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40) ceramics from oxides
Journal of Alloys and Compounds 339 (2002) 167–174
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Fabrication and characterization of lead lanthanum zirconate titanate (PLZT7 / 60 / 40) ceramics from oxides L.B. Kong*, J. Ma, R.F. Zhang, T.S. Zhang School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Block N4, B2, Nanyang 639798, Singapore Received 21 August 2001; accepted 19 November 2001
Abstract Lead lanthanum zirconate titanate (PLZT7 / 60 / 40) ceramics were fabricated from their oxide mixture via a direct sintering process without involving the calcination step. Different levels of excess PbO (0, 10 and 20 mol%) were used to check its effect on microstructure and thus the electrical properties of the PLZT ceramics. The reaction and densification behavior of the oxide mixtures for the PLZT ceramics were monitored by using a dilatometer. The electrical properties of the PLZT ceramics and their relationships with the processing parameters and excess PbO levels were characterized and discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxide materials; Ferroelectrics; Ceramics; Microstructure; Dielectric response
1. Introduction Lead lanthanum zirconate titanate (PLZT) ceramics, with variable dopant concentrations and Zr / Ti ratios, have been shown to exhibit a variety of ferroic phases such as ferroelectric (FE), antiferroelectric (AFE) and paraelectric (PE) phases. Due to these advantages, PLZT ceramics have received much attention from researchers all over the world [1–5]. The 7%-La-doped Pb(Zr 60 Ti 40 )O 3 ceramic (PLZT7 / 60 / 40) is one of the most important candidates for piezoelectric applications due to its extremely high piezoelectric coefficients (d 33 5710 pC / N, k p 50.72), as reported in the literature [6–8]. PLZT powders were conventionally prepared by solidstate reaction processing using oxides as the starting materials [6–9], involving several steps such as mixing, calcination and sintering. Calcination at temperatures ranging from 900 to 1000 8C is used to synthesize the desired compound before sintering is performed. The synthesized powders usually contain unwanted agglomerates harmful to the subsequent densification behavior, the microstructural uniformity and the final electrical properties of the PLZT ceramics. Synthesis of PLZT powders in this way also suffers from the problem of PbO loss because PbO easily evaporates at a high temperature. In addition, this process is very time consuming; one run *Corresponding author. E-mail address: [email protected] (L.B. Kong).
usually takes more than a week. To lower the calcination temperature, many wet-chemistry-based methods have been developed to prepare ultra-fine powders for PLZT ceramics. They include chemical co-precipitation, sol–gel process, and hydrothermal reaction [10–13]. The ultra-fine powders are helpful in reducing sintering temperature and ensuring the homogeneity of the final PLZT ceramics. However, from the experimental procedure point of view, chemistry based processes are not essentially different from the conventional solid-state reaction method. Furthermore, wet-chemistry-based methodologies mostly use expensive and environment-sensitive chemicals, making the process difficult to deal with when compared with the conventional solid-state reaction process. Reactive sintering is a promising technique for preparation of multi-component ceramics, in which the reactions between constituent phases take place during the sintering process at high temperatures. It is advantageous because of the simplicity of its experimental operations and high productivity. Preparation of lead containing ceramics such as PbTiO 3 (PT), PbZrO 3 (PZ), Pb(Zr, Ti)O 3 (PZT) and Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN) via reaction sintering has been reported by Shrout et al. [14]. They found that enhanced densification of PZT could be achieved by partially reacting the component powders. The enhanced densification effect was believed to be beneficial from a reactive sintering process that occurred during the final stages of perovskite formation. In this paper, we report an alternative approach to
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fabricating PLZT (7 / 60 / 40) ceramics via reaction sintering from their constituent oxides by skipping the calcination step. This process is of significance in reducing costs of ferroelectric ceramics and can be extended to other multi-component ceramic products.
