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Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics
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Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics Yangxi Yan a, Zhimin Li a,n, Maolin Zhang a, Peng Sun a, Yonghao Xu b, Yujun Feng c,nn a
School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071, China School of Electrical Engineering and Automation, Henan Polytechnic University, Jiaozuo 454003, China c Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2016 Received in revised form 26 August 2016 Accepted 26 August 2016
Fe-doped (Pb0.99Nb0.02)[(Zr0.70Sn0.30)0.52Ti0.48]0.98O3 (PNZST) ceramics were prepared via conventional solid state reaction method, and the effect of Fe doping on their structural and electrical properties was investigated in detail. Results showed that Fe3 þ cations could dissolve into readily the B-sites of perovskite structure for the PNZST ceramics with the less amount of Fe content ( r0.8 wt%), resulting in the full densification after sintered at 1300 °C. Meanwhile, Fe doping caused a structure transform from the tetragonal to the rhombohedral. The better electric properties for PNZST ceramic with 0.6 wt% Fe content were obtained, i.e. piezoelectric constant d33 ¼380 pC/N, electromechanical coupling factor kp ¼ 0.57, mechanical quality factor Qm ¼225, dielectric constant εr ¼ 1190, loss tangent tan δ ¼0.007 and curie temperature Tc ¼318 °C. & 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: Fe doping Piezoelectric property Ferroelectric Dielectric property
1. Introduction Lead-based ferroelectric ceramics have been widely investigated due to their outstanding ferroelectric and piezoelectric properties, which make them to become excellent candidate materials for the device applications such as multilayer capacitors, transducers and actuators [1–3]. One of the typical lead-based ferroelectrics is lead zirconate titanate system (Pb(Zr1 xTix)O3, PZT). In general, the properties of PZT ceramics are modified with donor or acceptor dopants to satisfy the specific requirements, because the piezoelectric constant (d33) and dielectric constant (εr) are affected by the domain structures and the defects of materials [4–7]. For PZT ceramics, high-valent additives or donors would induce a “soft” piezoelectric behavior which are appropriate for positioning actuator applications, while lower-valent additives or acceptors would induce a “hard” behavior which are particularly suitable for ultrasonic motor applications [8,9]. Garg et al. [10] reported that Nd doping could improve the piezoelectric and dielectric properties of PZT ceramic in which the d33, εr, and kp were 415 pC/N, 1030 and 0.49, respectively. Meanwhile, the PMS-PZT ceramics modified with Sr2 þ showed the optimum kp of 0.53 and Qm of 1115 [11]. Zhong et al. found that n
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (Y. Feng).
nn
0.1 mol% WO3 doped PMN-PT ceramic exhibited a better d33 (466 pC/N) and kp (0.564) [12]. Although the properties of PZT ceramics can be modified by donor or acceptor dopants, as mentioned above, it is still difficult to obtain the ceramic with both the high piezoelectric constant (Z300 pC/N) and the moderate mechanical quality factor (200 500) for the application in acoustic transducer [13–15]. The PbNb[Zr, Sn, Ti]O3 (PNZST) material has been extensively studied for the ferroelectric and anti-ferroelectric phase transition [16–18], but little investigation has been done for the piezoelectric property. In our previous work, it had been found that the PNZST ceramics showed higher dielectric and piezoelectric constants, with lower the mechanical quality factor [19]. Additionally, the substitution of Fe on PZT could decrease the tan δ and increase the Qm [20]. Hence, in this work, Fe3 þ cation was introduced to PNZST, and the effect of Fe doping on the mechanical quality factor, piezoelectric constant and dielectric constant of the PNZST ceramics was investigated in detail.
2. Experiment details The general formula of the studied materials was (Pb0.99Nb0.02) [(Zr0.70Sn0.30)0.52Ti0.48]0.98O3 þx wt% Fe2O3, where x ¼0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.2. Reagent-grade oxide powders of Pb3O4 (purity 97.0%), ZrO2 (99.0%), TiO2 (98.0%), SnO2 (99.0%), Nb2O5 (99.5%) and Fe2O3 (99.0%) were used as the raw materials. They were
http://dx.doi.org/10.1016/j.ceramint.2016.08.167 0272-8842/& 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: Y. Yan, et al., Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.167i
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Fig. 1. SEM images of the surface of as-sintered PNZST samples with different Fe content.
