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Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate
Materials Chemistry and Physics xxx (2013) 1e7
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Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate Raghavendra Sagar, Shivanand Madolappa, Nagbasavanna Sharanappa, R.L. Raibagkar* Department of Post Graduate Studies and Research in Materials Science, Gulbarga University, Gulbarga 585 106, Karnataka, India
h i g h l i g h t s Nano crystalline solid solution of (Ba1-xPrx)(Zr0.52Ti0.48)O3 (x¼0.1 and 0.2). AC conduction mechanism. Complex impedance (Z*) spectroscopy. n-type non-degenerated semiconductor. NTC thermistor device.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 December 2011 Received in revised form 12 February 2013 Accepted 3 March 2013
Praseodymium (Pr) doped barium zirconium titanate with nominal composition (Ba1xPrx)(Zr0.52Ti0.48)O3 (x ¼ 0.1 and 0.2) were synthesized using solid state reaction method. X-ray analysis conform the formation of cubic phase Pr-doped barium zirconium titanate along with minor pyrochloric phase. The increase in grain size after primary investigation reveals the influence of Pr ions on the domain structure and its microstructure. In order to correlate the effect of the chemical composition with the conduction mechanism, different AC electrical parameters have been addressed. The frequency dependant tangent loss of the sample was less for both the ceramics. The temperature dependence results show that the dielectric parameters and resistivity increases as Pr-content in the ceramic increases; this is attributed to the grain size and dipole dynamics. Complex impedance (Z*) plots show frequency dependent behavior as the response for the grain resistance mechanisms. This mechanism has been represented by an equivalent circuit. The temperature dependence of the electrical conductivity and Seebeck coefficient showed n-type non-degenerated semiconductor in the measured temperature range. The temperature dependent conductivity measurement suggests a novel negative temperature coefficient of resistance behavior of the samples. Furthermore, the frequency dependent conductivity plot shows increasing behavior. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Powder diffraction Microstructure Electrical properties Dielectric properties Transport properties
1. Introduction After successful investigation of primary structural and electrical properties of Samarium (Sm) and Neodymium (Nd) ions doped barium zirconium titanate (BZT) in our previous studies [1,2], the research of same compound with other rare earth ions doped is necessary in order to understand better amongst all substituted BZT’s for practical technological applications. Barium zirconium titanate Ba(ZrxTi1x)O3 (BZT), as an important member of modified BaTiO3-based ceramics, has received extensive * Corresponding author. Tel.: þ91 8472 263295; fax: þ91 8472 263206. E-mail addresses: [email protected], [email protected] (R.L. Raibagkar).
attention, due to its prominent electrical properties including high voltage resistance. Because it is derived from two perovskite lattices i.e., BaTiO3 and BaZrO3, many researchers have reported enhanced electrical and dielectric properties after the substitution of zirconium in BT [3e12]. Furthermore, BZT offers high temperature resistance and chemical stability than (Ba1xSrx)TiO3 due to substitution of Zr4þ [13]. By altering the Zr/Ti-ratio, one can easily engineer the domain structure, which changes the structuree property relation of the BZT ceramics [14]. High sintering temperature, inhomogeneous composition and internal stress within the microstructure are matter of concerns, which greatly influences electrical properties of BZT. However, reports on detailed studies of Zr4þ rich BZT solid solution exhibiting excellent electrical behavior are indeed less. Here, the electrical behavior is mainly depending
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
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on the Zr4þ content, which varies the room temperature resistivity of the BZT. Electrical properties of BZT can be tuned by proper substitution of typical rare earth ion, with smaller ionic radius than that of barium [15,16]. Since, the influence of rare earth elements on the electrical properties of BT has been widely studied, the work dealing with a typical rare earth element doped-BZT are indeed limited [17,18]. Considering the uses of barium based solid solutions in device applications and less work on transport properties of barium zirconium titanate solid solutions, the objective of this research was set to seek a kind of suitable barium based material as a potential candidate for NTC thermistor. In this paper, we report the synthesis of Pr-doped BZT solid solution, with nominal stoichiometry (Ba0.9Pr0.1)(Zr0.52Ti0.48)O3 (BPZT10) and (Ba0.8Pr0..2)(Zr0.52Ti0.48)O3 (BPZT20). Phase structure and temperature dependent resistivity measurement of BPZT with two different concentrations have been studied. The main objective of present work is to understand the effect of Pr3þ ions on the electrical and thermoelectric behavior of BZT ceramics toward developing NTC thermistor device. 2. Experimental procedure 2.1. Synthesis BPZT-ceramic compounds were synthesized by well-known solid state reaction method. The starting materials BaCO3, Pr6O11, TiO2, and ZrO2 (all Aldrich make 99.99%) were weighed in stoichiometric proportions. The samples were calcined at 900 C to remove impurities like carbon present, using temperature controlled programmable muffle furnace. At 1050 C, the distinct phases of Pr-doped BaTiO3 and Pr-doped BaZrO3 were formed. Finally, the calcined powders were pressed isostatically into circular pellets at pressure of 100 MPa using a hydraulic press and sintered at 1400 C for 8 h in air to obtain final product.
