Chapter 4 Lead zirconate titanate ceramics from different raw materials and with lead
4.2 Comparison of PZT ceramics prepared from industrial‐used and highly pure raw
Chapter 4 Lead zirconate titanate ceramics from different raw materials and with lead nonstoichiometry
4.1 Introduction
As has been mentioned, the final properties of PZT ceramics are affected by raw materials and processing factors (Hiremath, Kingon et al. 1983). In the Chapter 2, effects of mixing methods on these highly pure raw materials have been described. The preparation process of PZT ceramics has been optimized. In this section, comparison of PZT prepared from two sources of raw materials (industrial raw materials and highly pure raw materials) is made in terms of sintering behavior, microstructure and electrical properties to have an overall impression of the total effect of impurities.
4.2 Comparison of PZT ceramics prepared from industrial‐used and highly pure raw materials
4.2.1 Differential Thermal Analysis
Figure 4.1 presents DTA curves of PbO‐ZrO2‐TiO2 synthesized from different raw materials. These curves show the expected transitions of PbO and the formation of PbTiO3 at approximately 600 °C (Chandratreya, Fulrath et al. 1981). The transition temperature of PbO to form PbTiO3 varied in the two mixtures. The HM mixture showed a transition temperature of 625°C, while that temperature of the IM mixture shifted to a lower value, 587°C. It is known that polymorphic forms exist in TiO2 such as rutile phase and anatase phase. From Table 2.1, the modification of HM TiO2 was rutile, different from the modification of IM TiO2, anatase. The anatase phase transforms to rutile at above 900°C and is considered to be metastable (Chen, Cheng et al. 1990), which could explain the shift in the formation temperature of PbTiO3. A detailed discussion will be presented in the following chapter.
4.2.2 X‐ray Diffraction Analysis
XRD diffraction patterns of the powder mixtures calcined at 850°C are shown in Figure 4.2. The XRD results indicate that a single perovskite phase existed in both PZT samples prepared from different raw materials. In addition, the splitting of the (200) diffraction peak is observed. Usually
34 4.2 Comparison of PZT ceramics prepared from industrial-used and highly pure raw materials
the (200) line is used for phase identification. It splits into two in the tetragonal structure and no splitting occurs in the case of rhombohedral structure. Three peaks suggest the coexistence of rhombohedral and tetragonal phases (Chen, Long et al. 2003). Thus, the diffraction peak at (200) together with the other two split peaks confirm the compositions of these samples were within the morphtropic phase boundary (Chen, Long et al. 2003). Since the composition of Pb(Zr0.53Ti0.47)O3 was designated to be within the MPB, the desired phases were achieved after the calcination.
400 500 600 700 800 900
-0.03
(002)T (200)T(200)R(002)T (200)T
HM
4.2.3 In situ sintering behavior
The green samples with a composition of Pb(Zr0.53Ti0.47)O3 were prepared from calcined HM and IM powder and sintered in TOMMI device at 1280°C for 1h with a heating rate of 10 K/min to study the sintering behavior. Their shrinkage curves are illustrated as a function of temperature in Figure 4.3a. Shrinkage of a sample made from IM materials with1.5 wt% additional PbO is also plotted for comparison. The onset temperatures of shrinkage increase from 745 °C to 865 °C and to 960 °C for sample IM with excess PbO, sample IM and HM, respectively. In addition, the temperature that HM sample requires to be fully densified is approximately 1280°C, which is much higher than that of IM sample (1100°C) and IM sample with 1.5wt% additional PbO (1000°C). The large difference in sintering activity between the samples can be seen from the densification rates as well (Figure 4.3b), which is calculated from time derivative of the shrinkage data. The densification rate of sample HM is much lower than that of sample IM and reaches its maximum at 1190 °C whereas sample IM shows the highest densification rate at 950 °C. The differences in the sintering behavior between the HM and IM samples are attributed the differences in raw materials. Since the particle sizes of those two samples are similar (refer to Table 2.5), the reason for the distinctive sintering behavior of the two samples is explained by the impurity level. As listed in Table 2.1, all the impurity concentrations in IM raw materials are higher than that in HM raw materials.