120 °C for 30 min. The grain morphologies on the surface of the as-sintered pellets were observed by scanning electron microscope (SEM, FEI Quanta 250 FEG, Hillsboro, Oregon, U.S.). The phase purities of the samples were analyzed by X-ray diffraction (XRD, Model D/MaxIIIC, Rigaku, Japan) using Cu Kα radiation in the 2θ range from 15° to 70°. The quantities kp and Qm were derived via the resonanceantiresonance method using an impedance analyzer (Agilent 4294, Japan). The room-temperature dielectric constant εr and loss tangent tan δ at 1 kHz were measured directly using the impedance analyzer. The piezoelectric coefficient was measured with a piezo-d33 meter (Model ZJ-3A, Institute of Acoustics, Chinese Academy of Science, China). The relative dielectric permittivity εr and loss tangent tan δ were measured with an LCR meter (E4980A, Agilent, Palo Alto, CA, U.S.) at a frequency of 1 kHz with a heating rate of 3 °C/min from 25 °C to 450 °C. Fig. 2. XRD patterns of the as-prepared PNZST ceramics with different Fe content.
stoichiometrically mixed and ball-milled in ethanol for 5 h. After ball-milling, dried powders were calcined at 850 °C for 2 h in air, and then ball-milled again for 5 h. The calcined powders were added with 5 wt% polyvinyl alcohol (PVA) solution, and the mixtures were pressed into the disks with a diameter of 12 mm at 100 MPa. The samples were sintered at 1300 °C for 3 h in a sealed alumina crucible. The sintered pellets were polished to a thickness of 1 mm, and the Ag electrode paste was coated and fired at 650 °C for 30 min. The as-prepared samples were poled at 30 kV/cm at
3. Results and discussion Fig. 1 shows the SEM images of the as-sintered surfaces of PNZST samples with different Fe content. As can be seen, the average grain size of pure PNZST is about 3 mm, and the average grain sizes of samples increase with the increasing Fe content. When Fe content increases to 0.8 wt%, the average grain size of the sample is approximately 13 mm. But there gives rise to abnormal grain boundaries accompanied with a high porosity and a sharp reduction in grain size, when the Fe content increases to 1.2 wt%. Therefore, the addition of Fe2O3 can influence significantly the
Please cite this article as: Y. Yan, et al., Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.167i
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Fig. 3. (a) d33 and kp and (b) Qm and tan δ as a function of the Fe content.
Fig. 4. (a) the temperature-dependent dielectric behavior for Fe-doped PNZST ceramics at 1 kHz, and (b) εr, εr and Tc as a function of the Fe content.
Fig. 5. (a)The polarization-electrical field (P-E) hysteresis loops of Fe-doped PNZST ceramics at room temperature, and (b) the tendency of related parameters.
grain growth and bulk density of PNZST ceramics. When the Fe content does not exceed its solubility limit in PNZST, it is believed that Fe3 þ cations can uniformly dissolve into the B-sites of the perovskite structure to improve the ion mobility, which is favorable for the grain growth and the densification of the material [21,22]. However, further increase of Fe content to the amount exceeding the solubility limit will hinder the grain growth because of the segregation of redundant Fe ions at the grain boundaries, leading to the inferior crystallization behavior. The XRD patterns of the as-prepared PNZST ceramics with different Fe content are shown in Fig. 2. Results indicate that all the samples are pure perovskite structure, and that no pyrochlore or other phase is observed. Also, it is believed that the splitting of the (200) and (002) peaks suggests the tetragonal distortion for the pure PNZST ceramic. One can find that the splitting becomes gradually weak as Fe content increases, which indicates that the
perovskite structure gradually changes from the tetragonal to the rhombohedral across the morphotropic phase boundary (MPB). The radius of Fe3 þ (0.645 Å) is smaller than that of Zr4 þ (0.79 Å) and Ti4 þ (0.68 Å) [23], so Fe3 þ cations can occupy the B-sites of PZT-based lattice in the form of acceptor. As a result, the tolerance factor of perovskite structure is expected to increase due to the incorporation of Fe, resulting in the evolution of the rhombohedral structure from the tetragonal structure [23]. The piezoelectric constant d33 and electromechanical coupling factor kp as a function of Fe content are shown in Fig. 3(a). It can be seen that the d33 and kp values firstly decrease suddenly, and then increase with increasing Fe content, in the case of Fe content less than 0.6 wt%. When the Fe content increases further, the values of d33 and kp decrease again. Therefore, the optimum d33 of 380 pC/N and kp of 0.57 were obtained for the sample with 0.6 wt% Fe content.
