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Effect of Fe and Fe–Ba substitution on the piezoelectric and dielectric properties of lead zirconate titanate ceramics
Materials Science and Engineering B 176 (2011) 242–245
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Effect of Fe and Fe–Ba substitution on the piezoelectric and dielectric properties of lead zirconate titanate ceramics S.R. Sangawar ∗ , B. Praveenkumar, H.H. Kumar, D.K. Kharat PZT Centre, Armament Research and Development Establishment, Pune 411021, India
a r t i c l e
i n f o
Article history: Received 20 May 2010 Received in revised form 9 September 2010 Accepted 1 December 2010 Keywords: PZT Ceramics Solid-state reaction Mechanical quality factor Piezoelectric SONAR transducer
a b s t r a c t Polycrystalline samples of Fe and Fe–Ba doped lead zirconate titanate (PZT) ceramics near the morphotrophic phase boundary have been synthesized by a solid-state reaction technique. Preliminary X-ray analysis of the compound confirms that there is no change in the crystal structure of PZT on co-doping with Fe and Ba. The maximum mechanical quality factor Qm was found to be 1000 for Fe doped material and 880 for Fe–Ba doped material. The electromechanical coupling factor for Fe and Fe–Ba doped samples were 0.535 and 0.495 respectively. The corresponding values for the piezoelectric charge constant d33 were 135 and 250 pC/N respectively. These results are discussed in terms of position occupied by dopants in to the lattice and their corresponding microstructures. These Fe–Ba doped PZT materials could be likely candidates for high power ultrasonic and underwater SONAR transducer systems. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Pb(Zr,Ti)O3 (PZT) ceramics with a composition around the morphotrophic phase boundary have been studied extensively to understand its electromechanical properties. These ceramic materials are cost effective and have greater durability under adverse atmosphere conditions [1,2]. They are found to be useful for sensors, actuators and high power transducer applications and offers linearity, higher mechanical and dielectric strengths achieved through optimized microstructure [3]. Piezoelectric charge coefficient and dielectric constant of PZT ceramics are mainly because of its domain structure, but also by the presence of defects that can modify the domain wall mobility with appropriate quantity of acceptor and donor dopants which classifies PZT as hard and soft type/class. Hard PZT ceramics [4] can withstand high levels of electrical excitation which are well suited for high power generators and transducers, while the significant feature of soft PZT is the high sensitivity and permittivity which are well suited for various sensors and hydrophone applications [5]. Most of the earlier studies [6–8] have been carried out by using conventionally prepared powders for optimizing piezoelectric properties and very little effort have been made to study the role of substituents in modifying and optimizing the mechanical quality factor (Qm ) of PZT along with higher piezoelectric charge
∗ Corresponding author. Tel.: +91 020 25881828; fax: +91 020 25893102. E-mail address: [email protected] (S.R. Sangawar). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.12.003
coefficient (d33 ). As reported earlier [9] the substitution of Fe on PZT decreases the dielectric constant and dissipation factor while increasing the mechanical quality factor and frequency constant. This could be achieved mainly because of Fe substitution, which helped in inhibiting grain growth. Obtaining both high piezoelectric charge coefficient and mechanical quality factor has not been studied extensively so far, whereas piezoelectric and mechanical quality factors were studied individually by few researchers [10–12]. In the present study, an attempt has been made by substituting Fe and Fe–Ba dopant on PZT to obtain high mechanical quality factor, piezoelectric charge coefficient and dielectric properties for high power generators and SONAR systems. 2. Experimental The materials investigated in the present work were hard PZT material with following composition Pb(Zr0.53 Ti0.47 )O3 + (1 wt%) Fe2 O3 and Pb(Zr0.53 Ti0.47 )O3 + (1 wt%) Fe2 O3 + (1 wt%) BaCO3 and hereafter designated as PZTF and PBZTF respectively. AR grade (>99% pure) PbO (D’Waldies), ZrO2 (Loba Chemie), TiO2 (Loba Chemie), BaCO3 (Loba Chemie) and Fe2 O3 (Edwards & Illford) powders were used. Powders were mixed in appropriate molar ratios and ball milled for 24 h using distilled water. Dried powder was calcined at 1000 ◦ C in closed alumina crucibles for 2 h. Powder was again ball milled for 24 h to reduce particle size. Particle size of both the milled powders was measured by Particle size analyzer (Model 2000MU, Malvern) and was found to be ∼1.2 m. X-ray diffractograms (XRD) of both the powder samples were recorded at room temperature with Cu K␣ radiation using an X-ray powder diffrac-
S.R. Sangawar et al. / Materials Science and Engineering B 176 (2011) 242–245
243
7.72 7.70
PBZTF PFZT
7.68 7.66
Density (g/cc)
7.64 7.62 7.60 7.58 7.56 7.54 7.52 7.50 7.48 1225
1250
1275
1300
Sintering Temperature (ºC) Fig. 1. XRD patterns of PZTF and PBZTF samples.
