C. Alemany

(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.

Automatic determination of complex constants of piezoelectric lossy materials in the radial mode

An iterative automatic method is described for the characterization of lossy piezoelectric materials in the radial resonance mode based on the use of the more general expression for the complex admittance. From the experimental data of Y at four adequately selected frequencies, the constants of the material are determined with the necessary accuracy to reproduce the piezoelectric behaviour of the sample around resonance. The IEEE-176 Standard procedure has been automatized for the initial estimation of the real parts of the elastic constants. The method is applicable even to those materials in which said standard does not allow one to determine the piezoelectric constants.

Automatic iterative evaluation of complex material constants in piezoelectric ceramics

An automatic iterative method by which material constants of piezoelectric ceramics may be determined in complex form is described. Measurements are reported on samples with different shapes: thickness expansion and shear plates, and length expansion bars. With the constants provided by this method, the profiles for the conductance, resistance, susceptance and reactance are calculated, for the same sample, in the frequency range containing resonance and anti-resonance, and very good agreement is obtained with respect to the experimental data.

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.

Theoretical treatment of ferroelectric composites using Monte Carlo calculations

Measurements of ferroelectric composites made from lead zirconate titauate (PZT) ceramic particles and an epoxy resin as matrix have allowed one to establish the constitutional parameters of the composite that characterize the pyroelectric and piezoelectric properties of these materials. The concept of the degree of heterogeneity is introduced to complete the description of these composites, which do not present a defined connectivity. A model is proposed based on consideration of the randomness of the particle distribution in the matrix by using the Monte Carlo method in the calculation of the active volumetric fraction of ceramic. An equivalent circuit of the composites is used to describe their dielectric properties. The model allows one to estimate the effect of the degree of heterogeneity on the properties of the composite. The results of the calculations compare well with the experimental values. Thus the model opens possibilities for the prediction of the properties and design of two‐phase ferroele...

Piezoelectric ceramics based on Bi3TiNbO9 from mechanochemically activated precursors

Abstract Ceramics based on the Aurivillius type structure compound Bi 3 TiNbO 9 were processed by natural sintering and hot pressing of amorphous precursors obtained by mechanochemical activation of oxides and carbonates mixtures. Synthesis, grain growth and sintering take place in a single thermal treatment at moderate temperatures in comparison with ceramics processed from crystalline precursors. The influence of the processing parameters on the ceramic texture and microstructure at mesoscopic scale were studied by XRD and quantitative optical microscopy. It was possible to obtain both isotropic and textured ceramics. The occurrence of abnormal grain growth was observed under some conditions. An ample electrical characterisation of the ceramics was carried out comprising dielectric, ferroelectric and piezoelectric properties. The influence of the microstructure on the properties and the interest of the materials as high temperature piezoelectrics are discussed.

Contributions to the knowledge of calcium-modified lead titanate ceramics

Abstract It will be summarized here the work of the Ferroelectric Materials Group of this Institute on ceramics of calcium modified lead titanate during the past few years.

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.

Determination of the frequency dependence of characteristic constants in lossy piezoelectric materials

The frequency dependence of the elasto - piezo - dielectric constants of piezoelectric materials is usually neglected. By means of a new procedure of piezoelectric characterization based on the characterization in the overtones of resonant modes we can measure accurately the constants involved in the piezoelectric behaviour of the materials in a wide range of frequencies. The characterization in the overtones of resonant modes is possible by using an automatic iterative method, described below, that takes account of the number of the overtone to obtain the appropriate solution of the equation system. The proposed procedure has been checked for the radial resonances in materials that exhibit very different behaviour and very good agreement is obtained in all the overtones not overlapped with the thickness extensional mode of resonance.

Pulsed hysteresis loops on ferroelectric thin films

An alternative method to obtain hysteresis loops for ferroelectric thin films is described that overcomes the drawbacks of the classical methods. In this method, a pulsed signal, obtained by mixing a sinusoidal voltage of low frequency with a square voltage of variable period and width, is applied to a (Pb0.76Ca0.24)TiO3 thin film. Electric field is applied to the sample for a short time and so, contribution of extra charges to the hysteresis loops is very small. The experimental loops measured only need a small compensation.