Please cite this article as: Y. Yan, et al., Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.167i
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When the Fe content was lower, the oxygen vacancies arising from Fe3 þ substitution would produce a “pinning effect” on the domain rotation, which thereby made the materials “hard” [24], in agreement with the results in Fig. 2. In general, the d33 and kp increase with the increase of grain size and homogeneity, and the change in structure and dopant amounts [25,26]. MPB also plays a significant role in enhancing the piezoelectric properties of perovskite-structured piezoelectric ceramics. Because the compositions at the MPB region have the tetragonal and rhombohedral structure, there are a total of 14 spontaneous polarization directions, of which the rhombohedral structure has 8 spontaneous polarization directions o111 4, and the tetragonal structure has 6 spontaneous polarization directions o001 4. During the poling process, the sample at the MPB region can obtain the best crystal orientation due to a large number of spontaneous polarization directions, which is beneficial to get the abnormal high piezoelectric properties [27]. As mentioned above, the optimum of d33 and kp of the sample with 0.6 wt% Fe content are attributed to the better grain morphology, higher density and MPB phases structure. Fig. 3(b) shows the mechanical quality factor Qm and dielectric loss tan δ as a function of the Fe content. As can be seen, the Qm of PNZST ceramic increases firstly, and then decreases with increasing Fe content. The maximum response of Qm (230) is obtained in the case of x ¼0.8, which is possibly due to the existence of more oxygen vacancies which efficiently pin and inhibit the domain wall motion. However, when the Fe content is more than 0.8 wt%, the Qm decreases sharply, which is apparently because of the inferior crystallization behavior and insufficient densification. Accordingly, the tan δ exhibits a contrary change tendency compared to Qm, with the smallest value of 0.0065 in the case of x ¼0.8. Fig. 4(a) shows the temperature-dependent dielectric behavior at 1 kHz for Fe-modified PNZST ceramics. The curie temperature (Tc) of pure PNZST is around 313 °C, and Tc gradually shifts to higher temperature with increasing Fe content. For the sample with 1.2 wt% Fe content, the corresponding Tc is around 322 °C. This phenomenon is similar to that observed in BSPT and PZT ceramics [28–31]. Also, the dielectric peak is found to be broadened with increasing Fe content, which is due to the substitutional fluctuations and structural disorder in the arrangement of cations at the A-sites and/or B-sites [30]. In addition, it can be noted that the dielectric constant increases drastically above Tc with the dopant level x Z0.6. The increase of Fe-doped level can enhance the space charge polarization, which may not follow the external field, leading to the large increase in dielectric constant above Tc [28]. The room-temperature dielectric constant (εr) and dielectric peak (εmax) measured from Fig. 4(a) are plotted in Fig. 4 (b). It is found that the highest εr (1358) and εmax (41,578) are achieved for the sample with 0.2 wt% Fe content. The polarization-electrical field (P-E) hysteresis loops of Femodified PNZST ceramics were collected at room temperature, as shown in Fig. 5(a). The representative parameters measured from Fig. 5(a), including Pr and Ec of all the samples are plotted in Fig. 5 (b). As can be seen in Fig. 5(a), the P-E loop of the pure PNZST is almost symmetric. However, the P-E loop gradually becomes obvious asymmetric with increasing Fe content. The asymmetric P-E loops could be observed in many acceptor-doped ferroelectrics, such as Fe-doped PbTiO3 and Mn-doped BaTiO3 after poled [32,33]. The asymmetry of the P-E loop is due to the preferentially oriented defect dipoles formed by the acceptor dopant cations and oxygen vacancies along the poling direction [34]. Furthermore, the Ec of the sample increases slightly from 12.8 kV/cm to a maximum value of 15 kV/cm at x¼ 0.4, then decreases substantially to 11.7 kV/cm at x¼1.2. The Pr value firstly decreases to 28 μC/cm2 for the sample with 0.4 wt% Fe content, and then increases to 35 μC/cm2 in the case of x ¼0.6. The initial
decrease in Pr may be attributed to the defects caused by the acceptor type substitution of Fe3 þ on the B-site cations in the perovskite lattice [28]. As the Fe content further increases, the concentration of defects keeps relatively stable, but the increase in Pr should be mainly caused by the oxygen vacancies induced by Fe doping, which facilitate the sintering process, hence improving the electrical performance [21,22]. In addition, when the Fe content is above 0.6 wt%, the Pr, Ec and the squareness of the hysteresis loop obviously decreases, which is possibly due to the fact that the high doping level makes the redundant Fe3 þ cations to segregate at grain boundaries and hinders the grain growth.
