Liliana Mitoseriu

Ferroelectric properties of dense nanocrystalline BaTiO3 ceramics

Dense BaTiO3 ceramics with 50?nm average grain size obtained by spark plasma sintering were investigated. The dielectric data show a broad ferro?para phase transition with a maximum permittivity of at 390?K and 1?kHz. The local ferroelectric switching behaviour was investigated by piezoresponse force microscopy. Typical piezoelectric hysteresis loops were recorded at different positions of the sample. The present results provide experimental evidence for polarization switching at the local scale, indicating that the critical grain size for the disappearance of ferroelectric behaviour in dense, bulk BaTiO3 nanocrystalline ceramics is below 50?nm.

Functional properties of BaTiO3–Ni0.5Zn0.5Fe2O4 magnetoelectric ceramics prepared from powders with core-shell structure

In the present work, diphasic ceramic composites with core-shell nanostructures formed by Ni0.50Zn0.50Fe2O4 core and BaTiO3 shell were investigated. Their properties were compared with those of composites prepared by coprecipitation. The core-shell structure was confirmed by microstructural powder analysis. Homogeneous microstructures with a good phase mixing and percolated dielectric phase by the magnetic one were obtained from coprecipitated powders. Less homogeneous microstructures resulted in ceramics produced from the powder prepared by core-shell method, with isolated small ferrite grains besides large ferrite aggregates embedded into the BaTiO3 matrix. Both the ferroelectric and magnetic phases preserve their basic properties in bulk composite form. However, important differences in the dielectric relaxation and conduction mechanisms were found as result of the microstructural difference. Extrinsic contributions play important roles in modifying the electric properties in both ceramics, causing spa...

Analysis of ferroelectric switching in finite media as a Landau-type phase transition

A detailed analysis of polarization reversal in ferroelectrics has been performed, in the framework of the Landau model for phase transitions. Some important characteristics of homogeneous switching have been emphasized and later used in studying the more general case of inhomogeneous switching. The two extremes of switching current correspond to the inflexion points of the dielectric hysteresis loop. Hysteresis loops of poled ferroelectric samples are expected to include negative-susceptibility regions, for high-frequency applied electric fields. The switching current minimum is eliminated by the experimental method used for recording the switching responses. Equivalent Landau coefficients and electric fields have been defined, in order to integrate the size effects and inhomogeneity contribution to switching of the global order parameter. We correlated the size effects on the critical parameters of the switching (the coercive field) and the ferroelectric-to-paraelectric phase transition (the Curie temperature). Polarization reversal in small-size ferroelectrics can be regarded as a diffuse phase transition, whereas its character is closer to normal for large-size samples. The size dependencies of the reversal speed and maximum current result from the size dependencies of the equivalent Landau coefficients and electric field inducing reversal.

First-order reversal curves diagrams for the characterization of ferroelectric switching

A method to describe the switching characteristics of the ferroelectrics is proposed, using the first-order reversal curve (FORC) diagrams. On these diagrams, the reversible and irreversible contributions to the ferroelectric polarization can be clearly separated. They are extremely sensitive to the changes of the hysteresis loops induced by degradation of the ferroelectric polarization, such as fatigue. Sharp in the fresh state, the FORC distribution becomes wide with its maximum shifted towards higher fields after 109 switching cycles. A strong increase of the reversible component was found in the fatigue state. With appropriate interpretation, these diagrams could be valuable as “fingerprints” of the switching characteristics of the ferroelectric systems in a particular state.

Analysis of the composition-induced transition from relaxor to ferroelectric state in PbFe2/3W1/3O3–PbTiO3 solid solutions

The low-field dielectric response of the relaxor-ferroelectric (1−x)PbFe2/3W1/3O3–xPbTiO3 ceramics with various x, was investigated. The permittivity data were analyzed with empirical laws that describe the diffuse phase transitions in relaxors. A change of the character of the phase transition was found with increasing x, from a total diffuse, characteristic of relaxors, to a sharp one, typical of ferroelectrics. The deviations from the Curie–Weiss law of the dielectric constant data in the paraelectric phase were used to calculate a local order parameter within a modified-Landau theory for relaxors. The nonzero values of the local order parameter far above the Curie region indicate the thermal stability of the polar nanoregions in the relaxor state. The temperature dependence of the local order parameter clearly shows the evolution of the system from a short range ordered to a long range ordered ferroelectric, with increasing the PbTiO3 addition.The low-field dielectric response of the relaxor-ferroelectric (1−x)PbFe2/3W1/3O3–xPbTiO3 ceramics with various x, was investigated. The permittivity data were analyzed with empirical laws that describe the diffuse phase transitions in relaxors. A change of the character of the phase transition was found with increasing x, from a total diffuse, characteristic of relaxors, to a sharp one, typical of ferroelectrics. The deviations from the Curie–Weiss law of the dielectric constant data in the paraelectric phase were used to calculate a local order parameter within a modified-Landau theory for relaxors. The nonzero values of the local order parameter far above the Curie region indicate the thermal stability of the polar nanoregions in the relaxor state. The temperature dependence of the local order parameter clearly shows the evolution of the system from a short range ordered to a long range ordered ferroelectric, with increasing the PbTiO3 addition.

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 ...

Intrinsic/extrinsic interplay contributions to the functional properties of ferroelectric-magnetic composites

Recent literature data about the preparation and the functional properties of two-phase ceramic composites comprising of a ferroelectric perovskite and a magnetic ferrite are presented and critically compared, together with new results found in BaTiO3–(Ni,Zn)Fe2O4 composites. The phase assemblage and the spontaneous strain of the ferroelectric perovskite, the microstructural aspects and the phase percolation problem, the functional properties (resistivity, magnetic, dielectric, ferroelectric and magnetoelectric coupling) and the use of effective field models for predicting the permittivity and permeability of these composites are discussed in detail.

Phase coexistence in Pb(Fe2/3W1/3)O3–PbTiO3 solid solutions

The crystalline symmetry of the solid solution formed by a relaxor and a ferroelectric material with formula: (1−x)Pb(Fe2/3W1/3)O3–xPbTiO3, with various x was investigated, at T=300 K. A continuous change from pseudocubic to tetragonal symmetry, with cell parameters dependent on the PbTiO3 amount, was found. In a large range of compositions, from x=0.20 to x=0.37, both phases coexist, with different relative amounts.

Local switching properties of dense nanocrystalline BaTiO3 ceramics

The switching properties of dense BaTiO3 ceramics with 50 nm average grain size were investigated at local scale by piezoresponse force microscopy. Large areas with low piezoelectrical activity beside islands with strong piezoresponse were found. The application of electric fields induces stable domain structures and changes in the polarization state far away from the probing area, probably via trans-granular dipole interactions. Piezoelectric hysteresis loops were recorded on various positions, even in regions with initial zero piezoresponse, which possibly showed a superparaelectric behavior. The results are incontestable proof that 50 nm BaTiO3 ceramics retain ferroelectricity at a local scale.