Chapter 4 Raw materials and lead nonstoichiometry 35
800 1000 1200
0.84 IM+1.5mol% PbO a)
800 1000 1200
0.000
IM+1.5mol% PbO
-[d(L/L0)/dt]/(L/L0)
Temperature [°C]
b)
Figure 4.3 Scaled width L/L0 (a) and densification rate dL/dt (b) of PZT samples made from different raw materials at a heating rate of 10 K/min (HM‐highly pure materials, IM‐ industrially
Figure 4.4 SEM images of pure PZT samples quenched at a linear shrinkage of 10% made from industrially used raw materials (a) and highly pure raw materials (b). (Arrows indicate secondary phase)
It is well known that a small amount of liquid phase can significantly stimulate rapid densification and reduce the sintering temperatures (Marion, Hsueh et al. 1987). Liquid phase sintering in PZT containing excess PbO has already been reported and its effect is attributed to the low melting point of lead oxide (Snow 1974; Fan and Kim 2001). Thus, the similar sintering behavior of IM samples with and without PbO excess indicates that liquid phase has been formed even in stoichiometric IM sample. Figure 4.4 shows the microstructure of samples IM and HM which were quenched after a linear shrinkage of 10% during the sintering with a heating rate of 10 K/min.
Secondary phases were observed at grain boundaries and triple junctions as pointed out by arrows in Figure 4.4a. Unlike the IM sample, the HM sample showed no secondary phase after quenching (Figure 4.4b). Although – due to its small size– it was difficult to measure the composition using EDX, a brighter color of the secondary phase than the grains was observed using the QBSD detector of SEM. It indicated a PbO‐rich phase, which has already been found in PZT samples with PbO excess or additives purposely introduced by other researchers (Snow 1974). Therefore, it is assumed that the impurities in IM mixture formed a melt phase with the PbO at low temperature.
36 4.2 Comparison of PZT ceramics prepared from industrial-used and highly pure raw materials
The effect of secondary phase on sintering of PZT is emphasized by Figure 4.3b, as the densification rate of sample IM reaches a maximum which is about two times higher than that of sample HM.
4.2.4 Sintering behavior with uniaxial load
The comparison of sintering with and without load of PZT samples from different raw materials is shown in Figure 4.5. With the load the strain in the axial direction, i.e. the height change of the cylindrical samples, is increased while the strain in the radial direction is significantly decreased compared to free sintering. During cooling the strain rate is nearly zero. Samples sintered without load showed exactly the same shrinkage in axial and radial direction. Note that the used external stress of 0.4 MPa is smaller than the sintering stress which is usually between 1 and 10 MPa for the used powders. Previous experiments on many ceramic systems have shown that the anisotropy which is caused by small uniaxial stresses during loading dilatometry depends on the sintering mechanism: very small plastic deformation is observed for solid state sintering whereas a drastic increase by more than an order of magnitude was found for all liquid phase sintered systems (Scherer 1986). The anisotropy observed in the loading experiments indicates that sintering of both types of samples, HM and IM, was dominated by a liquid phase mechanism. Although a liquid phase sintering mechanism is expected for sample IM (referring to the previous section) it is surprising for sample HM exhibiting such anisotropic sintering behavior.
750 900 1050 1200
0.6
Scaled strain L/L0
Width with load Width/Height without load Height with load a)
900 1050 1200 1350
0.7 Width/Height without load Height with load
Scaled strain L/L0
Temperature [°C]
b)
Figure 4.5 Axial and radial strain of PZT samples with or without uniaxial load of 0.4 MPa at a heating rate of 10 K/min made from industrially used raw materials (a) and highly pure raw materials (b)
4.2.5 Dielectric and piezoelectric properties
The dielectric and piezoelectric properties of HM and IM samples are given in Table 4.1. Within the experimental error, the mechanical quality factor shows no significant variation between these two samples. IM PZT exhibits a lower coupling factor, dielectric loss factor as well as lower
Chapter 4 Raw materials and lead nonstoichiometry 37
piezoelectric coefficient d33 than HM sample. However, the dielectric constant of IM sample is much higher than that of HM PZT. Previous investigations have demonstrated that the dielectric and piezoelectric characteristics can be easily tailored by doping (Hammer, Monty et al. 1998). The smaller coupling factor and piezoelectric coefficient imply a “soft” characteristic of IM sample comparing to HM sample, whereas the higher dielectric constant suggests the “hard” side of IM sample as well. The combination of “soft” and “hard” characteristics of IM sample could be attributed to the different impurities in the raw materials. In order to understand the mixed behavior, further investigations on the impurities will be presented in the next chapter. Pure IM 16.5±0.4 51.3±0.2 17.4±1.6 96±10 270±0.2 863±11
Pure HM 13.4±0.4 53.3±0.9 20.6±0.8 84±15 322±5 754±22
4.2.6 Ferroelectric properties
-3 -2 -1 0 1 2 3
Electrical field [kV/mm]
Figure 4.6 Comparison of hysteresis loops obtained from HM PZT and IM PZT
Differences on the hysteresis loops were also observed. Figure 4.6 plots the polarization hysteresis (P‐E) loops of both samples. The shape of the P–E loops differs significantly with the ceramic compositions. A well saturated loop is observed with the highly pure sample, while a less “square”