L. Pardo

(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.

Stress-induced suppression of piezoelectric properties in PbTiO3:La thin films via scanning force microscopy

Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity o...

Synthesis and sintering improvement of Aurivillius type structure ferroelectric ceramics by mechanochemical activation

An oxide mixture of composition 3Bi 2 O 3 :Nb 2 O 5 :2TiO 2 has been mechanochemically activated in a laboratory mill for different times from 3 to 336 h. The as-milled powder and an unmilled mixture of identical composition were annealed at different temperatures up to the formation of the Aurivillius-type oxide Bi 3 NbTiO 9 , and examined by X-ray diffraction (XRD) and differential thermal analysis (DTA). The sample milled for 336 h shows complete amorphization. Its DTA curve exhibits two exothermic process at 370 and 560°C, corresponding to the formation of a metastable fluorite phase and the Aurivillius-type oxide, respectively. The crystallization temperature of this Aurivillius phase from the unmilled oxide mixture is reported to be 1050°C, whereas from the amorphous powder obtained by 336 h of mechanochemical activation this temperature becomes as low as 600°C. Ceramics of this composition must be prepared by hot-pressing in order to obtain low porosities, owing to the lamellar morphology of the conventionally crystallized powder, which gives rise to textured anisotropic materials. Non-textured ceramics were obtained both from the conventionally crystallized and the amorphous powder by natural sintering at 1100°C. Ceramics obtained from amorphous powder show lower porosity and higher electromechanical coupling factors.

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.

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...

Room temperature stabilisation of γ-Bi2VO5.5 and synthesis of the new fluorite phase f-Bi2VO5 by a mechanochemical activation method

Mechanochemical activation followed by annealing at moderate temperatures results in the stabilisation at room temperature of the high ionic conductor oxide γ-Bi2VO5.5 belonging to the Bi–VV–O2 system, while in the Bi–VIV–O2 system a non-previously reported fluorite-type f-Bi2VO5 phase is isolated. All α-, β- and γ-Bi2VO5.5 and f-, α- and β-Bi2VO5, as well as amorphous powders with 2Bi2O3∶V2O5 and Bi2O3∶VO2 compositions, are studied by X-ray powder diffraction at ambient and high temperatures, thermal analysis and scanning electron microscopy.

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).