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Electrical, dielectric and pyroelectric behavior of neodymium substituted barium zirconium titanate
Solid State Sciences 14 (2012) 211e215
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Electrical, dielectric and pyroelectric behavior of neodymium substituted barium zirconium titanate Raghavendra Sagar, Shivanand Madolappa, R.L. Raibagkar* Department of Post Graduate Studies and Research in Materials Science, Gulbarga University, Gulbarga 585 106, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2011 Received in revised form 8 September 2011 Accepted 5 November 2011 Available online 13 November 2011
Neodymium (Nd) substituted barium zirconium titanate with nominal composition (Ba1xNdx) (Zr0.52Ti0.48)O3 [x ¼ 0.1, and 0.2] were synthesized using solid state reaction method. X-ray analysis confirmed the formation of perovskite structure along with minor pyroclore phase of neodymium. The change in grain size revealed the influence of Nd-ions on the microstructure. The sintered samples exhibited negative temperature coefficient of resistance (NTCR) and superior semiconducting behavior. Addition of Nd3þvaries the room temperature resistivity of Ba(Zr0.52Ti0.48)O3. As the concentration of Ndion increased, the value of temperature dependent dielectric constant decreased whereas the Curie temperature of the ceramics shifted toward higher temperature side showing diffuse phase transition. This is attributed to decrease in average grain size. Temperature dependent pyroelectric current exhibited combination of primary and secondary pyroelectric effect. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Nd-substitution Perovskite Microstructure NTCR behavior Diffused phase transition Primary and secondary pyroelectric effects
1. Introduction The study of barium based ABO3etype solid solutions has so far been of great interest from many years due to their possibility to optimize several electrical properties for manufacturing number of devices [1,2]. The potential applications of ABO3 include multilayered ceramic capacitors, thermistors, piezoelectric devices, optoelectronic elements and semiconductors [3,4]. Ba(Ti1yZry)O3 (BZT) is one among ABO3 materials, which has received extensive research attention due to its excellent electrical and dielectric behavior. By appropriate doping of Zr4þ-ions in BaTiO3 matrix, the electrical and dielectric behavior under influence of field can be tuned desirably [5e7]. Numerous efforts have been made to improve the dielectric behavior of BZT system. Impurity doping is one among the common ways to improve the materials performance [8e10]. Substitution of rare earth ions in BZT influences the dielectric properties i.e., at lower doping concentration of rare earth ion, the dielectric constant (3 r) of BZT varies exponentially [11e14]. BZT-based materials with moderate 3 r and low dielectric loss (tand) have been obtained by substituting rare earth ions up to lower concentration. Recent studies on rare earth ions doping in BZT ceramics
have demonstrated the effects on electrical and dielectric properties, which finds potential applications in various microwave devices [15e18]. Considering the applications of barium based solid solutions in electronic devices and scanty work reported on Nd-doped BZT, the objective of the research was set to study the electrical, dielectric and pyroelectric behavior. In this paper, we report the structural and microstructural analysis of ceramics having typical stoichiometric composition (Ba1xNdx)(Zr0.52Ti0.48)O3 [x ¼ 0.1 (BNZT10), and 0.2 (BNZT20)]. Temperature dependent resistivity (rdc) dielectric constant (3 r) and pyroelectric current (Ip) measurements of BNZT10 and BNZT20 samples have been studied. 2. Experimental procedure 2.1. Synthesis The ceramic compounds were synthesized by well-known solid state reaction method. The starting materials BaCO3, Nd2O3, TiO2, and ZrO2 (all Aldrich make 99.99%) were weighed in stoichiometric proportions according to the nominal formulation:
ð1 xÞBaO þ xNdO1:5 þ 0:52ZrO3 þ 0:48TiO2 /ðBa1x Ndx Þ * Corresponding author. Tel.: þ91 8472 263295; fax: þ91 8472 263206. E-mail address: [email protected] (R.L. Raibagkar). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.11.006
ðZr0:52 Ti0:48 ÞO3
212
R. Sagar et al. / Solid State Sciences 14 (2012) 211e215
