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.

Broadband Dielectric Response in Hard and Soft PZT

Li Jin

Lead zirconate titanate (Pb(Zr1-xTix)O3 or PZT) ferroelectric ceramics have been widely used in transducers, actuators, and sensors, since they posses high dielectric and piezoelectric properties with a relatively high temperature of operation. Commercially used PZT ceramics are always modified by different dopants and are divided into the "soft" (donor doped) and "hard" (acceptor doped) groups. Compared with the undoped composition, hard PZT often shows lower but more stable properties after ageing. In contrast, soft PZT shows higher properties and insensitivity to ageing. The difference in properties between the soft and hard PZT ceramics is rather large, even though the doping level is limited to a very low value (on the order of 1 mole %). The large difference of the physical properties between them mainly originates from the contributions from domain wall motion rather than properties of the crystal lattice. However, the mechanisms of hardening and softening are not well understood. In order to understand better the hardening and softening mechanisms, in this thesis the different contributions to the dielectric properties of soft and hard PZT ceramics are studied by means of a broadband dielectric spectroscopy from 10 mHz to 20 GHz. Properties at THz and infrared frequencies where only crystalline lattice contributes to the dielectric response were also investigated in collaboration with another group. In the frequency range below 20 GHz, the different contributions to the permittivity by domain wall motion were revealed in hard and soft materials. In order to correlate the properties to the microscopic structure of hard and soft PZT ceramics, the domain structures were also investigated by transmission electron microscopy. Piezoelectric spectroscopy was employed to help separating different contributions at frequencies below 100 Hz. The main results of this work are: The microwave dielectric dispersion of all PZT ceramics (including undoped, soft and hard PZT ceramics), which is characterized by a rapid decrease of the permittivity and a loss peak in the GHz frequency range, is contributed by both domain wall motion and piezoelectric grain resonances. These two mechanisms are separated by gradual poling of samples. The dispersion related to the domain wall motion appears at a higher frequency than the one related to grain resonance and constitutes the main contribution to the microwave dielectric properties of unpoled samples. Above the GHz frequency range, the dielectric properties of hard and soft PZT ceramics are rather close and approach the upper limit value of their intrinsic properties, which are identified by dielectric properties determined by THz dielectric spectrum. The contributions by domain wall motion make up more than 50% of the quasistatic dielectric properties (measured at 100 kHz) in all studied samples. Below GHz frequency range, another dielectric dispersion due to the domain wall creep, which manifests itself by a logarithmic decrease of permittivity with increasing frequency, is indentified in both soft and hard PZT ceramics in a relatively broad frequency region over at least eight decades. The difference of the dielectric properties between the hard and soft PZT mainly results from this logarithmic dispersion, since soft PZTs exhibit stronger logarithmic dispersion than hard PZTs. By controlling the doping kind and crystalline symmetry, the slope of the logarithmic function can be adjusted. Large values are observed in the system with disordered distribution of defect dipoles (soft, donor doped materials) while nearly zero slope could be observed in hard, acceptor doped materials with tetragonal structure with defect dipoles well aligned with polarization within domains. Rapid increase of both permittivity and loss below 1 Hz, which cannot be described by the logarithmic function, is observed in hard PZT ceramics. To interpret this behavior, results of dielectric spectroscopy were complemented by piezoelectric measurements in the frequency range from 10 mHz to 100 Hz. The absence of the strong dispersion in the piezoelectric properties and presence in the dielectric response strongly indicates that its origin is in charge transport (such as hopping conductivity) rather than in motion of domain walls. Nanodomains, observed in Fe3+-doped hard PZT with composition at morphotropic phase boundary do not lead to the high dielectric and piezoelectric properties, as would be expected from some theoretical models. Acceptor doping rapidly decreases the domain size in PZT ceramics with compositions in rhombohedral, tetragonal and MPB regions. This effect could not be interpreted by the decrease of the grain size according to the commonly assumed parabolic relationship between the domain size and grain size. The small domain size is rather related to the presence of oxygen vacancies which break continuity of polarization.

