Poled Composites with Nd-Mn-Doped PZT Fibers Under Bipolar Electrical Load

Using the sol-gel process, Nd-Mn-doped PZT fibers were produced. The PZT was doped with 2 mol% neodymium and 1.1 mol% manganese. For characterization, the fibers were embedded in a polymer. The resulting 1-3 composites were poled with constant electric field. Strain and polarization were measured by applying a bipolar sinusoidal voltage of high amplitude. Instead of the expected shifted butterfly-shaped strain hysteresis, an asymmetric strain-field relation was observed. It is characterized by a rather linear region in direction of the poling field and an inflated region without strain switching for reversed polarity. Within the temperature range from room temperature to 80degC, the strain switching seems to be suppressed. Measurements of the piezoelectric coefficient at superimposed electric field prove the blocking of strain switching. Cyclation experiments with sesquipolar load show a pronounced linearity of the strain loops that declines after more than 2 times 10⁴ cycles.     

BaTiO3–P(VDF-HFP) nanocomposite dielectrics—Influence of surface modification and dispersion additives

We report on BaTiO₃–polymer composites as dielectrics for film capacitors. BaTiO₃ was synthesized by a sol–gel soft-chemistry method leading to spherical nanoparticles with a high degree of surface hydroxyl groups which turned out to be important for the bonding of surfactant molecules. As surfactants, n-octylphosphonic acid and 2,3,4,5,6-pentafluorobenzyl phosphonic acid were used to inhibit particle agglomeration and to improve the wetting behaviour with the polymer. The phosphonic acid-coated BaTiO₃ nanoparticles were dispersed in solutions of poly(vinylidefluoride-co-hexafluoropropylene). Composite films were prepared by the spin-coating technique. A systematic study was performed on the influence of varying oxide fractions, different surfactants and the effect of additional dispersion aids such as sodium dodecyl sulphate or BYK-W 9010 on the quality and dielectric properties of the films obtained. The chemical adjustment of the 2,3,4,5,6-pentaflourobenzyl phosphonic acid within the fluorinated organic host form a more uniform particle distribution and increase relative permittivity of the resulting composite material compared to the unflourinated surfactant. Additionally, an enhancement of the relative permittivity can be realized by adding of dispersants. These two components can increase the relative permittivity by factor 5 compared to the pure polymer material.     

Study of the pyroelectric behavior of BaTi<Subscript>1−<Emphasis Type="BoldItalic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript>O<Subscript>3</Subscript> piezo-ceramics

In this work, the thermal square wave method at single-frequency (TSWM) was applied to functionally graded BaTi_1−x Sn _x O₃ samples with a tin gradient of 0.075 ≤ x ≤ 0.15. The samples were fabricated by sintering of bi-, tri- and tetramorph green bodies. The polarization at the back side with a higher tin content was in opposite direction to the polarization of the surface with a lower tin content. This was attributed to the appearance of a space charge layer. The determined depth profiles of the pyroelectric coefficient illustrate the appearance of a nearly constant polarization in the region of lower tin concentration of tri- and tetramorph samples. The pyroelectric coefficient profiles were in good agreement with the averaged over sample thickness pyroelectric coefficient obtained separately by the quasistatic method.     

Bipolar strain hysteresis of poled composites with Nd–Mn-doped PZT fibres

Nd–Mn-doped PZT fibres were produced using the sol–gel process. The PZT was doped with 2 mol% neodymium and an amount of 1.1 mol% or 0.75 mol% manganese. The fibres were investigated with respect to microstructure, composition after sintering and phase content. Strain and polarisation were measured after imbedding the fibres in a polymer matrix.The resulting 1–3-composites were poled with constant electric field. Measurements of strain and polarisation were done using a sinusoidal voltage of high amplitude. Instead of a shifted strain hysteresis (butterfly loop) an asymmetric strain–field relation was observed. The asymmetry depends on the direction of the applied voltage. For the half wave with voltage parallel to former poling voltage the strain curve is linear. For the other half wave the strain curve inflates and there is a region with no change of strain. Possible explanations for the asymmetric strain behaviour are discussed.     

Modelling and characterization of piezoelectric and polarization gradients

A gradient of the piezoelectric properties in monolithic ceramics can be useful to minimize mechanical stresses inside piezoelectric bending actuators or at interfaces between layers with different electromechanical properties, and increase the bandwidth of ultrasonic transducers. Piezoelectric gradients can be prepared by different techniques. We use a powder pressing technique to prepare a gradient of the chemical composition, which is transformed to the piezoelectric gradient by the poling process. Conventional mixed-oxide powders of BaTiO₃ with different amount of Sn from 7.5 up to 15 mol% and Mn-doped BaTiO₃ were used to produce lead-free ceramic samples. We describe the poling process using a equivalent circuit approach. The ferroelectric behavior of the different layers are described by the Preisach model. In the Ba(Ti,Sn)O₃ system the properties can be changed from ferroelectric to paraelectric depending on the Sn content. A gradient of polarization and piezoelectric properties can be observed in monolithic ceramics with a tin gradient. The profile of the polarization distribution was measured by the thermal wave method. A gradient of polarization where the sign of polarization changes can be induced in a system consisting of piezoelectric hard and soft materials. We describe a special poling process using our poling model. The principle is demonstrated in a two layer model system.