Properties and phase transitions in lead free piezoelectrics

Piezoelectric materials are used in a wide range of applications, with lead zirconate titanate (PZT) ceramics being the dominant family of materials, due to high piezoelectric coefficients, dielectric permittivity and coupling factors. The high properties have been found in compositions in the proximity of a morphotropic phase boundary (MPB). The MPB exhibits a weak temperature dependence so that the stability of the properties is attained over a large temperature range. However, environmental problems rising from the toxicity of lead have driven scientists around the world to search for lead free piezoelectric materials. These new materials should exhibit properties comparable to those of PZT and the efforts are at the present concentrated on solid solutions with an MPB. In this thesis an emphasis was made on constructing the phase diagrams and detecting an MPB in two of the most promising families of lead-free solid solutions: bismuth sodium titanate based and potassium sodium niobate based ceramics and single crystals. By investigating the structure of the phases, using X-ray and neutron diffractions and Raman spectroscopy, and measuring the dielectric and piezoelectric properties at temperatures from -266 °C to 550 °C, the phase transitions were assigned and phase diagrams constructed. Both systems (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 (0 ≤ x ≤ 0.09) (BNBT) and (1-x)(K0.5Na0.5)NbO3-xLiNbO3 (0.02 ≤ x ≤ 0.1) (KNLN), reveal similar MPBs to the one found for PZT. However, in PZT the MPB is pseudovertical up to about 350 °C whereas the MPB in BNBT is vertical only till 75 °C and in KNLN is till -50 °C. This difference in the temperature stability range is crucial in most applications. Other interesting findings in the BNBT phase diagram correspond to the phase sequence at the temperature range from 200 to 300 °C, where ferroelectric phase transforms into a mixed phase (antiferroelectric or paraelectric) with polar regions. As temperature rises, the polar regions gradually disappear. Whereas dielectric and piezoelectric properties of BNBT could compete with those of PZT in some compositions, and fairly simple processing of the ceramics is used, the fabrication of KNLN ceramics has shown to be complex and reproducible properties difficult to obtain. Even after the critical processing conditions were identified in this thesis, and reproducibility was achieved, properties were significantly lower than for PZT. Exceptional values of the thickness coupling coefficients were measured on potassium niobate (KN) and KNLN single crystals rotated away from the Z axis by θ = 45° about the orthorhombic Y axis, [001]C-cut. Monodomain and domain-engineered single crystals cut at this angle showed very high coefficients (kt ≤ 70%), which are extremely stable over a large temperature range and after thinning to thickness below 60 µm. Together with low permittivity, these properties make KN single crystals a good candidate for high-frequency applications. The higher permittivity of KNLN makes it an attractive crystal for applications at lower frequencies. Another possible application of lead free piezoelectrics could be in biological systems. For this reason, preliminary in-vitro biocompatibility study of KNLN and PZT ceramics was conducted, showing similar microscopic attachment of cells to the two ceramics and slightly lower viability values when compared to non coated tissue culture plates. Laboratory attempts to introduce KNLN ceramics as an implant in the mineral of the bone, hydroxyapatite (HA), ended with a strong chemical reaction between the two. However, some piezoelectric response could be measured from the composite of HA and KNLN.