2. Experimental procedure Commercially available PbO (.99.9% purity, Aldrich, USA), La 2 O 3 (.99.9% purity, Aldrich, USA), ZrO 2 (.99% purity, Alfa Aesar, USA) and TiO 2 (.99.9% purity, Aldrich, USA) powders were used as the starting materials with the nominal composition of (Pb 0.93 La 0.07 )(Zr 0.60 Ti 0.40 ) 0.9825 O 3 (PLZT7 / 60 / 40). Different levels of excess PbO (0, 10 and 20 mol%) were chosen to examine the effect of PbO on the reaction and densification process of the oxide mixture during sintering at high temperature. The excess PbO also compensated for the lead evaporation during sintering process. The oxide mixtures were ball milled for 4 h using stabilized ZrO 2 balls and vials as milling medium. The slurries were dried at 80 8C and then uniaxially pressed into 10-mm diameter green pellets using a hardened stainless steel die at 50 MPa. The pellets were directly sintered at temperatures from 1000 to 1250 8C for 2 h, with heating at 5 8C / min and a cooling rate of 10 8C / min. X-ray diffraction analysis of the powders was performed using a Rigaku ultima1 type diffractometer (XRD) with Cu Ka radiation. The sintering behavior of the milled powders was monitored by a Setaram setsys 16 / 18 type dilatometer with a heating rate of 10 8C / min. All the sintered samples were polished and covered with silver paste as the electrode for electrical measurement. The microstructures of the sintered samples were characterized using a JEOL JSM-6340F type field emission scanning electronic microscope (FESEM). For SEM observation, the sintered samples were carefully polished and thermally etched at a temperature 50 8C less than the sintering temperature. The density of the PLZT ceramics was measured using a Mirage MD-200S type electronic densimeter. The dielectric and ferroelectric properties of the sintered PLZT samples were measured using an HP 4194A impedance / gain phase analyzer and Radiant Technologies RT6000HVS type standard ferroelectric tester, respectively. For piezoelectric measurement, the PLZT ceramics were poled at 120 8C for 1 h at an electric field of 30 kV/ cm in a bath of silicon oil. Piezoelectric parameters of the PLZT ceramics were calculated from measured impedance–frequency curves.
3. Results and discussions Fig. 1 shows the sintering behaviors of the oxide mixtures up to 1200 8C. All the samples demonstrate a
Fig. 1. Sintering behaviors of the oxide mixtures with different excess of PbO: (a) 0, (b) 10 and (c) 20 mol%.
volumetric expansion starting at |800 8C and peaking at |830 8C, with maximum expansion of 18, 21 and 18% for the samples with 0, 10 and 20 mol% excess PbO, respectively. This expansion is a characteristic of sintering reaction of the starting oxides towards the PLZT phase, which is due to the fact that PLZT has a larger molar volume than the sum of the individual components [14]. Such expansion has also been observed in other lead containing ferroelectric materials and bismuth titanate (Bi 4 Ti 3 O 12 ) via sintering reaction process [14,15]. In addition to the large expansion, there is a small expansion observed beginning at |600 8C and maximizing at |700 8C, which is believed to be caused by the burning of the PVA additive. It is noticed in the sintering behavior curves that the presence of excess PbO greatly influences the densification of the oxide mixtures. The sample without excess PbO shows a different densification pattern from those with excess PbO, in which significant shrinkage only occurs
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after |1100 8C. In fact, the sample still holds a volume larger than its green pellet up to 1200 8C. In contrast, the samples with excess PbO experience a constant densification after 900 8C, with final linear shrinkage of 10 and 15% for 10 and 20 mol% excess PbO, respectively. These results can be attributed to the presence of liquid phase caused by the excess PbO due to the low melting point of PbO (|850 8C) [15]. Densification without excess PbO is advanced by solid-state sintering, where contact points between particles are formed so that a rigid skeleton is established. The rigid skeleton prevents the samples from further densification [16]. With the presence of liquid phase, however, densification is performed by the rapid rearrangement of fine PLZT particles that are surrounded by a layer of PbO liquid phase. The liquid can prevent particles from forming a rigid skeleton, which leads to easier densification. Fig. 2 shows the measured density of the PLZT ceramics as a function of the sintering temperature. The microstructures of the PLZT ceramics with different levels of excess PbO and sintered at different temperatures are shown in Figs. 3 and 4, respectively. For the sample without excess PbO sintered at 1000 8C for 2 h, the measured density is only 6.5 g / cm 3 (80% of the theoretical density). A slight increase in density is observed as the sintering temperature increases up to 1100 8C. However, a density jump occurs from 1100 to 1150 8C and almost fully dense ceramics are achieved thereafter. This observation is in good agreement with the result shown in Fig. 1(a). Densification is inhibited below 1150 8C by the rigid skeleton formed as a result of contacted points. Once the rigid skeleton is destroyed at higher temperature (.1150 8C), densification is performed. Further increase in sintering temperature to 1250 8C leads to a slight decrease in the
Fig. 2. Measured density of the PLZT ceramics as a function of sintering temperature.
