- Email: [email protected]
Dilatometric and dielectric behaviour of Sm modified PCT ceramics
ARTICLE IN PRESS
Physica B 355 (2005) 280–285 www.elsevier.com/locate/physb
Dilatometric and dielectric behaviour of Sm modified PCT ceramics Sarabjit Singha, O.P. Thakura,, Chandra Prakasha, K.K. Rainab a Electroceramics Group, Solid State Physics Laboratory, Lucknow Road, Delhi-110054, India Materials Research Laboratory, School of Physics and Materials Science, Thapar Institute of Engg. & Technology, Patiala-147004, India
b
Received 3 August 2004; received in revised form 2 November 2004; accepted 3 November 2004
Abstract Samarium modified PCT ceramics with composition (Pb0.76xSmxCa0.24)(Ti0.98Mn0.02)O3; x ¼ 0–0.08 in steps of 0.02 were prepared by conventional mixed-oxide method. Detailed dilatometric studies were carried out for green specimens in order to study sintering behaviour. Change in the dilatometric behaviour is correlated with the XRD results of powders calcined at different temperatures. Dielectric constant was observed to increase with increasing Sm concentration, which has been attributed to reduced tetragonality and better densification on Sm substitution. SEM micrographs have revealed the grain size of the samples. Ferroelectric hysteresis behaviour was studied for all the compositions. r 2004 Elsevier B.V. All rights reserved. PACS: 77; 77.84.s Keywords: Dilatometric study; Modified PCT; Samarium substitution; Dielectric properties
1. Introduction Lead titanate, PbTiO3 (PT), is a ferroelectric material with a perovskite structure, which has been prepared by chemical [1–3] and solid-state Corresponding author. Tel.: +91 11 23921692; fax: +91 11 23913609. E-mail addresses: [email protected] (O.P. Thakur), [email protected] (Chandra Prakash).
reaction methods [4]. In recent years, there is an increasing interest in this material with compositional modifications at Pb and Ti sites due to their potential applications such as infrared sensors, electro-optic devices, ferroelectric memories and so on. These materials include compounds (Pb1xAx)(Ti1yBy)O3, where a number of both isovalent and aliovalence substitution for both Pb (A site) and Ti (B site) are possible. Lead may be substituted by isovalent cations including
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.11.010
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
Ca2+, Ba2+, Sr2+ and Cd2+ as well as aliovalence substitutions such as rare-earth ions Sm3+ and Y3+ while maintaining the perovskite crystal structure responsible for the strong ferroelectric behaviour. In B-site substitution, cations including Mn4+, Fe3+, Sb5+, Ni2+, etc., may replace the Ti4+ ions in the crystal structure and can serve to compensate charge due to aliovalence A-site substitutions. Among these materials, (Pb,Ca)TiO3, PCT, ceramic has recently attracted much attention for use in pyroelectric devices and ferroelectric memories, particularly as nonvolatile ferroelectric random access memories (NVFRAMs) due to the promise of high speed, radiation hardness, high remnant polarization, high dielectric constant and low power consumption [5–7]. Pure lead titanate has a large tetragonal distortion at room temperature, c/a ¼ 1.064, which introduces stress in the material upon cooling through phase transition, producing cracks in the material [8]. Substitution of Ca for Pb introduces shrinkage of the lattice in c-axis of the tetragonal phase in the perovskite structure of PbTiO3. Also it is difficult to pole lead titanate due to its high domain energy, high transition temperature and high coercive field. Lead titanate ceramics modified with Sm and Ca are found to possess higher piezoelectric anisotropy [9,10]. Electromechanical coupling factor of PT for thickness vibration, kt, is much larger than that for planer extensional vibration, kp (ktbkp). It is generally considered that 24 mol% concentration of Ca (x ¼ 0.24) in Pb1xCaxTiO3 (PCT) ceramics allows cracking to be avoided while maintaining better ferroelectric properties [11]. Dilatometric measurements are generally employed to determine the thermal strain generated during the course of heat treatment. The information about the phase transitions in materials, sintering behaviour, thermal expansion coefficient (a) and their anisotropies are obtained from this technique. Large dimensional change can occur during a solid-state reaction due to the diffusion of ions, which results in dimensional changes of the sample [12]. A macroscopic technique like dilatometry can give indication of these events during the commencement of a solid-state reaction. Here, we report the detailed dilatometric study supported
281
by a perovskite phase evolution by XRD analysis and dielectric studies of the present system.