Fig. 2. Density vs sintering temperature for PZTF and PBZTF samples.
tometer (Model PW-3020, Philips) for phase formation studies. The binder polyvinylalcohol (PVA) was added to milled powder and disks (20 × 2 mm) were compacted using uniaxial press at 150 MPa pressure. Compacts were sintered in the temperature range of 1250 to 1300 ◦ C in a lead rich atmosphere. Microstructural studies of the sintered compacts were carried out using Scanning Electron Microscope (SEM) (Model No. JSM-6360, JEOL). Sintered compacts were electroded using silver paste and subsequently poled at 3 kV/mm in silicone oil bath at 100 ◦ C for 30 min. Dielectric and piezoelectric properties of poled samples were measured 24 h after the poling. Capacitance was measured by LCR Bridge (Model 4262A, Hewlett Packard) at frequency of 1 kHz and piezoelectric charge coefficient (d33 ) was measured using d33 Meter (Model CPDT3300, Channel Products), dielectric constant (ε), mechanical quality factor (Qm ) and coupling coefficient (Kp ) were calculated by standard formulae using the measured values of capacitance, resonance and anti-resonance frequencies.
temperature. The densification of ceramics is produced entirely by solid state diffusion processes. The decrease in density with sintering temperature may be attributed to rise in the number of oxygen vacancy concentration at higher temperatures. The scanning electron micrographs of flat surfaces of sintered PZTF and PBZTF samples are shown in Fig. 3(a) and (b). The specimens are sintered at optimized sintering condition 1250 ◦ C and grain size is measured by linear intercept method. It is observed that grain size decreases with addition of Ba2+ substitution in PBZTF
3. Results and discussion Fig. 1 shows XRD patterns of Fe (PZTF) and Fe–Ba (PBZTF) doped PZT ceramics. It is seen that, both PZTF and PBZTF samples are polycrystalline and perovskite in nature. The phases are pure ˚ could and no secondary phases were observed. Fe3+ (RFe = 0.78 A) occupy the B-site of PZT lattice as acceptor owing to the similar ˚ and Ti4+ (RTi = 0.68 A) ˚ while the ionic radius of Zr4+ (RZr = 0.79 A) ˚ occupies the A-site of PZT partially subisovalent Ba2+ (RBa = 1.34 A) ˚ Isovalent substitutions of Pb2+ ions by stituting Pb2+ (RPb = 1.20 A). Ba2+ do not create any oxygen vacancies on the contrary acceptor doped substitutions of Zr4+ or Ti4+ by Fe3+ causes oxygen vacancies. Our results indicate that Fe and Ba contribute to the alteration of phase formation from coexistence to tetragonal and additional Ba with Fe resulted in splitting of peaks. Calculation of XRD data reveals decrease in the unit cell volume from 0.0663 nm3 (PBZTF) to 0.0654 nm3 (PZTF). Fig. 2 illustrates the behavior of densities of sintered samples as a function of sintering temperature for both PZTF and PBZTF samples. It is evident from the graph that PBZTF has higher density than PZTF samples. Doping effect of Fe3+ is less in increasing the densification as compared to combined Fe3+ and Ba2+ ions due to shrinkage and distortion of the unit cell caused by the formation of oxygen vacancies in the lattice. In PBZTF, the effect of Fe3+ is slightly reduced by partial substitution of Pb2+ by Ba2+ , for maintaining the unit cell size with no shrinkage [13]. Densities of all the samples are optimum at 1250 ◦ C and then decreases with increase in sintering
Fig. 3. SEM of sintered (a) PZTF and (b) PBZTF samples.
