Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005

High Temperature Behavior of Nano-Structured Ceramic Composites Studied by Mechanical Spectroscopy

Mehdi Mazaheri

Silicon nitride based ceramics (SiAlONs); tetragonal polycrystalline zirconia (3Y‐TZP); alumina and their composites reinforced with different amount of multi‐ walled carbon‐nanotubes (CNTs) have been processed by Spark Plasma Sintering (SPS). High temperature mechanical spectroscopy measurements were performed in each material between room temperature and 1600 K. In each of these materials, anelastic and viscoplastic relaxation phenomena were investigated and responsible mechanisms were explained. In particular, grain boundary (GB) sliding, which is responsible for high temperature plasticity in fine grained ceramics, gives rise to a peak or an exponential increase in the high temperature mechanical loss. Peak and exponential background depend on two forces, which control the GB sliding: a friction force due to the GB viscosity and a restoring force due to the elasticity of the surrounding grains. In the present thesis, ceramics and composites have been processed, which allow one to study the role played by these forces on the thermo‐ mechanical behavior of these refractory materials. SiAlON ceramics were chosen for studying the viscous force due to the inter‐granular glassy phase. In zirconia and alumina, it is shown how the CNT reinforcements may improve the restoring force and consequently the creep resistance. Different grades of SiAlON ceramics, processed with different sintering aids (Ca2+, Y3+, Yb3+), with oxygen rich (CaO, Y2O3 and Yb2O3) and nitrogen rich (CaH2, YN and YbN) compounds, have been studied. Mechanical spectroscopy has been used to analyze the behavior of the residual glassy phase, present after sintering, either as grain‐boundary glassy (GB) films or glass pockets located in GB triple junctions. The mechanical loss spectra show a relaxation peak, which is due to a relaxation phenomenon (the so called “α‐relaxation”) associated with the glass transition in the amorphous phase. The peak position depends on the glass viscosity and the peak height is mainly affected by the glassy phase amount and the SiAlON bimodal microstructure, namely equiaxed versus elongated grains. Moreover the peak height depends on the restoring force provided by the neighboring grains, which limit the GB sliding process. As a matter of fact, a good correlation between the mechanical loss peak and plastic deformation in a compression test has been observed. It is concluded that the elongated grains in the bimodal microstructure provide a higher restoring force, which limits the GB sliding of smaller equiaxed grains. In the case of equiaxed fine grained oxide ceramics, such as zirconia and alumina, the restoring force has been increased by CNT additions, which can reinforce the GBs. 3Y‐TZPcomposites with a homogenous distributions of CNTs, ranging within 0.5 – 5 wt%, were processed by SPS. A significant improvement in room temperature fracture toughness and shear modulus as well as creep performance at high temperature were obtained, and interpreted as due to the GB reinforcing role of the CNTs. To support this interpretation, high‐resolution electron microscopy and Raman spectroscopy have been carried out. Moreover, a remarkable enhancement of the electrical conductivity up to ten orders of magnitude due to CNT additions has been obtained with respect to the pure ceramics. The isothermal spectrum of the 3Y‐TZP composites (measured at 1600 K) is composed of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at lower frequency. This is interpreted as primarily due to GB sliding. The absence of a well‐marked peak in monolithic 3Y‐ TZP is justified by considering that the restoring force decreases at low frequencies or high temperatures. Therefore, GB sliding is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. With CNT additions the mechanical loss decreases and the relaxation peak is better resolved with respect to the background. This is interpreted by the pinning effect of CNTs on GBs, providing addition source of restoring force, which can hinder GB sliding at high temperature, resulting in a creep resistance improvement. Similarly to 3Y‐TZP based composites, alumina specimens (3 different types of alumina powders) reinforced with different amount of CNTs were sintered by SPS. It is shown how the initial particle size of the powders may affect the dispersion of CNTs. Measurement of different properties, such as hardness, fracture toughness and mechanical loss at high temperature, have evidenced the crucial role of the CNT dispersion in the obtained mechanical properties.

EPFL2013

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