1. Introduction
Relaxor ferroelectric materials have the high dielectric constant, broad ferroelectric-paraelectric transition (the diffuse phase transition) and strong frequency dependence of dielectric properties [1] [2] . So far, researchers have been interesting on the Pb-based relaxors Pb(
)O3 because they are promising materials for multilayer capacitors, transducer and actuators [2] -[4] .
The addition of small amounts of such relaxor materials was found to be beneficial for the electrical properties of PZT-based ceramics due to the formation of fine and uniform rhombohedral domains along tetragonal ones [4] [5] . Recently, it was observed that both the dielectric and piezoelectric properties of these PZT-relaxor materials are strongly influenced by the addition of other additives such as La, ZnO, CuO, MnO2 and Fe2O3 [2] [4] [6] -[8] . The Fe2O3 addition is probably one of the most frequently used acceptors in ferroelectrics. The effects of Fe2O3 on PZT and PZT-based ternary ceramics have been studied [7] -[10] . However, the effect of Fe2O3 addition on the quaternary PZT-PZN-PMnN was scarcely reported.
Recently, we studied the effect of Zr/Ti ratio content on some physical properties of 0.8Pb(ZrxTi1-x)]O3 – 0.125Pb(Zn1/3Nb2/3)O3 – 0.075Pb(Mn1/3Nb2/3)O3 [11] . We found that the electromechanical coupling factor (kp), the piezoelectric constant (d31) and the mechanical quality factor (Qm) of the ceramics are enhanced with the increase of Zr/Ti ratio. At Zr/Ti ratio of 48/52, the ceramics has the optimal electromechanical properties, kp = 0.62, d31 = 140 pC/N, Qm = 1112. According to our results [12] showed that 0.7 wt% Li2CO3 was quite effective in lowering the sintering temperature of 0.8PZT-0.125PZN-0.075PMnN ceramics from 1150˚C down to 930˚C, with the retention of good piezoelectric properties.
In this study, we investigated the effect of Fe2O3 doping on structure and electrical properties of Fe2O3-doped 0.8Pb(Zr0.48Ti0.52)O3 – 0.125Pb(Zn1/3Nb2/3)O3 – 0.075Pb(Mn1/3Nb2/3)O3 + 0.7 wt% Li2CO3 ceramics.
2. Experimental Procedure
The general formula of the studied material was 0.8Pb(Zr0.48Ti0.52)O3 – 0.125Pb(Zn1/3Nb2/3)O3 – 0.075Pb(Mn1/3Nb2/3)O3 + 0.7 wt% Li2CO3 + x wt% Fe2O3 (PZT-PZN-PMnN), where x is 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 and are denoted by M0, M1, M2, M3, M4, M5, M6, respectively. Reagent grade oxide powders (purity ³ 99%) of PbO, ZnO, MnO2, Nb2O5, ZrO2, TiO2, Li2CO3 and Fe2O3 were used as starting materials. Firstly, the powders of (Zn,Mn,Fe)Nb2(Zr,Ti)O6 were prepared by reactions of ZnO, MnO2, Nb2O5, ZrO2, TiO2 and Fe2O3 at temperature 1100˚C for 2 h. Then (Zn,Mn,Fe)Nb2(Zr,Ti)O6 and PbO were weighed and milled for 8 h. The powders were calcined at temperature 850˚C for 2 h, producing the PZT-PZN-PMnN compound. Thereafter, predetermined amounts of Li2CO3 were mixed with the calcined PZT-PZN-PMnN powder, and then, powders milled for 16 h. The ground materials were pressed into disk 12 mm in diameter and 1.5 mm in thick under 100 MPa. The samples were sintered at 950˚C for 2 h.
The crystal structure of the sintered samples was examined by X-ray diffraction (XRD, D8 ADVANCE). The density of samples was measured by Archimedes method. The synthesized pellets were poled in a silicone oil bath at 120˚C by applying the DC electric field of 30 kV∙cm−1 for 20 min then cooling down to room temperature (RT). They were aged for 24 h prior to testing.