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Fig. 3. XRD patterns of the PLZT ceramics with excess PbO sintered at (a) 1000 8C and (b) 1250 8C, with 10 mol% excess PbO sintered at (c) 1000 8C and (d) 1250 8C, and 20 mol% excess PbO sintered at (e) 1000 8C and (f) 1250 8C.
measured density, which can be attributed to the loss of the lead component because of the high volatility of PbO at the elevated temperature. In contrast, with the presence of excess PbO, fully dense samples can be achieved at low temperature due to the liquid sintering. The samples with 10 and 20% excessive PbO possess similar variation in density as a function of sintering temperature. To confirm whether perovskite phase is formed during the reaction sintering process, the sintered samples were examined by using X-ray diffraction. Representative XRD patterns of the PLZT ceramics sintered at different temperatures are shown in Fig. 3. It is noticed that a single phase with perovskite structure is observed in the samples sintered at 1000 8C and this phase is stable up to 1250 8C. No PbO is detected in the XRD patterns probably because of its lower diffraction intensity compared with PLZT. Similar results are observed in the samples sintered at temperatures from 1050 to 1200 8C, which are not shown here. Figs. 4–6 illustrate the microstructure of the PLZT ceramics sintered at different temperatures. Grain size of the PLZT ceramics estimated from the SME observations as a function of sintering temperature is shown in Fig. 7. For the samples without excess PbO, grain size increases from 1.2 to 1.6 mm as the sintering temperature increases from 1050 to 1100 8C. A slight increase in grain size is observed between 1100 and 1200 8C, whereas the grain size greatly increases when sintered at 1250 8C. At low temperature (below 1150 8C), the gain size of the samples with excess PbO is smaller than that of those without
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Fig. 4. Cross-sectional microstructures of the PLZT ceramics without excess PbO sintered at: (a) 1050 8C, (b) 1100 8C, (c) 1150 8C, (d) 1200 8C and (e) 1250 8C for 2 h.
excess PbO, especially in the case of 20 mol%. This observation is also consistent with the sintering behavior shown in Fig. 1. At low temperature, solid state sintering in the mixture without excess PbO forms point contacts between particles, which inhibits densification but is beneficial to grain growth [16]. With the presence of excess PbO, densification is favored by the liquid phase sintering, but the grain growth is inhibited because of the liquid layer which surrounds the particles and increases their diffusion path. At a higher temperature (,1150 8C), the three groups are governed by a similar sintering mechanism.
Fig. 8 shows the dielectric constant (measured at room temperature and 1 kHz) of the PLZT ceramics as a function of the sintering temperature. The electrical properties of the PLZT samples sintered at 1000 8C were not measured because of their poor mechanical strength. The dielectric constant increases from 1050 to 1200 8C, reaches a maximum value at 1200 8C and shows a decrease at 1250 8C. There is no significant difference in dielectric constant among the three samples. The low dielectric constant of the samples without excess PbO sintered at low temperature may be attributed to the low density and small grain
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Fig. 5. Cross-sectional microstructures of the PLZT ceramics with 10 mol% excess PbO sintered at: (a) 1050 8C, (b) 1100 8C, (c) 1150 8C, (d) 1200 8C and (e) 1250 8C for 2 h.