244
S.R. Sangawar et al. / Materials Science and Engineering B 176 (2011) 242–245 1100
PZTF PBZTF
280 260 240 220 200 180 160 140
Mechanical Quality Factor (Qm)
Piezoelectric charge coefficient (d33 )
300
PZTF PBZTF
1050 1000 950 900 850 800 750 700 650
120
1225
1250
1275
1300
0 Sintering Temperature ( C)
100 80
Fig. 5. Qm vs sintering temperature for PZTF and PBZTF samples.
1225
1250
1275
1300
Sintering Temperature (ºC) Fig. 4. d33 vs sintering temperature for PZTF and PBZTF samples.
and it is around 1 m compared to 2 m grain size of PZTF samples. Ba2+ substitution decreases the grain size and increases the density because of chemical diffusion of isovalent ions substituting the lead ions and optimized the compositional homogeneity. It also shown that PBZTF sample has lower and controlled grain size which attributes to narrow particle size distribution. The decrease in grain size increases the density of PBZTF samples as mentioned earlier. In general, PZT will have higher density because of higher concentration of Pb ions. When single and/or co-doping of respective cation substitution, will replace Pb and Zr/Ti ions in PZT, causing decrease in density of PZT ceramics. SEM studies revealed the formation of micro pores among the grains, and confirms decrease in density for both the samples. The microstructural parameters influence the electrical and mechanical properties of the PZT ceramics. The splitting of peaks observed in XRD could be attributed to the change in grain morphology. Fig. 4 shows the variation of piezoelectric charge coefficient (d33 ) with sintering temperature for both PZTF and PBZTF samples. It was observed that PBZTF have higher d33 than the PZTF samples. This may be due to the following reasons observed in PBZTF samples: (i) higher density, (ii) no shrinkage and distortion of unit cell, (iii) less hindrance for the orientation of dipoles, (iv) higher poling efficiency due to the presence of Ba2+ ions and decrease in conductivity (v) decrease in specific resistivity [14]. Whereas in PZTF samples the addition of Fe3+ results in creation of oxygen vacancies and decreases unit cell size and it hinders the orientation of dipoles which results in difficulty in poling. It is revealed that PBZTF has lower conductivity than PZTF because of substitution of Ba2+ ions, which subsequently increases the efficiency of poling and results in higher d33 . It was also observed from the graph that d33 increases with sintering temperature till it reaches the optimum conditions (1250 ◦ C) and then decreases for both PZTF and PBZTF because of its density. Fig. 5 shows the variation of mechanical quality factor (Qm ) with sintering temperature for both PZTF and PBZTF samples. Mechanical quality factor is defined as the reciprocal proportion of the energy consumed by the material during the electrical and mechanical energy conversion. The value of Qm is found to be less for PBZTF compared to PZTF samples for all the sintering temperature. As reported earlier [15] whenever the internal friction between the domain wall is increased (lower grain size), the domain wall motion increases and produce micro hysteresis, which further results in decrease in local stresses and domain switching. Consequently higher domain wall motion in PBZTF samples reduces the Qm values. Similarly space charge caused by impurities also affects Qm and