The piezoelectric properties were determined with the resonance and antiresonance frequency using an impedance analyzer (HP 4193A and RLC HIOKI 3532). Dielectric measurements (capacitance and loss factor) were measured in the temperature range of 30 to 350˚C at frequencies 1 kHz by using a programmable temperature controller and an impedance analyzer (RLC HIOKI 3532).
3. Results and Discussion
3.1. Effect of Fe2O3 Addition on the Structure and Microstructure of PZT-PZN-PMnN Ceramics
Figure 1 shows X-ray diffraction patterns (XRD) of the PZT-PZN-PMnN ceramics at the different contents of Fe2O3. All samples have pure perovskite phase with tetragonal structure. When increasing of Fe2O3 content, the tetragonality c/a ratio increases as shown in Figure 2.
Figure 3 shows the SEM micrographs of the fracture surface of the samples as Fe2O3 addition. It is seen from the micrographs that the grain size grows with the increase of Fe2O3 addition (Figure 5). Below the Fe2O3 solubility of 0.25 wt%, the grain sizes increase and the grain boundaries present regular shapes. However, when the addition of Fe2O3 is higher than 0.25 wt%, a few cavities appeared between the grains.
Figure 4 shows the variation of density of PZT-PZN-PMnN samples at the different Fe2O3 contents. It can be seen that the density of the PZT-PZN-PMnN samples as a function of the Fe2O3 content. With Fe2O3 content increased, the density of PZT-PZN-PMnN samples increased. It achieves a maximum value (ρ = 7.86 g/cm3) at Fe2O3 content of 0.25 wt%, and then decreases. The variation in density of the ceramic samples is in good accordance with the microstructure. When the addition is less than 0.25 wt%, Fe3+ ions can incorporated into the perovskite and cause the increase of the density; however, the excess addition causes the aggregation at the grain
Figure 1. The XRD patterns of PZT-PZN-PMnN ceramics with different contents of Fe2O3.
Figure 2. The lattice constant and the tetragonality c/a ratio of PZT-PZN-PMnN ceramics with different contents of Fe2O3.
Figure 3. Microstructures of samples with the different Fe2O3 contents.
boundaries, thus leading to the decrease of the density of the samples [7] [8] . The variation in density may affirm the solubility limit of Fe2O3 in PZT-PZN-PMnN lattice.
The grain size, the density of ceramic have a strong effect on dielectric, piezoelectric and ferroelectric properties of ceramic materials. The relationships between the grain size and the density of ceramic and electrical properties are discussed in the next section.
Figure 4. Density of the PZT-PZN-PMnN ceramics with different amounts of Fe2O3.
3.2. Effect of Fe2O3 Content on the Electrical Properties of PZT-PZN-PMnN Ceramics
Figure 5 shows the room temperature dielectric constant er at 1kHz of PZT-PZN-PMnN ceramics as function of the Fe2O3 contents. The er increases with the Fe2O3 content increases and reaches highest value (1400) at x = 0.25. With contents x > 0.25, the dielectric constant er decreased. This is related to grain size of ceramics. When the addition of Fe2O3 is lower than the solution limit, the drastic increase in grain size appears, which compensates for the effect of oxygen vacancy on pinning the motion of domains, thus leading to the increase of er [2] [6] -[8] . When the addition is above 0.25 wt%, the occurrence of the cavities between the grains becomes obvious, which causes the decrease of the er [7] [8] .
Figure 6 shows the forms of feroelectric hysteresis loops of the samples measured at room temperature. From the changes of the shape of these loops with Fe2O3 contents, the remanent polarization Pr and the coercive field Ec were determined, as shown in Figure 7. A sharp increase in Pr was observed for M0-M4 samples, reaches the highest value (37 mC/cm2) with M4 sample, and then decreases. This result is in good agreement with the studied dielectric and piezoelectric properties of the samples. While, the coercive field Ec increases with increasing of Fe2O3 content. This results (increased Ec) clearly indicate the “hard” characteristics with addition of Fe2O3, mainly caused by Fe ions substitution in B site leads to the creation of oxygen vacancies, which pin the movement of the ferroelectric domain walls [2] [7] [8] .