4. Conclusions Fe-modified PNZST ceramics were prepared via the traditional solid-state reaction. Microstructures of all the as-prepared samples indicated that the average grain size increased with increasing Fe content in the case of Fe content less than 0.8 wt%. The results of XRD showed that the samples with the Fe content x r0.4 had the tetragonal structure, and that the coexistence of the tetragonal and rhombohedral structures was found for the sample with 0.6 wt% Fe content. The dielectric, ferroelectric, and piezoelectric properties of PNZST ceramics were significantly influenced by Fe doping. The optimum electrical properties of Fe-doped PNZST ceramics were d33 ¼380 pC/N, kp ¼ 0.57, Qm ¼ 225, εr ¼ 1190, tanδ ¼0.007 and Tc ¼318 °C in the case of x ¼0.6. The results of Fe-doped PNZST ceramics are practically useful for the application in transmittingreceiving acoustic transducers.
Acknowledgments Authors would like to acknowledge the financial supports of the National Natural Science Foundation of China (No. 51602240), Fundamental Research Funds for the Central Universities (Nos. JB161403, JB161405 and JB161406), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JQ6252) and Ningbo Natural Science Foundation (Nos. 2016A610029, 2015A610037 and 2015A610109).
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Contents lists available at ScienceDirect
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Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics Yangxi Yan a, Zhimin Li a,n, Maolin Zhang a, Peng Sun a, Yonghao Xu b, Yujun Feng c,nn a
School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071, China School of Electrical Engineering and Automation, Henan Polytechnic University, Jiaozuo 454003, China c Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 March 2016 Received in revised form 26 August 2016 Accepted 26 August 2016
Fe-doped (Pb0.99Nb0.02)[(Zr0.70Sn0.30)0.52Ti0.48]0.98O3 (PNZST) ceramics were prepared via conventional solid state reaction method, and the effect of Fe doping on their structural and electrical properties was investigated in detail. Results showed that Fe3 þ cations could dissolve into readily the B-sites of perovskite structure for the PNZST ceramics with the less amount of Fe content ( r0.8 wt%), resulting in the full densification after sintered at 1300 °C. Meanwhile, Fe doping caused a structure transform from the tetragonal to the rhombohedral. The better electric properties for PNZST ceramic with 0.6 wt% Fe content were obtained, i.e. piezoelectric constant d33 ¼380 pC/N, electromechanical coupling factor kp ¼ 0.57, mechanical quality factor Qm ¼225, dielectric constant εr ¼ 1190, loss tangent tan δ ¼0.007 and curie temperature Tc ¼318 °C. & 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: Fe doping Piezoelectric property Ferroelectric Dielectric property
1. Introduction Lead-based ferroelectric ceramics have been widely investigated due to their outstanding ferroelectric and piezoelectric properties, which make them to become excellent candidate materials for the device applications such as multilayer capacitors, transducers and actuators [1–3]. One of the typical lead-based ferroelectrics is lead zirconate titanate system (Pb(Zr1 xTix)O3, PZT). In general, the properties of PZT ceramics are modified with donor or acceptor dopants to satisfy the specific requirements, because the piezoelectric constant (d33) and dielectric constant (εr) are affected by the domain structures and the defects of materials [4–7]. For PZT ceramics, high-valent additives or donors would induce a “soft” piezoelectric behavior which are appropriate for positioning actuator applications, while lower-valent additives or acceptors would induce a “hard” behavior which are particularly suitable for ultrasonic motor applications [8,9]. Garg et al. [10] reported that Nd doping could improve the piezoelectric and dielectric properties of PZT ceramic in which the d33, εr, and kp were 415 pC/N, 1030 and 0.49, respectively. Meanwhile, the PMS-PZT ceramics modified with Sr2 þ showed the optimum kp of 0.53 and Qm of 1115 [11]. Zhong et al. found that n
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (Y. Feng).