it is more observed in fine grained materials. It is important to note that Qm of ∼880 is achieved along with d33 of ∼250 pC/N in PBZTF. PBZTF materials with high value of Qm and d33 are highly used for high power transducer applications. The dielectric loss factor for PZTF and PBZTF samples are 0.003 and 0.005 respectively. Dielectric constant as a function of sintering temperature of PZTF and PBZTF samples are shown in Fig. 6. It is observed that PBZTF samples have higher dielectric constant than PZTF samples. The domains are separated from one another by narrow regions, termed as domain walls. The domain wall movement in the polycrystalline ceramics depends upon the individual grains which further contain multiple domains of different ferroelectric orientations. The increase in domain wall motion, higher poling efficiency and fine grain size are the reasons for achieving higher dielectric constant in PBZTF samples. The increase in dielectric constant with respect to reduced grain size is due to increase in the number of defect and electric dipoles. It seems that there are more number of defects present in the fine grained materials. And also dipole–dipole interaction depends on the location of dipoles (grain core or grain boundary). While the grain size of the confined system is reduced, the fraction of dipoles at the interfaces increases significantly and dielectric constant is increased. It was also observed from the graph that dielectric constant increases with sintering temperature till it reaches optimum condition (1250 ◦ C) and subsequently decreases for both PZTF and PBZTF because of its decrease in density and coarse grain size. Fig. 7 shows the variation of dielectric constant with temperature at 1 kHz for both the compositions. As in normal ferroelectrics,
Fig. 6. ε vs sintering temperature for PZTF and PBZTF samples.
S.R. Sangawar et al. / Materials Science and Engineering B 176 (2011) 242–245
245
ceramics prepared by high temperature solid-state reaction exhibits good homogeneity and confirms tetragonal phase. The doping effect of Fe3+ is less in increasing the densification as compared to combined Fe3+ and Ba2+ ions due to shrinkage and distortion of the unit cell caused by the formation of oxygen vacancies in the lattice. Dielectric and piezoelectric properties of PZTF and PBZTF ceramics were studied. Their properties were correlated with microstructure. Subsequently small grain size resulted in higher piezoelectric and dielectric properties. Higher piezoelectric charge coefficient (∼250 pC/N) was obtained without reduction in mechanical quality factor (∼880) in PBZTF samples, is the vital research work in the field of dopant substitution studies of PZT ceramics. Acknowledgement Authors are thankful to Director, ARDE for his valuable support and permitting to publish this paper. Fig. 7. ε vs temperature for PZTF and PBZTF (sintered at 1250 ◦ C) at 1 kHz.
the dielectric constant of both the samples increases gradually with increasing temperature up to the transition temperature (Tc ) and subsequently it decreases. Dielectric constant increases with temperature because of interfacial polarization becoming more dominant compared to the dipolar polarization. After Curie temperature is reached, dielectric constant decreases due to the phase transition from ferroelectric to the paraelectric phase. Here dielectric peak is observed to be broadened in PBZTF samples. This broadening can be due to arrangement of cations in the A and Bsites by Ba2+ and Fe3+ ions respectively which was discussed earlier. The increase in the εmax value in PZTF samples implies that the substitution of Ba2+ ion decreases the dipole moment value at higher temperature. It was also observed that PZTF samples have higher Curie temperature (645 K) than PBZTF (630 K) samples. 4. Conclusions In this study, Fe and Fe–Ba doped PZT in the area of morphotrophic phase boundary have been investigated. PBZTF
