Films minces relaxeur-ferroélectriques à base de Pb(Mg1/3NB2/3)O3:élaboration, propriétés diélectriques et électromécaniques

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

Depolarization of multidomain ferroelectric materials

Depolarization in ferroelectric materials has been studied since the 1970s, albeit quasi-statically. The dynamics are described by the empirical Merz law, which gives the polarization switching time as a function of electric field, normalized to the so-called activation field. The Merz law has been used for decades; its origin as domain-wall depinning has recently been corroborated by molecular dynamics simulations. Here we experimentally investigate domain-wall depinning by measuring the dynamics of depolarization. We find that the boundary between thermodynamically stable and depolarizing regimes can be described by a single constant, Pr/ε0εferroEc. Among different multidomain ferroelectric materials the values of coercive field, Ec, dielectric constant, εferro, and remanent polarization, Pr, vary by orders of magnitude; the value for Pr/ε0εferroEc however is comparable, about 15. Using this extracted universal value, we show that the depolarization field is similar to the activation field, which corresponds to the transition from creep to domain-wall flow.Aerospace Structures & MaterialsNovel Aerospace Material

Hydration and structure development of ternary binder system as used in self-levelling compounds

Self-levelling flooring compounds (SLC) are applied in thin layers (less than 1cm) to level floors before further finishing cover such as parquet or plastic cover. They are composed of a ternary binder system based on Portland cement (PC), calcium aluminate cement (CAC) and calcium sulfate to provide fast set. This mixed binder is modified by additives in order to obtain specific properties such as flow properties or defoaming. In total, more than 10 components make up the formulations of SLCs. SLCs are one of the most complex systems in the field of cementitious materials. It is known that ettringite is the main hydration product of mixed binder systems but the hydration development at early ages is little known. The main objective of this work is thus to clarify the mechanisms of the development of the hydration at early ages of ternary binder systems modified by additives. Two formulations were studied: a system in which the mixed binder system is dominated by PC and one in which it is dominated by CAC. Both systems exhibit the formation of ettringite, but the formation of this phase is mainly due to the hydration of monocalcium aluminate CA from the CAC. Thus, the PC formulation is not robust. Parameters such as the minor phases of the PC and the type of the calcium sulfate have a major impact on the hydration development. As CA is a minor constituent of this mixed binder system, the amount of ettringite formed is low. Thus, the PC dominated formulation reacts relatively slowly. The CAC dominated formulations react faster because the CA content is higher. The porosity is lower and internal and external parameters such as the minor phases of the PC and evaporation affect much less the development of hydration. As the CAC dominated system forms more ettringite, the chemical shrinkage is higher but the autogenous expansion measured is lower than that observed for the PC dominated formulations. Calcite is also added in the formulation in large amount as a filler to fill the porosity. The formation of monocarboaluminate from the reaction of calcite is also observed

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

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

Properties of ferroelectric PbTiO3 thin films

PbTiO3 thin films were prepared from alkoxide solution precursors and crystallized onto Pt/TiO2/SiO2/Si substrates. Microscopy observations revealed that the complexity of the domain walls structure decreased with the grain size. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in details. A shift of the temperature of the dielectric permittivity maximum due to stresses in the plane of films has been observed. Electrostrictive M and Q coefficients were estimated by measuring strain as a function of the ac electric field amplitude. The d(33) vs E-dc loops are rectangular with a maximum weak field piezoelectric d(33) coefficient equal to 65 pm/V. The dielectric permittivity and piezoelectric nonlinearities can be explained by taking into account domain-walls contributions. Dielectric and piezoelectric aging was investigated. It was found that both coefficients follow logarithmic time dependence, with comparable rates. The aging behavior in the PT films is thus qualitatively closer to that in ceramics than in thin films of lead zirconate titanate.(C) 2002 American Institute of Physics

Electromechanical properties and self polarization in relaxor Pb(Mg1/3Nb2/3)O-3 thin films (vol 89, pg 1393, 2001)

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

Preparation and electromechanical properties of Pb(Mg1/3Nb2/3)O-3 thin film

Pb(Mg1/3Nb2/3)O-3 (PMN) alkoxide precursor solutions were synthesized and used to prepare thin layers on TiO2/Pt/TiO2/SiO2/Si substrates by spin coating. Many parameters like homogeneity of the solution and appropriate processing must be controlled to minimize the pyrochlore formation. Optimization of the processing allowed highly (111)-oriented thin films to be obtained. Dielectric measurements and TEM investigations were performed and showed that PMN thin films exhibit relaxer behavior

Relaxor behavior and electromechanical properties of Pb(Mg1/3Nb2/3)O-3 thin films

Pb(Mg1/3Nb2/3)O-3 (PMN) alkoxide precursor solutions were synthesized and used to prepare thin films by spin coating on TiO2/Pt/TiO2/SiO2/Si substrates. Many parameters like the use of homogeneous and stable precursor solutions and appropriate processing were used to greatly reduce the presence of the nonferroelectric pyrochlore phase. Transmission electron microscopy investigations, dielectric, electrostrictive, and direct current field induced piezoelectric measurements were carried out and have shown that PMN thin films exhibit a relaxor-like behavior. (C) 1998 American Institute of Physics. [S0003-6951(98)03142-8]