size, while the similar low value of low dielectric constant for the samples containing excess PbO is believed to be caused by the excess PbO. Fig. 9 shows representative P-E hysteresis loops of the PLZT samples sintered at 1200 8C for 2 h. All the samples demonstrated a well-developed symmetric hysteresis loop. The remnant polarization of the PLZT ceramics as a function of sintering temperature is shown in Fig. 10, showing a trend similar to that of dielectric constant. All the dielectric and ferroelectric properties of the PLZT ceramics as a function of sintering temperature can be
explained in terms of microstructure and grain size of the samples. Ferroelectric properties of ferroelectric ceramics, such as dielectric constant and remnant polarization, are dependent on grain size of the materials. This is because an increase in grain size reduces the volume fraction of grain boundaries, and as a result, the coupling effect between the grain boundaries and the domain wall, which makes domain reorientation more difficult and severely constrains the domain wall motion, will decrease [17,18]. Thus, the domain wall mobility will increase, leading to an increase
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Fig. 6. Cross-sectional microstructures of the PLZT ceramics with 20 mol% excess PbO sintered at: (a) 1050 8C, (b) 1100 8C, (c) 1150 8C, (d) 1200 8C and (e) 1250 8C for 2 h.
in dielectric constant with grain size. The achievable domain alignment will also increase, and hence result in the increase in saturated and remnant polarization. This explains the observations in the change of dielectric constant (Fig. 8) and remnant polarization (Fig. 10) of the PLZT ceramics as a function of sintering temperature from 1050 to 1200 8C. The decrease in dielectric and ferroelectric properties for the samples sintered at 1250 8C may be attributed to the loss of PbO. In addition, although the powder mixture with 20% excess PbO shows a larger shrinkage as shown in Fig. 1(c), the ceramics derived from
this powder do not give rise to better properties than those with 10% excess PbO. In this respect, 10% excess PbO is sufficient to obtain good electrical properties for the fabrication of PLZT ceramics. Piezoelectric coefficient of the PLZT ceramics as a function of sintering temperature is shown in Fig. 11, with a trend similar to those of dielectric constant and remnant polarization. The 1200 8C-sintered PLZT samples without excess PbO, with 10 and 20% PbO excess demonstrate piezoelectric coefficients of 658, 682 and 659 pC / N, respectively. These values together with the dielectric and
L.B. Kong et al. / Journal of Alloys and Compounds 339 (2002) 167 – 174
Fig. 7. Grain size of the PLZT ceramics as a function of sintering temperature.
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Fig. 10. Remnant polarization of the PLZT ceramics with sintering temperature.
Fig. 8. Dielectric constant at 1 kHz of the PLZT ceramics as a function of sintering temperature. Fig. 11. Piezoelectric coefficient of the PLZT ceramics as a function of sintering temperature.
ferroelectric properties of the PLZT ceramics are comparable with the literature results. The one-step reactive sintering process is believed to be applicable to other multiple component ceramic materials.
4. Conclusions
Fig. 9. Representative P-E hysteresis loops of the PLZT ceramics with different excess of PbO sintered at 1150 8C for 2 h: (a) 0, (b) 10 and (c) 20 mol%.
Lead lanthanum zirconate titanate (PLZT7 / 60 / 40) ceramics can be prepared directly from their oxide mixtures. The electrical properties of the PLZT ceramics sintered at 1200 8C for 1 h are in a good agreement with those reported in the literature. The experimental results also suggest that 10% excess PbO is the optimized level to obtain PLZT ceramics with acceptable electrical prop-
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erties. This simple one-step process will be favorable for cost reduction and increased productivity of not only ferroelectric but also other ceramic materials.
References [1] L.M. Brown, K.S. Mazdiyasni, J. Am. Ceram. Soc. 55 (11) (1972) 541–544. [2] Y. Yoshikawa, K. Tsuzuki, J. Am. Ceram. Soc. 75 (9) (1992) 2520–2528. [3] M.A. Akbas, W.E. Lee, J. Eur. Ceram. Soc. 15 (1995) 57–63. [4] G.H. Haertling, J. Am. Ceram. Soc. 82 (4) (1999) 797–818. [5] Q.Y. Jiang, E.C. Subbarao, L.E. Cross, J. Appl. Phys. 75 (11) (1994) 7433–7443. [6] G.H. Haertling, C.E. Land, Ferroelectrics 3 (1972) 269–280. [7] G.S. Snow, J. Am. Ceram. Soc. 56 (9) (1973) 479–480. [8] D. Viehland, X. Dai, J.F. Li, Z. Xu, J. Appl. Phys. 84 (1) (1998) 458–471.