EPFL2011

Films minces relaxeur-ferroélectriques à base de Pb(Mg1/3NB2/3)O3

Zian Kighelman

Relaxor Pb(Mg1/3Nb2/3)O3(PMN) and its solid solutions with ferroelectric PbTiO3 (PT) are of considerable interest from both the applications and the scientific point of view. In the past, many attempts were made to prepare and study the properties of these material in thin film form. However, due to difficulties in the preparation of pure phase films with high PMN content, there exists very little knowledge on the properties of these important materials. It is the goal of this thesis to prepare PMN and PMN-PT films without second any phases, to study in detail their structure, dielectric and electromechanical properties, and to see how these properties compare with those of the bulk materials. The studies were carried out along three major directions: PMN, 0.9PMN-0.1PT and PT chemical solution precursor syntheses, thin films preparation, dielectric and electromechanical properties characterization and their interpretation. The synthesis of precursors used for the preparation of PMN, 0.9PMN-0.1PT and PT films is reported in detail. Careful control and optimization of the precursors was important to obtain thin films without second any phases. The presence of new compounds and reaction mechanisms were established in the course of developing reproducible and stable precursors solutions. Crystallization of pure PMN and 09PMN-0.1PT films requires high temperatures (∼800°C) and lead excess in the precursor solutions making processing difficult. The correct choice of seeding layers that favor perovskite phase nucleation and growth minimizing substrate instabilities at high temperatures was essential. This approach allowed us to find a narrow processing window that lead to films without pyrochlore and other second any phases. Important PMN and 09PMN-0.1PT processing parameters are reported. It is shown that crystallographic orientation and microstructure could be modified by controlling the perovskite phase nucleation and growth with different seeding layers (TiO2 and PbTiO3). PMN film epitaxial growth on conductive SrTiO3 single crystals allowed an original transmission electron microscopy study. The results showed that PMN in thin film and bulk forms shows the same structural characteristics (morphology and size of polar regions). However, temperature dependence of unit cell parameters of PMN and 09PMN-0.1PT films seems to be qualitatively different from these obtained in bulk materials. PMN and 0.9PMN-0.1PT thin films show typical relaxor characteristics (dielectric permittivity dependence on frequency and temperature, presence of polar regions, superstructure, ferroelectric hysteresis at low temperature) however significant differences exist between films and bulk materials. Most of these differences (low permittivity, presence of the self-polarization) are at least in part related to the properties of the substrate used, as well as the conditions of films preparation. The presented results clearly show the influence of the measuring signal and processing parameters (AC and DC filed amplitude, sintering temperature, substrate quality) on the dielectric response of the films. Due to the small thickness of the films, the non-linearity of the dielectric properties of PMN and 0.9PMN-0.1PT could be studied over a field range never before used in the characterization of bulk samples. The choice of measuring conditions (field strength) can significantly alter the dielectric behavior of the films, and the use of large fields can explain certain results published in the literature. By investigating the AC and DC field dependence of the dielectric permittivity for oriented films it was possible to show that the decrease of the permittivity with the field, at strong fields, is consistent with the model of coalescence of neighboring polar regions. For the first time, the electromechanical properties of PMN and 09PMN-0.1PT films were characterized in details. Longitudinal electrostrictive (M and Q) and induced piezoelectric (d33) coefficients were determined for both compositions. Compared to bulk materials, it seems that thin film properties are reduced selectively. The reduction of dielectric permittivity and, consequently, the electrostrictive coefficient M is large, whereas the value of the Q coefficient is close to that in bulk materials. The preparation, dielectric, electrostrictive and piezoelectric properties of PbTiO3 films, as end member of the (1-x)PMN-(x)PT binary system, were characterized in detail. Stresses in the plane of the film, which are due to the clamping of the film by the substrate, are most likely responsible for the decrease in the temperature of the dielectric permittivity maximum. It was found that clamping of the film by the substrate does not have a strong influence on the value of the electrostrictive coefficients. The contribution of domain-walls displacement to dielectric and piezoelectric properties was investigated in detail and found to be non-negligible.

EPFL2002

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

Dragan Damjanovic, Nava Setter

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.