r ¼ R
A d
where R ¼ resistance, A ¼ area of the pellet (pr2) and d ¼ thickness of the sample. The thermoelectric power (TEP) of BPZT samples was calculated by adjusting thermal gradient (DT) across the samples. The resulting thermal electro-motive force (DE) across the pellet was measured by a digital micro-voltmeter and thermoelectric power (S) was estimated. 3. Results and discussion The room temperature X-ray diffraction (XRD) pattern of BPZT10 and BPZT20 ceramics are shown in Fig.1. The reflection peaks’ corresponding to 2q-values were indexed on a cubic system along with pyrochloric phase of Pr2Ti2O7. On the basis of least squares’ fitting between observed and calculated inter-planar distance (d), we found that, the pattern of BPZT10 and BPZT20 reflects the formation of single phase perovskite. Here, when Pr3þ was introduced into BaTiO3eBaZrO3, due to higher stress energy of Zr4þ and Ti4þ, Pr3þ cations occupy both the sites. Increase in cell volume of the samples also endorsed the substitution of Pr-ions in BZ and BT matrix. The lattice parameters and cell volume of BPZT10 and BPZT20 are given in Table 1. The pure Ba(Zr0.52Ti0.48)O3 (BZT) sintered at lower temperature (1050 C) showed two distinct phases of BT and BZ peaks [19]. It is well-known that as the peak width increases, the crystallite size decreases. Peak broadening is originated from variations
2.2. Characterizations The phase identification was exercised using X-ray diffraction (XRD) measurements with a Philips, PANalytical PRO Diffractometer using CuKa-radiation with Ni-filter (l ¼ 1.54056 A) at room temperature, at the scanning rate of 2 min1. Cell parameters including cell volume and lattice parameters of the samples were refined by the least squares fitting method based software UNITCELL-97. The crystallites size was estimated using Scherrer’s formula. The surface morphology of the samples was examined using scanning electron microscope (SEM, Model JSM/4048/SM JEOLJapan). The grain sizes of the samples were evaluated by a line intercept method. The dielectric and impedance properties of the samples were studied using impedance analyzer (WAYNE KERR, 43,100) from which measurement of capacitance (C) tangent loss (tan d) and Impedance (Z), at various temperature range of 50 Ce 400 C were studied in order to calculate various related dielectric parameters. DC resistivity is measured using two probe method. A standard experimental unit for this measurement was employed. Thin polished and silver coated pellet (10 mm diameter and 2 mm thickness) was placed in between two spring-loaded foils of the electrode. A constant voltage source (DC voltage ¼ 10 V) was used to apply potential across the pellet. The temperature was measured with a chromel alumel thermocouple, a tip of which was maintained in close proximity to the sample for maximum accuracy. The value of current was measured using nanoammeter during cooling in the temperature range from 40 C to 500 C at the interval of 10 C and the resistance was calculated. Resistivity (r) of all samples were estimated using the relation
Fig. 1. X-ray diffractograms of the BPZT10 and BPZT20 ceramics sintered in air.
Please cite this article in press as: R. Sagar, et al., Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
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Table 1 Structural and electrical parameters of BPZT10 and BPZT20 ceramics. Parameters
BPZT10
BPZT20
Deviation in parameters
a ( A) Cell volume ( A)3 Crystallite size (nm) Grain Size (mm)
4.05 66.43 10.21 0.42 1.34 4.65 3.10 1547 6195 7.06 7.18 1.01
4.07 64.48 14.27 0.47 1.37 4.80 3.18 1593 4304 5.24 5.46 1.1
0.02 1.95 4.06 0.05 0.03 0.02 0.17 204 16 1.82 1.72 0.09
3731 1 103 to 3.096 106
3736 1 103 to 7.463 105
5 e
g s1 (ms) s2(ms) Rg (U) Rgb( U) Cg (pF) Cgb(pF) High resistivity (rhigh) (Um1 109) B-constant (323e573 K) Seebeck coefficient (V/K)
in lattice spacing, caused by lattice strain. These salient features are clearly evidenced in the present set of compounds. Fig. 2 illustrates the SEM micrographs of BPZT compounds. Both the compounds showed porous and non-uniformly distributed grains due to the characteristic of matter transport mechanism between the grains during the sintering process. At initial stages of solid state reaction, the carbonates and oxides were well mixed and thoroughly grinded in order to reduce the powder particle sizes. The heat promoted slow kinetics of inter-diffusion in the contact points between the particles with irregular morphologies. This diffusion resulted into irregular shaped grains due to an elastic deformation caused by the surface energy reduction in the contact interface by its orientation and mobility [20,21]. The average grain sizes of the samples are given in Table 1. Fig. 3(a) presents the frequency dependent plots of the real part (3 0 ) and imaginary part (3 00 ) of the dielectric constant for Pr doped BZT samples at 50 C. It is observed that the value of 3 0 for both samples decreases as the frequency increases and attains lower values before increasing at higher frequencies. The high value of the dielectric constant at low frequencies is due to the accumulation of charges at the interfaces between the sample and the electrodes, i.e., MaxwelleWagner polarization and interfacial polarization [22]. As the frequency increases, the dipoles in the samples reorient themselves fast enough to respond to the applied electric field resulting increase in 3 0 at higher frequencies. On the other hand, at higher frequencies, it has been observed that the 3 0 value increases as the Pr content changes from 0.1 to 0.2, which is attributed to the increase in the grain size. The SEM micrographs of the BPZT10 and BPZT20 shows more aggregated grains within its microstructure. The increase in 3 0 as a function of concentration is because of the grain aggregates which induces the internal stress within the grains. When a fine-grained ceramic are subjected to field, the grain is subjected to an internal stress which depend on the orientation of all the surrounding grains. Thus, the increase in 3 0 is observed. According to Yung Park et al. [23], the stress system would tend to suppress the spontaneous deformation and force the grain back toward the cubic state. Recent, Diez-Betriu et al. [24] also revealed the increase in grain size as a function of Pr-content along with increase in the value of dielectric constant. Fig. 3(b) shows the variation of 3 as a function of temperature of BPZT10 and BPZT20 ceramics. As Pr is doped in ceramics, the Curie temperature of ceramics shifted toward higher temperature due to the internal stresses developed within the ceramics from substitution. Meanwhile, temperature dependant 3 exhibited broad peak indicating the diffuse phase transition from ferroelectric to
Fig. 2. SEM micrographs of the BPZT10 and BPZT20 ceramics samples.