2. Experimental details Powders with compositions (Pb0.76xSmxCa0.24) (Ti0.98Mn0.02)O3; x ¼ 0–0.08 in steps of 0.02; were synthesized by calcining at 850 1C for 4 h after ball milling of the raw materials [PbO, TiO2, Sm2O3, MnO2 and CaO powders (Aldrich, 499.9% purity)] in planetary ball mill using distilled water as wetting agent and Zirconia balls as a grinding media for 4 h. Excess PbO (2 mol%) was added to counteract the volatization of lead oxide during sintering. After ball milling and calcination, all the powders were sieved and compacted in the form of rods using cold isostatic press (CIP) at a pressure of 200 MPa. These rods were sintered in closed alumina crucibles with heating rate of 5 1C/min at 1150 1C for 4 h. A lead-rich atmosphere was maintained with PbZrO3 powder to minimize the lead loss during firing. Several slices in a disc shape were cut from the sintered rods using a precision diamond raw. Density of sintered rods was measured by Archimedes method. Phase relations for the sintered body with the substitution of Sm were identified using XRD. Lattice parameters were calculated using XLAT software. Microstructural study for fractured surfaces of the samples was done using SEM (Leo 1430, Japan). Shrinkage behaviour of the green sample was studied by using dilatometer (Orton, 1600D) from room temperature to 1150 1C. For electrical characterization, discs of dia 10 mm and thickness 0.5 mm were coated with gold using Turbo sputter coater (Denton Vacuum). Dielectric properties were measured at room temperature at 10 kHz using LCR meter HP 4284A. Ferroelectric hysteresis loops were recorded using an automated PE loop tracer based on modified Sawyer–Tower circuit working at 50 Hz frequency.
3. Results and discussion XRD analysis shows all the samples to be singlephase with tetragonal structure. Table 1 shows the
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282
variation of density and tetragonality (c/a) as a function of samarium concentration in the system Pb0.76xSmxCa0.24Ti0.98Mn0.02O3. The experimental density increases with increasing samarium content. Tetragonality decreases with increasing samarium substitution, which indicates that with samarium substitution, PCT composition is approaching towards cubic structure resulting in hard and dense ceramics. This behaviour is in good agreement with the results reported by Prakash et al. [13]. The typical dilatometric behaviour of green sample of composition; Pb0.70Sm0.06Ca0.24Ti0.98Mn0.02O3, was recorded to study the variation of percentage linear change (DL/L) with increase in temperature. The shrinkage behaviour is illu-
strated in Fig. 1, which indicates the onset of shrinkage around 850 1C and much higher near 1100 1C. Up to 850 1C, the sample undergoes normal expansion. Some change in the expansion behaviour was observed around 450, 550 and 700 1C as shown in figure inserted. These changes were further investigated by the room temperature XRD study. The XRD patterns obtained for the powders heated at 450, 550 1C for 5 min were recorded and are shown in Fig. 2. XRD Patterns show the onset of the formation of perovskite phase at 450 1C and the peaks corresponding to perovskite phase become dominant at 550 1C and also with the soaking time (4 h). The change in the
Table 1 Physical and structural parameters for (Pb0.76xSmxCa0.24)(Ti0.98Mn0.02)O3 system x
Density (g/cc)
Tetragonality (c/a)
Grain size (mm)
0 0.02 0.04 0.06 0.08
6.80 6.85 6.90 6.95 7.00
1.057 1.055 1.049 1.037 1.028
1.2 3.2 3.0 2.2 2.8
Perovskite
(d)
0.0
(c)
°
Intensity (arb. units)
d(∆L/L )/dT
-0.1 0.010
-0.2
0.005 -0.3 0.000 -0.4
(b)
(a)
-0.005 200 400 600 800
-0.5 20
0
200
400
600
800
1000 1200
30
40
50
60
2θ (degrees)
Temperature (°C) Fig. 1. Shrinkage rate of green compact of the same composition.
Fig. 2. XRD patterns for the powders (a) green (as mixed) and heated at (b) 450 1C/5 min. (c) 550 1C/5 min.and (d) 550 1C/4 h having composition x ¼ 0.06.