The proportions were grinded homogeneously and then calcined at 900 C and 950 C for 8 h. 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. 2.2. Characterizations The BNZT ceramics were structurally characterized by X-ray diffraction (XRD) with a Philips, PANalytical PRO Diffractometer using Cu-Ka-radiation with Ni-filter (l ¼ 1.54056 Å) at room temperature, at the scanning rate of 2 /min. 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 (JSM/4048/SM JEOL-Japan). The grain sizes of the samples were evaluated by a line intercept method. DC-resistivity in the temperature range of 50 Ce500 C was obtained by standard two-probe method, by coating silver paste on both the surfaces of the pellets so as to understand the behavior of resistance as a function of temperature. Temperature dependent 3 r was studied in the temperature range 40 Ce600 C. Temperature dependent pyroelectric current (Ip) was measured by circuit constructed in our laboratory using operational amplifier (IC741). 3. Results and discussion Fig. 1 illustrates the XRD pattern for the BNZT10 and BNZT20 ceramics sintered in a conventional furnace at 1400 C for 8 h. On the basis of least squares’ fitting between observed and calculated inter-planar distance (d), we found that, the patterns of BNZT10 and BNZT20 reflects the formation of single phase perovskite along with minor phase of Nd2Ti2O7. The Nd-doped Ba(Zr0.52Ti0.48)O3 crystallize in cubic structure, as conformed by the “JCPDS” card No. 36-0019, with a space group Pm3m. Here, when Nd3þ was introduced into Ba(Zr0.52Ti0.48)O3, due to higher stress energy of Zr4þ and Ti4þ, Nd3þ cations occupy both the sites. The lattice parameters and cell volume of BNZT10 and BNZT20 are given in Table 1. Pure Ba(Zr0.52Ti0.48)O3 sintered at lower temperature 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 variation in lattice spacing, caused by lattice strain. These salient features are clearly evidenced in the present set of compounds. Fig. 2 shows the scanning electron micrographs (SEM) of the pure and Nd-doped BZT ceramic samples. The microstructure of all the samples show porous clustered and non-uniformly distributed grains. These morphological characteristics were obtained due to the matter transport mechanism between the grains during sintering. At initial stages of sintering, heat treatment performed at 1400 C for 8 h, established the contact points between the oxides and promoted a slow kinetics of inter-diffusion 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 [20]. The morphology was also due to lower sintering temperature, allowing the asymmetrical material transport [21]. Low temperature sintering allowed formation of grains of irregular shapes as a consequence of the variations on the kinetics of movement from boundary to boundary. Also, the grain-boundary energy was dependent on the grain-boundary orientation and grain-boundary mobility [22]. A decrease in average grain size after the substitution of Nd-ions in Ti4þ- and Zr4þ-site shrunk the ceramic microstructure. This result is in consistent with the report by Chou et al., that
Fig. 1. Room temperature X-ray diffraction patterns of BNZT10 and BNZT20 ceramics.
the average grain size of BZT was reduced greatly after doping of lanthanum ions [15]. The change in resistivity as a function of temperature for BNZT10 and BNZT20 ceramics is shown in Fig. 3. The graph clearly shows an NTCR behavior, i.e., high resistivity at lower temperatures (<100 C), which was attributed to multiple grain domains, and presence of pores/voids, as also observed in SEM images. Smaller grain boundaries offer high resistance to the charge carriers. Above 180 C, the linear decrease in resistivity as a function of temperature is due to the oxygen vacancies, which act as mobile charge species, creating free electrons, i.e.,
Vo 5V0o þ e0 V0o 5V00o þ e0 These electrons were obtained from Ti4þ and Zr4þ in the form of
Ti4þ þ e0 5Ti3þ
Table 1 Structural and electrical parameters of BNZT10 and BNZT20 ceramics. Parameters
BNZT10
BNZT20
a (Å) Cell volume (Å)3 Crystallite size (nm) Grain Size (mm) High Resistivity (rhigh) (Um) B-constant (323e573 K) Diffusivity (g)
4.024 65.15 10.99 0.58 1.88 109 294 1.9
4.029 65.40 13.66 0.30 1.4 1010 576 1.2
R. Sagar et al. / Solid State Sciences 14 (2012) 211e215
213
Fig. 3. Variation of dc-resistivity as a function of temperature for BNZT10 and BNZT20.