paraelectric phase. The diffusivity (g) coefficient of both the ceramics is given in Table 1. Moreover, the diffuse transition behavior increased with increasing Pr content, indicating a composition induced diffuse phase transition. The diffuse phase transition is generally due to broadening of dielectric constant and deviation from CurieeWeiss law in the vicinity of Tc. The variation of loss tangent (tan d) as a function of frequency is given in Fig. 4. The figure shows a kink at 280 kHz for BPZT ceramics. This kink separates the polarization mechanism occurring due to the electrode effect from the grain boundaries mechanism, which is not noticeable in the 3 0 or 3 00 plots. However, it is observed that the dispersion process occurring due to the electrode mechanism is stronger than the one resulting from the grain boundaries. This kind of broad peak occurs when the hopping frequency of electric charge carriers becomes approximately equal to that of the external applied AC electric field [11]. The frequency dependant real (Z0 ) and imaginary (Z00 ) components of impedance for BPZT ceramics used in this work are
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Fig. 5. Variation of the real part (Z0 ) and imaginary part (Z00 ) of the impedance for BPZT10 and BPZT20 ceramics as a function of frequency at 50 C.
Fig. 3. (a) Variation of the real part (3 0 ) and imaginary part (3 00 ) of the dielectric constant for Pr doped BZT samples as a function of frequency at 50 C. (b) Temperature dependant dielectric constant of BPZT10 and BPZT20 ceramics.
shown in Fig. 5. Z0 values decrease with increasing frequency for BPZT ceramics. Furthermore, it is observed that Z0 increases as the Pr content varies from 0.1 to 0.2. The increase in Pr content allows forming more grain aggregates, which increases the area
Fig. 4. Variation of the tan d versus frequency for BPZT10 and BPZT20 at room 50 C.
of potential barrier between the grains. This grain aggregates offers more resistance for charge carriers. Hence, the real part of the impedance seems to behave in the same fashion as the dielectric constant; it increases with the addition of Pr in the ceramics. In order to further investigate the relaxation mechanisms, Nyquist plots (Z00 versus Z0 ) are used. Fig. 6 shows the Nyquist (Z*) plots of BPZT10 and BPZT20 at 400 C. The points on the plots are the experimental data. The plots were polynomial fitted to get the impression of semicircular arc. The fitted results of BPZT10 and BPZT20 are in good agreement between the plots and fitted curves. Each of the plots has two impressed semicircles which correspond to bulk contribution at higher frequencies and grain boundary contribution at lower frequencies, respectively. The semicircle at lower frequencies is not easy to be distinguished because of the overlapping of the two semicircles. The centers of the semicircles are located below the real axis, indicating a certain degree of depression. This decentralization obeys a non-Debye type relaxation behavior which typically represents a phenomenon with a distribution of relaxation times. In order to analyze and interpret the impedance spectrum, it is helpful to have an equivalent circuit model that provides a practical illustration of the electrical properties of the respective regions; therefore, each semicircle is characterized by a parallel combination of RC element are also given in Fig. 6. The equivalent circuits for BPZT10 and BPZT20 is represented as series R1/CPE1 and R2/CPE2, where R1 and R2 are the resistances of grain and grain boundary, CPE1 and CPE2 are the effect of constant phase elements from grain and grain-boundary resistance, respectively. The values of bulk resistance and capacitance with their relaxation time (s) obtained from complex impedance spectrum are given in Table 1. From impedance spectra it can be observed that the total impedance of the BPZT ceramics increases as Pr content decreases. The increment in the impedance is attributed to the conduction mechanism at the grain and grain boundaries. The peak frequency for grain boundaries is much lower than that for grains due to their large resistance and capacitance compared with those of grains. Therefore, in the impedance spectra, we attribute the higher frequency response, which corresponds to the small arc in Fig. 6, to the grains, and the lower one to the grain boundaries. Comparing the peak values of the two responses, and from Table 1, it is seen that the resistance of grain boundaries is at least 2.5 times larger than that of grains. However, two nearly equal capacitance values imply that Cg and Cgb has a weak frequency dependence which can be
Please cite this article in press as: R. Sagar, et al., Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
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Fig. 6. Nyquist plots with equivalent circuits for BPZT10 and BPZT20 ceramics at 400 C.
revealed from the resistance (R) and relaxation time (s). Also, the little different capacitances of grains and grain boundaries give rise to arcs of approximately similar semicircles. Furthermore, 3 is constant when Cgb is frequency independent. This result is excellently consistent with our experiment, namely, we observed a constant dielectric constant in the wide frequency range due to the nearly unchanged Cgb. During the heat treatment of BPZT ceramics, oxygen atoms accumulated at the interfaces generates many electron traps at the grain boundary surfaces [25]. These trapped electrons jump the grain barrier and flow into the grain boundary layers, filling the traps first, and then accumulating at the grain boundary interfaces to form spatial charges [26]. Nevertheless, the BPZT10 ceramic with small grain size consists less charge carriers decreases due to the continued electron loss at the grain boundaries. On the other hand, our previous study shows that the grain size of BZT decreases as the rare earth ions content increases [1,2] which indicates that Pr doped BZT used in this work has the smallest grains. Fig. 7(a) shows the frequency dependant ac conductivity behavior of BPZT ceramic samples. The plot shows conductivity increases with increase in temperature. The magnitude of conductivity is more in BPZT10 sample and decreases drastically at x ¼ 0.2. For BPZT10 ceramics, frequency dependent behavior of the conductivity is observed and becomes sensitive as a function of frequency, which is generally due to the hopping of charge carriers in finite clusters frequency. The jump of conductivity in the studied temperature range indicates the enhancement of mobile charge carriers through the grain boundary which will supports the conclusion drawn from complex impedance spectra. In BZT materials oxygen vacancies are considered to be one of the mobile charge carriers and mostly, the ionization of oxygen vacancies create conduction electrons, a process which is defined by KrögereVink notation [11]. The excess electron and oxygen vacancies are formed in the reduction reaction,
this the chemical inhomogeneity, may be due to the difference in the ionic environment of Ba2þ and Pr3þ. Furthermore the sharing in the A site of perovskite may also contributes to the conduction mechanism
1 Oxo / O2 þ Vo þ 2e 2 and bond to Ti4þ/Zr4þ in the form Ti4þ/Zr4þþ þe_4Ti3þ. The formation of oxygen vacancies, may be due to neutral ðVox Þ state, which it is able to capture two electrons and it is neutral in the lattice, singly ionized ðVo Þ state and double ionized Vo state, which it is not trap any electron and it is twofold positively, can be thermally activated to enhance the conduction process. Double charge oxygen vacancies ðVo Þ are the most mobile charge in BPZT perovskite and play important role in conduction. In addition to
Fig. 7. (a) Variation of the ac conductivity versus frequency for BPZT10 and BPZT20 at room 50 C. (b) Variation of dc-resistivity as a function of temperature for BPZT10 and BPZT20 ceramics.