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
Perovskite Sm2O3
Intensity (arb units)
(d)
(c)
(b)
(a)
20
30
40
50
60
2θ (degrees)
350
0.014
300
0.012
250
0.010
tanδ
Fig. 3. XRD patterns for the powders heated at (a) 550 1C, (b) 650 1C, (c) 750 1C and (d) 850 1C for 4 h. having composition x ¼ 0.06.
ε'
expansion behaviour thus can be explained on the basis of the signature of phase progression during heating of the powder as recorded by XRD technique [14]. At the early stage of the reaction, interdiffusion of cation and the vacancies in between the reacting materials takes place, leading to the formation of product at the contact point of reactants. Also the formed product has different lattice parameters compared to the reactants. This change in lattice parameter causes strain at the interface between reactant and the product, leading to the fracture at the interface. In this process, fine particles with lot of voids in between are produced, resulting in thermal expansion of the compact at reaction initiation temperature. Also the difference in the XRD patterns for the samples heat-treated at 550, 650, 750 and 850 1C for 4 h were recorded and the results are shown in Fig. 3. It can be seen that the powder heated at 850 1C for 4 h shows the complete perovskite phase formation, so it is thus used in this study as calcination temperature. The sample shrinks around 17% as the temperature reaches up to 1150 1C. Then it shows further shrinkage up to approximately 18% during the soaking time and it was observed that shrinkage was almost constant after 200 min soaking period, which implies that the soaking for 240 min is sufficient for getting dense samples. Therefore, dilatometric study can be one of the tools for deciding the sintering temperature and soaking duration. The variation of room temperature values of dielectric constant (e0 ) and dissipation factor (tan d) measured at 10 kHz with increasing amount of samarium concentration is shown in Fig. 4. An increase in dielectric constant with increasing Sm content is observed. This increase is due to mainly three reasons; (1) due to the compensation of A-site vacancies by Sm3+; (2) increase in density and (3) decrease in tetragonality. Sm acts as donor, these donors are usually compensated by A-site vacancies, which results in enhanced domain reorientation, thereby increasing dielectric constant [15]. Also, increase is expected with increase in density. According to literature [16], dielectric constant of
283
200 0.008 150 0.006 100
0.00
0.02
0.04
0.06
0.08
x (mol%) Fig. 4. Room temperature variation of e0 and tan d with samarium substitution (x).
ARTICLE IN PRESS
30
0.08
x=0
0.06
20
0.02 P (µC/cm2)
10
0.04
0 -10
24
22
20
21
16
20
12
19
8
18 17
4 0.00
0.02
0.04 x
0.06
0.08
Fig. 6. Dependence of coercive field (EC) and remnant polarization (Pr) on the amount of Sm substitution.
increasing Sm content up to 6 mol%. It is well reported in the literature that with increase in grain size, EC decreases as each grain is mechanically clamped by its surrounding. This clamping effect, in addition to the mechanical stresses accompanying 901 domain rotations, tends to impede the polarization reversal process, and hence, apparently increases EC [20]. Also there is a drastic change observed in the values of Pr for 6 mol% Sm and 8 mol% Sm concentrations, which is an indication of softening nature of the PCT ceramics after 6 mol% Sm substitution as Ec is also decreasing for 8 mol% Sm substitution. For 8 mol% Sm, the grain size again increases and more uniformity was observed in the microstructure. The microstructures for 6 and 8 mol% Sm are shown in Fig. 7.
4. Conclusions
-20 -30 -100
23
Pr (µC/cm2)
PZT ceramics decreases with increase in porosity. The author has explained it in terms of depolarizing effect of pores. In our results, dielectric constant increases as the porosity decreases. Our observations thus get supported by the model of depolarizing effect of pores as suggested by Okazaki [16]. Also in our case, decrease in tetragonality is observed with increasing Sm3+ substitution. The influence of decrease in tetragonality with increase in dielectric constant has been reported in the literature [8,17–19]. The value of dielectric loss decreases with increasing Sm content, which is obvious because there is an increase in density with increasing Sm3+ substitution. Fig. 5 illustrates P–E hysteresis plots for all the compositions, which show well-defined ferroelectric behaviour. Values of remnant polarization (Pr) and coercive field (EC) determined from the P–E loops are shown in Fig. 6. From the figure, it can be seen that there is a decrease in the value of coercive field (EC) up to 2 mol% Sm followed by the increase up to 6 mol%. This behaviour can be explained with the grain size variation (determined from SEM micrographs and shown in Table 1), which also show increase up to 2 mol% Sm followed by the decrease with further
EC (kV/cm)
S. Singh et al. / Physica B 355 (2005) 280–285
284
-75
-50
-25
0
25
50
75
E (kV/cm) Fig. 5. Behaviour of ferroelectric hysteresis loops.
