B. Jimenez

(Bi3TiNbO9)x(SrBi2Nb2O9)1−x aurivillius type structure piezoelectric ceramics obtained from mechanochemically activated oxides

Ferroelectric ceramics of (Bi3TiNbO9)x(SrBi2Nb2O9)1−x compositions are proposed in this work for use as high temperature piezoelectrics. Novel compositions with x>0.40 have been processed and studied. This type of compound grows in lamellar crystals, resembling their layered Aurivillius type structure, with the perovskite c-axis perpendicular to the major surfaces, which tend to pile up with the c-axis parallel to the applied pressure when ceramics are obtained by hot pressing. Such texture does not favor the ferro-piezoelectric properties. Isotropic ceramics with low porosity were processed by natural sintering of a mechanochemically activated oxide mixture. Optical microscopy and computer-aided image analysis and measurements were used in the characterization of the ceramic microstructure. Ceramics with transition temperatures in the range of 420°C≲Tt≲900°C can be obtained. Ferro-piezoelectric characterization shows that ceramics with x=0.60 present Tt=700°C, d33=11 pC N−1 and Kt=9.45%, which make them good candidates to be tested as high temperature piezoelectrics.

Influence of calcium on the ferroelectricity of modified lead titanate ceramics

Abstract Lead titanate ceramics modified with 24, 30, 35 and 40 mole% of calcium have been obtained. The thermal behaviour of permittivity as well as that of pyroelectric coefficient present a larger broadening the larger the Ca content, which could be adscribed to an increasing disorder on the Ca and Pb arrangement. On the other hand the diminishing of tetragonality with increasing Ca amount causes the decrease of coercive field, which favors the 90° domain reorientation; the moderate strains produced on poling could justify the minimum poling reversal effects on piezoelectricity.

Sodium niobate ceramics prepared by mechanical activation assisted methods

Abstract Sodium niobate is an interesting material that is nowadays considered to be used as a lead-free piezoelectric ceramic. However, it is very difficult to process fully dense ceramics of this composition starting from crystalline powdered precursors. It seems a priority to develop a new synthesis route for the processing of high quality ceramics. A new synthesis method based on high-energy milling, not yet explored for this type of materials, has been applied to obtain NaNbO 3 . Stoichiometric mixtures of analytical grade Na 2 CO 3 and Nb 2 O 5 were mechanically activated in a vibrating-type mill. A progressive broadening of the different X-ray diffraction peaks was observed when the milling time was increased. The starting products did not become amorphous. The thermal behaviour of the milled samples was studied by high temperature in situ X-ray powder diffraction and thermal analysis techniques. Ceramic materials were processed by natural sintering and hot-pressing or a combination of both. Ferro-piezoelectric properties were measured to assess their practical use as piezoceramics.

Dielectric and mechanoelastic relaxations due to point defects in layered bismuth titanate ceramics

Complex permittivity and Youngs modulus provide relevant information on the role of point defects in the dielectric and mechano-elastic properties of ferroelectric materials. Low-frequency measurements as a function of the temperature performed on Bi4Ti3O12 (BIT) have shown that point and dipole defects are frozen close to domain walls. Low-temperature dipole defect relaxation processes take place with characteristic times (τ0) of the order of 10-11 s and 10-12 s and activation energies (Ea) of 0.70 eV and 0.65 eV for dielectric and mechano-elastic relaxations, respectively. At higher temperatures new dielectric relaxation peaks appear that can be attributed to jumps of de-iced oxygen vacancies (τ010-11 s, Ea = 1.08 eV, T300 °C) and to vacancy migration (τ010-15 s, Ea = 1.90 eV, T450 °C). Elastic relaxation peaks are also present close to 300 °C whose activation energy (1.50 eV) and characteristic time (10-15 s) suggest a vacancy migration process. Close to 500 °C with Ea = 2.30 eV and τ010-17 s another relaxation peak, which should correspond to domain wall viscous motion near the phase transition temperature, is observed. The Youngs modulus has a smooth step at T300 °C that we attribute to a change in the mobility of oxygen vacancies with respect to the domain walls. Below 300 °C the vacancies are frozen in the domain walls and they are de-iced and distributed throughout the material at temperatures above 300 °C. The experimental results show that the material is softer when the vacancies are linked to domain walls than when they are distributed throughout the material. The diffusion of vacancies back to the domain wall traps at room temperature takes a long time (days).