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Fig. 7(b) illustrates the temperature dependant conductivity of BPZT10 and BPZT20 ceramics. Decrease in the resistivity is noticed with an increase in the temperature suggesting the NTCR behavior. At low temperature, smaller grain boundaries offered high resistance to the flow of charge carriers [26]. SEM images also endorse the presence of multiple grain domains and porous microstructure. The linear decrease in resistivity as a function of temperature was due to the creation of oxygen vacancies, which acted as a mobile charge species, creating free electron, i.e.
Vo 5Vo0 þ e0 Vo0 5Vo00 þ e0 These electrons were obtained from Ti4þ and Zr4þ in the form of
Ti4þ þ e0 5Ti3þ Zr4þ þ e0 5Zr3þ
The color centers were formed due to trapped electrons of Ti4þ or Zr4þ ions or oxygen vacancies, which were thermally activated into conduction band at 353 K due to hopping of electrons [27]. The resistivity of pure BZT sample is 37.85 MU m1 at 323 K. As seen from Table 1, the room temperature resistivity of both the samples is extremely high compared to pure BZT sample, revealing NTCR behavior. The Pr3þ substituted in ceramics behaved as a donor creating energy level below conduction band. The rise in temperature provided sufficient energy to these Pr3þ cations trapped in donor level for moving toward conduction band. Thus, dcresistivity of the samples reduced with increase in temperature. B-constant value was calculated by following standard formula [28] and is given in Table 1.
3 R1 ln 6 R2 7 7 B Const ¼ 6 41 15 T1 T2 2
where R1 and R2 are the resistances measured at temperature T1 and T2, respectively. The values of B-constant reported in our study are comparable with the values reported earlier by other researchers as shown in Table 2 [29]. However, the structural stability and nonvolatile behavior of our sample have merits over others. Thus, B-values obtained endorsed BPZT as a good quality NTC material for the fabrication of thermistor device [30]. Fig. 8 shows the temperature dependent Seebeck coefficient (S) of the samples. The S values were negative in the whole temperature range, indicating electrons as major charge carriers in both the compounds. The maximum value of S is at lower temperatures and tends toward nearly zero with increase in temperature and become constant, which is due to solid state reaction taking place between the cations. The absolute value of S slightly decreases with increasing Pr-concentration. Since, the Pr-substitution affects the conduction band, i.e. Pr-ion widens the conduction band through variable rang hopping of the charge carriers and the effective mass of the carrier is reduced, thereby to cause a decrease in the value S. The S value is in good agreement with that of non-degenerated Table 2 B-constant values of some ceramic samples reported earlier. Compositions
B-constant
(Ba0.8Sr0.2)TiO3 (Ba0.97Sn0.03)TiO3 (Ba0.95Pb0.05)TiO3
822 967 905
Fig. 8. Seebeck coefficient vs. temperature for the BPZT samples.
semiconductor in the higher temperature ranges above 300 C, as evidenced in dc-electrical resistivity measurements. 4. Conclusions Pr-doped barium zirconium titanate samples were prepared by well known solid state reaction technique. XRD pattern indicates formation of single phase compound along with minor pyrochloric phase or Pr2Ti2O7 and interaction of Pr with both BZT lattice. The frequency dependant dielectric studies reviled low loss material with almost stable dielectric behavior as a function of frequency. The complex impedance spectra described influence of bulk resistance on the electrical behavior of the synthesized ceramics. Both compositions exhibited an NTCR behavior for the fabrication of highly sensitive thermistor. The decreases in resistivity as a function of temperature suggest oxygen vacancies/ions or defects are responsible for long range motion of charge carriers. Seebeck coefficient measurement revealed that both BPZT samples are n-type non degenerated semiconductors and the electron carriers are successively doped up to x ¼ 0.2. These findings also suggested that BPZT samples are promising candidates as n-type thermoelectric oxides. The difference between the lattice parameter and B-constant of the samples is quite low i.e., 0.02 A and 5, respectively. Thus, preparing the samples in various fractional weight percentages will not give any drastic change in structure and/or NTCR behavior. Acknowledgments We wish to acknowledge Department of Science and Technology (DST) New Delhi, India, for granting INSPIRE Fellowship vide No. DST/INSPIRE Fellowship/2011 dated 29 June 2011 to one of the authors (RS). Authors (NG and RLR) wish to thank University Grants Commission, New Delhi for sanction of UGC MRP project vide no. F.37-177/2009 (SR) and (SM) RFSMS fellowship vide No.F.4-1/ 2006(BSR)/11-129/2009. Thanks for Dr. P. M. Dongre, Head, Dept of Biophysics, and Mr. Chetan, Dept of Physics, University of Mumbai, Mumbai for extending XRD facility. References [1] R. Sagar, S. Madolappa, R.L. Raibagkar, NTCR behavior of Sm-substituted barium zirconium titanate nanocrystalline solid solution, Solid State Commun. 151 (2011) 1949e1952.