100
Sintering schedule could be devised on the basis of detailed dilatometric study for better densification and reproducible properties. The simultaneous addition of Ca and Sm not only reduce the lattice anisotropy (c/a), but also exhibit welldefined hysteresis behaviour and there is an increase in the value of dielectric constant.
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
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carrying out SEM and XRD measurements, respectively.
References
Fig. 7. SEM micrographs for the sample x ¼ 0.06 and 0.08.
Acknowledgements The authors gratefully acknowledge the help of Mr. D.S. Rawal and Mr. Anshu Goyal for
[1] J.B. Blum, S.R. Gurcovich, J. Mater. Sci. 20 (1985) 4479. [2] E.C. Paris, E.R. Leite, E. Longo, J.A. Varela, Mater. Lett. 37 (1998) 1. [3] M. Kakihana, T. Okubo, M. Arima, O. Ochiyama, M. Yashima, M. Yoshimura, Chem. Matter. 9 (1997) 451. [4] B. Jaffe, W.R. Cook Jr., H. Jaffe, In: Piezoelectric Ceramics, Academic Press, New York, 1971. [5] S.J. Kang, D.H. Chang, Y.S. Yoon, Thin Solid Films 373 (2000) 53. [6] J. Mendiola, M.L. Calzada, P. Ramos, M.J. Martin, F.A. Rueda, Thin Solid Films 315 (1998) 195. [7] M. Es-Souni, M. Abed, A. Piorra, S. Malinowski, V. Zaporojtchenko, Thin Solid Films 389 (2001) 99. [8] S.Y. Chu, C.Z. Chen, Sensors Actuators A 89 (2001) 210. [9] Y. Ito, H. Takeuchi, S. Jyomura, K. Nagatsuma, S. Ashida, Appl. Phys. Lett. 35 (1979) 595. [10] H. Takeuchi, S. Jyomura, E. Yamamoto, Y. Ito, J. Acoust. Soc. Am. 72 (1982) 1114. [11] J. Mendiola, B. Jimenez, C. Alemany, L. Pardo, L. del Olmo, Ferroelectrics 94 (1989) 183. [12] S. Kim, G.S. Lee, T.R. Shrout, S. Venkataramani, J. Mater. Sci. 21 (1991) 4411. [13] C. Prakash, O.P. Thakur, Mater. Lett. 57 (2003) 2310. [14] V.V. Bhat, M.V.R. Rao, A.M. Umerji, Mater. Res. Bull. 38 (2003) 1081. [15] C. Prakash, A. Bhalla, Ferroelectrics 262 (2001) 321. [16] K. Okazaki, Ferroelectrics 41 (1982) 77. [17] S. Chu, C. Chen, Mater. Res. Bull. 35 (2000) 2317. [18] P. Duran, J.F. Lozano, F. Capel, C. Moure, J. Mater. Sci. 24 (1969) 447. [19] K. Prasad, R.N.P. Chowdhary, S.N. Chowdhary, R. Sati, Bull. Mater. Sci. 19 (1996) 505. [20] K. Carl, Ferroelectrics 9 (1975) 23.
Physica B 355 (2005) 280–285 www.elsevier.com/locate/physb
Dilatometric and dielectric behaviour of Sm modified PCT ceramics Sarabjit Singha, O.P. Thakura,, Chandra Prakasha, K.K. Rainab a Electroceramics Group, Solid State Physics Laboratory, Lucknow Road, Delhi-110054, India Materials Research Laboratory, School of Physics and Materials Science, Thapar Institute of Engg. & Technology, Patiala-147004, India
b
Received 3 August 2004; received in revised form 2 November 2004; accepted 3 November 2004
Abstract Samarium modified PCT ceramics with composition (Pb0.76xSmxCa0.24)(Ti0.98Mn0.02)O3; x ¼ 0–0.08 in steps of 0.02 were prepared by conventional mixed-oxide method. Detailed dilatometric studies were carried out for green specimens in order to study sintering behaviour. Change in the dilatometric behaviour is correlated with the XRD results of powders calcined at different temperatures. Dielectric constant was observed to increase with increasing Sm concentration, which has been attributed to reduced tetragonality and better densification on Sm substitution. SEM micrographs have revealed the grain size of the samples. Ferroelectric hysteresis behaviour was studied for all the compositions. r 2004 Elsevier B.V. All rights reserved. PACS: 77; 77.84.s Keywords: Dilatometric study; Modified PCT; Samarium substitution; Dielectric properties
1. Introduction Lead titanate, PbTiO3 (PT), is a ferroelectric material with a perovskite structure, which has been prepared by chemical [1–3] and solid-state Corresponding author. Tel.: +91 11 23921692; fax: +91 11 23913609. E-mail addresses: [email protected] (O.P. Thakur), [email protected] (Chandra Prakash).