nn
0.1 mol% WO3 doped PMN-PT ceramic exhibited a better d33 (466 pC/N) and kp (0.564) [12]. Although the properties of PZT ceramics can be modified by donor or acceptor dopants, as mentioned above, it is still difficult to obtain the ceramic with both the high piezoelectric constant (Z300 pC/N) and the moderate mechanical quality factor (200 500) for the application in acoustic transducer [13–15]. The PbNb[Zr, Sn, Ti]O3 (PNZST) material has been extensively studied for the ferroelectric and anti-ferroelectric phase transition [16–18], but little investigation has been done for the piezoelectric property. In our previous work, it had been found that the PNZST ceramics showed higher dielectric and piezoelectric constants, with lower the mechanical quality factor [19]. Additionally, the substitution of Fe on PZT could decrease the tan δ and increase the Qm [20]. Hence, in this work, Fe3 þ cation was introduced to PNZST, and the effect of Fe doping on the mechanical quality factor, piezoelectric constant and dielectric constant of the PNZST ceramics was investigated in detail.
2. Experiment details The general formula of the studied materials was (Pb0.99Nb0.02) [(Zr0.70Sn0.30)0.52Ti0.48]0.98O3 þx wt% Fe2O3, where x ¼0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.2. Reagent-grade oxide powders of Pb3O4 (purity 97.0%), ZrO2 (99.0%), TiO2 (98.0%), SnO2 (99.0%), Nb2O5 (99.5%) and Fe2O3 (99.0%) were used as the raw materials. They were
http://dx.doi.org/10.1016/j.ceramint.2016.08.167 0272-8842/& 2016 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: Y. Yan, et al., Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.167i
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Fig. 1. SEM images of the surface of as-sintered PNZST samples with different Fe content.
120 °C for 30 min. The grain morphologies on the surface of the as-sintered pellets were observed by scanning electron microscope (SEM, FEI Quanta 250 FEG, Hillsboro, Oregon, U.S.). The phase purities of the samples were analyzed by X-ray diffraction (XRD, Model D/MaxIIIC, Rigaku, Japan) using Cu Kα radiation in the 2θ range from 15° to 70°. The quantities kp and Qm were derived via the resonanceantiresonance method using an impedance analyzer (Agilent 4294, Japan). The room-temperature dielectric constant εr and loss tangent tan δ at 1 kHz were measured directly using the impedance analyzer. The piezoelectric coefficient was measured with a piezo-d33 meter (Model ZJ-3A, Institute of Acoustics, Chinese Academy of Science, China). The relative dielectric permittivity εr and loss tangent tan δ were measured with an LCR meter (E4980A, Agilent, Palo Alto, CA, U.S.) at a frequency of 1 kHz with a heating rate of 3 °C/min from 25 °C to 450 °C. Fig. 2. XRD patterns of the as-prepared PNZST ceramics with different Fe content.
stoichiometrically mixed and ball-milled in ethanol for 5 h. After ball-milling, dried powders were calcined at 850 °C for 2 h in air, and then ball-milled again for 5 h. The calcined powders were added with 5 wt% polyvinyl alcohol (PVA) solution, and the mixtures were pressed into the disks with a diameter of 12 mm at 100 MPa. The samples were sintered at 1300 °C for 3 h in a sealed alumina crucible. The sintered pellets were polished to a thickness of 1 mm, and the Ag electrode paste was coated and fired at 650 °C for 30 min. The as-prepared samples were poled at 30 kV/cm at
3. Results and discussion Fig. 1 shows the SEM images of the as-sintered surfaces of PNZST samples with different Fe content. As can be seen, the average grain size of pure PNZST is about 3 mm, and the average grain sizes of samples increase with the increasing Fe content. When Fe content increases to 0.8 wt%, the average grain size of the sample is approximately 13 mm. But there gives rise to abnormal grain boundaries accompanied with a high porosity and a sharp reduction in grain size, when the Fe content increases to 1.2 wt%. Therefore, the addition of Fe2O3 can influence significantly the
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Fig. 3. (a) d33 and kp and (b) Qm and tan δ as a function of the Fe content.