To determine piezoelectric properties of ceramics, resonant vibration spectra of samples were measured at room temperature. From these resonant spectra, piezoelectric parameters of samples were determined. Figure 8 shows the electromechanical coupling factor (kp, kt), the piezoelectric constant (d31), the mechanical quality factor Qm and dielectric loss tanδ change as a function of the amount of Fe2O3. The mechanical quality factor (Qm) and the dielectric loss (tanδ) of the Fe2O3-doped PZT-PZN-PMnN ceramics markedly improved, as shown in Figure 8. As the Fe2O3 content in the PZT-PZN-PMnN ceramics was increased up to 0.25 wt%, the Qm value increased steadily up to 1450 while dielectric loss tanδ decreased steadily down to the lowest value (0.25 %) because the Fe ions at the (Ti, Zr, Nb) sites in the lattice acted as acceptors. More specifically, the substitution of Fe ions in the B-sites of the perovskite structure increases the number of oxygen vacancies. As mentioned above, these oxygen vacancies induce a space charge and internal field inside the PZT-PZN-PMnN grains, which inhibits the motion of the domain walls, thereby increasing the Qm value and decreasing the tanδ value [2] [7] [8] [10] . However, when the Fe2O3 content exceeded a certain threshold value (>0.25 wt%), the Qm value decreased and the tanδ value increased, apparently due to the solubility limit of Fe. When the amount of Fe2O3 added to PZT-PZN-PMnN ceramics exceeded the solubility limit, the excessive Fe ions could not incorporate into the lattice of perovskite structure, which accumulated at the grain boundaries and reduced the physical properties of ceramic materials [2] [7] [8] . As can be seen in Figure 8, the kp, kt and the d31 show a similar variation with increasing Fe2O3 content. When the content of Fe2O3 is lower than 0.25 wt%, the kp, kt and the d31 are increased with increasing Fe2O3 content. The optimized values for kp of 0.64, kt of 0.51 and d31 of 155 pC/N were obtained at x = 0.25. This is probably related to characteristics of the increasing grain size. As is well known, the increased grain size makes domain reorientation easier and severely promotes domain wall motion, which could increase the piezoelectric properties [2] [7] [8] [11] [12] .
Figure 5. Room-temperature dielectric constant er and average grain size of ceramics with different amounts of Fe2O3.
Figure 6. Hysteresis loops of Fe2O3-doped PZTPZN-PMnN ceramic samples.
Figure 7. The Pr and the Ec as a function of Fe2O3 contents.
4. Conclusions
We have investigated the effect of Fe2O3 addition on structure and electrical properties of 0.8Pb(Zr0.48Ti0.52) – 0.125Pb(Zn1/3Nb2/3) – 0.075Pb(Mn1/3Nb2/3)O3 ceramics. The results of this study are summarized as follow:
All samples have pure perovskite phase with tetragonal structure; the c/a ratio increases with increasing Fe2O3 contents.
The dielectric, piezoelectric and feroelectric properties of ceramics were markedly improved by Fe2O3 addition. At the Fe2O3 content of 0.25% wt, electrical properties of ceramics are best: the density of 7.86 g/cm3, the
Figure 8. The kp, kt, d31, Qm, and tanδ as a function of Fe2O3 contents.
electromechanical coupling factor (kp) of 0.64 and (kt) of 0.51, the dielectric constant, εr of 1400, the dielectric loss (tand) of 0.003, the mechanical quality factor (Qm) of 1450, the piezoelectric constant (d31) of 155 pC/N, and the remanent polarization (Pr) of 37 mC/cm2, which makes it as a promising material for high power piezoelectric devices.
NOTES
*Corresponding author.