PZTs vs. Lead Metaniobates
There are significant differences between Lead Zirconate Titanates (PZTs) and Lead Metaniobates. These differences are not simply compositional, and they affect both the properties and how the materials should be used. The following discussion will attempt to shed light on what is known and what is speculated about the differences.
The most typical piezoceramics are based on a family of materials known as PZTs. These are modifications of Lead Zirconate Titanate, a material with the crystal structure known as Perovskite. The basic formula is ABO3. This crystal structure is very stable (even on geologic time scales) and can accommodate substitutions by virtually every element in the periodic table. PZTs are based on Pb(Zr0.52Ti0.48)O3 where Lead (Pb) is on the ‘A’ site and a combination of Zirconium (Zr) and Titanium (Ti) occupy the ‘B’ site. This formulation can be adjusted with dopants on either the ‘A’ or ‘B’ sites or by the balance between Zirconia and Titania to modify properties. These ceramic materials behave nicely in the sense they can be reacted, formed, and fired into desired configurations easily and controllably. They lend themselves to mass production and statistical process control. The resulting family of materials have been extensively studied and exploited for decades.
Lead Niobates, on the other hand, are not so well-behaved. Lead Niobates are a family of materials with the basic formula (AB2O6) and are based on PbNb2O6. The stable crystal form for this family is not Perovskite but a structure known as Francombite, which is not even piezoelectric. To force it into a state which is piezoelectric requires a specific (and unusual) series of dopants and processing steps. These result in a material which is not normally seen in nature, i.e. it's ‘metastable’. It does not make good ceramic in the sense that it is not fully dense nor does it have a well-controlled grain structure like PZT.
Complex ceramics are all made in much the same fashion. Constituent chemicals (usually oxides) are mixed in the proper proportions, chemically reacted, ground to fine particles, mixed with a binder, formed into the desired shape, and fired to achieve the final finished part. The chemical reaction is known as calcining. This step creates both the final chemical composition and the structural crystal phase. For Lead Mets, the calcining step is used to lock into the metastable phase rather than a nice stable configuration like Francombite. Thus, control of the calcine operation is much more critical than for PZTs.
Lead Mets can be poled by a variety of methods just like PZTs but are typically poled hotter and at higher voltages. Historically Lead Mets have been left with residual poling oil in the porosity because it is extremely difficult to completely remove. This residual oil dramatically affects properties including dielectric constant, dissipation, frequency constant, and Piezo activity. Because the level of oil has such a large effect and the removal of a viscous fluid from microscopic pores is, at best, problematic, the final properties of Lead Mets have tended to vary much more than PZTs. Because of this, it is more difficult to control variability with Lead Mets than with PZTs.
In relation to PZTs, Lead Metaniobates have high Curie points, low Piezo activities, and very low Q's. It turns out this is a nice combination of properties for certain applications such as NDT transducers. It is a terrible combination of properties for high drive or high sensitivity applications. For applications which require rapid ring-down or broad bandwidth, low mechanical Qs are important. In the PZT family the lowest Qs correspond to the lowest Curie points and highest dielectric constants. Lead Mets allow very low Qs (even lower than traditional soft PZTs) with higher Curie points and lower dielectric constants.
While Lead Mets are capable of handling fairly high outputs they should not be used where high power or high efficiency is needed. They are not suitable for radial mode parts due to the large anisotropy in coupling. They tend to higher dielectric losses than PZTs due to the high level of residual oil. The acoustic impedance of Lead Mets is significantly lower than PZTs.