Fig. 4. (a) the temperature-dependent dielectric behavior for Fe-doped PNZST ceramics at 1 kHz, and (b) εr, εr and Tc as a function of the Fe content.
Fig. 5. (a)The polarization-electrical field (P-E) hysteresis loops of Fe-doped PNZST ceramics at room temperature, and (b) the tendency of related parameters.
grain growth and bulk density of PNZST ceramics. When the Fe content does not exceed its solubility limit in PNZST, it is believed that Fe3 þ cations can uniformly dissolve into the B-sites of the perovskite structure to improve the ion mobility, which is favorable for the grain growth and the densification of the material [21,22]. However, further increase of Fe content to the amount exceeding the solubility limit will hinder the grain growth because of the segregation of redundant Fe ions at the grain boundaries, leading to the inferior crystallization behavior. The XRD patterns of the as-prepared PNZST ceramics with different Fe content are shown in Fig. 2. Results indicate that all the samples are pure perovskite structure, and that no pyrochlore or other phase is observed. Also, it is believed that the splitting of the (200) and (002) peaks suggests the tetragonal distortion for the pure PNZST ceramic. One can find that the splitting becomes gradually weak as Fe content increases, which indicates that the
perovskite structure gradually changes from the tetragonal to the rhombohedral across the morphotropic phase boundary (MPB). The radius of Fe3 þ (0.645 Å) is smaller than that of Zr4 þ (0.79 Å) and Ti4 þ (0.68 Å) [23], so Fe3 þ cations can occupy the B-sites of PZT-based lattice in the form of acceptor. As a result, the tolerance factor of perovskite structure is expected to increase due to the incorporation of Fe, resulting in the evolution of the rhombohedral structure from the tetragonal structure [23]. The piezoelectric constant d33 and electromechanical coupling factor kp as a function of Fe content are shown in Fig. 3(a). It can be seen that the d33 and kp values firstly decrease suddenly, and then increase with increasing Fe content, in the case of Fe content less than 0.6 wt%. When the Fe content increases further, the values of d33 and kp decrease again. Therefore, the optimum d33 of 380 pC/N and kp of 0.57 were obtained for the sample with 0.6 wt% Fe content.
Please cite this article as: Y. Yan, et al., Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.167i
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When the Fe content was lower, the oxygen vacancies arising from Fe3 þ substitution would produce a “pinning effect” on the domain rotation, which thereby made the materials “hard” [24], in agreement with the results in Fig. 2. In general, the d33 and kp increase with the increase of grain size and homogeneity, and the change in structure and dopant amounts [25,26]. MPB also plays a significant role in enhancing the piezoelectric properties of perovskite-structured piezoelectric ceramics. Because the compositions at the MPB region have the tetragonal and rhombohedral structure, there are a total of 14 spontaneous polarization directions, of which the rhombohedral structure has 8 spontaneous polarization directions o111 4, and the tetragonal structure has 6 spontaneous polarization directions o001 4. During the poling process, the sample at the MPB region can obtain the best crystal orientation due to a large number of spontaneous polarization directions, which is beneficial to get the abnormal high piezoelectric properties [27]. As mentioned above, the optimum of d33 and kp of the sample with 0.6 wt% Fe content are attributed to the better grain morphology, higher density and MPB phases structure. Fig. 3(b) shows the mechanical quality factor Qm and dielectric loss tan δ as a function of the Fe content. As can be seen, the Qm of PNZST ceramic increases firstly, and then decreases with increasing Fe content. The maximum response of Qm (230) is obtained in the case of x ¼0.8, which is possibly due to the existence of more oxygen vacancies which efficiently pin and inhibit the domain wall motion. However, when the Fe content is more than 0.8 wt%, the Qm decreases sharply, which is apparently because of the inferior crystallization behavior and insufficient densification. Accordingly, the tan δ exhibits a contrary change tendency compared to Qm, with the smallest value of 0.0065 in the case of x ¼0.8. Fig. 4(a) shows the