reaction methods [4]. In recent years, there is an increasing interest in this material with compositional modifications at Pb and Ti sites due to their potential applications such as infrared sensors, electro-optic devices, ferroelectric memories and so on. These materials include compounds (Pb1xAx)(Ti1yBy)O3, where a number of both isovalent and aliovalence substitution for both Pb (A site) and Ti (B site) are possible. Lead may be substituted by isovalent cations including
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.11.010
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
Ca2+, Ba2+, Sr2+ and Cd2+ as well as aliovalence substitutions such as rare-earth ions Sm3+ and Y3+ while maintaining the perovskite crystal structure responsible for the strong ferroelectric behaviour. In B-site substitution, cations including Mn4+, Fe3+, Sb5+, Ni2+, etc., may replace the Ti4+ ions in the crystal structure and can serve to compensate charge due to aliovalence A-site substitutions. Among these materials, (Pb,Ca)TiO3, PCT, ceramic has recently attracted much attention for use in pyroelectric devices and ferroelectric memories, particularly as nonvolatile ferroelectric random access memories (NVFRAMs) due to the promise of high speed, radiation hardness, high remnant polarization, high dielectric constant and low power consumption [5–7]. Pure lead titanate has a large tetragonal distortion at room temperature, c/a ¼ 1.064, which introduces stress in the material upon cooling through phase transition, producing cracks in the material [8]. Substitution of Ca for Pb introduces shrinkage of the lattice in c-axis of the tetragonal phase in the perovskite structure of PbTiO3. Also it is difficult to pole lead titanate due to its high domain energy, high transition temperature and high coercive field. Lead titanate ceramics modified with Sm and Ca are found to possess higher piezoelectric anisotropy [9,10]. Electromechanical coupling factor of PT for thickness vibration, kt, is much larger than that for planer extensional vibration, kp (ktbkp). It is generally considered that 24 mol% concentration of Ca (x ¼ 0.24) in Pb1xCaxTiO3 (PCT) ceramics allows cracking to be avoided while maintaining better ferroelectric properties [11]. Dilatometric measurements are generally employed to determine the thermal strain generated during the course of heat treatment. The information about the phase transitions in materials, sintering behaviour, thermal expansion coefficient (a) and their anisotropies are obtained from this technique. Large dimensional change can occur during a solid-state reaction due to the diffusion of ions, which results in dimensional changes of the sample [12]. A macroscopic technique like dilatometry can give indication of these events during the commencement of a solid-state reaction. Here, we report the detailed dilatometric study supported
281
by a perovskite phase evolution by XRD analysis and dielectric studies of the present system.
2. Experimental details Powders with compositions (Pb0.76xSmxCa0.24) (Ti0.98Mn0.02)O3; x ¼ 0–0.08 in steps of 0.02; were synthesized by calcining at 850 1C for 4 h after ball milling of the raw materials [PbO, TiO2, Sm2O3, MnO2 and CaO powders (Aldrich, 499.9% purity)] in planetary ball mill using distilled water as wetting agent and Zirconia balls as a grinding media for 4 h. Excess PbO (2 mol%) was added to counteract the volatization of lead oxide during sintering. After ball milling and calcination, all the powders were sieved and compacted in the form of rods using cold isostatic press (CIP) at a pressure of 200 MPa. These rods were sintered in closed alumina crucibles with heating rate of 5 1C/min at 1150 1C for 4 h. A lead-rich atmosphere was maintained with PbZrO3 powder to minimize the lead loss during firing. Several slices in a disc shape were cut from the sintered rods using a precision diamond raw. Density of sintered rods was measured by Archimedes method. Phase relations for the sintered body with the substitution of Sm were identified using XRD. Lattice parameters were calculated using XLAT software. Microstructural study for fractured surfaces of the samples was done using SEM (Leo 1430, Japan). Shrinkage behaviour of the green sample was studied by using dilatometer (Orton, 1600D) from room temperature to 1150 1C. For electrical characterization, discs of dia 10 mm and thickness 0.5 mm were coated with gold using Turbo sputter coater (Denton Vacuum). Dielectric properties were measured at room temperature at 10 kHz using LCR meter HP 4284A. Ferroelectric hysteresis loops were recorded using an automated PE loop tracer based on modified Sawyer–Tower circuit working at 50 Hz frequency.