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Please cite this article in press as: R. Sagar, et al., Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
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Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate Raghavendra Sagar, Shivanand Madolappa, Nagbasavanna Sharanappa, R.L. Raibagkar* Department of Post Graduate Studies and Research in Materials Science, Gulbarga University, Gulbarga 585 106, Karnataka, India
h i g h l i g h t s Nano crystalline solid solution of (Ba1-xPrx)(Zr0.52Ti0.48)O3 (x¼0.1 and 0.2). AC conduction mechanism. Complex impedance (Z*) spectroscopy. n-type non-degenerated semiconductor. NTC thermistor device.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 December 2011 Received in revised form 12 February 2013 Accepted 3 March 2013
Praseodymium (Pr) doped barium zirconium titanate with nominal composition (Ba1xPrx)(Zr0.52Ti0.48)O3 (x ¼ 0.1 and 0.2) were synthesized using solid state reaction method. X-ray analysis conform the formation of cubic phase Pr-doped barium zirconium titanate along with minor pyrochloric phase. The increase in grain size after primary investigation reveals the influence of Pr ions on the domain structure and its microstructure. In order to correlate the effect of the chemical composition with the conduction mechanism, different AC electrical parameters have been addressed. The frequency dependant tangent loss of the sample was less for both the ceramics. The temperature dependence results show that the dielectric parameters and resistivity increases as Pr-content in the ceramic increases; this is attributed to the grain size and dipole dynamics. Complex impedance (Z*) plots show frequency dependent behavior as the response for the grain resistance mechanisms. This mechanism has been represented by an equivalent circuit. The temperature dependence of the electrical conductivity and Seebeck coefficient showed n-type non-degenerated semiconductor in the measured temperature range. The temperature dependent conductivity measurement suggests a novel negative temperature coefficient of resistance behavior of the samples. Furthermore, the frequency dependent conductivity plot shows increasing behavior. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Powder diffraction Microstructure Electrical properties Dielectric properties Transport properties
1. Introduction After successful investigation of primary structural and electrical properties of Samarium (Sm) and Neodymium (Nd) ions doped barium zirconium titanate (BZT) in our previous studies [1,2], the research of same compound with other rare earth ions doped is necessary in order to understand better amongst all substituted BZT’s for practical technological applications. Barium zirconium titanate Ba(ZrxTi1x)O3 (BZT), as an important member of modified BaTiO3-based ceramics, has received extensive * Corresponding author. Tel.: þ91 8472 263295; fax: þ91 8472 263206. E-mail addresses: [email protected], [email protected] (R.L. Raibagkar).
attention, due to its prominent electrical properties including high voltage resistance. Because it is derived from two perovskite lattices i.e., BaTiO3 and BaZrO3, many researchers have reported enhanced electrical and dielectric properties after the substitution of zirconium in BT [3e12]. Furthermore, BZT offers high temperature resistance and chemical stability than (Ba1xSrx)TiO3 due to substitution of Zr4þ [13]. By altering the Zr/Ti-ratio, one can easily engineer the domain structure, which changes the structuree property relation of the BZT ceramics [14]. High sintering temperature, inhomogeneous composition and internal stress within the microstructure are matter of concerns, which greatly influences electrical properties of BZT. However, reports on detailed studies of Zr4þ rich BZT solid solution exhibiting excellent electrical behavior are indeed less. Here, the electrical behavior is mainly depending
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
Please cite this article in press as: R. Sagar, et al., Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
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on the Zr4þ content, which varies the room temperature resistivity of the BZT. Electrical properties of BZT can be tuned by proper substitution of typical rare earth ion, with smaller ionic radius than that of barium [15,16]. Since, the influence of rare earth elements on the electrical properties of BT has been widely studied, the work dealing with a typical rare earth element doped-BZT are indeed limited [17,18]. Considering the uses of barium based solid solutions in device applications and less work on transport properties of barium zirconium titanate solid solutions, the objective of this research was set to seek a kind of suitable barium based material as a potential candidate for NTC thermistor. In this paper, we report the synthesis of Pr-doped BZT solid solution, with nominal stoichiometry (Ba0.9Pr0.1)(Zr0.52Ti0.48)O3 (BPZT10) and (Ba0.8Pr0..2)(Zr0.52Ti0.48)O3 (BPZT20). Phase structure and temperature dependent resistivity measurement of BPZT with two different concentrations have been studied. The main objective of present work is to understand the effect of Pr3þ ions on the electrical and thermoelectric behavior of BZT ceramics toward developing NTC thermistor device. 2. Experimental procedure 2.1. Synthesis BPZT-ceramic compounds were synthesized by well-known solid state reaction method. The starting materials BaCO3, Pr6O11, TiO2, and ZrO2 (all Aldrich make 99.99%) were weighed in stoichiometric proportions. The samples were calcined at 900 C to remove impurities like carbon present, using temperature controlled programmable muffle furnace. At 1050 C, the distinct phases of Pr-doped BaTiO3 and Pr-doped BaZrO3 were formed. Finally, the calcined powders were pressed isostatically into circular pellets at pressure of 100 MPa using a hydraulic press and sintered at 1400 C for 8 h in air to obtain final product.