temperature-dependent dielectric behavior at 1 kHz for Fe-modified PNZST ceramics. The curie temperature (Tc) of pure PNZST is around 313 °C, and Tc gradually shifts to higher temperature with increasing Fe content. For the sample with 1.2 wt% Fe content, the corresponding Tc is around 322 °C. This phenomenon is similar to that observed in BSPT and PZT ceramics [28–31]. Also, the dielectric peak is found to be broadened with increasing Fe content, which is due to the substitutional fluctuations and structural disorder in the arrangement of cations at the A-sites and/or B-sites [30]. In addition, it can be noted that the dielectric constant increases drastically above Tc with the dopant level x Z0.6. The increase of Fe-doped level can enhance the space charge polarization, which may not follow the external field, leading to the large increase in dielectric constant above Tc [28]. The room-temperature dielectric constant (εr) and dielectric peak (εmax) measured from Fig. 4(a) are plotted in Fig. 4 (b). It is found that the highest εr (1358) and εmax (41,578) are achieved for the sample with 0.2 wt% Fe content. The polarization-electrical field (P-E) hysteresis loops of Femodified PNZST ceramics were collected at room temperature, as shown in Fig. 5(a). The representative parameters measured from Fig. 5(a), including Pr and Ec of all the samples are plotted in Fig. 5 (b). As can be seen in Fig. 5(a), the P-E loop of the pure PNZST is almost symmetric. However, the P-E loop gradually becomes obvious asymmetric with increasing Fe content. The asymmetric P-E loops could be observed in many acceptor-doped ferroelectrics, such as Fe-doped PbTiO3 and Mn-doped BaTiO3 after poled [32,33]. The asymmetry of the P-E loop is due to the preferentially oriented defect dipoles formed by the acceptor dopant cations and oxygen vacancies along the poling direction [34]. Furthermore, the Ec of the sample increases slightly from 12.8 kV/cm to a maximum value of 15 kV/cm at x¼ 0.4, then decreases substantially to 11.7 kV/cm at x¼1.2. The Pr value firstly decreases to 28 μC/cm2 for the sample with 0.4 wt% Fe content, and then increases to 35 μC/cm2 in the case of x ¼0.6. The initial
decrease in Pr may be attributed to the defects caused by the acceptor type substitution of Fe3 þ on the B-site cations in the perovskite lattice [28]. As the Fe content further increases, the concentration of defects keeps relatively stable, but the increase in Pr should be mainly caused by the oxygen vacancies induced by Fe doping, which facilitate the sintering process, hence improving the electrical performance [21,22]. In addition, when the Fe content is above 0.6 wt%, the Pr, Ec and the squareness of the hysteresis loop obviously decreases, which is possibly due to the fact that the high doping level makes the redundant Fe3 þ cations to segregate at grain boundaries and hinders the grain growth.
4. Conclusions Fe-modified PNZST ceramics were prepared via the traditional solid-state reaction. Microstructures of all the as-prepared samples indicated that the average grain size increased with increasing Fe content in the case of Fe content less than 0.8 wt%. The results of XRD showed that the samples with the Fe content x r0.4 had the tetragonal structure, and that the coexistence of the tetragonal and rhombohedral structures was found for the sample with 0.6 wt% Fe content. The dielectric, ferroelectric, and piezoelectric properties of PNZST ceramics were significantly influenced by Fe doping. The optimum electrical properties of Fe-doped PNZST ceramics were d33 ¼380 pC/N, kp ¼ 0.57, Qm ¼ 225, εr ¼ 1190, tanδ ¼0.007 and Tc ¼318 °C in the case of x ¼0.6. The results of Fe-doped PNZST ceramics are practically useful for the application in transmittingreceiving acoustic transducers.
Acknowledgments Authors would like to acknowledge the financial supports of the National Natural Science Foundation of China (No. 51602240), Fundamental Research Funds for the Central Universities (Nos. JB161403, JB161405 and JB161406), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JQ6252) and Ningbo Natural Science Foundation (Nos. 2016A610029, 2015A610037 and 2015A610109).
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Please cite this article as: Y. Yan, et al., Effect of Fe substitution on the piezoelectric, dielectric and ferroelectric properties of PNZST ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.167i