3. Results and discussion XRD analysis shows all the samples to be singlephase with tetragonal structure. Table 1 shows the
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
282
variation of density and tetragonality (c/a) as a function of samarium concentration in the system Pb0.76xSmxCa0.24Ti0.98Mn0.02O3. The experimental density increases with increasing samarium content. Tetragonality decreases with increasing samarium substitution, which indicates that with samarium substitution, PCT composition is approaching towards cubic structure resulting in hard and dense ceramics. This behaviour is in good agreement with the results reported by Prakash et al. [13]. The typical dilatometric behaviour of green sample of composition; Pb0.70Sm0.06Ca0.24Ti0.98Mn0.02O3, was recorded to study the variation of percentage linear change (DL/L) with increase in temperature. The shrinkage behaviour is illu-
strated in Fig. 1, which indicates the onset of shrinkage around 850 1C and much higher near 1100 1C. Up to 850 1C, the sample undergoes normal expansion. Some change in the expansion behaviour was observed around 450, 550 and 700 1C as shown in figure inserted. These changes were further investigated by the room temperature XRD study. The XRD patterns obtained for the powders heated at 450, 550 1C for 5 min were recorded and are shown in Fig. 2. XRD Patterns show the onset of the formation of perovskite phase at 450 1C and the peaks corresponding to perovskite phase become dominant at 550 1C and also with the soaking time (4 h). The change in the
Table 1 Physical and structural parameters for (Pb0.76xSmxCa0.24)(Ti0.98Mn0.02)O3 system x
Density (g/cc)
Tetragonality (c/a)
Grain size (mm)
0 0.02 0.04 0.06 0.08
6.80 6.85 6.90 6.95 7.00
1.057 1.055 1.049 1.037 1.028
1.2 3.2 3.0 2.2 2.8
Perovskite
(d)
0.0
(c)
°
Intensity (arb. units)
d(∆L/L )/dT
-0.1 0.010
-0.2
0.005 -0.3 0.000 -0.4
(b)
(a)
-0.005 200 400 600 800
-0.5 20
0
200
400
600
800
1000 1200
30
40
50
60
2θ (degrees)
Temperature (°C) Fig. 1. Shrinkage rate of green compact of the same composition.
Fig. 2. XRD patterns for the powders (a) green (as mixed) and heated at (b) 450 1C/5 min. (c) 550 1C/5 min.and (d) 550 1C/4 h having composition x ¼ 0.06.
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
Perovskite Sm2O3
Intensity (arb units)
(d)
(c)
(b)
(a)
20
30
40
50
60
2θ (degrees)
350
0.014
300
0.012
250
0.010
tanδ
Fig. 3. XRD patterns for the powders heated at (a) 550 1C, (b) 650 1C, (c) 750 1C and (d) 850 1C for 4 h. having composition x ¼ 0.06.
