Lavinia Curecheriu

Grain size effect on the nonlinear dielectric properties of barium titanate ceramics

The nonlinear dielectric properties of dense BaTiO3 ceramics with grain size of 1 μm–90 nm were investigated. In the finest ceramics, the permittivity reduces below 1000 and a remarkable nonhysteretic linear dc-tunability [e(E)] is obtained at high field, above 40 kV/cm. The observed behavior was explained by considering the nanostructured ceramic as a composite formed by ferroelectric grains, whose nonlinearity is reducing, and by low-permittivity nonferroelectric grain boundaries, whose volume fraction increases when decreasing the grain size. Reducing the grain size in ferroelectric dense materials is an alternative route to accomplish the application requirements: nonhysteretic tunability and permittivity below 1000.

High-field dielectric properties and Raman spectroscopic investigation of the ferroelectric-to-relaxor crossover in BaSnxTi1−xO3 ceramics

BaSnxTi1−xO3 solid solutions with compositions in the range x = 0–0.20 were studied by combining analysis of the field-induced dielectric and ferroelectric properties with Raman spectroscopic investigations. By combining techniques, the detection of specific features related to the ferroelectric-to-relaxor crossover with increasing Sn content is possible. Detailed tunability analysis of the x = 0.05 composition indicated that multiple components contribute to the dc-field induced permittivity response; these components are active in different temperature and field ranges and could be assigned to a few polarization mechanisms. First order reversal curves (FORC) for the material clearly show a transition from ferroelectric-to-relaxor behavior with increasing x, confirming the conclusions from the Raman and dielectric studies. This was evidenced by the shift of the FORC distribution over coercivities toward zero field values. Raman measurements allow the identification of the separate phases with varying Sn ...

Tailoring non-linear dielectric properties by local field engineering in anisotropic porous ferroelectric structures

A method to control the nonlinear dielectric properties in porous anisotropic ceramics is proposed. The local field distributions in anisotropic porous ceramic structures were determined and the field-induced permittivity was estimated in parallel and perpendicular configurations (with respect to the pore orientation vs. the applied field direction). The predicted tunability behavior agrees well with the measured values obtained for Pb(Zr0.52Ti0.48)Nb0.024O3 ceramics with different anisotropic porosity levels. The paper demonstrates the concept of engineered local fields in porous microstructures for tailoring the permittivity and tunability values and the possibility to increase tunability with reducing permittivity for small porosity levels.

Temperature dependence of tunability of Ba(SnxTi1−x)O3 ceramics

The electric field dependence of the dielectric constant (dc-tunability) as a function of temperature for a few compositions of Ba(Sn,Ti)O3 ceramics was investigated. Dense and homogeneous BaSnxTi1−xO3 ceramics (ferroelectric for x = 0.05 and x = 0.10, relaxor for x = 0.15 and 0.20) with high dielectric constant and low dielectric losses were prepared by solid state reaction. Their dc-tunability characteristics were investigated at a few temperatures from 20–120 °C, including the Curie range. The tunability data were discussed in terms of the Johnson model completed with a Langevin term that describes the “extrinsic” contribution to the nonlinear ɛ(E) dependences.

Investigation of the ferroelectric–relaxor crossover in Ce-doped BaTiO3 ceramics by impedance spectroscopy and Raman study

BaCe x Ti1− x O3 (x = 0.06, 0.10, and 0.20) solid solutions were prepared via conventional the solid-state reaction and sintered at 1500°C for 4 h, resulting in dense single phase ceramics with homogeneous microstructures. The electric field dependence of permittivity of the BaCe x Ti1− x O3 ceramics was investigated in detail, together with the ferroelectric–paraelectric phase transition features. A transformation from normal to relaxor ferroelectrics was observed by increasing the Ce concentration. For low-Ce content, a substitution of Ce on both A and B site positions was proposed from the dielectric study and confirmed by Raman analysis.