Piezoelectric PMN-PT ceramics from mechanochemically activated precursors

Piezoelectric Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) ceramics with 0.2 and 0.35 PT have been prepared from mechanochemically activated powders. Pressureless sintering in air and hot pressing were tested. PbZrO 3 packing was necessary to limit lead oxide losses and to avoid the formation of second phases at the surface when temperatures above 1000 °C were used. Ceramics sintered at 1200 °C presented densities of 90% and a grain size of 4 μm. Hot pressed ceramics hardly showed porosity and had submicron (0.1-0.5 μm) grain size, which make the mechanoactivated powders very suitable for templated grain growth matrices. Electrical and electromechanical characterisation was accomplished. The 0.2 PT composition combined relaxor dielectric behaviour with piezoelectricity after poling. The 0.35 PT ceramics presented a first order ferroelectric-paraelectric transition at 171 °C and a complex transverse piezoelectric coefficient of -(181.8-13.6) pC N -1 at room temperature.

Rayleigh type behavior of the Young's modulus of unpoled ferroelectric ceramics and its dependence on temperature

The dependence on stress of the low frequency Youngs modulus and mechanical losses of unpoled ferroelectric ceramics has been studied as a function of temperature. The Youngs modulus of unpoled Pb(Zr,Ti)O3 (PZT) showed a Rayleigh type dependence, analogous to the one already described for the longitudinal piezoelectric coefficient. This has been associated to ferroelectric/ferroelastic domain wall movements across, and their pinning/depinning on, randomly distributed defects. The Rayleigh coefficient was found to increase with temperature. The activation energy of the Rayleigh process was obtained, which must be related to the pinning energy. The Youngs modulus of Mn doped 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 showed no stress dependence.

Electric and ferro-piezoelectric properties of (SBN)1−x(BTN)x ceramics obtained from amorphous precursors

Abstract Ceramics of compositions within the solid solution system (SrBi 2 Nb 2 O 9 ) 1− x (Bi 3 TiNbO 9 ) x have been obtained for 0≤ x ≤1. The preparation method for the ceramic precursor powder consists of mechanochemical activation by milling of the oxides and carbonates mixture until amorphization. Sintered ceramics obtained from amorphous powders are not textured and have high densifications (up to 98% of theoretical density). Ferro-piezoelectric characterisation of the ceramics has been carried out and the electric properties of ceramics with x =0.50 and x =0.25 compositions were exhaustively studied. Dielectric characterisation shows several anomalies in the dielectric constant and a lineal variation of the Curie temperature as a function of the composition. The anomaly at temperatures close to 500°C, more significant for x =0.25 composition, is, surely, originated by the contribution of any dipolar contribution (pole-defects) to the dielectric constant. The Arrhenius plots of the D.C. conductivity show different behaviour in the heating and cooling run of the first thermal cycle, being less pronounced in subsequent cycling. For x =0.50 composition on heating, four slopes can be clearly drawn, but only one slope appears on cooling from high temperature. Explanations are given based on the point defect thermal evolution and their relation with extended defects (boundary grains and ferroelectric domains).

Transition between the relaxor and ferroelectric states for (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 with x=0.2 and 0.3 polycrystalline aggregates

The transition between the relaxor and ferroelectric states for 0.8Pb(Mg1∕3Nb2∕3)O3–0.2PbTiO3 and 0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 polycrystalline aggregates has been studied by measurements of the dielectric and elastic properties as a function of temperature. The ferroelectric to relaxor transition seems to present a regime of ferroelectric fluctuations that is sharply interrupted by the appearance of the polar nanodomains. It also presents thermal hysteresis, not only in the transition temperature, Tc, but in the characteristic time scale of the ferroelectric fluctuations that is longer on cooling than on heating. The latter effect is much more significant for 0.8Pb(Mg1∕3Nb2∕3)O3–0.2PbTiO3 (Tc=346K on heating), than for 0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 (Tc∼408K on heating). This indicates that there is a sharp slowing down of the kinetics of the phase transition below 346 K.

Evidence of ferroelastic–ferroelastic phase transition in BiMoxW1−xO6 compounds

Through mechanoelastic, thermal expansion, and dielectric measurements as a function of the temperature on ceramics of BiMoxW1−xO6 (x=0, 0.10, and 0.25) solid solutions, anomalies in the corresponding parameters are found below the ferroparaelectric phase transition temperature. One of these anomalies, that corresponding to the temperature close to 940 K, contains mechanoelastic features that strongly suggest that we are dealing with a ferroelastic–ferroelastic phase transition. The behavior of thermal expansion and dielectric constant suggests that this phase transition scarcely affects the macroscopic ferroelectric properties in that temperature range. The origin of the proposed ferroelastic–ferroelastic phase transition could be attributed to the tilting of BO6 octahedra.

Ferropiezoelectricity of calcium modified lead titanate ceramics

When a ceramic with grain Boundaries rich in OPb is poled Beyond 4O kV cm−1 at ferroelectric phase, the highest anisotropy of electromechanical coupling factor is obtained. An attempt to explain this Behaviour is made in terms of domain switching and the strains and cracks generated during cooling down poling. Apart from composition, two more factors are revealed as contributing to the disappearance of radial coupling in Ca-modified lead titanate ceramics: microtexture and electric poling strategy.