ε'
expansion behaviour thus can be explained on the basis of the signature of phase progression during heating of the powder as recorded by XRD technique [14]. At the early stage of the reaction, interdiffusion of cation and the vacancies in between the reacting materials takes place, leading to the formation of product at the contact point of reactants. Also the formed product has different lattice parameters compared to the reactants. This change in lattice parameter causes strain at the interface between reactant and the product, leading to the fracture at the interface. In this process, fine particles with lot of voids in between are produced, resulting in thermal expansion of the compact at reaction initiation temperature. Also the difference in the XRD patterns for the samples heat-treated at 550, 650, 750 and 850 1C for 4 h were recorded and the results are shown in Fig. 3. It can be seen that the powder heated at 850 1C for 4 h shows the complete perovskite phase formation, so it is thus used in this study as calcination temperature. The sample shrinks around 17% as the temperature reaches up to 1150 1C. Then it shows further shrinkage up to approximately 18% during the soaking time and it was observed that shrinkage was almost constant after 200 min soaking period, which implies that the soaking for 240 min is sufficient for getting dense samples. Therefore, dilatometric study can be one of the tools for deciding the sintering temperature and soaking duration. The variation of room temperature values of dielectric constant (e0 ) and dissipation factor (tan d) measured at 10 kHz with increasing amount of samarium concentration is shown in Fig. 4. An increase in dielectric constant with increasing Sm content is observed. This increase is due to mainly three reasons; (1) due to the compensation of A-site vacancies by Sm3+; (2) increase in density and (3) decrease in tetragonality. Sm acts as donor, these donors are usually compensated by A-site vacancies, which results in enhanced domain reorientation, thereby increasing dielectric constant [15]. Also, increase is expected with increase in density. According to literature [16], dielectric constant of
283
200 0.008 150 0.006 100
0.00
0.02
0.04
0.06
0.08
x (mol%) Fig. 4. Room temperature variation of e0 and tan d with samarium substitution (x).
ARTICLE IN PRESS
30
0.08
x=0
0.06
20
0.02 P (µC/cm2)
10
0.04
0 -10
24
22
20
21
16
20
12
19
8
18 17
4 0.00
0.02
0.04 x
0.06
0.08
Fig. 6. Dependence of coercive field (EC) and remnant polarization (Pr) on the amount of Sm substitution.
increasing Sm content up to 6 mol%. It is well reported in the literature that with increase in grain size, EC decreases as each grain is mechanically clamped by its surrounding. This clamping effect, in addition to the mechanical stresses accompanying 901 domain rotations, tends to impede the polarization reversal process, and hence, apparently increases EC [20]. Also there is a drastic change observed in the values of Pr for 6 mol% Sm and 8 mol% Sm concentrations, which is an indication of softening nature of the PCT ceramics after 6 mol% Sm substitution as Ec is also decreasing for 8 mol% Sm substitution. For 8 mol% Sm, the grain size again increases and more uniformity was observed in the microstructure. The microstructures for 6 and 8 mol% Sm are shown in Fig. 7.
4. Conclusions
-20 -30 -100
23
Pr (µC/cm2)
PZT ceramics decreases with increase in porosity. The author has explained it in terms of depolarizing effect of pores. In our results, dielectric constant increases as the porosity decreases. Our observations thus get supported by the model of depolarizing effect of pores as suggested by Okazaki [16]. Also in our case, decrease in tetragonality is observed with increasing Sm3+ substitution. The influence of decrease in tetragonality with increase in dielectric constant has been reported in the literature [8,17–19]. The value of dielectric loss decreases with increasing Sm content, which is obvious because there is an increase in density with increasing Sm3+ substitution. Fig. 5 illustrates P–E hysteresis plots for all the compositions, which show well-defined ferroelectric behaviour. Values of remnant polarization (Pr) and coercive field (EC) determined from the P–E loops are shown in Fig. 6. From the figure, it can be seen that there is a decrease in the value of coercive field (EC) up to 2 mol% Sm followed by the increase up to 6 mol%. This behaviour can be explained with the grain size variation (determined from SEM micrographs and shown in Table 1), which also show increase up to 2 mol% Sm followed by the decrease with further
EC (kV/cm)
S. Singh et al. / Physica B 355 (2005) 280–285
284
-75
-50
-25
0
25
50
75
E (kV/cm) Fig. 5. Behaviour of ferroelectric hysteresis loops.
100
Sintering schedule could be devised on the basis of detailed dilatometric study for better densification and reproducible properties. The simultaneous addition of Ca and Sm not only reduce the lattice anisotropy (c/a), but also exhibit welldefined hysteresis behaviour and there is an increase in the value of dielectric constant.
ARTICLE IN PRESS S. Singh et al. / Physica B 355 (2005) 280–285
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carrying out SEM and XRD measurements, respectively.
References
Fig. 7. SEM micrographs for the sample x ¼ 0.06 and 0.08.
Acknowledgements The authors gratefully acknowledge the help of Mr. D.S. Rawal and Mr. Anshu Goyal for
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