r ¼ R
A d
where R ¼ resistance, A ¼ area of the pellet (pr2) and d ¼ thickness of the sample. The thermoelectric power (TEP) of BPZT samples was calculated by adjusting thermal gradient (DT) across the samples. The resulting thermal electro-motive force (DE) across the pellet was measured by a digital micro-voltmeter and thermoelectric power (S) was estimated. 3. Results and discussion The room temperature X-ray diffraction (XRD) pattern of BPZT10 and BPZT20 ceramics are shown in Fig.1. The reflection peaks’ corresponding to 2q-values were indexed on a cubic system along with pyrochloric phase of Pr2Ti2O7. On the basis of least squares’ fitting between observed and calculated inter-planar distance (d), we found that, the pattern of BPZT10 and BPZT20 reflects the formation of single phase perovskite. Here, when Pr3þ was introduced into BaTiO3eBaZrO3, due to higher stress energy of Zr4þ and Ti4þ, Pr3þ cations occupy both the sites. Increase in cell volume of the samples also endorsed the substitution of Pr-ions in BZ and BT matrix. The lattice parameters and cell volume of BPZT10 and BPZT20 are given in Table 1. The pure Ba(Zr0.52Ti0.48)O3 (BZT) sintered at lower temperature (1050 C) showed two distinct phases of BT and BZ peaks [19]. It is well-known that as the peak width increases, the crystallite size decreases. Peak broadening is originated from variations
2.2. Characterizations The phase identification was exercised using X-ray diffraction (XRD) measurements with a Philips, PANalytical PRO Diffractometer using CuKa-radiation with Ni-filter (l ¼ 1.54056 A) at room temperature, at the scanning rate of 2 min1. Cell parameters including cell volume and lattice parameters of the samples were refined by the least squares fitting method based software UNITCELL-97. The crystallites size was estimated using Scherrer’s formula. The surface morphology of the samples was examined using scanning electron microscope (SEM, Model JSM/4048/SM JEOLJapan). The grain sizes of the samples were evaluated by a line intercept method. The dielectric and impedance properties of the samples were studied using impedance analyzer (WAYNE KERR, 43,100) from which measurement of capacitance (C) tangent loss (tan d) and Impedance (Z), at various temperature range of 50 Ce 400 C were studied in order to calculate various related dielectric parameters. DC resistivity is measured using two probe method. A standard experimental unit for this measurement was employed. Thin polished and silver coated pellet (10 mm diameter and 2 mm thickness) was placed in between two spring-loaded foils of the electrode. A constant voltage source (DC voltage ¼ 10 V) was used to apply potential across the pellet. The temperature was measured with a chromel alumel thermocouple, a tip of which was maintained in close proximity to the sample for maximum accuracy. The value of current was measured using nanoammeter during cooling in the temperature range from 40 C to 500 C at the interval of 10 C and the resistance was calculated. Resistivity (r) of all samples were estimated using the relation
Fig. 1. X-ray diffractograms of the BPZT10 and BPZT20 ceramics sintered in air.
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Table 1 Structural and electrical parameters of BPZT10 and BPZT20 ceramics. Parameters
BPZT10
BPZT20
Deviation in parameters
a ( A) Cell volume ( A)3 Crystallite size (nm) Grain Size (mm)
4.05 66.43 10.21 0.42 1.34 4.65 3.10 1547 6195 7.06 7.18 1.01
4.07 64.48 14.27 0.47 1.37 4.80 3.18 1593 4304 5.24 5.46 1.1
0.02 1.95 4.06 0.05 0.03 0.02 0.17 204 16 1.82 1.72 0.09
3731 1 103 to 3.096 106
3736 1 103 to 7.463 105
5 e
g s1 (ms) s2(ms) Rg (U) Rgb( U) Cg (pF) Cgb(pF) High resistivity (rhigh) (Um1 109) B-constant (323e573 K) Seebeck coefficient (V/K)
in lattice spacing, caused by lattice strain. These salient features are clearly evidenced in the present set of compounds. Fig. 2 illustrates the SEM micrographs of BPZT compounds. Both the compounds showed porous and non-uniformly distributed grains due to the characteristic of matter transport mechanism between the grains during the sintering process. At initial stages of solid state reaction, the carbonates and oxides were well mixed and thoroughly grinded in order to reduce the powder particle sizes. The heat promoted slow kinetics of inter-diffusion in the contact points between the particles with irregular morphologies. This diffusion resulted into irregular shaped grains due to an elastic deformation caused by the surface energy reduction in the contact interface by its orientation and mobility [20,21]. The average grain sizes of the samples are given in Table 1. Fig. 3(a) presents the frequency dependent plots of the real part (3 0 ) and imaginary part (3 00 ) of the dielectric constant for Pr doped BZT samples at 50 C. It is observed that the value of 3 0 for both samples decreases as the frequency increases and attains lower values before increasing at higher frequencies. The high value of the dielectric constant at low frequencies is due to the accumulation of charges at the interfaces between the sample and the electrodes, i.e., MaxwelleWagner polarization and interfacial polarization [22]. As the frequency increases, the dipoles in the samples reorient themselves fast enough to respond to the applied electric field resulting increase in 3 0 at higher frequencies. On the other hand, at higher frequencies, it has been observed that the 3 0 value increases as the Pr content changes from 0.1 to 0.2, which is attributed to the increase in the grain size. The SEM micrographs of the BPZT10 and BPZT20 shows more aggregated grains within its microstructure. The increase in 3 0 as a function of concentration is because of the grain aggregates which induces the internal stress within the grains. When a fine-grained ceramic are subjected to field, the grain is subjected to an internal stress which depend on the orientation of all the surrounding grains. Thus, the increase in 3 0 is observed. According to Yung Park et al. [23], the stress system would tend to suppress the spontaneous deformation and force the grain back toward the cubic state. Recent, Diez-Betriu et al. [24] also revealed the increase in grain size as a function of Pr-content along with increase in the value of dielectric constant. Fig. 3(b) shows the variation of 3 as a function of temperature of BPZT10 and BPZT20 ceramics. As Pr is doped in ceramics, the Curie temperature of ceramics shifted toward higher temperature due to the internal stresses developed within the ceramics from substitution. Meanwhile, temperature dependant 3 exhibited broad peak indicating the diffuse phase transition from ferroelectric to
Fig. 2. SEM micrographs of the BPZT10 and BPZT20 ceramics samples.
paraelectric phase. The diffusivity (g) coefficient of both the ceramics is given in Table 1. Moreover, the diffuse transition behavior increased with increasing Pr content, indicating a composition induced diffuse phase transition. The diffuse phase transition is generally due to broadening of dielectric constant and deviation from CurieeWeiss law in the vicinity of Tc. The variation of loss tangent (tan d) as a function of frequency is given in Fig. 4. The figure shows a kink at 280 kHz for BPZT ceramics. This kink separates the polarization mechanism occurring due to the electrode effect from the grain boundaries mechanism, which is not noticeable in the 3 0 or 3 00 plots. However, it is observed that the dispersion process occurring due to the electrode mechanism is stronger than the one resulting from the grain boundaries. This kind of broad peak occurs when the hopping frequency of electric charge carriers becomes approximately equal to that of the external applied AC electric field [11]. The frequency dependant real (Z0 ) and imaginary (Z00 ) components of impedance for BPZT ceramics used in this work are
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Fig. 5. Variation of the real part (Z0 ) and imaginary part (Z00 ) of the impedance for BPZT10 and BPZT20 ceramics as a function of frequency at 50 C.