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
K. Okazaki, K. Nagata, J. Am. Ceram. Soc. 56 (1973) 82–86. F. Kulsar, J. Am. Ceram. Soc. 42 (1959) 343–349. T. Yamamoto, Am. Ceram. Bull. 71 (1992) 978–985. N. Wongdamnern, N. Triamnak, A. Ngamjanurojana, Y. Laosiritaworn, S. Ananta, R. Yimnirun, Ceram. Int. 34 (2008) 731–734. B. Praveenkumar, H.H. Kumar, D.K. Kharat, B.S. Murty, Mater. Chem. Phys. 112 (2008) 31–34. C. Cheon, J.S. Park, J. Mater. Sci. 16 (1997) 2043–2046. K. Raman, M. Lopez, J. Phys. D: Appl. Phys. 39 (2006) 4466–4471. R.S. Nasar, M. Cerqueira, E. Longo, E.R. Leite, J.A. Varela, A. Beltran, J. Andres, J. Mater. Sci. 34 (1999) 3659–3667. T.B. Weston, A.H. Webster, V.M. McNamara, J. Am. Ceram. Soc. 52 (1969) 253–257. G. Li, G. Heartling, Proceedings of the IEEE International Symposium on Applications of Ferroelectrics, 1992, pp. 569–572. H. Ouchi, J. Am. Ceram. Soc. 51 (1968) 169–176. C. Moure, M. Villegas, J.F. Fermandez, P. Duram, Ferroelectrics 127 (1992) 113–118. B. Jaffe, W.R. Cook Jr., H. Jaffe, Piezoelectric Ceram. (1971) 148–183. M. Takahashi, Jpn. J. Appl. Phys. 9 (1970) 1236–1246. S. Takahashi, M. Takahashi, Jpn. J. Appl. Phys. 11 (1972) 31–35.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Effect of Fe and Fe–Ba substitution on the piezoelectric and dielectric properties of lead zirconate titanate ceramics S.R. Sangawar ∗ , B. Praveenkumar, H.H. Kumar, D.K. Kharat PZT Centre, Armament Research and Development Establishment, Pune 411021, India
a r t i c l e
i n f o
Article history: Received 20 May 2010 Received in revised form 9 September 2010 Accepted 1 December 2010 Keywords: PZT Ceramics Solid-state reaction Mechanical quality factor Piezoelectric SONAR transducer
a b s t r a c t Polycrystalline samples of Fe and Fe–Ba doped lead zirconate titanate (PZT) ceramics near the morphotrophic phase boundary have been synthesized by a solid-state reaction technique. Preliminary X-ray analysis of the compound confirms that there is no change in the crystal structure of PZT on co-doping with Fe and Ba. The maximum mechanical quality factor Qm was found to be 1000 for Fe doped material and 880 for Fe–Ba doped material. The electromechanical coupling factor for Fe and Fe–Ba doped samples were 0.535 and 0.495 respectively. The corresponding values for the piezoelectric charge constant d33 were 135 and 250 pC/N respectively. These results are discussed in terms of position occupied by dopants in to the lattice and their corresponding microstructures. These Fe–Ba doped PZT materials could be likely candidates for high power ultrasonic and underwater SONAR transducer systems. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Pb(Zr,Ti)O3 (PZT) ceramics with a composition around the morphotrophic phase boundary have been studied extensively to understand its electromechanical properties. These ceramic materials are cost effective and have greater durability under adverse atmosphere conditions [1,2]. They are found to be useful for sensors, actuators and high power transducer applications and offers linearity, higher mechanical and dielectric strengths achieved through optimized microstructure [3]. Piezoelectric charge coefficient and dielectric constant of PZT ceramics are mainly because of its domain structure, but also by the presence of defects that can modify the domain wall mobility with appropriate quantity of acceptor and donor dopants which classifies PZT as hard and soft type/class. Hard PZT ceramics [4] can withstand high levels of electrical excitation which are well suited for high power generators and transducers, while the significant feature of soft PZT is the high sensitivity and permittivity which are well suited for various sensors and hydrophone applications [5]. Most of the earlier studies [6–8] have been carried out by using conventionally prepared powders for optimizing piezoelectric properties and very little effort have been made to study the role of substituents in modifying and optimizing the mechanical quality factor (Qm ) of PZT along with higher piezoelectric charge