[9] R.P. Brodeur, K.W. Gachigi, P.M. Pruna, T.R. Shrout, J. Am. Ceram. Soc. 77 (11) (1994) 3042–3044. [10] W.K. Lin, Y.H. Chang, Mater. Sci. Eng. A 186 (1994) 177–183. [11] K. Kitaoka, H. Kozuka, T. Yoko, J. Am. Ceram. Soc. 81 (5) (1998) 1189–1196. [12] M. Cerqueira, R.S. Nasar, E. Leite, R.E. Longo, J.A. Varela, Mater. Lett. 35 (1998) 166–171. [13] R.N. Das, A. Pathak, P. Pramanik, J. Am. Ceram. Soc. 81 (12) (1998) 3357–3360. [14] T.R. Shrout, P. Patet, S. Kim, G.S. Lee, J. Am. Ceram. Soc. 73 (7) (1990) 1862–1867. [15] M. Hammer, M.J. Hoffmann, J. Am. Ceram. Soc. 81 (12) (1998) 3277–3284. [16] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, Wiley, New York, 1976, Chapter 9. [17] D. Damjanovic, M. Demartin, Appl. Phys. Lett. 68 (21) (1996) 3046–3048. [18] C.A. Randall, N. Kim, J.P. Kucera, W. Cao, T.R. Shrout, J. Am. Ceram. Soc. 81 (3) (1998) 677–688.
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Fabrication and characterization of lead lanthanum zirconate titanate (PLZT7 / 60 / 40) ceramics from oxides L.B. Kong*, J. Ma, R.F. Zhang, T.S. Zhang School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Block N4, B2, Nanyang 639798, Singapore Received 21 August 2001; accepted 19 November 2001
Abstract Lead lanthanum zirconate titanate (PLZT7 / 60 / 40) ceramics were fabricated from their oxide mixture via a direct sintering process without involving the calcination step. Different levels of excess PbO (0, 10 and 20 mol%) were used to check its effect on microstructure and thus the electrical properties of the PLZT ceramics. The reaction and densification behavior of the oxide mixtures for the PLZT ceramics were monitored by using a dilatometer. The electrical properties of the PLZT ceramics and their relationships with the processing parameters and excess PbO levels were characterized and discussed. 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxide materials; Ferroelectrics; Ceramics; Microstructure; Dielectric response
1. Introduction Lead lanthanum zirconate titanate (PLZT) ceramics, with variable dopant concentrations and Zr / Ti ratios, have been shown to exhibit a variety of ferroic phases such as ferroelectric (FE), antiferroelectric (AFE) and paraelectric (PE) phases. Due to these advantages, PLZT ceramics have received much attention from researchers all over the world [1–5]. The 7%-La-doped Pb(Zr 60 Ti 40 )O 3 ceramic (PLZT7 / 60 / 40) is one of the most important candidates for piezoelectric applications due to its extremely high piezoelectric coefficients (d 33 5710 pC / N, k p 50.72), as reported in the literature [6–8]. PLZT powders were conventionally prepared by solidstate reaction processing using oxides as the starting materials [6–9], involving several steps such as mixing, calcination and sintering. Calcination at temperatures ranging from 900 to 1000 8C is used to synthesize the desired compound before sintering is performed. The synthesized powders usually contain unwanted agglomerates harmful to the subsequent densification behavior, the microstructural uniformity and the final electrical properties of the PLZT ceramics. Synthesis of PLZT powders in this way also suffers from the problem of PbO loss because PbO easily evaporates at a high temperature. In addition, this process is very time consuming; one run *Corresponding author. E-mail address: [email protected] (L.B. Kong).
usually takes more than a week. To lower the calcination temperature, many wet-chemistry-based methods have been developed to prepare ultra-fine powders for PLZT ceramics. They include chemical co-precipitation, sol–gel process, and hydrothermal reaction [10–13]. The ultra-fine powders are helpful in reducing sintering temperature and ensuring the homogeneity of the final PLZT ceramics. However, from the experimental procedure point of view, chemistry based processes are not essentially different from the conventional solid-state reaction method. Furthermore, wet-chemistry-based methodologies mostly use expensive and environment-sensitive chemicals, making the process difficult to deal with when compared with the conventional solid-state reaction process. Reactive sintering is a promising technique for preparation of multi-component ceramics, in which the reactions between constituent phases take place during the sintering process at high temperatures. It is advantageous because of the simplicity of its experimental operations and high productivity. Preparation of lead containing ceramics such as PbTiO 3 (PT), PbZrO 3 (PZ), Pb(Zr, Ti)O 3 (PZT) and Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN) via reaction sintering has been reported by Shrout et al. [14]. They found that enhanced densification of PZT could be achieved by partially reacting the component powders. The enhanced densification effect was believed to be beneficial from a reactive sintering process that occurred during the final stages of perovskite formation. In this paper, we report an alternative approach to
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fabricating PLZT (7 / 60 / 40) ceramics via reaction sintering from their constituent oxides by skipping the calcination step. This process is of significance in reducing costs of ferroelectric ceramics and can be extended to other multi-component ceramic products.