where R1, and R2 are the values of resistance at temperature T1 ¼ 100 C and T2 ¼ 200 C. The obtained B-constant values endorse BNZT20 as a good quality NTCR material for the fabrication of thermistor device [23]. Fig. 4 shows the variation of 3 r as a function of temperature for BNZT10 and BNZT20 ceramics. At temperature above 300 C, the 3 r increases with temperature up to Curie temperature (Tc) and then decreases which is normally expected in most of perovskite materials. The Nd-ion affects the transition temperature of BNZT10 and BNZT20 ceramics by shifting the Tc above 10 C due to the internal stresses developed within the ceramics from substitution. Hiroshima et al. have reported a close relation between the Curie temperature and internal stresses developed within the constrained grains at the phase transition temperature [24]. The internal stress shift Tc toward higher temperature because of non-uniform grain size effect. This change in internal stress is anticipated in the pellets for the reason of change in sintering temperature. In large grained BNZT ceramics, the internal stress concentration is more and enough to form cracks at grain boundaries, due to which Tc shift toward higher temperature side and observed at 550 C and 560 C for BNZT10 and BNZT20 respectively. Fig. 2. SEM micrographs of BNZT10 and BNZT20 ceramics.
Zr4þ þ e0 5Zr3þ Hence, the color centers were formed due to these trapped electrons of Ti4þ or Zr4þ ions or oxygen vacancies, which are easily activated into conduction band by thermal energy. The Nd3þsubstitution behaves as a donor impurity source creates donor level below conduction band. Increase in temperature provides sufficient energy to these trapped electrons in donor levels for moving toward conduction band. Thus, dc-resistivity decreases with increase in temperature. The resistivity of BZT sample was 37.85 MU at 50 C as compared with BNZT10 and BNZT20 (Table 1). B-constant value was calculated by the following standard formula and is given in Table 1.
R1 R2 B Const ¼ 1 1 T1 T2 ln
Fig. 4. Temperature dependent dielectric behavior of pure and Nd-doped BZT ceramics.
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R. Sagar et al. / Solid State Sciences 14 (2012) 211e215
However, 3 r of BNZT ceramics exhibited a broad peak. This broadness indicates the diffuse phase transition. Moreover, the diffuse phase transition behavior increased with increasing Nd content, signifying a composition-induced diffuse phase transition. The diffuse phase transition is generally due to broadening of 3 r and deviation from CurieeWeiss law in the vicinity of Tc, as can be observed in Fig. 4. A modified CurieeWeiss law was proposed to describe the diffusivity of the phase transition by the equation as [25]
1 3
1 3m
¼
ðT Tm Þg C1
where g and C1 are assumed to be constants. The parameter g gives information about degree of diffusivity. The plot of ln(1/3 1/3 m) as a function of ln(T Tm) for the two sample is shown in Fig. 5. A linear relationship is observed for both the samples. The slope of the curves was used to determine the value of g, which is given in Table 1. It is found that as x increases from 0.1 to 0.2, g decreases drastically from 1.9 to 1.2. This indicates that the diffuseness of the phase transition reduces with the increasing of Nd content. It is because of the fact that the average grain size of synthesized ceramics decreases as Nd content increases. Hence, the Nd-substitution in BZT ceramics showed a strong diffuse phase transition and also a relaxor-type diffuse phase permittivity. Temperature dependent pyroelectric current (Ip) of the Ndsubstituted BZT samples are shown in Fig. 6. At lower temperatures, Ip was negative in all the samples. There is sudden change in the slope of the curve around 180 C. The sign of Ip changes from negative to positive and reaches maximum value upto transition temperature where the value of 3 r is also observed to be higher. Ip decreases on the positive side above the transition temperature. The room temperature value of Ip observed for BZT ceramics was 4.125 107 Amps, where as for the BNZT10 and BNZT20 ceramics, the value of Ip are 7.32114 107 Amps and 1.06 106 Amps respectively. In the perovskite compounds, the sign of total pyroelectric current depends on the dominance of primary or secondary pyroelectric effects. The negative values of the total pyroelectric current of the present samples in lower temperatures clearly indicate the dominance of primary pyroelectricity, which is akin to that of the other perovskite compounds [26]. The positive sign of the
Fig. 6. Pyroelectric current as a function of temperature for Nd-doped BZT ceramics.