Tunable gold-chitosan nanocomposites by local field engineering

A method to control the permittivity and tunability in composite materials formed by conductive nanoparticles embedded in a flexible nonlinear dielectric matrix is proposed. The local field distributions in composite structures were determined and the field-induced permittivity was estimated for different volume fraction of conductive particles. The predicted tunability behavior agrees well with the observed trends obtained for gold nanoparticles embedded in chitosan matrix. The paper demonstrates the concept of engineered local fields in nanocomposites by using metallic nanoparticles as fillers in polymer matrix for tailoring the permittivity and tunability values.

Non-linear dielectric properties of BiFeO3 ceramics

The nonlinear dielectric properties of BiFeO3 ceramics as a function of temperature were investigated. The present results demonstrate a high dielectric tunability in quite low range of the bias electric fields over a broad temperature interval around the room temperature. The non-linear permittivity-field response results from multiple contributions, whose weights are different at various temperatures and in different field ranges.

BaTiO3–ferrite composites with magnetocapacitance and hard/soft magnetic properties

BaTiO3–ferrite multiphase composites were prepared starting from di-phase mixtures of α-Fe2O3 and BaTiO3 powders. During the sintering step, the formation of small amounts of secondary phases with multifunctional character as BaFe12O19 or Ba12Ti28Fe15O84 was promoted. The resulting multiphase ceramic compounds show interesting dielectric, magnetic and small magnetocapacitance effect at low temperature. The coexistence of different magnetic phases with contrasting coercivities (hard/soft) was detected by the presence of ‘wasp-waisted’ M(H) hysteresis loops and first-order reversal curve analysis. The present approach demonstrates that active materials can be realised by controlling in situ reactions at the interfaces in ferroelectric–magnetic oxide composites.

Novel magnetoelectric ceramic composites by control of the interface reactions in Fe2O3@BaTiO3 core-shell structures

In the present work, composite ceramics of ferroelectric BaTiO3 (BT) with magnetic αFe2O3 were prepared from powders synthesized by two different methods: (a) core-shell powders of Fe2O3@BT and (b) Fe2O3-BT composites from mixed powders with the same composition. The M(H) loops at room temperature show a “wasp-waisted” character, with multiple components, determined as result of the formation of magnetic phases with contrasting coercivities (hard BaFe12O19 and soft Ba12Ti28Fe15O84 phases) in different ratios, as indicated by the magnetic first-order-reversal curves analysis. The core-shell composite ceramics generally showed slightly improved dielectric properties and smaller conductivity. The high field properties of composite ceramics show a strong nonlinearity in both cases, together with a reduction of zero field permittivity, making from these composites possible tunable materials interesting for applications. Their multifunctional character is enhanced by the presence of a complex magnetic character...

Using multi-walled carbon nanotubes in spark plasma sintered Pb(Zr0.47Ti0.53)O3 ceramics for tailoring dielectric and tunability properties

The addition of small amounts (below 0.1 wt. %) of multi-walled carbon nanotubes (MWCNTs) to Pb(Zr0.47Ti0.53)O3 (PZT) ceramics prepared by spark plasma sintering is proposed as a method of tailoring the electrical properties, which are expected to be modified with respect to the pure PZT, both as result of the presence of 1-D conductive fillers in the ceramic product and via the microstructural modifications of ceramics induced during the sintering. The addition of even small amounts of carbon nanotubes strongly reduced the sinterability of PZT ceramics and resulted in the porous and fine-grained microstructures (relative density of 73% for a MWCNT addition of 0.5 vol. % by comparison with 91% in the pure PZT, produced in the same conditions). A monotonous decrease of permittivity with increasing the MWCNT level from ∼830 in pure PZT to ∼627 for x = 0.5 vol. %, at a fixed frequency f = 1kHz, and low dielectric losses below 2% have been observed. Tunability increases with respect to the values of dense PZT...