2. Experimental procedure Commercially available PbO (.99.9% purity, Aldrich, USA), La 2 O 3 (.99.9% purity, Aldrich, USA), ZrO 2 (.99% purity, Alfa Aesar, USA) and TiO 2 (.99.9% purity, Aldrich, USA) powders were used as the starting materials with the nominal composition of (Pb 0.93 La 0.07 )(Zr 0.60 Ti 0.40 ) 0.9825 O 3 (PLZT7 / 60 / 40). Different levels of excess PbO (0, 10 and 20 mol%) were chosen to examine the effect of PbO on the reaction and densification process of the oxide mixture during sintering at high temperature. The excess PbO also compensated for the lead evaporation during sintering process. The oxide mixtures were ball milled for 4 h using stabilized ZrO 2 balls and vials as milling medium. The slurries were dried at 80 8C and then uniaxially pressed into 10-mm diameter green pellets using a hardened stainless steel die at 50 MPa. The pellets were directly sintered at temperatures from 1000 to 1250 8C for 2 h, with heating at 5 8C / min and a cooling rate of 10 8C / min. X-ray diffraction analysis of the powders was performed using a Rigaku ultima1 type diffractometer (XRD) with Cu Ka radiation. The sintering behavior of the milled powders was monitored by a Setaram setsys 16 / 18 type dilatometer with a heating rate of 10 8C / min. All the sintered samples were polished and covered with silver paste as the electrode for electrical measurement. The microstructures of the sintered samples were characterized using a JEOL JSM-6340F type field emission scanning electronic microscope (FESEM). For SEM observation, the sintered samples were carefully polished and thermally etched at a temperature 50 8C less than the sintering temperature. The density of the PLZT ceramics was measured using a Mirage MD-200S type electronic densimeter. The dielectric and ferroelectric properties of the sintered PLZT samples were measured using an HP 4194A impedance / gain phase analyzer and Radiant Technologies RT6000HVS type standard ferroelectric tester, respectively. For piezoelectric measurement, the PLZT ceramics were poled at 120 8C for 1 h at an electric field of 30 kV/ cm in a bath of silicon oil. Piezoelectric parameters of the PLZT ceramics were calculated from measured impedance–frequency curves.
3. Results and discussions Fig. 1 shows the sintering behaviors of the oxide mixtures up to 1200 8C. All the samples demonstrate a
Fig. 1. Sintering behaviors of the oxide mixtures with different excess of PbO: (a) 0, (b) 10 and (c) 20 mol%.