pyroelectric current at higher temperature is generally suggestive of the dominance of secondary pyroelectric effect. However, the rise in the pyroelectric current is too strong to be totally suspected to be secondary effect. It may also be due to a sudden release of stored charges associated with the microstructure. The secondary pyroelectricity originates from the contribution caused by piezoelectric, elastic stiffness and thermal expansion effects. Thus, it is possible to evaluate the primary and secondary pyroelectric coefficients separately from the pyroelectric current measurements. 4. Conclusions Nd-substituted Ba(Zr0.52Ti0.48)O3 samples were prepared by the solid state reaction. XRD pattern revealed the presence of polycrystalline BNZT perovskite phase along with Nd2Ti2O7 pyrochlore phase and interaction of Nd with BZT lattice. Both the compositions exhibited an NTCR behavior, for the fabrication of highly sensitive thermistor. The decrease in resistivity as a function of temperature suggests oxygen vacancies/ions or defects are responsible for long range motion of charge carriers. The diffuse phase transition behavior of the ceramics becomes more remarkable at higher Nd content, implying the composition-induced diffuse transition. A modified CurieeWeiss law describes the diffuseness of a phase transition. The nature of pyroelectric current as a function of temperature depicted the existence of both primary and secondary effect in the ceramics. An attempt will be made to prepare the said samples by one of the standard chemical techniques and comparison of the data with significant observations will be reported elsewhere. Acknowledgments Authors (RS and SM) would like to acknowledge University Grants Commission (UGC), New Delhi, India, for granting Non-SAP RFSMS fellowship vide No. F.4-1/2006 (BSR)/11-129/2009(BSR). References
Fig. 5. The plot of ln(1/3 1/3 m) as a function of ln(T Tm) for BNZT10 and BNZT20.
[1] T.R.N. Kutty, N.S. Hari, J. Phys. D: Appl. Phys. 28 (1995) 371e374. [2] A. Kozyrev, A. Ivanov, O. Soldatenkov, E. Gol’man, A. Prudan, V. Loginov, Tech. Phys. Lett. 25 (10) (1999) 836e837. [3] N.V. Giridharan, M. Subramanian, R. Jayavel, Appl. Phys. A. 83 (2006) 123e126. [4] U. Weber, G. Greuel, U. Botteger, S. Weber, D. Hennings, R. Waser, J. Am. Ceram. Soc. 84 (2001) 759e766.
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D. Hennings, A. Schnell, J. Am. Ceram. Soc. 65 (1982) 539e544. S.M. Neirman, J. Mater. Sci. 23 (1988) 3973e3980. Z.Y.C. Ang, R. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 2655e2657. S. Halder, T. Schneller, U. Bottger, R. Waser, Appl. Phys. A. 81 (2005) 25e29. F. Moura, A.Z. Simoes, B.D. Stojanovic, M.A. Zaghete, E. Longo, J.A. Varela, J. Alloys Compd. 462 (2008) 129e134. F. Moura, A.Z. Simoes, L.S. Cavalcante, M.A. Zaghete, J.A. Varela, E. Longo, J. Alloys Compd. 466 (2008) L15eL18. W. Cai, C. Fua, J. Gao, H. Chen, J. Alloys Compd. 480 (2009) 870e873. S.B. Reddy, K.P. Rao, M.S.R. Rao, J. Alloys Compd. 481 (2009) 692e696. W. Cai, J. Gao, C. Fu, L. Tang, J. Alloys Compd. 87 (2009) 668e674. A. Dixit, S.B. Mujumdar, A.S. Savvinov, R.S. Katiyar, R. Guo, A.S. Bhalla, Mat. Lett. 56 (2002) 933e940. X. Chou, J. Zhai, H. Jiang, X. Yao, J. Appl. Phys. 102 (2007) 0841061e0841066. C. Ostos, L. Metres, M.L. Martinaz-Sarrion, J.E. Garcia, A. Albarda, R. Perez, Sol. St. Sci. 11 (2009) 1016e1022.
