Chapter 5 Systematic study of the impurity effect on lead zirconate titanate ceramics
5.2 Impurities originating from customary raw materials
5.2.2 Sintering behavior of highly pure PZT doped with impurities
Table 5.1 Particle size (d50) of the individual powders after ball milling
PbO ZrO2 TiO2
IM HM IM HM IM HM
d50 (μm) 6.52 5.96 0.99 1.14 0.96 1.52
Phase Massicot Massicot Monoclinic Monoclinic Anatase Rutile
Differences in the TG curve are also observed. A distinct weight loss is found right before the formation of the PbTiO3 phase in the HM sample as well as in the mixture where HM ZrO2 was replaced by IM ZrO2. However, such weight loss is not evidenced in the curve of the mixture in which HM TiO2 was replaced by IM powder. According to Härdtl’s and Rau’s report, PbTiO3 has a PbO vapor pressure approximately two order of magnitude lower than pure PbO (Härdtl and Rau 1969). Therefore, it is assumed that formation of PbTiO3 at lower temperature reduced the PbO partial pressure and consequently the evaporation of PbO during calcination. Since even small changes in PbO concentration have a large impact on sintering activity as discussed in the previous chapter, the relative higher concentration of lead oxide content in the sample with TiO2 replaced by IM component is considered to account for its improved densification behavior (refer to Figure 5.1).
5.2.2 Sintering behavior of highly pure PZT doped with impurities
To study the effects of the impurities, they were added into the highly pure materials with twice the concentration identified in IM materials. Their concentrations are as follows: Al‐0.076 mol%;
Ba‐0.0094 mol%; Ca‐0.08 mol%; Fe‐0.029 mol%; Hf‐0.73 mol%; Na‐0.21 mol%; Si‐0.18 mol%; Y‐0.13 mol%. Table 5.2 gives the ionic radii of the various constituents in doped PZT ceramics. According to ionic radii and valencies, the impurities were roughly grouped into two categories to reduce the number of experiments. One of low valency and large ion radius (Na, Ba, Ca) was considered to replace Pb ‐ if incorporated into the PZT lattice. The other group (Fe, Al, Y, Hf, Si) was formally assumed to substitute Zr/Ti in the PZT lattice. This second group was further divided into two groups of middle (Fe, Al, Y) and high valency (Hf, Si) impurities respectively. When the impurities were added, the concentration of PbO, TiO2 and ZrO2 was adapted to the stoichiometric concentration, according to the formal attribution of the impurities to their respective sites in the PZT lattice.
Chapter 5 Systematic study of impurity effect 53
Table 5.2 Ionic radii of the various constituents in the doped PZT ceramics
Ionic Species Ionic radius /Å Ionic Species Ionic radius/Å
Pb2+ 1.43 Zr4+ 0.86
Na+ 1.32 Ti4+ 0.75
Ba2+ 1.56 Si4+ 0.54
Ca2+ 1.26 Hf+ 0.85
Y3+ 1.04 Fe3+ 0.72
Al3+ 0.68
All impurities Hf, Si Na, Ba, Ca Fe, Al, Y HM 800
900 1000 1100 1200
TOnset T50
Temperature [°C]
Figure 5.3 Temperatures for the onset of shrinkage Tonset and 50 % of total shrinkage (T50) of PZT samples doped with different groups of impurities
Figure 5.3 illustrates the sintering temperatures of PZT doped with grouped impurities. The sample with all the impurities exhibits a similar Tonset but a lower T50 temperature than the HM sample. Doping the HM sample with the Na/Ba/Ca group leads to a significant decrease in T50. In this group, it is believed that Na incorporated at a Pb site in the PZT lattice and increased the concentration of oxygen vacancies (Lee and Lee 2006). Because oxygen vacancies were the rate‐
limiting species in the system, the increased concentration further enhanced both the diffusion flux and the densification rate (Atkin and Fulrath 1971). Ba and Ca have the same valency as Pb. It is known that isovalent substitution usually has little influence on PZT ceramic during the period of sintering (Huang, Chen et al. 2004). Thus, Na is assumed to mainly account for the effect of doping Na/Ba/Ca. On the other hand, doping with Fe/Al/Y shows nearly no improvement of densification kinetics in this study, although Fe and Al are usually reported to substitute Zr/Ti and be compensated by oxygen vacancies similar to Na as discussed above (Weston, Webster et al. 1969;