volumetric expansion starting at |800 8C and peaking at |830 8C, with maximum expansion of 18, 21 and 18% for the samples with 0, 10 and 20 mol% excess PbO, respectively. This expansion is a characteristic of sintering reaction of the starting oxides towards the PLZT phase, which is due to the fact that PLZT has a larger molar volume than the sum of the individual components [14]. Such expansion has also been observed in other lead containing ferroelectric materials and bismuth titanate (Bi 4 Ti 3 O 12 ) via sintering reaction process [14,15]. In addition to the large expansion, there is a small expansion observed beginning at |600 8C and maximizing at |700 8C, which is believed to be caused by the burning of the PVA additive. It is noticed in the sintering behavior curves that the presence of excess PbO greatly influences the densification of the oxide mixtures. The sample without excess PbO shows a different densification pattern from those with excess PbO, in which significant shrinkage only occurs
L.B. Kong et al. / Journal of Alloys and Compounds 339 (2002) 167 – 174
after |1100 8C. In fact, the sample still holds a volume larger than its green pellet up to 1200 8C. In contrast, the samples with excess PbO experience a constant densification after 900 8C, with final linear shrinkage of 10 and 15% for 10 and 20 mol% excess PbO, respectively. These results can be attributed to the presence of liquid phase caused by the excess PbO due to the low melting point of PbO (|850 8C) [15]. Densification without excess PbO is advanced by solid-state sintering, where contact points between particles are formed so that a rigid skeleton is established. The rigid skeleton prevents the samples from further densification [16]. With the presence of liquid phase, however, densification is performed by the rapid rearrangement of fine PLZT particles that are surrounded by a layer of PbO liquid phase. The liquid can prevent particles from forming a rigid skeleton, which leads to easier densification. Fig. 2 shows the measured density of the PLZT ceramics as a function of the sintering temperature. The microstructures of the PLZT ceramics with different levels of excess PbO and sintered at different temperatures are shown in Figs. 3 and 4, respectively. For the sample without excess PbO sintered at 1000 8C for 2 h, the measured density is only 6.5 g / cm 3 (80% of the theoretical density). A slight increase in density is observed as the sintering temperature increases up to 1100 8C. However, a density jump occurs from 1100 to 1150 8C and almost fully dense ceramics are achieved thereafter. This observation is in good agreement with the result shown in Fig. 1(a). Densification is inhibited below 1150 8C by the rigid skeleton formed as a result of contacted points. Once the rigid skeleton is destroyed at higher temperature (.1150 8C), densification is performed. Further increase in sintering temperature to 1250 8C leads to a slight decrease in the
Fig. 2. Measured density of the PLZT ceramics as a function of sintering temperature.
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Fig. 3. XRD patterns of the PLZT ceramics with excess PbO sintered at (a) 1000 8C and (b) 1250 8C, with 10 mol% excess PbO sintered at (c) 1000 8C and (d) 1250 8C, and 20 mol% excess PbO sintered at (e) 1000 8C and (f) 1250 8C.
measured density, which can be attributed to the loss of the lead component because of the high volatility of PbO at the elevated temperature. In contrast, with the presence of excess PbO, fully dense samples can be achieved at low temperature due to the liquid sintering. The samples with 10 and 20% excessive PbO possess similar variation in density as a function of sintering temperature. To confirm whether perovskite phase is formed during the reaction sintering process, the sintered samples were examined by using X-ray diffraction. Representative XRD patterns of the PLZT ceramics sintered at different temperatures are shown in Fig. 3. It is noticed that a single phase with perovskite structure is observed in the samples sintered at 1000 8C and this phase is stable up to 1250 8C. No PbO is detected in the XRD patterns probably because of its lower diffraction intensity compared with PLZT. Similar results are observed in the samples sintered at temperatures from 1050 to 1200 8C, which are not shown here. Figs. 4–6 illustrate the microstructure of the PLZT ceramics sintered at different temperatures. Grain size of the PLZT ceramics estimated from the SME observations as a function of sintering temperature is shown in Fig. 7. For the samples without excess PbO, grain size increases from 1.2 to 1.6 mm as the sintering temperature increases from 1050 to 1100 8C. A slight increase in grain size is observed between 1100 and 1200 8C, whereas the grain size greatly increases when sintered at 1250 8C. At low temperature (below 1150 8C), the gain size of the samples with excess PbO is smaller than that of those without
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Fig. 4. Cross-sectional microstructures of the PLZT ceramics without excess PbO sintered at: (a) 1050 8C, (b) 1100 8C, (c) 1150 8C, (d) 1200 8C and (e) 1250 8C for 2 h.
excess PbO, especially in the case of 20 mol%. This observation is also consistent with the sintering behavior shown in Fig. 1. At low temperature, solid state sintering in the mixture without excess PbO forms point contacts between particles, which inhibits densification but is beneficial to grain growth [16]. With the presence of excess PbO, densification is favored by the liquid phase sintering, but the grain growth is inhibited because of the liquid layer which surrounds the particles and increases their diffusion path. At a higher temperature (,1150 8C), the three groups are governed by a similar sintering mechanism.