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[17] E. Delgado, C. Ostos, M.L.M. Sarrion, L. Mestres, P. Prieto, Phys. Stat. Sol 4 (11) (2007) 4099e4106. [18] S.B. Reddy, M.S.R. Rao, K. Prasad Rao, Appl. Phys. Lett. 91 (2007) 0229171. [19] R. Sagar, S. Madolappa, R.L. Raibagkar, BioNano Frontier, Spl. Issue (2010) 221e225. [20] M.N. Rahaman, Sintering of Ceramics, FL. CRC Press, Taylor and Francis Group, Boca Raton, 2008, 55e106. [21] S.J. Kang L, Sintering-densification, Grain Growth and Microstructure. Elsevier, Amsterdam, 2005, 39e91. [22] Z.C. Xia, S.L. Yuan, W. Feng, L.J. Zhang, G.H. Zhang, J. Tang, L. Liu, S. Liu, G. Peng, D.W. Niu, L. Chen, Q.H. Zheng, Z.H. Fang, C.Q. Tang, Sol. Stat. Commun. 128 (2003) 291e294. [23] A. Feteira, J. Am. Ceram. Soc. 92 (5) (2009) 967e983. [24] T. Hiroshima, K. Tanaka, T. Kimura, J. Am. Ceram. Soc. 79 (1996) 3235e3242. [25] H. Chen, C. Yang, C. Fu, J. Shi, J. Zhang, W. Leng, J. Mater. Sci. Mater. Electron. 19 (2008) 379e382. [26] G.S. Murugan, K.B.R. Varma, J. Electroceram 8 (2002) 37e48.
Contents lists available at SciVerse ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Electrical, dielectric and pyroelectric behavior of neodymium substituted barium zirconium titanate Raghavendra Sagar, Shivanand Madolappa, R.L. Raibagkar* Department of Post Graduate Studies and Research in Materials Science, Gulbarga University, Gulbarga 585 106, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2011 Received in revised form 8 September 2011 Accepted 5 November 2011 Available online 13 November 2011
Neodymium (Nd) substituted barium zirconium titanate with nominal composition (Ba1xNdx) (Zr0.52Ti0.48)O3 [x ¼ 0.1, and 0.2] were synthesized using solid state reaction method. X-ray analysis confirmed the formation of perovskite structure along with minor pyroclore phase of neodymium. The change in grain size revealed the influence of Nd-ions on the microstructure. The sintered samples exhibited negative temperature coefficient of resistance (NTCR) and superior semiconducting behavior. Addition of Nd3þvaries the room temperature resistivity of Ba(Zr0.52Ti0.48)O3. As the concentration of Ndion increased, the value of temperature dependent dielectric constant decreased whereas the Curie temperature of the ceramics shifted toward higher temperature side showing diffuse phase transition. This is attributed to decrease in average grain size. Temperature dependent pyroelectric current exhibited combination of primary and secondary pyroelectric effect. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Nd-substitution Perovskite Microstructure NTCR behavior Diffused phase transition Primary and secondary pyroelectric effects
1. Introduction The study of barium based ABO3etype solid solutions has so far been of great interest from many years due to their possibility to optimize several electrical properties for manufacturing number of devices [1,2]. The potential applications of ABO3 include multilayered ceramic capacitors, thermistors, piezoelectric devices, optoelectronic elements and semiconductors [3,4]. Ba(Ti1yZry)O3 (BZT) is one among ABO3 materials, which has received extensive research attention due to its excellent electrical and dielectric behavior. By appropriate doping of Zr4þ-ions in BaTiO3 matrix, the electrical and dielectric behavior under influence of field can be tuned desirably [5e7]. Numerous efforts have been made to improve the dielectric behavior of BZT system. Impurity doping is one among the common ways to improve the materials performance [8e10]. Substitution of rare earth ions in BZT influences the dielectric properties i.e., at lower doping concentration of rare earth ion, the dielectric constant (3 r) of BZT varies exponentially [11e14]. BZT-based materials with moderate 3 r and low dielectric loss (tand) have been obtained by substituting rare earth ions up to lower concentration. Recent studies on rare earth ions doping in BZT ceramics
