductivity (0.78 eV) and DC conductivity (0.77 eV) is smaller in the case of glassceramic. The lower activation energies (than that of pure BW) associated with the present sample may be attributed to the mechanisms that have extrinsic origin. The possible error in the activation energy calculation is associated with the conductivity measurement which is found to be less than 1%.
S D
2 Ea s 5 s0 exp ]] kT
where s0 is the pre-exponential factor and Ea , k and T are the activation energy for conduction, Boltzmann constant and absolute temperature, respectively. For both the glass and glass-ceramic (the one heat-treated at 720 K / 6 h) the activation energy
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Table 1 Bulk resistance (R b ), relaxation frequency ( f0 ) and bulk capacitance (Cb ) of the as-quenched and heat-treated (720 K / 6 h) sample at various temperatures T (K)
R b (V)
f0 (Hz)
Cb (F)
As-quenched 470 520 570 610 650 690 720 750
7.67310 5 1.22310 5 2.57310 4 1.03310 4 4.55310 3 1.92310 3 6.80310 2 2.90310 2
3.53310 4 2.15310 5 9.49310 5 2.49310 6 7.01310 6 1.33310 7 1.96310 7 2.09310 7
5.85310 212 6.05310 212 6.52310 212 6.17310 212 4.98310 212 6.19310 212 1.19310 211 2.61310 211
4.21310 3 3.53310 4 1.56310 5 5.66310 5 1.22310 6 2.49310 6 5.07310 6 9.67310 6 1.52310 7 2.09310 7 3.09310 7
1.00310 211 9.69310 212 1.01310 211 1.01310 211 1.09310 211 1.11310 211 1.10310 211 1.06310 211 1.13310 211 1.34310 211 1.35310 211
Heat-treated at 720 K / 6 h 420 3.75310 6 470 4.64310 5 510 1.00310 5 550 2.77310 4 580 1.18310 4 610 5.74310 3 640 2.84310 3 670 1.55310 3 695 9.23310 2 720 5.63310 2 745 3.80310 2
Fig. 7. Arrhenius plot of bulk and DC conductivity of the sample heat-treated at 720 K / 6 h.
4. Conclusions In conclusion, glasses and glass-ceramics of good optical quality in the composition (12x)Li 2 B 4 O 7 – xBi 2 WO 6 (0#x#0.35) were fabricated. The DC and bulk conductivities of both glasses and glassceramics have been evaluated via impedance spectroscopy and demonstrated that in these materials both electronic and ionic conduction processes are active. The thermal activation energies for the electronic conduction for the as-quenched glass and glass-ceramic are 0.59 eV and 0.77 eV, respectively, whereas the activation energies for ionic conduction for the glass and glass-ceramic are, respectively, 0.17 eV and 0.01 eV. The present findings demonstrate the effectiveness of impedance spectroscopy in rationalizing the dielectric behavior of glass-ceramic diphasic composites.
Acknowledgements
Fig. 6. Arrhenius plot of bulk and DC conductivity for the as-quenched sample.
The authors thank Department of Science and Technology, Government of India for financial grant. We also acknowledge Dr. G.N. Subbanna for his help in carrying out electron microscopy.
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