∗ Corresponding author. Tel.: +91 020 25881828; fax: +91 020 25893102. E-mail address: [email protected] (S.R. Sangawar). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.12.003
coefficient (d33 ). As reported earlier [9] the substitution of Fe on PZT decreases the dielectric constant and dissipation factor while increasing the mechanical quality factor and frequency constant. This could be achieved mainly because of Fe substitution, which helped in inhibiting grain growth. Obtaining both high piezoelectric charge coefficient and mechanical quality factor has not been studied extensively so far, whereas piezoelectric and mechanical quality factors were studied individually by few researchers [10–12]. In the present study, an attempt has been made by substituting Fe and Fe–Ba dopant on PZT to obtain high mechanical quality factor, piezoelectric charge coefficient and dielectric properties for high power generators and SONAR systems. 2. Experimental The materials investigated in the present work were hard PZT material with following composition Pb(Zr0.53 Ti0.47 )O3 + (1 wt%) Fe2 O3 and Pb(Zr0.53 Ti0.47 )O3 + (1 wt%) Fe2 O3 + (1 wt%) BaCO3 and hereafter designated as PZTF and PBZTF respectively. AR grade (>99% pure) PbO (D’Waldies), ZrO2 (Loba Chemie), TiO2 (Loba Chemie), BaCO3 (Loba Chemie) and Fe2 O3 (Edwards & Illford) powders were used. Powders were mixed in appropriate molar ratios and ball milled for 24 h using distilled water. Dried powder was calcined at 1000 ◦ C in closed alumina crucibles for 2 h. Powder was again ball milled for 24 h to reduce particle size. Particle size of both the milled powders was measured by Particle size analyzer (Model 2000MU, Malvern) and was found to be ∼1.2 m. X-ray diffractograms (XRD) of both the powder samples were recorded at room temperature with Cu K␣ radiation using an X-ray powder diffrac-
S.R. Sangawar et al. / Materials Science and Engineering B 176 (2011) 242–245
243
7.72 7.70
PBZTF PFZT
7.68 7.66
Density (g/cc)
7.64 7.62 7.60 7.58 7.56 7.54 7.52 7.50 7.48 1225
1250
1275
1300
Sintering Temperature (ºC) Fig. 1. XRD patterns of PZTF and PBZTF samples.
Fig. 2. Density vs sintering temperature for PZTF and PBZTF samples.
tometer (Model PW-3020, Philips) for phase formation studies. The binder polyvinylalcohol (PVA) was added to milled powder and disks (20 × 2 mm) were compacted using uniaxial press at 150 MPa pressure. Compacts were sintered in the temperature range of 1250 to 1300 ◦ C in a lead rich atmosphere. Microstructural studies of the sintered compacts were carried out using Scanning Electron Microscope (SEM) (Model No. JSM-6360, JEOL). Sintered compacts were electroded using silver paste and subsequently poled at 3 kV/mm in silicone oil bath at 100 ◦ C for 30 min. Dielectric and piezoelectric properties of poled samples were measured 24 h after the poling. Capacitance was measured by LCR Bridge (Model 4262A, Hewlett Packard) at frequency of 1 kHz and piezoelectric charge coefficient (d33 ) was measured using d33 Meter (Model CPDT3300, Channel Products), dielectric constant (ε), mechanical quality factor (Qm ) and coupling coefficient (Kp ) were calculated by standard formulae using the measured values of capacitance, resonance and anti-resonance frequencies.