Fig. 3. (a) Variation of the real part (3 0 ) and imaginary part (3 00 ) of the dielectric constant for Pr doped BZT samples as a function of frequency at 50 C. (b) Temperature dependant dielectric constant of BPZT10 and BPZT20 ceramics.
shown in Fig. 5. Z0 values decrease with increasing frequency for BPZT ceramics. Furthermore, it is observed that Z0 increases as the Pr content varies from 0.1 to 0.2. The increase in Pr content allows forming more grain aggregates, which increases the area
Fig. 4. Variation of the tan d versus frequency for BPZT10 and BPZT20 at room 50 C.
of potential barrier between the grains. This grain aggregates offers more resistance for charge carriers. Hence, the real part of the impedance seems to behave in the same fashion as the dielectric constant; it increases with the addition of Pr in the ceramics. In order to further investigate the relaxation mechanisms, Nyquist plots (Z00 versus Z0 ) are used. Fig. 6 shows the Nyquist (Z*) plots of BPZT10 and BPZT20 at 400 C. The points on the plots are the experimental data. The plots were polynomial fitted to get the impression of semicircular arc. The fitted results of BPZT10 and BPZT20 are in good agreement between the plots and fitted curves. Each of the plots has two impressed semicircles which correspond to bulk contribution at higher frequencies and grain boundary contribution at lower frequencies, respectively. The semicircle at lower frequencies is not easy to be distinguished because of the overlapping of the two semicircles. The centers of the semicircles are located below the real axis, indicating a certain degree of depression. This decentralization obeys a non-Debye type relaxation behavior which typically represents a phenomenon with a distribution of relaxation times. In order to analyze and interpret the impedance spectrum, it is helpful to have an equivalent circuit model that provides a practical illustration of the electrical properties of the respective regions; therefore, each semicircle is characterized by a parallel combination of RC element are also given in Fig. 6. The equivalent circuits for BPZT10 and BPZT20 is represented as series R1/CPE1 and R2/CPE2, where R1 and R2 are the resistances of grain and grain boundary, CPE1 and CPE2 are the effect of constant phase elements from grain and grain-boundary resistance, respectively. The values of bulk resistance and capacitance with their relaxation time (s) obtained from complex impedance spectrum are given in Table 1. From impedance spectra it can be observed that the total impedance of the BPZT ceramics increases as Pr content decreases. The increment in the impedance is attributed to the conduction mechanism at the grain and grain boundaries. The peak frequency for grain boundaries is much lower than that for grains due to their large resistance and capacitance compared with those of grains. Therefore, in the impedance spectra, we attribute the higher frequency response, which corresponds to the small arc in Fig. 6, to the grains, and the lower one to the grain boundaries. Comparing the peak values of the two responses, and from Table 1, it is seen that the resistance of grain boundaries is at least 2.5 times larger than that of grains. However, two nearly equal capacitance values imply that Cg and Cgb has a weak frequency dependence which can be
Please cite this article in press as: R. Sagar, et al., Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.03.009
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Fig. 6. Nyquist plots with equivalent circuits for BPZT10 and BPZT20 ceramics at 400 C.
revealed from the resistance (R) and relaxation time (s). Also, the little different capacitances of grains and grain boundaries give rise to arcs of approximately similar semicircles. Furthermore, 3 is constant when Cgb is frequency independent. This result is excellently consistent with our experiment, namely, we observed a constant dielectric constant in the wide frequency range due to the nearly unchanged Cgb. During the heat treatment of BPZT ceramics, oxygen atoms accumulated at the interfaces generates many electron traps at the grain boundary surfaces [25]. These trapped electrons jump the grain barrier and flow into the grain boundary layers, filling the traps first, and then accumulating at the grain boundary interfaces to form spatial charges [26]. Nevertheless, the BPZT10 ceramic with small grain size consists less charge carriers decreases due to the continued electron loss at the grain boundaries. On the other hand, our previous study shows that the grain size of BZT decreases as the rare earth ions content increases [1,2] which indicates that Pr doped BZT used in this work has the smallest grains. Fig. 7(a) shows the frequency dependant ac conductivity behavior of BPZT ceramic samples. The plot shows conductivity increases with increase in temperature. The magnitude of conductivity is more in BPZT10 sample and decreases drastically at x ¼ 0.2. For BPZT10 ceramics, frequency dependent behavior of the conductivity is observed and becomes sensitive as a function of frequency, which is generally due to the hopping of charge carriers in finite clusters frequency. The jump of conductivity in the studied temperature range indicates the enhancement of mobile charge carriers through the grain boundary which will supports the conclusion drawn from complex impedance spectra. In BZT materials oxygen vacancies are considered to be one of the mobile charge carriers and mostly, the ionization of oxygen vacancies create conduction electrons, a process which is defined by KrögereVink notation [11]. The excess electron and oxygen vacancies are formed in the reduction reaction,
this the chemical inhomogeneity, may be due to the difference in the ionic environment of Ba2þ and Pr3þ. Furthermore the sharing in the A site of perovskite may also contributes to the conduction mechanism
1 Oxo / O2 þ Vo þ 2e 2 and bond to Ti4þ/Zr4þ in the form Ti4þ/Zr4þþ þe_4Ti3þ. The formation of oxygen vacancies, may be due to neutral ðVox Þ state, which it is able to capture two electrons and it is neutral in the lattice, singly ionized ðVo Þ state and double ionized Vo state, which it is not trap any electron and it is twofold positively, can be thermally activated to enhance the conduction process. Double charge oxygen vacancies ðVo Þ are the most mobile charge in BPZT perovskite and play important role in conduction. In addition to
Fig. 7. (a) Variation of the ac conductivity versus frequency for BPZT10 and BPZT20 at room 50 C. (b) Variation of dc-resistivity as a function of temperature for BPZT10 and BPZT20 ceramics.
