Pb(Mg1/3Nb2/3)O-3 and (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) relaxor ferroelectric thick films: Processing and electrical characterization

The lead magnesium niobate [Pb(Mg1/3Nb2/3)O-3 or PMN], and its solid solutions with lead titanate (PbTiO3 or PT), are of great interest because of their high electromechanical properties. At large PMN content, these materials exhibit relaxor characteristics with large electrostrictive strains and a large permittivity, while compositions near the morphotropic phase boundary present very interesting piezoelectric properties. So far, properties of these materials in ceramic, thin film and single-crystal form have been investigated. In this paper, we report on preparation and properties of pyrochlore free PMN and 0.65PMN-0.35PT thick films ( thickness = 10 to 20 mum). The films were prepared from ethyl cellulose ink by screen printing on alumina substrate. The influence of various parameters, such as powder characteristics, inks formulation and films sintering conditions, on films densification are discussed. The dielectric and electromechanical properties of the films were examined. Relaxor-like behaviour was clearly demonstrated in PMN films. The maximum relative permittivity for PMN film was 10000 (at 0.1 kHz), which is lower than in bulk ceramics ( 17800 at 0.1 kHz) prepared under the same conditions. For 0.65PMN-0.35PT, the maximum relative permittivity was around 15500 against 24000 in the bulk. Several parameters, which might be responsible for the lower permittivity, are discussed. Poled 0.65PMN-0.35PT thick films exhibit relatively large piezoelectric response (d(33) up to 200 pm/V) and unipolar strains approaching 0.1%, making these films of interest for various actuator and transducer applications.

Properties and phase transitions in lead free piezoelectrics

Naama Klein

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.

EPFL2009

Dielectric and electromechanical properties of ferroelectric-relaxor 0.9 Pb(Mg1/3Nb2/3)O-3-0.1PbTiO(3) thin films

Dragan Damjanovic, Zian Kighelman, Nava Setter

Pyrochlore free 0.9Pb(Mg1/3Nb2/3)O-3-0.1PbTiO(3) thin films were prepared from alkoxide-based solution precursors. Preferential (111) crystallographic orientation was obtained on TiO2/Pt/TiO2/SiO2/Si substrates by spin coating. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in detail. Films show relaxor-like behavior, but with dielectric permittivity which is low (around 4300 at peak and at 340 Hz with E-ac=1.6x10(6) V/m) compared to bulk ceramics and single crystals. Several parameters which might be responsible for this lower permittivity are suggested. The ac and dc field dependences of the dielectric response have been investigated. Electrostrictive coefficients, M-11(7.76x10(-18) m(2)/V-2) and Q(11)(1.9x10(-2) m(4) C-2), were determined by measuring strain and polarization as a function of the electric field (E-ac). The maximum field induced piezoelectric d(33) coefficient is 100 pm/V and electrostrictive strains up to 1.2x10(-3) (with an ac electric field of 140 kV/cm) were measured. (C) 2001 American Institute of Physics.

2001

Electromechanical properties and self-polarization in relaxor Pb(Mg1/3Nb2/3)O-3 thin films

Dragan Damjanovic, Zian Kighelman, Nava Setter

Pyrochlore free Pb(Mg1/3Nb2/3)O-3 (PMN) thin films were prepared from alkoxide-based solution precursors. The influence of different seeding layers and chemical solution on the microstructures is shown. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in detail. Films show relaxor-like behavior, but with dielectric permittivity which is low (around 4000 at peak) compared to bulk ceramics and single crystals. Several parameters which might be responsible for this lower permittivity are suggested. Electrostrictive coefficients, M and Q, were determined by measuring strain S and polarization P as a function of the electric field (E-ac). At large fields (>2.6x10(6) V/m), S vs P-2 appears to deviate from linear behavior possibly suggesting that the electrostrictive coefficient Q becomes nonlinear in this field range. Investigated as-prepared PMN films exhibit piezoelectric response in the absence of a dc electric field (d(33)=8-20 pm/V). The value of the associated self-polarization in the films is estimated and its presence confirmed by zero-field pyroelectric measurements. The self-polarization and the piezoelectric coefficient are strong functions of the ac field amplitude. Asymmetry of S vs E-ac and d(33) vs E-dc loops are related to the self-polarization. An ac field induces shift in P vs E loops along the field axis. This increase in the coercive field is associated, through a simple model, with the presence of the self-polarization. (C) 2001 American Institute of Physics.

2001

Properties and phase transitions in lead free piezoelectrics

Naama Klein

EPFL2009