have demonstrated the effects on electrical and dielectric properties, which finds potential applications in various microwave devices [15e18]. Considering the applications of barium based solid solutions in electronic devices and scanty work reported on Nd-doped BZT, the objective of the research was set to study the electrical, dielectric and pyroelectric behavior. In this paper, we report the structural and microstructural analysis of ceramics having typical stoichiometric composition (Ba1xNdx)(Zr0.52Ti0.48)O3 [x ¼ 0.1 (BNZT10), and 0.2 (BNZT20)]. Temperature dependent resistivity (rdc) dielectric constant (3 r) and pyroelectric current (Ip) measurements of BNZT10 and BNZT20 samples have been studied. 2. Experimental procedure 2.1. Synthesis The ceramic compounds were synthesized by well-known solid state reaction method. The starting materials BaCO3, Nd2O3, TiO2, and ZrO2 (all Aldrich make 99.99%) were weighed in stoichiometric proportions according to the nominal formulation:
ð1 xÞBaO þ xNdO1:5 þ 0:52ZrO3 þ 0:48TiO2 /ðBa1x Ndx Þ * Corresponding author. Tel.: þ91 8472 263295; fax: þ91 8472 263206. E-mail address: [email protected] (R.L. Raibagkar). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.11.006
ðZr0:52 Ti0:48 ÞO3
212
R. Sagar et al. / Solid State Sciences 14 (2012) 211e215
The proportions were grinded homogeneously and then calcined at 900 C and 950 C for 8 h. 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. 2.2. Characterizations The BNZT ceramics were structurally characterized by X-ray diffraction (XRD) with a Philips, PANalytical PRO Diffractometer using Cu-Ka-radiation with Ni-filter (l ¼ 1.54056 Å) at room temperature, at the scanning rate of 2 /min. 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 (JSM/4048/SM JEOL-Japan). The grain sizes of the samples were evaluated by a line intercept method. DC-resistivity in the temperature range of 50 Ce500 C was obtained by standard two-probe method, by coating silver paste on both the surfaces of the pellets so as to understand the behavior of resistance as a function of temperature. Temperature dependent 3 r was studied in the temperature range 40 Ce600 C. Temperature dependent pyroelectric current (Ip) was measured by circuit constructed in our laboratory using operational amplifier (IC741). 3. Results and discussion Fig. 1 illustrates the XRD pattern for the BNZT10 and BNZT20 ceramics sintered in a conventional furnace at 1400 C for 8 h. On the basis of least squares’ fitting between observed and calculated inter-planar distance (d), we found that, the patterns of BNZT10 and BNZT20 reflects the formation of single phase perovskite along with minor phase of Nd2Ti2O7. The Nd-doped Ba(Zr0.52Ti0.48)O3 crystallize in cubic structure, as conformed by the “JCPDS” card No. 36-0019, with a space group Pm3m. Here, when Nd3þ was introduced into Ba(Zr0.52Ti0.48)O3, due to higher stress energy of Zr4þ and Ti4þ, Nd3þ cations occupy both the sites. The lattice parameters and cell volume of BNZT10 and BNZT20 are given in Table 1. Pure Ba(Zr0.52Ti0.48)O3 sintered at lower temperature 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 variation in lattice spacing, caused by lattice strain. These salient features are clearly evidenced in the present set of compounds. Fig. 2 shows the scanning electron micrographs (SEM) of the pure and Nd-doped BZT ceramic samples. The microstructure of all the samples show porous clustered and non-uniformly distributed grains. These morphological characteristics were obtained due to the matter transport mechanism between the grains during sintering. At initial stages of sintering, heat treatment performed at 1400 C for 8 h, established the contact points between the oxides and promoted a slow kinetics of inter-diffusion 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 [20]. The morphology was also due to lower sintering temperature, allowing the asymmetrical material transport [21]. Low temperature sintering allowed formation of grains of irregular shapes as a consequence of the variations on the kinetics of movement from boundary to boundary. Also, the grain-boundary energy was dependent on the grain-boundary orientation and grain-boundary mobility [22]. A decrease in average grain size after the substitution of Nd-ions in Ti4þ- and Zr4þ-site shrunk the ceramic microstructure. This result is in consistent with the report by Chou et al., that
Fig. 1. Room temperature X-ray diffraction patterns of BNZT10 and BNZT20 ceramics.