Atkin and Fulrath 1971). It is however disputed which site (Pb or Zr/Ti) Y would substitute (Yoon,
54 5.2 Impurities originating from customary raw materials
Kim et al. 1993; Beitollahi and Abedini 2006). If Y together with Fe and Al substituted Zr/Ti, oxygen vacancy concentration and hence the densification rate should have been increased. If Y had substituted Pb the positive charge could have been compensated by substitution of a neighboring Zr/Ti atom by Fe or Al. Therefore, only small net effect of co‐doping Y, Fe and Al could result from charge compensation. Atkin demonstrated a similar pairwise substitution with co‐doping of Al and Nb in PZT (Atkin and Fulrath 1971). Doping with Al or Nb kept the grain size small and expedited densification, whereas codoping did not lead to improvements on sintering.
The prevailing effect of impurities on the sintering temperatures results from doping with Hf and Si, which almost takes up the total effect of adding all impurities at one time. On one hand, it is assumed that Hf simply substituted Zr atoms in the PZT lattice and did not change the sintering kinetics, as it is chemically similar to Zr. On the other hand, SiO2 could form liquid phase with PbO during sintering as a eutectic exists in their binary phase diagram (Smart and Glasser 1974). Figure 5.4 displays the SEM image of the fracture surface of the sample doped with Hf and Si. An intergranular fracture mode was observed and a secondary phase was identified along the grain boundaries. EDS measurement showed that this secondary phase had a high concentration of PbO and contained Si, which indicates that PbO and Si may have formed a melt phase during sintering.
Si is, therefore, mainly ascribed to the decrease of sintering temperature when adding both impurities. The assumption is confirmed by an additional experiment where only Si was introduced (as will be discussed in the following section). Palkar et al reported the use of Si as sintering aid to improve the sintering behavior of PbTiO3 (Palkar, Purandare et al. 2000). It was observed that it was possible to obtain a relative density of 95% at 1000°C for1 h using 2 mol% of Si.
Figure 5.4 SEM picture of fracture surface of sample doped with Hf and Si (one grain is partly covered with secondary phase)
Chapter 5 Systematic study of impurity effect 55
In the experiments described above, the impurities were doped in the PZT system with a stoichiometric composition. However, the composition is nonstoichiometric in the industrial production chain, because usually the impurities are not taken into account when weighing the raw materials. If they are incorporated in the PZT lattice, deviations from stoichiometric composition are expected. It is seen from Table 2.2 that HfO2 dominated among the impurities in the IM raw materials. Since it is believed that Hf replaces Zr/Ti and it has nearly double molecular weight than ZrO2, about 0.7 mol % PbO excess could result from not considering this contribution.
In the previous chapter, it was already shown that small concentrations of excess PbO could drastically decrease sintering temperature. As plotted in Figure 5.5, when the impurities were doped without considering stoichiometry, i.e. similar to the industrial process, densification was significantly enhanced with a reduction of T50 by more than 50 K compared to the sample with stoichiometric composition. This phenomenon verifies that the PbO concentration is one of the most important parameters, controlling sintering kinetics of PZT.
All impurities (non)All impurities (s) Pure HM 800
900 1000 1100 1200
Temperature [°C]
TOnset
T50
Figure 5.5 Temperatures for the onset of shrinkage Tonset and 50 % of total shrinkage (T50) of pure (HM) PZT samples without impurities and stoichiometric (s) and nonstoichiometric (non) samples doped with all impurities