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Fig. 7(b) illustrates the temperature dependant conductivity of BPZT10 and BPZT20 ceramics. Decrease in the resistivity is noticed with an increase in the temperature suggesting the NTCR behavior. At low temperature, smaller grain boundaries offered high resistance to the flow of charge carriers [26]. SEM images also endorse the presence of multiple grain domains and porous microstructure. The linear decrease in resistivity as a function of temperature was due to the creation of oxygen vacancies, which acted as a mobile charge species, creating free electron, i.e.
Vo 5Vo0 þ e0 Vo0 5Vo00 þ e0 These electrons were obtained from Ti4þ and Zr4þ in the form of
Ti4þ þ e0 5Ti3þ Zr4þ þ e0 5Zr3þ
The color centers were formed due to trapped electrons of Ti4þ or Zr4þ ions or oxygen vacancies, which were thermally activated into conduction band at 353 K due to hopping of electrons [27]. The resistivity of pure BZT sample is 37.85 MU m1 at 323 K. As seen from Table 1, the room temperature resistivity of both the samples is extremely high compared to pure BZT sample, revealing NTCR behavior. The Pr3þ substituted in ceramics behaved as a donor creating energy level below conduction band. The rise in temperature provided sufficient energy to these Pr3þ cations trapped in donor level for moving toward conduction band. Thus, dcresistivity of the samples reduced with increase in temperature. B-constant value was calculated by following standard formula [28] and is given in Table 1.
3 R1 ln 6 R2 7 7 B Const ¼ 6 41 15 T1 T2 2
where R1 and R2 are the resistances measured at temperature T1 and T2, respectively. The values of B-constant reported in our study are comparable with the values reported earlier by other researchers as shown in Table 2 [29]. However, the structural stability and nonvolatile behavior of our sample have merits over others. Thus, B-values obtained endorsed BPZT as a good quality NTC material for the fabrication of thermistor device [30]. Fig. 8 shows the temperature dependent Seebeck coefficient (S) of the samples. The S values were negative in the whole temperature range, indicating electrons as major charge carriers in both the compounds. The maximum value of S is at lower temperatures and tends toward nearly zero with increase in temperature and become constant, which is due to solid state reaction taking place between the cations. The absolute value of S slightly decreases with increasing Pr-concentration. Since, the Pr-substitution affects the conduction band, i.e. Pr-ion widens the conduction band through variable rang hopping of the charge carriers and the effective mass of the carrier is reduced, thereby to cause a decrease in the value S. The S value is in good agreement with that of non-degenerated Table 2 B-constant values of some ceramic samples reported earlier. Compositions
B-constant
(Ba0.8Sr0.2)TiO3 (Ba0.97Sn0.03)TiO3 (Ba0.95Pb0.05)TiO3
822 967 905
Fig. 8. Seebeck coefficient vs. temperature for the BPZT samples.
semiconductor in the higher temperature ranges above 300 C, as evidenced in dc-electrical resistivity measurements. 4. Conclusions Pr-doped barium zirconium titanate samples were prepared by well known solid state reaction technique. XRD pattern indicates formation of single phase compound along with minor pyrochloric phase or Pr2Ti2O7 and interaction of Pr with both BZT lattice. The frequency dependant dielectric studies reviled low loss material with almost stable dielectric behavior as a function of frequency. The complex impedance spectra described influence of bulk resistance on the electrical behavior of the synthesized ceramics. Both compositions exhibited an NTCR behavior for the fabrication of highly sensitive thermistor. The decreases in resistivity as a function of temperature suggest oxygen vacancies/ions or defects are responsible for long range motion of charge carriers. Seebeck coefficient measurement revealed that both BPZT samples are n-type non degenerated semiconductors and the electron carriers are successively doped up to x ¼ 0.2. These findings also suggested that BPZT samples are promising candidates as n-type thermoelectric oxides. The difference between the lattice parameter and B-constant of the samples is quite low i.e., 0.02 A and 5, respectively. Thus, preparing the samples in various fractional weight percentages will not give any drastic change in structure and/or NTCR behavior. Acknowledgments We wish to acknowledge Department of Science and Technology (DST) New Delhi, India, for granting INSPIRE Fellowship vide No. DST/INSPIRE Fellowship/2011 dated 29 June 2011 to one of the authors (RS). Authors (NG and RLR) wish to thank University Grants Commission, New Delhi for sanction of UGC MRP project vide no. F.37-177/2009 (SR) and (SM) RFSMS fellowship vide No.F.4-1/ 2006(BSR)/11-129/2009. Thanks for Dr. P. M. Dongre, Head, Dept of Biophysics, and Mr. Chetan, Dept of Physics, University of Mumbai, Mumbai for extending XRD facility. References [1] R. Sagar, S. Madolappa, R.L. Raibagkar, NTCR behavior of Sm-substituted barium zirconium titanate nanocrystalline solid solution, Solid State Commun. 151 (2011) 1949e1952.
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Please cite this article in press as: R. Sagar, et al., Synthesis, structure and electrical studies of praseodymium doped barium zirconium titanate, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.03.009