Fig. 8 shows the dielectric constant (measured at room temperature and 1 kHz) of the PLZT ceramics as a function of the sintering temperature. The electrical properties of the PLZT samples sintered at 1000 8C were not measured because of their poor mechanical strength. The dielectric constant increases from 1050 to 1200 8C, reaches a maximum value at 1200 8C and shows a decrease at 1250 8C. There is no significant difference in dielectric constant among the three samples. The low dielectric constant of the samples without excess PbO sintered at low temperature may be attributed to the low density and small grain
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Fig. 5. Cross-sectional microstructures of the PLZT ceramics with 10 mol% excess PbO sintered at: (a) 1050 8C, (b) 1100 8C, (c) 1150 8C, (d) 1200 8C and (e) 1250 8C for 2 h.
size, while the similar low value of low dielectric constant for the samples containing excess PbO is believed to be caused by the excess PbO. Fig. 9 shows representative P-E hysteresis loops of the PLZT samples sintered at 1200 8C for 2 h. All the samples demonstrated a well-developed symmetric hysteresis loop. The remnant polarization of the PLZT ceramics as a function of sintering temperature is shown in Fig. 10, showing a trend similar to that of dielectric constant. All the dielectric and ferroelectric properties of the PLZT ceramics as a function of sintering temperature can be
explained in terms of microstructure and grain size of the samples. Ferroelectric properties of ferroelectric ceramics, such as dielectric constant and remnant polarization, are dependent on grain size of the materials. This is because an increase in grain size reduces the volume fraction of grain boundaries, and as a result, the coupling effect between the grain boundaries and the domain wall, which makes domain reorientation more difficult and severely constrains the domain wall motion, will decrease [17,18]. Thus, the domain wall mobility will increase, leading to an increase
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Fig. 6. Cross-sectional microstructures of the PLZT ceramics with 20 mol% excess PbO sintered at: (a) 1050 8C, (b) 1100 8C, (c) 1150 8C, (d) 1200 8C and (e) 1250 8C for 2 h.
in dielectric constant with grain size. The achievable domain alignment will also increase, and hence result in the increase in saturated and remnant polarization. This explains the observations in the change of dielectric constant (Fig. 8) and remnant polarization (Fig. 10) of the PLZT ceramics as a function of sintering temperature from 1050 to 1200 8C. The decrease in dielectric and ferroelectric properties for the samples sintered at 1250 8C may be attributed to the loss of PbO. In addition, although the powder mixture with 20% excess PbO shows a larger shrinkage as shown in Fig. 1(c), the ceramics derived from
this powder do not give rise to better properties than those with 10% excess PbO. In this respect, 10% excess PbO is sufficient to obtain good electrical properties for the fabrication of PLZT ceramics. Piezoelectric coefficient of the PLZT ceramics as a function of sintering temperature is shown in Fig. 11, with a trend similar to those of dielectric constant and remnant polarization. The 1200 8C-sintered PLZT samples without excess PbO, with 10 and 20% PbO excess demonstrate piezoelectric coefficients of 658, 682 and 659 pC / N, respectively. These values together with the dielectric and
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Fig. 7. Grain size of the PLZT ceramics as a function of sintering temperature.
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Fig. 10. Remnant polarization of the PLZT ceramics with sintering temperature.
Fig. 8. Dielectric constant at 1 kHz of the PLZT ceramics as a function of sintering temperature. Fig. 11. Piezoelectric coefficient of the PLZT ceramics as a function of sintering temperature.
ferroelectric properties of the PLZT ceramics are comparable with the literature results. The one-step reactive sintering process is believed to be applicable to other multiple component ceramic materials.
4. Conclusions
Fig. 9. Representative P-E hysteresis loops of the PLZT ceramics with different excess of PbO sintered at 1150 8C for 2 h: (a) 0, (b) 10 and (c) 20 mol%.
Lead lanthanum zirconate titanate (PLZT7 / 60 / 40) ceramics can be prepared directly from their oxide mixtures. The electrical properties of the PLZT ceramics sintered at 1200 8C for 1 h are in a good agreement with those reported in the literature. The experimental results also suggest that 10% excess PbO is the optimized level to obtain PLZT ceramics with acceptable electrical prop-
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erties. This simple one-step process will be favorable for cost reduction and increased productivity of not only ferroelectric but also other ceramic materials.
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