the average grain size of BZT was reduced greatly after doping of lanthanum ions [15]. The change in resistivity as a function of temperature for BNZT10 and BNZT20 ceramics is shown in Fig. 3. The graph clearly shows an NTCR behavior, i.e., high resistivity at lower temperatures (<100 C), which was attributed to multiple grain domains, and presence of pores/voids, as also observed in SEM images. Smaller grain boundaries offer high resistance to the charge carriers. Above 180 C, the linear decrease in resistivity as a function of temperature is due to the oxygen vacancies, which act as mobile charge species, creating free electrons, i.e.,
Vo 5V0o þ e0 V0o 5V00o þ e0 These electrons were obtained from Ti4þ and Zr4þ in the form of
Ti4þ þ e0 5Ti3þ
Table 1 Structural and electrical parameters of BNZT10 and BNZT20 ceramics. Parameters
BNZT10
BNZT20
a (Å) Cell volume (Å)3 Crystallite size (nm) Grain Size (mm) High Resistivity (rhigh) (Um) B-constant (323e573 K) Diffusivity (g)
4.024 65.15 10.99 0.58 1.88 109 294 1.9
4.029 65.40 13.66 0.30 1.4 1010 576 1.2
R. Sagar et al. / Solid State Sciences 14 (2012) 211e215
213
Fig. 3. Variation of dc-resistivity as a function of temperature for BNZT10 and BNZT20.
where R1, and R2 are the values of resistance at temperature T1 ¼ 100 C and T2 ¼ 200 C. The obtained B-constant values endorse BNZT20 as a good quality NTCR material for the fabrication of thermistor device [23]. Fig. 4 shows the variation of 3 r as a function of temperature for BNZT10 and BNZT20 ceramics. At temperature above 300 C, the 3 r increases with temperature up to Curie temperature (Tc) and then decreases which is normally expected in most of perovskite materials. The Nd-ion affects the transition temperature of BNZT10 and BNZT20 ceramics by shifting the Tc above 10 C due to the internal stresses developed within the ceramics from substitution. Hiroshima et al. have reported a close relation between the Curie temperature and internal stresses developed within the constrained grains at the phase transition temperature [24]. The internal stress shift Tc toward higher temperature because of non-uniform grain size effect. This change in internal stress is anticipated in the pellets for the reason of change in sintering temperature. In large grained BNZT ceramics, the internal stress concentration is more and enough to form cracks at grain boundaries, due to which Tc shift toward higher temperature side and observed at 550 C and 560 C for BNZT10 and BNZT20 respectively. Fig. 2. SEM micrographs of BNZT10 and BNZT20 ceramics.
Zr4þ þ e0 5Zr3þ Hence, the color centers were formed due to these trapped electrons of Ti4þ or Zr4þ ions or oxygen vacancies, which are easily activated into conduction band by thermal energy. The Nd3þsubstitution behaves as a donor impurity source creates donor level below conduction band. Increase in temperature provides sufficient energy to these trapped electrons in donor levels for moving toward conduction band. Thus, dc-resistivity decreases with increase in temperature. The resistivity of BZT sample was 37.85 MU at 50 C as compared with BNZT10 and BNZT20 (Table 1). B-constant value was calculated by the following standard formula and is given in Table 1.
R1 R2 B Const ¼ 1 1 T1 T2 ln
Fig. 4. Temperature dependent dielectric behavior of pure and Nd-doped BZT ceramics.