temperature. The densification of ceramics is produced entirely by solid state diffusion processes. The decrease in density with sintering temperature may be attributed to rise in the number of oxygen vacancy concentration at higher temperatures. The scanning electron micrographs of flat surfaces of sintered PZTF and PBZTF samples are shown in Fig. 3(a) and (b). The specimens are sintered at optimized sintering condition 1250 ◦ C and grain size is measured by linear intercept method. It is observed that grain size decreases with addition of Ba2+ substitution in PBZTF
3. Results and discussion Fig. 1 shows XRD patterns of Fe (PZTF) and Fe–Ba (PBZTF) doped PZT ceramics. It is seen that, both PZTF and PBZTF samples are polycrystalline and perovskite in nature. The phases are pure ˚ could and no secondary phases were observed. Fe3+ (RFe = 0.78 A) occupy the B-site of PZT lattice as acceptor owing to the similar ˚ and Ti4+ (RTi = 0.68 A) ˚ while the ionic radius of Zr4+ (RZr = 0.79 A) ˚ occupies the A-site of PZT partially subisovalent Ba2+ (RBa = 1.34 A) ˚ Isovalent substitutions of Pb2+ ions by stituting Pb2+ (RPb = 1.20 A). Ba2+ do not create any oxygen vacancies on the contrary acceptor doped substitutions of Zr4+ or Ti4+ by Fe3+ causes oxygen vacancies. Our results indicate that Fe and Ba contribute to the alteration of phase formation from coexistence to tetragonal and additional Ba with Fe resulted in splitting of peaks. Calculation of XRD data reveals decrease in the unit cell volume from 0.0663 nm3 (PBZTF) to 0.0654 nm3 (PZTF). Fig. 2 illustrates the behavior of densities of sintered samples as a function of sintering temperature for both PZTF and PBZTF samples. It is evident from the graph that PBZTF has higher density than PZTF samples. Doping effect of Fe3+ is less in increasing the densification as compared to combined Fe3+ and Ba2+ ions due to shrinkage and distortion of the unit cell caused by the formation of oxygen vacancies in the lattice. In PBZTF, the effect of Fe3+ is slightly reduced by partial substitution of Pb2+ by Ba2+ , for maintaining the unit cell size with no shrinkage [13]. Densities of all the samples are optimum at 1250 ◦ C and then decreases with increase in sintering
Fig. 3. SEM of sintered (a) PZTF and (b) PBZTF samples.
244
S.R. Sangawar et al. / Materials Science and Engineering B 176 (2011) 242–245 1100
PZTF PBZTF
280 260 240 220 200 180 160 140
Mechanical Quality Factor (Qm)
Piezoelectric charge coefficient (d33 )
300
PZTF PBZTF
1050 1000 950 900 850 800 750 700 650
120
1225
1250
1275
1300
0 Sintering Temperature ( C)
100 80
Fig. 5. Qm vs sintering temperature for PZTF and PBZTF samples.
1225
1250
1275
1300
Sintering Temperature (ºC) Fig. 4. d33 vs sintering temperature for PZTF and PBZTF samples.
and it is around 1 m compared to 2 m grain size of PZTF samples. Ba2+ substitution decreases the grain size and increases the density because of chemical diffusion of isovalent ions substituting the lead ions and optimized the compositional homogeneity. It also shown that PBZTF sample has lower and controlled grain size which attributes to narrow particle size distribution. The decrease in grain size increases the density of PBZTF samples as mentioned earlier. In general, PZT will have higher density because of higher concentration of Pb ions. When single and/or co-doping of respective cation substitution, will replace Pb and Zr/Ti ions in PZT, causing decrease in density of PZT ceramics. SEM studies revealed the formation of micro pores among the grains, and confirms decrease in density for both the samples. The microstructural parameters influence the electrical and mechanical properties of the PZT ceramics. The splitting of peaks observed in XRD could be attributed to the change in grain morphology. Fig. 4 shows the variation of piezoelectric charge coefficient (d33 ) with sintering temperature for both PZTF and PBZTF samples. It was observed that PBZTF have higher d33 than the PZTF samples. This may be due to the following reasons observed in PBZTF samples: (i) higher density, (ii) no shrinkage and distortion of unit cell, (iii) less hindrance for the orientation of dipoles, (iv) higher poling efficiency due to the presence of Ba2+ ions and decrease in conductivity (v) decrease in specific resistivity [14]. Whereas in PZTF samples the addition of Fe3+ results in creation of oxygen vacancies and decreases unit cell size and it hinders the orientation of dipoles which results in difficulty in poling. It is revealed that PBZTF has lower conductivity than PZTF because of substitution of Ba2+ ions, which subsequently increases the efficiency of poling and results in higher d33 . It was also observed from the graph that d33 increases with sintering temperature till it reaches the optimum conditions (1250 ◦ C) and then decreases for both PZTF and