Lead zirconate titanate ceramics are the “gold standard” materials for transducers, actuators and other applications of ferroelectric ceramic materials. Unfortunately, lead is toxic, and there has been an effort to find a lead-free alternative with strong piezoelectric properties, driven at least in part by the European RoHS regulations. Finding suitable substitute materials has proven challenging, enough so that the EU has issued a RoHS exemption, which allows PZT to continue to be used.
It is a reprieve, but there is a growing body of work to find lead-free ferroelectric materials. There are two systems under investigation, the potassium sodium niobate system (called KNN) and the bismuth sodium titanate-barium titanate based system (BNT–BT). One of the challenges facing researchers is to understand the mechanisms by which the piezoelectric property develops in lead-free ferroelectrics.
The strongest piezoelectric response happens in PZT at the so-called morphotropic phase boundary, which occurs in the solid solution at about 50 mol%. The structure is tetragonal on the titanium-rich side of the boundary and rhombohedral on the zirconium-rich side. The difference between the two structures is very small, but it is the basis for creating a strong piezoelectric response at the MPB.
A paper published last week in Physical Review Letters reports on a major breakthrough in understanding the mechanisms by which the piezoelectric property is induced in lead-free BNT–BT piezoelectric ceramics.
The Iowa State University team led by associate professor, Xiaoli Tan, investigated the behavior of lead-free piezoelectric compositions in the (Bi½Na½)TiO3-BaTiO3 solid solution, and Tan explained in an interview that the structural instability of this system makes the material highly responsive to external stimuli, like an applied electric field.
Piezoelectric properties are induced in sintered ceramic by applying an electric field in what is known as the poling process. Polycrystalline ferroelectric materials are comprised of domains, all of which are individually piezoelectric, but because they are randomly oriented, the net effect cancels out. By poling, the domains are aligned and the material develops a bulk piezoelectric property. In PZT, applying an electric field induces ferroelectric domain alignment, but the MPB does not change in response.
Tan’s group used transmission electron microscopy to study the responses in-situ of three solid solution compositions-5.5, 6 and 7 mol% BaTiO3-to an applied electric field. Using this unique in-situ, they tracked the changes in crystal structure and domain morphology in real time and discovered that MPBs (and the consequent piezoelectricity) are not stable in the presence of an electric field-they can be created, destroyed or replaced.
According to the paper, “The creation/destruction of MPBs during poling strongly correlates with the piezoelectric property measured from bulk samples.”
In the lead-free system the group studied, applying an electric field induces a transformation to two new phases simultaneously—a rhombohedral phase and a tetragonal phase. They found that the phases can coexist, transform or disappear, as shown in the phase diagram above, and that the MPB can form out of a single phase during poling. The paper explains, “[The phase diagram] unambiguously demonstrates the importance of poling-induced phase transitions in interpreting the microscopic origin of macroscopic piezoelectric behaviors.”
This work shows that the means exist to exert more control over the development of the piezoelectric properties in a composition range. “Previously, with PZT, the MPB had to be there,” Tan said. “We discovered that by starting with a lead-free composition, we can manipulate and adjust the poling field to control the piezoelectric properties.” As Tan says, “It gives us extra knobs to turn.”
Full details are in the paper (which includes supplementary material), “Creation and Destruction of Morphotropic Phase Boundaries through Electrical Poling: A Case Study of Lead-Free (Bi½Na½)TiO3-BaTiO3 Piezoelectrics,” C. Ma, H. Guo, S. Beckman and X. Tan, Physical Review Letters, 2012, Vol. 109 (doi: 10.1103/PhysRevLett.109.107602).