214
R. Sagar et al. / Solid State Sciences 14 (2012) 211e215
However, 3 r of BNZT ceramics exhibited a broad peak. This broadness indicates the diffuse phase transition. Moreover, the diffuse phase transition behavior increased with increasing Nd content, signifying a composition-induced diffuse phase transition. The diffuse phase transition is generally due to broadening of 3 r and deviation from CurieeWeiss law in the vicinity of Tc, as can be observed in Fig. 4. A modified CurieeWeiss law was proposed to describe the diffusivity of the phase transition by the equation as [25]
1 3
1 3m
¼
ðT Tm Þg C1
where g and C1 are assumed to be constants. The parameter g gives information about degree of diffusivity. The plot of ln(1/3 1/3 m) as a function of ln(T Tm) for the two sample is shown in Fig. 5. A linear relationship is observed for both the samples. The slope of the curves was used to determine the value of g, which is given in Table 1. It is found that as x increases from 0.1 to 0.2, g decreases drastically from 1.9 to 1.2. This indicates that the diffuseness of the phase transition reduces with the increasing of Nd content. It is because of the fact that the average grain size of synthesized ceramics decreases as Nd content increases. Hence, the Nd-substitution in BZT ceramics showed a strong diffuse phase transition and also a relaxor-type diffuse phase permittivity. Temperature dependent pyroelectric current (Ip) of the Ndsubstituted BZT samples are shown in Fig. 6. At lower temperatures, Ip was negative in all the samples. There is sudden change in the slope of the curve around 180 C. The sign of Ip changes from negative to positive and reaches maximum value upto transition temperature where the value of 3 r is also observed to be higher. Ip decreases on the positive side above the transition temperature. The room temperature value of Ip observed for BZT ceramics was 4.125 107 Amps, where as for the BNZT10 and BNZT20 ceramics, the value of Ip are 7.32114 107 Amps and 1.06 106 Amps respectively. In the perovskite compounds, the sign of total pyroelectric current depends on the dominance of primary or secondary pyroelectric effects. The negative values of the total pyroelectric current of the present samples in lower temperatures clearly indicate the dominance of primary pyroelectricity, which is akin to that of the other perovskite compounds [26]. The positive sign of the
Fig. 6. Pyroelectric current as a function of temperature for Nd-doped BZT ceramics.
pyroelectric current at higher temperature is generally suggestive of the dominance of secondary pyroelectric effect. However, the rise in the pyroelectric current is too strong to be totally suspected to be secondary effect. It may also be due to a sudden release of stored charges associated with the microstructure. The secondary pyroelectricity originates from the contribution caused by piezoelectric, elastic stiffness and thermal expansion effects. Thus, it is possible to evaluate the primary and secondary pyroelectric coefficients separately from the pyroelectric current measurements. 4. Conclusions Nd-substituted Ba(Zr0.52Ti0.48)O3 samples were prepared by the solid state reaction. XRD pattern revealed the presence of polycrystalline BNZT perovskite phase along with Nd2Ti2O7 pyrochlore phase and interaction of Nd with BZT lattice. Both the compositions exhibited an NTCR behavior, for the fabrication of highly sensitive thermistor. The decrease in resistivity as a function of temperature suggests oxygen vacancies/ions or defects are responsible for long range motion of charge carriers. The diffuse phase transition behavior of the ceramics becomes more remarkable at higher Nd content, implying the composition-induced diffuse transition. A modified CurieeWeiss law describes the diffuseness of a phase transition. The nature of pyroelectric current as a function of temperature depicted the existence of both primary and secondary effect in the ceramics. An attempt will be made to prepare the said samples by one of the standard chemical techniques and comparison of the data with significant observations will be reported elsewhere. Acknowledgments Authors (RS and SM) would like to acknowledge University Grants Commission (UGC), New Delhi, India, for granting Non-SAP RFSMS fellowship vide No. F.4-1/2006 (BSR)/11-129/2009(BSR). References
Fig. 5. The plot of ln(1/3 1/3 m) as a function of ln(T Tm) for BNZT10 and BNZT20.
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