PBZTF because of its density. Fig. 5 shows the variation of mechanical quality factor (Qm ) with sintering temperature for both PZTF and PBZTF samples. Mechanical quality factor is defined as the reciprocal proportion of the energy consumed by the material during the electrical and mechanical energy conversion. The value of Qm is found to be less for PBZTF compared to PZTF samples for all the sintering temperature. As reported earlier [15] whenever the internal friction between the domain wall is increased (lower grain size), the domain wall motion increases and produce micro hysteresis, which further results in decrease in local stresses and domain switching. Consequently higher domain wall motion in PBZTF samples reduces the Qm values. Similarly space charge caused by impurities also affects Qm and
it is more observed in fine grained materials. It is important to note that Qm of ∼880 is achieved along with d33 of ∼250 pC/N in PBZTF. PBZTF materials with high value of Qm and d33 are highly used for high power transducer applications. The dielectric loss factor for PZTF and PBZTF samples are 0.003 and 0.005 respectively. Dielectric constant as a function of sintering temperature of PZTF and PBZTF samples are shown in Fig. 6. It is observed that PBZTF samples have higher dielectric constant than PZTF samples. The domains are separated from one another by narrow regions, termed as domain walls. The domain wall movement in the polycrystalline ceramics depends upon the individual grains which further contain multiple domains of different ferroelectric orientations. The increase in domain wall motion, higher poling efficiency and fine grain size are the reasons for achieving higher dielectric constant in PBZTF samples. The increase in dielectric constant with respect to reduced grain size is due to increase in the number of defect and electric dipoles. It seems that there are more number of defects present in the fine grained materials. And also dipole–dipole interaction depends on the location of dipoles (grain core or grain boundary). While the grain size of the confined system is reduced, the fraction of dipoles at the interfaces increases significantly and dielectric constant is increased. It was also observed from the graph that dielectric constant increases with sintering temperature till it reaches optimum condition (1250 ◦ C) and subsequently decreases for both PZTF and PBZTF because of its decrease in density and coarse grain size. Fig. 7 shows the variation of dielectric constant with temperature at 1 kHz for both the compositions. As in normal ferroelectrics,
Fig. 6. ε vs sintering temperature for PZTF and PBZTF samples.
S.R. Sangawar et al. / Materials Science and Engineering B 176 (2011) 242–245
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ceramics prepared by high temperature solid-state reaction exhibits good homogeneity and confirms tetragonal phase. The doping effect of Fe3+ is less in increasing the densification as compared to combined Fe3+ and Ba2+ ions due to shrinkage and distortion of the unit cell caused by the formation of oxygen vacancies in the lattice. Dielectric and piezoelectric properties of PZTF and PBZTF ceramics were studied. Their properties were correlated with microstructure. Subsequently small grain size resulted in higher piezoelectric and dielectric properties. Higher piezoelectric charge coefficient (∼250 pC/N) was obtained without reduction in mechanical quality factor (∼880) in PBZTF samples, is the vital research work in the field of dopant substitution studies of PZT ceramics. Acknowledgement Authors are thankful to Director, ARDE for his valuable support and permitting to publish this paper. Fig. 7. ε vs temperature for PZTF and PBZTF (sintered at 1250 ◦ C) at 1 kHz.
the dielectric constant of both the samples increases gradually with increasing temperature up to the transition temperature (Tc ) and subsequently it decreases. Dielectric constant increases with temperature because of interfacial polarization becoming more dominant compared to the dipolar polarization. After Curie temperature is reached, dielectric constant decreases due to the phase transition from ferroelectric to the paraelectric phase. Here dielectric peak is observed to be broadened in PBZTF samples. This broadening can be due to arrangement of cations in the A and Bsites by Ba2+ and Fe3+ ions respectively which was discussed earlier. The increase in the εmax value in PZTF samples implies that the substitution of Ba2+ ion decreases the dipole moment value at higher temperature. It was also observed that PZTF samples have higher Curie temperature (645 K) than PBZTF (630 K) samples. 4. Conclusions In this study, Fe and Fe–Ba doped PZT in the area of morphotrophic phase boundary have been investigated. PBZTF
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